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Ultra high field (UHF) MRI presents several advantages to human imaging, but problems arise due to its magnetic susceptibility, says Richard Bowtell, who presented a lecture on MRI and spectroscopy of human subjects at UHF in London last week. Ultra high field (UHF) MRI presents several advantages to human imaging, but problems arise due to its magnetic susceptibility, says Richard Bowtell, who presented a lecture on MRI and spectroscopy of human subjects at UHF at a series of seminars in London last week. The event, Futures for Imaging in Clinical Research, was held by GlaxoSmithKline (GSK) at their new Clinical Imaging Centre in the Hammersmith Hospital site of the Imperial College London.

Professor Bowtell helped construct one of the first 3T human imaging systems and was involved in its exploitation in early fMRI experiments. His current research focuses on the development of the first ultra-high field 7T human imaging system in the UK. In his talk he presented the results of his team's experience with a Philips 7Tesla Achieva scanner at the University of Nottingham. The system was installed in November 2004 and became operational in October 2005.

The UHF system, says Bowtell, has so far shown several advantages:

• Higher spatial resolution with shorter scan time
  
• Fewer trials with subtler effects
  
• Easier access to T2 contrast
  
• Improved arterial spin-labelling
  
• Increased susceptibility effects - access to new contrast mechanism in phase imaging
  
• Great chemical shift effects - high spectrum resolution

Problems encountered were the need for shorter echo times; potential loss of T1 contrast; image distortion due to signal drop-out, and spatial variation of signal intensity and contrast. Inhomogeneity, says Bowtell, is the main disadvantage presented by ultra high field scanners, as the field offsets are proportional to the signal strength.

"Field distortion leads to geometrical distortion and signal drop-out", he explains. So how to address this obstacle and reduce inhomogeneity effects? Active and passive shimming (dynamic shimming) might be one of the answers, as it reduces the scanner's sensitivity. Another one would be insert gradient coils to produce stronger gradients, allowing for a technique called diffusion-weighted echoplanar imaging (EPI).

"Reduced T2 at 7T also allows generation of contrast with shorter TE [time to echo]", says Bowtell, "making it possible to use shorter TR [repetition time] and reducing imaging time."

Processing is not automatic because some aspects of it still need adjusting. "The relationship between complex cortical structures and field variation is not well understood", the researcher points out. "The level of localization of contrast to structure remains uncertain.