Pushing ULF MRI instrumentation to the limit
May 24, 2017 - Rainer Körber
At PTB we are developing novel techniques to image current flow in the brain. If the imaged currents are intrinsic, for example when your brain is processing sound or other stimuli, it is called neuronal current imaging (NCI), if they are impressed from the outside by electrodes, it goes by the name of current density imaging (CDI). If we can get it to work, we will be able to image directly and without ambiguity where currents flow in the brain. With this information from NCI and CDI, we and our colleagues could advance immensely functional imaging.
We know from phantom measurements that we need at least a factor of about 2 improvement in the signal-to-noise ratio to be able to successfully demonstrate NCI. But this pushed our previous system already to its limits. To make the next step, we built one of the most sensitive SQUID systems to date with a noise level of about 150 aT/sqrt(Hz), an improvement of more than a factor of 3. We achieved this by upgrading our dewar with special superinsulation and non-electrically conducting heat shields.
Now, we are getting close to the constraint set by the human body. As human tissue is electrically conducting, it also creates magnetic field noise and this represents the ultimate limit. It remains still a big challenge to retain the extremely low noise level in our new SQUID system with all MRI, NCI and CDI equipment operated. Nevertheless, it looks like that this ultra-sensitive performance should make NCI possible. We will know soon whether this is the case.