Korean J Radiol.  2014 Dec;15(6):746-756. 10.3348/kjr.2014.15.6.746.

Dual-Energy Computed Tomography Arthrography of the Shoulder Joint Using Virtual Monochromatic Spectral Imaging: Optimal Dose of Contrast Agent and Monochromatic Energy Level

Affiliations
  • 1Department of Radiology, Research Institute of Radiological Science, Severance Hospital, Yonsei University College of Medicine, Seoul 120-752, Korea. hotsong@yuhs.ac
  • 2Department of Orthopedic Surgery, Severance Hospital, Yonsei University College of Medicine, Seoul 120-752, Korea.
  • 3Department of Radiology, Gangnam Severance Hospital, Yonsei University College of Medicine, Seoul 135-720, Korea.
  • 4GE Healthcare, Seoul 135-820, Korea.

Abstract


OBJECTIVE
To optimize the dose of contrast agent and the level of energy for dual-energy computed tomography (DECT) arthrography of the shoulder joint and to evaluate the benefits of the optimized imaging protocol.
MATERIALS AND METHODS
Dual-energy scans with monochromatic spectral imaging mode and conventional single energy scans were performed on a shoulder phantom with 10 concentrations from 0 to 210 mg/mL of iodinated contrast medium at intervals of 15 or 30 mg/mL. Image noise, tissue contrast, and beam hardening artifacts were assessed to determine the optimum dose of contrast agent and the level of monochromatic energy for DECT shoulder arthrography in terms of the lowest image noise and the least beam hardening artifacts while good tissue contrast was maintained. Material decomposition (MD) imaging for bone-iodine differentiation was qualitatively assessed. The optimized protocol was applied and evaluated in 23 patients.
RESULTS
The optimal contrast dose and energy level were determined by the phantom study at 60 mg/mL and 72 keV, respectively. This optimized protocol for human study reduced the image noise and the beam-hardening artifacts by 35.9% and 44.5%, respectively. Bone-iodine differentiation by MD imaging was not affected by the iodine concentration or level of energy.
CONCLUSION
Dual-energy scan with monochromatic spectral imaging mode results in reduced image noise and beam hardening artifacts.

Keyword

Dual energy computed tomography; Gemstone spectral imaging; Shoulder arthrography

MeSH Terms

Analysis of Variance
Artifacts
Contrast Media/*diagnostic use
Female
Humans
Male
Middle Aged
Phantoms, Imaging
Shoulder Joint/pathology/*radiography
Signal-To-Noise Ratio
*Tomography, X-Ray Computed
Contrast Media

Figure

  • Fig. 1 Reconstructed coronal CT image (A) and schematic drawing (B) of shoulder phantom. CT image was obtained in 120 kVp conventional scan mode with articular space filled with 180 mg/mL diluted iodine contrast medium (window width 1600, window level 3800). Tiny bony lesion (circle), two shallow/deep tendon injuries (arrowheads), cartilage defect (short arrow), and complex injury of bone and cartilage (long arrow) were created to simulate shoulder joint lesions. BC = bone cortex, LB = labrum

  • Fig. 2 Measurements of beam hardening artifacts. CT images showing 72 keV virtual monochromatic spectral images that were reconstructed in three planes. Articular space was filled with contrast medium diluted at 180 mg iodine per mL of saline. A. One of three regions of interest (ROIs) in axial plane. B. One of three ROIs in reconstructed sagittal plane. C. Three ROIs in reconstructed coronal plane (green boxes).

  • Fig. 3 Bone-iodine differentiation in virtual monochromatic spectral (VMS) imaging mode. A. Coronal image with material decomposition (MD) imaging in VMS imaging mode at 120 mg/mL iodine. Bone and iodine were separated based on their characteristic attenuation curves and displayed in different colors (blue for cortical bone and orange for iodine). B. VMS scatterplot showing clustered distribution of pixels and gating for MD imaging. Red dots on scatterplot show calculated densities of iodine under assumption that each pixel contains only calcium carbonate and iodine in regions of interest (ROIs) drawn in contrast medium (Io1, Io2, and Io3). Blue dots represent bone densities at each pixel in ROIs within cortical bone (Bo1, Bo2, Bo3, and Bo4). Threshold values for color-mapping were set such that every pixel with > 40 mg/cm3 of iodine and < 1500 mg/cm3 of calcium carbonate (bone cortex) was color-coded orange.

  • Fig. 4 Comparison of CT numbers obtained from 72 keV virtual monochromatic spectral (VMS) and 120 kVp conventional CT images to evaluate image contrast between tissues. Soft tissue includes tendon and simulated labrum made of agar. CT number of iodine contrast agent increased as concentration of iodine increased until maximum CT number (3071 Hounsfield units [HU]) was reached, but those of soft tissue and bone were not significantly affected by iodine concentration or scan mode (17.6 to 24.2 HU for soft tissue, and 1620.8-1720.4 HU for bone). At 60 mg/mL iodine concentration, image contrast was 1381 for 72 keV VMS and 1235 for 120 kVp conventional CT images between iodine contrast agent and soft tissue, and was 268 for VMS and 360 for conventional CT images between iodine contrast and bone.

  • Fig. 5 Comparison of beam-hardening artifacts between 72 keV virtual monochromatic spectral (VMS) images and conventional CT images. At any given iodine concentrations, VMS images produced less beam hardening artifacts than conventional CT images (p < 0.05). *Beam hardening artifacts on 72 keV VMS images at 0, 30, and 60 mg/mL of diluted iodine concentration were not different (p = 0.74 by analysis of variance). At all iodine concentrations for conventional CT images and between 60 and 180 mg/mL for 72 keV VMS images, beam hardening artifacts significantly decreased as iodine concentration decreased (p < 0.05 for each decrease).

  • Fig. 6 Comparison of beam-hardening artifacts between 165 mg/mL and 60 mg/mL iodine concentration for 72 keV virtual monochromatic spectral (VMS) images. Reconstructed coronal images from 72 keV VMS images. A. Iodine concentration was 165 mg/mL. Difference in CT number between iodinated contrast media and tendon ranged from 2950 to 3050 Hounsfield units (HU). Beam hardening artifacts was 27.5. B. Iodine concentration was 60 mg/mL. Difference in CT number between iodinated contrast media and tendon ranged from 1280 to 1380 HU. Beam hardening artifacts was 8.0. Window width was 400 and window level was 40 for both images.

  • Fig. 7 Clinical evaluation of shoulder dual-energy CT arthrography with virtual monochromatic spectral (VMS) imaging: improvement after protocol optimization. A. 62-year-old man with intermittent pain in his left shoulder underwent shoulder CT arthrography using VMS imaging with non-optimized protocol (300 mgI/mL with 65 keV monochromatic energy level). B. 51-year-old man with chronic pain in his left shoulder underwent shoulder CT arthrography using VMS imaging with optimized protocol (60 mgI/mL with 72 keV monochromatic energy level). In comparison with image before protocol optimization (A), image with optimized protocol (B) showed fewer beam hardening artifacts, reduced image noise, and enhanced tissue contrast. Window width was 1400 and window level was 380 for both images.

  • Fig. 8 Clinical evaluation of shoulder dual-energy CT (DECT) arthrography with virtual monochromatic spectral (VMS) imaging: bone-iodine differentiation using material decomposition. A, B. 59-year-old woman who underwent shoulder DECT arthrography with non-optimized protocol (300 mgI/mL with 65 keV monochromatic energy level). C, D. 65-year-old man who underwent shoulder DECT arthrography with optimized protocol (60 mgI/mL with 72 keV monochromatic energy level). Material decomposition images (B, D) show successful bone-iodine differentiation regardless of protocol used. Iodine is color-coded red, and bone cortex is green or blue. Window width was 900 and window level was 50 for both images.


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