Photon-Counting CT: A Game Changer in MSK Imaging?

This is the third article in a series focusing on advances in MSK imaging. Read the first and second stories.

Musculoskeletal (MSK) imaging often deals with submillimeter structures best depicted using high-resolution imaging systems, such as computed tomography (CT).

With the introduction of photon-counting CT, radiologists can achieve unprecedented spatial resolution of individual trabeculae of cancellous bone. This is because photon-counting CT converts X-rays directly to electrical signals in a single step, rather than requiring two steps, as in conventional CT, according to Kishore Rajendran, PhD.

Dr. Rajendran is an assistant professor of radiology and research associate consultant at the Mayo Clinic’s CT Clinical Innovation Center in Rochester, MN. The institution was an early adopter of photon-counting CT.

“Unlike dual-energy CT, where we often face the dilemma of whether to acquire images at high spatial resolution or in dual-energy ‘spectral’ mode, we are now able to routinely acquire spectral data at high spatial resolution using photon-counting CT,” Dr. Rajendran said. “This streamlines our approach to protocol design and simplifies our workflow, as all MSK CT exams can be performed at high spatial resolution with spectral data always available.”

Dr. Rajendran was the senior author on the subject in an American Journal of Roentgenology article offering a clinical perspective on the use of photon-counting CT in MSK imaging.

His co-author, Cynthia H. McCollough, PhD, professor of medical physics and biomedical engineering and director of the Mayo Clinic’s CT Clinical Innovation Center, said photon-counting CT measures—or counts—each individual X-ray and its energy. This is something conventional detectors cannot do, she noted.

“The benefits of photon-counting detectors include improved spatial resolution, reduced image noise, increased iodine or calcium signal and the availability of multi-energy information in all scan modes,” she said. “Studies have shown not only improved image quality but also reduced radiation dose.”

Photon-counting CT improves the quality of CT for all applications, but is particularly beneficial in MSK, cardiac, vascular, lung and inner-ear imaging, Dr. McCollough said.

Francis Baffour, MD, associate medical director of the Mayo Clinic CT Clinical Innovation Center and lead author of a Radiology study on photon- counting CT for multiple myeloma, noted that higher energy X-rays have greater potential to reduce metal artifact.

“With photon-counting CT detectors, high-energy threshold images can be leveraged to produce images with reduced metal artifacts and beam hardening effects,” he said.

In the Radiology study, he and his fellow researchers found that imaging features of myeloma were depicted more clearly on photon-counting detector images compared with energy-integrating detector images.

“We believe that the lower image noise and higher intrinsic spatial resolution of the photon-counting detector images led to sharper lesion margins and better image contrast,” he said. “In phase two of the study (with photon- counting detector images at thinner image sections of 0.6 mm and deep learning denoising), the radiologists detected more lytic lesions compared to energy-integrating scans.”