Fine Tuning Dual Energy CT’s Critical for Clarity

Dual energy CT scans are fairly common in hospitals and clinics in part because they can reduce the effects of metal artifacts on the images. But, according to a new study, there is a crucial trade-off. What is that trade-off and how can clinicians adjust for it?

This new study addresses those questions and concludes that, among other actions, clinicians who rely on dual energy CT scans need to be aware of how different approaches to reading these scans may affect the clarity and usefulness of the information they pull from their patient CT scans.

The study, “Impact of Computed Tomography Metal Artefact Reduction Protocol on Periprosthetic Tissue Characterisation after Total Hip Arthroplasty – A Cadaveric Study,” appears in the June 2, 2022, edition of Journal of Orthopaedic Research.

Mishelle Korlaet, co-author and Computed Tomography Lead Radiographer at the South Australian Health and Medical Research Institute in Adelaide, Australia, explained the issue to OTW, “Metal artefact reduction has always been a challenge for CT [computed tomography], particularly in dense hip prostheses. There are many papers that investigate the benefit of dual energy CT on metal artefact reduction alone but none (that we could find) that examined the effect on surrounding tissues.”

“We adopted the Digital Enhanced Cordless Telecommunications protocols based on recommendations from the vendor. Subsequent to this, we received feedback from radiologists and orthopaedic specialists that the images looked ‘odd’ and ‘flat’ compared to the original protocols. It seemed pertinent to find out if this was correct or if we needed to retrain our brains to the different image impression.”

The research team used bilateral total hip arthroplasty on a fresh frozen cadaveric pelvis with simulated osteolytic cavities. They then used computed tomography along with projection-based metal artifact reduction and noise equivalence to create images of the hip. Next they applied image subtraction, segmentation, region of interest histograms, and line profiles to characterize tissue density and separation.

“Tissue densities were heavily dependent on the energy profile of the protocol,” wrote the researchers. “Cancellous bone ranged from 182 to 45 HU and cement from 1012 to 131 HU using 140 kVp compared to dual-energy with weighted high energy tube, respectively. Spectral separation between cancellous bone, osteolytic defect, and cement was reduced for all protocols compared with 140 kVp. Spectral overlap was most severe using dual-energy with heavily weighted high-energy tubes. Dual-energy algorithms reduced trabecular contrast within the cancellous bone and cortical edge response.”

Digital Enhanced Cordless Telecommunications Reduced Contrast Resolution

“Our most important finding,” said Korlaet to OTW, “was that the Digital Enhanced Cordless Telecommunications protocol recommended by the vendor may reduce metal artefact, but it reduces contrast resolution also. This makes the diagnosis of osteolysis/prosthetic loosening more challenging (and this is the whole purpose of the imaging).”

“This finding has altered our practice. We no longer use the Digital Enhanced Cordless Telecommunications protocol for our postop hip imaging. We use the 140 kVp, single energy protocol with iterative metal artefact reduction algorithms instead of the (then) vendor recommended Digital Enhanced Cordless Telecommunications.”

Dr. Andrew Dwyer, a radiologist and director at the South Australian Health and Medical Research Institute, commented to OTW, “Techniques for computed tomography metal artefact reduction have revolutionized the possibilities for imaging of orthopaedic hardware. In recent years, CT scanners with dual energy capability have become commonplace across both hospital and community settings and promoted by vendors as improving metal artefact reduction. Clinicians need to be increasingly aware of how different approaches may affect their patient’s imaging.”

“This study shows that some techniques such as dual energy, especially weighted to higher energies, can have a detrimental effect on imaging cement and the fine low-contrast detail in trabecular bone. These are critical for accurate assessment of implant migration, loosening and osteolysis. Our results should inform radiographers, radiologists, and orthopaedic surgeons in their choice of imaging protocols and to challenge the evidence behind vendor claims for new technologies.”