Calorimetry in Computed Tomography Beams.
H Heather Chen-Mayer1, Ronald E Tosh1, Fred B Bateman1
1National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.
Summary
A new portable calorimeter was developed for measuring absorbed dose in medical computed tomography (CT) scans. While accurate for high-energy X-rays, excess heat complicated measurements in CT beams, necessitating lower-atomic-number sensors for future designs.
Area of Science:
- Medical Physics
- Radiological Dosimetry
- Instrumentation
Background:
- Accurate absorbed dose determination is crucial for patient safety in medical imaging.
- Computed tomography (CT) procedures require specialized dosimetry methods due to their unique beam characteristics.
- Existing dosimetry techniques may have limitations in direct realization of absorbed dose for CT.
Purpose of the Study:
- To construct and evaluate a portable calorimeter for direct absorbed dose realization in CT procedures.
- To compare calorimeter measurements with ionization chamber readings in both clinical accelerator and CT scanner beams.
- To identify design considerations for future calorimeters intended for CT dosimetry.
Main Methods:
- A portable calorimeter with polystyrene core and thermistors was designed for insertion into a high-density polyethylene phantom.
- The calorimeter and a calibrated ionization chamber were used to measure absorbed dose in 6 MV X-ray and 120 kVp CT X-ray beams.
- Measurements were performed at various dose rates, and data were analyzed in both frequency and time domains to assess calorimeter response and excess heat.
Main Results:
- The calorimeter showed good agreement with the ionization chamber for 6 MV X-rays, with negligible excess heat.
- For the 120 kVp CT beam, calorimeter response was significantly affected by excess heat (factor of ~15 increase in temperature rise).
- The thermistor's high-atomic-number composition dominated the calorimeter's response in the CT beam, indicating a need for different sensor materials.
Conclusions:
- The developed portable calorimeter is suitable for absorbed dose measurements in high-energy X-ray beams but requires modification for CT applications.
- Excess heat generated within the calorimeter components is a significant challenge for CT dosimetry.
- Future CT-compatible calorimeters should utilize low-atomic-number temperature sensors to improve accuracy and reliability.
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