Imagine yourself selecting a new PC for your home office. Should you choose the system with the fastest modem or processor? The largest RAM? Or is system memory the most important parameter?

Unfortunately, it's not that simple; none of these specifications can stand alone as an accurate measure of system performance. And those evaluating the clinical performance of various digital x-ray detector technologies are facing an even tougher challenge. The specifications we once relied on to measure the diagnostic utility of x-ray imaging-spatial resolution and MTF, for instance - are inadequate as stand-alone gauges of digital-system performance.

That's why physicists and researchers are using an effective new way to measure digital x-ray image quality - Detective Quantum Efficiency (DQE), the measure of the combined effect of the noise and contrast performance of an imaging system, expressed as a function of object detail.

Quantum and electronic noise are unavoidable in a digital imaging chain. The effect, often expressed as signal-to-noise ratio (SNR), can vary widely from system to system.

Signal             Useful image formation
----------   =   -----------------------
Noise              Erratic Information

High SNR, or low system noise, is therefore key to capturing the greatest proportion of useful image information - in short, to digital x-ray image quality (Fig. 1). Often, the only way to compensate for poor SNR is to increase radiation dose, an unacceptable trade-off.

That's why physicists and researchers are using an effective new way to measure digital x-ray image quality - Detective Quantum Efficiency (DQE), the measure of the combined effect of the noise and contrast performance of an imaging system, expressed as a function of object detail.

Contrast performance describes a system's ability to capture and display an object's actual contrast - especially critical when imaging such inherently low-contrast objects as breast microcalcifications and pulmonary nodules.

Most digital x-ray detectors have a wide dynamic range to permit capture of a wide range of low-to-high signal intensities, as well as very high contrast resolution to permit the display of thousands of shades of gray. In fact, that's one of digital x-ray's great advantages over conventional film/screen imaging: Digital x-ray detectors can successfully image areas that might be under- or overexposed on film, and can further improve their as automatic contrast enhancement and window/leveling.

How critical is spatial resolution to a digital x-ray system's performance?

Clearly, it's an important parameter – but not nearly as important as one might think. That's because there's a point of diminishing return; while noise remains constant in a given system, the amount of signal captured per pixel dwindles with pixel size. As a result, the finer the matrix, the lower the Signal-to-Noise-Ratio (SNR) at each pixel. This, combined with the lower inherent contrast of small objects, often limits detectability.

Determining the optimum balance between pixel size and noise is therefore crucial to the developers of digital detectors – and to the clinicians contributing to the technology's evolution.

It's the combination of very low noise and superior contrast performance that allows some digital x-ray systems to offer such significant improvements in the detectability of low-contrast objects - a quality that is best quantified by a single parameter, Detective Quantum Efficiency (DQE). As one medical physics expert recently reported, The DQE (f) has become the de facto benchmark in the comparison of existing and emerging x-ray detector technologies.

DQE especially affects one's ability to view small, low-contrast objects. In fact, in many imaging situations, it's more important to detecting small objects than is limiting spatial resolution (LSR) - the parameter traditionally used to determine how small an object one can visualize. Even if a digital system has very high LSR, it can't take advantage of the resolution if it has low DQE, which prevents the detection of very small objects (Fig. 2).

A study comparing film/screen and digital imaging demonstrates that a digital system with high DQE can improve one's ability to detect small, low-contrast objects – even though the digital system may have substantially lower Limiting Spacial Resolution (LSR) than film.

Reducing radiation dose is another potential advantage of digital x-ray technology; and high DQE should make significant contributions to this equation. Compared with film/screen imaging, a digital detector with high DQE has the potential to deliver significant object-detectability improvements at an equivalent dose, or to permit object detectability comparable to film's at reduced dose.

Equally important, high DQE provides the requisite foundation for advanced digital applications - dual-energy imaging, tomosynthesis, and low-dose fluoro, for instance. Combined with advanced image-processing algorithms and fast acquisition and readout capability, high DQE is key to making such applications as these clinically practical in the years to come.

Maximizing DQE should therefore be the key goal for developers of digital detectors. And it was just that for the designers of GE's digital x-ray technology.

As a result, GE digital detectors typically exhibit a higher DQE than not only film/screen, but also computed radiography and flat-panel, Selenium-based imaging systems (Fig. 3).

1. JM Boone, "Spectral modeling and compilation of quantum fluence in radiography and mammography," in Physics of Medical Imaging, James T. Dobbins III, John M. Boone, Editors, Proceedings of SPIE Vol. 3336, 592-596 (1988).

2. Kodak curve: Van Metter and Dickerson. Objective performance characteristics of a new asymmetric screen-film system. Medical Physics 21:9, pages 1483-90.

Fuji curve: Ogawa et al. Proceedings of SPIE, Vol. 2432, 1995, pages 421-431.

Sterling curve: Denny L. Lee et al. Proceedings of SPIE, Vol. 3336, 1998, pages 14-23.