12_-_Mammography_II

Compression of a suspicious are shows no visible mass
Contrast
• Maximum contrast with scatter:
S C S = C 0 1 + P where S is the amount of scatter P is the amount of primary radiation S/P is the scatter to primary ratio and C 0 = ∆P/P is the maximum subject contrast without scatter
Film processor testing
Radiation dosimetry
• Mean glandular dose preferred index since glandular tissue is most always the site of carcinogenesis • Speed of screen-film system and film OD preferred by radiologist are major factors • Should not exceed 3.0 mGy (300 mrad) per film
Spot compression
• Smaller compression paddle area (~ 5 cm diameter) reduces even further the breast thickness in a specific area and redistributes the tissue for improved contrast and anatomic rendition • Valuable in delineating anatomy and achieving minimum thickness in an area of the breast presenting with suspicious findings on previous images
• Limitations of magnification include:
– Geometric blurring caused by the finite focal spot size – Small focal spot limits tube current, and extends exposure times. Even slight breast motion will cause blurring during the long exposure times
Mammography II
Compression, scattered radiation, & magnification Screen-film cassettes & processing Quality assurance
Compression
• Firm compression reduces overlapping anatomy and decreases tissue thickness of the breast. This results in:
Compression (cont.)
• Compression paddle should match the size of the image receptor, be flat and parallel to the breast support table, and not deflect more than 1 cm at any location when compression is applied • A right-angle edge at the chest wall produces a flat, uniform breast thickness when compressed with a force of 10 to 20 newtons (22 to 44 pounds)
Geometric magnification
Magnification (cont.)
• Advantages of magnification include:
– Increased effective resolution of the image receptor by the magnification factor – Reduction of effective image noise – Reduction of scattered radiation
Illustration of the need for the 0.1 mm focal spot in magnification studies
Screen-film cassettes
• Most cassettes are made of low-attenuation carbon fiber and have a single high-definition phosphor screen used with a single emulsion film • The screen is positioned in the back of the cassette • Light spread varies with the depth of x-ray absorption
– Fewer scattered x-rays – Less geometric blurring of anatomic structures – Lower radiation dose to the breast tissues
• Uniform breast thickness lessens exposure dynamic range and allows the use of higher contrast film
– Most x-rays interact in the layers of the screen closest to the film, preserving spatial resolution
Film-screen system for mammography
Film processor QA
Linear grid (typically 5:1), cellular grid for 2D scatter rejection, and the air gap (intrinsic to magnification procedures)
2D copper scatter grid with air interspaces transmits 87% of primary photons (Argonne National Labs, 2006)
– 6 cylindrical nylon fibers of decreasing diameter – 5 simulated calcification groups (Al2O3 specks) of decreasing size – 5 low contrast disks, of decreasing diameter and thickness, that simulate masses
Mammography phantom
• Used to determine adequacy of overall imaging system (including film processing) in terms of detection of subtle radiographic findings, and to assess reproducibility of image characteristics (e.g., contrast and optical density) over time
• • • • Must be done daily, prior to first patient use Solution levels and temperatures checked Base + fog must be within +0.03 of baseline Mid-density must be within ±0.15 of baseline • Density difference (contrast) must be within ±0.15 of baseline
Magnification
• Magnification is achieved by:
– Placing a breast support platform to a fixed position above the detector – Selecting the small focal spot – Replacing the antiscatter grid with a cassette holder, and – Using an appropriate compression paddle
Wax insert composition and “undegraded” radiograph
来自百度文库Phantom QA
• Should be done weekly; must be done monthly • Minimum of 4 largest fibers, 3 largest speck groups, and 3 largest masses must be visible • Number of test objects of each group type visible in image should not decrease by more than one half • Background OD should be at least 1.4 and not vary more than ±0.20 from operating level • Density difference due to a 4.0 mm acrylic disc should be at least 0.40 and not vary more than ±0.05 from established operating level
Scatter depends on breast thickness and field area
Antiscatter grid
• Parallel linear grids with a ratio of 4:1 or 5:1 are commonly used • Aluminum and carbon fiber are common interspace materials • Grid frequencies range from 30 to 50 lines/cm for moving grids, and up to 80 lines/cm for stationary grids
Phantom (cont.)
• Intended to mimic attenuation characteristics of a “standard breast” of 4.2 cm compressed thickness composed of 50% adipose tissue and 50% glandular tissue • Wax insert contains:
−1
Scatter
• The amount of scatter in mammography increases with increasing breast thickness and breast area, and is relatively constant with kVp • The fraction of scattered radiation in the image can be reduced by use of antiscatter grids or air gaps, as well as “vigorous” compression