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Chest Imaging

by Noah Lechtzin, MD, MHS

Imaging includes use of x-rays, MRI, nuclear scanning, and ultrasonography. There are no absolute contraindications to undergoing noninvasive imaging procedures except for MRI. The presence of metallic objects in the patient's eye or brain precludes MR imaging. Presence of a permanent pacemaker or internal cardioverter-defibrillator is a relative contraindication. Additionally, gadolinium, when used as a contrast agent for MRI, increases risk of nephrogenic systemic fibrosis in patients with stage 4 or 5 chronic kidney disease.

X-Ray Techniques

X-ray techniques that are used to image the chest include plain x-rays, fluoroscopy, high-resolution and helical (spiral) CT, and CT angiography.

Chest x-ray

Plain chest x-rays and fluoroscopy are used to provide images of the lungs and surrounding structures.

Plain chest x-rays provide images of structures in and around the thorax and are most useful for identifying abnormalities in the heart, lung parenchyma, pleura, chest wall, diaphragm, mediastinum, and hilum. They are usually the initial test done to evaluate the lungs. The standard chest x-ray is taken from back to front (posteroanterior view) to minimize x-ray scatter that could artifactually enlarge the cardiac silhouette and from the side of the thorax (lateral view). Lordotic or oblique views can be obtained to evaluate pulmonary nodules or to clarify abnormalities that may be due to superimposed structures, although chest CT provides more information and has largely superseded these views. Lateral decubitus views may be used to distinguish free-flowing from loculated pleural effusion, but CT or ultrasonography can provide more information. End-expiratory views can be used to detect small pneumothoraxes. Screening chest x-rays are often done but are almost never indicated; one exception is in asymptomatic patients with positive tuberculin skin test results, in whom a single posteroanterior chest x-ray without a lateral view is used to make decisions regarding treatment for pulmonary TB. Portable (usually anteroposterior) chest x-rays are almost always suboptimal and should be used only when patients are too ill to be transported to the radiology department.

Chest fluoroscopy is the use of a continuous x-ray beam to image movement. It is useful for detecting unilateral diaphragmatic paralysis. During a sniff test, in which the patient is instructed to forcibly inhale through the nose (or sniff), a paralyzed hemidiaphragm moves cranially (paradoxically) while the unaffected hemidiaphragm moves caudally.


Computed tomography

CT defines intrathoracic structures and abnormalities more clearly than does a chest x-ray. Conventional (planar) CT provides multiple 10-mm–thick cross-sectional images through the thorax. Its main advantage is wide availability. Disadvantages are motion artifact and limited detail from volume averaging of tissue within each 10-mm slice.

High-resolution CT (HRCT) provides 1-mm–thick cross-sectional images. HRCT is particularly helpful in evaluating interstitial lung diseases (eg, lymphangitic carcinomatosis, sarcoidosis, fibrosing alveolitis) and bronchiectasis. Chest CT is normally done at full inspiration. Aeration of the lungs during imaging provides the best views of the lung parenchyma, airways, and vasculature, and of abnormal findings such as masses, infiltrates, or fibrosis. Obtaining HRCT images at full expiration as well as full inspiration can help. Expiratory imaging can increase visibility of air trapping, which is typical of obliterative bronchiolitis. Images obtained with the patient in the prone position can help differentiate dependent atelectasis (which changes with changes in body position) due to lung disorders that cause ground-glass attenuation in the dependent posterior parts of the lungs, which persists despite changes in patient position (eg, fibrosis due to idiopathic pulmonary fibrosis, asbestosis, or systemic sclerosis).

Helical (spiral) CT provides multiplanar images of the entire chest as patients hold their breath for 8 to 10 sec while being moved continuously through the CT gantry. Helical CT is thought to be at least equivalent to conventional CT for most purposes. Its main advantages are speed, less radiation exposure, and an ability to construct 3-dimensional images. Software can also generate images of bronchial mucosa (virtual bronchoscopy). Its main disadvantages are less availability and the requirement for breath-holding, which can be difficult for patients with symptomatic pulmonary disease. Newer multidetector CT technology allows more rapid scanning of the entire chest with imaging of thin slices at high resolution.

CT angiography uses a bolus of IV radiopaque dye to highlight the pulmonary arteries, which is useful in diagnosis of pulmonary embolism. Dye load is comparable to that with conventional angiography, but the test is quicker and less invasive. Several studies have confirmed CT angiography provides sufficient accuracy for the detection of pulmonary emboli, so it has largely replaced conventional pulmonary angiography and, except in patients unable to tolerate contrast agents, ventilation/perfusion (V/Q) scanning.


Magnetic Resonance Imaging

MRI has a relatively limited role in pulmonary imaging but is preferred over CT in specific circumstances, such as assessment of superior sulcus tumors, possible cysts, and other lesions that abut the chest wall. In patients with suspected pulmonary embolism in whom IV contrast cannot be used, MRI can sometimes identify large proximal emboli but usually is limited in this disorder. The use of MRI to evaluate pulmonary hypertension is being studied, and this practice may become more common.

Advantages include absence of radiation exposure, excellent visualization of vascular structures, lack of artifact due to bone, and excellent soft-tissue contrast. Disadvantages include respiratory and cardiac motion, the time it takes to do the procedure, and the occasional presence of absolute or relative contraindications.

Ultrasonography

Ultrasonography is often used to facilitate procedures such as thoracentesis and central venous catheter insertion. Endobronchial ultrasonography (EBUS) is increasingly being used in conjunction with fiberoptic bronchoscopy to help localize masses and enlarged lymph nodes. Diagnostic yield of transbronchial lymph node aspiration is higher using EBUS than conventional unguided techniques. Ultrasonography is also very useful for evaluating presence and size of pleural effusions and is now commonly used at the bedside to guide thoracentesis.

Nuclear Scanning

Nuclear scanning techniques used to image the chest include V/Q scanning and positron emission tomography (PET).

V/Q scanning

V/Q scanning uses inhaled radionuclides to detect ventilation and IV radionuclides to detect perfusion. Areas of ventilation without perfusion, perfusion without ventilation, or matched increases and decreases in both can be detected with 6 to 8 views of the lungs.

V/Q scanning is most commonly used for diagnosing pulmonary embolism but has largely been replaced by CT angiography. Split-function ventilation scanning, in which the degree of ventilation is quantified for each lobe, is used to predict the effect of lobar or lung resection on pulmonary function; postsurgical forced expiratory volume in 1 sec (FEV 1 ) is estimated as the percentage of uptake of ventilation tracer in the healthy fraction of the lungs multiplied by preoperative FEV 1 (in liters). A value of < 0.8 L (or < 40% of that predicted for the patient) indicates limited pulmonary reserve and a high likelihood of unacceptably high perioperative morbidity and mortality.


PET

PET uses radioactively labeled glucose (fluorodeoxyglucose) to measure metabolic activity in tissues. It is used in pulmonary disorders to determine whether lung nodules or mediastinal lymph nodes harbor tumor (metabolic staging) and whether cancer is recurrent in previously irradiated, scarred areas of the lung. PET is superior to CT for mediastinal staging because PET can identify tumor in normal-sized lymph nodes and at extrathoracic sites, thereby decreasing the need for invasive procedures such as mediastinoscopy and needle biopsy. Current spatial resolution of PET is 7 to 8 mm; thus, the test is not useful for lesions < 1 cm. PET reveals metastatic disease in up to 14% of patients in whom it would not otherwise be suspected. The sensitivity of PET (80 to 95%) is comparable to that of histologic tissue examination. False-positive results can occur with inflammatory lesions, such as granulomas; slowly growing tumors (eg, bronchoalveolar carcinoma, carcinoid tumor, some metastatic cancers) may cause false-negative results. Newer combined CT-PET scanners may become the most cost-effective technology for lung cancer diagnosis and staging.


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