Experimental validation of an OSEM-type iterative reconstruction algorithm for inverse geometry computed tomography

Abstract

Iterative reconstruction methods have emerged as a promising avenue to reduce dose in CT imaging. Another, perhaps less well-known, advance has been the development of inverse geometry CT (IGCT) imaging systems, which can significantly reduce the radiation dose delivered to a patient during a CT scan compared to conventional CT systems. Here we show that IGCT data can be reconstructed using iterative methods, thereby combining two novel methods for CT dose reduction. A prototype IGCT scanner was developed using a scanning beam digital X-ray system -an inverse geometry fluoroscopy system with a 9,000 focal spot x-ray source and small photon counting detector. 90 fluoroscopic projections or "superviews" spanning an angle of 360 degrees were acquired of an anthropomorphic phantom mimicking a 1 year-old boy. The superviews were reconstructed with a custom iterative reconstruction algorithm, based on the maximum-likelihood algorithm for transmission tomography (ML-TR). The normalization term was calculated based on flat-field data acquired without a phantom. 15 subsets were used, and a total of 10 complete iterations were performed. Initial reconstructed images showed faithful reconstruction of anatomical details. Good edge resolution and good contrast-to-noise properties were observed. Overall, ML-TR reconstruction of IGCT data collected by a bench-top prototype was shown to be viable, which may be an important milestone in the further development of inverse geometry CT. IntroductionComputed tomography (CT) is an indispensable tool in medicine, and its importance is likely to increase in the coming years. Computed tomography (CT) has played an important role in significantly improving the outcome in many diseases, including cancer and heart disease. For example, for most types of cancer CT has become the dominant imaging method used for diagnoses, progress monitoring, and biopsy guidance among other applications throughout staging, treatment, and follow-up examinations; it is not uncommon for a cancer patient to receive ten CT scans during this time.According to the National Cancer Institute (NCI), CT is the largest contributor to medical radiation exposure received by the U.S. population [1]. Other recent publications predict that a significant number of cancers will be induced by the radiation from CT scans, especially in more radiation-sensitive subjects. While pediatric cancer is a small fraction in the overall cancer problem, children are particularly vulnerable to the effects of radiation exposure. Although the benefits of CT greatly outweigh the risk, it is important to work to reduce the radiation exposure from CT scanning. Simulations and preliminary experiments suggest that inverse geometry CT (IGCT) in conjunction with iterative reconstruction can accomplish this goal. In conventional geometry x-ray imaging systems, a pointsource X-ray tube is projected onto an extended detector. In contrast, inverse geometry imaging systems utilize an extended, multi-focal spot X-ray source, essentially a "spatially resol...

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