Program Information
The Effect of Dose On Low-Contrast Detectability in MVCT
B Harstad*, E Chao , J Shea , E Schnarr , Accuray Incorporated, Madison, WI
Presentations
SU-I-GPD-J-52 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: To assess the low-contrast detectability performance of TomoTherapy® MVCT at a variety of dose levels.
Methods: The imaging beam of a TomoTherapy system was adjusted from factory settings to yield an imaging dose approximately two and four times the normal imaging dose. MVCT images were acquired of the low-contrast section of a CT phantom (QRM ConeBeam Phantom). Portions of each image containing a single object size and contrast were presented to 25 people in a blind survey where they were asked if they could distinguish an object.
Results: At 2% contrast, standard dose yielded detection rates of 100%, 95%, and 12% for object sizes 32mm, 16mm, and 8mm respectively. Doubling the dose yielded detection rates of 98% and 58% for object sizes 16mm and 8mm respectively. At 1.5% contrast and standard dose, 99%, 56%, and 0% of objects were detected with sizes of 32mm, 16mm, and 8mm. Doubling the dose increased the detection rates to 100%, 94%, and 42%. At four times dose, detection rates increased further to 100% and 71% for 16mm and 8mm objects. False positive rates were 2%, 2%, and 5% for standard, double, and four times doses.
Conclusion: The measured low contrast performance was better than the manufacturer’s specifications of 3%/3cm, even at the standard dose. This is probably due in part to the use of the QRM phantom which has a smaller diameter than the TomoPhantom (16cm vs. 30cm). However, it may also be due to the more careful low-contrast measurements which are not possible with the TomoPhantom since the removable contrast objects are outlined by an air gap. Increasing the dose further improves the low-contrast detections.
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