Choosing a Digital Mammography System: a-Se, CsI and CR Detectors Compared

How a digital mammography system converts X-rays to images

In a direct-conversion detector, a layer of amorphous selenium turns X-ray photons into electrical charge, which is read by a thin-film transistor array. There is no intermediate light stage, so the signal spreads little sideways and sharpness is determined largely by the pixel pitch.

In an indirect detector, a CsI scintillator converts X-rays into light, which a photodiode array on amorphous silicon turns into charge. Needle-like CsI crystals guide the light and limit spread. Both families are widely used in full-field digital mammography, and the difference in practice depends on the specific product, not only on the technology label.

Mammography flat panel detector vs CR vs DR

In CR vs DR mammography, CR uses a reusable phosphor plate that is exposed in the unit and then scanned in a separate reader. DR uses a flat panel that reads out within the unit. DR generally gives faster workflow, since no plate handling is needed, and avoids the artefacts associated with plate handling and reader wear, while CR may be a lower-entry-cost path for departments upgrading from film.

Both approaches can meet quality requirements, but tomosynthesis needs fast readout, which in practice requires a flat panel. If you plan 3D, treat CR as a stop-gap and plan for DR.

Amorphous selenium detector mammography: what to consider

Selenium detectors are valued for high spatial resolution. Questions to ask include the pixel size, the readout speed, how the detector behaves after high-dose exposures, temperature and humidity requirements, and warranty on the detector itself, which is one of the most expensive components.

Also ask for DQE (detective quantum efficiency) curves measured under the standard conditions used by your physicist, and for dose-to-signal data. DQE describes how efficiently the detector uses the incident dose, but it is a function of spatial frequency and dose, so a single quoted number is incomplete.

• Pixel size and active area
• DQE curves at stated dose and spatial frequencies
• Readout speed, especially for tomosynthesis
• Detector warranty and expected lifetime
• Environmental requirements (temperature, humidity)

Detector options on the Villa Melody III line

The Villa Melody IIID can be configured with an a-Si/CsI flat-panel detector or an optional a-Se detector. The Melody IIID TS is offered with an a-Si/CsI detector or an optional a-Se detector in Standard or Fast versions; with the Fast a-Se detector the listed scan times are 2.5 s, 4 s and 7.7 s.

Because the manufacturer offers more than one detector, ask for clinical sample images and dose data for the exact detector you are quoted, and ask whether the detector can be changed later. Our article on mammography machine maintenance explains calibration and detector care.

Practical advice for biomedical engineers and buyers

Include detector calibration, replacement policy and expected lifetime in the contract, and verify that acceptance tests (uniformity, noise, artefacts) are part of installation. Compare vendors by the complete chain, from detector to AEC to display, because image quality depends on all of it. For an overview of how the whole unit functions, see how a mammography machine works.

Beckfinity's specialists can help you specify the detector and the rest of the configuration; request a quote or consultation to discuss it.