In high-end CT scanning, timing means everything
As technology races ahead, contrast application techniques must keep up the pace
By: Wendy Despain
New CT technology is so fast and accurate that it can capture images of a heart between contractions. Scanners just reaching the market can obtain perfect cardiac images of patients with less than perfect heartbeats. But to do this, contrast media have to be in the right place at the right time, and faster scanners still depend on the same slow human circulatory system to move the contrast into the appropriate location for optimal imaging. From an injection in the arm, the contrast bolus has to make its way to the heart, usually flushed into place by saline.
As scanners cover more area in less time, techniques for administering contrast media are being adjusted to keep pace. The contrast protocols recommended for cardiac studies on 16-slice scanners are not appropriate for 40- and 64-slice scanners. With only a handful of the highest end scanners in clinical use, designers and early adopters are testing, speculating, and improvising to find the right protocols to make the best use of the technology.
Haw Loke Gie, clinical marketing manager for Siemens CT, emphasized the role that contrast techniques play in the use of new scanners.
"Vendors have to be very careful when they push it and say 'Oh, I can scan the whole body in three seconds; it's going to be great.' No, it's not going to be great; it's going to be a headache. Nobody can light that," she said.
When new scanner technology enters the market, manufacturers provide guidelines for contrast protocols. Clinicians then refine those protocols based on their own experience and patient requirements. The basic formula at Siemens is injection flow rate x scan time = total contrast.
The length of time required to capture an image directly influences the contrast load. On four-slice scanners, for example, a cardiac scan takes about 40 seconds. The flow rate used for injecting contrast is about 4 cc per second. Cardiac studies on a four-slice scanner therefore require at least 160 cc of contrast medium. As scan times become shorter, the volume decreases. Some 16-slice scanners can complete a cardiac scan in 20 seconds. If the flow rate remains at 4 cc per second, the total contrast could be reduced to 80 cc.
The newest generation of CT scanners can accomplish an entire heart scan in less than 10 seconds. According to Siemens, preliminary tests show that cardiac scans can have good results with just 45 cc of contrast, although most clinicians adjust that to 60 cc for better density.
"You need to get a certain minimum volume in there to image the coronaries, we know from experience that this minimum is somewhere between 60 cc and 80 cc. At some point, we won't be able to lower the contrast any more," said Robb Young, CT product manager for cardiology at Toshiba.
All vendors are targeting between 60 cc and 80 cc of contrast for their top-of-the-line scanners. But that's just the starting point. In clinical use, some additional volume is added or subtracted depending on patient size and fitness.
"A 100-pound elderly woman does not need the same contrast load as a 250-pound guy who's 6 feet tall," said Phillip Prather, director of global cardiology CT for Philips.
Philips takes figuring contrast volume one step further with its "cardiac cookbook," which provides specific techniques and assistance for fine-tuning all the variables in the contrast equation, with information specific to Philips' products. The cardiac cookbook for 16-slice scanners will soon be available on the company's Web site, and a version for the new 40-slice scanner is in development.
With lower total amounts of contrast, the saline flush should become even more important. Injecting small amounts of contrast alone doesn't effectively move it into the right position on time. With 16-slice CT scanners, a saline flush became an integral part of the contrast process. The bolus of saline is injected after one, and sometimes two, varying injections of contrast. Higher slice machines could simplify this process. Since the contrast doesn't need to linger as long in the heart, a single contrast bolus can be pushed into position with one larger saline bolus.
Without the saline, these smaller injections of contrast can become too dilute by the time they reach the heart. And if the contrast isn't expelled from the right ventricle, it can cause image artifacts that obscure the small arteries usually of interest in cardiac scans.
Other problems result more directly from timing errors. If the scan starts too early, just as the contrast is arriving at the heart, arteries can appear smaller than they really are when the image is postprocessed. This can result in serious miscalculations if the image is used to help plan revascularization procedures.
If scanning is delayed too long, image artifacts can arise from diluted contrast in the cardiac veins. The window of opportunity for optimal scans is very small, as contrast circulates quickly through cardiac arteries and into cardiac veins.
IMPROVEMENTS ON THE WAY
With contrast timing so crucial on the latest generation scanners, all manufacturers are taking steps to make the process easier. To that end, they're eliminating the need for a test bolus to estimate circulation rates.
"The disadvantage to that method is that you've just added another 20 cc to whatever you're going to do for your real scan. You also have to do a lot of manual calculation, and with any CT scanning, the more automated you can make it the easier it is," Young said.
A bolus-tracking system allows the scanner to automatically begin the scan when it detects the right levels of contrast density. Only one injection and one scan, without a test bolus, are needed, he said.
The new generation of CT scanners also cuts down on the effect heart rates have on image quality.
"The faster system allows image acquisition during the time when the heart rate in most patients is relatively stable," said Dr. Dennis Foley, a professor of radiology at the Medical College of Wisconsin.
Another source of motion artifacts in cardiac images is even more difficult to avoid with earlier generation scanners.
"Patients have a residual, involuntary motion of the diaphragm that occurs after a certain breath-hold," Prather said. "So the benefit of the larger slice-count machine is that it should provide better coverage for each revolution, and the result is lower breath-holds for the patient."
Toshiba is also automating as much of the process as possible, Young said.
"We're including more of this protocol recommendation on our user interface. Coaching on breathing, for instance: We include this in our multisegment reconstruction software so the doctor or the tech can see what the heart rate is, press 'accept,' and it automatically scans the patient," he said.
With scanners that capture complete images in seconds, automation is the only way to obtain reliably good image quality. All manufacturers include automation in their systems from start to finish, and, as a result, they are confident about the consistent quality of images.
"You can actually be a little less rigorous in patient prep and still have 'brochure-quality' images, as I like to call them," Young said.
Future advances in CT will continue to influence contrast protocols. Someday, breath-holds will be eliminated altogether, Young said.
"That is something that we are seriously looking at right now. We're doing investigations with our 256-slice," he said. "With that, you're talking about a very short breath-hold, under two seconds and even less."
Although this work is purely experimental, it's clear that managing contrast will be a whole new problem in that scenario. Instead of chasing a bolus, the scan would occur only after the entire heart had reached maximum contrast levels, he said.
With these decreasing time frames, Siemens is exploring the possibility of intra-arterial injections to more accurately place contrast.
"We're looking at selective injection into the arteries rather than using the vein, which is, of course, more invasive," Haw said. "But then you start getting a better result in terms of less venous contamination, and in some areas you may need that because the CT is so fast. But we don't know if that's the answer for now; it's still under research."
In the more immediate future, Philips is working on a program that automates everything compiled in the cardiac cookbooks.
"You load in the patient's height and weight and it provides the exact contrast injection protocol for that patient," Prather said. "It makes sense to understand what's needed for better scanning and patient care and for providing better support to our customers in getting the best image quality at the lowest dose and least contrast."
Ms. Despain is a Montana-based freelance writer.