mammography

what is the x-ray spectrum

what is the aim of mammography units

•AIM of mammography units is to image soft tissue only!

• •Very small differences in physical density that are the source of contrast for imaging;

• •Significant sign of some breast Ca are very small ‘micro-calcifications’, beyond capability of most methods

•Design of equipment must allow for this functionality

•µ value of soft tissue is very low: need photons of very low energy to image

what are the challenges of mammography units

•Mammography units operate in the low kV range of 20-45

•Thermionic electrons do not have enough energy to cross to anode

•This produces a beam of low quality: low intensity

•Problem: longer exposure times = movement unsharpness

•Breast tissue has very little subject contrast

what are the solutions

•Shorter distance between cathode and anode

•Short SID- 45-60cm

•Focal spot (smaller): 0.3mm for general mammos and 0.1mm for magnification views = reduces geometric unsharpness

•Need AEC control due to varying breast composition/size

•High-voltage generator must supply near DC high voltage with a ripple less than 5%

•High tube current reduces exposure time required and risk of movement unsharpness

•High tube current: increased heat, therefore use rotating anode

what’s the anode and filtration of mammography units

Molybdenum:

•High melting point and low atomic number (Z = 42)

•Used as X-ray tube anode and filter

•Anode: produces characteristic X-ray peaks at 17.6 kV and 19.7 kV

•Filter: attenuation k-edge at 20 kV

•X-ray spectrum:

• •relatively narrow range of energies near 20 kV

• •Optimum with respect to image contrast and radiation dose…especially for smaller breasts

what is rhodium

Rhodium:

•Moly-moly spectrum not optimum for all

•High melting point and higher atomic number (Z=45) than molybdenum

•Shifts both the characteristic radiation and k-edge energies up to higher values

•Optimum for larger/denser breasts

why is compression needed

•Variation in thickness from the chest wall to the nipple creates a wide range of exposures to the receptor;

•Adequate visualisation of micro-calcifications requires total blurring to be limited to approximately 0.15mm;

•Exposure times can be several seconds (very long compared to most other radiographic procedures);

•Almost any patient motion during the exposure can be detrimental to image quality;

•Physical compression and stabilisation of the breast was developed to improve image quality both with respect to contrast and reduced blurring.

whats the evolution of breast compression

•1960s: conventional X-ray tubes with tungsten anodes

•1970s: specific mammography X-ray systems were developed, some type of compression and breast stabilisation was included by the X-ray beam cone and sometimes with soft components in contact with the breast

•1980: the introduction of continuing flat plate development was a major contribution to image quality

what is flat-plate compression

•Major contribution is reshaping of the breast into a more uniform thickness spread over a slightly larger area;

•Uniform thickness reduced range of X-ray exposure to the receptor and loss of contrast due to latitude limitations;

•Spreading of breast tissue, especially thicker regions, reduced overlapping of objects & structures that could interfere with visualisation;

•Especially significant for imaging tissues up to the chest wall.

•Display and measurement of thickness: exposure factor selection

•AEC sensor location: selection desired density

summary

•Mammography units have modifications of the tube to meet the goal of X-ray imaging of soft tissue.

•This is primarily through changes to the anode material and the use of filtration to create a ‘mono-energetic beam’.

•Compression plays an important role in obtaining high-quality images