Evan Thomas MS, Presenter: Nothing to Disclose

Matthew C. Larrison MD, Abstract Co-Author: Nothing to Disclose

Richard A. Popple PhD, Abstract Co-Author: Consultant, Varian Medical Systems, Inc

John B. Fiveash MD, Abstract Co-Author: Research Grant, Varian Medical Systems, Inc

High-Z objects can compromise the accuracy and visual integrity of CT scans. This artifact hinders radiation treatment planning in two main ways: degradation of visual quality of the images due to photon-starved regions around the high-Z object which prevents proper delineation of both target and organ-at-risk contours in affected CT slices, and adulteration of the Hounsfield Unit (HU) values. If HU values are unreliable, so will the electron density values upon which radiation treatment plans are computed and optimized. We undertook this study to determine whether dual energy CT (DECT) offered a plausible solution to these problems.

We designed a custom phantom to observe the effects of a variety of metal artifact-causing objects (ACOs). The phantom was first scanned on a radiation therapy CT simulator, w/wo high-Z objects. The phantom was then scanned on a Discovery 750HD 140/80 kVp rapid-switching DECT, w/wo high-Z objects, using a metal-artifact subtraction (MARS)  algorithm. We chose HU based auto-contouring and volume computation of ROIs because of its ability to simultaneously illustrate the effect of streak artifact on contour delineation and HU values. Each scan was imported into treatment planning software. The mean differences between each reconstructed volume and the theoretical volume were then statistically compared.

The DECT scans, with and without the MARS algorithm employed, outperformed the traditional scanner in rendering the ROI contours and true HU values. Volume reconstruction data for each ROI is provided in Figure 1 along with each corresponding high-Z object. DECT w/MARS reduced the relative error of volume reconstruction from an average of 18.1% for the traditional scanner to just 4.56%. DECT w/MARS was the only scan whose volume reconstruction was not significantly different from the scans without the presence of high-Z objects (p = 0.09 and 0.08 for DECT).

DECT w/ MARS represents a promising method of coping with high-Z artifact. Our initial studies reveal that it is a useful tool in improving contouring ability and restoring proper HU values in photon-starved regions of affected CT slices. We are currently undertaking further analysis to verify DECT’s in improving dosimetry for affected patients.

We believe DECT is an effective tool for resolving certain treatment planning difficulties in patients with metal streak artifact.

Thomas, E, Larrison, M, Popple, R, Fiveash, J, Utilization of Dual Energy CT to Improve Treatment Planning for Patients with Metal Streak Artifact.  Radiological Society of North America 2012 Scientific Assembly and Annual Meeting, November 25 - November 30, 2012 ,Chicago IL. http://archive.rsna.org/2012/12037053.html