WO2002075347A1

WO2002075347A1 - Method for analysing a sample by producing an imaging sequence

Info

Publication number: WO2002075347A1
Application number: PCT/DE2002/000712
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: DE10112880A1

Abstract

The invention relates to a method for analysing a sample, comprising at least one imaging sequence, whereby at least one excitation pulse (AP) and at least one rephase pulse (RP) are transmitted into said sample. The inventive method is characterised in that at least one gradient-echo-signal (GES1, GES2, GES3) is detected after the excitation pulse (AP), and at least one rephase pulse (RP1) is subsequently transmitted.

Description

Method for examining a sample by generating an imaging sequence

description

The invention relates to a method for examining a sample with generation of at least one imaging sequence, wherein at least one excitation pulse (AP) and at least one rephasing pulse (RP) are irradiated into the sample.

The term "sample" is meant in its broadest meaning in the present case 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 nuclei perform a precession movement with a characteristic angular frequency around the direction of the magnetic field (Larmor- Frequency). The Larmor frequency depends on the strength of the magnetic field and on the magnetic properties of the substance, especially the gyromagnetic constant γ of the core. The gyromagnetic constant γ is a characteristic quantity for each atom type. The atomic nuclei have a magnetic moment μ = γ xp, 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 applying the uniform magnetic field, an excitation field B _x is additionally generated. The excitation field Bi 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 plane 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 examine 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 image consists of pixels, a volume data set consists of voxels. One 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. The neuronal activation manifests itself in an increase in blood flow in activated brain areas, with a decrease in the deoxyhemoglobin concentration. Deoxyhemoglobin (DOH) is a paramagnetic substance that reduces magnetic field homogeneity and thus accelerates signal relaxation. Oxyhemoglobin has one 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 time). 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 has a high signal-to-noise ratio and with which a fast Signal acquisition is possible.

According to the invention, this object is achieved in that at least one gradient echo signal is detected after the excitation pulse, and that at least one rephasing pulse RPi is then irradiated.

It is particularly advantageous here that at least one spin echo signal SE is recorded after the rephasing pulse.

The method is advantageously carried out such that at least one further rephasing pulse is radiated in after the detection of the spin echo signal.

In order to record as much data as possible, it is advantageous that between the irradiation of the excitation pulse and the first rephasing pulse, several gradient echo signals with different signs are recorded.

A particularly effective detection can take place in that gradient-echo signals with a positive sign and gradient-echo signals with a negative sign alternate with one another.

Advantageously, gradient echoes with a sequence sequence [GE (1,1), GE (2,1), GE (3,1), ..., GE (N, 1)] are used to acquire the gradient echo signals. be generated. It is also advantageous to carry out the method in such a way that after the first rephasing pulse, spin echo signals with the sequence sequence [SE (1,1), SE (2,1), ..., SE (N, 1)] are generated become.

To generate image information, it is expedient for N repetitions of the recording sequence to take place in order to record at least one image in the form of an N x N matrix.

Images which contain information about the relaxation time T ₂ as well as about the relaxation time T ₂ ^* can be achieved according to a particularly preferred embodiment of the invention in that in the image of the N x N matrix echo signals in a sequence [GE (1 , 1), GE (2,1), GE (3,1), ..., GE (N, 1), SE (1,1), SE (2,1), SE (3, 1), ..., SE (N, 1), GE (1,2), GE (2,2), GE (3,2), ..., SE (1,2), SE (2,2), SE (3,2), ..., SE (N, 2), ... GE (1, N), GE (2, N), GE (3, N), ..., SE (1, N), SE (2, N), SE (3, N), ..., SE (N, N)] ....

It is particularly advantageous here that the spin echo signals are rearranged in such a way that spin echo signals which were recorded at the same echo time T _E 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 spin echo signals of the Sequence sequence [SE (1,1), SE (1,2), SE (1, 3), ... SE (1, N)] corresponds.

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.

Other advantages, special features and practical Further developments of the invention result from the subclaims and the following illustration of a preferred exemplary embodiment 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. The gradient field G _s creates a layer selected a sample to be examined. Therefore, the gradient field G _{s is} also referred to as a 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.

A further gradient field is shown below the first gradient field G _s , which corresponds to a phase coding 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. In order to enable the signals to be reproduced in the form of an image, the reading gradient G _{R is used} to display a plurality, shown one above the other in FIG. Recording sequences carried out.

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 from, for example, 1 to 10 milliseconds, a duration of 2 to 3 milliseconds being 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 Gs ¹ applied during the excitation pulse. As a result, the further slice selection gradient G _s ² acts as a rephasing gradient.

Gradient echo pulses GE 1.1, GE 2.1, GE 3.1, ... are then generated. The relaxation time T _{2 is} determined here.

A rephasing pulse RPi, preferably a 180 ° pulse, is then irradiated. It is expedient that the rephasing pulse RPi 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 RPi there is a spin echo excitation SE (l, i). Instead of the one spin echo excitation SE (1, i) shown, several spin echo excitations can also take place, but the illustrated case of a single spin echo excitation is preferred.

Following the spin echo excitation SE (l, i), there is a gradient echo excitation G _B (4) with the opposite sign.

An echo signal is then recorded.

After observing the echo signal, a further spin echo excitation SE (2, i) takes place after each gradient echo excitation G _s (4). 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. Another rephasing pulse then takes place, whereupon an echo signal is generated and measured.

The further course of the sequence of the field shown in the top line continues 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.

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, an 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 to be suppressed by essentially the same phase position between different 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. This avoids convolution of the signal with a T ₂ ^* decay function. 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

AP excitation pulse

B ₀ - polarization field

Bl excitation magnetic 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

RPi 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 (AP) and at least one rephasing pulse (RP) are irradiated into the sample, characterized in that after the excitation pulse (AP) at least one gradient echo signal (GESl , GES2, GES3) is detected and that at least one rephasing pulse (RPI) is then irradiated.

2. The method according to claim 1, which also means that after the rephasing pulse (RPI) at least one spin echo signal is detected.

3. The method according to claim 2, so that at least one further rephasing pulse (RP2) is radiated in after the detection of the spin echo signal (SES).

4. The method according to one or more of the preceding claims, d a d u r c h g e k e n n z e i c h n e t that between the irradiation of the excitation pulse

(AP) and the first rephasing pulse (RPI) a plurality of gradient echo signals with different signs are recorded.

5. The method of claim 4, d a d u r c h g e k e n n z e i c h n e t that gradient echo signals with a positive sign (GES2) and gradient echo signals with a negative sign

(GESl, GES3) take turns.

6. The method according to claim 5, characterized in that for detecting the gradient echo signals gradient echoes with a sequence sequence [GE (1,1), GE (2,1), GE (3,1), ..., GE (N, 1)]. be generated.

7. The method according to one or more of the preceding claims, characterized in that after the first rephasing pulse (RPI) spin echo signals with the sequence sequence [SE (1,1), SE (2,1), ..., SE ( N, 1)] are generated.

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 for the recording of at least one image in the form of an N x N matrix N repetitions of the recording sequence take place.

9. The method according to claim 8, characterized in that in the image of the N x N matrix echo signals in a sequence [GE (1,1), GE (2,1), GE (3,1), ..., GE (N, 1), SE (1,1), SE (2,1), SE (3,1), ..., SE (N, 1), GE (1,2), GE (2,2 ), GE (3,2), ..., SE (1,2), SE (2,2), SE (3,2), ..., SE (N, 2), ... GE ( 1, N), GE (2, N), GE (3, N), ..., SE (1, N), SE (2, N), SE (3, N), SE (N, N)] ... become.