CROSSTALK: COMBINING DIAGNOSTIC APPROACHES FOR CASE SUCCESS


18F-NaF PET Combined with CT or MRI for Optimal Bone Imaging


Based on similar cross-sectional imaging proper- ties, PET can be overlaid with CT or MRI to create “fused” images. A dedicated softwarec allows PET data to be aligned with CT or MRI images acquired with other scanners.9 The combination of the func- tional information from PET with the structural information from CT or MRI provides optimal assessment of the imaged area. PET/CT or PET/ MRI fusion is particularly helpful for assessment of subchondral bone and enthesis. These regions are common sites of lesions for which both CT and MRI havesomelimitations. For subchondral bone assessment, the detection of


abnormalities on CT relies mostly on changes in the bone density, but sclerosis is very common and not specific for lameness causing pathology, and resorp- tion is typically only detected in the later stages of subchondral injury. MRI with the use of fluid-sensi- tive sequences can detect earlier changes in bone than CT. This is particularly true for the trabecular sub- chondral bone where fluid accumulation can easily be recognized. However, MRI is much more limited in the assessment of the dense compact subchondral bone as changes in the compact subchondral bone are only detected with CT and MRI when the lesion has progressed to significant resorption. Standing MRI image quality is also more greatly affected by patient motion compared to standing PET. The value of 18F-NaF for assessment of early


changes in the compact bone was established in the initial equine PET pilot study as focal increased 18F-


NaF uptake was detected in the sagittal ridge of the flexor surface of the navicular bone of a horse where no abnormality had been detected with CT, MRI, and scintigraphy. This horse had a resorptive lesion at limb (Fig. 4). The combination of PETwith CTwas proven partic-


ularly helpful for assessment of the fetlock in a popu- lation of sport horses.10 The most common sites of increased uptake were the medial subchondral bone of the proximal phalanx and the dorsomedial aspect of the distal third metacarpal bone. These are common sites of lesions, and the presence of increased uptake was associated with lameness in this population, based on diagnostic analgesia. In addition to recogniz- ing sites of subchondral bone activity, PET can help determine the clinical significance of other CT find- ings. For example, a focal subchondral defect appa- rent on CT without associated increased 18F-NaF uptake suggests that the lesion is not active (Fig. 5). Diagnosing enthesopathies is also challenging be-


cause of a requirement to detect changes in dense bone. PET seems particularly capable of detecting abnormalities at the attachments of the collateral lig- ament of the distal interphalangeal joint (Fig. 6). Increased uptake can also be recognized with PET at the attachment of the distal sesamoidean impar liga- ment on the distal phalanx. PET also identified the


Fig. 9. Sagittal (left column) and transverse (right column) 18F-FDG images of the feet of 4 different horses. A, B, Normal horse, marked 18F-FDG uptake is present in the coronary band (arrow) with low uptake in the dorsal hoof wall (arrowheads). C, D, Horse with early acute laminitis; there is increased 18F-FDG uptake in the dorsal hoof wall (arrowheads). E, F, Horse with severe acute laminitis with protrusion of the distal phalanx through the sole. There is loss of 18F-FDG uptake at the coro- nary band (arrow). Marked increased 18F-FDG uptake is pres- ent in the distal dorsal hoof wall (arrowheads) and at the dorsal aspect of the sole at the site of protrusion of the distal phalanx (long arrow). G, H, Horse with quiescent chronic laminitis. There is decreased 18F-FDG uptake at the coronary band (arrow). The dorsal hoof wall appears distorted but presents a normal low 18F-FDG uptake (arrowheads).


attachment of the chondrosesamoidean ligament on the distal phalanx as a possible site of injury. This lesion has been recognized in a population of horses with foot lameness.5 In themajority of cases, this find- ing was associated with other navicular apparatus lesions, but in a few cases, the chondrosesamoidean enthesopathy was considered the primary source of lameness. Another common issue in the diagnosis of


AAEP PROCEEDINGS / Vol. 68 / 2022 163


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