There are many different protocols for performing CT scans of the chest, and this is why clinical detail in the request or referral for imaging is so important, as the presumed diagnosis will make a difference as to which protocol is selected.
Probably the most frequently used protocol - used to stage bronchial malignancy. Normally performed without oral contrast (although this may be given when staging upper GI malignancies, when the protocol is otherwise virtually the same) and with IV contrast. The first sequence is performed through the chest in the arterial phase of contrast enhancement and the second sequence is performed through the liver in the portal venous phase of enhancement. With the advent of spiral scanners the slices are contiguous and can be reformatted at varying slice thickness (usually in the order of 5mm). The images are generally viewed on soft tissue and lung windows to look for the primary lesion and evidence of nodal and other metastases.
CT pulmonary angiography (CTPA) is used to look for evidence of pulmonary embolism. This generally uses software which triggers on a rise in the amount of IV contrast reaching the main pulmonary artery so that ideally, the pulmonary vessels are well-delineated. Pulmonary emboli are then seen as filling defects in the vessels. It is rapid, good at showing large emboli and is without a doubt the investigation of choice when the patient is haemodynamically compromised and diagnosis will change management. It is being used more frequently in place of V/Q (or Q) scanning. However just like V/Q scanning its sensitivity and specificity depends upon the technology and protocol used, an issue often unknown to non-specialist clinicians. An important factor to remember particularly in young women is that the breast dose (and therefore the risk of inducing cancer) is much higher with CT than with V/Q scanning and ideally the latter should be the investigation of choice in young females. In addition, the iodinated contrast poses some risk to the developing fetus. A baby exposed in utero to contrast will require post-natal thyroid function tests so for this reason also, pregnant women are probably better investigated by V/Q scanning as a first line.
Because of the difficulty in diagnosing pulmonary embolism, this investigation has the potential for overuse. Best practice is to use a validated clinical scoring system such as the Wells Score for clinical risk of pulmonary embolism to guide the decision as to whether to request a CTPA. However, many unsuspected pulmonary malignancies or infections are detected on such scans and indeed the CTPA makes a definitive diagnosis possible in almost 15% of requests. This issue is very problematical for practising clinicians where clinical assessment including CXR (which is at least half as sensitive for sub-lobar consolidation and much less sensitive for early malignancy) does not result in a definitive diagnosis and pulmonary embolism remains in the differential diagnosis of the patients symptoms.
High resolution computed tomography (HRCT)
Used to look for interstitial lung disease. The slices obtained are thinner giving more parenchymal detail, but are not contiguous (1mm slices every 10mm is a standard protocol), therefore making it possible that small neoplasms could be missed. The liver is also not imaged and IV contrast is not given.
- ↑ Scatarige JC, Weiss CR, Diette GB, Haponik EF, Merriman B, Fishman EK. Scanning systems and protocols used during imaging for acute pulmonary embolism: how much do our clinical colleagues know? Acad Radiol. 2006;13(6):678-85
- ↑ Perelas A, Dimou A, Saenz A, Rhee JH, Teerapuncharoen K, Rowden A, Eiger G. Incidental findings on computed tomography angiography in patients evaluated for pulmonary embolism. Annals of the American Thoracic Society. 2015 May; 12(5):689-95.(Link to article – subscription may be required.)