Chapter 11 Intensive Care Unit

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CHAPTER 11 • Intensive Care Unit Imaging

Major advances have occurred in ICU radiology over the last two decades as technological progress has perfected digital filming techniques, accelerated acquisition and processing speeds, deployed ultraso- nography to the bedside, and dramatically enabled improved imaging communications to and from the ICU point of care. Clinical data and background information can be rapidly reviewed by both clini- cian and radiologist, and digital images can now be viewed remotely on almost any computer, por- table X-ray machine, or handheld electronic device. This technological revolution has brought a host of improvements. Among them: 1. “Hard copy” films are no longer lost or out of chronological order. 2. Delays in availability have decreased. 3. It is now possible to manipulate image bright- ness and contrast and to compare new images side-by-side with previous ones. 4. Geographically separated physicians can simul- taneously view a study. 5. Physicians no longer need to leave the ICU to view studies. There are two important disadvantages of the digital revolution. First, although the situation is rap- idly improving, the expensive high-resolution displays necessary to see the smallest details are not widely available; hence, studies are often examined on sub- optimal screens. Second, the frequent meetings of the intensivist with radiologist that nearly always occurred when hard copy X-ray films were used have all but vanished. Although “throughput” efficiency may be enhanced, such isolation is unquestionably detrimental. Failure to connect face-to-face often deprives the radiologist of important clinical infor- mation to aid in effective consultation, may result in clinicians overlooking subtle but important findings, and eliminates a valuable educational function.

and exposure technique. One simple measure to improve the ability to interpret CXRs is to reposi- tion overlying devices (e.g., ECG monitoring wires, ventilator and IV tubing, external pacing pads, and nasogastric or orogastric tubes) out of the field of the radiograph. Orientation of the patient with respect to the radiographic beam is of critical importance. Kyphotic, lordotic, and rotated projections impact the apparent dimensions of intrathoracic structures and detection of pathology. The use of “gravity- dependent” radiopaque markers on the corners of portable films helps clarify a patient’s position. The AP technique blurs and magnifies the anterior medi- astinum and great vessels, in some cases by as much as 20%. Obese patients present particular challenges in separating what is normal from what is not, espe- cially when filmed supine (Fig. 11-1). Moreover, apart from the AP requirement itself, radiographs obtained in supine patients exaggerate apparent cardiovascular dimensions because of augmented venous filling, higher diaphragms, and reduced lung volume. For example, the azygous vein distends in the supine normal subject but collapses in the upright position (Fig. 11-2). Conversely, supine films often render imperceptible a small pneumothorax or pleural effusion. Rotation produces artifactual hemi- diaphragm elevations and depressions. In diffuse infiltrative processes, lateral positioning accentuates asymmetry—making the dependent lung appear more affected. Film penetration may emphasize or diminish parenchymal lung markings. Consistency in exposure technique is critical to allow day-to-day comparison of radiographs. A properly exposed CXR should reveal vertebral interspaces in the retrocar- diac region. Films on which these interspaces are not visualized are underpenetrated, exaggerating parenchymal markings and making visualization of any air bronchograms more difficult. Changes in lung volume influence the appear- ance of parenchymal infiltrates, especially in mechanically ventilated patients and in those receiving positive end-expiratory pressure (PEEP). Infiltrates seen on a CXR obtained in full inspira- tion on the ventilator usually appear less dense than when viewed in partial inspiration. Similarly, many patients will have a “less-infiltrated” appear- ing CXR following the application of higher PEEP. Unfortunately, there is no predictable relationship between the level of PEEP applied and its impact on the appearance of the film. To facilitate comparison, therefore, serial films ideally should be exposed with the patient in the same position, during the same

CHEST RADIOGRAPHY Technique

Although the CT has displaced the bedside film from its former diagnostic prominence, the simple portable film suffices to answer many questions that require repeated follow up and do not require CT precision. Bedside radiography, therefore, retains a strong place for many applications. However, the usefulness of the portable anterior–posterior (AP) CXR is largely determined by positioning