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August 28, 2006

Contrasting View — Faster Scan Time Can Shrink Contrast Needed for a Sharp Exam
By Jim Knaub
Radiology Today
Vol. 7 No. 17 P. 22

The way David Piazzo, BSHA, RT, sees it, the mind-set driving contrast injection for a CT exam needs to change from “how much will it take?” to “how little can you get away with?”

That concept—driven by reducing the amount of contrast patients’ kidneys need to process—was the topic of Piazzo’s presentation at the annual meeting of the American Healthcare Radiology Administrators in Las Vegas earlier this month.

Piazzo, who recently took an applications specialist position with contrast injector manufacturer E-Z-EM, Inc. after a career as both a technologist and an imaging administrator, said many facilities commonly use 120 to 150 milliliters (mL) of contrast for nearly every contrast-enhanced CT study they perform, even though the amount of contrast needed can be significantly reduced for most exams. By using dual-head injectors, one chamber filled with contrast and the other with saline to push the reduced amount of contrast through the circulatory system, Piazzo said it’s feasible to reduce contrast use to 40 to 60 mL for many common exams. While an abdomen or pelvic scan may still require 100 mL of contrast, a coronary CT scan can be effective—arguably, more effective—with 60 mL.

“As an employee of a company that makes contrast injectors, it is my hope that we can inject only enough contrast that will keep the area of the patient that is currently in the primary beam of the scan opacified and move that contrast through the patient with the beam of the scan,” Piazzo said. “If we could do that, we could do virtually any study with 15 mL or 20 mL of contrast. And the dual-head injector with saline on board is the first tool that lets us move in the direction of moving contrast through the patient as we’re doing the study. We no longer have to think in terms of keeping the area of interest opacified for the length of time of the scan.”

Background
As recently as 1999, the scan time for an abdomen was 55 seconds and there was a 45- or 50-second delay between contrast injection and starting the scan for those studies, Piazzo told the audience.

“At the time, most injectors held 150 mL of contrast and that was pretty much the amount you used for virtually every study,” Piazzo said.

As multislice scanners entered the market, manufacturers touted faster exam times with each successive doubling of the new scanners’ detector rows. Piazzo said the realization started to settle in that scanning times were shorter than the amount of time technologists were injecting contrast.

“I’ll never forget the first time I did a study in a new [multislice] scanner and we were done scanning and the injector we were using was still injecting contrast,” Piazzo said. “I remember the other tech and I looking at each other and saying, ‘Can we cut it off?’ We had never crossed that time barrier before.”

As the number of slices increased and scan time started to dip under the 60-second mark, Piazzo said more people began reducing the amount of contrast used for exams.

“Some forward-thinking departments began wondering if they could use less contrast for chest exams than for abdomens,” he added.

The dual-head injector facilitates that by connecting two injectors to the coil and catheter, allowing the technologist performing the exam to first put the calculated amount of contrast into the patient and immediately follow with saline to push the bolus to the area the technologist wants to opacify to perform the study. Piazzo used an example of cardiac CT angiography to illustrate the idea.

Cardiac Flow
“Think about cardiac flow,” Piazzo said. “The blood that is inside the heart is moving at 10 times the speed of the blood that’s in the subclavian [vein]. The blood that’s coming from the antecubital [vein] and brachial and subclavian [vein] is pulsing at the rate of the patient’s cardiac flow rate for that vessel making its way toward the heart. Once it’s in the heart, which is also fed by other vessels that empty into the superior vena cava [SVC] and the inferior vena cava [IVC] … there it tears off at a much faster flow rate. So the amount of contrast that you have in the heart at the point that your single head injector stops injecting is the amount of contrast that’s actually going to do your chest CT. The rest of the contrast in the subclavian [and behind] is never going to make it to the heart while you are scanning.

“The scan is going to be done before it gets there. So if you subtract that 20 mL or 30 mL of contrast that is in the subclavian, you’re now down to 60 mL that actually created your images. Why not just load 60 mL of contrast and push it with 40 mL or 45 mL of saline? That way, all of the contrast we put into the patient gets pushed into the heart, gets passed through the pulmonary axis, and becomes part our study, but the patient doesn’t have 30 more mL of contrast in their vasculature that never made it into the study but their kidneys still have to process.”

While the extra contrast may not be a major problem in most patients, in patients with compromised kidney functions, it becomes a real risk or even a deterrent to imaging.

“Radiocontrast-induced nephropathy represents a common cause of increased risk to patients,” Piazzo said. “And for at-risk patients, physicians must consider using less, or other, means of imaging that don’t continue to burden their patients’ kidneys.”

Developing and using protocols that reflect the time needed to reach the imaging area and the amount need to cover the entire areas of interest for each study’s duration is the key to reducing contrast use. Piazzo again used a cardiac CT angiography as an example.

“If we’re going to do a cardiac study, as an example, it takes eight to 10 seconds to acquire images for it,” Piazzo said. “We don’t need to inject contrast from the start of the study until the acquisition phase. All we need to do is inject for 10 seconds using the flow rate at 5 mL per second—which is pretty much the minimum flow rate required to get the contrast to the left side of the heart in great enough volume that coronaries will opacify. If we can inject for 10 seconds at 5 [mL] per [second], that’s 50 mL of contrast. Give yourself 2 mL or 3 mL leeway and you can see that we can push this volume of contrast with the saline on the other side of the injector through the heart allowing us to do the scan with much less contrast [pushed by saline] than if we had done it only with contrast.”

Different Study, Different Amount
While still needing 80 to 100 mL of contrast for abdomen and pelvic studies, Piazzo is confident that for nearly every other type of study with contrast—chests, heads, necks, livers, kidneys, abdominal aortic aneurysm screening—facilities can save 30% to 40% of contrast currently used for these exams. He also pointed out that when using a single-head injector, the technologist routinely throws away 10 mL of contrast every time he or she disconnects the disposable coil tubing.

The manufacturers of new multislice CT systems advise users how to calculate the timing and amount needed. As new scanners develop—two 256-slice scanners are already being used in Japan—injection protocols will change accordingly, Piazzo said.

“They scan the heart in 0.4 seconds,” Piazzo said. “What is that going to mean to contrast injection? You’ve got to find a way to get a big chunk of dense contrast into the heart so that it’s completely opacified but only for half a second. Then you want to wash it out very quickly. The challenges on the horizon for those of us involved with contrast injection are going to be really interesting. More and more departments are using not just 5 [mL] per [second] flow rates but 6 per, 7 per, or 8 per flow rates for neuro studies of the brain and for cardiac perfusion studies.”

Piazzo also pointed out that shorter injection times and particularly increased flow rates usually mean another change: utilizing a slightly larger catheter, an 18-gauge instead of a 20-gauge.

“If you’re injecting the new contrasts and you’re using a 20-gauge catheter, you’re probably not going to get the contrast delivered at 5 mL per second,” he cautioned. “The injector is going to slow down to keep the pressure down. You’re probably not going to get enough contrast there fast enough to get a good, crisp study.”

While the details differ by exam, Piazzo is confident that shift to “how little contrast can we get away with?” is underway.

— Jim Knaub is editor of Radiology Today.