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Clinical Radiology 65 (2010) 155–166

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Clinical Radiology

journal homepage: www.elsevierheal th.com/journals /crad

Pictorial Review

Understanding chest radiographic anatomy withMDCT reformationsA.R. Sussmann, J.P. Ko*

Department of Radiology, Thoracic Imaging, NYU Langone Medical Center, New York, NY, USA

article information

Article history:Received 4 May 2009Received in revised form29 September 2009Accepted 8 October 2009

* Guarantor and correspondent: J. P. Ko, 560 FirNY 10016, USA. Tel.: þ1 212 263 5229; fax: þ1 2

E-mail address: jane.ko@nyumc.org (J.P. Ko).

0009-9260/$ – see front matter � 2009 The Royal Codoi:10.1016/j.crad.2009.10.005

Chest radiograph interpretation requires an understanding of the mediastinal reflections andanatomical structures. Computed tomography (CT) improves the learning of three-dimensional(3D) anatomy, and more recently multidetector CT (MDCT) technology has enabled the crea-tion of high-quality reformations in varying projections. Multiplanar reformations (MPRs) ofvarying thickness in the coronal and sagittal projections can be created for direct correlationwith findings on frontal and lateral chest radiographs, respectively. MPRs enable simultaneousvisualization of the craniocaudal extent of thoracic structures while providing the anatomicdetail that has been previously illustrated using cadaveric specimens. Emphasis will be placedon improving knowledge of mediastinal anatomy and reflections including edges, lines, andstripes that are visible on chest radiographs.

� 2009 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.

Introduction

The chest radiograph is the most commonly orderedradiological study. Superposition of structures on the chestradiograph poses a challenge for interpretation. More recenteducational manuscripts mainly focus on correlating radio-graphic findings with axial CT sections.1–3 While axialsections help one understand the anatomical causes for thespecific radiographic reflections and interfaces, multiplanarreformations (MPRs), maximum intensity projection (MIP),and volume-rendered images from multidetector CT (MDCT)data can be altered in terms of section thickness and displaysettings to clarify the craniocaudal course of structures ofinterest and to show anatomical detail previously elucidatedthrough linear tomography and cadaveric specimens.4–8

The aim of the present review is to improve the under-standing of radiographic chest anatomy and hence identifi-cation of disease by correlating coronal and sagittal CT

st Ave, IRM 236, New York,12 263 7348.

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reformations with posteroanterior and lateral projections,respectively, in nearly all of the cases from the same patient.Given the broad nature of this topic, we have selected morechallenging radiographic anatomy focusing on the hilarregions, pulmonary and systemic vasculature, and mediastinalreflections. However, it is worth noting that slight differencesin the thoracic configuration are evident between the recon-structions and radiographs secondary to the supine posi-tioning and differences in the degree of patient inspiration.

Terminology

Descriptive terms have been applied to characterize thereflections of the pleura and lung around the mediastinum.3

A ‘‘line’’ typically measures less than 1 mm in width, anexample being when the visceral and parietal pleuralreflections of each lung meet. ‘‘Edge’’ has been used inregions where structures of two different densities comeinto contact with each other, particularly when the lungabuts the mediastinum. A ‘‘stripe’’ is typically greater than1 mm in thickness and occurs when a mediastinal structurehas air on both sides. A recess can become a stripe,3 as willbe further discussed.

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Table 1Frequency of visibility of selected structures on the lateral radiographa

Structure Frequency (%)

Lungs and airwaysOrifice of RUL bronchus 50Orifice of LUL bronchus 77Posterior wall of bronchus intermedius 95Anterior wall of bronchus intermedius 6Anterior and posterior walls of RLL bronchus 8Anterior and distal posterior wall of LLL

bronchus and posterior wall of LMB43

RML bronchus 4

Heart and vasculatureRight pulmonary artery 96Left pulmonary artery 86Sharp-edged aorta and main pulmonary artery 10Posterior portion of aortic arch 73Rounded or angular cardiac incisura 18

Mediastinal reflectionsRetrosternal and/or parasternal stripes 90Posterior tracheal or tracheoesophageal stripes 45Left subclavian artery/innominate-right subclavian

artery complex10

RUL, right upper lobe; LUL, left upper lobe; LLL, left lower lobe; LMB, leftmain bronchus; RML, right middle lobe.

a As originally catalogued by Proto and Speckman.7,8

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Airways and Hilar structures

Upper lobe bronchi and bronchus intermedius

On the lateral radiograph, the orifice of the left upperlobe (LUL) bronchus is readily seen as a rounded lucency

Figure 1 Bronchus intermedius (BI) and left upper lobe bronchial (LULB(short white arrows), termed the intermediate stem line, projects throughbronchus (RULB) (R). The left lower lobe bronchus (LLLB; posterior aspeclobe bronchus (RLLB; black arrowheads). Right middle lobe bronchus supthe right hilum (second from left image) demonstrates the BI (curved blartery (PA; white curved arrow) anteriorly. The LULB orifice (second fromsuperior pulmonary vein inferiorly indicated by long black arrow; left inteindicated by white arrowhead). The LLLB is indicated by a short black arrowand right-sided MPRs illustrates the posterior wall of the BI (arrowheads) porifice. The RULB is indicated by the black arrow.

with the left pulmonary artery superiorly, the left superiorpulmonary vein inferiorly, and the left interlobar arteryposteriorly.9 The orifice of the right upper lobe (RUL)bronchus is not visualized as often as that of the LULbronchus. Discernible on roughly 50% of lateral films andnot as well circumscribed in comparison to its left-sidedcounterpart (Table 1), the RUL bronchus orifice appears asa round lucency above the orifice of the LUL bronchus(Fig. 1). The RUL orifice is best defined at its anterosuperioraspect, which abuts the truncus anterior. The entirecircumference of the RUL bronchus orifice becomes sharplymarginated when adenopathy is present.7

The posterior wall of the bronchus intermedius (Fig. 1)contacts aerated lung in the azygo-oesophageal recess and,therefore, is visible on 95% of lateral radiographs asa vertical or slightly oblique stripe extending inferiorly fromthe posterior aspect of the RUL bronchus and projectingover the mid- to posterior portion of the LUL bronchusorifice (Fig. 1). This has been named the ‘‘intermediate stemline.’’ The anterior wall of the bronchus intermedius isusually abutted, and therefore also obscured, by the rightpulmonary artery (Table 1), making it visible in fewer than10% of films.7

Upper hilar anatomy

The right and left suprahilar vessels and bronchi havedistinct arrangements on the frontal radiograph (Fig. 2). Onthe right, the truncus anterior courses inferior to the RULbronchus at the hilum and then passes anterosuperior tothe bronchus as it exits the hilum. The truncus anterior lies

) orifice. On the lateral film (left image), the posterior wall of the BIthe orifice of the LULB (L), located below that of the right upper lobe

t marked by white arrowheads) projects posterior to the right lowererior aspect is indicated by black arrows. Sagittal 5 mm MPR throughack arrow), RULB orifice (open arrowhead), and the right pulmonaryright image, long black arrowhead) is surrounded by vasculature (leftrlobar PA posteriorly indicated by short black arrow, left PA superiorly

head, centre image. Semitransparent overlay (right image) of the leftrojecting over the mid to posterior aspect of the LULB (white arrows)

Figure 2 Anatomy of the right superior hilum. On frontal chest radiograph (left image), the RUL apical segmental bronchus (asterisk) passesbetween the more medial truncus anterior (arrowhead) and the lateral branches of the superior pulmonary vein (SPV; arrow). In a differentpatient, coronal 5 mm MPR (second from left image) demonstrates the RUL (arrow) and apical segmental bronchi (arrowhead) while a thick15 mm MIP image (second from right image) shows the truncus anterior (arrowhead) and the right SPV (arrow). Superimposition (right image)of a semi-transparent version of the MPRs depict their relationship (RUL bronchus indicated by an asterisk; truncus anterior indicated by anarrowhead; branches of the right SPV are indicated by an arrow; blue overlay indicates the SPV; red overlay indicates the right pulmonary andinterlobar arteries).

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anterior and medial to the apical segmental RUL bronchus,while the right superior pulmonary vein is positionedanterolaterally in relation to the bronchus as it coursesobliquely towards the hilum. This pattern can sometimes bediscerned on the radiograph,9 but is demonstrated to betteradvantage on coronal CT reformations (Fig. 2). The leftsuperior pulmonary vein on frontal radiograph lies medialto both the LUL pulmonary artery branches and LULbronchi.9 Mediastinal fat can obscure the left suprahilararea on the chest film.

Pulmonary veins

The right superior pulmonary vein receives blood fromboth the veins of the RUL and RML, which join at thesuperior right aspect of the left atrium. The left superiorpulmonary vein typically is formed by the confluence of theLUL and the lingular veins at the superior left atrium.10 Notuncommonly, on either or both sides, the superior pulmo-nary veins may be joined by the inferior pulmonary vein ontheir respective side to form a single, shared confluence.9

On the lateral radiograph, in patients with prominentpulmonary veins, such as those with congestive heartfailure, a pulmonary vein or a venous confluence visualized

Figure 3 Lateral chest radiograph (left image) shows an oval opacity (withby the right lung abutting the confluence of the right middle lobe and rigfrom left image) shows the lung abutting the right SPV (arrowheads) thatoverlay (right image) of this MPR onto a sagittal 5 mm MPR through the lthe confluence seen on lateral radiograph.

end-on may appear as a rounded opacity near the posterioraspect of the left atrium (Fig. 3). Without prior knowledgeone can misinterpret these structures as lung nodules.11 Onfrontal radiographs the inferior pulmonary veins appear asthin, linear opacities coursing superomedially toward theheart in an orientation more horizontal than that of thepulmonary arteries (Fig. 4) and can be mistaken for diseaseentities by the unwary reader.

Mediastinal reflections

Aortopulmonary window and reflection

The aortopulmonary (AP) window is the region belowthe aortic arch and superior to the left pulmonary arterythat is bounded laterally by the parietal pleura of the leftlung, anteriorly by the posterior aspect of the ascendingaorta, medially by the trachea, and posteriorly by theanterior aspect of the descending aorta.3 The AP windowhouses fat, lymph nodes, the ligamentum arteriosum, andthe left recurrent laryngeal nerve. On the frontal radio-graph, the lateral aspect of the AP window creates a concavereflection that extends from the aortic arch to the leftpulmonary artery and curves towards the mediastinum.

in the circle) that projects over the posterior left atrium and is formedht superior pulmonary veins (SPV). Sagittal 5 mm MPR mage (secondreceives blood from the right upper lobe pulmonary vein (arrow). An

eft atrium (second from right image) clarifies the structures that form

Figure 4 Frontal chest radiograph (a, left image) and coronal 15 mm MIP image (a, right image) demonstrate the horizontal course of the inferiorpulmonary veins (solid arrowheads) in comparison to the pulmonary arteries (open arrowheads). The right paraspinal line (a portion of which islabelled with small black arrows) is an undulating opacity formed by the lung abutting the vertebral bodies and adjacent soft tissues. Thedescending aortic interface (larger black arrows) is located to the left of the spine. (b) Frontal chest radiograph (left image) from a differentpatient and corresponding coronal (right image) 5 mm MPR demonstrate obscuration of part of the margin of the descending aortic interface(arrowheads) secondary to a soft-tissue density lesion, a long-standing probable proteinaceous bronchogenic cyst.

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Seen in about 49% of normal frontal radiographs,3 the APreflection is more anterior and begins more superiorly thanthe AP window. The reflection becomes convex in thepresence of adenopathy (Fig. 5). The AP reflection repre-sents the pleura in contact with mediastinal fat that isanterolateral to the transverse segment of the aortic archand the left pulmonary artery. The AP reflection is anteriorto the posterior aspect of the ascending aorta. The APreflection curves mildly towards the mediastinum as it cour-ses inferiorly to the level of the left main bronchus and thengently courses towards the left at the level of the mainpulmonaryartery (Fig. 5). Occasionally when mediastinal fat isprominent, this normal reflection can be difficult to differen-tiate from adenopathy.3,12,13 Pneumomediastinum can lead tothe elevation and lateral displacement of the AP reflection.2

Azygos vein and left superior intercostal veins

The azygos vein begins at the L1–L2 level and is thecontinuation of the right ascending lumbar vein. The azygos

vein courses cranially in the right anterolateral para-vertebral region. Around the level of T4–T5, the vessel as theazygos arch passes over the right mainstem bronchus andtruncus anterior and empties into the superior vena cava(SVC). On the frontal film, the azygos vein is visible as anelliptical density at the right tracheobronchial angle(Fig. 6).14 Intrathoracic pressure can markedly affect theazygos vein capacity and, hence, size on plain film. Forinstance, the azygos vein will appear larger when a patientis in the supine rather than the upright position.3 Theazygos vein enlarges when there is physiological ormechanical obstruction to the flow of blood through theSVC, other central veins, and right heart. Varices and azygoscontinuation of the inferior vena cava can lead to azygosdistention.15,16 The azygos vein is very rarely seen on thelateral films of healthy individuals.

The left superior intercostal vein (LSIV) lies in theposterior paravertebral upper mediastinum, drains thesecond through fourth intercostal spaces in the posteriormediastinum, and travels inferiorly until it reaches the T3 or

Figure 5 AP window and reflection. (a) Frontal chest radiograph (left image) and 5 mm coronal MPR show the AP reflection (centre image,arrows) located anterior and superior to the AP window (right image, arrowheads). The AP reflection courses along the transverse arch to thelevel of the main pulmonary artery. (b) On 5 mm axial and sagittal MPRs, the AP window is shaded. The ligamentum arteriosum (arrows) coursesbetween the anterior aspect of the proximal descending thoracic aorta and the superior aspect of the proximal left main pulmonary artery. (c)Frontal chest radiograph (left) and corresponding coronal (right) 5 mm MPR from a patient with sarcoidosis demonstrate convexity of the lateralmargin of the AP window (arrowheads) secondary to calcified, enlarged lymph nodes.

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T4 vertebral level.3,16 The LSIV then courses anteriorly,typically along the left superolateral aspect of the aorticarch, and ultimately empties into the left brachiocephalicvein (Fig. 7). The posteroanterior segment of a distendedLSIV on a frontal film can be visualized end-on as a tiny,round opacity called the ‘‘aortic nipple’’ (Table 2).16 The LSIVcan be seen on end on the frontal radiograph along the

periphery of the aortic arch at points ranging clockwisefrom the right-superior to left-inferior aspects.

Azygo-oesophageal and supra-azygos recesses

Inferior to the azygos arch, the azygo-oesophageal recessis the space behind the heart that creates a midline interface

Figure 6 (a) Frontal chest radiograph (left image) and coronal 5 mm MPR (right image) show the azygo-oesophageal recess (small arrowheads) andsupraazygos recess (arrows), below and above, respectively, the azygos arch (larger arrowhead). (b) Axial 5 mm CT section (left image) in a differentpatient shows the azygo-oesophageal recess (arrowheads) and the left sided pre-aortic recess (long white arrow) formed by the left side of theoesophagus (open arrowhead) and mediastinal tissues abutting the left lower lobe. When the oesophageal lumen is distended with air, right (openarrowhead) and left (arrow, centre image) inferior oesophagopleural stripes are formed. The posteroinferior junction line (arrows, right image) is

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Figure 7 LSIV. On frontal chest film and coronal 5 mm MPR, the location of the LSIV (arrow) is demonstrated by inadvertent catheterization bya peripherally inserted central catheter. The LSIV is positioned lateral to the aortic arch (arrowheads).

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when aerated right lower lobe abuts the mediastinal fat,azygos vein, and oesophagus on the frontal radiograph. Thisinterface extends from the diaphragm to the arch of theazygos vein and is visualized in 51.5% of radiographs (Fig. 6;Table 2). This interface in adults typically curves toward thepatient’s left (levoconvex).3,17 When air is present in theoesophageal lumen, a right inferior oesophagopleural stripeis visualized. The stripe represents the oesophageal walland adjacent mediastinal soft tissues that are surroundedby aerated lung laterally and by intraluminal air in theoesophagus (Table 2).

The left-sided counterpart of the azygo-oesophagealrecess, the pre-aortic recess, is seen less frequently, in 7.5%of chest radiographs (Table 2).3 The interface created bylung in the pre-aortic recess becomes the left inferioroesophagopleural stripe when air fills the oesophageallumen (Fig. 6).9 Uncommonly, the posterior right and leftpleura meet behind the oesophagus and anterior to thedescending aorta and azygos vein, forming the poster-oinferior junction line (Fig. 6). This finding is seen inless than 1% of individuals but occurs to a greater degreein patients with emphysema and lung hyperinflation(Table 2).18

The medial right lung and associated pleural layers abutthe mediastinal fat superior to the azygos arch and form thesupra-azygos recess. The azygos arch has a long horizontalpath in the anteroposterior plane, and the interface,therefore, can be formed by contact with the lung anywherealong the course of the azygos arch (Fig. 6). Given that theazygos vein is below the supra-azygos recess, enlargementof the supra-azygos recess argues against the presence of anenlarged azygos vein and rather suggests adenopathy as thecause of a large right tracheobronchial opacity.3,6

formed by contact of the visceral and parietal pleura posterior to the oesophlarge opacity at the expected region of the azygos vein shadow azygos, at theMPRs from the same patient confirm a markedly dilated azygos arch (arrowheads, right image) of an interrupted inferior vena cava.

Descending aortic interface and paraspinal lines

The descending aortic interface (Fig. 4) is an edge createdby the aerated left lung when it abuts the left lateral aspectof the descending thoracic aorta on the frontal radiograph.Therefore, space-occupying lesions in the medial aspect ofthe left lung can obscure part or all of this interface. Moremedially, the left paraspinal reflection is formed when lungabuts the pleura and fat in the paravertebral regions (Fig. 4)at the level of the aortic arch and extends caudally to theinferior-most left lower lobe. The left paraspinal reflectionis seen in approximately 31% of radiographs and describedmore commonly as a line, related to the Mach band effect.Mach bands are optical edge-enhancement effects that arecreated by inhibition of the lateral retina and occur partic-ularly when a reflection is convex towards the lung.3,19 Theleft paraspinal line may appear as an edge.3,6 The rightparaspinal line typically extends from the T8 to T12 verte-bral bodies.2 The angle of the pleura created by the left-sided descending thoracic aorta and also the amount ofparaspinal fat likely account for less frequent visualizationof the right paraspinal line. Abnormalities in the spine andparaspinal soft tissues, such as adenopathy, haematoma,mass, and extramedullary haematopoiesis displace theparaspinal lines laterally.

Posterior tracheal and tracheo-oesophageal (TE) stripes

The posterior tracheal stripe is seen on 50% of lateralprojections. The anterior aspect of the posterior trachealstripe is formed by air within the tracheal lumen thatcontacts the posterior wall of the trachea. The posterioraspect of the stripe represents the posterior wall of the

agus. (c) Frontal chest radiograph from a different patient depicts a veryright tracheobronchial angle (arrows). (d) Corresponding coronal 5 mmhead, left image), in this case secondary to azygos continuation (arrow-

Table 2Frequency of visibility of select structures on the posteroanterior radiographa

Structure Frequency (%)

Oesophageal walls 29Aortic nipple 9Aortopulmonary interface 49Anterior junction line 24–57Posterior junction line 32Right paratracheal stripe 83Supra-azygos recess 23Azygo-oesophageal recess 52Pre-aortic recess 8Posteroinferior junction line <1%Left paraspinal line 31

a Taken from Proto,3 Gibbs et al.,2 and Curtis and Fisher.18

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trachea and surrounding tissue abutting the aerated retro-tracheal RUL (Fig. 8). This stripe therefore extends only tothe level of the azygos arch and, hence, the level of theaortic arch.8,9

The TE stripe, seen on 50% of films, is seen when theposterior tracheal wall, intervening mediastinal tissue, andanterior oesophageal wall are surrounded anteriorly andposteriorly by air in the tracheal and oesophageal lumens,respectively.9,20 Unlike the posterior tracheal stripe, the TEstripe extends caudal to the superior aspect of the aorticarch (Fig. 8).7,20

Thus, the differentiation of the TE stripe from theposterior tracheal stripe can be difficult. The thickness ofthe posterior tracheal/TE stripe has been reported onaverage to measure 2 mm, with a range of 1–5.5 mm.7

Thickening in this region raises the question of adenopathyor tumour infiltration, such as from oesophageal cancer(Fig. 8).

Figure 8 Lateral radiograph (left image) show a TE stripe (white arrows), fin the oesophagus, in combination with the posterior tracheal stripe. Exte(arrows), as seen in the 5 mm sagittal MPR (right image) while the uppestripe on radiograph represents a combination of both the posterior trac

Paratracheal stripes

On the frontal radiograph, the right paratracheal stripe(Table 2) is composed of the right wall of the trachea,adjacent visceral and parietal pleura of the RUL, and anymediastinal fat.1,3,21 The right paratracheal stripe isbordered laterally by the RUL and medially by air in thetracheal lumen. It typically measures 4 mm or less (Fig. 9).21

Adenopathy lateral to the right tracheal wall will increasethe thickness of the right paratracheal stripe (Fig. 9).22

Paratracheal adenopathy located anterior to the para-tracheal stripe would fail to increase the thickness of theright paratracheal stripe. The left paratracheal reflection,seen in 31% of frontal radiographs, occurs when the LULinserts itself anterior to the left subclavian artery and abutsthe lateral aspect of the left common carotid artery andadjacent mediastinal soft tissues.23 The left subclavianartery typically forms the most lateral reflection of theupper left mediastinum (Fig. 9).

Anterior and posterior junction lines

The anterior junction line (AJL) is the thin apposition ofright and left visceral and parietal pleura anterior to themediastinum and posterior to the sternum. The AJL beginsbelow the manubrium and courses inferiorly in a slightlyoblique manner from right to left to the upper aspect of theheart. At this point the pleura diverge into the right and leftinferior recesses and abut the heart and adjacent medias-tinal fat (Fig. 10).3,24 The AJL becomes a stripe when medi-astinal soft tissue from thymus in young individuals ormediastinal fat interposes between the right and left pleura.The right and left superior recesses are formed when the

ormed by contact of the posterior wall of the air-filled trachea with airnsion inferior to the level of the aortic arch is indicative of a TE striper segment is likely a combination of both. The upper segment of theheal and TE stripes.

Figure 9 (a) Frontal chest radiograph and coronal 5 mm MPR depict the right paratracheal stripe (arrows). The lateral left subclavian artery isindicated by arrowheads. SVC, superior vena cava; IVC, inferior vena cava. (b), Coronal 5 mm MIP (left image) and axial 5 mm section (top) froma different patient depict the interface between the left subclavian artery (arrowheads) and the LUL. The right paratracheal stripe (long arrows, rightimages) is posterior to the SVC interface with the right lung (short arrow). (c) Frontal chest radiograph (left image) from a patient with sarcoidosisdepicts thickening of the paratracheal stripe medial aspect demarcated by (arrowheads) secondary to lymphadenopathy demonstrated on corre-sponding 5 mm coronal MPR (right image).

Figure 10 Anterior and posterior junction lines. Frontal chest radiograph (left image) and volume-rendered CT image (centre image) depict theAJL (arrowheads), right and left superior recesses (white arrows), and PJL (black arrows). Left and right inferior recesses (long white arrows) areshown. Coronal 4 mm MPR (right image) demonstrates the superior (solid arrowheads) and inferior (open arrowheads) recesses of the PJL(arrows), which is above the aortic arch.

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pleura and adjacent air-filled lung contact the retro-manubrial fat (Fig. 10). A convexity of the AJL, superiorrecesses, or inferior recesses is suggestive of a mass in theselocations.

The posterior junction line (PJL) is formed by thecontact of right and left pleura posterior to the oesoph-agus and anterior to the vertebrae at the T3 through T5levels.25 On the frontal radiograph, the PJL appears asa vertical line or stripe projected in the vicinity of thetracheal air column, coursing from above the manubriuminferiorly to the level of the aortic and azygos arches. The

Figure 11 Parasternal and retrosternal stripes. Lateral film (left image)contours of the parasternal (arrowheads) stripe created by the posterioradiograph (second from right image) and sagittal MPR (right image) demwhen lung touches adjacent retrosternal mediastinal fat.

PJL, therefore, projects above the AJL (Fig. 10). When thereis not enough space between the vertebral bodies and theoesophagus, lung on one or both sides will then abut theoesophageal walls and adjacent mediastinal soft tissue,forming an edge. If air is present in the oesophageal lumenin this scenario, the oesophageal wall will project asa stripe in the region of the posterior junction line.Superior and inferior right and left recesses, as with theAJL, occur with the PJL (Fig. 10). Focal convexity in the PJLraises suspicion for adenopathy or oesophageal, neuro-genic, and aortic masses.3,25

and 5 mm sagittal MPR (second from left image) show the lobulater aspects of the anterior ribs contacting the adjacent lung. Lateralonstrate the straighter retrosternal stripe (arrowheads), which occurs

Figure 12 Cardiac incisura. Lateral chest radiograph and 5 mm sagittal MPRs show the cardiac incisura being formed by the exclusion of the leftlower lung from the chest wall by the cardiac apex (centre image, arrowheads) and pericardial fat pad (right image, arrowheads).

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Retrosternal and parasternal reflections

On the lateral radiograph, the parasternal and retro-sternal reflections have been termed ‘‘stripes.’’ Indenta-tions on the adjacent lung by the posterior portions of theribs as well as by any associated costal cartilage, intercostalmusculature, and internal mammary vessels create theparasternal stripe. The morphology of the parasternalstripe is usually lobulate. Areas that are convex towardsthe lungs are typically related to the costochondral carti-lage (Fig. 11).8 The internal mammary artery and vein andvery small lymph nodes reside in the vicinity of the par-asternal stripes. When disease is present in the parasternalregion, a focal convexity towards the lung will projectbetween rib levels.

The retrosternal pleural reflection courses parallel to theoverlying sternum and, unlike the parasternal stripe, doesnot undulate. The retrosternal reflection has also beentermed a stripe by some and occurs when mediastinal fat isinterposed between the chest wall and the anteromedialportions of both lungs (Fig. 11; Table 1).8 Its mean width is

Figure 13 Lateral chest radiograph and corresponding volume-renderedheads) coursing superiorly and posteriorly over the tracheal air column (riarrows) is defined by air within the abutting LUL.

1.5 mm, with a range of 1–3 mm. The retrosternal stripe,parasternal stripe, or both reflections can be visualized in90% of normal films.8

Cardiac incisura

Near its base, the anterior portion of the left lung isusually displaced from the anteromedial chest by epicardialfat and/or by the cardiac apex itself. The resultant interfacebetween lung and interposed fat/heart on the lateralprojection is called the cardiac incisura. This finding variesin appearance; rounded, angular, or straight configurationshave been described on the lateral chest film.8 Awareness ofthis normal finding helps minimize misdiagnosis of a masslesion on the lateral radiograph (Fig. 12).

Left subclavian artery

The left lateral margin of the left subclavian artery is seenabutting the LUL as a gently curving interface that extendssuperiorly from the level of the aortic arch to the left clavicle

CT image demonstrates the left subclavian artery reflection (arrow-ght image, arrows). The posterior aspect of the aortic arch (left image,

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(Fig. 9).26,27 Either the left subclavian or the right innomi-nate/subclavian arteries appear as a reflection travellingposterosuperiorly from the aortic arch and crossing the air-filled trachea (Fig. 13).7 Although it can be difficult todifferentiate between these two reflections, the leftsubclavian interface is more posterior and projects over theposterior aspect of the tracheal air column. Additionally, thecourse of the left subclavian artery from the aortic archappears more linear as opposed to the convex contour of theinnominate artery. Focal convexity in a vascular interfacewith the adjacent lung raises question of a mass, includinganeurysm.

Conclusion

Our understanding of thoracic structures and theirspatial orientation has been facilitated by the developmentof cross-sectional imaging. Multiplanar CT reformations canillustrate the sources and craniocaudal extents of radio-graphic lines, stripes, and reflections and hence act aspowerful tools for learning radiographic anatomy.

Acknowledgments

The authors thank Martha Helmers of Photoradiology inthe Department of Radiology at NYU Langone Medical Centerfor her meticulous attention to detail pertaining to imagepreparation. The authors also acknowledge Emilio Vega, RTRin the Department of Radiology at NYU Langone MedicalCenter for his assistance with post-processing techniques.

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