A computed tomography (CT) scan is a noninvasive imaging modality that uses rotating X-ray beams and detectors to produce high-quality cross-sectional imaging of anatomic structures. The word “tomography” comes from the words “tomos” meaning “slice” and “graph” meaning “recording,” referring to the cross-sectional image slices recorded with CT imaging. A CT scan may also be referred to as a computerized axial tomography (CAT) scan.
How to Use It
CT revolutionized the fields of radiology and medicine when it was introduced in the 1970s. Similar to a chest X-ray, CT scanning produces grayscale images of anatomic structures based on the different radiodensities of tissues. See Chapter 16: Chest X-Ray for more details. However, unlike conventional radiography (taken in the frontal/coronal or sagittal planes) in which body structures are superimposed on imaging, CT scans allow visualization of cross-sectional slices of the desired anatomy (in the axial/transverse plane). CT scans also provide significantly greater contrast resolution than conventional radiography, which allows for improved tissue differentiation and characterization. ,
CT has numerous applications in patient care. For example, a noncontrast head CT is indicated in patients presenting with acute mental status changes, focal neurologic deficits, or head trauma. It can detect acute intracranial hemorrhage, intracerebral edema, hydrocephalous, and skull fractures. A chest CT can further characterize abnormalities detected on chest X-rays, including pleural abnormalities, chest masses/nodules, and pleural or pericardial fluid collections. It is also used in the diagnostic evaluation of interstitial lung disease and is the best noninvasive method to assess for drug-induced lung disease. , CT pulmonary angiography is the preferred imaging modality to diagnose pulmonary embolism (PE). CT is able to detect a wide range of intra-abdominal pathologies, including appendicitis, mesenteric ischemia, diverticulitis, pancreatitis, abdominal abscesses, and volvulus. Additionally, CT scans are often obtained to determine the location and severity of traumatic injuries and to help determine sources of bleeding.
How It Is Done
CT scans are performed in both outpatient and inpatient settings and are often used in the diagnosis of medical emergencies. Unlike X-rays, which may be performed portably, CT scans cannot be performed at the bedside. Hospital emergency rooms may have adjacent radiology suites equipped with CT scanners to provide rapid diagnostic information; however, patients should be stabilized before they are transported to the radiology suite.
CT scans are noninvasive and painless and can often be completed within minutes. However, they are more expensive than plain radiographs and expose patients to much higher amounts of ionizing radiation. For example, patients undergoing an abdominal and pelvis CT are exposed to approximately 10 mSv or the equivalent of 3 years of background radiation.
The CT scanner consists of a sliding table where the patient is moved through a donut-shaped gantry that contains an X-ray source that rapidly rotates around the patient. The X-ray beam passes through the patient at multiple angles before reaching corresponding detectors. Similar to chest X-rays, the amount of X-ray attenuation depends on the radiodensity of the tissue the X-ray beam is passing through. See Chapter 16: Chest X-Ray for more details. The X-rays that pass though the patient to the detector are then transmitted to a computer that reconstructs cross-sectional images based on the degrees of attenuation at different points. While the axial/transverse view is standard ( Fig. 17.1 ), modern CT technology allows for images to be reformatted and displayed in any anatomic plane, with sagittal ( Fig. 17.2 ) and coronal/frontal ( Fig. 17.3 ) reformations being the most common.
Procedural sedation for a CT scan is generally not required for adult patients. One exception is an agitated or combative patient who may require sedation to remain still for the scan. Patients undergoing a CT scan with contrast are often required to fast for several hours before the scan. This is due to the risk of nausea, vomiting, and potential aspiration that can occur after administration of intravenous (IV) iodinated contrast. However, there is limited evidence to support fasting before IV contrast administration, and readers are encouraged to follow their local institutional policies.
Findings of a normal CT scan are specific to the region of the body being imaged. In general, a normal CT scan should show healthy tissues, organs, vasculature, lymph nodes, and bones without evidence of abscesses, masses, hematomas, hemorrhage, or free air. Findings should be interpreted in the context of the patient’s history and current medical problems.
If a patient has an external portable infusion pump (e.g., insulin pump), it should not be directly exposed to the primary X-ray beam during the CT scan, as it may, albeit rarely, cause pump malfunction. If the pump will be exposed, options include temporary discontinuation (when safe) or moving the pump to another part of the body that will not be exposed. However, the risk of pump malfunction after direct X-ray exposure is extremely low (unlike magnetic resonance imaging [MRI] exposure) and should never preclude a CT scan that is medically indicated. See Chapter 41: Magnetic Resonance Imaging for more details.
Patients with a PE detected on CT pulmonary angiography often require anticoagulation treatment. Initial anticoagulation options to treat a PE include unfractionated heparin, low-molecular-weight heparin, fondaparinux, or direct oral anticoagulants. Patients who have persistent arterial hypotension or obstructive shock as a result of a high-risk/massive PE should receive immediate reperfusion therapy with systemic fibrinolysis (alteplase 100 mg IV infused over 2 hours) if there are no contraindications. ,
A noncontrast head CT should be performed in patients with suspected stroke. Most ischemic strokes will not show immediately on head CT. However, a head CT is required to exclude the presence of hemorrhage prior to fibrinolytic administration ( Figs. 17.4 and 17.5 ), which remains a mainstay of therapy for the treatment of ischemic stroke. The recommended dose of alteplase for ischemic stroke is a total of 0.9 mg/kg (maximum 90 mg) with 10% of the total dose given as an IV bolus over 1 minute and 90% of the total dose given as an infusion over 60 minutes.