GE uses ACC meeting to debut MRI scanner for cardiac imaging

Signa Select will explore potential of cardiovascular MRI

GE Medical Systems made a move at this week's American College of Cardiology meeting to advance the clinical applications of MRI technology with the debut of its most recent work in cardiovascular MRI. The Milwaukee vendor showed its work-in-progress Signa Select scanner, a version of its 1.5-tesla platform that has been optimized for conducting cardiovascular MRI studies.

GE lifted the wraps on Signa Select at a symposium held March 29 at the Atlanta conference. Scheduled to give presentations were representatives from five luminary sites that have been conducting work with the new scanner, in areas ranging from cardiac perfusion to ventricular function to coronary artery imaging. GE also displayed a mock-up of the system in its ACC booth, as well as static and dynamic images collected with the system.

By taking aim at the heart, GE and other developers of cardiac MRI technology are hoping to take on some of the numerous heart-related imaging exams conducted worldwide. Cardiovascular disease is the number-one cause of death in most industrialized countries, but MRI is rarely used for cardiac imaging due to the the modality's historical limitations.

In particular, the long imaging times of MRI exams have been poorly suited for imaging the rapid motion of the heart, resulting in motion artifacts on cardiac MRI studies. Linking the MRI scanner to an electrocardiogram, a procedure known as cardiac gating, is one potential solution, but strong magnetic fields used in MRI often disturb ECGs, and this reduces the accuracy and reproducibility of cardiac gated exams, according to Kevin M. King, general manager for cardiovascular global MR at GE.

Although the technical challenges have been substantial, cardiac MRI holds such promise that it continues to attract investment from vendors. The modality's ability to visualize fine anatomical structures makes it a good match for imaging heart tissue. MRI has a wider field-of-view and better tissue contrast than echocardiography, and is far less invasive than cardiac catheterization. It does not involve radiation, unlike nuclear medicine or cardiac cath studies.

Cardiac MRI proponents believe that, if the technology proves itself, MRI could ultimately handle the simultaneous assessment of morphology, function, tissue character, and blood flow, applications that require a battery of different tests.

"MRI has excellent temporal and spatial properties, as well as the ability to look at contrast, the differences in soft tissue," King said. "In terms of its comprehensiveness, everything from looking at large structures all the way down to evaluating very fine structures like vessel wall morphology is now within the realm of possibility."

Although GE has been involved in cardiac MRI development for years, the company began the Select program in earnest about two years ago. To solve the problems related to MRI's limitations in cardiac imaging, GE employed principles used in its Six Sigma quality improvement program, which is being applied company-wide.

One issue related to the speed of MRI scanning was that of gradient performance. By increasing gradient amplitude and slew rates, GE dramatically improved the performance of the scanner to make it better suited for cardiac studies, according to King. Signa Select's gradients produce peak amplitudes of up to 40 mtesla/m and generate slew rates of up to 150 mtesla/m/msec.

The more powerful gradients have enabled GE to develop a technique it refers to as real-time interactive imaging, a sort of MR fluoroscopy mode. With the technique, Signa Select simultaneously acquires, reconstructs, and displays images at frame rates of up to 15 fps. This could be useful in applications such as localizing coronary arteries, King said.

Not a dedicated scanner. GE is sensitive to the characterization of Signa Select as a dedicated cardiac scanner, and with good reason: Few hospitals would probably be willing to spend millions on a scanner restricted to a procedure as experimental as cardiac MRI. Instead, the vendor is labeling Select as a cardiac-optimized system; in other words, a scanner that can conduct routine bread-and-butter MRI applications, but that also has special capabilities to support cardiovascular studies.

Signa Select is based on GE's Signa Horizon 1.5-tesla platform, with special gradients and clinical analysis packages designed to support cardiovascular imaging. New Signa Select scanners are based on the short-bore magnets that GE introduced last year, but older systems can also upgrade to the platform in accordance with GE's Continuum upgrade program without installing the short-bore magnet. GE plans to begin marketing Signa Select as soon as the system receives Food and Drug Administration 510(k) clearance, which was applied for earlier this year. GE declined to release a list price for Signa Select.

GE's development of cardiovascular MRI technology was spurred by demand from clinical users, who want to see the technology become more clinically useful, and the company has relied heavily on contributions made by the 10 clinical development sites where Signa Select is installed, King said. The site with the most experience with Signa Select, the National Institutes of Health, has had a system installed for about a year and a half.

Future developments include optimizing the system with MRI contrast agents. GE is working with Epix Medical of Cambridge, MA, on a nonexclusive basis to explore cardiac imaging with that firm's MS-325 agent (SCAN 1/21/98). GE is open to working with other contrast developers to improve contrast-enhanced coronary artery imaging, King said.

Ultimately, MRI may change the clinical diagnosis of cardiovascular disease in the same way that it's changed clinical practice regarding other anatomical areas. And that may be bad news for other imaging modalities that have staked out positions in cardiac imaging.

"What we are driving toward is a comprehensive tool, and users will ultimately determine how it's used," King said. "They tell us that they hope to be substituting MR for other studies. The goal here is not to increase utilization; the goal is to optimize it."