Date(s) - 11/25/2014
Dose reduction in CT has been achieved through various methods since the first measurement of CTDI nearly 30 years ago.1 The method of TCM, first published in 1981,2 resulted in dose reduction approaching 40% when compared to scans without modulated tube current .3 Several other techniques such as collimation, AEC and protocol optimization have also resulted in significantly reduced CT dose.4 The newest dose reduction technique is the iterative reconstruction algorithm. Based on mathematics derived in 1917,5 and used for image reconstruction on the first CT scanners in the 1970s, this method has only now become feasible in the clinic. Numerous studies claim that iterative reconstruction techniques can reduce patient dose by 50% to 75% when compared to a scan with only TCM employed.6-8
It was the aim of this dissertation to investigate the dose reduction capabilities of AIDR-3D on postmortem subjects as surrogates for patients using a systematic and reproducible in-vivo dosimetry method. Additionally, impacts of iterative reconstruction on image quality were explored through phantom and postmortem subject experiments. Finally, a subjective image quality analysis study was conducted to test the limits of image acceptability with relaxed noise tolerance levels using AIDR-3D.
The overall organ dose reductions provided by the Standard setting of AIDR-3D ranged from 21%-46% for a CAP protocol, when compared to TCM scans, for normal weight to morbidly obese postmortem subjects. Investigations of increased noise tolerance in body protocols resulted in additional dose reductions on the order of 39-66% for the CAP protocol. Overall, the added dose reduction of AIDR-3D vs. FBP and the increased noise level acceptance can yield an average of 75% dose reduction from current scans performed without iterative reconstruction.
Using an accurate dosimetry method and performing a robust subjective image quality assessment allowed for the characterization of the AIDR-3D mechanism for dose reduction and clinical image quality. Though anthropomorphic phantom and retrospective scanner dose index evaluations of iterative reconstruction algorithm dose reduction have been performed, the use of human subjects and direct organ dosimetry offers new perspective. The use of postmortem subjects for CT dosimetry has been validated through this dissertation work. This method offers unparalleled accuracy and insight into the performance of modern day scanners equipped with complex noise and dose reduction reconstruction algorithms.