Norm of Computed Tomography of the Body (Spiral [Helical], Electron Beam [EBCT, Ultrafast], High Resolution [HRCT])
Negative. No tumor, malformations, or pathologic activity.
No coronary artery stenosis, no pulmonary embolism. EBCT has the potential to replace ventilation-perfusion scanning as the primary screening diagnostic test for pulmonary emboli.
Traditional CT Usage
Determination of the extent of primary and secondary neoplasms of the neck; evaluation of bony and inflammatory abnormalities of the spine and joints, including neoplasms, fractures, dislocations, and congenital anomalies; localization of foreign bodies in the soft tissues, hypopharynx, or larynx; assessment of airway integrity after trauma; evaluation of retropharyngeal abscesses; investigation of suspected tracheal, thymic, mediastinal, and hilar masses; evaluation of problems identified on chest radiographs; staging of bronchogenic carcinoma and gastrointestinal tumors; detection of aortic aneurysm or aortic dissection; detection, localization, and characterization of lung disease; detection of mediastinal or diaphragmatic herniation; evaluation of musculoskeletal or soft-tissue trauma or neoplasms; evaluation of suspected congenital or other abnormalities of specific body organs such as the liver, gallbladder, pancreas, kidneys, adrenal gland, and spleen; identification and localization of sites of hemorrhage; assessment of the organs and structures of the peritoneal cavity and pelvis; and sometimes used to provide imaging identification and guidance for invasive procedures such as abscess drainage or amebic liver abscess, percutaneous biopsy, or aspirate for cytologic or histologic study.
Used with contrast for imaging the coronary arteries and coronary artery bypass grafts; for diagnosis of aortic malformations and diseases, pulmonary emboli, and other lung diseases; and for quantifying ventricular mass and volume. Used primarily in asymptomatic clients who have risk factors for heart disease.
Spiral CT Usage
Procedure of choice for lung evaluations for cancer; greater than 90% sensitive and specific for pulmonary embolism, except when subsegmental emboli are found; superior to ultrasonography for detection of deep venous thrombosis; 100% sensitive and 98% specific for detection of aortic injury; 80–85% sensitive for detection of metastatic liver disease; rapid evaluation for ischemic stroke; preferred CT method for children because of reduced length of radiation exposure and reduced dose of contrast material. Spiral CT equipment often is selected as a replacement for older CT equipment; thus usage would be as described above for traditional CT.
High-Resolution CT Usage
Procedure of choice for lung evaluations for chronic infiltrative lung diseases, and for vascular evaluations, such as brain imaging for vascular malformations.
Description of Computed Tomography of the Body (Spiral [Helical], Electron Beam [EBCT, Ultrafast], High Resolution [HRCT])
Computed tomography (CT) is a radiographic scan that may be performed with or without contrast on virtually any portion of the body. CT is classified as a reconstructive imaging procedure because it produces a picture of the contents of a portion of the body based on the differing densities and composition of body tissues. The picture is obtained by projection of x-rays along all possible lines in the plane of the body. An x-ray detector records the intensity of the x-rays from multiple angles as it is transmitted through the tissue. A computer then reconstructs the differing intensities into pixels that appear in differing shades for differing tissues and represent an anterior-to-posterior “slice” across the plane of the body. CT is used to detect very minor differences in radiographic contrast, providing radiography that portrays boundaries between tissues that are normally indistinguishable to radiographic examination. The tissue-contrast differentiation of CT is superior to that of conventional radiography.
Electron beam CT (EBCT, Ultrafast CT) was developed specifically for evaluating the heart and other structures in the chest, such as the lungs and blood vessels. This type of CT uses an electron beam magnetically directed to take a rapid sequence of images at the speed of light, thus providing detailed information about how the heart functions throughout the cardiac cycle. Because the images can be taken so quickly, the test takes less time than a regular CT. EBCT can detect plaque and stenosis and can also detect minute amounts of calcific deposits, which can progress to coronary artery lesions.
Spiral (also called helical) CT, first available in the early 1990s, is an improvement in the CT technology that provides much improved resolution in a much shorter time than older CT imaging methods. Spiral CT enables the collection of multiple overlapping pictures taken in a continuous spiral pattern that can be fused to give a three-dimensional picture of the body. Because of the continuous nature of the imaging, less contrast material is needed, and the procedure takes only a few minutes.
High-resolution CT (HRCT) improves on traditional CT technology by providing optimized spatial resolution of body structures and better differentiation of normal from abnormal blood vessels.
The newest equipment, called “Dual Modality Imaging” and also known as “3-D Body Scan,” combines CT with functional imaging modalities such as PET or SPECT for improved imaging results. In this technique the cross-sectional CT images are fused with the metabolically-differentiated PET images to produce a single three-dimensional image that provides better detection of early heart disease, cancer, and brain disorders than either modality alone. (See Dual modality imaging)
Professional Considerations of Computed Tomography of the Body (Spiral [Helical], Electron Beam [EBCT, Ultrafast], High Resolution [HRCT])
Consent form IS required if contrast material will be injected.
Allergic reaction to dye (itching, hives, rash, tight feeling in the throat, shortness of breath, bronchospasm, anaphylaxis, death); renal toxicity; hematoma or infection at the injection site for CT with contrast.
CT with contrast: Previous allergy to iodine, shellfish, or radiographic dye; renal insufficiency. CT is contraindicated in clients who are unable to remain motionless while lying in a supine position.
During pregnancy, risks of cumulative radiation exposure to the fetus from these and other previous or future imaging studies must be weighed against the benefits of the procedures. Although formal limits for client exposure are relative to this risk: benefit comparison, the United States Nuclear Regulatory Commission requires that the cumulative dose equivalent to an embryo/- fetus from occupational exposure not exceed 0.5 rem (5 mSv). Radiation dosage to the fetus is proportional to the distance of the anatomy studied from the abdomen and decreases as pregnancy progresses. An abdominal CT (10-slice) exposes the first trimester fetus to 2.6 rad, but the week 35 fetus to only 1.7 rad. For pregnant clients, consult the radiologist/radiology department to obtain estimated fetal radiation exposure from these procedures.
- For CT with contrast, see Client and Family Teaching.
- Remove radiopaque objects such as jewelry, snaps, and electrocardiographic leads with snaps (if possible).
- Establish intravenous access for injection of the dye and prepare emergency equipment for a possible hypersensitivity reaction.
- Obtain radiographic contrast medium, if the procedure will be performed with contrast.
- Have emergency equipment readily available if the procedure will be performed with contrast.
- If contrast medium will be injected, just before beginning the procedure, take a “time out” to verify the correct client, procedure, and site.
- The client is positioned supine, with his or her head secured and resting on a headrest on a motorized handling table. For spinal studies, the lumbar spine is straightened by flexing the knees and providing a footrest.
- The client must lie motionless as the table slowly advances through the circular opening of the scanner. The CT scanner sends a narrow beam of x-rays across the area to be imaged in a linear fashion. While a client is being scanned, the nonabsorbed x-rays are detected at the same time as the beam is transmitting. This linear scan sequence is repeated at many different angles around the client's body. The data collected consist of a series of profiles that reflect the area visualized at different angles.
- If contrast medium is to be used, it is injected intravenously at this time, and the scan is repeated. The client is observed for rash or respiratory difficulty, which may indicate reaction to the contrast medium. Reactions usually develop within 30 minutes if the client is allergic to the dye.
- For Ultrafast and Spiral CT, the client may be asked to hold his/her breath for short periods of time. Operating in the multislice scan mode, the scanner takes several pictures as the table is advanced by a 2-mm step.
- Replace the electrocardiographic (ECG) leads if they were removed.
- For CT with contrast, observe for side effects such as headache, nausea, and vomiting. Resume previous diet if no side effects have been noted.
Client and Family Teaching
- You must lie motionless during the scan. Because this can be a frightening test, it should be described carefully to the client before he or she enters the CT room.
- If contrast medium will be used, fast from food and fluids for 4 hours before the CT scan.
- For Ultrafast and Spiral CT imaging, you will have to hold breath for several seconds.
- A sensation of burning may be felt from the injection of the contrast medium.
Factors That Affect Results
- Unavoidable internal motion of body organs such as the heart and lungs or intentional movement by the client contributes to the appearance of “tuning fork”-like streaks across the picture.
- Radiopaque objects such as jewelry and snaps obscure visualization.
- The literature contains differing opinions concerning findings of segmental emboli cloud when pulmonary embolus is suspected.
- For chest examinations, the average breast dose from EBCT is comparable to that of conventional CT scanners, despite differences in dose distribution.
- EBCT has the potential to replace ventilation-perfusion scanning as the primary screening diagnostic test for pulmonary emboli.
- See also Cerebral computed tomography.