1 Department of Radiology 2 Department of Anaesthesia, Royal Victoria Hospital, Belfast BT12 6BA, UK
* Corresponding author. E-mail: peter.farling{at}dnet.co.uk
Preoperative assessment of the central airway is concerned primarily with the detection and evaluation of laryngotracheal stenosis. There are many causes of laryngotracheal stenosis; however, stenosis secondary to a thyroid goitre is one of the more common types requiring evaluation prior to general anaesthetic. A postero-anterior chest and lateral thoracic inlet radiograph have been the primary investigative tools used to assess the degree of tracheal compression and deviation in both the transverse and antero-posterior planes.1 In many patients this type of imaging will suffice; however, computed tomography (CT) will more comprehensively detail the extent of tracheal stenosis and the degree of retrosternal extension. Compression of other structures by retrosternal extension of a thyroid goitre can result in a number of syndromes, including superior vena caval syndrome, cerebral hypoperfusion from arterial compression and thyrocervical steal, phrenic and recurrent laryngeal nerve palsies, Horner's syndrome, pleural effusions, chylothorax and pericardial effusions.2 The effects of many of these may be seen on plain films.
Until the recent advent of multislice (multidetector) CT technology, CT images have been obtained with single-slice spiral scanners. Although imaging by this technique is superior to plain radiographs in assessing the degree of laryngotracheal stenosis and retrosternal extension, standard axial CT images may underestimate the craniocaudal extent of the disease and give inadequate representation of airway stenosis orientated obliquely to the axial plane.3 Sagittal and coronal reformatted images can be obtained in an attempt to overcome these problems. However, the quality of these images has suffered in the past as a result of the inherent limitations of single-slice CT technology.
Although magnetic resonance imaging (MRI) can produce exquisite imaging of the airway in any plane without incurring a large dose of radiation to the patient, the images are more susceptible to artifact from respiratory and cardiac movement. In addition, MRI has less spatial resolution than CT and therefore is less accurate in the assessment of a high-grade or tortuous stenosis.
Recent improvements in imaging of the airway have been afforded by the advent of multislice CT scanners. These machines enable multiple thin sections of the patient to be imaged during one rotation of the CT gantry around the body. At present, with the latest commercially available CT technology, this typically involves imaging 16 slices per gantry rotation. However, this is a rapidly evolving field and recently 40-slice and 64-slice CT scanners have become available. At our institution a 16-slice scanner is used. This has a gantry rotation time of 0.5 s, which enables 32 slices to be imaged per second. The overall effect of this technology is that the slice thickness (collimation) can be reduced, which causes a resultant increase in the z axis resolution (the axis along the craniocaudal length of the patient). Thus, image voxels are obtained which are almost isotropic (the same resolution in three dimensions). Voxel is the notation for volume pixel, which is the smallest distinguishable box-shaped part of a 3D image. In practice, this means that, even with routine imaging studies, excellent 2D reformatted images (sagittal and coronal sections) or three-dimensional images can be obtained. In addition, because multiple slices are being obtained per gantry rotation, a greater anatomical coverage can be obtained in a shorter breathhold for the patient, resulting in a reduced likelihood of artifact from patient movement or respiration that might degrade image quality. Advances in computer hardware and software have enabled rapid post-processing of the CT volume dataset so that, for 3D imaging, external rendered (CT tracheobronchogram) and internal rendered (virtual bronchoscopy) images can be obtained and interrogated within a few minutes of the study finishing. External rendering demonstrates the external surface of the airway and its relationship to adjacent structures. It has been shown to improve the evaluation of the shape, length and degree of airway stenosis.4 Internal rendering (virtual bronchoscopy) produces images similar to those seen at conventional bronchoscopy. On the CT workstation, it is possible to fly-through the airway in conjunction with viewing axial, coronal and sagittal 2D images on the same screen. Having concomitant 2D imaging helps with orientation within the tracheobronchial tree, and when strictures are a result of neoplastic disease, it permits differentiation between intraluminal tumour growth and extraluminal extrinsic compression.5
Although virtual bronchoscopic images are visually impressive, the technique has its limitations. It cannot show the true colour or vascularity of the mucosa, and this limits the detection of subtle mucosal lesions. Retained mucus and secretions may also cause false-positive findings.6 Therefore it has not yet been proven as a primary tool for detecting endobronchial malignancy. The major benefit of virtual bronchoscopy in relation to conventional bronchoscopy is its ability to visualize the major airway distal to a stenosis that cannot be traversed by the bronchoscope.
In this issue, the case report by Toyota et al. describes how multislice CT provided non-invasive imaging of the airway to facilitate fibrescopic endotracheal intubation in a patient with severe tracheal stenosis secondary to a malignant thyroid goitre.7 This is certainly a valid use of such technology for patients in whom severe laryngotracheal stenosis is suspected. Detailed non-invasive imaging of the stenosis influenced decision-making with regard to mode of anaesthetic induction and intubation. The virtual bronchoscopy images can be stored as a cinematic file and later reviewed by the anaesthetist on a compatible computer or reviewed on the CT workstation with the radiologist. In patients where plain films do not indicate a severe stenosis or significant retrosternal extension, more caution is required when requesting a CT for preoperative assessment. This is not only because of the cost of the examination, but more importantly because of the radiation dose received by the patient. The radiation dose received by a patient from a standard CT scan of chest is approximately equivalent to the dose received from 400 chest radiographs or the natural background radiation received over a period of 3.6 yr.8 If the patient has known malignant disease, this is not of prime concern as it is likely that the patient will require CT for staging, and once the CT volume has been acquired with a multislice scanner, no further imaging is needed to perform virtual bronchoscopy. However, if the patient is being imaged for benign disease, there will be a significant, although difficult to quantify, increased lifetime risk of cancer from such a radiation dose. Therefore, as with all CT examinations, adequate justification is required so that the perceived benefits outweigh the risks of using ionizing radiation.
Anaesthetists should continue to explore the potential provided by modern imaging techniques in order to improve the quality of preoperative investigations.
References
1 Farling PA. Thyroid disease. Br J Anaesth 2000; 85: 1528
2 Anders HJ. Compression syndromes caused by substernal goitres. Postgrad Med J 1998; 74: 3279[Abstract]
3 Boiselle PM, Ernst A. Recent advances in central airway imaging. Chest 2002; 121: 165160
4 Remy-Jardin M, Remy J, Artaud D et al. Volume rendering of the tracheobronchial tree: clinical evaluation of bronchographic images. Radiology 1998; 208: 76170[Abstract]
5 Hoppe H, Dinkel H-P, Walder B et al. Grading airway stenosis down to the segmental level using virtual bronchoscopy. Chest 2004; 125: 70411
6 Eliashar R, Davros W, Gramlich T et al. Evaluating laryngotracheal stenosis in a canine model with virtual bronchoscopy. Ann Otol Rhinol Laryngol 2000; 109: 90612[ISI][Medline]
7 Toyota K, Uchida H, Ozasa H, Motooka A, Sakura S, Saito Y. Preoperative airway evaluation using mult-slice three-dimensional computed tomography for a patient with severe tracheal stenosis. Br J Anaesth 2004; 93: 8657
8 Making the Best Use of a Department of Clinical Radiology, 5th edn. London: Royal College of Radiologists, 2003