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By Steven G. Turowski, Michael Loecher, Mukund Seshadri and Richard
Mazurchuk Roswell Park Cancer Institute, Buffalo, NY
Published in
Bioscience Technology April 2007
Small animal magnetic resonance imaging (MRI) techniques are
currently one of the premier research tools available to probe
and validate structural and functional relationships at the biosystem,
cellular or molecular level.
In fact, a growing number of MRI facilities dedicated to imaging
small animal models of disease now exist in a variety of
environments encompassing pharmaceutical, medical and basic science research. Preclinical MRI studies are
typically performed at high magnetic field strengths, yielding high signal-to-noise ratios (SNRs)
and soft tissue contrast compared to other available modalities.
A majority of preclinical studies, especially those that involve
characterization of disease progression and response to therapy
in transgenic animal models, require an elaborate experimental
design using large cohorts of animals. The acquisition of these large MRI data sets can be
expensive, time consuming and labor intensive. Therefore,
automation techniques to improve throughput, increase efficiency
and/or improve accuracy would represent a significant advance,
especially with regard to screening and phenotyping animals.
Specifically, this article describes the use of a novel MRI-compatible device (SQUIGGLE motor, New Scale Technologies,
Rochester, NY) that could allow researchers to remotely:
(1) administer agents to live animals in a MRI
environment without image artifact,
(2) re-position samples/animals in a dynamic fashion
during data acquisition, and
(3) tune and impedance match RF coils at their
resonant frequency.
To demonstrate the potential utility of this device in small
animal imaging, studies were carried out in a 4.7T MRI scanner dedicated
for small animal imaging research.
Motors and MRI
Traditional electromagnetic motors contain ferrous metal and
therefore represent a safety hazard in areas containing strong
magnetic fields (i.e., contraindicated in MRI environments).
Electromagnetic motors also generate their own magnetic and RF
fields during operation that could result in RF arcing, causing
hardware damage and undesirable image artifacts. In addition,
motor operation may be influenced by static and gradient
magnetic fields used during MRI data acquisition, causing the
motor to function unpredictably or to become permanently damaged. To
overcome these problems, our laboratory has made use of a
piezoelectric motor or SQUIGGLE motor. This miniature ultrasonic
motor does not generate magnetic fields and can be constructed
entirely of non-ferrous materials. The potential utility of the
SQUIGGLE motor in small animal imaging-related applications,
including remote administration of contrast agents to animals
and dynamic repositioning of the animals within the MR scanner,
was investigated.
The piezoelectric motor
The SQUIGGLE motor consists of four piezoelectric ceramic
plates bonded to a non-magnetic metal tube, threaded on the
inside. A matching threaded screw is inserted into the tube
(Figure 1). Two-phase drive signals cause the piezoelectric
plates to vibrate at an ultrasonic frequency of 40 kHz to 200
kHz, matching the first bending resonant frequency of the tube.
The motion of the plates is synchronized to make the tube
vibrate in an orbital, “hula hoop” motion. This causes the screw
to rotate and translate. The position and speed of the screw can
be controlled with high precision.
Figure 1: The piezoelectric SQUIGGLE® motor can be made entirely
of non-ferrous materials and does not generate an
electromagnetic field when operated.
Contrast media injection apparatus
We first used the SQUIGGLE motor to inject contrast media into a
mouse during a MRI scan with the motor placed in the bore of our
magnet in close proximity to the animal.
Our previous set up for performing infusions included a
modified ferrous-containing infusion pump interfaced to a
computer for accurate control of injection volume and delivery
rates. Although functional, the device is characterized by two
limitations. First, the modified infusion pump cannot precisely
deliver low doses in a linear time-dependent manner. Second, the
configuration generally requires > 2-3 meters of PE 50 catheter
tubing to deliver injections to an animal placed at magnetic
field isocenter during scanning. As a result, the dead volume in
our catheter is non-trivial and approximates an injected dose
volume for mice (~ 0.26 ml), resulting in an increased blood
volume that could confound MRI results.
To overcome these problems, we developed a novel setup using the
SQUIGGLE motor to drive a 1cc syringe connected to a PE 50
catheter (< 50 cm in total length) at a controlled speed of about 1
mm/sec (0.0185 cc/sec) when placed within the bore of a magnet
(Figure 2).
Due to the open-loop configuration of
the SQUIGGLE motor, a miniature LCD camera system and a digital
time stamp were used to visualize syringe movement within the
bore of the scanner. This enabled precise determination of the
injection volume of the contrast agent as a function of time.
The SQUIGGLE motor allowed precise delivery of low doses of the MRI contrast media in real time during data acquisition.
Figure 2: Setup for the SQUIGGLE motor based infusion system.
Automated sample re-positioning to improve image quality and
throughput
Acquiring data close to magnetic field isocenter minimizes
artifact in MR images. However, this is not possible without
repositioning the sample for each "slice." To circumvent this
problem, we used the SQUIGGLE motor to dynamically reposition a
live animal (mouse) inside the scanner during data acquisition.
The SQUIGGLE motor was able to dynamically translocate the
animal along the longitudinal
z-axis of the magnet
with precision. This not only increased
the effective field of view (FOV) along the z-axis, but also improved the
signal-to-noise ratio (SNR) and the overall image quality.
Generally, RF and gradient coil homogeneity is limited by the
geometric shape, size and coil construction, i.e., the RF and
magnetic field homogeneity diminishes as the distance from
magnetic field isocenter increases. Using the SQUIGGLE motor to
dynamically re-position a sample to optimize image quality or to translocate the sample in a
precisely controlled fashion could permit semi-automated MRI
cancer screening or morphologic phenotyping of large cohorts of
animals.
Whole body small animal imaging possesses the benefit of
allowing for precise sample placement, as well as observation of
whole body processes as they relate to contrast/drug injection,
metastatic spread or simply scanning multiple points of
interest, all without having to physically move the animal. We
currently have the ability to scan a relatively homogeneous FOV
over a spherical diameter of 40 mm without repositioning the sample or the animal. Whole-animal scanning would require either a larger (and
more expensive) RF and gradient coils with reduced SNR, or a
mechanism to move the animal through the coil at a constant,
precisely controlled rate. Our preliminary work in this area has
yielded significant promise in this area of application for the SQUIGGLE motor.
Automated capacitor tuning and impedance matching
Finally, it should be possible to design an automated
tuning/matching device using the SQUIGGLE motor for MRI
applications. Currently, the procedure involves the use of a
"tuning wand" to manually match the frequency and the coil
impedance for each sample prior to data acquisition. We believe
that an automated system based on the SQUIGGLE motor would
significantly improve the accuracy and reduce the amount of time
involved for this procedure, especially for large scale
screening studies.
Conclusion
Based on our work to date, piezoelectric motors such as the
SQUIGGLE motor hold great promise for use in MRI environments
and to improve the efficiency and quality of preclinical MRI
data acquisitions.
New Scale Technologies’ engineering team works with OEM
customers to integrate SQUIGGLE motor
systems into your next-generation medical systems and devices designs.
Contact us today.
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