Low-field techniques put a new spin on MR

Instrumentarium and Nycomed have provided some details about theirjoint R&D effort in low-field magnetic resonance imaging (SCAN4/12/89). The two companies are keeping mum, however, on the exactnature of a special MR contrast agent under development.

The low-field MRI technique is called Overhauser MRI (OMRI).OMRI requires the use of an electronic paramagnetic contrast agentin conjunction with an ultra-low MRI system. Researchers fromboth companies discussed their work at the 1990 Radiological Societyof North America meeting.

"We consider this a major event in medical imaging, equivalentto the introduction of sodium iodide in 1918," said KlaesGolman, managing director of Nycomed Innovation in MalmÕo,Sweden. "The images are somewhere between what we're usedto in nuclear medicine and conventional MRI."

Nycomed, a major supplier of contrast agents for medical imaging,is based in Oslo, Norway. Instrumentarium of Helsinki manufacturesdental x-ray, mammography and low-field MRI equipment. NycomedInnovation is a wholly owned subsidiary of Nycomed, involved inMR research.

The physics underlying OMRI involve the phenomenon of electronspin resonance (ESR), the electronic equivalent of nuclear spinresonance in conventional MRI. In OMRI, additional RF pulses areused to excite the electron spins of the contrast agent. Thisin turn produces a nuclear Overhauser effect whereby the electronspins of the agent are coupled to neighboring nuclear spins. Intheory, the technique can amplify conventional nuclear magneticresonance signals by several orders of magnitude, Golman said.

"If you want to study (hydrogen), which I believe we will,a simplified Overhauser equation predicts a signal enhancementof 330 times," he said. "If you want to study othernuclei, you can get even further increases. Sodium would increaseby a factor of 2000, for instance."

OMRI requires a very low field strength. Because less powerfulradio-frequency pulses are required for excitation in low-fieldMRI, additional RF pulses can be used in the sequence chain withoutexceeding established specific-absorption-rate limits.

Since the saturation frequency of electrons is about 600 timeshigher than that of protons, the RF power needed to perform OMRIat 0.01 tesla is roughly equivalent to the power employed in conventionalproton imaging at 4 tesla, said Raimo E. Sepponen, clinical applicationsmanager for Instrumentarium.

The key to OMRI, however, is its contrast agent. Because thecompound has a very low ESR line width, large amounts of powerare not needed to saturate its electron spins, Sepponen said.While he also declined to identify the agent, Sepponen confirmedthat the compound is biocompatible, "like gadolinium."

INSTRUMENTARIUM IS DEVELOPING separate low-field enhancement techniqueson its own. One technique, magnetization transfer (MT) imaging,is similar to OMRI in that it depends on transferring the energyof one pool of dipole spins to the pool of nuclear spins fromwhich the image is derived. MT imaging also takes advantage ofthe low-field MRI's lower RF power requirements.

Additional RF pulses are used to saturate the spins of protein-boundprotons in MT imaging. Through a poorly understood cross-relaxationprocess, some of this energy in turn saturates the spins of neighboringfree water protons, resulting in a net decrease in signal intensity.

Because the signal loss is proportional to the amount of proteincontained within a tissue, the contrast between protein-densetissues and water- or fat-dense tissues is accentuated, with thesignal of tissues containing only small amounts of protein remainingunaffected. Thus, MT images often offer improved contrast betweenlesions--which typically have a high water content--and normalmuscle, liver and brain.

"It's kind of an electronic negative contrast agent,"said Sepponen. "The same T1 and T2 relaxation processes apply.Magnetization transfer just enhances the conventional contrastparameters."

Instrumentarium is also developing an RF-intensive, low-fieldtechnique called spin-lock imaging. While MT contrast providesinformation regarding protein concentration, spin-lock imagingoffers insights into the size distribution of macromolecules.Both techniques have the potential to greatly improve MRI's abilityto characterize tissues, Sepponen said.

MT and spin-lock imaging can be performed on Instrumentarium'snew 0.1-tesla Mega-4 MRI system (SCAN 10/24/90). OMRI will requirea scanner functioning at a lower field strength, however, withcurrent work being performed on a Mega-4 system modified to runat 0.01 tesla.