Improved region of interest imaging
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As minimally invasive procedures gain preference over invasive surgeries, the devices required to perform such interventional procedures get smaller and more complex. Along with placing the imaging detector in the best place to view the pathology under treatment, providing the interventionalists with high resolution images without significantly increasing patient exposure to radiation is very important for success of such procedures. The focus of this dissertation is to develop and explore ways to make optimal use of a high resolution region of interest imager. Five different methods were evaluated. In most of the interventional procedures, it has been observed that the actual site of intervention does not cover the full field of view of commercial detectors like the flat panel detector and x-ray image intensifier. In order to position the interventional devices in complex vasculature, along with high resolution in the region of interest, it is important to provide the best view to the interventionalists. The gantry orientation that minimizes vessel foreshortening is defined as optimal viewing angle. A method to calculate gantry angles to achieve the optimal viewing angle for three different Toshiba gantries is described here. Detectors with small pixel sizes show improved resolution performance. But the quantum noise resulting from fewer x-ray photons absorbed in each pixel affects the image quality adversely. Increasing detector exposure under permissible patient exposure limits can be achieved using High Level Control (HLC) during fluoroscopy. HLC uses medium focal spot. When used with a high resolution detector, geometric blurring due to medium focal spot degrades image quality. A method referred to as the High Definition (HD) mode that uses the smaller focal spot to provide higher detector exposure without affecting the continuity of the fluoroscopic procedure is introduced here. Software noise reduction algorithms that preserve the spatial and temporal resolution can be used to reduce the effect of quantum noise on image quality. Temporal filtering improves the image quality while preserving spatial resolution, but is susceptible to motion blur. Motion blur can be avoided by implementing adaptive spatiotemporal filtering. An object detection based adaptive temporal filtering method which detects motion of an object of interest and varies the temporal filter accordingly is proposed. Another adaptive temporal filter based on the similarity between consecutive frames is also described. An image denoising filter based on the class of filters called "coherence filters" where only similar pixels are averaged, is adapted to suit the pixel size and noise characteristics of the MAF. To save patient dose during interventional procedures, region of interest fluoroscopy was proposed in 1992 taking advantage of the fact that actual intervention is carried out over a small portion of the full field of view. A method of moving the beam-attenuator aperture to follow the object of interest such that it always remains in the region of interest (ROI) during a fluoroscopic procedure is discussed here. This increases the flexibility to use ROI fluoroscopy in situations where the treatment location is in different parts of the full field of view over time. The MAF is detector designed mainly for x-ray imaging. The high sensitivity and variable gain allow operation in quantum limited region with virtually no instrumentation noise. This is used to acquire both emission (nuclear) and transmission (x-ray) images in single photon counting (SPC) mode. It is shown how each of these techniques provide improved use of region of interest fluoroscopy with optimized viewing of the object of interest and optimized image quality. High definition mode allows the use of small focal spot with increased exposure during fluoroscopy. The implemented adaptive temporal filters show improvement in the signal to noise ratio for x-ray images. The coherence filter implementation shows that it preserves the edges as well as reduces the effect of quantum noise in the image quality. Moving the beam attenuator allows varying locations of the ROI during ROI fluoroscopy. Functionality of the MAF is further improved by using it for both emission and transmission imaging in the SPC mode.