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Magnetic resonance imaging also possible at frequencies below 1 kHz

18.06.2012

In magnetic resonance imaging (MRI), there is a clear trend towards increasingly high magnetic fields – from currently 1.5 T up to 7 T – for tomographs in hospitals; this corresponds to Larmor frequencies from 63.8 MHz to 298 MHz. This trend is based on the fact that magnetization – and thus also the achievable image resolution – increases with an increasing magnetic field. On the other hand, successful imaging at magnetic fields below the magnetic field of the Earth (~ 50 µT, this corresponds to frequencies of ~ 2 kHz) has – in the past few years – been presented by different research groups. Our development is aimed at achieving imaging at an even lower frequency range which is corresponding to the frequency range arising in the human brain during neuronal activity. These frequencies lie in the range from DC to approx. 1 kHz. For that purpose, an imaging prototype system was manufactured at PTB and successfully tested at Larmor frequencies of 100 Hz and 731 Hz. At these frequencies, the brain activity can directly influence magnetic resonance imaging and would then allow direct, functional imaging of brain activity.

 

Prototype of a magnetic resonance imaging set-up inside a magnetically shielded room
Photo: PTB

A basic requirement of magnetic resonance imaging below the Earth's magnetic field (~ 50 µT, this corresponds to a frequency of ~ 2 kHz) is a sufficient suppression of the Earth's magnetic field and of other external interference fields from the environment. At a frequency of 1 kHz, the environmental interferences measured at the PTB site amount to approx. 1-2 pT/√Hz and, at 100 Hz, to approx. 15 pT/√Hz. Therefore, all MRI experiments were carried out inside a specially designed magnetically shielded room (MSR). It is based on the commercially available MSR AK3b from Vaccumschmelze Hanau (VAC) and consists of 2 layers of Mu-metal and an eddy-current shield made of copper-plated aluminium; the room is accessible and its internal dimensions are: 2.5 x 2.5 x 2.3 m3. A specially designed coil system around the Mu-metal layers allows excellent demagnetization of the MSR. After demagnetization, the residual magnetic fields in the measuring volume (1 m3) are smaller than 1.5 nT, with a gradient < 20 pT/cm.

As a magnetic field sensor, a 1-channel SQUID system, constructed by the PTB, was used. It consists of a first-order axial gradiometer with a pickup coil of 20 mm in diameter and a gradiometer base length of 120 mm .This is made from superconducting niobium wire and connected to a SQUID current sensor. The SQUID itself is accommodated inside a superconducting niobium shield to protect it against the relatively high polarization fields of up to 50 mT. The SQUID system is operated in a fiber glass helium cryostat which has been especially modified for this task and has a noise level of approx. 1.9 fT/√Hz at 1 kHz.

The SQUID system was supplemented by a coil arrangement, composed of the polarization coil for magnetizing the samples as well as the imaging coils. The polarization coil allows magnetic fields of up to a maximum of 50 mT to be generated. This field is then switched off very quickly and is no longer active during the imaging sequence. The imaging coils generate, on the one hand, the detection field (2 µT for a frequency of 100 Hz or 17 µT, respectively, for a frequency of 731 Hz) as well as the required gradient fields (~ 30 μT/m). The complete set-up can be seen in the photo.

This set-up allowed 2D magnetic resonance images of different samples to be generated at frequencies of 731 Hz and 100 Hz. These samples consisted of small containers with liquids which had T1 and T2 relaxation times similar to those of the brain. The required measuring times were in the order of approx. 1 hour.

In the case of a further increase in the measuring sensitivity, the measuring set-up could measure brain activity directly. To achieve this, the sensitivity must still be improved by at least one order of magnitude. This could be achieved by different measures (noise reduction, polarization increase, improvement of the current sources for imaging coils).

The method presented here for the measurement of brain activity by means of low-field MRI would then allow activity to be directly mapped onto an anatomical image. Thus, the inaccurate estimation of the localization of brain activity encountered in MEG/EEG measurements could be avoided and neuroscience would have direct access to the brain activity’s points of generation.

 

Scientific publication

Ingo Hilschenz, Rainer Körber, Hans-Helge Albrecht, Antonino Cassara, Tommaso Fedele, Stefan Hartwig, Hans-Jürgen Scheer, Lutz Trahms, Jürgen Haase and Martin Burghoff
MAGNETIC RESONANCE IMAGING AT FREQUENCIES BELOW 1 kHz
J MRI, 2012, accepted.

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