An apparatus being developed for electron paramagnetic resonance (EPR) imaging operates in the resonance- frequency range of about 1 to 2 MHz — well below the microwave frequencies used in conventional EPR. Until now, in order to obtain sufficient signal-to-noise radios (SNRs) in conventional EPR, it has been necessary to place both detectors and objects to be imaged inside resonant microwave cavities.
EPR imaging has much in common with magnetic resonance imaging (MRI), which is described briefly in the immediately preceding article. In EPR imaging as in MRI, one applies a magnetic pulse to make magnetic moments (in this case, of electrons) precess in an applied magnetic field having a known gradient. The magnetic moments precess at a resonance frequency proportional to the strength of the local magnetic field. One detects the decaying resonance-frequency magnetic-field component associated with the precession. Position is encoded by use of the known relationship between the resonance frequency and the position dependence of the magnetic field.
EPR imaging has recently been recognized as an important tool for noninvasive, in vivo imaging of free radicals and reduction/oxidization metabolism. However, for in vivo EPR imaging of humans and large animals, the conventional approach is not suitable because (1) it is difficult to design and construct resonant cavities large enough and having the required shapes; (2) motion, including respiration and heartbeat, can alter the resonance frequency; and (3) most microwave energy is absorbed in the first few centimeters of tissue depth, thereby potentially endangering the subject and making it impossible to obtain adequate signal strength for imaging at greater depth. To obtain greater penetration depth, prevent injury to the subject, and avoid the difficulties associated with resonant cavities, it is necessary to use lower resonance frequencies. An additional advantage of using lower resonance frequencies is that one can use weaker applied magnetic fields: For example, for a resonance frequency of 1.4 MHz, one needs a magnetic flux density of 0.5 Gauss — approximately the flux density of the natural magnetic field of the Earth.
In the present apparatus, a superconducting quantum interference device (SQUID) is used in detecting the EPR signal. Even without a cavity resonator, in the frequency range of about 1 to 2 MHz, it is possible to obtain a sufficient SNR by use of a simple pickup coil placed near the surface of the object to be imaged. The pickup coil is part of a flux transformer coupled to the SQUID. The signal is detected by either a commercial SQUID array amplifier device (which has a typical bandwidth of about 1 MHz) or a custom microwave SQUID (which has a typical bandwidth of about 10 MHz). To increase the SNR, the output of the microwave SQUID can be fed to a cryogenic high-electron-mobility transistor amplifier.
The figure depicts the layout of the electromagnetic coils and the cryostat (which houses the SQUID circuitry) of the present apparatus. These coils are identical to those of a low-magnetic- field-strength SQUID MRI apparatus developed in the same laboratory. In one mode of operation, the polarizing coils are used to prepolarize the specimen by applying a magnetic field stronger than the nearly homogeneous magnetic field generated by the Helmholtz coils; the prepolarizing field is then rapidly turned off immediately before starting to record EPR imaging data. The polarizing coils are positioned and oriented so as not to induce measurable flux in the pickup coil. It is important to note that in some cases, the EPR signal may be strong enough to be detectable even in the absence of prepolarization.
This work was done by Inseob Hahn, Peter Day, Konstantin Penanen, and Byeong Ho Eom of Caltech and Mark Cohen of UCLA Center for Cognitive Neuroscience for NASA’s Jet Propulsion Laboratory. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
Innovative Technology Assets Management
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
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