Program Information
Impact of Object Size and Imaging Dose On MVCT and KVCT Image Noise
E Chao*, J Shea , B Harstad , Accuray Incorporated, Madison, WI
Presentations
SU-I-GPD-J-27 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: To evaluate the impact of object size and imaging dose on MVCT and kVCT image noise.
Methods: MVCT and kVCT images were acquired of a CT image quality phantom (QRM ConeBeam Phantom) which contains low-contrast objects at a wide variety of contrasts and object sizes. The 16cm diameter phantom was scanned with, and without, a body ring that extends the diameter to a 35cm by 25cm ellipse. The kVCT images were acquired using a fan-beam CT over a range of dose levels (mAs). The MVCT images were acquired on a TomoTherapy® system and the imaging beam dose rate was adjusted from the factory defaults by changing the pulse rate and the RF power.
Results: Both MVCT and kVCT image noise scale inversely with the square-root of the delivered dose. When noise was reduced, more low contrast objects were visible. When scanning the pediatric-head sized 16cm diameter phantom alone, the MVCT image noise was a factor of 5 higher than the kVCT. Including the body ring, the MVCT image noise was only a factor of 1.8 higher than the kVCT. The kVCT image noise was much more sensitive to the addition of the body ring, increasing by a factor of 4.5. The MVCT image noise increased by only 1.7 when introducing the body ring, which could be compensated by a factor of 3 increase in dose.
Conclusion: The kVCT image noise is sensitive to object size, necessitating a dose control (mAs) in order to compensate. MVCT is less sensitive to object size, and TomoTherapy systems have not historically provided a method to adjust dose. However, a method to adjust dose in MVCT would have a similar benefit as in kVCT.
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