Hologic - Digital Detectors

Characterizing Detector Performance

There are a number of parameters that characterize the overall image quality obtained with digital mammography and general radiography detectors. The most important are:

Pixel Size
In indirect-conversion digital detectors, as the pixel size is decreased a limit is reached beyond which further reductions in pixel size does not improve resolution. This resolution limit is a function of the light scattering in the scintillator.

In direct-conversion digital detectors, spatial resolution is only limited by the size of the pixel. The size of the pixel in these detectors can be made arbitrarily small to make the resolution performance extend to very high spatial frequencies.

For example, a system with a 100-micron pixel cannot adequately resolve spatial frequencies above 5-line pairs/mm (lp/mm). Indirect-conversion methods can scatter light over several pixels, further limiting the effective resolution of the system, more so than indicated by pixel size alone.

The pixel size for the Hologic digital mammography detector is 70 microns, and the Hologic digital radiography detector is 139 microns. Because of the design of these systems, this represents their true resolution characteristic.

Images acquired using Selenia's 70 micron direct conversion detector compared to a 70 micron indirect conversion detector demonstrate higher contrast and improved visualization of fine details

The Modulation Transfer Function (MTF) measures the ability of a system to faithfully transfer incoming signals and record the information. Having a high MTF over the useful range means the output of the detector provides a good representation of the original information.

The MTF of Hologic detectors is superior over the normal diagnostic range compared to the MTF of indirect methods including conventional screen-film and computed radiography systems.

For a number of reasons, screen-film resolution is linked to system speed. This is not the case for Hologic DirectRay detectors because the dose requirements are established by the desired signal/noise ratio in the resulting image. In other words, with Hologic detectors, greater resolution does not require greater patient dose.

Dynamic Range
Screen-film systems have a limited dynamic range, which prevents visualization with equal clarity of all breast tissue regions from the chest wall to the skin line. Digital detectors offer greatly improved performance. It has been shown that for an ideal detector with no inherent detector noise, 3,100 gray levels are discernable in a typical digital mammographic image.

Dose Efficiency
x-ray quantum detection efficiency is a measure of the percentage of x-rays that hit the detector that are absorbed. Systems with higher quantum efficiency can produce higher quality images, at lower dose.

Hologic direct conversion selenium detectors are better than indirect conversion methods in terms of Detective Quantum Efficiency (DQE). With no signal spreading, the DQE of the Hologic DirectRay detector are governed mainly by the inherent limits of the pixel size. That means by eliminating information scatter, DQE values close to the theoretical limit are achieved with Hologic detectors.

The Hologic Direct-to-Digital Difference

Ultimately, x-ray image quality depends on the signal profile that is captured. By eliminating light scatter; Hologic detectors achieve a leap in image quality.