TU-E-BRA-08: A Comprehensive Study on the Relationship between Image Quality and Imaging Dose in Low-Dose Cone Beam CT.

PURPOSE

To investigate image quality as a function of number of projections and tube load per projection in compressive sensing (CS) based low-dose cone beam CT (CBCT), and achieve optimal low-dose scan protocols in image guided radiation therapy (IGRT).

METHODS

We have performed CS-based CBCT reconstruction with different combinations of number of projections (from 46 to 364) and mAs (from 0.2 to 2.4 mAs/view), which covers the whole clinically relevant range. Image quality is assessed in each case. On this basis, optimal scan protocols are analyzed according to various IGRT applications.

RESULTS

Image quality degrades ∼10% when the imaging dose decreases from 400 to 100 total mAs, further ∼10% from 100 to 40 total mAs, and another ∼80% below 40 total mAs. Image quality on iso-low-dose lines at 36.8, 72.6, 109.2 and 145.6 total mAs varies 17.16%, 13.69%, 11.99% and 5.74% in terms RMSE with various scanning protocols.

CONCLUSIONS

1. In CS-based CBCT, image quality has little degradation with imaging dose> 100 total mAs. Optimal low-dose scan protocols likely fall in the range of 40-100 total mAs. 2) At a constant low-dose level, the scan protocol that with super sparse views (projection number < 50) is the most challenging case. 3) The optimal scan protocol is the combination of a medium number of projections and a medium level of mAs/view. This is more evident when the dose is ∼72.8 total mAs or below, and when the ROI is a low-contrast or high-resolution object. 4) The clinically acceptable lowest imaging dose level is task dependent. In our study, 72.8mAs is a safe dose level for visualizing low-contrast objects, while 12.2 total mAs is sufficient for detecting high-contrast objects of diameter greater than 3 mm. This work is supported in part by NIH (1R01CA154747-01), Varian Medical Systems through a Master Research Agreement, and the Thrasher Research Fund.