Chapter 1 Thoracic Radiology
Imaging Methods, Radiographic Signs, and Diagnosis of Chest Disease
EXAMINATION TECHNIQUES AND INDICATIONS
The plain chest radiograph is the most commonly performed imaging procedure in most radiology practices, constituting between 30% and 50% of studies. The standard routine chest radiograph consists of an erect radiograph made in the posteroanterior projection and a left lateral radiograph, both obtained at full inspiration.
The target film distance is 6 feet. Chest radiographs should be exposed using a high kilovoltage peak (kvp) technique, usually in the range of 100 to 140 kvp (Fig. 1-1). With this technique, a grid or air gap is required to reduce scatter radiation. The main advantage of this technique is that the bony structures appear less dense, permitting better visualization of the underlying parenchyma and the mediastinum. The only drawbacks are the decreased detectability of calcified lesions and loss of bony detail.
Figure 1-1 Standard posteroanterior and lateral chest radiographs obtained at 140 kvp, a 12:1 grid, and an automated phototimed exposure. Notice the visibility of retrocardiac vessels and mediastinal structures and the companion shadow of the left clavicle (arrowheads).
Additional views of the chest may be required in special instances (Table 1-1). Shallow oblique radiographs (15 degrees) may be useful in confirming the presence of a suspected nodule. Forty-five-degree oblique radiographs are recommended for the detection of asbestos-related pleural plaques. Apical lordotic views (Fig. 1-2) project the clavicles above the chest, improving visualization of the apices and the middle lobe, particularly in cases of middle lobe atelectasis. Expiration chest radiographs can be used to detect air trapping or to confirm small pneumothoraces. Lateral decubitus radiographs are commonly used to determine the presence or mobility of pleural effusion. These views can also be obtained to detect small pneumothoraces, particularly in patients who are confined to bed and unable to sit or stand erect. Bedside portable examinations may account for up to 50% of chest radiographs obtained for hospital patients.
|Lordotic||Apical and middle lobe disease|
Figure 1-2 In an apical lordotic view, the clavicles are projected above the apices of the lungs. There is excellent demonstration of right middle lobe collapse.
The diagnostic quality of these images is usually limited because of the increased exposure time needed, which results in respiratory motion. Because the target film distance is considerably less than 6 feet, magnification occurs, particularly of the heart and anterior structures. Many very ill patients, including patients in intensive care units, must be radiographed at the bedside, resulting in radiographs with limited diagnostic information.
During the past decade, rapid advances in electronics and computer technology have created new possibilities for x-ray imaging, including specific receptor systems independent of film that permit image information to be recorded in digital form and displayed on picture archiving and communication system (PACS) workstations. These systems include photostimulable phosphor computed radiography (PPCR) systems and selenium-based digital chest systems. A new generation of direct readout x-ray detectors based on thin film transistor arrays has emerged, offering unsurpassed image quality from a compact digital detector.
Storage PPCR systems employ a reusable imaging plate in place of the traditional screen film detector. These were first introduced in the middle to late 1980s and have been used for bedside radiography. The linear response of photostimulable phosphors over an extremely wide range of radiation exposures makes their application particularly good for portable radiography. A generation of digital x-ray systems based on flat panel detectors has emerged that provides good image quality and very rapid direct access to digital images. Most of these systems use large-area, thin-film transistor arrays. They offer compact packaging and direct connection to digital imaging networks. Image quality from digital acquisition systems is equivalent or better than standard film radiography.
Fluoroscopy has become rather obsolete with the widespread application of computed tomography (CT) (Table 1-2). Fluoroscopy is mainly restricted to the evaluation of diaphragmatic motion. The patient is placed in an oblique position so that both hemidiaphragms can be visualized simultaneously. In patients with diaphragmatic paralysis, the affected hemidiaphragm moves up during a rapid inspiratory maneuver (e.g., a sniff).
|Major airways, trachea|
Computed tomography typically is used as a diagnostic study, usually after a standard chest radiograph is obtained or when the chest radiograph result is considered to be abnormal (Box 1-1). Indications for CT include staging of lung carcinoma; a solitary pulmonary nodule, mass, or opacity; diffuse infiltrative lung disease; widened mediastinum, a mediastinal mass, or other abnormality of the mediastinum; an abnormal hilum; pleural abnormalities or the need to differentiate pleural from parenchymal abnormalities; chest wall lesions; trauma; and diagnosis of pulmonary embolism. CT may also be used for the detection of occult disease. Indications include detection of metastatic disease in tumors with a propensity for metastases to the lungs; hemoptysis or suspected bronchiectasis; evaluation of the thymus in patients who have myasthenia gravis; evaluation of patients with endocrine abnormalities that are associated with a suspected lung tumor or parathyroid adenoma; search for an unknown source of infection, especially in the immunocompromised population; evaluation of the pulmonary parenchyma in patients with normal chest radiographs and suspected diffuse infiltrative lung disease or emphysema; and suspicion of aortic dissection and other vascular abnormalities.
Box 1-1 Common Indications for Computed Tomography
ABNORMAL CHEST RADIOGRAPH
OCCULT DISEASE: NORMAL CHEST RADIOGRAPH
CT scans should be performed during deep inspiration at total lung capacity. For routine helical CT of the chest, contiguous 2.5- to 3-mm sections are recommended. High-resolution CT (HRCT) using thinner 1- to 1.25-mm sections can be used to study the fine details of the pulmonary parenchyma. A short scan time of 0.8 to 1 second is necessary to reduce the effect of motion. On routine studies, the field of view should be adjusted to the size of the thorax, but smaller fields of view may be selected for smaller anatomic parts that require study.
The routine approach is to obtain at least three window settings, used for the lung parenchyma, the mediastinum, and the bony structures. Suggested settings for the mediastinum are window level of +30 to +50 and window width of +350, and settings for the lung are a window width of +1500 and a window level of 2500 to 2700. The algorithm of reconstruction may be modified for the mediastinum or lung. For the mediastinum, a smoothing or standard algorithm is recommended. This is also sufficient for routine studies of the lung. However, HRCT requires an algorithm with high spatial resolution that corresponds to the bone algorithm on most scanners.
With thorough knowledge of mediastinal and hilar anatomy, contrast material may not be required for routine CT of the thorax, especially with a thinner slice thickness of 2.5 mm or less. However, contrast enhancement may be necessary for the evaluation of known or suspected vascular abnormalities (e.g., aortic aneurysm or dissection, pulmonary embolism), for evaluation of an abnormal hilum, or for abnormalities of the pleura. Approximately 100 to 150 mL at an injection rate of 2-4 mL/sec using an agent with 30% to 40% concentration of iodine is recommended. In hemodynamically normal individuals, the transit time of contrast from an antecubital vein to the right heart is about 3 seconds, 6 seconds to the pulmonary arteries, 9 seconds to the left heart, and 12 to 15 seconds to the major arteries. Although transit times vary among patients, we recommend as a routine a delay of at least 25 seconds between the onset of the injection and the first image. A power contrast injector should be used. Bolus tracking techniques allow for improved contrast opacification.
Improvement in scanner technology has led to the introduction of spiral or helical volumetric CT (Box 1-2). These CT scanners acquire data continuously and as the patient is transported through the scanner during a single breath hold (Fig. 1-3). Multidetector helical CT (MDCT) revolutionized thoracic imaging by providing near-isocubic volumetric scanning. Initial multidetector imaging involved four slice detectors that are still popular today. However, this technology has expanded to 16- and 64-slice detectors as well as dual source scanners allowing even shorter scan times. With the newer technology, a patient’s entire thorax is scanned in less than 10 seconds. MDCT slice acquisition provides a thinner slice thickness of an entire original dataset, with the elimination of interscan gaps and minimal respiratory motion. Capabilities include multiplanar imaging with little or no stair-stepping artifact on coronal, sagittal, and three-dimensional images. The greatest impact of MDCT in imaging the thorax involves reconstruction of the vasculature system and airways, and it provides comprehensive imaging of trauma patients (Fig. 1-4). Pulmonary embolism studies are obtained in shorter imaging sessions, improving on motion artifact and resolution of smaller subsegmental vessels.
Box 1-2 Indications for Helical and MDCT Computed Tomography
Figure 1-3 Helical CT scan principle.
(From Kalender WA, Seissler W, Klotz E, Vock P: Spiral volumetric CT with single-breath-hold technique, continuous transport, and continuous scanner rotation. Radiology 176:181–183, 1990).
Positron Emission Tomography
Increased metabolism of neoplastic cells can be detected by 2-[18F]fluoro-2-deoxy-d-glucose (FDG) positron emission tomography (PET) imaging, and FDG-PET is therefore useful in the detection of malignancy in pulmonary nodules, masses, and lymph nodes. PET is routinely used for the evaluation of a single pulmonary nodule equal to or greater than 1 cm in diameter and for staging and restaging of neoplasms, such as lung carcinoma, breast cancer, lymphoma, and melanoma, which commonly involve the thorax. A false-negative result for a pulmonary nodule may occur if the nodule is less than 1 cm. Tumors of relatively low metabolic activity, such as carcinoid tumor and bronchioloalveolar cell carcinoma, also may produce false-negative results. Because FDG-PET images the rate of glycolysis in tissues within the body, false-positive results can be seen in cases of infection and inflammation. False-negative results have been obtained for patients with focal infections such as mycobacterial disease and organizing pneumonia.
The implementation of FDG-PET has significantly improved the radiologic staging of lung cancer (Fig. 1-5). The characterization of lymph node disease by CT is limited by the use of size criteria to detect abnormal nodes. Lymph nodes are interpreted as abnormal if their short-axis diameter exceeds 1 cm. Because of reliance on size criteria, enlarged lymph nodes due to inflammatory or infectious disease are frequently misinterpreted as neoplastic. Early metastases in small nodes are not detected by CT. FDG-PET improves the specificity of lymph node disease detection by better identifying tumor involvement based on increased metabolic activity rather than anatomic enlargement. However, PET also has limitations when it comes to lymph node size. Detection of metastatic foci may also be limited by PET because of camera resolution. Lymph nodes with metastases that measure 5 mm in diameter or less may go undetected. False-positive results can be obtained in cases of granulomatous disease or silicosis. In the staging of lung cancer, mediastinoscopy should be performed on any patient with positive lymph nodes on FDG-PET to avoid erroneously overstaging a patient who has “hot” reactive nodes.
Figure 1-5 Dual FDG-PET/CT scan of lung cancer. A, Fusion image from dual PET/CT scan shows increased FDG uptake in the left upper lobe nodule. B, Subcentimeter hilar lymph node can be seen on the CT scan (arrow). Because its size is less than 1 cm, this lymph node is not normally identified on CT. C, Fusion image from a dual PET/CT scan demonstrates increased FDG uptake in this node (arrow). On surgical resection, the lymph node was found to contain metastatic adenocarcinoma.
Because of its relatively low spatial resolution, FDG-PET imaging should be interpreted with cross-sectional imaging such as a CT. Studies have shown that interpretation of PET improves when CT images are also available. The added application of fusion imaging of PET and CT by computer registration or dual PET/CT scanning has lead to even better radiologic sensitivity and specificity for detecting lymph node metastases and recurrent tumor. With dual PET/CT imaging, the CT scan is used for attenuation correction, thereby providing the high spatial resolution needed to better localize areas of increased FDG uptake on PET.
Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) has not had extensive application in imaging the thorax, mainly because of problems due to motion artifacts caused by cardiac and respiratory movements. The normal lung does not produce an MR signal because of magnetic susceptibility effects. However, MRI does provide excellent images of the mediastinum and the chest wall, and it does permit direct imaging in the coronal, sagittal, and axial planes. General indications for MRI in the chest include evaluation of the mediastinum or vascular structures in patients in whom contrast media is contraindicated; diagnosis of aortic dissection and congenital abnormalities of the aorta; evaluation of superior sulcus tumors; imaging of chest wall lesions and brachial plexus abnormalities; staging of lung carcinoma with particular reference to direct chest wall and mediastinal invasion; and evaluation of posterior mediastinal masses (Box 1-3).
Box 1-3 Indications for Magnetic Resonance
Some general recommendations can be made in regard to technique. Techniques can be varied according to the clinical indication. Typically, a body coil is used, and images are obtained in the axial plane using two different spin-echo sequences. With high field-strength magnets, electrocardiographic (ECG) gating should be employed. T1-weighted, multislice, single-echo (i.e., echo time [TE] values of 15 to 30 msec) sequences are always obtained, and a T2-weighted sequence with two echoes (i.e., TE of 60 to 100 msec) is obtained in most instances. T1-weighted images give information concerning diagnosis of masses and provide the best information about vascular anatomy. The T2-weighted images may render fluid collections distinguishable from solid masses and may help separate tumor from fibrosis (Fig. 1-6). Gadolinium contrast administration is often helpful in distinguishing benign from malignant conditions.
Figure 1-6 MRI of a bronchogenic cyst. A, T1-weighted image shows low signal intensity of the right paratracheal mass. The low signal intensity results from the water content of the cyst. B, On the T2-weighted image, the cyst has greater signal intensity than fat or muscle because of the long T2 value of water.
In addition to ECG gating, several other techniques can be used to limit or correct motion artifacts. Respiratory compensation and presaturation (i.e., destroying the magnetization of incoming blood by repeatedly imposing radiofrequency pulses to the areas adjacent to the image volume) to eliminate the artifacts related to blood motion are frequently used. Rapid scanning techniques (i.e., gradient recalled acquisition of the steady-state [GRASS] or fast low-angle shot [FLASH]) that allow for acquisition of single or multiple images during a single breath hold have been developed. These techniques use decreased flip angles, gradient refocused echoes, and short repetition time (TR) and TE values.
Because MRI is often used as a problem-solving procedure, it needs to be correlated carefully with CT scans. For this reason, images are usually obtained in the transaxial plane. However, it is possible to have direct MRI in the sagittal and coronal planes. The benefits of imaging in the sagittal and coronal planes are that they better elucidate structures oriented longitudinally, and they reduce the chance of misinterpretation of findings due to volume averaging.
Fast imaging techniques, sometimes referred to as cine MRI, are available for imaging vascular structures and diagnosing vascular abnormalities. They are discussed in more detail by Miller in Cardiac Radiology: The Requisites, which is part of a series dealing with vascular diseases and cardiac imaging.
Trachea and Main Bronchi
The trachea is a midline structure that usually is 6 to 9 cm long. The wall contains horseshoe-shaped cartilage rings at regular intervals, but the posterior wall is membranous. The upper limits for coronal and sagittal diameters are 25 and 27 mm for men and 21 and 23 mm for women. The lower limit of normal in both dimensions is 13 mm in men and 10 mm in women. The trachea divides into two major bronchi at the carina. The carinal angle usually is about 60 degrees, but a wide range of 40 to 75 degrees can be seen in normal adults. The right main bronchus has a more vertical course than the left, and its length is considerably shorter. The air columns of the trachea, both major bronchi, and the intermediate bronchus are usually visible on well-exposed standard radiographs of the chest in the frontal projection (Figs. 1-7 and 1-8). The right lateral and posterior walls of the trachea are identifiable on posteroanterior and lateral chest radiographs as vertically oriented linear opacities, called the right paratracheal and posterior tracheal stripes. They are described in more detail in the “Mediastinum” section.
Figure 1-7 Tracheal and bronchial anatomy on standard posteroanterior (A) and lateral (B) views, which show the trachea (t), carina (c), right main bronchus (r), left main bronchus (l), right paratracheal stripe (arrowhead), right intermediate bronchus (large black arrow), and posterior paratracheal stripe (arrowhead in B).
Figure 1-8 Anteroposterior tomogram shows the anatomy of the tracheobronchial tree, including the venous confluence (black arrows), trachea (t), carina (c), right main bronchus (r), and left main bronchus (l). White arrow is the right paratracheal stripe, and the black arrow is the right intermediate bronchus, and the large arrow head is the venous confluence.
Lobar Bronchi and Bronchopulmonary Segments
Table 1-3 summarizes the bronchopulmonary segments of the right and left lung.
|Right Lung Segments||Left Lung Segments|
|Upper Lobe||Upper Lobe|
|1. Apical||1 and 2. Apical posterior|
|2. Anterior||3. Anterior|
|3. Posterior||4. Superior lingula|
|Middle Lobe||5. Inferior lingula|
|4. Lateral||Lower Lobe|
|5. Medial||6. Superior|
|Lower Lobe||7 and 8. Anteromedial basal|
|6. Superior||9. Lateral basal|
|7. Medial basal||10. Posterior basal|
|8. Anterior basal|
|9. Lateral basal|
|10. Posterior basal|
The bronchus to the right upper lobe (Fig. 1-9) arises from the lateral aspect of the mainstem bronchus, approximately 2.5 cm from the carina. It then divides into three branches—the anterior, posterior, and apical—each supplying a segment of the right upper lobe. The intermediate bronchus continues distally for 3 to 4 cm from the takeoff of the right upper lobe bronchus and bifurcates to become the bronchi to the middle and lower lobes. The middle lobe bronchus arises from the anterolateral wall of the intermediate bronchus almost opposite the origin of the superior segmental bronchus of the lower lobe. It then bifurcates into lateral and medial segments.
Figure 1-9 A, Anatomy of right bronchial tree, including the bronchial segments. B and C, Segmental anatomy viewed from the medial and lateral surfaces of the right lung.
The superior segmental bronchus is the first segment originating in the lower lobe. It arises from the posterior aspect of the lower lobe bronchus immediately beyond its origin and directly posterior to the takeoff of the middle lobe bronchus. Four basal segments subsequently arise from the root bronchus of the right lower lobe: anterior, lateral, posterior, and medial segments. This is the order of the basal bronchi from the lateral to the medial aspect of the hemithorax on a standard posteroanterior radiograph.
The left upper lobe bronchus (Fig. 1-10) arises from the left main bronchus and then bifurcates or trifurcates. The upper division is the main left upper lobe bronchus and the lower division is the lingular bronchus. The upper division almost immediately divides into two segmental branches, the apical posterior and anterior. The lingular bronchus is analogous to the middle lobe bronchus of the right lung. The lingular bronchus then bifurcates into superior and inferior divisions or segments.
Figure 1-10 A, Anatomy of the left bronchial tree, including the bronchial segments. B and C, Segmental anatomy viewed from the medial and lateral surfaces of the left lung.
The divisions of the left lower lobe bronchus are in name and anatomic distribution identical to the right lower lobe bronchus, except that there are usually three basal bronchi: anteromedial, lateral, and posterior. The distribution from lateral to medial on the frontal radiograph is anteromedial, lateral, and posterior. The lingular bronchus, like its corollary on the other side, the middle lobe bronchus, usually comes off directly anterior to the takeoff of the superior segmental bronchus of the lower lobe.
The main pulmonary artery originates in the mediastinum at the pulmonic valve and passes upward, backward, and to the left before bifurcating within the pericardium into the short left and long right pulmonary arteries (Figs. 1-11 and 1-12). The right pulmonary artery courses to the right behind the ascending aorta before dividing behind the superior vena cava and in front of the right main bronchus into a right upper branch (i.e., truncus anterior) and the descending or interlobar branch. The interlobar artery subsequently divides into segmental arteries to the right middle and right lower lobes. The higher left pulmonary artery passes over the left main bronchus. It may give off a separate branch to the left upper lobe or, more commonly, continues directly into a vertical left interlobar or descending pulmonary artery from which the segmental arteries to the left upper and lower lobes arise directly. The left descending or interlobar artery lies posterior to the lower lobe bronchus.
Figure 1-11 Central pulmonary vasculature: main pulmonary artery (1), right pulmonary artery (2), truncus anterior (3), right interlobar artery (4), left pulmonary artery (5), right superior pulmonary vein (7), right inferior pulmonary vein (8), left superior pulmonary vein (9), inferior pulmonary veins (10), and left atrium (14).
(From Genereux GP: Conventional tomographic hilar anatomy emphasizing the pulmonary veins. Am J Roentgenol 141:1241–1257, 1983.)
Figure 1-12 Central pulmonary vasculature. A, Left pulmonary artery (lp), left interlobar artery (li), right interlobar artery (ri), and sternum (s). Small black arrows on right indicate the right superior pulmonary vein that forms the upper border of the right hilum and a V configuration with the right interlobar artery. Lower black arrow on the right points to the horizontal course of the inferior pulmonary vein. White arrow on the left indicates the left superior pulmonary vein. B, Lateral view. Anterior portion of the hilar structures is made up mostly by the right pulmonary artery (upper black arrow). The left pulmonary artery (white arrow) is seen as a longitudinal structure arching over and passing posterior to the left upper lobe bronchus (lower black arrow). LD, left hemidiaphragm; RD, right hemidiaphragm; S, stomach bubble.
The upper limit of normal diameters for the pulmonary arteries have been determined in normal subjects on the basis of CT scans: main pulmonary artery, 28.6 mm; left pulmonary artery, 28 mm; and proximal right pulmonary artery, 24.3 mm. The right interlobar artery can often be measured on standard radiographs, with the intermediate bronchus serving as the medial border. The mean diameter is approximately 13 mm for men and approximately 12.5 mm for women. Another method for estimating changes in arterial caliber is the artery-to-bronchus index. Normally, the ratio of pulmonary artery to bronchus size at any point distal to the takeoff of the upper lobe bronchi is approximately 1.3:1 to 1.4:1. On CT scans, the more peripheral arteries can be visualized in the bronchovascular bundles, and the arterial bronchial index is approximately 1:1.
The right superior pulmonary vein drains the segmental veins of the right upper lobe and descends medially into the mediastinum to the upper and posterior aspect of the left atrium. After passing under the middle lobe bronchus, the middle lobe vein usually joins the left atrium at the base of the superior pulmonary venous confluence. The left superior pulmonary vein drains the left upper lobe and lingula and courses in an oblique fashion medially into the mediastinum to join the superior part of the left atrium. In the lower lobes, the right and left inferior pulmonary veins have a horizontal rather than an oblique course and drain into the left atrium medially. They form inferior pulmonary venous confluences.
The pulmonary hila, or roots, of the lungs contain bronchi, pulmonary and systemic arteries and veins, autonomic nerves, lymph vessels, and lymph nodes.
Standard Posteroanterior and Lateral Chest Radiography
The hila can be conveniently divided into upper and lower zones, and specific anatomic structures can be identified in each area (see Fig. 1-12). The upper part of the right hilum consists of the right superior pulmonary vein and the truncus anterior branch of the right pulmonary artery. A short segment of the upper lobe bronchus and the end-on anterior segmental artery and bronchus can often be identified. The lower portion of the right hilum is formed by the interlobar artery, which descends in a vertical manner and lies lateral to the intermediate bronchus. The horizontally oriented inferior pulmonary vein lies posteroinferior to the hilum. On the left side, the upper part of the left hilum is formed by the distal left pulmonary artery and the left superior pulmonary vein. The proximal left pulmonary artery is almost always higher than the highest point of the right interlobar artery, with the left hilum therefore being higher than the right. The lower portion of the left hilum is formed by the distal interlobar or descending artery and more caudally by the left inferior pulmonary vein. The air columns of the lingular and left lower lobe bronchus may be identified.
Occasionally, the venous confluences may be extremely prominent and produce vascular pseudotumors. This is particularly common in the right retrocardiac area when the inferior right venous confluence is prominent (see Fig. 1-8).
Understanding hilar anatomy on the lateral projection is critical (Fig. 1-13). The tracheal air column is always clearly visible and ends caudally in a rounded radiolucency that represents the distal mainstem or proximal left upper lobe bronchus seen end on. The right pulmonary artery is projected as a circular opacity anterior to this bronchus. The left pulmonary artery is tubular in configuration and arches over the left mainstem or left upper lobe bronchus. The right upper lobe bronchus can be identified approximately 1 cm above the left upper lobe bronchus. Between the right and left upper lobe bronchi, which are seen end on, is a thin, vertical, white line representing the posterior wall of the bronchus intermedius that courses inferiorly. It separates the lumen of the bronchus intermedius from the aerated right lung and the azygoesophageal recess posteriorly. The area beneath the left mainstem bronchus is sometimes referred to as the inferior hilar window. It should be clear and radiolucent. An opacity, particularly a rounded opacity, in this area suggests the presence of hilar or subcarinal adenopathy. Abnormalities of the hilum on standard radiographs may be increased opacity or changes in size, shape, or lobulation.
Figure 1-13 Hilar anatomy in the lateral view shows the trachea (t), right upper lobe bronchus (large white arrow), left upper lobe bronchus (white open arrowhead), right pulmonary artery (small white arrow), left pulmonary artery (large white arrowhead), posterior wall right intermediate bronchus (long white arrow), and inferior hilar window (curved white arrow).
The pulmonary hila are probably best evaluated with CT. They can be visualized with or without the use of intravenous contrast medium. However, dense opacification of the pulmonary or the hilar vessels simplifies interpretation. The bronchial tree is best assessed at wide windows (i.e., 1500 to 2000 Hounsfield units [HU]). Visualization of hilar structures is also improved by thin (2-3 mm) sections. The anatomy of the hila is illustrated in Figure 1-14, and mediastinal anatomy is shown later (see Figs. 1-23 and 1-24).
Figure 1-14 CT hilar anatomy. Sequential CT sections demonstrate anatomy of the major airways and central pulmonary vessels. ABSB, anterior basal segmental bronchus; AJL, anterior junction line; B1, apical segmental bronchus upper lobe; B2, anterior segmental bronchus upper lobe; B3, posterior segmental bronchus upper lobe; B6, superior segmental bronchus-right lower lobe and left lower lobe; BI, bronchus intermedius; LB, lingular bronchus; LBSB, lateral basal segmental bronchus; LDPA, left descending (interlobar) pulmonary artery; LLB, left lower lobe bronchus; LLLB, left lower lobe bronchus; LMB, left main bronchus; LULB, left upper lobe bronchus; MBSB, medial basal segmental bronchus; PBSB, posterior basal segmental bronchus; RIA, right interlobar artery; RLLB, right lower lobe bronchus; RMB, right main bronchus; RMLB, right middle lobe bronchus; RULB, right upper lobe bronchus; T, trachea.
Figure 1-23 Sequential CT slices from the thoracic inlet to the diaphragm show normal mediastinal and hilar anatomy. A, The slice shows the trachea (TR), thyroid gland (Th), right subclavian vein (RSV), right subclavian artery (RSA), esophagus (E), left subclavian vein (LSV), and left common carotid artery (LCCA). B, The slice shows the right brachiocephalic vein (RBV), brachiocephalic artery (BCA), left brachiocephalic vein (LBV), and left subclavian artery (LSA). C, The slice shows the right brachiocephalic vein and superior vena cava junction (RBV-SVC). D, The slice shows the superior vena cava (SVC) and aorta (Ao). E, The slice shows the azygos vein (AzV). F, Slice shows the descending aorta (DAo). G, The slice shows the right superior pulmonary vein (RSPV), left main pulmonary artery (LMPA), internal mammary vessels (IM), sternum (S), tracheal carina (C), and main pulmonary artery (MPA). I, The slice shows the left superior pulmonary vein (LSPV). J, The slice shows the right pulmonary artery (RPA), descending (interlobar pulmonary artery) (DLPA), and left main bronchus (LMB). K, The slice shows a right atrial appendage (RAA). L, The slice shows the right interlobar artery (RIA), right atrium (RA), pulmonary outflow tract of the right ventricle (POTRV), and left coronary artery (LCA). M, The slice shows the left atrium (LA), right inferior pulmonary vein (RIPV), right ventricle (RV), left ventricle (LV), and left inferior pulmonary vein (LIPV). N, The slice shows the pericardium (PC) and coronary sinus (CS). O, The slice shows the inferior vena cava (IVC). P, The slice shows the liver (L) and stomach (ST). Q, The slice shows the crus of the diaphragm (CR), the liver (L), and the stomach (S). A normal-sized right paratracheal lymph node (small black arrowhead), aorticopulmonary (anteroposterior window) lymph node (large black arrowhead), and subcarinal lymph node (curved white arrow) can be seen.
Figure 1-24 MRI of mediastinal and hilar anatomy. A-G, Sequential axial images from the thoracic inlet to the cardiac apex. H-M, Coronal images proceeding from anterior to posterior. N-Q, Sagittal images from right to left. Ao, aorta; AAo, ascending aorta; APW, aorticopulmonary window; BA, brachial artery; BCA, brachiocephalic artery; DAo, descending aorta; E, esophagus; LA, left atrium; LBV, left brachiocephalic vein; LCCA, left common carotid artery; LMB, left main bronchus; LMPA, left main pulmonary artery; LSA, left subclavian artery; LSPV, left superior pulmonary vein; LV, left ventricle; MPA, main pulmonary artery; POT, pulmonary outflow tract; RA, right atrium; RBI, right bronchus intermedius; RBV, right brachiocephalic vein; RIA, right interlobar artery; RIB, right intermediate bronchus; RMB, right main bronchus; RMPA, right main pulmonary artery; RSPV, right superior pulmonary vein; RTA, right truncus anterior artery (right upper lobe branch); RV, right ventricle; SVC, superior vena cava; TR, trachea.
Magnetic Resonance Imaging
MRI has several advantages in imaging the hila. Contrast is not required because flowing blood within hilar vessels generates no signal and can be easily differentiated from the lymph nodes and masses in the hila. The anatomy is identical to that described on CT in the axial plane. MRI also has the advantage of direct imaging in the coronal and sagittal planes.
The spatial resolution of MRI is less than that of CT, and on T1-weighted, spin-echo images, signal may be generated from soft tissues in the normal hilum that can be confused with enlarged nodes or masses. This signal is most likely caused by focal hilar fat and normal-sized lymph nodes. For these reasons, CT with contrast is the preferred method for evaluating the hila. However, MRI may be useful, particularly in patients who cannot tolerate intravenous contrast.
The pulmonary acinus is often considered an anatomic and functional unit of the lung parenchyma (Fig. 1-15). It refers to the gas-exchanging unit of the lung and is defined as that portion of the lung distal to the terminal bronchiole (i.e., the last purely conducting airway), which is composed of the respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli. There has been considerable debate about whether the acinus is radiologically visible. Experimentally, the acinus can be filled with bronchographic contrast medium, and the radiographic opacities that are produced are nodular opacities with a rosette appearance and a diameter of approximately 6 to 10 mm. However, it is debatable whether such “acinar shadows” can be identified with confidence in disease processes creating opacification in the lungs in living patients.
The secondary lobule is defined as the smallest discrete portion of the lung that is surrounded by connective tissue septa (Fig. 1-16). It is composed of three to five terminal bronchioles with their accompanying airways and parenchyma. The shape is usually polyhedral, and it usually is 1 to 2.5 cm in diameter. The secondary lobule has been recognized by some investigators as the radiographically visible basic structural unit of the lung. It is certainly the unit of the lung that is readily identified on HRCT. However, the distribution of lobules is not uniform throughout the lung, and the septa are better developed and more numerous in the lateral and anterior surfaces of the lower lobes. The secondary pulmonary lobule consists of core structures, which are the bronchus and accompanying pulmonary artery, and peripheral structures within the interlobular septa, which are the pulmonary veins and lymphatics.
Figure 1-16 A, Secondary pulmonary lobule. Schematic drawing shows the pulmonary arteriole and airway in the center of the lobule. Pulmonary veins lie in the interlobular septum. B, Photograph of cut surface of inflated fixed lung. The margin of the secondary pulmonary lobule is formed by the interlobular septum, which is continuous with the pleural surface (single arrow). The pulmonary arteriole and airway are seen in the center of the lobule (three arrows), the pulmonary veins in the septa (two arrows).
(A, From Netter FH: Atlas of Human Anatomy. Basel, Novartis, 1989; B, from Groskin SA: Heitzman’s the Lung: Radiologic Pathologic Correlations, 3rd ed. St. Louis, Mosby, 1993.)
In diffuse infiltrative (interstitial) lung diseases (see Chapter 7), the lobular architecture can often be readily identified on HRCT, and the relationship of the disease process to the center or the periphery of the lobule may be helpful in the diagnosis. However, lobular architecture is impossible to appreciate on standard chest radiographs.
The pleurae consist of the parietal and visceral layers. The pleura is not of sufficient thickness to be visible on standard chest radiography. The pleura becomes visible when it is thickened, particularly over the lateral surfaces of the lungs and over the convexity, but such thickening cannot be appreciated along the mediastinal or diaphragmatic surfaces.
Between the lobes, contiguous layers of visceral pleura, called the interlobar fissures, separate individual lobes and can be visualized on standard chest radiographs (Fig. 1-17) and on CT. The fissures may or may not be complete, and incomplete fissures allow collateral air drift or spread of disease from one lobe to the other. The major or oblique fissures separate the upper and, on the right, the middle lobe from the lower lobes. They extend from about the level of the fifth thoracic vertebra obliquely downward and forward, roughly paralleling the sixth rib to the diaphragm a few centimeters behind the anterior costophrenic angle. The minor or horizontal fissure separates the anterior segment of the right upper lobe from the middle lobe and lies in a horizontal plane at about the level of the fourth rib anteriorly. On a lateral chest radiograph, the posterior extent of the minor fissure is sometimes projected behind the hilum and the right major fissure due to the undulating course of the fissures. The position of the interlobar fissures is critical in the diagnosis of pulmonary volume changes such as lobar collapse. It is uncommon to see the normal major fissure on a frontal projection, and if it is visualized, it usually indicates thickening or fluid within the fissure or an abnormal position of the fissure due to volume loss and atelectasis.
Figure 1-17 Interlobar fissures. Posteroanterior (A) and lateral (B) chest radiographs of a patient with congestive heart failure show a minor fissure (small arrowheads) and major fissures (large arrowheads).
The pleural fissures can usually be identified on CT (Figs. 1-18 and 1-19