Computed Tomography

Abstract

Computed tomography (CT), also known as computed axial tomography (CAT), is a volumetric imaging modality that is based on X-ray absorption. Unlike projection X-ray imaging (Chap. 2), CT allows the reconstruction of a two- or three-dimensional absorber map. CT vastly exceeds projection X-ray imaging in soft tissue contrast, but the spatial resolution of a clinical whole-body CT scanner is significantly lower than that of plain X-ray imaging. Nonetheless, CT can reveal small tumors, structural detail in trabecular bone or the alveolar tissue in the lungs. CT was introduced in 1971, and it is the first imaging modality where the computer is essential in the image reconstruction: a series of X-ray projections undergoes a transformation that yields the cross-sectional image. Since the introduction of the first CT scanners, major progress has been made in contrast, image quality, spatial resolution, and acquisition time. Modern clinical CT scanners are very fast and can produce a 2D cross-sectional image in less than a second. Spatial resolution can be as low as 100 $\upmu$m in-plane, and specialized CT microscopes provide voxels of less than 10 $\upmu$m. On the other hand, clinical CT scanners are expensive, ranging in the millions of dollars. This translates into a relatively high cost per CT scan, which prevents its more widespread adoption.