Mammography Flashcards
1
Q
Difference between general radiography and mammography: X-ray production
A
- Lower tube voltage is used (25-34kV)
- Anode material used is molybdenum and/or rhodium (characteristic x-rays of Mo = 17.9 and 18.5 keV, Rh = 20.2 and 22.7 used in thicker breasts) producing lower energy x-rays (vs 70-100keV with tungsten anodes)
- Anode angle:
- Small anode angle (~0-10 deg vs 20) to ensure small focal spot (0.3mm vs 1.2mm) and increase resolution
- X-ray tube is metal with a beryllium window, rather than glass housing, to reduce off-focus radiation.
- Entire x-ray tube is angled 20 deg to make effective focal spot small
2
Q
Difference between general radiography and mammography: imaging technique
A
- Filtration: minimises low and high energy x-rays (Mo, Ag, Rh)
- Grids: moving carbon fibre grids
- bucky factor 2
- grid ratio 5:1
3
Q
Difference between general radiography and mammography: Patient position
A
- Breast is compressed for uniform tissue thickness
- Decreases beam hardening as it passes through breast tissue
- Immobilises breast
- Ensures even irradiation due to uniform optical density
- Reduces dose if thickness is uniform
- Reduces scatter
- Lower tube voltages can be used if images tissue is thinner
- Chest wall is placed on cathode side to take advantage of heel effect
4
Q
Difference between general radiography and mammography: Image
A
- Contrast is mainly a result of Photoelectric effect (rather than Compton’s scatter)
5
Q
Contact Mammography
A
- Tube current ~ 100mA
- Exposure times 1-2 seconds
- Tube voltage ~25-34kV
6
Q
Magnification mammography
A
- Tube current 25mA
- Exposure time up to 6 seconds
- Tube voltage is same 25-34kV
- No grid is used, due to airgap used
- Air gap reduces scatter
7
Q
Use of Magnification mammography
A
- Used to followup suspicious lesions
- Breast is imaged closer to the xray tube and air-gap is employed between the breast and the detector, though the source to detector distance is the same
- Tube currents are reduced compared with contact mammo
8
Q
Advantages of magnification mammography
A
- Increase in effective resolution by magnification factor
- Reduction in scatter due to employment of air gap
9
Q
Disadvantages of magnification mammography
A
- Due to low tube currents and small focal spot, exposure times are prolonged movement arterfact
- Because breast is close to x-ray source increased patient dose
- Geometric blurring occurs due to finite focal spot
- Area of breast imaged is reduced
10
Q
Mean Glandular dose
A
Absorbed dose to the glandular tissue of the breast
11
Q
Mean glandular dose factors
A
- Breast type: more glandular tissue will lead to increased dose
- kVp: increased kVp will decrease the dose due to increase beam penetrance, BUT will reduce contrast
- filtration: will reduce the dose
- grid: bucky factor of 2 à double the dose
- Breast thickness: increased breast thickness will increase the MGD due to increased exposure time
- Compression: will reduce MGD
12
Q
Digital methods applied to mammography
A
- Film digitisation
- Films are scanned and digitised to enable viewing on PACS consoles
- Allows computer-aided diagnosis
- Computed radiography
- Photo-luminescent phosphor cassettes are used, which release light once scanner, photodiodes convert light to signal and image is formed
- Poorer spatial resolution (<5lp/mm) but better contrast resolution
- Digital radiography:
- X-ray phosphor screen converts attenuated photons to light, converted to digital signal via photodiodes
- Better spatial (>5-6lp/mm) and contrast resolution
-
CCD – stereotactic biopsies
- Charge couple devices are used in stereotaxic mammographic biopsies of breast lesions, allowing immediate viewing of acquired images
- Field of view is limited to 5x5cm, with demagnification of the image
- 2 views of the breast are acquired, and calculation of the position of the lesion can be made from the amount of shift of the lesion on the 2 images
- biopsy needle gun then be positioned to accurately acquire tissue sample.
-
Tomosynthesis
- Series of digital images are taken at different angles to the breast to allow tomographic slices to be reconstructed
- Improves lesion visibility by reducing the impact of tissue overlap as a result of breast compression
- Slices can be 1mm thick
13
Q
Advantages of digital over Film mammography
A
Allows use of computer aided diagnosis
- No more lost films
- Better contrast resolution
- Can manipulate images
14
Q
Disadvantages of digital over film
A
- Inferior spatial resolution
- Full size MMG image is large file to store (>100MB file)
15
Q
Impact of system geometry on spatial resolution
A
- Magnification (bringing the breast close to the x-ray tube with a fixed source to detector distance) will increase the effective spatial resolution of the system
- However, there will be increased geometric blurring due to the finite small focal size
- Also, in magnification MMG, the tube current is reduced to 25mA (from 100mA), which will increase exposure time increased risk of movement artefact and increase dose (even though no grid is used in mag mmg).
