Metal artifact reduction using dual-layer spectral CT

Computed tomography (CT) is the preferred imaging exam for implant-related complications. But its diagnostic utility may be reduced or compromised by artifacts from metal implants. Dual-layer spectral CT shows promise for reducing metal artifacts, according to research from University Hospitals Cleveland Medical Center of Case Western Reserve University in Ohio.

A quantitative and qualitative image analysis of 15 implants in 12 patients, published in the June 1, 2019, online edition of Skeletal Radiology, showed that dual-source dual-energy CT reduced metal artifacts more effectively than conventionally acquired CT images, especially for small implants. The authors reported that 160 and 180 keV levels had the best overall diagnostic image quality compared to other virtual monochromatic and conventional polychromatic images.

Principal investigator Christos Komas, MD, a musculoskeletal radiologist, and co-authors conducted the study to evaluate the utility of dual-layer spectral CT compared to conventional polychromatic CT by quantitatively measuring signal-to-noise ratio (SNR) and artifact size, and qualitatively assessing image quality indices.

They used a prototype spectral detector CT scanner designed by Philips Healthcare with two detectors, a top layer constructed with 1-mm Yttrium-based garnet scintillator and a bottom layer composed of gadolinium oxysulphide. Dual-source dual-energy CT utilizes two separate X-ray tubes operating at high- and low-kVp energy levels. With its two detector layers, two high- and low-energy data sets from the spectrum of projected X-rays can be simultaneously registered.

Twelve consecutive patients with implant related complications who underwent CT scans of the neck, thorax, abdomen, and extremities participated in the study. Collectively, these patients had seven shoulder implants, five hip implants, and three spine implants. Seven shoulder CTs without contrast, two routine abdominal CTs with and without contrast, one pelvic CT without contrast, 1 CT angiography, and one transcatheter aortic valve implantation scans were performed.

The researchers compared 120 kVp polyenergetic images with iterative reconstruction with virtual monoenergetic reconstructions from 40 to 200 keV at 20-keV interval for each metal implant. They also measured the size of each implant. Both types of images were compared quantitatively using SNR and artifact reduction software.

Two musculoskeletal radiologists independently reviewed the two sets of images, scoring six different variables of image quality on a 5-point Likert scale. These included overall image quality, degree of streak artifact, conspicuity of metal-bone interface, soft tissue details, and trabecular bone visualization at 1 cm and 5 cm.

Dr. Komas and colleagues reported that “metal artifacts were effectively reduced in virtual monoenergetic reconstructions when compared to conventional polychromatic images.” There was a maximum reduction of 21% in artifact size at 200 keV in comparison to 120 kVp polychromatic images.

The severity of metal-induced artifact was dependent upon the size of the implant. There was a significant or near significant improvement in three out of six qualitative image indices of small devices with the dual-layer spectral CT images. However, improvements were non-significant with respect to larger devices.

REFERENCE

1. Kosmas C, Hojjati M, Young PC, et al. Dual-layer spectral computerized tomography for metal artifact reduction: small versus large orthopedic devices. Skeletal Radiol. Published online June 1, 2019. doi: 10.1007/s00256-019-03248-3