SCIENTIFIC EXHIBIT
The Ribs: Anatomic and Radiologic Considerations1 Yasuyuki Kurihara, MD Yoshiko K. Yakushiji, MD Junichi Matsumoto, MD Tohru Ishikawa, MD Kazuaki Hirata, MD CME FEATURE This article meets the criteria for 1.0 credit hour in category 1 of the AMA Physician’s Recognition Award.
To obtain credit, see the questionnaire on pp 147–154.
LEARNING OBJECTIVES After reading this article and taking the test, the reader will:
• Be able to differentiate between the right and left ribs on lateral chest radiographs and count ribs on chest CT scans.
• Be familiar with the essen-
The ribs are essential structures of the osseous thorax and provide information that aids in the interpretation of radiologic images. Techniques for making precise identification of the ribs are useful in detection of rib lesions and localization of lung lesions. The big rib sign and the vertical displacement sign can be used to differentiate the right and left ribs on lateral chest radiographs. The clavicle, the xiphoid process, and the sternal angle may be used as anatomic landmarks for rib counting on computed tomographic scans. For rib counting on lateral chest radiographs, the sternal angle or the 12th rib may be used. Anatomic rib variants include developmental deformities, cervical rib, and short rib and may mimic true rib diseases. Detection of thoracic deformities such as funnel chest (pectus excavatum) and barrel-shaped thorax requires an awareness of the strong correlation between the transverse appearance of the thorax and costal shape. Shadows around the rib cage (eg, rib companion shadows, sharp lines along the lower margin of the ribs, rib overlying shadows) may mimic pleural and extrapleural disease on frontal chest radiographs. It is imperative that the radiologist be familiar with normal rib anatomy, normal rib variants, and the radiologic appearance of the ribs to prevent misdiagnosis.
tial anatomic variants of the ribs.
• Understand the correlation between the transverse appearance of the thorax and costal shape.
• Be familiar with the characteristic shadows that accompany the ribs and mimic pleural and extrapleural disease. Index terms: Ribs, 471.13, 471.14, 471.92 • Thorax, CT, 471.1211 • Thorax, radiography, 471.11 RadioGraphics 1999; 19:105–119 1From
the Departments of Radiology (Y.K., Y.K.Y., J.M., T.I.) and Anatomy (K.H.), St Marianna University School of Medicine, 2-16-1 Sugao, Miyamae-Ku, Kawasaki City, Kanagawa, Japan 216. Presented as a scientific exhibit at the 1997 RSNA scientific assembly. Received March 16, 1998; revision requested April 29 and received June 22; accepted June 22. Address reprint requests to Y.K.
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Figure 2. Vertical displacement sign. Schematic demonstrates how the vertical divergence of the x-ray beam results in the progressive vertical separation of the paired ribs as one moves in either the cephalic or caudal direction from the midthorax.
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INTRODUCTION
Although the ribs are clearly visible on chest radiographs and computed tomographic (CT) scans, radiologists seldom pay attention to their appearance. Actually, the ribs provide useful information that aids in the interpretation of these images. In this article, we discuss and illustrate identification of the ribs; anatomic rib variants; and the radiologic appearance of the ribs with anatomic correlation, including correlation between the transverse appearance of the thorax and costal shape and characteristic shadows that accompany the ribs at radiography. ■
IDENTIFICATION OF THE RIBS
Differentiating the Right and Left Ribs on Lateral Chest Radiographs
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Diseases of the ribs and diaphragm are often identified only at lateral chest radiography. Therefore, it is important to use a reliable technique for differentiating the right and left ribs. Two techniques—the “big rib” sign and the vertical displacement sign—can be used to localize the right and left ribs at lateral chest radiography. The big rib sign (1) is a well-known technique that exploits the difference in magnification between the right and left sides on lateral chest radiographs. The side of the rib cage farther from the film is magnified more than the side closer to the film. On a well-positioned left lateral chest radiograph, the right ribs appear larger than
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Figure 1. Big rib sign. On a left lateral chest radiograph, the comparable portions of the posterior and anterior ribs appear very different in size (arrowheads). The posterior rib (right) is farther from the film and is magnified more than the anterior rib (left), which is in contact with the film.
the left ribs (Fig 1). This difference in rib size is more easily detected posteriorly where the x-ray beam is tangential to the ribs but can be appreciated at all corresponding points along the curvature of the two ribs. In addition, rotating the patient may enhance or reduce the magnification effect because, with such rotation, the xray beam is transmitted through different portions of the ribs, which are thin medially and thick laterally. In a left lateral projection, when
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b. Figure 3. Vertical displacement sign. (a) Left lateral chest radiograph (magnified view) shows one pair of ribs projecting at the horizontal level (H). In the inferior pair of ribs (I), the posterior rib (arrow) is slightly lower than the anterior rib. In contrast, in the cranial rib pair (S), the posterior rib (arrowhead) is higher than the anterior rib. Thus, the posterior ribs are farther from the film and are on the right. (b) Left lateral chest radiograph (magnified view) obtained at a slightly different angle shows the upper anterior rib (arrowhead) cephalad and the lower anterior rib (arrow) caudad to the posterior ribs (ie, the anterior ribs are on the right). (c) Left lateral chest radiograph (magnified view) demonstrates how the vertical displacement sign is usually applicable even when the posterior ribs are partially superimposed. The higher rib (arrowhead) and lower rib (arrow) could be identified even when they overlapped.
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the posterior portions of the right and left ribs appear comparable in size, the hemidiaphragm traceable to the most anterior ribs is the right hemidiaphragm. Otherwise, the significantly larger ribs are the right ribs, which are farther from the film. The big rib sign is very useful but is not perfect because the magnification difference between the right and left ribs is only 10%. For example, if the width of a rib is 5 mm, the observed
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difference between sides is only 0.5 mm, which is not always sufficient to enable differentiation of the two sides. Furthermore, the big rib sign is not applicable when the posterior ribs are superimposed. In 1997, we presented a new technique called the vertical displacement sign, which is an easy, reliable, and precise method for differentiating the right and left ribs on lateral radiographs (2) (Figs 2, 3). The vertical displacement sign is based
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Figure 5. Rib counting at CT with the xiphoid process as an anatomic landmark. Axial CT scan shows the xiphoid process (*), the seventh rib (7), and the relative position of the lower ribs (6–9). It is important to identify the transverse portion of the costal cartilage (arrows), at which point one stops counting down and starts counting up (ie, moves from medial to lateral). 5 = fifth rib.
primarily on the vertical divergence of the x-ray beam rather than on the magnification of the ribs. Because the right rib cage is farther from the film, the projection of the right ribs on a lateral radiograph will fan out and diverge in a vertical direction to a greater degree than that of the left ribs. Thus, the right side can be distinguished from the left by the vertical displacement of the paired ribs. At our facility, the distance between the x-ray target and the film in lateral chest radiography is 200 cm (~78 in). If the intercostal space is about 3 cm (~1.2 in), the slight vertical divergence at the most posterior aspect of the rib cage results in a 3-mm separation, whereas the greater vertical divergence at a more cephalic or caudal location results in a larger separation. The vertical displacement sign is usually applicable even when the posterior ribs are partially superimposed. It is usually possible to identify which rib is higher or lower even when they overlap. The vertical displacement sign can be used as an alternative when the big rib sign is not applicable. ●
Counting Ribs on Chest CT Scans
Three methods are commonly used to count ribs on CT scans. These three methods make use of the clavicle, the xiphoid process, or the sternal angle as an anatomic landmark to determine the precise location of the ribs. In 1990, Bhalla et al (3) presented a useful method for counting ribs on CT scans in which the bilateral clavicles were used as anatomic landmarks. The first step was to identify the first rib on the axial image that demonstrated the medial
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Figure 4. Rib counting at CT with the clavicle as an anatomic landmark. Axial CT scan shows the medial third of the clavicle (*) and the relative position of the first two ribs (1, 2).
third of the clavicle (Fig 4). Bhalla et al also used the costovertebral articulation as a landmark for counting vertebral bodies. Identifying the first rib in relation to the clavicles is easy and precise; however, counting ribs from the clavicles is tedious for middle and lower rib lesions. Furthermore, counting thoracic vertebrae may be imprecise owing to partial volume effect. In 1993, Kim et al (4) presented a new technique that made use of the proximal xiphoid process as an anatomic landmark. The seventh costal cartilages articulate with the anterior or anterolateral portion of the xiphoid process, and the sternal ends of these cartilages can be identified on an axial chest CT scan that demonstrates the proximal xiphoid process (Fig 5).
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c. d. Figure 6. Rib counting at CT with the sternal angle as an anatomic landmark. Sequential axial chest CT scans of the ribs and sternum show identification of the sternal angle (*) with the second costal cartilage and rib (2) followed by the counting of the third, fourth, and fifth ribs (3–5) in numeric order (a). The fifth rib is traced as the reference rib in sequential planes (b – d). Note that the rib moves slightly anterior in the caudal planes. The target rib with the metastatic lesion (arrowhead in d) is identified by counting ribs in numeric order from fifth (5) to fourth (4) in the same axial plane.
This method is particularly useful because the ribs can be counted on an abdominal CT scan. However, identification of the seventh costal cartilages near the xiphoid process can be difficult. Furthermore, the order of the costal cartilages and ribs on an axial image can be confusing. In 1995, we presented a third method for counting ribs on CT scans. In this method, the primary anatomic landmark is the sternal angle, which is the joint between the sternal body and the manubrium (5). The second costal cartilages attach to the sternal angle, which appears as an oval to square structure on the mediastinal win-
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dow that is smaller than the remaining portion of the sternum. The central portion of the angle has a higher attenuation than the sternal body or manubrium because it lacks bone marrow. The second costal cartilages are relatively high in attenuation and are connected to the second ribs laterally (Fig 6a). Our method for counting ribs on a CT scan (Fig 6) comprises the following steps: (a) Identify the sternal angle with the second rib and
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a. b. Figure 7. Rib counting at lateral chest radiography with the sternal angle as an anatomic landmark. (a) Diagram of the ribs and sternum (lateral view) shows the second costal cartilage attached to the sternal angle (*), which is characterized by a slight anterior convex angulation. The second rib (darkened area) courses upward obliquely. (b) Lateral chest radiograph shows the second rib (arrowheads) and costal cartilage attached to the sternal angle (*).
costal cartilage. (b) Count the next three or four ribs in numeric order from front to back. (c) Select one rib as the reference rib and trace it on sequential images. Contiguous scans usually show slight anterior movement of the reference rib in the next caudal section and posterior movement in the next cephalic section because the ribs slope caudally from back to front. (d) Identify the target rib by tracing the reference rib on sequential images and counting the ribs in numeric order on the same image.
Counting Ribs on Lateral Chest Radiographs
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Counting ribs on a lateral chest radiograph is sometimes necessary to determine which ribs or thoracic vertebrae are involved in a disease process. One method is to use the sternal angle as an anatomic landmark (Fig 7) as described earlier. The sternal angle is easily identified on a lateral radiograph, which demonstrates a slight angulation of the anterior border of the sternum at the sternal angle. The sternal angle
also appears as an interface between the cortex of the body and the manubrium of the sternum. Once the sternal angle is located, it is easy to identify the second rib and cartilage (6). Unfortunately, this method is not always useful for identifying the thoracic vertebrae because the area around the vertebrae is usually not sufficiently demonstrated on a lateral chest radiograph to enable recognition of the structures. Furthermore, the overlapping bilateral ribs are very hard to distinguish. We recommend using the 12th rib on a lateral radiograph as an anatomic landmark for identifying the vertebrae and ribs (Fig 8). The 12th (or lowest) rib is usually clearly seen as the most caudal arch. Once this rib is identified, the number of vertebrae is automatically determined. ■
ANATOMIC VARIANTS OF THE RIBS
Developmental Deformities of the Ribs
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There are various types of congenital anomalies and deformities of the ribs, including developmental fusion of two or more ribs, articula-
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Figure 8. Determining the number of vertebrae with the 12th rib as an anatomic landmark. Lateral chest radiograph demonstrates the 12th (ie, the lowest) rib as the most caudal arch (arrow). The L1 vertebral body is compressed.
Figure 9. Bifid (forked) rib. Posteroanterior chest radiograph (magnified view) shows a bifid or forked deformity of the anterior end of the fifth rib (* and white arrows). An associated spurlike process or incomplete fusion of the ribs (black arrow) is seen.
lies are usually of little or no clinical significance; however, one should be familiar with them to distinguish them from true rib diseases. ●
Figure 10. Cervical rib. Posteroanterior chest radiograph (magnified view) shows a supernumerary bone arising from the seventh cervical vertebra (arrow).
tion or bridge formation between two ribs, and bifid rib (forked rib) (Fig 9). These morphologic anomalies and anatomic variants occur in 0.15%–0.31% of the population, have a female predilection, and occur more frequently on the right side than on the left side (7). These anoma-
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Cervical Rib
A cervical rib is a supernumerary or accessory rib arising from the seventh cervical vertebra. This abnormality occurs in approximately 0.5% of the population and is more common in females than in males (8) (Fig 10). Although a cervical rib is usually asymptomatic, it is the most important anatomic rib variant from a clinical standpoint because it can cause thoracic outlet syndrome by compression of the brachial plexus or subclavian vessels. This syndrome is usually associated with pain in the hand when the arm is elevated, difference in pulse intensity between the two arms when the affected extremity is in a certain position, and Raynaud phenomenon. Angiography is helpful in the diagnosis and essential in the evaluation of this syndrome in patients with a cervical rib. ●
Short Rib
Frontal chest radiography sometimes reveals a shortened midthoracic rib arch in patients who have not experienced trauma or undergone surgery (Fig 11). Short rib is diagnosed if the lateral margin of the affected rib is more than 4 mm
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a. b. Figure 12. Costal shape in the normal thorax. (a) Photograph of a right rib shows the smooth, hemispheric curvature of the rib. (b) Posteroanterior chest radiograph shows the overlying arc of the ribs.
medial to a tangent drawn between the lateral margins of adjacent ribs. According to Sheflin (9), short rib occurs in approximately 16% of the population. In addition, short rib is more common on the right side than on the left (8% vs 1%) and is seen bilaterally in 7% of patients. This variant has been reported in only the sixth, seventh, and eighth ribs. Shortening of the rib arch does not appear to be clinically significant. The explanation for this normal variant may involve rib development and maturation: Early fusion of the epiphyseal center could cause a shortening of the rib arch.
CORRELATION BETWEEN TRANSVERSE APPEARANCE OF THE THORAX AND COSTAL SHAPE
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There is a strong correlation between costal curvature and the transverse appearance of the rib cage on a frontal chest radiograph. The frontal appearance of the ribs provides important information regarding thoracic deformity. Figure 11. Short rib. Posteroanterior chest radiograph shows a short midthoracic rib arch (arrow).
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c. Figure 13. Funnel chest. (a, b) Schematic (a) and posteroanterior chest radiograph (b) show accentuation of the downward angulation of the anterior portions of the ribs, which run almost parallel to each other. (c) Lateral chest radiograph shows depression of the sternum (arrow).
ally; therefore, the curvature of each rib on a frontal chest radiograph is also hemispheric and smooth (Fig 12). ●
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Costal Shape in the Normal Thorax
Axial images of a normal thorax show an almost completely oval rib cage. The lateral margins of the ribs are hemispheric and symmetric bilater-
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Funnel Chest
Funnel chest (pectus excavatum) is a depression of the sternum. Lateral chest radiography clearly shows the position of the sternum, which allows the diagnosis to be made without difficulty. This deformity demonstrates several characteristics at frontal chest radiography, including an indistinct border of the right side of the heart, decreased heart density, and displacement of the heart. The appearance of the bilateral ribs is also unique: The frontal view usually accentuates the downward angulation of the anterior portions of the ribs, which run almost parallel to each other (10) (Fig 13). The posterior portions of the ribs sometimes angle slightly upward.
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a. b. Figure 14. Barrel-shaped thorax in a patient with diffuse panbronchiolitis (Asian bronchiolitis). (a) Axial chest CT scan shows a barrel-shaped thorax characterized by a relatively long anteroposterior diameter of the osseous thorax. (b, c) Posteroanterior chest radiograph (b) and schematic (c) show the straight, downward angulation of the lateral portion of the ribs (arrows in b), which become elongated as the sagittal diameter of the thorax increases.
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Barrel-shaped Thorax
Barrel-shaped thorax, characterized by a relatively large sagittal diameter of the osseous thorax, is frequently seen in patients with chronic obstructive pulmonary disease. A large anteroposterior diameter is also observed on lateral chest radiographs in patients with emphysema (11). The appearance of the osseous thorax on axial images in patients with barrel-shaped thorax is more square than oval or spherical (Fig 14a). On frontal radiographs, the lateral portion of each rib appears elongated and straight and points straight downward (Fig 14b, 14c).
SHADOWS THAT ACCOMPANY THE RIBS
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Frontal chest radiographs show several characteristic shadows around the rib cage that may mimic pleural and extrapleural disease. Recognition of these characteristic shadows is important to prevent misdiagnosis.
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Rib Companion Shadows
Thin, smooth, water-density shadows that parallel the ribs and measure 1–5 mm in diameter project adjacent to the inferior and inferolateral margins of the first and second ribs and the axillary portions of the lower ribs (Fig 15). These companion shadows of the first and second ribs occur in 35% and 31% of the population, respectively (12).
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a. b. Figure 16. Rib companion shadows. (a) Posteroanterior chest radiograph reveals thick companion shadows (arrows). (b) Anteroposterior chest radiograph fails to demonstrate these shadows.
Figure 15. Rib companion shadows. Posteroanterior chest radiograph (magnified view) shows rib companion shadows projecting adjacent to the inferior and inferolateral margins of the second rib (arrows).
Gluck et al (13) showed that most rib companion shadows are associated with fat. The authors found differences in the thickness of companion shadows between obese patients
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and healthy subjects. They also measured the thickness of companion shadows in obese patients before and after weight reduction. The mean thickness decreased significantly (P < .001) from almost 3.0 mm before dieting to approximately 1.2 mm afterward. Companion shadows are now thought to be caused by the parietal pleura and the soft tissues (principally fat) immediately external to the pleura in tangential projections and should not be interpreted as local pleural thickening (14). Rib companion shadows along the inferior margin of the second ribs have some interesting radiographic features. First, the shadows are not always similar on posteroanterior and anteroposterior images. The anteroposterior view sometimes fails to show a rib companion shadow, whereas the posteroanterior view reveals thick companion shadows (Fig 16). Second, a sagittal reformatted CT scan of the thorax shows the posterior or posterolateral portion of the second rib located at the superoposterior aspect of the lung (Fig 17). The rib-lung interface is not tangential to the x-ray beam and should be invisible on a frontal chest radiograph.
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Figure 17. Rib companion shadows. Sagittal reformatted CT scan of the thorax shows the posterior or posterolateral portion of the second rib (2) located at the superoposterior aspect of the lung. Note the slightly protruding soft tissues in the second intercostal space (arrow), which may cause the rib companion shadow below the second rib. 1 = first rib, 3 = third rib, 4 = fourth rib.
Therefore, it is doubtful that the rib companion shadow is caused by fat located between the second rib and the parietal pleura. We propose that the shadow along the second rib represents the fat and muscles located in the intercostal spaces because a sagittal reformatted CT scan frequently demonstrates slight protruding soft tissues in the intercostal space (Fig 17). These protruding soft tissues are probably composed of extrapleural fat, intercostal muscles, and subcostal muscles.
Sharp Lines along the Lower Margin of the Ribs
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Sharp, thin lines are sometimes seen along the lower margins of the ribs on frontal chest radiographs (Fig 18). These lines are caused by the costal groove, in which the intercostal arteries and veins run alongside the nerves (Fig 19). The lower edge of the costal groove is seen as a thin, sharp hairline and should not be confused with a
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Figure 18. Sharp lines along the lower costal margin. Posteroanterior chest radiograph shows sharp, thin lines along the lower margin of the ribs (arrows).
pneumothorax (15). All but the 1st and 12th ribs have costal grooves. However, these grooves can vary in thickness and size between patients and cannot always be identified at chest radiography.
Short Linear Opacities on the Lateral Portion of the Ribs
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We frequently encounter two types of short, linear opacities projecting over the lateral portion of the ribs bilaterally. One type projects over the anterolateral portion of the ribs (anterior overlying opacity), whereas the other projects over the posterolateral portion of the ribs (posterior overlying opacity) (Fig 20). We find these opacities on 68% of chest radiographs, and they have no gender or side predilection. Anterior overlying opacities are more common than posterior overlying opacities (64% vs 8% of cases) and are frequently seen on the sixth, seventh, and eighth ribs. A posterior overlying opacity is usually seen on the ninth rib. Although these opacities are common, they have received little attention. Only Gilmartin (16) discussed them, and he suggested that they might represent portions of the serratus anterior
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a. b. Figure 19. Costal groove. (a) Photograph of a right rib shows the normal anatomy of the costal groove (arrows). (b) Radiograph of the same rib shows a thin, sharp hairline caused by the cortex of the costal groove (arrows).
Figure 20. Rib overlying shadows. Posteroanterior chest radiograph shows two types of short, linear opacities projecting over the lateral portion of the ribs. White arrows indicate anterior overlying opacity, black arrow indicates posterior overlying opacity.
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muscle that are visibly superimposed on the lung on frontal radiographs and are seen crossing the anterior ends of the ribs obliquely. However, two radiographic findings are inconsistent with Gilmartin’s hypothesis. First, the opacities are always confined to the rib shadow and do not extend beyond it. If the opacities are part of the serratus anterior muscle, the margin of the muscle should be outside the margin of the rib. Second, the opacities seem too sharp and distinct to be caused by an interface between muscle and soft tissue. There is neither abundant fat nor air around the serratus anterior muscle. The superior and inferior rib margins should be visible on the lateral portion of the ribs and should create the medial border of anterior and posterior overlying shadows. The ribs are curved, slightly twisted structures. Therefore, on frontal images, the superior and inferomedial margins should cross between the external and internal margins at the lateral portion of the ribs.
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a. b. c. Figure 21. Rib overlying shadows. (a) Photograph of a right eighth rib shows a blue catheter along the superior margin and a guide wire along the inferomedial margin. (b) Radiograph of the same rib shows the catheter (thick line) and the guide wire (thin line) crossing between the external and internal margins at the lateral portion of the rib. The course of the guide wire at the lateral portion of the rib is very similar to the medial border of the anterior overlying opacity. The course of the catheter corresponds to the posterior overlying opacity. (c) Schematic demonstrates the relationship between the rib margins and the overlying opacities. The gray area corresponds to the overlying shadow, which results from overlap of the bone structure; the blue line indicates the superior margin of the rib; and the red line indicates the inferomedial margin of the rib.
An x-ray beam that penetrates outside the crossed inferomedial and superior margins on the lateral portion of the ribs should penetrate thick bone structures better than an x-ray beam that penetrates inside the margins. This phenomenon would result in overlying opacity. The inferomedial margin of the rib would create the medial border of the anterior overlying shadow,
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and the superior margin would create the medial border of the posterior overlying shadow (Fig 21). ■
SUMMARY
To evaluate chest radiographs and CT scans with precision, it is necessary to be familiar with the radiologic appearance of ribs and their anatomic correlates. There are several techniques for making precise identification of
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the ribs. The big rib sign and the vertical displacement sign are used to differentiate the right and left ribs on lateral chest radiographs. For rib counting on CT scans, any of three anatomic landmarks may be used: the clavicle, the xiphoid process, or the sternal angle. For rib counting on lateral chest radiographs, the sternal angle or the 12th rib may be used. One should be familiar with normal rib variants to distinguish them from true rib diseases. To detect deformities of the osseous thorax on frontal chest radiographs, it is important to understand the correlation between the transverse appearance of the thorax and costal shape. The shadows that accompany the ribs may mimic pleural and lung disease. Rib companion shadows represent the fat and muscles in the intercostal space. A sharp line along the lower margin of the rib on chest radiographs is caused by the costal groove. Rib overlying shadows are probably caused by the margins of the twisted ribs. ■ REFERENCES 1. Naidich JB, Naidich TP, Roger AH, Schwartz K, Goldman MA, Pudlowski RM. The big rib sign: localization of basal pulmonary pathology in lateral projection utilizing differential magnification of the two hemithoraces. Radiology 1979; 131:1–8. 2. Kurihara Y, Nakajima Y, Kurihara YY, et al. The vertical displacement sign: a more reliable technique for differentiating the left and right ribs on the lateral chest radiograph. Thorac Imaging 1997; 22. 3. Bhalla M, McCauley DI, Golimbu C, Leitman BS, Naidich DP. Counting ribs on chest CT. J Comput Assist Tomogr 1990; 14:590–594. 4. Kim JS, Im J, Cho S, Lee SK, Park KS, Kim DY. Rib counting on CT using the sternal approach. J Comput Assist Tomogr 1993; 17:358–362.
5. Kurihara Y, Nakajima Y, Ishikawa T, Galvin JR. Counting ribs on chest CT scans: the easiest way (letter). AJR 1995; 165:487. 6. Kurihara Y, Kurihara YY, Ishikawa T, Galvin JR. Counting thoracic vertebrae on lateral chest radiographs (letter). AJR 1996; 166:992. 7. Kohler A, Zimmer EA. Borderlands of normal and early pathological findings in skeletal radiology. 3rd ed. New York, NY: Grune & Stratton, 1968. 8. Fisher MS. Eve’s rib (letter). Radiology 1981; 140:841. 9. Sheflin JR. Short rib(s). AJR 1995; 165:1548– 1549. 10. Juhl JH. The cardiovascular system. In: Juhl JH, ed. Essentials of roentgen interpretation. 4th ed. Hagerstown, Md: Harper & Row, 1981; 1041–1115. 11. Reich SB, Weinshelbaum A, Yee J. Correlation of radiographic measurements and pulmonary function tests in chronic obstructive pulmonary disease. AJR 1985; 144:695–699. 12. Felson B. A review of over 30,000 normal chest radiograms. In: Felson B, ed. Chest roentgenology. 1st ed. Philadelphia, Pa: Saunders, 1973; 494–501. 13. Gluck MC, Twigg HL, Ball MF, et al. Shadows bordering the lung on radiographs of normal and obese persons. Thorax 1972; 27:232–238. 14. Proto AV. Conventional chest radiographs: anatomic understanding of newer observations. Radiology 1992; 183:593–603. 15. Keats TE. The shoulder girdle and thoracic cage. In: Keats TE, ed. Atlas of normal roentgen variants that may simulate disease. 5th ed. St Louis, Mo: Mosby–Year Book, 1992; 321– 378. 16. Gilmartin D. The serratus anterior muscle on chest radiographs. Radiology 1979; 131:629– 635.
This article meets the criteria for 1.0 credit hour in category 1 of the AMA Physician’s Recognition Award. To obtain credit, see the questionnaire on pp 147–154.
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