An approximately 75-year-old female with history of bilateral total knee arthroplasty (TKA) presented with pain in both knees. X-rays of both knees were unremarkable except for mild radiolucency at the tip of the stem of the tibial component of the right TKA. 99mTc HMDP bone SPECT/CT was performed to determine the cause of symptoms and the presence of loosening or infection in the prosthetic joints.
The study was conducted on Symbia Pro.spectaTM[a] SPECT/CT 3 hours following 15.6 mCi (578 MBq) intravenous (IV) injection of 99mTc HMDP. Following anterior and posterior planar static acquisitions, a SPECT/CT acquisition of both knees was performed. CT was performed with 110 kV with 181 reference mAs. 1-mm-reconstructed CT slices were obtained for evaluation and fusion with SPECT. Iterative metal artifact reduction (iMAR)[b] was performed on CT using BR44 S3 kernel. Single-bed SPECT was performed with 60 stops per detector and 20 seconds per stop. SPECT was reconstructed using OSEM3D 18i4s with 128 x 128 matrix. Furthermore, xSPECT BoneTM reconstruction was performed using xSPECTTM-CG using CT data with iMAR. SPECT/CT data was reviewed using syngo®.via.
Figure 1: Anterior X-ray shows bilateral TKA without any clearly defined lysis around the prosthetic margins. No obvious varus or valgus misalignment visualized.
The images clearly define the focal hypermetabolism in the bone-prosthesis interface below the tibial plate of the tibial component of the left TKA. The CT with iMAR clearly shows the lucency below the plates, as well as around the tibial stem of the left TKA, especially at its tip. These lucencies, along with the focal hypermetabolism at the tibial plate and the stem tip, suggest early loosening of the tibial component of the left TKA. The patellar hypermetabolism predominantly at the lateral margin is related to patellofemoral arthropathy related to impingement of the resurfaced patellar articular margin to the femoral component of the left TKA. Thin-slice CT with iMAR was instrumental in accurate evaluation of the bone prosthetic margins and the related lucency and associated bone changes due to the elimination of beam-hardening artifacts. This makes interpretation of the prosthesis-related bone changes and the correct localization of the focal hypermetabolism related to bone stress and prosthetic loosening possible. High-quality CT with thin slices and metal artifact reduction is ideal for orthopedic SPECT/CT as reflected by the artifact-free CT images and ease of interpretation in this clinical example.
The right TKA shows significant metal-related artifacts, especially in the femoral component on the standard CT images, which are significantly reduced with iMAR. The focal hypermetabolism localized to the lateral margin of the lateral femoral condyle at the point of impingement of the lateral femoral component to adjacent bone reflects the possibility of early loosening. The patellar uptake reflects patellofemoral osteoarthropathy due to impingement of the resurfaced patellar articular surface to the underlying femoral component of the right TKA.
xSPECT Bone also provides sharp definition with high contrast of the focal hypermetabolism related to the stress points of the left and right TKA. Improved resolution and higher contrast using xSPECT Bone, along with high-quality, virtually artifact-free CT, is instrumental for accurate evaluation of the intensity of bone stress to attribute focal hypermetabolism to loosening rather than reactive bone changes.
Bone SPECT/CT has been established as an accurate and cost-effective modality for diagnosis of TKA loosening and related causes of pain and prosthetic joint failure.1 Improved visualization and quantification of bone SPECT/CT with xSPECT Bone and xSPECT Quant, along with improved CT visualization with metal artifact reduction on Symbia Pro.specta SPECT/CT, provides an optimum imaging approach for the diagnostic evaluation of TKA-related pathologies.
In this clinical case, early loosening of the tibial component of the right TKA and that of the femoral component of the left TKA were clearly and sharply defined by xSPECT Bone with iMAR. Specifically, iMAR aided in the creation of zone maps that defined cortical bone, spongiosa, soft tissue, fat, and air in order to improve SPECT resolution.
iMAR significantly improved the image quality of the CT in the presence of TKA-related metal artifacts and was instrumental in the proper visualization and localization of the focal hypermetabolism and the related CT findings of periprosthetic lucency. Moreover, the combination of xSPECT Quant with xSPECT Bone provided quantitative indices like SUVmax, which further defined the extent of focal hypermetabolism compared to normal bone, with SUVmax in the right tibial plateau as high as 10.15. xSPECT Bone has been shown to improve evaluation of skeletal pathology including prosthetic joints.2 However, the combination of xSPECT Bone with xSPECT Quant using iMAR provides further improvement in diagnostic confidence as shown in a study by Braun et al.3
In this study, 22 patients with TKA were evaluated with SPECT/CT on Symbia IntevoTM with xSPECT Bone and xSPECT Quant with iMAR, and the presence or absence of loosening was confirmed during 1-year follow-up. Overall, periprosthetic SUVmax was lower with xSPECT Quant (OSCGM reconstruction without CT-based zone map corrections) compared to that of xSPECT Bone combined with xSPECT Quant (CT-based zone map corrections incorporated into quantitative values; mean SUVmax ± SD: 9.14 ± 3.31 versus 12.61 ± 4.79). SUVmax cut-off values determining uptake levels with the highest accuracy for prediction of loosening were highest with xSPECT Bone using iMAR (cut-off SUVmax 13.78 without iMAR and 14.78 with iMAR) but significantly lower for xSPECT Quant without CT zone map (cut-off SUVmax 9.40 without iMAR and 9.61 with iMAR). Overall diagnostic accuracy for prosthetic loosening was highest with xSPECT Bone and Quant combination (84.8%) compared to xSPECT Quant (77.4%). The addition of iMAR increased the accuracy of not only xSPECT Bone and Quant combination from 84.8 to 93.9% but also for xSPECT Quant (from 77.4 to 81.8%).
The present case example is thus an ideal combination of xSPECT Bone, xSPECT Quant, and iMAR to provide high diagnostic accuracy. The SUVmax of 11.15 at the left tibial plateau, which was deemed the site of loosening of the tibial component, matches the SUVmax values associated with the TKA loosening in the study by Braun et al.
Scanner: Symbia Pro.specta
15.6 mCi (578 MBq) 99mTc HMDP
181 ref mAs
1 bed position/30 stops per detector, 20 seconds per stop
32 x 0.7 mm
128 x 128 matrix, OSEM3D 18i4s, xSPECT Bone xSPECT-CG