CT, also called computerized tomography or computed axial tomography, is an x-ray scan that produces images similar to those used in conventional radiography using a special scanner system. Conventional x-rays pass though the body and produce an image of bone, soft tissues, and air. With CT scans, a computer provides rapid complex calculations that determine the extent to which tissues absorb multiple x-ray beams. CT is unique because it can produce cross-sectional images (i.e., “slices”) of anatomic structures without superimposing tissues on each other. Additionally, CT can discern the different characteristics of tissue structures within solid organs. Agents may be used for delineation of blood vessels, the opacification of certain tissue (e.g., kidneys), demonstration of bowel, and blood flow patterns.
For a CT scan, the patient lies on a motorized table positioned inside a doughnut-shaped frame called the gantry (Figure 10.3). The gantry contains the x-ray tubes, which rotate around the patient during the scan. By rotating the narrow-beamed x-ray source around the patient’s body, multiple attenuation readings are gathered and processed by the computer. The display, similar to a conventional x-ray, demonstrates varying densities that correspond to the absorption of x-rays by the patient’s anatomy. As with traditional x-ray techniques, bones appear white, and gas and fat appear black. However, with CT, discrete differences in attenuation can be quantified. This means a CT scan can demonstrate minor differences in density and composition in shades of gray. A CT scan can differentiate tumors from soft tissues, air space from CSF, and normal blood from clotted blood.
By interpreting the scan, structures are identified by appearance, shape, size, symmetry, and position. Usually, space-occupying lesions show characteristic displacement of surrounding viscera. Scans can be performed at different levels and planes and in different slice thicknesses to isolate small lesions. Often, hollow viscera (e.g., intestines) and blood vessels need to be accentuated with the use of contrast media.
Spiral CT scanners, also known as helical CT scanners, are a modification of the conventional CT technique. A spiral scan employs a continuous corkscrew pattern that produces a three-dimensional raw data set. This allows for three-dimensional reconstruction and CT angiography. Multirow scanners are capable of producing up to 64 image slices simultaneously. Following image acquisition on a multirow or spiral CT scanner, several postprocessing techniques can be applied to the data sets. This computer manipulation allows for:
CT angiography, which allows the vascular system to be viewed in three dimensions without the visualization of overlying structures. Considered a complement to true angiography, the CT angiography technique has the advantage of requiring only an IV needle puncture rather than an arterial puncture.
Shaded surface display, which is a computer-generated surface rendering. The resultant images have the perception of depth, which may be of particular value to surgeons, especially during reconstruction (e.g., posttrauma) procedures.
CT scans can be performed on any body part and can isolate any abdominal organ. Typical CT applications include the following studies:
Abdominal: to include the liver, pancreas, gallbladder, kidneys, adrenals, spleen, retroperitoneum, and abdominal blood vessels
Pelvic: to include the urinary bladder, uterus, ovaries, distal colon, and prostate
Spinal: to include the vertebrae, spinal structures, and tissues of the spine
Head (cranial/brain): to include the sinuses, orbits, mastoids, internal auditory canals, and facial bones
Neck: to include the cervical spine, upper airway, carotid vessels, salivary and thyroid glands, parotid and submandibular glands, and vocal cords
Chest (thoracic): to include the lungs, mediastinum, and heart
Heart (calcium score screening heart scan): to identify calcium deposits in plaque
Joints and specific bones
CT-guided biopsy
Fee-for-service screening test may be available to evaluate the heart, lungs, colon, or the entire body
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