Normal radiographic anatomy of thoracic structures - BIR Publications

The British Journal of Radiology, 78 (2005), 398–404 DOI: 10.1259/bjr/20497987

E

2005 The British Institute of Radiology

Normal radiographic anatomy of thoracic structures: analysis of 1000 chest radiographs in Japanese population H ABIRU, MD, K ASHIZAWA, MD, R HASHMI, MD and K HAYASHI, MD Division of Radiological Science, Department of Radiology and Radiation Biology, Nagasaki University Graduate School of Biomedical Sciences, 1-7-1 Sakamoto, Nagasaki 852-8501, Japan

Abstract. The purpose of this paper was to study the frequency of visualization and characteristics of normal thoracic structures on posteroanterior (PA) chest radiographs in Japanese population. 1000 consecutive normal PA chest radiographs of men and women ranging in age from 20 years to 90 years were reviewed. Frequency of visualization and configuration of structures including (1) fissure lines such as major, minor, vertical fissure line, and accessory fissures, (2) vascular structures including normal apical opacity, aortic nipple, and descending aortic interface, and (3) other structures including air in the oesophagus, aortic pulmonary stripe, and diaphragm were studied. On PA chest radiographs: (1) minor fissure, superolateral major fissure, superomedial major fissure, vertical fissure line, superior accessory fissure, and inferior accessory fissure were visualized in 74.7%, 19.7%, 15.4%, 1.6%, 2.9% and 13.1%, respectively. (2) Normal apical opacity was seen in 3.7%, while aortic nipple was seen in 0.9%. Descending aortic interface was obliterated in 13.7%. (3) Air in the oesophagus and aortic pulmonary stripe were seen in 8.9% and 17.7%, respectively. Hemidiaphragm was obliterated in 10.3% on the right, and in 32.4% on the left. Scalloping of the diaphragm was seen in 10.6% on the right, 6.5% on the left, and 4.3% bilaterally. Frequency of visualization and characteristics of various normal anatomical structures on chest radiographs in Japanese population differ from those reported previously from the West. Familiarity with these normal thoracic structures and variations is important for our daily image interpretation.

Although the use of chest CT has greatly increased over the past several years, chest radiography remains the most frequently performed imaging examination. A good understanding of normal anatomy and variations is essential for the interpretation of chest radiographs. Important normal anatomical structures on posteroanterior (PA) chest radiographs include fissure lines such as minor fissure [1], superolateral and superomedial major fissures [2, 3], vertical fissure line [4], superior and inferior accessory fissures [1, 5], and vascular structures such as normal apical opacity [6], aortic nipple [7, 8], and descending aortic interface [9]. Most of the data regarding these normal structures have been provided from the West (Europe and USA) [1–8]. There have been few reports describing the normal radiographic anatomy and variations of the thoracic structures in Japanese population [9]. Therefore we reviewed normal PA chest radiographs and analysed the radiographic anatomy in detail among Japanese population.

with thoracic deformities such as pectus excavatum and scoliosis were excluded. The chest radiographs were obtained in erect posture with focus to film distance of 1.5 m using 90,100 kVp, 150 mA, 10,20 ms, 1.6 mm focal spot, Fuji HR4 or Fuji HGM screen, 12:1 grid with 105 lines, and Fuji UR1 film (Fuji Medical Systems, Tokyo, Japan). All radiographs were reviewed by two chest radiologists. PA radiographs were examined for the visualization and characteristics of: (1) fissure lines including minor fissure, superolateral major fissure, superomedial major fissure, vertical fissure line, superior accessory fissure, and inferior accessory fissure (Figure 2); (2) vascular structures including normal apical opacity, aortic nipple, and descending thoracic aortic interface; and (3) other structures including air in the oesophagus, aortic pulmonary stripe and diaphragm (Figure 3). The following is the brief explanation of each item including the definition and points of our investigation.

Methods and materials We evaluated 1000 consecutive normal PA chest radiographs of Japanese adults obtained between January and May in 1996. The patients were 482 men and 518 women, ranging in age from 20 years to 90 years (average 49 years). There was almost the same distribution of patients in different age groups of both sexes (Figure 1). The chest radiographs were obtained as a part of routine work-up or follow-up study for non-chest diseases. Patients Received 21 July 2004 and in revised form 11 October 2004, accepted 23 November 2004. Address correspondence to Dr Kazuto Ashizawa.

398

Figure 1. A bar graph showing distribution of patient’s age. The British Journal of Radiology, May 2005

Normal radiographic anatomy of thoracic structures

(2) Superolateral major fissure was defined as a curving line or edge with a lateral convexity at the upper lateral right and/or left hemithorax [2]. (3) Superomedial major fissure was defined as an obliquely oriented relatively short straight line. On the right it was found in the vicinity of the right tracheobronchial angle, while on the left it was found in the vicinity of the aortic knob. When both superomedial major fissures were seen on the same PA view, the left usually extended slightly higher than the right [3]. (4) Vertical fissure line was defined as a straight or slightly curved vertical line with a lateral convexity near the right and/or left costophrenic angle [4]. (5) Superior accessory fissure was defined as a line parallel and inferior to the minor fissure. As it is difficult to distinguish between left minor fissure and left superior accessory fissure without CT, left superior accessory fissure was excluded from evaluation [1, 5]. (6) Inferior accessory fissure was defined as an oblique line near the right and/or left cardiophrenic angle [1, 5]. Figure 2. Schematic drawing of fissure lines. 1: Minor fissure. 2: Superolateral major fissure. 3: Superomedial major fissure. 4: Vertical fissure line. 5: Superior accessory fissure. 6: Inferior accessory fissure.

Fissure lines Figure 2 shows schematic drawing of the 6 fissure lines. (1) Minor fissure. When visualized, following features of the minor fissure were evaluated: number, angle (lateral side higher, medial side higher, or horizontal), shape (convex upward, convex downward, flat, or sigmoid), and length of visualization (dividing the fissure into three equal parts) [1].

Vascular and other structures Schematic drawing of vascular and other structures is shown in Figure 3. (1) Normal apical opacity was defined as a homogeneous, round opacity with unsharp margin, approximately at the midpoint between the spine and the inner margin of the first anterior rib above the clavicle [6]. (2) Aortic nipple was defined as a small ‘‘nipple’’ projecting from the lateral aspect of the aortic knob [7, 8]. (3) Obliteration of the descending aortic interface was defined as non-visualization of any portion of the interface longer than 1 cm [9]. The interface was divided into three equal parts: superior, middle, and inferior portions. (4) Air in the oesophagus was defined as collection of air below the aortic knob with triangular appearance [10]. (5) Aortic pulmonary stripe was defined as an increased density with an oblique, lateral edge that extends across the outline of the aortic knob superiorly and the left pulmonary artery inferiorly [11]. (6) Diaphragm. The difference in height between the right and left hemidiaphragm, and focal obliteration and smooth arcuate elevation (the so-called scalloping) of the diaphragm were examined. To evaluate focal obliteration, the diaphragm was divided into three equal parts (medial, middle, and lateral) [1, 12].

Results Fissure lines

Figure 3. Schematic drawing of vascular and other structures. 1: Normal apical opacity. 2: Aortic nipple. 3: Descending thoracic aortic interface. 4: Air in the oesophagus. 5: Aortic pulmonary stripe. 6: Diaphragm.

The British Journal of Radiology, May 2005

Visualization of fissure lines in our study and in reported data is shown in Table 1, and characteristics of minor fissure are shown in Table 2. The minor fissure was visualized in 74.7%. It was visualized as one line in 49.2% (Figure 4a), as two lines in 24.2% (Figure 4b), as three lines in 1.2%, and as four lines in 0.1%. Lateral side of the fissure was higher in 24.8%, while medial side was higher in 28.2%. In 21.7% the fissure was horizontal. The fissure was convex upward in 32.3%, convex downward in 2.6%, flat in 34.5%, and sigmoid shaped in 5.3%. The minor fissure was visible for more 399

H Abiru, K Ashizawa, R Hashmi and K Hayashi Table 1. Visualization of the six fissure lines

Minor fissure Superolateral major fissure Superomedial major fissure Vertical fissure Superior accessory fissure Inferior accessory fissure

Right

Left

Bilateral

Overall

74.7 5.1 4.5 0.7 2.9 7.1

8.5 (6.0) [2] 9.7 0.8 — (5–14) [5] 5.2 (1.0) [5]

6.1 (4.0) [2] 1.2 0.1 — (12.0) [5] 0.8 (0.6) [5]

19.7 (14.0) [2] 15.4 (8.0) [3] 1.6

(44,56) [1] (4.0) [2]

(30.0) [5] (5–6.6) [5]

13.1

Numbers are our data expressed in percentage and those in parentheses are reported data in percentage.

Table 2. Characteristics of the minor fissure Number 1 2 3 4

line lines lines lines

Angle 49.2 24.2 1.2 0.1

Lateral side higher Medial side higher Horizontal

Shape 24.8 28.2 21.7

Length

Convex upward Convex downward Flat Sigmoid shape

32.3 2.6 34.5 5.3

More than two-thirds One-third to two-thirds Less than one-third

32.0 29.6 13.1

Numbers are expressed as percentages.

than two-thirds of its length in 32.0%, a third to two-thirds in 29.6%, and less than one-third in 13.1%. Superolateral major fissure was visible in 19.7%, including 5.1% on the right, 8.5% on the left, and 6.1% bilaterally (Figure 5). Superomedial major fissure was seen in 15.4%, including 4.5% on the right, 9.7% on the left, and 1.2% bilaterally (Figure 6). Vertical fissure line was visible in 1.6%, including 0.7% on the right (Figure 7), 0.8% on the left, and 0.1% bilaterally. Right superior accessory fissure was visualized in 2.9% (Figure 8). Inferior accessory fissure was visible in 13.1%, including 7.1% on the right (Figure 9), 5.2% on the left, and 0.8% bilaterally.

Visualization of normal apical opacity, aortic nipple, air in the oesophagus, and aortic pulmonary stripe in our data and in reported data is shown in Table 3.

The normal apical opacity was visible in 3.7%, including 1.2% on the right, 1.9% on the left (Figure 10), and 0.6% bilaterally. The aortic nipple was visualized in 0.9% (Figure 11). Focal obliteration of the descending aortic interface was observed in 13.7%, including 1.0% at the superior portion, 3.7% at the middle, 4.1% at the inferior portion, 0.2% at the superior to middle, and 4.3% at the middle to inferior portions (Figure 12) (Table 4). Two portions of the aortic interface were obliterated in four cases. All parts of the descending aortic interface were clearly seen in 86.3%. Air in the oesophagus was seen in 8.9% (Figure 13). The aortic pulmonary stripe was visible in 17.7% (Figure 14). In 94.2%, the right hemidiaphragm was higher than the left, while in 1.4% the left hemidiaphragm was higher (Table 5). In the remaining 4.4%, right and left hemidiaphragms were visualized at the same level. Hemidiaphragm was obliterated in 10.3% on the right and in 32.4% on the left. In the right hemidiaphragm, focal

(a)

(b)

Vascular and other structures

Figure 4. Posteroanterior chest radiographs showing minor fissures in (a) a 67-year-old man and (b) a 54-year-old woman. (a) The minor fissure with shape of convexity upward is visible as one line (arrows). Medial side of it is higher than lateral and its length is more than two-thirds. Focal obliteration of the fissure is seen medially but outside of the pulmonary artery. (b) The minor fissure is seen as two lines running parallel to each other (arrows). 400


Normal radiographic anatomy of thoracic structures

Figure 5. Posteroanterior chest radiographs showing superolateral major fissures in a 25-year-old man. Fissures are seen as curving contours with lateral opacity and medial lucency bilaterally (arrows). The left superolateral major fissure extends higher than the right.

Figure 7. Posteroanterior chest radiograph showing vertical fissure line in a 30-year-old man (arrows).

Figure 6. Posteroanterior chest radiograph showing superomedial major fissure in a 68-year-old man. Fissures appear as short straight lines bilaterally (arrows).

obliteration was seen medially in 8.8%, in middle in 0.6%, and laterally in 0.2%. In the left hemidiaphragm, focal obliteration was seen medially in 21.2%, in middle in 3.5%, and laterally in 0.7%. Smooth arcuate elevation of the diaphragm was seen in 10.6% on the right, 6.5% on the left, and 4.3% bilaterally. The number of smooth arcuate elevation of the diaphragm on the right and left were one in 11.5% and 8.3%, two in 3.2% (Figure 15) and 2.1%, three in 0.1% and 0.2%, and four in 0.1% and 0.2%, respectively.

factor to higher frequency of visualization of most of the fissures. According to an old study by Felson, the minor fissure was absent or poorly developed in 20% of anatomical dissections, while it was visualized in only 56% on radiograph [1]. Frequency of visualization of the minor fissure was 74.7% in our study; this approximates the data of anatomical dissection.

Discussion We evaluated 1000 consecutive normal PA chest radiographs and reported the frequency of visualization and characteristics of various normal structures among Japanese population. There has been considerable improvement in the image quality of chest radiographs since the era when most of the papers referred to in this report were published. This improvement in image quality could be a contributing The British Journal of Radiology, May 2005

Figure 8. Posteroanterior chest radiograph showing right superior accessory fissure in a 28-year-old woman. The fissure lies inferior to and parallel to minor fissure (arrows). 401

H Abiru, K Ashizawa, R Hashmi and K Hayashi

Figure 9. Posteroanterior chest radiograph showing right inferior accessory fissure in a 64-year-old man as a thin line extending from the diaphragm obliquely upward toward hilum (arrows).

Table 3. Visualization of vascular and other structures Right Normal apical opacity Aortic nipple Air in oesophagus Aortic pulmonary stripe

Left

Bilateral Overall

1.2 (12.8) 1.9 (16.8) 0.6 [6] [6]

3.7

Figure 11. Posteroanterior chest radiograph showing aortic nipple in a 28-year-old man. A small nipple around the aortic knob is seen (arrow).

The apical opacity has been only reported by Proto and Challiff [6]. This is important and should be differentiated from intrapulmonary nodule, in particular early lung cancer occurring in the apical region. The visualization of the normal apical opacity in their study was astonishingly high when compared with our data. This may be due to

0.9 (1.4–9.5) [7] 8.9 (15.0) [10] 17.7


Recognition of superolateral and superomedial major fissures is essential for the daily interpretation of chest radiographs. Proto and Ball first reported these fundamental chest radiographic findings [2, 3]. Their importance is twofold. First, they should not be mistaken for some pathological condition. Second, they can serve as a landmark for the localization of intrapulmonary lesion.

Figure 10. Posteroanterior chest radiograph showing normal apical opacity in a 41-year-old man. This opacity is seen above the clavicle and between the lateral margin of the spine and the inner margin of the first anterior rib (arrows).

402

Figure 12. Posteroanterior chest radiograph showing focal obliteration of the descending aortic interface at inferior portion in a 50-year-old man (arrows).


Normal radiographic anatomy of thoracic structures Table 4. Rate and portion of obliterated descending aortic interface Portion of obliteration One portion Superior Superior to middle Middle Middle to inferior Inferior Two portions Superior and middle to inferior Superior and inferior Overall

Obliteration rate 1.0 0.2 3.7 4.3 4.1 0.3 0.1 13.7

Numbers are expressed in percentage.

higher incidence of atherosclerotic tortuous subclavian artery in their patients, but may need re-evaluation. The descending thoracic aortic interface can be obliterated in various pathological conditions. These include cardiomegaly, aortitis (such as Takayasu arteritis), and pectus excavatum. Inflammatory or neoplastic lesions adjacent to the descending thoracic aorta may also obliterate the interface. Obliterated descending aortic

Figure 14. Posteroanterior chest radiograph showing aortic pulmonary stripe in a 23-year-old man. This stripe is seen as an oblique contour extending across the outline of the aortic knob from superior medium to the left hilum (arrows).

interface can be the first clue to the diagnosis of mediastinal lymphadenopathy or mass, mediastinal vascular disease, pleural effusion, oesophageal lesions, intrathoracic extension of retroperioneal lesions, pneumonia, and lung cancer [9]. One should be aware, however, that the similar appearance can be a normal finding. The descending aortic interface is most often obliterated at middle to inferior portion. Smooth arcuate elevation of the diaphragm, the socalled scalloping, is one of the fundamental normal chest radiographic findings, and should not be mistaken for a mass lesion of the diaphragm or the liver. The frequency of the visualization of smooth arcuate elevation was high

Table 5. Visualization and characteristics of the diaphragm Level


Left higher

Same

94.2 (91) [1]

1.4

4.4

Portion of obliteration

Right

Left

Medial Middle Lateral Medial to Middle Middle to Lateral Medial and Lateral All portions Overall

8.8 0.6 0.2 0.5 0.1 0.1 0.0 10.3

21.2 3.5 0.7 6.2 0.2 0.1 0.5 32.4

Scalloping

Figure 13. Posteroanterior chest radiograph showing air in the oesophagus in a 60-year-old woman. Segmental air in the oesophagus is visible as a triangular lucency below aortic knob (arrowheads). Right and left pleuroesophageal stripe are also seen (arrows).

Right higher

Right

Visualization 10.6 (4.2) of scalloping [1] Number of scalloping 1 11.5 2 3.2 3 0.1 4 0.1

Left

Bilateral

Overall

6.5 (0.2) 4.3 (1–1.1) 21.4 (5.5–10.9) [1] [1, 12] [1, 12]

8.3 2.1 0.2 0.2


403

H Abiru, K Ashizawa, R Hashmi and K Hayashi

data are helpful in understanding normal thoracic structures. Familiarity with these normal structures and their variations on chest radiographs in the Japanese population is important in image interpretation in our daily work.

References

Figure 15. Posteroanterior chest radiograph showing the right diaphragm in a 77-year-old woman. The right diaphragm with two smooth arcuate elevations is seen.

in our study when compared with some others [1, 12]. In our study, hemidiaphragm was obliterated in 10.3% on the right and in 32.4% on the left. Focal obliteration was more often seen in medial portion especially on left side, probably because of obscuration by the heart. These results are in agreement with Felson’s reports [1]. Although focal obliteration of the diaphragm can easily be recognized as abnormal, we should recognize that medial portion of the left as well as right diaphragm is occasionally obliterated on normal chest radiograph. The obliteration of medial portion of the right hemidiaphragm may be attributed to the phrenic nerve that descends between the pericardium and the parietal pleura, along the lateral aspect of inferior vena cava on the right side [13, 14]. In conclusion, we evaluated 1000 consecutive normal PA chest radiographs of Japanese adults, and showed frequency of visualization and characteristics of various normal anatomical structures. Our data have some differences from the data of previously published reports. It is not clear whether the differences are due to different racial and physical characteristics, different radiographic techniques or accidental. Although the number of the radiographs we evaluated may not be large, we believe our

404

1. Felson B. Chest roentgenology. Philadelphia, PA: WB Saunders, 1973. 2. Proto AV, Ball JB Jr. The superolateral major fissures. AJR Am J Roentgenol 1983;140:431–7. 3. Proto AV. The chest radiograph: anatomic considerations. Clin Chest Med 1984;5:213–46. 4. Friedman E. Further observations on the vertical fissure line. Am J Roentgenol Radium Ther Nucl Med 1966;97:171–3. 5. Godwin JD, Tarver RD. Accessory fissure of the lung. AJR Am J Roentgenol 1985;144:39–47. 6. Proto AV, Chaliff MI. Apical opacity: a normal finding on posteroanterior chest radiographs. Radiology 1986;161:429– 32. 7. Ball JB Jr, Proto AV. The variable appearance of the left superior intercostal vein. Radiology 1982;144:445–52. 8. Friedman AC, Chambers E, Sprayregen S. The normal and abnormal left superior intercostal vein. AJR Am J Roentgenol 1978;131:599–602. 9. Takahashi K, Shinozaki T, Hyodo H, Ogawa C, Ohsawa T. Focal obliteration of the descending aortic interface on normal frontal chest radiographs: correlation with CT findings. Radiology 1994;191:685–90. 10. Proto AV, Lane EJ. Air in the esophagus: a frequent radiographic finding. AJR Am J Roentgenol 1977;129:433– 40. 11. Keats TE. The aortic-pulmonary mediastinal stripe. Am J Roentgenol Radium Ther Nucl Med 1972;116:107–9. 12. Kobayashi T, Watanabe T, Nakanishi F, Moriya K. Normal variants of the diaphragm: scalloping and dromedary. Rinsho Hoshasen 1979;24:361–6 [in Japanese]. 13. Ojiri H, Ujita M, Ariizumi M, Tada S. CT anatomy of phrenic nerve, supradiaphragmatic inferior phrenic artery, and inferior pulmonary ligament. Rinsho Hoshasen 1993;38:47–53 [in Japanese]. 14. Ujita M, Ojiri H, Ariizumi M, Tada S. Appearance of the inferior phrenic artery and vein on CT scans of the chest: a CT and Cadaveric study. AJR Am J Roentgenol 1993;160:745–7.
