Weight-Bearing CT Offers a New Horizon in Imaging for Orthopaedic Surgery

Editor’s note: This article is the first in a two-part series on weight-bearing CT. Part two, which covers indications, limitations, and future directions for this technology, will will be published in an upcoming issue of AAOS Now.

Interest in weight-bearing CT (WBCT) imaging is not new. Researchers have already used different kinds of loading frames to simulate weight-bearing. Previous studies identified two concerns with the frames that can lead to minimizing deformities. First, the loading frame can only simulate partial weight-bearing. Second, no muscle activation is generated, as would be the case in standing position.

Although CT imaging has been available since the early 1970s, it was not until the early 1990s that helical CT technology revolutionized the use and applications of CT scanning, allowing minimization of motion and respiratory artifacts and production of overlapping axial images quickly. The acquisition mode shifted from one cut at a time to a continuous patient translation with rotation of the radiograph source around the patient. Following this innovation, algorithms and detectors improved tremendously, making high-resolution, multiplanar 3D imaging possible with less noise and better resolution.

Multidetector CT was developed in 2008 and provided the basics for four-dimensional (4D) kinematic CT of the joints. The fourth dimension is time, and image acquisition can be done during joint movement.

More recently, cone beam CT (CBCT) was made available. This technology was adopted from radiation oncology. A greater number of detectors was used—1,000 compared with up to 64 in multidetector CT—and a pyramid shaped (cone) radiograph source was implemented to allow full volumetric acquisition of images from different angles in one rotation without moving the patient.

The simple mechanical configuration of CBCT allows it to be used across many clinical settings, including breast imaging, dental imaging, radiation therapy, and musculoskeletal imaging. CBCT has also been used intraoperatively in “O-ram” systems, which feature a C-arm with 3D capability and fluoroscopy. This technology has helped surgical navigation in pedicle screw placement, open reduction and internal fixation of acetabular fractures, and syndesmotic injury management.

WBCT is one of the applications of cone beam technology. It is small and easily installed in a clinical area. To obtain an image, have the patient stand between a radiograph source and a flat panel detector (Fig. 1). The gantry motor is mounted on two vertical motion towers. This allows height and tilt adjustment. The radiograph source rotates along the outer diameter and the flat panel detector along the inner diameter of the gantry. This construction offers simultaneous scanning of both extremities. A bench can be added, and the patient can be seated during image acquisition (Fig. 2).

The patient steps into the CT scanner and, depending on the area of interest to be scanned, the gantry can be moved up to the knee or pelvis. The gantry can be flipped 90 degrees for upper-extremity CBCT scanning.

It is difficult to compare the radiation dose of a standard CT scan to a WBCT scan due to multiple factors. For example, WBCT provides images of both extremities, whereas regular CT can scan only one extremity at a time. Another challenge is radiation measurement in terms of central versus peripheral radiation dose. However, it has been shown that WBCT can produce excellent visualization at a low radiation dose.

Richter et al reviewed more than 11,000 WBCT scans from 4,987 patients over a period of 5.6 years. The yearly average radiation dose was 4.3 uSv for WBCT compared with 4.8 uSv in the radiograph and conventional CT scan group. There was 10 percent less radiation in the WBCT group.

The WBCT scan group also used 77 percent less time to complete scans compared with the radiograph/conventional CT group (3.3 minutes versus 16 minutes per patient). Although reimbursement can vary among payers and countries, in that study, WBCT scanning had an overall increased financial profit to the institution of 44,682 Euros (51.12 Euros per patient).

Holbrook et al investigated the radiation dose in 68 pediatric patients who underwent WBCT versus 48 patients who underwent conventional CT scanning. Variable indications for CT scanning included fracture, tarsal coalition, and Lisfranc injury. In the WBCT group, the average radiation dose was 0.63 mGy for patients weighing less than 100 pounds and 1.1 mGy for those weighing more than 100 pounds. In the conventional CT group, the average radiation dose was 7.92 mGy for patients less than 100 pounds and 10.37 mGy for those more than 100 pounds. Again, a significant reduction in radiation dose was found for WBCT versus conventional CT.

This article reviewed WBCT scan design, evolution, and technique, showing how this technology has overcome previous conventional scanning obstacles related to lack of weight-bearing. Part 2 will review the clinical applications of WBCT in different orthopaedic subspecialties, its current use, limitations, and future orientations in musculoskeletal imaging.

Naji S. Madi, MD, is an assistant professor of foot and ankle surgery in the Department of Orthopaedic Surgery at West Virginia University in Morgantown, West Virginia.

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