Abstract
BackgroundQuantum noise intrinsically limits the quality of fluoroscopic images. The lower is the X-ray dose the higher is the noise. Fluoroscopy video processing can enhance image quality and allows further patient’s dose lowering. This study aims to assess the performances achieved by a Noise Variance Conditioned Average (NVCA) spatio-temporal filter for real-time denoising of fluoroscopic sequences. The filter is specifically designed for quantum noise suppression and edge preservation. It is an average filter that excludes neighborhood pixel values exceeding noise statistic limits, by means of a threshold which depends on the local noise standard deviation, to preserve the image spatial resolution. The performances were evaluated in terms of contrast-to-noise-ratio (CNR) increment, image blurring (full width of the half maximum of the line spread function) and computational time. The NVCA filter performances were compared to those achieved by simple moving average filters and the state-of-the-art video denoising block matching-4D (VBM4D) algorithm. The influence of the NVCA filter size and threshold on the final image quality was evaluated too.ResultsFor NVCA filter mask size of 5 × 5 × 5 pixels (the third dimension represents the temporal extent of the filter) and a threshold level equal to 2 times the local noise standard deviation, the NVCA filter achieved a 10% increase of the CNR with respect to the unfiltered sequence, while the VBM4D achieved a 14% increase. In the case of NVCA, the edge blurring did not depend on the speed of the moving objects; on the other hand, the spatial resolution worsened of about 2.2 times by doubling the objects speed with VBM4D. The NVCA mask size and the local noise-threshold level are critical for final image quality. The computational time of the NVCA filter was found to be just few percentages of that required for the VBM4D filter.ConclusionsThe NVCA filter obtained a better image quality compared to simple moving average filters, and a lower but comparable quality when compared with the VBM4D filter. The NVCA filter showed to preserve edge sharpness, in particular in the case of moving objects (performing even better than VBM4D). The simplicity of the NVCA filter and its low computational burden make this filter suitable for real-time video processing and its hardware implementation is ready to be included in future fluoroscopy devices, offering further lowering of patient’s X-ray dose.
Highlights
Quantum noise intrinsically limits the quality of fluoroscopic images
For a mask size of 7 × 7 × 7 pixels, the moving average filter produced a line spread function with FWHM 77% larger than that produced via Noise Variance Conditioned Average (NVCA) with same mask size (2σ threshold)
For low-threshold levels, the spatial resolution obtained with NVCA filter resulted comparable to that for video denoising block matching-4D (VBM4D)
Summary
Quantum noise intrinsically limits the quality of fluoroscopic images. The lower is the X-ray dose the higher is the noise. It is an invaluable tool for interventional radiology procedures, such as orthopedic surgery, angioplasty, pacemaker and defibrillator implantation [1, 2], for diagnostic exams, such as investigations of gastrointestinal tract or blood vessels, for the assessments of joints and implanted prosthesis [3, 4] and image-guided radiotherapy [5] Since these applications require extended and unpredictable exposure times, patient radiation dose must be kept acceptably low, resulting in a degradation of image quality. A better image quality could be only achieved by increasing the number of photons per pixel This cannot be pursued in fluoroscopy, since it would require an unacceptable increase of patient’s dose, as exposure time cannot be reduced or even limited. There is the need to improve fluoroscopic image quality by means of image processing techniques
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