Mammography Flashcards
mammography equipment
- angled tube head
- c-arm design
- fixed focus-detector distance
- compression device
- fixed field size
- grids
- AEC
difference between conventional and mammography x-ray HOUSING tubes
conventional: glass envelope
mammo: metal tube
difference between conventional and mammography x-ray ANODE
conventional: tungsten
mammo: grounded Mo, Rh anode
difference between conventional and mammography x-ray axis of rotation
C: horizontal
M: vertical
difference between conventional and mammography x-ray filters
C: Al filter dose reduction
M: Mo or Rh filters for spectral shaping
difference between conventional and mammography x-ray anode angle
C: 7-16deg
M: 0deg; tube tilt 26deg
mammography x-ray tube operating voltage
23-40kVp
dual filaments in a focusing cup
0.3mm (contact) and 0.1mm (magnification) focal spot size
why use small focal spot
- minimize geometric blurring
- maintains spatial resolution
typical tube currents
100mA +/- 25mA for large focal spot
25mA +/- 10mA for small focal spot
what track is used for anode
Mo/Rh targets, sometimes combination with Tungsten
reason for choosing Mo and Rh as anode targets
charateristics xray production
Mo - 17.5 and 19.6 keV
Rh - 20.2 and 22.7 keV
why is tube tilted by 26deg
minimize effective focal spot size
what are the 3 factors that xray beam spectrum depend on
- anode material
- selected filter
- kV
why is low kVp used
- minimise compton scattering and maximise photoelectric effect -> enhance differential absorption by various tissues of the breast
- reduce radiation dose to breast -> glandular tissue of breast is high radiosensitive
use of a filter of ____ element as x-ray tube target is designed to
same; suppress the lower and higher energy bremsstrahlung x-ray and allow transmission of characteristics x-ray energy
what does the lower and higher energy bremsstrahlung xray contribute to
lower - radiation dose
higher - loss of contrast
xray tube window is made of
Berylium
breast thickness and its recommended target, filter and kVp
fatty breast ~4cm thick:
- Mo target
- 30micron Mo filter
- 24-26kVp
glandular breast ~5-7cm thick:
- Mo target
- 25micron Rh filter
- 27-31kVp
> 7cm:
- Rh target
- 25micron Rh filter
- 27-31kVp
what does HVL depend on
- kvp
- compression paddle thickness
- added tube filtration
- target material
- age of tube
relationship between kvp, HVL, atomic number targets and filters
HVL increases with higher KVP and higher atomic number targets and filters
AEC modes of operation
- auto time
- auto kvp
- full automatic
why is there a need for breast compression
- reduces overlapping anatomy and decreases tissue thickness
- less scatter, more contrast, less geometric blurring of the anatomic structures, less motion and lower radiation dose to the tissues
- better visualisation of tissues near the chest wall
what is spot compression
- apply compression to a smaller area of tissue using a small compression plate or cone
what does scatter radiation do to the images
degrades subject contrast and detail
relationship between scatter, breast thickness and area
scatter increases with breast thickness and breast area
main variables of a mammo imaging system
- contrast
- sharpness
- dose
- noise
why and how is half field xray beam used in mammo
- angled such that the central ray comes down along the chest wall edge perpendicular to the detector
- eliminated unnecessary dose to the patient’s torso
- prevents non-visualisaion of tissue in the chest wall region
explain the anode heel effect
- lower xray intensity is on the anode side of the field and should be placed at where nipple edge is.
- more tissue at chest wall, so require more intense xray beam
- a more uniform exposure is achieved
what is the optimal viewing conditions of a mammo image
soft copy:
- ambient light intensity <20lux
- display monitor 5MP
- magnifiers available to view fine deatils such as microcalcifications
hard copy:
- films are exposed to high optical densities to achieve high contrast
- luminance of mammo viewbox should be at least 3000cd/m^2
- film masking - preserve visibility of low contrast details
advantages of analog mammo over DM
- more affordable
- superior spatial resolution
- can be transformed into digital images with CR and saved as DICOM
- no digital detector -> which is a fragile and expensive part
- easier to maintain
disadvantage of analog mammo over DM
- uses film so need a CR reader to convert image to digital –> takes a longer time to receive the image
- 2 systems working together (machine and CR reader, if one breaks, the entire process is down
- difficult to archive images unless have CR
advantages of DM over analog in terms of lower patient dose and reduced recalls
- lower dose
- wider dynamic range
- higher contrast resolution
- better SNR
- reduced false positive and increased PPV
- image manipulation tools
advantages of DM over analog in terms of improved efficiency
- simplifies storage and retrieval of images
- elimination of lost films
- images can be easily transferred electronically to a central location (PACS) for diagnosis
- availability of CAD
advantages of DM over analog in terms of enhanced patient care and productivity
- elimination of need to leave exam room to process films
- eliminates processing issues
- faster interventional procedures
- faster patient throughput
disadvantage of DM over analog
- more expensive
- delicate digital detector
- more sensitive to ambient temp
- spatial resolution is limited to pixel size
- harder to service
does digital or film screen have a better contrast resolution
digital - wider latitude = greater dynamic range = more shades of grey
what target material is used for digital mammo
- tungsten (W) is better as the kV is more for penetration since the algorithm drives contrast
- W offers lower dose with longer scale contraste
what filtration is used for digital mammo
Rh and Ag
what are the 2 types of digital mammo
direct and indirect
direct: convert xray directly to electronic signals
indirect: convert xray -> light -> electronic signals
what are some image quality measurements
- modulation transfer function (MTF)
- detector quantum efficiency (DQE)
explain MTF
- measure of signal transfer over a range of spatial frequencies -> image sharpness
- high MTF = more transfer = sharper image and superior contrast
explain DQE
- measure of dose efficiency
- high DQE = superior image quality and reduced patient dose
- high DQE = lower image in noise = reduced missed cancers
- high DQE = flexibility to reduce dose
what is the preferred target filtration combination of a full field digital mammo
W/Rh
what is tomosynthesis
- set of rapidly acquired low-dose projections taken at multiple angles through the compressed breast
- then used computer reconstructions to create 3D images of the breast
compare narrow and wider sweeping angle for tomosynthesis
- wider sweep angle = gives more complete blurring outside of the focal plane
- narrow sweep angle = lesion margins appear sharper
advantages of DBT compared to 2D
- better depiction of the smallest calcifications
- better delineation of the lesion border
- less compression
disadvantage of DBT compared to 2D
- motion artifacts more likely to occur because of longer exposure time
- large calcifications cause significant artifacts
- reconstructed images lengthens interpretation time
how is breast dose calculated
mean glandular dose (MGD) - measured in mGy or mrad
MGD (mrad) = DgN x ESE
what is the acceptable dose
for a 50-50 glandular adipose tissue breast, with 4.2cm compression thickness, MGD must be less than 3mGy per view
factors affecting breast dose
- photon energy
- breast composition and thickness
- equipment
what is DgN
factor that converts ESE to mean dose
depends on:
- HVL, filter, target, kVp
- breast composition and thickness
what are some patient related artefacts
- motion
- skin line
- hair
- anti-perspiration
- superimposition of body parts
- eye glasses
- jewelry
what are some hardware-related artefacts
- dust/cracks on compression paddle
- under exposure
- field inhomogeneity
- collimation misalignment
- noise obscuring calcification
- grid lines
- grid misplacement
- vibration artifact
- detector-associated:
ghosting
gouging
horizontal line artifact
dead pixel
unread lines
what are some software processing artefacts
- high density
- loss of edge
- breast within breast
- vertical processing bars
- detector interface line
- skin line processing
what is a storage related artefact
reconstruction artifact
why is laser printer sensitometry QC performed
- ensure that the film sensitometry is operating within tolerance level
why is SMPTE QC performed
- to determine whether the contrast and brightness setting of the monitors are acceptable
- to check for limitations in spatial resolution and aliasing of displays
- to check for non-uniformities, scratches and other defects
what is ACR phantom used for
- to assess the quality and consistency of the mammographic images
what do SNR and CNR measure
- both are a measurement of image quality
SNR: ratio of a signal power to the noise power corrupting the signal
CNR: ability of an imaging modality to distinguish between various structures of an acquired image by the perception of differences in their apparent signal intensities
what is a compression thickness indicator QC for
to ensure that the indicated compression thickness is within tolerance
what is the flat field uniformity test
- an analysis of the homogeneity of detector field
- to ensure that the images are uniform in intensity and free of artifacts
what is gain and geometry calibration for
to ensure that the system is calibrated properly
