The lungs and mediastinum Modalities
Chest radiographs with supplementary methods of examination
This is the most frequently performed radiographic examination. It can be performed rapidly, and can give much valuable information when the indications are correct.
This examination is the first choice in most diseases in the chest where imaging techniques are us ed to reach a diagnosis. Using the plain chest
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Figure 1.
Normal chest x-ray
a) PA view
b) Lateral view
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radiograph as a starting point, supplementary radiographs using special projections may be indicated.
A plain chest radiograph consists of frontal and lateral views (Fig. 1a, b) in a standing patient when this is possible. Use of two projections makes it easier both to find and to localize pathological processes. The frontal view is taken with full inspiration and posteroanterior (PA) beam direction. The heart thus comes closest to the film, and magnification of the cardiac shadow caused by ray divergence is minimized. In order to further reduce the effect of ray divergence, the distance from the tube is at least 1.5 metres. When bedridden patients are examined using a mobile apparatus, the film must be placed behind the back of the patient, and the distance from the tube must be shorter. The relative magnification of the cardiac shadow in this circumstance must be considered when comparing standard (posteroanterior) and portable (anteroposterior) radiographs.
The most important examinations that can be obtained to supplement the frontal and lateral radiographs are oblique views, lateral decubitus views, expiratory frontal views, overpenetrated films, lordotic views, fluoroscopy, and tomography.
Oblique views with the left and right sides, respectively, turned forwards towards the film, can provide valuable additional information on pleural thickening and are also useful when poorly defined opacities are seen in the frontal view, but not in the lateral view. These may be caused by summation phenomena, which can be confused with opacities. The oblique views may clarify the diagnosis.
Lateral decubitus views are usually taken with a horizontal direction of the x-ray beam, i.e. a frontal view of a patient lying on his side. The objective is to identify small amounts of pleural fluid which collect and become visible at the most dependent region of the pleural space.
Expiratory views are used to identify a small pneumothorax. By reducing the volume of the pleural space during expiration, a small amount of "trapped" air in the pleural cavity will be forced to increase in breadth so that the surface of the lung is pushed further away from the chest wall, thus becoming visible.
For overpenetrated films, higher energy rays are used. These can provide additional information about conditions in the mediastinum behind the heart shadow and about the soft tissues along the vertebral column.
Lordotic views can help to clarify uncertain findings in the apex of the lungs that are hidden behind the clavicle and first rib in the standard frontal view. The patient stands with his back to the film cassette and bends backwards to that his back is in lordosis. The clavicles are projected above the apex of the lung, and in some cases opacities may become more visible and possible to localize (Fig. 2 a, b).
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Figure 2.
Advantages of a "lordotic view"
a) PA view of chest shows an opacity projected over first right rib (arrow)
b) Lordotic view shows that the opacity is completely extrathoracic, and caused by an osteochondroma arising from the transverse process of the 7th cervical vertebra.
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Fluoroscopy is used to localize and adjust special projections in order to depict uncertain opacities. It may also be useful for assessing the mobility of the diaphragm. Paresis of the phrenic nerve causes the dome of the affected side of the diaphragm to be elevated. When the patient sniffs deeply through the nose with the mouth closed, fluoroscopy demonstrates descent of the diaphragm on the normal side. On the affected side, increased intra-abdominal pressure causes the relaxed, paretic hemidiaphragm to display rapid upward or inverse movement as proof of phrenic nerve paralysis. Paralysis of the phrenic nerve may be a sign of tumor invasion of the mediastinum.
Tomography involves sectional imaging. In conventional tomography, the tube and film are moved in a pendular fashion during exposure. This technique provides a sectional picture where the contours situated in the plane of the axis of movement are in focus. The localization and demarcation of opacities are often shown more clearly on tomograms than on standard radiographs, and the structures in the hilar regions and near the mediastinum can often be analyzed better. The method has now been
replaced by computed tomography (CT).
Under ideal working conditions, the radiologist assesses the referral note and the chest radiograph as soon as the films are developed, and then makes a decision on whether supplementary films are necessary. The simplest supplementary examinations are carried out before the patient leaves the department. The patient may often provide additional clinical information when the referral note is incomplete.
In digital radiography of the chest, the appropriate information appears in digital electronic form and is displayed on a monitor. Shades of grey can be adjusted on the monitor before it is transferred to film. Contrast can be optimized for lung tissue, the mediastinum, or the skeleton. This technique is particularly useful in bedside postoperative films to "save" an incorrectly exposed film, and to obtain comparable exposures from day to day.
Bronchography
For this procedure, local anaesthesia is given by inhalator, after which a soft catheter is passed into the main bronchus on the side to be examined. Fluoroscopy is used for guidance, both at the level of the larynx and at the level of the carina. The bronchial tree is made visible by administering an iodized contrast medium in aqueous suspension, which lines the walls of the bronchial branches (Fig. 3 a-c).
The main indications are the demonstration of bronchiectasis, bronchial anomalies, and occasionally a fistula communicating with the pleural cavity.
Because of the use of bronchoscopy and/or high resolution CT, the use of bronchography has greatly diminished.
Pulmonary angiography is used to demonstrate the pulmonary arteries and veins (Fig. 4). Using fluoroscopic guidance, ECG and pressure monitoring, a catheter is passed into the pulmonary artery. After injection of contrast medium, a series of film sequences are acquired to follow the passage of the contrast bolus through the pulmonary circulation. The main indications are suspected pulmonary embolism, vascular anomalies, or malformations.
The trachea and bronchi receive their nourishment via the bronchial arteries which originate from the upper part of the descending aorta. In
chronic haemoptysis it may be necessary to examine the vessels supplying the bronchi using bronchial arteriography. Occlusion of the bleeding vessels via the catheter may be carried out.
Computed tomography (CT)
The advantage of CT is that sectional imaging of high quality in the transverse plane is made possible, using a method that is simple and without discomfort. CT has better contrast resolution than conventional techniques, and the organ structures in the mediastinum are clearly demarcated. Measurement of attenuation values sometimes permits characterization of tissues (fat, clear fluid, etc). It is often necessary to visualize the vascular structures in the mediastinum. The CT sections are then exposed immediately after the injection of a contrast bolus into a vein in the forearm.
The good contrast resolution increases the possibility of demonstrating small round shadows in the lungs, for example in patients with presumed solitary metastases where thoracotomy is being considered. CT has to a large extent replaced conventional tomography, for example in staging lung cancer. By using a special computer program (high resolution CT), a detailed representation of pulmonary infiltrates and diffuse conditions of the parenchyma can be obtained. CT is also valuable for demonstrating localised fluid or air-filled cavities, and for defining diseases of the thoracic wall.
CT sections are also of great value for assessing the depth of lesions prior to needle biopsy and for adjustment of external radiotherapy.
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Figure 5.
Bronchial carcinoma right upper lobe.
a) Chest x-ray shows a large, well-defined expansive process in right apex.
b) T1-weighted MR image (coronal section) shows growth into the chest wall between the ribs, and into the mediastinum where air channels and vascular structures appear as signal-free structures.
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Magnetic resonance imaging (MRI)
This technique is used in selected cases where conventional chest radiograph combined with CT has not been sufficient to make a diagnosis. The advantage of MRI is the possibility of making sections in the coronal and sagittal planes in addition to the transverse plane. MRI provides excellent definition of the mediastinal structures, as the high signal of mediastinal fat on T1-weighted images provides good contrast. Moreover, the low or absent signal of vascular structures and airways permits discrimination of these structures without the need for contrast media (Fig. 5 a, b). MRI is of special value if expansive lesions in the mediastinum and hilar region are suspected, and in the case of occlusion or aneurysm of mediastinal vessels. MRI is of little value for assessment of details in lung parenchyma, and calcifications in the pleura, lung, and hilar regions will generally not be visible.
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Figure 6.
a) ECG gated spin echo MR image in the transaxial plane demonstrates an aneurysm of the sinuses of Valsalva in a patient with Marfan's syndrome.
b) ECG gated spin echo MR image in the coronal plane demonstrates an aneurysm of the ascending aorta in a patient with Marfan's syndrome. |
MRI has now be en available for more than ten years, but its clinical role in the thorax is still evolving. Moreover, the indication for preferential use of MRI rather than CT remains controversial. MRI can be considered comparable or preferable to CT for the following indications:
1. Thoracic aortic disease including aortic dissection and aneurysm (Fig. 6)
2. Thoracic venous disease including superior vena caval syndrome (Fig. 7), and suspected brachiocephalic venous occlusions
3. Pulmonary arterial diseases such as pulmonary arteriovenous malformation
4. Staging of lung cancer for specific questions such as chest wall invasion (Fig. 8), superior sulcus tumors (Fig. 9), invasion of the pericardium and cardiovascular structures (Fig. 10), differentiation of recurrent tumor vs fibrosis after surgery or radiation therapy
5. Staging and post therapy follow-up of lymphoma
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Figure 7.
Breath hold gradient echo images in the transaxial (a) and coronal (b) planes in a patient with occlusion of the superior vena cava due to lung cancer.
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Figure 8.
ECG gated spin echo images with T1-weighting (a), T2-weighting (b), and T1-weighting after administration of gadodiamide injection (Omniscan) (c). After contrast enhancement the sites of chest wall invasion (arrows) are clearly defined. Chest wall invasion is also evident on the T2-weighted images because of the increased signal intensity of the chest wall muscles. Central low intensity region is tumor necrosis.
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 Figure 9.
ECG gated spin echo image in the sagittal plane shows extension of a Pancoast (superior sulcus) tumor into the neck and encasement of subclavian artery and adjacent brachial plexus.
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 Figure 10.
ECG gated spin echo image in the coronal plane demonstrates inoperability of lung cancer due to encasement and invasion of the posterior portion of the aortic arch. |
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a) ECG gated spin echo image
in the transaxial plane shows a paracardiac mass (lymphoma). The mass is separated from the cardiac structures by the low intensity pericardial line.
b) ECG gated spin echo image in the transaxial plane shows a paracardiac mass invading the right and left atrial walls. |
6. Evaluation of brachial plexopathy
7. Mediastinal masses, especially paracardiac masses (Fig. 11) and posterior mediastinal masses with possible intraspinal extension
8. Inconclusive CT scan, which is usually caused by inadequate vascular opacification or contraindications to the use of iodinated contrast media
A variety of MRI techniques are applicable in the assessment of thoracic diseases. Most MRI studies of the thorax require the use of prospective or retrospective electrocardiographic (ECG) gating. ECG gated T1 weighted spin echo images are used to evaluate both vascular and nonvascular pathology. MR contrast medium, gadolinium chelates, can be used to increase the signal of lung and mediastinal lesions. The gradient echo technique shows bright signal of the blood pool; consequently, this technique is frequently used to evaluate vascular abnormalities (Fig. 7).
Since ultrasound waves do not penetrate through aerated alveoli, the use of ultrasound in the diagnosis of disease of the chest is confined mostly to assessment of the heart by echocardiography (which is usually performed in the cardiology department), pleural effusion and specific parts of the mediastinum.
Pleural fluid can become loculated. This makes the drainage of fluid difficult. Demonstration of the loculation(s) using ultrasound facilitates drainage with a minimum of punctures. Samples of tissue may also be collected through ultrasonographically guided needle punctures.
Radioactive isotope scanning is frequently used for the evaluation of suspected pulmonary embolism.
An intravenous injection of radioactive particles is administered for perfusion scintigraphy. The size of these particles is such that they are "trapped" by the pulmonary capillaries. A scan is acquired of the isotope-containing lungs using a gamma camera. The areas of perfused lungs emit radiation. Areas that emit relatively less or no radiation are considered to be underperfused or nonperfused.
Reduced radioactivity may be due to pulmonary embolism, but also to other conditions, such as interlobar pleural fluid, emphysematous
Figure 12.
Normal findings and normal variations in chest x-ray.
1. cervical rib; 2. medial border of scapula; 3. azygos lobe; 4. rib bridge; 5. interlobar fissure between upper and middle lobes; 6. bifid rib; 7. nipple; 8. breast; 9. calcified rib cartilage; 10. air in fundus of stomach
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bullae, pneumonia, etc. In order to distinguish embolism from other conditions with reduced perfusion, ventilation scintigraphy is also performed. During this procedure, a radioactive gas, such as xenon-133, is inhaled. In the presence of embolism (unlike the other conditions entailing reduced perfusion mentioned above), ventilation of the effected areas will usually be maintained. A combination of reduced perfusion and normal ventilation is an indication of the presence of embolism.
Alf Kolbenstvedt, Arnulf Skjennald and Charles B. Higgins