Measurement of the volume CT dose index on spiral CT scanning with a real‐time ionization chamber

TL;DR

This study introduces a new method to directly measure CT radiation dose during spiral scans using a real-time ionization chamber and compares it with existing methods.

Contribution

A direct measurement technique for volume CTDI on spiral CT scanning using a real-time ionization chamber is developed and validated.

Findings

CTDIvolSpiral showed excellent agreement with CTDIvolAxial, with differences < -2.0%.

Differences between CTDIvolDisplayed and CTDIvolSpiral were < 11.9% for clinical protocols with automatic exposure control.

Excluding the dual-energy twin-beam protocol reduced differences to < 2.0% and < -3.9%.

Abstract

The measurement of computed tomography dose index 100 (CTDI100), which is feasible only through axial scanning, requires that the clinical spiral protocols be replaced with those for axial scanning. The real‐time ionization chamber detects the integral of radiation dose rate profile, enabling the direct verification of the volume CTDI on spiral CT scanning (CTDIvolSpiral).

This study aimed to develop a direct measurement technique for CTDIvolSpiral and compare its accuracy with that measured using axial scanning (CTDIvolAxial) or that displayed on the console (CTDIvolDisplayed).

A CTDI phantom with a real‐time ionization chamber was placed on the headrest or examination table. CTDI100Axial was measured with following parameters: tube voltage = 120 kV, effective mAs = 100, and rotation time = 1.00 s. The parameters for measuring CTDI100Spiral were set identical to those used for axial scanning, except for rotation times = 0.33, 0.50, and 1.00, pitch = 0.35, 0.50, 0.75, 1.00, 1.25, and 1.50, and the scanning range = 15 cm. CTDI100Spiral was extracted from the integral of radiation dose rate profile. CTDIvolSpiral was subsequently calculated and compared with CTDIvolAxial and CTDIvolDisplayed. Finally, CTDIvolSpiral was measured for 10 clinical protocols and compared with CTDIvolDisplayed.

The differences between CTDIvolAxial and CTDIvolDisplayed, CTDIvolSpiral and CTDIvolDisplayed, and CTDIvolAxial and CTDIvolSpiral for the head and body phantoms were all < −2.0%. The differences between CTDIvolDisplayed and CTDIvolSpiral for scans on clinical protocols with and without automatic exposure control were < 11.9% and < 10.5%, respectively; these large differences were observed in the dual‐energy twin‐beam protocol. Excluding this protocol yielded differences between the measurements with and without automatic exposure control of < 2.0% and < −3.9%, respectively.

The results showed an excellent agreement between CTDIvolAxial and CTDIvolSpiral, supporting the use of the clinical spiral CT scanning to verify CTDIvolDisplayed using a real‐time ionization chamber.

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