DIAGNOSTIC PROCEDURES

ROUTINE EXAMINATION (see Plates 3-4 to 3-6 )

In most imaging centers, radiographs are no longer recorded on film but rather on digital imaging receptors. The imaging data can then either be transferred directly to a computer (digital radiography) or recorded on an imaging plate, similar in appearance to the traditional x-ray cassette. The data are placed into a “reader” and converted to an image (computed radiography). The images can then be printed on radiographic film but are more frequently stored in a digital database called a PACS (Picture Archiving and Communication System) and viewed on a computer monitor on which the contrast and brightness can be adjusted and the image magnified and annotated.

Frontal and lateral chest radiographs are the mainstay of chest radiography. Frontal radiographs are most frequently obtained with the patient facing the image receptor and the x-ray beam passing from posterior to anterior (PA projection). Almost all lateral chest radiographs are obtained with the patient's left side nearest the image receptor to minimize magnification of the heart. These are usually obtained with the image receptor 72 inches from the x-ray tube to decrease overall radiographic magnification. A relatively high beam energy of 125 to 140 kVp is usually used to increase film latitude (i.e., lengthen the gray scale). This makes the ribs less noticeable and lung pathology easier to see. In some instances, a frontal radiograph alone will suffice and has the advantage of decreasing radiation exposure because the lateral radiograph typically gives a higher radiation dose to the patient. Examples where one might choose to forego the lateral film include evaluating a patient for a positive purified protein derivative (PPD) result or after seeing a readily visible lesion. The lateral projection, however, may be very useful for lesion localization, evaluation of the spine, identification of pleural effusions, distinguishing a vessel seen on end from a nodule, and identification of calcium within the heart. Plate 3-4 demonstrates correct positioning for PA and lateral radiographs with corresponding images.

Hospitalized patients, especially those in the intensive care unit, frequently cannot be readily brought to the radiology department for PA and lateral radiographs and are usually evaluated with a single portable anteroposterior (AP) radiograph where the cassette is placed behind the patient's back (see Plate 3-4 ). These studies are usually performed with the x-ray tube at a distance of 40 inches from the image receptor and at lower beam energy (80-90 kVp). The patient is rarely more than semi-upright and usually cannot take a deep breath, resulting in poorer image quality than studies performed in the radiology department. The AP projection and shorter source to image distance result in significant magnification of the cardiac silhouette and the lower beam energy makes the images have more contrast. Plate 3-4 demonstrates a standard supine AP chest radiograph.

Oblique views of the chest can serve to help localize lesions within the lung or determine whether a perceived lesion is inside the thorax or on the chest wall. In practice, however, they are rarely obtained except for evaluation of the rib cage.

Lateral decubitus chest radiographs are useful for evaluation of pleural fluid and can be used to identify the presence of even very small pleural effusions as well as to assess the amount of free-flowing fluid. In the lateral decubitus view, the patient lies on one side, and the x-ray beam passes horizontally through the patient (see Plate 3-5 ). As a general rule, both decubitus views should be obtained if possible because it may be difficult to determine how much free-flowing fluid there is on the down side when the effusion is large and to evaluate the underlying lung parenchyma on the up side when fluid shifts from lateral to medial. When an upright chest radiograph cannot be obtained, a lateral decubitus radiograph is the best way to look for a pneumothorax without resorting to computed tomography (CT). In this case, the side of interest is the up side of the chest. Plate 3-5 demonstrates proper positioning for a decubitus chest radiograph and a corresponding image.

The AP lordotic view of the chest was widely used in the past to evaluate the lung apices in patients in whom a suspicious opacity was seen on the standard PA view. By projecting the clavicles cephalad, the apices may be better visualized. In practice, however, the question of whether a perceived lesion is real is frequently not completely answered by the lordotic view, and in most places, it has been replaced by the more expensive but also more definitive CT scan. Furthermore, unless there is obvious calcium within a lesion, the lordotic view does not answer the question of what the lesion is.

Another imaging procedure that has largely been replaced is chest fluoroscopy, although it remains a quick way to confirm that a perceived lesion is actually a confluence of shadows, saving the patient from undergoing a CT scan. Chest fluoroscopy is also useful for evaluation of diaphragmatic motion and identifying a paralyzed diaphragm. If the diaphragm is paralyzed, it will move paradoxically when the patient forcefully sniffs.

COMPUTED TOMOGRAPHY (see Plate 3-6 )

CT has revolutionized the diagnosis of thoracic disease not only by earlier detection of disease but also by much more accurate characterization of disease severity and extent. Modern units, termed multidetector row (MDCT) scanners , are capable of imaging the entire volume of the chest in less than 10 seconds, allowing 1-mm-thick high-resolution scans in a single breath-hold (see Plate 3-6 ). For this reason, virtually all CT scans performed on a MDCT scanner provide high-resolution detail of the parenchyma, although at a higher radiation dose than the spaced scans of a high-resolution chest CT, which is still used to evaluate and monitor patients with diffuse parenchymal lung disease. This ability of the MDCT to provide thin sections of the entire lung provides detailed images for the evaluation of solitary pulmonary nodules. Because the reconstructed images from MDCTs in the sagittal and coronal planes are equal in resolution to the axial source images, these multiplanar reconstructions are especially useful for the evaluation of the aorta, the tracheobronchial tree, and the pulmonary vasculature (see Plate 3-6 ).

As a result of the rapid speed of scan acquisition during maximal intravascular contrast levels, CT has become the primary method for the evaluation of suspected pulmonary embolism. The ability to acquire the CT scan in correlation with the patient's electrocardiogram has allowed motion-free images of the heart and coronary arteries to be obtained noninvasively. An additional advantage of the rapid acquisition times possible with current MDCT scanners is the ability to image the chest dynamically during expiration, thereby providing an assessment of obstructive airways disease due to tracheobronchial or small airway pathology.

The radiation dose of a CT scan, however, is substantially greater than that of radiographs; therefore they should not be used unless the value of the information to be gained outweighs the potential harmful effects of ionizing radiation.

CONTRAST EXAMINATIONS

Contrast bronchography for the detection of tracheal and bronchial masses and in the evaluation for bronchiectasis has been completely supplanted by MDCT, but Plates 3-7 and 3-8 demonstrate the normal bronchial anatomy.

Pulmonary Angiography

Although still considered the gold standard in the radiologic evaluation of pulmonary vascular anatomy, catheter pulmonary angiography has all but been replaced by CT pulmonary angiography in the evaluation of acute pulmonary embolism. Conventional pulmonary angiography is performed by percutaneous catheterization of the pulmonary artery via a femoral or upper extremity venous access and still has a role, albeit somewhat limited, in the preoperative evaluation of chronic thromboembolic pulmonary hypertension and in the diagnosis and transcatheter embolization of pulmonary arteriovenous malformations. Rarely, pulmonary angiography is performed for the evaluation of congenital abnormalities such as agenesis, aplasia, or hypoplasia of the pulmonary arteries, as in the evaluation of the minority of patients who have massive hemoptysis thought to arise from a pulmonary arterial source, such as patients with suspected pulmonary artery aneurysms. Two- and three-dimensional reconstructions of the pulmonary vasculature obtained from MDCT scans provide equivalent information and have limited the use of pulmonary angiography for mostly therapeutic indications (see Plate 3-9 ).

RADIONUCLIDE IMAGING

Ventilation-Perfusion Scintigraphy

Although CT pulmonary angiography has emerged as the primary imaging modality in the evaluation of suspected acute pulmonary embolism, ventilationperfusion (V-Q) scanning remains a very sensitive method of evaluation for pulmonary embolism and is still used in selected situations for this indication. V-Q scanning after chest radiography remains of value in patients with contraindications to intravenous iodinated contrast administration and may be the more appropriate imaging study in younger individuals evaluated for possible pulmonary embolism because it subjects patients to a lower radiation dose than does CT. In patients with pulmonary hypertension who are being evaluated for possible chronic thromboembolic pulmonary hypertension, a normal lung perfusion scan can effectively exclude this diagnosis. Finally, V-Q scans are occasionally performed for the preoperative assessment of patients considered for lobar or lung resection because they can help assess the relative contribution of the affected lobe to overall pulmonary function, thereby accurately predicting the anticipated level of pulmonary disability after pulmonary resection.

Positron emission tomography (PET) (see Plate 3-10 ) using fluorine-18-labeled fluorodeoxyglucose (FDG) has high accuracy in the distinction of benign from malignant solitary pulmonary nodules. FDG-PET has a high sensitivity for malignant nodules larger than 10 mm in diameter, with most PET-negative lesions requiring only follow-up imaging evaluation. Whole-body FDG-PET is now used routinely in the staging of lung cancer, with a higher accuracy for the detection of mediastinal and hilar lymph node involvement and high sensitivity for the detection of bone, liver, adrenal, and distant metastases.

MAGNETIC RESONANCE IMAGING

Magnetic resonance imaging is a technique that does not require ionizing radiation but instead relies on the measurement of energy released by tissue protons that have been placed in an external magnetic field. Two essential characteristics of tissue, termed T1 and T2 relaxation times , are used to evaluate tissues in health and disease. In general, whereas T1-weighted scans of the chest are useful for anatomic evaluation of the heart and mediastinum, providing excellent delineation of vascular from adjacent structures without the need for intravascular contrast, T2-weighted images are more useful for tissue characterization because they are sensitive to the greater water (i.e., proton) content of tumors. As with MDCTs, images in the direct axial, sagittal, and coronal planes are obtained.