X-Ray Detection
Radiography
• Few high-quality images are made in a study
• Orthopedic
• Chest
• Abdomen
• (Mammography)
Radiation Units
• EXPOSURE
– Amount of radiation in an x-ray beam capable of
ionizing air
• Total charge of electrons liberated per unit
mass by x-ray photons
• Coulombs per kg (C/kg) or Roentgens (R )
• 1R =2.58 10-4 C/kg
• Defined only for photons of E <3MeV
Absorbed Dose
• Amount of energy (E) absorbed per unit mass (M)
– D=E/M
• SI units Grays
1 Rad = 100 erg/g = 0.001 J/kg = 0.001 Gray [Gy]
[SI units]
1erg=1e-7 joule
• Depends on material so material and locations should
always be specified
• Radiation risk is related to absorbed dose
Effective dose equivalent
Effective dose equivalent HE [Sv] (Sievert) takes
into account sensitivity of organ exposed:
HE  wi Hi
i
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)
f factor – Conversion factor between exposure (X)
and absorbed dose D = f X
air, muscle, soft tissue f~ 1
bone f ~4
Biological 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
• Exposed area: The larger the exposed area the greater the damage
(collimators, shields!)
• 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.]
Analog X-ray imaging
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Film
Intensifying screens
Scatter removal
Image intensifiers
Photographic 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/Io)
– D > 2 = "black", D = 0.25 … 0.3 = "transparent (white)" with standard light box
(diagnostic useful range ~ 0.5 - 2.5)
Film characteristic curve
• Relationship between film exposure and optical density D
• Film characteristics:
– Fog: D for zero exposure
– Sensitivity (speed S):
Reciprocal of exposure XD1 [R]
S  1/XD1
– Linear region
XD1
Film blackening
Blackening is related to the number of photons
reaching the film
Measured in OD
OD=-log (I/Io)
The eye has a log
response to ligth
Film 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
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
• One x-ray photon can generate multiple optical photons
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% (calcium tungstate)
– Rare earth phosphors: LaOBr:Tb, Gd2O2S:Tb
Screen / 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
Film emulsion
Light-tight
cassette
Foam
X ray
X-ray photons
Crystals
Phosphor screen
Film
Light spread
Characteristics 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
photoreflective layer
fluorescent screen
photosensitive layer
film substrate
Image 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
Applications
Fluoroscopy
• Allow real time observation of dynamic
activities: barium moving through the GI tract
• Low dose xray
Mammography
• 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.
Mammography 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 source
• Voltage ~ 25-30 kVp
• Target material Mo, Rh (characteristic peaks)
• Filtering:
Target Mo, Filter Mo
Target Rh, Filter Rh
Anti-scatter grid
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Significant Compton interaction for low Ep (37-50% of all photons).
Linear grid: Scatter-to-primary (SPR) reduction factor ~5
Recently crossed grid introduced
Grids are moved during exposure
Longer exposure
breast
lead
septa
detector
X-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
X-ray phase contrast