Advanced Biomedical Imaging

Lecture 3

Dr. Azza HelalDr. Azza HelalA. Prof.A. Prof. of Medical Physics of Medical Physics

Faculty of MedicineFaculty of MedicineAlexandria UniversityAlexandria University

X ray Machine X ray Machine Slide 5

X ray tube

Slide 13

Effective Effective focal spot:focal spot:

Face of Face of target is target is slanted slanted from from verticalvertical(6-15(6-1500).).

As angle As angle increased increased effective effective focal spot focal spot increased but increased but (actual focal (actual focal spot)spot) remains remains the same.the same.

11 . .Focal spotFocal spot

Actual focal spotActual focal spot Effective focal spotEffective focal spot

A small focal spot produces:A small focal spot produces:

• Sharper images, Sharper images, • but unable to dissipate heat rapidly,but unable to dissipate heat rapidly,• can not sustain high current required for short can not sustain high current required for short

exposure required to decrease patient motion,exposure required to decrease patient motion,• used in mammography.used in mammography.

A large focal spot:A large focal spot:

• can tolerate more heat, can tolerate more heat, • but un-sharpness increases,but un-sharpness increases,• used in fluoroscopy.used in fluoroscopy.

A A quality filterquality filter: thickness of material placed in : thickness of material placed in radiation beam to radiation beam to attenuate lower energiesattenuate lower energies photons (responsible for patient dose) more than photons (responsible for patient dose) more than higher energies (responsible for image).higher energies (responsible for image).

This can be done through photoelectric interaction process.

2. Filters2. Filters Slide 2

So filtration:-↑photon energy , ↑ exit dose / entry dose ratio so ↑ film dose / skin dose ratio, but it decreases the contrast.

Uα1/E3

As u (attenuation coefficient) is α to 1/E3, it attenuates low energy photons & does not affect high energy photons

3. Collimators & Grids 3. Collimators & Grids

I=I0e-ux

Slide 10

The problem is scattered radiation?

Primary radiation carries information to be imaged & but Scattered radiation obscures it.

S/P depend on thickness of part, it is 4:1 in PA chest & 9:1 in lateral pelvis.

Large patient creates more scatter. It ↓ contrast which produced by primary beam.

To reduce amount of scatter produced by patientTo reduce amount of scatter produced by patientField sizeField size cones & diaphragm → ↓ volume of cones & diaphragm → ↓ volume of

scattering tissues → ↓ scatter & scattering tissues → ↓ scatter & ↑ contrast↑ contrast..

Compression of patient:Compression of patient: moving overlying moving overlying tissues laterally reduces volume of scatter & tissues laterally reduces volume of scatter & reduce patient dose. ↓ scatter & reduce patient dose. ↓ scatter & ↑ contrast↑ contrast

Kv :Kv : ↑Kv → ↓ μ, ↓ scatter in pt and ↑ forward ↑Kv → ↓ μ, ↓ scatter in pt and ↑ forward scatter reach film. scatter reach film. ↓ contrast↓ contrast

To To reducereduce amount of scatter after it left pt: amount of scatter after it left pt:

Grid between patient & film, air gap & flat metal filter. Grid between patient & film, air gap & flat metal filter.

1. Grid: anti-scatter grid: 1. Grid: anti-scatter grid:

thin strips of leadthin strips of lead sandwiched between plastic, sandwiched between plastic, carbon fiber & AL. carbon fiber & AL.

Lead absorb 90% of scattered rays while allow 70% of 1ry beam Lead absorb 90% of scattered rays while allow 70% of 1ry beam to pass through gap to reach film. to pass through gap to reach film. ↑ contrast↑ contrast

The beam becomes The beam becomes more penetratingmore penetrating as low energy beam can as low energy beam can not reach the film so the mean of not reach the film so the mean of the energy increasethe energy increase..

Slide 7

Grid ratio (GR)= Depth / Width of interspaced channelDepth / Width of interspaced channel

– GR, efficiency to absorb scatter radiation & scatter / primary rays, contrast. , & dose

Contrast improvement factor = = Contrast with grid / contrast without grid=3-5.Contrast with grid / contrast without grid=3-5.

Bucky factor or grid factor =Exp necessary with grid/Exp necessary without grid=2-3Exp necessary with grid/Exp necessary without grid=2-3

Frequency is the number of strips per inch. High ratio & frequency more dose required. No grid is used with thin part of body (extremities) & child

Film moved 30 cm a way from patient so much oblique Film moved 30 cm a way from patient so much oblique scatter will miss the film & so scatter will miss the film & so contrast improvescontrast improves

Need ↑Kv or mAS as image magnified (mammography). Need ↑Kv or mAS as image magnified (mammography). Patient dose ↑Patient dose ↑ by grids, air gap, grids require higher by grids, air gap, grids require higher

dose than air gap so they are not used in pediatrics.dose than air gap so they are not used in pediatrics.

3.3. Flat metal filter:Flat metal filter: placed on cassette, absorb softer & placed on cassette, absorb softer & obliquely traveling scatter > harder direct rays.obliquely traveling scatter > harder direct rays.

2. Air gap

A) x ray cassetteA) x ray cassette is a flat light box with film between a is a flat light box with film between a pair of screen.pair of screen.

Front of cassette Front of cassette (AL low Z=13)(AL low Z=13) to minimize to minimize attenuation of beam so decrease required pt dose. attenuation of beam so decrease required pt dose.

Back of cassette is Back of cassette is thin lead sheetthin lead sheet to absorb remnant to absorb remnant radiation so no back scatter irradiated pt nor screen.radiation so no back scatter irradiated pt nor screen.

4. Screen film cassette4. Screen film cassetteSlide 2

B) Intensifying screen:

phosphor crystals (↑z)

Crystals absorb x-ray and emit light of intensity α intensity of x rays. It converts 1 photon to 1500 light photons.

So allows

Low pt dose & High contrast

But with Noise & low resolution

C) FilmsC) Films

Silver halide crystalsSilver halide crystals

exposed to light → electrons attach to silver ions →silver metal. The more silver metal the more film blackening.

Screen is more sensitive than film exposed alone: why?

– The screen phosphor layer is more effective than film emulsion at absorbing x-ray.

Use of screen & film reduces pt dose by a factor of 50.

Film is used alone to obtain Film is used alone to obtain sharp imagesharp image in thin part where in thin part where fine detailsfine details is required ( is required (dentaldental))

Quantity:

It is α Kv2 mAs & z & exp time.

Quality: (penetrating power)

It is affected by KV & tube filtration

Tube filtration:

Affects both quantity & quality, it affect quality by removal of low energy photons.

Factors affect x ray emission: (quantity & quality)Factors affect x ray emission: (quantity & quality)

1.1. Heat with x ray Heat with x ray 2.2. Patient dosePatient dose

1.1. Heat with x ray: Heat with x ray: 99.5% of energy converted to heat

It is avoid by using:

Rotating anode

Copper anode & oil circulating through anode.

Limitation of x ray tubeLimitation of x ray tube

2. Patient dose: (limiting factor)2. Patient dose: (limiting factor)

X ray is absorbed by patient. X ray is absorbed by patient.

Emerged x-ray beam carries pattern of intensity Emerged x-ray beam carries pattern of intensity dependent on thickness & composition of organsdependent on thickness & composition of organs

Acceptable patient dose of radiation is a limiting factor; Acceptable patient dose of radiation is a limiting factor;

it should be it should be as low as possible as low as possible

Minimum exit dose emerges from pt is required to Minimum exit dose emerges from pt is required to produce satisfactory image. produce satisfactory image.

Factors affect Patient doseFactors affect Patient dose

Effect of tube Kv: Effect of tube Kv: ↑ ↑ Kv → ↓u → Kv → ↓u → → → ↓ skin dose↓ skin dose, ↑ penetration & ↑ , ↑ penetration & ↑

proportion of high energy photon reach film screen. proportion of high energy photon reach film screen. So So entrance doseentrance dose decreases to acquire same exit dose. decreases to acquire same exit dose.

Effect of tube mAEffect of tube mA mA increases no of photons, and so signals but also mA increases no of photons, and so signals but also

amount of exposure (dose). amount of exposure (dose). ↑ patient dose↑ patient dose

Effect of Focus – Film distance on patient dose:Effect of Focus – Film distance on patient dose: ↑↑FFD, beam diverge and no of photons distributes over FFD, beam diverge and no of photons distributes over

larger area larger area →↓ →↓ patient dosepatient dose ↑ ↑ noisenoise in image, to compensate more x rays is needed to in image, to compensate more x rays is needed to

maintain maintain (MAS ↑ but it (MAS ↑ but it ↑ skin dose↑ skin dose))..

U U αα ZZ33/E/E33

So to reduce scatter radiation reaching the film:So to reduce scatter radiation reaching the film: – Use low KV,Use low KV,– Moving grid, Air gap. Moving grid, Air gap. – Conning, Conning, – placing thin sheet of zinc on film cassette, placing thin sheet of zinc on film cassette,

To reduce patient exposure:To reduce patient exposure: – adding 2mm Al filter, adding 2mm Al filter, – increase KV ( increase penetration, if obese)increase KV ( increase penetration, if obese)– decrease mA decrease mA – ↑↑x-ray target –object distance,x-ray target –object distance,– small object film distance,small object film distance,– compress patient.compress patient.

QuestionsQuestions

1. Enumerate the factors that affect patient dose in x ray image?

2. Enumerate the factors that affect x ray quantity & factors that affect x ray quantity & quality?quality?

3. Enumerate the factors thatfactors that used to reduce the effect of reduce the effect of scatter radiation on the film? scatter radiation on the film?

4. What are the advantages of using the grids?5. What is the GR (grid ratio)?6. Mention one case in which grid is not recommended?