DE102007045172B3 - Magnetic resonance tomography system operating method for patient, involves carrying out physiological synchronized compensation-measurement-sequence before metrological detection of spin-echo-signals for determination of organ position - Google Patents

Abstract

The method involves impressing susceptibility dependency by echo-signals before detection of the echo-signals by an evolution period (TI) that is additionally included between a high frequency-stimulation pulse and a high frequency-rephasing pulse for developing and dephasing of transversal magnetization. A physiological synchronized compensation-measurement-sequence is carried out before metrological detection of spin-echo-signals for determination of an organ position. The spin-echo-signals are evaluated or rejected as a measuring signal depending on the determined organ position.

Description

The The invention relates to a method for operating a magnetic resonance tomograph to the spatially resolved Spin resonance measurement on a living object, which is in a static Magnetic field B0 is arranged, resulting in an alignment of the Spins of the object and a longitudinal net magnetization Mz along the magnetic field direction Z results, by means of at least a high frequency excitation pulse in resonance a deflection of the Net magnetization around a desired flip angle is generated in which a transverse magnetization component Mxy created or changed whose T2 * relaxation is susceptibility-weighted for performance Measurements is recorded, the location of the thereby metrologically detected Volume element (voxel) of the object-by several, at least temporarily superimposed on the static magnetic field B0 Gradient magnetic fields is determined and wherein the start and / or Preparation sequences of a measurement to a detected physiological Movement is / are synchronized.

magnetic Resonance Imaging are well known in the art and essentially include Gradient coils for generating a plurality, in particular orthogonal Magnetic fields, in particular in a Cartesian coordinate system, where usually a coil or coil arrangement is provided to provide a strong static Magnetic field B0 along a Z-direction of the selected coordinate system to produce, for example, with field strengths of several Tesla. For this are usually superconducting coil arrangements used.

Perpendicular as well as parallel to the magnetic field direction B0 thus generated about that In addition, further coils or coil arrangements provided to the static magnetic field perpendicular as well as at least one parallel to it To generate magnetic field, these magnetic fields in particular as Gradientenmagneffelder be formed, d. H. ie magnetic fields, their magnetic field strength over one of the coordinate axes changes. By the overlay is achieved so that the resonance frequency or precession frequency of Spins dependent changes from the variable location magnetic field, resulting in spatially resolved measurements can be realized.

The Measuring principle to use such a magnetic resonance tomograph sectional images Being able to achieve through living objects is based on that the spins, especially hydrogen, of the living object within the static magnetic field B0, first along the magnetic field direction and thus align along the Z axis.

By a high-frequency excitation pulse which is at the so-called Lamorfrequenz the spins is tuned and usually basically programmable at least in the amplitude and the envelope is, a deflection of the spins can take place from their equilibrium, such that the net magnetization Mz generated in the homogeneous magnetic field B0 the so-called flip angle is deflected, so that a transverse Magnetizing component Mxy within the XY plane of the selected coordinate system is present. Here, the flip angle is essentially of the RF excitation pulse dependent and thus also application-specific programmable.

The thus generated transverse magnetization component Mxy is timed not stable and relaxed due to different processes with different ones Relaxation times, whereby the different processes are superimposed.

the relevant Those skilled in the art are well aware of these processes and are referred to as T1, T2 and T2 * relaxations designated. Here, the T1 relaxation corresponds to that at the magnetization component Mxy again in the direction of the Z-axis back flips, whereas T2 relaxation is due to dephasing of the individual Spins is based within the XY plane and leads to a weakening of the Signal leads, which is based on the radiation of an electromagnetic wave due to the rotation of the transverse magnetization component in the XY plane.

Farther are superimposed on the T2 signal dephasing dephasings by macroscopic and microscopic magnetic field inhomogeneities in the examined tissue or the examined object, that is to say on differences based in the magnetic susceptibility of the tissue. The effective relaxation rate, both T2 relaxation and susceptibility effects is called T2 * relaxation.

Next the programming of at least temporarily acting gradient magnetic fields to achieve a spatial resolution in the examined object during measurement preparation and / or signal readout, what the expert in principle adequately is known, there are various possibilities, so-called preparatory and measurement sequences of pulses for driving the respective coils (magnetic field coils and / or high frequency coils) to selectively select the T1-, T2- or T2 * -Relaxationszeiten make metrologically detectable. In this context is also of a corresponding weighting in terms of T1, T2 or T2 * spoken during the measured value acquisition.

Susceptibility-weighted measurements are thereby of particular advantage, since these are based on magnetic field inhomogeneities within the examined tissue and thus are very sensitive to the various possible tissue types. Measurements on the other hand are very susceptible to errors, so that such measurements are often overlaid by metrological artifacts, which make evaluation of the images more difficult.

• – ein hohes Signal-Rausch-Verhältnis
• – eine möglichst vollständige Aufnahme des gesamten Herzens
• – Unabhängigkeit der Bildgebungstechnik von zeitlichen Faktoren wie Aufnahmegeschwindigkeit und eingeschränkter Wiederholbarkeit
• – die Ausschaltung unerwünschter Einflüsse durch Atembewegung, Herzbewegung, Pulsation und Blutfluss. Deshalb muss die Bildgebung zwingend mit der physiologischen Bewegung synchronisiert werden.
• – Quantifizierbarkeit suszeptibilitätsbedingter Gewebezustände.

Central challenges of magnetic resonance (MR) imaging of the cardiovascular system include unimpaired reproducibility, spatial resolution in the millimeter range and, in particular, the compelling need for detailed geometric imaging of the anatomy. In addition, a susceptibility-weighted MR imaging of the cardiovascular system requires imaging techniques that can reliably record, display or quantify very small susceptibility-related signal differences between normal and abnormal tissue types. This requires:

• - a high signal-to-noise ratio
• - a complete recording of the entire heart
• - Independence of the imaging technique of temporal factors such as recording speed and limited repeatability
• - The elimination of unwanted influences by respiratory movement, heart movement, pulsation and blood flow. Therefore, imaging must be synchronized with the physiological movement.
• - Quantification of susceptibility-related tissue states.

Current clinical MR imaging techniques use either intrinsic contrast mechanisms (eg, T1 and T2 contrast) or differences in the kinetics of contrast passage to delineate between healthy and diseased tissue. In contrast-agent-based studies, extravascular paramagnetic contrast agents (eg Gd-DTPA) are predominantly intravenously injected with bolus. Side effects of contrast agents are the subject of current discussion on the clinical use of contrast-based MRI examinations ( 12 . 13 ). Depending on the nature and concentration of the contrast agent, the contrast agent passage causes a shortening of the T1 or T2 relaxation times, which leads to changes in the signal intensity. To image the very short passage of the contrast agent bolus, ultra-fast contrast enhanced imaging techniques are used to meet the imposed time requirements of 1 or 2 cardiac cycles (RR interval) of temporal resolution.

One Another intrinsic contrast mechanism is based on oxygen saturation of the blood. This method uses the dependence of the MR signal intensity on the degree of oxygen saturation of the blood, also called the BOLD effect (GOLD: blond oxygenation level dependent). Functional GOLD techniques are for Clinical applications are therefore attractive because they do not require contrast agent injection because blood itself acts as a tracer. Thus, in place exogenous contrast agent blood as direct information carrier (endogenous Contrast agent) for imaging the cardiovascular system become. However, it can cause image artifacts due to blood flow come.

conventional Contrast-enhanced MR imaging is due to the above high speed requirements either not synchronized or required with the cardiac cycle a temporal resolution from 1-2 Cardiac cycles (R-R intervals) to track contrast passage. This speed requirement is at the expense of the signal-to-noise ratio (SNR) as well as the contrast-to-noise ratio (CNR) and reduced the realizable spatial resolution per unit time to about 3 to 4 mm pixel edge length.

Consequently is the diagnostic image quality the conventional, contrast-enhanced, and cardiac cycle synchronized bolus techniques despite numerous device technology and MR methodological innovations borderline and their reliable use clinically questionable.

aggravating In addition, the administration of contrast agents in combination with the use of small image matrices very pronounced image artifacts (Gibbs ringing, susceptibility artifacts) can cause. In addition, conventional techniques are due to the time restrictions only on a few image levels (approx. 3-4 Layers). In addition, the images are influenced by respiratory movement, since the image capture over a period of over 20 s takes place while the breath hold phase is over about 15-20 s extends. Another limitation is that repeat studies only after complete leaching of the contrast agent can be carried out and thus waiting times of about 20 minutes, due to the half-life the contrast agent residence time, are necessary. Also is the Number or chronological sequence of repeat examinations limited by the contrast agent dose.

• – Bildverzerrungen und Signauslöschungen, die auf die Empfindlichkeit gegenüber B0-Inhomogenitäten zurückzuführen sind, bewirken eine verzerrte und damit räumlich ungenaue Abbildung der Anatomie, insbesondere an Übergängen zwischen Lunge und Myokardgewebe.
• – Die begrenzte räumliche Auflösung (typischerweise 4×4×5) mm³ Pixelgröße) erschweren eine detailgetreue Abbildung kleiner pathologischer Areale.
• – Sättigungs- und Blutflusseffekte überlagern die GOLD-Information und beeinträchtigen stark die Reproduzierbarkeit der Bildgebung.

GOLD MRI, which uses blood as an intrinsic contrast agent, requires a corresponding susceptibility weighting - also called T2 * weighting - conventionally realized by majority with fast echo planar imaging (EPI) or segmented gradient echo techniques. however The intrinsic properties of these techniques affect the image quality and thus the reproducibility of GOLD-weighted perfusion imaging by e.g. For example, the following negative properties:

• - Image distortions and signal cancellations due to sensitivity to B0 inhomogeneities cause a distorted and thus spatially inaccurate imaging of the anatomy, especially at transitions between lung and myocardial tissue.
• - The limited spatial resolution (typically 4 × 4 × 5) mm ³ pixel size) complicate a detailed reproduction of small pathological areas.
• - Saturation and blood flow effects overlay the GOLD information and severely affect imaging reproducibility.

These Störmerkmale are at high magnetic field strengths from Z. B. 3.0 T or more stronger as pronounced on conventional 1.5 T MR systems and require new methodological approaches.

A method for performing susceptibility-weighted measurements with a magnetic resonance tomograph is e.g. B. from the document US 5,300,886 A. known. Here, the weighting is achieved by varying the position of the induced echo, working with gradient echoes. This known method is adversely affected by the aforementioned effects and artifacts.

task The invention is a method for operating a magnetic resonance tomograph to provide the acquisition susceptibility weighted Allows measurements, without the attendant disadvantages of known imaging techniques to show that usually for the suszeptibilitätsgewichteten Measurements are used on magnetic resonance imaging, such. Eg echo planar imaging or the segmented gradient-echo technique. It is still the task with such a procedure and the programmed Sequences for the control of RF pulses and the coil arrangements to achieve an imaging that allows artifacts through blood flow to suppress, Allows measurements of free breathing of a living object, Measurements synchronous to the cardiac cycle allowed, avoids image distortion and achieves a high imaging speed, in particular with the achievement of another, so-called fat suppression.

So should it continue to be possible preferably with the inventive method, to realize an imaging technique that uses high-resolution 2D or 3D representations, in particular of the heart and the cardiovascular system with high and diagnostic usable image quality implements a wide range of applications. Especially this should include applications such as For example, the study of the endothelial function of vessels that Recognize stress-induced angina pectoris, imaging and quantification the iron content of myocardial tissue as well as the mapping of myocardial tissue oxygen saturation of the blood, the differentiation between arteries and veins and the detection of myocardial perfusion deficits.

According to the invention this Task solved by that in preparation for metrological detection with a not layer-selective RF pulse globally inverts the net magnetization Mz and subsequently with a slice-selective RF pulse in the slice to be imaged of the object the original one Net magnetization Mz is restored. Here is the layer selectivity achieved by at least temporarily additional activation of the gradient magnetic field in the Z direction of the homogeneous B0 field.

hereafter is waited for a first evolution time, in which the blood in the layer to be imaged is replaced by blood, its magnetization was inverted with the first non-slice selective RF pulse. There is a possibility, this to choose first evolutionary time in such a way that after the exchange of the blood in the observed layer of the living object, the magnetization component Mz of the in the Magnetization of inverted blood is relaxed in time, that this Mz component takes the amount zero.

Becomes at this moment the measurement by irradiation of an RF excitation pulse Started, so creates a spin flip, so carries the flowing blood at the site of investigation, d. H. the selected one Volume element (voxel) of the object, not the recorded measurement signal at.

in this connection the said first evolution time can be waited unused or it may in a special embodiment of the method this Time can also be used to other desired sequences of high-frequency pulses or to switch gradient magnetic fields.

It is then further provided according to the invention, that the metrological detection of multiple spin-echo signals from a desired Volume element (voxel) of the object takes place, which after an RF excitation pulse by a Sequence of mutually equidistant high-frequency rephasing pulses be generated.

The detection of several spin-echo signals per se is well known to the person skilled in the art and is based on the fact that after an RF excitation pulse for generating a spin flip around a desired flip angle due to a plurality of equidistant high frequency rephasing pulses are irradiated into the object to reverse dephasing of the transverse Mxy magnetization components, that is to produce a rephasing. In this case, according to the known spin-echo technique, a first echo signal is produced after twice the time between the excitation pulse and the rephasing pulse, when all previously dephased spins are again in phase. This results in a significant electromagnetic signal in the measuring coil, the so-called spin-echo signal.

there the spin-echo technique has been developed in the art, just to exclude susceptibility influences, since by reversing the dephasing effect of the magnetization component Mxy susceptibility effects are suppressed and so with the so-called spin-echo technique or the fast spin echo technique, which generates multiple spin echoes be, basically pure T2 relaxations can be observed.

These Technology that is not sensitive to susceptibility differences is known, but the invention is currently exploiting, since this provides a better, especially distortion-free, image quality. For this purpose, it is provided according to the invention for the Metrological acquisition of several spin-echo signals from one desired Volume element of the object after an RF excitation pulse first a additional wait for the second evolution time (tau) before the first RF rephasing pulse takes place to record spin echo signals. So this is one way Time between the RF excitation pulse and the first rephasing pulse generated so that the total time between RF pulse and first rephasing pulse over that usual time goes beyond these pulses, used in the usual spin-echo technique is going to spin the echoes metrologically to capture.

hereby can be achieved according to the invention, that by inhomogeneities the magnetic field generated dephasing, d. H. thus Suszeptibilitätsbedingte Develop dephasings and thus contribute to the measured signal, although in itself the spin-echo technique Susceptibility is independent. With the procedure thus carried out So you can take advantage of the advantages of the spin-echo technique offers, namely For example, the distortion-free recordings and in combination with the aforementioned preparation sequence blood artifacts simultaneously avoid and a susceptibility weighting introduce Furthermore is it inventively provided before the metrological detection of the echo signals a physiological synchronized compensation measurement sequence to determine one by the respiratory movement of the living object conditional organ position. Therefor can, for example, by appropriate programming of Gradientmagneffelder metrologically, an area of the living object is captured, the sensitive responds to the respiratory movement of the living object such as the metrological detection of a profile, in particular through a spatially selective Excitation and data extraction at the transition between moving organs and their environment (eg liver interface, Diaphragm and lungs), so that in principle z. B. the diaphragm position metrologically can be detected and so the opportunity exists, depending from the thus determined organ position, ie z. B. of the diaphragm, the indulging in recorded spin-echo signals depending on the current state of motion as a measurement signal to evaluate or discard and resume.

So can the spin-echo signals continuously under free breathing of the living Object are recorded, being automatically detected the respective organ position that in the compensation measurement sequence is determined, only such measuring signals are evaluated, which is assigned to a specific organ position, that is to say in a specific especially always the same moment of respiratory movement arise. Spin echo signals outside this specific time can for the Measurement are discarded and therefore do not carry in the imaging at. Metrology can do this a temporal acquisition window to capture the spin echoes only opened at the echoes that correlate with organ position.

in this connection it is basically irrelevant, at which time such a compensation measurement sequence, d. H. Control of the high-frequency coil or the Gradientenmagneffelder carried out is and at most affects the determination of the acquisition window out. The only important thing is that the detected organ position at a time before recording the spin echo signals to At the time of recording these echo signals, an evaluation criterion for the Rating or for to have the discarding of these signals available, in particular So to open the acquisition window or close.

So it can be in one execution be provided of the method that the Kompensationsmesssequenz carried out becomes after the two, the magnetization inverting high-frequency pulses and before the RF excitation pulse for generating the spin flip for the spin echo imaging sequence.

In another alternative embodiment it is also intended to perform this compensation measurement sequence within the additional introduced second evolution time (tau) after the RF excitation pulse for generation of the spin flip for the echo measurement. In particular, this variant of the process provides the advantage that the evolution time can be used on the one hand a development of the generated by the inhomogeneities of the magnetic field Dephasing the transverse magnetization Mxy, and on the other hand, this compensation measurement sequence within this time to push in and therefore not waste this evolutionary time let pass. So, in total, the measured value acquisition be accelerated.

Of the Innovative value of the invention is to create one at a time suszeptibilitätsgewichteten, Distortion-free imaging technique free of disturbances due to blood flow, respiratory movement or coherences between even and odd echo groups, as previously used in this Combination of suitable modules for the preparation, weighting and selection of the magnetization not exist. The resulting image information, including improved spatial resolution opened in the (sub-) millimeter range New opportunities for one wide range of applications of cardiovascular MRI including examination the endothelial function of vessels, detection stress-induced angina, imaging and quantitation of the iron content of myocardial tissue, the mapping of the oxygen saturation of the Blood in the myocardium, the detection of myocardial perfusion deficits as well as the differentiation between arteries and veins due to the limitations described so far only partially or not at all accessible were.

• – Endogene vs. exogene Kontrastmechanismen: Suszeptibilitätswichtung ist für klinische Anwendungen attraktiv, weil sie keine Konstrastmittelinjektion erfordert, da Blut oder andere Gewebe- oder Körperbestandteile als Tracer oder Marker fungieren. Somit können an Stelle exogener Kontrastmittel Blut oder andere Gewebe- oder Körperbestandteile als direkte Informationsträger, d. h. als endogene Kontrastmittel, zur Abbildung bestimmter physiologischer Parameter (z. B. Myokardperfusion, Sauerstoffsättigung, Eisengehalt, Endothelfunktion) ausgenutzt werden.
• – Verzerrungsfreie Bildgebung: Spin-Echo-Techniken unterliegen nicht den qualitativen Einschränkungen wie sie bei konventioneller Echo-Planar-Bildgebung oder anderen Gradienten-Echo basierten Techniken auftreten, da sie per se keine intrinsische Empfindlichkeit gegenüber B0-Inhomogenitäten aufweisen. Damit sind Spin-Echo basierte Techniken selbst im Fall der hier erfindungsgemäß aufgeprägten T2*-Wichtung verzerrungsfrei. Dieser Vorteil kommt insbesondere bei hohen Magnetfeldstärken zum Tragen, da diese eine deutlich erhöhte Ausprägung makroskopischer und mikroskopischer Suszeptibilitäts- bzw. B0-Gradienten aufweisen und deshalb die EPI- und Gradientenecho basierte Bildgebung verstärkt unter drastischen Bildverzerrungen und Signalauslöschungen, die die Diagnostik erheblich beeinträchtigen, leidet.
• – Beliebig wählbare Suszeptibilitätswichtung: Die T2*-Wichtung kann über die geeignete Wahl der zweiten Evolutionszeit an die Verhältnisse unterschiedlicher Gewebe und Organteile bzw. unterschiedlicher Magnetfeldstärken beliebig angepasst werden (0 ≤ zweite Evolutionszeit ≤ ∞)
• – Wiederholbarkeit der Untersuchungen: Mit der Kombination aus Spin-Echo Technik und Suszeptibilitätswichtung, also Blut, Gewebe oder als endogenem Kontrastmittel, ermöglicht das neue Verfahren, eine beliebige Reproduzierbarkeit der Messungen ohne Verträglichkeitsbeschränkungen durch etwaige Halbwertszeiten oder unzulässige Konzentrationen der Kontrastmittel oder deren Verweildauer.
• – Vermeidung von Fluss- und Blutpulsationsartefakten: Mit der Double-Inversion-Recovery (Schwarzblutbildgebung) wird das Blutsignal im Ventrikel unterdrückt und somit sämtliche Störeinflüsse, die durch gerichteten Blutfluss entstehen, eliminiert. Erst diese Form der Vorbereitung der Magnetisierung ermöglicht eine zuverlässige T2*-Bildgebung aufgrund der sehr geringen Signalintensitäts- oder T2* Unterschiede von wenigen Prozent zwischen gesundem und pathologischem Gewebe. Optional können Signalbeiträge, die auf Fett zurückgehen, zusätzlich unterdrückt werden.
• – Eignung für die Hochfeld-MRT: Änderungen in der Sauerstoffsättigung führen zu sehr geringen Signalintensitätsänderungen in der BOLD-gewichteter Bildgebung. Für Magnetfeldstärken von 1.5 T wurden Signalintensitätsänderungen von ca. 2–5% berichtet (11). Höhere Feldstärken offerieren eine erhöhte Empfindlichkeit gegenüber mikroskopischen und makroskopischen Suszeptibilitätsunterschieden. Im Unterschied zum SNR besitzt die Empfindlichkeit gegenüber Suszeptibilitätsgradienten keine lineare Abhängigkeit von der magnetischen Feldstärke, sondern folgt annähernd einer quadratischen Funktion. Eine Verdoppelung der Feldstärke von 1.5 T auf 3.0 T resultiert in einer Vervierfachung der Empfindlichkeit gegenüber mikroskopischen Suszeptibilitätsgradienten. Deshalb bietet die vorgeschlagene Technik an Hochfeld-MR-Systemen eine verbesserte Detektionsmöglichkeit für suszeptibilitätsbedingte Signaländerungen.
• – Hohe Bildgebungsgeschwindigkeit: Die vorgeschlagene Technik ist kompatibel mit der parallelen Bildgebung.
• – Vermeidung von Einflüssen durch Atembewegung: Konventionell werden Atemanhaltetechniken benutzt. Deren Einsatz ist unzureichend, da die konventionelle dynamische T2*-gewichtete Bildgebung über die Dauer einer Atemanhaltephase hinausgeht. Außerdem beeinträchtigen Atemanhaltetechniken den Patientenkomfort, die Reproduzierbarkeit der Bildqualität und damit die Güte der Diagnostik.

In its entirety, the invention combines the advantages of the individual above-mentioned gradient and high-frequency modules for the preparation, weighting and readout of the magnetization. These are in detail:

• - Endogenous vs. Exogenous Contrast Mechanisms: Susceptibility weighting is attractive for clinical applications because it does not require contrast agent injection because blood or other tissue or body components act as tracers or markers. Thus, instead of exogenous contrast agents, blood or other tissue or body components may be utilized as direct information carriers, ie as endogenous contrast agents, for imaging certain physiological parameters (eg myocardial perfusion, oxygen saturation, iron content, endothelial function).
• - Distortion-free Imaging: Spin-echo techniques are not subject to the qualitative limitations of conventional echo-planar imaging or other gradient-echo-based techniques because they do not have inherent intrinsic sensitivity to B0 inhomogeneities. Thus, spin echo-based techniques are distortion-free even in the case of the T2 * weighting imposed here according to the invention. This advantage is particularly useful at high magnetic field strengths, since these have a significantly increased expression of macroscopic and microscopic susceptibility or B0 gradients and therefore the EPI and gradient echo-based imaging increasingly under drastic image distortions and signal cancellations, which significantly affect the diagnosis suffers ,
• - arbitrarily selectable susceptibility weighting: the T2 * weighting can be adjusted by the appropriate choice of the second evolution time to the conditions of different tissues and organ parts or different magnetic field strengths (0 ≤ second evolution time ≤ ∞)
• - Repeatability of examinations: With the combination of spin-echo technique and susceptibility weighting, ie blood, tissue or as endogenous contrast agent, the new method allows any reproducibility of the measurements without compatibility restrictions by any half-lives or impermissible concentrations of the contrast media or their residence time.
• - Avoidance of flow and blood pulsation artefacts: Double-inversion recovery (black blood imaging) suppresses the blood signal in the ventricle, eliminating any interference caused by directional blood flow. Only this form of preparation of magnetization allows reliable T2 * imaging due to the very low signal intensity or T2 * differences of a few percent between healthy and pathological tissue. Optionally, signal contributions due to fat can be additionally suppressed.
• - High Field MRI Ability: Changes in oxygen saturation lead to very small signal intensity changes in BOLD-weighted imaging. For magnetic field strengths of 1.5 T, signal intensity changes of approximately 2-5% have been reported (11). Higher field strengths offer increased sensitivity to microscopic and macroscopic susceptibility differences. In contrast to the SNR, the susceptibility gradient has no linear dependence on the magnetic field strength, but follows approximately a quadratic function. Doubling the field strength from 1.5 T to 3.0 T results in a fourfold increase in sensitivity to microscopic susceptibility gradients. Therefore, the proposed technique on high-field MR systems offers improved de possibility for susceptibility-induced signal changes.
• High imaging speed: The proposed technique is compatible with parallel imaging.
• - Avoidance of influences by breathing movement: Conventionally, breath-hold techniques are used. Their use is inadequate because conventional T2 * weighted dynamic imaging extends beyond the duration of a breath hold phase. In addition, breath-hold techniques compromise patient comfort, reproducibility of image quality and thus the quality of the diagnostic.

Therefore Here is the imaging with the help of navigator techniques (pencil beam) reacted under free breathing. This approach is of existential Significance for avoiding respiratory movement artifacts. In The result is free breathing imaging for mapping selected selections of the heart or under complete coverage of the heart with 2D or be enabled with 3D imaging. In addition, the mapping of T2 * -weighted images to the quantification be extended by means of T2 * maps, since the approach of the free Breathing in contrast to breathing techniques no temporal restriction of the Imaging time dictated.

An embodiment of the invention is in the 1 shown. This shows the juxtaposition of preparation and measurement sequences both for the suppression of blood artifacts, for compensation, or consideration of the respiratory movement as well as for the recording of spin echo measurement signals.

recognizable In this case, in each case the control or a measurement signal (RF Receive) within a coil for generating RF excitation or preparation pulses, as well as the control of coils for the generation of gradient magnetic fields, for a location selectivity in the investigation of an object. Basically the coil for generating the RF pulses also be used to capture the measurement signals, ie the echoes, or alternatively a separate receiver coil.

Thus, it can be seen here that in a sequence I. to achieve the suppression of Blufflussartefakten initially a non-slice-selective radio-frequency pulse 1 is irradiated into the object to globally effect the inversion of the net magnetization component Mz to -Mz. Subsequently, a second slice-selective radio-frequency pulse 2 in order to achieve a further inversion within the investigated layer of the living object, and thus to return to the original state of the net magnetization component Mz. The slice selectivity is achieved here by switching on a gradient magnetic field along the Z direction of the homogeneous magnetic field, as here by switching on the so-called slice encoding gradient by a programmed pulse 3 he follows.

Then, a first evolution time TI is waited here after which a high-frequency excitation pulse 4 layer-selectively into the living object with simultaneous switching of the layer-coding gradient with a pulse 5 is produced.

The time interval and thus the duration of the first evolution time TI between the inverting slice-selective radio-frequency pulse 2 and the excitation RF pulse 4 for generating a spin flip is dimensioned such that at the moment of the RF excitation pulse 4 the blood moved into the layer to be examined by the heartbeat has relaxed so far with the previously inverted net magnetization component Mz that its net magnetization component in the Z direction is zero in absolute value, ie Mz = 0, so that the Spins of this replaced blood do not spin-flip through the excitation pulse 4 and thus no signal contribution due to this blood is to be expected in the subsequent measurement sequence.

The 1 further shows that after the RF excitation pulse 4 an evolution time Tau is awaited, after which a first rephasing pulse 6 is irradiated in the living object to repeatedly reverse the dephasing of the Mxy magnetization component by periodic sequence of further rephasing pulses and thus to generate the so-called spin-echo signals.

In this case, the insertion of the additional evolution time Tau, which causes a total time between pulse 4 and first rephasing pulse 6 generated over the equidistant time interval between the individual rephasing pulses 6 goes beyond that before the first rephasing pulse 6 can build a dephasing of the transverse magnetization component Mxy, which is caused exclusively by inhomogeneities of the entire magnetic field at the site of the investigation. Accordingly, this dephasing proportion is retained in the individual subsequent generated spin echo signals and can be determined by the evaluation of the spin echo signals, so that susceptibility-weighted imaging is possible even with the spin-echo technique otherwise insensitive to susceptibility differences.

The execution according to the 1 shows here further that there is the possibility, before the RF excitation pulse 4 to carry out a preparation sequence II, ie to program a sequence of radiofrequency pulses and gradient fields, with which it is possible to investigate an organ position dependent on the respiratory movement, in particular within a column investigated instead of a complete layer of the living object and thus obtain a criterion as to whether when the spin-echo signals are recorded with free breathing of the living object, the correspondingly recorded spin-echo signals are to be utilized or the imaging is to be used.

In this case, in an alternative not shown, the programmed sequence II may also follow the RF excitation pulse 4 , ie be carried out within the evolution time Tau.

The inventive method thus combines the advantages of the individual programmed sequences, like the oppression of blood artifacts in the sequence I, the possibility of compensation the respiratory movement by selection of contributing measured values from the entire acquired readings as well as imaging on the basis the well-known spin-echo technique, which is characterized as distortion-free, here however under introduction of the Evolution time under consideration suszeptibilitätsbedingter Dephasing of the measuring signal.

in this connection there is still the possibility of off the acquisition window, d. H. the window, which temporally open is to capture the readings, to avoid coherences and Interference between odd and even echo groups one of these two groups by means of an additional Gradient pulse along the reading direction from the acquisition window to push, so that only the remaining echo group contributes to the signal.

How is the 1 Furthermore, Lets, is here each of the programmed sequences, ie on the one hand, the sequence for the suppression of blood artifacts that compensate for respiratory movement as well as those for recording the spin echo each preferably synchronized to the heart movement of the living object. In the embodiment in which the sequence II is programmed to compensate for the respiratory movement within the second evolution time Tau, another only requirement is that at the end of this sequence performed there be a net dephasing of zero at the site of the examination to ensure this in that due to the inserted sequence for compensation of the respiratory movement no artefacts arise in the imaging.

Claims (3)

A method for operating a magnetic resonance tomograph for spatially resolved spin resonance on a living object, which is arranged in a static magnetic field B0, whereby an alignment of the spins of the object and a longitudinal net magnetization Mz along the magnetic field direction Z results, by means of at least one high-frequency excitation pulse a spin-flip around a desired flip-angle is generated in resonance, in which a transverse magnetization component Mxy is generated or changed, the T2 * -Relaxation is detected to carry out susceptibility-weighted measurements, wherein the location of the thereby metrologically detected volume element of the object by a plurality characterized at least temporarily superimposed on the homogeneous magnetic field B0 gradient magnetic fields and wherein the start and / or preparation sequences of a measurement is synchronized to one or more detected forms of physiological movement / be characterized t that in preparation for the metrological detection with a non-slice selective RF pulse globally inverts the net magnetization Mz and then. with a slice-selective RF pulse in the slice of the object to be imaged, the original net magnetization Mz is restored, after which a first evolution time is waited in which the blood in the slice to be imaged is replaced by blood whose magnetization inverts with the first non slice-selective RF pulse and wherein the metrological detection of a plurality of spin-echo signals from a desired volume element of the object, which are generated after an RF excitation pulse by a sequence of mutually equidistant high-frequency Rephasierungimpulsen, wherein the echo signals prior to their detection impressed a susceptibility dependency is caused by a second evolution time additionally inserted between the high-frequency excitation pulse and the first high-frequency rephasing pulse for developing the dephasing generated by inhomogeneities of the magnetic field generated by the RF excitation pulse Magnetizing Mxy and wherein prior to the metrological detection of the echo signals, a physiologically synchronized compensation measurement sequence for determining a caused by the respiratory movement of the living object organ position is performed, depending on the thus determined organ position, the recorded spin echo signals as Measurement signal evaluated or discarded. A method according to claim 1, characterized in that the compensation measurement sequence is performed after the two magnetization inverting high-frequency pulses and before the RF excitation pulse. Method according to claim 1, characterized in that that the compensation measurement sequence is carried out within the additionally inserted second Evolution time after the RF excitation pulse.