Reducing radiation dose to selected organs by selecting the tube start angle in MDCT helical scans: a Monte Carlo based study

Abstract

Purpose: Previous work has demonstrated that there are significant dose variations with a sinusoidal pattern on the peripheral of a CTDI 32 cm phantom or on the surface of an anthropomorphic phantom when helical CT scanning is performed, resulting in the creation of "hot" spots or "cold" spots. The purpose of this work was to perform preliminary investigations into the feasibility of exploiting these variations to reduce dose to selected radiosensitive organs solely by varying the tube start angle in CT scans.

Methods: Radiation dose to several radiosensitive organs (including breasts, thyroid, uterus, gonads, and eye lenses) resulting from MDCT scans were estimated using Monte Carlo simulation methods on voxelized patient models, including GSF's Baby, Child, and Irene. Dose to fetus was also estimated using four pregnant female models based on CT images of the pregnant patients. Whole-body scans were simulated using 120 kVp, 300 mAs, both 28.8 and 40 mm nominal collimations, and pitch values of 1.5, 1.0, and 0.75 under a wide range of start angles (0 degree-340 degrees in 20 degrees increments). The relationship between tube start angle and organ dose was examined for each organ, and the potential dose reduction was calculated.

Results: Some organs exhibit a strong dose variation, depending on the tube start angle. For small peripheral organs (e.g., the eye lenses of the Baby phantom at pitch 1.5 with 40 mm collimation), the minimum dose can be 41% lower than the maximum dose, depending on the tube start angle. In general, larger dose reductions occur for smaller peripheral organs in smaller patients when wider collimation is used. Pitch 1.5 and pitch 0.75 have different mechanisms of dose reduction. For pitch 1.5 scans, the dose is usually lowest when the tube start angle is such that the x-ray tube is posterior to the patient when it passes the longitudinal location of the organ. For pitch 0.75 scans, the dose is lowest when the tube start angle is such that the x-ray tube is anterior to the patient when it passes the longitudinal location of the organ.

Conclusions: Helical MDCT scanning at pitch 1.5 and pitch 0.75 results in "cold spots" and "hot spots" that are created both at surface and in-depth locations within patients. For organs that have a relatively small longitudinal extent, dose can vary considerably with different start angles. While current MDCT systems do not provide the user with the ability to control the tube start angle, these results indicate that in these specific situations (pitch 1.5 or pitch 0.75, small organs and especially small patients), there could be significant dose savings to organs if that functionality would be provided.

Figure 1

The dose distribution along the z axis on the surface of a CTDI 32 phantom undergoing a CT scan with nominal beam width of 28.8 mm and pitch 1.5. The locations of x-ray beam on, x-ray beam off, start image data, and end image data are also shown. Double headed arrows indicate regions of overscan.

Figure 2

Diagram illustrating the tube position and corresponding surface dose distribution along the z axis under two scenarios with different tube start angles at pitch 1.5. An illustrative patient with schematic indication of the location of the breasts is shown at the bottom as well. This figure shows that the dose to the breasts of the patient can be affected by selecting different tube start angles.

Figure 3

The organ dose variation curves for Baby phantom from a simulated CT scan with various pitch and collimation settings: (a) Pitch 1.5 and 28.8 mm nominal collimation, (b) pitch 1 and 28.8 mm nominal collimation, (c) pitch 0.75 and 28.8 mm nominal collimation, (d) pitch 1.5 and 40 mm nominal collimation, (e) pitch 1 and 40 mm nominal collimation, and (f) pitch 0.75 and 40 mm nominal collimation. It should be noted that the same OCTA may correspond to different tube start angles for different organs.

Figure 4

The maximum dose reduction for individual organs from three GSF phantoms for a 40 mm collimation pitch 1.5 simulated CT scan as a function of both organ size in the longitudinal dimension and its distance to the isocenter in AP dimension.

Figure 5

The organ dose variation curves for F7 phantom from a simulated CT scan with 40 mm nominal collimation and various pitch values: (a) Pitch 1.5 (b) pitch 0.75.

Figure 6

Diagram illustrating the tube position, corresponding surface dose distribution along the z axis under two scenarios with different tube start angles, and a patient, at pitch 0.75. It was shown that the dose to the breasts of the patient can be affected by selecting different tube start angles.