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CT of the Limb – When and How?
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Introduction
Recent technological improvements using large-bore computed tomography (CT) scanners and development of several standing CT systems have significantly increased the use of CT in equine practice.
CT of the limb: how
Two different types of CT machines, using different acquisition systems, are currently available on the equine market: fan beam/helical CT and cone beam CT (CBCT). Fan beam CT uses a rotating X-ray generator and multirow curved X-ray detector mounted on a high-speed rotating gantry while the patient or the CT is moving at a constant speed, thus generating a helical CT scan. CBCT uses a flat panel X-ray detector and an X-ray generator turning around the area of interest over up to 360 degrees. Disadvantages of CBCT compared with fan beam CT are motion susceptibility, scatter, decreased contrast resolution and HU variability, which limit the evaluation of soft tissues with CBCT systems.
CT of the limbs can be performed under general anaesthesia or in the standing horse. The use of large-bore helical CT permits scanning of the head, the entire cervical spine, the cranial thoracic spine (up to T4 or T5 depending on the size of the horse and the system used), the proximal and distal limb, including elbow [1], shoulder, stifle [2] and coxofemoral joints, and the pelvis in adult Warmblood horses. Fan beam CT is most used under general anaesthesia, although the head, cranial cervical spine and distal limb can be examined in the standing horse [3] (Qalibra CT for distal limb, head and cranial cervical spine or human CT machines with a custom-made horse table for head and cranial cervical spine).
Several types of CBCT systems (Pegaso and human systems) [4] can be used for CT of the equine limb under general anaesthesia. A dedicated equine robotic system (Equimagine) [4,5] is on the market for use in the standing horse, and is capable of scanning the head, cervical spine and (distal) limb. Hallmarq recently developed a distal limb CBCT scanner for use in the standing horse.
CT of the limb: when
Advanced imaging (CT or MRI) is indicated when lameness has been localised by diagnostic analgesia but radiographic and ultrasonographic findings are equivocal. CT provides us with a very detailed 3D image of the bone with excellent spatial resolution. Therefore, CT is specifically indicated for the identification of small chip fractures, avulsion fractures, enthesophytes, osteophytes and small resorptive bone lesions, lesions which can be missed and/or underestimated on (standing low-field) MRI. One major disadvantage of CT compared with MRI is that it is not capable of identifying bone marrow lesions.
CT is the modality of choice for imaging of (complex) fractures, allowing the determination of the exact fracture configuration and helping in surgical planning. Multiplanar reconstruction and 3D reconstruction allow for excellent visualisation of the fracture and all involved fracture fragments.
Compared with MRI, CT has a lower contrast resolution and, therefore, a less detailed image of the soft tissues is obtained. However, the use of soft tissue reconstruction kernels in combination with intra-arterial contrast administration permits a good visualisation of the soft tissues in the distal limb [6,7]. Therefore, CT (under general anaesthesia) is also a valuable option for horses that are uncooperative for standing MRI, especially for more proximal regions such as the proximal metacarpal/metatarsal region, carpus and/or tarsus; however, high-field MRI remains the gold standard for the detection of soft tissue lesions. CT arthrography allows visualisation of the outline of the articular cartilage [7] and intra-articular soft tissues in the carpus [8] and stifle [2], such as intra-articular ligaments and menisci.
Large-bore CT makes scanning of the proximal limb in adult Warmblood horses possible, up to elbow [1] and shoulder in the forelimbs and stifle and pelvis in the hindlimbs, body regions which cannot be examined with MRI (except for the stifle).
Thanks to the speed of CT acquisition, the technique can also be used in horses with multi-limb lameness, complex lameness localised to several regions in the same limb or as a screening technique in horses with lameness that has not responded to diagnostic analgesia.
In conclusion, CT of the limb is a useful technique for advanced imaging in horses. It is mandatory to know the strengths (excellent bone detail, fractures) and weaknesses (inferior soft tissue image and no bone marrow lesion) of CT compared with MRI to make the right choice of imaging modality for each patient.
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Affiliation of the authors at the time of publication
Via Nova Equine Services, Berkenbroekstraat 1, 3960 Bree, Belgium
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