Room: Stars at Night Ballroom 2-3
Purpose: To further reduce imaging dose of proton CT (pCT) by employing a modulated fluence field achieving a prescribed, non-homogeneous image noise map, by exploiting the flexibility of modern pencil beam scanning delivery.
Methods: We present a three-step method for optimization of pencil beam fluence fields: (1) To obtain required variance levels at the detector, that yield the given image noise prescription, we use an iterative approach which respects constraints such as positivity. (2) Detector variance levels at unit fluence are calculated from a prior pCT scan, or from Monte Carlo simulations using a prior x-ray CT. (3) The required fluence modulation is estimated by finding a linear combination of relative pencil beam fluences, that equals the ratio of the variance levels obtained in steps (1) and (2). We prescribed a non-convex region-of-interest (ROI) of low noise surrounded by a high-noise region and tested the optimized fluence modulation by randomly selecting individual proton histories in experimental pencil beam data acquired with the phase II prototype pCT scanner.
Results: The obtained image noise map reproduced the non-convex shape of the ROI. The agreement in terms of variance was better than 16% within the ROI. Due to the finite pencil beam size and filtering operations during reconstruction, image noise gradients were blurred compared to sharp gradients in the prescription. The average fluence over the whole phantom was reduced from 130 mmâ?»Â² to 89 mmâ?»Â² while preserving the noise level in the ROI.
Conclusion: We present the first fluence modulation for pCT of a non-circular ROI. The accuracy of the resulting image noise map was satisfactory and systematic limitations were investigated. By means of fluence modulation, the average fluence could be reduced by a factor of 1.5, which correlates with the expected imaging dose saving.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by the German Research Foundation (DFG) project #388731804 and the DFG's Cluster of Excellence Munich-Centre for Advanced Photonics (MAP), by the Bavaria-California Technology Center (BaCaTeC) and by the Franco-Bavarian university cooperation center (BayFrance).