A method to convert digital mammograms acquired on one system to appear as if acquired using another system is
presented. This method could be used to compare the clinical efficacy of different systems. The signal transfer properties
modulation transfer function (MTF) and noise power spectra (NPS) were measured for two detectors - a computed
radiography (CR) system and a digital radiography (DR) system. The contributions to the NPS from electronic, quantum
and structure sources were calculated by fitting a polynomial at each spatial frequency across the NPS at each dose. The
conversion process blurs the original image with the ratio of the MTFs in frequency space. Noise with the correct
magnitude and spatial frequency was added to account for differences in the detector response and dose. The method was
tested on images of a CDMAM test object acquired on the two systems at two dose levels. The highest dose images were
converted to lower dose images for the same detector, then images from the DR system were converted to appear as if
acquired at a similar dose using CR. Contrast detail curves using simulated CDMAM images closely matched those of
real images.
A number of different technologies are available for digital mammography. However, it is not clear how differences in the physical performance aspects of the different imaging technologies affect clinical performance. Randomised controlled trials provide a means of gaining information on clinical performance however do not provide direct comparison of the different digital imaging technologies. This work describes a method of simulating the performance of different digital mammography systems. The method involves modifying the imaging performance parameters of images from a small field of view (SFDM), high resolution digital imaging system used for spot imaging. Under normal operating conditions this system produces images with higher signal-to-noise ratio (SNR) over a wide spatial frequency range than current full field digital mammography (FFDM) systems. The SFDM images can be 'degraded’ by computer processing to simulate the characteristics of a FFDM system. Initial work characterised the physical performance (MTF, NPS) of the SFDM detector and developed a model and method for simulating signal transfer and noise properties of a FFDM system. It was found that the SNR properties of the simulated FFDM images were very similar to those measured from an actual FFDM system verifying the methodology used. The application of this technique to clinical images from the small field system will allow the clinical performance of different FFDM systems to be simulated and directly compared using the same clinical image datasets.
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