Mammography: Breast Examination

Mammography

• Radiographic (X-ray) procedure optimized for the examination of the breast
• Highly effective at detecting early-stage breast cancer
• Diagnostic mammography investigates symptomatic patients
• Screening is for asymptomatic women in selected age groups
• Screening examination consists of one or two views of each breast
  • Cranio-caudal (CC)
  • Medial-lateral oblique (MLO)
• Can be used for pre-surgical localization and guidance of biopsies

Task Challenges

• Spatial Resolution: Excellent spatial resolution is required to see tiny microcalcifications in the range of 0.15mm
• Achieved via: Specialized film/screen combinations, dedicated digital detectors, X-ray tube with small focal spots, and magnification views
• Contrast Resolution: Excellent contrast resolution is required to see minimal attenuation between normal glandular tissue and breast cancer
• Achieved via: Low Kvp Technique (maximizes PE interactions), compression (minimizes scatter), grid (minimizes scatter), special film or digital detector, and tomosynthesis
• Noise: Low noise is required for accurate detection of subtle contrast differences and tiny structures
• Achieved via: Appropriate mAs (balance b/n noise and dose), and compression (reduces breast thickness)
• Dose: Low dose is required, as the breast is radiosensitive
• Resolution, contrast and noise requirements have to have balanced
• Achieved via: Appropriate kVp and mAs, appropriate target and filter material, compression, automatic exposure control, and collimation and shielding

Radiological Requirements

• Sufficient spatial resolution with details possibly as fine as 50 µm adequately visualized
• Adequate contrast in image with low-energy X ray spectra
• Broad dynamic range due to composition of the breast and age-dependent changes in the breast
• Lowest absorbed dose compatible with adequate diagnostic image quality

Mammographic Equipment

• Specialized gantry accommodates the breast with rotation and vertical movement
• Specialized beam geometry improves visualization of the chest wall edge
• The X-ray generator has high frequency and near constant potential waveform
• The X-ray tube has a rotating anode, dual focus 0.3/0.1 mm, and beryllium exit window (low attenuation)
• FID (focus image distance) is generally in the range 60 to 65 cm

Further Equipment

• Standard X-ray tube is not suited for mammography
• Mammography requires excellent soft tissue contrast between glandular tissue, neoplasm and fat
• Requires excellent spatial resolution to detect microcalcifications
• Anode uses Molybdenum or Rhodium target with k-edge for ideal characteristic x-rays
• Greater percentage of Photoelectric Effect results in better contrast
• Tungsten target is more common in digital mammo
• Focal Spot is 0.1 to 0.3 mm as opposed to 1-1.2 mm in a conventional x-ray tube, and results in greater spatial resolution

Parameters

• kVp is typically 25-35kVp, above k-edge of target material to get characteristic x-ray production
• Higher kVp decreases contrast, but may be necessary for thicker/denser breasts
• mA- typically in the 100 mA range
• Tube power is 2.5 to 3.5 kW
• Imaging time is typically 1-2 seconds, longer imaging time increases chance of motion blur
• Filtration done by Beryllium window on x-ray tube
• Added filtration can be matched to anode (target material)
• Thin molybdenum or rhodium (25 to 30 micrometer)
  • HVL approx. 0.3 mm Al
• X ray spectrum should provide a range of energies that give an appropriate compromise between radiation dose and image quality for tissues under examination
• X ray spectrum determined by target material, filter material, and tube voltage (kV)
• For screen-film mammography optimum beam energy lies between 18 and 23 keV depending on breast thickness and composition
• Characteristic X rays from molybdenum and rhodium are suitable
• Higher energies may be more optimal for digital mammography

Equipment and Filters

• Metallic filters are used in mammography
• Molybdenum (Mo) filter (30 to 35 µm thick) is commonly employed with Mo anode -Greater attenuation of X rays at low energies and above K-absorption edge of Mo at 20 keV
• Mo characteristic X rays from the target and X rays of similar energy produced by bremsstrahlung pass through the filter
• The resultant spectrum is enriched with X rays in the range 17 to 20 keV
• Higher energies are desirable for imaging thick, dense breasts
• Use of Mo/Rh (molybdenum/rhodium) and Rh/Rh target/filter combinations

Compression Paddle

• Compression should be firm but not painful
• Reasons for applying compression: reduces superposition of tissues, decreases ratio of scattered to transmitted radiation reaching image receptor, decreases the distance from any plane within the breast to the image receptor reducing geometric unsharpness, lower overall attenuation allowing radiation dose to be reduced, provides more uniform attenuation over the image reducing the exposure range which must be recorded, and provides clamping action reducing anatomical motion during the exposure reducing image unsharpness
• Material: Low density plastic with minimal effect on x-ray beam (e.g. scatter) and stiff to provide uniform compress force with even distribution
• Drawback: Patient discomfort 25-40 lbs of compression force

Grids

• Scattered radiation reduces image quality
• Grid significantly decreases ratio of scattered to transmitted radiation reaching the image receptor
• Focused linear grids (integral part of the system)
• Grid moves during exposure to blur the image of the grid septa
  • Motion must be uniform and of sufficient amplitude to avoid nonuniformities in the image
• Bucky factor (increase in dose due to use of grid) can be as large as 2 to 3 and justified by improvement in image quality.

Imaging and Set Up

• Contact mammography set-up: -Source-to-image receptor distance (SID) -Source-to-object distance (SOD) -Object-to-image receptor distance (OID)
• Magnification mammography set-up and Heel effect exists
• Source-to-image receptor distance (SID) is typically 65 cm
• Source-to-object distance (SOD) should be as close to SID as possible and good compression
• Object-to-image receptor distance (OID) should be as close to 0 as possible with good compression
• Objects closer to detector have less magnification

Automatic Exposure Control

• All modern mammography units are equipped with automatic exposure control (AEC)
• Essential in order to provide the optimum dose to the image receptor
• Target optical density for screen-film mammography
• Target SNR (signal-to-noise ratio) or preferably SDNR (signal difference-to- noise ratio) for digital mammography
• For screen-film mammography and cassette-based digital systems AEC sensor is located behind image receptor avoid a shadow on the image
• Sensor terminates exposure when pre-set amount of radiation is received
• Location of sensor adjustable and positioned below appropriate region of the breast
• AEC is generally microprocessor controlled
• Controls correction for reciprocity law failure of film
• Uses automatic selection of exposure parameters (kV, filter, target) depending on breast thickness and composition and sensing of breast thickness (compression device) and attenuation (short (typically <100 ms) X ray pre-exposure)

Digital Mammography

• The digital detector can act as the AEC sensor
• Pre-exposure concept is typically used
• An entire low-dose image is captured by the digital detector, which is then analyzed to determine the overall SDNR or minimum SDNR over a small (~1cm2) region-of-interest (roi) in the image
• Target, filter and kV are selected automatically to give desired SDNR when the main exposure is performed
• Digital detectors can be operated at a wide range of input dose levels, to optimize imaging according to a priority of SDNR, low dose or a combination
• Development in this area is on-going and used for the location of the edges or critical areas of breast identified automatically