Imaging Studies - Diagnostic Evaluation

IMAGING STUDIES

Imaging studies, including x-rays, computed tomography (CT) scans, magnetic resonance imaging (MRI), contrast studies, and radioisotope diagnostic scans may be part of any diagnostic workup, ranging from a determination of the extent of infection in sinusitis to tumor growth in cancer.

Chest X-Ray

Normal pulmonary tissue is radiolucent; therefore, densities pro-duced by fluid, tumors, foreign bodies, and other pathologic con-ditions can be detected by x-ray examination. A chest x-ray may reveal an extensive pathologic process in the lungs in the absence of symptoms. The routine chest x-ray consists of two views—the posteroanterior projection and the lateral projection. Chest x-rays are usually taken after full inspiration (a deep breath) because the lungs are best visualized when they are well aerated. Also, the diaphragm is at its lowest level and the largest expanse of lung is visible. If taken on expiration, x-ray films may accentuate an otherwise unnoticed pneumothorax or obstruction of a major artery.

Computed Tomography

CT is an imaging method in which the lungs are scanned in suc-cessive layers by a narrow-beam x-ray. The images produced pro-vide a cross-sectional view of the chest. Whereas a chest x-ray shows major contrast between body densities, such as bones, soft tissues, and air, CT scans can distinguish fine tissue density. CT may be used to define pulmonary nodules and small tumors adjacent to pleural surfaces that are not visible on routine chest x-ray, and to demonstrate mediastinal abnormalities and hilar adenopathy, which are difficult to visualize with other techniques. Contrast agents are useful when evaluating the mediastinum and its contents.

Magnetic Resonance Imaging

MRIs are similar to CT scans except that magnetic fields and radiofrequency signals are used instead of a narrow-beam x-ray. MRIs yield a much more detailed diagnostic image than CT scans. MRI is used to characterize pulmonary nodules, stage bronchogenic carcinoma (assessment of chest wall invasion), and evaluate inflammatory activity in interstitial lung disease, acute pulmonary embolism, and chronic thrombolytic pulmonary hy-pertension (Kauczor & Kreitner, 2000).

Fluoroscopic Studies

Fluoroscopy is used to assist with invasive procedures, such as a chest needle biopsy or transbronchial biopsy, performed to iden-tify lesions. It also may be used to study the movement of the chest wall, mediastinum, heart, and diaphragm, to detect di-aphragm paralysis, and to locate lung masses.

Pulmonary Angiography

Pulmonary angiography is most commonly used to investigate thromboembolic disease of the lungs, such as pulmonary emboli and congenital abnormalities of the pulmonary vascular tree. It involves the rapid injection of a radiopaque agent into the vascu-lature of the lungs for radiographic study of the pulmonary vessels.

It can be performed by injecting the radiopaque agent into a vein in one or both arms (simultaneously) or into the femoral vein, with a needle or catheter. The agent also can be injected into a catheter that has been inserted in the main pulmonary artery or its branches or into the great veins proximal to the pulmonary artery.

Radioisotope Diagnostic Procedures (Lung Scans)

Several types of lung scans—ventilation-perfusion scan, gallium scan, and positron emission tomography—are used to detect normal lung functioning, pulmonary vascular supply, and gas exchange.

A ventilation-perfusion lung scan is first performed by inject-ing a radioactive agent into a peripheral vein and then obtaining a scan of the chest to detect radiation. The isotope particles pass through the right side of the heart and are distributed into the lungs in amounts proportional to the regional blood flow, mak-ing it possible to trace and measure blood perfusion through the lung. This procedure is used clinically to measure the integrity of the pulmonary vessels relative to blood flow and to evaluate blood flow abnormalities, as seen in pulmonary emboli. The imaging time is 20 to 40 minutes, during which the patient will lie under the camera with a mask fitted over the nose and mouth. This is followed by the ventilation component of the scan. The patient takes a deep breath of a mixture of oxygen and radioactive gas, which diffuses throughout the lungs. A scan is performed to de-tect ventilation abnormalities in patients who have regional dif-ferences in ventilation. It may be helpful in the diagnosis of bronchitis, asthma, inflammatory fibrosis, pneumonia, emphy-sema, and lung cancer. Ventilation without perfusion is seen with pulmonary emboli.

A gallium scan is a radioisotope lung scan used to detect in-flammatory conditions, abscesses, adhesions, and the presence, location, and size of tumors. It is used to stage bronchogenic can-cer and record tumor regression after chemotherapy or radiation. Gallium is injected intravenously, and scans are taken at 6, 24, and/or 48 hours to evaluate gallium uptake by the pulmonary tissues.

Positron emission tomography (PET) is a radioisotope study with advanced diagnostic capabilities. It is used to evaluate lung nodules for malignancy. PET scans can detect and display meta-bolic changes in tissue, distinguish normal tissue from tissues that are diseased (such as in cancer), differentiate viable from dead or dying tissue, show regional blood flow, and determine the distrib-ution and fate of medications in the body (Shuster, 1998). PET scans are more accurate in detecting malignancies than CT scans (Coleman, 1999; Graeber, Gupta & Murray, 1999) and have equivalent accuracy in detecting malignant nodules when com-pared to invasive procedures such as thoracoscopy (Lowe, Fletcher, Gobar et al., 1998).