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What's inside the human brain?

Anatomically selective measurement of metabolites in vivo thanks to new technique

PTB-News 1.2017
Especially interesting for


Florian Schubert
Department 8.1 Medical Metrology
Phone: +49 (0)30 3481-7477

Scientific publication

P. Waxmann, R. Mekle, F. Schubert, R. Brühl, A. Kühne, T. Lindel, F. Seifert, O. Speck, B. Ittermann: A new sequence for shaped voxel spectroscopy in the human brain using 2D spatially selective excitation and parallel transmission. NMR Biomed. 29, 1028−1037 (2016)

Magnetic resonance spectroscopy (MRS) allows the non-invasive deter-mination of metabolites in the human brain in vivo. However, it is usually limited to rectangular volumes. This makes it difficult to quantify the metabolites selectively in a certain cerebral structure in order to establish a precise neurochemical profile of this structure. A new pulse sequence combined with a multichannel transmission technique allows the excitation of the magnetic resonance signal in principle in volumes of any shape, hereby making MRS anatomically selective.

In vivo MR spectrum with fitted result from an anatomically shaped voxel excited by means of the SHAVE method which only contains white cerebral matter (i.e. is not contaminated by gray matter). The individual peak positions in the spectrum represent metabolites, their amplitudes represent their concentrations. In the right corner of the diagram, the voxel can be seen in the brain image (yellow).

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.