Major Factors Affecting Image Quality in Standard Film-Screen Radiography and Digital Radiography

Standard analog film-screen radiography has been employed for the production of chest radiographic images for many years, but has largely been supplanted by digital techniques. With film-screen radiography, an x-ray photon is absorbed in a screen containing a photostimulable coating, resulting in the production of multiple light photons that, in turn, expose the actual film. This method differs from that used by digital radiographic techniques for image creation; the differences between film-screen and digital radiographic techniques are reviewed below. Nevertheless, the following discussion of technical factors impacting imaging quality applies to both methods.

Radiographic Views and Techniques

Routine Examination

A routine chest radiographic examination in an ambulatory patient usually consists of posteroanterior (PA) and left lateral projections. The utility of routine lateral radiographs, however, has been questioned. After analyzing more than 10,000 radiographic chest examinations obtained routinely in a hospital-based population, Sagel and associates concluded that the lateral radiograph could be safely eliminated in the routine examination of patients 20 to 39 years of age. Conversely, they and others have concluded that the lateral radiograph should be obtained in patients with suspected chest disease and in screening examinations of patients 40 years of age or older ( Fig. 18-2 ).

The utility of the lateral radiograph.

A, The posteroanterior radiograph shows no specific abnormalities. B, The lateral radiograph identifies a mass ( arrows ) projected over the hilum. C, Axial chest CT confirms the presence of a mass ( arrow ) in the right lower lobe posterior to the bronchus intermedius ( arrowhead ) . The lesion was found to represent bronchogenic carcinoma.

(Courtesy Michael Gotway, MD.)

Expiratory Views

Conventional radiographs are made at total lung capacity (i.e., full inspiration), thereby permitting the greatest volume of lung to be evaluated for possible pathology and providing the most contrast between intrapulmonary air and normal and abnormal intrathoracic structures ( Fig. 18-3 ). However, localized or generalized air trapping in the lung or pleural space is more easily detected (and sometimes only detected) on a radiograph made during expiration. The normal lung diminishes in volume and increases in density with expiration. Areas of trapping usually retain their lucency and volume regardless of the phase of respiration. With unilateral or localized air trapping, mediastinal shift ( Fig. 18-4 ) and failure of normal elevation of the hemidiaphragm on the affected side are frequently apparent only on expiration. Small pneumothoraces, difficult to visualize and frequently overlooked on inspiratory radiographs, appear larger and more apparent on the expiratory study. As the thorax and underlying lung diminish in volume, the lung becomes denser, whereas the pneumothorax remains essentially unchanged in size, thus occupying a greater proportion of the deflated hemithorax, thereby being outlined more clearly by denser pulmonary parenchyma ( Fig. 18-5 ).

Comparison of radiographic appearances of thorax between expiratory and inspiratory radiographic techniques.

A, Frontal chest radiograph performed with expiratory technique shows basal opacities ( arrows ) bilaterally. The mediastinum is wide, and the heart size is difficult to assess. B, Repeat frontal chest radiograph performed at full inspiratory volume made shortly following ( A ) now shows clearing of the basal opacities; the heart size is clearly normal and the mediastinum now shows normal width.

(Courtesy Michael Gotway, MD.)

Unilateral hyperlucent lung in an 8-year-old boy with a foreign body in the right main-stem bronchus.

A, The inspiration radiograph shows mild hyperlucency of the right lung. B, The expiration film shows that the right hemidiaphragm remains fixed in a low position, the right lung remains lucent, and the mediastinum is shifted to the left (air trapping). A foreign body was removed.

(Courtesy Michael Gotway, MD.)

Inspiration and expiration chest radiography of pneumothorax.

A, On the inspiratory image, the right pneumothorax is difficult to visualize but is faintly seen in the right upper thorax ( arrows ). B, On the expiratory image, the right pneumothorax ( arrows ) is outlined against denser lung and appears larger than on the inspiration ( A ) study.

(Courtesy Michael Gotway, MD.)

Other causes of localized or unilateral lucency on the radiograph are differentiated from air trapping by the expiratory study. Among these are technical causes such as patient rotation ( eFig. 18-11 ), miscentered x-ray beam or grid, and “the anode-heel effect,” an artifact of asymmetrical generation by the anode of the x-ray tube. Chest wall abnormalities, either congenital or postsurgical ( Fig. 18-6 ), can produce unilateral lucency. Undetected areas of atelectasis with compensatory overexpansion of portions of the lung and primary vascular disease, such as pulmonary embolus (see eFigs. 57-7 and 57-9 ), may also produce lucency. None of these causes of pulmonary lucency will show trapped air on the expiratory radiograph.

“Unilateral hyperlucent lung” simulating air trapping.

Frontal chest radiograph shows lucency throughout the left thorax compared with the right, simulating air trapping (compare with Fig. 18-4B ). Note, however, that the left thorax shows neither abnormally increased (unlike Fig. 18-4B ), nor decreased, lung volume. The left axillary surgical clips are a clue to the cause of left lung hyperlucency: left mastectomy for previous breast malignancy. The removal of left thoracic soft tissue creates less tissue density along the path of travel of the x-ray photons through the patient’s left side relative to the right, resulting in relative lucency of the left thorax compared with the right.

(Courtesy Michael Gotway, MD.)

Decubitus Views

Decubitus radiography is performed by placing the patient in the recumbent position, usually lying on one side and then the other. The x-ray exposure is made with a horizontal beam in either the AP or PA projection.

The technique is useful for determining the presence or absence of free fluid in the pleural space or parenchymal cavities, for estimating the size of effusions, and for diagnosing pneumothorax in patients who are unable to sit or stand. Free fluid gravitates to the dependent portion of the thorax, which in the decubitus patient lies against the lateral rib cage of the dependent hemithorax ( Fig. 18-7 ) or the mediastinum of the contralateral side; pneumothorax behaves in the opposite fashion. In the typical AP supine chest radiograph, free fluid will layer posteriorly and manifest as an increase in density involving the entire hemithorax. Air within the pleural space will collect anteriorly and is often difficult to detect. The proper use of decubitus radiography will demonstrate the presence of free fluid, pneumothorax, or both ( Figs. 18-8 and 18-9 ). When a portable study must be performed with the patient in bed, it may be technically difficult to obtain high-quality radiographs of the dependent portion of the hemithorax due to the underlying bed, clothes, or other inconveniences. For this reason, bilateral decubitus studies may be obtained. Even in patients in whom PA and lateral erect radiographs can be performed, subpulmonic effusions may be difficult to detect, and no fluid meniscus may be visualized. Unless the fluid is completely loculated, a decubitus study will demonstrate its presence and size (see Fig. 18-9 ). As little as 20 mL of fluid can be seen in the decubitus position.

The value of decubitus radiography.

This patient had right pleuritic chest pain. A, A frontal chest radiograph shows no specific abnormalities; the right costophrenic angle is sharp. B, A right lateral decubitus radiograph shows a small right pleural effusion ( arrows ) . Note that decubitus imaging causes collapse of the ipsilateral lung.

(Courtesy Michael Gotway, MD.)

Comparison of pneumothorax appearance at upright and lateral decubitus chest radiography.

A, Upright inspiratory frontal chest radiograph shows the typical appearance of a pleural line ( arrows ) over the apex of the right lung, representing pneumothorax. B, Left lateral decubitus chest radiograph shows the gas in the pleural space has migrated in a nondependent fashion over the entire peripheral right thorax, creating a pleural line ( arrows ) over both the apical and costophrenic regions.

(Courtesy Michael Gotway, MD.)

The value of decubitus radiography in a patient with right thoracic pain.

A, Frontal chest radiograph shows apparent elevation of the right hemidiaphragm, but the right costophrenic angle is sharp. B, Lateral chest radiograph reveals obscuration of the posterior portion of the right hemidiaphragm, but a clear meniscus indicating right pleural effusion is not seen. C, Right lateral decubitus radiograph reveals the presence of a large, free pleural effusion ( arrows ). The apparent elevation of the hemidiaphragm on the conventional posteroanterior and lateral radiographs is due to subpulmonic effusion.

(Courtesy Michael Gotway, MD.)

Fluoroscopy

Fluoroscopy at one time was commonly employed either as the primary method of radiologic examination of the chest or as an adjunct study to standard radiographs. Its use over the past several decades has diminished considerably. However, there remain a few situations in which fluoroscopy can provide information that is difficult to obtain by other means, such as for the detection of abnormalities of diaphragmatic motion that result from conditions affecting the phrenic nerve (Video 18-1 ). The major use of chest fluoroscopy in current medical practice is as a guide for interventional procedures such as catheter angiography and needle or transbronchial biopsies.

Bronchography

Contrast bronchography, formerly a fairly common thoracic examination, has been replaced by fiberoptic bronchoscopy and either CT or high-resolution computed tomography (HRCT) of the lung. In patients with suspected bronchiectasis, HRCT and CT are as sensitive and are safer, more easily obtained, and much more pleasant to undergo than bronchography.

Pulmonary Angiography

Pulmonary angiography has traditionally been employed primarily for the detection or exclusion of pulmonary embolism ( eFig. 18-14 ; see Fig. 57-7 , Fig. 57-11 , Fig. 57-16 ,Video 18-2 ). Although it has been traditionally regarded as the “gold standard” for making the diagnosis of pulmonary embolism, it has limitations and tends to be underutilized. Other situations in which pulmonary angiography has been employed include the diagnosis and embolization of pulmonary arteriovenous malformations ( eFig. 18-15 ; see Fig. 19-14 ; and Chapter 61 ), pulmonary arterial foreign body retrieval, pulmonary vasculitis ( eFig. 18-16 ; see Chapter 60 ), and occasionally, the delineation of the anatomy of pulmonary vessels before lung surgery. The study is performed by rapidly injecting intravenous contrast material through a catheter and imaging the lung while the contrast material traverses the arteries and veins. Most frequently, the catheter is inserted into the femoral vein percutaneously and then guided into the pulmonary artery under fluoroscopic control.

Angiograms may be done with the catheter in a main branch of the pulmonary artery or more selectively in distal vessels. The radiographic contrast material is injected at the rate of approximately 10 to 25 mL/sec, depending on the vessel size. The volume of contrast depends on the location of the catheter. Between 40 and 60 mL of contrast material is usually injected into the right or left main pulmonary artery; half this amount of contrast is needed with digital subtraction techniques. Selective injections made within the more distal pulmonary arteries require smaller volumes. If the angiogram is performed for the detection or exclusion of pulmonary embolism after an abnormal but nondiagnostic perfusion scan, the location of the perfusion defect on the scan may act as a “road map” for the angiographer, and selective or superselective injections proximal to the area of perfusion deficit may be performed.

Unless the angiogram is done with a balloon occlusion technique, serial recording is necessary. In the past, rapid film changers were used with an exposure rate of 3 or 4/sec, but such “cut-film” techniques have been replaced by digital subtraction angiography. Filming is usually carried out for several seconds after injection to permit visualization of the pulmonary veins, the left side of the heart, and the thoracic aorta.

It is frequently necessary to image in several different projections to confirm or exclude pulmonary embolism. Oblique or lateral projections may be desirable. If the angiographic suite contains biplane equipment, the number of necessary contrast material injections can be diminished by imaging in two orthogonal planes after a single injection of contrast medium. Magnification angiography may show emboli in vessels that are too small to be seen clearly on standard studies.

For a definite diagnosis of pulmonary embolism, it is necessary to visualize the embolus as either a filling defect (see eFig. 18-14 ) within a vessel or the trailing edge of a thrombus. Other abnormalities should not be regarded as diagnostic of embolism.

Complications and death are rare, fortunately. Significant complications are seen in less than 5% of patients, and 0.1% to 0.5% of patients die undergoing angiographic studies to evaluate suspected pulmonary embolism. Minor arrhythmias are not infrequent when the catheter tip traverses the right ventricle. Ventricular fibrillation has been reported, as has refractory shock. Electrocardiographic monitoring and immediate availability of defibrillating equipment are mandatory. Reactions to contrast material are most frequently minor and primarily consist of urticaria and vasovagal responses. However, patients can occasionally suffer major reactions, with bronchospasm and cardiopulmonary collapse. Renal failure after administration of iodinated contrast material is a well-recognized complication, and preexisting renal disease manifested by elevated serum creatinine levels represents a relative contraindication to angiography. In all circumstances, the amount of contrast material injected should be the minimum required to produce diagnostic opacification of the pulmonary vessels. The use of nonionic or low-osmolar contrast material diminishes the number of minor reactions and increases patient comfort during the procedure. It is now routine in most practices.

Aortography and Bronchial Angiography

Aortography is generally accomplished by the retrograde passage of a catheter from the femoral artery to the aorta or its branches after percutaneous insertion ( eFig. 18-17 ). The study of the aorta and its branches plays a limited role in diseases of the lung. The imaging modalities of choice for diagnosing suspected aortic abnormalities, including aortic dissection, are CT, MRI, and, in acute cases, transesophageal ultrasonography; aortography is seldom required. The diagnosis of pulmonary sequestration once depended on the demonstration of anomalous systemic blood supply by aortography but, in modern practice, chest CT aortography and magnetic resonance angiography (MRA) are often diagnostic and easily demonstrate the abnormal vascular supply ( eFig. 18-18 ). Patients with severe hemoptysis may require bronchial arteriography and embolization of the bronchial artery or arteries supplying the bleeding site ( eFig. 18-19 ). These studies require selective catheterization of the bronchial arteries. Angiographic technique is, for the most part, individualized to fit the particular clinical condition being studied.

Angiography can be performed with a technique termed “digital subtraction angiography.” With digital subtraction angiography, an early image from the angiogram is recorded with an image intensifier and a high-resolution television camera and is digitized and stored. A later image in which vessels are opacified is handled in a similar manner. The first image is then subtracted from the second, and the resulting image is displayed. The background body structures are subtracted, leaving an image of the contrast-filled vessel (see eFigs. 18-17 and 18-19 ). Because the body background does not obscure the final image, angiography can be accomplished with smaller amounts of contrast than otherwise necessary. Motion of structures in the area being studied may make subtraction of images difficult.

Physical Principles

CT is based on the precise measurement of attenuation of a thinly collimated x-ray beam. Differential x-ray beam attenuation by different tissues forms the basis of image contrast on CT images. The advantages of CT are its axial tomographic (“slicelike”) format and its high sensitivity to differences in density between different tissues.

To obtain CT images, x-rays—produced by a modified standard x-ray tube—are passed through the patient, and the transmitted photons are measured by a series of x-ray detectors. The detectors produce an electric current that is proportional to the intensity of the incident x-ray beam. The strength of this current is measured and digitized by an analog-to-digital converter and thus is available for computer manipulation. The reduction in the intensity of the beam as it passes through the patient’s body is termed “attenuation” and is due to scattering and absorption of the x-ray photons by tissue. The attenuation (more precisely, the linear attenuation coefficient) of each point within the body can be calculated by the scanner’s computer, provided that multiple measurements of x-ray attenuation can be made from different angles. The details of the x-ray beam and detector motion by which these multiple measurements are obtained vary considerably among scanners.

Spiral, or helical, CT scanners, and more recently, multislice CT scanners, use a continuously rotating gantry (containing the tube and detector array) and are capable of imaging the entire thorax in a single breath-hold.

Because the x-ray attenuation measurements are stored in the computer, the image can be enhanced or manipulated mathematically with specific reconstruction algorithms. For example, the technique of high resolution CT (HRCT) combines narrow collimation and image reconstruction with a high-spatial-frequency algorithm to produce increased sharpness in the final stage.

Image Display

The reconstructed CT image is made up of a matrix of picture elements or “pixels.” The x-ray attenuation of each pixel is normalized to that of a test object containing pure water. The CT number is the ratio of tissue attenuation minus water attenuation relative to water attenuation multiplied by 1000 and is expressed in Hounsfield units (HU). The CT number for normal lung ranges from −700 to −900 HU, whereas soft tissues have CT numbers ranging from −100 (fat) to +100 HU (blood clot), with water measuring 0 HU and most soft tissues in the 20 to 60 HU range. The range of CT numbers encountered in patients is approximately 2000, ranging from −1000 (air) to +1000 (bone). However, although the computed image contains 2000 number levels, which could correspond to 2000 shades of gray, the human eye can perceive only about 16 to 20 distinct shades of gray. It is thus necessary when displaying CT images to restrict the image display to a small fraction of the actual range of attenuation values.

This restriction is done first by combining similar CT numbers into a single gray shade and second by setting the window width and window level display settings. The window width is the range of densities that will be displayed as shades of gray; all higher pixel values are shown as white, and all lower ones are shown as black. The window level is simply the median pixel value about which the display range is centered. Thus, to view the lungs, an appropriate window level is −700 HU, with a window width of approximately 1200 HU, which may be adjusted depending on user preference. For viewing soft tissues, pleural space, mediastinum, or hila, a window level of 20 to 40 HU with a width of approximately 400 HU is preferred. These are usually referred to, respectively, as “lung window” and “mediastinal or soft-tissue window” settings, and both should be displayed for a chest CT study.

Spiral and Multislice Computed Tomography

The terms “spiral” and “helical” CT are essentially equivalent; they refer to the path the rotating x-ray beam describes as a result of its continuous rotation around the longitudinal axis of the patient as the patient is transported though the CT gantry on a continuously moving table. In many patients, the entire thorax can be scanned during a single breath-hold. Spiral CT has the advantages of (1) volumetric imaging, which ensures contiguous image reconstruction and allows multiplanar or three-dimensional reconstructions to be performed; (2) more rapid scanning; and (3) more rapid contrast infusion with denser opacification of vessels.

Volumetric CT refers to the ability of modern CT scanners to acquire scan data continuously between two points (in the thorax, typically from the cervico-thoracic junction to the diaphragm), rather than, as with older CT technology, using a “stop-and-shoot” method. With the latter technique, individual image “slices” could be summed to create a volume of imaged tissue but were limited by misregistration between the individual image slices, particularly between successive individual breathholds. Owing to the rapid acquisition times of modern CT scanners, the entire thorax can now be imaged completely in a single breathhold, leading to a single “block” of tissue data that can be reconstructed into practically any desired slice thickness and imaging plane desired.

Multislice CT (MSCT) markedly improves on the advantages offered by single-slice spiral CT. MSCT scanners acquire information using multiple channels during a single tube rotation, thereby dramatically increasing the data acquisition rate. Current MSCT scanners may acquire up to 320 images per tube rotation for a given collimation, whereas routine single-slice spiral CT scanners would only acquire 1 image per tube rotation for a similar collimation. The speed advantage of MSCT scanning is obvious, and this tremendous speed allows for rapid imaging of large volumes of tissue in practically any phase of intravenous contrast enhancement. The image data sets provided by MSCT scanners also allow for improved-quality reformatted images ( eFig. 18-22 ,Videos 18-3 and18-4 ). Furthermore, the imaging data acquired with MSCT scanning allow for “retrospective reconstruction” of narrow section images (e.g., “thin sections” or “high-resolution” images) following scan completion.

With MSCT, the ability to reconstruct narrow section images retrospectively has transformed the process of nodule characterization. In the era of single-slice CT scanning, when narrow section imaging was used to characterize a pulmonary nodule, particularly by the detection of calcium or fat within the nodule, either the patient would be held within the department and the radiologist contacted to review the scan to determine if narrow section imaging was required (with the associated detrimental effect on departmental throughout), or the patient would be called back at a later time for further imaging (with the associated patient and referring physician inconvenience). With MSCT scanning, CT protocols that acquire narrow sections (often on the order of 1 mm or less) are devised but are reconstructed using wider sections (often 5 mm) for routine interpretation, which allows for optimized computer performance and data handling; however, the narrow sections have been acquired and are available for review, if needed. In the foregoing example, if narrow sections are needed for pulmonary nodule characterization, the narrow section data may be retrospectively reconstructed and reviewed ( Fig. 18-10 ). In fact, many radiology departments prospectively reconstruct both wider section widths for routine viewing and narrow section widths for specialized applications and save both series to the picture archiving communication system; such an approach strikes an excellent balance between the need for smaller data sets to facilitate routine data handling, while still allowing for specific questions to be addressed later through the use of thin-section imaging without reimaging the patient.

Volumetric multislice CT imaging of a pulmonary nodule detected at chest radiography.

A, “Routine” axial 5-mm CT image shows a soft-tissue nodule in the left upper lobe. Vaguely increased attenuation is seen within the nodule, suggesting calcification, but the finding is not well seen. B, Lung window images show that the nodule is circumscribed. C, Retrospectively reconstructed thin-section (1-mm) soft-tissue image more clearly shows a small focus of calcification ( arrow ) within the lesion, suggesting a benign etiology.

(Courtesy Michael Gotway, MD.)

Computed Tomography Scan Protocols

CT scans are obtained with different parameters, depending on the indication for the examination ( Table 18-1 ). Variables include scanning range, patient position, scan section thickness, table transport speed/pitch, and intravenous contrast injection rate, timing, and volume. These parameters are briefly reviewed later. The influence of table transport speed/pitch on CT protocols has evolved with the proliferation of MSCT scanners. Consideration of these variables is more relevant to cardiovascular imaging and is not discussed here. The interested reader is referred to excellent reviews on the topic of MSCT scan parameters and the technical aspects of MSCT scanning. Owing to MSCT’s rapid scan capability and ability to retrospectively reconstruct narrow section data, in recent years there has been some degree of CT protocol “simplification” compared with the single-slice CT era. For example, single-slice high-resolution CT protocols previously included noncontiguous supine and prone 1-mm inspiratory scans, as well as noncontiguous supine expiratory scanning. Using such a protocol, it was not uncommon to mistake a pulmonary vessel for a lung nodule; the inability to review adjacent sections immediately cranial and caudal to the vessel, owing to the noncontiguous nature of single-slice high-resolution CT protocols, often resulted in a pulmonary vessel simulating a nodule. This high-resolution CT protocol was distinct from the “routine” single-slice CT chest protocol, employing contiguous imaging, that was required to clarify the nature of this potential abnormality. Current MSCT high-resolution CT protocols, however, employ contiguous 1-mm supine imaging routinely, supplemented with noncontiguous 1-mm prone inspiratory and supine expiratory imaging, thus alleviating the difficulties associated with noncontiguous imaging techniques while simultaneously maintaining the benefits of thin-section imaging in the characterization of diffuse lung diseases.

Medical Imaging and Radiation

The major drawback of the increased utilization of CT is radiation exposure. In 1980–1982, radiation from medical imaging accounted for approximately 18% of the per-capita effective radiation dose in the United States, but this value increased to 54% by 2006, due largely to increased radiation doses resulting from CT scanning and, to a much lesser extent, nuclear medicine procedures. It was recently reported that CT represents about 17% of all radiologic procedures in the world but accounts for more than 40% of the collective dose. Given the continued increased use of CT since 2006, it is likely that these numbers are now even greater.

The two primary terms for describing the radiation imparted by a CT scan are adsorbed and effective dose. The adsorbed dose, measured in grays (Gy), is the energy absorbed per unit mass of a specific organ. In other words, it is the radiation imparted to a specific organ and thus accounts for the future risk of developing a malignancy at that site. Effective dose, measured in sieverts (Sv), is an overall estimate of total-body radiation exposure; this unit attempts to provide a measure of the overall potential harm caused by radiation exposure. Effective dose is defined as the radiation dose that must be delivered to the whole body to yield the same biologic consequences (specific for cancer induction only) as the dose actually received by the exposed organs. This is calculated by summing the individual adsorbed doses to each organ, each of which is multiplied by a weighting factor reflecting individual organ radiosensitivities. Effective dose is useful for gross comparisons between different modalities and techniques. It should be understood that effective dose is not an actual measure of the radiation imparted to a patient during a given CT examination, although many published articles use this unit (often expressed as milliSieverts [mSv] or in the form of the older unit, millirems [mrem]) in this fashion. This misconception is compounded by the recording of a “patient dose report” as part of the CT scan report ( eFig. 18-23 ). Rather, the actual radiation dose imparted to a patient during a CT examination depends directly on the size and shape of the actual patient. The units reported by the CT scanner, often the CT Dose Index (usually expressed as the CTDI _vol , reported as an absorbed dose, in mGy) or the Dose-Length Product (DLP, reported as an effective dose, in mSv, and equal to the CTDI _vol multiplied by the irradiated scan length), are measures of scanner radiation output, not patient dose. Calculating the radiation dose imparted to a patient during a given CT scan requires complex equations that take into account patient size, shape, body composition, and the organs irradiated, as well as scanning range. With knowledge of the patient size, body region scanned, CT scanner output, and scan length, a reasonable approximation of patient dose for a given CT scan is possible, but it remains imperative to understand that the units of scanner output—the CTDI and DLP—are not measures of patient radiation dose. Therefore, these parameters should not be used to estimate potential cancer risk or radiation-related deterministic effects (e.g., skin injury) for individual patients undergoing medical imaging procedures.

Approximate effective doses (scanner outputs) for CT are given in Table 18-2 and compared with nonmedical radiation exposures. Of note, the values given are representative measurements but, in fact, there is a wide range of exposures dependent on CT technique and method of estimation. Also, indications such as pulmonary embolism, which require narrower sections, may involve a relatively greater amount of radiation exposure compared with “standard” chest CT protocols. In recent years, concerted efforts to curtail medical radiation exposure have resulted in a number of advances that have significantly reduced the amount of radiation delivered to the patient, chiefly through reduction of CT-related exposures. A thorough review of these methods is beyond the scope of this brief discussion but includes increasing public and provider awareness through efforts such as the “ImageGently” campaign conducted by the Alliance for Radiation in Safety in Pediatric Imaging and the ACR’s “ImageWisely” (radiation safety in adult medical imaging) effort; technical advances in CT scanner equipment (new reconstruction algorithms and automated exposure controls that more efficiently apply the radiation exposure to the patient, such as automatically reducing tube current through thinner body regions); encouraging the use nonionizing radiation imaging alternatives for selected disorders; and active interventions by radiologists, including limiting scanning range and reducing x-ray tube current and voltage, among other considerations. It is now relatively commonplace that exposures associated with newer CT protocols are significantly less than the values reported in Table 18-2 (Video 18-5 ).

Considerable controversy regarding the genesis and magnitude of adverse effects resulting from radiation exposure in humans exists and has been the subject of debate for decades. The rapid rise in the use of CT scanning in recent years, as well as several well-publicized patient overexposure events at CT scanning, have fueled this debate. Nevertheless, several points can be agreed on by investigators on both sides of this debate:

• 1.
  

  Radiation is potentially harmful to biologic systems and exposure can induce cancer, among its other effects ;

  
  
• 2.
  

  There is relatively clear evidence for radiation-induced carcinogenesis for exposures exceeding 100 mSv ;

  
  
• 3.
  

  Radiation is a relatively weak carcinogen, and excess radiogenic cancer risk, if any, is considerably less than spontaneous cancer risk ;

  
  
• 4.
  

  There are no parameters that allow a radiation-induced malignancy to be distinguished from a “naturally occurring” one ;

  
  
• 5.
  

  The risk of radiation-induced malignancy is generally inversely related to age and relatively higher in women compared with men ;

  
  
• 6.
  

  The risk for radiation-induced carcinogenesis from a CT scan is generally far less than the benefit of the information gained from appropriately indicated CT studies.

The primary source of controversy between those who believe low-level radiation exposures—exposures less than 100 mSv, which are typical for the exposures associated with medical imaging—may result in the development of malignancy and those who claim no such evidence exists lies with the linear nonthreshold (LNT) model for radiation-induced carcinogenesis. The LNT theory holds that a straight-line relationship between radiation dose and cancer risk exists, and there is no threshold dose below which radiation is not carcinogenic. Proponents of the LNT theory hold that a linear model best describes the relationship between radiation exposure and cancer induction, and no evidence to support a departure from the LNT model exists ; by contrast, critics of the LNT model for radiation-induced carcinogenesis risk prediction insist that the LNT model was developed to establish standards for protection of occupationally exposed individuals and overestimates the incremental risk for cancer development at low-level radiation exposures, and, therefore, it is inappropriate to project individual cancer risks from medical radiation exposures using this model. Nevertheless, many of the widely publicized reports estimating the number of cancer-related deaths resulting from CT scanning have used the LNT methodology.

Estimates for the risk of radiation-induced carcinogenesis in humans primarily stem from epidemiologic studies of exposed patient cohorts, particularly Japanese survivors of the atomic bombings in 1945; other data sources include occupational radiation exposures and radiation accidents, such as the Three Mile Island and Chernobyl accidents. The LNT model is often used to extrapolate the elevated rate of malignancy found among these exposed cohorts to patients receiving the relatively low-level radiation exposures, typical of medical imaging, in the effort to estimate individual cancer risks; in addition, both the National Council on Radiation Protection and Measurements and the International Commission on Radiological Protection advise against the use of the effective dose paradigm and LNT theory in this fashion. Furthremore, it should be understood that the LNT model does not provide for the ability to “sum” separate radiation exposures from temporally separated radiation-utilizing procedures into a “cumulative” cancer risk. In other words, regardless of the patient’s previous radiation exposures, the imaging study the patient is about to undergo only adds the potential incremental risk for cancer development attributable to that examination ; the radiation exposures from separate examinations are not additive. This notion carries significant implications regarding the recent trend toward the development of radiation “dose registries,” whereby patient medical imaging radiation exposures are tracked in a database with the intention of using such information to make future medical imaging decisions.

Physical Principles

MRI is performed by magnetizing the patient’s tissue slightly, generating a weak electromagnetic signal by applying a radiofrequency pulse, and mapping that signal spatially by manipulating its frequency and phase in a location-dependent manner with magnetic field gradients. MRI does not require mechanical motions of the scanner and therefore can image directly in nonaxial planes.

Although MRI uses electromagnetic radiation, the energy levels used in MRI are quite low (i.e., nonionizing). MRI appears to be remarkably free of significant bioeffects. Potential safety hazards in MRI relate to the extremely strong static magnetic field and rapidly switched gradient magnetic fields used and to the possibility of tissue heating from radiofrequency energy absorbed by the body. Tissue heating is a theoretic concern but, in practice, is not significant when conventional MRI techniques are used. Theoretically, the rapidly varying gradient magnetic fields could stimulate electrically excitable tissue, but this is not a problem with routine MRI techniques. In contrast, the powerful static magnetic field is a major safety hazard because the magnetic forces near a whole-body magnetic resonance (MR) imager are strong enough to cause significant projectile hazards. For example, an ordinary steel oxygen cylinder brought into an MRI examination room will fly into the bore of the magnet with a terminal velocity of about 45 mph. The possibility of displacements or torques on metallic implants within patients also must be considered. Although the margin of safety is high, there are rare documented instances of harm to patients from dislodging intracranial aneurysm clips or intraocular metallic foreign bodies. Finally, the magnetic field can operate reed relays in cardiac pacemakers and cause a change in the pacing mode. Accordingly, strict security around MRI facilities is essential to prevent patients with certain types of metallic implants from entering the scanner and to prevent medical personnel from carrying into the scan room objects that could become projectiles.

MRI produces extremely high contrast between different types of soft tissue. This soft-tissue contrast is based on intrinsic properties of the tissues, but it can be modified and exploited by appropriate use of operator-selectable imaging techniques. The tissue properties normally utilized in MRI are as follows: the concentration of protons available to produce an MR signal (“proton density”), the presence of motion or blood flow, and two properties known as T1 and T2, time constants that describe how quickly an MR signal can be generated from a tissue (T1) and how quickly the MR signal, once generated, decays away (T2). In general, pathologic tissues have long T1 times and appear dark on those MR images whose appearance is conditioned primarily by T1 effects (“T1-weighted images”). Usually, pathologic tissues also have long T2 times and appear bright on T2-weighted images. The reason for this opposite behavior is that “T1” and “T2” describe processes that have opposite effects on the intensity of the MR signal. Flowing blood also can appear either bright or dark on MR images, depending on the examination technique used. The art of performing an MRI examination depends on appropriate manipulations of imaging techniques to capitalize on differences in proton density, flow, T1, and T2 between normal and pathologic tissue. In addition, the use of gadolinium-based MRI contrast materials, which alter the T1 and T2 of tissues semiselectively, has become important for numerous clinical applications, particularly MR angiography (MRA). MRI also can display other tissue properties such as molecular self-diffusion, temperature, and metabolites (MR spectroscopy), but these are less important for thoracic applications, although diffusion imaging and MR spectroscopy have recently shown some promise in lung cancer evaluation and staging.

Techniques

Motion during MRI causes artifacts that can degrade image quality severely. Motion compensation techniques, such as electrocardiographic gating and respiratory compensation, are widely used in chest MRI. High-speed techniques permit imaging in a matter of seconds, even fast enough to stop cardiac motion.

Direct MRI in sagittal, coronal, and oblique planes can provide superior depiction of structures that are oriented in the long axis of the body, such as the aorta, and of edges of structures that lie within the axial plane, such as lesions in the aortopulmonary window or lung apex.

Flow has profound effects on the MR image. Two basic effects from flow are seen in MRI; they are called “time-of-flight effects” and “spin-phase effects.” The manifestations of flow effects in MR images strongly depend on the type of flow that is present and on the specific MRI techniques used. The clinical implication of this is that one can make the MR image of blood either bright (“white-blood images”) or dark (“black-blood images”). Most types of vascular pathology can be demonstrated with either white-blood or black-blood images, but there are usually clinical advantages to using one approach or the other in specific situations. Flow velocity can be estimated with MRI techniques, and thus noninvasive estimates of blood flow are potentially available.

Although high-speed imaging techniques are now available with MRI, it still is not an appropriate method to use for wide-ranging screening examinations of the entire trunk for metastatic disease. Such examinations normally should be performed with modern CT scanners. It usually is more rewarding to ask an anatomically focused question when requesting an MRI study.

Screening and “Routine” Chest Radiographs

It is now generally agreed that screening chest radiographs are not indicated except in specific high-risk populations. Estimates as to possible iatrogenic disease caused by ionizing radiation from screening examinations, forecasts of possible genetic consequences, and financial concerns outweigh the medical value of such examinations. In 1973, the Department of Health, Education, and Welfare, in conjunction with the ACR and the American College of Chest Physicians, recommended the discontinuation of screening examinations. In 1985, recommendations of the ACR were more explicit: chest radiographs obtained for routine examination, preemployment screening, prenatal or obstetric screening, and hospital admission, as well as repeated examinations of patients with positive tuberculin tests and a negative initial chest radiograph, should be eliminated. From their analysis of more than 10,000 chest radiographic examinations obtained in a hospital-based population, Sagel and associates concluded that routine screening examinations done solely because of hospital admission or scheduled surgery are not warranted in patients younger than 20. Even in selected high-risk populations, radiographic screening for carcinoma of the lung has failed to significantly increase longevity.

The value of the routine hospital admission chest radiograph is still controversial. Although the yield is low in asymptomatic patients, the examination can be extremely valuable as a baseline study for comparison with radiographs taken during or after the course of hospitalization in patients who develop pulmonary symptoms.

Detection of Lung Cancer and Assessment of Solitary Pulmonary Nodules

Although chest radiographs are clearly useful in the initial evaluation of patients suspected of having lung cancer, they are of limited accuracy in showing small or early cancers. At the time of their initial diagnosis, it is not uncommon for lung cancers to be visible retrospectively on prior radiographs. Although additional radiographic studies, such as oblique views, are cost-effective for the initial evaluation of “nodular opacities” (see eFig. 18-13 ) that do not clearly represent a lung nodule and are often of value in determining that a “nodular opacity” is not a true nodule, studies by CT are superior for the detailed evaluation of a known lung nodule.

Evaluation of Intensive Care Unit Patients

Chest radiography is generally the most common imaging study performed in the most critically ill patients in the hospital—those in the ICU. ICU radiographs are taken with portable and, therefore, suboptimal technique; nonetheless, the information they provide is useful and may alter patient management.

The overall incidence of abnormalities found on chest films in ICU patients is quite high; in a study of more than 1000 consecutive medical or surgical ICU films obtained routinely or after a change in clinical status, malposition of a monitoring device or a marked change in apparent cardiopulmonary status was found in 65%. In a study of patients in a respiratory ICU, chest radiographs showed at least one new, clinically unsuspected finding in 35% of patients that, in 29% of these patients, led to a change in management. Significant findings are even more likely when the study is obtained to assess a change in clinical status. In intubated patients in a medical ICU, 43% of radiographs showed significant worsening of a known process or development of a new abnormality. The value of obtaining chest radiographs on a daily basis in ICU patients is less clear. Strain and colleagues found that routine morning radiographs revealed unsuspected abnormalities that led to a change in management in only about 15% of medical ICU patients, although this percentage was considerably higher (57%) in patients with pulmonary or unstable cardiac disease. Graat and coworkers prospectively assessed the value of 2457 routine chest radiographs performed on patients in a combined medical and surgical ICU and found only 5.8% of routine chest radiographs showed new or unexpected findings; only 2.2% of patients required a change in management. Hall and colleagues found that 18% of mechanically ventilated patients in a medical/surgical ICU had at least one radiograph that showed a clinically significant and unsuspected finding. The value of daily chest radiography in patients who have undergone thoracotomy and pulmonary resection has also been challenged. Daily chest radiography in nonhypoxic patients who had recently undergone thoracotomy and pulmonary resection changed care in only 27% of patients, whereas chest radiography altered management in 79% of such patients who were hypoxic. Finally, a meta-analysis of 8 studies totaling 7078 patients found that elimination of daily routine chest radiography did not affect hospital or ICU mortality and there was no difference in length of stay in either the ICU or hospital, or the period re­­quired for mechanical ventilation, between patients undergoing routine daily chest radiography and those receiving “on-demand” imaging. Similarly, Hejblum and associates concluded that an on-demand strategy for ordering chest radiographs for mechanically ventilated patients in the ICU is associated with a 32% reduction in the use of chest radiography compared with a routine daily strategy, but both strategies result in a similar rate of therapeutic or diagnostic intervention. On the basis of these results, the ACR has recommended that portable chest radiography should be obtained only for clinical indications, not routine assessment, when monitoring a stable patient or for a patient undergoing mechanical ventilation in the ICU. Furthermore, routine chest radiography is not recommended for stable patients admitted to the ICU for cardiac monitoring or for stable patients admitted to the ICU for extrathoracic disease only. Chest radiography is still recommended for the assessment of patients following initial support device placement, including endotracheal intubation, central venous catheter insertion, pulmonary arterial catheter placement, nasogastric tube placement, and thoracostomy tube insertion.

Indications in Acute Lung Disease

Solitary Pulmonary Nodules

Assessment of a solitary pulmonary nodule (SPN) seen on chest radiographs is a common indication for CT. CT is used to confirm that the SPN is real, to confirm that the SPN is solitary, to attempt further noninvasive characterization of the lesion, to guide percutaneous biopsy of the lesion, and to provide staging information if the SPN is found to represent a carcinoma. The likelihood of malignancy in such nodules varies from less than 10% in mass screenings to about 50% among resected nodules.

In general, CT of a patient with an SPN should be performed with thin-section volumetric imaging to provide detailed analysis of nodule morphology and to allow identification of the presence of fat or calcium (see Fig. 18-10 ,Video 18-6 ) within the nodule, and when the latter is present, the pattern of calcification; this is now fairly routine in the era of MSCT technology.

In up to 20% of instances, a “lung nodule” visible on chest radiographs actually represents an artifact, chest wall lesion ( eFig. 18-24 ), or pleural abnormality; in some cases, CT is essential to determine the true nature of the opacity. CT can be useful to define the morphology of the SPN and suggest whether it is benign, likely malignant, or indeterminate, in other words, having neither benign nor malignant characteristics. In some patients, a specific diagnosis of lesions such as rounded atelectasis ( Fig. 18-11 , ), a mucous plug, or arteriovenous malformation (Video 18-8 ) can be made on the basis of CT findings, indicating the benign nature of the lesion. Other CT appearances suggest the presence of malignancy ( eFig. 18-25 ). Malignant features include a spiculated (see eFig. 18-25D ) or irregular contour; the presence of air bronchograms within the nodule (see eFig. 18-25A ), bubbly air collections (“pseudocavitation”), or cavities; and a diameter larger than 2 cm ( Fig. 18-12 , see eFig. 18-25A ). Benign features include the presence of several patterns of calcification; “benign” patterns include diffuse, central, laminated, and chondroid, or “popcorn,” calcification ( Fig. 18-13 ). In about 30% of benign nodules, calcium not readily visible on chest radiographs can be seen on thin-section CT ( eFigs. 18-26 and eFigs. 18-27 ). Carcinomas may occasionally show calcification ( eFig. 18-28 ), although often in an eccentric or stippled pattern. The presence of fat within an SPN, indicated by a low CT attenuation coefficient ( Fig. 18-14 ), strongly suggests the presence of hamartoma or lipoid pneumonia ( eFig. 18-29 ); such nodules can be safely followed with serial radiographs.

Chest CT shows typical findings of rounded atelectasis.

A, Frontal chest radiograph shows volume loss in the left lower lobe associated with an oblong mass ( arrow ) and left-sided pleural abnormality. B, Axial chest CT in lung windows shows a subpleural left lower lobe mass ( arrow ) associated with left lower lobe volume loss (posterior displacement of left major fissure, small volume in the left lower lobe). Note “spiraling” appearance of left lower lobe bronchovascular bundles as they extend into the mass ( arrowheads ); this is the so-called comet-tail sign. C, Axial chest CT displayed in soft-tissue windows shows other features consistent with rounded atelectasis including abnormally thickened pleura ( arrowheads ) and contact of the mass ( arrow ) with the abnormal pleura.

(Courtesy Michael Gotway, MD.)

Chest CT of bronchogenic carcinoma.

Axial CT displayed in lung windows shows a spiculated lesion within the left upper lobe.

(Courtesy Michael Gotway, MD.)

Chest CT of hamartoma.

Chest CT showing “popcorn” (chondroid) calcification within a left upper lobe nodule ( arrow ), consistent with hamartoma.

(Courtesy Michael Gotway, MD.)

Chest CT of a hamartoma.

CT image showed low attenuation within the solitary pulmonary nodule (typical CT numbers within the lesion ranged from −90 to −100 HU), consistent with fat.

(Courtesy Michael Gotway, MD.)

Malignant tumors tend to show greater enhancement than benign nodules after the rapid injection of iodinated contrast material. Because the degree of enhancement depends on the amount and rapidity of contrast infusion, it is important to use a consistent technique. Using a specific contrast enhancement CT protocol, a threshold for enhancement of 15 HU or more is typically seen with malignancy, hamartoma, and some inflammatory lesions. Enhancement of less than 15 HU almost always indicates a benign lesion, usually a granuloma. Therefore, whereas positive results (enhancement of >15 HU at any time point during the study) are nonspecific, negative results are quite useful ( eFig. 18-30 ). This technique has been shown to have a sensitivity of 98% and a specificity of 58% in diagnosing carcinoma. More importantly, the negative predictive value of this technique is approximately 96%. CT nodule enhancement studies are most appropriately used for patients who have indeterminate nodules (i.e., those without typical benign or malignant appearances). A similar use for dynamic, contrast-enhanced MRI has been reported, although data are limited. The use of fluorodeoxyglucose positron emission tomography (PET) imaging (see Chapter 21 ) has also been shown to be useful for distinguishing benign from malignant nodules (see Fig. 21-1 ) and is generally favored over contrast-enhanced CT for additional imaging characterization of indeterminate lung nodules. In a meta-analysis, PET was shown to be 94% sensitive and 83% specific for the differentiation of benign from malignant solid nodules 1 to 3 cm in size. The addition of PET-CT tends to increase sensitivity with no change in specificity compared with PET alone. False-positive PET results may be seen with inflammatory processes, particularly granulomatous diseases such as tuberculosis (see eFig. 53-6C and D ), histoplasmosis, and sarcoidosis. Tumors that may be ¹⁸ PET-negative include minimally invasive adenocarcinoma (see Fig. 21-2 , eFig. 53-7A ), carcinoid tumors, and some metastases (e.g., renal cell carcinoma). In addition, small nodules may not be of a sufficient size to produce a positive result with PET scanning (see eFig. 53-7F ). The sensitivity of PET imaging significantly decreases with nodules measuring less than 8 to 10 mm in diameter (see eFig. 53-7F ). As with any imaging modality, the correlation of the PET results with morphologic features on CT, prior radiographic examinations, and the clinical presentation will improve the determination of the likelihood of malignancy.

Multiple Pulmonary Nodules

CT is the favored procedure for identifying multiple pulmonary nodules or masses. It detects more and smaller metastases ( Fig. 18-15 ,Video 18-9 ) than any other imaging technique, and 2- to 3-mm nodules are routinely visible. This is most relevant for patients with extrathoracic malignancies, in whom the detection of metastatic disease has a major impact on initial staging and assessment of response to therapy. The major sources of controversy regarding the use of CT for evaluation of possible metastases have been its limited specificity and cost-benefit ratio.

Chest CT in a patient with metastatic cervical carcinoma.

Maximum-intensity projected CT shows numerous bilateral pulmonary nodules ( arrows ) that were not visible on the chest radiograph. CT is more sensitive than radiography for detection of small nodules of any cause.

(Courtesy Michael Gotway, MD.)

CT is clearly more sensitive than chest radiographs in diagnosing multiple pulmonary nodules in patients with suspected metastasis. The sensitivity of CT for detecting surgically proven nodules ranges from 50% to 75% (i.e., 50% to 75% of all resected nodules are seen on preoperative CT scans), whereas the sensitivity of chest radiographs is about 25%. In some studies, most nodules (55% to 60%) seen on CT and subsequently resected have proved to be benign (granulomas, intrapulmonary lymph nodes, and the like). In other studies, most CT-detected nodules (80% to 95%) have proved to be metastatic lesions. In an older study from the pre-MSCT era involving radiologic-surgical correlation, Peuchot and Libshitz evaluated 84 patients with previously documented extrathoracic malignancies and newly identified pulmonary nodules. These authors noted important limitations in both the sensitivity and specificity of CT. Of a total of 237 nodules resected, CT was able to identify only 173 (73%); 207 (87%) were metastatic tumors, 21 (9%) were benign, and 9 (4%) were bronchogenic carcinomas. Of 65 nodules interpreted as solitary on chest radiographs, CT disclosed multiple nodules in 46%, and 84% of these additional nodules were metastases. Although exquisitely sensitive for small nodule detection, CT is not infallible. In a study of 53 preoperative patients underoing 60 metastectomy surgeries from 1996-2004, in whom nodules detected at chest CT were compared with pathologically confirmed metastases, 46% to 47% of surgically palpable lung metastases were not reported at preoperative CT. Although there are intrinsic biases affecting such reports, including the possibility that surgical palpation will detect benign nodules and may overlook some malignant nodules that will subsequently manifest as growing nodules at chest CT, as well as the inherent difficulties correlating the location of nodules detected at chest CT with pathologic specimens, the results of this study nevertheless highlight the limitations of chest CT for small nodule detection and characterization.

Accordingly, CT results must be interpreted in light of the clinical characteristics of the patient. Primary tumors that commonly spread to the lungs, more advanced malignancies, and the absence of any simulators of metastases (silicosis, sarcoidosis, and prior granulomatous infection) would favor malignancy in pulmonary nodules detected by CT. Furthermore, smaller and less numerous nodules are more likely to be benign. The findings on CT are in themselves not specific, however, and follow-up may be necessary to demonstrate the growth or stability of small nodules.

Lung Cancer Staging

In patients with lung cancer, accurate anatomic staging is essential for planning the therapeutic approach. CT is used in both assessment of primary tumor extent and detection of lymph node metastases. However, its accuracy in both situations is limited. Lung cancer staging is reviewed in detail in Chapter 53 .

Computed Tomography

The primary goal of CT is to help distinguish patients who likely have resectable tumors from those who do not. Pulmonary malignancy is considered to be likely unresectable (stage T4) if the primary tumor (1) involves the trachea ( eFig. 18-31 ) or carina; (2) invades the mediastinum ( eFig. 18-32 ) with involvement of mediastinal structures ( eFig. 18-33 ); (3) invades the chest wall with involvement of great vessels, brachial plexus (see eFig. 53-18 ), or vertebral column ( eFig. 18-34 ); or (4) results in a malignant pleural effusion or pleural implants ( eFig. 18-35 , see eFigs. 53-4 and eFig. 53-5 ). Note, however, that acceptable morbidity and mortality have been reported following en bloc vertebral body resection with reconstruction for neoplasms invading the spine, particularly following induction chemotherapy, but such extensive surgeries are relatively restricted to larger academic or high-volume centers. The presence of satellite nodules in the same lobe as the primary tumor (see eFig. 53-19 ) is now designated as T3 in the seventh edition of the TNM Classification of Malignant Tumors and does not preclude surgical resection.

Tracheal (see eFig. 18-31 ) or carinal invasion ( eFig. 18-36 ) can be suggested with CT but requires biopsy confirmation. The CT diagnosis of chest wall (see eFig. 53-16 ) or mediastinal (see eFig. 18-32 ) invasion can be a problem, and many CT scans suggesting chest wall invasion are relatively nonspecific and can be seen with a number of inflammatory disorders. The sensitivity and specificity of CT for diagnosing T4 mediastinal or chest wall invasion are about 60% and 90%, respectively, although some findings (e.g., vertebral body destruction, encasement of mediastinal structures, mediastinal fat infiltration) are virtually diagnostic. Limited chest wall or mediastinal invasion is considered potentially resectable by many surgeons, but knowledge of tumor extent is nonetheless an important factor when planning therapy.

CT findings suggesting mediastinal invasion include (1) replacement of mediastinal fat by soft-tissue attenuation; (2) compression or displacement of mediastinal vessels by tumor; (3) tumor contacting more than 90 degrees of the circumference of a structure such as the aorta or pulmonary artery (the greater the extent of circumferential contact, i.e., 180 degrees, the greater the likelihood of invasion); (4) more than 3 cm of contact between tumor and the mediastinum; and (5) obliteration of the mediastinal fat plane normally seen adjacent to most mediastinal structures (see eFig. 18-32 and eFig. 18-37 ). However, individually, these findings are unreliable for differentiating mediastinal invasion from anatomic contiguity.

CT is of little value in the diagnosis of malignant pleural effusion because this diagnosis requires cytologic confirmation. However, in some patients, CT may show diffuse or nodular pleural thickening with (see eFig. 53-4 ) or without (see eFig. 18-35 and eFig. 53-5 ) pleural liquid. PET may show tracer uptake in patients with malignant pleural effusion (see eFig. 53-4 ), even when CT does not show evidence of nodular pleural thickening and when cytologic analysis is negative.

The diagnosis of mediastinal lymph node metastasis by CT is determined largely by node size, although the preservation of fat within lymph nodes often reflects the absence of pathologic infiltration, whereas necrosis may imply pathologic infiltration even within normal-sized lymph nodes. By convention, a nodal short axis diameter of 1 cm or greater is considered to be abnormal in all node stations ( Fig. 18-16 ), except the subcarinal space. In general a tumor cannot be detected in normal-sized lymph nodes by CT, and there are no characteristic appearances allowing benign and malignant causes of nodal enlargement to be distinguished (see eFigs. 53-1 and 53-3 ). Pooled information from 35 studies published between 1991 and June 2006 evaluating the performance of CT scanning for noninvasive staging of the mediastinum has shown that this size threshold has a sensitivity of only about 51% and a specificity of 86% for diagnosing mediastinal lymph node metastases in patients with lung cancer. Furthermore, the accuracy of CT for detecting involvement of individual node groups is low, perhaps as low as 40%. In a more recent review of the accuracy of mediastinal lymph node staging using CT, a combination of studies evaluating 7368 patients found the median sensitivity and specificity for mediastinal lymph node metastases detection at chest CT of 55% and 81%, respectively.

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