Computed Tomography

CT produces a two-dimensional, cross-sectional image by rotating an x-ray beam around the area of the patient's anatomy of interest. Each cross section requires a few seconds of radiation exposure, and the typical scan comprises 20 or more such sections. patient immobility is required during each exposure to produce a high-quality image, but the patient can move between sections, provided that alignment with the imaging equipment is not changed. The x-ray tube and detectors are housed in a circular scanning tunnel in which the patient lies during the study. The scanning procedure may be noisy, some heat is generated, and the patient can become claustrophobic and frightened. Although small children often sleep through an examination if they have recently been fed, older children frequently and some adults need to be sedated to tolerate this examination.

CT is commonly used for diagnostic purposes, for example, to evaluate the status of a neoplastic process in the thorax or abdomen. It has proved valuable in studies of vascular malformations and tumors. Caregivers involved in imaging patients undergoing this examination should be mindful that some patients have limited reserve because of their disease process (e.g., compression of the upper airway or vena cava by a neoplastic process). CT is occasionally also used for invasive therapeutic procedures instead of fluoroscopy, for example, for visualized drainage of a liver abscess. Contrast media are often used in conjunction with CT to enhance the quality of the image. If contrast media are to be introduced into the gastrointestinal system in patients who are sedated or anesthetized, it is usually by nasogastric tube. When the airway is not protected in this situation, there is a risk of aspiration. ^[31] It has also been suggested that the incidence of adverse sequelae associated with the use of contrast media is higher in patients undergoing CT versus other types of radiologic studies because of limited access to the patient and consequent compromised monitoring and treatment if reactions do occur.^[15]

Issues confronting anesthesiologists requested to anesthetize patients undergoing CT include inaccessibility of the patient during the procedure and control of movement artifact. Monitoring interference is not a problem in CT, however, as it is with MRI, as discussed later. With anesthetized patients, care needs to be taken to ensure that the sides of the scanning tunnel do not occlude or dislodge the breathing circuit or monitoring leads during the procedure. As with other radiographic procedures, ionizing radiation exposure occurs during CT scans. The patient can be monitored visually through a lead glass window, supplemented if necessary by closed-circuit television. Should the patient's condition merit closer monitoring by the anesthesiologist, radiation monitoring badges and protective lead aprons and thyroid shields must be worn.

The anticipated procedure may necessitate special anesthetic techniques. Ablation of bony metastases may be conducted under CT guidance, and continuous post-procedure regional analgesia may be beneficial, as noted previously. Selection of the regional anesthetic technique and timing of placement of the catheter (if a catheter-based technique is selected) are dependent on the patient's condition, as well as the available space and equipment. For example, the CT room is not typically a suitable location for placement of an epidural catheter. If a recovery room or preanesthetic area is available in the radiology suite, it might be a convenient, safe location for placement of a regional anesthetic catheter. If such areas are not conveniently located or sufficiently equipped, these techniques might have to be performed in the operating suite before or after the CT-guided procedure.

Certain pulmonary neoplasms, usually metastases not suitable for surgical resection, are amenable to CT-guided RF ablation.^{[32] [33]} These procedures have been performed under both sedation/analgesia and general anesthesia. Reported complications of RF ablation of pulmonary masses include pleural effusion, pneumothorax, pneumonia,

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