Radiology of the Chest: Techniques

TECHNIQUES

Conventional Radiography

The Posteroanterior and Lateral Chest Radiograph

The simplest conventional study of the chest is a posteroan-terior and lateral chest radiograph taken in a radiographicunit specially designed for these studies. The x-rays travel through the patient and expose a receptor from which the image is recorded. Most commonly, digital receptors are used, although a receptor utilizing an intensifying screen and radiographic film remains in some use as well. Com-puted radiography and large field-of-view image intensi-fiers are two types of digital receptors. The digital images may be printed on film by laser printers but are generally viewed on monitors. The two views of a chest radiograph are taken in projections at 90 degrees to each other with the patient’s breath held at the end of a maximum inspira-tion. The first view is obtained as the patient faces the re-ceptor with the x-ray beam source positioned 6 feet behind him. Because the x-ray beam travels in a posterior-to-anterior direction, this view is called a posteroanterior (PA) chest radiograph. Another view is then obtained with the patient turned 90 degrees and the left side against the receptor and arms overhead. The x-ray beam travels from right to left through the patient, and this is called a left lat-eral view. Anatomic features of the chest that are readily identifiable on conventional radiographs are shown in Figures 4-1 and 4-2.

Other Radiographic Projections

In some clinical situations, patients may not be able to stand or sit upright for the conventional PA and lateral ra-diographs, and an image must be taken with the patient’s back turned to the receptor and the x-ray beam traversing the patient in an anterior-to-posterior direction. These ra-diographs are called anteroposterior (AP) radiographs. They may be taken in the x-ray department but are more commonly obtained as portable studies at the patient’s bedside.

Images may also be obtained with the patient lying on one side in a decubitus position with the x-ray beam tra-versing the patient either PA or AP along a horizontal plane. These images are designated as lateral decubitus im-ages (see Figure 4-63c). A left lateral decubitus radiograph indicates that the left side of the patient is dependent against the table. A right lateral decubitus radiograph indi-cates that the right side of the patient is dependent against the table.

The Portable Chest Radiograph

If the clinical situation prevents the patient from coming to the radiology department, a chest radiograph may be ob-tained at the patient’s bedside, and these are almost always AP radiographs. The AP portable radiograph does not pro-vide as much information as PA and lateral chest radi-ographs for a number of reasons. Because it is a single view, lesions are not as easily or accurately localized along the AP axis of the thorax. The patients for whom these images areobtained are usually quite ill and cannot be positioned as well as patients traveling to the x-ray department. An ill pa-tient may not be able to cooperate by holding his breath at full inspiration. A mobile x-ray generator is typically not as powerful as a fixed x-ray generator, and longer exposure times therefore are necessary to obtain sufficient exposure. The quality of portable chest radiographs, therefore, is often inferior to that of PA and lateral radiographs, as a result of both respiratory and cardiac motion. X-ray grids are used to reduce scatter radiation and improve image quality. Grids are used for most conventional chest radiographs done in radiology departments where fixed equipment is present. Grids are not usually used for portable radiographs, and the result is a high proportion of scattered x-rays, which degrade the image. Paradoxically, the portable radiograph may be more expensive than a conventional PA and lateral chest ra-diograph, owing to extra labor and equipment costs in ob-taining a bedside radiograph.

Computed Tomography of the Chest

For CT examinations of the chest, intravenous contrast ma-terial is frequently administered for opacification of arteries and veins within the mediastinum and hila to facilitate the recognition of abnormal masses or lymph nodes. Anatomic features of the chest that are readily identifiable on CT scans are shown in Figures 4-3 and 4-4.

Nuclear Medicine Perfusion Imaging of the Chest

Nuclear medicine techniques used in evaluating diseases of the thorax include ventilation-perfusion (V/Q) scanning and scanning with tumor-seeking radiopharmaceuticals for tumor staging. The V/Q scan may be used for patients with suspected pulmonary thromboembolism and who have con-trast allergy or renal failure. The V/Q scan is noninvasive, and when results are negative, fewer than 10% of patients have pulmonary thromboembolism. The ventilation study is typi-cally performed with the patient inhaling 10 to 30 mCi of xenon-133 while images are obtained with a scintillation camera (Figure 4-5A). Wash-in images are obtained for two consecutive 120-second periods, an equilibrium image is ob-tained, and then wash-out images are obtained over 30- to 60-second periods in posterior and left and right posterior oblique projections. This portion of the study takes about 15 minutes. The perfusion scan is obtained by intravenously injecting 2 to 4 mCi of technetium-99m-labeled macroaggre-gated albumin containing 200,000 to 700,000 particles. The particles range in size from 10 to 100 m, and they lodge in capillaries and capillary arterioles, accurately reflecting pul-monary blood flow (Figure 4-5B). The scintillation camera is set so that it obtains anterior, posterior, both posterior oblique, and both anterior oblique projections for 750,000 counts per image. The perfusion study takes about 30 minutes to perform.

Positron Emission Tomography/Computed

Tomography Imaging of the Chest

Tomography is also available for radionuclide imaging. A PET scanner resembles a CT scanner and uses positron emitters (fluorine-18 [F-18] or carbon-11 [C-11]). Today, the most widely used positron emitter is F-18-fluoro-deoxyglucose (FDG), which is used as a metabolic tracer. The raised metabolic rate can be used to distinguish neo-plasm and inflammation from normal tissue. Although PET provides tomographic images, the spatial resolution (0.7 to 1.0 cm) is somewhat inferior to that of CT. This spatial res-olution is improved by utilizing PET/CT fusion imaging in which a patient receives both a PET scan with F-18 FDG as well as a CT with or without contrast. These images can then be overlaid, or fused (Figure 4-6), to combine the spa- tial resolution of CT with the localization power of ra-dionuclide imaging.

Magnetic Resonance Imaging of the Chest

The principles and applications of MR are described earily. Anatomic features of the chest that are readily identifiable on MR images are shown in Figures 4-7 and 4-8.

Ultrasonography of the Chest

Ultrasound is described in detail in earily. Ultrasound of the chest is typically performed to evaluate fluid collections within the pleural space. Ultrasound may be used to guide thoracentesis, especially when the fluid collection is small or loculated. Less frequently, ultrasound is utilized to guide per-cutaneous biopsy of mediastinal or peripleural lung lesions. Advances in image fusion also allow fusion of ultrasound im-ages with a separately performed CT examination, which can be useful for ultrasound-guided biopsies in the thorax.