Electricity: measuring and testing – Particle precession resonance – Using an electron resonance spectrometer system
Reexamination Certificate
2000-04-10
2003-06-03
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using an electron resonance spectrometer system
C324S318000
Reexamination Certificate
active
06573720
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to electron paramagnetic resonance (EPR) resonance spectroscopy systems and, more particularly, relates to improved resonators for use in such systems.
Resonator coils are used to detect magnetic resonance responses from infusible or implantable spin probes in living objects after pulsed excitation using radiofrequency, in the range 60-400 MHz, of the sample placed in a magnetic field. With the addition of gradient fields to the stationary magnetic field, the responses are collected for image reconstruction purposes.
2. Description of the Relevant Art
In electron paramagnetic resonance (EPR) spectroscopy, a uniform magnetic field Bo is applied to an object to align the magnetic moments of the electrons along the Z-axis of the applied magnetic field (C. P. Poole Jr,
Electron Spin Resonance,
2nd Ed., Wiley, NY). An EPR spectroscopy system is described in a co-assigned patent application to Murugesan et al. (Ser. No. 08/504,616 filed Jul. 20, 1995) which is hereby incorporated by reference for all purposes. For a single electron system the spins are in either of the two levels, a higher and lower energy level. Electromagnetic radiation of appropriate frequency can cause absorption of energy by inducing transitions between the two states, a process called electron paramagnetic resonance (EPR). This process is similar to nuclear magnetic resonance where the nuclear spin systems such as protons are studied in a similar manner.
The frequency of operation is related to the magnetic moment of the spin system, and the applied magnetic field or the frequency of the incident RF radiation and is given by the equation:
2&pgr;&ngr;
0
=&ngr;
e
B
0
where &ngr;
0
is the frequency of operation, B
0
is the applied magnetic field, and &ngr;
e
is the gyromagnetic ratio of the spin system. Since the g for electrons is nearly 660 times greater in magnitude than that of a proton, a correspondingly lower applied magnetic field compared to proton NMR is necessary for a given frequency of operation.
After radiofrequency irradiation pulses of suitable duration and intensity are used to irradiate a sample containing unpaired electrons in a resonator coil, weak resonance signals, called “free induction decays,” (FIDs) which decay in amplitude as a function of time, are detected by the same resonator coil, which also serves as a receiver. The FID is converted to a resonance absorption signal by mathematical transformation called the Fourier transformation.
For proton NMR, the pulse widths of irradiation are in the microsecond range and the FIDs last for times ranging from milliseconds to seconds. For free electron spin probes such as free radicals, the corresponding excitation pulse widths are typically in the range of 10 to 100 ns, and the FIDs last between 100 ns to 20 &mgr;s.
The purpose of the resonator coil is to deliver the transmitted power into the spin system to create and receive resonance signals.
Coupling the input power efficiently into the spin system and recovering the resonance signals at optimal strength will have a significant influence in the quality of images obtained. Several resonators exist for Electron Paramagnetic Resonance (EPR) practiced at frequencies in the range of 9 GHz and above (Pfenninger et. al., “General Method for Adjusting the Q-factor of EPR Resonators,”
Rev. Sci. Instr.
vol 68, 4857-4865, 1995; Prisner et. al., “Pulsed 95 GHz High Field EPR Heterodyne Spectrometer with High Spectral and Time Resolution,”
Appl. Magn. Reson.
7, 167-183 1994). These resonators are not suitable for studies involving living objects due to the small volumes available in which to place the object under study. The resonators used in MRI using protons as spin probes are suitable in terms of size but not in terms of electrical characteristics such as the resonator dead time (R S Withers and G C Liang, U.S. Pat. No. 5,276,398 dated Jan. 4, 1994).
The dead time is the time taken for the noise originating from the intense input power to the resonator housing the object under study to return to thermal noise levels and is given by the equation:
T
D
=t Q/
2&pgr;&ngr;
0
where t is the time constant for the noise dissipation, Q is the quality factor of the resonator and &ngr;
0
is the operating frequency (Pfenninger et. al., “General Method for Adjusting the Q-factor of EPR Resonators,”
Rev. Sci. Instr.
vol 68, 4857-4865, 1995). Only after the resonator dissipates the noise associated with the intense input power can the rapidly decaying weak signals associated with the resonance absorption be recovered.
The dead times are not an important factor in MRI, since the signals associated with the resonance absorption of the nuclei are long lived (in the order of seconds) permitting dead times in the order of micro- to milliseconds. In addition, in MRI, since the spectral bandwidth is extremely narrow (<10 kHz), resonators of high Q-values are desirable for enhanced sensitivity (R S Withers and G C Liang, U.S. Pat. No. 5,276,398 dated Jan. 4, 1994).
For time domain EPR experiments, the lower the frequency, the longer is the time required for the receiver to recover after the dead time. In addition, the spectral band width of EPR is in the order of 5-10 MHz. These two factors limit the resonator Q-values to less than 50.
The input power (P) to be given to the spin system to provide a given magnitude of the magnetic field B
1
is dependent on both the Q-value of the resonator and the volume V of the resonator and is given by the expression:
P
=constant (
V/Q
).
Accordingly, spin systems in resonators with lower Q values provide correspondingly lower resonance signals compared to resonators having higher Q values. In addition, the increased volumes necessary for studying living objects necessitates the use of resonators with correspondingly increased volumes. This causes an added demand on the input power to achieve a given magnetic component (B
1
) of the RF (Prisner et. al., “Pulsed 95 GHz High Field EPR Heterodyne Spectrometer with High Spectral and Time Resolution,”
Appl. Magn. Reson.
7, 167-183 1994).
Lowering the Q-value by overcoupling has been suggested to be desirable over resistively lowering the Q for resonators for EPR experiments conducted at X-band frequency so as to minimize the Q-value without decreasing the effective B
1
field at a given input power (Rinard et. al., “Relative Benefits of Overcoupled Resonators vs. Inherently Low-Q Resonators for Pulsed Magnetic Resonance,”
J. Magn. Reson. A
108, 71-81, 1994). Two helical coils connected in parallel and overcoupled to lower the Q have been implemented for NMR (Chingas, “Overcoupling NMR Probes to Improve Transient Response,”
J. Magn. Reson.
54, 153-157, 1983). However, since the demands of time domain RF EPR in terms of decreased dead times, effective B
1
field, RF penetration and enhanced sensitivity, the available designs were found not to be adequate.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a family of resonator coils necessary to accommodate and irradiate objects of interest of varying dimensions, such as living objects, containing free radical spin probes and induce an EPR signal which can also be recovered by the resonator. Such a resonator has the capability of facilitating the enhanced dissipation of noise to thermal noise levels associated with the input power from the RF pulse, and recovering weak and rapidly decaying FIDs. In addition, the lowering of the Q values by overcoupling, instead of resistively damping provides enhanced B
1
fields thereby increasing the sensitivity of detection of the resonance signals after pulsed excitation.
According to one aspect of the invention, a radiofrequency (RF) coil design suitable for detecting time domain electron paramagnetic resonance (EPR) responses from spin probes after pulsed excitation using radiofrequency irradiation (60-400 MHz) is provided. The coil is configured in an array of numerous surface coils of appropriate
Afeworki Mobae
Cherukuri Murali K.
Cook John
Devasahayam Nallathamby
Harrington Frank S.
Lefkowitz Edward
The United States of America as represented by the Department of
Townsend & Townsend & Crew LLP
Vargas Dixomara
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