5. Mammo Flashcards
difference in mammo XR
- lower energy monoenergetic beam to enhance attenuation differences
- increase in spatial resolution to see microcalculations
mammo – optimal kVp
ideal kVp: 16-23 keV ; voltage of 25-30 kVp is used (Dx is usually 50-120)
-PE dominates at low kvP; PE proportional to Z^3 and indirectly with energy 1/E^3
mammo – target, characteristic x-rays
mammo uses molybednum or rhodium anode
moly: 18 keV ; rhodium: 20.2 keV
why is moly used for mammo
low atomic number, generate low K characteristic x-rays
peak characteristic x-rays with low Bremsstrahlung
k-edge filtration
filter placed outside of tube to create mono-energetic beam (<15 keV and >20 filtered out)
when is Rh vs Mo used?
Rh used for denser breasts since it’s higher energy
why would Mo anode and Rh filter be used?
intermediate energy spectrum
would you ever use Rh target (21 kev) and Mo filter (20 kev K edge)?
NO
when is Mo anode used with aluminium filter?
harder beam to penetrate denser breasts
increasing the filter k edge will result in beam ? and contrast ?
increases average energy of beams
reduces contrast
mammo – focal spot size
0.1-3 small; better spatial resolution
general x ray 0.6-1.2 mm
problems with small focal spot size
cannot tolerate heat well, must have lower mA (and requires longer exposure time)
mA 50 for 0.1 mm focal size
mA 100 for 0.3 mm focal size
mammo – exposure time
longer than xray
mammo ~1 sec
abdominal x ray 0.05 sec
CXR 0.005 sec
heel effect and mammography; which side does cathode go on?
cathode goes on chest wall
tube angled up 20 degrees
effective anode angle
effective anode angle = anode + tube tilt angle
diagnostic tube for mammo vs xr?
mammo: beryllium exit window (less attenuation)
Dx: glass
reason for breast compression in mammo?
reduced thickness/scatter so lower kVP can be used
lower kVP/scatter so improved contrast
improved tissue penetration, so less dose/scatter
reduced thickness so less mAs/dose needed
no motion so less motion/focal blur and spatial resolution
less geometric magnification
less tissue overlap
antiscatter grid in mammo
4-5 grid ratio (height/width)
smaller grid ratio and lower kVp (reduces scatter)
problems with grid
increased dose with grid since grid removes scatter
bucky factor for mammo vs XR
mammo: 2
XR: 5
mag view of breast?
move breast away from detector and closer to the source/focal spot
cutting distance to source in half doubles the mag
is there a grid in mag view?
no. instead the air gap is used as a grid
the air gap allows off axis scatter to miss the target
major changes with mammography and XR
smaller focal spot less mA (to not melt focal spot) longer exposure time to compensate for increase distance between breast tissue and image receptor
difference between mammo and mag view
no grid, even smaller focal spott, increase exposure time, less mA
optical density in mammo
high maximum optical density, most useful at periphery of breast (skin) where thickness decreases
air kerma? relationship with distance
density of xrays ; decreases via inverse square law with distance
entrance air kerma
radiation incident on patient
receptor air kerma
radiation used to generate image
which is bigger, entrance vs receptor air kerma?
receptor AK less than entrance
air kerma in mammo
higher than in general XR
usually because source to image distance is much smaller in mamms compared to DX
digital mammo vs analog with spatial resolution?
digital systems have lower spatial resolution
is there a specific line pair requirement MQSA for digital?
no; manufacturers specifications
digital mammography dose?
less than analog ~15% because of better beam quality
digital mammography noise?
fixed after exposure is taken
dark noise: electronic fluctuations within detector element; proportional to temperature of detector
flat field test
imaging acrylic to improve image quality and calibrate digital detectors
ghosting in mammo
residual image from prior exposure tpically from high attenuating objects (like lead) placed in beam
reasons lead is not allowed on flat panel digital systems
pixels gone bad
square or streak
digital artifacts in mammo
ghosting, pixels gone bad
digital breast tomosynthesis
stacked images taken in 15 prjections that decreases the effects of tissue superimposition
improves sensitivity/specificity
pros/cons of DBT
pros: better margins/shapes
cons: density of a mass may appear decreased ; intralesional fat may be benign but should prompt biopsy as cancers may loo like they are engulfing fat
mildly increased dose
PPV
positive predictive value
proportion of people with a postive study who have breast cancer
positive exam / (all positive exams–cancer and overcalls)
benchmark for PPV1
positive screening – BR 0, 3-5 or call backs
benchmark -4.4.%
benchmark for PPV2
callbacks where biopsy recommended – BR 4-5
benchmark - 25.4%
benchmark for PPV3
results of biopsy; positive biopsy rate , biopsy yield of malignancy
benchmark: 31%
how often is mammo accredited/certified?
3 years
who monitors MQSA
FDA
who is responsible for QA and QC per MQSA?
interpreting physician
minimum megapixels on a mammo workstation
3 MP
target range for medical audit: recall rate
5-7%
target range for medical audit: cancers/1000 screened
3-8
processor QC frequency
daily
darkroom cleanliness frequency
daily
viewbox conditions frequency
weekly
phantom evaluation frequency
weekly
repeat analysis frequency
quarterly
compression test frequency
semi-annually
darkroom fog frequency
semi-annually
screen-film contrast frequency
semi-annually
mammo requirements
240 mammograms in the last 2 years of training (during a 6 month period)
3 months of formal training
60 documented hours of mammo education
how many line pairs/mm for the anode-cathode direction and in the left right direction?
anode-cathode: 13 LP/min
left-right: 11 LP/min
MQSA requirmeents for screen film and manufacture spects for digital?
12 lp/mm for screen-film
manufacture specs for digital ~7 lp/mm
breast phantom specs
4.2 cm of breast tissue is 50% adipose/glandular
average compressed breast thickness/glandularity in the US
60 mm, ~15% glandularity
phantom mammo limit
3 mGy with grid
1 mGy w/o a grid
