THE HEART AND MEDIASTINUM
Imaging of the heart will be considered under the following headings:
Simple X-ray
Screening
Cardiac catheterisation
Angiocardiography
Coronary arteriography
Ultrasound
Isotope scanning
Computed tomography
MRI.
Simple X-rays
A simple X-ray of the chest is usual as the first imaging investigation in cases of heart disease, because it yields vital information concerning the size of the heart, enlargement of individual chambers and condition of the lung fields. All these features are important in the assessment of the nature of the specific heart disease and its severity. An initial chest X-ray also forms a baseline against which future progress or deterioration can be measured.
Size of heart. This is measured by the cardiothoracic (CT) ratio. The maximum transverse diameter (TD) of the heart is compared with the maximum transverse diameter of the thorax (Fig. 3.1). In normal adults this is less than 50% in a film taken at the standard
Shape of heart. The cardiac contour has characteristic appearances in specific conditions depending on the chambers mainly enlarged.
Left ventricular enlargement is seen in hypertension, aortic valve disease and other conditions where the main burden is on the left
Fig. 3.1 The assessment of cardiac enlargement. The cardiac diameter should be the maximum cardiac diameter (r + 1). The transverse thoracic diameter is measured in a variety of ways. Here it is measured as the maximum internal diameter of the thorax.
ventricle. It manifests by enlargement of the apical region of the heart in both the PA and lateral projections (Figs 3.2A and 3.3).
Left auricular enlargement is seen characteristically in mitral valve disease, when it enlarges backwards and to the right, appearing as added density superimposed on the central part of the heart shadow in the PA view. It projects backwards and slightly upwards in the lateral view, presenting a marked impression on the barium filled oesophagus (Figs 3.2B and 3.4).
■Right ventricular enlargement may also be seen in mitral disease because of the increased pulmonary resistance secondary to the pulmonary congestion. It is also seen in many congenital cardiac lesions associated with pulmonary stenosis or left to right shunts, and in pulmonary conditions with chronic airways obstruction. The enlarged right ventricle is best seen in the lateral view where it fills in the normal retrosternal space, but is also identifiable when gross in the PA view where it straightens the left border and elevates the apex of the heart (Fig.
(B)
Fig. 3.2 Diagram showing directions of cardiac enlargement in standard PA and lateral views in (A) left ventricular, (B) left auricular and (C) right ventricular enlargement.
Fig. 3.3 Left ventricular enlargement in hypertension. The apex enlarges downwards and to the left.
Fig. 3.4 Backward displacement of barium-filled oesophagus by enlarged left auricle.
The lung fields
The appearance of the lung fields is of great importance in cardiac assessment since alterations in pulmonary haemodynamics are a feature of many forms of heart disease. Three types of change can be identified:
1. Congestion. This is due to pulmonary venous hypertension, which follows left heart lesions, resulting in back pressure on the lungs. Left ventricular failure or mitral disease are typical causes. The characteristic features are diversion of blood from the lower to the upper zones of the lung in the erect PA film. Normally the upper zone vessels appear smaller than those in the lower zone, but with pulmonary venous hypertension they become more prominent. As pressure rises pulmonary oedema develops involving the interstitial or alveolar spaces or both. Most characteristic are septal lines at the costophrenic angles representing fluid in the interlobular tissue planes (Fig. 3.6), and lamellar effusions in the parietal subpleural spaces or in the fissures. Alveolar oedema is often perihilar with blurring and haziness of the central lung areas Chat’s wing shadows’), but may be more widespread (Fig. 3.7). Pleural effusions may also develop, most commonly on the right, and may loculate, particularly in the fissures.
Fig. 3.5 Mitral stenosis. Note enlargement of the pulmonary conus and small aortic knuckle. The enlarged left auricle in this case projects as a rounded opacity to the right as well as backwards.
2. Pulmonary plethora. This is seen in conditions of high pulmonary flow mainly due to congenital left to right heart shunts and the degree
Fig. 3.6 Kerley B lines. Thickened interlobular septa in a patient with mitral valve disease seen as horizontal lines in the costophrenic angle (septal lines).
of plethora roughly parallels flow, provided pulmonary arterial pressure remains normal. Both arteries and veins become more prominent, particularly the arteries, with end on vessels close to the hilum being particularly well seen and distal vessels extending out to the lung periphery. Extreme pulmonary arterial hypertension may complicate any shunt whether at atrial, ventricular or aortopulmonary level (Eisenmenger syndrome), with the shunt then becoming right to left. Major degrees of pulmonary plethora with giant central arteries are seen where the shunt is of long standing before hypertension develops, as in some cases of ASD (atrial septal defect) where patients may reach their twenties before this occurs.
Fig. 3.7 Acute intra-alveolar pulmonary oedema with a bat’s wing distribution around the hila.
Pulmonary arterial hypertension (PAH) may also arise from the increased resistance produced by severe pulmonary venous hypertension, or from so-called primary PAH. It may also arise acutely from massive pulmonary embolus or from chronic multiple small pulmonary emboli. Finally it may be seen in chronic pulmonary disease with chronic airways obstruction.
3. Pulmonary oligaemia is seen when there is obstruction to the pulmonary outflow at or below the pulmonary valves, especially with a right to left shunt as in Fallot’s tetralogy (see Fig. 3.11)
Pericardial effusion
Pericardial effusions may be classified as:
1. Inflammatory
a. Tuberculous
b. Rheumatic
c. Suppurative
d. Viral
Fig. 3.8 Massive pericardial effusion enlarging heart and masking both hila.
2. Non-inflammatory
a. Heart failure
b. Uraemia
c. Myocardial infarction
d. Haemopericardium
i. Traumatic
ii. Post-cardiac or aortic rupture
3. Malignant
The radiological diagnosis can be difficult unless the amount of fluid exceeds 200 ml. However, above this amount the appearances are fairly typical. The heart becomes globular with masking of the hila (Fig. 3.8). In doubtful cases the diagnosis can be confirmed by ultrasound. CT or MRI will also readily show small effusions.
Constrictive pericarditis may follow viral or tuberculous pericarditis and is occasionally seen following collagen diseases and haemopericardium. There is thickening and rigidity of the pericardium leading to constriction and impaired filling of the heart. The right heart is mainly involved, leading to the clinical features of right heart failure. The heart may appear normal in size, but pericardial calcification, best seen in lateral view, is present in 50% of cases (Fig. 3.9).
Fig. 3.9 Constrictive pericarditis. Lateral view showing extensive pericardial calcification spreading over the front of the right ventricle and also encircling the heart in the atrioventricular groove. There is no calcium at the back as fluid cannot collect there.
Screening
Cardiac calcification is better seen at screening with an image intensifier than on a simple film. Calcification is most commonly seen in the mitral or aortic valves, but may also be seen in atheromatous coronary arteries, in the mitral annulus, or in a left atrium containing mural thrombus.
Screening the heart is essential for cardiac catheterisation, angiocardiography, left ventriculography, and coronary arteriography.
Cardiac catheterisation
This procedure requires the introduction of a catheter into the heart and manipulation of its tip under screen control so as to enter different chambers of the heart or to pass through abnormal defects or communications.
Fig. 3.10 Selective angiocardiograms. (A) The catheter tip is sited in the right ventricular outflow tract, just below the pulmonary valves (lateral view). (B) The catheter tip is sited in the main pulmonary artery (lateral view).
Right heart catheterisation. This can be performed percutaneously or after surgical exposure of a vein in the arm or groin, and passage of a catheter from there to the right atrium. The tip is manipulated into the right ventricle or beyond into the pulmonary artery or lung fields (Figs 3.10 and 3.11). If there is an atrial septal defect, ventricular septal defect, or patent ductus present, the catheter may be passed to the left atrium, left ventricle or aorta through the defect. The site of the catheter tip can be confirmed by taking pressure recordings during the investigation and also by taking blood samples which are examined for oxygen saturation. The pressure recordings and oxygen saturation levels are of vital importance in the diagnosis of the different forms of congenital heart disease.
Fig. 3.11 Right ventricular angiogram of Fallot’s tetralogy (AP view) showing hypoplastic RV outflow tract and deformed small pulmonary valve. The left ventricle and aorta are also filling through a ventricular septal defect (VSD). Diagram: A – aorta; P = pulmonary artery; RV = right ventricle; PV ■ pulmonary valve; OT = outflow tract.
In many cases simple cardiac catheterisation, by obtaining intracardiac pressures and oxygen saturations in the affected cardiac chambers, is sufficient for precise diagnosis. In other cases, angiocardiography is also performed to obtain an accurate anatomical diagnosis.
Left heart catheterisation. The usual technique of left heart catheterisation is for the radiologist to introduce a catheter percutaneously into the femoral artery and to pass it under screen control into the aortic arch and through the aortic valves into the left ventricle. Pressures are obtained from inside the ventricle and recorded, as is a withdrawal pressure trace into the aorta.
Angiocardiography
Angiocardiography may be performed from either the right or the left side of the heart. In venous angiocardiography the catheter tip was sited either in the superior or inferior vena cava, and a bolus of contrast medium injected at high pressure. Rapid films were taken demonstrating its passage through the various chambers of the heart.
Right heart angiocardiography is now more usually performed by siting the catheter tip in the right atrium. In many congenital heart conditions selective angiocardiography is performed with the catheter tip sited in the right ventricle or pulmonary outflow tract (Figs 3.10 and 3.11). The left side of the heart can also be shown by following the contrast through the lungs to the left auricle and ventricle on serial films. However contrast values are not so good as in direct left heart ventriculography.
For left heart angiocardiography the catheter tip is sited in the left ventricle by the method of transfemoral catheterisation just described. Injections are made through the catheter and rapid serial films taken. The left ventricle is best studied by video-filming, and this method is essential in the study of the ischaemic heart in coronary disease. Left ventricular function is assessed radiologically by noting the adequacy of left ventricular contraction and the presence of areas of dyskinesia. Mitral incompetence can be demonstrated at left ventriculography by opacification of the left auricle and the degree of incompetence quantified.
The aortic valves may be studied by injections made into the root of the aorta. With aortic incompetence there will be regurgitation
into the left ventricle; with aortic stenosis the narrowed jet of blood from the ventricle will be shown as a defect in the opacified aorta.
Coronary arteriography
Coronary arteriography involves direct injection of the coronary arteries. Specially shaped catheters are introduced percutaneously from the femoral artery and passed into the coronary ostia. Contrast medium is injected and video-films obtained. With modern apparatus it is possible to obtain excellent quality angiograms demonstrating stenotic or other lesions of the coronary vessels (Fig. 3.12). This is essential if surgery is being considered in patients with ischaemic cardiac symptoms. Arteriography can also be used to assess the results of coronary surgery.
Fig. 3.12 Selective left coronary arteriograin. (A) Normal appearance in AP view.
It is also possible to dilate an atheromatous stenosis during coronary arteriography using specially designed catheters with attached dilatable balloons.
Fig. 3.12 (B) Selective right coronary arteriogram. AP view shows stenosis of main right coronary artery (arrow).
Echocardiography
Ultrasound has developed into one of the most important techniques, for cardiac diagnosis.
M-mode (Figs 1.9,1.10), once widely used, has now been supplanted by two-dimensional echocardiography (2DE) (Figs 1.9, 1.11) which is also referred to as cross-sectional echocardiography (CSE).
2DE uses a sector scanner and by making the ultrasound beam oscillate very rapidly backwards and forwards through an arc of 80° the information from a large number of M scans is combined to produce an accurate moving image of the section scanned. A moving real-time image is thus obtained of the different sections or sectors of the heart being scanned. This can be studied at the time or at leisure on a videorecording.
Figure 3.13 illustrates diagrammatically the standard ‘long-axis’ and ‘short-axis’ views used. The 2DE provides definitive information on intracardiac anatomy in most neonates and children with congenital heart lesions (Fig. 3.14). It also provides definitive information in over 70% of adults’with acquired heart lesions (Fig. 1.11).
Fig. 3.13 CSE scanning of the heart. I The heart shown diagrammatically with the standard scanning planes indicated. (A) The long-axis view. (B) Short-axis view through the cavity of the left ventricle below the level of the mitral valve. (C) Short-axis view more cranially than in (B). II Long-axis view. HI Short-axis views. Ao ■ aorta, La = left atrium, Lv = left ventricle, L. ax. = long-axis view, Pa = pulmonary artery, Ra = right atrium, Rv = right ventricle, S. ax. = short-axis view.
Fig. 3.14 Modified apical four-chamber echocardiogram of a patient with a secundum atrial septal defect. The right-sided chambers are considerably enlarged. LA = left atrium, RA = right atrium,
The limiting factor in other adults is the rib cage obstructing ultrasound access. In such cases the more invasive transoesophageal echocardiogram may be indicated (see below).
Doppler principles in the form of pulsed and continuous wave Doppler are used to obtain flow profiles of direction and average velocity of flow and thus show physiological aspects of intracardiac shunts and valvular disease. This information is obtained from a small area of interest and does not show flow images. Colour flow mapping (CFM) gathers Doppler shift information from multiple sample volumes along each CSE scan line. These are colour-coded electronically to produce a display of flow direction and velocity. This will provide physiological information on areas of abnormal flow associated with intracardiac shunts or leaking or stenosed valves.
Transoesophageal echocardiography (TEE). This technique requires passage of an oesophagoscope with an ultrasound transducer at its tip which can be angled and placed at different levels behind the heart. This enables high quality CSE or CFM images to be obtained in cases where the conventional techniques are difficult or unsuccessful.
Radionuclide scanning
There are two main forms of cardiac scanning with radionuclides:
Myocardial imaging
Nuclear angiography.
Myocardial imaging can be achieved in two ways. Infarct scanning uses an isotope (“Tcm pyrophosphate) which accumulates in damaged myocardium, whilst perfusion scanning uses several isotopes, the most popular being thallium-201, which accumulates iormal but not in damaged or ischaemic myocardium (Fig. 3.15).
The main clinical indication for infarct scanning is to assess patients with inconclusive or equivocal evidence of a recent myocardial infarct. It is also used for prognostic purposes as there is some correlation between the size of the infarct abnormality and long term prognosis.
Nuclear angiography can also be done in two different ways.
First-pass technique involves rapid i.v. injection of a bolus of a simple radionuclide (99Tcm pertechnetate). Its passage through the cardiac chambers is then recorded. The method is most useful for the study of intracardiac shunts.
Multigated equilibrium studies (MUGA) follow injection of an isotope which remains fixed within the vascular space (“Tcm-labelled human serum albumen or red blood cells) thus labelling the total blood pool. Cardiac movement is then assessed by Unking the recorder to ECG gating over several hundred cardiac cycles, the data being accumulated into one totalised cycle. The images can then be transferred to a continuous loop of video film for viewing in cine mode.
Fig. 3.15 Myocardial ischaemia. 201Thallium citrate. (A) anterior image after exercise showing a large inferior defect. (B) 3 hours later, at equilibrium. Normal image. Thus there is exercise-induced transient myocardial ischaemia.
Abnormalities of ventricular function, particularly those due to ischaemic heart disease and cardiomyopathy are readily assessed by this method. Computer manipulation of the data also enables ventricular ejection fractions to be obtained.
Computed tomography
CT provides an excellent method of showing cardiac anatomy in the axial plane together with the great vessels and adjacent mediastinal structures. However intravenous contrast enhancement is usually necessary for most diagnostic purposes. This permits chamber identification and will show such features as intraventricular thrombus or the neck of a cardiac aneurysm (Fig. 3.16). Patency of coronary artery bypass grafts can also be shown by this technique.
MRI
Cardiac MRI is now well established and its use is expanding rapidly. The technique can provide information on both morphology and function. Its main attractions include high contrast between flowing blood and myocardium without the need for contrast medium or invasive techniques or radiation. It also readily provides multiplanar images including axial, coronal, sagittal and oblique views. As with CT the great vessels and adjacent mediastinal structures are also well shown. Images of the whole thoracic aorta can be easily obtained.
With modern techniques blood flow patterns caow be recognised and it is possible to quantify stenotic and regurgitant lesions as well as volume flow for cardiac output shunts etc.
Fig. 3.16 Left ventricular aneurysm. Contrast enhancement demonstrates neck of apical and posterior aneurysm communicating with left ventricular cavity.
Fig. 3.17 Oblique sagittal gated spin-echo MR image in a child with coarctation of the aorta (arrowed), aa = ascending aorta, da = descending aorta. (Courtesy of the Trustees of the
Fig. 3.18 Aortic stenosis on an oblique gated, gradient echo image (TE 22 ms) through the aortic arch during peak systole. Note signal loss, with a maximum measured length of
Fig. 3.19 Hypertrophic cardiomyopathy on transverse gated spin-echo images (TE 40 ms). (A) End-diastole. (B) End-systole. The myocardium is markedly thickened, with an associated pericardial effusion (straight arrows), la = left atrium, lv = left ventricle. (Adapted with permission from Jenkins & Isherwood 1987.)
Various cardiac lesions identified by MRI are illustrated in Figures 3.17, 3.18 and 3.19.
Ultrasound is of course cheaper and more easily available than MRI and is also radiation-free and non-invasive. It remains therefore the primary investigation of choice in most cardiac cases. MRI however is invaluable in cases where ultrasound fails or gives equivocal results.
DESEASEASES of the MEDIASTINUM
A number of mediastinal reflections are visible at conventional radiography that represent points of contact between the mediastinum and adjacent lung The presence or distortion of these reflections is the key to the detection and interpretation of mediastinal abnormalities Anterior mediastinal masses can be identified when the hilum overlay sign is present and the posterior mediastinal lines are preserved Widening of the right paratracheal stripe and convexity relative to the aortopulmonary window reflection indicate a middle mediastinal abnormality Disruption of the azygoesophageal recess can result from disease in either the middle or posterior mediastinum Paravertebral masses disrupt the paraspinal lines, and the location of masses above the level of the clavicles can be inferred by their lateral margins, which are sharp in posterior masses but not in anterior masses The divisions of the mediastinum are not absolute, however, referring to the local anatomy of the mediastinal reflections in an attempt to more accurately localize an abnormality may help narrow the differential diagnosis Identification of the involved mediastinal compartment helps determine which imaging modality might be appropriate for further study
In the era of cross-sectional imaging, mediastinal abnormalities can easily he identified However, these abnormalities often manifest initially at conventional radiography Chest radiography is a very common examination, and radiographic identification of an unexpected mediastinal mass is important Knowledge of the normal mediastinal reflections that can be appreciated at conventional radiography is crucial to identifying a mediastinal mass These mediastinal reflections can also help identify the location of a mass, thereby aiding in differential diagnosis and possibly influencing the choice of modality for further assessment
Dividing the Mediastinum
The mediastinum is often divided into convenient compartments in an attempt to develop a differential diagnosis. However, there are no physical boundaries between compartments that limit disease.
Anatomists divide the mediastinum into four parts. The mediastinum is divided into superior and inferior compartments by an imaginary line traversing the manubriosternal joint and the lower surface of the fourth thoracic vertebra. The inferior compartment is further subdivided into three parts: the middle mediastinum, which contains the pericardium and its contents as well as the major vessels and airways; the anterior mediastinum, which lies anterior to the middle mediastinum and posterior to the sternum; and the posterior mediastinum, which lies posterior to the middle mediastinum and anterior to the thoracic vertebral column (1). A popular modification of this method divides the entire mediastinum into anterior, middle, and posterior compartments but does not recognize a separate superior compartment (2).
The Felson method of division is based on findings at lateral chest radiography. line extending from the diaphragm to the thoracic inlet along the back of the heart and anterior to the trachea separates the anterior and middle mediastinal compartments, whereas a line that connects points
Heitzman (4) divided the mediastinum into the following anatomic regions: the thoracic inlet, the anterior mediastinum, the supraaortic area (above the aortic arch), the infraaortic area (below the aortic arch), the supraazygos area (above the azygos arch), and the infraazygos area (below the azygos arch).
In any method used to divide the mediastinum, the divisions are theoretic rather than physical. Therefore, disease can spread from one compartment to another, and some diseases do not occur exclusively in any one compartment. It is often more instructive to determine precisely where an abnormality lies. However, for ease of classification and for practicality, we have adopted the modified anatomic method of dividing the mediastinum (ie, anterior, middle, and posterior compartments with no separate superior compartment).
Anterior Mediastinum
Anatomy
The anterior mediastinum is bounded anteriorly by the sternum; posteriorly by the pericardium, aorta, and brachiocephalic vessels; superiorly by the thoracic inlet; and inferiorly by the diaphragm (Fig 1). Its contents include the thymus, lymph nodes, adipose tissue, and internal mammary vessels (1,5,6). The thyroid gland (if it extends into the mediastinum) is traditionally considered an anterior mediastinal compartment structure. Disease of any of the contents of the anterior mediastinum may result in a mass; thus, knowledge of the normal contents of the anterior mediastinum aids in developing a differential diagnosis once a mass has been identified. Masses may be subdivided into (a) prevascular masses and (b)precardiac masses that are in contact with the diaphragm (Table 1).
Table 1. Anterior Mediastinal Masses
· Prevascular masses Lymphadenopathy Retrosternal goiter
· Thymic lesions (thymoma, carcinoma, hyperplasia, cysts, thymolipoma)
· Germ cell lumor Precardiac masses in contact with the diaphragm
· Epicardial fat pad
· Diaphragmatic hump
· Morgagni hernia
· Pleuropericardial cysts*
· Lymph node enlargement Rare lesions
· Lymphatic malformations
· Hemangiomas
Figure 1. Drawing illustrates the anterior mediastinum (outlined in black).
Anterior Junction Line
The anterior junction line is seen at posteroanterior chest radiography. The line is formed by the anterior apposition of the lungs and consists of the four layers of pleura separating the lungs behind the upper two-thirds of the sternum (Fig 2). There is a variable amount of fat between these layers that can affect the thickness of the anterior junction line (5), which can be seen in approximately 25% of examinations. The line runs obliquely from upper right to lower left and does not extend above the manubriosternal junction. These properties help differentiate the anterior junction line from the posterior junction line (discussed later) (7).
Figure 2a. Anterior junction line, (a) Posteroanterior chest radiograph demonstrates the anterior junction line (arrow), (b) Computed tomographic (CT) scan shows the four layers of pleura that constitute the anterior junction line (arrow). The interface between aerated lung and pleura allows the line to be appreciated at conventional radiography (cf a).
Figure 2b. Anterior junction line, (a) Posteroanterior chest radiograph demonstrates the anterior junction line (arrow), (b) Computed tomographic (CT) scan shows the four layers of pleura that constitute the anterior junction line (arrow). The interface between aerated lung and pleura allows the line to be appreciated at conventional radiography (cf a).
Anterior mediastinal masses in the prevascular region can obliterate the anterior junction line, although it is usually the preservation of more posterior lines at radiography that helps identify the location of an anterior mediastinal mass.
The hilum overlay sign (3) is present when the normal hilar structures project through a mass, such that the mass can be understood as being either anterior or posterior to the hilum (Fig 3).
Preservation or disruption of posterior mediastinal lines can help further clarify the location of the mass.
Figure 3a. Hilum overlay sign in a patient with lymphoma, (a) Posteroanterior chest radiograph clearly depicts the hila (white arrow), which indicates that the mass is either anterior or posterior to the hila. In addition, the descending aorta is clearly seen (black arrow), indicating that the mass is not within the posterior mediastinum, (b) Chest CT scan demonstrates an anterior mediastinal mass. The anterior junction line is obliterated, whereas the lung interfaces with the hilar vessels (arrow) and aorta (arrowhead) are preserved.
Figure 3b. Hilum overlay sign in a patient with lymphoma, (a) Posteroanterior chest radiograph clearly depicts the hila (white arrow), which indicates that the mass is either anterior or posterior to the hila. In addition, the descending aorta is clearly seen (black arrow), indicating that the mass is not within the posterior mediastinum, (b) Chest CT scan demonstrates an anterior mediastinal mass. The anterior junction line is obliterated, whereas the lung interfaces with the hilar vessels (arrow) and aorta (arrowhead) are preserved.
The craniocaudal location and tissue density of a mass may also help in developing a differential diagnosis. Anterior mediastinal masses that are in contact with the diaphragm include an epicardial fat pad, pleuropericardial cyst, and Morgagni hernia (Table 1). Epicardial fat pads obliterate the cardiac silhouette and are of relatively low density (Fig 4). The presence of bowel gas within an anterior mediastinal mass that is in contact with the diaphragm is diagnostic for a Morgagni hernia.
Figure 4a. Epicardial fat pad. (a) Posteroanterior chest radiograph shows loss of the cardiac silhouette at the border of the right side of the heart and an epicardial fat pad with relatively low density (arrow), (b) CT scan shows the fat pad (arrow) as an area of homogeneous fat attenuation adjacent to the right border of the heart.
Figure 4b. Epicardial fat pad. (a) Posteroanterior chest radiograph shows loss of the cardiac silhouette at the border of the right side of the heart and an epicardial fat pad with relatively low density (arrow), (b) CT scan shows the fat pad (arrow) as an area of homogeneous fat attenuation adjacent to the right border of the heart.
The difficulty of limiting the differential diagnosis to one specific compartment is typified by thyroid disease. The thyroid gland is conventionally included in the anterior mediastinum. This gland is intimately related to the trachea, and a retrosternal goiter may not be limited to the anterior mediastinum, since it can travel along the course of the trachea into the middle and posterior mediastinum. Therefore, this enlargement may disrupt the middle and posterior mediastinal lines (discussed later) (Fig 5). Although involvement of other compartments may be seen when a goiter extends into the mediastinum, above the level of the clavicles it may be possible to appreciate the anterior location of the goiter by assessing its lateral margin. Posterior masses above the level of the clavicles have an interface with lung and therefore typically have sharp, well-defined margins; in contrast, anterior masses above the level of the clavicles do not have an interface with lung, so that their margins are not usually sharp.
Figure 5a. Right-sided retrosternal goiter, (a) Posteroanterior chest radiograph demonstrates a thyroid goiter (arrow) extending into the middle mediastinum, obliterating the right paratracheal stripe, and causing deviation of the trachea to the left (black arrowhead). Above the level of the clavicles, the margins of the mass are not sharp (white arrowhead), indicating that the mass has an anterior mediastinal component, (b) CT scan shows the mass (arrow) between the trachea and right lung, a location that explains the obliteration of the right paratracheal stripe seen in a. There is no contact between the anterior component of the mass and the lung (arrowhead) at the level of the clavicular heads, a relationship that continues above the level of the clavicles. This finding explains why the lateral border of the anterior mediastinal component above the level of the clavicles is not sharp in a.
Figure 5b. Right-sided retrosternal goiter, (a) Posteroanterior chest radiograph demonstrates a thyroid goiter (arrow) extending into the middle mediastinum, obliterating the right paratracheal stripe, and causing deviation of the trachea to the left (black arrowhead). Above the level of the clavicles, the margins of the mass are not sharp (white arrowhead), indicating that the mass has an anterior mediastinal component, (b) CT scan shows the mass (arrow) between the trachea and right lung, a location that explains the obliteration of the right paratracheal stripe seen in a. There is no contact between the anterior component of the mass and the lung (arrowhead) at the level of the clavicular heads, a relationship that continues above the level of the clavicles. This finding explains why the lateral border of the anterior mediastinal component above the level of the clavicles is not sharp in a.
Middle Mediastinum
Anatomy
The middle mediastinum is bounded anteriorly by the pericardium, posteriorly by the pericardium and posterior tracheal wall, superiorly by the thoracic inlet, and interiorly by the diaphragm (Fig 6). Its contents include the heart and pericardium; the ascending and transverse aorta; the superior vena cava (SVC) and inferior vena cava (IVC); the brachiocephalic vessels; the pulmonary vessels; the trachea and main bronchi; lymph nodes; and the phrenic, vagus, and left recurrent laryngeal nerves (1,5,6). Knowledge of the contents of this compartment facilitates the development of a differential diagnosis for middle mediastinal masses (Table 2). However, as will be demonstrated later, the theoretic boundaries of mediastinal compartments are not clear-cut, and knowledge of the local anatomy of an interrupted mediastinal line is much more helpful in identifying a possible alternative diagnosis.
Table 2. Middle Mediastinal Masses
· Lymphadenopathy
· Aortic arch aneurysm
· Enlarged pulmonary artery
· Foregut duplication cysts (bronchogenic, esophageal, neurenteric)
· Pericardial cyst
· Tracheal lesions
Figure 6. Drawing illustrates the middle mediastinum (outlined in black)
The aortopulmonary (AP) window is a middle mediastinal space bounded superiorly by the inferior margin of the aortic arch; inferiorly by the superior margin of the left pulmonary artery; anteriorly by the posterior wall of the ascending aorta; posteriorly by the anterior wall of the descending aorta; medially by the trachea, left main bronchus, and esophagus; and laterally by the left lung (7). The AP window contains lymph nodes, the left recurrent laryngeal nerve arising from the vagus nerve, the left bronchial arteries, the ligamentum arteriosum, and fat.
Right Paratracheal Stripe
The right paratracheal stripe is seen projecting through the SVC (Fig 7a). It is formed by the trachea, mediastinal connective tissue, and paratracheal pleura and is visible due to the air-soft tissue interfaces on either side (Fig 7b). The right paratracheal stripe should be uniform in width. In one study, this stripe was visible in 94% of patients, with a normal width ranging from 1 to
The azygos vein lies at the inferior margin of the right paratracheal stripe at the tracheobronchial angle (Fig 7a, 7c). There have been reports of a normal size range for the azygos vein, including upper limits of
Figure 7a. Right paratracheal stripe, (a) Posteroanterior chest radiograph shows the right paratracheal stripe (arrow). The azygos vein is seen at the inferior margin of the stripe at the tracheobronchial angle (arrowhead), (b) CT scan shows the right wall of the trachea with medial and lateral air-soft tissue interfaces caused by air within the tracheal lumen and right lung (arrow). These interfaces create the right paratracheal stripe (cf a). Note the position of the SVC (arrowhead), which explains why the paratracheal stripe is seen projecting through the SVC at radiography, (c) CT scan obtained at the level of the azygos arch shows that the azygos vein (arrow) disrupts the lung-tracheal wall interface at the tracheobronchial angle.
Figure 7b. Right paratracheal stripe, (a) Posteroanterior chest radiograph shows the right paratracheal stripe (arrow). The azygos vein is seen at the inferior margin of the stripe at the tracheobronchial angle (arrowhead), (b) CT scan shows the right wall of the trachea with medial and lateral air-soft tissue interfaces caused by air within the tracheal lumen and right lung (arrow). These interfaces create the right paratracheal stripe (cf a). Note the position of the SVC (arrowhead), which explains why the paratracheal stripe is seen projecting through the SVC at radiography, (c) CT scan obtained at the level of the azygos arch shows that the azygos vein (arrow) disrupts the lung-tracheal wall interface at the tracheobronchial angle.
Figure 7c. Right paratracheal stripe, (a) Posteroanterior chest radiograph shows the right paratracheal stripe (arrow). The azygos vein is seen at the inferior margin of the stripe at the tracheobronchial angle (arrowhead), (b) CT scan shows the right wall of the trachea with medial and lateral air-soft tissue interfaces caused by air within the tracheal lumen and right lung (arrow). These interfaces create the right paratracheal stripe (cf a). Note the position of the SVC (arrowhead), which explains why the paratracheal stripe is seen projecting through the SVC at radiography, (c) CT scan obtained at the level of the azygos arch shows that the azygos vein (arrow) disrupts the lung-tracheal wall interface at the tracheobronchial angle.
The right paratracheal stripe can be widened due to abnormality of any of its components, from the tracheal mucosa to the pleural space. Paratracheal masses, most commonly lymphadenopathy, can obliterate the right paratracheal stripe by interrupting the air-soft tissue interface between the trachea and lung (Fig 8). Fig 9
Figure 8a. Lymphadenopathy. (a) On a collimated posteroanterior chest radiograph, the right paratracheal stripe is not seen, having been obliterated by a right paratracheal mass (arrowheads), (b) CT scan demonstrates right paratracheal lymphadenopathy (arrow), which obliterates the air-soft tissue interface between the right lung and the tracheal wall. This finding explains the obliteration of the right paratracheal stripe in a.
Figure 8b. Lymphadenopathy. (a) On a collimated posteroanterior chest radiograph, the right paratracheal stripe is not seen, having been obliterated by a right paratracheal mass (arrowheads), (b) CT scan demonstrates right paratracheal lymphadenopathy (arrow), which obliterates the air-soft tissue interface between the right lung and the tracheal wall. This finding explains the obliteration of the right paratracheal stripe in a.
Figure 9. AP window reflection. On a posteroanterior chest radiograph, the AP window reflection (arrowhead) extends from the aortic knob to the left pulmonary artery and has a normal concave appearance. The aortic-pulmonary reflection (arrow) is a more anterior line and extends from the aortic arch to the level of the left main bronchus.
Mediastinal Reflections at the AP Window
The AP window is bounded by the aortic arch superiorly and the pulmonary artery interiorly, with its lateral aspect seen as the aortic-pulmonary window reflection due to the interface between the left lung and the mediastinum (Fig 9). At radiography, the “edge” of the window extends from the aortic knob to the left pulmonary artery. This edge should have a concave or straight border with the adjacent lung, with a straight border being considered normal unless previous studies have demonstrated a concave border (7).
There are two other lines that have been described as being in proximity to this region, but these lines are separate and distinct from the AP window mediastinal reflection. Anterior to the AP window reflection, the aortic-pulmonary reflection extends from the aortic arch to the level of the left main bronchus, where it usually continues as the border of the left side of the heart (7,11). This edge represents the interface between the lung and the mediastinum along the main pulmonary artery and toward the aortic arch. A number of configurations of the aortic-pulmonary reflection have been described (11); however, this reflection is not always seen. A preaortic recess may be seen at the posterior aspect of the AP window (7). This mediastinal reflection is created by an interface between the left lung and the mediastinum anterior to the descending aorta and is usually straight or concave relative to the lung in its upper extent. It is considered to be the equivalent of the azygoesophageal recess (discussed later) on the left.
An abnormal convex contour of the AP window suggests a mediastinal abnormality, most commonly lymphadenopathy (Fig 10), although such a contour may occasionally represent a normal variant caused by the accumulation of fat. Similarly, excess fat within the mediastinum can cause apparent mediastinal widening at chest radiography (5,12). Vascular abnormalities such as an aortic arch aneurysm can also distort the AP window (Fig 11).
Figure 10a. AP window lymphadenopathy. (a) Chest radiograph shows the AP window with an abnormal convex border (arrow), (b) CT scan demonstrates lymphadenopathy (arrow), which accounts for the distortion of the AP window in a.
Figure 10b. AP window lymphadenopathy. (a) Chest radiograph shows the AP window with an abnormal convex border (arrow), (b) CT scan demonstrates lymphadenopathy (arrow), which accounts for the distortion of the AP window in a.
Figure 11a. Aneurysm of the aortic arch, (a) Posteroanterior chest radiograph demonstrates the AP window with a convex border (arrow), (b) CT scan reveals an aneurysm (arrow) arising laterally from the aortic arch, a finding that accounts for the abnormality seen in a.
Figure 11b. Aneurysm of the aortic arch, (a) Posteroanterior chest radiograph demonstrates the AP window with a convex border (arrow), (b) CT scan reveals an aneurysm (arrow) arising laterally from the aortic arch, a finding that accounts for the abnormality seen in a.
Pitfalls in Assessing the Middle Mediastinum
A variety of normal vascular variants may be mistaken for middle mediastinal disease at chest radiography. A right-sided aortic arch, seen in 0.5% of the general population (13), may mimic paratracheal lymphadenopathy because it obliterates the right paratracheal stripe; however, the absence of the aortic knuckle on the left should help correctly identify this variant (Fig 12). A left-sided SVC may create an additional mediastinal line lateral to the aortic arch at radiography (Fig 13). This variant courses anterior to the left hilum and drains into the coronary sinus. A left-sided SVC is present in 0.3% of the general population and in 4.3% of patients with congenital heart disease (14), although some series have reported a prevalence of 11% in the latter group (15). Another normal variant is azygos continuation of the IVC, in which the usual development of the IVC does not occur and the azygos vein provides an alternate route for systemic venous return to the heart. This anatomic variant results in an enlarged azygos vein, which may be mistaken for lymphadenopathy (Fig 14).
Figure 12a. Right-sided aortic arch, (a) Posteroanterior chest radiograph demonstrates an abnormality in the right paratracheal region (arrow) with loss of the paratracheal stripe. Note, however, the absence of the aortic knuckle on the left, (b) CT scan shows a right-sided aortic arch (arrow), which explains the findings in a.
Figure 12b. Right-sided aortic arch, (a) Posteroanterior chest radiograph demonstrates an abnormality in the right paratracheal region (arrow) with loss of the paratracheal stripe. Note, however, the absence of the aortic knuckle on the left, (b) CT scan shows a right-sided aortic arch (arrow), which explains the findings in a.
Figure 13a. Left-sided SVC. (a) Collimated posteroanterior chest radiograph shows an additional line (arrow) lateral to the aortic arch, (b) Venogram demonstrates a left-sided SVC, which explains the finding in a. (c, d) CT scans obtained at the levels of the aortic arch (c) and pulmonary trunk (d) show the left-sided SVC (arrow), which drains into the coronary sinus.
Figure 13b. Left-sided SVC. (a) Collimated posteroanterior chest radiograph shows an additional line (arrow) lateral to the aortic arch, (b) Venogram demonstrates a left-sided SVC, which explains the finding in a. (c, d) CT scans obtained at the levels of the aortic arch (c) and pulmonary trunk (d) show the left-sided SVC (arrow), which drains into the coronary sinus.
Figure 13c. Left-sided SVC. (a) Collimated posteroanterior chest radiograph shows an additional line (arrow) lateral to the aortic arch, (b) Venogram demonstrates a left-sided SVC, which explains the finding in a. (c, d) CT scans obtained at the levels of the aortic arch (c) and pulmonary trunk (d) show the left-sided SVC (arrow), which drains into the coronary sinus.
Figure 13d. Left-sided SVC. (a) Collimated posteroanterior chest radiograph shows an additional line (arrow) lateral to the aortic arch, (b) Venogram demonstrates a left-sided SVC, which explains the finding in a. (c, d) CT scans obtained at the levels of the aortic arch (c) and pulmonary trunk (d) show the left-sided SVC (arrow), which drains into the coronary sinus.
Figure 14a. Azygos continuation of the IVC. (a) Collimated posteroanterior chest radiograph shows enlargement of the azygos vein at the inferior margin of the right paratracheal stripe (arrowheads), a finding that mimics lymphadenopathy. (b) CT scan also shows enlargement of the azygos vein (arrow). This finding is the result of azygos continuation of the IVC.
Figure 14b. Azygos continuation of the IVC. (a) Collimated posteroanterior chest radiograph shows enlargement of the azygos vein at the inferior margin of the right paratracheal stripe (arrowheads), a finding that mimics lymphadenopathy. (b) CT scan also shows enlargement of the azygos vein (arrow). This finding is the result of azygos continuation of the IVC.
Posterior Mediastinum
Anatomy
The posterior mediastinum is bounded anteriorly by the posterior trachea and pericardium, anteroinferiorly by the diaphragm, posteriorly by the vertebral column, and superiorly by the thoracic inlet (Fig 15). As discussed previously, the true anatomic posterior boundary is the vertebral column; however, with respect to mediastinal disease, masses in the paraspinal regions are usually included in the posterior mediastinum. The contents of the posterior mediastinum include the esophagus, descending aorta, azygos and hemiazygos veins, thoracic duct, vagus and splanchnic nerves, lymph nodes, and fat (1,5,6). Drawing illustrates the posterior mediastinum (outlined in black).
As with the anterior mediastinum, disease involving any of the contents of the posterior mediastinum may result in a mass, and knowledge of the normal anatomy aids in developing a differential diagnosis (Table 3).
Table 3. Posterior Mediastinal Masses
· Esophageal lesions, hiatal hernia
· Foregut duplication cyst
· Descending aortic aneurysm
· Neurogenic tumor
· Paraspinal abscess
· Lateral meningocele
· Hxtramedullary hematopoiesis
Azygoesophageal Recess
The azygoesophageal recess is the interface between the right lung and the mediastinal reflection inferior to the arch of the azygos vein, with the esophagus lying anteriorly and the azygos vein posteriorly within the mediastinum. At radiography, this interface is seen as a line, or, more accurately, an edge (Fig 16). In its upper third, as it deviates to the right at the level of the carina to accommodate the azygos vein arching forward, the line is usually straight or concave relative to the right lung. In children and young adults, a convexity to the right may be seen (16). In its middle third, the line has a variable appearance: It is usually straight, but in the region of the right pulmonary veins a minimal convexity to the right may be seen in adults. In its lower third, the line is usually straight (7,16). If there is air within the esophagus, the right esophageal wall (and any adjacent paraesophageal tissue) may be seen as a stripe. If the left lung forms an interface with the esophagus, the left esophageal wall may have a similar appearance.
Figure 16a. Azygoesophageal recess reflection, (a) Posteroanterior chest radiograph shows the azygoesophageal line (arrowheads), (b) CT scan shows the azygoesophageal recess (white arrow) formed by the esophagus anteriorly (black arrow) and the azygos vein posteriorly (arrowhead). The azygoesophageal line in a represents the interface between this recess and the lung.
Figure 16b. Azygoesophageal recess reflection, (a) Posteroanterior chest radiograph shows the azygoesophageal line (arrowheads), (b) CT scan shows the azygoesophageal recess (white arrow) formed by the esophagus anteriorly (black arrow) and the azygos vein posteriorly (arrowhead). The azygoesophageal line in a represents the interface between this recess and the lung.
The azygoesophageal recess reflection is a pre-vertebral structure and is, therefore, disrupted by prevertebral disease. It has an interface with the middle mediastinum; thus, the resulting line seen at radiography can be interrupted by abnormalities in both the middle and posterior compartments.
Again, the divisions of the mediastinum are theoretic rather than physical and do nut limit disease occurrence. Identifying the close anatomic relations of a mass is often more instructive. In the subcarinal region, left atrial enlargement, subcarinal lymphadenopathy, esophageal disease, and bronchogenic cysts (Fig 17) may cause deviation of the azygoesophageal line. More inferior to the subcarinal region, the azygoesophageal recess may be disrupted by esophageal disease and hiatal hernia.
Figure 17a. Bronchogenic cyst, (a) Posteroanterior chest radiograph demonstrates a subcarinal abnormality with increased opacity (*), splaying of the carina, and abnormal convexity of the upper and middle thirds of the azygoesophageal line (arrowheads), (b) Corresponding CT scan helps confirm a subcarinal mass (arrow), which proved to be a bronchogenic cyst.
Figure 17b. Bronchogenic cyst, (a) Posteroanterior chest radiograph demonstrates a subcarinal abnormality with increased opacity (*), splaying of the carina, and abnormal convexity of the upper and middle thirds of the azygoesophageal line (arrowheads), (b) Corresponding CT scan helps confirm a subcarinal mass (arrow), which proved to be a bronchogenic cyst.
Posterior Junction Line
The posterior junction line is a posterior mediastinal line that is seen above the level of the azygos vein and aorta and that is formed by the apposition of the lungs posterior to the esophagus and anterior to the vertebral bodies, usually the third to fifth thoracic vertebrae (Fig 18). It can occasionally be seen more inferiorly if the lungs come in contact posterior to the esophagus in the lower thorax. Like the anterior junction line, it consists of four layers of pleura. Unlike its counterpart, however, the posterior junction line can be seen above the suprasternal notch and lies almost vertical, whereas the anterior junction line deviates to the left (7).
Figure 18a. (a) Collimated posteroanterior chest radiograph shows the posterior junction line (arrow) projecting through the tracheal air column, (b) CT scan shows the posterior junction line (arrow), which is formed by the interface between the lungs posterior to the mediastinum and consists of four pleural layers.
Figure 18b. (a) Collimated posteroanterior chest radiograph shows the posterior junction line (arrow) projecting through the tracheal air column, (b) CT scan shows the posterior junction line (arrow), which is formed by the interface between the lungs posterior to the mediastinum and consists of four pleural layers.
Prevertebral disease superior to the level of the aortic arch may obliterate the posterior junction line (Fig 19). Further clues to the location of a mass in this region can be inferred from the lateral margins of the line above the level of the clavicles (see “Anterior Junction Line”).
Figure 19a. Bronchogenic cyst, (a) Posteroanterior chest radiograph shows a mass (arrow) obliterating the posterior junction line. Note that the mass extends above the level of the clavicle and has a well-demarcated outline due to the interface with adjacent lung (arrowhead), (b) CT scan helps confirm the posterior location of the mass (arrow), which proved to be a bronchogenic cyst.
Figure 19b. Bronchogenic cyst, (a) Posteroanterior chest radiograph shows a mass (arrow) obliterating the posterior junction line. Note that the mass extends above the level of the clavicle and has a well-demarcated outline due to the interface with adjacent lung (arrowhead), (b) CT scan helps confirm the posterior location of the mass (arrow), which proved to be a bronchogenic cyst.
Paraspinal Lines
The paraspinal lines are created by the interface between lung and the pleural reflections over the vertebral bodies. The left paraspinal line is much more commonly seen than the right. The descending aorta holds the pleural reflection off the vertebral body, allowing the lung-soft tissue interface to be more tangential to the x-ray beam and, therefore, to be visualized as a line (Fig 20a, 20b). On the right, the pleural reflection is more often oblique to the x-ray beam and therefore less commonly seen (Fig 20c). The amount of mediastinal fat also affects these lines.
Figure 20a. (a) On a collimated posteroanterior chest radiograph, the left paraspinal line (arrow) is seen separate and distinct from the vertebral body (black arrowhead) and the descending thoracic aorta (white arrowhead), (b) CT scan shows the left paraspinal line. The descending aorta holds the pleural reflection (arrow) away from the vertebral body, which allows the lung-soft tissue interface to be more tangential to the x-ray beam and therefore visualized as a line, (c) Collimated posteroanterior radiograph shows the right paraspinal line (arrow).
Figure 20b. (a) On a collimated posteroanterior chest radiograph, the left paraspinal line (arrow) is seen separate and distinct from the vertebral body (black arrowhead) and the descending thoracic aorta (white arrowhead), (b) CT scan shows the left paraspinal line. The descending aorta holds the pleural reflection (arrow) away from the vertebral body, which allows the lung-soft tissue interface to be more tangential to the x-ray beam and therefore visualized as a line, (c) Collimated posteroanterior radiograph shows the right paraspinal line (arrow).
Figure 20c. (a) On a collimated posteroanterior chest radiograph, the left paraspinal line (arrow) is seen separate and distinct from the vertebral body (black arrowhead) and the descending thoracic aorta (white arrowhead), (b) CT scan shows the left paraspinal line. The descending aorta holds the pleural reflection (arrow) away from the vertebral body, which allows the lung-soft tissue interface to be more tangential to the x-ray beam and therefore visualized as a line, (c) Collimated posteroanterior radiograph shows the right paraspinal line (arrow).
The paraspinal lines are disruf d by paravertebral disease—which commonly includes diseases originating in the intervertebral disks and vertebrae— and by neurogenic tumors.
The left paraspinal line and the lateral margin of the descending aorta should be clearly distinguished from one another. This differentiation is demonstrated in Figure 21, which shows a paraspinal abscess effacing the left paraspinal line while the aorta maintains an air-soft tissue interface with lung and is, therefore, still visible. In contrast, Figure 22 shows a descending aortic aneurysm with deviation of the lateral margin of the aorta. It should be remembered that the paraspinal lines also project below the level of the diaphragm at radiography, and disruption of the lines in this location can also be identified (Fig 23).
Figure 21a. Paraspinal abscess, (a) Posteroanterior chest radiograph shows a mass (arrow) effacing the left paraspinal line. The lateral wall of the descending aorta is seen as a separate entity (arrowhead), (b) CT scan shows a paraspinal abscess (arrow) effacing the paraspinal lines. The air-soft tissue interface between the lung and aorta remains intact (arrowhead), thereby preserving the normal radiographic appearance of the lateral aortic wall (cf a).
Figure 21b. Paraspinal abscess, (a) Posteroanterior chest radiograph shows a mass (arrow) effacing the left paraspinal line. The lateral wall of the descending aorta is seen as a separate entity (arrowhead), (b) CT scan shows a paraspinal abscess (arrow) effacing the paraspinal lines. The air-soft tissue interface between the lung and aorta remains intact (arrowhead), thereby preserving the normal radiographic appearance of the lateral aortic wall (cf a).
Figure 22a. Descending aortic aneurysm, (a) Posteroanterior chest radiograph shows lateral displacement of the lateral margin of the descending thoracic aorta due to an aortic aneurysm (arrowheads), (b) CT scan also demonstrates the aneurysm (arrow).
Figure 22b. Descending aortic aneurysm, (a) Posteroanterior chest radiograph shows lateral displacement of the lateral margin of the descending thoracic aorta due to an aortic aneurysm (arrowheads), (b) CT scan also demonstrates the aneurysm (arrow).
Figure 23a. Neurogenic tumor, (a) Posteroanterior chest radiograph shows a small mass (arrow) disrupting the left paraspinal line inferiorly. (b) Coronal T2-weighted magnetic resonance (MR) image helps confirm a left paraspinal mass (arrow).
Figure 23b. Neurogenic tumor, (a) Posteroanterior chest radiograph shows a small mass (arrow) disrupting the left paraspinal line inferiorly. (b) Coronal T2-weighted magnetic resonance (MR) image helps confirm a left paraspinal mass (arrow).
Further Assessment
Once a mediastinal mass has been identified, it can be assessed with cross-sectional imaging, which can help confirm its location and further characterize the disease. CT is most often used in the assessment of mediastinal masses, with MR imaging usually being used as an adjunct to CT. MR imaging has high contrast resolution and multiplanar capability, thereby providing additional information as to the location and extent of the abnormality, and is the preferred modality in evaluating neurogenic tumors because it provides information regarding the nature and extent of intraspinal involvement. In addition, MR imaging can further characterize tissue, is useful in showing the cystic nature of mediastinal lesions that appear solid at CT, and can help assess the mediastinum in patients who have contraindications to iodinated contrast material. If a posterior mediastinal mass is suspected, MR imaging may be the imaging modality of choice. However, this modality does not demonstrate calcification as well as CT and has poorer spatial resolution.
Summary
Many mediastinal reflections can be appreciated at conventional radiography, and their presence or distortion is the key to the interpretation of mediastinal abnormalities Anterior mediastinal masses can be identified when both the hilum overlay sign and preservation of the posterior mediastinal lines are present Widening of the right paratracheal stripe and convexity relative to the AP window reflection both indicate abnormality in the middle mediastinum Disruption of the azygoesophageal recess can be caused by disease in either the middle or posterior mediastinum Paravertebral masses disrupt the paraspinal lines, and the location of masses above the level of the clavicles can be inferred by their lateral margins Posterior masses have sharp margins due to their interface with lung, whereas anterior masses do not
Although the divisions of the mediastinum are not absolute, attempting to more accurately localize an abnormality with reference to the local anatomy of the mediastinal reflections may help narrow the differential diagnosis Identification of the involved compartment helps determine appropriate further imaging
