Prog Med Phys.  2021 Dec;32(4):165-171. 10.14316/pmp.2021.32.4.165.

Effect of Total Collimation Width on Relative Electron Density, Effective Atomic Number, and Stopping Power Ratio Acquired by Dual-Layer Dual-Energy Computed Tomography

Affiliations
  • 1Department of Radiation Oncology, Seoul National University Hospital, Seoul, Korea
  • 2Institute of Radiation Medicine, Seoul National University Medical Research Center, Seoul, Korea
  • 3Biomedical Research Institute, Seoul National University Hospital, Seoul, Korea
  • 4Interdisciplinary Program in Bioengineering, Seoul National University, Seoul, Korea
  • 5Department of Radiation Oncology, Seoul National University College of Medicine, Seoul, Korea
  • 6Robotics Research Laboratory for Extreme Environments, Advanced Institute of Convergence Technology, Suwon, Korea

Abstract

Purpose
This study aimed to evaluate the effect of collimator width on effective atomic number (EAN), relative electron density (RED), and stopping power ratio (SPR) measured by dual-layer dual-energy computed tomography (DL-DECT).
Methods
CIRS electron density calibration phantoms with two different arrangements of material plugs were scanned by DL-DECT with two different collimator widths. The first phantom included two dense bone plugs, while the second excluded dense bone plugs. The collimator widths selected were 64 mm×0.625 mm for wider collimators and 16 mm×0.625 mm for narrow collimators. The scanning parameters were 120 kVp, 0.33 second gantry rotation, 3 mm slice thickness, B reconstruction filter, and spectral level 4. An image analysis portal system provided by a computed tomography (CT) manufacturer was used to derive the EAN and RED of the phantoms from the combination of low energy and high energy CT images. The EAN and RED were compared between the images scanned using the two different collimation widths.
Results
The CT images with the wider collimation width generated more severe artifacts, particularly with high-density material (i.e., dense bone). RED and EAN for tissues (excluding lung and bones) with the wider collimation width showed significant relative differences compared to the theoretical value (4.5% for RED and 20.6% for EAN), while those with the narrow collimation width were closer to the theoretical value of each material (2.2% for EAN and 2.3% for RED). Scanning with narrow collimation width increased the accuracy of SPR estimation even with highdensity bone plugs in the phantom.
Conclusions
The effect of CT collimation width on EAN, RED, and SPR measured by DL-DECT was evaluated. In order to improve the accuracy of the measured EAN, RED, and SPR by DL-DECT, CT scanning should be performed using narrow collimation widths.

Keyword

Dual-layer dual-energy computed tomography; Collimation width; Cone-beam artifact; Effective atomic number; Relative electron density; Stopping power ratio

Figure

  • Fig. 1 Two arrangements of CIRS phantoms. (a) Arrangement 1 (Arr1) included two dense bone plugs. (b) Arrangement 2 (Arr2) had no dense bone plugs.

  • Fig. 2 Computed tomography (CT) images acquired with wide collimation for (a) phantom arrangement 1 (Arr1) and (b) arrangement 2 (Arr2). CT images acquired with narrow collimation for (c) phantom Arr1 and (d) phantom Arr2. Window width and level were 200 HU and 15 HU, respectively.

  • Fig. 3 Stopping power ratio (SPR) residual for (a) arrangement 1 (Arr1) and (b) arrangement 2 (Arr2) for comparison of the accuracy of SPR estimation. inh., inhale; exh., exhale; trab. trabecular.


Reference

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