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ANATOMY OF THE THORAX AND SHOULDER GIRDLE DISPLAYED BY MAGNETIC RESONANCE IMAGING

James D. Collins, MD, Marla L.Shaver, MD, Poonam Batra, MD, and Katheleen Brown, MD Los Angeles, California

In 1971, radiographic anatomy of the human body was added to the gross anatomy course at UCLA.1 Radiographic contrast studies and plain anatomical displays were formulated into teaching packages for all organ systems. Residents presented each package to first-year medical students in the dissection laboratory to augment the teaching of anatomy. In November 1984, magnetic resonance imaging was instituted in the radiology department. Imaging the chest produced coronal and axial planes which displayed the muscles and soft tissues of the thorax. In 1986, the authors presented their study of MR anatomy of the chest and shoulder girdle to the American Association of

Anatomists.2

The purpose of this presentation is to demonstrate the anatomy of the thorax and shoulder girdle as displayed by magnetic resonance, correlated with regional anatomy, with emphasis on soft tissue structures.

Key Words * gross anatomy * MRI * thorax * chest* shoulder girdle * MRI soft tissues * medical student teaching * chest wall * MRI nerves

From the Department of Radiological Sciences, UCLA School of Medicine, Los Angeles, California. Requests for reprints should be addressed to Dr James D. Collins, Department of Radiological Sciences, UCLA Medical Center, Los Angeles, CA 90024.

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Magnetic resonance imaging (MRI) of soft tissues displays normal and abnormal anatomic images of the surface and connective tissues that cannot be imaged with conventional radiographic techniques. The advent of the cursor lines, depicting the area of tissue selected for imaging, enables the radiologists at UCLA to offer a new modality for teaching anatomy. Cursor lines may be rotated 180 degrees on the axial, the coronal, and sagittal planes of anatomy. The images may be magnified and displayed without reconstruction. When the cursor lines are placed on the lateral chest wall of the mid-coronal MR of the chest, the pleural surface and adjacent muscles of the chest wall are demonstrated in the oblique coronal plane. Applied to the anatomical planes of the thorax, MR imaging displays anatomy that assists the teaching of medical students, anatomists, and radiologists. David Maxwell from the UCLA Department of Anatomy prepared a template list of anatomical structures to determine if the authors' team could demonstrate the thoracic anatomy with MRI. Volunteers were chosen for imaging. The images and prepared list were matched beyond expectations.

MATERIALS AND METHODS

All images were recorded with a spin echo of TE = 28 and TR = 500, using a 0.3 Tesla Fonar permanent magnet. Axial (transverse), coronal, and sagittal planes were selected to image the anatomy for comprehensive learning. Anatomical landmarks were maintained for orientation. Images chosen for display were labeled

JOURNAL OF THE NATIONAL MEDICAL ASSOCIATION, VOL. 83, NO. 1

TABLE. ANATOMICAL NOMENCLATURE DIRECTORY CHEST AND SHOULDER GIRDLE 28. Aorta 55. Brachial vein F. First of the Series 1. Pectoralis major muscle 29. Pulmonary artery 56. Axillary artery 2. Pectoralis minor muscle 57. Median nerve 30. Trachea 31. Aorta (descending) 3. Trapezius muscle 58. posterior humeral circumflex 32. Subclavian artery 4. Spine of the scapula artery 33. Vertebral artery 59. Scalenus anterior muscle 5. Clavicle 34. Brachial plexus 60. Ulnar nerve 6. Supraspinatus muscle 35. Mandible 7. Deltoid muscle 61. Subracromial bursa 36. Spinalis thoracis muscle 8. Serratus anterior muscle 62. Radial nerve 9. External oblique muscle 37. Brachiocephalic artery 63. Scalenus medius muscle 10. Latissimus dorsi muscle 38. Subcutaneous tissue 64. Biceps muscle 11. Deltoid muscle (posterior) (lymphatics, etc) 65. Right primary bronchus 39. Diaphragm 12. Infraspinatus muscle 66. Left primary bronchus 13. Subclavius muscle 40. Liver 67. Ligamentum arteriosum 14. Acromion process 41. Spleen 68. Bronchial arteries 42. Stomach 69. Axillary nerve 15. Body of the scapula 43. Right lung 16. Coracoid process 70. Inferior vena cava 71. Spinal cord 44. Left lung 17. Teres major muscle 72. Rib 45. Vertebral body (thoracic) 18. Teres minor muscle 73. Spinalis thoracis and cervicis 19. Coracobrachialis muscle 46. Right pulmonary artery 47. Intervertebral disc space muscles 20. Triceps muscle (long head) 74. Left ventricle 21. Triceps muscle (lateral head) 48. Thoracic duct 22. Brachialis muscle 49. Brachial artery 75. Pericardial fat pad (right) 50. Intercostal artery 76. Pericardial fat pad (left) 23. Subscapularis muscle 77. Subclavian vein 24. Rhomboid muscle 51. Intercostal muscle 52. Superior vena cava 78. Longus collis muscle 25. Humerus 79. Biceps muscle (short head) 53. Right atrium 26. Humerus (head) 27. Intertubercular sulcus 54. Left atrium 80. Biceps muscle (long head)

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with numbers and assigned to a directory (Table) to assist location of the anatomical structure on each image. Serial images were enlarged to enhance visual association with the gross anatomy prior to dissection. All images were labeled, and mounted on clear transparent plastic panels. The panels were then placed on large radiograph view boxes and backlighted for display. Images selected for presentation are displayed from the mid-coronal and posterior coronal planes. Oblique sagittal images were positioned from the left mid-coronal image.

RESULTS

Figure 1. Thoracic duct (arrows) crosses the angle between the internal jugular vein and the subclavian vein. Aorta (28), subclavian (77).

JOURNAL OF THE NATIONAL MEDICAL ASSOCIATION, VOL. 83, NO. 1

The reader should refer to the nomenclature directory (Table) for Figures 1 through 13 for specific structures. Not every structure is labeled in each figure, and the structures labeled are intended to orient and guide. In general, the lungs, blood vessels, joint capsules, tendon insertions, and scarred tissues have low signals (black) in MR Imaging. The vascular supply of nerves are continuous like a spider web enveloping the nerve sheaths.6 Small nerves have a relative intermediate high signal in fatty tissue.

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MRI OF THE THORAX AND SHOULDER GIRDLE

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(black arrows). Pectoral rows).

nerves

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Figure 3. Note the low signals (white arrows) marginating the high signals of the nerves.

crosses the left

Nerves are composed of phospholipids and displayed as a high signal. When the nerves are embedded in fatty tissue; the blood supply marginates the nerve from the fat. Magnetic resonance imaging detects the visual display of the nerve and locates the low signal of the blood supply. Lymph nodes have a relative gray signal and are located in specific anatomical locations. Chyle (fat) has a high signal and allows location of the thoracic duct. These features are important for understanding the anatomy displayed in the following MR images. Figures 1 through 8 demonstrate the surface anatomy of the chest wall and shoulder girdle. Figure 1 is the first of the series and orients the reader to the

subclavian artery and internal jugular vein, and the ligamentum arteriosum joins the aorta and pulmonary artery. The vagus nerve is a plexus of

image positions. The high signals (white) demonstrate deposited fat and nerve signals.7'8 The low signals (black) demonstrate a variety of vascular structures coursing through

the mediastinum and muscular tissue. Bones are marginated by low signals of the periostium, cortex, and cartilage. Cartilage may be located when sufficient fat surrounds the margins. In Figyure 1, the thoracic duct

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intermediate signals within the high signals of fat surrounding the aorta, left lateral wall of the trachea, left carotid artery, and pulmonary artery. Serial images two through eight display the muscular structures as they appear from the left medial oblique to the lateral oblique position. The spine of the scapula separates the supraspinatus from the infraspinatus muscle in Figure 2. The trapezius muscle attaches to the scapula and clavicle. The low signals of blood vessels and intermediate high signals of the nerves overlay the muscles. The axillary artery, axillary vein, and brachial plexus are positioned in the axillary fat between the subscapularis muscle and the pectoralis muscles. Figure 3 images the teres major and minor muscles. The subclavius muscle is inferior to the clavicle. The anterior deltoid muscle appears superior to the pectoralis muscle. The triceps muscles are inferior to the subcutaneous tissue of the chest wall. The axillary

JOURNAL OF THE NATIONAL MEDICAL ASSOCIATION, VOL. 83, NO. 1

MRI OF THE THORAX AND SHOULDER GIRDLE

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Figure 5. Musculocutaneous nerve (arrow), humerus (25).

Figure 4. Supraspinatus muscle (6), coracobrachialis muscle (19), humerus (25).

nerve is inferior to the supraspinatus muscle and the superior body of the scapula. The intermediate signals of the vascular plexus and the lymphatics course within the subcutaneous tissue (38). Figure 4 displays the coracobrachialis muscle attaching to the coracoid process (16). The brachial plexus parallels the brachial artery and vein. Intermediate high signals of the axillary nerve and low signals of the vascular supply are inferior to the scapula and overlay the subscapularis muscle. The nerve and vascular supply to the deltoid muscle intersects the overlay of the pectoralis muscle group. Figure 5 demonstrates the low signal of the fibrous intermuscular septum between the long and lateral heads of the triceps. The musculocutaneous nerve (arrow) is identified between the long and short heads of the biceps. The last of the pectoralis minor muscle is demonstrated as it attaches to one of the ribs of the chest wall.

JOURNAL OF THE NATIONAL MEDICAL ASSOCIATION, VOL. 83, NO. 1

Figure 6 demonstrates the posterior humeral circumflex artery and nerve. The nerve supply to the deltoid muscle lies superior to the axillary artery (56). The acromioclavicular ligament joins the acromion process (14) to the clavicle (5). Figures 7 through 13 are posterior coronal views of the left posterior chest and shoulder girdle. The series was obtained in the prone position for coronal plane imaging. The images are displayed from the posterior to the anterior projection. In Figure 7, the spinalis thoracis muscle (73) of the erector spinalis group marginates the spinous process of the thoracic spine. The latissimus dorsi muscle (10) is outlined laterally merging with the teres major muscle (17) and rhomboid major muscle (24). In Figure 8, the superior margin of the scapula separates the trapezius muscle and the supraspinatus muscles. The deltoid muscle, lungs, and ribs appear. The signal of the intercostal nerves are inferior to the ribs. In Figure 9, the spine of the scapula (4) separates the supraspinatus and the infraspinatus muscle. The long head of the triceps (20) and the teres major (17) and

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MRI OF THE THORAX AND SHOULDER GIRDLE

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minor muscles (18) intersect. The serratus anterior muscle lies parallel to the latissimus muscle (10). In Figure 10, the intercostal arteries arise from the medial aspect of the thoracic aorta and course across the thoracic vertebra. The hemiazygous vein (arrow) joins the azygous vein which parallels the aorta. Nerve roots exit to the left of the first thoracic vertebral body. Figure 11 demonstrates the left atrium (54) in the arms

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JOURNAL OF THE NATIONAL MEDICAL ASSOCIATION, VOL. 83, NO. 1

30

MRI OF THE THORAX AND SHOULDER GIRDLE

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In Figure 13, the left ventricle lies on the bed of the left diaphragm. The convexed left auricle seems to merge with the coronary sinus (arrow) crossing the left ventricle. The coracobrachialis muscle forms a confluence with the teres major and pectoralis major muscles (1). The trapezoid ligament is superior to the coracoid process. The high signal of the pericardial recess lies between the pulmonary artery and aorta (28). Focal high signals of the fat on the left lung represent rib costochondral cartilage. Vertebral arteries are demonstrated as the brachiocephalic and subclavian arteries arch laterally.

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JOURNAL OF THE NATIONAL MEDICAL ASSOCIATION, VOL. 83, NO. 1

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MRI OF THE THORAX AND SHOULDER GIRDLE

DISCUSSION AND CONCLUSION

Chest and shoulder girdle anatomy as displayed by MRI has allowed the authors' team to prepare exhibits for teaching surface anatomy to medical students, visiting physicians, and residents in radiology2-57-9 In the past, the experienced radiologist interpreted the plain film by recognizing changes in the normal density of the presented anatomy. The displayed abnormal density represented a possible differential diagnostic list. Applying linear tomography to the area of the abnormal density usually revealed patterns suggesting a diagnosis. The correlation' of the radiographic abnormality to the clinical history was, in most cases, convincing to the clinician. However, the fascial planes of anatomy were not always clearly identified by plain radiograph study. Computerized axial tomography (CAT) added an extra study demonstrating axial density display of organ systems.'I Reconstruction was often required to display the sagittal plane and contrast injections were given to outline vascular structures and the intestinal tract. Computerized tomography (CT) did not always clearly display the separation of vasc-ular structures and tumor masses. Contrast was injected into the patients to separate vascular structures from mass densities. The contrast did not entirely distinguish between an abnormal density and a blood vessel of the same density. Proton densities separate organ systems. Magnetic resonance separates proton densities within organ systems. Multiplane imaging of MR does not require reconstruction. Increasing the signal-to-noise ratio with water bags sharpens the details of images and allows identification of small tissues. 1'12 Positive and negative mode imaging supported by visual display techniques can be used to image without contrast materials.8"13 These various techniques are possible in many MR Units. The authors applied these techniques in our studies of disease. Radiological imaging is an important adjunct to teaching anatomy. Magnetic resonance

imaging is a powerful tool in identifying small tissues for teaching and for clinical studies.

Literature Cited 1. Clemente CD. Anatomy: A regional atlas of the human body. 3rd ed. Baltimore, MD: Urban & Schwartzenberg; 1987. 2. Collins D, Batra P, Brown K, Shaver M. Anatomy of the thorax and shoulder girdle as displayed by magnetic resonance. Anat Rec. 1986;214(3):24A. 3. Collins JD. Shaver ML, Batra P. Brown K. Anatomy of the abdomen, back and pelvis as displayed by MRI: Part One. J Natl Med Assoc. 1989;81(6):680-684. 4. Collins JD, Shaver ML, Batra P, Brown K. Anatomy of the abdomen, back and pelvis as displayed by MRI: Part Two. J Natl Med Assoc. 1989;81(7):809-813. 5. Collins JD, Shaver ML, Batra P, Brown K. Anatomy of the abdomen, back and pelvis as displayed by MRI: Part Three. J NatI Med Assoc. 1989;81 (8);857-861. 6. Sunderland S. Blood supply of the nerves to the upper limb In man. Archives of Neurology and Psychology. 1945;53:91 -106. 7. Collins JD, Shaver ML, Batra P, Brown K. Why can we see nerves on MRI? Presentation at the American Association of Anatomists 100th Annual Meeting; 1987 and the 92nd Annual Convention of the National Medical Association; 1987. 8. Collins JD, Shaver ML, Batra P. Brown K. Nerves on magnetic resonance imaging. J Natl Med Assoc. 1989;81 (2):129-134. 9. Collins JD, Shaver ML, Batra P, Brown K. Anatomy of the upper and lower extremity muscle and tendon insertions as displayed by magnetic resonance imaging. Anat Rec. 1 988;224(4):24A. 10. Collins JD, Batra P, Brown RK, Winter J, King W. Computerized chest tomography in asbestos workers suspected of having pleural disease. J Natl Med Assoc. 1 987;79(3):273-277. 11. Cameron L, Ord VA, Fullerton GD. Characterization of proton NMR relaxation times in normal and pathological tissues by correlation with other tissue parameters. Magn Reson Imaging. 1984;2:97-106. 12. Collins JD, Shaver ML, Kovacs BJ, et al. Enhancing magnetic resonance images using water bags. J Natl Med Assoc. 1990;3:197-200. 13. Masih S, Bakhda RK, Collins JD. Pelvic fused kidneys: Magnetic resonance imaging and intravenous pyelogram correlation. J Natl Med Assoc. 1988;(8):925-927.

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