Xray and mammo Flashcards
Xray yield proportional to?
Z squared
Auger e heavy vs light elements
Heavy elements more likely x rays
Lighter elements more likely Auger
Secondary ionization electrons are called?
Delta rays
Why use plastic to shield beta emitters?
High Z = more bremstrahlung
Y90
Low Z plastic minimizes brems
mA vs kVp on intensity
mA = quantity, current at cathode
kVp = kinetic energy given to electrons, defines maximum energy
Xray production increases linearly with mA
increasing kVp by 15 % will double the intensity of spectrum
Heat math
tube power =
heat units =
Power = kV x mA
heat units = kVp x mA x seconds
130kv x 190mA = 24,700 watts or 24,700 J for a 1 second exposure
Watt = joule per second
“average energy”
affected by?
rough guess?
attenuation at target, exiting window, collimation
Standard Tungsten target with normal filtration, average energy is 1/3 - 1/2 the maximum energy (kVp)
kVp and mA spectrum changes
kVp moves the peak of the curve, increasing kVp increases average energy (and max energy)
mA increases size (area) of/under the curve
increased kVp and entrance skin dose
Entrance skin dose will change as the square of the change in kVp (tube voltage)
characteristic peaks move?
characteristic peaks go away?
Move = changing target material
Characteristic peaks disappear = kVp dropped below threshold for k shell e
HVL depends on?
DOES NOT DEPEND ON?
average photon energy (more energy = further)
higher Z target anode material GREATER HVL
More filtration, higher average energy GREATER HVL
Less filtration, lower average, LOWER HVL
mAs has no effect
10th HVL ?
“TVL”
attenuate 90%
used for shielding calcs
Average brems energy?
1/3 kVp selected
DEXA methods/trivia
relies on?
methods?
transmission measurements made at 2 different photon energies
filter that drops k-edge into middle of spectrum
switch tube voltage between low and high (70 and 140)
mammo focal spot size?
0.3 and 0.1 mm
general xray focal spot size?
0.6 and 1.2mm
limitation of portable xray anode?
usually stationary (doesn’t rotate), limits tube rating
Target angle and focal spot
SMALLER ANGLE = SMALLER FOCAL SPOT
(better spatial res)
heel effect worse on?
anode side
Heel effect worse with?
SMALLER angle
SMALLER SID
LARGER FOV (less uniform when spread out)
Mammo app of heel effect (position)?
Chest wall Cathode
Nipple Anode
mA/kVp and focal spot?
High mA, low kVp = WIDER
High kVp = SMALLER
more photons spread out more, blooming
higher kVp, moving faster, spread LESS
CLASSICAL/COHERENT interaction
what is?
energy change?
effect on dose?
low energy electron basically bouncing off an outer shell electron
NO ENERGY LOST
DIRECTION CHANGE
DON’T CAUSE IONIZATION
ADDS A TINY BIT OF DOSE
DOESN’T CONTRIBUTE TO IMAGE
Clinical setting of classical/coherent
LOW ENERGY, MOSTLY in MAMMO
15 % of photon interaction below 30keV
COMPTON “BAD GUY”
What is?
3 actors when its done
HIGH ENERGY incoming xray hits an outer shell e, knocks it out, loses some energy and heads off in new direction
IONIZED atom - bad
Free electron - bad, 2ary interactions
scattered photon bad - fogs image
COMPTON interactions depend on/probability of a ?
DOESNT depend on Z of the atom
DOES depend on density of material
DOMINANT contributor to scatter/fog
MAJOR SOURCE OF OCCUPATIONAL EXPOSURE
PE dominates at?
What is?
LOWER ENERGY RELATIVE to COMPTON
incoming xray gives all energy to inner shell e
ALL OR NOTHING, incomin xray is TOTALLY ABSORBED
–> characteristic xray or Auger (auger dominates in soft tissue, lower Z)
Energy of PE interaction?
need energy higher than inner shell binding
peak energy for PE is around this binding energy
Xrays with more energy pass through
INVERSE to THRID POWER
HIGHER Z makes PE more likely
ALSO INVERSE TO THIRD POWER
PE and image contrast
HIGHER Z = greater chance of PE = more xray absorption
Probability of PE and cubes
DIRECTLY PROPORTIONAL TO Z cubed
INVERSELY PROPORTIONAL TO ENERGY CUBED
K edge
spike in attenuation corresponding to the K shell binding energy
Lower energy are easily attenuated (no contrast)
Higher pass through everything (no contrast)
K edge barium iodine example
kVp selected?
K edge of barium = 37 keV, iodine = 33 keV
Select kVp of 65-90
average energy around these (1/3 - 1/2 kVp)
PEDS XRay
Grid?
kVp?
mAs?
NO GRID
lower kVp ~ 65 (kids are small, don’t need much to penetrate)
(adult cxr kVp = 120-140)
same or just less mAs
attenuation in tissue depends on 3 things?
Effective Z in tissue
X ray beam quality (energy)
Tissue density
ABOVE 30 kEv this interaction dominates?
BELOW?
Above = COMPTON
Below = PE
Noise
photon starvation
Factors that increase or decrease noise
Post processing?
Not a good answer. raw data is still crap
Factors that increase or decrease noise
Field size/collimation
smaller field/collimation decrease photons, increase noise
therefore mAs usually increased with collimation
(in this situation book says answer is collimation decreases noise)
Scatter primarily depends on ?
Collimation - less field less scatter
Thickness of body part
Energy of beam (Compton dominates >26kVp in soft tissue, 35kVp in bone)
Grid ratio
height to distance between
Grid effect on dose?
bucky factor?
increases dose (abc)
mAs with grid/mAs without (usually 2-3)
“Grid cut off”
too many photons blocked –> quantum mottle
grid not aligned/positioned correctly.
kVp and mAs with noise
both increase exposure but kVp going up will increase Compton scatter potentially more noise.
mAs better answer for decreasing noise
SDD and noise?
Increase in noise with increased SDD, described by inverse square law
noise vs mottle
noise includes scatter
mottle = photon starvation
Spatial resolution quant?
Spatial frequency?
Unsharpness?
line pairs per mm
spatial frequency = spatial resolution
unsharpness means loss of spatial res
Types of unsharpness
Motion (decrease exposure time)
System unsharpness = limiting factor of detector
Geometric unsharpness
System unsharpness
Film?
Computed radiography (CR) ?
Digital radiography (DR) ?
Film = size of the grain of photographic chemical
CR = size of laser used to read the phosphor plate
DR = size of individual thermoluminescent transistor
Geometric unsharpness (–> blur)
focal spot
SOD
ODD
MAG
focal spot - smaller = less blur
SOD - further = less blur
ODD - closer = less blur
mag –> blur
Magnification
formula?
SOD + ODD
SOD
MTF?
ability to maintain contrast as a function of spatial resolution
curve with MTF on Y and spatial res on X
As spatial res increases, MTF, ability to maintain contrast, decreases
Nyquist frequency?
spatial resolution = half of nyquist frequency
How little you can sample something and still be able to tell what it is…
Harder and harder to keep up signal when looking at smaller and smaller line pairs (harder and harder to have contrast when looking at smaller things)
Detective Quantum efficiency
estimate of required exposure level necessary to create an optimal image
“prediction of dose”
ideally = 1.0 (all radiation energy absorbed and converted to image) at any spatial resolution
Better the DQE, the less radiation needed to maintain spatial resolution
high DQE, low dose
DQE factoids
- Is a measurement of EFFICIENCY
- Directly proportional to MTF
- Inversely proportional to SNR
- Better at LOW SPATIAL RES
- Usually better, around 0.45 for DR, CR or film 0.25
HIGH DQE = HIGH DOSE LOW DQE = LOW DOSE
MORE JUICE PER SQUARE INCH to see fine details
Spatial res and film
probably only advantage of film, made of a continuous thing, no pixels, small advantage in spatial res
Random things that improve contrast
those that decrease scatter
Grid and collimation
Image receptor contrast
digital imaging
Pixel depth = determined by number of bits
number of shades of gray available for imaging
more bits = more contrast resolution
Window width and contrast
narrower window = more contrast
Level determines BRIGHTNESS
Primary factor influencing contrast in plain film vs digital
Plain film primary factor for contrast is kVp
Digital systems primary factor for contrast is LUT
Look Up Table?
histogram of known input intensities and corresponding grayscale
allows rescaling for adjustment to present optimal image (underexposed scaled darker, overexposed scaled lighter)
Means less important to adjust kVp when using digital
MEANS WIDER DYNAMIC RANGE IN DIGITAL SYSTEMS
“Brightness”, film vs digital
FIlm, brightness means how many xrays are blocked
bones block a lot, they’re bright
Digital imaging, brightness = level
level up to see lung
Dynamic range
narrow in film
easy to under or overexpose
Digital
larger dynamic range and linear response to exposure
Dynamic range and dose
with digital
can underexpose a little and fix later
kVp less important, can go up by 15% and cut mA in half to decrease dose
“pixel pitch”
distance from center of one pixel to another
pixel pitch, density and spatial res
Better spatial res =
higher pixel density (pixels per unit area)
lower pixel pitch
AEC mechanism
ionization chamber BETWEEN PATIENT AND RECEPTOR
charge produced in chamber, calibrated to know how much charge to terminate an exam based on body part
AEC controls?
QUANTITY of radiation reaching receptor, NO effect on kVp
Bit depth
number of bits determining number of shades of gray that can be displayed on a monitor
2 to the X number of bits = number of gray shades
Classification of digital detectors
CR
DR
CR - storage phosphor (computer think laser), type of indirect
DR - Flat panel detectors direct or indirect
Direct vs indirect
INDIRECT = SCINTILLATORS
XRAYS –> LIGHT –> CHARGE
DIRECT = photoconductors
XRAYS –> CHARGE
CR = storage phosphor
mechanism
electons in phosphor interact with xrays, change to metastable state (in ‘conduction band’)
storage phosphor holds ‘LATENT IMAGE’
CASSETTE then docking station, readout
“photostimulable luminescense”
Phosphor readout in CR
laser
red light laser liberates trapped electrons, return to their shells and release energy as BLUE-GREEN LIGHT
blue green light signal directed to a photodetector which then converts it to an electronic signal
signal digitized and divided into a matrix
DR
no cassette
Flat panel detectors
what most people mean when they say “digital detector”
faster than CR or film
INDIRECT or DIRECT
DR Indirect
INDIRECT THINK SCINTILLATOR
INDIRECT THINK LIGHT inbetween XRAYS and CHARGE
Xray activates Cesium Iodide
light emitted, converted by a photodiode into a charge
Charge captured and transmitted by Thin-Film-Transistor (TFT) to the workstation
Lateral dispersion of light
INDIRECT method
LOSS of spatial res
phosphors
WORSE WITH GREATER THICKNESS OF CRYSTAL
Types of indirect
CR = INDIRECT = PHOSPHOR
DR INDIRECT = CS scintillator (and photodiode, TFT)
DIRECT think?
PHOTOCONDUCTOR
NO LIGHT IN BETWEEN
XRAY –> CHARGE
AMORPHOUS SELENIUM
DIRECT rough mech
DIRECT THINK SELENIUM
charge applied across selenium, same direction as x rays
Xrays absorbed by selenium, electrons released, travel to surface of selenium and neutralize some of applied charge
NO LAT DISPERSION
charges are drawn to charge storage capacitor connected to TFT
pattern of charges scanned and converted to a digital signal
“Fill Factor”
DR vs CR
Area of detector sensitive to Xrays (in relation to entire detector area)
HIGHER = more EFFICIENT
DR - electric field shaping allows near 100% fill factor
CR WORSE
DQE
DR better fill factor = more efficient = HIGHER DQE
Factors specific to Spatial Res of CR
SMALLER LASER?
THICKER PHOSPHOR?
SAMPLING FREQUENCY?
SMALLER LASER BETTER
THICKER PHOSPHOR WORSE
HIGHER SAMPLING FREQ = SMALLER PIXEL PITCH = BETTER
Spatial res of DR
BETTER than CR WHY
avoid lateral dispersion
Centralized vs Decentralized workflow
CR Centralized (Cassettes have to be brought somewhere)
DR Decentralized. image acquired, adjusted and transferred to PACS by the tech, in the room
Ideal average energy for mammo
16-23 keV
therefore set kVp to 25-30 keV
Mammo targets
molybdenum or rhodium (vs tungsten)
K edge filtration
goal?
what’s filtered out?
goal = creating a mono-energetic beam
moly used, 20keV k edge, energies lower than 15 and higher than 20 filtered out
Rhodium
similar, slightly stronger spectrum than moly
Rh vs Moly
Denser breasts?
Mo anode with Rh filter?
Rh/Rh for denser breasts (higher energy)
Mo/Rh intermediate
mammo focal spot size
effect on mA
.1 and .3 mm in mammo
general .6 and 1.2
smaller spot gets hotter, have to lower mA
mA 50 for 0.1 and 100 for 0.3
lower mA means longer exposure
Spatial res trivia
Screen film mammo
15 lp/mm
Spatial res trivia
digital mammo
7 lp/mm
Spatial res trivia
Digital radiograph
3 lp/mm
Spatial res trivia
CT
0.7 lp/mm
Spatial res trivia
MRI
0.3 lp/mm
Heel effect compensation in mammo
cathode side = chest wall
ANGLING TUBE UP TO ABOUT 20 degrees
Effective anode angle = ?
Anode angle + tube angle
mammo tube exit window vs general
General uses PYREX
Mammo uses Beryllium
PYREX attenuates energies used in mammo
Compression
Less scatter = lower kVp can be used
lower kVp and less scatter = improved contrast
Thinner = less mAs, less dose
no motion
Closer to Bucky = less mag
less motion and mag = better spatial resolution
mammo grid ratio
low kVp and compression already lower scatter, so lower grid ratio used
usually 4-5
standard xray 6-16
Mammo and mag
distance to source cut in half –> double mag
mag view air gap
NO GRID NECESSARY
increased object to detector distance means scatter scatters further away from detector
mag view and focal spot
mA
exposure time
smaller focal spot used to improve spatial res
mA decreased so not to melt anode
exposure time increased, compensation for boob to detector distance
Digital vs analog mammo dose
Digital - 1.6 mGy
analog - 1.8 mGy
Dark noise
electronic fluctuations within the detector element
effect proportional to temp of detector
coolers
flat field test
imaging a large piece of acrylic
improve image quality
calibrate digital detectors
Digital artifacts
Ghosting
residual from prior exposure
lead is not allowed on flat panel digital systems
Digital artifacts
Bad pixels
square or a streak
PPV =
proportion of people who have a positive study and actually have breast cancer
PPV think all we care about is positive tests
positive test, has cancer
——————————————
positive test with cancer + positive test, no cancer
PPV1 ?
benchmark
ANYTHING other than continued screening (BR0, BR3, BR4, BR5)
4.4% cancer within 1 year
PPV 2 = ?
benchmark?
Cases where biopsy was recommended (BR4 or 5)
25.4% dx cancer within 1 year
PPV 3 = ?
Benchmark
Results of biopsy, PBR pos biopsy rate
31%
MAMMO MEM SHIT
target recall rate for audit
5-7%
MAMMO MEM SHIT
cancers per 1000 screened
3-8
MAMMO MEM SHIT
Processor QC
Daily
MAMMO MEM SHIT
Darkroom cleanliness
DAILY
MAMMO MEM SHIT
Viewbox conditions
WEEKLY
MAMMO MEM SHIT
PHANOM EVAL
WEEKLY
MAMMO MEM SHIT
Repeat analysis
Quarterly
MAMMO MEM SHIT
Compression test
SEMI annually
MAMMO MEM SHIT
darkroom fog
Semi annually
MAMMO MEM SHIT
screen film contrast
SEMI annually
MQSA
sites accredited and certified q?
MONEY TO?
3 years
MQSA = FDA
Mammo education reqs
240 during a 6 month period in last 2 years
3 months of formal training
60 documented hours of mammo education
Mean glandular dose
boob phantom spex
4.2 cm of compressed breast that is 50/50 adipose and glandular
MQSA DOSE
UNDER 300 millirads
3 mGy
ONLY FOR PHANTOM
MEASURED WITH A GRID
WITHOUT GRID = 1mGy
diagonal corduroy on a CR Xray
Moiré type artifact can be seen in computed radiography (CR) systems if grids with low grid frequencies are used that are oriented parallel to the plate readers scan lines.
