Radiographs vs. Computed Tomography: When and How to Use CT in General Practice?
Conventional radiology is certainly the first imaging step performed in veterinary medicine in many pathological conditions of small animal patients. Erroneously, only ultrasonography is considered to be operator dependent. A radiographic study needs to have not only correct exposure parameters, but also positioning of the patient is essential to obtain a diagnostic quality of the examination. So, even conventional radiology is operator dependent, and it is very important to have a systematic approach when viewing a radiographic study to avoid missing important lesions.
Critical patients, where anaesthesia can be an additional risk, are often evaluated with conventional radiology and ultrasonography (e.g., airway disorders, pericardial effusion, trauma, hemoabdomen, uroabdomen). After stabilization of the patient, if extra information is required, advanced imaging technique such as computed tomography (CT) is needed. For the general practitioner, sometimes it can be tricky to know in advance what to expect from a CT study. So, basically, when and how is it useful to refer a patient for a CT scan?
The high contrast resolution, the absence of superimposition of structures and the possibility to reconstruct the acquired images in different planes are the most important advantages of this advanced imaging technique. Moreover, multislice CT machines allow acquiring large volumes (e.g., total body scan) in a few seconds, decreasing the risk of the anaesthesia for the patient.
In patients affected by airway disorders, attention needs to be paid not only to the lung parenchyma, but also to the pleural space and tracheobronchial structures. In cases of severely dyspnoeic patients, bronchial diseases and thromboembolic disorders need to be considered and included in the differential diagnoses. In these situations, conventional radiography can be inconclusive and CT of the thorax can provide essential information to the clinician. The lung infiltrates, the diameter of the bronchial structures and related vessels are more easily evaluated with CT rather than conventional radiology.
Thoracic masses or lung lobe torsions need to be carefully evaluated, and CT is important also to have a precise presurgical evaluation.
In patients with thoracic trauma and/or severe dyspnea, multislice CT can also be considered as a diagnostic option in awake patients and be less stressful than conventional radiography.
The combination of conventional radiology and ultrasonography can certainly be helpful in allowing the clinician to reach the final diagnosis in many situations. However, also for abdominal diseases, CT has superior diagnostic power as compared to the conventional imaging techniques.
CT angiography is essential when evaluating patients affected by congenital or acquired portosystemic shunts; this technique allows evaluation with extreme precision of anomalous abdominal vessels and to have a precise presurgical evaluation when this is needed.
Hydro-CT has been proposed to evaluate the gastrointestinal study. In vomiting patients, where ultrasound of the abdomen has been inconclusive, water administration via orogastric tube allows distension of the stomach and better delineation of gastric lesions.
Uro-CT is very useful when evaluating patients affected by congenital or acquired urological disorders, where conventional radiography was inconclusive. CT allows depicting vascular, nephrographic and excretory phases with exquisite resolution and can be essential in the clinical workup of incontinent patients (e.g., ectopic ureters) or in patients affected by uroabdomen, to evaluate the site of urinary leakage.
Even if CT has a lower contrast resolution as compared to MRI, in some situations, it is useful to consider referring a neurological patient for a CT study of the brain or spine. Brain space-occupying lesions, undetectable with conventional imaging modalities, can be seen with CT. If the brain-blood barrier is also damaged, post-contrast CT study can show enhancement of the lesion, which can be better defined.
Degenerative disc disease and herniation is a common pathology affecting mainly chondrodystrophic breeds. Conventional radiology (except myelography) doesn't allow evaluation of the site of disc herniation and the degree of spinal cord compression. CT scan of the vertebral column allows, most of the time, detection of the site of disc herniation and is also useful in neoplastic infiltrations of the spinal cord or in cases of vertebral fractures.
CT certainly represents the gold standard imaging technique for staging of oncological patients. In cases of space-occupying lesions, CT allows assessment of the size, volume, extension of the mass and its relation with the adjacent organs; moreover regional lymphadenopathy can be evaluated and CT-guided biopsy can be performed. Metastases checked with CT have a higher diagnostic accuracy as compared to conventional radiology; the lack of superimposition of structures and the higher contrast resolution allow in fact the detection of pulmonary lesions up to 1–2 mm in size (normally not detectable with conventional radiology).