Chest CT Scan
Chest CT Scan
Computed tomography (CT) of the chest is performed to diagnose a variety of symptoms, including vascular, cardiac, airway disease as well as cancers that can occur in the thoracic region of the body.
The purpose of the CT scan is to noninvasively and quickly visualize internal organs which may be damaged by disease or trauma.
Pregnant women usually defer these types of scans due to the amount of radiation received during the examination; however due to an emergent situation, (e.g., trauma or pulmonary embolism) it may be necessary to scan a gravid woman. Proper shielding is always recommended in these situations. Radiation dose should always be limited to the amount necessary to adequately penetrate the selected body part. Lead shielding will also provide some protection to patients, although most radiation to other organs outside of the chest is from scattering within the body, which cannot be reduced by shielding. The greatest radiation protection is a well-trained technologist who understands the type of exam and who can asses the body habitus (size and shape) of the patient to determine the correct amount of radiation needed for the patient. Patients with a mild allergy to iodine may be premedicated to prevent sequlae of the administration of contrast. If a patient has a severe allergy, then a scan without IV contrast may be undertaken or a different imaging modality may be used to answer the clinical question. Diabetic patients who are taking Glucophage or Glucovance should stop taking the medication at the time of or before the scan and 48 hours after. The medications should only be reinstated after normal renal status has been established through lab values. Stopping and restarting of medications should only be done after consultation with the attending nephrologist.
A brief list of these conditions, which can be imaged by CT, includes infection, trauma, aortic dissections, staging for cancer, and assessment of interstitial lung disease. The entire chest is imaged, unless a specialized limited exam is ordered to focus on a smaller area. The technique for CT examination of the chest is similar to other anatomic regions of the body. The patient lies on a special table, which moves a patient through an opened gantry, which contains a rotating combination of a x-ray tube and a complex array of sensitive detectors. A radiologic technologist gives the patient instructions to lie still and hold the breath while the images are being acquired. This limits the number of artifacts caused by voluntary and involuntary motion. X rays are passed through a patient. As they pass through a patient, the energy of the beam is attenuated, or decreased by the density of the tissue they pass through. The denser the tissue, the fewer number of x rays that pass through the body. Sensitive detectors, which continually rotate around the patient, translate the beam into numbers, which then are translated into shades of gray and viewed as a final image for the radiologist to interpret.
The patient may be scanned with or without IV contrast depending on which type of pathology is to be demonstrated. If a tumor or vascular pathology is suspected, intravenous contrast is usually given. Certain exams do not require IV contrast to get high quality images. These exams include low-dose lung cancer screening, lung nodule work ups, and high-resolution chest exams for conditions such as asbestosis. If a lung nodule is the indication for the exam, a non-contrast study may be performed first, to confirm if there is calcium in the nodule. If calcium is present in the pattern, then no further examination is required because that is an indicator of a benign lesion. Occasionally, oral contrast will be used to demonstrate a hiatal hernia or a barium paste will be ingested to demonstrate esophageal lesions or stricture. As scanners become faster, patients do not have to hold the breath for as long a period. With current multislice technology, a standard chest CT can be completed in less than ten seconds. This is a great benefit to patients who are short of breath, due to any number of lung conditions, including emphysema and chronic obstructive pulmonary disease (COPD). As computer speed and detector sensitivity increases, examination time will continue to decrease with lower cost and radiation exposure.
If IV contrast is to be used the patient should remain NPO (not given anything by mouth) for at least four hours. Occasionally to view possible esophageal pathology, patients will ingest oral contrast to help delineate possible mediastinal lymph nodes or gastric tumors. Patients should also remove any metal jewelry or clothing accessories to prevent artifacts from decreasing the diagnostic quality of the images. Certain lab values may be necessary before the scan is undertaken. These would include the BUN (blood uria nitrogen) and creatinine levels to asses the patients renal status. A high creatinine could contraindicate the use of IV contrast as the kidneys are the main organs which excrete the contrast. If a patient is on renal dialysis, then the values are usually higher, and the scan is prior to that day's dialysis treatment.
If the patient received IV contrast, a small bandage will be placed over the injection site, and it should be treated with the same care one would receive after donating blood. Patients should also drink plenty of clear fluids to help flush the iodinated contrast out of their kidneys to prevent any kidney damage.
If the patient is in renal failure then it will be necessary to dialyze the patient soon after the procedure. These arrangements should be made before hand with the knowledge and consent of the consulting nephrologist.
If the patient received any oral contrast it will pass through the GI tract with the next day or two. As the contrast used for CT is much thinner than the standard barium used for GI flouroscopy studies there should be no problems with constipation.
Several complications are possible, though are relatively unlikely as long as care is taken during the procedure. These precautions would include the careful supervision of the injection of the IV contrast as an infiltration could result in a painful collection of contrast around the IV site. If the patient has a known allergy to iodine, then premedication will be necessary to prevent any sort of histamine reaction. The reaction can be mild, such as hives, to life-threa-tening if the patient were to have a larygospasm. With newer and safer non-ionic contrast, reactions have become less frequent, however this does not mean that technologists should be any less vigilant for signs of a mild reaction to the contrast media. There is also a small risk of renal failure in certain high-risk patients.
The results are given in a written report that is sent to multiple locations, including the patient's referring physician, the file room of the radiology department, and the patient's permanent medical record if he or she was an inpatient at the time of the study. A verbal report will occasionally be used as well to help speed up the communication of results to the appropriate physicians. If there are previous examinations, the radiologist will consider if there has been a change as judged by the appearance of any pathology on the images.
Computed tomography— The technique of imaging a patient with ionizing radiation to diagnose disease process.
Non-ionic intravenous contrast— A injectable x-ray dye that mixes with the blood to enhance structures, aiding their ability to be judged as health or diseased.
Health care team roles
The main team member involved in the CT of the chest is the radiologic technologist who has been trained to perform CT exams. If the physician ordering the procedure can confirm the lab values before the patient arrives, and if the patient is an inpatient, ensuring that the patient's IV line is patent and safe for use will streamline the examination.
American College of Radiology. Manual on Contrast Media. 4th Edition. Reston, VA: 2001.
Horton, K.M. and E.K. Fishman. "Spiral CT of the Esophagus and Stomach." In Spiral CT: Principles, Techniques and Applications. E.K. Fishman and R.B. Jeffery, eds. New York: Lippincott-Raven Press 1998, 221-230.