A noninvasive medical examination or procedure that uses specialized X-ray equipment to produce cross-sectional images of the body. Each cross-sectional image represents a “slice” of the person being imaged, like the slices in a loaf of bread. These cross-sectional images are used for a variety of diagnostic and therapeutic purposes.
Computed tomography (CT), sometimes called "computerized tomography" or "computed axial tomography" (CAT).
CT scans can be performed on every region of the body for a variety of reasons (e.g., diagnostic, treatment planning, interventional, or screening). Most CT scans are performed as outpatient procedures.
HOW IT WORKS
Makes use of computer-processed combinations of many X-ray measurements taken from different angles to produce cross-sectional (tomographic) images (virtual "slices") of specific areas of a scanned object, allowing the user to see inside the object without cutting.
A motorized table moves the patient through a circular opening in the CT imaging system.
While the patient is inside the opening, an X-ray source and a detector assembly within the system rotate around the patient. A single rotation typically takes a second or less. During rotation the X-ray source produces a narrow, fan-shaped beam of X-rays that passes through a section of the patient's body.
Detectors in rows opposite the X-ray source register the X-rays that pass through the patient's body as a snapshot in the process of creating an image. Many different "snapshots" (at many angles through the patient) are collected during one complete rotation.
For each rotation of the X-ray source and detector assembly, the image data are sent to a computer to reconstruct all of the individual "snapshots" into one or multiple cross-sectional images (slices) of the internal organs and tissues.
Radiocontrasts for X-ray CT are, in general, iodine-based. Often, images are taken both with and without radiocontrast.
WHEN ITS USED
Head (to detect infarction, tumors, calcifications, haemorrhage and bone trauma)
Lungs (for detecting both acute and chronic changes in the lung parenchyma, that is, the internals of the lungs, not shown in normal x-rays)
Computed tomography angiography (contrast CT to visualize arterial and venous vessels throughout the body. This ranges from arteries serving the brain to those bringing blood to the lungs, kidneys, arms and legs)
Heart (to detect, diagnose or follow up coronary artery disease)
Abdominal and pelvic (diagnosis and staging of cancer, as well as follow up after cancer treatment to assess response and to diagnose acute abdominal pain)
Extremities (to image complex fractures, especially ones around joints, because of its ability to reconstruct the area of interest in multiple planes)
RISKS AND BENEFITS
CT is regarded as a moderate- to high-radiation diagnostic technique.The radiation dose for a particular study depends on multiple factors: volume scanned, patient build, number and type of scan sequences, and desired resolution and image quality.
CT completely eliminates the superimposition of images of structures outside the area of interest (advantage)
data from a single CT imaging procedure consisting of either multiple contiguous or one helical scan can be viewed as images in the axial, coronal, or sagittalplanes, depending on the diagnostic task = multiplanar reformatted imaging
READING A HEAD CT
Hypodense (dark) structures can indicate edema and infarction
Hyperdense (bright) structures indicate calcifications and haemorrhage
Disjunction in bone windows is bone trauma
Tumors can be detected by the swelling and anatomical distortion they cause, or by surrounding edema
(Magnetic resonance imaging (MRI) of the head provides superior information as compared to CT scans when seeking information about headache to confirm a diagnosis of neoplasm, vascular disease, posterior cranial fossa lesions, cervicomedullary lesions, or intracranial pressure disorders.)
READING A LUNG CT
Bronchial wall thickening can be seen on lung CTs, and generally (but not always) impies inflammation of the bronchi.
An incidentally found nodule in the absence of symptoms (sometimes referred to as an incidentaloma) may raise concerns that it might represent a tumor, either benign or malignant.