MR angiography challenges CT, ultrasound

MR angiography challenges CT, ultrasound – magnetic resonance

Donna Zsinko

Magnetic resonance angiography (MRA), the non-invasive capability for studying blood flow, could eventually replace various vascular imaging procedures.

Many teaching hospitals and freestanding facilities are adopting MRA or using it in addition to ultrasound, computed tomography and conventional angiography. Once specific applications are determined and reimbursement is approved, community hospitals as well as teaching hospitals will be purchasing MRA.

“The 1990s are turning around the clinical disappointment of the ’80s with new and innovative diagnostic techniques that are proving MRI to be the tool it was originally promoted to be,” said Gary Reed, president of the Lebanon, N.J.-based Integration Resources Inc., a consulting firm specializing in medical imaging. MRA’s role as a screening tool has expanded since its entrance into the market. MRA has become accepted as a reliable imaging method in the areas of carotid artery disease and intracranial aneurysms.

The technique is being used to evaluate arteriovenous malformations, intrinsic vessel disease, intracranial circulation and other vascular problems.

“While MRA is a rather expensive procedure when done alone, there is a cost benefit if the needed MRA is an added series with a needed MRI,” said Dr. Thomas J. Masaryk, who specializes in neuroradiology at the Cleveland Clinic Foundation, Cleveland, Ohio. “Depending on the specialty, MRA should be considered a reliable replacement procedure for CT or ultrasound.”

“Doing four patient procedures per day allows me to break even,” said Dr. James Zelch, who owns Regional MRI, Cleveland, Ohio. Zelch examines up to eight MRA patients a day with a Hitachi mid-field MRP system and has replaced aortograms with MRA.

“I get artifact-free images on the carotid MRA studies and they are taking eight to ten minutes per outpatient procedure. The system provides quality images and is proven to be cost-efficient,” said Dr. Zelch.

“New MRA 3D techniques will provide superior images of the smaller vessels in the brain, and new software and expanded memory for post-processing can isolate an individual artery,” said Dr. John Huston III, senior associate consultant, Diagnostic Radiology, Mayo Clinic, Rochester, Minn. Improvements in how MRA is applied and its image quality will further expand its role in clinical research, according to Dr. Huston.

MRA’s ability to provide quality images non-invasively in an outpatient setting also makes it an attractive alternative to conventional angiography. Teaching hospitals and privately owned, freestanding MRI facilities are performing between one and eight MRA procedures a day. Each MRA screening or diagnosis takes two to ten minutes, a considerably shorter time than conventional angiography.

Other major advantages of MRA over conventional angiography are that it does not require contrast media, and projection views can be studied from any specified angle. Patients also can avoid X-ray exposure and the expense of post-procedure care.

Although MRA has come a long way since Siemens introduced the first product in August 1989, users and manufacturers agree that it has not replaced the image quality of conventional angiography. MRA, when used with conventional angiography, is a useful screening tool; used together, they help eliminate complications, according to Dr. Huston.

“Conventional angiography is the gold standard for diagnosis,” Reed said. “The current (angiography) procedures use well-established and refined imaging techniques that include echo, nuclear medicine thallium studies, and angio in the cath lab.

“MR of the heart has a difficult way ahead of it. However, MRA for the entire body moves up on the quality acceptance scale.”

More clinical studies need to be done on MRA for it to gain widespread acceptance, according to Sheldon Schaffer, director of marketing, at Hitachi Medical Systems of America, Twinsburg, Ohio. “Once clinical confidence reaches 100%, MRA will take off,” he said.

Strong market anticipated

All existing MRI units (2,200 installed base) could potentially be retrofitted with MRA packages and new MRIs could be sold with MRA as standard enhancements (MR unit installations have averaged 450 per year). The annual MRI market is estimated at $700 million.

A facility could spend an additional $50,000 to $400,000 to enhance its MR unit with MRA hardware and software, according to Reed. Two types of MRA packages are available: flash-imaging (gradients at high speed), ranging from $50,000 to $75,000, and echo planer (stop action of heart), which is offered on the Advanced NMR/General Electric system for about $400,000.

“The total MRA market has the potential to reach 400 new units per year,” said Anne Deery, communications manager, MR Division, Siemens Medical Systems, Inc., Iselin, N.J. “Radiologists have accepted MRA and are working with manufacturers to improve the image quality and make it better accepted by vascular surgeons.”

There is disagreement over whether MRA will be used successfully on low- and mid-field strength systems, which account for 48% of the MR market. Some users said that MRA image quality has no relationship to field strength and therefore can be used with lower strength magnets.

“MRA is now demanded on high-field systems (1.0 Tesla and above) and .5T units are finding it hard to compete without it,” Deery said.

“MRA quality has no relationship to field strength; it has to do with gradient echo imaging quality and computer software–the infrastructure of the magnet, not the super structure,” Dr. Zelch said.

But in the view of Dr. Val Runge, director, Magnetic Resonance Imaging & Spectroscopy Center at the University of Kentucky, Lexington,” no quality MRA work has been done on units that have a field strength less than 1.0 Tesla.”

The best MRA imaging results are the product of an MRI machine that operates at high-field strength and is equipped with computer hardware for pulse sequences, a coil to help integrate technique and 3D time-of-flight (TOF) methods that are easy to set up, according to Dr. Runge. TOF angiography provides images from the inflow of fully magnetized blood into saturated stationary tissue.

Candidates for MRA include people with neurological and cardiac disorders, high-risk patients and pediatric patients. The technique also is being used to evaluate injuries to or disease of the neck, abdomen and extremities.

There are about three million stroke victims currently living in the United States who may benefit from an MRA screening. Stroke killed 147,470 Americans in 1989 and is the third leading cause of death in the U.S., following heart attacks and all cancer-related deaths. Using MRA to screen patients may help prevent these statistics from rising.

In 1990, the leading nonsurgical procedure was arteriography and angiocardiography using contrast material (1,735,000 individual procedures) according to the International Classification of Disease, 9th Revision, Clinical Modification (ICD-9-CM) statistics for patients discharged from short-stay hospitals by selected nonsurgical categories.

As the image quality of MRA continues to improve, MRA could replace these specialty procedures.

MRA potential

Conventional Magnetic resonance

angiography Imaging

Installed base 4,700 2,200

Annual unit sales 500 450

Annual $ market $630 million $700 million

Avg. cost per unit $1.1 million $1.4 million

Technical charge per unit $750 $800

Annual procedures 6.8 million 4.8 million

Studies per day per system 4 12

Source: Integration Resources Inc., Lebanon, N.J.

Siemens takes lead

The manufacturers of MRI are the major players in the MRA market. They include General Electric Medical Systems, Waukesha, Wis.; Hitachi Medical Systems of America; Picker International, Highland Heights, Ohio; Resonex Corp., Sunnyvale, Calif.; Siemens Medical Systems; and Toshiba America Medical Systems, Tustin, Calif.

Siemens introduced MRA as a standard package on the MAGNETOM MRI system. Worldwide, the company has sold more than 120 packages, which have been used to perform more than 55,000 MRA exams.

The standard MRA package provides 2D and 3D TOF sequences, which are easy to set up and quick to acquire and post-process. Integrated post-processing with on-line MIP also is standard technology on the system.

“To make this application technically possible, strong gradients, high duty cycles for high resolution studies, and the MR angiography coil (WIP) help integrate the application more fully,” Deery said. Siemens has exhibited its MRA capability at trade shows of the American Society of Neuroradiology, Radiological Society of North America, and Society of Magnetic Resonance Imaging.

In late 1990, General Electric began marketing a multisequence vascular package with pulse sequences manufactured by Advanced NMR Systems, Inc., Wilmington, Mass., that runs on the G.E. Signa Advantage 1.5T MRI system. In 1989, G.E. entered into an agreement with Advanced NMR to develop ultrafast scanning equipment and techniques to reduce scan time and provide stop action images of the heart. The MRA add-on package has been retrofitted on MRI units at Massachusetts General Hospital. The package and MRI unit are being marketed to hospitals, small and large clinics, free-standing centers and mobile MRI owners. G.E. says that the MRA market potential is “very high and growing.”

Hitachi’s MRA is an add-on to the MRP MRI system. The MRI system, designed with permanent mid-field (.2T, .3T) magnet technology, can be easily sited in existing structures. The design of the gantry provides a larger rectangular patient opening, accommodating greater patient volume, and solenoid RF coils provide a field of view with uniform signal intensity.

“This (permanent magnet) system is more economical to operate when compared to the super-conductive magnets,” Schaffer said.

The success of MRA as a screening tool depends on how the physician uses it. When evaluating vascular disease with MRA, there are key MRI approaches that need to be thoroughly understood and employed. Therefore, manufacturers need to educate users on how to set up and interpret clinical findings from spin echo imaging, phase imaging, MRA and diffusion/perfusion techniques.

MRA is still in the development stage with methods of pulse technique, 3D TOF projections with maximum intensity projections (MIP), 2D TOF projections, and phase contrast.

Manufacturers and users are keeping close tabs on MRA development and application.

COPYRIGHT 1992 J.B. Lippincott Company

COPYRIGHT 2004 Gale Group