Development and Enhancement of a Real-Time Skin Dose Tracking System dor Fluoroscopically- Guided Interventions
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X-ray image guided interventions are increasingly used for the treatment of vascular pathologies. Some of the interventions can be prolonged resulting in a high skin dose to the patient. A real-time dose tracking system (DTS) has been developed that calculates the skin dose distribution during interventional procedures and provides a color map of the dose distribution on a 3D patient graphic for immediate feedback to the interventionalist. The purpose of the work in this thesis is to develop improvements for the DTS to make it more accurate in special circumstances and to incorporate additional features. These special circumstances include the variation of backscatter with non-uniform fields, small field-of-view imaging with a microangiographic camera, cone-beam CT and the use of a head holder in neurointerventional procedures. The facility to calculate kerma-area-product and cumulative air kerma also was added to the DTS. Radiation backscattered from the patient can contribute substantially to skin dose in fluoroscopically-guided interventions. Since the shape of the primary x-ray beam and its intensity can vary, use of a single constant backscatter factor is not appropriate. We devised a method to determine the backscatter spatial distribution through convolution of a backscatter-to-primary point-spread function (PSFn) with the primary beam distribution using the DTS. The backscatter distributions calculated using the convolution method were validated with Monte-Carlo-derived distributions for uniform fields and with Gafchromic film for non-uniform x-ray fields obtained using region of interest (ROI) attenuators and compensation filters, both with homogenous acrylic and non-homogenous head phantoms. The DTS was modified to automatically account for the change in backscatter for the very small field of view (FOV) of the high-resolution micro-angiographic fluoroscope (MAF) and to provide separated dose distributions for the MAF and the standard flat panel detector (FPD). To validate the reduction of integral dose to the patient when using the MAF compared to when using the FPD, a comparison of kerma-area-product (KAP) per image frame was done. The DTS has been extended to include cone-beam computed tomography (CBCT) for neurointerventions. In addition to the entrance dose, both the exit primary and the scatter from the many overlapping projections are added for every projection in the CBCT scan to obtain a total dose mapping. The dose mapping of the DTS was compared to that measured with Gafchromic film wrapped around a CTDI-head phantom with good agreement. To improve the accuracy of skin dose calculation for neuro interventions, a correction was developed for the head-holder which is used to immobilize and support the head. Attenuation corrections considering the path length of each ray through the curved holder were successfully incorporated in the DTS software. Finally, the DTS was modified to calculate the KAP and cumulative air kerma (CAK) for fluoroscopic interventions for uniform x-ray fields and non-uniform x-ray fields generated when the compensation filters are used. The calculated KAP and CAK had excellent agreement with the values displayed by the fluoroscopy machine. This capability eliminates the need for a separate transmission ionization chamber at the x-ray collimator as currently used. There are many different configurations and exposure techniques used in interventional fluoroscopically guided procedures and the DTS must be able to provide an accurate estimate of skin dose for all situations. The work performed here expands the capabilities of the DTS and the exposure conditions for which dose can be accurately calculated in real-time enabling the interventionalist to proceed during a procedure with added confidence in his/her ability to manage patient radiation risk.