Chapter 8 Mammography Flashcards
cancer detection rates with mammo
-4/1000
mammo views of the breast
craniocaudal
medio-lateral oblique
size and glandularity for average compressed breast
60 mm thick
15 % glandularity
what are microcalcifications
specks of calcium hydroxyapatite that have high attenuation coefficients
why is detection and characterization of microcalcification hard?
small dimension (0.1 mm diameter)
spatial resolution in mammo compared to human eye
superior to human eye at 25 cm viewing distance
how is mammo contrast increased?
using low-energy photons
how is noise reduced?
high radiation intensities at image receptor
digital mammo contrast, noise, and resolution vs other x-ray medical x-ray imaging modality?
mammo is superior
density of calcification vs adipose
fibroglandular vs carcinoma
2.2 g/cm3 vs 0.93 g/cm3 adipose tissue
1.04 g/cm3 fibroglandular vs 1.05 g/cm3 carcinoma
most common target material
molybdenum (Z= 42)
tungsten (Z= 74) is becoming more common, with no characteristic x-ray and more bremstrahlung
focal spot size
0.3 mm
small focal spot 0.1 mm for magnification mammo
why is a berrylium x-ray tube window used?
Z=4
minimizes x-ray beam attenuation
how is heel effect used?
used to increase radiation intensity at chest wall by pointing anode towards nipple
how is spectral shape obtained?
using K-edge filters (Mo, Rh, Ag)
tube voltages in mammo
25-35 kV
-avg x-ray beam energy is governed with the K edge filters
tube currents
100 mA in contact mammography
25 mA in magnification mammo
exposure times
1 s
power loading in mammo
3 kW in contact mammo
<1 kW in magnification mammo
vs CT 100 kW
what do the filters do?
remove low energy x-rays that only contribute to dose
Mo/Mo target/filter
Mo K edge is 20 keV
-filter attenuates photons above K edge
Mo/Rh target/filter
Rh K edge is 23 keV
-allows x-rays up to 23 keV (higher average energy than Mo/Mo)
characteristic Mo x-ray energies
19.6 keV and 17.5 keV
filters used with tungsten targets
Rh or Ag
Ag K-edge is 25 keV
what photons do the filters remove?
low energy photons and those above their k-edge
average energy and HVL of a W/Ag combination vs W/Rh combination
W/Ag is higher
With W targets, the average energy of transmitted photons is always “slightly less than” the K-edge energy of the K-edge filter
beam qualities of W targets vs Mo targets
W is higher
for what breast thickness do we use the different target/filter combinations and what is respective HVL?
Mo/Mo, < 65 MM, hvl 0.35 MM aL
Mo/Rh, > 65 mm, HVL 0.45 mm Al
W/Rh, < 65 mm, HVL 0.5 mm Al
W/Ag, > 65 mm, HVL 0.6 mm Al
scatter to primary ratio
1:1
does PE or compton dominate?
PE, opposite of other radiography
source to image distance
65 cm (shorter than 100 cm in other radiography)
longer SIDs would be unacceptable exposure times; shorter SIDs would increase focal spot blur and distortion
why use breast compression?
-immobilizes breast, minimizing motion blur
-spreads out breast tissue to decrease tissue overlap
-compression brings breast closer to imaging plane, minimizing image magnification and reducing focal spot blur
-compression results in increased penetration, which reduces exposure times and radiation doses
compression force
25-45 lb
grids in mammo
grid ratio 5:1
grid line densities of 50 lines/cm
other radiographic imaging often uses 10:1 grid
what does HTC grid do?
-high transmission cellular grid
-honeycomb pattern
-improve scatter removal because of 2D structure
-improve primary transmission because interspace material is air with negligible transmission
bucky factor of mammo grids
2
doubles dose
benefit of contrast outweighs risk of dose
other than grid, what else can mammo use?
air gap in magnification mammo
reduces mAs because primary photon losses don’t occur due to a grid
what does digital mammo use?
scintillators (CsI) or photoconductors (Se)
CsI vs Se pros and cons
both absorb >90% of mammo photons
CsI= faster and more reliable
Se = better resolution
use of photostimulable phosphors?
have been used, but have worse resolution and inferior clinical detection
are scintillators phosphors?
all scintillators can be phosphor but not all phosphors are scintillators
-phosphor shows luminescence after being stimulated
-scintillators show luminscence after being stimulated with ionizing radiaiton specifically. Also scinntillation is prompt, phosphorence is delayed fluorescnece.
pixel sizes in mammo
50-100 um
of pixels in mammo
15 million
does digital mammo always use AEC?
yes
for processing vs for presentation images
for processing: after application of detector corrections
for presentation: after application of vendor specific enhancements that help enhance subcutaneous tissue and skin
luminance required for mammo monitors
600 cd/m2, view in dark room
size of monitors
5 MP-8 MP
-5 MP requires using zoom
contrast ratio for monitor
brightest to darkest pixel
> 300:1 required for mammo
optimal viewing distance for mammo
50 cm
how should monitors be calibrated?
DICOM grayscale standard display function
how is magnification achieved?
move breast away from detector, toward focal spot
air gap reduces scatter from reaching the film and eliminates need for a grid
-removing grid allows mAs to be reduced from 100 to 70
-usually magnify x 2
-SNR improves
-only part of breast is imaged
focal spot size in magnification mammo
0.1 mm to maximize sharpness
tube currents for magnification mammo
25 mA
exposure times in magnification mammo
3 s
what does magnification mammo improve visualization of?
mass margins
fine calcifications
what is breast tomosynthesis?
removes overlap of breast tissue in image by creating tomographic imaging
-only shows lesions in a given plane
-acquires a number of projection images, each at a different angle (x-ray tube moves in arc around breast)
-typical arc is 15 degrees, with one image taken at each degree
-compression is used to minimize motion artifact
length of breast tomosynthesis exam
5 s
slice thickness of breast tomosynthesis image, how many tomographs?
45 tomographs, 1 mm each
radiation dose of breast tomo
comparable to contact mammo
advantages of breast tomo
-detects 40% more invasive cancers
-more promising than dedicated breast CT systems
-similar in plane resolution as 2D imaging, which is better than that of breast CT
breast biopsy
-patient is prone
-two views of breast are acquired +/- 15 degrees from normal
-biopsy needle gun captures a sample
advantages of upright vs prone biopsy
upright has better lesion visibility, faster lesion targeting, reduced procedure time
what is computer aided detection?
-uses algorithms to flag possible findings in digital mammo
sensitivity of CAD
90%
false positive rate
2 false positives/image
-attempts to characterize each identified lesion
-assigns likelihood of malignancy
what is MQSA?
-mammography quality standards act
-requires all mammo facilities in US to be certified every 3 years
ACR phantom to assess image quality
- has:
6 fibers
5 speck groups
5 masses
to pass, image must show 4 fibers, 3 speck groups, and 3 masses
MSQA requirement for dose
average glandular dose for 4.2 cm thick breast phantom must be < 3 mGy/image (with grid)
1 mGy without grid
-if fails, the machine is taken out
how do mammo scintillators minimize blur?
made of columns
MSQA limiting resolution requirement
12 lp/mm for screen-film
7 lp/mm for digital (14 pixels/mm)
limiting factor for resolution in digital mammo
pixel size (70 um)
what does higher beam quality do in mammo?
-reduce contrast
-reduce dose
-reduce exposure time, minimizing blur
now tungsten being used- higher quality- but contrast reductions are offset with image processning
what does breast dose depend on?
-breast thickess and glandularity
average glandular dose for 60 mm thick breast
1-2 mGy per view
-left and right breast doses are never added together
i.e. if each breast gets 3 mGy the patient dose is 3 mGy
what does increasing breast thickness by 10 mm do to average glandular dose?
double it
agd for 80 mm breast
> 3 mGy/view
by how much do screening programs reduce fatality rate?
20%
radiation risk from mammo
equivalent to risk of dying in an accident when traelling 1000 miles by a car
one million 50-year old women receiving a glandular dose of 3 mGy corresponds to a risk of 6 fatal breast cancers using BEIR VII risk estimates
how many cancers are identified by screening 1 million women?
4,000, a quarter of them fatal
benefits of digital mammo over screen-film
ability to process image
-linear response over wide dynamic range
smallest microcalcifications seen in mammograms
0.1 mm
representative HVL for mammo
0.5 mm Al
reducing SID on mammo would increase what?
focal spot blur
of pixels in digital mammogram
20 million
screening a million 50 yo women most likely results in how many fatal-induced cancers?
10
4000 cancers detected per million
