Mammography


Equipment

Mammography machine

 

Angled Tube Head

Due to the anode heel effect, the x-ray beam is not uniform in the direction parallel to the anode-cathode axis of the x-ray tube. This is used in mammography by aligning the cathode over the chest wall end (thicker area to penetrate, higher energy beam) and the anode over the nipple end (thinner area to penetrate, lower energy beam).

C-Arm Design

The x-ray set is a c-arm. The whole gantry rotates so that the tube and breast table remain opposite each other.

Fixed Focus-Detector Distance (FDD)

The set is designed for a single examination and the focus-detector distance (FDD) or focus-to-film distance (FFD) of 65-66cm is considered optimum. This set FDD is a compromise between lower patient doses (lower doses with higher FFDs) and higher film doses (lower exposures with higher FFDs). Also, higher FDDs require longer exposures for a fixed mA resulting in more movement unsharpness.

Compression Device

The maximum force applied should be no greater than 200 N (approx. 20 kg weight). Standard compression forces are normally between 100 - 150 N. The compression plate is angled so that more of the breast is in contact with the compression paddle.

Fixed Field Size

Unlike in general radiography, only one type of examination is done meaning collimation creating fixed field sizes are all that are required.

Grids

Moving anti-scatter grids are used in normal mammography imaging. For magnification views, the breast support table is above the film to give magnification factors of around 1.8. In this case the large air gap between the breast and the film works to reduce scatter and so no grid is needed.

Automatic Exposure Control (AEC)

The AEC detector is placed behind the cassette (unlike in general radiography in which the AEC detector is placed between the patient and the film). The AEC detectors terminate the x-ray exposure either when the number of photons detected or the energy deposited in the device reaches a predetermined level.

A problem faced by AECs is beam hardening. A harder beam will reach the AEC chamber for a thicker breast (more attenuation of the softer energy photons) and it may terminate the exposure sooner than for a standard film. This is corrected for by several methods.

  • Two ion chambers: one placed above and the other with a filter in between. The ratio of the output of each is calculated and the beam hardening can then be determined and corrected for.
  • Fluorescent screen and photomultiplier tube: this detects the number of photons and, therefore, is not affected by beam hardening.
  • Solid state detectors

Target / filter material

  • Need good differentiation of low contrast structures
  • Need very high spatial resolution for micro-calcifications
Target

Need material that produces characteristic x-rays with energies of 17-20 keV (20-30 keV for larger breasts) to produce the best contrast. The commonly used material is Molybdenum (characteristic x-rays at 17.5 and 19.6 keV).

Filter

A filter with a k-edge of an energy just above the characteristic energies is used to remove the higher energy x-ray photons and make the beam as monoenergetic as possible. Molybdenum has a k-edge of 20 keV, just high enough so that the large increase in attenuation (k-edge) doesn't fall into the characteristic energies produced at the molybdenum target.

Alternatives

Mostly MoMo (molybdenum target, molybdenum filter) but this does not give high enough energies for larger breasts.

  • Rhodium has a k-edge at 23.3 keV and we can use a molybdenum target and rhodium filter (MoRh) to increase the amount of x-rays with energies in the range of 20 - 23.3 keV.
  • Rhodium characteristic x-rays are at 20.2 - 22.7 keV. When used as a target this produces a beam with a mean energy that is higher than for MoMo and for MoRh.
  • Tungsten (W) target and Rhodium filter. The x-ray output is reduced as no characteristic x-rays are produced (and, therefore, longer exposure times) but tungsten is much cheaper. It is mostly used in breasts with implants or that have been treated with radiotherapy as they are much larger and denser.

 

  Contrast Radiation dose
Highest MoMo MoMo
  MoRh MoRh
  RhRh RhRh
Lowest WRh WRh

 

The mean energy of the spectrum decreases from WRh to MoMo. Lower energy photons have a higher probability of interacting with matter and, therefore, produces better contrast. However, the lower the energy, the greater the absorption, the more energy is deposited in the matter, and the higher the dose.

Summary

  • General use: MoMo
  • Dense breasts: MoRh or RhRh

 

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Spatial Resolution

A very high resolution is required to see microcalcifications. This is achieved via:

  • The film-screen combination
  • Focal spot size
  • Compression
  • Anti-scatter grid

Film and Screens

The reciprocity law states that:

Exposure = mA x s

Following this, the same exposure can be created by altering either the current or the exposure time. However, at very low mA doses or long exposure times this relationship fails (reciprocity failure). This creates problems in mammography where very soft radiation is used. When using a screen-film radiography, if the dose rate is very low, it is possible for the time between exposures in one grain to be so long that the grain becomes 'de-sensitised' and for the grain to return to its initial state before the next exposure occurs.

The film used has a fine grain structure, single screens and single-sided emulsion films to maximise spatial resolution.

In mammography there is not a lot of difference in the attenuation coefficients of the different tissues imaged. Therefore, the film needs a small latitude so that a small change in exposure gives a large change in optical density.

Small Focal Spot Sizes

Broad focal spot size = 0.3 mm

Fine focus focal spot size = 0.1 to 0.15 mm

From a point source, objects are easily resolved as separate on the film. However, with increasing focal spot size, the radiation comes from all parts of the source. The radiation creating the image does not provide a sharp image, but has blurring at the edges. If the objects are too close together they can appear as one or an extra 'object' can be created.

Compression

Typical compression force is 100 - 150 N

The compression force:

  • Lowers patient radiation dose
  • Reduces scatter
  • Spreads the tissues out so that there is less overlaying of features
  • Reduces geometric unsharpness by moving some tissue closer to the image receptor
  • Reduces movement unsharpness by holding the breast still

Anti-Scatter Grids

In mammography, moving grids are used for all contact (broad focus) images. For magnification images using a fine focal spot size an air gap technique is used to reduce the amount of scattered radiation reaching the receptor meaning a grid is not required


Altering Parameters

Parameters need to be altered to provide optimal imaging of different breasts. Two factors need to be taken into consideration:

  1. Thickness of breast
  2. Composition of breast

1. Thickness

In large breasts:

  • More radiation absorbed - higher doses needed
  • More scatter
  • Increased beam hardening (lower contrast)
  • Longer exposure needed at 28 kV MoMo, therefore, movement artefacts may occur

Thinnest breasts: MoMo at 25 kV

Thickest breasts: MoRh or even WRh for very thick breasts at 32 kV

2. Composition

With more dense breasts, higher doses are needed due to extra attenuation and more beam hardening. Due to beam hardening, the AEC may cut off the exposure prematurely (the measured exposure will be of a higher intensity). To ensure this doesn't happen, one of two methods may be used:

  1. A pre-exposure determines whether the breast is as dense as expected for this thickness by looking at the dose rate and beam hardening.
  2. Adjustment on dose rate based on measuring the dose detected at the start of the examination and then adjusting the dose and exposure time as necessary.

Σ  Summary
  • Angled tube head
    • Cathode over chest wall (thickest part of breast) to exploit anode heel effect (higher energy radiation at thickest part of breast)
  • Focus-to-film distance
    • Fixed at 65-66cm
  • Target-filter material:
    • General use - MoMo at 25 kV
    • Dense breasts - MoRh or RhRh at 32 kV
  • Compression
    • Maximum pressure of 200N
    • Lowers patient radiation dose
    • Reduces scatter
    • Spreads the tissues out so that there is less overlaying of features
    • Reduces geometric unsharpness
    • Reduces movement unsharpness
  • Anti-scatter grids used
    • Except in magnification view where air-gap used
  • Focal spot
    • Broad focal spot size = 0.3 mm
    • Fine focus focal spot size = 0.1 to 0.15 mm

Next page: Fluoroscopy


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