MR reaches Antarctica, but focus stays on ice

Sedation, movement, and contrast haven't been issues for operators of Antarctica's only MR scanner. Their highly cooperative target is sea ice.

Three researchers from Victoria University in Wellington, New Zealand, spent the 2002-03 polar research season using MR to characterize pockets of liquid brine found in ice in Antarctica's McMurdo Sound. By studying brine structure and dynamics and the thermal conductivity of the ice, the physicists hope to gain a better understanding of the influence of sea ice on climate changes.

With the application of a new NMR probe, researchers measured sea brine diffusivity and gathered information about brine pocket anisotropy (whether the sea brine exhibits properties with different values when measured in different directions).

"We are NMR specialists trying to find out what NMR can tell us about this very important material," said Paul Callaghan, Ph.D., who was joined by fellow physicists Mark Hunter and Ocean Mercier on the three-week expedition.

The probe operates at temperatures ranging from -20¼C at the surface to -1.8¼C at the sea (the melting point of ice). The Antarctic weather proved more of a challenge for the crew than for the equipment.

"That's a whole other story about tents, extreme cold weather clothing, food, blizzards, boredom, and sheer physical exhaustion," Callaghan said.

The portable, 65-microTesla probe was developed over the past year by physicists from Massey University in New Zealand. It was designed so that probe insertion and measurements would cause the least possible disturbance to the sample- new-season ice added to a glacier in the previous winter. Given the nature of its subject, the MR scanner used in this research works with a much lower field strength and frequency than a standard scanner used for humans.

Callaghan and colleagues drill ring-shaped holes in the sea ice and slip the sleeve-shaped probe, 15 cm in diameter, around a 10-cm core of ice. The probe, 1 meter in length, can be lowered as much as 2 meters into the ice sheet. A portable battery-operated spectrometer allows the team to travel up to 1 km from the field camp, based near the edge of the glacier's tongue.

Brine is the target sample simply because brine is liquid and the remaining ice is solid. Because of its very short relaxation time, solid ice gives no NMR signal.

According to Callaghan, the measurements from the NMR probe were successful, and brine diffusivities were measured at depths down to 140 cm. The complete analysis of data from the NMR measurements will be submitted for publication within the next six months.