to avoid in equine patients. Most CT scanners have beam hardening reduction software that may or may not be available to the operator. Photon starvation is caused by beam hardening between two dense objects. This is of particular importance in horses when two limbs are placed through the gantry at the same time. Even if the operator reduces the field of view to include only one limb, the effect of the pair will be visible in the images.
Motion produces image blurring or mismapping of anatomy. These can have negative impacts on the identification of fragments if the blurring causes margins to become inconspicuous or in fracture evaluation when a hypoattenuating area such as fracture gap can be mismapped to a different region.
Photon starvation is seen in areas of high attenuation, particularly associated with metal implants. Insufficient photons reach the detector, and during reconstruction noise is greatly magnified in these areas creating streaks in the image.
Clinical Indications
In anatomically accessible areas, CT has the potential to provide additional and useful information for the identification and characterization of all fractures, whether they are managed conservatively or with surgical intervention. The benefits must be weighed against the potential risks associated with acquisition such as general anaesthesia and moving the horse to or through the scanner.
CT is considered the gold standard for fracture diagnosis and evaluation of three‐dimensional configuration. Complex, comminuted, articular fractures, small, minimally displaced fractures of long bones or simple fractures in complicated anatomic regions are best evaluated with cross‐sectional CT imaging with or without 3D or surface rendering. In humans and horses, CT has been shown to be more sensitive than radiographs for identifying fractures and recognizing comminution [117–121].
The three‐dimensional nature of CT has proved integral to presurgical planning and has been reported for the central tarsal bone [122], distal phalanx [123, 124], navicular bone [124] and proximal phalanx [125]. This is also the case in the authors experience for third carpal bone fractures (Figure 5.11); further applications are documented throughout the book. It has been repeatedly shown to give better spatial information and thus recognition of fracture configuration and complexity and the structure of affected bones and fragments [126]. In addition, areas with complex anatomy or shape, such as the distal phalanx, where dimensions vary according to orientation, and cases with multifocal pathology are only adequately assessed by CT [123, 126, 127].
Osseous trauma of the skull is better evaluated with CT than plain radiographs with respect to identification [128], classification and surgical planning [129], although small fractures maybe missed if inappropriate window parameters are chosen [130] (Chapter 36). The basics of acquisition, i.e. thin slice thickness, and appropriate reading, i.e. bone algorithms, are essential [131]. CT can also differentiate between structures that radiographically mimic fractures such as suture lines or overlapping sinuses.
Figure 5.11 Evaluation and surgical planning of two‐third carpal bone fractures. (a) Dorsal 35° proximal–dorsodistal oblique radiograph demonstrating a parasagittal plane fracture of the radial facet and corresponding dorsal plane reformatted CT image revealing the fracture line to extend from the middle carpal joint to the distal subchondral bone plate. A lag screw was therefore placed in a central position in the bone. (b) Flexed dorsal 35° proximal–dorsodistal oblique radiograph demonstrating a dorsal plane fracture of the radial facet and corresponding sagittal plane reformatted CT image demonstrating the fracture to be located in the proximal third of the bone. The surgical implant was therefore placed proximally in the bone at the mid‐point of the fracture.
Small, portable CT machines can be used during surgical procedures. CT‐assisted surgery of navicular bone and distal phalangeal fractures has increased surgical accuracy and reduced surgery time. Barium paste as markers for orientation applied to the hoof wall [124], and surgical skin staples [122] have been used as surface locators.
Limitations
CT is an excellent determinant of bone morphology but does not provide information about biological activity. This can be inferred by interpretation of the complement of morphological changes but does not reflect the level of activity as seen in nuclear medicine studies (scintigraphy or positron emission tomography [PET] scanning) or provide a visual map of intra‐osseous fluid accumulation as shown by fluid‐sensitive MRI sequences.
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,