DE10260372A1 - Imaging arrangement and method for spatially resolved image display - Google Patents

Abstract

For problem-free detection and display of microscopic pathological structures adjacent to blood vessels in the human or animal body, in particular of lymphatic tissue, arteriosclerotic deposits in the blood vessels, an arrangement and a method are proposed according to which a magnetic resonance imaging device for obtaining data for a spatially resolved image representation of the Magnetic resonance behavior of atomic nuclei is used in a selected viewing area in the body, the device being designed and programmed such that the device is exposed to high-frequency and magnetic field gradient echo pulse sequences which generate magnetization in the body so that the magnetization of one in at least one spatial direction flowing medium in the body can be weakened by dephasing the spins of atomic nuclei in the medium, an MR contrast medium being supplied to the body becomes.

Description

The invention relates to an imaging Arrangement and a method for spatially resolved image display in particular of human or animal bodies. The arrangement and that Procedures are particularly for Magnetic resonance (MR) tomography on human and animal body suitable.

Lymphatic vessels in the mammalian and human bodies have been identified in the past with the help of x-ray procedures under direct Puncture of the lymphatic vessels and lymph nodes with simultaneous administration of X-ray contrast media shown (direct lymphography). These interventions are for the patient very painful and lead often side effects.

To represent lymphatic Tissue can be used instead of X-ray diagnostics magnetic resonance imaging can also be used. This technique is fundamental just like X-ray technology for imaging the lymphatic vessels and lymph nodes due to the multi-planar layer guidance and the high soft tissue contrast well suited. In “Interstitial MR Lymphography with a Conventional Extracellular Gadolinium-based Agent: Assessmentin Rabbits "; S.G. Ruehm, C. Corot, J.F. Debatin; Radiology; 2001; 218: 664-669 the subcutaneous administration of gadoterate meglumine in rabbits described to represent the lymphatic system.

In experimental studies a good differentiation of metastases with novel contrast media in the lymphatic system and healthy lymphoid tissue.

Depending on the used Contrast agents have the following disadvantages:

• (a) If the contrast medium is given interstitially, only a part of the lymphatic system can be visualized.
• (b) Experimental (unauthorized), iron-based contrast media show a very slow uptake in the lymph nodes. It is therefore necessary to order the patient to be examined twice, namely once to administer the contrast medium and once to carry out the examination. However, ordering the patient twice is often not practical in everyday clinical practice. In addition, these contrast media result in unfavorable contrast properties, namely a negative contrast (signal drop in the target organ) and susceptibility artifacts. For example, very small superparamagnetic iron oxide particles (USPIO) coated with dextran can be used. The particles are stored in phagocytes in functional lymph node tissue, but not in metastatic tissue in which phagocytosis does not take place. Maximum concentration of this contrast medium within the lymph nodes is only reached after about 24-48 h after the administration. The target structures are dark, since these substances, as negative contrast agents, significantly reduce the spin-spin relaxation time T ₂ and in particular T ₂ * due to their susceptibility effect. (c) In the case of systemic administration of lymphotropic contrast media, the long residence time in the blood (intravascular contrast media) results in a difficult delineation of the lymph nodes that are usually located directly next to the blood vessels. Examples of these contrast media are given in: "Magnetic Resonance in Medicine";PARinck; 4th edition; Blackwell Wissenschafts-Verlag, Berlin, 2001. The display of arteriosclerotic deposits in the vascular wall, so-called plaques, is also of particular interest mostly intravascular contrast agents cause an increase in the signal inside the blood vessels, it is almost impossible to distinguish a plaque in the vessel wall immediately after the administration of the contrast agents. The arteriosclerotic deposits are very difficult to distinguish from the blood vessels immediately after the administration into the blood system it is necessary, as is necessary to visualize lymph nodes, to wait a long time between administration of the contrast medium and imaging using an imaging technique. Special contrast media can be used to display the plaques. To differentiate the plaques from the inside of the blood vessel, it can be used that the contrast medium remains in the plaques longer than in the blood stream, so that after a waiting time of about 12 - 48 h the plaques are still signal-intensified, but the blood is again displayed with less signals. As for lymphography, it is often not justifiable for the presentation of plaques in everyday clinical practice to observe such a long waiting time between administration and admission, since a patient would then have to be called in twice for the examination.

When examining a body, in particular a human or animal body, with a nuclear magnetic resonance experiment, either a so-called spin echo sequence with 90 ° -180 ° excitation or instead of this excitation or in addition to this a measurable signal sequence also with a gradient pulse sequence (gradient echo method ) be achieved. In this case, after the 90 ° high-frequency pulse, a gradient pulse is first switched, for example in the x direction. A gradient pulse is then applied in the -x direction, ie with an inverted gradient sign. As a result, the initially dephased spins refocus to a measurable signal, which can be represented as a sum vector in a coordinate system rotating with ω ₀ .

For a spatially resolved representation of nuclear spins in a field to be examined (FOV: field-of-view) in a body, the nuclear spins must be assignable to individual spatial elements. For this purpose, the effect is used that the Larmor frequency ω (x, t) is dependent on the magnetic field strength B (x, t). In the case of a slice scanning, using a magnetic field gradient in the form of a gradient pulse during the 90 ° pulse in the z direction, only a thin layer (“slice”) is excited in which the magnetic field strength B ₀ just corresponds to the Larmor frequency ω _0. As a result, only nuclei A simple assignment of nuclei, for example in the z direction, is achieved, which are located within the layer under resonance conditions. To achieve a spatial resolution in the x and y direction, further gradient pulses are along along the 90 ° pulse the x or y direction switched: In the y direction, a gradient circuit is inserted after the 90 ° pulse ("phase"). The location information in the y direction is contained in the phase shift of the precessing nuclear spins, which is caused by this temporarily switched gradient (phase coding). The location information is made available from the response signal by Fourier Transform (FT) analysis. In order to achieve different phase shifts in the y direction and thus to obtain clear location information of the nuclear spins in this spatial direction, gradient circuits are inserted in successive pulse sequences, which are successively increased or decreased in increments, the phase gradients with gradient slopes between two maximum values + G and -G can be varied. In the same way, a first gradient circuit is inserted in the x direction (“read”), which contains the required information about the spatial resolution of the nuclear spins in the x direction. To generate an echo signal, a second read gradient circuit with different polarity is then the first inserted, which after a dephasing of the spins in the xy plane due to the first read gradient circuit leads to a refocusing of the nuclear spins due to the switching of the second pulse, so that a response signal is generated, since the read gradient circuit depending on the location in x - If there are different magnetic fields in the range B ₀ ± ΔB during the refocusing, the signals originating from different locations can be separated using the different frequencies in the range ω ₀ ± Δω (frequency coding).

The measurement can be accelerated can be achieved by exciting the nuclear spins with an RF pulse become a toggle of the net magnetization of less leads as 90 ° (flip angle α <90 °).

While the individual layers in the body to be examined the previously described 2D-FT can be examined in succession also a three-dimensional representation of a body to be examined in a single pulse sequence without stratification (3D-FT): For this purpose, the aforementioned pulse sequences for the phase and the readout gradient pulse applied. The slice gradient pulse during the RF pulse is additionally followed downstream slice gradient pulse with inverted polarity, where the second slice gradient pulse in successive pulse sequences increased in increments between two maximum values + G and –G or is lowered.

To visualize blood vessels (angiography) Various techniques have been used, some of which are included consist of the signal from the blood vessels in an acquisition sequence to suppress and in another with flow compensation, i.e. without dephasing the moving nuclear spins (signal carriers). To differentiate the vessels from the surrounding resting tissue then becomes a difference between the two shots formed a good contrast between the vessels and the surrounding tissue, leaving the blood vessels bright being represented. A comparison the procedure is in “Black Blood Angiography "; W.Lin, M.Haacke, R.R. Edelman; in: "Magnetic Angiography, Concepts and Applications "(ed .: E.J. Potchen, E.M. Haacke, J.E. Siebert, A.Gottschalk), Mosby, St. Louis (1993) included.

• a) Schon in der klassischen MR-Messsequenz, der Spin-Echo-Sequenz, werden bewegte Spins intrinsisch unterdrückt, da diejenigen Spins, die zwischen dem 90°-Anregungspuls und dem 180°-Refokussierungspuls die Messschicht verlassen, nicht zum MR-Signal beitragen. Dieser Laufzeiteffekt wird mit zunehmender Echozeit TE, die bei einer klassischen Spin-Echo-Sequenz mit 90°- und 180°-Puls doppelt so lang ist wie die Zeitspanne zwischen 90°-Puls und 180°-Puls, oder verringerter Schichtdicke stärker. Diese Technik ist jedoch für Gradientenechosequenzen ohne 180°-Pulse nicht geeignet. Daher kann dieser Effekt für eine schnelle Aufnahmezeit nicht genutzt werden: Es ist technisch allerdings fast unmöglich, in praktikablen Messzeiten dreidimensionale Volumina mit Spin-Echo-Sequenzen darzustellen, während dies mit schnellen Gradientenechoverfahren, speziell mit der mittels Kontrastmitteln verstärkten Magnetresonanz-Angiographie, in wenigen Sekunden gelingt.
• b) In konventionellen Magnetresonanz-Verfahren zur Unterdrückung von Blutgefäßsignalen wird das Signal der Blutgefäße außerhalb der Bildgebungsschicht gesättigt, indem beispielsweise parallel zur Messschicht sogenannte Saturationsschichten positioniert werden. Da hier keine 180°-Pulse bei der Signalauslese eingesetzt werden, kann dieses Sättigungsverfahren mit nahezu jeder beliebigen Bildgebungstechnik in der Magnetresonanztechnik kombiniert werden. Grundsätzlich wird mit diesem Verfahren der Vorteil ausgenutzt, dass Blut gegenüber anderen Geweben eine sehr lange T₁-Relaxationszeit hat und in der direkt auf die Sättigung folgenden Signalauslese in der Messschicht nur gesättigtes Blut vorhanden ist, das nahezu kein Signal liefert. In einer Variante dieses Verfahrens wird die Magnetisierung nur außerhalb der Messschicht invertiert. Danach wird solange gewartet, bis die Längsmagnetisierung der Signalträger im Blut (entlang der z-Achse) aufgrund der T₁-Relaxation einen Nulldurchgang aufweist. Die in die Messschicht eingeflossenen Signalträger im Blut tragen dann nicht zum MR-Signal bei.
• c) In Gradientenechobildern wurde schon relativ früh beobachtet, dass schnell aus einer Gefäßverengung strömendes Blut in einer bestimmten Verwirbelungszone (Jet) eine artifizielle Signalreduktion hervorruft. Dieser Effekt beruht darauf, dass bewegte Spins unter Einwirkung von Gradienten, die zur Ortskodierung in der MR-Bildgebung benötigt werden, eine zusätzliche Phase akkumulieren, die von der Bewegungsgeschwindigkeit der Spins abhängig ist. Im Jet treten innerhalb eines Bildelements (Pixel) viele verschiedene Phasen auf, so dass die phasenkohärente Addition der MR-Signale zu einer Reduktion des Summensignals im MR-Bild führt. Dieses Phänomen wird als intravoxel incoherent motion bezeichnet und ist auch aus der diffusionsgewichteten MR-Bildgebung bekannt. Dieser Effekt kann noch dadurch verstärkt werden, dass Gradienten so zur Bildgebung hinzugefügt werden, dass die Ortskodierung unbeeinflusst bleibt, während die geschwindigkeitsabhängige Phase maximiert wird. Diese Technik wird Black Blood Angiography genannt. Sie wird in Verbindung mit Spin-Echo-Sequenzen implementiert, um deren zusätzliche Signalunterdrückung auszunutzen. Umgekehrt kann dieser Effekt auch zur Darstellung von Blutgefäßen verwendet werden, indem zwei Datensätze ohne und mit zusätzlicher Gradientenschaltung voneinander subtrahiert werden (Rephase/-Dephase Imaging).

Since the beginning of clinical MR imaging, methods have been used with which MR signals from moving signal carriers can be suppressed. The main methods for suppressing moving MR signal carriers are discussed below:

• a) Even in the classic MR measurement sequence, the spin-echo sequence, moving spins are intrinsically suppressed, since those spins that leave the measurement layer between the 90 ° excitation pulse and the 180 ° refocusing pulse do not contribute to the MR signal , This transit time effect becomes stronger with increasing echo time TE, which is twice as long in a classic spin-echo sequence with 90 ° and 180 ° pulse as the time span between 90 ° pulse and 180 ° pulse, or reduced layer thickness. However, this technique is not suitable for gradient echo sequences without 180 ° pulses. Therefore, this effect cannot be used for a fast recording time: It is, however, technically almost impossible to display three-dimensional volumes with spin echo sequences in practical measuring times, while in a few with fast gradient echo methods, especially with magnetic resonance angiography enhanced with contrast agents Seconds succeed.
• b) In conventional magnetic resonance methods for suppressing blood vessel signals, the signal of the blood vessels outside the imaging layer is saturated by, for example, positioning so-called saturation layers parallel to the measuring layer. Since no 180 ° pulses are used for signal readout, this saturation process can be combined with almost any imaging technique in magnetic resonance technology. Basically, the advantage is gained with this procedure uses the fact that blood has a very long T ₁ relaxation time compared to other tissues and that in the signal reading immediately following saturation, only saturated blood is present in the measuring layer, which delivers almost no signal. In a variant of this method, the magnetization is only inverted outside the measuring layer. Then wait until the longitudinal magnetization of the signal carriers in the blood (along the z-axis) has a zero crossing due to the T ₁ relaxation. The signal carriers in the blood that have flowed into the measuring layer then do not contribute to the MR signal.
• c) In gradient echo images, it was observed relatively early that blood flowing rapidly from a vasoconstriction causes an artificial signal reduction in a certain swirl zone (jet). This effect is based on the fact that moving spins, under the influence of gradients, which are required for the spatial coding in MR imaging, accumulate an additional phase which is dependent on the speed of movement of the spins. Many different phases occur in the jet within a picture element (pixel), so that the phase-coherent addition of the MR signals leads to a reduction in the sum signal in the MR image. This phenomenon is known as intravoxel incoherent motion and is also known from diffusion-weighted MR imaging. This effect can be enhanced by adding gradients to the imaging in such a way that the spatial coding remains unaffected while the speed-dependent phase is maximized. This technique is called black blood angiography. It is implemented in conjunction with spin echo sequences to take advantage of their additional signal suppression. Conversely, this effect can also be used to visualize blood vessels by subtracting two data sets without and with additional gradient switching (rephase / dephase imaging).

To visualize blood vessels used in clinical practice either the spin echo method, which is made even more efficient by built-in dephasing gradients (Black Blood Angiography), or the Rephase / -Dephase method, the main one is used for positive imaging of the peripheral arteries. So far, neither technique has been combined with contrast media been used. Rather, the intrinsic contrast has an effect here of the moving blood for blood vessel imaging out. With the rephase / dephase method, this means that in the Subtract the two records Vessels only bright then appear if enough fresh blood flows into the measuring layer.

However, there are also investigations known by means of Black Blood Angiography, for example for illustration of arteriosclerotic deposits in blood vessels. So in “Extracranial Carotid Arteries: Evaluation with 'Black Blood MR Angiography'; R.R. Edelman, H.P. Mattle, B.Wallner, R.Bajakian, J.Kleefield, B.Kent, J.J.Skillman, J.B.Mendel, D.J.Atkinson; Radiology; 1990; 177: 45-50, a comparison the Bright Blood Angiography to the Black Blood Angiography for illustration of pathological changes the carotid artery. Black Blood Angiography is said to be opposite Bright Blood angiography has the advantage of being very malfunctioning can be represented exactly. To represent lesions Black Blood Angiography used a 2D spin echo method since gradient echo sequences were not suitable, the moving nuclear spins to suppress, although the layers examined were saturated. To oppression too achieve the echo time TE is extended Need to become. This should have been however, to a decrease in the resolution of the structures on the Blood vessels and reduced contrast between the blood vessel and muscle tissue.

With the previously known methods for oppression from blood vessels it basically avoid their appearance if there is no contrast agent in the bloodstream is available. However, it will shorten the relaxation times Given contrast medium, it is in the layers shown almost impossible in a human or animal body, pathological Structures in the lymphatic vessels and of arteriosclerotic deposits in the blood vessels without recognize more, especially if they are relatively small.

The present invention lies hence the task of finding means with which in particular small pathological structures easily recognized and displayed can be. Above all, a high-contrast, overlay-free and clear Representation of adjacent to blood vessels in human or animal body lying, not moving structures may be possible. In particular, should Metastases in the lymphatic tissue and in plaques can be identified easily and quickly and be representable.

The problem is solved by the imaging arrangement specified in claim 1 and that in claim 9 specified procedures. Advantageous embodiments of the invention are specified in the subclaims.

If specified below and in the claims is that magnetic field gradient echo pulse sequences in a given Direction of space, it is to be understood that the sequences in one or two arbitrary or all three spatial directions can be switched. In the same way, stating that the magnetization of flowing Medium can be weakened in a spatial direction, to understand that the magnetization in any one or two spatial directions or can be weakened in all three spatial directions.

A magnetic resonance (MR) contrast medium, which is absorbed by the body to be examined, is used to display pathological structures, some of which are microscopic in size, in the lymphatic system and in the blood vessels using magnetic resonance imaging. For the image display, a magnetic resonance imaging device is used to obtain data for a spatially resolved image display of the magnetic resonance behavior of the atomic nuclei in a selected viewing area in a body. For this purpose, the device is designed and programmed so that the device can expose the body to high-frequency and magnetic field gradient echo pulse sequences that generate magnetization in the body. The magnetization of signal carriers (spins of atomic nuclei, in particular ¹ H nuclei), which are located in a medium flowing in at least one spatial direction, in particular blood, are weakened by dephasing the spins of the atomic nuclei in this flowing medium, so that a representation of itself structures in the immediate vicinity of the flowing medium are considerably simplified, even if they are microscopically small, since this makes blood appear dark. It is only by administering an MR contrast agent that it is possible to find the fine structures sought and to recognize them with certainty.

By realizing the invention can for example lymph nodes of a certain region in the human or animal body or the whole body with high spatial resolution are shown because, on the one hand, the moving signal carriers the blood vessels are suppressed and on the other hand, the target structures of contrast media are increasingly displayed so that they stand out particularly well. The one from the blood vessels signal intensity becomes selective according to the invention suppressed so that, for example, the lymph nodes are shown in close proximity to large blood vessels and delimited against the vessel can be. The same applies to arteriosclerotic deposits, so-called plaques, in the blood vessel walls.

Conventional saturation methods, which were developed to suppress moving signal carriers, cannot be used to delimit the lymph nodes and plaques in combination with contrast media, since the saturated magnetization in the presence of the contrast media recovers within a few milliseconds and is available in the subsequent signal readout. This effect is caused by the massive T ₁ shortening, which is dependent on the contrast medium concentration. In contrast to this, the dephasing of the nuclear spins according to the invention can be easily achieved by using gradient pulse sequences in the presence of contrast media and is therefore superior to them. Compared to the rephase / dephase imaging method, the method according to the invention is about a factor of 2 faster, since the rephase part of the method can be dispensed with.

• a) Sie sollen mit der gewählten Sequenz zu einer Signalverstärkung im MR-Bild führen.
• b) Sie sollen sich in der Zielstruktur anreichern, d.h. im lymphatischen Gewebe bzw. in arteriosklerotischen Ablagerungen. Hierzu ist es selbstverständlich erforderlich, dass die Kontrastmittel zur Darstellung von Lymphknoten lymph-gängig und zur Darstellung von Plaques plaquegängig sind.
• c) Sie sollten sich auch im Blutgefäßsystem anreichern.

MR contrast agents are used to display, in particular, lymphatic tissue and arteriosclerotic deposits in blood vessels, which may be advantageously tailored to the respective application. The contrast media should preferably meet the following conditions:

• a) With the selected sequence, they should lead to a signal amplification in the MR image.
• b) They should accumulate in the target structure, ie in the lymphatic tissue or in arteriosclerotic deposits. To do this, it is of course necessary that the contrast media are lymphatic to display lymph nodes and plaque to display plaques.
• c) They should also accumulate in the blood vessel system.

For the detection of metastases of the Lymphatic systems are, for example, those already mentioned before Suitable iron oxide particles in the form of the USPIO. However, need coated Iron oxide particles a longer one Time to accumulate in the lymph nodes. They are also contrast agents because of the negative contrast not suitable for the representation of the lymphatic vessels.

Compounds containing gadolinium, in particular, can be used advantageously. For lymphography, polymeric compounds, such as those from L.Harika, R.Weissleder, K.Poss, C.Zimmer, MIPapisov, TJBrady in: "MR Lymphography with a Lymphotropic T ₁ -Type MR Contrast Agent: Gd-DTPA-PGM";MRM;1995; 33: 88-92 and by G.Staatz, CCNolte-Ernsting, A.Bucker et al. In: "Interstitial T ₁ -Weighted MR Lymphography with Use of the Dendritic Contrast Agent Gadomer-17 in pigs"; Rofo. Fortschr. Born Röntgenstr. New imaging traversing .; 2001; 173: 1131-1136 described compounds, as well as lipophilic compounds which form aggregates or micelles, such as those of B.Misselwitz, J.Platzek, B.Raduechel, JJOellinger, HJWeinmann in: “Gadofluorine 8: Initial Experience with a New Contrast Medium for Interstitial MR Lymphography ';Magma;1999; 8: 190-195 and by G.Staatz, CCNolte-Ernsting, GBAdam et al. in: "Interstitial T ₁ -Weighted MR Lymphography: Lipophilic Perfluorinated Gadolinium Chelates in Pigs";Radiology;2001; 220: 129-134.

Particularly suitable are those compounds which accumulate in the lymphoid tissue within a very short time after administration. These are preferably gadolinium complexes provided with polar residues, for example sugar residues, and fluorinated side chains, which are aggregated to micelles with a size of 4-6 nm. Such compounds are described, for example, in WO 02/14309 A1. With these contrast media, the MR examination can be carried out after the administration can be done within a few minutes to an hour. These special gadolinium compounds can also be used to display arteriosclerotic deposits (plaques).

Compounds of other paramagnetic metal ions can also be used, for example compounds of Mn (II), Dy (III) and Fe (III). Gd (III), Mn (II) and Fe (III) compounds act as positive contrast agents, since these agents reduce the longitudinal relaxation time T ₁ , so that those parts in an MR image into which the contrast agent has been recorded , are lightened. In contrast, Dy (III) compounds, like iron oxide particles, act as negative contrast agents, since their susceptibility effect lowers T ₂ and in particular T ₂ *, so that the parts in an MR image into which these contrast agents have been recorded are darker appear. In this respect, the latter compounds are not as suitable as Mn (II) and Fe (III) compounds.

Instead of the aforementioned contrast media, other types of contrast media can also be used, for example nitrogen oxides which, like the metal ions mentioned, are paramagnetic. Furthermore, gas-filled microbubbles are also proposed, which can be filled with nitrogen or perfluoropropane, for example. Such systems are exemplary in US 6,315,981 A described.

Furthermore, instead of paramagnetic or superparamagnetic substances, diamagnetic compounds which do not contain ¹ H but instead other signal carriers, for example fluorocarbon compounds, can also be used as contrast agents. Instead of ¹ H-MRI is a ^{1 9} F-MRI is performed in this case, since the ^{1 9} F-atomic nucleus has a nuclear spin of ½, wherein the gyromagnetic ratio for ¹⁹ F different from the apparent to ¹ H is so that these atomic nuclei form an image contrast in the MR image. These should be those connections that are included in the target structures. If these compounds have a long circulation time in the blood, the target structures can be made selectively visible with the present invention without the blood vessels preventing the recognition of these structures.

The MR contrast medium can be human or animal body especially intravenously be administered. In principle, however, the contrast medium can also intra-arterial, percutaneous, especially subcutaneous, also oral, intraperitoneal, intramuscular or given in some other way.

To weaken the magnetic resonance signals of spins in a flowing medium, the effect is used according to the invention that the spins of the atomic nuclei contained in the observation area in the body to be examined dephase during the movement, while this does not apply to non-moving spins. This can be achieved by suitable switching of the magnetic field gradient pulses. To determine the conditions under which the signals are attenuated, the following equation is used for the phase of the respective nuclear spins, which is location-dependent and time-dependent and which is a function of the location x within a gradient field, the time-dependent gradient field strength G (t) and the Time t after excitation of the atomic nuclei with a high-frequency pulse is:

The constant γ is the gyromagnetic ratio and is for the protons mainly used in magnetic resonance imaging in practical units 2π · 42.577 MHz / T.

If the excited atomic nuclei move with a motion component parallel to the spatial direction of the gradient while a gradient is switched on, then the location x at which the atomic nuclei are currently located at time t also becomes time-dependent. Therefore equation [1] can be reformulated as follows:

By developing into a Taylor series and neglecting higher terms, the following relationship results:

x ₀ is the starting point of the atomic nucleus when moving during a gradient pulse sequence, and ν ₀ is the constant velocity of the flowing medium. As an abbreviation for the time integral, multiplied with γ, M ₀ and M _{1 are} introduced, so that the following relationship results: φ (t) = x 0 · M 0 + ν 0 · M 1 , [2a]

M ₀ is known as the zero-order gradient moment and M, as the first-order gradient moment. Time-dependent gradient moments of the i-th order M _i (t), which have already been neglected in equation [2a], are defined as follows:

Gradient circuits in which M ₀ (zero-order gradient moment) is zero are required to generate echo signals generated by gradient circuits, since the nuclear spins only rephase under this condition. Conventional gradient circuits in which M ₁ (first-order gradient moment) is zero are referred to as flow-compensated, since nuclear spins that move at a constant speed in a flowing medium do not experience any additional dephasing caused by the movement, so that they are imaged appear bright. From the definition of the gradient moments M _{i it} follows that for short times only the low moments contribute to the signal phase, while higher moments scale with t ⁱ and thus remain small.

maximiert wird, wobei
γ das gyromagnetische Verhältnis der Atomkerne,
G(t') eine zeitabhängige Gradientenfeldstärke in dieser Raumrichtung und
t die seit dem Einstrahlen eines Hochfrequenzpulses zur Anregung der Atomkerne verstrichene Zeitspanne
sind.It follows from the above explanations that M ₁ must be as large as possible in order to suppress the signals from moving nuclear spins, since the dephasing is particularly large in this case. This means that the magnetic resonance signals of the medium flowing in the at least one spatial direction in the body can be weakened by flow dephasing gradient pulses in that a gradient moment M ₁ (t) in this spatial direction according to the following relationship:

is maximized, whereby
γ the gyromagnetic ratio of the atomic nuclei,
G (t ') a time-dependent gradient field strength in this spatial direction and
t the time elapsed since the irradiation of a high-frequency pulse to excite the atomic nuclei
are.

By considering gradient moments higher Order according to equation [3] with i> 1 can dephasing of such flowing media can also be achieved, which not only have a constant speed, but while gradient circuits are also accelerated or decelerated.

In a preferred embodiment of the invention, the magnetization of the medium flowing in the at least one spatial direction in the body is weakened by dephasing the spins in that gradient moments of the i-th order M _i (t), in particular gradient moments of the first order M ₁ (t), in this spatial direction can be maximized.

For example, if there are atomic nuclei within a picture element with speeds within a speed interval from 0 to ν _max with the same frequency and if the associated nuclear spins are mapped with the same signal intensity, the sum signal of these nuclear spins disappears exactly when the phase caused by M ₁ is exactly 2π (i.e. 360 °), so that the following relationship then applies: 2π = ν Max · M 1 [4] where ν _{max is} a maximum velocity of the flowing medium in the body to be examined, up to which dephasing is not effectively achieved.

Even for speeds greater than ν _max , the sum signal remains very small, so that ν _max can be interpreted as a limit speed below which signal suppression in the flowing medium does not work effectively. This means that the signal of moving nuclear spins is retained and is not suppressed if these nuclear spins move at a speed that is less than ν _max . Knowledge of the typical velocities in the blood vessels to be examined is therefore of interest in order to be able to use the invention effectively. Since blood moves very slowly in venous structures, these structures are generally clearly recognizable, since the nuclear spins contained therein are not dephased and thus suppressed. However, this is not disadvantageous for the display of lymphatic tissue and plaques, since these are rather adjacent to the arteries. If venous flow is also to be suppressed, then must stronger and / or longer gradient circuits are used.

In general, a predetermined gradient circuit will have a non-vanishing gradient torque of the first order M ₁ , so that the gradient circuit is not flow-compensated. Gradient circuits of this type are usually used to accommodate non-moving signal carriers. However, to suppress a slow flow, gradient moments are required that are not achieved by typical imaging gradients.

Because of the special gradient circuits can be used, which lead to river dephasing, in an embodiment according to the invention conventional 2D or 3D gradient echo pulse sequences, in particular flow compensated gradient echo pulse sequences are applied in the flow dephasing gradient pulses are inserted.

If the gradients in an existing gradient echo pulse sequence that is used for imaging are to be modified, for example in a flow-compensated gradient echo pulse sequence, in such a way that a large first-order gradient moment M ₁ arises, then a new sequence, the flow dephasing gradient pulse sequence, can be added to the existing imaging gradient echo pulse sequence which fulfills the condition according to equation [4] and thus leads to a maximization of M ₁ . This condition is necessary so that the spatial coding of the magnetic resonance signals remains unaffected, so that the following relationship is fulfilled:

This condition can be clearly interpreted by the fact that + M ₀ then represents the area under a gradient-time curve. A simple way of fulfilling this relationship is to use bipolar gradient pulses, ie two gradients of different polarity, the strength and length of which can be different, but in particular the pulses can be equally strong and long.

For example, the nuclear spins in a Direction of space by switching a gradient pulse with the time integral A be dephased by a certain amount. By switching later a second gradient pulse with the time integral -A in the same In the spatial direction, non-moving signal carriers are completely rephased, but not moving signal carriers.

As an alternative to the variant in which the flow dephasing gradient pulses are added to a flow-compensated gradient echo pulse sequence, a non-flow-compensated gradient echo pulse sequence can also be assumed in a further embodiment of the invention. After adding the further pulse sequence, however, the above-mentioned conditions must be fulfilled, according to which M is _0-0 (equation [5]) and M ₁ is maximized according to equation [4].

When using contrast media to better display microscopic structures in lymph nodes or arteriosclerotic deposits, the selected pulse times for the gradient moments must be very short, since the relaxation times are very short due to the use of the contrast media. However, if very short gradient pulses are used, a correspondingly high gradient field strength must be switched in the short period of time that is available. When implementing motion-sensitive gradient pulses, the following technical boundary conditions must therefore still be observed: Gradient systems that consist of current-carrying coils are used to generate gradient pulses. These coils are driven by a current amplifier. These amplifiers can only provide finite power, so that the amount of the gradient field strength is limited in practice. Today, the gradient field strength in clinical magnetic resonance tomographs is limited, for example, to 30-40 mT / m: | G (r) | ≤ G Max [6a]

However, this value may change in the future lie higher.

Since the coil turns of the gradient system represent an inductance, a minimum time is also required according to Lenz's rule to switch to a maximum gradient field strength. This minimum period of time, like the maximum gradient field strength, is of course dependent on the respective technical possibilities, so that a reduction in the time required depends on technical progress. The rise time is usually given in the form of a slew rate s _max :

The above-mentioned conditions according to equations [6a] and [6b] can, for example, be realized simply by using a flow dephasing gradient pulse sequence with long pulses. For example, the first-order gradient moment M _{1 can be used} for a bipolar gradient pulse give his through M 1 (t; G bipolar , t ramp , t plateau , t sep ) = γG bipolar · (T ramp + tplateau) · (2nd tramp + t plateau + t sep ) [7] in which
G _bipolar : the maximum gradient field strength,
t _ramp : the rise / fall time when switching on / off the gradient field,
t _plateau : the time period during which the G _{bipolar is} reached, and
t _sep : the time between two gradient pulses
are. To explain these parameters in more detail, click 1 directed.

The gradient pulses for flow dephasing should be kept as short as possible for contrast medium-assisted examinations according to the invention. In particular, the gradient pulses additionally used for the flow dephasing should be as short as possible, since a shortening of the longitudinal relaxation time T ₁ caused by the contrast medium inevitably goes hand in hand with a shortening of the transverse relaxation time T ₂ . If long gradient pulses were switched under these conditions, the echo times TE for signal readout would be extended, so that consequently an accelerated T ₂ decay would result in a strong signal loss for both moving and stationary signal carriers.

In a preferred embodiment the gradient pulse sequence comprises flow dephasing gradient pulses in the three (orthogonal to one another in the Cartesian coordinate system standing) spatial directions. The gradient echo pulse sequences in the respective spatial directions are there by inserting the Flow dephasing gradient pulses in imaging gradient echo pulse sequences educated.

Of course, flow dephasing gradient pulses can also be used in only one or only two spatial directions in imaging gradient echo pulse sequences added become. This can be advantageous, for example, if the medium is flowing should not be weakened in the spatial directions in which the flow dephasing gradient pulses not inserted become. It may be of particular interest to suppress the aorta by Flow dephasing gradient pulses are switched in the z direction.

The gradient echo pulse sequences can be chosen in any way, provided that only M ₀ = 0 and M _{1 are} as maximum as possible, since the exact shape of these gradient pulses is irrelevant for the implementation of the present invention. However, the time required to insert the additional flow dephasing gradient pulses should be short in order to minimize the echo times of the sequence. This is necessary because the signals of all structures enriching a contrast medium in a body have a shortened T ₂ decay, which would lead to a massive signal loss with long echo times.

• 1. In einer ersten Implementierung werden bipolare Gradientenpulse in Frequenz- und Phasenkodierrichtung vor der Signalauslese und in Schichtselektionsrichtung nach der Hochfrequenzanregung zwischen die eigentlichen Bildgebungsgradienten zusätzlich eingefügt (siehe hierzu auch 2a und 2b). Die Parameter G_bipolar, t_ramp und t_sep können beispielsweise fest vorge geben werden, so dass sich für eine minimale Plateauzeit t_plateau = 0 ms ein maximaler Wert für ν_max ergibt. Um auch größere Gradientenmomente erster Ordnung M₁ (und damit kleinere ν_max) realisieren zu können, kann die Gradientenechopulssequenz beispielsweise so programmiert werden, dass mit zunehmender Echozeit TE > TE_min symmetrisch Plateauzeiten t_plateau gemäß
  
  eingefügt werden. Damit können das Gradientenmoment erster Ordnung M₁ gemäß Gleichung [7] und die Geschwindigkeit ν_max, oberhalb der eine massive Unterdrückung der Signale zu erwarten ist, indirekt über die Echozeit TE gemäß Gleichung [4] eingestellt werden.
• 2. In einer optimierten Implementierung werden die gesamten Bildgebungsgradienten einer vorgegebenen Pulssequenz, die zwischen Hochfrequenzanregung und Signalauslese verwendet werden, so neu berechnet, dass die zusätzlichen Gradientenbeiträge zur Maximierung des Gradientenmoments erster Ordnung M₁ ein über eine Grenzgeschwindigkeit ν_max vorgegebenes Gradientenmoment erster Ordnung M₁ realisieren und gleichzeitig das Gradientenmoment nullten Ordnung M₀ des ursprünglichen Gradientenzuges nicht verändern (siehe hierzu 1c). Hierbei ist es häufig erforderlich, die Echozeit TE des Gradientenzuges zu verlängern. Allerdings sind die mit dieser Implementation realisierten Echozüge immer kürzer als die unter 1. beschriebenen Züge, da hier Bildgebungs- und Flussdephasiergradientenpulse gleichzeitig und nicht nacheinander ausgespielt werden. Das bei vorgegebenen Randbedingungen gemäß den Gleichungen [6a] und [6b] kürzeste Gradiententiming wird in einem solchen Ansatz durch numerische Optimierung gefunden.

In principle, the invention can be implemented in two embodiments. Two different variants of river dephasing are used for this. Both variants have in common that a gradient echo pulse sequence is switched which fulfills the conditions according to which M ₀ = 0 (equation [5]) and M ₁ is maximized according to equation [4]. In addition, the secondary conditions formulated in equations [6a] and [6b] must be observed:

• 1. In a first implementation, bipolar gradient pulses are additionally inserted between the actual imaging gradients in the frequency and phase coding direction before the signal readout and in the slice selection direction after the high-frequency excitation (see also here 2a and 2 B ). The parameters G _bipolar , t _ramp and t _sep can, for example, be predetermined, so that a maximum value for ν _max results for a minimum plateau time t _plateau = 0 ms. In order to also be able to realize larger first order gradient moments M ₁ (and thus smaller ν _max ), the gradient echo pulse sequence can be programmed, for example, such that with increasing echo time TE> TE _min symmetrical plateau times t _plateau
  
  be inserted. The first order gradient moment M ₁ according to equation [7] and the speed ν _max above which massive suppression of the signals can be expected can thus be set indirectly via the echo time TE according to equation [4].
• 2. In an optimized implementation, the entire imaging gradients of a predetermined pulse sequence to be used between RF excitation and signal readout are recalculated so that the additional Gradientenbeiträge to maximize Gradientenmoments first-order M ₁ a above a threshold speed ν _max predetermined gradient moment of the first order M ₁ and at the same time do not change the zero-order gradient moment M _{0 of} the original gradient train (see here 1c ). It is often necessary to extend the echo time TE of the gradient train. However, the echo trains realized with this implementation are always shorter than the trains described under 1., since here imaging and flow dephasing gradient pulses are played out simultaneously and not in succession. The given boundary conditions according to equations [6a] and [6b] shortest gradient timing is found in such an approach through numerical optimization.

Basically, the difference between the two methods is greatest at high limit speeds ν _max of nuclear spins, which can be dephased with relatively short and weak gradient pulses, while the flow dephasing gradient pulse sequences make the essential contribution to gradient timing compared to the imaging gradient echo pulse sequences at low speeds , so that the echo times differ only slightly.

So there are basically two Method of suppression of signals in moving media by dephasing nuclear spins to disposal, in which the gradient echo pulse sequences to be used are flow dephasing gradient pulse sequences in at least one spatial direction, the gradient echo pulse sequences in the respective spatial direction by inserting respective flow dephasing gradient pulses formed in imaging gradient echo pulse sequences or according to the aforementioned Boundary conditions can be calculated. Through the inserted or newly calculated sequences are dephased nuclear spins, which move in the spatial directions in which the flow dephasing gradient pulses are effective.

Basically, the selection of the Imaging data can be designed as desired. An advantageous pulse sequence is the so-called FLASH sequence (Fast Low Angle Shot), in which an excitation pulse with a flip angle α <90 °, for example 25 °, is irradiated and gradient pulses are used for refocusing. additional Gradients are used for imaging and river dephasing. The for Data acquisition time required is reduced when the excitation pulse radiated with a flip angle a <90 ° becomes.

To speed up the recording are in principle Multipulse sequences can also be used, for example EPI (echo planar imaging). Only one excitation pulse is radiated into these sequences and in succession a plurality of gradient pulses for spatially resolved imaging switched so that refocusing signals with each readout gradient pulse be preserved. Because with data in a gradient echo pulse sequence, for example for a series or an entire matrix in k-space (measurement data obtained before Conversion into the location-coded image data by Fourier transformation) be included. EPI is advantageous from the point of view that the data is scanned quickly. However, in in this case the disadvantage that when recording many regions of the body, especially in the abdominal area, artifacts show the modifications, for example, segmented EPI.

In a main variant of the present invention, the data are recorded point by point with separate gradient echo pulse sequences in such a way that a new excitation pulse is irradiated for each point. This procedure is somewhat more time-consuming than the methods in which multipulse sequences are used. However, the method is much more robust than a method with multipulse sequences. EPI also has the disadvantage that image smearing and signal loss occur during the relatively long echo times when contrast agents are used that accelerate the T ₂ * decay.

In an alternative procedure according to the invention, an imaging sequence can also be used, in which a transverse magnetization is first generated by initially spinning the spins aligned in the z direction at least partially by irradiating a 90 ° pulse or a pulse with a flip angle α <90 ° are folded over into the xy plane, then the spins are dephased with a suitable flow dephasing gradient pulse for moving spins, in which M ₀ = 0, and the spins are finally folded back into the z direction by a second 90 ° pulse. The magnetization thus stored in the z-direction is impressed with the dephasing as an additional contrast, so that it can be read out with any imaging sequence. Compared to the FLASH sequence, however, this embodiment has the disadvantage that the rapid T ₁ relaxation caused by the contrast media levels the contrast impressed in the z direction again.

Furthermore, it is also conceivable that additionally a 180 ° pulse is irradiated for refocusing. Very disadvantageous about this The procedure, however, is that the data acquisition is a clear one longer It takes time as if only a readout gradient was switched on for refocusing. Besides, will this means that much more energy is radiated into the body to be examined. this leads to to an adverse burden on the object under examination.

To further accelerate the recording technique the data acquisition can also be reduced by not the maximum amount of data in one of the Fourier transforms subject data matrix is recorded in k-space. For example can in one embodiment only half of the data volume and the other half are filled with zeros. In another embodiment only 80% of the lines are recorded in k-space. The rest Part is again filled with zeros. In all such cases a limited resolution accepted the figure. In many cases, however, this is for the clinical Diagnostics, at least for a first orientation examination, sufficient.

• – einen statischen Magnet, insbesondere einen supraleitenden Elektromagnet,
• – Gradientenvorrichtungen zur Erzeugung von Gradientenpulsen in drei orthogonal aufeinander stehenden Raumrichtungen, diese Vorrichtungen werden von stromdurchflossenen Spulen gebildet,
• – eine Sendevorrichtung zur Erzeugung von Hochfrequenzsignalen, es handelt sich hierbei insbesondere um eine RF-Sendespule,
• – eine Empfangsvorrichtung für Hochfrequenzsignale, es handelt sich hierbei vorzugsweise um eine RF-Empfangsspule,
• – eine Vorrichtung zur Ansteuerung der Gradientenvorrichtungen und der Sendevorrichtung, es handelt sich hierbei um Verstärker, sowie um programmierbare Vorrichtungen, mit denen die Gradientenpulssequenzen generiert werden können, ferner handelt es sich hierbei auch um programmierbare Vorrichtungen, mit denen die Sende- und Empfangsspulen angesteuert werden können,
• – eine Auswertevorrichtung und
• – eine Anzeigevorrichtung.

The device according to the invention has the following essential features:

• A static magnet, in particular a superconducting electromagnet,
• - Gradient devices for generating gradient pulses in three orthogonal to each other Spatial directions, these devices are formed by current-carrying coils,
• A transmitting device for generating high-frequency signals, in particular an RF transmitting coil,
• A receiving device for high-frequency signals, this is preferably an RF receiving coil,
• A device for controlling the gradient devices and the transmitting device, these are amplifiers, as well as programmable devices with which the gradient pulse sequences can be generated, furthermore there are also programmable devices with which the transmitting and receiving coils are controlled can,
• - An evaluation device and
• - a display device.

The sending device and the receiving device can in a preferred embodiment of the Invention can be realized by a common device. In this case is additional a changeover switch is provided to control these devices serves and switches between the send mode and the receive mode.

Serve to explain the invention in more detail following figures, which are described in the context of the individual examples become. They show in detail:

1 : a schematic representation of a gradient echo pulse sequence;

2a : A schematic representation of a flow-compensated gradient echo pulse sequence for recording two-dimensional MR data without special gradient circuits to suppress moving MR signal carriers;

2 B : a schematic representation of a gradient echo pulse sequence for recording two-dimensional MR data in a first embodiment according to the invention with inserted flow dephasing gradient pulses (marked in dark);

2c : a schematic representation of a gradient echo pulse sequence for recording two-dimensional MR data in a second embodiment according to the invention with newly calculated flow-flow dephasing gradient pulses which are used simultaneously for imaging and for suppressing moving MR signal carriers;

3 : Single images, taken with a 3D gradient echo pulse sequence with flow dephasing in one spatial direction, of a Copenhagen rat after administration of a lymphatic MR contrast medium;

4 : Comparison of individual images, taken with a 3D gradient echo pulse sequence, from a Copenhagen rat, without and with flow dephasing gradient circuits in all three spatial directions;

5 : High-resolution gradient echo MR images of a Watanabe rabbit 12 h after administration of a plaque-common MR contrast medium without signal carriers moving with and with signal suppression in all three spatial directions;

6 : High-resolution gradient echo MR images as in 5 . 28 h after administration of the plaque-common MR contrast medium.

In 1 a schematic representation of a gradient echo _{pulse sequence} is initially shown to illustrate the parameters G _bipolar , t _ramp , t _plateau and t _sep in a plot of G (t) (gradient field strength) over time t. The meaning of the individual parameters is explained in more detail above.

In 2 gradient echo pulse sequences are shown for image display.

In 2a a sequence is initially shown which does not have any flux dephasing gradient pulses for dephasing moving nuclear spins, but rather a sequence with flux compensation, ie a sequence in which M ₀ and M _{1 are} each zero. The representation shows the gradient switching in the three spatial directions over time.

The upper display ("G _silice ") shows the gradient _{pulse sequence} for the _{slice selection} in the z direction. An excitation RF pulse is radiated in during the first gradient pulse. A specific slice is selected by this slice gradient pulse, since only there is the The subsequent pulses in the z direction with reversed or the same polarity serve to refocus the defocus caused by the first pulse and to set the condition that both M ₀ and M _{1 become} zero.

In the lower representation (“G _phase ”), pulses for phase encoding the nuclear spins are shown schematically. With each repetition of the pulse sequence shown, the size and polarity of this phase gradient pulse is incrementally changed between two extreme values — G _phase and + G _phase .

The readout gradient pulses are shown in the middle representation (“G _read ”). As in the case of the slice gradient pulses, the pulse sequence is calculated in such a way that the conditions of M ₀ = 0 and M ₁ = 0 are met - Gradient pulses are frequency-coded depending on their location, and during the last pulse the signal of the nuclear spins is generated in the xy-plane through refocusing that is recorded.

In 2 B is a schematic representation of the gradient echo pulse sequence for recording two-dimensional MR data in a first embodiment according to the invention. On the one hand, this sequence contains the sequence 2a , which has no flow dephasing gradient pulse sequences, but only flow-compensated imaging gradient echo pulse sequences. In addition, gradient circuits are shown in dark color, which have also been inserted into the imaging sequences and which serve to dephasize the nuclear spins in moving media without the imaging gradient echo pulse sequences being influenced. In this case, the first-order gradient moments M _{1 were} inserted in all spatial directions (slice, phase and readout), so that the signals of nuclear spins which move in any spatial direction during the measurement are suppressed.

In 2c is a schematic representation of the gradient echo pulse sequence for recording two-dimensional MR data in a second embodiment according to the invention. In this sequence they are originally in 2a Imaged gradient echo pulse sequences can no longer be recognized separately. These gradient echo pulse sequences have arisen from recalculation taking into account flow dephasing gradient pulse sequences.

The examples described below have been implemented on clinical MR tomographs.

Example 1:

In a first variant, inserted bipolar gradient echo pulse sequences were used on a 1.5 Tesla whole-body tomograph (Magnetom Vision, Siemens, Erlangen), which has a maximum gradient field strength G _max = 25 mT / m and a maximum increase rate s _max = 42 T / (m s) had.

A 3D FLASH sequence was used as the starting sequence used whose parameters for optimized the imaging of small animals (high spatial resolution) was.

In order to demonstrate the effectiveness of suppressing the signal from blood vessels, a Copenhagen rat with stimulated lymph nodes was administered with a contrast agent in an animal experiment. The contrast medium was chosen so that it remained in the blood stream for a long time and massively reduced the relaxation times T ₁ and T _{2 there} . It was a gadolinium complex with a fluorinated side chain, which has the following chemical name: [10- {(RS) -1 - [({[(5S) -6- {4 - [(heptadecafluorooctyl) sulfonyl] piperazin-1- yl} -5 - {[(alpha-D-mannopyranos-1-O-yl) oxy] acetylamino} -6-oxohexan-1-yl] carbamoyl} methyl) -carbamoyl-kappa O] ethyl} -1.4, 7,10-tetraazacyclododecane-1,4,7-triacetato (3 -) - kappa N1, kappa N4, kappa N7, kappa N10, kappa 01, kappa 04, kappa O7] gadolinium. 50 μmol Gd / kg body weight were injected iv.

In a first experiment, selective bipolar flow dephasing gradient pulse sequences were first inserted in one spatial direction. The recordings received are in 3 recognizable: The conditions for the recording were as follows: echo time TE = 14.0 ms, size of the field of view FOV = 60 × 120 mm ² ; Layer thickness SL = 0.32 mm; Matrix: 104 x 256; BW = 150 Hz / pixel; flip-winket α = 15 °; Recording time TA = 3 min 42 s.

In the upper picture in 3 the inguinal lymph nodes of the rat are clearly visible through a strong contrast enhancement (arrows). It can be seen that blood vessels that run in the direction of the inserted gradient pulses are shown with little signal. Since the contrast medium used was absorbed by the lymphatic system, in which the speed of movement of the signal carriers is very slow compared to the blood flow, a signal-rich display of the lymph nodes was achieved.

In the lower picture in 3 it can also be seen that the aorta (open arrows) which runs in the readout direction shows no signal because of the suppression in the readout direction. In contrast to this, the kidney vein running perpendicular to it (closed arrow) was not suppressed, since a corresponding flow dephasing gradient pulse sequence was not switched in this direction. However, the signal from the renal vein could also have been significantly reduced if suitable flow dephasing gradient pulse sequences had also been switched in this direction. This would, however, have required relatively large first order gradient moments M ₁ , since the speed of the signal carriers in the veins is relatively low, so that it would have been necessary to reduce ν _max .

In 4 are images taken with a 3D gradient echo pulse sequence, without and with flow dephasing gradient circuits in comparison to one another, again taken on a Copenhagen rat. In this case, bipolar gradient pulses were inserted in all three spatial directions. The corresponding recordings are on the right side of 4 to see. On the left are reproductions which were obtained without the insertion of flow dephasing gradient circuits.

In this experiment, the movement sensitivity ν _{max was changed} in the range from 2.56 cm / s to 36.5 cm / s by varying the echo time TE from TE _min = 9.4 ms to 18 ms. The other parameters were: G _bipolar = 20 mT / m; t _ramp = 0.6 ms; t _plateau = ½ (TE - TE _min ) = 0 to 8.6 ms; t _sep = 3.7 ms; TR = 19.1 ms to 25.5 ms; Size of the field of view FOV = 40 × 80 mm ² ; Layer thickness SL = 0.5 mm; Matrix: 128 x 256; BW = 150 Hz / pixel; flip angle α = 25 °; Recording time TA = 2 min 29 s.

The pulse sequence was implemented that with increasing echo time TE (from top to bottom in the recording sequence) ever lower speeds were sufficient to move the signal Suppress spins.

Since a prolonged echo time TE also causes a reduction in the MR signal via an increased T ₂ decay, the experiment was carried out for all echo times even without the usual gradient for flow dephasing.

The figure shows a comparison of the two series of images, which shows how the MR signal in the aorta was increasingly suppressed with decreasing ν _max , so that the iliac lymph nodes lying next to the aorta were more and more distinguishable from the aorta. A clear demarcation from the aorta was only possible below 10 cm / s.

Example 2:

A second variant was implemented on a 1.5 Tesla whole-body tomograph (Magnetom Symphony, Siemens, Erlangen) with a maximum gradient field strength G _max = 30 mT / m and a maximum increase rate s _max = 120 T / (m · s).

In an animal experiment, the aforementioned intravascular gadolinium contrast medium [10 - {(RS) -1 - [({[(5S) -6- {4 - [(heptadecafluorooctyl) sulfonyl] piperazin-1-yl} - was 5 - {[(alpha-D-mannopyranos-1-O-yl) oxy] acetylamino} -6-oxohexan-1-yl] carbamoyl} methyl) carbamoyl-kappa O] ethyl} -1,4,7,10 -tetraazacyclododecan-1,4,7-triacetato (3 -) - kappa N1, kappa N4, kappa N7, kappa N10, kappa O1, kappa O4, kappa O7] -adolinium in an amount of 0.1 mmol / kg body weight iv injected, which accumulates in plaques. With a given speed sensitivity of ν _max = 10 cm / s, depending on the spatial resolution, echo times between 8.0 ms and 9.5 ms were realized. Image data were recorded with and without flow dephasing in all three spatial directions after a long time after administration.

In 5 are shown the image data obtained after 12 h after the administration and in 6 the image data acquired after 28 h after administration of the contrast medium.

Even after 28 h, the contrast medium signal in the blood vessels was still so strong that plaques could only be clearly identified in the flow-dephased image data:
In the 5 shown recordings were taken under the following conditions: TR = 16 ms; TE = 9.4 ms; FOV = 135 x 180 mm ² ; SL = 2 mm; Matrix: 307 x 512; BW = 245 Hz / pixel; α = 30 °; TA = 2 min 37 s.

In this figure are high-resolution gradient echo MR images without (left pictures) or with (right pictures) signal suppression moving signal carrier shown. While with river dephasing the inside of the blood vessels (arrows) dark against the bright, contrasting plaque could not be in the recording without river dephasing possible, Identify plaques.

For inclusion in 6 the following parameters were applied: TR = 14 ms; TE = 8.5 ms; FOV = 200 x 200; SL = 2 mm; Matrix: 205 x 256; BW = 245 Hz / pixel; α = 30 °; TA = 1 min 32 s.

This figure shows images of the rabbit 28 hours after the administration of the plaque-common gadolinium complex, the left image being obtained without and the right one with signal suppression moving signal carriers in all three spatial directions (ν _max = 10 cm / s). As in 5 could only be separated from the blood vessels in the flow-dephased measurement.

Claims (15)

Imaging device, comprising a) one Magnetic resonance imaging device for obtaining data for a spatially resolved image display the magnetic resonance behavior of atomic nuclei in a selected observation area in a body, being the device is designed and programmed so that the body through the device high frequency and magnetic field gradient echo pulse sequences can be exposed, which generate a magnetization in the body, so that the Magnetization of a medium flowing in at least one spatial direction in the body can be weakened by dephasing the spins of atomic nuclei in the medium, and b) one of the body MR contrast medium taken. Arrangement according to claim 1, characterized in that the magnetization of the medium flowing in the at least one spatial direction in the body can be weakened by dephasing the spins in that gradient moments of the i-th order M _i (t) can be maximized in this spatial direction according to the following relationship:

where i is an integer greater than zero, γ is the gyromagnetic ratio of the atomic nuclei, G (t ') is a time-dependent gradient field strength in this spatial direction and t is the time elapsed since the irradiation of a high-frequency pulse to excite the atomic nuclei. Arrangement according to claim 2, characterized in that the magnetization of the medium flowing in the at least one spatial direction in the body can be weakened by dephasing the spins in that first-order gradient moments M ₁ (t) in this spatial direction according to the following relationship:

are maximizable. Arrangement according to one of the preceding claims, characterized characterized in that the gradient echo pulse sequences in the respective Spatial directions by insertion from river dephasing gradient pulses to flow compensated imaging Gradient echo pulse sequences can be generated. Arrangement according to claim 4, characterized in that M ₁ satisfies the following relationship: M 1 (T; G bipolar , t ramp , t plafeau , t sep ) = γG bipolar · t ramp + t plateau ) * (2 tramp + t plateau + tsep) [7] Arrangement according to one of the preceding claims, characterized characterized that the device - a static magnet, - gradient devices to generate gradient pulses in three orthogonal to each other standing spatial directions, - one Transmitter device for generating high-frequency signals, - a receiving device for high frequency signals, - a device for controlling the gradient devices and the transmitting device, - an evaluation device and - one Display device. Arrangement according to one of the preceding claims, characterized characterized that the MR contrast medium a human or animal body intravenous is administrable. Arrangement according to one of the preceding claims, characterized characterized in that the MR contrast medium is lymph and / or plaque common. Process for spatially resolved image display of the magnetic resonance behavior of atomic nuclei in a selected one Viewing area in a body, in the data from the observation area by means of a magnetic resonance tomography device can be obtained by the body using high frequency and magnetic field gradient echo pulse sequences exposed to create a magnetization in the body, so that the magnetization of medium flowing in at least one spatial direction in the body is weakened by dephasing the spins of atomic nuclei in the medium and that the body an MR contrast medium is supplied becomes. A method according to claim 9, characterized in that the magnetization of the medium flowing in the at least one spatial direction in the body is weakened by dephasing the spins by maximizing gradient moments of the i-th order M _i (t) in this spatial direction according to the following relationship:

where i is an integer greater than zero, y is the gyromagnetic ratio of the atomic nuclei, G (t ') is a time-dependent gradient field strength in this spatial direction and t is the time elapsed since the irradiation of a high-frequency pulse to excite the atomic nuclei. A method according to claim 10, characterized in that the magnetization of the medium flowing in the at least one spatial direction in the body is weakened by dephasing the spins by maximizing first order gradient moments M ₁ (t) in this spatial direction according to the following relationship:

Method according to one of claims 9 - 11, characterized in that that the gradient echo pulse sequences in the respective spatial directions by inserting from river dephasing gradient pulses to flow compensated imaging Gradient echo pulse sequences are generated. A method according to claim 12, characterized in that M ₁ satisfies the following relationship: M 1 (t; G bipolar , t ramp , t plateau , t sep ) = γG bipolar · (T ramp + t plaleau ) * (2 tramp + tplateau + t sep ) [7] Method according to one of claims 9 - 13, characterized in that that the MR contrast medium is administered intravenously to a human or animal body becomes. Method according to one of claims 9 - 14, characterized in that that the MR contrast medium is lymph and / or plaque common.