Capturing the Body in Motion: How 4D CT Acquires Dynamic Imaging of the Musculoskeletal System

Technology may provide insight into the evolution of musculoskeletal disease and help prevent osteoarthritis

Abhijit J. Chaudhari, PhD
Shadpour Demehri, MD
Francis Baffour, MD

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

The human body is engineered for motion, yet most radiologic imaging captures us at rest. 

“Our bodies are made to move. Movement is life. However, when someone gets a CT scan, they are asked to lie stationary on a bed,” said Abhijit J. Chaudhari, PhD, director, UC Davis Center for Molecular and Genomic Imaging and professor in the Department of Radiology, UC Davis Health, in California. “Outside of, say, cardiac imaging and blood flow, we have previously had only limited ability of imaging physical motion while it is happening using CT. We have been capturing pictures at different orientations, and from those pictures, we are extrapolating the motion that happens in the middle.”

With the ongoing development of four-dimensional computed tomography (4D CT), these technological limitations could soon change and allow radiologists more insight into abnormal motion, particularly within the musculoskeletal system. 

The Fourth Dimension of Time

4D CT is a high-resolution volumetrically acquired imaging of peripheral joints during motion in real time, which captures the fourth dimension of time. Its current application in MSK imaging is to better detect and diagnose biomechanical derangement associated with peripheral joint disease. 

Shadpour Demehri, MD, professor of radiology and radiological science at Johns Hopkins Medicine, Baltimore, explained the history of the technology and the anatomy for which it is ideally suited: 

“In early 2000, vendors were thinking of increasing the size of the CT bore for brain and heart imaging. In CT angiogram efforts, it was realized that the peripheral joint can be positioned in a large bore CT scanner and its motion can be captured,” he said. “The literature has explored its use on several joints, mostly the knees, as well as wrist and ankle, and to a lesser extent hip and shoulder.” 

4D CT images are acquired as the patient performs specific motions, and then played back as a video for radiologists to observe the motions in detail. 

“The addition of time to conventional 3D CT has been a major development forward,” Dr. Chaudhari said. “As patients perform motions on the scanner, we can watch exactly how tissues may be involved in creating painful processes. This revolutionizes the way we understand a lot of disease processes.” 

Early Disease Detection and Prevention

The major benefit of 4D CT for patients is earlier detection of biomechanical derangements, facilitating earlier medical intervention and helping prevent osteoarthritis, the end stage of most biomechanical derangements. 

“The typical progression of instability, or bones not moving the way they should, usually goes from a ligament injury to abnormal motion, and then eventually to osteoarthritis,” said Francis Baffour, MD, assistant professor of radiology, Mayo Clinic, Rochester, MN. “If some of these things can be picked up earlier in the disease process, surgeons can perhaps prevent or delay that end stage osteoarthritis through physical therapy or surgery.” 

For the field of radiology, 4D CT offers increased knowledge about both normal and abnormal motion. 

“After so many years of visualizing the human body, there are still many unknowns in terms of what happens in our bodies during motion,” Dr. Chaudhari said. “With these imaging techniques, we can start to understand normal physiology and biomechanics.”  

Additional knowledge of normal motion will lead to greater understanding of abnormal motion and the evolution of musculoskeletal diseases, Dr. Chaudhari explained. 

“Once we understand normal biomechanics, then we can move to abnormal biomechanics. In the context of MSK disease, abnormal biomechanics start early, and accrue over time, leading to diseases such as osteoarthritis. 4D CT has potential to allow us to examine these processes much earlier and understand them better,” he said.

“The addition of time to conventional 3D CT has been a major development forward. As patients perform motions on the scanner, we can watch exactly how tissues may be involved in creating painful processes. This revolutionizes the way we understand a lot of disease processes.” 

Abhijit J. Chaudhari, PhD

Establishing Protocols is Important Next Step

As 4D CT capability advances, Dr. Demehri suggested that one productive next step is the creation of protocols and standards for its use with specific pathologies and patient groups. 

“This is an evolving technology with interesting and exciting possibilities; however, it incurs a lot of radiation and high expenses. We want to make sure it’s applied to the right patient population, as it’s not an ideal modality for everyone with musculoskeletal diseases,” he said. “The next step should be to come up with guidelines or protocols on the patients who would benefit from it, and pathologies, starting with the wrist and knee, that are best suited for it.” 

On the topic of 4D CT’s high radiation dose, Dr. Baffour explained how this might be mitigated moving forward, perhaps by allowing 4D CT to be used on more central areas close to radiation-sensitive organs.

“Advanced CT techniques for radiation dose reduction and reconstruction techniques that can mitigate noise and artifacts from low scanning doses will need to be optimized so that 4D CT can be safely performed at central sites like the torso,” he said. “Cutting-edge updates in CT detector technology and novel techniques in AI denoising, as well as image reconstruction, have shown promising results for low dose scanning at central sites.”

Further developments in the design of 4D CT machines could also allow more standardized patient movements. 

“Improving the platform for imaging of the peripheral joint would make motions more controlled, and the imaging will be more reliable,” Dr. Demehri said. 

Improved scanner designs might also allow for imaging the body in various positions.

“One of the major questions is how reliably anatomic motion can be replicated on a CT scan. Lying on one’s back and emulating standing, or placing force on the knee, are probably unrealistic,” said Dr. Chaudhari. “To accommodate sitting or standing, the scanner designs will have to change for that.”

In order to ensure the availability of 4D CT to patients who need it, Dr. Chaudhari emphasized the importance of demonstrating its clinical value. 

“In MSK imaging, CT may be a complementary procedure whereas MR is more commonly performed,” he said. “4D CT will need to demonstrate clinical value that will justify the time, dose and cost of a separate exam.” 

For More Information

Access the Radiology study, “Four-dimensional CT Analysis of Wrist Kinematics during Radioulnar Deviation.”

Access the Radiology commentary, “Musculoskeletal CT Imaging: State-of-the-Art Advancements and Future Directions."

Read previous RSNA News stories on musculoskeletal imaging: