Fast CT scans increase risk of contrast timing errors

The potential advantages of multislice CT for contrast-enhanced imaging are clear. The improved spatial and temporal resolution offered by MSCT means that high-quality images can be obtained far more quickly. Phases of enhancement are well defined, the volume of contrast required for vessel imaging can be reduced, and, depending on which phase is being imaged, the enhancement may well be more homogeneous.

Achieving optimal results is by no means certain, though. The shorter the scan duration, the more critical it becomes to get the protocol right. Injection timing, contrast volume, and scan delay must all be considered carefully, Euthimios Agadakos told delegates at an ECR 2008 session devoted to CT and contrast media.

"When targeting for ultimate contrast attenuation in a particular phase, these short scan times give very little room for error. In multislice CT, synchronizing acquisition to the desired phase is essential," said Agadakos, senior radiographer at the General Hospital "Laiko" in Athens, Greece.

Protocols used with single-slice CT cannot simply be carried over to MSCT imaging, Agadakos said. Faster acquisition times over identical volumes mean a greater risk of outrunning the contrast bolus. A chest CT study on a conventional system, for example, takes approximately 36 seconds. Switching to 64-slice CT will allow the entire abdomen and chest to be imaged in just five seconds.

Setting the time delay between contrast injection and scanning requires careful calculation. For all examinations performed on MSCT scanners with eight or more detector rows, an additional "diagnostic delay" is added to the conventional scan delay. The overall time lag can be determined using a test bolus or automated bolus-triggering software.

Saline flushing is recommended to increase the level and duration of enhancement. The saline bolus is injected immediately after the contrast to drive the tail of the contrast bolus into the central volume. Saline flushing can also improve the bolus geometry.

Getting the injection strategy right is also important. For MSCT studies of around 20 seconds, the injection duration should equal the scanning duration for optimal enhancement. Fast acquisitions, where imaging is completed in 15 seconds or less, are more complex, according to Agadakos. His advice is to set a single injection parameter and then vary the remaining parameters according to the patient.

Yet more care is required when optimizing contrast protocols for multiphase imaging, a common strategy for liver studies. Hypervascular lesions can be distinguished from normal parenchyma in the late arterial phase, while hemangiomas, fibrotic lesions, and small cysts may be identified during delayed phase imaging. Get the scan timing or contrast volume wrong, and pathology may be missed.

Optimal arterial enhancement in the liver can be achieved with relatively low doses of iodinated contrast. Imaging the parenchyma, however, will require a higher dose. Raising the injection rate increases the degree of separation between the peaks of hepatic arterial and parenchymal enhancement, so improving the conspicuity of vascular lesions."To sum up, faster scanning is better, but only when you need to demonstrate the target organ in just one phase," Agadakos said. "On the other hand, we need to know how much faster we can scan [to] appreciate the effects of technical parameters and the duration of arterial and parenchymal enhancement."

-By Paula Gould