CURRENT STATUS OF CARDIOVASCULAR IMAGING BY TRANSMISSION COMPUTED TOMOGRAPHY

CURRENT STATUS OF CARDIOVASCULAR IMAGING BY TRANSMISSION COMPUTED TOMOGRAPHY*William R. BrodyDepartments of Radiology and Electrical EngineeringStanford University School of MedicineStanford, CaliforniaAbstractAlthough the technological advances in transmission computed tomography (CT)have been dramatic, cardiovascular applications of CT are currently limited by in-adequate temporal resolution to resolve moving structures (e.g., the myocardium),difficulties in timing the scan exposure to coincide with maximum contrast en-hancement of vascular structures and cardiac chambers, and limited field of viewperpendicular to the axial plane. The modes of cardiac CT operation are reviewed:static (single scan), dynamic (multiple scans in rapid succession), gated andrapid (scans <500 msec). Modest improvements in gated and dynamic scanning areexpected with existing technology and optimization of reconstruction algorithms.Future advances in CT technology should be directed toward the development ofrapid scanning systems for imaging the myocardium and toward higher resolutionscanners for imaging vascular structures.IntroductionSince the introduction of the first device for computed tomography, peoplehave speculated on the potential application of CT for imaging the cardiovascularsystem. With the advance of CT technology into third and fourth generation pro-duction models, cardiovascular applications have become feasible. However, theuse of CT for imaging the heart and blood vessels is in its embryonic state, pend-ing better understanding of ways in which temporal and spatial resolution of CTcan be improved to advance the state of the art. It is the purpose of this paperto outline the current status of cardiovascular computed tomography, indicatingthe limitations of current methods, and to point out directions for the future ofthis exciting application.Computed tomography offers two important advantages for cardiovascular imaging:exceedingly fine density resolution (the ability to measure small differences inthe x -ray attenuation coefficient of tissues), and a cross -sectional imaging for-mat. The major impact of these two advantages is the ability of CT to createblood pool images which outline the heart and blood vessels following the intra-venous injection of modest amounts of iodine -containing contrast agents (0.15-0.25 g iodine /kg). Without this iodine sensitivity, cardiovascular CT would beof limited value, since the differences in the x -ray attenuation coefficients ofblood and muscle are too small to be distinguished by CT without exogenous con-trast medium (iodine) administration.While the use of intravascular iodine for contrast enhancement is common to allforms of CT, there are certain aspects of contrast enhancement that are peculiarto cardiovascular applications. First, a bolus injection is preferred to continu-ous infusion, to permit maximum enhancement of vascular structures. Bolus intra-venous iodine injections introduce serious difficulties in timing the CT exposureto correspond with the appropriate time when the blood level of iodine is maximalin the region of interest. Second, the dynamic change in iodine concentration(and hence x -ray attenuation) following bolus injection provides important physio-logic information about tissue perfusion that can be utilized with a speciallydesigned scanner.The use of a cross -sectional imaging format is not necessarily optimal. Forexample, the tortuous course of arteries is often better appreciated from twoorthogonal projection images rather than a series of axial cross sections. None-theless, the cross -sectional format of CT is important for removing interferencefrom surrounding bone and soft tissues, which normally would obscure the informa-tion contained in the low concentrations of iodine which are imaged with CT.Future applications for blood vessel imaging have been proposed to utilize someof the features of CT imaging for sensitive iodine detection, coupled with a pro-jectional imaging format.* This work was supported by a Grant -in -Aid from the American Heart Associationand with funds contributed in part by the Santa Clara County Heart Association.