What's inside the human brain?
Anatomically selective measurement of metabolites in vivo thanks to new technique
Modern magnetic resonance tomographs provide innocuous – radiationfree – imaging. In addition, they permit the concentrations of metabolites to be measured non-invasively inside the human body. Quantitative magnetic resonance spectroscopy thus contributes to finding out where, how and why neurological and psychiatric diseases occur, and it will, in the future, be very helpful in diagnosing them and controlling the therapy applied.
Research on this technique intensively deals with triggering even more precisely those anatomic target regions in which a neurochemical profile is to be measured. To date, the measuring signal in cerebral MRS could only be obtained from a rectangular target volume (voxel) in the cerebral region of interest. This limitation results from the interaction between the magnetic field's gradients which determine the voxel geometry and the radio frequency pulses which excite the spin system to be detected. Rectangles are, however, difficult to adjust to the twists of the cerebral cortex or other anatomic structures that usually exhibit irregular shapes. If the MRS voxel is selected so as to be small enough to be specific to a particular structure, the signal-to-noise ratio (SNR) is, however, lower, which affects the precision of the measurement. If the voxel is enlarged, then a large amount of cerebral matter is scanned which, in turn, results in a biased measurement.
At PTB, a new MRS sequence (SHAVE – SHAped Voxel Excitation) has therefore been developed; it allows the excitation of anatomically adjusted volumes. The core of the procedure consists in the multichannel transmit technique for the radio frequency excitation of the volume which is realized by means of an 8-channel transmit array combined with an 8-channel radio frequency coil. Their individual channels emit radio frequency pulses whose amplitude and phase can be manipulated independently of each other. By combining such pulses, it is in principle possible to excite any kind of target volume shape. With this new measurement techniques, it was possible to obtain MR spectra from voxels without any undesirable proportion of gray matter.
Another advantage of this method was the implementation of a two-shot procedure to localize the third direction in space. In this way, the MR signal can be read out with a very short echo time. As a result, the metabolite resonance to be determined hardly has any time to decay and thus provides strong signals. It is therefore also possible to detect metabolites with a very low SNR due to their short transverse relaxation time.
To make this new method easier to apply in hospitals, the workflow will be optimized and adapted to functionally highly differentiated (e.g. cortical and subcortical cerebral) structures which are difficult to trigger.