PHYSICS - X-ray Flashcards
Relationship of frequency and wavelength to photon energy
frequency is proportional, wavelength is inversely proportinal; via Planck’s equation
Effect of higher Z on K-shell binding energy
increased K-shell binding energy (stronger nuclear attraction with higher Z)
Specific ionization (SI) relationships
SI is proportional to charge and inversely proportional to velocity
Specific ionization definition
number of ion pairs generated per unit path length
Inverse square law
beam intensity is proportional to 1 / d^2; beam intensity is x-rays produced PER SECOND
Projection naming related to source or detector
source (e.g. PA = source is posterior to patient, LAO = source is left-anterior to patient)
X-ray tube leakage limit
<1 mGy per 1 hour at 1 meter
Ideal anode characteristics
good conductor of heart and electricity, high melting point, high Z
Thermionic emission definition
process of electrons boiling off filament and moving toward anode
Primary x-ray-generating interaction in tungsten
Bremsstrahlung
Primary electron interaction in tungsten
excitation (release of heat)
K-shell binding energy of Ag
-25 keV; Ag = silver
K-shell binding energy of iodine
-33 keV
K-shell binding energy of barium
-37 keV
Change in position of characteristic peaks
target material has been changed
Bremsstrahlung radiation is the result of electron interaction with…
nucleus
Effect of increased Z material on Bremsstrahlung production
increased Bremsstrahlung production (vs. characteristic x-ray production)
Majority of x-rays in general radiology are…
Bremsstrahlung
Majority of x-rays in mammography are…
characteristic x-rays
Relationship of target material Z to Auger electron production
lower Z material => more Auger electron production (vs. characteristic x-rays)
mA is proportional to…
number of x-rays produced PER SECOND (mAs is obtained by multiplying by the # of seconds of the exposure)
mAs is proportional to…
total number of x-rays produced
Increase number of photons by 2x by…
doubling mAs or increasing kVp by 15%
Result of using a kVp less than the K-shell binding energy
no characteristic x-ray production (from the K-shell)
Average beam energy with a tungsten target
approximately 1/2 of kVp
3 ways to increase average beam energy
increase kVp, use target material with a higher Z, beam filtration (“hardening”)
How to: lower dose while maintaining a constant exposure
increase kVp by 15%, half mAs
Standard kVp for a chest radiograph (with grid)
120 kVp; with grid = not portable
Reduce voltage ripple by…
using a “three-phase generator” or “high frequency inverter generator”
2 standard deviations encompass what percentage?
95%
Primary contributor to patient dose (amongst photon interactions)
photoelectric effect
End-result of photoelectric absorption
original photon absorbed, photoelectron produced, characteristic x-ray or Auger electron production
Relationship of Compton scatter to Z of patient tissue
Compton scatter is independent of Z (only affected by 1/E and material density)
End-result of Compton scatter
free electron (Compton electron), new direction and lower energy of photon (which may cause more Compton interactions)
Major determinant of image contrast
photoelectric absorption
Dominant photon interaction at higher kVp
Compton scatter (photoelectric effect dominates at lower kVp; e.g. mammo)
How to: decrease scatter (creation or reaching detector)
collimation (smaller FOV), thinner object, air gap, grid
Delta rays
x-rays created when ejected electrons have enough energy to cause additional ionizations
Ideal kVp for a DSA
70 kVp
Ideal kVp for a CTA
100 kVp
Ideal kVp for barium fluoro
90-110 kVp
Half value layer definition
thickness of a material at which beam intensity (or air kerma) is reduced by one-half; expressed in mm
Effect of higher average beam energy on HVL
larger HVL (and vice-versa for lower average beam energy)
For a given kVp, monoenergetic or polyenergetic beams have a higher HVL?
monoenergetic beams have a higher HVL at a given kVp
Determinants of focal spot size
filament length, focusing cup charge, anode angle
How to: decrease heel effect
larger anode angle, increase SID, smaller FOV
Inherent filtration definition
attenuation occurring within the anode itself
Added filtration definition
attenuation occurring with the use of a filter
Most common situations for use of a copper filter
pediatrics and IR
Effect of collimation
smaller FOV (area of exposure) => fewer photon-patient interactions => lower dose, less scatter => increased contrast
Standard grid ratio for general radiography
10:1
Standard grid ratio for mammography
5:1
Typical Bucky factor
5 for general radiography; therefore a grid typically increases dose 5x
Effect of adding a grid
reduced scatter => increased contrast, higher dose, longer exposure required
Artifact: radiograph overpenetrated centrally and underpenetrated peripherally (left and right sides)
grid cutoff artifact (upside-down)
Linear attenuation coefficient
describes attenuation per unit length of tissue; varies with kVp and tissue properties (Z and density); expressed in cm^-1; different for ice, water, and water vapor
Effect of increasing kVp on _ (LAC)
smaller _ (less beam attenuation per unit length of tissue)
Effect of increasing tissue Z on _ (LAC)
larger _ (more beam attenuation per unit length of tissue)
Effect of increasing tissue density on _ (LAC)
larger _ (more beam attenuation per unit length of tissue)
_ (LAC) for photon energies at the k-edge is increased or decreased?
increased _ (more beam attenuation per unit length of tissue at the k-edge)
Relationship between _ (LAC) and HVL
inversely related; materials with a higher _ have a smaller HVL
Mass attenuation coefficient
describes attenuation per unit mass of tissue; expressed in g^-1; same for ice, water, and water vapor
Effect of decreasing SOD (same SID)
increased magnification, increased blurring, smaller FOV, decreased scatter (air gap); results in increased contrast (less scatter) and decreased spatial resolution
Area of exposure (FOV) is proportional to…
SOD^2
Kerma area product (KAP)
equal to AK * area exposed; independent of distance (SOD)
Quantum mottle increases with…
SID (greater SID => increased quantum mottle; inverse square law)
Flat panel detector (FPD) systems
indirect DR and direct DR; does NOT include CR and photon counting systems
Effects of using a screen in film screen (vs. no screen)
increased speed (sensitivity) => decreased dose, decreased spatial resolution
Components of film and screen layers
film layer contains a base and a single or double emulsion; screen layer contains a phosphor and a reflective layer
Size and shape of silver halide crystals influences…
speed, contrast, and resolution
Emulsion type, general radiography vs. mammography
general uses a double emulsion, mammo uses a single emulsion
Photostimulable phosphor
used in CR; activated electrons are excited into a metastable state; composed of oxysulfides (e.g. GdOS)
Effect of decreasing laser spot size (CR)
increased spatial resolution
Spatial resolution in CR systems is dependent on…
laser spot size, phosphor density and thickness, rate of light sampling
Scintillator properties in indirect DR
thallium-doped CsI (or NaI)
Role of photodiodide in indirect DR
converts light into charge separation (which can be measured by TFT array)
Drawback of CCDs
requires demagnification (minification) because size is limited to a few cm^2
aSi vs. aSe
aSi is used in indirect DR, TFT array exists within aSi; aSe is used in direct DR, converts x-ray into charge separation
Number of possible grayscale values per pixel
2^n, where n = # of bits per pixel
Penetration is related to…
kVp; extent to which x-rays penetrate through the patient
Exposure is related to…
mAs; total number of photons used to image patient
Image is too white and spine is not visible
under-penetrated
Image is too black and spine is too easily seen
over-penetrated
Impact of increasing pixel density on fill factor
decreased fill factor => higher dose required
Determinants of spatial resolution
focal spot size, geometric magnification/blurring, motion blur, detector properties
Limitations of spatial resolution in film screen and digital systems
film screen is focal spot size; digital systems is pixel size
Modulation transfer function (MTF)
measure of spatial resolution for an imaging system
Detector quantum efficiency (DQE)
measure of signal to noise for an imaging system
Effect of a higher DQE system on patient dose
same image quality can be obtained using a lower dose in a higher DQE system
DQE of direct DR, indirect DR, and film screen systems
direct DR > indirect DR > film screen; better DQE means a lower dose can be used to obtain the same image quality
Modifications for pediatric imaging
no grid, lower kVp (thinner object), same or decreased mAs
Typical kVp for chest x-ray (no grid)
90 kVp; no grid = portable
Typical SID for a PA chest radiograph
72” (or 182 cm)
Typical SID for AP CXR, AXR, skull, C-spine, or extremities
40” (or 100 cm)
Typical SID for mammography
26” (or 65-70 cm)
Spatial resolution for screen film mammo
13 lp/mm in parallel direction and 11 lp/mm in the perpendicular direction; relative to anode-cathode axis
Spatial resolution for digital mammo
7 lp/mm
Spatial resolution for screen film radiography
6 lp/mm
Spatial resolution for digital radiography
3 lp/mm
Artifact: focal area of darkening
fogging; addition of charge to detector by something other than the x-ray beam; may affect all or part of image; e.g. cassette left in x-ray room or bracytherapy seeds within patient
Artifact: two superimposed images
double exposure a.k.a. twin artifact; cassette used twice without erasing the plate in between (or without changing screen film); part of image double exposed = incomplete erasure
Artifact: decreased image density peripherally
delayed scanning (CR); image degradation begins peripherally
Tube power formula
kVp * mA; measured in Watts (or Joules/second)
Tube window material (general radiography)
pyrex glass (vs. beryllium in mammo)
Major source of occupational exposure
Compton scatter
Relationship of quantum mottle to SID
quantum mottle is proportional to SID^2 (inverse square law)
Relationship between FOV and anode angle
increased anode angle => increased FOV (area of exposure)
Artifact: one side of image appears more exposed than the other
grid cutoff artifact (off-level or not centered)
Artifact: thin white line through image
dust/dirt on CR plate reader (“light guide”)
Artifact: tiny focal white dot(s)
dust/dirt on imaging plate
Artifact: alternating lines throughout image
Moire artifact; seen with stationary grids, low density grids, or misaligned in plate reader; use higher density grid or moving grid
If question stem mentions “obesity”…
remember that scatter is increased in bigger patients
Focused grid
septa diverge away from patient (vs. a parallel grid); optimized for a particular SID
Subject contrast
determined by beam characteristics and tissue characteristics (basically the same stuff that determines PE effect probability); loss of subject contrast means decrease PE absorption
Image contrast
determined by subject contrast, detector properties, post-processing
Equilibrium between PE effect and Compton scatter (soft tissue and bone)
25 keV for soft tissue, 40 keV for bone
DEXA (acronym)
Dual Energy X-ray Absorptiometry; 2 kVp’s utilized to quantify bone mineral density (70-100 and 140 kVp)
DEXA: osteoporosis
T-score < or equal to -2.5
DEXA: osteopenia
-2.5 < T-score < -1.0 (note that a score of -1.0 would be considered normal)
DEXA: what is T-score?
compared to health 30 y/o adult
DEXA: what is Z-score?
compared to an age-matched control
K-shell electrons are in a low or high energy state relative to M-shell electrons?
low energy state (require greater energy to remove)
Speed of light
3 x 10^8 m/s
K-shell binding energy of Tungsten
70 keV
RIS (acronym)
Radiology Information System; workflow management (order entry, preauth, billing, distribute reports, coding)
