November 02, 2018

Making digital radiography more accessible

Simplifying the MTF calculation during the optical design process for a less expensive and time-intensive design

Making digital radiography more accessible

The digital radiography market is growing—and it’s no wonder since flat-panel and other detectors present numerous benefits to the healthcare field. For example, patients are exposed to less radiation with digital x-rays than with traditional film x-rays. In addition, using sensors instead of film means that results can be read immediately versus waiting for film to process. Higher quality images can be lightened, darkened, or enlarged for greater clarity. What’s more, password-protected digital patient files are more secure than film printouts stored in filing cabinets.

The importance of MTF in developing digital detectors

Modulation Transfer Function (MTF) is an important method of describing the performance of an optical system. A consequence of applying Fourier theory to image forming optical systems, MTF describes the contrast in the image of a spatial frequency presented in the scene being viewed.

For imaging systems, the performance specification is often MTF at a given spatial frequency. This is especially important for systems with digital detectors, where spatial frequencies beyond a certain value are not needed and mid-range frequency performance is desired.

Optimizing directly on MTF is difficult, though. The calculation is computationally expensive and tends to behave badly in the early stages of a design, so that other optimization methods are needed until the system is very close to its final form.


The Contrast Optimization feature in OpticStudio largely solves these problems. Instead of calculating the full MTF, it measures the phase differences in the exit pupil across a distance corresponding to the desired spatial frequency in the MTF. This quantity can be used to construct a merit function during optimization that has a minimum in the same location as the MTF. The method is much faster and more well-behaved than a direct optimization on the MTF value—and can help optical designers get to a high-confidence digital detector design, faster. Our customers tell us that Contrast Optimization can help speed MTF by up to 30 times faster.

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