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Chapter 20 How is radiation used to maintain human health? © John Wiley & Sons Australia, Ltd

CHAPTER 20 How is radiation used to maintain human health?

Contents Ionising radiation

Ionising radiation and living things

How much radiation is too much?

Absorbed dose

Equivalent dose

Effective dose

Radioactivity as a diagnostic tool

Choosing the right isotope

Labelling with isotopes

Targeting body organs

Obtaining the image — SPECT

Some applications of radioisotopes

Safety issues

X-rays in medical diagnosis

What are X-rays?

Production of X-rays

Use and detection of X-rays

Effect of X-radiation on the body

Hard and soft X-rays

Using conventional X-rays as a diagnostic tool

How is a CAT scan produceed?

Comparing CAT scans and conventional X-rays

Safety first

Positron emission tomography (PET)

Positron–electron interactions

How a PET scan is obtained

Magnetic resonance imaging (MRI)

How MRI works

The use of MRI in medical diagnosis

Imaging methods working together

Comparison of imaging techniques

Chapter review

Summary

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Questions

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CHAPTER 20 How is radiation used to maintain human health?

Using computer analysis, the data from images of ‘slices’ through the body can be combined to produce a three-dimensional image of the area under investigation.

REMEMBER

Before beginning this chapter, you should be able to:

■ recall that all matter is made up of atoms

■ explain the arrangement of particles in an atom, in particular that atoms have a central nucleus containing protons and neutrons

■ recall that energy cannot be created, just transformed from one type to another or transferred from one object to another.

■ recall the features of a wave, including speed, frequency and wavelength

■ distinguish between transverse and longitudinal waves

■ outline the properties of waves, including reflection, refraction and scattering

■ describe what is meant by critical angle and outline the conditions needed for total internal reflection

■ recall the nature of alpha, beta and gamma radiation

■ describe what is meant by half-life.

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KEY IDEAS After completing this chapter, you should be able to:

■ describe how x-rays are produced

■ describe the use of x-rays and CAT scans in medical imaging and diagnosis

■ recognise and name radioisotopes that are used to obtain scans of organs

■ describe the use of PET scans in medical diagnosis

■ explain how MRI works and how it is used in medical diagnosis

■ compare the information obtained from images produced by, x-rays, CAT, PET and MRI scans

■ make simple interpretations of images produced by ultrasound, x-rays, CAT, PET and MRI scans

■ discuss the advantages and disadvantages of imaging techniques in medical diagnosis

The models used by physicists to explain the behaviour of electromagnetic radiation, sound waves and the nucleus of the atom have provided the basis for huge improvements in medical diagnosis and treatments during the last century. In this chapter we will look at how ultrasound, x-rays and some radioisotopes are used in medical imaging and diagnosis. We will also look at the use of optical fibres in endoscopes and the use of lasers in medical treatment.

Ionising radiation Ionising radiation is the collective name given to α and β particles, neutrons that have been released from the nucleus, γ rays and x-rays. These forms of radiation are grouped together because they have high energies and therefore similar effects on matter. Other forms of electromagnetic radiation, such as radio waves, microwaves and visible light, have lower energies and do not interact with matter in the same way. They are non-ionising radiations.

X-rays are the only type of ionising radiation that are not formed by changes in the nucleus. They result from large energy losses by electrons.

Ionising radiation has sufficient energy to knock an electron from its orbit around a nucleus. Once an electron has been knocked away from the nucleus, the atom has more positive charges (protons) than negative charges (electrons), giving the atom an overall positive charge. Atoms that have an overall charge are called ions, hence the name ionising radiation.

Sometimes the electron that is knocked from the atom is part of a bond between one atom and another. This causes the bond to be broken, and can result in the molecule being split in two. Each piece of the molecule would then have a charge. The charged pieces are called free radicals.

Both ions and free radicals are chemically very reactive. This may result in new chemical reactions taking place inside the substance that was exposed to the ionising radiation.

Ionising radiation and living things The chemical changes resulting from the production of ions and free radicals in living cells can have a range of effects. The cytoplasm (the part of the cell which surrounds the nucleus) has a high water content, therefore it is often water molecules that are broken. This results in the production of H+ and OH– ions, which are chemically very reactive. The ions may react with important molecules, causing damage to DNA, or the mechanisms for

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controlling cell division, or the production of molecules necessary for the life of the cell. Cells undergoing division when they are irradiated are particularly at risk.

Often the cell is able to repair itself, but sometimes the chemical changes cause the cell to die. This is not a problem for a living thing unless a large number of cells are damaged.

If the mechanism for cell division is damaged, the cell may begin to reproduce uncontrollably, forming a cancer. If DNA in the ovaries, the testes, or in an unborn fetus is damaged, genetic mutations may be passed on. Usually these mutations are recessive and are not exhibited.

How much radiation is too much? This seemingly simple question has a very complicated answer. The damage caused by radiation depends on the type of radiation, the rate at which it is received, the part of the body exposed, the general health of the individual and many other factors.

Absorbed dose One measure of the amount of radiation received is the absorbed dose. This is the amount of energy absorbed by each kilogram of the tissue being irradiated. The unit of absorbed dose is the gray, which is given the symbol Gy (1 Gy = 1 joule kg–1).

energy absorbedabsorbed dose

mass=

Unfortunately, the number of grays absorbed by a person does not provide much information about the extent of the damage to that person. The penetrating power of the type of radiation is important. For example, alpha (α) particles are stopped in a short distance. They pass on all their energy in a short space, causing a great deal of localised damage. Beta (β) particles are more penetrating, so the damage they cause is less severe in any one area but is more widespread. Neutrons, β rays and x-rays are far more penetrating than either α or β particles. They spread their energy over a large range.

The damage caused by ionising radiations of different penetration powers in a block of carbon (as observed through an imaginary microscope)

Equivalent dose To take into account the different styles of damage caused by the various forms of ionising radiation, another measure of the amount of radiation, equivalent dose, was developed. The units for equivalent dose are sieverts (Sv).

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equivalent dose (Sv) = absorbed dose (Gy) × quality factor

The quality factor is determined by the type of radiation that delivered the energy.

TABLE 20.1 Quality factors for different types of radiation

Type of radiation Approximate quality factor

γ rays and x-rays 1

β particles 1

Slow neutrons 3

Fast neutrons 10

α particles 10 to 20

One sievert of radiation causes the same amount of biological damage, no matter what type of radiation is used.

SAMPLE PROBLEM 20.1

A 60 kg person absorbs 0.054 J of energy due to ionising radiation.

a. Calculate the absorbed dose. b. What would be the dose equivalent if the energy was delivered by γ rays? c. What would be the dose equivalent if the energy was delivered by α particles? (Take the quality factor to

be 20.) d. Which would cause more biological damage to the person?

Solution: a.

1

energy absorbedabsorbed dose

mass0.054 J60 kg

9 10 Gy−

=

=

= ×

b. 4

4

dose equivalent absorbed dose quality factor

9 10 Gy 1

9 10 Sv

0.9 mSv

−

−

= ×= × ×= ×=

c. 4

2

dose equivalent absorbed dose quality factor

9 10 Gy 20

2 10 Sv

20 mSv

−

−

= ×= × ×= ×=

d. The dose equivalent of 20 mSv delivered by the α particles would cause about 20 times more damage than the 0.9 mSv delivered by the γ rays.

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REVISION QUESTION 20.1 On average, each crew member on the Apollo space missions received a dose of 12 mSv while in space. The exposure was mainly electrons and gamma rays. Estimate an astronaut’s mass and determine how much energy each astronaut absorbed.

Effective dose Radiation affects different parts of the body in different ways. Each organ or tissue in the body has a different sensibility to radiation doses. For example, the head is less sensitive than the chest.

The effective dose is a number that is calculated for an individual patient. This number takes into account the absorbed dose, the quality factor (relative harm level) of the different types of radiation, and the sensitivity of each organ or tissue type to the different types of radiation. It also takes into account the fact that different parts of the body will not receive the same amount of radiation when undergoing a medical procedure.

The calculation of the effective dose helps to estimate the risk to a patient from a procedure. The actual risk to an individual patient will also depend on such factors as the size and age of the patient.

The effective dose for a patient is the total of the equivalent doses for all the different parts of the body. Effective dose is measured in sieverts.

Radioactivity as a diagnostic tool The best known use of radioactive materials in medicine is probably in the ‘radiotherapy’ treatment of cancer. Less well known is the use of a radioactive material inside the body to diagnose disease. This use of radioactive material in the body may seem very risky because of the danger associated with radioactivity. In fact the use of radioisotopes and, more recently, PET (positron emission tomography) to image organs and study their function has become a very common, effective and safe means of diagnosis. The image in the figure at left, taken using a radioactive tracer, shows a tumour in the patient’s left kidney.

For the purposes of medical diagnosis, radioactive substances may be introduced into the body and used to target areas of interest. The radiation produced is measured and used to determine the health of the organ or section of the body under investigation.

SAMPLE PROBLEM 20.2

A 20 mg sample of iodine-123 is to be used as a radioactive tracer in the body. The half-life of the iodine-123 is 13 hours.

a. How long will it take for 17.5 mg to decay? b. Calculate how much iodine-123 will remain after 26 hours.

Solution: a. In one half-life, 10 mg of iodine-123 will decay. This will leave 10 mg of iodine-123.

In the second half-life, 5 mg of iodine-123 will decay, leaving 5 mg of iodine-123.

In the third half-life, 2.5 mg of iodine-123 will decay.

Altogether, 17.5 mg (10 + 5 + 2.5 mg) of iodine will have decayed in 3 half-lives or 39 hours.

b. 26 hours is two half-lives (2 × 13 hours).

After one half-life, 10 mg of iodine-123 will decay, leaving 10 mg of iodine-123.

After two half-lives, 5 mg of iodine-123 will decay, leaving 5 mg of iodine-123.

So 5 mg of iodine-123 will remain after 26 hours.

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REVISION QUESTION 20.2

A radioisotope with a half-life of 13 hours is used in the diagnosis of a patient. A check 52 hours later reveals that 1.0 mg of the radioisotope remains.

a. What mass of the radioisotope was used in the diagnosis? b. How much of the radioisotope will remain after a further 52 hours?

SAMPLE PROBLEM 20.3

A sample of a radioisotope has a half-life of 10 minutes.

a. Calculate the time it will take the radioisotope’s activity to drop from 8.0 MBq (mega becquerels) to 4.0 MBq.

b. Calculate the time it will take for its activity to be 1.0 MBq.

Solution:

a. When half the sample has decayed, the activity will also halve. This assumes that the atoms formed are not radioactive. Hence the time needed to reduce the activity to 4.0 MBq is one half-life, or 10 minutes.

b. Halving the activity each half-life means that three half-lives have passed before the activity is 1.0 MBq. The time taken is 30 minutes.

REVISION QUESTION 20.3 A sample of a radioisotope with a half-life of 8 days has an activity of 8 MBq 16 days after it is placed in safe storage.

a. What was the activity of the sample when it was placed in safe storage? b. What is the activity of the sample after a further 16 days? c. How long will it take after the sample is placed in safe storage for its activity to decrease to 1

MBq?

Choosing the right isotope When a radioisotope is introduced into the body, other factors in addition to the half-life of the radioisotope need to be considered. The radioisotope is removed from the patient’s body by processes such as respiration, urination and defecation. However, some patients metabolise the chemical to which the radioisotope is attached more quickly than others, so it is important that the characteristics of the particular patient are considered when dosages are being determined.

The half-life of the radioisotope must be long enough to allow useful readings to be taken after it has been taken up by the targeted organ. Generally, if the radioisotope remains in the patient’s body for a long period of time, its half-life should be comparable to the time taken to carry out the investigation, to minimise the dose to the patient. When the radioisotope is excreted in about the same time as is needed for the investigation, a longer half-life radioisotope can be safely used.

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Radioisotopes that emit alpha particles are not used in the diagnosis of disease because the alpha particles cause damaging ionisation inside the body. Beta particles travel further than alpha particles before they are absorbed but their ionisation damage is much less. They are used in therapy but not in diagnosis of disease.

The most useful radioisotopes for nuclear medicine are those that emit gamma radiation only. Technetium-99m and iodine-123 are two such isotopes. A gamma-emitting radioisotope inside the body can be detected outside the body because gamma radiation is very penetrating. Common radioisotopes used in medical diagnosis are listed in table 20.2.

Labelling with isotopes The radioisotope chosen needs a way of getting to the target organ. First, it must be chemically attached to a compound that would normally be metabolised by the organ of interest. When this compound is ‘labelled’ with the radioisotope (they are chemically attached) it is called a radiopharmaceutical. The radiopharmaceutical that is used must not alter the functioning of the organ or the area being examined. It must be sterile, non-toxic and compatible with the body. For example, glucose is a compound that is readily absorbed by the brain. Hence glucose is labelled to become a suitable radiopharmaceutical for imaging brain function.

TABLE 20.2 Radioisotopes used in medical diagnosis

Radioisotope Production site

Half-life Function

Chromium-51 Nuclear reactor 27.70 days

To label red blood cells and measure gastro-intestinal protein loss

Iodine-131 Nuclear reactor 8 days To diagnose and treat various diseases associated with the thyroid gland; used in the diagnosis of the adrenal medullary; used for imaging some endocrine tumours

Iodine-123 Cyclotron 13 hours To monitor thyroid function, evaluate thyroid gland size and detect dysfunction of the adrenal gland; to assess stroke damage

Molybdenum-99 Nuclear reactor 65.94 hours

Used as the ‘parent’ in a generator to produce technetium-99m, which is the most widely used isotope in nuclear medicine

Technetium-99m

‘Milked’ from molybdenum-99

6 hours To investigate bone metabolism and locate bone disease; assess thyroid function; study liver disease and disorders of its blood supply; monitor cardiac output, blood volume and circulation clots; monitor blood flow in lungs; assess blood and urine flow in kidneys and bladder; investigate brain blood flow and function; estimate total body plasma and blood count

Thallium-201 Cyclotron 3.05 days To detect the location of damaged heart muscles

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PHYSICS IN FOCUS

Producing radioisotopes: the medical cyclotron

The effectiveness of nuclear medicine for diagnosis of disease has depended on the ability to:

• produce radioisotopes • detect the gamma radiation produced.

The production of radioisotopes became possible with the development of the cyclotron by E. O. Lawrence in 1931. From the mid-1940s, a range of radioisotopes from the United States and later from the United Kingdom was available.

The radioisotopes needed in nuclear medicine in Australia are made at the nuclear research reactor based at Lucas Heights in the south of Sydney, or in a cyclotron under the control of ANSTO (Australian Nuclear Science and Technology Organisation). Cyclotrons are needed to make radioisotopes for positron emission tomography (PET), a diagnostic technique discussed later in this chapter (pages 428–429). PET facilities are presently found in hospitals in Brisbane, Melbourne and Sydney.

A cyclotron for radioisotope synthesis in a clinical medical centre

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PHYSICS IN FOCUS

Radioisotopes emitting gamma radiation

Both iodine-123 and technetium-99m are valuable radioisotopes because they decay by the emission of gamma radiation only.

Iodine-123 is more expensive than iodine-131, which has been used in the investigation of the thyroid gland. However, iodine-131 emits both gamma and beta radiation, leading to larger radiation doses than desirable. The half-life of 8 days for iodine-131 is relatively long, resulting in exposure of the patient to radiation well after the testing has been carried out. By contrast, iodine-123 has a short half-life of 13 hours.

Technetium-99m has a short half-life of only 6 hours so it must be produced in the hospital where it is to be used. A purpose-built generator system is used to obtain the technetium-99m when it is needed. The generator contains the ‘parent’ isotope molybdenum-99, which decays to the excited ‘daughter’ isotope technetium-99m. The technetium-99m is flushed from the molybdenum using a saline solution and the molybdenum remains in the generator as it is chemically attached to a central column (the figure at left shows a cross-section through a technetium generator).

The technetium-99m is said to be ‘milked’ from the molybdenum. This operation usually happens daily, allowing the technetium sufficient time to build up. As the molybdenum has a half-life of approximately 66 hours it must be replaced weekly because, by that time, the rate of production of technetium is too low to be of value.

Technetium-99m has the added advantage that it readily attaches to different compounds to form radioactive tracers.

A cross-section through a typical technetium generator used in hospitals to generate technetium-99m

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Targeting body organs As indicated in table 20.2 (page xxx), particular radioisotopes are chosen to target particular organs. The radiopharmaceutical is injected into the bloodstream, inhaled or taken orally and its passage through the body can be traced by measuring the radiation it emits.

Sometimes an image is taken after a period of up to several hours, to allow the radioisotope to accumulate in the target organ. This image measures the amount of radiation emitted from the target organ and shows where the radioisotope has accumulated.

In other situations, a series of images are taken over a period of time, starting from when the radioisotope is first introduced. This type of investigation shows the distribution of the radioisotope and the rate of absorption or excretion by various organs. The images may be taken over a few minutes for a heart or lung study, or over a period up to half an hour for kidney or bladder investigation.

In analysing the images, radiographers identify ‘hot spots’ with a higher than normal concentration of radioisotope and ‘cold spots’ showing a lack of radioisotope. These areas often indicate disease (see the figure at left).

Radiopharmaceuticals can also be used for bone scans with images taken by a gamma camera.

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Obtaining the image — SPECT. The image is obtained by measuring the amount of gamma radiation coming out of the patient’s body using a gamma camera. The gamma camera is stationary and collects gamma radiation over a large area. It converts the gamma rays into light flashes (scintillations) which are transformed into amplified electrical signals. These signals are decoded and converted to an image on a computer screen. This process is known as single photo-emission computed tomography (SPECT).

A gamma camera used for radioisotope imaging

Some applications of radioisotopes

Thyroid investigations

The thyroid gland metabolises iodine. A dilute solution of sodium iodide tagged with iodine-123 is given to the patient to drink and the radioisotope’s accumulation is measured over a 10 minute to 48 hour time interval. An image of the goitre may be obtained, as in the figure above left, or the uptake of the isotope may be graphed and compared with a standard, as in the figure at left.

Thyroid investigations now commonly use technetium-99m, which is also taken up readily by the thyroid but is also more readily released than iodine-123.

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Uptake of iodine-123 by the thyroid gland

The heart

Human serum albumen is labelled with technetium-99m and injected into the patient. The passage of the radiopharmaceutical through the heart chambers is monitored to measure the efficiency of the heart as a pump.

Thallium-201, as part of thallium chloride, can be injected and monitored to assess damage caused by a stroke or to measure the effect on the heart of exercise or drugs (see the figure below).

Performance of heart muscle using thallium-201. A series of images produces ‘slices’ through a chamber in the heart. The top row is images taken during exercise and the bottom row is taken when at rest.

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Bones, lungs and brain

Technetium-99m is used in imaging the bones, lungs and brain.

Polyphosphate ions are labelled with technetium-99m and injected, accumulating in bone within an hour. The image shows the function of the bone. Areas of increased blood flow show up as ‘hot spots’. Such areas are frequently associated with disease. Bone imaging often shows up bone tumours and stress fractures earlier than standard X-rays, which show the structure of the skeleton (see the figure below).

(a) An X-ray of a broken leg (b) A bone scan showing (i) a healthy skeleton and (ii) a skeleton with tumours (Note: the white spot on the right arm shows where the isotope was injected.)

Brain studies using technetium-99m as a tracer measure blood flow through the brain, allowing dementia and stroke damage to be identified (see the following figure).

To study the blood flow in the lungs, technetium-99m attached to albumen is mixed with saline solution and injected into the veins in the arm. It becomes trapped in the fine capillaries in the lung and allows a map to be made of the functioning capillaries. Any blockage in the lung, perhaps due to a clot, shows as a region without any radioactive tracer. This is called a perfusion study.

To enable the health of the airways to be studied, the patient inhales an aerosol labelled with technetium-99m. This ventilation study shows, over about half an hour, ‘cold spots’ where the radioisotope has not accumulated because the airway is blocked (see the figure below).

Lung studies (a)(i) A normal perfusion study and (ii) ventilation study of the lungs (b) Front view of lung scans of a patient with a blockage in the left pulmonary artery: (i) the perfusion scan shows no blood flow to the left lung; (ii) the ventilation scan shows both lungs as the airway is not blocked.

Blood

To determine the volume of blood in the body, a measured quantity of a radioisotope is administered and, after a period of time, a sample of blood is taken. If the activity of the tracer in the blood is measured, the dilution of the

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tracer and hence the volume of blood in the body can be calculated. This procedure, known as dilution analysis, is valuable in investigating disorders such as anaemia, assessing stroke damage and monitoring blood loss as a result of an accident.

Safety issues In hospitals, the general public, medical teams and patients must be protected from overexposure to radioactive material. Strict guidelines are implemented to control and monitor exposure to radiation.

Areas where work is carried out with ionising radiation are clearly marked as controlled areas with limited access. Equipment is checked regularly to make sure it does not leak radioactive material. Personnel distance themselves from radioactive material where possible and wear monitors to measure their exposure to radioactive sources. These monitors are checked regularly.

A radiation warning sign. The trefoil is the internationally recognised sign indicating a controlled area.

X-rays in medical diagnosis X-rays are used frequently in medicine and dentistry. It is likely that you or someone you know has had an X-ray at some time; for example, to check the development of teeth at the dentist or at a hospital for a suspected broken bone.

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What are X-rays? X-rays are electromagnetic waves of very high frequency and very short wavelength, in the range 0.001 nm to 10 nm. Because of their high frequency, and hence high energy, they can penetrate flesh and may cause ionisation of atoms they encounter on the way through.

When X-rays pass through the body, the body tissue absorbs energy and the intensity of the beam is reduced. The more dense material, such as bone, absorbs more X-radiation.

An X-ray of the lungs showing damage due to tuberculosis

Production of X-rays X-rays are emitted from a cathode-ray tube when the cathode rays strike the glass of the tube. Similar principles are used to produce X-rays for medical diagnosis.

An X-ray tube

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A cross-section of an X-ray tube is shown in the figure above. The tube is highly evacuated and a very high voltage, from 25 000 to 250 000 volts, is applied between the anode and cathode. The cathode is a filament of wire through which a current is passed. Electrons are emitted from the hot filament and a metal focusing cup directs the electrons towards the anode. The very high voltage between the cathode and anode accelerates the electrons to the anode. The anode is usually made of tungsten that can withstand the high temperatures generated. When the electrons strike the tungsten they are absorbed and some of their energy is converted to X-rays. By placing the tungsten target at an angle to the incoming electron beam, the X-rays emitted from the tungsten can be sent in a predetermined direction.

Tungsten is usually used for the target as it has a very high melting point of about 3400°C and emits X-rays when struck by electrons. This is not a very efficient way to produce X-rays as only about 1% of the energy reaching the target is converted to X-rays. The rest is converted to thermal energy in the target — enough to heat a cup of water to boiling point in one second. Hence it is important to prevent the target from overheating or melting. Copper — a good conductor of heat — is used for the anode mountings; and oil, circulating in the outer region near the anode, helps the cooling by convection. Rotating the target at a rapid rate, approximately 3600 revolutions per minute, also allows the heat produced to be distributed over a large area.

Use and detection of X-rays Since X-rays cannot be focused, the images from X-rays are shadows of objects placed in the beam. To obtain a sharp image it is necessary to have an object that is as still as possible and illuminated by an X-ray beam of small cross-sectional area, with the detecting plate as close to the object as possible. In this way, blurring of the image is minimised and the shadow is sharper. X-rays may also be scattered by surrounding tissue, which will affect the sharpness of the image. This is illustrated, using light, in the figure below.

Obtaining a sharp shadow image. (a) A narrow source produces a sharp shadow. (b) An extended source or large distance between object and screen results in a shadow that is less sharp. (c) Cloudy water scatters light and produces a blurry image.

A narrow beam of X-rays can be obtained by shaping the target to allow the beam of electrons to strike it over a reasonable area while significantly reducing the width of the X-ray beam, as illustrated in the figure at left.

The X-ray beam is directed at the part of the patient being imaged. Some tissues absorb X-rays very well and cast a shadow on the detecting screen. Bone is more dense than soft tissue and absorbs X-rays. Consequently, bones produce a clear image when X-rayed.

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Electrons hitting the target over a wide area produce a narrow beam of X-rays.

X-rays may be detected on a photographic film or by an image intensifier. The photographic film is used when only a record of the image is required. An image intensifier allows direct viewing of the X-ray image. X-rays strike a phosphor screen that produces light. This light stimulates a photocathode to produce electrons that are accelerated to strike an output phosphor screen, producing more light than was generated originally and intensifying the image up to 1000 times. The image produced can be viewed directly by the eye, a movie camera or a TV camera. The viewing area can be altered while the X-ray process is occurring.

AS A MATTER OF FACT In shoe shops in the first half of the twentieth century, a fluorescent screen was used to observe X-rays passing through a person’s foot to see if their toes were squashed by shoes that were too tight. These screens were banned from about 1960 because of the danger from exposure to scattered radiation — a danger that clearly outweighed any benefit in shoe fitting, as is obvious from the figure below.

A fluorescent screen did not produce a bright enough image to view easily. Rather than increasing the intensity of the X-ray beam, the fluorescent screen was replaced, in medical diagnosis, by the X-ray intensifying technique using phosphor screens and photocathodes.

(a) A shoe fluoroscope showing a considerable amount of scattered X-radiation (dashed lines) striking other parts of the boy’s body, for example, the reproductive organs. (b) X-rays are used to produce an image of the boy’s feet.

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Effect of X-radiation on the body If the intensity of X-radiation striking the body is great enough it may be absorbed and cause electrons to be removed from atoms or molecules (ionisation). The effect may be harmful, which is why X-radiation is often referred to as ‘harmful ionising radiation’. One reaction that may occur is the ionisation of water molecules in the body and the formation of hydroxyl and hydrogen free radicals. These free radicals may alter base structures and sequences in the DNA in chromosomes, causing mutations. This may affect not only the person exposed to the radiation, it may also be passed on to that person’s children.

Radiation which can cause damage to the body includes alpha (α), beta (β) and gamma (γ) radiation as well as X-rays.

REMEMBER THIS The amount of radiation present can be expressed as a dose equivalent in units of sieverts (Sv).

Government bodies set dose limits that are considered to be safe, but they vary from country to country. For the general population, the limit may be 1 mSv per year. For children under 16, it is usually about 0.5 mSv per year. These appear to be conservative values as the limit for radiation workers is generally set at 20 mSv per year. These values are in addition to the background radiation from the Earth and cosmic rays, which amounts to a value under 3 mSv. Approximate values for radiation from various sources are listed in table 20.3.

TABLE 20.3 Radiation from various sources

Source Radiation received

Dental X-ray <10 μSv

Chest X-ray 20 μSv

Pelvic X-ray 70 μSv

Mammogram <4000 μSv

‘Barium meal’ X-ray 3000 μSv

CAT scan of head 2000 μSv

CAT scan of chest 8000 μSv

Aircraft crew additional annual exposure 2000 μSv

Hard and soft X-rays An X-ray beam consists of a range of frequencies. A thin sheet of material placed in the path of the X-ray beam acts as a filter and absorbs more low frequency rays than high frequency rays. The beam will then have a greater proportion of high frequency (high energy) rays, which are more penetrating than the low frequency rays. The beam is said to consist of hard X-rays.

By contrast, a beam of X-rays with lower frequency rays has less energy, is less penetrating and is said to consist of soft X-rays. Note that the higher the frequency of the rays, the shorter their wavelength.

Hard X-rays are preferred for imaging as they penetrate the body and are absorbed by material such as bone, allowing images of the bone to be observed. Soft X-rays are not useful for imaging as they will not penetrate the body. They expose the patient to additional useless and possibly harmful X-radiation.

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Using conventional X-rays as a diagnostic tool The effects that X-rays have on the tissues of the body depend on the frequency (and therefore energy) of the X-rays and the time of exposure to them. For diagnostic purposes, the optimal frequency is around 7 × 1018 Hz, resulting in the best contrast between different tissues. At this frequency the X-rays are absorbed by the tissues and electrons are released. The extent of the X-ray absorption depends on the cube of the number of protons in the nuclei of the atoms encountered. For example, bone that has a high atomic density (high number of protons in the nuclei), attenuates the beam about 11 times more than the surrounding tissue and hence produces a strong X-ray shadow and allows a very good image of the bone to be obtained.

Atomic density values are high for bone, moderate for soft tissue and low for air. Hence the skeleton is imaged very well by X-rays.

Imaging parts of the body

To image soft tissue, an artificial contrast medium that absorbs X-rays readily may be introduced. For investigations of the circulatory system, iodine in a compound is introduced into the bloodstream. To X-ray the gastrointestinal tract, which is composed of soft tissue, a ‘barium meal’ consisting of a thick suspension of barium sulfate is swallowed by the patient or introduced into the intestines through the anus. The barium compound absorbs X-rays and gives a clear image, as shown in the figure below.

X-ray of the abdomen with barium sulfate used to provide a white contrast to image the bowel

A chest X-ray is the most common way of detecting lung cancer or tuberculosis. The X-ray must be taken from several different directions to overcome the problem that the heart sometimes obstructs a clear view of the lungs.

The teeth and jaw are X-rayed to detect tooth decay, and the position of crowded teeth or wisdom teeth before surgery or orthodontal treatment is recommended.

Someone who has swallowed a foreign object may be X-rayed to locate its position.

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An X-ray image of a breast showing a tumour

For imaging the breast, which is an area of continuous soft tissue, careful choice of the X-ray beam and film detector provides high resolution. Molybdenum targets in the tube and low voltage maximise photoelectric attenuation. High tube current and short exposure time minimise image blur due to movement by the patient.

A better technique for imaging soft tissue is computed axial tomography (CAT) scanning, which detects small differences in X-ray attenuation.

CAT scans in medical diagnosis

Computed axial tomography scanning (or CAT scanning) uses X-rays to obtain an image of a cross-section through the body. Very slight differences in X-ray attenuation can be measured and so soft tissue can be accurately imaged. Sometimes the name of the technique is abbreviated to computed tomography scanning (or CT scanning).

AS A MATTER OF FACT

Godfrey N. Hounsfield was born in England in 1919 and educated as an electrical engineer. A long career in medical research and engineering led to his invention of the computed axial tomography scanner, for which he earned the Nobel Prize for Medicine in 1979.

‘Tomography’ comes from the Greek word tomos, meaning ‘slice’. The CAT scanner produces an image of a slice through the object it is examining. Hounsfield analysed the data by computer, using a technique that was originally developed for use in astronomy.

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CAT scan applets

How is a CAT scan produceed? A CAT scanner consists of an X-ray tube that is rotated around the patient being imaged. The tube and detection mechanism are mounted on a frame called a gantry. The part of the patient’s body being scanned is positioned in a gap in the gantry. An image is obtained in the plane being examined. The patient, on a bed, is moved slowly through the gantry so that a series of images of ‘slices’ through the body may be obtained.

The X-ray source must produce a very narrow beam so that the path of the X-rays can be carefully controlled. To produce the narrow beam the tube voltages are high and consequently a lot of heat must be conducted away from the anode in the tube generating the X-rays. This requirement, coupled with the tube movement during scanning, means that tubes fail and have to be replaced after a few months of use. The cost of such replacement is high.

The beam is filtered to remove some soft X-rays that are not needed. This ensures that the beam is relatively uniform in frequency and the dose to the patient’s skin is reduced. The X-rays are detected by an array of several hundred detectors. The detectors convert the X-radiation directly into electrical signals that go to multiple integrated-circuit amplifiers.

A modern CAT scanner

The patient is accurately positioned in the gantry so that a plane of the body can be scanned. A beam of X-rays is sent through the patient and detected on the other side. The tube is then rotated, usually 1°, and another beam transmitted and detected. This process is repeated until an angle of 180° has been swept out.

The data from the scan are collected, displayed and reconstructed using a powerful computer and software. The computer analyses the absorption of the X-rays at each measured point in the slice. For example, if X-ray beam absorption is measured at 160 distinct points along each scanning path and 1° increments in angle are used, approximately 29 000 distinct pieces of data about X-ray absorption are obtained. The reconstruction, which is explained in simplified form in the figure below, is the result of around one million computations. The image can be displayed on a TV screen or stored in the computer memory and used with other data to produce an image in

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a different plane. Using computer analysis, the data from images of ‘slices’ through the body can be combined to produce a 3-D image of the area under investigation.

Creating a CAT scan image

In recent years, full body CAT scans have been advertised for those who want to detect problems before symptoms appear. The medical profession has criticised this offer on several grounds: people are exposed to unnecessary radiation; potential problems may not be detected; and abnormalities, which are harmless, may be found. People may either be given false security or false alarms. For further information about full CT body scans, enter ‘CT scans’ or ‘full body scans’ in a search engine.

Images showing positions of (a) transverse and (b) longitudinal ‘slices’ of the upper leg (femur) taken by a CAT scan (c) Two transverse slices through the femur showing positions of a tumor (d) Two longitudinal slices showing the same tumor

Comparing CAT scans and conventional X-rays CAT scans are significantly more expensive than conventional X-rays. They are, however, a superior diagnostic tool to X-rays when fine detail is needed.

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CAT scans provide detail to distinguish between areas where the density difference is quite small and a dense material shields the area. For example, in the brain the density range is only a few per cent but the bony skull is so dense that it absorbs most of the X-rays. A conventional X-ray will therefore provide an image of the dense skull rather than the brain tissue inside. However, by taking X-ray images from many angles in a CAT scan, the material along the path of the X-ray beams can be distinguished clearly. The method of obtaining and analysing the image makes it possible to see behind bone using a CAT scan.

Conventional X-rays are valuable when there is high natural contrast between the tissues to be viewed. The contrast is high for bone, moderate for soft tissue and low for air. Hence X-rays are good for diagnosing bone problems such as fractures, dislocation and arthritis. Conventional X-rays can also be used to image the digestive tract if an artificial contrast medium, such as a barium meal, is introduced.

Because very good soft tissue resolution is possible with CAT scans, soft tissue damage due to bone fracture or ruptured spinal discs can be investigated. CAT scans are also used to scan the liver and kidneys to obtain resolution better than 1 mm (meaning that differences separated by 1 mm can be detected). CAT scans are preferred for imaging the lungs. Although conventional X-rays give adequate routine lung screening, CAT scans provide clearer detail.

Safety first X-rays of sufficient intensity are a harmful ionising radiation, whereas ultrasound does not produce any ionisation. Hence ultrasound is safe for use with foetuses. Ultrasound could in fact be used many times with the same patient without any harmful effects. The specific gain from a CAT scan or X-ray would need to be considered in each case before exposing a patient to the X-ray doses involved.

Often the less expensive, quick and portable imaging technique using X-rays or ultrasound may give an initial diagnosis which could lead to further testing for tissue damage or internal bleeding by ordering a CAT scan.

Positron emission tomography (PET) Positron emission tomography, known as PET, is used to diagnose and monitor brain disorders, investigate heart and lung functioning and detect the location and spread of tumours. Using particular radiopharmaceuticals, a cross-sectional image through an organ can be obtained or a region of the body can be imaged, allowing the function of an area to be determined. A PET image of the brain shows the patient’s responses to factors such as noise, illumination and changes in mental concentration.

A PET scan showing a build-up of fluid in the lungs (pulmonary edema)

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PET scans

Positron–electron interactions Certain radioisotopes decay by the emission of positrons — positively charged beta particles. That is, they are positively charged electrons formed when a proton disintegrates to form a neutron and a positron. Radioisotopes that are deficient in neutrons often decay in this way. For example, carbon-11 decays to boron-11 emitting a positron (β+): 11 116 5C B +→ + β

When a positron meets an electron they ‘annihilate’ each other, converting their combined energy and mass into two gamma rays. The energy of each of these gamma rays is 0.51 MeV (mega electron volts). This process is sometimes called ‘pair annihilation’.

How a PET scan is obtained To obtain a PET scan, a suitable pharmaceutical is labelled with a positron-emitting radioisotope. The radiopharmaceutical is usually injected into the patient, but sometimes the chemical is inhaled. After a short period of time the radiopharmaceutical will have accumulated in particular areas of the body and begun to decay by the emission of positrons. These positrons travel a short distance, of the order of a few millimetres, before they encounter electrons in the body. Pair annihilation takes place and two gamma rays are produced. The gamma rays travel in opposite directions from the site of annihilation and emerge from the body, where they are detected by gamma cameras.

Gamma cameras surround the patient in the section being scanned, so that gamma rays can be detected from all angles. Pairs of gamma rays travelling in opposite directions are detected and their relative intensity measured. By comparing these measurements with known attenuation coefficients for gamma rays passing through tissue, the position of the decaying radioisotope can be approximately determined. In this way, an image is produced showing where radioisotopes accumulate. A PET imaging system detecting emissions from a region of the brain is illustrated in the figure below.

(a) Some PET images of brain activity (b) Cross-section showing pairs of gamma rays travelling in opposite directions and reaching detectors

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Isotopes used in PET

Common isotopes used in PET are listed in table 20.4. As can be seen from the table, the half-lives of isotopes suitable for PET are very small. The isotopes must be created on the day of use and, except for fluorine-18, must be made at the site of use. A cyclotron is needed for their production. This is a serious limitation, as the cost of an on-site cyclotron and facility for producing radiopharmaceuticals is extremely high. In Victoria, the Austin Hospital, the Royal Melbourne Hospital and the Peter McCallum have on-site cyclotrons for their own PET facilities. The largest cyclotron in Victoria, at Bundoora, is privately owned and produces radioisotopes for PET facilities in a number of hospitals, including some in other states. Fortunately the longer half-life of fluorine-18 means tracers can be labelled with fluorine-18 and shipped to nearby hospitals from a central location.

TABLE 20.4 Common isotopes used in PET

Radioisotope Symbol Half-life

Carbon-11 116C 20.4 min

Nitrogen-13 137 N 10.0 min

Fluorine-18 189F 109.8 min

Oxygen-15 158O 2.13 min

Magnetic resonance imaging (MRI) Magnetic resonance imaging (MRI) makes use of the effect of a strong magnetic field on nuclei in the body to obtain images of organs and tissue. The images show significant clear contrast between different types of soft tissue, making MRI scans suitable for examining the brain, spinal cord, muscle, tendons, cartilage and joints.

A patient having a scan using MRI

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How MRI works Our bodies consist of a relatively small variety of elements combined in a large variety of compounds. The protons in the nuclei of hydrogen atoms and isotopes such as carbon-13, fluorine-19, sodium-23 and phosphorus-31 respond to a strong external magnetic field by aligning themselves so that they are parallel to the field. Hydrogen is the most commonly imaged element in MRI because it is the most abundant of these elements in the body.

When a patient is placed inside an MRI machine, the machine generates a strong magnetic field. A pulse of low-energy electromagnetic radiation in the radio frequency range is beamed into the patient. The aligned protons resonate with the radio frequency, absorbing a small amount of energy. When the pulse is turned off, the protons release the absorbed energy as a radio frequency pulse. The intensity and duration of the radio frequency pulse is analysed by computers, enabling an image of a ‘slice’ through the patient’s body to be obtained.

The contrast between different tissues on the images exists because they have different concentrations of hydrogen and different hydrogen compounds.

The greater the density of hydrogen protons in the tissue, the larger the signal and the brighter the image. Air and outer bone contain little or no hydrogen, so they appear dark in an MRI scan. Water, however, contains many mobile hydrogen protons, so it produces a strong signal. Cerebrospinal fluid in the brain and spinal cord has a large amount of water that is not bound to other molecules, so it shows up brightly on an MRI scan.

Images using MRI and showing clear contrast of soft tissue (a) MRI of the abdomen showing liver cancer (b) MRI image of a normal skull

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The types of hydrogen compounds in the tissue also determine how easily the protons can release energy to neighbouring nuclei. Therefore the type of tissue influences the intensity of the signal. The type of tissue can be identified by examining the time it takes for protons in the tissue to release the energy absorbed from the radio frequency pulse.

The use of MRI in medical diagnosis MRI is considered the best diagnostic technique for obtaining sharp and crisp images of tissues. It depicts soft tissue so well that it is the preferred choice for imaging the brain and spine, where it is able to show suspected tumours and slipped discs. Cancerous tumours contain different amounts of water from normal tissue or are surrounded by watery tissue; they can be distinguished in an MRI scan because of the different brightness.

AS A MATTER OF FACT

Cardiac MRI allows investigation of congenital abnormalities and coronary heart disease to be carried out. Improvements in the speed of MRI have made abdominal imaging possible. Early MRI machines took 10 minutes to scan 24 ‘slices’ of the body and this can now be done in under 1 second. Injection of a contrast agent into the blood, combined with rapid imaging techniques, now allows blood flow in the kidneys to be examined and narrowing of the arteries due to fatty plaques to be seen.

Functional MRI allows brain activity to be investigated while the patient is awake and able to think and respond to stimuli. There is an increased flow of oxygenated blood to areas that are stimulated. Knowledge of the magnetic properties of oxygenated blood allows the parts of the brain involved in the activity to be identified and studied (see the figure below). Parts of the brain may be able to be studied prior to surgery.

Brain activation showing increased blood flow to reward centres of the brain

Imaging methods working together Medical imaging to obtain both functional and structural images is often needed for adequate diagnosis. For example, CAT scans are used to obtain structural images. Radioisotopes, on the other hand, allow functional information to be gathered. For example, a nuclear medicine image may show tumours but not very much normal tissue. Hence it may be difficult to determine the position of the tumour relative to other structures. If a CAT scan is obtained at the same time, the location of the tumour can be established precisely.

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Comparison of imaging techniques Table 20.5 provides some comparison between imaging techniques. Improvements are being made in the machines used in all these imaging methods, and students are advised to search the internet for the latest advances. At the time of printing, the white boxes represent the preferred method for imaging the organ or tissue indicated.

TABLE 20.5 Comparing imaging techniques

X-rays CAT SPECT and PET MRI

Cost of machine (capital cost)

Least expensive

Quite expensive Quite expensive Very expensive

Mobility of machines

Small portable machines available

Fixed machines Fixed machines Very few mobile machines

Spatial clarity (ability to see fine detail)

0.1 mm 0.25 mm 5–15 mm 0.3–1 mm

Time for examination

Very fast Moderate May be long, depending on tracer and procedure

Relatively long but some procedures are now quite short

Comfort and safety

Small dose of ionising radiation

Usually higher dose of ionising radiation than for X-rays

Moderate dose of ionising radiation from radioisotopes

Some claustrophobia from lying inside the bore containing the magnetic field. Patients with metallic implants cannot be scanned.

Imaging soft tissue of abdomen

Image poor — needs contrast medium

Good for whole abdomen scan

Good for growth of tumours and functional study of liver and kidneys

Good clarity for specific areas e.g. kidneys

Imaging soft tissue of joints

Poor contrast Good — preferred to MRI when extra bone detail is needed

Poor clarity but good for functional information

Excellent for studying muscles, tendons and cartilage

Imaging heart and circulation

Contrast medium is needed

Limited use with digital imaging techniques

Good for blood flow studies

Good clarity and ability to measure blood flow

Imaging chest Adequate for routine lung screening

Better detail than X-rays

Good for functional studies of blood and air flow

Not good for imaging air spaces

Imaging brain and spinal cord region

Limited use as bone blocks most waves

Good and preferred to MRI for details of bone of spine

PET scans are useful for showing function

Excellent for giving good contrast between tissues

Imaging bone Given very good clarity

Good when more complicated structures must be viewed

Good for whole body bone cancer and early diagnosis of stress fractures

Signal is weak so of limited use.

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Chapter review

Summary ■ The effect of radiation exposure can range from nausea to death. The amount of radiation energy received

by each kilogram of living tissue is measured in gray (Gy), but this value does not take into account the type of radiation that has been absorbed. Each type of radiation has a different effect because of its ionising power.

■ Dose equivalent measures the radiation energy absorbed by each kilogram of biological tissue and its effect by taking into account the form of radiation energy absorbed. Dose equivalent is measured in sievert (Sv). The Australian average annual radiation dose is 2 mSv, most of which is from background radiation.

■ Gamma radiation from radioisotopes is detected and used to make an image of an organ. This process is known as SPECT (single photon emission computed tomography). Radiopharmaceuticals to which radioisotopes have been attached are taken up by particular organs in the body. The rate at which the radioisotope accumulates in the target organ indicates the health of the organ.

■ The half-life of the radioisotope and length of time needed for the procedure must be considered when choosing an appropriate radioisotope.

■ PET uses radioisotopes that are positron emitters. Positrons and electrons annihilate each other in the body, producing two gamma rays. Detecting the position from which the gamma rays originate enables the position of the positron emitter to be mapped.

■ X-rays are produced by the collision of electrons with a target material. Soft X-rays are less penetrating and have lower frequency than hard X-rays.

■ A CAT scan is produced by the computer analysis of the attenuation of X-rays moving around a slice of the body. CAT scans can distinguish soft tissue with small differences in density and can produce an image of tissue behind bone.

■ PET scans indicate the biochemistry, metabolism and function of a particular area. PET scans are used for studying the brain and heart, detecting cancers at an early stage and monitoring cancers during treatment.

■ MRI scans make use of the magnetic effects of a strong external magnetic field on certain nuclei, particularly hydrogen, together with pulses of radio waves, to produce images of internal body tissue.

■ MRI scans show soft tissue clearly, making them suitable for imaging the brain and spinal cord.

Questions

Effects of radiation

1. Why can the formation of free radicals and ions be damaging to living cells?

2. A 30 kg child receives 3 mGy of radiation. How much energy did the child absorb?

3. An adult (60 kg) absorbs the same amount of energy as the child in question 19. What is the adult’s absorbed dose?

4. What is the dose equivalent of the child in question 19, assuming the energy was delivered by γ radiation?

5. What is the dose equivalent of the adult in question 20, assuming the energy was delivered by α radiation? Assume a quality factor of 20.

6. Why is α radiation given a higher quality factor than γ radiation?

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7. How much energy, absorbed via γ rays, would cause the death of an 80 kg person within 48 hours due to vascular system damage?

8. Why is dose equivalent often a more useful measure than absorbed dose?

9. It is more dangerous for pregnant women to be exposed to high radiation levels than for other people. Why?

10. A particularly concerned man is keen to minimise his exposure to background radiation. What advice could you give him on the lifestyle changes he should make?

11. Australians receive on average 2.0 mSv of radiation each year. Assuming this radiation is all beta particles with energy of 1.0 MeV, how many beta particles pass in or out of your body every second? (Hint: Estimate your body mass and find out how many joules of radiation you receive each year. Find out how many joules of energy there are in a 1.0 MeV beta particle, then find out how many beta particles pass through your body every year, then every second.)

12. Ionising radiation can cause cancer, yet it also can cure cancer. Explain this contradictory statement.

Radioactivity as a diagnostic tool

13. Carbon-11 has a half-life of 20 min while bromine-75 has a half-life of 100 min. If samples of these isotopes initially have the same activity, show on the same graph how their activities vary with time.

14. A small amount of iodine-131, which has a half-life of 8 days, is used to treat a patient with a thyroid condition. Sixteen days later, an amount of 6.0 mg remains.

a. How much iodine-131 was used in the treatment? b. How much of the radioisotope will remain after another 16 days? c. When is iodine-123 preferred to iodine-131 even though it is more expensive?

15. A sample of a radioisotope has a half-life of 2.0 minutes.

a. Calculate the time it will take the activity to drop from 4.0 MBq (mega becquerels) to 1.0 MBq. b. Calculate the time it will take for its activity to be 0.25 MBq.

16. A particular isotope has a half-life of 100 days. Discuss the suitability of this isotope for use in medical diagnosis.

17. Describe the problems associated with using a radioisotope of very short half-life for medical diagnosis.

18. a. Choose two specific radioactive isotopes used in medical diagnosis and outline where they would be used in the body. Justify your answer.

b. Explain why α-emitting radioisotopes are not used for medical imaging.

19. Identify a radioactive tracer study in which the tracer:

a. mixes with the substance under investigation b. is accumulated in the organ of interest.

20. Explain why technetium-99m is such an ideal radioisotope for medical imaging.

21. The middle figures on page 427 show two different types of studies of lungs.

a. Contrast the studies. b. Relate the type of study to the disease diagnosed.

22. The bottom figures on page 426 show an X-ray of a leg and a bone scan of the body.

a. Compare the X-ray image with the bone scan. b. Explain why there are differences in the images obtained.

23. The function of the lungs can be studied using a radioactive gas. The choices are xenon-133 or krypton-81m and their properties are listed in the table following. Evaluate the claim that ‘Xenon should be used in preference to krypton for investigations of lung function’.

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Isotope Emission products Half-life

Xenon-133 β, γ 5.3 days

Krypton-81m γ 13 seconds

X-rays in medical diagnosis

24. a. With the aid of a labelled diagram, give a description of the way in which X-rays are produced. b. Explain why the X-rays usually pass through a thin filter before they are used to image the patient.

25. a. Outline how the attenuation of X-rays changes for different materials in the body. b. Describe and account for the appearance of an X-ray image of part of the body containing bone,

muscle and air spaces.

26. X-rays can be classified as hard or soft.

a. How are hard X-rays different from soft X-rays? b. Why are hard X-rays preferred for imaging the human body?

CAT scans in medical diagnosis

27. Describe the differences between the ways in which CAT scans and conventional X-ray images are produced.

28. Use a table to summarise situations in which CAT scans are a superior diagnostic tool to X-rays and ultrasound.

Positron emission tomography (PET)

29. How are the radioisotopes used in PET scans different from those that are not used in PET scans?

30. a. What is a positron? b. How are positrons obtained? c. Identify issues associated with positron–electron interaction and describe how this interaction is used

in medical diagnosis.

31. Describe how a radioisotope of your choice is used in a PET investigation. In your answer you should name the isotope, state what radiation is emitted and how it is monitored. You should describe what measurements are made and how they are used to obtain a result. You should also mention any precautions or safety procedures.

Magnetic resonance imaging (MRI)

32. Describe how an external magnetic field influences a hydrogen proton.

33. Why are hydrogen nuclei imaged more than any other nuclei in MRI?

34. Describe two different pieces of information that can be analysed during an MRI scan when the low-energy radio frequency pulse is turned off.

35. Why is MRI useful for imaging cancerous tumours in the brain?

Comparison of imaging techniques

36. Using the internet or other sources: a. Find a scanned image of at least two healthy body parts or organs (the image should have been

obtained using radioisotopes). b. Find a scanned image of the diseased counterpart of the body parts or organs.

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c. Compare the images and outline why the differences are obvious in the images. d. (In a search engine, use phrases such as ‘radiopharmaceuticals’, ‘nuclear medicine images’, ‘CAT

scans’, ‘CT scans’, ‘PET images’, ‘MRI’, ‘bone scans’ or ‘brain scans’. Go to a particular hospital web site and search for their nuclear medicine department.)

37. Compare the advantages and disadvantages of X-ray scans, CAT scans, ultrasound and MRI scans for each of the following purposes:

a. imaging the brain b. imaging bone c. imaging the heart and circulation.

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