iPAT Squared offers big-time boost for 3T MR imaging

Parallel imaging has gone exponential in Siemens' latest version of iPAT. The advanced iPAT Squared technology allows four, eight, or even 16 times the data capture of systems without parallel imaging, according to the company. There is a catch, however: Parallel imaging is a signal hog, and that can cause problems at all but the highest field strengths.

"Typically, parallel imaging at 1.5T has not been used beyond a factor of three, at least for body work, because as you go higher in iPAT factors, you lose a lot of signal," said Dr. Vivian Lee, vice chair of research at New York University School of Medicine. "At 1.5T, the images just can't tolerate the signal loss that goes with a factor of four or higher."

This places iPAT Squared firmly in the domain of 3T, where Lee and her colleagues have been applying it. They have seen several advantages, particularly in very fast 3D imaging for time-resolved or very fast contrast-enhanced studies. The NYU team has obtained promising studies in the lower extremities when scanning for peripheral vascular disease.

"Time-resolved imaging below the calf has given us temporal information about how the vessels are filling and whether their collaterals are exhibiting retrograde flow. It almost mimics x-ray angiography," Lee said.

The group has had similar results in the chest for identifying vascular malformations. The advanced form of parallel imaging may also have clinical implications for whole-body MR angiograms.

"We want to chase the bolus from the head all the way to the toes, so we have to be quite fast. iPAT Squared might be able to do that, too," she said.

Speed is less a factor than resolution in brain imaging. The NYU radiologists have focused heavily on improving image quality, using a 12-channel head coil. The better quality of images derives from the ability of iPAT Squared to minimize artifacts, said Dr. Edmond A. Knopp, section chief of neuroradiology at NYU. The most obvious improvement addresses susceptibility artifacts from the skull base and sinuses.

"The greater the parallel imaging factor, the less artifact," he said.

Proponents of 3T have been promoting this field strength for the past few years as the future benchmark for clinical MR scanning. Much of their argument has rested on the development of new technologies that will take advantage of the increased signal-to-noise these scanners deliver. Lee and Knopp are optimistic that improved parallel imaging may be the needed technology. But its potential is just being recognized, Lee said.

The NYU radiologists have been running iPAT Squared on their Siemens' Trio whole-body scanner only since early spring. Their 3T scanner is outfitted with a TIM (total imaging matrix) body coil, which provides the coil density necessary to take advantage of the advanced form of parallel imaging.

"You need the coil piece; without it you've got nothing," Knopp said.

Siemens' conventional iPAT allows phase encoding in just one direction. TIM's multichannel capacity, in combination with iPAT Squared software, supports phase encoding in two directions, said Charles Collins, Siemens MR radiology segment manager.

"With these two phase-encoding directions, you can square the iPAT factor," he said.

Like any 3D acquisition, the resulting 3D data set is isotropic, so the data can be postprocessed in any plane. The key contribution from iPAT Squared is the speed with which the data can be acquired.

"We were able to do isotropic imaging before. With iPAT Squared we can make more images in a more reasonable time," Lee said.