PHYSICS - Mammo Flashcards
Ideal beam properties for mammography
low energy, nearly mono-energetic beam
Typical kVp for mammo
25-30 kVp => average beam energy of 16-23 keV
Dominant photon-tissue interaction in mammo
photoelectric effect (dominates at low energies)
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
Majority of x-rays in mammo are…
characteristic x-rays (vs. Bremsstrahlung in general radiography)
Target/anode combinations for dense breasts
Rho/Rho or W/Ag
X-ray spectrum in W/Ag setup
composed of Bremsstrahlung (characteristic x-rays of W are removed by Ag filter)
K-shell binding energy of Moly
-20 keV
K-shell binding energy of Rho
-23 keV
K-shell binding energy of silver (Ag)
-25 keV
Focal spot size (mammo)
0.3 mm (non-mag views) and 0.1 mm (mag views)
Mag view changes
smaller focal spot, air gap, no grid, smaller compression paddle
Effect of smaller focal spot
higher resolution, more anode heating => lower mA => longer exposure required => higher chance of motion
mA for non-mag and mag views
100 mA and 50 mA, respectively; due to smaller focal spot for mag views
Heel effect
higher intensity beam on cathode side, and vice-versa
Tube window material (mammography)
beryllium (vs. pyrex glass in general radiography)
Standard compression force
25 lb (or 111 Newtons)
Effect of compression on contrast
reduced tissue thickness => less Compton scatter + more photons reach detector => kVp can be reduced => increased contrast
Effect of compression on spatial resolution
reduced tissue thickness => more photons reach detector => AEC reduces exposure time (lower mAs) => lower chance of motion => increased resolution
Grid ratio (mammo)
5:1 (vs. 10:1 for general radiography)
Bucky factor (mammo)
2 (vs. 5 for general radiography)
Scatter reduction in mammo is accomplished by…
compression (thinner object), air gap, grid, lower kVp
Mag view - spatial resolution factors
smaller focal spot increases resolution, but decreased SOD results in increased geometric blurring + longer exposure required due to heating increases chance of motion => net effect is INCREASED resolution (better)
Mag view - patient dose factors
decreased SOD (air gap) => increased dose (by inverse square law), but grid removal results in decreased dose => net effect on dose is unclear
Mag view - contrast factors
decreased scatter reaching detector (air gap) => increased contrast
Benefits of single emulsion film
less parallax, less crossover
Drawback of single emulsion film
increased dose (relative to double emulsion)
Film placed before screen to…
minimize halation (spread of light) which increases blurring
Limiting factors for spatial resolution in film screen and digital mammo
focal spot size for film screen and pixel size for digital
Artifact: residual image from prior exposure burned into detector
ghosting; seen with highly attenuating objects; “burned into detector”
Air gap distance
15-30 cm
“Contact mode”
a.k.a. normal non-mag view; breast is in contract with grid/detector
ACR breast phantom characteristics
4.2 cm thick, 50% glandular; contains 6 fibers, 5 masses, 5 specks
Mammo view naming convention
based on path of penetration; e.g. ML view = source is medial, detector is lateral
Benefits of tomosynthesis
improved contrast and specificity, decreased recall rate, increased cancer detection rate
Drawbacks of tomosynthesis
increased acquisition time => risk of motion artifact
MQSA acronym + year + who created
Mammography Quality and Standards Act (MQSA), 1992, created by congress
MQSA is enforced by…
the FDA, or separately by the VA for VA facilities
MQSA regulates…
regulates dose, training, QC, and facilities
Average glandular dose for single image of breast phantom (with grid)
<3 mGy (applies to phantom, not actual patients); this is an MQSA regulation
Average glandular dose for single image of breast phantom (no grid)
<1 mGy (applies to phantom, not actual patients); this is an MQSA regulation
Lay report due within…
30 days
Referring physician report due within…
30 days
PPV1 benchmark
cancers in patients called back (BR-0, BR-3, BR-4, or BR-5); 4%
PPV2 benchmark
cancers in patients recommended for biopsy (BR-4 or BR-5); 25%
PPV3 benchmark
cancers in patients who underwent biopsy; 31%
Effect of lower average beam energy (e.g. Mo/Mo) on AGD
lower beam energy => higher average glandular dose (lower energy x-rays do not penetrate)
Mammo QC - digital machines
follow manufacturer’s recommendations (as opposed to all the dumb QC for screen film)
MQSA requirement for spatial resolution
12 lp/mm for screen film, manufacturer resolution specifications for digital (usually 7 lp/mm)
Who sets and enforces mammo quality standards?
FDA sets standards (MQSA); ACR enforces mammo quality standards
