A novel MRI technique called spiral scanning may offer an alternativeto other fast MR imaging methods, such as echo-planar imaging(EPI), without requiring additional hardware for the scanner. Spiral scanning employs the same gradients used for
A novel MRI technique called spiral scanning may offer an alternativeto other fast MR imaging methods, such as echo-planar imaging(EPI), without requiring additional hardware for the scanner.
Spiral scanning employs the same gradients used for conventionalMRI while offering potential added advantages in avoiding motionartifacts, according to a Stanford University research team.
"Spiral scanning will impact virtually any area wherespeed is important," said Dr. Robert J. Herfkens, directorof body MRI at Stanford. "You can make a list of the thingspeople want to do with echo-planar imaging and apply spiral scanningto them. All of the dynamic cardiac studies will be done withit eventually. It's a very cheap way of getting speed."
Spiral scanning in MR should not be confused with fast techniquesdeveloped for CT imaging, which utilize continuous gantry rotationand scanning along the length of the patient (SCAN 3/27/91). Whatspirals in spiral MRI is the way in which the MR data are collectedand formed into "k-space," a term for the raw data mapfrom which a single MRI image slice is processed.
In all conventional and most fast MRI techniques, data areadded line by line into two-dimensional k-space. In spiral scanning,the gradient pulses are manipulated so as to collect a spiralof data. The spiral trajectory of data collection begins in thecenter of the k-space map and progresses to the outside in a circularfashion and at a constant linear velocity.
Multiple spiral collections are then weaved together to completethe data map. The more spirals added, the better the quality ofthe processed image.
"We cover all of the k-space in the same time as in EPI,but we do it coarsely and then we interleave," said Dr. AlbertMacovski, a professor of electrical engineering and diagnosticradiology at Stanford.
Macovski holds a patent on the technique and leads the spiralMRI research effort.
The technique is not without drawbacks. Although collecting thedata with spiral trajectories is relatively straightforward, turningthat gathered information into an image is not.
While EPI collects k-space data one line at a time, specialgradients are manipulated very rapidly, essentially allowing allthe lines of k-space to be sampled during a single repetitiontime (TR). The resulting data are readily processed into images.
In contrast to EPI, where it is relatively easy to processthe data but harder to get it in the first place, processing thedata from spiral scanning is "tremendously difficult,"said Dr. Steven E. Harms, director of MRI at Baylor Universityin Dallas. The biggest advance made by the Stanford group maybe the creation of software that can turn spiral data into good-qualityimages.
"They're able to process that data and do a very goodjob at it," Harms said. "You could conceivably do spiralscanning on conventional MR scanners just by beefing up the processingend. That's considerably less expensive than buying a new gradientset."
Spiral MRI also produces images that look good because theyhave a reduced sensitivity to motion, said Dr. Jens Frahm, headof biomedical NMR research at the Max Planck Institute in Gottingen,Germany. Although the physical reasons for this are not fullyunderstood, it is thought to be related to the way the gradientsare arranged to produce the spiral trajectory.
"The waveforms more or less resemble what people haveused for motion compensation," Frahm said.
Image processing of the spiral data is currently performedoff-line on a workstation, with images made available at the rateof roughly one image every minute to a minute and a half, saidCraig H. Meyer, a doctoral candidate in Macovski's laboratory.
While more work is needed to develop spiral MR, its potentialcould throw a damper into efforts to commercialize EPI technology.Advanced NMR Systems is filing an application for Food and DrugAdministration approval of an EPI package developed in conjunctionwith GE Medical Systems (SCAN 9/25/91). Spiral scanning doesn'toffer equivalent speed to EPI, but it may provide a large portionof the same clinical utility.
"Until I see what's behind this door with spiral scanning,I'm not willing to spend any money on anything," Harms said.