What effect have technological advances in CT and MR had in contrast delivery methodology? How has the newest contrast delivery technology adapted to the changes? What are the implications for the latest generation of injectors of the current understanding of contrast-induced nephropathy (CIN)?

In general, the impact of these advances has affected CT more than MRI. Contrast delivery technology has adapted to changes while making its own progress in simplifying workflow for radiologists and technologists. As far as CIN goes, it appears that the jury is still in deliberations.

FAST FORWARD CT

The increase in the number of detectors in CT scanners has decreased scan times, says Joseph Schoepf, MD, director of CT research and development at the Medical University of South Carolina and associate professor of radiology. The 64-slice CT reportedly can scan a heart in 10 seconds, a significant decrease from previous scanner generations. The dual-source CT is reported to be able to reduce scan time by another 50%: heart scans will take 5 seconds and entire chest scans will take about 10 seconds. “That means that contrast media injection protocols become more challenging because there’s significantly less room for error,” Schoepf says. “You need to make sure that during that particular period when you do your very short scan, you have optimum contrast attenuation at a target organ.”

Schoepf says that CT angiography (CTA) is one of the most challenging applications for contrast enhancement. In order to accommodate the faster scan times, it is necessary to increase the flow rate of the contrast agent to achieve the same attenuation in the target vessel that you would need for a 16-, 8-, or 4-slice scanner. A faster scanner also makes the proper timing of the bolus imperative because the machines are less forgiving when it comes to finding the optimal phase of contrast enhancement. For some applications, radiologists at the Medical University of South Carolina use automated bolus triggering or a test bolus where a small amount of contrast material is injected into the body and target anatomy is scanned multiple times so a peak time of contrast attenuation can be gauged.

To optimize contrast enhancement, a number of devices have been introduced to work with the latest imaging technology. Traditionally, single syringe injectors were used to introduce contrast material. With the more challenging contrast media protocols required by 64-slice and even 16-slice CT, most institutions that perform CT angiography use dual syringe injectors that utilize the contrast material by flushing the tubing as well as the arm of the patient with saline and keep the bolus more compact, allowing for a more homogenous attenuation in the target vasculature. The saline also flushes out the contrast from the superior vena cava in the right heart, reducing artifact in the process. That is particularly important for the visualization of the coronary arteries during CTA where dense artifacts in the right heart or superior vena cava can create the appearance of nonexistent lesions. Some physicians even use the saline to ensure that the patient IV is properly placed because a misplaced bolus is very painful for the patient and can be a reason for litigation.

There is one problem with this dual phase approach to contrast injections, according to Schoepf. It can be too efficient at flushing the contrast material out of the superior vena cava and right heart and the visualization of those structures can be lost. This makes it impossible to see pathology in the right heart. For this reason, the radiologists at the Medical University of South Carolina developed a tri-phase protocol where the bolus of iodine is introduced, followed by a mixture that is mostly saline but has enough contrast to make right heart structures and potential pathologies visible, but not so strong as to cause artifacts. The final phase is a pure chaser of saline.

REDUCED CONTRAST MEDIA

Safety First: FDA

Using the appropriate vascular access devices when administering contrast media as part of CT or MRI studies is paramount to avoid serious patient injury. Devices that are not designed to tolerate high pressures can and do burst. The US Food and Drug Administration received more than 250 reports of such events before issuing a reminder of the danger in June 2004. The FDA recently issued a Sentinel Event Alert to reiterate the dangers.

The ruptured devices included central venous catheters, small gauge peripheral catheters, implanted ports, extension tubing, and intravenous administration sets. Some of these ruptures have resulted in device fragmentation, sometimes with embolization or migration that required surgical intervention; extravasation of contrast media; loss of venous access requiring device replacement; and contamination of the room and personnel with blood and contrast media. The FDA issued the following recommendations to help prevent the rupture of vascular access devices when they are used with power injectors:

  1. Check the labeling of each vascular access device for its maximum pressure and flow rate. If none is provided, assume device is NOT intended for power injection and do not use.
  2. Know the pressure limit setting for your power injector and how to adjust it.
  3. Ensure that the pressure limit set for the power injector does not exceed the maximum labeled pressure for the vascular access device(s).

New multislice CT technology requires significantly less contrast material than previous generations. According to Schoepf, a CT angiogram takes between 70 and 90 cc of contrast, down from 150 cc. The decrease in contrast material is most dramatic in CT pulmonary angiography: In the past, 150 cc of contrast material was needed. Now, only about 50 cc is required. This reduction is important to the elderly population who are at risk for fluid overload. Many physicians believe that the reduction of contrast translates into decreasing the risk of CIN.

“The pathophysiology and etiology of CIN are not well understood,” Schoepf says. “That’s one of the problems with CIN: nobody really knows, for sure, what causes it. Some people argue that if you reduce the amount of contrast material, that doesn’t really help you prevent cases of CIN because 30 cc of contrast is about as nephrotoxic as 150 cc of contrast. In other words, it may not be dose dependent. We operate under the assumption that it is dose dependent, but we don’t know that for sure.”

Unlike CT scanners, technological advances in MRI have not had a significant effect on contrast material delivery, although there are studies that suggest less contrast can be used in brain studies when performed on a 3T scanner. “The number of patients in the studies is relatively small, so I’m not sure what impact [those studies] have had so far,” says David Bluemke, MD, PhD, clinical director of MRI at John Hopkins University. “To obtain equivalent contrast enhancement in the brain, at 3T compared to 1.5, it looks like we can potentially reduce the contrast for 3T.”

Since these studies are relatively new, the protocol at Hopkins has not changed and the protocols for the 3T scanners are similar to those of the 1.5. Before the 3T was released, there was significant testing to make sure the magnet strength that almost doubles that of its predecessor would not adversely affect contrast mixture. It has not and the same solution is used. Even with the more powerful machines, scan times have not changed.

Bluemke says that John Hopkins Hospital purchases a new MRI annually. When it acquires a new unit, it also purchases power injectors that are compatible with both 1.5 and 3T units. Technically, the injectors are portable but the electrical requirements of the units and the accompanying electrical connections as well as the setup complicate matters.

Angiographic Delivery Devices

Automatic power injection devices have been in use at Wake Forest University Baptist Medical Center (WFUBMC) for 3 years, according to Darla Page, RTR, the cardiac catheterization laboratory manager at the hospital. Unlike standard contrast injectors, these units provide a mechanical delivery. The device also provides the ability to stop delivery the moment the artery is visualized. Using traditional techniques to visualize a right coronary artery would require 8 cc of contrast, but the power injector with its ability to deliver the contrast under pressure—and stop on visualization—calls for 3 cc, Page reports. She also said that diagnostic catheterizations that required between 110 cc and 125 cc of contrast now use between 50 cc and 75 cc.

According to Page, the emphasis in the WFUBMC catheterization laboratory is to reduce the volume of contrast to the patient. “Because, obviously, by reducing the contrast that you give, you decrease the nephrotoxicity,” Page notes. “We were looking at care of the patient.”

Stephen Krcmar is staff writer for Decisions in Axis Imaging News.