When imaged with X‐ray technology, soft tissues have low intrinsic subject contrast thus generating images with low contrast resolution. This is further exacerbated when soft tissues abut high‐density bone surfaces, e.g. cartilage over subchondral bone or the deep digital flexor tendon over the navicular bone. Modern and appropriate image processing mitigates these effects and, in general, soft tissue imaging is fair to good in conventional scanners. Contrast media can also help by increasing subject contrast and should be considered when excellent bone and soft tissue or cartilage imaging is required.
Availability of CT remains limited and most require general anaesthesia. Standing CT offers shorter acquisition time than MRI; however, the reliance on changes in bone density before a discrete fracture line can be identified means, that as a screening tool, there remains the possibility of false negatives.
Principles of Interpretation
Image production relies on the same attenuation coefficients as radiography. Thus, a lack of attenuation due to the presence of a fracture is self‐evident with a hypoattenuating or dark region on the processed image. Occult fractures are defined by the presence of a sharp hypoattenuating line within the trabecular bone pattern and a break in continuity of the cortex [132].
Magnetic Resonance Imaging
General Principles
MRI is a cross‐sectional, multiplanar modality that has transitioned from expensive and logistically difficult to ubiquitous in equine practice. The multiplanar imaging capability, improved contrast resolution, capacity to assess both bone and soft tissue and ability to identify injury to trabeculae make it an excellent modality for detecting fractures that are not depicted radiographically. This is also the case for radiographically negative studies in areas with complex anatomy and substantial superimposition, e.g. the tarsus [133]. In man, it is the preferred modality for assessment of stress fractures [23] where it has been demonstrated to be the most sensitive and specific imaging test in the lower limb [134]. It is also the only modality that can identify bone marrow lesions (BMLs) which enables occult bone injury to be identified, although this is not always definitive and false positives can occur [135]. Trabecular bone trauma can be identified with MRI which can be difficult to appreciate radiographically [12]. Scintigraphy and MRI grades for stress fractures in human patients are closely correlated [23], but MRI provides more diagnostic information including identification of fracture lines and periosteal oedema. MRI has also been instrumental in early recognition of subchondral fractures [136].
MRI is based chiefly on the presence and properties of hydrogen atoms in tissue. Their large magnetic moment and abundance in the body, including in water and fat, makes this clinically useful. Following injury or disease, the amount of water can alter markedly which increases the sensitivity of MRI to these processes. The rudimentary components are the magnetic moments of hydrogen nuclei (protons), the magnetic field strength of the magnet and the resultant net magnetic moment (net magnetization vector). Acquisition involves the focus area being placed in a magnet, which applies a strong magnetic field (B 0), and a radiofrequency (RF) coil placed over the region of interest. An electromagnetic RF energy pulse, synchronized to the precessional (Larmor) frequency for hydrogen, causes absorption of energy and displacement of the magnetic moment from equilibrium. Following the RF pulse, a gradient is used to produce a small, known variation in the magnetic field. Subsequent emission of energy (relaxation), which restores equilibrium, is proportional to the number of excited protons in the tissue volume. Protons may lose energy by dissipation into the surrounding molecular environment (T1 recovery), transfer between protons (T2 decay) or due to inhomogeneities of the magnetic field (T2* decay). Differing proton density and relaxation methods between tissues creates contrast. Multiple repetitions of the RF pulse enable the signal in an entire volume of tissue to be recorded by a receiver coil and, following a complex of mathematical processes, slices of cross‐sectional images are formed. Sagittal, transverse and dorsal planes are acquired as standard. However, MRI is multiplanar and images can be acquired in any slice plane without changing the position of the region of interest. A number of textbooks delve into the physics of MR image generation, and interested readers are referred to these for further information. [1, 137, 138].
Contrast resolution in MRI is high compared to radiography, ultrasonography and CT. Multiple factors contribute to the spatial resolution, including field and gradient strengths, matrix size and slice thickness. The magnetic field strength is measured in Tesla (T). In general, greater field strengths create images with improved contrast and more signal. Both high field (1.0–3.0 T) systems, which require general anaesthesia, and low field (0.27 T) standing MRI (sMRI) systems are available. Though sMRI units are purpose built, some institutions will use these scanners in horses under general anaesthesia. The SNR increases in a nearly linear relation to magnetic field strength [139].
The sequence generated is based on the pattern and timing of acquisition parameters. The main sequences used are spin echo (SE), fast spin echo (FSE) and gradient recalled echo (GRE). Their values pertaining to the specific tissue type is different, and each has a trade‐off in terms of acquisition time, spatial resolution and SNR. SE and FSE have higher contrast resolution than GRE, but this has a higher resolution relative to acquisition time and provides a more robust scan for sMRI if patient motion becomes challenging. Most manufacturers have proprietary sequences, particularly high field scanners intended for human use, which are developed to optimize imaging of a specific tissue type. Users must understand for which tissue or tissues proprietary sequences were developed, or understand with which traditional sequence they are most closely aligned, e.g. fluid‐sensitive sequence with higher anatomic detail.
Fat suppression can be achieved using a short tau inversion recovery (STIR) sequence or fat saturation. The latter is not possible in sMRI units. Fat suppression is essential in a fracture study; once the high signal from fat is eliminated from the image, any remaining hyperintensity pertaining to a possible fracture is clearly discernible.
Image contrast is generated through tissue weighting. T1 weighting (T1W) has high signal and good anatomical detail, but due to the increased shades of grey the contrast is reduced. T2 weighting (T2W) has lower signal than T1W or proton density weighting (PDW) but greater contrast resolution between normal and abnormal tissue. T2* weighting (T2*W) is susceptible to magnetic field inhomogeneities and ferrous materials, but since it is created using a GRE sequence it is rapidly acquired with thinner slices. It is also fluid sensitive and creates phase cancellation artefact that is helpful for ascertaining the presence of intra‐osseous fluid accumulation. PDW signal intensity and contrast are connected to the mobile population of protons within the tissue. They have good resolution and tissue contrast and can delineate between articular cartilage and synovial fluid.
Each sequence gives different information. The signal intensity of tissue on a number of sequences needs to be ascertained in order to characterize a lesion. In assessing human fractures, a T1W SE or PDW SE is utilized for the anatomic detail it affords and a fluid‐sensitive sequence, such as a STIR or fat‐suppressed T2W SE sequence, for emphasizing contrast differences between normal and abnormal tissues. In sMRI of horses, a T1W 3D or GRE, depending upon the area, and a fluid‐sensitive sequence (ideally both STIR and T2*W) are principally employed using the same rationale.
Technical Considerations
Appreciation of artefacts is necessary in order to avoid interpretation errors. Absence of patient motion is important. Many fracture evaluations will employ sMRI, but it is necessary for horses to be sufficiently comfortable to stand square without resting pain. Immobility is essential to avoid phase mismapping and loss of image quality. The team involved in patient handling, sedation and acquisition have a substantial bearing on end image quality.
Phase cancellation or chemical shift artefact is the result of the differing precessional frequencies of protons
