GRADIENTS

FIELD OF THE INVENTION

The invention concerns a diffusion encoded magnetic resonance imaging method. This magnetic resonance imaging method acquires magnetic resonance signals that are diffusion weighted.

BACKGROUND OF THE INVENTION

Diffusion-weighted magnetic resonance imaging (DWI) provides image contrast that is dependent on molecular motion of water. Usually DWI uses a 7 -weighted acquisition sequence with the addition of pulsed linear gradient magnetic fields with zero time integral. This renders the magnetic resonance imaging method sensitive to diffusion. Diffusion Weighted Imaging (DWI) in MRI uses the loss of phase coherence within the imaging voxel - induced by the random walk of the spins under the influence of a linear gradient - to deduce micro-structural information. Since diffusion is typically dyadic (i.e. equal # of spins moving in opposed directions), the entire spin ensemble within the imaging voxel does not experience a net phase accrual, but only a loss in total signal magnitude due to dephasing. Hence, classical DWI cannot utilize the MRI-phase information per se.

SUMMARY OF THE INVENTION

An object of the invention is to provide a magnetic resonance imaging method that has a better signal-to-noise ratio as compared to currently known DWI methods.

This object is achieved by the magnetic resonance imaging method as defined in Claim 1.

An insight of the present invention is that the even order dependence of the spatially non-linearly varying magnetic field induces a coherent phase accumulation in the excited spins in an object, notably in a patient's tissue to be examined. The spatially nonlinearly varying magnetic field may be quadratic, fourth order, sixth order or even higher even order dependent on at least one spatial coordinate. The coherent phase accumulation due to the spatially non-linear even order magnetic field induces a diffusion encoding of the conventional diffusion encoding by a bipolar pair of spatially linear gradients. The conventional diffusion encoding is based in different signal loss due to dephasing dependent on the gradient strength. The present invention achieves a markedly better signal level and signal-to-noise ratio of the diffusion encoded magnetic resonance signal because it relies on a coherent phase change of the net magnetization and not on a de-phasing and thus a loss of signal. It is to be noted however, that although in each spatial dimension a quadratically spatially varying magnetic field induces a coherent phase accumulation in an individual dimension, the ensuing concommitent fields in the other spatial diredftion cause cancellation of the net coherent phase accumulation. This is due to the fact the the mangetic field has zero Laplacian. The fourth order and higher even order spatially varying fields with their concimmittent field induce a non-zero net coherent phase accumulation. Classical DWI methods extract information about microstructural tissue complexity from the signal decrease of the MR-magnitude as a function of b-value. Utilization of linear gradients for motion encoding prevents theoretically the use of the MR-phase. Rather, the diffusion information is encoded in the MR-magnitude via global spin dephasing due to Brownian motion with zero net phase shift. This dogma is overturned when considering quadratic gradient fields in space. We demonstrate in theory, experiment, and simulation that the diffusion process leads to a net phase shift with minimal loss in signal magnitude when imaging at the minimum of the quadratic gradient.

These and other aspects of the invention will be further elaborated with reference to the embodiments defined in the dependent Claims.

In a preferred implementation of the invention, the spatially non-linear even order temporary magnetic field is combined with a temporary spatially linear gradient magnetic field. From the magnetic resonance signals encoded by the non-linear and linear gradient magnetic field bulk motion of spins can be separated from diffusion motion.

In another preferred implementation, the spatially nonlinearly varying magnetic field is modulated in time. Magnetic resonance signals with a coherent phase accumulation are acquired at different values of the modulation rate of the spatially nonlinearly varying magnetic field. From the accumulated coherent phase as a function of the modulation rate the (apparent) diffusion coefficient as well as the fractal dimension that characterises anomalous diffusion can be calculated. Anomalous diffusion imaging is known per se from the paper 'From diffusion-weighted MRI to anomalous diffusion imaging' in MRM59(2008)447-455 by M.T Hall and T.R. Berrick. The present invention appears to the modulation rate of the spatially nonlinearly varying magnetic field of the coherent accumulated phase. In fact, the ratio of accumulated phase for different modulation rate is nearly linearly dependent on the fractal dimension of the studied material, e.g. brain tissue of the patient to be examined. The fractal dimension of the anomalous diffusion represents microstructure of the object being examined. Notably, the anomalous diffusion may show differences in microstructure of brain tissue. Also anomalous diffusion, notably it fractal dimension, may be employed for tumour characterisation and for therapy response.

The application of a spatially quadratic magnetic field can be achieved in practice by the magnetic field of a single coil loop, or a multiple of coil loops. For example, at a modulation rate of about 30ms and an electrical current of 20A using fifty windings causes the accumulated coherent phase to be about lrad in free water.

In brief, the invetion concerns the magnetic resonance diffusion encoding imaging method based on a non-linear gradient magnetic field pulseof 4 ^th order or 6 ^th order or an even order of at least 8 ^th order in spatial position.The magnetic resonance signals have a coherent phase accumulation due to the spatially nonlinearly varying magnetic field pulse.

These and other aspects of the invention will be elucidated with reference to the embodiments described hereinafter and with reference to the accompanying drawing wherein

Figure 1 shows (a) Concept of DWI2, (b,c,d) experimental results in a water phantom, and (e) phase distribution from MC simulations for linear and quadratic coil configuration.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In this section the coherent phase accumulation is demonstrated for only one spatial direction (along the z-axis). As mentioned above, in volumeric space the net phase accumulations due to the concommittent fields cancel in the total net phase accumulation. Here, for simplicity only the phase accumulation due to a qudratically varying field in one dimension is illustrated, while ignoring concomittent field conntributions. DWI ² is based upon the idea that a diffusing spin in a spatially varying 2nd order magnetic field (i.e.

Β(ζ)=β*ζ2) will experience only positive phase accrual independent of the direction of motion. Thus, DWI ² can explore the MRI-phase information. Furthermore, the absence of a linear component (i.e. B(z)=a*z) in the proximity around z=0 leads to minimal loss in signal magnitude. Thus, in DWI ² the SNR remains minimally altered when imaging at this preferred position. This opens the gateway to entirely new concepts for diffusion weighted imaging, in particular to elevated sensitivity to micro-architectural complexity currently hidden due to limited SNR.

Experiments used a classical spin-echo sequence with TE=50ms and TR=500ms at 3T (Philips Medical Systems). As shown in the figure, a water sample (a) was placed along the z-symmetry axis of two Helmholtz coils (inner 0=10mm, # 150 windings for each coil, length of each coil=10mm, separation of 8.6mm between coils, 00.3mm for copper wire) which could be operated in linear or quadratic mode. Imaging was done at z=0 with transverse image orientation to explore the effect of the linear or quadratic field on the diffusion of the water molecules. One single sinusoidal gradient pulse was applied after the π/2-pulse and before the π-pulse at v=100Hz with a current of 1 A. Pre-emphasis of the gradient pulse was adjusted to ensure a net pulse area of zero. A magnetostatic field simulation (FEMM 4.2) of the quadratic coil configuration yields β=5300Τ/ηι2 at z=0.

Analytic calculus predicts a net phase-change φ=γϋβ/(2ν2) = (-)0.16 [rad] (with D the water diffusion coefficient and γ the gyromagnetic ratio). Monte Carlo simulations (Camino software [2], #100000 particles, pure water, β=7560Τ/ιη2, v=30Hz) were performed to investigate the correctness of the analytic calculus.

The resulting images of the MR-magnitude and MR-phase for both coil configurations (linear, quadratic) are shown in (b) without (OA) and with application of a current (1A). The linear coil configuration leads - as expected - to a significant reduction in magnitude (c) without any statistically significant phase change (d). This is corroborated by analytic calculus. On the contrary, the quadratic coil configuration leads to no statistically significant loss in magnitude (as expected), while a statistically significant negative phase drift is measured. The measure phase shift of (-)0.24±0.1 lrad agrees within errors with the theoretically expected of (-)0.16rad. Monte Carlo simulations (e) confirmed the net phase shift for the quadratic field configuration (cpAVG=-l .44rad) with excellent agreement to the analytic expression ((ptheory=-1.45rad).

Classical DWI is a powerful method for gaining insight into tissue microstructure. Its diagnostic value is currently explored for various applications, from diagnosis over response to therapy to prediction of outcome. Intrinsically, DWI uses the decrease of the MR- magnitude signal as a function of b-value for extracting the mono-exponential or bi- exponential ADC value(s). More complex diffusion information are obtained via stretched exponentials [3], or ADC-values at shorter diffusion time scales via oscillating gradients [4]. Still, all those methods utilize the MR-magnitude information. The inevitable loss in signal characterizing microstructure. DWI ² overcomes this limitation by using 2nd order spatial field changes for encoding the diffusion information. As expected from theory - and experimentally demonstrated - a quadratic field gradient leads to a minimal loss in signal magnitude in combination with a net shift in phase when operating in close proximity of z=0 for Β(ζ)=βζ2. Monte Carlo simulations of freely diffusing water molecules show excellent agreement between theory and simulation results, further demonstrating the validity of the proposed method. The essential conservation of signal for DWI ² leads therefore to a paradigm shift for DWI as currently inaccessible domains can now be explored.