Dhiraj Sinha1 and Shao Ying Huang1
1Engineering Product Development, Singapore University of Technology and Design, Singapore, Singapore
Synopsis
An MRI system at
small physical dimensions can be developed using a piezoelectric-microcantilever
system with the capability of sensing
magnetic fields in the range of microtesla to picotesla at room
temperature conditions . The RF magnetic field induces voltage in the
piezoelectric material which is amplified by the microcantilever which also
filters out the signal around its resonant frequency. The cantilever vibration
is measured using an optical detection system or by using capacitance to
impedance converter. A thick block of piezoelectric material is used as a
transmitter in order to replace the transmitting coil.
Purpose: The objective of current research is to develop a
miniaturised MRI system at low values of magnetic fields.
Methods: The major challenges in development of
miniaturised MRIs are associated with fabrication of efficient radio transmitters
and sensitive receivers at
low dimensions1. Here, we present a novel design of an MRI system having
a volume in the range of a couple of cm3 operating at a few hundred
kilohertz with magnetic field in the range of millitesla. The receiver of the
MRI system has a piezoelectric-microcantilever system having the
characteristics of an integrated sensor, filter and amplifier which is
difficult to achieve using conventional RF technology and can sense magnetic fields in picotesla range at room temperature conditions (Fig.1). The RF magnetic
field induces voltage in the piezoelectric material which is amplified by the
microcantilever which also filters out the signal around its resonant
frequency. The cantilever vibration is measured using an optical detection
system or by using capacitance to impedance converter. A thick block of
piezoelectric material is used as a transmitter in order to replace the
transmitting coil.
A sample having a net magnetic moment when subjected to
a static magnetic field B precesses
at a certain frequency f which is
given by2 where B is the
magnetic field and is the gyromagnetic ratio which is expressed
by , where, q is charge and m is mass of the corresponding
particle. For Hydrogen atom, the value of is 42.576 MHz/Tesla
which means that for a magnetic field of 1 Tesla, the frequency of radio waves
would be 42.574 Tesla. The general value of static fields in present day MRIs
are about 3T which sets the resonant frequency at 127.72 MHz. If we reduce the
magnetic field to 1 mT, the resonant frequency would be of the order of 127.72
KHz. To detect a signal of 127 KHz, we would need an LC circuit with an
inductance of at least 1.5 mH and capacitance 1 mF. The losses in a capacitor of these values
are extremely high. The best way would be to use capacitances in the range of
nF which would mean increasing inductances in the mH regime which would result
in larger coils and higher level of Ohmic losses in the inductor. The best
quality factor which we can get is 200. In order to sense the magnetic field,
arrays of highly-sensitivity MEMS cantilevers mounted on piezoelectric bases as
radio receivers are being tested3. The cantilevers are fabricated
from doped silicon or other materials, each with a resonant frequency close to
the precession frequency of the hydrogen atom in the biological sample. Such a
system can pick up very low power radio signals in the range of picotesla
regime and have the overall volume which is in the range of 25 mm3.
The transmitting circuit comprises of a stack of
piezoelectric material of dimensions 18 mmX5mmX5mm. According to the Van Dyke model, a
piezoelectric material has a finite value of capacitance, inductance and
resistance4. Above its resonant frequency, it behaves like an
inductor. Fig 2 shows the phase changes associated with a transition from
capacitive to inductive behavior of the piezoelectric stack around its
resonant frequencies. The system behaves
like an inductor of 0.2 mH at around 300 KHz. The corresponding inductance
for a one turn coil of diameter 2 cm is
0.14 mH. The magnetic flux density generated by
such a coil excited by current of 1 A at a distance of 1 cm along a square
plane of 1cm3 is shown in Fig. 3. The magnetic flux density is non
uniform but has a uniformity in the central region of the plane which shows a
value of 6 mT. Considering the value of the inductance
of the piezoelectric stack, they can easily replace such coils in order to
generate similar magnetic field patterns. Thus, a complete MRI apparatus can be
developed using piezoelectric stacks as transmitters, piezoelectric coupled
microcantilevers as receivers and ultra-low magnetic fields (Fig.1 )
Results: A compact MEMS based radio receiver and transmitter with
volume in the range of a couple of cm3 is possible using piezoelectric stack with a microcantilever
mounted on it.
Discussion: These experiments show that
piezoelectric-cantilever system comprising of arrays of cantilevers of
different resonance frequencies can be used in place of transmitting and
receiving coils of an MRI system in order to diagnose samples of low physical dimensions.
Conclusion: A MEMS based radio receiver comprising
piezoelectric material with microcantilever can be used for ultra-low MRI based
applications.
Acknowledgements
No acknowledgement found.References
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