BONY THORAX

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BONY THORAX




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Sternum


The sternum, or breastbone, is directed anteriorly and inferiorly and is centered over the midline of the anterior thorax (Figs. 9-1 to 9-3). A narrow, flat bone about 6 inches (15 cm) in length, the sternum consists of three parts: manubrium, body, and xiphoid process. The sternum supports the clavicles at the superior manubrial angles and provides attachment to the costal cartilages of the first seven pairs of ribs at the lateral borders.





The manubrium, the superior portion of the sternum, is quadrilateral in shape and is the widest portion of the sternum. At its center, the superior border of the manubrium has an easily palpable concavity termed the jugular notch. In the upright position, the jugular notch of the average person lies anterior to the interspace between the second and third thoracic vertebrae. The manubrium slants laterally and posteriorly on each side of the jugular notch, and an oval articular facet called the clavicular notch articulates with the sternal extremity of the clavicle. On the lateral borders of the manubrium, immediately below the articular notches for the clavicles, are shallow depressions for the attachment of the cartilages of the first pair of ribs.


The body is the longest part of the sternum (4 inches [10.2 cm]) and is joined to the manubrium at the sternal angle, an obtuse angle that lies at the level of the junction of the second costal cartilage. The manubrium and the body contribute to the attachment of the second costal cartilage. The succeeding five pairs of costal cartilages are attached to the lateral borders of the body. The sternal angle is palpable; in the normally formed thorax, it lies anterior to the interspace between the fourth and fifth thoracic vertebrae when the body is upright.


The xiphoid process, the distal and smallest part of the sternum, is cartilaginous in early life and partially or completely ossifies, particularly the superior portion, in later life. The xiphoid process is variable in shape and often deviates from the midline of the body. In the normal thorax, the xiphoid process lies over the 10th thoracic vertebra and serves as a useful bony landmark for locating the superior portion of the liver and the inferior border of the heart.



Ribs


The 12 pairs of ribs are numbered consecutively from superiorly to inferiorly (Fig. 9-4; see Figs. 9-1 and 9-2). The rib number corresponds to the thoracic vertebra to which it attaches. Each rib is a long, narrow, curved bone with an anteriorly attached piece of hyaline cartilage, the costal cartilage. The costal cartilages of the first through seventh ribs attach directly to the sternum. The costal cartilages of the 8th through 10th ribs attach to the costal cartilage of the 7th rib. The ribs are situated in an oblique plane slanting anteriorly and inferiorly so that their anterior ends lie 3 to 5 inches (7.6 to 12.5 cm) below the level of their vertebral ends. The degree of obliquity gradually increases from the 1st to the 9th rib and then decreases to the 12th rib. The first seven ribs are called true ribs because they attach directly to the sternum. Ribs 8 to 12 are called false ribs because they do not attach directly to the sternum. The last two ribs (11th and 12th ribs) are often called floating ribs because they are attached only to the vertebrae. The spaces between the ribs are referred to as the intercostal spaces.



The number of ribs may be increased by the presence of cervical or lumbar ribs, or both. Cervical ribs articulate with the C7 vertebra but rarely attach to the sternum. Cervical ribs may be free or articulate or fuse with the first rib. Lumbar ribs are less common than cervical ribs. Lumbar ribs can lend confusion to images. They can confirm the identification of the vertebral level, or they can be erroneously interpreted as a fractured transverse process of the L1 vertebra.


Ribs vary in breadth and length. The first rib is the shortest and broadest; the breadth gradually decreases to the 12th rib, the narrowest rib. The length increases from the 1st to the 7th rib and then gradually decreases to the 12th rib.


A typical rib consists of a head, a flattened neck, a tubercle, and a body (Figs. 9-5 and 9-6). The ribs have facets on their heads for articulation with the vertebrae. The facet is divided on some ribs into superior and inferior portions for articulation with demifacets on the vertebral bodies. The tubercle also contains a facet for articulation with the transverse process of the vertebra. The 11th and 12th ribs do not have a neck or tubercular facets. The two ends of a rib are termed the vertebral end and the sternal end.




From the point of articulation with the vertebral body, the rib projects posteriorly at an oblique angle to the point of articulation with the transverse process. The rib turns laterally to the angle of the body, where the bone arches anteriorly, medially, and inferiorly in an oblique plane. Located along the inferior and internal border of each rib is the costal groove, which contains costal arteries, veins, and nerves. Trauma to the ribs can damage these neurovascular structures, causing pain and hemorrhage.



Bony Thorax Articulations


The eight joints of the bony thorax are summarized in Table 9-1. A detailed description follows.



The sternoclavicular joints are the only points of articulation between the upper limbs and the trunk (see Fig. 9-3). Formed by the articulation between the sternal extremity of the clavicles and the clavicular notches of the manubrium, these synovial gliding joints permit free movement (the gliding of one surface on the other). A circular disk of fibrocartilage is interposed between the articular ends of the bones in each joint, and the joints are enclosed in articular capsules.


Posteriorly, the head of a rib is closely bound to the demifacets of two adjacent vertebral bodies to form a synovial gliding articulation called the costovertebral joint (Fig. 9-7, A; see Fig. 9-6). The 1st, 10th, 11th, and 12th ribs each articulate with only one vertebral body.



The tubercle of a rib articulates with the anterior surface of the transverse process of the lower vertebra at the costotransverse joint, and the head of the rib articulates at the costovertebral joint. The head of the rib also articulates with the body of the same vertebra and articulates with the vertebra directly above. The costotransverse articulation is also a synovial gliding articulation. The articulations between the tubercles of the ribs and the transverse processes of the vertebrae permit only superior and inferior movements of the first six pairs. Greater freedom of movement is permitted in the succeeding four pairs.


Costochondral articulations are found between the anterior extremities of the ribs and the costal cartilages (see Fig. 9-7, B). These articulations are cartilaginous synchondroses and allow no movement. The articulations between the costal cartilages of the true ribs and the sternum are called sternocostal joints. The first pair of ribs, rigidly attached to the sternum, form the first sternocostal joint. This is a cartilaginous synchondrosis type of joint, which allows no movement. The second through seventh sternocostal joints are considered synovial gliding joints and are freely movable. Interchondral joints are found between the costal cartilages of the sixth and seventh, seventh and eighth, and eighth and ninth ribs (see Fig. 9-7, C). These interchondral joints are synovial gliding articulations. The interchondral articulation between the 9th and 10th ribs is a fibrous syndesmosis and is only slightly movable.


The manubriosternal joint is a cartilaginous symphysis joint, and the xiphisternal joints are cartilaginous synchondrosis joints that allow little or no movement (see Figs. 9-3, B, and 9-7, B and C).



RESPIRATORY MOVEMENT


The normal oblique orientation of the ribs changes little during quiet respiratory movements; however, the degree of obliquity decreases with deep inspiration and increases with deep expiration. The first pair of ribs, which are rigidly attached to the manubrium, rotate at their vertebral ends and move with the sternum as one structure during respiratory movements.


On deep inspiration, the anterior ends of the ribs are carried anteriorly, superiorly, and laterally while the necks are rotated inferiorly (Fig. 9-8, A). On deep expiration, the anterior ends are carried inferiorly, posteriorly, and medially, while the necks are rotated superiorly (Fig. 9-8, B). The last two pairs of ribs are depressed and held in position by the action of the diaphragm when the anterior ends of the upper ribs are elevated during respiration.




DIAPHRAGM


The ribs located above the diaphragm are best examined radiographically through the air-filled lungs, whereas the ribs situated below the diaphragm must be examined through the upper abdomen. Because of the difference in penetration required for the two regions, the position and respiratory excursion of the diaphragm play a large role in radiography of the ribs.


The position of the diaphragm varies with body habitus: It is at a higher level in hypersthenic patients and at a lower level in asthenic patients (Fig. 9-9). In sthenic patients of average size and shape, the right side of the diaphragm arches posteriorly from the level of about the 6th or 7th costal cartilage to the level of the 9th or 10th thoracic vertebra when the body is in the upright position. The left side of the diaphragm lies at a slightly lower level. Because of the oblique location of the ribs and the diaphragm, several pairs of ribs appear on radiographs to lie partly above and partly below the diaphragm.



The position of the diaphragm changes considerably with the body position, reaching its lowest level when the body is upright and its highest level when the body is supine. For this reason, it is desirable to place the patient in the upright position for examination of the ribs above the diaphragm and in a recumbent position for examination of the ribs below the diaphragm.


The respiratory movement of the diaphragm averages about 1½ inches (3.8 cm) between deep inspiration and deep expiration. The movement is less in hypersthenic patients and more in hyposthenic patients. Deeper inspiration or expiration and greater depression or elevation of the diaphragm are achieved on the second respiratory movement than on the first. This greater movement should be taken into consideration when the ribs that lie at the diaphragmatic level are examined.


When the body is placed in the supine position, the anterior ends of the ribs are displaced superiorly, laterally, and posteriorly. For this reason, the anterior ends of the ribs are less sharply visualized when the patient is radiographed in the supine position.



BODY POSITION


Although in rib examinations it is desirable to take advantage of the effect that body position has on the position of the diaphragm, the effect is not of sufficient importance to justify subjecting a patient to a painful change from the upright position to the recumbent position or vice versa. Even minor rib injuries are painful, and slight movement frequently causes the patient considerable distress. Unless the change in position can be effected with a tilting radiographic table, patients with recent rib injury should be examined in the position in which they arrive in the radiology department. An ambulatory patient can be positioned for recumbent images with minimal discomfort by bringing the tilt table to the vertical position for each positioning change. The patient stands on the footboard, is comfortably adjusted, and is then lowered to the horizontal position.



TRAUMA PATIENTS


The first and usually the only requirement in the initial radiographic examination of a patient who has sustained severe trauma to the rib cage is to take AP and lateral projections of the chest. These projections are obtained not only to show the site and extent of rib injury, but also to investigate the possibility of injury to the underlying structures by depressed rib fractures. Patients are examined in the position in which they arrive, usually recumbent on a stretcher. The recumbent position is necessary to show the presence of air or fluid levels using the decubitus technique.









Sternum


The position of the sternum with respect to the denser bony and soft tissue thoracic structures makes it a difficult structure to radiograph satisfactorily. Few problems are involved in obtaining a lateral projection, but because of the location of the sternum directly anterior to the thoracic spine, an AP or PA projection provides little useful diagnostic information. To separate the vertebrae and sternum, it is necessary to rotate the body from the prone position or to angle the central ray medially. The exact degree of required angulation depends on the depth of the chest, with deep chests requiring less angulation than shallow chests (Fig. 9-10 and Table 9-2).




Angulation of the body or the central ray to project the sternum to the right of the thoracic vertebrae clears the sternum of the vertebrae but superimposes it over the posterior ribs and the lung markings (Fig. 9-11). If the sternum is projected to the left of the thoracic vertebrae, it is also projected over the heart and other mediastinal structures (Fig. 9-12). The super imposition of the homogeneous density of the heart can be used to advantage (compare Figs. 9-11 and 9-12).




The pulmonary structures, particularly in elderly persons and heavy smokers, can cast confusing markings over the sternum, unless the motion of shallow breathing is used to eliminate them. If motion is desired, the exposure time should be long enough to cover several phases of shallow respiration (Figs. 9-13 and 9-14). The milliampere (mA) must be relatively low to achieve the desired milliampere-second (mAs).




If a female patient has large, pendulous breasts, they should be drawn to the sides and held in position with a wide bandage to prevent them from overlapping the sternum and to position the sternum closer to the image receptor (IR). This positioning is particularly important in the lateral projection, in which the breast can obscure the inferior portion of the sternum.



Radiation Protection


Protection of the patient from unnecessary radiation is a professional responsibility of the radiographer (see Chapter 1 for specific guidelines). In this chapter, the Shield gonads statement indicates that the patient is to be protected from unnecessary radiation by restricting the radiation beam using proper collimation. In addition, the placement of lead shielding between the gonads and the radiation source is appropriate when the clinical objectives of the examination are not compromised.



image PA OBLIQUE PROJECTION



RAO position




NOTE: This position may be difficult to perform on trauma patients. Use an upright position if necessary or possible.






Position of part:



• Adjust the elevation of the left shoulder and hip so that the thorax is rotated just enough to prevent superimposition of the vertebrae and sternum.


• Estimate the amount of rotation with sufficient accuracy by placing one hand on the patient’s sternum and the other hand on the thoracic vertebrae to act as guides while adjusting the degree of obliquity. The average rotation is about 15 to 20 degrees (Fig. 9-15).



• Align the patient’s body so that the long axis of the sternum is centered to the midline of the grid.


• Place the top of the IR about 1½ inches (3.8 cm) above the jugular notch.


• Shield gonads.


• Respiration: When breathing motion is to be used, instruct the patient to take slow, shallow breaths during the exposure. When a short exposure time is to be used, instruct the patient to suspend breathing at the end of expiration to obtain a more uniform density.


NOTE: On trauma patients, obtain this projection with the patient supine, and use the LPO position and AP oblique projection.


Mar 4, 2016 | Posted by in GENERAL RADIOLOGY | Comments Off on BONY THORAX
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