target="_blank" rel="nofollow" href="#ulink_9f7205e5-3fdb-5b21-be27-31c8e34ddd9d">Figure 5.12 Four‐year‐old Thoroughbred racehorse with acute onset right forelimb lameness and pain on palpation of the dorsoproximal aspect of the proximal phalanx. (a) Dorsopalmar radiograph on day one: no abnormalities evident. Same day T1W GRE (b) and STIR FSE (c) dorsal plane sMRI depicting sagittal area of T1W hypointensity and intense STIR hyperintensity in the proximal third of the bone (arrows) compatible with a short incomplete proximal phalangeal fracture. Dorsopalmar (d) and lateromedial (e) radiographs taken six weeks post‐operatively. A sharp radiolucent line can be seen in the subchondral bone of the proximal phalanx (arrow), and periosteal new bone is evident dorsally (arrows).
Lack of pathological correlation in many areas of equine MRI means that interpretation is frequently subjective. This is particularly relevant to the parasagittal grooves of the metacarpal and metatarsal condyles. Fissures have been described which may represent normal variation in condylar groove morphology or a genuine fissure fracture. The presence of intra‐osseous fluid accumulation surrounding the hyperintense area provides further evidence of significance.
Principles of Interpretation
As with other modalities, the diagnosis of fracture requires evidence of osseous discontinuity. Osseous trauma on MRI is associated with other changes in tissue composition, most importantly, the presence of bone marrow signal alteration (fluid) that can result from injury even in the absence of a visible fracture. Histological evidence suggests that less severe trauma can cause marrow oedema without obvious injury to the cellular elements, while more severe trauma causes microfracture and haemorrhage [12]. In man, T1W SE and STIR sequences consistently demonstrate prominent signal abnormalities at fracture sites including patients with subtle radiographic signs [147]. The high sensitivity of MRI for recent fractures is due to the fracture line being highlighted by intra‐osseous fluid accumulation [148]. The pattern of intra‐osseous fluid accumulation has been described as like a footprint left by the injury [149] (Figure 5.14).
Figure 5.13 Six‐year‐old eventer with acute onset moderate right forelimb lameness with a positive response to local analgesia of the medial and lateral palmar metacarpal nerves at a proximal metacarpal level. (a) T2*W GRE transverse plane sMRI image at the level of the proximal metacarpus. A large triangular zone of high fluid signal is present in the palmar medial aspect of the third metacarpal bone. The zone of high fluid is demarcated by phase cancellation artefact. (b) Radiograph taken six weeks post injury. A linear radiolucent fracture line is evident in the palmar medial cortex of the third metacarpal bone. No abnormalities were detected on radiographs taken two weeks post injury.
An acute non‐displaced trabecular fracture may present as a discrete hypointense linear, solid or broken lesion in T1W images [150] surrounded by intra‐osseous fluid accumulation, i.e. STIR hyperintensity [151]. Where a fracture gap is present, there is a hyperintense line on T1W, T2*W and STIR sequences in compact and/or trabecular bone along with decreased T1W signal intensity and increased T2*W and STIR signal intensity in the trabecular bone. Occult fractures have been variably described, ranging from diffuse trabecular intra‐osseous fluid accumulation, intra‐osseous speckled or linear regions of low signal intensity on T1W images to irregular areas of high signal intensity in corresponding areas on fluid‐sensitive sequences [132]. Compression fractures of trabecular bone can present simply as a zone of intra‐osseous fluid accumulation.
Figure 5.14 T2* GRE dorsal plane sMRI image of a metatarsophalangeal joint. The phase cancellation artefact delineating the fluid signal associated with a lateral condylar fracture leaves a ‘footprint’.
Pathological changes in the bone surrounding fractures can include sclerosis (detected as reduced 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,