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fMRI Imaging Techniques
Nature 458, 925-928 (16 April 2009) Functional Neuroscience: How to get ahead in imaging Nathan Blow
MRI is at the core of neuroimaging today. To create detailed anatomical and functional images, MRI systems take advantage of the ability of very large cylindrical magnets, ranging in strength from 1.5 to 15 tesla, to align the protons found in water throughout the body. Smaller, localized radio-frequency electromagnetic fields are then generated to push those protons out of alignment. The displaced protons generate signals that are detected by the MRI instrument and translated into an image. Whether built for imaging humans or small animals, MRI instruments are defined by the field strengths of their magnets — and bigger is better. The signal from magnetic resonance is inherently weak and it can be difficult to detect, which tends to limit resolution. So developers are constantly trying to increase the field strengths of their magnets and boost the signal. Small-animal imaging is based on making systems with magnets of smaller diameter but much higher field strength than you would get in clinical practice. Most animal-imaging magnets range in field strength from 4.7 T to 15 T, with 7 T being the "workhorse field strength in animal imaging." Human MRI instruments, on the other hand, use 0.5–3 T magnets for clinical applications and up to 9.4 T for research applications. Part of the challenge in moving human scanners up in field strength to match their small-animal counterparts comes from body sizes. Maintaining field strength homogenously over a large volume becomes more and more challenging as the field strength increases.". For this reason, human scanners that are operating at ultra-high field strengths are mostly using smaller diameter magnets, similar to animal scanners, to image small human samples such as tissues rather than performing whole-body scans. Maintaining homogenous field strength is not the only issue that magnet builders face in their quest to boost the signal. Another challenge for all systems is that as you go up in field strength, you must address issues related to magnetic susceptibility.. At higher field strengths, a magnetic field gradient can occur at the interface of materials with very different magnetic susceptibilities, such as tissue, bone and a void (in the sinuses). This can lead to artefacts in the images that must be accounted for during image analysis. Currently, 3 T MRI systems are the standard for high-end human neuroimaging. But research-grade 7 T MRI instruments for human studies have come a long way. First generation of 7 T systems were monsters that needed 400 tons of shielding, but the second-generation systems are actively shielded, so now in principle you don't need any iron to shield the magnet. But many researchers say that, even with the advances, 7 T systems still need more engineering work. Whether 7 T will eventually become a clinically robust field strength for human MRI is not clear. "Ever since the beginning of MRI there has always been discussion about what field strength. First there was the decision between using 0.5 T and 1.5 T, and then between 3 T and 4 T magnets. Now, he says, a 7 T debate might start. "Some of the images from the brain at 7 T are truly amazing. "We are seeing things that we have never seen at either 1.5 T or 3 T." Even as new 7 T human systems and 11.7 T and higher animal systems from companies such as Bruker Biospin and Varian in Palo Alto, California, expand in use within the neuroscience community, bigger magnets are being designed for cutting-edge animal and human imaging. In autumn 2008, the Martinos Center installed a 15 T magnet designed by Varian and Magnex Scientific in Walnut Creek, California, for imaging mice and rats, which Wald says is now at field strength and should be generating its first images in the coming weeks. The machine, used in conjunction with knockout mice with genes that have been 'turned off', should allow scientists to understand neural disease progression more effectively and even test potential drug therapies, says Wald. Field strengths for human MRI magnets may reach double digits in the coming years. High-strength magnets come with big technical challenges and hefty price tags. A complete imaging system based on a 16 T magnet for small-animal imaging cost tens of millions of dollars, and magnets for imaging humans often cost similar amounts, Advances in radio-frequency-coil design could provide a more economical route to higher sensitivity. Radio-frequency coils are used as both transmitters to oscillate protons and as detectors to receive the signal. Ten years ago, single-channel coils were used for this purpose. But today, most instruments have multi-channel coils, allowing the parallel acquisition of data during a scan. Re3searchers are working to get a better signal from scans at lower field strengths with a 96-channel head coil. A protocol that used to run as two 8.5-minute scans, can now be run in a single 3.5-minute scan. Faster scans mean less time in the instrument for subjects, which presents new opportunities for the researchers at the larger imaging centres who routinely scan thousands of people a year. As magnet field strengths increase, coil developers will have to keep improving sensitivity. "The detection side of it will continue to be hard, but we know what to do and how to do it now. The transmit side on the other hand is potentially most challenging. For radio-frequency transmitting, the technical challenges increase with field strength because the human body starts to distort the radio frequency at high field strengths affecting the radio-frequency transmission. Some progress has been made on this front already, with researchers working on the idea of parallel transmission using multiple radio-frequency transmitters that are simultaneously excited. By using parallel transmission it is possible to eliminate the homogeneity problems caused by the shorter wavelengths generated at higher frequencies. (end of paraphrase)
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