in water being arranged with oxygen and hydrogen in fat being arranged with carbon. When they are in phase their signals add together, and when they are out of phase their signals cancel out. This results in a dark line at the interface of fat and water which is extremely useful in highlighting the presence of intra‐osseous fluid accumulation on T2*W GRE sequences.
Susceptibility artefact is produced by agents that disrupt the local magnetic field due to their ability to become magnetized, e.g. ferromagnetic materials or blood degradation products. This results in dephasing at the agent's interface resulting in signal loss or void and is most prominent on gradient echo sequences as the gradient reversal is unable to compensate for the phase difference. Implants also cause distortion of the magnetic field and can complicate interpretation.
Within each voxel, the signals received are averaged creating the potential for volume averaging artefacts. Increased slice thickness and the poorer resolution of sMRI exacerbate this process [140]. A common example occurs in the metacarpal/metatarsal condyles where the curvature and thin articular cartilage can be susceptible to volume averaging artefacts.
Clinical Indications
The decision to use MRI in the equine fracture patient is multifactorial, but prior regionalization of the injury is a prerequisite. Lesion location, patient comfort level and the type of system available are all determinants. In the absence of definitive radiographic findings, the commonality of fracture location in horses in training (carpus, fetlock and pastern) means that sMRI can provide a safe method to determine the presence, suspicion or absence of features supportive of a fracture (Figure 5.12). MRI has also proved beneficial in sports horses for fractures when there are discrete clinical findings, but radiographs have been negative [141] or following localization with diagnostic analgesia, again with negative radiographic and ultrasonographic findings (Figure 5.13). In addition to assisting in diagnosis, MRI also gives an insight into the health of subchondral bone [142]. When considering the bone stress injury continuum, a BML depicting stress reaction at a predilection site for an exercise‐related fracture can represent prodromal damage [88, 143]. Following the bone’s normal pathogenetic response, a discernible fracture line may, in time, become evident [144] and demonstrate a lesion that requires surgical intervention. MRI under general anaesthesia is not usually indicated in suspected equine fractures.
Limitations
The principal limitations in equine fracture detection are lesion location, acquisition time, motion artefact and low SNRs associated with STIR sequences in sMRI. The low signal intensity of normal compact bone complicates the detection of subtle non‐displaced cortical fractures [145]. This is particularly important if secondary signs of fracture such as intra‐osseous fluid accumulation are not identified. In addition, the low signal intensity of compact bone, tendon and ligament can make avulsed bone fragments difficult to identify [146]. In general, identification of any small osseous or osteochondral fragment can be difficult if the fragment is near to compact bone or intact collagen. The requirement for multiple coil placement for the evaluation of long fractures in sMRI has both time and sedation implications [145].
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
