Chest CT Scan Health Article

Definition

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.

Purpose

The purpose of the CT scan is to noninvasively and quickly visualize internal organs which may be damaged by disease or trauma.

Precautions

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 adequate- ly 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 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.

Description

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.