Patent Application
20080211499
The following relates to the magnetic resonance arts. It finds particular application in multi-nuclear magnetic resonance spectroscopy observing 13C or 15N nuclei bonded to 1H nuclei, and to imaging or other data analysis employing data collected by such spectroscopy, and will be described with particular reference thereto. However, it finds application more generally in multi-nuclear magnetic resonance spectroscopy observing substantially any nuclear species bonded to another hetero-nuclear species, and to chemical shift imaging or other data analysis employing data collected by such spectroscopy.
Magnetic resonance spectroscopy can provide information on the chemical bonding of the imaged nuclear species, through observation of chemical shifts caused by the chemical environment. For example, when observing the 13C nuclear species, chemical shift information can distinguish the C, CH, CH2, and CH3 chemical configurations. Such chemical information can be used for various applications, such as tracking the metabolism of an injected tracer.
A complication arises due to scalar coupling, also called J-coupling, between the nuclei. For example, J-coupling causes the resonance line from a CH2 or other chemically bonded configuration to split in frequency space to form a multiplet. The presence of spin coupling therefore reduces the signal-to-noise ratio, and the overlap of multiplets from different chemically shifted resonances obscures the spectrum.
Secondary radio frequency irradiation is applied at the magnetic resonance frequency of the coupled second nuclear species during readout of the observed magnetic resonance to remove the effects of scalar coupling (J-coupling) between the nuclei in order to simplify the spectrum and increase the signal-to-noise ratio. The process is referred to simply as “decoupling”. For example, 13C nuclei are commonly bonded with 1H and show strong couplings with J˜100-200 Hz. When observing 13C, the J-coupling with hydrogen can be suppressed by applying secondary radio frequency irradiation at the 1H magnetic resonance frequency during the readout phase of the magnetic resonance spectroscopy sequence, so that only a single line is observed for each chemically bonded configuration.
The use of decoupling in performing magnetic resonance spectroscopy of 13C, 15N, and other nuclear species of interest has led to its widespread application and has generated many methods that optimize its bandwidth, efficiency, and power for collapsing couplings across the full width of the proton spectrum. However, the existing techniques have certain disadvantages. The suppressed J-coupling information is inherently lost, and the applied secondary radio frequency irradiation substantially increases the specific absorption rate (SAR) of the magnetic resonance spectroscopy sequence.
Increased SAR is highly disadvantageous in medical applications where the observed subject is a human patient. Increased SAR is also of particular concern in high-field magnetic resonance scanners, such as 3T scanners and the more recently developed 7T scanners, where the increased central magnetic field requires the use of increased radio frequency power levels to produce the same degree of nuclear excitation.
The secondary radio frequency irradiation used for decoupling should be sufficiently broadband to span the frequency range of the chemical shift spectrum of the coupled species. For example, to decouple protons over a chemical shift range of 6.5 ppm, the secondary radio frequency irradiation should encompass a decoupling bandwidth of 830 Hz at 3T (128 MHz 1H magnetic resonance frequency), and should encompass a decoupling bandwidth of 1936 Hz at 7T (298 MHz 1H magnetic resonance frequency). Thus, a relatively broadband secondary radio frequency irradiation is called for, which is applied throughout the readout phase in order to suppress J-coupling effects. The relatively broadband nature and extended time over which the secondary radio frequency irradiation is applied leads to substantially heightened SAR.
The present invention contemplates improved apparatuses and methods that overcome the aforementioned limitations and others.
According to one aspect, a magnetic resonance spectroscopy method is disclosed. A first nuclear species magnetic resonance is excited, the excitation encompassing either the whole volume of the subject, or a smaller volume of interest restricted by the application of selective radio frequency pulses and magnetic field gradients. A spin echo signal from the first nuclear species magnetic resonance is generated, and is either read out directly, or encoded spatially with additional magnetic field gradient pulses. The first nuclear species and a second coupled hetero-nuclear species are decoupled using a pulse applied on a secondary radio frequency channel during the generation of the spin echo. Decoupling to suppress hetero-nuclear J-coupling is not applied during the signal read out. At least the spin echo generation, the reading, and the radio frequency pulse applied on the secondary radio frequency channel are repeated for a plurality of different spin evolution times (Δ) to spectrally encode data with a modulation due to hetero-nuclear J-coupling.
According to another aspect, a magnetic resonance apparatus is disclosed, including a magnetic resonance scanner, and a controller controlling the magnetic resonance scanner to perform the magnetic resonance spectroscopy method set forth in the preceding paragraph.
According to another aspect, a magnetic resonance apparatus is disclosed. A means is provided for acquiring heteronuclear J-modulated data. A processor processes the acquired heteronuclear J-modulated data to extract at least one of (i) coupling information, (ii) chemical shift information, or (iii) spatial information when this is separately encoded with magnetic field gradients.
According to another aspect, a magnetic resonance spectroscopy method is disclosed. A first nuclear species magnetic resonance is excited, the excitation encompassing either the whole volume of the subject, or a smaller volume of interest restricted by the application of selective radio frequency pulses and magnetic field gradients. A spin echo signal from the first nuclear species magnetic resonance is generated, and the spin echo is either read out directly, or encoded spatially with additional magnetic field gradient pulses. During the generation of the spin echo, J-coupling of the first nuclear species with a second coupled hetero-nuclear species is spectrally encoded by the application of broadband decoupling to the second nuclear species over the spin evolution interval (Δ). Decoupling to suppress hetero-nuclear J-coupling is not applied during the signal read out. Signal acquisition with spectral encoding is repeated using a plurality of different decoupling time intervals (Δ) to generate hetero-nuclear J-modulated data containing both chemical shift and J-coupling information. The hetero-nuclear J-modulated data are processed to extract at least one of the chemical shift information, the J-coupling information, or spatial information when this is separately encoded with magnetic field gradients.
One advantage resides in reduced SAR in multi-nuclear magnetic resonance spectroscopy with suppressed J-coupling.
Another advantage resides in acquiring multi-nuclear magnetic resonance spectroscopy including separated chemical shift and J-coupling information.
Another advantage resides in suppression of J-coupling with concomitant retention of high signal-to-noise ratio in one-dimensional chemical shift spectra acquired by multi-nuclear magnetic resonance spectroscopy.
Numerous additional advantages and benefits will become apparent to those of ordinary skill in the art upon reading the following detailed description of the preferred embodiments.
The invention may take form in various components and arrangements of components, and in various process operations and arrangements of process operations. The drawings are only for the purpose of illustrating preferred embodiments and are not to be construed as limiting the invention.
With reference to
Magnetic field gradient coils 28 are arranged in or on the housing 12 to superimpose selected magnetic field gradients on the main magnetic field in at least the observed region of the observed subject 16. Typically, the magnetic field gradient coils include coils for producing three orthogonal magnetic field gradients, such as x-gradients, y-gradients, and z-gradients. At least two radio frequency coils 30, 32 (or alternatively a single coil capable of being tuned to at least two different radio frequencies) are disposed in the bore of the scanner 10.
One or more of the radio frequency coils, namely the local coil 30 in
Additionally, one or more of the radio frequency coils, namely the local coil 32 in
In this Detailed Description, the example of 13C is used as the observed nuclear species, and the example of 1H is used as the chemically bonded second nuclear species. However, it is to be appreciated that either or both of the observed first nuclear species and the chemically bonded second nuclear species can be other species. For example, the observed first nuclear species can be 15N and the bonded second nuclear species can be 1H. For hetero-nuclear spectroscopy, the first nuclear species and the second nuclear species typically have different atomic number (Z) values. For example, carbon has Z=6 while hydrogen has Z=1.
During magnetic resonance spectroscopy data acquisition, a radio frequency power source 38 operating at the magnetic resonance frequency of the observed first nuclear species (e.g., 13C) is coupled to the local coil 30 through radio frequency switching circuitry 40 to inject radio frequency excitation pulses at the magnetic resonance frequency of the observed first nuclear species into the observed region of the observed subject 16 so as to excite magnetic resonance in spins of the first nuclear species (e.g., 13C). Optionally, a magnetic field gradients controller 44 operates the magnetic field gradient coils 28 to spatially localize the magnetic resonance excitation to a slab or other localized region. The radio frequency power source 38 further operates the local coil 30 to generate one or more spin echoes, for example by applying one or more inversion pulses at the magnetic resonance frequency to invert the excited magnetic resonance of the first nuclear species so as to generate one or more spin echoes. Optionally, the magnetic field gradient controller 44 operates the magnetic field gradient coils 28 to apply one or more spatial encoding magnetic field gradient pulses. During the magnetic resonance readout phase of the pulse sequence, the switching circuitry 40 disconnects the radio frequency transmitter 38 from the local coil 30, and connects a radio frequency receiver 46 to the local coil 30 to acquire magnetic resonance data from the observed region of the observed subject 16. The acquired magnetic resonance data are stored in a data buffer 48.
A second, decoupling radio frequency transmitter 50 operates the local coil 32 at a magnetic resonance frequency of the second nuclear species (e.g., 1H) during the generating of the spin echo, but not during the reading out of the spin echo, so as to decouple the first and second nuclear species to generate heteronuclear J-modulation. Under the control of a scanner controller 54, low power adiabatic radio frequency pulses are applied at the magnetic resonance frequency of the second nuclear species (e.g., 1H) to invert the spin states of the decoupled nucleus simultaneously with a spin-refocusing pulse at the magnetic resonance frequency of the first nuclear species (e.g., 13C) to generate a spin echo, and using a variable time delay (A) to encode spin evolution under the influence of heteronuclear J-coupling. The variable time delay is incremented by an amount sufficient to sample a spectral bandwidth in the J-coupling dimension larger than the widest multiplet expected to be encountered. For the example of 13C observe nuclei and 1H coupled nuclei, a —CH3 group will generate a quartet with a width of 3J Hz; accordingly, increment times of less than or equal to ⅙ J are suitable.
A sufficient number of transients are collected, with the transients distributed over the whole (chemical shift and J-coupling) data acquisition, to yield the desired signal to noise ratio in a final one-dimensional chemical shift spectrum. For example, if 128 transients correspond to a desirable 13C signal-to-noise ratio, then collecting sixteen time increments with eight averages, or thirty-two time increments with four averages each, is suitable. The signal-to-noise ratio of the final decoupled one-dimensional spectrum obtained from the data matrix is proportional to the square root of the total number of transients used to acquire the data matrix, so the distribution of transients and encodings can be assigned fairly flexibly.
A spectral shift and J-coupling data/image processor 60 performs data and optional image processing, such as: processing the collected magnetic resonance data with a series of complex one-dimensional spectral fast Fourier transform operations for each encoded data line to recover chemical shift and J-coupling data; performing optional image reconstruction using one or more spatial fast Fourier transform operations; phase correcting the data to remove both zero order and first order phase errors; transposing the data array and reconstructing the J-spectrum domain of the nuclear magnetic resonance (NMR) data set with a complex ID FFT operation for each data line; rotating the data matrix through an angle selected to align the elements of each coupled multiplet transverse to the chemical shift axis; projecting a decoupled spectrum onto the chemical shift axis; scaling the decoupled spectrum to correct a chemical shift scale while allowing for the angle of rotation used to make the projection; applying spectral peak detection to the decoupled spectrum to classify the resonances; extracting traces or projecting regions from the rotated data matrix to reveal coupling patterns that lie parallel to the J-spectrum axis; peak detecting along the J-spectrum axis to determine the multiplicity of the coupling pattern; assigning the decoupled spectral lines to suitable chemical configurations such as —C, —CH, —CH2, —CH3 or —N, —NH, —NH2, —NH3 groupings, or so forth. The resulting processed data or images are suitably stored in a data/images memory 62, displayed on a user interface 64, printed, communicated over the Internet or a local area network, stored on a non-volatile storage medium, or otherwise used. In the embodiment illustrated in
The optional pre-saturation 102 is followed by an excitation operation 104 during which spins of the observed first nuclear species (e.g., 13C) are excited to produce magnetic resonance. The excitation can use a 90° or other flip-angle radio frequency pulse at the magnetic resonance frequency of the first nuclear species.
A spin echo generating operation 110 generates one or more spin echoes of the excited magnetic resonance of the first nuclear species. The spin echo can be generated, for example, by applying one or more 180° inversion pulses at the magnetic resonance frequency of the first nuclear species. A decoupling operation 112 is performed during a selected portion of the generating of the spin echo. The decoupling operation 112 applies one or more radio frequency pulses, optionally adiabatic pulses, at the resonance frequency of the second nuclear species (e.g., 1H) so as to perturb the J-coupling between the first and second nuclear species for a selected time interval denoted herein as the spin evolution time interval A. In a readout operation 116, the generated spin echo is read out by the radio frequency receiver 46, and the read out data is stored in the data buffer 48. To limit SAR, the decoupling operation 112 does not extend into the readout 116.
Optionally, one or more spatial localization and/or encoding operations 120 are performed during one or more of the excitation 104, spin echo generating 110, readout 116, or during intervals therebetween. For example, spatial encoding magnetic field gradients can be applied between the spin echo generation and the readout to provide spatial phase encoding.
A repeat operation 124 repeats at least the spin echo generating 110, decoupling 112, and readout 116, for a number of transients. Depending upon the rate of decay of the magnetic resonance, the repeating 124 may also repeat the excitation 104 and optionally also the pre-saturating 102. The transients can include averaging repetitions of transients having the same spin evolution time interval A so as to increase the signal-to-noise ratio for the chemical shift spectrum. The transients also include transients with different spin evolution time intervals A to provide modulation data in the J-coupling dimension. The plurality of transients acquired by the repetitions 124 define collected heteronuclear J-modulated data 130.
With reference to
With continuing reference to
With returning reference to
One suitable embodiment of the automated phase correction 136 employs a data line collected with Δ=0. The absolute value of the complex data line at Δ=0 is computed to produce a spectrum A(v) which is a suitable approximation to the absorption part |S(v)| of the correctly phased spectrum. That is, A(v)≅|S(v)|. From the absolute value spectrum |S(v)|, an approximation to the dispersion part of the correctly phased spectrum is computed using a Hilbert transform. Taken together the absolute value spectrum |S(v)| and its Hilbert transform H(v) form an approximation to the correctly phased spectrum Sa(v)=|S(v)|+iH(v). A function of the difference Sa(v)−exp(−i(Z+Fv))S(v) is minimized so that estimates of the both the zeroeth and first order phase correction parameters are obtained. The function used for the minimization can, for example, be a least squares criterion, a least first power, Chebyshev or other like criteria.
A variation of the automated phase correction procedure is to take the phase difference between the Hilbert approximation to the correctly phased spectrum and the actual spectrum, and use this phase difference to correct the other transients of the data matrix.
A second spectral FFT 140 employing a complex one-dimensional FFT operation for each data line is applied, after transposing the matrix, to the separated absorption mode data and dispersion mode data. The real and imaginary parts of the two data sets are then combined so as to reconstruct the two dimensional magnetic resonance data set (J-spectrum) in pure absorption mode. The recovered J-spectrum domain data is, however, tilted by 45° because decoupling was not applied during the readout 116.
As noted previously, the J-spectrum recovered by the second spectral FFT 140 is tilted by 45° because the decoupling was not applied during the readout 116. Accordingly, a rotation operation 144 rotates the J-spectrum by 45° to recover the untilted J-spectrum having horizontal and vertical axes corresponding to chemical shift and J-coupling, respectively.
In a variation of the procedure used to correctly align the chemical shift and J-coupling axes before projection and analysis, an affine transformation can be applied to the J-spectrum to skew the data matrix so that the two axes become orthogonal. This approach has the advantage that data matrices with an intrinsic difference in frequency resolution between the chemical shift and J-coupling axes may also be transformed for onward processing.
In a variation of the procedure the projection of the decoupled spectrum is obtained by summing the data lines of the untilted J-spectrum. This modification has the advantage of preserving the full signal-to-noise ratio of the acquired data.
With continuing reference to
In some embodiments, the processing 150 includes automated multiplet type recognition employing pattern recognition on the untilted two-dimensional J-spectrum of
The amplitudes for the different multiplet pattern possibilities in the spectral domain can also be estimated from the signal power of the pseudo-echo using Parseval's Theorem that states that the total signal power (noise and signal) is the same in the two domains. Shift and J coupling parameters can be estimated using the constrained spectral line fitting for each multiplet type on the trace. Triplets can be distinguished from quartets and doublets on the basis of the overall power in the least square residuals, since triplet patterns typically have a poor fit to either true doublets or true quartets. Doublets and quartets can be distinguished on the basis of the second moment of the least square residuals taken about the multiplet center. The outer lines of a true quartet contribute strongly to this measure when compared with a doublet template fitted to the inner two lines.
While the pattern recognition is advantageously applied to the untilted J-spectrum (such as the example untilted J-spectrum of
With continuing reference to
It is to be appreciated that the pulse sequences of
In performing the decoupling, adiabatic radio frequency pulses, or pulse packets have the advantage of providing decoupling with low power and hence low SAR. For example, if protons are to be decoupled over a chemical shift range of 6.5 ppm (spanning a decoupling range of 830 Hz at 3T, or 1936 Hz at 7T), conventional pulsed decoupling calls for radio frequency field strengths of the same order as the chemical shift range. On the other hand, a low-power adiabatic pulse can invert the same proton range with a radio frequency field strength of only about 400-500 Hz. The J-modulation employed in the method of the first embodiment disclosed herein uses only inversion of the decoupled nucleus spin state and evolution of the transverse coherence during time interval 2Δ, or in the method of the second embodiment, the use of broadband decoupling during the first time interval Δ, followed by a period of evolution for the transverse coherence during the second time interval Δ. These procedures introduce substantially less SAR than does decoupling over the readout phase of the magnetic resonance sequence.
The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB2006/051886 | 6/13/2006 | WO | 00 | 12/14/2007 |
| Number | Date | Country | |
|---|---|---|---|
| 60691114 | Jun 2005 | US |
