DE10112879A1 - Method for examining a sample by generating and determining echo signals - Google Patents

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

The invention relates to a method for examining a Sample with at least one imaging sequence, in which Sample at least one excitation pulse and several Rephasing pulses are radiated, and at least one Echo signal arises and is determined.

The term "sample" is used in this case meant broadest meaning and includes living and not living material.

Various methods are known in which one Sample using one excitation pulse and several Rephasing pulses is examined.

In nuclear magnetic resonance imaging it is known to an excitation of echo signals from a sample, information to get over the sample.

The generic method is in the Nuclear magnetic resonance imaging preferably used to spectroscopic information or image information to get about a substance. A combination of the nuclear magnetic resonance imaging using techniques of Magnetic resonance imaging  MRI) gives a spatial picture of the chemical Composition of the substance.

Magnetic resonance imaging is one sophisticated imaging method used worldwide in clinical Use is. On the other hand is magnetic Resonance imaging also outside of medical Area is a very important investigation tool for Industry and research. Applications are, for example Food inspections, quality controls, preclinical investigation of drugs in the pharmaceutical industry or the investigations of geological structures such as pore sizes in rock samples for petroleum exploration.

The special strength of magnetic resonance imaging stems from the fact that very many parameters affect nuclear magnetic resonance signals. By changing this carefully and in a controlled manner Parameters appropriate experiments can be carried out which 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.

Nuclear magnetic resonance imaging uses atomic nuclei, which possess a magnetic moment through an external one applied magnetic field aligned. The cores lead 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 magnetic properties the substance, especially the gyromagnetic Constant γ of the nucleus. The gyromagnetic constant is γ a characteristic for each atom type. The Atomic nuclei have a magnetic moment µ = γ × p, where p denotes the angular momentum of the nucleus.

A substance to be examined, or a person to be examined, is subjected to a uniform magnetic field in nuclear magnetic resonance imaging. The uniform magnetic field is also referred to as the polarization field B _o 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 ₁ is additionally generated. The excitation field B ₁ 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.

This exists particularly in medical research Need, information about anatomical structures, spatial distributions of substances as well as over the Brain activity or in a broader sense information about Blood flow or deoxyhemoglobin concentration changes in to obtain animal and human organs.

Magnetic resonance spectroscopy (MRS) enables the measurement the spatial density distribution of certain chemical Components in one material, especially biological Tissue.

Fast magnetic resonance imaging (Magnetic Resonance Imaging - MRI) in connection with Magnetic resonance spectroscopy Spectroscopy - MRS) makes it possible to local distributions of metabolic processes. For example becomes a regional hemodynamics with changes in Blood volumes and blood conditions as well as changes in Metabolism in vivo depending on one Brain activity determined, see: 5. Posse et. al .: Functional Magnetic Resonance Studies of Brain Activation;  Seminars in Clinical Neuropsychiatry, Vol. 1, No 1, 1996; p. 76-88.

An experimental study of hemodynamics is presented in: "The variability of human BOLD hemodynamic responses: Aguirre; Neurolmage, 1998, Vol. 8 (4), p. 360 369, further in "J. Rajapakse, F. Kruggel, D. Y. 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 from Cramon; Human brain mapping 6: 283- 300, 1998 "with proposed Gauss and Poisson Functions.

Using NMR imaging methods, layers or Volumes selected under the appropriate irradiation of high frequency pulses and the application of magnetic Deliver a measurement signal to gradient fields, which digitized and as a one- or multi-dimensional field is stored in the measuring computer.

From the recorded raw data is a by or multi-dimensional Fourier transform the desired Image information obtained (reconstructed).

A reconstructed slice image consists of pixels, a volume data set consists of 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 of the order of 1 mm ² , those of a voxel of 1 mm ³ . The geometries and dimensions can be variable.

Because for experimental reasons never with slice images be assumed from a strictly two-dimensional level the term voxel is often used here, which says that the image planes have a thickness.

With functional nuclear magnetic resonance it is possible to capture dynamic changes and thereby to monitor processes over time.

With functional nuclear magnetic resonance imaging (functional Magnetic Resonance Imaging - fMRI) images are generated, that involve local changes.

It is also known, with functional nuclear magnetic resonance, or with functional nuclear magnetic resonance Imaging to investigate neuronal activation. The neuronal activation manifests itself in an increase in Blood flow in activated areas of the brain, causing a Decrease in deoxyhemoglobin concentration comes. Deoxyhemoglobin (DOH) is a paramagnetic substance which reduces the magnetic field homogeneity and thus the  Signal relaxation accelerates. Oxyhemoglobin has one magnetic susceptibility, which is essentially the Tissue structure in the brain corresponds to that of magnetic Field gradients across a boundary between oxyhemoglobin-containing blood and tissue are very small. The DOH concentration drops due to one increasing blood flow triggering brain activity, so signal relaxation in the active areas of the brain slowed down. The protons are primarily excited of hydrogen in water. A localization of Brain activity is made possible by an examination is used with functional NMR methods, which the Measure the NMR signal with a time delay (echo time). This is also called a susceptibility sensitive measurement designated. The biological mechanism of action is in the Literature under the name BOLD effect (Blood Oxygenation Level Dependent Effect) known and leads susceptibility sensitive magnetic Resonance measurements at a field strength of a static, for example 1.5 Tesla strong magnetic field, up to approx. 5% increases in image brightness in activated Brain regions. Instead of the endogenous contrast agent DOH other contrast agents can also occur that change evoke in susceptibility.

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

The invention is based on the object to develop generic method that a high  Has signal-to-noise ratio and with which a fast Signal acquisition is possible.

According to the invention, this object is achieved in that after the excitation pulse at least one rephasing pulse is irradiated that after the rephasing pulse or more spin echo suggestions are made after that or an echo signal is detected from the spin echo excitations, and that after the detection of the echo signal one or several further spin echo excitations take place.

It is particularly advantageous that after or another spin echo excitation Rephasing pulse is irradiated.

Another improvement in the signal-to-noise ratio can be done in a particularly simple and advantageous manner achieve that between the irradiation of the Rephasing pulse and the detection of the echo signal exactly a spin echo excitation takes place.

It is also used to increase the signal-to-noise ratio advantageous that between the detection of the echo signal and the irradiation of the further rephasing pulse exactly a spin echo excitation takes place.

It is advisable to carry out the method in such a way that the Spin echo excitation (SE1) between the radiation of the Rephasing pulse and the detection of the echo signal takes place and the spin echo excitation (SE2), which between the  Detection of the echo signal and irradiation further Rephasing pulse (RP2) takes place, the same sign exhibit.

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

For the elimination of external influences and thus for It is advisable to increase the measuring accuracy all gradient echoes within a recording sequence Encoded signals with essentially the same phase position become.

It is to further reduce interference advantageous that all within a recording sequence Spin echo signals with essentially the same phase position be encoded.

Image information is obtained advantageously in that the recording sequence is repeated at least once.

To further improve the signal-to-noise ratio it is appropriate that successive Acquisition sequences essentially the same phase exhibit.

Furthermore, it is expedient that gradient echo signals, which were recorded with the same echo time T _E , are displayed as an 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 by the fact that images in the form of an N × N Matrix in a sequence [GE (1,1), SE (2,1), GE (3.1),. , ., GE1, N, SE (2, N), GE (3, N),. , ., SE (N, N)] be recorded.

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 rearranged so that echo signals, which at a same echo time TE were taken as one picture being represented.

Furthermore, it is expedient that in order to take an image in Form an N × N matrix N repetitions of the Recording sequence take place.

It is also expedient to carry out the method in such a way that the image of the N X 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, especially a repeated two-dimensional echo Planar imaging method, which consists of a repeated Use of two-dimensional echo planar image coding consists.

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

By repeating the echo planar coding several times is a course of the Signal drop in the sequence of reconstructed Single images shown.

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 presentation of a  preferred embodiment of the invention based on the Drawing.

The drawing shows a sequence diagram of a preferred one Implementation 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.

The top line is the created, respectively resulting field RF in a the time dependency of the Field reproducing, corresponding to a pulse sequence, Line shown.

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 _o . The magnetic field B _o 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. The gradient field G _S is therefore also referred to as a slice selection gradient. In order to be able to better differentiate the different gradients, 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, a plurality of recording sequences, shown one above the other in FIG. 1, are carried out with the reading gradient G _R.

In detail, the procedure is carried out as follows. First, there is a net magnetization of the examining sample by one in the top line Excitation pulse AP shown on the left, preferably one 90 ° pulse, excited. The excitation pulse AP has a duration from, for example, 1 to 10 milliseconds, where 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 1 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 slice, the slice selection gradient G _S is preferably applied again at the same time. 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 shown Spin echo excitation also includes several spin echo excitations done, but the case shown is 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 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, another is done Spin echo excitation. Although a single spin echo excitation is preferred, several spin echo excitations can also respectively. If there are several spin echo excitations, they are preferably opposite to each other Sign.

In the particularly preferred case of a single spin It is echo excitation before the echo signal is read out particularly advantageous, even after detecting the Echo signal to make exactly one spin echo excitation. It is particularly useful that this spin echo excitation has the same sign as the first spin echo Stimulation.

This is followed by another rephasing pulse RP2, whereupon an echo signal is generated and measured.  

By repeating the individual measurements, a Time development recorded.

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 × N _x ) image matrix.

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

In the simplest case, that is an area to be examined each is to be represented as an N × N matrix Recording sequence N excitation pulses. In this case the Recording sequence 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 appropriate however not necessary.

The invention provides for artifacts in the Essentially the same 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. 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.

Corresponding central areas of the impulse space, with a 0-phase coded, data can be for a Phase correction of measurement data of further recording sequences be used. This allows preliminary measurements of the investigating samples can be avoided. Suppression of lipid signals is advantageous. there is preferably a frequency selective lipid presaturation applied.  

LIST OF REFERENCE NUMBERS

B ₀

polarization field
fMRI functional magnetic resonance imaging
G _P

gradient
G _R

gradient
G _S

gradient
MRI Magnetic Resonance Imaging
MRS magnetic resonance spectroscopy
M _t

Net magnetic moment, transverse magnetic magnetization
M _z

net magnetization
T ₂

relaxation
T _e

echo time
SE spin echo signals
SE ₁

Spin echo excitation
SE ₂

Spin echo excitation
GE gradient echo signals
RF Resulting field
Gs slice selection gradient
RP ₁

rephasing pulse
RP ₂

further rephasing pulse

Claims (16)

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 one or more spin echo excitations take place after the rephasing pulse, that an echo signal is detected after the spin echo excitation and that one or more further spin echo excitations take place after the detection of the echo signal. 2. The method according to claim 1, characterized characterized that after the or another spin echo excitation Rephasing pulse is irradiated. 3. The method according to one or both of claims 1 or 2, characterized in that between the irradiation of the rephasing pulse and the detection of the echo signal exactly one spin echo Excitation (SE1) takes place. 4. Method according to one or more of the preceding Claims, characterized,  that between the detection of the echo signal and the Irradiation of the further rephasing pulse (RP2) exactly one spin echo excitation (SE2) takes place. 5. The method according to claims 3 and 4, characterized characterized that the spin echo Excitation (SE1) between the irradiation of the Rephasing pulse and the detection of the echo signal takes place and the spin echo excitation (SE2) that between the detection of the echo signal and the irradiation of the further rephasing pulse (RP2) takes place, the same Show signs. 6. Method according to one or more of the preceding Claims, characterized, that spin echo signals (SE) and gradient echo Signals (GE) are recorded alternately. 7. The method according to claim 6, characterized characterized that within a Recording sequence of all gradient echo signals with im Essentially the same phase position can be encoded. 8. The method according to one or both of claims 6 or 7, characterized in that all spin echo signals within a recording sequence encoded with essentially the same phase position become. 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 according to claim 9, characterized characterized that on each other following recording sequences essentially the same Have 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 N × N matrix in one Sequence sequence [GE (1,1), SE (2,1), GE (3,1),. , ., GE1, N, SE (2, N), GE (3, N),. , ., SE (N, N)] are recorded. 14. The method according to one or more of the preceding claims, characterized in that a slice selection gradient (G _S 1) is present on the sample during the excitation of the sample to be examined by the excitation pulse. 15. The method according to claim 14, characterized in that after the application of the first slice selection gradient (G _S 1), 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 1).