signal intensity on all sequences), BML (increased signal intensity on fat supressed images) or bone resorption (most typically detected as increased signal intensity on all sequences). The fracture plane itself can vary in appearance depending on the sequence, fracture configuration, width and location [145].
Monitoring Fracture Healing
Healing is monitored by assessment of fracture gap, margins, degree of displacement, periosteal proliferation and degree of mineralization along with the changes in associated bone marrow signal. The persistence of increased bone marrow signal intensity is not a clear indicator of a lack of progression (healing) since it is known that STIR hyperintensity can persist despite resolution of lameness. Furthermore, mature fibrous tissue as seen in delayed or non‐union fractures can have mixed T2 signal but is generally T1 hyperintense, making the degree of mineralization difficult to assess.
Bone stress injuries in humans have been graded according to MRI features of the periosteal surface, bone marrow and the presence of a fracture line. Depending on anatomical location, these can be used to develop management strategies and return to exercise [144].
Positron Emission Tomography
PET is a cross‐sectional, nuclear medicine emission technique that is often used in combination with other imaging modalities such as CT or MRI. It is a recent addition to equine diagnostic imaging but has broader use in human medicine. A radioactive, positron emitting material is administered systemically in order to map physiologically active anatomic regions in a tomographic fashion resulting in cross‐sectional images.
The positron emitting radionuclide fluorine‐18 (18F) is incorporated into a biologically active molecule, such as fluorodeoxyglucose, a glucose analogue that is associated with high cellular metabolic activity. This is the most common usage in human PET scanning. In horses, for purposes of mapping skeletal activity, 18F‐sodium fluoride (18F‐NaF) can be used. This works on the same principles as 99mTc‐MDP nuclear scintigraphy studies where the radionuclide is taken up by exposed mineral matrix in osseous tissues. 18F‐NaF is a small molecule with rapid distribution when administered intravenously. The half‐life of 18F is 109 minutes. These factors allow for scanning to occur relatively soon after intravenous injection (30–60 minutes) and for the horse to clear to a safe level of radioactivity relatively rapidly (five to six hours depending on regional radiation safety regulations). Dosage is based on extrapolation from humans; however, the group at the University of California, Davis, has found that the total dose can be reduced to ~15 mCi per horse without reducing image quality (M. Spriet, personal communication). 18F positrons have a much higher energy (511 keV) than X‐rays or gamma rays used in radiography or technetium scintigraphy: its implications must be understood for radiation safety.
Human PET scanners are often coupled with a CT scanner to allow fusion of the high anatomic detail of the latter with the functional images provided by the former. The physical construct of the human scanners is typically a PET scanner in series with a CT scanner. This arrangement would be a major limitation to equine use. This is circumvented by a novel PET, purpose‐built scanner developed in concert with UC Davis that can accommodate a horse limb and can be coupled with CT images acquired by a different machine. Originally, the equipment was used in horses under general anaesthesia, but recently the group developed a PET scanner for standing, sedated horses, which is in use at Santa Anita Racetrack. Software also allows for semi‐automated fusion of the PET images with either MRI or CT images acquired at a different time. This particular scanner has an 8 cm detector length that can translate over 14 cm, resulting in an acquisition time of 3–10 minutes depending on the area being scanned.
Clinical indications for musculoskeletal PET scanning in the horse are similar to those for nuclear scintigraphy with the obvious caveat that the region of interest must physically fit into the scanner. Thus, PET scanning can be used for the investigation of fractures and stress remodelling, assessment of crack and other osseous defect significance and the investigation of subchondral injuries. There is also interest in assessing its potential to identify prodromal pathology that could predispose (race‐)horses to catastrophic fractures. To date, there are few publications documenting its use in horses [152–154].
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