Biomedical Imaging I

BMI I FS05– Class 3 “X-Ray Instrumentation” Slide 1

Biomedical Imaging IBiomedical Imaging I

Class 2 – X-Ray Imaging II: Instrumentation and Applications

09/21/05


X-Ray GenerationX-Ray Generation

BMI I FS05 – Class 3 “X-Ray Instrumentation” Slide 3

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 electrons

Electrons are accelerated toward the anode target (A) by applied high-voltage (Vtube = 40 – 150 kV); kinetic electron energy: Ke = e HVusually rated in “peak-kilo voltage” kVp

Typical: Vtube = 40 – 150 kVp, Itube = 1-1000mA

Deceleration of electrons on target creates "Bremsstrahlung"

+-

kVp, Itube

CA

VC, If

+-


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

The maximum energy for the produced photon is given by:

BremsstrahlungBremsstrahlung

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

Most of the energy is converted into heat, ~0.5 % is x-ray

'e eh K K

,maxp e tubeE h K eV

K

K’

h

Nucleus


Bremsstrahlung intensityBremsstrahlung intensity

Overall Bremsstrahlung intensity I:

The produced x-ray power Px (in[W]) is given by:

Material constant k = 1.1×10-9 for Tungsten (Z=74).

2tube tubeI V I

2

/ : x-ray production efficiency

x tube tube tube tube tube

x tube tube

P k Z V I kZ V P P

P P kZ V

Electrical power consumption of tube: Ptube = Itube Vtube [W]


Bremsstrahlung spectrumBremsstrahlung spectrum

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

Ib(E) Z(Emax E)

Actual spectrum deviates from ideal form due to

Absorption in window / gas envelope material and absorption in anode

Multienergetic electron beam

Ep h

I

Ep,max,layer 1


Characteristic radiationCharacteristic radiation

Narrow lines of intense x-ray at characteristic energies are superimposed on the continuous bremsstrahlung spectrum.

Caused by removal of inner shell electrons and subsequent filling of hole with electrons from higher shell under emission of x-ray at shell-energy difference

Lines are named after the lower shell involved in the process; the upper shell involved is denoted by Greek letters: n = 1 -transitions, n = 2 -transitions, ...

-

--

-

--- -

--

-

h

KLM

-

Continuum0

K

L

M

N

E [keV]

K-lines

L-lines

K

K

K

0.5

3

11

70


X-ray spectraX-ray spectra

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

Maximum photon energy: Ep[keV] hmax e kVp

Characteristic radiation occurs only for anode voltages

e kVp > IK,L,M,…

74W


X-ray tube designX-ray tube design

Cathode w/ focusing cup, 2 filaments (different spot sizes)

Anode

Tungsten, Zw 74, Tmelt 2250 ºC

Embedded in copper for heat dissipation

Angled (see next slide)

Rotating to divert heat


Reduction of anode heatingReduction of anode heating

Anode angle of 7º…15º results in apparent or effective spot size Seffective much smaller than the actual focal spot of the electron beam (by factor ~10)

Seffective depending on image location

Rotation speed ~ 3000 rpm

Decreases surface area for heat dissipation from w (r2 r1) to (r2

2 r12); generally by a

factor of 18-35.


Limitations of anode angleLimitations of anode angle

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

"Heel effect" causes inhomogeneous x-ray exposure


Magnification and image blurMagnification and image blur

Geometric magnification given by

Reduction of M by minimizing B, i.e. placing patient next to film. Finite target thickness can lead to variations in M.

Blurring of edges and fine structures due to finite source size leads to penumbra p:

loss of spatial resolution

A

BA

O

IM

A

Bsp


X-ray dosimetry IX-ray dosimetry I

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 = 0.001 J/kg = 0.001 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

Threshold: No minimum level is known, below which damage occurs

Exposure time: Because of recovery, a given dose is less harmful if divided

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

Variation in Species / Individuals: LD 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 [Nat. Counc. on Rad. Prot. and Meas.]


X-Ray DetectionX-Ray Detection


RadiographyRadiography

Few high-quality images are made in a study

Orthopedic

Chest

Abdomen

(Mammography)


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

Film Composition:

Transparent plastic substrate (acetate, polyester)

Both sides coated with light-sensitive emulsion (gelatin, silver halide crystals 0.1-1 mm). Exposure to light splits ions atomic silver appears black (negative film)

Blackening depending on deposited energy (E = I t)

Optical density (measure of film blackness) for visible light:

D = log (Iincident/Itransmitted)

D 2 = "black", D = 0.25 … 0.3 = "transparent (white)" with standard light box (diagnostic useful range ~ 0.5 - 2.5)


Film characteristic curve (H and D curve) IFilm characteristic curve (H and D curve) I

Relationship between film exposure 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


Film characteristic curve IIFilm characteristic curve II

Film characteristics continued:

Film gamma (maximum slope):

Contrast C D/log X

Latitude: Range of exposure causing appreciable values of D 0.5…2.5

Compromise between maximum gradient and latitude causes under- / overexposed regions in the image.

2 1

2 1 maxlog log

D D

X X

Film gamma Contrast, latitude


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


Fluorescent screensFluorescent 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.

CaWO4: 5%

Rare earth phosphors: LaOBr:Tb, Gd2O2S:Tb, Y2O2S:Tb: 12 - 18%


Screen / Film CombinationsScreen / Film Combinations

Sandwiching phosphor and film in 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


Characteristics of Fluorescent ScreenCharacteristics of Fluorescent Screen

Fluorescence wavelengths are chosen to match spectral sensitivity of film:

CaWO2: 350nm-580nm, peak @ 430 nm (blue)Rare earths: Gd: green

La: blue

Dual-coated film, two screen layers

Optically reflective layers cassette

fluorescent screen

photosensitive layer

photoreflective layer

film substrate


FluoroscopyFluoroscopy

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

Angiography

GI tract


Image IntensifierImage Intensifier

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 phosphor screen (CsI)

Emission of low-energy electrons by photomissive layer (antimony)

Acceleration (to enhance brightness) and focusing of electrons on output phosphor screen (ZnCdS)

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


ApplicationsApplications


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

Need for good image contrast of various tissue types.

Simple x-ray shadowgram from a quasi-point source.


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.


Mammography sourceMammography source

Voltage ~ 25-30 kVp

Target material Mo, Rh (characteristic peaks)

Filtering:

Target Mo, Filter Mo Target Rh, Filter Rh


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 ~ 3.5-5:1. Only scatter correction in one dimension. Scatter-to-primary (SPR) reduction factor ~5

Recently crossed grid introduced

Grids are moved during exposure

Longer exposure

detector

breast

leadsepta


X-ray projection angiographyX-ray projection angiography

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

Biomedical Imaging I

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