WO2017074204A1

WO2017074204A1 - Electron paramagnetic resonance imaging using multiple harmonics

Info

Publication number: WO2017074204A1
Application number: PCT/PL2016/000119
Authority: WO
Inventors: Tomasz Czechowski; Mikołaj BARANOWSKI
Original assignee: "Novilet" Sp. Z O.O.
Priority date: 2015-10-27
Filing date: 2016-10-26
Publication date: 2017-05-04

Abstract

Obtaining projections with the electron paramagnetic resonance imaging method using the analysis of multiple harmonics and the procedure application system, comprising measurement of the EPR signal during impact on the sample in the resonator with the multi-component magnetic field, is characterised in that the projection is obtained in the following steps: determination of harmonics of the EPR signals, determination of the k-th derivative of the EPR absorption signal, determination of final, not overmodulated form of projection through inverse Fourier transform of projection P ^k obtained through the analysis of n harmonics; performance of deconvolution of inverse Fourier transform of the projection P ^k through the analysis of n harmonics. To implement the method described above the system is used equipped with: master control device (1), microprocessor control system (2), high-frequency generator (3), amplifier (4), bridge (5), resonator (6), high-frequency pre-amplifier (7), detector (8), filters of the first, second, third and n-th harmonics (9, 10, 11, 12), low frequency amplifiers (13, 14, 15, 16), ADC unit (17), signal splitter (18), low-frequency generator unit (19), memory (20), power adapters (21), second modulation coils (22), G_x gradient coils, G_y and G_z (23, 24 and 25), scanning coils (26) Bo main field coils (27) and phase shifter (28).

Description

ELECTRON PARAMAGNETIC RESONANCE IMAGING USING MULTIPLE HARMONICS

The object of the invention is obtaining projections with the electron paramagnetic resonance imaging method using the analysis of multiple harmonics and the procedure application system, particularly in testing living organisms.

The EPR (Electron Paramagnetic Resonance) imaging may provide data on the spatial distribution and pharmacokinetics of free radicals and oxygen concentration in tissues. Detection of oxygen in tissues may be very important information in cancer treatment in oncology. Since free radicals are only found at low concentrations in tissues and have a very short lifetime, it is assumed that at this stage of study it is necessary to introduce a free radical from the outside, with an appropriately long lifetime and at a suitable concentration; in this case its EPR line width depends to a large extent on oxygen concentration in its surroundings. Standard EPR spectrometers operating at microwave frequencies are not useful for EPR imaging due to the depth of penetration of electromagnetic radiation into living matter and the potential effect of heating of the tested object. Consequently, in practical applications, lower frequencies in the radio frequency (RF) range are used, which results in decreased signal-to-noise (SIN) ratio.

In laboratory conditions, the imaging of small biological objects is currently based on impulse methods as well as on continuous wave (CW) methods.

The usefulness of impulse methods is limited due to a very short duration of spin - spin T₂ relaxation times for most radicals. The significantly increased widening of resonance lines related to it makes such methods useless in the EPR imaging. An additional problem is shortened relaxation time T₂, resulting from the presence of the field gradient, which - at significant gradient values - will cause free induction decay (FID) during the spectrometer's dead time. This effect makes it difficult, or even impossible, to perform a measurement. To markedly reduce the effect of CT dead time it was proposed to use the Frank or Chu sequences, which enable imaging. This method may only be used with radicals with long relaxation times. The images generated with this method are characterised by low spatial resolution, especially when it comes to large, human size objects.

On the other hand, application of continuous wave methods does not involve such limitations. In the CW technique, phase-sensitive detection is used, which enables one to separate useful signals from noise. For this purpose, the parameter affecting the detected signal is subject to specific frequency modulation (e.g. magnetic field, frequency, ...). Determining the size and shape of the signal is achieved through demodulating the signal at modulating frequency, or trough demodulating its harmonics. This technique makes it possible to use bandpass filters with a substantial quality factor and centre frequency matching modulation frequency. With this, substantial narrowing of the bandwidth is achieved and the signal-to-noise (S/N) ratio is improved. Standard CW imaging methods are based on the use of the magnetic field gradient which is constant during scan time. After each measurement, gradient orientation is rotated by a fixed angle, depending on the number of projections, while the rotation angle changes within the 0°-180° range for 2D imaging. The minimum measurement time for a single projection is between 1÷2 s; however, given the low signal-to-noise (S/N) ratio in the presence of gradient, signal accumulation is required which, for in vivo imaging, extends measurement time. The value of the S/N parameter of recorded projections is affected by the value of the 2nd modulation amplitude, namely it goes up as the 2nd modulation amplitude increases. It must be noted that the obtained image shows only spatial distribution of the radical, but it fails to offer any information about its environment. Since the EPR method is much more sensitive in terms of detecting changes in the local environment of radicals than the MRI (Magnetic Resonance Imaging), the EPR imaging is, in some cases, more useful than the MRI imaging technique. To obtain data about the radical environment, it is necessary to obtain information not only about the spatial distribution, but also about the spectral distribution of such free radical for each projection. To this end, an additional spectral- spatial imaging technique is used separately for each projection. In practical terms, this means that the measurement duration will increase a number of times due to performing at least several additional measurements for each gradient orientation. Another limitation is the need to apply the 2nd modulation, the amplitude of which is smaller than 1/3 of the line width of the radical subject to imaging. Otherwise, a disadvantageous effect of overmodulation is observed, which will significantly contribute to the low S/N ratio.

A method used to considerably speed up measurements is the Rapid Scan (RS) of the magnetic field. The 2nd modulation is not used in this method; instead a rapid scanning of the magnetic field is applied (at frequencies in the range of 1-100 kHz), which can be performed following the sinusoidal or triangular pattern. Detection is performed directly; consequently, an absorption spectrum is obtained rather than its first derivative. This results in shorter measurement times of a single projection even down to 5 us. In practice, due to the low signal-to-noise ratio, the recorded spectrum should be accumulated. In spite of this, the traditional continuous wave method is being successfully replaced by the RS method due to its ability to markedly reduce measurement times, which means an over 100 times higher S/N ratio.

Lately, the rotational gradient method has been used in the RS-based imaging, which significantly reduces measurement time. The method utilities gradient rotation in the course of magnetic field scanning, while the gradient rotation frequency should be at least 4 times higher than the scanning frequency. In practical terms, this limitation necessitates using relatively low scanning frequencies of 1 kHz.

The new method of projection measurement in the EPR imaging is free of such limitations as the need to apply modulation amplitude smaller than 1/3 of the radical line width; the new method involves detection and analysis of more harmonics of the field modulated signal.

It is an objective of the invention is ways of generating projections in in-vitro and in- vivo imaging (EPRI).

Another objective of the present invention is a system for applying the method, based on a traditional EPR technique with modified Radio Frequency components, enabling detection of a selected number of harmonics of the field modulated frequencies.

The essence of the invented method involves obtaining projections by following the procedure below:

a/ harmonics of EPR signals are determined, using reference generators for phase- sensitive detectors with frequency filters matching harmonics of the EPR absorption signals, recorded during sample scanning with a variable-period magnetic field, sinusoidal, triangular or sawtooth in shape, in the presence of rapidly changing modulation of the magnetic field and in the presence of the magnetic field gradient;

b/ based on the measured harmonics, the k-th derivative of the EPR absorption signal is determined based on the formula:

where P^k - is the Fourier transform of the k-th derivative of the EPR absorption spectrum resulting from the analysis of "n" harmonics; Η_n - is the sum of the Fourier transform of measured "n" harmonics multiplied by a suitably selected complex coefficient, while the "n" number is in the range of 1 to 1000, and the "k" parameter - in the range of 0 to 999, - is the sum of filter functions for "n" harmonics in order to obtain the k-th derivative of the absorption spectrum depending on the amplitude of rapidly changing modulation of the magnetic field, multiplied by a suitably selected complex coefficient;

c/ the final, not overmodulated form of projection is determined through inverse Fourier transform of projection P^k obtained through the analysis of n harmonics;

d/ deconvolution is performed of inverse Fourier transform of the projection P^k obtained through the analysis of n harmonics to obtain radical or radicals density distribution against the direction defined by the spatial orientation of the magnetic field gradient vector.

Preferably, the multi-component magnetic field consists of an external magnetic field, slowly changing scanning field, gradient field produced by the system of gradient coils, and a rapidly changing modulating field.

Preferably, the slowly changing scanning field is produced by coils generating the external magnetic field.

Moreover, preferably, the slowly changing scanning field is produced by scanning coils.

Preferably, the sum of Fourier transform of recorded n harmonics H„ is realized by making the sum of the Fourier transform of the filtered recorded n harmonics.

Preferably, the measured and/or analysed harmonic frequencies of rapidly changing modulation of the magnetic field are marked in the range of 1-999.

Also preferably, projections mark the EPR signal recorded in the presence of the magnetic field gradient after deconvolution with a spectrum recorded without the gradient.

Moreover, preferably, projections mark the EPR signal recorded in the presence of the magnetic field gradient after deconvolution with a theoretically derived spectrum.

Preferably, the generated gradient field is constant during measurement of harmonics of the EPR absorption signal. Also preferably, the generated gradient field is variable during measurement of harmonics of the EPR absorption signal.

Preferably, the modulation amplitude of the rapidly changing magnetic field is established within the range of 0.001mT - 1000mT.

The essence of the system according to the invention is that the master control device is bi-directionally connected to the microprocessor control system, to the input/output of which the ADC unit and memory are bi-directionally connected, while the first output of the microprocessor control system is connected to the input of the low frequency generator unit, and the other output is connected to the input of the high frequency generator, while the first output of the high frequency generator is connected to the input of the phase shifter, which is connected to the first input of the detector, while the other output of the high frequency generator is connected to the input of the power amplifier, the output of which is connected to the bridge input, to the output/input of which the resonator is bi-directionally connected and to the output of which the input of the high frequency preamplifier is connected, the output of which is connected to the input of the detector, the output of which is connected to the input of the splitter, while "n" splitter outputs are connected to "n" inputs of phase sensitive detectors with harmonic filters, to the other input of which the input of the low frequency amplifiers is connected, the outputs of which are connected with n independent inputs to the ADC unit, the output of which is connected to the input of the memory, while yet another output of the low frequency generator unit is connected to the input of the gradient power amplifier, main field, scanning and 2nd modulation, the first output of which is connected to the input of the 2nd modulation coils, the second output is connected to the input of G_x gradient coils, the third output is connected to the input of G_y gradient coils, the fourth output is connected to the input of G_z gradient coils, the fifth output is connected to the input of the scanning coils, while the sixth output is connected to the input of the B₀ main field coils.

Preferably, the "n" number of phase detectors with harmonic filters connected to the low frequency amplifiers marks the filter of the first harmonic connected to the input of the low frequency amplifier; then the phase sensitive detector with the filter of the 2nd harmonic connected to the input of the low frequency amplifier, then the phase sensitive detector with the filter of the 3rd harmonic connected to the input of the low frequency amplifier, then the phase sensitive detector with the filter of n-th harmonic connected to the input of the low frequency amplifier.

An embodiment of the present invention is illustrated by means of drawings, where Fig. 1 shows detection system using hardware signal processing and the bridge. Demodulation of the high frequency signal is synchronous.

The test sample is placed in the multi-component magnetic field, comprising: constant external magnetic field, gradient field which is either constant or variable, scanning magnetic field: sinusoidal, triangular or sawtooth-shaped, changing over the T period, and a rapidly changing modulating field.

The Fourier transform recorded on the n-th harmonic EPR spectrum may be linked with the Fourier transform of the k-th derivative EPR absorption spectrum using the data deconvolution procedure in the Fourier conjugate space u-domain. With this, the Fourier transform of the n-th harmonic in u-domain can be detennined as:

where is the Fourier transform of the k-th derivative of the EPR absorption

spectrum, while the filtering part for the analysis of the first derivative of

absorption spectrum (k=l) is determined as:

where A is modulation amplitude of the rapidly changing magnetic field, and J_n+1 and J_n-1 are Bessel functions of the first kind. In order to determine the form of the first derivative of absorption spectrum, the deconvolution procedure is performed:

(3)

However, given the zero spot of the filtering factor the noise-related effects are significantly amplified. However, the zero spots of the filtering part exist in various areas of domain u for various harmonics, so it is possible to recreate the correct values P by applying the formula (3) to various harmonics and for each value of parameter u. Therefore, the first derivative of the EPR absorption spectrum can be reconstructed by analysing many harmonics of the EPR spectrum in domain u, using the formula:

(4) where, is the n-th harmonic of the EPR absorption spectrum, x_n is

the complex coefficient for the n-th harmonic,

the maximum harmonic used in the analysis, while R is the parameter generated by noise. The value of x_n coefficients is selected with a view to minimise the R parameter value, using the following equation:

where, σ_η is a standard deviation determining the noise level of the recorded EPR absorption signal harmonics. With the R parameter minimisation, it is possible to recreate the k-th derivative of the absorption spectrum through inverse Fourier transform of the function P^k{u). In the case of obtaining other absorption spectrum derivative as a result of the analysis, the filtering part is changed, which is determined using the formula (2).

The sample is placed in the resonator, which is located in the area of the constant, external magnetic field, rapidly changing magnetic field, and specific orientation of the magnetic field gradient After producing in the sample area of a time-varying scanning field, detection of EPR signal harmonics is possible.

Having determined, using the familiar method, the EPR signal, there occurs recording of all the harmonics needed to arrive at the correct projection shape, namely the k-th derivative of the ERP absorption spectrum, determined with the formula (4), measured in the presence of the field gradient, before or after the data deconvolution procedure. Based on established time projections, using the familiar method, the image of its distribution is recreated in ID, 2D or 3D space, as well as the functional spectral-spatial images ID, 2D, 3D or 4D. In the system of the invention, the master device 1 is bi-directionally connected to the microprocessor control system 2, to the input/output of which ADC unit 17 and memory 20 are bi-directionally connected, while the first output of the microprocessor control system 2 is connected to the input of the low frequency generator unit 19 and the other output is connected to the input of the high frequency generator 3, while the first output of the high frequency generator 3 is connected to the input of the phase shifter 28, which is connected to the first input of the detector 8, while the other output of the high frequency generator 3 is connected to the input of the power amplifier 4, the output of which is connected to the bridge input 5, to the output/input of which the resonator 6 is bi-directionally connected, and to the output of which the input of the high frequency preamplifier 7 is connected, the output of which is connected to the input of the detector 8, the output of which is connected to the input of the splitter 18, while "n" splitter outputs 18 are connected to "n" inputs of phase sensitive detectors with harmonic filters to the other output of which low frequency resonator unit 19 is connected,

to the output of which the input of low frequency amplifiers 13, 14, 15, 16 are connected, the outputs of which are connected through n independent inputs with the ADC unit 17, the output of which is connected to the input of the memory 20, while yet another output of the low frequency generator unit 19 is connected to the input of the power adapter of the gradient, main field, scanning and 2nd modulation 21, the first output of which is connected to the input of the 2nd modulation coils 22, the second output is connected to the input of G_x gradient coils 23, the third output is connected to the input of G_y gradient coils 24, the fourth output is connected to the input of G_z gradient coils 25, the fifth output is connected to the input of the scanning coils 26, while the sixth output is connected to the input of the B₀ main field coils 27 (Fig. 1).

Once the sample has been placed in the resonator 6, located between the electromagnet pole pieces 27, gradient coils 23, 24, and 25, as well as scanning coils 26, then the low frequency generator unit 19 is activated, which should be tuned to the middle section of the band in which the experiment takes place to arrive at EPR harmonics, detected using the 2nd modulation method. The output of low frequency generator unit 19 is connected to 2nd modulation coils 22, being fed from the power adapter unit 21. The coils modulate the external magnetic field B0 generated by the main field coils 27, which has much higher frequency than the magnetic field generated by the scanning coils 26, which is controlled by the microprocessor control system 2, which is controlled by the master control device 1. At the same time the microprocessor controller 2 programmes the resonance frequency generated by the generator 3, and then fed into the high-frequency amplifier 4, which is connected to the bridge 5, which feeds the high-frequency signal to the resonance circuit where the object under study is placed. Subsequently, the Electron Paramagnetic Resonance signal generated in the resonator 6 is sent to the bridge 5; subsequently it is amplified using the low-noise high-frequency pre-amplifier 7, subject to detection in the detector 8 using the signal generated by the high-frequency generator 3, following phase correction in the phase shifter 28, and then divided into n signals in the signal splitter 18. Next, the signals generated in the signal splitter 18 are filtered at n parallel phase detectors with filters corresponding to the n harmonics of signal 9-12. controlled using the low-frequency generator unit 19, and then amplified by low- frequency amplifiers 13-16, until the optimum level is achieved for the fast ADC units 17, where the signal is converted into digital, and then stored in the memory 20, sent to the micro-controller 2 and the PC I, where the EPR signal is digitally detected and analysed with harmonic frequencies and specialised software.

List of signs

(1) - master control device

(2) - microprocessor control system

(3) - high-frequency generator

(4) - amplifier

(5) - bridge

(6) - resonator

(7) - high-frequency pre-amplifier

(8) - detector

(9) - phase-sensitive detector with the first harmonic filter

(10) - phase-sensitive detector with the second harmonic filter

(11) - 3rd harmonic filter

(12) - n-th harmonic filter

(13) - low-frequency amplifier

(14) - low-frequency amplifier

(15) - low-frequency amplifier

(16) - low-frequency amplifier

(17) - ADC unit

(18) - signal splitter

(19) - low-frequency generator unit

(20) - memory

(21) - power adapters

(22) - second modulation coils

(23) - G_x gradient coils

(24) - G_y gradient coils

(25) - G_z gradient coils

(26) - scanning coils

(27) - B₀ main field coils

(28) - phase shifter

Claims

Claims 1. A method to obtain projections with the electron paramagnetic resonance imaging method using the analysis of multiple harmonics and the procedure application system, comprising measurement of the EPR signal during impact on the sample in the resonator with the multicomponent magnetic field, wherein the projection is obtained in the following steps: a/ harmonics of the EPR signals are determined, using reference generators for phase-sensitive detectors with frequency filters matching harmonics of the EPR absorption signals, measured during sample scanning with a variable-period magnetic field, with preferably sinusoidal, triangular or sawtooth shape, in the presence of rapidly changing magnetic field modulation and in the presence of the magnetic field gradient; b/ based on the measured harmonics, the k-th derivative of the EPR absorption signal is determined based on the formula:

where P^k - is the Fourier transform of the k-th derivative of the EPR absorption spectrum resulting from the analysis of "n"; harmonics; H„ - is the sum of the Fourier transform of recorded "n" harmonics multiplied by a suitably selected complex coefficient, while the "n" number is in the range of 1 to 1000, and the "k" parameter - in the range of 0 to 999, is the sum of filter functions for "n"

harmonics in order to obtain the k-th derivative of the absorption spectrum depending on the amplitude of the rapidly changing modulation of the magnetic field, multiplied by a suitably selected complex coefficient; c/ the final, not overmodulated form of projection is determined through inverse Fourier transform of projection P^feobtained through the analysis of n harmonics; d/ deconvolution is performed of inverse Fourier transform of the projection P^k through the analysis of n harmonics to obtain radical(s) density projection against the direction defined by the spatial orientation of the magnetic field gradient vector.

The method according to claim 1, is characterised in that the multicomponent magnetic field consists of an external magnetic field, slowly changing scanning field, gradient field created by the system of gradient coils, and a rapidly changing modulating field.

3. The method according to claim 1 or 2, is characterised in that the slowly changing scanning field is created by coils generating the external magnetic field.

4. The method according to claim 1 or 2, is characterised in that the slowly changing scanning field is created by scanning coils.

5. The method according to claim 1, is characterised in that the sum of Fourier transform of recorded n harmonics H_n is realized by making the sum of the Fourier transform of the filtered recorded n harmonics.

6. The method according to claim 1 or 5 is characterised in that the recorded and/or analysed harmonic frequencies of rapidly changing modulation of the magnetic field are marked in the range of 1-999.

7. The method according to claim 1 , is characterised in that projections mark the EPR signal recorded in the presence of the magnetic field gradient after deconvolution with a spectrum registered without the gradient

8. The method according to claim 1, is characterised in that projections mark the EPR signal recorded in the presence of the magnetic field gradient after deconvolution with a theoretically derived spectrum.

9. The method according to claim 1, is characterised in that the generated gradient field is constant during measurement of hannonics of the EPR absorption signal.

10. The method according to claim 1, is characterised in that the generated gradient field is variable during measurement of harmonics of the EPR absorption signal.

11. The method according to claim 1, is characterised in that the modulation amplitude of the rapidly changing magnetic field is established within the range of O.OOlmT - lOOOmT.

12. The system to record projections in the electron paramagnetic resonance imaging method, which includes: master device, microprocessor control system, ADC units, memory, low frequency generator unit, high frequency generator, power amplifier unit, resonator, high frequency pre-amplifier, detector, signal spliter, harmonic filters, low frequency amplifier, ADC units, 2nd modulation coil, scanning coils, G_x gradient coils, G_y gradient coils, G_z gradient coils, main field coils, detection and control system, is characterised in that the master device (1) is bi-directionally connected to the microprocessor control system (2), to the input/output of which ADC unit (17) and memory (20) are bi-directionally connected, while the first output of the microprocessor control system (2) is connected to the input of the low frequency generator unit (19) and the other output is connected to the input of the high frequency generator (3). while the first output of the high frequency generator (3} is connected to the input of the phase shifter (28), which is connected to the first input of the detector (8), while the other output of the high frequency generator (3} is connected to the input of the power amplifier (4), the output of which is connected to the bridge input (5), to the output/input of which the resonator (6) is bi-directionally connected, and to the output of which the input of the high frequency preamplifier (7) is connected, the output of which is connected to the input of the detector (8), the output of which is connected to the input of the splitter (18), while "n" splitter (18) outputs are connected to "n" inputs of phase sensitive detectors with harmonic filters to the other output of which low frequency generator unit (19) is

connected, to the output of which the input of low frequency amplifiers (13, 14, 15, 16) are connected, the outputs of which are connected through n independent inputs with the ADC unit (17), the output of which is connected to the input of the memory (20), while yet another output of the low frequency generator unit (19) is connected to the input of the power adapter of the gradient, main field, scanning and 2nd modulation (21), the first output of which is connected to the input of the 2nd modulation coils (22), the second output is connected to the input of G_x gradient coils (23), the third output is connected to the input of G_y gradient coils (24), the fourth output is connected to the input of G_z gradient coils (25), the fifth output is connected to the input of the scanning coils (26), while the sixth output is connected to the input of the B₀ main field coils (27).

13. The system according to claim 12, is characterised in that the "n" number of phase detectors with harmonic filters connected to the low frequency amplifiers marks the filter of the first harmonic (9) connected to the input of the low frequency amplifier (13); then the phase sensitive detector with the filter of the 2nd harmonic (10) connected to the input of the low frequency amplifier (14), then the phase sensitive detector with the filter of the 3rd harmonic (11) connected to the input of the low frequency amplifier (15), then the phase sensitive detector with the filter of the n-th harmonic (12) connected to the input of the low frequency amplifier (16).