MDCT Technology – Introduction
Substantial changes in computed tomography (CT) protocols are essential as radiologists migrate from 4-slice to 16-slice and more recently towards 64-slice multidetector CT (MDCT) systems. MDCT scanners have progressively increased the number of detectors and reduced scan acquisition times. The latest 64-slice CT systems with gantry rotation times of 0.33 seconds and a spatial resolution of 0.4 mm are now in clinical use. Cardiac imaging, first made feasible with ECG-gated 16-slice CT, will become a routine, highly reproducible examination with 64-slice CT. It is projected that cardiac CT will be the largest growth area for CT imaging over the next five years.
Both 16-slice and 64-slice CT scanners acquire data as isotropic voxels. This means that images can be viewed in all imaging planes with similar spatial resolution leading to routine utilization of 3 dimensional visualization tools. High resolution CT angiography (CTA) for detection of vascular occlusion and stenoses in the carotids, abdominal or thoracic aorta, and the peripheral circulation has replaced conventional diagnostic angiography. CT pulmonary angiography for diagnosing pulmonary embolism has, in most institutions, replaced ventilation-perfusion scans. High resolution MDCT imaging of the liver, pancreas, and kidneys not only improves lesion discrimination but provides a vascular imaging map useful in planning surgical procedures.
As the number of CT applications expands with newer generations of CT systems, practicing radiologists face significant challenges. Not only must they develop and implement new protocols such as cardiac CT or peripheral angiography, but they need to revise established protocols in view of CT technical refinements. Given the abbreviated scan times associated with 16-slice and even more with 64-slice CT, accurately timed contrast delivery becomes critical. We can time our acquisition to coincide with arterial or venous phase but when should this be performed? How much contrast and what concentration provide the highest quality diagnostic images? Exactly what scanning parameters, reconstruction intervals, and rendering techniques should be applied for each clinical question? Only when studies are tailored to a specific indication do we perform the best possible examination and provide relevant information to the referring physician.
Modifications of contrast administration and scan timing need adjustments according to the type of CT scanner. As scan times shorten, with 16- and 64-slice CT, it is necessary to delay the onset of scanning in order to image during peak aortic enhancement particularly for CTA applications. Smaller volumes of contrast material may be used but the magnitude of enhancement is reduced, so it becomes necessary to compensate by increasing the injection rate and increasing the concentration of contrast material. Therefore, an ideal technique with a short scan duration decreases the amount of contrast material, increases the injection rate, and uses a high concentration of contrast material.
Additional improvements in arterial enhancement can be achieved by the use of a saline flush. The magnitude of arterial enhancement can be increased with a compact contrast bolus, and contrast volumes may be decreased up to 20%. Contrast material that would remain in the intravenous line or brachiocephalic vein is thereby flushed into the vascular system. Artifacts from dense contrast material in the superior vena cava and right atrium can be eliminated, markedly improving cardiac and pulmonary imaging.
Philip Costello, MD, FACR
Professor and Chair of Radiology
Medical University of South Carolina
Charleston, South Carolina, USA
Multidetector CT Protocols
Developed for GE, Philips, Siemens,Toshiba Scanners
Springer 2005, 2006