System and method for performing in vivo imaging and oxymetry and FT microscopy by pulsed radiofrequency electron paramagnetic resonance

Claims

What is claimed is:

1. A fast response pulsed radiofrequency (RF) electron paramagnetic resonance (EPR) system, with the system utilizing a system clock signal, comprising:

a pulse generating sequential, non-overlapping transmit, diplexer, and receive gating pulses

an ultra-fast excitation pulse forming subsystem including:

an RF signal generator for providing an RF signal having a frequency of between about 200 MHz and about 400 MHz;

a beam splitter, coupled to the output of the RF signal generator for splitting said RF signal into a reference RF signal and an excitation signal RF signal;

a phase shifter, coupled to said beam splitter to receive said transmitted RF signal, for controllably either passing or phase-shifting said RF excitation signal by 180.degree.;

a gating circuit, coupled to said phase shifter and including a gate coupled to receive a transmit gating pulse from said pulse generator having a duration of about 10 to 90 nanoseconds, for transmitting a received RF excitation signal when said transmit gating pulse is asserted, to form an excitation pulse having a duration of about 10 to about 90 nanoseconds with rise times of less than about 2 nanoseconds;

an ultra-fast data acquisition system including:

a gated preamplifier, having a signal input port and having a control input coupled to receive a receive gating pulse, said gated preamplifier amplifying RF radiation received at said signal input port only when said receive gating pulse is received and said gated preamplifier being isolated from RF radiation received at said signal input port when said receive gating pulse is not received, with said gated preamplifier for amplifying EPR response RF radiation received at said signal input port to form an EPR response signal;

demodulating means, coupled to receive said reference RF signal and said EPR response signal, for demodulating said EPR response signal to form an EPR parameter signal;

an ultra-fast, sampling and summing unit, coupled to said demodulating means, for averaging a series of EPR parameter signals to increase signal to noise ratio, said sampling and summing unit including a high-speed sampler to digitize each received EPR parameter signal and a summing means, coupled to receive each digitized EPR parameter signal, for generating a running sum of said digitized EPR parameter signals;

a resonator for inducing paramagnetic resonance in a sample when an excitation pulse is received, for detecting EPR response RF radiation emitted from the sample due to paramagnetic resonance, and for outputting EPR response RF radiation;

a diplexer, coupled to said pulse generator to receive said excitation pulse, coupled to said resonator to receive the EPR response RF radiation, coupled to the signal input port of said gated preamplifier, and having a control input for receiving a diplexer gating pulse of a preset duration, said diplexer for coupling said ultra-fast pulse forming subsystem to said resonator when said diplexer gating pulse is received, for isolating said pulse forming system from said ultra-fast data acquisition system when said diplexer gating pulse is not received, and for providing said EPR response RF radiation from the resonator to the input signal port of said gate preamplifier subsequent to receiving said diplexer gating pulse.

2. The system of claim 1 wherein said resonator is characterized by a Q parameter, where the bandwidth of the resonator response is inversely-proportional to the magnitude of Q and the resonator ring-down time is proportional to Q, said system further comprising:

Q-switching means, coupled to said resonator and said timing controller to receive a Q-switching pulse, for increasing resonator Q and decreasing ring-down time for said resonator when a Q-switching pulse is asserted;

and wherein said pulse generator generates a Q-switching pulse of about 20 nanoseconds immediately after said transmit pulse is received at said resonator.

3. The system of claim 1 further comprising:

a DC magnet field for generating a constant magnetic field to induce magnetization in said sample;

a gradient magnet for forming a gradient in said constant magnetic field.

4. A method for measuring EPR parameters utilized to perform in vivo measurement or imaging of oxygen tension in a living sample, with a gated RF amplifier for amplifying response radiation generated by the sample, said method comprising the steps of:

providing a paramagnetic contrast agent which interacts with in vivo oxygen in the living sample to increase relaxation rate to improve imaging of oxygen;

introducing said paramagnetic contrast agent into a living sample to be imaged;

providing a magnetic resonator;

placing said living sample within the magnetic resonator;

generating a first series of RF excitation pulses, having an RF frequency between about 200 and 400 MHz separated by time intervals greater than about 4 microseconds;

coupling each RF excitation pulse in said first series to said resonator to induce EPR in said sample while isolating the gated RF amplifier from said resonator;

coupling said gated RF amplifier to said resonator when said response radiation is generated in response to each excitation pulse in said first series to generate a first series of corresponding EPR response signals based on the interaction of in vivo oxygen with said paramagnetic contrast agent in time intervals between said first series of RF excitation pulses;

digitizing and summing said first series of EPR response signals to obtain accurate values of EPR response signals; and

processing said accurate value of said EPR response signals to generate a first series of EPR parameter signals.

5. The method of claim 4 further comprising the steps of:

generating a second series of RF excitation pulses separated by time intervals greater than about 4 microseconds;

phase-shifting said second series of RF excitation pulses by 180.degree. to generate phase-shifted pulses;

coupling each-phase shifted RF excitation pulse in said second series to said resonator to induce EPR in said sample while isolating said gated RF amplifier from said resonator;

coupling said gated RF amplifier to said resonator when said response RF radiation is generated in response to each phase-shifted pulse in said second series to generate a second series of corresponding EPR response signals based on the interaction of in vivo oxygen with said paramagnetic contrast agent in time intervals between said RF excitation pulses in said second series;

digitizing and subtracting said second series of EPR response signals from said first series of EPR response signals to subtract systematic noise and DC bias to obtain accurate values of said EPR response signals; and

processing said accurate values of said EPR response signals to generate a second series of EPR parameter signals.

6. The method of claim 4 further comprising the steps of:

generating a first gradient magnetic field along a first axis prior to generating said first series of RF excitation pulses and maintaining said field until after said first series of EPR response signals have been generated to form a first projection of said sample; and

generating a second gradient magnetic field along a second axis;

generating a second series of RF excitation pulses, subsequent to generating the second gradient magnetic field, separated by time intervals greater than about 4 microseconds;

coupling each RF excitation pulse in said second series to said resonator to induce EPR in said sample while isolating said gated RF amplifier from said resonator;

coupling said gated RF amplifier to said resonator when said response RF radiation is generated to generate a corresponding second series of EPR response signals, based on the interaction of in vivo oxygen with said paramagnetic contrast agent, in time intervals between said RF excitation pulses in said second series;

digitizing and subtracting said second series of EPR response signals from said first series of EPR response signals to subtract systematic noise and DC bias to obtain accurate values of said EPR response signals; and

processing said accurate values of said EPR response signals to generate a second series of EPR parameter signals and form a second projection of said sampler.

7. A method for measuring EPR parameters utilized to perform pulsed EPR measurement or imaging of a sample placed within a magnetic resonator which excites the sample when an RF radiation pulse is received to induce the sample to emit response RF radiation subsequent to excitation, with a gated RF amplifier for amplifying response RF radiation to form an EPR response signal, said method comprising the steps of:

generating a corresponding first series of pseudo-random numbers having either a first or second value;

generating a first series of RF excitation pulses, having an RF frequency between about 200 and 400 MHz and separated time intervals greater than about 4 microseconds;

phase-shifting each pulse in said first sequence by 180.degree. if a corresponding number in said first series has said second value and transmitting each pulse without phase-shift if a corresponding number in said first sequence is equal to said first value to generate a first series of phase-processed excitation pulses pulses;

coupling each phase-processed RF excitation pulse in said first series to said resonator to induce EPR in said sample while isolating said gated RF amplifier from said resonator;

coupling said gated RF amplifier to said resonator when said response RF radiation is generated in response to each phase-processed pulse in said first series to generate a corresponding first set of EPR response signals based on paramagnetic resonance in said sample in time intervals between said first series of phase-processed RF excitation pulses;

digitizing and summing said first series of EPR response signals to obtain accurate values of said EPR response signals; and

performing a Hadamard transformation on obtained EPR response signals to obtain free induction decay parameters.

8. A method for measuring EPR parameters utilized to perform in vivo measurement of free radicals in a sample comprising the steps of:

providing a living sample;

providing a magnetic resonator;

placing said living sample within the magnetic resonator;

generating a first series of RF excitation pulses, having an RF frequency between about 200 and 400 MHz and separated time intervals greater than about 4 microseconds;

coupling each RF excitation pulse in said first series to said resonator to induce EPR in said sample while isolating the gated RF amplifier from said resonator;

coupling said gated RF amplifier to said resonator when said response radiation is generated in response to each excitation pulse in said first series to generate a first set of corresponding EPR response signals, based on in vivo paramagnetic resonance of free radicals in said sample, in time intervals between said first series of RF excitation pulses;

digitizing and summing said first series of EPR response signals to obtain accurate values of said EPR response signals; and

processing said accurate values of EPR response signals to generate a first series of EPR parameter signals.

9. The method of claim 8 further comprising the steps of:

generating a second series of RF excitation pulses separated intervals greater than about 4 microseconds;

phase-shifting said second series of RF excitation pulses by 180.degree. to generate phase-shifted RF excitation pulses;

coupling each-phase shifted RF excitation pulse in said second series to said resonator to induce EPR in said sample while isolating said gated RF amplifier from said resonator;

coupling said gated RF amplifier to said resonator when said response RF radiation is generated in response to each phase-shifted pulse in said second series to generate a corresponding second series of EPR response signals, based on in vivo paramagnetic resonance of free radicals in said sample, in time intervals between said phase-shifted RF excitation pulses in said second series;

digitizing and subtracting said second series of EPR response signals from said first series of EPR response signals to subtract systematic noise and DC bias to obtain accurate values of said EPR parameters; and

processing said accurate values of said EPR response signals to generate a second series of EPR parameter signals.

10. The method of claim 8 further comprising the steps of:

generating a first gradient magnetic field along a first axis prior to generating said first series of RF excitation pulses and maintaining said field until after said first series of EPR response signals have been generated to form a first projection of said sample; and

generating a second gradient magnetic field along a second axis;

generating a second series of RF excitation pulses, subsequent to generating the second gradient magnetic field, separated by time intervals greater than about 4 microseconds;

coupling each phase shifted RF excitation pulse in said second series to said resonator to induce EPR in said sample while isolating said gated RF amplifier from said resonator;

coupling said gated RF amplifier to said resonator when said response RF radiation is generated to generate a corresponding second series of EPR response signals, based on in vivo paramagnetic resonance of free radicals in said sample, in time intervals between said RF excitation pulses in said second series;

digitizing and subtracting said second series of EPR response signals from said first set of EPR response signals to subtract systematic noise and DC bias to obtain accurate values of said EPR response signals; and

processing said accurate values of said EPR response signals to generate a second series of EPR parameter signals and form a second projection of said sample.

11. The method of claim 8 further comprising the steps of:

providing a spin trapping agent; and

introducing said spin trapping agent into said sample to stabilize said free radicals for imaging.

12. A method for measuring EPR parameters utilized to perform pulsed EPR measurement or imaging of a sample placed within a magnetic resonator which excites the sample when an RF radiation pulse is received to emit response RF radiation subsequent to excitation, with a gated RF amplifier for amplifying response RF radiation to form an EPR response signal, said method comprising the steps of:

generating a corresponding first series of pseudo-random numbers having either a first or second value;

generating a first series of RF excitation pulses, having an RF frequency between about 200 and 400 MHz separated by time intervals greater than about 4 microseconds;

modulating the amplitude each RF excitation pulse in said first sequence to an OFF value if a corresponding number in said first series has said first value and to an ON value if a corresponding number in said first sequence has said second value to generate a first series of modulated RF excitation pulses;

coupling each RF excitation pulse in said first series of modulated RF excitation pulses to said resonator to induce EPR in said sample while isolating the gated RF amplifier from said resonator;

coupling said gated RF amplifier to said resonator when said response RF radiation is generated in response to said first series of modulated RF excitation pulses to generate a corresponding first set of EPR response signals based on paramagnetic resonance in said sample in time intervals between said first series of modulated RF excitation pulses;

digitizing and summing said first series of EPR response signals to obtain accurate values of said EPR response signals; and

performing a Hadamard transformation on obtained EPR response signals to obtain free induction decay parameters.

13. A method for measuring EPR parameters utilized to perform RF FT EPR microscopy of free radicals or a paramagnetic contrast agent in a living sample placed within a magnetic resonator which excites the sample when an RF radiation pulse is received to emit response RF radiation subsequent to excitation, with a gated RF amplifier for amplifying response RF radiation to form an EPR response signal, said method comprising the steps of:

generating a first series of RF excitation pulses, having an RF frequency between about 200 and 400 MHz separated by time intervals greater than about 4 microseconds;

coupling each RF excitation pulse in said first series to said resonator to induce EPR in said sample while isolating said gated RF amplifier from the resonator;

coupling said gated RF amplifier to said resonator when said response RF radiation is generated in response to each excitation pulse in said first series to generate generating a first set of EPR response signals based on in vivo paramagnetic resonance of free radicals in said sample in time intervals between said first series of RF excitation pulses;

digitizing and summing said first series of EPR response signals to obtain accurate values of EPR response signals; and

processing said first series of EPR response signals to generate a first series of EPR parameter signals.

14. The method of claim 13 further comprising the steps of:

generating a second series of RF excitation pulses separated by time intervals greater than about 4 microseconds;

phase-shifting said second series of RF excitation pulses by 180.degree. to form phase-shifted RF excitation pulses;

coupling each phase-shifted RF excitation pulse in said second series to said resonator to induce EPR in said sample while isolating the gated RF amplifier from said resonator;

coupling said gated RF amplifier to said resonator when said response RF radiation is generated in response to each excitation pulse in said second series phase-shifted RF excitation pulses to generate a second series of EPR response signals based on paramagnetic resonance of in vivo free radicals in time intervals between said RF excitation pulses in said second series;

digitizing and subtracting said second series of EPR response signals from said first series of EPR response signals to subtract noise and DC signals to obtain accurate values of said EPR response signals

processing said accurate values of said EPR response signals to generate a second series of EPR parameter signals.

15. The method of claim 13 further comprising the steps of:

generating a first gradient magnetic field along a first axis prior to generating said first series of RF excitation pulses and maintaining said field until after said first series of EPR response signals have been generated to form a first projection of said sample; and

generating a second gradient magnetic field along a second axis;

generating a second series of RF excitation pulses, subsequent to generating the second gradient magnetic field, separated by time intervals greater than about 4 microseconds;

coupling each phase shifted RF excitation pulse in said second series to said resonator to induce EPR in said sample while isolating said gated RF amplifier from said resonator;

coupling said gated RF amplifier to said resonator when said response RF radiation is generated in response to each excitation pulse in said first second to generate a corresponding second series of EPR response signals based on in vivo paramagnetic resonance of free radicals in said sample in time intervals between said RF excitation pulses in said second series;

digitizing and subtracting said second series of EPR response signals from said first set of EPR response signals to subtract noise and DC signals to obtain accurate values of said EPR parameters; and

processing said accurate values of said EPR response signals to generate a second series of EPR parameter signals and form a second projection of said sample.

16. The method of claim 13 further comprising the steps of:

providing a spin trapping agent;

introducing said spin trapping agent into said sample to stabilize said free radicals for imaging.

17. A fast response pulsed radiofrequency (RF) electron paramagnetic resonance (EPR) imaging system for forming an EPR image of a sample, said imaging system coupled to an RF signal generator that provides an RF excitation signal and an RF reference signal, with the RF signal having a frequency range of about 200 to 400 Mhz and receiving a system clock signal, said system comprising:

a pulse generator, having an input coupled to receive the system clock signal and said RF excitation signal, for generating sequential, non-overlapping transmit, diplexer, receive, and Q-switching gating pulses;

a gating circuit, coupled to receive RF radiation and coupled to receive a transmit gating pulse having a duration of about 10 to 90 nanosecond, for transmitting said an RF signal when said transmit gating pulse is asserted, to form an RF excitation pulse having a duration of about 10 to about 90 nanoseconds with rise times of less than about 2 nanoseconds;

an ultra-fast data acquisition system including:

a gated amplifier, having a signal input port and having a control input for receiving a receive gating pulse, said gated amplifier for amplifying RF radiation received at said signal input port only when said receive gating pulse is received and said gated amplifier being isolated from RF radiation received at said signal input port when said receive gating pulse is not received, with said gated amplifier for amplifying EPR response RF radiation received at said signal input port to form an EPR response signal;

demodulating means, coupled to receive said EPR response signal and said RF reference signals, for demodulating said EPR response signal to form an EPR parameter signal;

an ultra-fast, sampling and summing unit, for averaging a series of EPR parameter signals to increase signal to noise ratio, said sampling and summing unit including a high-speed sampler to digitize each received EPR parameter signal and summing means, coupled to receive each digitized EPR parameter signal, for generating a running sum of said digitized EPR parameter signals; and

a resonator for inducing paramagnetic resonance in a sample when an excitation pulse is received, for detecting EPR response RF radiation emitted from the sample due to paramagnetic resonance, and for outputting EPR response RF radiation;

a diplexer, coupled to said to receive said RF excitation pulse, coupled to said resonator to receive the EPR response RF radiation, coupled to the signal input port of said gated amplifier, and having a control input for receiving the diplexer gating pulse of a preset duration, said diplexer for coupling said RF excitation pulse to said resonator when said diplexer gating pulse is received, for isolating said RF excitation pulse from said ultra-fast data acquisition system when said diplexer gating pulse is not received, and for providing said EPR response RF radiation from said resonator to the input signal port of said gate amplifier subsequent to receiving said diplexer gating pulse; and

Q-switching means, coupling said resonator to said diplexer and coupled to said pulse generating circuit to receive said Q-switching gating pulse, for increasing resonator Q decreasing the ring-down time of the resonator.

18. The system of claim 17 further comprising:

a phase shifter, coupled to receive said RF signal and having an output coupled to said gating circuit, for controllably either passing or phase-shifting said RF signal by 180.degree..

BACKGROUND OF THE INVENTION

This invention describes a fast response pulsed Radiofrequency (RF) Electron Paramagnetic Resonance (EPR) spectroscopic technique for in-vivo detection and imaging of exogenous and endogenous free radicals, oxygen measurement and imaging and other biological and biomedical applications.

The main emphasis is the use of low dead-time resonators coupled with fast recovery gated preamplifiers and ultra fast sampler/summer-summer/processor accessory. Such a spectrometer will be practical in high resolution detection and imaging of free radicals possessing narrow line widths. This method avoids factors compromising the imaging speed and resolution inherent in the existing Continuous Wave (CW) EPR imaging methods, where modulation and saturation broadening and artifacts of object motion are problems.

It is also possible to perform Fourier imaging and hence to produce image contrasts based on relaxation when using special narrow-line free radical probes.

The response of tumors to radiation therapy and chemotherapeutic agents depends upon the oxygen tension. Hence, for an effective cancer therapy, measurement of molecular oxygen in tumors is vital. Also in general medicine measurement of the oxygen status of ischemic tissue in circulatory insufficiency, be it acute as in stroke or myocardial infarction, or chronic as in peripheral vascular disease associated with numerous diseases such as diabetes, hyperlipidemias, etc., becomes an important tool for assessment and treatment of diseases. Although a variety of techniques are available for measuring oxygen tension in biological systems, polarographic technique is perhaps the one most widely used in clinical applications. However, this is an invasive technique. Besides patients' discomfort, the tissue damage caused by the probe electrodes leads to uncertainty in the values measured, especially at low oxygen concentration (<10 mm Hg).

Magnetic Resonance Imaging (MRI) enjoys great success as a noninvasive technique. NMR imaging based on perfluorinated organic compounds has been used to study blood oxygenation of animal brains. Binding of oxygen to hemoglobin is also used as a marker in MRI of human brains to monitor oxygenation changes. However, these techniques lack sufficient sensitivity for routine applications.

Overhauser magnetic resonance imaging (OMRI), based on the enhancement of the NMR signal due to the coupling of the electron spin of an exogenously administered free radical with the water protons, is also attempted for in-vivo oxymetry. Here again the sensitivity is limited, since the organic free radicals used have low relaxivity since they don't possess the free sites for water binding as in the case of gadolinium based contrast agents. The Gd based contrast agents, however, have relaxation times that are too far for efficient spin polarization transfer. On the other hand, EPR oxymetry compared to MRI or OMRI is very sensitive for oxygen measurements, since it is based on the direct dipolar interaction of the paramagnetic oxygen molecule with the free radical probe.

EPR is generally performed at microwave frequencies (9 GHz). The use of microwave frequency results in substantial tissue heating, and unfortunately, severely limits tissue penetration. Low-frequency EPR has been attempted to achieve better tissue penetration. All of these studies except the last one (from this lab) were done using the Continuous Wave (CW) method.

Although low-frequency EPR offers the potential for greater in-vivo tissue penetration, its use in continuous wave-based methods is severely limited by lack of sensitivity resulting from the physically imposed Boltzmann factor. Furthermore, sensitivity enhancement by signal averaging as done with CW methods, may not be effective since CW methods are band limited. Pulse EPR techniques, however, as presented in this application, advantageously utilize the very short electron relaxation times to rapidly enhance the signal to noise ratio, which immediately leads to speed and sensitivity advantage in pulse EPR detection and imaging.

Further, the absence of any modulation in the method leads to true line widths, whereas in the CW methods finite modulation can, in the case of narrow lines, lead to artifacts and, therefore, can severely limit the resolution achievable. Power saturation is another factor that greatly limits the resolution when detecting and imaging narrow line systems. Also for in vivo studies, any movement of the subject being studied poses severe problems in CW methods. Further, relaxation weighted imaging for contrast mapping is feasible mainly with the pulsed methods. Most of these advantages of pulse techniques over CW method are well established in MRI.

Application of pulse techniques to EPR has serious limitations. The very advantage of short relaxation time, which can in principle lead to virtual "real time" imaging, poses a challenge to the state of the art electronics for ultra fast excitation and data acquisition. Instrumental dead-time problems become very severe, especially at low frequencies, since the ringing time constant, t=2 Q/w (where Q is the resonator quality factor and w is the carrier frequency), allows acquisition of signals only after a significant interval following excitation, which can lead to loss of sensitivity.

The current invention addresses all of these problems and outlines pulsed EPR methodologies at radiofrequency region for in-vivo imaging of free radicals and oxygen measurement and imaging of oxygen using suitable paramagnetic agents.

Apart from oxygen measurements, using appropriate free radical probes one can perform rapid imaging to map out blood vessels (for example, cardiac and cerebral angiography), study tissue characteristics and free radical metabolic intermediates in situ with or without using spin traps. Use of free radical probes also provides the ability to use administered paramagnetic contrast agents for imaging both normal and diseased tissues.

This invention has additional advantages as follows. Firstly, the magnetic field used is only about of 10 mT, orders of magnitude less than in MRI. Secondly, the sensitivity achievable is much higher than OMRI. Lastly, sensitivity enhancement, image resolution and imaging speed and T1 and T2 weighted imaging modalities are far superior to CW RF EPR.

SUMMARY OF THE INVENTION

According to one aspect of the invention, a pulsed EPR FT imaging and spectroscopy system includes an excitation system for forming 20 to 70 nanoseconds RF excitation pulses of about 200 to 400 MHz. A gated data acquisition system with very low dead time generates EPR response signals. A pulse sequence with a repetition rate of about 4 to 5 microseconds is sampled and summed to provide a signal having a high signal-to-noise ratio.

Another aspect of the present invention is a novel RF FT EPR technique for measuring oxygen tension in vivo in biological systems in conjunction with a suitable narrow-line oxygen-sensitive free radical ( See Anderson, G. Ehnholm, K. Golman, M. Jurjgenson, I. Leunbach, S. Peterson, F. Rise, O. Salo and S. Vahasalo: Overhause MR imaging with agents with different line widths, Radiology 177, 246 (1990); Triarylmethyl radicals and the use of inert carbon free radicals in MRI, World Intellectual Property Organization, International Bureau, International Patent Classification A61K 49/00, C07D519/00, C07B 61/02//C07D 493/04, International Publication, No. WO 91/12024, (22.08.1991) and for the detection and imaging of endogenous and exogenous free radicals. The subject of study, placed in a suitable resonator of low Q, high filling factor and coupled to a RF pulse excitation system (vide infra) is given an injection of the free radical probe and immediately thereafter it is subjected to an intense short RF pulse. The time response of the RF signal, which will be oxygen dependent and/or the signature of the free radical present, is acquired using a very fast acquisition system. The signal-to-noise ratio is enhanced to an extent of 60 dB in just one second by coherent averaging using an ultra fast averager (vide infra). The spatial resolution in three-dimension is obtained by using a set of three-axis gradient coil system.

According to another aspect of this invention, stochastic excitation or pseudo stochastic excitation with subsequent Hadamard transformation will be used where a large bandwidth is to be excited, instead of using a compressed high power pulse. This will avoid sample heating considerably, because the power required for stochastic excitation is at least an order of magnitude less than in the conventional pulsed techniques. The principle and application of Hadamard transformation is well documented and illustrated in NMR spectroscopy and imaging literature. The RF carrier is modulated by a pseudorandom binary sequence which is generated in a shift register and the values of the sequence are used to modulate the RF phase for each sampling interval t by +90.degree. or -90.degree.. The pseudo-noise sequence thus generated will be repeated in a cyclic fashion after a given number of values. The acquisition of the response and phase cycling follow standard procedures. A Hadamard transform of the response produces the FID free induction decay which, upon complex Fourier transform, yields a spectrum or a single projection when gradients are present.

According to another aspect of the invention, by using free radical probes of long relaxation time gradient switching can be used to perform slice-selective EPR tomography as in MRI, as well as all other imaging modalities used in MRI. Additionally, high gradients can be used to perform EPR microscopy.

Other advantages and features of this invention will be made apparent from the following drawings and descriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall block diagram of the spectrometer and imager;

FIG. 2A is a schematic diagram of the phase-shifter used in a preferred embodiment;

FIG. 2B is a schematic diagram of the high-speed gates used in a preferred embodiment;

FIG. 2C is a schematic diagram of the diplexer used in a preferred embodiment;

FIG. 2D is a schematic diagram of the gated preamp used in a preferred embodiment;

FIGS. 2E-F are schematic diagrams of a Q-circuit and an equivalent Q-circuit, respectively, utilized in the resonator of the preferred embodiment;

FIG. 2G is a schematic diagram of the quadrature detector used in a preferred embodiment;

FIG. 2H is a layout diagram of the ultra-fast data acquisition subsystem used in the preferred embodiment;

FIG. 2I is a block diagram of the summing part of the ultra-fast data acquisition subsystem of FIG. 2H;

FIG. 3 is a timing diagram relating to the operation of the preferred embodiment;

FIGS. 4A-D are timing diagrams for using the system to implement a Hadamard excitation scheme;

FIG. 5 is a flow chart giving the details of generating an image.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a block diagram of the spectrometer/imager. RF power from a Hewlett-Packard (Palo Alto, Calif.) signal generator model HP8644A, 1 is split by a two-way-zero degree power splitter (model ZSC-2-1W, Minicircuits, Brooklyn, N.Y.) 2 into two ports, one serving the reference arm and the other the transmitter side. The reference side is gated using RF gate 4b. The required gate timing is provided by pulse generator 6 in the form of a cluster of four Digital Delay Generators (model 535, Stanford Research Systems, Sunnyvale, Calif.). For synchronization, the first of the delay generators utilizes the system clock generated by the RF signal generator 1, a trigger input (10 MHz), thereby reducing the jitter of the delay outputs to less than 25 ps rms. The time base drift between the various delay generators is eliminated by daisy chaining the reference output of the first DG535 with the reference input on the other DG535's. An appropriate level of reference signal for mixing is selected using the variable attenuator 5a.

The other arm of the splitter is directed through a 0.degree./180.degree. phase shifter 3 which can be software controlled using timing pulses from the generator 6. The transmitter pulse is gated through 4a, a gating circuit further amplified by a home-made RF amplifier 7a (25 db) and still further amplified by a power amplifier (ENI 5100L, 100 W) 7b. The optimization of the RF power level is accomplished using a set of attenuators 5b and 5c. The amplified pulses are coupled with the diplexer T/R switch 9 through a pair of crossed diodes 8 for protection from the reflected pulse generator. The diplexer switch 9 receives the timing signal from 6 and the RF pulse is delivered to the resonator 12.

The magnetic induction response from the object in the resonator is first taken through a specially designed gated preamplifier 10 with a low-noise high-gain (45 dB) capability and a very short saturation recovery time. The preamplifier gate switching is also controlled by pulse generator 6. The output of the preamplifier is further amplified using amplifiers 11 and 7c with suitable attenuation in between by attenuators 5d and 5e to avoid saturation.

The reference signal from 4b and the amplified induction signal from 7c are mixed using a double-balanced quad mixer 17. The real and imaginary parts are passed through two identical low-pass filters 18a and 18b, before sampling, using a specially designed ultra-fast sampler/summer/averager 19. The averaged signal is processed in a Silicon Graphics computer 20 which also controls the overall spectrometer/imager as shown by the bus connection in FIG. 1.

The resonance condition is set by changing the current in the DC magnet 13 using the power supply 14, which is addressed by the computer.

For imaging, the spatial/spectral distribution of the spin is frequency encoded by using a set of 3- axes orthogonal field gradient coils 15. The gradient steering is done by software control of the gradient power supply 16. The overall process of generating the image/spectrum is summarized in FIG. 6.

The various components/modules depicted in FIG. 1 will now be described. In the preferred embodiment, the RF signal generator 1 is a Hewlett Packard model 8644A--Synthesized Signal Generator and the splitter 2 is a Minicircuits ZSC-2-1W (1-650 MHz).

The phase shifter 3 is depicted in FIG. 2A and has been designed and built for the removal of systematic noise. A gating pulse C provided by the pulse generator may have either negative polarity (to induce a 180.degree. phase shift) or positive polarity (to induce a 0.degree. phase shift) where the polarity is controlled by the host computer 20.

RF from the transmitter can, despite the good isolation between the transmitter (Tx) and receiver (Tx) provided by the diplexer and the various gatings, leak into the receiver. This leakage can arise from pulse breakthrough while the transmitter is on and/or from direct radiation into the receiver from within the spectrometer's electronics. This results in unwanted dc output from quad mixer 17. If uncorrected this can lead to a large dc bias and result in a spurious spike at zero frequency upon Fourier Transformation.

With the phase shifter set at 0.degree. phase a group of FIDs, say 1000, is accumulated. Then the phase of the RF pulse is changed 180.degree. by a pulse given from the pulse generator 6, and another 1000 FIDs are accumulated. The resultant FID signals are unaffected, except for the change in sign, and thus these are subtracted from the previous group leading to a total collection of 2000 FIDs. The unwanted dc biases, from the RF leakage and the amplifiers' drift, and other systematic noises do not change in sign and thus they get subtracted. Thus, the phase shifter, besides removing the unwanted systematic noises, also helps to reduce the data collection time by half.

The gates 4a are depicted in FIG. 2B and should possess a very high on-off ratio (typically 100-120 dB) to avoid any RF leakage through the gate to the sample. Further, the rise time of the gate should be very short, since pulses of the order of 10 to 20 ns are used in RF FT-EPR in contrast to pulses of tens of microsecond or millisecond in NMR. Even a two ns rise time will make a 10 ns pulse and can distort the desired square wave pulse. Also, the gate opening and closing glitches should be minimal to avoid any amplification by the power amp 7b. To meet these demands of ultra-fast excitation needed for the RF FT-EPR, the special gates depicted in FIG. 2B have been designed and built.

The attenuators 5 are Kay Electronics model 839, and the pulse generator 6 is a cluster of Stanford Research Systems DG535 four-channel digital delay/pulse generators; the RF Amplifier 7a is a 10-400 MHz, 5 dBm in OP-AMP +25 dBm out--Motorola MHW 590; the RF Power Amplifier 7b is an ENI Model 5100L Watt, 50 dB; and the cross-diodes 8 are IN 9153 diodes. These cross diodes 8 disconnect the transmitter from the probe (tank circuit) and the preamplifier during the receiving mode to reduce the noise.

The diplexer 9 is depicted in FIG. 2C. A major requirement for a sensitive RF FT-EPR spectrometer is to design a suitable technique to couple the transmitter, probe and receiver. During the transmit cycle high RF power of the transmitter should be delivered to the sample in the probe without damaging or overloading the sensitive receiver, and during the receiving mode any noise originating from the transmitter must be completely isolated. This is not trivial since the EPR signal of interest is i