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CT Dose Metrics Applied to Cone Beam CT Using a Clinical SPECT/CT System

Z Lu

Z Lu*, University of Chicago, Chicago, IL

SU-E-I-5 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: CTDI includes a series of common dose descriptors for CT scanners. Due to rapidly advancing CT technologies such as cone beam CT, the CTDI metrics face practical and methodological limitations. We applied the CT dose metrics on a SPECT/CT system using both the conventional narrow beam and the cone beam. The purpose of the work is to explore new strategies of overcoming the CTDI limitations.

Methods: The SPECT/CT system utilized for this study was a clinical system. It combined a SPECT camera with a flat panel CT detector. During CT imaging, the X-ray beam width was 32 cm at the detector position, exceeding the length of the pencil ion chamber normally utilized for CTDI100 measurements. We applied the equilibrium dose - Deq method (described in AAPM Report No. 111) using a 0.6 cc Farmer-type ion chamber in the cone beam mode. The Deq results were then compared with the conventional CTDI100 measurements done with a 10 cm long pencil ion chamber in the narrow beam mode when a 1/15th aperture was added in the collimator that reduced the X-ray beam width down to 2.2 cm.

Results: The Deq measured in the cone beam mode was consistent with the CTDIw measured in the narrow beam mode. The difference was only 3.5% for a technique of 120 kVp, 20 mA and 12 sec with a 32 cm diameter body phantom. The difference was 16.5% for a technique of 120 kVp, 80 mA and 24 sec with a 16 cm diameter body phantom.

Conclusion: The equilibrium dose - Deq is a robust CT dose metric for all scanning modes including cone beam CT. This methodology provides a unified and self-consistent approach for CT scanning of fan beam as well as cone beam.

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