Mammography

Mammography uses low-energy X-rays to image breast tissue and detect breast cancer early. Breast tissue however has very low subject contrast as it is composed of soft tissue which has small differences in physical characteristics of density and atomic number.

Principles of soft tissue imaging

Recall the factors which affect contrast:

Subject contrast

• Tissue
  • Effective atomic number
  • Density, thickness
• Beam quality
  • kVp
  • Filtration
  • Target material

Image contrast

• Subject + imaging system
• Detector properties
• Image processing
• Viewing monitor
• Adjustment of window width and level

1.2. Linear attenuation coefficient and photon energy

The linear attenuation coefficient (μ) increases with density (ρ), increases with atomic number (Z) and decreases with increasing photon energy (E). With increased keV, the difference between attenuation coefficients of varying tissues becomes less, thus reducing subject contrast. Hence, the maximum difference between tissue attenuation and therefore subject contrast occurs in the low keV range.

1.3. Photoelectric effect

Mammography principally utilises the photoelectric effect (PE) with L-shell electrons to enhance the contrast between tissues. Recall:

PE ∝ Z3/E3

Lower x-ray beam energy increases the likelihood of PE absorption. Hence, a lower tube potential is needed to maximise low subject contrast.

A typical exposure in mammograph is 25 kV and 100 mA (2.5 kW power rating).

Although microcalcifications have high atomic number (recall PE ∝ Z³/E³), it’s small size (0.01 mm) may result in low contrast. Additionally, it is easily masked by noise (similar grainy texture)

2. Mammography equipment

2.1. Important performance characteristics

• High contrast resolution – need good differentiation of low contrast structures
• High spatial resolution – for microcalfications
• High signal to noise ratio
• Reduced absorbed dose to breast tissue

2.2. Anode material

Molybdenum (Mo) is often used as;

• Z = 42
• Characteristic radiation has useful energies for soft tissue imaging
• K_α radiation = 17.4 keV, K_β radiation = 19.6 keV

2.3. Filtration

Filtration removes the lowest energy (soft) x-rays from the beam which do not contribute to image quality and unnecessary increase entrance surface dose and image degradation from scatter.

There are two types of filtration:

• inherent filtration from components in the x-ray tube, i.e. window, housing, cooling oil (equivalent to 0.5 – 1.0 mm Al)
• added filtration from interchangeable metal sheets (Al, Cu, etc.)

In mammography, special k-ede filters are used to remove higher energy x-ray photons, and make the beam as monoenergetic as possible.

2.4 X-ray tube

• Metal envelope – Vacuum of mammography tube is maintained inside metal envelope rather than glass used in general radopgraphy. Typically 65 cm.. This has the advantage of reducing off-focal (or extra focal) radiation that would otherwise detract from the ability to demonstrate subtle differences in subject contrast. This reduces blur, improves contrast, decrease noise and decrease patient dose.
• Beryllium exit window – The tube exit window is made of a thin layer of beryllium (0.5 – 1mm) rather than glass used in general radiography. Beryllium (Z = 4) minimises X-ray beam attenuation, allowing transmission of all but the lowest energy Bremsstrahlung radiation (< 5 keV) hence the need for additional filtration (i.e. Mo and Rh filters)
• Focal spot size – generally small focal spot sizes are used to allow high spatial resolution of fine features such as microcalcfications, rather than focal spots of 0.6 – 1 mm used in general radiography.
  • Broad focus – A 0.3 mm focal spot is used for contact images where the breast is placed near the image receptor.
  • Fine focus – A 0.1 mm focal spot is used for magnification work, where the breast is positioned about 30 cm above the image receptor.
• Tube tilt – The effective anode angle required to provide full coverage on the large field of view (24 x 30 cm) at a source to image distance of 65 cm is about 24°
  This is achieved by having: a tube tilt of 6° and an anode angle of 16°. When the x-ray tube is angled, the vertical ray is the central ray. Mammography systems utilise a “half-field” x-ray beam geometry, which is achieved by collimation at the x-ray tube head

• Focus to Detector Distance (also called Source to Image Distance, SID) is fixed. Typically 65 cm.

• Auto-collimation – (18×24, 24×30, mag and spot) i.e. fixed field sizes

2.5. Compression

Compression helps to spread out the normal dense fibroglandular tissue of the breast allowing for easier detection of abnormalities. The compression paddle is composed of 3mm of perspex. Standard compression forces range from 100 – 150 N. 

Benefits

1. Immobilises breast tissue – reduces motion blur
2. Spreads out tissue – reduces superimposition of anatomy, allowing for lesions to be detected easier
3. Uniform breast tissue – creates a homogeneous image intensity by
4. Decreases tissue thickness – This allows use of lower voltages which improves subject contrast. Contrast is also improved by reducing scattered radiation. This also reduces exposure times minimising patient blur and dose. Decreased tissue thickness also means less attenuation and therefore less absorbed dose (this is greatest appreciated in dense breasts).
5. Reduces breast to image receptor distance (object to image distance) – Improves spatial resolution (due to reduced geometric unsharpness i.e. smaller penumbra)

For the smallest penumbra i.e. lowest geometric unsharpness:

• Maximise SID (fixed)
• Minimise focal spot size (fixed)
• Minimise object to image receptor distance (compression!)

Magnification increases geometric unsharpness.

2.6. Anti-scatter grids

Moving grids are used for all contact (broad focus) images, to reduce scatter and improve image contrast. Grids are placed between the breast and the image receptor.

Linear grids have lead strips and low attenuation interspace material (e.g. carbon fibre)

• Provides scatter rejection in one dimension

High transmission cellular grids have a focussed cellular pattern (e.g. honeycomb pattern) and is constructed of copper and air

• Provides scatter rejection in two dimensions

For magnification images (fine focus) an air gap technique is used (instead of a grid) to reduce the amount of scattered radiation reaching the receptor.

2.7. Automatic exposure control

AEC is used due to the wide variation of breast composition and thickness. The AEC detector is placed behind the detector (unlike in general radiography where it is placed between the patient and the receiver). 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

2.8. Image receptor

In modern systems flat panel detectors are used rather than film. Direct flat panel detectors are more commonly used than indirect detection systems as the material used for the semiconductor layer (amorphous selenium a-Se) has a low atomic number, therefore low absorption efficiency. Direct detectors have a much better spatial resolution, which is an important consideration

Whilst film has better limiting spatial resolutions, direct flat panel detectors have higher modulation transfer function/contrast at lower spatial frequencies. Computed radiography image receptors comparatively has the lowest MTF. Additionally, flat panel detectors allow for manipulation of contrast and brightness, image processing (e.g. smoothing or edge enhancement), along with the numerous advantages of digital radiography (improved image access/transmission/retrieval/storage), lower average radiation doses, improved sensitivity in dense breasts.

Recall MTF = ratio of output contrast to input contrast.

Digital dectors also have a wide dynamic range, i.e. contrast is achieved over a wide range of exposures. Film on the hand has a narrow dynamic range or latitude (contrast only changes in a limited linear portion of the curve).

Contact and Magnification Modes