Mammography Systems and X-Ray Tube Design

Modern Mammography Goals

• The primary goals of modern mammography are to:
  • Enhance low contrast detection
  • Minimize patient dose
  • Optimize image quality

Modern Mammography Systems

• Modern mammography systems have:
  • Special x-ray tube
  • Breast compression devices
  • Optimized screen/film detector systems

X-Ray Tube Design

• The x-ray tube has:
  • Cathode with dual filaments in a focusing cup
  • Produces 0.3 mm and 0.1 mm focal spot sizes
  • Operates at low voltage (below 35 kVp)
  • Tube current: 100 ± 25 mA for 0.3 mm focal spot, 25 ± 10 mA for 0.1 mm focal spot
• Anode:
  • Made of Molybdenum (Mo) or Rhodium (Rh) targets
  • Produces k-characteristic x-ray energies (17.5 & 19.6 keV for Mo, 20.2 & 22.7 keV for Rh)
  • X-ray tube is tilted to provide adequate coverage for 60-70 cm source to image distance

Heel Effect

• The heel effect causes a significant drop in x-ray intensity towards the anode side of the x-ray beam
• The cathode is placed over the chest wall (thicker part of the breast) and the anode over the nipple part (thinner part)

Focal Spot Considerations

• Two focal spot sizes are used:
  • Large (0.3 mm) for routine imaging
  • Small (0.1 mm) for magnification imaging
• The effective focal spot is smaller than the actual focal spot due to the line focus principle
• The focal spot length at the reference axis is less than the focal spot length at the chest wall, causing more geometric blurring at the chest wall

Tube Port, Tube Filtration, and Beam Quality

• The optimum x-ray energy for high subject contrast is at the lowest radiation (15-25 keV)
• The tube port is made of 1 mm thick beryllium (Z=4) for low attenuation and good structural integrity
• Filters:
  • Added tube filters of the same element as the target reduce low and high x-ray energy in the spectrum
  • Allow characteristic x-ray energies to be transmitted

Radiation Dosimetry

• The risk of carcinogenesis from radiation dose to the breast is a concern
• Monitoring of dose is essential and required yearly by MQSA
• The average glandular dose is the preferred dose index
• Factors affecting breast dose:
  • kVp: higher kVp increases beam penetrability but decreases inherent subject contrast
  • Breast thickness: increased breast thickness requires an increased dose
  • Compression devices: vigorous compression lowers breast dose by reducing thickness
  • Target/filter: Rh/Rh combination can significantly reduce the dose to thick breast
  • Speed of screen: faster screen film reduces the dose at the expense of spatial resolution
  • Film processing conditions: higher OD achieved on film will increase the dose
  • Antiscatter grid: increases the dose but improves image quality
  • Tissue composition of the breast: glandular tissue will have a higher breast dose due to increased attenuation and a greater tissue mass at risk

MQSA Regulations

• The MQSA limits the average glandular dose to 3 mGy (300 rad) per film for a compressed breast thickness of 4.2 cm and a breast composition of 50% glandular and 50% adipose tissue using the MQSA approved mammography phantom
• The average glandular dose for this phantom is typically 1.5 to 2.2 mGy per view or 3 to 4.4 mGy for two views for a film optical density of 1.5-2

Stereotactic Breast Biopsy

• Biopsy systems provide the capability to localize in 3D and physically sample suspected lesions on mammograms
• Two views are required (+15° and -15°) to measure the shift of a lesion and determine its distance from the detector
• Charged coupled device (CCD) camera coupled to x-ray phosphor screens is used for this purpose

Full Field Digital Mammography

• Advantages of digital mammography:
  • Wide dynamic range (1000:1) compared with screen-film mammography (40:1)
  • Dynamic image manipulation
  • Ability to post-process
  • The soft copy read accompanied by computer-aided detection (CAD) to provide the second opinion
  • 3D imaging
• Regulation requirements:
  • For a facility to perform mammography legally under MQSA, it must be certified and credited
  • The accreditation body verifies that the mammography facility meets the standards set forth by the MQSA

Quality Control Phantom

• The phantom is composed of:
  • Acrylic disk (4 mm thick, 10 mm diameter)
  • Acrylic body (mimics 4.2 cm compressed breast of 50% adipose and 50% glandular tissue composition)
  • Wax insert with 6 cylinder nylon to simulate speculations, 5 Al2O3 specks to simulate calcification, and 5 low contrast disks to simulate masses
• A mammography phantom is a test object that simulates the radiographic characteristics of compressed breast tissues containing disease
• To pass the MQSA test, at least 4 cylinders, 3 calcification groups, and 3 masses must be clearly visible at an average glandular dose of less than 3 mGy.