Risk-minimizing tube current and tube voltage modulation for CT: A simulation study

Background

The optimal tube voltage in clinical CT depends on the patient's attenuation and the imaging task. Although the patient's attenuation changes with view angle and longitudinal position of the X-ray tube, the tube voltage remains constant throughout the scan in current clinical practice. In general, the optimum tube voltage increases with patient diameter. For iodine-enhanced scans, the tube voltage is ideally low to increase contrast. However, 70 kV, the lowest clinically available tube voltage today, can not always be used due to tube current restrictions.

Purpose

To determine the additional relative reduction in effective dose of a tube voltage modulation in addition to a tube current modulation for unenhanced and iodine-enhanced CT scans.

Methods

For patient models based on CT scans, the effective dose was simulated per projection for different voltages using Monte Carlo simulations. Using these dose data and analytical estimations of noise and iodine contrast, tube voltage and tube current curves were optimized for circular scans. For unenhanced scans, the dose-weighted noise was minimized, and for iodine-enhanced scans, the dose-weighted contrast-to-noise ratio (CNRD) was maximized. The effective dose values of the optimized tube voltage and tube current curves (riskTCTVM) were compared at the same noise or same contrast-to-noise ratio (CNR) to a pure tube current modulation minimizing the effective dose (riskTCM) and to conventional mAs-minimizing tube current modulation (mAsTCM).

Results

For unenhanced scans, riskTCTVM reduces the effective dose by less than $1\%$ compared to riskTCM at its optimal tube voltage. For iodine-enhanced scans, the effective dose benefit increases with the availability of low tube voltages and the eccentricity of the patient's anatomy. For a lowest voltage of 70 kV, we found average effective dose benefits of riskTCTVM to riskTCM of less than $3\%$ for thorax and abdomen, $6\%$ for the pelvis, and $14\%$ for the shoulder. For a lowest voltage of 50 kV, we found average effective dose benefits of $7\%$ for the thorax, $11\%$ for the abdomen, $16\%$ for the pelvis, and $28\%$ for the shoulder. However, the maximum requested tube current was multiple times higher than for mAsTCM at 70 kV. Only for eccentric anatomies in the pelvis and the shoulder, riskTCTVM could lower tube current demands for a lowest available voltage of 70 kV.

Conclusions

For unenhanced scans, tube voltage modulation in addition to a modulated tube current yields a negligible effective dose benefit. However, for iodine-enhanced circular scans, all studied anatomical regions from shoulder to pelvis would benefit from tube current and tube voltage modulation if X-ray generators with voltages down to 50 kV were available at sufficient tube power. For a lowest voltage of 70 kV, riskTCTVM can considerably reduce the effective dose for eccentric anatomies in the shoulder and the pelvis.