WO2008135878A1

WO2008135878A1 - Multifrequency magnetic resonance device and method

Info

Publication number: WO2008135878A1
Application number: PCT/IB2008/051571
Authority: WO
Inventors: Muhammed Yildirim; Rudolf M. J. N. Lamerichs; Jochen Keupp
Original assignee: Koninklijke Philips Electronics N.V.; Philips Intellectual Property & Standards Gmbh
Priority date: 2007-05-03
Filing date: 2008-04-24
Publication date: 2008-11-13

Abstract

The invention relates to a device for magnetic resonance imaging of a body (14) placed in an examination volume. According to the invention, the device (1) is arranged to a) excite nuclear magnetization from a nuclear spin system having two or more resonance lines by subjecting at least a portion of the body (14) to an MR imaging pulse sequence comprising RF pulses, the RF pulses being generated at two or more different carrier frequencies (f1, f2, f3) corresponding to the resonance frequencies of said nuclear spin system; b) acquire MR signals from the body (14); c) reconstruct an MR image from the acquired MR signals.

Description

MULTIFREQUENCY MAGNETIC RESONANCE DEVICE AND METHOD

FIELD OF THE INVENTION

The invention relates to a device for magnetic resonance imaging of a body placed in an examination volume.

Furthermore, the invention relates to a method for MR imaging and to a computer program for an MR device.

BACKGROUND OF THE INVENTION

In magnetic resonance imaging (MRI) pulse sequences consisting of RF pulses and switched magnetic field gradients are applied to an object (a patient) placed in a homogeneous magnetic field within an examination volume of an MR device. In this way, phase and frequency encoded magnetic resonance signals are generated, which are scanned by means of RF receiving antennas in order to obtain information from the object and to reconstruct images thereof. Since its initial development, the number of clinically relevant fields of application of MRI has grown enormously. MRI can be applied to eveiy part of the body, and it can be used to obtain information about a number of important functions of the human body. The pulse sequence, which is applied during an MRI scan, plays a significant role in the determination of the characteristics of the reconstiiicted image, such as location and orientation in the object, dimensions, resolution, signal-to-noise ratio, contrast, sensitivity for movements, etcetera. An operator of an MRI device has to choose the appropriate sequence and has to adjust and optimize its parameters for the respective application.

SUMMARY OF THE INVENTION

So-called molecular imaging and diagnostics (MID) is rapidly developing during the last years. MID is sometimes defined as the exploitation of specific molecules for image contrast and for diagnosis. This definition refers to the in-vivo measurement and characterization of cellular and molecular level processes in human subjects and to the analysis of biomolecules to screen, diagnose and monitor the human health status and to assess potential risks. An important prerequisite for molecular imaging is the ability to image specific molecular targets.

It is important to note in this context that in particular ¹⁹F MRI has a high potential in the field of MID and also in pharmaceutical research. ¹⁹F MRI allows the direct quantification of nano particles, which can be used as contrast agents in MID. These nano particles contain ¹⁹F based molecules, such as, e.g., PFOB (perfluoro-octyl bromide). The particles are coated with a functionalized protective and stabilizing lipid layer. Depending on the functional groups on the lipid layer the nano particles will bind to protein markers specific to a disease and accumulate at the sites within the body of the patient where the disease is progressing. The accumulated nano particles will show up as bright spots in a corresponding ¹⁹F MR image. For an accurate detection and localization of a disease within the body, a precise determination of the position of the bright spots caused by the accumulated nano particles is required.

However, ¹⁹F MRI and contrast agent quantification is frequently complicated by strong chemical shift artifacts induced by multi-line spectra of the ¹⁹F nuclear spins with a shift range of around 100 ppm. This problem equally occurs in MRI of other nuclei like "¹P or C.

One important problem associated with broad chemical shift spectra is that the existence of multiple resonance lines in a wide chemical shift range makes it necessary to apply excitation RF pulses during the imaging procedure in such a way that the excitation energy transferred from the RF pulses into the nuclear spin system is distributed over the complete spectral range. This is required in order to be able to make use of all resonance lines of the spectrum for imaging. An excitation bandwidth which does not cover the complete spectral range of the examined nuclear spin system results in a reduced signal intensity and, consequently, in an insufficient image sensitivity.

The excitation properties of a given RF pulse, i.e. the excitation profile in terms of flip angle and the spectral bandwidth, is determined by the characteristics of the pulse, i.e. duration, shape, frequency, peak-to-peak power, and phase. In general, the excitation bandwidth of the RF pulse increases as the pulse duration decreases. For a larger spectral bandwidth to be excited, undesirable off-resonance effects become significant, while exciting the whole spectrum becomes more difficult and inefficient. Among other constraints, specific absorption rate (SAR) has to be considered. In order to adapt the excitation bandwidth of the RF pulse to the broad chemical shift spectiiim of, e.g., PFOB, a short pulse duration of about 100-500 μs (depending on the pulse shape) has to be chosen. On the other hand, for achieving a sufficiently large flip angle (e.g. 90°) for all spectral lines in the spectrum, an adequate amount of energy has to be transferred to the spin system. For completely exciting a typical chemical shift spectrum with a shift range of around 100 ppm, an RF pulse amplitude Bj in excess of 30 μT is required. Bj values of this magnitude are impractical for MR scanners that are presently in clinical use. The corresponding heating rate (SAR) is intolerable for the examined patient and exceeds the allowable limits.

Therefore, it is readily appreciated that there is a need for an improved device for magnetic resonance imaging which provides optimized broadband excitation efficiency and image sensitivity. It is consequently an object of the invention to provide an MR device that enables imaging of a nuclear spin system exhibiting a broad spectral range while keeping the B_J amplitude and the SAR within tolerable and allowable limits.

In accordance with the present invention, an MR device for magnetic resonance imaging of a body placed in an examination volume is disclosed, which comprises means for establishing a substantially homogeneous main magnetic field in the examination volume, means for generating switched magnetic field gradients superimposed upon the main magnetic field, means for radiating RF pulses towards the body, control means for controlling the generation of the magnetic field gradients and the RF pulses, means for receiving and sampling magnetic resonance signals, and reconstruction means for forming MR images from the signal samples. The invention proposes that the MR device is arranged to a) excite nuclear magnetization from a nuclear spin system having two or more resonance lines by subjecting at least a portion of the body to an MR imaging pulse sequence comprising RF pulses, the RF pulses being generated at two or more different carrier frequencies corresponding to the resonance frequencies of said nuclear spin system; b) acquire MR signals from the body; and c) reconstruct an MR image from the acquired MR signals.

The MR device of the invention provides a solution to the above-described problems associated with broadband excitation. The technique of the invention can be described as "multi-offset excitation", meaning that RF excitation pulses having different carrier frequencies are irradiated in order to excite the spin system. The different carrier frequencies of the excitation RF pulses correspond to the frequencies of the resonance lines in the chemical shift spectrum of the examined nuclear spin system. In this way, each individual spectral component can be excited on-resonance and narrow-band. Since the RF pulse components can be designed independently, the resulting excitation profile can be tailored to precisely match the spectiiim. This is particularly valuable in the case of imaging probes which show a fixed and a priori known chemical shift spectiiim. The total MR signal obtained is increased in accordance with the invention as compared to conventional techniques that employ only a single earner frequency. This is because the nuclear spins associated with a particular resonance line in the spectiiim will attain precisely the desired flip angle. Hence, an optimized image sensitivity is achieved according to the invention. Furthermore, the required duration of the individual RF excitation pulse components is determined by the widths of the individual resonance lines (or the width of closely lying groups of resonance lines) in accordance with the invention and not - as in the conventional single- frequency excitation techniques - by the width of the whole spectrum. Thus, the invention keeps the necessary RF pulse amplitude Bj and the SAR well within tolerable and allowable limits.

According to a preferred embodiment of the invention, the two or more RF excitation pulses having different carrier frequencies are irradiated simultaneously. In this case, the RF excitation pulse is a composite pulse resulting from a superposition of the different frequency components. The resulting RF pulse can be generated, e.g., as a combined waveform irradiated over a standard RF transmission channel which is available by default in conventional MR scanners. This can be achieved by employing an appropriate programmable RF waveform generating means in the device of the invention. In order to maximize excitation efficiency, the different components of the composite RF excitation pulse may have different excitation bandwidths. The different components may have, e.g., envelopes of Gaussian shapes with different pulse durations. The excitation bandwidths of the RF excitation pulses should practicably correspond to the widths of the different resonance lines or the width of closely lying groups of resonance lines of the examined nuclear spin system. The resulting different durations of the RF pulse components determine the waveform of the resulting composite excitation RF pulse. The total RF excitation pulse duration is determined by the pulse component with the narrowest excitation bandwidth.

The device of the invention may further be arranged to generate RF excitation pulses at carrier frequencies corresponding to two or more selectable spectral lines of the examined nuclear spin system. In this way, specific resonance lines can be excited selectively or excluded arbitrarily from excitation and thereby from contributing to the final image. It is an advantage of this embodiment of the invention that individual resonance lines of the MR spectrum can be switched on and off in the acquired image at the discretion of the user of the MR device. This enables, e.g., a differentiation between different contrast agents which have characteristic resonance lines in their respective chemical shift spectra.

In a practical embodiment of the invention, the MR device comprises two or more physically separate RF transmission channels and a corresponding number of independent RF coils for radiating the different RF excitation pulse components towards the examined body. In this embodiment, a set of indpendent RF coils instead of a single coil is used to transmit the individual frequency components of the RF pulses. Each RF coil is used to transmit a single RF pulse component. The RF pulses having different earner frequencies are superimposed in the same spatial region in the examination volume of the MR apparatus. This embodiment does advantageously not require the application of composite RF excitation pulses according to the invention, which possibly require longer pulse durations and advanced RF waveform generating technology.

The invention not only relates to a device but also to a method for magnetic resonance imaging of at least a portion of a body placed in an examination volume of an MR device. The method comprises the following steps: a) exciting nuclear magnetization from a nuclear spin system having two or more resonance lines by subjecting at least a portion of the body to an MR imaging pulse sequence comprising RF pulses, the RF pulses being generated at two or more different carrier frequencies corresponding to the resonance frequencies of said nuclear spin system b) acquiring MR signals from the body; c) reconstructing an MR image from the acquired MR signals.

A computer program adapted for carrying out the imaging procedure of the invention can advantageously be implemented on any common computer hardware, which is presently in clinical use for the control of magnetic resonance scanners. The computer program can be provided on suitable data carriers, such as CD-ROM or diskette. Alternatively, it can also be downloaded by a user from an Internet server.

BRIEF DESCRIPTION OF THE DRAWINGS

The enclosed drawings disclose preferred embodiments of the present invention. It should be understood, however, that the drawings are designed for the purpose of illustration only and not as a definition of the limits of the invention. In the drawings

Fig. 1 shows an MR scanner according to the invention;

Fig. 2 illustrates the generation of a composite RF excitation pulse according to the invention; Fig. 3 illustrates the waveform of a composite RF excitation pulse in accordance with the invention tailored for the excitation of the ¹⁹F nuclear spins in PFOB.

DETAILED DESCRIPTION OF THE EMBODIMENTS In Fig.l a magnetic resonance imaging device 1 in accordance with the present invention is shown as a block diagram. The apparatus 1 comprises a set of main magnetic coils 2 for generating a stationary and homogeneous main magnetic field, and three sets of gradient coils 3, 4 and 5 for superimposing additional magnetic fields with controllable strength and having a gradient in a selected direction. Conventionally, the direction of the main magnetic field is labelled the z-direction, the two directions perpendicular thereto the x- and y-directions. The gradient coils 3, 4, 5 are energized via a power supply 6. The apparatus 1 further comprises an RF coil arrangement for generating RF fields in an examination volume 7. The RF coil arrangement comprises six independent resonator elements 8, 9, 10, 1 1, 12, 13 which are arranged adjacent to each other on a cylindrical surface around the examination volume 7. The resonator elements 8, 9, 10, 1 1, 12, 13 are used for emitting RF pulses to a body 14. The resonator elements 8, 9, 10, 1 1, 12, 13 are used to generate RF excitation pulses at different carrier frequencies according to the invention. This means that, e.g., coils 8 and 1 1 are used for a first carrier frequency, coils 9 and 12 are used for a second carrier frequency, and coils 10 and 13 are used for a third earner frequency, wherein each carrier frequency corresponds to a resonance line in the spectiiim of the examined nuclear spin system (for example a molecular species containing several ¹⁹F nuclei). The coils 8, 9, 10, 1 1, 12, 13 are mutually decoupled (e.g. by appropriate decoupling means such as capacitors or inductances coupled between the respective resonator elements, not shown) in order to make sure that an RF signal irradiated via one of the coils is not absorbed by another coil. Each of the six resonator elements 8, 9, 10, 1 1, 12, 13 is connected to an RF switching module 15. Via the switching module 15 the relevant resonator element 8, 9, 10, 1 1, 12, 13 is connected to either a signal transmission channel 16 associated with the respective resonator element or with a signal reception unit 17, that is, in dependence of the mode of operation of the device (either transmit mode or receive mode). For each resonator element 8, 9, 10, 1 1, 12 ,13 the MR device 1 has an individual transmission channel 16 (each including an RF oscillator, a power amplifier and a modulator for determining the duration of the RF irradiation) and a receiver unit 17 including a sensitive pre-amplifier, a demodulator and a digital sampling unit. The transmission channels 16 and the power supply 6 for the gradient coils 3, 4 and 5 are controlled by a control system 18 to generate the MR imaging pulse sequence in accordance with the above-described invention. The control system is usually a microcomputer with a memory and a program control. For the practical implementation of the invention it comprises a programming with a description of an imaging procedure wherein RF pulses are generated simultaneously at the first frequency via the resonator elements 8, 1 1, at the second frequency via the resonator elements 9,12, and at the third frequency via the resonator elements 10, 13. The receiver unit 17 is coupled to a data processing unit 19, for example a computer, for transformation of the received magnetic resonance signals into an image. This image can be made visible, for example, on a visual display unit 20. Fig. 2 illustrates the generation of a composite RF excitation pulse B₁Ct). The

RF pulse B₁Ct) is a superposition of three oscillating components having different carrier frequencies fi, ft, and ft, wherein each of the oscillating signals is modulated by an individual envelope E₁Ct), E₂Ct), and E^Ct). The envelopes E₁Ct), E₂Ct), and E^Ct) are of Gaussian shape and have different durations. Thus, the excitation bandwidth of the RF pulse B₁Ct) is different for each frequency component ft, f₂, and ft. The composite pulse B₁Ct) can be generated in the way illustrated in Fig. 1, i.e. by irradiating RF signals oscillating at the frequencies fj, f₂, and ft over separate transmission channels. It is also possible to generate the RF pulse B₁Ct) directly and to radiate the signal to the body of the examined patient over a single transmission channel. In this case, an appropriate (programmable) RF waveform generating means would be required.

Fig. 3a shows the RF excitation pulse B₁Ct) (in μT) more detailed. The diagram shows the x- and y-components of the B₁ field as a function of time (in milliseconds). The chemical shift spectrum of the examined nuclear spin system is shown in Fig. 3b together with excitation profile of the RF excitation pulse B₁Ct) shown in Fig. 3a. The spectiiim is determined by the resonance frequencies of ¹⁹F nuclei contained in PFOB, which is the imaging probe in the depicted embodiment. The spectrum of PFOB comprises five resonance lines at -25 ppm, -20 ppm, -15 ppm, 20 ppm and 40 ppm. In order to excite the whole spectrum, the RF pulse B₁Ct) includes carrier frequencies ft at -20 ppm, ft at 20 ppm, and ft at 40 ppm. The envelopes E₁Ct), E₂(t), and E*(t) are selected such the duration Of E₁Ct) is 2 milliseconds resulting in a broader excitation bandwidth covering the resonance lines of PFOB at -25 ppm, -20 ppm, and -15 ppm, while the duration of E₁Ct) and E₂(t) is 5 milliseconds covering to the resonance lines at 20 ppm and 40 ppm, respectively. As can be seen in Fig. 3b, the range of the PFOB chemical shift spectrum is 65 ppm. The spins associated with the individual resonance lines of this broadband spectiiim are flipped quite uniformly by about 90°. However, as Fig. 3a shows, the Bj field amplitude does not exceed 8 μT, while the maximum pulse duration is about 5 ms (determined by the excitation bandwidths at 20 ppm and 40 ppm).

After all, as Figs. 3a and 3b illustrate, the technique of the invention enables an RF excitation that is adaptive to the MR spectiiim and is thus applicable to any contrast agent with sufficiently resolvable resonance lines. A uniform excitation of the whole spectiiim is achieved. Hence, the image sensitivity is maximum because all spectral components contribute to the acquired MR signal. No broadband RF pulses are required such that the SAR can be kept acceptably low.

Claims

CLAIMS:

1. Device for magnetic resonance imaging of a body ( 14) placed in an examination volume, the device ( 1 ) comprising means (2) for establishing a substantially homogeneous main magnetic field in the examination volume, means (3, 4, 5) for generating switched magnetic field gradients superimposed upon the main magnetic field, means (8, 9, 10, 1 1, 12, 13, 16) for radiating RF pulses towards the body ( 14), control means ( 18) for controlling the generation of the magnetic field gradients and the RF pulses, means ( 17) for receiving and sampling magnetic resonance signals, and reconstruction means ( 19) for forming MR images from the signal samples, the device ( 1 ) being arranged to a) excite nuclear magnetization from a nuclear spin system having two or more resonance lines by subjecting at least a portion of the body ( 14) to an MR imaging pulse sequence comprising RF pulses, the RF pulses being generated at two or more different carrier frequencies (fi, f?, U) corresponding to the resonance frequencies of said nuclear spin system; b) acquire MR signals from the body ( 14); c) reconstruct an MR image from the acquired MR signals.

2. Device of claim 1, wherein the device is further arranged to generate two or more RF excitation pulses having different carrier frequencies (fi, U-, U) simultaneously.

3. Device of claim 1 or 2, wherein said RF excitation pulses having different carrier frequencies (fi, f?, U) have different excitation bandwidths.

4. Device of claim 3, wherein the excitation bandwidths of the RF excitation pulses corresponds to the widths of the resonance lines or the widths of closely lying sub-sets of resonance lines of said nuclear spin system.

5. Device of any one of claims 1-4, wherein the device is arranged to generate RF excitation pulses at carrier frequencies (fi, f?, fθ corresponding to two or more selectable spectral lines of said nuclear spin system.

6. Device of any one of claims 1-5, wherein said RF excitation pulses have envelopes (Ei, E₂, E^) of Gaussian shape.

7. Device of any one of claims 1-6, wherein the device comprises two or more physically separate RF transmission channels ( 16) and a corresponding number of independent RF coils (8, 9, 10, 1 1, 12, 13) for radiating said RF excitation pulses having different carrier frequencies (fi, f?, f0 towards the body ( 14).

8. Device of any one of claims 1-6, wherein the said nuclear spin system contains ¹⁹F nuclei.

9. Method for MR imaging of at least a portion of a body of a patient placed in an examination volume of an MR device, the method comprising the following steps: a) exciting nuclear magnetization from a nuclear spin system having two or more resonance lines by subjecting at least a portion of the body to an MR imaging pulse sequence comprising RF pulses, the RF pulses being generated at two or more different carrier frequencies corresponding to the resonance frequencies of said nuclear spin system; b) acquiring MR signals from the body; c) reconstructing an MR image from the acquired MR signals.

10. Computer program for an MR device, comprising instructions for: a) generating an MR imaging pulse sequence comprising RF pulses, which RF pulses have two or more different carrier frequencies corresponding to the resonance frequencies of a nuclear spin system having two or more resonance lines; b) acquiring MR signals; c) reconstructing an MR image from the acquired MR signals.