Volumetric applications for Spiral CT in the thorax

Spiral computed tomography (CT) is a new technique for rapidly acquiring volumetric data within the body. By combining a continuous gantry rotation and table feed, it is possible to image the entire thorax within a single breath-hold. This eliminates the ventilatory misregistration seen with conventional thoracic CT, which can result in small pulmonary lesions being undetected. An additional advantage of a continuous data set is that axial sections can be reconstructed at arbitrary intervals along the spiral path, resulting in the generation of overlapping sections which diminish partial volume effects resulting from lesions that straddle adjacent sections. The independent control of collimator width and the table feed enables spiral CT scans to be acquired at variable pitch, enabling the operator to optimize resolution, coverage, and signal-to-noise. Though a minimal collimation of 7-8 mm is typically required to image the adult thorax in a single breath-hold, thoracic subvolumes up to 18 cm along the z axis can be imaged with 3 mm collimation or 6 cm can be imaged with 1 mm collimation. These "high resolution" volumetric data sets provide a unique opportunity for studying fourth and fifth branch order bronchial anatomy, relating these 2-3 mm branches to larger central bronchi. The contiguous nature of these data sets makes them optimally suited for two dimensional reformations along coronal, sagittal, oblique, or curved planes and for three dimensional renderings with shaded surface displays and maximum/minimum intensity projections. As a result, complex anatomical relationships are optimally appreciated. The rapid acquisition of spiral CT enables up to a 50% reduction in the total iodinated contrast dose required for routine thoracic CT scanning. This can be very important for imaging patients with cardiac and renal diseases and could reduce the cost of thoracic CT scanning. Alternatively, by combining a high flow peripheral intravenous iodinated contrast injection with a spiral CT acquisition, it is possible to obtain images of the vasculature, which demonstrate pulmonary arterial thrombi, aortic aneurysms and dissections, and congenital vascular anomalies in detail previously unattainable without direct arterial access. Three dimensional renderings of CT angiograms allow for accurate pre-operative planning for and postoperative assessment of vascular bypass with graft material. For these reasons, spiral CT is becoming the imaging modality of choice for many thoracic applications.