PROTOCOL FOR ACQUISITION OF 3D IMAGING OF BILATERAL FOOT, ANKLE, KNEE, AND HIP JOINTS WITH WEIGHT-BEARING CT (WBCT)

INTRODUCTION

MRI and CT can provide valuable 3D information on joint structures but are generally acquired in a non-functional position. The Multicenter Osteoarthritis Study (MOST) will address this serious limitation through using low-dose weight-bearing CT (WBCT) to obtain 3D images of the bilateral hips, knees, ankles/feet in a standing position, while also maintaining continuity with previous MOST exams by acquiring both 3D WBCT and knee radiographs using the Synaflexer frame used for all previous MOST knee x-rays. This imaging protocol takes about the same time as PA and lateral knee x-rays, while providing a wealth of additional information that will enable future opportunities to investigate novel questions about OA using 3D imaging of all the joints in the lower limb kinetic chain in a functionally loaded position.

OBJECTIVE

To standardize the 3D lower limb joints (hips, knees, feet/ankles) imaging procedures using WBCT and knee radiographs using the Planmed XFI scanner.

METHODS

The XFI permits efficient scans (46sec per scan for the hips, knees, and feet/ankles respectively) with excellent image quality (least artifact among WBCT scanners and resolution 150–300µm), with effective algorithms for correction of participant motion and metal artifacts, thereby minimizing the need to repeat scans. The gantry diameter of 85cm permits larger bodies or people with knee flexion contractures to be scanned and has a 43.2 × 43.2cm flat panel detector with pixel size 148µm and source distance of 80cm, providing superior imaging capabilities. Average effective radiation doses are acceptable (20µSv, 26.5µSv, and 629µSv for 3D WBCT of the feet/ankles, knees, and hips and 7µSv total for bilateral knee and hand radiographs), automatic exposure control (AEC) optimizes image quality vs. radiation dose.

RESULTS

Participants stand on a platform facing the vertical table, with their feet pressed against the Perspex Synaflexor frame with aluminum beads to position their feet in 5 degrees external rotation (10° between medial sides of feet) and their great toes at the edge of the positioner closest to the table. They stand with equal weight on each foot and with their patellae and front of their thighs pressed against the vertical table, holding handholds and with a Velcro strap around the table and their thighs for safety during the standing scan. The XFI and participant position is adjusted so that: the coronal laser passes through the greater trochanters, posterior to the knees and through the midfoot; the sagittal laser passes in the center between the lower limbs and the axial laser is positioned for each joint of interest. For an average size adult, 3D images of the hip are acquired at 120 kV, 450 mAs, bowtie filter on and using AEC, and knees, feet/ankles at 96 kV, 180 mAs. To scan the hips, the gantry is lowered until the axial laser is at the level 2 inches above the greater trochanters. Scout images are used to confirm inclusion of the hip region of interest, collimate the exposure and set the AEC. Following the hip scan, the gantry is lowered until the axial laser passes through the popliteal fossa crease. Following knee image acquisition, the gantry is lowered until the axial laser passes through the fifth metatarsal and the foot/ankle 3D image acquisition is completed.

Knee radiographs are acquired using 2D imaging protocols. The gantry is rotated to allow the participant to be positioned against an 80° angled Synaflexor frame placed against the x-ray detector. This frame was modified from the 90° Synaflexor frame to account for the horizontal beam angle of the XFI. For knees that would require a 10° beam angle to optimize imaging of the medial tibial plateau (MTP) on fixed-flexion x-ray, no additional adjustment is necessary. For knees that require a 5° or 15° x-ray beam angles, a 5° wedge is placed under the base of the modified Synaflexor frame to achieve those equivalent relative angles between the beam and the MTP when acquiring the standing bilateral fixed-flexion PA radiograph (Figure). Following this, lateral view radiographs of the left and then the right knee are acquired. The participant's leg of interest is positioned next to and parallel to the detector, with the tip of the great toe in contact a vertical Perspex sheet and the contralateral toes are level with the back of the heel, resulting in a 40-50°flexion angle of the knee being radiographed.

CONCLUSIONS

MOST4 will deliver a wealth of 3D weight-bearing imaging data for the bilateral hips, knees, feet, and ankles, using low-dose volumetric cone-beam WBCT technology at an acceptably low radiation dose.