WO2002075346A1

WO2002075346A1 - Magnetic resonance imaging method for analysing a sample by receiving spin and gradient echo signals

Info

Publication number: WO2002075346A1
Application number: PCT/DE2002/000711
Authority: WO
Inventors: Nadim Joni Shah; Karl Zilles
Original assignee: Forschungszentrum Jülich GmbH
Priority date: 2001-03-15
Filing date: 2002-02-28
Publication date: 2002-09-26
Also published as: DE10112879A1

Abstract

The invention relates to a method for analysing a sample, comprising at least one imaging sequence, whereby at least one excitation pulse and a plurality of rephase pulses are transmitted to a sample so that at least one echo signal arises and is detected. The inventive method is characterised in that at least one rephase pulse is transmitted after the excitation pulse, one or more spin-echo-excitations occur after the rephase pulse, an echo signal is detected after the spin-echo-excitations and one or more other spin-echo excitations occur after detection of the echo signal.

Description

METHOD OF IMAGING MAGNETIC RESONANCE FOR EXAMINING A SAMPLE BY MEANS OF SPIN AND GRADIENT Echo Signals

description

The invention relates to a method for examining a sample with at least one imaging sequence, wherein at least one excitation pulse and several rephasing pulses are irradiated into the sample, and at least one echo signal arises and is determined.

In the present case, the term “sample” is meant in its broadest meaning and includes living and non-living material.

Various methods are known in which a sample is examined by means of an excitation pulse and several rephasing pulses.

In nuclear magnetic resonance imaging it is known to obtain information about the sample by excitation of echo signals from a sample.

The generic method is in the

Nuclear magnetic resonance imaging is preferably used to obtain spectroscopic information or image information about a substance. A combination of nuclear magnetic resonance imaging with techniques of magnetic resonance imaging (Magnetic Resonance Imaging - MRI) gives a spatial picture of the chemical composition of the substance.

Magnetic resonance imaging is, on the one hand, a sophisticated imaging method that is in clinical use worldwide. On the other hand, magnetic resonance imaging is also a very important examination tool for industry and research outside of the medical field. Applications are, for example, investigations of food, quality controls, preclinical investigations of medicines in the pharmaceutical industry or the investigation of geological structures such as pore sizes in rock samples for petroleum exploration.

The particular strength of magnetic resonance imaging stems from the fact that a large number of parameters influence nuclear magnetic resonance signals. By carefully and controlled changing these parameters, suitable experiments can be carried out that show the influence of the selected parameter.

Examples of relevant parameters are

Diffusion processes, probability density distributions of protons or a spin-lattice relaxation time.

In nuclear magnetic resonance imaging, atomic nuclei that have a magnetic moment are aligned by an externally applied magnetic field. The cores make a precession movement around the direction of the magnetic field with a characteristic angular frequency (Larmor frequency). The Larmor frequency depends on the strength of the magnetic field and the magnetic properties of the substance, especially the gyromagnetisehen

Constant γ of the nucleus. The gyromagnetic constant γ is a characteristic quantity for each atom type. The atomic nuclei have a magnetic moment μ = γ x p, where p denotes the angular momentum of the nucleus.

A substance to be examined, or a person to be examined, is used in the

Nuclear magnetic resonance imaging subjected to a uniform magnetic field. The uniform magnetic field is also referred to as the polarization field B ₀ and the axis of the uniform magnetic field as the z-axis. The individual magnetic moments of the spins in the tissue precess with their characteristic Larmor frequency around the axis of the uniform magnetic field.

A net magnetization M _z is generated in the direction of the polarization field, the randomly oriented magnetic components canceling each other in the plane perpendicular to this (xy plane). After the uniform magnetic field has been applied, an excitation field Bi is additionally generated. The excitation field Bx is polarized in the xy plane and has a frequency that is as close as possible to the Larmor frequency. As a result, the net magnetic moment M _z can be tilted into the xy plane, so that a transverse magnetic magnetization M _t arises. The transverse component of the magnetization rotates in the xy Level with the Larmor frequency.

By varying the excitation field over time, different temporal sequences of the transverse magnetic magnetization M _t can be generated. Various layer profiles can be realized in connection with at least one gradient field.

In medical research in particular, there is a need to obtain information about anatomical structures, spatial distributions of substances as well as about brain activity or in a broader sense information about blood flow or deoxyhemoglobin concentration changes in animal and human organs.

Magnetic resonance spectroscopy (MRS) enables the measurement of the spatial density distribution of certain chemical components in a material, especially in biological tissue.

Rapid magnetic resonance imaging (MRI) in conjunction with magnetic resonance spectroscopy (MRS) makes it possible to investigate local distributions of metabolic processes. For example, regional hemodynamics with changes in blood volumes and blood conditions as well as changes in metabolism in vivo depending on brain activity are determined, see: S. Posse et. al. : Functional Magnetic Resonance Studies of Brain Activation; Seminars in Clinical Neuropsychiatry, Vol. 1, No 1, 1996; p. 76-88.

An experimental investigation of hemodynamics is presented in: "The variability of human BOLD hemodynamic responses: Aguirre; Neurolmage, 1998, Vol. 8 (4), p. 360-369, further in “J. Raj pakse, F. Kruggel, DY von Cramon, Neuronal and hemodynamic responses from functional MRI time-series: A comutational model, in "Progress in Connectionist-Based Information Systems (ICONIP'97)" (N. Kasabov, R. Kozma, K. Ko, R. O'Shea, G. Coghill, T. Gedeon, Eds.), P. 30-34, Springer, Singapore, 1997 "and" Modeling Hemodynamic Response for Analysis of Functional MRI Time-Series: Jagath c , Rajapakse, Frithjof Kruggel, Jose M. Maisog and D. Yves von Cramon; Human Brain Mapping 6: 283-300, 1998 "with suggested Gauss and Poisson functions.

NMR imaging methods are used to select layers or volumes which, under the appropriate irradiation of high-frequency pulses and the application of magnetic gradient fields, deliver a measurement signal which is digitized and stored in the measurement computer as a one- or multi-dimensional field.

The desired image information is obtained (reconstructed) from the recorded raw data by means of a one-dimensional or multi-dimensional Fourier transformation.

A reconstructed slice consists of pixels, a Volume data set from voxels. A pixel (picture element) is a two-dimensional picture element, for example a square. The image is composed of the pixels. A voxel (volume pixel) is a three-dimensional volume element, for example a cuboid. The dimensions of a pixel are on the order of 1mm ² , those of a voxel of 1mm ³ . The geometries and dimensions can be variable.

Since a strictly two-dimensional plane can never be assumed for layered images for experimental reasons, the term voxel is often used here too, which means that the image planes have a thickness.

Functional nuclear magnetic resonance makes it possible to record dynamic changes and thereby monitor a process over time.

Functional magnetic resonance imaging (fMRI) is used to generate images that contain local changes.

It is also known to investigate neuronal activation using functional nuclear magnetic resonance or functional nuclear magnetic resonance imaging. Neuronal activation manifests itself in an increase in blood flow in activated brain areas, with a decrease in deoxyhemoglobin concentration. Deoxyhemoglobin (DOH) is a paramagnetic substance that reduces the magnetic field homogeneity and thus the Signal relaxation accelerates. Oxyhemoglobin has a magnetic susceptibility, which essentially corresponds to the tissue structure in the brain, so that magnetic field gradients across a boundary between blood containing oxyhemoglobin and the tissue are very small. If the DOH concentration drops due to brain activity that triggers increasing blood flow, the signal relaxation in the active areas of the brain is slowed down. The protons of hydrogen in water are primarily excited. Localization of brain activity is made possible by using an investigation using functional NMR methods, which measure the NMR signal with a time delay (echo). This is also known as susceptibility-sensitive measurement. The biological mechanism of action is known in the literature under the name BOLD effect (Blood Oxygenation Level Dependent - Effect) and leads to approx. 5% increases in susceptibility-sensitive magnetic resonance measurements with a field strength of a static, for example 1.5 Tesla strong magnetic field the image brightness in activated brain regions. Instead of the endogenous contrast agent DOH, other contrast agents can also occur which cause a change in the susceptibility.

The known methods require preliminary examinations in order to obtain correction data for the images.

The invention has for its object to develop a generic method that a high Has signal-to-noise ratio and with which a quick signal detection is possible.

According to the invention, this object is achieved in that at least one rephasing pulse is radiated in after the excitation pulse, that one or more spin echo excitations take place after the rephasing pulse, that an echo signal is detected after the spin echo excitation (s), and that after the Detection of the echo signal, one or more further spin echo excitations take place.

It is particularly advantageous here that a further rephasing pulse is radiated in after the further spin echo excitations.

A further improvement in the signal-to-noise ratio can be achieved in a particularly simple and advantageous manner in that exactly one spin-echo excitation takes place between the irradiation of the rephasing pulse and the detection of the echo signal.

To increase the signal-to-noise ratio, it is also advantageous that exactly one spin-echo excitation occurs between the detection of the echo signal and the irradiation of the further rephasing pulse.

It is expedient to carry out the method in such a way that the spin echo excitation (SEI), which takes place between the irradiation of the rephasing pulse and the detection of the echo signal, and the spin echo excitation (SE2), which takes place between the Detection of the echo signal and irradiation of the further rephasing pulse (RP2) takes place, have the same sign.

In order to record the relaxation time T ₂ as accurately as possible, it is expedient that spin echo signals (SE) and gradient echo signals (GE) are acquired alternately.

In order to eliminate external influences and thus to increase the measuring accuracy, it is expedient that all gradient echo signals with essentially the same phase position are encoded within a recording sequence.

To further reduce interference, it is advantageous that all spin-echo signals with essentially the same phase position are also encoded within a recording sequence.

Image information is advantageously obtained by repeating the recording sequence at least once.

To further improve the signal-to-noise ratio, it is expedient for consecutive recording sequences to have essentially the same phase positions.

It is also expedient that gradient echo signals, which were recorded at the same echo time T _E are displayed as one image.

It is also advantageous that spin echo signals, which were recorded at the same echo time T _E , are displayed as one image.

A particularly advantageous embodiment of the invention is characterized in that images in the form of an NX N matrix in a sequence sequence [GE (1,1), SE (2,1), GE (3,1), ..., GE1 , N, SE (2, N), GE (3, N), ..., SE (N, N)] can be detected.

Both the relaxation time T ₂ and the relaxation time T ₂ ^* can be reliably determined in the manner shown.

It is particularly advantageous here that the echo signals are rearranged so that echo signals that were recorded at the same echo time T are displayed as one image.

It is also expedient for N repetitions of the recording sequence to take place in the form of an N x N matrix.

It is also expedient to carry out the method in such a way that the image of the NX N matrix echo signals of the sequence sequence [GE (1,1), SE (2,1), GE (3,1), ..., GE1, N , SE (2, N), GE (3, N), ..., SE (N, N)]. The imaging method is preferably a spectroscopic echo planar imaging method, in particular a repeated two-dimensional echo planar imaging method, which consists of a repeated application of a two-dimensional echo planar image coding.

Spatial coding takes place in the shortest possible time, which can be repeated several times during a signal drop and is preferably 20 to 100 ms.

The repeated repetition of the echo planar coding shows a course of the signal drop in the sequence of reconstructed individual images during a signal drop.

The relaxation time T ₂ is quantized using a plurality of images which are recorded at different echo times. For a given matrix size, the number of images is limited by the properties of the measuring apparatus and the value of T ₂ . In order to generate quantitative images, data must therefore be adapted based on a limited number of data points that may be noisy.

Further advantages, special features and expedient developments of the invention result from the subclaims and the following representation of a preferred embodiment of the invention with reference to the drawing.

The drawing shows a sequence diagram of a preferred embodiment of a method according to the invention.

In Fig. 1 different components of the sequence are shown one above the other in chronological order. Individual lines, each extending in a horizontal line, reflect the time dependence of individual parameters. The individual parameters are arranged one above the other in such a way that simultaneous events are directly one above the other.

In the top line, the created or resulting field RF is shown in a line representing the time dependence of the field and corresponding to a pulse sequence.

Below the line representing the time dependence of the field, three lines are shown which represent a time dependence of gradient fields G _s , G _P and G _R.

The first gradient field G _s preferably extends in a main direction of a uniform magnetic field B ₀ . The magnetic field B ₀ is also referred to as the polarization field and the axis of the uniform magnetic field as the z-axis. A layer of a sample to be examined is selected by the gradient field G _s . So that's why Gradient field G _s also referred to as slice selection gradient. In order to be able to differentiate the different gradients better, the designation G _{s is used} below for the slice selection gradient.

Below the first gradient field G _s an additional gradient field is shown which corresponds to a ^'phase encoding gradient G _P. The phase coding gradient G _P is preferably along a y axis. It is used to select lines of an impulse space to be examined.

A third gradient field is shown below the further gradient field, which corresponds to a reading gradient G _R. The reading gradient G _R is preferably along an x-axis. It is used to read out signals, in particular echo signals from the sample to be examined. A reproduction of the signals in the form to allow an image to be with the read gradient G _R more, in FIG. 1 one above the other illustrated performed acquisition sequences.

In detail, the procedure is carried out as follows.

First, a net magnetization of the sample to be examined is excited by an excitation pulse AP, preferably a 90 ° pulse, shown in the top line on the left. The excitation pulse AP has a duration of, for example, 1 to 10 milliseconds, with one Duration of 2 to 3 milliseconds is particularly preferred.

During the excitation of the sample to be examined by the excitation pulse, a first slice selection gradient G _s l is applied to the sample, which leads to a partial dephasing of the transverse magnetization.

Following the excitation pulse, the spins are rephased by a further slice selection gradient G _s 2 with a changed sign.

A time integral of the further slice selection gradient G _s is preferably half as large as the time integral of the first slice selection gradient G _s applied during the excitation pulse. As a result, the further slice selection gradient G _s acts as a rephasing gradient.

A rephasing pulse RP1, preferably a 180 ° pulse, is then irradiated. It is expedient that the rephasing pulse RP1 is irradiated with a phase shift of 90 ° with respect to the excitation pulse AP. To select a layer, the is preferably used simultaneously

Slice selection gradient G _{s created} again. The layer is in particular the same layer as before.

After the first rephasing pulse RP1 there is a spin echo excitation. Instead of the one spin echo excitation shown, several spin echo excitations can also take place, but the case shown is a single one Spin-echo excitation between the rephasing pulse RP1 and a detection of the echo signal is preferred.

Following the spin echo excitation SE 2.1, there is a gradient echo excitation GE 3.1 with the opposite sign.

The echo signal is then recorded.

By determining the reading gradient G _R , the desired echo signals are read out.

After observing the echo signal, a further spin echo excitation takes place. Although a single spin echo excitation is preferred, several spin echo excitations can also take place. If several spin echo excitations occur, they preferably have opposite signs.

In the particularly preferred case of a single spin echo excitation before the echo signal is read out, it is particularly advantageous to carry out exactly one spin echo excitation even after the detection of the echo signal. It is particularly expedient that this spin echo excitation has the same sign as the first spin echo excitation.

A further rephasing pulse RP2 then takes place, whereupon an echo signal is generated and measured. A time development is recorded by repeating the individual measurements.

The sequence of the field shown in the top line runs as long as it corresponds to a desired number of sampling points of a T ₂ relaxation curve.

After the first recording sequence described above, the method is repeated as often as it corresponds to the number N _y of lines of a desired (N _y x N _x ) image matrix.

Spin echo signals (SE) and gradient echo signals (GE) are recorded.

In the simplest case, in which an area to be examined is to be represented as an N x N matrix, each recording sequence contains N excitation pulses. In this case, the recording sequence is repeated N times.

While the axes shown in FIG. 1 are time axes, actual image coding takes place along the columns. The number of rephasing pulses is preferably identical to the desired number of sampling points on the T ₂ relaxation curve.

The number of repetitions shown is expedient, but not necessary.

The invention provides for artifacts due to an essentially identical phase position between different ones Suppress recording sequences.

A rearrangement of echo signals is made possible by the invention. This ensures that only those echo signals that correspond to a desired echo time T _E are reproduced in a desired level of the pulse space. In this way, a convolution of the signal with a, T ₂ ^* decay function can be avoided. This is particularly useful if the impulse space is passed through from central areas to central areas up to - opposite to the first areas - external areas. This ensures that the spatial resolution remains high in the entire pulse space.

Data corresponding to central areas of the pulse space and coded with a 0-phase can be used for phase correction of measurement data of further recording sequences. In this way, preliminary measurements of the samples to be examined can be avoided.

Suppression of lipid signals is advantageous. Frequency-selective lipid presaturation is preferably used.

LIST OF REFERENCE NUMBERS

B ₀ - polarization field fMRI - functional magnetic resonance imaging

G _P - gradient field

G _R - gradient field

G _s - gradient field

MRI - Magnetic Resonance Imaging

MRS - magnetic resonance spectroscopy

M _t - net magnetic moment, transverse magnetic magnetization

M _z - net magnetization

T ₂ - relaxation time

T _e echo time

SE spin echo signals

Be spin echo excitation

SE ₂ spin echo excitation

GE gradient echo signals

RF Resulting field

Gs slice selection gradient

RPx rephasing pulse

RP ₂ further rephasing pulse

Claims

claims

1. A method for examining a sample with at least one imaging sequence, wherein at least one excitation pulse and several rephasing pulses are irradiated into the sample, so that at least one echo signal is generated and determined, characterized in that after the excitation pulse at least one rephasing pulse is irradiated that after the Rephasing pulse or one or more spin echo excitations take place that an echo signal is detected after the spin echo excitation (s) and that one or more further spin echo excitations take place after the detection of the echo signal.

2. The method of claim 1, d a d u r c h g e k e n n z e i c h n e t that after the further spin echo excitation or a further rephasing pulse is irradiated.

3. The method according to one or both of claims 1 or 2, so that exactly one spin echo excitation (SEI) takes place between the irradiation of the rephasing pulse and the detection of the echo signal.

4. The method according to one or more of the preceding claims, characterized in that that exactly one spin echo excitation (SE2) takes place between the detection of the echo signal and the irradiation of the further rephasing pulse (RP2).

5. The method according to claims 3 and 4, characterized in that the spin echo excitation (SEI), which takes place between the irradiation of the rephasing pulse and the detection of the echo signal and the spin echo excitation (SE2), which occurs between the detection of the echo signal and the irradiation of the further rephasing pulse (RP2) takes place, have the same sign.

6. The method according to one or more of the preceding claims, that the spin echo signals (SE) and gradient echo signals (GE) are acquired alternately with one another.

7. The method as claimed in claim 6, so that all gradient echo signals having essentially the same phase position are encoded within a recording sequence.

8. The method according to one or both of claims 6 or 7, d a d u r c h g e k e n n z e i c h n e t that all spin echo signals are encoded with substantially the same phase position within a recording sequence.

9. The method according to one or both of claims 7 or 8, characterized in that the recording sequence is repeated at least once.

10. The method as claimed in claim 9, which also means that successive recording sequences have essentially the same phase positions.

11. The method according to one or more of claims 7 to

10, characterized in that gradient echo signals, which were recorded at the same echo time T _E , are displayed as an image.

12. The method according to one or more of claims 8 to

11, characterized in that spin echo signals, which were recorded at the same echo time T _E , are displayed as an image.

13. The method according to one or more of claims 6 to

12, characterized in that an image in the form of an NX N matrix in a sequence [GE (1,1), SE (2,1), GE (3,1), ..., GE1, N, SE (2nd , N), GE (3, N), ..., SE (N, N)] are recorded.

14. The method according to one or more of the preceding claims, characterized in that during the excitation of the sample to be examined by the excitation pulse on the Sample a layer selection gradient (G _s l) is present.

15. The method according to claim 14, characterized in that after the application of the first slice selection gradient (G _s l) a further slice selection gradient (D _s 2) is applied with a changed sign.

16. The method according to claim 15, characterized in that the further slice selection gradient (G _s 2) has a time integral which is substantially half as large as a time integral of the first slice selection gradient (G _s l).