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01-11-2004 | Physics

Digital detectors for mammography: the technical challenges

Authors: A. Noel, F. Thibault

Published in: European Radiology | Issue 11/2004

Abstract

This paper reviews the different techniques available and competing for full-field digital mammography. The detectors are described in their principles: photostimulable storage phosphor plates inserted as a cassette in a conventional mammography unit, dedicated active matrix detectors (i.e., flat-panel, thin-film transistor-based detectors) and scanning systems, using indirect and direct X-ray conversion. The main parameters that characterize the performances of the current systems and influence the quality of digital images are briefly explained: spatial resolution, detective quantum efficiency and modulation transfer function. Overall performances are often the result of compromises in the choice of technology.

Photodiode (or light-sensitive diode)

Semi-conductor element converting light energy into electrical current. The intensity of the current generated, and therefore the quantity of electricity produced, are proportional to the light intensity (itself proportional to the incident irradiation). In the case of indirect conversion detectors, the electronic charges produced are stored in a condenser. Detectors contain one photodiode per pixel.

Thin-film transistor (TFT)

Electronic component of an active matrix in which each element (one element per pixel) acts like a switch integrated in the reading circuit to determine the quantity of electronic charges produced by the photodiode and stored in the condenser.

Charge coupled device (CCD)

Electronic component converting light energy into electrical current. The quantity of electricity produced, proportional to the light intensity, is stored directly in the device. CCDs are used in indirect conversion detectors.

Structured scintillator

Scintillator whose crystalline structure is composed of needle-like elements that channel light down the length of the crystal and minimize lateral spread.

Spatial resolution

Spatial resolution is expressed in cycles per mm or, more commonly, in line-pairs per millimeter (lp/mm) and indicates the size of the smallest structure detectable on a test object measured under laboratory conditions, which increase contrast while reducing noise. It is measured by X-raying a phantom composed of periodic elements (alternating bars and spaces) of increasing frequency (bars of decreasing thickness).

Nyquist (or cut-off) frequency

Corresponds to a particular value of spatial resolution defined by the pixel size of the digital detector. According to the sampling theorem, the Nyquist frequency is equal to 1/[2 × pixel size (mm)]. It is expressed in lp/mm. Objects with a spatial frequency higher than the Nyquist frequency will either not be visualized or will be visualized incorrectly (aliasing).

Modulation transfer function (MTF)

Describe the ability of an imaging system to transfer the contrast of a structure to the final recorded image. In practice, this reflects the loss of contrast induced by the imaging system as a function of spatial resolution, i.e., spatial frequency expressed in line-pairs per millimeter. By definition, the MTF is equal to 1 for a spatial frequency of zero and decreases as far as 0 with increasing spatial frequency.

Spatial frequency

The frequency spectrum of an image can be obtained by its Fourier transform. More simply, by analogy with a periodic vibratory phenomenon specified by its frequency (number of cycles per unit of time, usually expressed in hertz, Hz), a pattern in the image is characterized by its repetition in space expressed in cycles or line-pairs per unit of length. An object with dimension d (for example, d=0.08 mm) contained in the image would be associated with a spatial frequency of 1/2d (1/2×0.08=6.25 lp/mm).

Aliasing

Phenomenon resulting from signal undersampling by a digital imaging system, either the detector or reading system (PSP laser beam). Objects with a spatial frequency higher than the Nyquist frequency of the imaging system are replicated as artifacts around the Nyquist frequency that superimpose on objects with lower frequencies. The low frequency noise level is therefore increased, which degrades low contrast object detection.

Detective quantum efficiency (DQE)

DQE characterizes the ability of a detector to use the transmitted photons (through the breast, bucky and grid) at the detector input. It is expressed as the ratio of the squares of the signal-to-noise ratio at the detector input and detector output. An ideal system, which would not add any noise, i.e., which would use all photons reaching the detector, would have a DQE equal to 1. A real detector is increasingly “better” as its DQE approaches 1. The DQE of a system is maximal for zero spatial frequency and decreases with increasing spatial frequency.

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Title: Digital detectors for mammography: the technical challenges
Authors: A. Noel
F. Thibault
Publication date: 01-11-2004
Publisher: Springer-Verlag
Published in: European Radiology / Issue 11/2004
Print ISSN: 0938-7994
Electronic ISSN: 1432-1084
DOI: https://doi.org/10.1007/s00330-004-2446-6

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Digital detectors for mammography: the technical challenges

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