x-ray imaging generation and detection of x-rays...

1

“X-Ray Instrumentation” Slide 1

X-Ray ImagingX-Ray Imaging

Modified from SUNY Downstate Medical Center BMI Lecture NotesReference textbook: Principles of Medical Imaging,

by Shung, Smith and Tsui

Generation and detection of X-rays,Instrumentation and Applications

Generation and detection of X-rays,Instrumentation and Applications


X-Ray GenerationX-Ray Generation

X-rays are generated when electrons with high energy strike a target made from materials like tungsten or molybdenum.

High energy electrons interact with the nuclei WHITE RADIATION

High energy electrons interact with the orbital electrons CHARACTERISTICRADIATION

2


X-ray tubeX-ray tube

Working Principle: Accelerated charge causes EM radiation: Cathode filament C is electrically heated (VC = ~10V / If = ~5 A) to boil off electronsElectrons are accelerated toward the anode target (A) by applied high-voltagekinetic electron energy: Ke usually rated in “peak-kilo voltage” kVp Typical: Vtube = 30 – 150 kVp, Itube = 1-1000mAThe tube voltage (kVp) is typically generated by transforming the AC line voltage to a higher voltage and then rectifying this voltage.Abrupt deceleration of electrons on target creates "Bremsstrahlung“ (white radiation)

+-

kVp, Itube

CA

VC, If+-


Bremsstrahlung / White radiationBremsstrahlung / White radiation

Continuous spectrum of EM radiation is produced by abrupt deceleration of charged particles (“Bremsstrahlung” is German for “braking radiation”).Deceleration is caused by deflection of electrons in the Coulomb field of the nuclei.(Negatively charged) electron passes near a (positively charged) nucleusThe electron is attracted towards the nucleus and deflected from its original pathThe electron may or may not lose some of its energy

If it does not: elastic scattering, NO x-ray photons producedIf it does: inelastic scattering, energy lost by the electron is emitted in the form of an x-ray photon

K

K’

hν

Nucleus

The energy of the generated x-ray photon is given by energy conservation:

The maximum energy for the produced photon is given by:

'e eh K Kν = −

,maxp e tubeE h K eVν= = =

3


Bremsstrahlung / White radiationBremsstrahlung / White radiation

Most of the energy is converted into heat, ~0.5-1% is x-ray.Probability of the electron to lose energy increases as the atomic number of the material (anode)increases.The electrons striking the target can interact with a number of nuclei before being stoppedThe electrons may carry different energies

� Bremsstrahlung spectrum

K

K’

hν

Nucleus


Bremsstrahlung intensityBremsstrahlung intensity

Overall Bremsstrahlung intensity I :

The produced x-ray power Px is given by:

Material constant k = 1.1×10-9 for W (Tungsten).

2tube tubeI V I∝

2

: x-ray production efficiencyx tube tube tube tube tube

tube

P k Z V I kZ V P P

kZ V

ηη

= = ==

4


Bremsstrahlung spectrumBremsstrahlung spectrum

The electrons striking the target can interact with a number of nuclei before being stopped or electrons may carry different energies therefore, the x-ray photons generated may have wide energy spectrum.


Bremsstrahlung spectrumBremsstrahlung spectrum

Theoretically, bremsstrahlung from a thick target creates a continuous spectrum from E = 0 to Emax with intensity Ib ∼ 1/E :

Actual spectrum deviates from ideal form due to absorption in window / gas envelope material and absorption in anode

Electrons may carry different energies therefore the X-ray photons generated may have wide energy spectrum

The efficiency for bremsstrahlung generation increases with Z, therefore heavy metals are used in x-ray tubes.

Ep = hν

I

Ep,max,layer 1

X-ray intensity in the tube

5


Tungsten Anode is desirable, because:

It has a high melting point,

It has little tendency to vaporize,

It is strong.


Characteristic radiationCharacteristic radiation

Narrow lines of intense x-ray at characteristic energies are superimposed on the continuous bremsstrahlung spectrum.These lines are caused by photons that are released when an electron is knocked out of an inner shell and replaced by one “dropping down” from a higher shell. The photon energy corresponds to the energy difference between the shells, causing distinct narrow lines in the spectrum.Characteristic radiation occurs only for anode voltages e × kVp > IK,L,M,…..

-

--

-

--- -

--

-

hν

KLM

-

The area under the spectrum represents the total number of x-rays

6


∆n = 1 → α-transitions, ∆n = 2 → β-transitions, ...


X-ray spectraX-ray spectra

X-ray for general diagnostic radiology produced at 40 – 150 kVp.

Maximum photon energy: Ep[keV] = hνmax = e × kVp.

Characteristic radiation occurs only for anode voltages

e × kVp > IK,L,M,…..

74W

7


X-ray tube designX-ray tube design

Cathode focusing cup for the electrons2 filaments (different spot sizes, Tungsten)

Anode Tungsten, Zw ==== 74, Tmelt ==== 2250 ºC Embedded in copper for heat dissipationAngled (see next slide)Rotating to divert heat


Reduction of anode heatingReduction of anode heating

Can increase heat dissipation by embedding of target (i.e., tungsten) in copper,

angled target surface.• Anode angle of 7º…15º results in apparent

or effective spot size Seffectivemuch smaller than the actual focal spot of the electron beam (by factor ~10)

• Seffective depends on image location

rotating target, • Rotation speed ~ 3000 rpm• Increases surface area for heat dissipation

from w × (r2 − r1) to π(r22 − r1

2); generally by a factor of 18-35. (Instead of a single focal spot, we now have the whole focal track, as shown in the figure)

8


Limitations of anode angleLimitations of anode angle

Restricting target coverage for a given source-to-image distance (SID)

"Heel effect" causes inhomogeneous x-ray exposure

lower

intensityhigher

intensity


X-ray tube ratings:X-ray tube ratings:

Factors affecting x-ray intensity are:

Filament temperature, controlled by the filament current (a few Amperes ac and dc),The potential difference between the anode and the cathode

150 kVp for chest imaging30 kVp for mammographs

The target material should have a high atomic number.

For a fixed filament current, the intensity, I, irradiated by the x-ray tube is:

F: rectification factor, equal to 1 for dc.

FkVvmAiZI pTUBE TUBE ⋅⋅⋅≅ 22 )()(

9


http://www.youtube.com/watch?v=I3s5HFQ2YME

http://www.youtube.com/watch?v=Bc0eOjWkxpU


X-Ray DetectionX-Ray Detection

10


FiltersFilters

X-rays generated by x-ray tubes are polychromatic,The radiation dose to the patient can be substantially reduced by filtering out the undesired portion of the x-ray photon.

FILTERS are thin sheet of metals placed between the patient and the source.

Aluminum (1-2 mm thick) is used to filter low-energy X-rays,Copper is used for filtering in the high-energy X-ray systems.The characteristic radiation produced by opper is 8keV, which is high enough to reach the patient and increase the skin dose. Thus, the aluminum layer is placed below the copper layer to absorb this radiation.

3mm thick Al can attenuate more than 90% of the x-ray energy at 20keV.

Spectra at 300 kV. Effect of physical material filters to reduce beam hardening (filtering out low energy photons).

J.P. Kruth, et. al, ‘Computed tomography for dimensional metrology,’ CIRP Annals - Manufacturing Technology, Volume 60, Issue 2, Pages 821–842, 2011.


Beam Restricters and GridsBeam Restricters and Grids

A beam restrictor is basically a sheet of lead with a hole in the center whose size and shape determines those of the X-ray beam. The basic function of a beam restrictor is to regulate the size and shape of the beam. A closely collimated beam can reduce patient exposure and generate less scattered radiation. There are three types of X-ray beam restrictors:

• aperture diaphragms,

• cones and cylinders,

• collimators.

11


Blurring of edges and fine structures due to finite source size leads to penumbra (partial shade / cloudiness / unsharpness; the blurred margin of an image) P.

The width of the penumbra, P, is related to the source diameter by the following equation:

D : the width of the source, L: the distances between the source and the restricter, I: the distances between the restricter and the detector.

Aperture Diagrams - Finite aperture size / penumbraAperture Diagrams - Finite aperture size / penumbra

To reduce the penumbra, the source should be made as small as possible and the diaphragm should be positioned as far away from the source as possible.

L

DIP =


Aperture Diagrams - Finite aperture size / penumbraAperture Diagrams - Finite aperture size / penumbra

12


MagnificationMagnification

Geometric magnification given by

Reduction of M by minimizing B, i.e. placing patient next to film.

A

BA

O

IM

+==


Cones and Cylinders / Collimators:Cones and Cylinders / Collimators:

Cones and cylinders are sometimes also used as beam restrictors, but they, along with the diaphragm, suffer from a major drawback: Only a limited number of beam sizes can be obtained.

The collimator is the most popular beam restrictor for two reasons:

The X-ray field size is adjustable, A light beam can be used to indicate the exact size of the field.

The X-ray beam size is adjusted by the movable aperture diaphragm.

The Xray field is illuminated by a light beam from a light bulb located within the collimator the same distance from the center of the mirror as the X-ray source.

13


Anti-scatter gridAnti-scatter grid

Significant Compton interaction for low Ep

(37-50% of all photons). Linear grid: Lead septa + interspace material. Septa focused toward source. Grid ratio (the ratio between the height of the lead strips and the width of the gap between) ~ 3.5-5:1. Only scatter correction in one dimension. Scatter-to-primary (SPR) reduction factor ~5. Grids are moved during exposure to increase image quality (reduce artifacts due to lead septa)Longer exposure needed (attenuation inside the gap material)

detector

breast

leadsepta

Scattered X-rays are noise that degrade image quality and increase patient exposure and therefore should be minimized. The most effective way of removing scatter radiation is the radiographic grid. The grid is composed of a series of lead foil strips separated by X-ray transparent spacers which are either aluminum or organic material. The grid blocks the scattered radiation while letting the primary radiation pass.


Image FormationImage Formation

The human eye cannot see the information carried by the x-rays directlyX-ray images have to be converted to visualizable information.

Exposing a photographic film to the x-rayConverting the x-ray photons to visible photons

14


Fluorescent screens (Intensifying screens)Fluorescent screens (Intensifying screens)

Scintillators ("phosphors") are used to convert X-ray energy to visible or near-infrared light through fluorescence

The light intensity emitted by screen is linearly dependent on x-ray intensity

Because Ep,x-ray ≈ (100 … 10,000)× Ep,vis one x-ray photon can generate multiple optical photons

Quantum detection efficiency (QDE): Fraction of incident x-rays that interact with screen (30-60%).

Conversion efficiency: Fraction of the absorbed x-ray energy converted to light. Calcium tungstate (CaWO4): 5% Rare earth phosphors : 12 - 18%

• Lanthanum, LaOBr, • Gadolinium, Gd2O2S, • Yttirium oxysulfide phosphor, Y2O2S:Tb


Image IntensifiersImage Intensifiers

The image produced on the image intensifying screen is typically very weak and can be visualized only when the room is darkened.To brighten the image, image intensifier can be used.Image intensifier tubes convert the x-ray image into a small bright optical image, which can then be recorded using a TV camera.

Conversion of x-ray energy to light in the phosphorousscreen,Emission of low-energy electrons by photoemissive layer (antimony)Acceleration (to enhance brightness) and focusing of electrons on output phosphorous screen (ZnCdS)

Quantum detection efficiencies ~60% - 70% @ 59 keV

15


Image IntensifierImage Intensifier

Photocathode coated with aphotoemissive material thatemits electrons when strikenby light photons


Photographic filmPhotographic film

Photographic film has low sensitivity for x-rays directly; a fluorescent screen (phosphor) is used to convert x-ray to light, which exposes film. Increased sensitivity to light was noted when silver (Ag) is combined with a halogen element. Such a combination is known as silver halide (ex: AgCl, AgBr, AgI). It is necessary to use a ‘binder’ an inert substance which will envelope the silver halide crystals - commonly called grains - holding them evenly suspended and attached to the support. Gelatin was found as a binding material with ideal properties. The combination of silver halides suspended in gelatin is known as silver halide emulsion.

Film Composition:Transparent plastic substrate (acetate, or polyester),Both sides coated with light-sensitive emulsion (silver halide crystals with grain sizes 0.1 -1 µm, mostly silver bromide and gelatine). The silver bromide crystal, upon receiving a light photon, yields a free electron that combines with a silver ion to form a silver atomatomic silver appears black (negative film) � exposure of film to light causes it to darken.Blackening depending on deposited energy (E = I (intensity of light) × t (exposure time)),

16


Silver Bromide (AgBr) CrystalSilver Bromide (AgBr) Crystal

Light photons

Silver Bromide Crystal The silver bromide crystal, upon receiving a light photon,

yields a free electron

Free electron + silver ionfree electron can combine with a silver ion to form a silver atom.

Silver atom(BLACK)Darkening the film


AgBr CrystalAgBr Crystal

(M. J. Langford)

17


Silver Image formation theory

Silver Image formation theory

(M. J. Langford)


The role of silver in photographyThe role of silver in photography

(M. J. Langford)

18


Film characteristicsFilm characteristics

Blackening depending on deposited energy (E = I×t)

Optical density (or photographic density):

(measure of film blackness) for visible light:

D = log10(I i/I t)=log10 opacity

D > 2: “black,”

D = 0.25 - 0.3: “transparent” (or “white”)

with standard light box

useful diagnostic range ~0.5 - ~2.5

FilmI i I t


Film characteristic curve IFilm characteristic curve I

Relationship between film exposure E (in Röntgen) and optical density D

Film characteristics:

Fog: D for zero exposure

Sensitivity (speed S): Reciprocal of exposure XD1 [R] that produces D of one:

Linear region

S= 1/XD1

XD1

Recall:1R= dose required to produce 2.08x109 ionization in 1 cm3 air (2.58×10-4 Coulomb/kg in airunder 760 mm Hg ambient pressure, 0 °C)

E

19


Film characteristic curve IIFilm characteristic curve II

Film characteristics continued: Film gamma γ (maximum slope):

Contrast: Difference in D between two adjacent regions, C = ∆D (related to latitude as well)Latitude: Range of log exposure that produces acceptable values of optical density for diagnosis purposes (D ≈ 0.5…2.5) (linear range)

Film gamma

Contrast, latitude

2 1

2 1 maxlog log

D D

X Xγ −=

−2 1

2 1log log

D D

E Eγ −=

−


Film sensitivity & resolutionFilm sensitivity & resolution

Tradeoff between sensitivity (S) and resolution (R):

Grain size: coarse: S↑ / R↓ fine: S↓ / R ↑Coating thickness: thick: S↑ / R↓ thin: S↓ / R ↑No. of coatings: dual: S↑ / R↓ single: S↓ / R ↑

20


Screen / Film CombinationsScreen / Film Combinations

Sandwiching phosphor and film in a light tight cassette.

Lateral light spread through optical diffusion limits resolution, can be minimized by absorbing dyes

Screen thickness is tradeoff between sensitivity and resolution

X ray

Phosphor screen

Film emulsion

Foam

Light spread

Light-tightcassette

Crystals

X-ray photons

Film


Limits of Analog Systems (Screen/film, intensifiers):Limits of Analog Systems (Screen/film, intensifiers):

Film has limited latitude (range of exposure for satisfactory film quality),

Film acts as detector, storage, display,

Development, storage,

Many steps involved, loss in image information,

Analog noise

21


Photomultiplier tube

X-ray photon

Scintillationcrystal

Photocathode

V1

V2

Vn

Anode

(grounded)

(1200 V)

Scintillation crystal coupled to a photomultiplier tube, like NaI emits light photons in proportion to the absorbed x-ray photon energy.

The photocathode is coated with a photoemission material that emits electrons when striken by light photons in proportion to the intensity of the light.

The electrons will be accelerated toward the first dynode (V1) which is covered by a material that emits secondary electrons when stricken by an electron.

The number of electrons are multiplied when they are propagating down the tube.

The output current is proportional to the number of x-ray photons.


Dynode:

Conventional dynode materials are BeO (Berlyllium oxide), MgO

The multiplication factor for a single dynode is given by

If N stages are provided in the multiplier section, the overallgain for the PM tube is δN.

Conventional dynode materials are characterized by a typical value of δ=5. Ten stages will therefore result in an overall tube gain of 510 or ~107.

photonsincident ofnumber emitted ronsphotoelect ofnumber

dephotocathoa of efficiency Quantum =

Photocathode:

electronsincident primary

emitted electronssecondary ofnumber =δ

Photomultiplier tube

22


Digital Image Detectors (CCD Based, 1)Digital Image Detectors (CCD Based, 1)

Charge coupled detector (CCD):

IC detector comprising a photodiode, a charging circuit, a capacitor and a charge transfer circuit (MOS capacitor).

Phosphor is optically coupled by lens or fiber taper to 1k×1k CCD array (real-time imaging).


Digital Image Detectors (CCD Based, 2)Digital Image Detectors (CCD Based, 2)

CsI : Cesium Iodide

23


Digital Image Detectors (non-CCD)Digital Image Detectors (non-CCD)CsI layer deposited directly on array of photodiodes with switching matrix [GE 2000, first FDA approved fully digital system (11 yrs, $130 million)]

Direct conversion of x-ray into charge (lead iodide, selenium, zinc cadmium telluride, thallium bromide)


25


RadiographyRadiography

Most commonly used clinical procedure

Radiologists are familiar with the procedureFully aotomated – easy to learn how to operate the machineImage resolution is goodPerformance is superior to other modalities in a number of situations

Lower density region � appears darker (less attenuation in the object, more X-ray intensity)Higher density region � appears lighter (more attenuation in the object, less X-ray intensity)


RadiographyRadiography

OrthopedicChestAbdomenMammography

26


FluoroscopyFluoroscopy

X-rays can be projected onto a fluorescent screenMoving object (eg. Contrast medium such as barium sulfate (nontoxic) in the digestive tract)

Lower x-ray levels are produced continuously and many images must be presented almost immediately

Total x-ray radiation dose received by the patient can be very high

The produced image is very weak � room has to be darkened to examine the image � İmage intensifying screen has to be used

GI tract imagingUlcers, tumors, obstructions, etc. could be diagnosed.


FluoroscopyFluoroscopy

27


X-ray projection angiographyX-ray projection angiography

Imaging the circulatory system. Contrast agent: Iodine (Z=53) compound; maximum iodine concentration ~ 350mg/cm3

Monitoring of therapeutic manipulations (angioplasty, atherectomy, intraluminal stents, catheter placement).Short intense X-ray pulses to produce clear images of moving vessels.

Pulse duration: • 5-10 ms for cardiac

studies.• 100-200 ms for cerebral

studies.


MammographyMammography

Detection and diagnosis (symptomatic and screening) of breast cancer,Pre-surgical localization of suspicious areas,Guidance of needle biopsies.

Breast cancer is detected on the basis of four types of signs on the mammogram:

Characteristic morphology of a tumor mass,Presentation of mineral deposits called microcalcifications,Architectural distortions of normal tissue patterns,Asymmetry between corresponding regions of images on the left and right breast,

28


MammographyMammography

Low energy X-rays (~20keV) should be used.

The difference between attenuation coefficients of soft tissues is more pronounced at low X-ray energy levels

X-ray tubes with Molybdenum are used since they have a characteristic radiation peaking at 17.4 and 19.6 keV.

Spatial resolution is better than 0.1 mm.

Short exposure time to lessen motion artifacts.

Energy

Mas

s at

tenu

atio

n co

eff.

( ).21 figurethein∆>∆


Mammography contrastMammography contrast

Image contrast is due to varying linear attenuation coefficient of different types of tissue in the breast (adipose tissue (fat), fibroglandular, tumor).

Contrast decreases toward higher energies ⇒ the recommended optimum for mammography is in the region 18 - 23 keV depending on tissue thickness and composition.

29


Mammography sourceMammography source

Voltage ~ 25-30 kVp Target material Mo, Rh (characteristic peaks)Filtering:

Target Mo, Filter Mo Target Rh, Filter Rh


Radiation Dose for Various X-Ray ProceduresRadiation Dose for Various X-Ray Procedures

X-ray procedure Exposure [mR]

Chest 20

Brain 250

Abdomen 550

Dental 650

Breast 54

Xeromammography 200

CT/slice 1000

30


X-ray dosimetryX-ray dosimetry

The radiation absorbed dose D [Rad] is defined as

Effective dose equivalent HE [Sv](Sievert) takes into account sensitivity of organ exposed:

[SI units]1 Rad = 100 erg/g = 0.01 J/kg = 0.01 Gray [Gy]

E i ii

H w H

H QF D

=

=

∑ i: indicates organ

w: relative organ sensitivity to radiation

QF: Quality Factor = danger of type of radiation QF(x-ray, gamma) = 1)


Biological effects of ionizing radiationBiological effects of ionizing radiation

Damage depends on deposited (= absorbed) energy (intensity × time) per tissue volume,No minimum level is known, Exposure time: Because of recovery, a given dose is less harmful if distributed in time,Exposed area: The larger the exposed area the greater the damage (collimators, shields!),Variation in Species / Individuals: LD (Lethal Dose) 50/30 (lethal for 50% of a population over 30 days, humans ~450 rads / whole body irradiation),Variation in cell sensitivity: Most sensitive are nonspecialized, rapidly dividing cells:(Most sensitive: White blood cells, red blood cells, epithelial cells,Less sensitive: Muscle, nerve cells)

Short/long term effects: Short term effects for unusually large (> 100 rad) doses (nausea, vomiting, fever, shock, death); long term effects (carcinogenic/genetic effects) even for diagnostic levels ⇒maximum allowable dose 5 R/yr and 0.2 R/working day [U.S. Nat. Counc. on Rad. Prot. and Meas.]

31


Effects of ionizing radiation on the living tissueEffects of ionizing radiation on the living tissue

Direct effects:

Effects on the macromolecules (for example, protein, RNA, DNA) of cells. The effects on the proteins can be repaired by the cell. However, effects on DNA can not be repaired yielding genetic mutation and death of the cell.

Indirect effects :

Effects on the water molecules. 80% of human body is made up of water. Water molecules are converted to other molecules (H and free radical OH ) with incoming radiation. The excess energy of these molecules may affect the other molecules and break their molecular bonds yielding toxic molecules (H2O2).

x-ray imaging generation and detection of x-rays...

Documents