PurposeThe modulation transfer function (MTF) and detective quantum efficiency (DQE) of x-ray detectors are key Fourier metrics of performance, valid only for linear and shift-invariant (LSI) systems and generally measured following IEC guidelines requiring the use of raw (unprocessed) image data. However, many detectors incorporate processing in the imaging chain that is difficult or impossible to disable, raising questions about the practical relevance of MTF and DQE testing. We investigate the impact of convolution-based embedded processing on MTF and DQE measurements.ApproachWe use an impulse-sampled notation, consistent with a cascaded-systems analysis in spatial and spatial-frequency domains to determine the impact of discrete convolution (DC) on measured MTF and DQE following IEC guidelines.ResultsWe show that digital systems remain LSI if we acknowledge both image pixel values and convolution kernels represent scaled Dirac δ-functions with an implied sinc convolution of image data. This enables use of the Fourier transform (FT) to determine impact on presampling MTF and DQE measurements.ConclusionsIt is concluded that: (i) the MTF of DC is always an unbounded cosine series; (ii) the slanted-edge method yields the true presampling MTF, even when using processed images, with processing appearing as an analytic filter with cosine-series MTF applied to raw presampling image data; (iii) the DQE is unaffected by discrete-convolution-based processing with a possible exception near zero-points in the presampling MTF; and (iv) the FT of the impulse-sampled notation is equivalent to the Z transform of image data.
Angioplasty is an interventional treatment for blood vessel stenosis, that uses a catheter navigated to obstructed vasculature under fluoroscopy to force open the blockage with a balloon or permanent wire stent. Accurate stent placement is critical to avoid stent shifting and enabling new blockages. Unfortunately, background structures such as bone and tissue interfere with stent visualization during angioplasty, making it difficult for physicians to visualize stents during placement and post-operative checkups. This contributes to the 25% restenosis rate in patients with permanent stents1. Multi-energy subtraction angiography (ESA) with fast kV switching may enhance nickel-stent visualization by suppressing background structures. The objective of this work was to develop an ESA protocol to enhance the visualization of nickel wire stents. The signal-to-noise ratio of nickel for a given radiation exposure was simulated using MATLAB® and validated against images of a deployed nitinol stent overlaid on a chest phantom. The stent was isolated from bone or soft tissue by applying a calculated weighting factor to the low energy image. We found that by manipulating the weighting factor, it is possible to selectively remove anatomical structures obstructing the view of the stent.
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