Applications of PET-MRI in musculoskeletal disease

Abstract

New integrated PET-MRI systems potentially provide a complete imaging modality for diagnosis and evaluation of musculoskeletal disease. MRI is able to provide excellent high-resolution morphologic information with multiple contrast mechanisms that has made it the imaging modality of choice in evaluation of many musculoskeletal disorders. PET offers incomparable abilities to provide quantitative information about molecular and physiologic changes that often precede structural and biochemical changes. In combination, hybrid PET-MRI can enhance imaging of musculoskeletal disorders through early detection of disease as well as improved diagnostic sensitivity and specificity. The purpose of this article is to review emerging applications of PET-MRI in musculoskeletal disease. Both clinical applications of malignant musculoskeletal disease as well as new opportunities to incorporate the molecular capabilities of nuclear imaging into studies of nononcologic musculoskeletal disease are discussed. Lastly, we discuss some of the technical considerations and challenges of PET-MRI as they specifically relate to musculoskeletal disease.

Level of evidence: 5 TECHNICAL EFFICACY: Stage 3 J. Magn. Reson. Imaging 2018;48:27-47.

Keywords: PET-MRI; bone; joint; molecular imaging; musculoskeletal disease.

© 2018 International Society for Magnetic Resonance in Medicine.

Figures

Figure 1

61-year-old female with a large pelvic mass discovered on CT and non-diagnostic biopsies taken during laparoscopic exploration at an outside institution. [a] T2-weighted fat-saturated (FS) and [b] FS post-gadolinium spoiled gradient recalled (SPGR) images demonstrate a large heterogeneously T2-hyperintense pelvic mass extending through the right sciatic notch with perilesional edema and extensive central necrosis. [c] Corresponding fused T2-weighted FS and [d] FS post-gadolinium SPGR FDG PET/MRI images demonstrate intense peripheral FDG activity. An FDG-avid area was targeted during subsequent CT-guided biopsy, with pathology showing a high-grade malignant peripheral nerve sheath tumor.

Figure 2

54-year-old female with multiple myeloma undergoing staging FDG PET/MRI. [a] Maximum intensity projection PET image shows normal FDG biodistribution throughout the skeletal bone marrow. [b] Sagittal T2-weighted FS MRI image of the thoracolumbar spine shows diffuse T2-hyperintense marrow infiltration, consistent with diffuse involvement by multiple myeloma. [c] Corresponding fused T2-weighted FS PET/MRI image image shows only normal low-grade marrow activity throughout the spine. Subsequent bone marrow biopsy demonstrated involvement by 80–90% clonal plasma cells.

Figure 3

43-year-old woman with multiple myeloma undergoing restaging FDG PET/MRI after chemotherapy. [a] Axial T1-weighted MRI image demonstrated a rounded, T1-hypointense marrow-replacing lesion in the posterior right iliac bone (arrow). [b] Corresponding fused T1-weighted PET/MRI image demonstrates no FDG activity within the lesion, consistent with a treated lesion showing resolution of FDG activity prior to normalization of MRI signal.

Figure 4

MRI, 18F-NaF PET and Fusion images of 3 patients with posttraumatic osteoarthritis. [a] Concordance is seen between a BML (blue arrowhead) and large osteophytes (red diamond arrows) on MRI and high 18F-NaF Uptake on PET. However, PET was able to provide additional information about metabolic activity in the knee. [b] Many small osteophytes identified on MRI (orange arrows) did corresponded to elevated metabolic bone activity on PET. Further, 18F-NaF was able to identify areas of increased metabolic activity (magenta line arrows) that appeared unremarkable on MRI. From Ref with permission.

Figure 5

[a] 18F-NaF and [b] FDG PET overlaid on a T2-weighted, fat suppressed FSE MRI image acquired simultaneously of a 52 yr-old male subject with OA. A large grade 3 BML (blue arrowhead) seen on MRI corresponded to considerably elevated 18F-NaF uptake (SUVmax =26.2) and only marginally elevated FDG uptake (SUVmax =1.4) relative to normal appearing bone on MRI. Imaging with multiple tracers may offer a better understanding the etiology of features associated with OA pain and progression such as BMLs. From Ref with permission.

Figure 6

Axial and Coronal [a] FDG PET, [b] T1-weighted MRI and [c] overlaid fusion images of the hand in early RA acquired on an integrated PET-MRI system. The highest uptake is seen at the palmer portion of metacarpophalangeal joint II which corresponds to synovial thickening and contrast enhancement on MRI. Reprinted by permission from SpringerNature: Clinical Rheumatology. (Miese et al. Hybrid 18F-FDG PET–MRI of the hand in rheumatoid arthritis: initial results) (2011).

Figure 7

Normal FDG PET and MRI patterns of lumbar spine (top row) and abnormal FDG PET and MRI patterns of a sciatica patient with a herniated disc (bottom row). The left sided L4–5 paracentral disc protrusion causes compression of descending L5 spinal nerve (red arrows), showing increased FDG uptake on the site of the nerve compression.

Figure 8

Simultaneous 18F-FDG PET-MRI of a [a] 71-yr-old and a [b] 59-yr-old patient with suspected spondylodiskitis. In both cases, MRI was inconclusive with typical hyperintense signal alterations on IR-FSE and moderate [a] to poor [b] postcontrast signal on T1-weighted MRI. Fused 18F-FDG PET-MRI images in case [a] showed increased uptake in the suspected discs (blue arrows) which signaled active inflammation and confirmed a diagnosis of spondylodiskitis, while the lack of elevated uptake (red arrows) in [b] excluded active inflammation resulting in a diagnosis of no spondylodiskitis but post fracture changes. This research was originally published in JNM. Fahnert et al. Use of Simultaneous 18F-FDG PET/MRI for the Detection of Spondylodiskitis. J Nucl Med. 2016;57:1396–1401.

Figure 9

[a] X-ray, [b] CT, [c] MRI and [d] 18F-NaF PET-MRI fusion of the ankle of a 58 yr-old female patient. X-ray images were negative while CT show sclerotic lesions in the dorsal calcaneus (red arrow) and degenerative changes in the talonavicular region but no signs of acute stress fractures. On MRI, a fracture line in the dorsal calcaneus with little edema is seen, due to an older stress fracture that corresponds to elevated 18F-NaF PET uptake (red arrow). Additionally, another stress fracture is shown on PET-MRI in the mediodorsal parts of the cuboid with bone marrow edema on MRI and elevated uptake on 18F-NaF PET (dotted arrow). Reprinted from Cronlein et al. Visualization of stress fractures of the foot using PET-MRI: a feasibility study. Eur J Med Res 2015;20:99 under the terms of the Creative Commons Attribution 4.0 International License.

Figure 10

Challenge of MRAC in a subject with total knee arthroplasty. [a] Source MRI image acquired with the conventional 3D gradient echo sequence, [b] attenuation coefficient (AC) map derived from the source MRI image, and [c] MAVRIC-SL metal artifact reduction MRI image. The severe signal loss artifact in the source MRI image near the metallic knee implant caused misclassification of metal (green arrow), femoral bone (yellow arrow), and soft tissues (red arrow) as air (white arrows) in the derived AC map. The attenuation coefficients of the misclassified materials are much higher than that of air, which can lead to significant underestimation of the tracer uptake in the reconstructed PET image.

Figure 11

[a] Hawkins compartmental model for pharmacokinetic analysis of 18F-NaF Uptake. [b] Dynamic analysis of tracer distribution (input function (IDIF)) and uptake (time activity curve (TAC)) allows for modeling of tracer kinetics. This permits a quantitative analysis of rate constants that describe bone perfusion (K1) as well as mineralization (k3).