Download - Biomedical Imaging I X-Ray Imaging, Instrumentation.

Biomedical Imaging I

X-Ray Imaging,Instrumentation

01/30 II.2

Interactions between X-rays and Matter

In the diagnostic range, below 200 keV, three mechanisms dominate the attenuation:

• Coherent scattering,• Photoelectric absorbtion,• Compton Scattering

For photon energies larger that 1 MeV another mechanism called Pair Production is the dominant interaction mechanism.

01/30 II.3

Coherent Scattering

Occurs in low energy radiation that is not sufficient to eject the electrons out of orbit.

It is the deflection of X-ray beams caused by atoms being excited by the incident radiation and then reemitting waves at the same wavelength.

Relatively unimportant in the energies used for diagnostic radiology.

E = h

--

-

--- -

--

-KLM

E ~ h

01/30 II.4

Photoelectric absorption ()The photon knocks an electron out of one of the inner shells of a target atom. The photon is destroyed in the process. Desirable interaction for imaging.

-

--

-

--- -

--

-

E = h

Ee

KLM

ContinuumZero

K

L

M

N

E

• The electron exits from its shell into the energy continuum (it leaves the field of the nucleus).

• This process is possible for a given shell only if E IK, L, M,...

• The process is most likely for E EK, L, M,...

(resonance)

• The cross section decreases with increasing photon energy

• Increases strongly with Z (Z5), decreases with E (1/E3.5)

e IE E E

Energy balance:

01/30 II.5

The remaining atom becomes a positively charged ion. Accompanying the ionization there occurs:

1)Characteristic radiation or fluorescent radiation in the form of X-ray photon will be emitted carrying an energy equal to the difference in energy between the outer shell electron and, for example, the L-shell electron.

2) Auger effect (an alternative to characteristic radiation)

Energy released by the outer shell electron is transferred to another orbital electron.

The orbital electron that acquires enough energy to escape is called Auger electron.

01/30 II.6

The photoelectric effect always yield three end products:

1)a photoelectron,

2) Characteristic radiation or Auger electrons

3) a positive ion.

The photoelectric absorbtion is the most desirable type of interaction in X-ray imaging.

X-ray photon is completely absorbed producing little scattered radiation (scattered radiation is dangerous for personnel and produce image noise)

01/30 II.7

Compton scattering ()Scattering process: Photon “bounces off” atom and “survives”, momentum and energy are exchanged. In a Compton scattering process, an x-ray photon interacts with one of the weakly bound electrons of the atom. This electron can be considered free because Ex-ray 1-100 keV >> EI few eV.

Inelastic scattering process:

x

y

x

y

’

e

' 2;

1 (1 cos )ee

E EE E E

m c

01/30 II.8

Compton Scattering

The fractional change in wavelength and photon energy with angle varies significantly with the initial energy of the photon.

01/30 II.9

Relative Importance of two major type of Interactions

01/30 II.10

Pair production ()

If the photon energy E exceeds the value 2 mec2 = 1.02 MeV, an electron-positron pair can be produced with destruction of the photon. The kinetic energy of the resulting particles is given by Ee = Ep = E - 2 mec2

This process can take place only in interaction with a nucleus, to account for conservation of momentum and energy. The cross section for pair production is proportional to Z2 and dominates the interaction at very high energies (>5 MeV).

01/30 II.11

Cross sections for Different Processes / Materials

Compton scattering

Pair production

Photoelectric absorbtion

K EDGE

L EDGE

X-Ray Generation

01/30 II.13

X-ray TubeWorking Principle: Accelerated charge causes EM radiation bombardment of a target material with a beam of fast electrons

Electrons are emitted thermally from a heated cathode (C) and are accelerated toward the anode target (A) by the applied voltage (aka potential) V (~kV).

+-

V

CA

01/30 II.14

BremsstrahlungContinuous 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

Most of the energy is converted into heat, <~1% is x-ray

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

The maximum energy for the produced photon is given by:

'h K K

maxE h K e V

K

K’

h

Nucleus

01/30 II.15

X-Ray Tube SpectrumBremsstrahlung creates a continuous spectrum ("white radiation") from E=0 to Emax with I 1/E.

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

absorbed by tubematerial

01/30 II.16

Characteristic radiationCharacteristic narrow lines of intense x-ray 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.Lines are named after the lower shell involved. n=1 -transitions, n=2 -transitions, ...

-

--

-

--- -

--

-

h

KLM

-

ContinuumZero

K

L

M

N

E [keV]

K-lines

L-lines

K

K

K

0.5

3

11

70

Tungsten (74W)

01/30 II.17

Realization of X-Ray Tube• Desired: Point source (less blurring) electrostatic focusing of

electrons (cathode geometry)• Limit: potential melting of anode can increase heat dissipation by

imbedding of target (i.e., tungsten) in copper, rotating target, angled target surface.

• Tubes come with two or more focal spots of different sizes (low power, sharp high power, blurry)

01/30 II.18

Spot Size / Heel EffectAnode angle reduces apparent spot size

Tradeoff: restricted usable area of image plane because of uneven intensity (“heel effect”)

01/30 II.19

Magnification and Image BlurGeometric magnification given by

Blurring of edges and fine structures due to finite source size causes penumbra of width

I A BM

O A

Bp s

A

p

, O

I

01/30 II.20

X-Ray Tube Spectra• Filtering: Selection of lines, suppression of low-energy x-ray: Cu, Al

(Mo in mammography)

• Tube window glass, metal (4Be in mammography)

• Voltage tuning

01/30 II.21

X-ray Tube ratingsFactors effecting X-ray Intensity:

• Filament temperature controlled by the filament current (few amperes ac or dc)• The potential difference between anode and cathode (150 kV peak for chest, 30 kV peak for mammography)• The target material (should have high atomic number)

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

I = Z x tube current (mA) x (kVp)2 x F

atomic number tube voltage rectification factor (1 for DC)

I

X-Ray Detection

01/30 II.23

RadiographyFew high-quality images are made in a study

• Orthopedic• Chest• Abdomen

• (Mammography)

01/30 II.24

Photographic Film• Photographic film can be exposed by x-ray directly.

• Increased sensitivity to light was noted when silver (Ag) is combined with a halogen element. Such a combination or comund 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, polyester)• Coated on both sides with light-sensitive emulsion (gelatin, silver halide

crystals (AgBr) 0.1-1 mm).

Exposure to light splits ions atomic silver appears black (negative film)

01/30 II.25

AgBr Crystal

01/30 II.26

Silver Image formation theory

(M. J. Langford)

01/30 II.27

The role of silver in photography

(M. J. Langford)

01/30 II.28

Film characteristics

• Blackening depending on deposited energy (E = It)• Optical Density (measure of film blackness):

D = log 10(Ii/It)=log10 opacity

• D > 2: “black,” D = 0.25 - 0.3: “transparent” (or “white”) with standard light box (useful diagnostic range ~0.5 - ~2.5)

Fil

mIi It

01/30 II.29

Film Characteristic Curve (H and D Curve) I

Gives relationship between film exposure and optical density DCharacteristics:

• Fog: D at zero exposure (higher the fog level faster the photographic mat)

• Sensitivity (speed S ): Reciprocal of X-ray exposure E in Röntgen ( R*) needed to produce a density D of 1 S = 1/E

• Linear region

1R= dose required to produce 2.08x109 ionization in 1 cm3 air (2.5810-4 Coulomb/kg in airunder 760 mm Hg ambient pressure, 0 °C)

D

E

01/30 II.30

Film Characteristic Curve II• Gamma (maximum slope)

• Latitude (range of exposure creating appreciable values of D [~0.5 - ~2.5])

• Contrast (curve gradient, D/log E Latitude)

2 1

2 1log log

D D

E E

Film gamma Contrast, latitude

01/30 II.31

Film Resolution

Light sensitivity is directly related to the grain size andthe number, thickness of sensitive layers (interaction volume)

In both cases, increasing sensitivity decreases resolution

Tradeoff between sensitivity resolution

Double sided Single sided

01/30 II.32

Grains

(M. J. Langford)

01/30 II.33

Low x-ray sensitivity of film Fluorescent screens (“phosphors”) are used to convert x-ray energy

to light. • Fluorescence:

The particles produced by x-ray interaction (electrons, photons) lose part of their energy by exiting the valence electrons in the medium, which upon relaxation emit light.

• Conversion efficiency: Fraction of the absorbed x-ray energy converted to light.

• CaWO2 (Calcium tungstate):20-50%

radiates ultraviolet and blue

• Rare earth phosphors: LaOBr (blue), Gd2O2S (green), Y2O2S:Tb: 12 - 18%

La: Lantanum, Gd: Gadolinium, Tb: Terbium, Yt: Ytrium• Quantum efficiency:

Fraction of incident x-rays that interact with screen (30 - 60%).

Fluorescent Screens

01/30 II.34

Screen / Film Combinations• Sandwiching phosphor and film in light-tight cassette or a film

between two screens. Major advantage: reduce the exposure required to form an image

• Resolution vs. sensitivity:• Most x-rays are absorbed close to the entrance surface. Lateral light

spread degrades spatial resolution. The light intensity emitted by screen is linearly dependent on x-ray intensity.

• Thicker screen increases sensitivity (larger interaction volume) but degrades resolution due to light scatter / lateral spread

X ray

Phosphor screen

Film emulsion

Foam

Light spread

Light-tightcassette

Crystals

X-ray photons

Film

Tradeoff between sensitivity resolution

01/30 II.35

Optimizing sensitivity:

• Fluorescence wavelengths are chosen to match spectral sensitivity of film:CaWO2: 350nm - 580nm, peak @ 430 nm (blue)Rare earths: green - blue

• Dual-coated film, two screen layers

• Optically reflective layers

Characteristics of Fluorescent Screen

cassette

fluorescent screen

photosensitive layer

photoreflective layer

film substrate Tradeoff between

sensitivity resolution dose image

quality

01/30 II.36

FluoroscopyLower x-ray levels are produced continuously and many images must

be presented almost immediately

• Angiography

01/30 II.37

Image IntensifierImage 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 input phosphor screen (CsI)• Emission of low-energy electrons by photo-emissive layer (Sb :

Antimony)• Acceleration (to enhance brightness) and focusing of electrons on

output phosphor screen (ZnCdS)• The ratio of image brightness of the two phosphors is called the

brightness gain of the intensifier tube

15 - 30 cm1,5-2 cm

01/30 II.38

Scintillation DetectorX-ray photon

Scintillationcrystal

Photocathode

V1

V2

Vn

Anode

(grounded)

(1200 V)

Scintillation crystal like NaI emit 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 striken 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.

01/30 II.39

Quantum efficiency of a photocathode = number of photoelectrons emitted

/ number of incident photons

Practical photocathodes show maximum quantum efficiencies of 20-30%

Photocathode :

Dynode:

Conventional dynode materials are BeO, MgO, Cs3Sb

The multiplication factor for a single dynode is given by

= number of secondary electrons emitted / primary incident electron

If N stages are provided in the multiplier section, the overall gain for the PM tube is N.

Conventional dynode materials are characterized by a typical value of =5. Ten stages will therefore result in an overall tube gain of 510 or 10 7.

01/30 II.40

Limits of Analog Systems

(Screen/film, intensifiers):

• Film has limited latitude,

• Film acts as detector, storage, display,

• Development, storage,

• Many steps involved, loss in image information,

• Analog noise

•

Fig. 1a:

In a conventional, digitized R&F imaging chain, the signal degradation that occurs with each component consumes more than 60% of the original x-ray signal.

Fig. 1b:

A digital detector replaces all these components, allowing the user to preserve more than 80% of the original signal – and to further enhance that signal automatically or explicitly.

•

Fig. 1a:

In a conventional, digitized R&F imaging chain, the signal degradation that occurs with each component consumes more than 60% of the original x-ray signal.

Fig. 1b:

A digital detector replaces all these components, allowing the user to preserve more than 80% of the original signal – and to further enhance that signal automatically or explicitly.

01/30 II.41

01/30 II.43

Digital Image Detectors (CCD Based, I)

• 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).

01/30 II.44

CCD must perform 4 tasks to generate an image:

• Generate Charge --> Photoelectric Effect

• Collect Charge --> pixels: an array of electrodes (called gates)

• Transfer Charge --> Apply a differential voltage across gates. Signal electrons move down vertical registers (columns) to horizontal register. Each line is serially read out by an on-chip amplifier.

• Detect Charge --> individual charge packets are converted to an output voltage and then digitally encoded

01/30 II.45

Digital Image Detectors (CCD Based, II)

01/30 II.46

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)

01/30 II.47

Putting it all together: MammographyUsed for detection and diagnosis (symptomatic and screening) of breast cancer, pre-surgical localization of suspicious areas, and 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

Need for good image contrast of various tissue types.Simple x-ray shadowgram from a quasi-point source. Structures are magnified depending on distance to breast-image receptor.

01/30 II.48

Mammography ContrastImage 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.

01/30 II.49

X-ray Projection AngiographyConcerned with diseases of the circulatory system. Contrast material is used to opacify vascular structures of interest. Contrast agent is an iodine-containing compound with maximum iodine (Z=53) concentration of ~350 mg/cm3.Important application is monitoring of therapeutic manipulations (angioplasty, atherectomy, intraluminal stents, catheter placement).Source produces short, intense pulses to produce clear images of moving vessels. Pulse duration ranges from 100-200 ms (for cerebral studies) to 5-10 ms (for cardiac studies).

Biological Effects of X-Ray

01/30 II.51

UnitsIntensity [W/cm2]: Power per unit area

= number of photons [n] photon energy [hn] / time [t] / area [A]

Roentgen [R]:Measure of energy (It): the amount of radiation that produces 2.5810-4 Coulomb [C] of charge separation in air @ standard conditions.

h

n

A

01/30 II.52

Two different materials, if subjected to the same exposure, will in general absorb different amounts of energy.

Because many important phenomena, including changes in physical properties or induced chemical reactions, would be expected to scale as the energy absorbed per unit mass of the material, a unit that measures this quantity is of fundamental interest.

Absorbed Radiation Dose [rad]: Defines the absorbed energy (dependent on target medium): 1 rad = 0.01 joule absorbed by 1 kg of material.

1 Gray [Gy] = 100 rad.

01/30 II.53

Determinants of Biological Effects• Damage depends on deposited (= absorbed) energy (intensity time) per

tissue volume.

• Threshold: No known minimum level below which no damage occurs.

• Exposure time directly effects

• Exposed area: The larger the exposed area the greater the damage (collimators, shields!).

• Variation in Species / Individuals:

• 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 or 0.2 R/working day [Nat. Counc. on Rad. Prot. and Meas.]

01/30 II.54

Radiation Dose for Various X-Ray Procedures

X-ray procedure/exposure

Exposure [mR]

Chest 20

Brain 250

Abdomen 550

Dental 650

Breast 54

Xeromammography 200

CT/slice 1000

01/30 II.55

Effects of ionizing radiation on the living tissueDirect 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).