Radiology of the Chest: Technique Selection

TECHNIQUE SELECTION

The number of diseases and clinical situations for which a chest radiograph may be indicated is so large that an ex-haustive listing of individual indications is prohibitive. As a general rule, however, conventional radiographs should be obtained for any patient with symptoms suggesting dis-ease of the heart, lungs, mediastinum, or chest wall. In ad-dition, a chest radiograph is indicated for patients with systemic diseases that have a high likelihood of secondary involvement of those structures. Examples of the former are pneumonia and congestive heart failure and of the lat-ter are a primary extrathoracic neoplasm and connective tissue disease.

In an acutely ill patient, the portable chest radiograph is an invaluable tool for monitoring the patient’s car-diopulmonary status. These radiographs are also used for monitoring of life-support hardware, such as central venous access catheters, nasogastric tubes, and endotra-cheal tubes.

Fluoroscopy provides real-time imaging of the chest. Flu-oroscopy may be used to evaluate the motion of the di-aphragm in a patient with suspected diaphragmatic paralysis. A paralyzed hemidiaphragm has sluggish motion as the pa-tient breathes, and as the patient takes in a quick breath of air, it moves paradoxically upward as the normal hemidi-aphragm moves downward (“sniff test”). Fluoroscopy and fluoroscopically positioned spot images are also useful for identification of calcification within a pulmonary nodule, within coronary arteries, or within cardiac valves. Fluoro-scopic guidance can also be used for percutaneous transtho-racic needle biopsy of lung masses.

Because the three dimensions of the thorax are captured on a single two-dimensional chest radiograph, superimposi-tion of structures within the thorax may result in confusing shadows. Because CT provides images without this overlap, it is frequently used to clarify confusing shadows identified on conventional radiographs (Table 4-1). These examina-tions are also used to detect disease that is occult because of small size or a hidden position. Because of its wider range of density discrimination, CT can demonstrate mediastinal and chest wall abnormalities earlier than is possible with conventional chest radiography. Abnormalities of hilar structures can be identified on CT scans because of the de-creased overlap of the complex structures of the hilum. CT scans of the chest are routinely ordered for oncology pa tients, both for evaluation of the extent of disease at presen-tation and for monitoring response to therapy or progres-sion of disease. CT is useful for evaluation of the lungparenchyma, as thin sections (1 to 2 mm thick) reveal great anatomic detail. Thin-section CT (or high-resolution CT [HRCT]) may enable detection of occult pulmonary parenchymal disease and may be used for following the course of known pulmonary disease (Table 4-2). HRCT is especially useful in the diagnosis of interstitial lung diseases. Additionally, CT angiography (CTA) of the chest is particu-larly useful for the evaluation of vascular pathology as well as pulmonary embolus (Table 4-3). Because intravenous contrast material can be administered, vascular structures may be evaluated and the technique may be useful in pa-tients with aortic dissection, aortic aneurysm, and superior vena caval obstruction. Because the cost of CT is approxi-mately 10 to 20 times that of a PA and lateral chest radi-ograph, CT is not practical for monitoring the course of diseases on a daily basis.

The diseases and situations for which nuclear medicine techniques are helpful are determined for the most part by the radioactive tracer, and these have been outlined in the technique section (Table 4-4).

MR imaging of the thorax is most commonly used for car-diovascular imaging, but there are indications for MR imag-ing in mediastinal and pulmonary parenchymal imaging as well (Table 4-5). MR is helpful when bronchogenic carcinoma is suspected of invading vascular structures, including the cardiac chambers, pulmonary arteries and veins, and the superior vena cava. In a patient with suspected Pancoast’s (superior sulcus) tumor (Figure 4-9), MR imaging is pre-ferred to CT because of the ability to obtain images in coronal and sagittal planes. The apex of the lung can be difficult to evaluate on axial images alone because of partial-volume effects.

Ultrasonography is useful for imaging the soft tissues of the chest wall, heart, and pericardium, as well as fluid collec-tions within the pleural space. Large, mobile pleural effusions are usually aspirated without sonographic guidance, because these collect predictably within dependent areas of the tho-rax. On the other hand, loculated pleural fluid collections may be difficult to aspirate without guidance, and the most appropriate entrance site may be marked with sonography for easier access. Ultrasonography has been used for guidance for biopsy of peripheral lung lesions as well.