New MRA technologies offer chance to boost vascular applications

June 12, 2002

Studies should see dramatic increase by 2005Patient tables might seem too ordinary, too basic to play an important role in the future of MR. But tables, and how they are used, may shape the acquisition of vascular studies.At the

Studies should see dramatic increase by 2005

Patient tables might seem too ordinary, too basic to play an important role in the future of MR. But tables, and how they are used, may shape the acquisition of vascular studies.

At the annual meeting of the International Society for Magnetic Resonance in Medicine in Honolulu, presenters described the use of tables as part of several techniques designed to make MR angiography competitive with x-ray-based digital subtraction angiography (DSA). Shoot and Scoot, WakiTrack, and AngioSURF are among the strategies that might be used to overcome obstacles to the widespread adoption of whole-body and peripheral MRA.

The roots of these techniques extend to the 1997 introduction by Philips Medical Systems of MobiTrak, a bolus-chasing technique that uses a moving table to step the patient through the bore of the magnet. Doing so aligns successive body regions in the field-of-view, facilitating peripheral runoff exams using MR. MobiTrak and other early techniques generated clinically relevant data, but their results were not equivalent to DSA, landing MRA in a state of clinical limbo.

The biggest problem has been venous contamination, especially in the lower body regions. In these exams, resolution typically drops off dramatically between the knees and toes, and modern approaches are designed to overcome this failing. In ISMRM presentations, Shoot and Scoot achieved encouraging results by manipulating k-space; WakiTrack used parallel data acquisition; and AngioSURF utilized a rolling table for optimal coverage.

These techniques have the potential to boost MR angiography to new heights. MRA holds only a minor stake in today's mix of MR procedures: about two million of the total 31 million MR exams done globally, according to GE Medical Systems. But GE analysts believe vascular applications could account for about 11 million of the 60 million expected in 2005.

"We believe that MR vascular will take off at a high rate," said Dave Weber, Ph.D., manager of GE's high-field MR business. "What's needed is to at least mimic the type of data that you get from conventional angio."

Researchers at university medical centers in collaboration with major vendors reported several evolutionary steps in this direction at the ISMRM meeting. Shoot and Scoot, developed at the Uniformed Services University of the Health Sciences in Bethesda, MD, in collaboration with GE, is designed to optimize data acquisition by decreasing the time between bolus detection and acquisition of distal runoff. Data are collected over two passes, according to Dr. Vincent Ho, a research radiologist at Uniformed Services. Tests of the system were run on a 1.5T Signa scanner optimized for cardiovascular scanning. The central 30% to 40% of k-space is acquired during the first pass in a reversed elliptical centric phase order. This pass concentrates on the abdomen. An elliptical centric order is used for subsequent stations down the body.

Typically, data are collected over three body regions or stations. Scanning is triggered by automatic bolus detection in the abdominal aorta, and table motion between stations begins at the start of each pulse sequence following the completion of data acquisition for the preceding station. The parameters for each station are set independently to achieve the best results with different matrix sizes, partitions, and coil selections.

Consequently, most of the contrast resolution that resides near the center of k-space is captured first. Resolution is preserved while scan time is cut nearly in half to 20 to 30 seconds per station, Ho said. Data acquisition is completed in the arterial phase, thereby avoiding venous contamination.

"This tailoring allows data collection from all three stations during the arterial phase of the contrast injection, yielding high-resolution images of proximal and distal stations with minimal venous contamination," Ho said. "Individual stations can be customized to the particular anatomy of the patient, maximizing signal to noise while minimizing the time spent acquiring data."

WakiTrack takes advantage of SENSE (sensitivity encoding), a parallel imaging method introduced by Philips Medical Systems several years ago. It allows the user to keep up with the passage of the gadolinium contrast bolus and permits faster contrast administration to preserve signal to noise, according to designer Jeffery Harold Maki, Ph.D.

"We take the time we save by getting to the lower station more quickly and invest it into higher resolution at the lower stations where vessels are the smallest," he said.

WakiTrack uses a phased-array body coil on the upper station and a phased-array spine coil behind the legs on the lower stations. Manual table movement is used to speed up data acquisition, which is less than four seconds between stations. Linear phase encoding is used to limit scan time at the abdominal station to 10 seconds. The middle station is completed in about 12 seconds, and scanning in the lower station is performed in about seven seconds to produce 1.6 x 1.8 x 1.2-mm voxels for imaging of the calves and ankles.

A primary advantage is the ability to capture data about arteries below the knee that are less than 3 mm in diameter with submillimeter resolution. Captured in such peripheral studies are the pedal arteries, which extend anteriorly outside the thin slab typically acquired during MR scans. All this is accomplished with a single injection of contrast medium on a 1.5T Philips scanner equipped with SENSE and set for data acquisition at three stations, Maki said.

The designers of AngioSURF (System for Unlimited Rolling Field-of-Views) have concentrated on the initial stations, with the objective of maximizing coverage for whole-body MR vascular exams, according to Dr. Mathias Goyen, a research radiologist at the University of Essen in Germany. Studies were conducted on a 1.5T Siemens Sonata using Bracco's new contrast medium, MultiHance, which is pending FDA review. The technology is optimized to produce high-resolution 3D whole-body MR angiograms from a continuous set of images with a single contrast bolus traveling from the carotid arteries to the feet. Data are acquired during a 72-second bolus track.

The rolling table platform that serves as the cornerstone of AngioSURF fits over an existing patient table and operates with the scanner's standard phased-array surface coil, which is inserted into the unit. Station-to-station positioning is performed manually, and 3D volumetric data are collected in four segments. Data are acquired while the region of interest is located in the isocenter of the magnet. Specificity and sensitivity were in the mid-90% range during preliminary study.

Goyen envisions the device expanding the role of MR into healthcare screening as an alternative to whole-body CT. Nonvascular applications of the moving table could also be used to survey the body for metastases following chemotherapy.