Thorax



Thorax



3.1 Thoracic Topography: Anterior and Posterior Views


The thorax is the part of the trunk between the neck and the abdominal cavity. It extends from the first rib to the diaphragm and is bounded by the sternum, twelve thoracic vertebrae, the twelve pairs of ribs, and the muscles that attach to these bones. The thorax contains the lungs surrounded by pleural spaces. It also contains the mediastinum, the block of tissue between the lungs consisting of the heart, esophagus, trachea, vessels, lymph nodes, connective tissue, and nerves. The heart is the approximate length of the body of the sternum. The great vessels and bifurcation of the trachea are behind the manubrium. The dome-shaped diaphragm extends over the liver on the right and spleen and stomach on the left. The apices of the lungs are above the first rib and clavicle. The parietal pleura that lines the pleural cavities extends more inferiorly than the lungs themselves in the recess between the diaphragm and the rib cage. The bases of the lungs during quiet respiration are near the seventh rib, whereas the costodiaphragmatic recess of the parietal pleura extends down to the ninth rib laterally. The right and left hemidiaphragms, pleural cavities, and lungs extend to a lower level in the posterior thorax than they do anteriorly.


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3.2 Posteroanterior and Chest X-Ray (Male and Female)


For a standard posteroanterior (PA) chest x-ray the patient “hugs” the x-ray recording plate. This protracts the shoulder girdles and moves the scapulae off the lungs. Having the x-ray beam pass from posterior to anterior through the patient, with the anterior chest adjacent to the recording plate, minimizes the magnification of the heart by the divergent beam. The lung fields appear dark because of their high air content. The larger pulmonary vessels (arteries and veins) are the white tubular densities near the lung roots. Note the air (darker area) in the midline trachea and in the stomach and splenic flexure of the colon under the left hemidiaphragm. Also note the clavicles, scapulae, and arch of each hemidiaphragm. The heart borders are seen clearly against the air-filled lungs. The right margin of the heart on the PA view is the right atrium. The left margin is the left ventricle. The right ventricle and left atrium do not contribute to the heart borders on a PA x-ray. They are better seen in a lateral view.


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3.5 Anterior Axillary CT and MRI


Both computed tomography (CT) and magnetic resonance imaging (MRI) are used for chest studies, although CT is more common. MRI is used for some heart studies and in cases in which patients may have allergies or other problems with receiving iodinated contrast used in CT. On the CT examination (A), the contrast was introduced into a left arm vein, and it is just entering the heart and lungs. The superior vena cava is bright white, and the white profiles in the lungs are blood vessels with some contrast. The very bright nodule at the left lung base medially is a calcified granuloma.


The window width and level of images can be adjusted on the computer screen to visualize either the soft tissue structures or the lung parenchyma. On this mediastinal window setting, the soft tissue structures of the mediastinum are visualized. The details of the lung parenchyma are poorly seen, and the lungs appear predominantly black. The window width and level can also be adjusted to make the white contrast in the blood vessels look brighter or less intense.


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3.8 Lateral Chest X-Ray


Routinely a left lateral chest radiograph (x-ray beam passing from right to left) is obtained to keep the heart closest to the image receptor. The patient’s arms are elevated to move the humeri and soft tissues of the arms out of the field of view. Lateral radiographs are used in conjunction with the PA view to evaluate the thorax in three dimensions and better localize any pathology that may be present. The roots of both lungs are superimposed on each other in the middle mediastinum. The right upper lobe bronchus is higher than the left upper lobe bronchus. Each is seen on end where the trachea ends. The distal arch of the aorta is seen posterior to the trachea. The clear space behind the sternum superiorly corresponds to the anterior mediastinum. The right ventricle is the most anterior heart chamber and makes up the anterior superior margin of the heart on the lateral view. The left atrium comprises the superior posterior border of the heart, and the left ventricle is the inferior posterior border of the heart. The contour of the left hemidiaphragm is not seen anteriorly because it is silhouetted by (against) the heart. The right hemidiaphragm contour can normally be followed along its entire course.


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3.9 Sagittal CT and MRI


In the CT reconstruction in A, the descending aorta, clavicle, and breast indicate that the section is to the left of the midline. Iodinated intravenous contrast is seen in the left ventricle, aortic arch, and descending thoracic aorta and two of the branch arteries of the aortic arch. The lungs appear black on this image viewed with a mediastinal window. The anterior clear space is the upper lobe of the left lung projecting over the anterior mediastinum. The heart and aorta are in the middle mediastinum, and the thoracic vertebral column (spine) is in the posterior mediastinum. In the upper abdomen portions of the left lobe of the liver, stomach, pancreas and left kidney are seen. B is a corresponding MRI sagittal section of the chest. Blood vessels can appear white on MRI without the injection of gadolinium contrast, depending on the pulse sequence used. On this image the ascending aorta and aortic arch are seen clearly. The heart is located anteriorly. Some blood vessels are seen in the left lung, and the left kidney is seen in the upper abdomen.


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3.10 Lung Anatomy


A primary (main stem) bronchus supplies each lung. Secondary (lobar) bronchi supply lobes, and tertiary (segmental) bronchi supply bronchopulmonary segments, the subdivisions of each lobe that are colored in the figures. The right primary bronchus is shorter than the left and has a more vertical orientation in line with the trachea. The right lung has three lobes, with a horizontal (minor) fissure separating the superior and middle lobes and an oblique (major) fissure separating the upper and middle lobes from the inferior lobe. The left lung has only two lobes separated by an oblique fissure. The lingual segment (lingula) of the left upper lobe is equivalent to the middle lobe of the right lung. Note that the inferior lobes of both lungs extend posteriorly to the superior lobes behind the middle lobe of the right lung and lingula of the left lung. At the root of the lungs the pulmonary arteries are above the pulmonary veins.


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Jan 10, 2016 | Posted by in RADIOGRAPHIC ANATOMY | Comments Off on Thorax

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