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From Thermal Response to CT Dose - a Calorimetric Journey in HDPE


H Chen-Mayer

H Chen-Mayer*, R Tosh, b zimmerman, NIST, Gaithersburg, MD

SU-E-T-99 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: : CT dose, normally measured with an ionization chamber, can be directly determined by measuring temperature increases in the sub-mK levels in a material, as shown in a recent proof-of-principle study using the proposed AAPM TG200 high-density polyethylene (HDPE) phantom. The present study set out to investigate radiation-induced chemical transformation (heat defect) that impacts the dose estimates.

Methods: The calorimeter employs two small thermistors (wired into a Wheatstone bridge and monitored via lock-in detection), embedded in a removable "core" element (1.5 cm dia. x 2 cm cylinder) inserted into a small channel of a cylindrical HDPE phantom (26 cm dia. X 10 cm) along the axis. To improve upon the earlier core which was made of PE, the present one is made of polystyrene (PS), which is expected to have no heat defect. Comparison measurements were made using the two cores and with a calibrated ionization chamber. The typical experimental run consisted of 30 2.8-s axial scans (16-slice CT scanner, 120 kV beam, 250 mA, 16x1.5 mm collimation), yielding an aggregate dose at the center of the HDPE phantom of ~0.5 Gy.

Results: The PS core data show a thermal response much closer to the nominal dose than the PE core data, suggesting that much of the difference could be attributable to excess heat from the larger thermistor beads in the PE core plus the heat defect in PE (5 to 10% by literature). However, variability in the traces due to heat transfer distortions limits the precision of the estimate at present.

Conclusion: The effect of heat defect mixed in with the excess heat produced from the non-phantom materials is observable in the present study. Further work is planned to address heat transfer within the phantom, which limits the precision of estimating heat defect corrections.

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