SIGNA™ Pulse of MR

Autumn 2022

Volume 33 – Autumn 2022

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Matthew F. Koff, PhD

Hospital for Special Surgery New York, NY

Hollis G. Potter, MD

Hospital for Special Surgery New York, NY

IN PRACTICE

HyperMAVRIC SL for evaluating total joint replacement surgeries

by Matthew F. Koff, PhD, Associate Scientist, and Hollis G. Potter, MD, Chairman, Department of Radiology and Imaging, Hospital for Special Surgery, New York, NY

Matthew F. Koff, PhD

Hospital for Special Surgery New York, NY

Hollis G. Potter, MD

Hospital for Special Surgery

New York, NY

The continued increase in total knee arthroplasty (TKA) and total hip arthroplasty (THA) procedures has led to a greater demand for post-surgery evaluation. Although radiography and CT are commonly used, these techniques have their limitations. MR imaging, on the other hand, provides the required high soft tissue contrast and permits multi-planar imaging without the use of ionizing radiation. Sequences such as HyperMAVRIC SL enable the generation of diagnostic-quality images near MR-Conditional implants, deliver shorter scanning times and allow the evaluation of osseous findings, osteolysis and implant loosening.

Your hips and knees are used every day of your life to go to work, to exercise and to play with pets and children. However, natural aging and traumatic injury may lead to joint degeneration of soft and hard tissues within the joint such as articular cartilage, ligaments, tendons and bone. This degeneration is commonly associated with pain, joint stiffness or reduced range of motion, all of which are hallmarks of degenerative joint disease or osteoarthritis (OA). While medications may be used to alleviate the pain, the current method for treatment for end-stage OA is total joint arthroplasty. A total joint arthroplasty (TJA) procedure entails replacing the articular cartilage and underlying bone within the joint with devices composed of metal and plastic, commonly cobalt-chromium and ultra-high molecular weight polyethylene (UHMWPE), respectively. As more Americans age, it is anticipated that over 800,000 arthroplasty procedures will be performed in the US alone by 2030.

Many TKA and THA devices function well following implantation. THA devices have been shown to have greater than 80% durability at 20 years and more than 90% of TKA devices are in place at 10 years. However, problems may occur even with these successful procedures. Approximately 20% of TKA patients report being “dissatisfied” or “very dissatisfied” with the surgical outcome. The source of the dissatisfaction is unclear and is not directly attributable to surgical technique or mechanical function of the TKA design. In patients with THA, particulate debris generated from the articulating bearing surfaces of the implant, such as metal-on-polyethylene, can cause bone loss (osteolysis) and subsequently lead to implant loosening. Alternative bearing surfaces in THA designs use ceramics and metals and have lower wear rates. However, ceramic devices can chip, fracture and/or squeak, while metal devices can lead to premature implant failure due to the development of adverse local tissue reactions (ALTRs) in the surrounding soft tissues, requiring revision. Direct visualization of hard and soft tissues in anatomic structures around TKAs and THAs is crucial for routine clinical care in patients.

Standard radiography is considered the gold standard when evaluating the presence of osteolysis in bone around arthroplasty. CT may be used as well, but there are notable limitations. Both imaging modalities use ionizing radiation and each may underestimate the size and location of osteolytic lesions near arthroplasty devices. Further, complete visualization of ALTRs in periprosthetic tissues is challenging when using radiography, because there is inadequate soft tissue contrast in the generated images. Magnetic resonance (MR) imaging generates images with high soft-tissue contrast and permits multi-planar imaging without the use of ionizing radiation, but historically, imaging near implanted metal devices has been challenging. The challenge is greater when a large amount of metal present, as is found in patients with arthroplasty. The metal of the implanted device distorts the local Bo field which, in turn, leads to in-plane and through-plane distortion, and the presence of pixel pile-up and signal voids in generated images. In 2009, a novel imaging technique called multi-acquisition variable resonance image combination (MAVRIC SL) was developed, which effectively mitigated nearly all in-plane and, in particular, through-plane distortion, leading to the creation of images with high diagnostic quality and improving the clinical utility of MR near orthopedic hardware. The MAVRIC SL acquisition is performed by acquiring numerous 3D image datasets (image frequency bins) offset from the dominant proton frequency, which are combined to generate a final output image. MAVRIC SL profoundly changed the utility of MR in the setting of imaging near orthopedic hardware, however, scanning times could be long because of the time it takes to acquire the data needed to cover all frequency bins, especially with materials that produce large off-resonance effects.

Figure 1. (A) The 3.5 mm HyperMAVRIC SL acquisition displayed the suture line from tendon repair in Gruen zone 1 with similar clarity to the (B) 1.3 mm HyperMAVRIC SL acquisition. The 3.5 mm HyperMAVRIC SL acquisition time (3:48 min.) reduced scan time by 40%, as compared to the 1.3 mm HyperMAVRIC SL acquisition (6:20 min.).

Further development of MAVRIC SL led to the creation of an initial calibration scan that could reduce the number of off-resonant bins needed for a specific implant known as the HyperMAVRIC SL technique. The reduction in the number of bins would then reduce overall scan time. A previous study substituted the time savings of a reduced-bin HyperMAVRIC SL acquisition with an acquisition of similar coverage, but with 1.3 mm isotropic voxels, to show improved visualization of soft tissues and periprosthetic bone in patients with THA as compared to a standard, 24 bin, MAVRIC SL acquisition.¹ Although better visualization was found, the 1.3 mm isotropic HyperMAVRIC SL scan was significantly longer than the standard MAVRIC SL acquisition.

Understanding the need for faster throughput in the busy clinical setting and the desire to perform a cursory evaluation of soft tissues around implanted orthopedic hardware, we have recently used a reduced-bin HyperMAVRIC SL acquisition with 3.5 mm-thick slices. Patients with THA were scanned using our institutional 1.3 mm isotropic HyperMAVRIC SL protocol, then scanned again using the HyperMAVRIC SL acquisition with a 3.5 mm slice thickness. The scan time of the 3.5 mm slice thickness acquisition was, on average, 44% shorter as compared to the 1.3mm isotropic acquisition. With this shorter acquisition, we were successful in evaluating patients for the presence of osseous findings in patients with THA.

We also found that the faster 3.5 mm HyperMAVRIC SL displayed anatomic features with similar image quality at the inter-medial aspect of the femoral head-neck junction, a primary location for synovial reactions in patients with THA.

We noted, however, that the higher through-plane resolution of the 1.3 mm HyperMAVRIC SL acquisition tended to have reduced blurring, which allowed for better visualization of pathology and permitted image reformatting.

Although the 1.3 mm HyperMAVRIC SL scan is preferred at HSS, a potential challenge for a general imaging center is the scan duration. These scans are typically over 7 minutes long (average acquisition of 7:38 min.) and may be difficult for some patients to remain still, especially if they are in pain. It may be suitable to perform two 3.5 mm HyperMAVRIC SL acquisitions in complementary planes to assess the soft tissues, while affording the patient a more agreeable imaging experience.

Figure 2. (A) The 3.5 mm HyperMAVRIC SL acquisition and the (B) 1.3 mm HyperMAVRIC SL acquisition displayed a synovial reaction (arrow) at the head-neck junction of a THA device. The 3.5 mm HyperMAVRIC SL acquisition time (3:30 min.) reduced scan time by 65% as compared to the 1.3 mm HyperMAVRIC SL acquisition (10:00 min.).

Figure 3. (A) The 3.5 mm HyperMAVRIC SL acquisition was not able to display the subtle presence of a fibrous interface between the lateral margin of the acetabular component and acetabular bone that was displayed with greater clarity on the (B) 1.3mm HyperMAVRIC SL acquisition.

Figure 4. Multi-planar reformat of the same patient, as displayed in Figure 3. (A) Acquired coronal, (B) axial reformat and (C) sagittal reformat.

We also used the HyperMAVRIC SL acquisitions to evaluate patients with orthopedic hardware in the knee. We found that the faster scanning protocol afforded by the HyperMAVRIC SL acquisition was able to reduce overall scan time by 14%, on average. In addition, the use of a smaller field of view (18 cm for HyperMAVRIC SL vs. 22 cm for our institution’s MAVRIC SL protocol), even with a larger slice thickness (1.23 mm for HyperMAVRIC SL vs. 1.0 mm for our institution’s MAVRIC SL protocol), produced results that displayed pathology with greater clarity than our institutional isotropic protocol. As shown in Figure 5, the 3.5 mm thick slice HyperMAVRIC SL images display osteolysis and implant loosening, a pathology commonly seen in patients with TJA, with a large amount of detail.

The 3.5 mm thick slice HyperMAVRIC SL acquisition was also applied to a patient with fracture fixation hardware of the distal femur. This acquisition better displayed the anterior cruciate ligament (ACL) and the articular cartilage, as compared to our institutional protocol.

Figure 5. (A) The HyperMAVRIC SL scan displayed better visualization of osteolysis (arrow) in the posterior aspect of the medial femoral condyle in a patient with TKA, as compared to (B) our institutional protocol. The HyperMAVRIC SL acquisition was 36 sec. faster than the institutional protocol acquisition.

Figure 6. (A) The HyperMAVRIC SL scan displayed better visualization of the ACL (arrow) and femoral trochlear cartilage (arrow heads), as compared to (B) our institutional protocol. The HyperMAVRIC SL acquisition was 65 sec. faster than the institutional protocol acquisition.

Our results found that HyperMAVRIC SL with a 3.5 mm slice thickness can reduce overall scan time while also generating images of suitable diagnostic quality. In patients with THA, the smaller voxels afforded by a 1.3 mm HyperMAVRIC SL acquisition may aid in the identification and diagnosis of pathology with limited anatomic coverage and permit image reformatting, but the faster 3.5 mm HyperMAVRIC SL acquisition is an alternative to rule out gross pathology. The faster acquisition associated with the 3.5 mm HyperMAVRIC SL may also be an attractive alternative for patients who may not be able to remain motionless for longer scan times. In patients with TKA, the slice thickness of HyperMAVRIC SL was 23% larger than our institutional protocol, but the in-plane resolution was smaller and produced excellent results in this study cohort.