Transmission Radiography

Radiography remains the most used technique for evaluating the position and movement of the diaphragm. Postero-anterior (P-A) and lateral views of the thorax at total lung capacity can be supplemented by radiographs at other static lung volumes and movement of the hemidiaphragms as assessed by fluoroscopy.

Limitations

As shown by three-dimensional reconstruction techniques using other imaging techniques, the shape of the human diaphragm is complex (1-3) (Figure 1), so transmission radiography, even with two views, can provide only qualitative estimates of shape. The silhouette of the diaphragm revealed by transmission radiography represents the most cephalad portion of the diaphragm apposed to the lung, rather than the chest wall from one side to the other in P-A views and from dorsal to ventral in lateral views. The most cephalad portion of the diaphragm, for example, the P-A view, may not lie in the same dorsal-ventral plane and, furthermore, may not even represent a contiguous line over the surface of the diaphragm. It clearly does not represent the curvature of a bundle of muscle in the diaphragm or necessarily any region of the diaphragm muscle. Furthermore, attempts to use radiographic changes at different lung volumes to assess changes in the length of the diaphragm require assumptions about the relative contribution of the central tendon and muscle. Parallax distortion and the necessity for exposure to ionizing radiation also cause difficulties with this technology.

Applications

Bearing in mind these important limitations, radiography has been used to assess diaphragm position and motion in clinical assessment and to derive estimates of diaphragm length in research studies. In addition, estimating lung volumes from P-A and lateral radiographs at full inflation may indirectly assist assessment of the diaphragm.

Position of Hemidiaphragm Domes at Total Lung Capacity

Normal subjects. In ~ 95% of normal adults the level of the dome of the right hemidiaphragm on postero-anterior radiographs taken standing at full inflation (total lung capacity) is projected in a plane ranging from the anterior end of the fifth rib to the sixth anterior interspace; and in only 5% is it at or below the level of the seventh rib (4). The height of the right dome tends to be higher in women, in subjects of heavy build, and in those older than 40 years of age. The plane of the right diaphragmatic dome tends to be about half an interspace higher than the left, although in ~ 10% of normal subjects both are at the same height or the left is higher than the right (5).

Disease. In bilateral diaphragm paralysis both domes are elevated at total lung capacity (and radiographic lung volume is reduced). This change is indistinguishable from volitional failure to fully inflate the lungs. In unilateral paralysis, elevation of the; paralyzed dome is obvious. When there is an ac quired enlargement of total lung capacity, as in severe emphysema, the domes are lower (the level of the right dome at the anterior end of the seventh rib or lower) with flattening and a larger radius of curvature, visible on both P-A and lateral radiographs. If a line is drawn on a P-A radiograph from the ver-tebrophrenic angle to the costophrenic angle, the maximum curvature of the dome, assessed at 90° to this line, should normally be at least 1.5 cm. In the most severe disease the domes may be flat or even inverted with loss of the zone of apposition, allowing visualization on the P-A radiograph of the insertions of the diaphragm into the lower ribs.

Excursion of hemidiaphragm domes during tidal breathing. High-speed cassette changers or video fluoroscopy can provide dynamic information. In a study of inspiratory-expi-ratory radiographs obtained during quiet tidal breathing in the erect position from 350 subjects, 30-80 years of age, and without evidence of respiratory disease, the mean tidal excursions of the domes of the right and left hemidiaphragms were found to be 3.3 and 3.5 cm, respectively (6). Tidal diaphragmatic movement averaged 0.5 cm less in women than in men. Despite similar mean values, unequal movement of the two hemi-diaphragmatic domes in an individual subject is common, most commonly being greater on the right (5).

In bilateral diaphragm paralysis individuals may breathe by active expiration below relaxation volume followed by passive inhalation, during which the diaphragm may descend, at least during early inspiration, leading to erroneous conclusions about diaphragm function (6, 7). Because relaxation volume decreases in the supine position, subjects are less likely to be able to breathe by active expiration below the relaxation volume and passive descent of diaphragm during inspiration is less likely to occur.

Unilateral diaphragm paralysis is easier to detect because there is paradoxic motion during tidal inspiration, with ascent of the paralyzed dome, contrasting with descent of the normal hemidiaphragm; this contrast can be amplified by the sniff test, which induces a vigorous, short-lived contraction in the normal hemidiaphragm. Because of normal variations in movement of the two diaphragms, relative weakness of one hemidi-aphragm (such as after cardiothoracic surgery) is difficult to detect radiographically.

Estimates of Change in Diaphragm Length from Radiographs at Different Lung Volumes

The length of the silhouette of the diaphragm domes, length of the diaphragm apposed to the lateral rib cage between the costophrenic angle and a skeletal landmark corresponding to its insertion, and the internal diameter of the rib cage can all be read on a P-A radiograph (8, 9). As discussed above, the silhouette may be formed from the dome in several dorsal-to-ventral planes, and therefore these measurements do not give the actual length of the diaphragm in a transverse section. But it has been argued that changes in these measurements in radiographs obtained at different lung volumes may still give a reasonable estimate of changes in diaphragm length (8). Similar measurements can also be made with a set of lateral radiographs. To estimate muscle fiber shortening, further assumptions must be made about the uniformity of shortening and the contribution of the central tendon to the total length. Using P-A radiographs at several lung volumes, estimated diaphragm shortening as lung volume increases in normal subjects has been predicted by a modified piston-and-cylinder model that

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Left

Figure 1. Three-dimensional reconstruction, using spiral computed tomography, of diaphragm contour at supine functional residual capacity in a normal subject (A) and a patient with hyperinflation due to chronic obstructive pulmonary disease (B). The silhouette of the domes projected on a transmission radiograph may arise from different planes. Scale in centimeters. Reprinted by permission from Reference 3.

Left

Figure 1. Three-dimensional reconstruction, using spiral computed tomography, of diaphragm contour at supine functional residual capacity in a normal subject (A) and a patient with hyperinflation due to chronic obstructive pulmonary disease (B). The silhouette of the domes projected on a transmission radiograph may arise from different planes. Scale in centimeters. Reprinted by permission from Reference 3.

allows for inspiratory expansion of the lower rib cage but neglects change in shape of the domes (9). Radiographic changes in diaphragm length over the vital capacity have been estimated in patients with chronic obstructive pulmonary disease (10-12). Nonradiographic methods can be used to measure the internal diameter of the rib cage (calipers or magnetometers, with chest wall thickness measured by ultrasound) and the length of the zone of apposition (ultrasound; see below); the accuracy of estimates of change in the diaphragm length by nonradiographic methods therefore will be influenced by whether the dome shape remains reasonably constant (or its change can be predicted) over the vital capacity (11, 12).

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