DE102008019862B4 - Magnetic resonance apparatus and method for operating a magnetic resonance apparatus - Google Patents

Abstract

Magnetic resonance device with a housing (1), which has magnetic coils for generating the magnetic fields required for magnetic resonance formation and an opening (2) for receiving at least part of the body of a patient (3), with a detection and evaluation device for imaging reproduction and with Radar transmitting and receiving device (5, 6) for detecting a movement in a part (8, 9) of the body (3) to be examined during magnetic resonance imaging, characterized in that the radar transmitting and receiving device (5, 6) is arranged within the housing that ultra-wideband radar signals emitted by the radar transmission and reception device (5, 6) are directed in the form of short radar pulses onto the part (8, 9) of the body (3) to be examined and that in the radar transmission and receiving device (5, 6) an evaluation device generates pulses by the radar pulses emitted by a reflecting Ge weave transition in the part (8, 9) of the body (3) to be examined reflected radar signals are compared by an autocorrelation function or cross-correlation function and that a ...

Description

The invention relates to a magnetic resonance apparatus having a housing which has magnetic coils for generating the magnetic fields required for magnetic resonance imaging and an opening for receiving at least a part of the body of a patient, with a detection and evaluation device for imaging and with a Radarsende- and -empfangseinrichtung for detecting movement in a part of the body to be examined during magnetic resonance imaging.

The invention further relates to a method for operating a magnetic resonance apparatus, with which a magnetic resonance imaging of a part of the body to be examined of a patient is performed, wherein during the magnetic resonance imaging with a Radarsend- and -empfangseinrichtung the movement in a part of the body to be examined is detected.

Magnetic resonance devices of this type have long been used for non-invasive medical diagnostics with non-ionizing radiation. The imaging with magnetic resonance devices, in particular magnetic resonance tomographs (magnetic resonance tomographs), allows the detection of disease foci, which are not detectable with other imaging methods, for example X-ray diagnostics. In particular, it is possible to establish diagnoses in soft tissues, for example in the brain or in glandular tissues, pathological changes, in particular tumors.

The imaging reproduction of body sections by means of magnetic resonance tomography usually requires a resting position of the patient during the recording.

In principle, consideration could also be given to mapping dynamic processes on the human body. In particular, it would be desirable to enable high-resolution magnetic resonance imaging of the human heart. High-resolution imaging of the coronary arteries using magnetic resonance imaging could avoid a number of problems with conventional contrast-enhanced X-ray imaging during catheterization (coronary angiography), particularly radiation exposure and mortality risk during surgery. In addition, magnetic resonance imaging could be used as an alternative imaging method in the application of cardiac catheters, for example, to allow biopsies from the heart muscle to set stents or perform other minimally invasive procedures on the heart.

For a cardiac examination by means of magnetic resonance imaging, there is the fundamental problem that, on the one hand, the heart is moving due to the activity of the heart and, on the other hand, the patient is carrying out breathing movements during the examination. Movement artifacts in particular, which disturb the evaluation of the magnetic resonance image or make it impossible, occur in particular due to the respiratory movement. It is therefore customary to take the images for magnetic resonance imaging while the patient is holding his breath in order to eliminate the respiratory movement. However, this method is not applicable to imaging the coronary arteries because image data acquisition can take several minutes, while the average patient can only hold his breath for a fraction of a minute.

In order to prevent the movement artifacts by the heart activity itself, the magnetic resonance image data acquisitions are triggered triggered by ECG signals (electrocardiogram signals). This navigation technique for magnetic resonance imaging of the heart is not very precise and moreover has the disadvantage that the ECG electrodes in conjunction with the RF pulses applied by the magnetic resonance system represent a risk of burns and with the switched gradient fields due to nerve stimulation a safety risk.

WO 2007/063516 A2 discloses a device for non-contact remote detection of the heartbeat in that the movement of the chest wall caused by the heartbeat is scanned without contact. This is possible optically, but also with electromagnetic radiation in the radar frequency range. The result is a cyclic signal of the heartbeat. This signal can be synchronized with an imaging device, such as a magnetic resonance device, by triggering the control of the imaging device with a phase of the recorded heartbeat. The trigger time is thereby determined by defining a characteristic area of the heartbeat as the zero point and calculating the trigger point after a predetermined time has elapsed. The predetermined time can be varied depending on the detected pulse rate.

In the article "Wearable System-on-a-Chip Pulse Radar Sensors for Health Care: System Overview" by the authors Zito et al. It is known to provide a portable meter with an ultra wide band radar transmitter which is aimed at the heart wall or chest wall to detect in real time the pulse rate and respiratory rate. The evaluation of the signals is carried out by cross-correlation of the emitted pulse with the received pulse. The system serves the Long-term monitoring of patients. A control of other devices is not provided here.

The object of the present invention is to design and operate a magnetic resonance apparatus such that motion artifacts can be avoided or corrected and control of the magnetic resonance apparatus with high accuracy is possible.

To achieve this object, a magnetic resonance apparatus of the type mentioned above is characterized in that the Radarsende- and -empfangseinrichtung is disposed within the housing that radiated from the radar transmitter and receiver ultra-wideband radar signals in the form of short radar pulses on the part to be examined Body are directed and that in the Radarsende- and -empfangseinrichtung an evaluation device pulses generated by the fact that the emitted radar pulses are compared with reflected by a reflective tissue transition in the part of the body to be examined radar signals by an autocorrelation function or cross-correlation function and that a mean time (τ ₁ , τ ₂ ) is adjusted for the reflected pulse so that the average transit time (τ ₁ , τ ₂ ) is always on one edge of the pulse formed in the evaluation device, whereby a measured amp Litude at a time (τ ₁ , τ ₂ ) is used as a measurement for the determination of the position, to utilize only in a same position of the reflective tissue junction in the part of the body to be examined detected magnetic resonance images.

To achieve the object, a method of the type mentioned above is characterized in that the Radarsende- and -empfangseinrichtung is disposed within the housing that emitted by the radar transmitter and receiver ultra-wideband radar signals in the form of short radar pulses on the part to be examined Body are directed and that in the Radarsende- and -empfangseinrichtung an evaluation device pulses generated by the fact that the emitted radar pulses are compared with reflected by a reflective tissue transition in the part of the body to be examined radar signals by an autocorrelation function or cross-correlation function and that an average transit time (τ ₁ , τ ₂ ) is adjusted for the reflected pulse so that the average transit time (τ ₁ , τ ₂ ) is always on one edge of the pulse formed in the evaluation device, whereby a measured amplitude to one m time (τ ₁ , τ ₂ ) serves as a measured value for the determination of the position in order to utilize magnetic resonance images acquired only in a same position of the reflecting tissue transition in the part of the body to be examined.

The present invention thus provides for combining the magnetic resonance imaging with a distance measurement with an ultra-wideband radar (UWB radar), with the ultra-wideband radar in particular position changes by the respiratory movement or by the heart movement are detectable. Since the radar transmitting and receiving device occupies a defined position within the housing of the magnetic resonance device, a measurement of the changing distance of a body part or tissue within a body part enables a position determination. This position determination can be exploited to carry out the magnetic resonance imaging only in one and the same position, for example of the heart in the heartbeat cycle, or the chest wall in the respiratory cycle, or to carry out a subsequent correction of the image data obtained in an alternative magnetic resonance image acquisition method.

The radar transmitter and receiver may be constructed with a combined transmit and receive stage and in this case have a single antenna. A simple construction results if, alternatively, a receiving stage separate from the transmitting stage, each with its own antenna, is provided. Furthermore, it is possible to use a plurality of radar transmitting and receiving devices in order to be able to carry out an improved and possibly extended position determination.

The radar transmitting and receiving device may be constructed in the housing of the magnetic resonance apparatus so that it is compatible with the high static magnetic field and the switched gradient fields of the magnetic resonance apparatus. In addition, when designing the UWB radar, care must be taken that the receiver input of the UWB device is not destroyed by the radiofrequency and gradient fields of the magnetic resonance device. For this purpose, the at least one antenna of the UWB radar device can be constructed of a material (eg copper) and designed in a manner known per se so that only minimal induction voltages occur due to the switched gradient fields.

In order to obtain measurement data with the required accuracy (position determination with an accuracy <1 mm) with the UWB radar, a relatively high sampling frequency must be selected. The frequency range of the UWB radar signals is expediently limited to 0 to 11 GHz. The maximum transmit power shall not exceed the permissible interference from electrical appliances (eg -41.3 dBm / MHz as determined by the FCC and ITU) exceed, in order to exclude a disturbance of narrow-band systems. For a narrowband receiver, such UWB signal then appears as noise, so that the UWB signal is not subject to frequency restrictions. An increase in the bandwidth or the sampling frequency is not recommended, because above 10 GHz, an increased attenuation of the signals in the tissue occurs.

Based on a sampling frequency of 9 GHz results in a maximum delay resolution of the reflection signals of about 111 ps. This corresponds to a motion resolution of about 1.7 cm, when ε _r = 1.

The motion resolution of a water-rich layer lying in the body (for example, the heart wall with ε _r ≈ 50, ε _r dielectric constant of the tissue) depends on the dielectric properties of the surrounding layers. In a favorable case, these are dielectric materials with a small ε _r (ε _r ≈ 1) (eg for fat layers and the pericardial layer), which is why an increase of the motion resolution by a maximum of √50 is to be expected the propagation of the signal through the various tissue layers occurring attenuation and dispersion effects can be neglected.

For a resolution <1 mm, a sampling frequency of more than 150 GHz would have to be provided. According to the invention, it is possible to work with a sampling frequency of <10 GHz (for example 8.95 GHz) and still achieve a resolution in the sub-millimeter range, if the distance determination on an edge of the reflected radar pulse is determined by determining the amplitude after a predetermined transit time is on the edge of the reflected radar pulse is made. In this way, movements with an amplitude of 1 mm can be reliably detected.

The invention will be explained in more detail below with reference to the accompanying drawings. Show it:

1 a frontal front view of a magnetic resonance device with an opening for receiving a patient;

2 a longitudinal section through the magnetic resonance apparatus according to 1 with a patient in it;

3 a schematic representation of the amplitude of movement of a chest wall and a heart, which are irradiated with a radar transmitting and receiving device;

4 a schematic representation of the resulting by the reflection on the chest wall or on the pericardial wall reflected radar pulses, as received and evaluated by a receiving stage of Radarsend- and -empfangseinrichtung;

5 a measurement curve for a movement amplitude of 1 mm;

6 a comparative representation of a test movement shown as a reference signal and their detection with a magnetic resonance apparatus and a UWB radar according to the present invention.

1 shows a conventional structure of a magnetic resonance device whose housing 1 in a known manner an end opening 2 in which a patient 3 - Depending on the task of investigation - completely or partially inserted. The magnetic resonance apparatus includes in a known manner not shown here excitation and detection coils and an evaluation that regularly a two-dimensional image representation of a corresponding to the position of the patient 3 formed section through the patient's body 3 reproduces.

According to the invention is in the opening 2 of the housing 1 an antenna mount 4 provided at the one transmitting antenna 5 a transmission stage and a receiving antenna 6 a receiving stage of a Radarsend- and -empfangseinrichtung are attached.

2 clarifies the position of the transmitting antenna and receiving antenna 5 . 6 necessary for a heart examination of the patient 3 are arranged.

2 illustrates a special feature of a magnetic resonance imaging, in which a magnetic resonance transmitting and receiving coil 7 is additionally used, which is placed on the chest of the patient. Even with this arrangement, the inventive use of a UWB radar is possible.

3 schematically shows that the radar signals of a Radarsendeantenne 5 on a chest wall 8th and a pericard (pericardium) 9 of a heart 10 a patient 3 reach. A middle position of the corresponding tissues, chest wall 8th and pericardium 9 is in 3 illustrated by solid lines. Dashed lines show the limit amplitudes for the movement of the chest wall 8th while breathing and the pericardium 9 at the heartbeat.

Accordingly, in 4 a first impulse 11 as on the chest wall 8th reflected signal and a second pulse 12 as at the pericardium 9 reflected pulse with predetermined by the movement amplitudes position shifts in each case a solid line and two dashed lines.

These pulses are thereby generated in the evaluation device that the emitted, very short radar pulse is passed into the receiving device of the Radarsende- and -empfangseinrichtung and there with the through the receiving antenna 6 received radar signals is compared by an autocorrelation function or cross-correlation function, whereby the in 4 illustrated pulse 11 . 12 as a peak of the autocorrelation function or cross-correlation function arises. As ultrabreitbandige radar signals corresponding pulses, such as Gaussian pulses, or a pseudo noises can be used.

The evaluation of the distance is now done in an advantageous manner in that the knowledge of the distance of the chest wall 8th from transmitting antenna 5 and the receiving antenna 6 - And thus the associated transit time τ ₁ - is known, and that a running time τ _{1 is} set, for which the reflected pulse 11 within its position variation always on one edge of the pulse 11 lies. The evaluation device thus samples the reflected pulse at time τ ₁ and determines its amplitude A (τ ₁ , t _x ). In 4 are the possible amplitudes at the time of inhalation (maximum proximity of the chest wall 8th to the transmitting antenna 5 and receiving antenna 6 ), in a middle position characterized by solid lines, at the time of exhalation (maximum distance of the chest wall 8th from the transmitting antenna 5 and receiving antenna 6 ) illustrates.

By the appropriate amplitude determination at the time τ ₁ , the desired resolution increase can be achieved.

In a similar way, the evaluation for the pericardium 9 reflected pulse 12 at time τ ₂ according to the same method.

5 has been detected with a UWB radar at a distance of 43 cm from a skin-tissue equivalent that deals with a sine function 13 with an amplitude of 1 mm and a frequency of 0.75 Hz.

The resulting measured curve 14 indicates that the movement of less than 1 mm can be detected with the UWB radar, using the expansion method as described by 4 has been explained.

6 shows a comparison representation of a reference function (reference), which has been recorded with a magnetic resonance device (MR) and with a UWB radar (UWB). The measurements were performed with a planar, tissue-equivalent phantom simulating the passive electrical properties of the skin. The phantom has been moved with a stepper motor in different ways. For clarity, the curves have been pulled apart in the ε-direction, since they would run very much without this representation. The illustration shows that the movement of a body part with a sufficient resolution can be tracked with a UWB radar, so that the position data measured by the UWB radar can be used both for triggering the magnetic resonance device and for the subsequent correction of the image data of the magnetic resonance device.

Claims (7)

Magnetic resonance device with a housing ( 1 ), the magnetic coils for generating the magnetic fields required for a magnetic resonance and an opening ( 2 ) for receiving at least part of the body of a patient ( 3 ), with a detection and evaluation device for imaging reproduction and with a Radarsend- and -empfangseinrichtung ( 5 . 6 ) for detecting a movement in a part to be examined ( 8th . 9 ) of the body ( 3 ) during magnetic resonance imaging, characterized in that the radar transmitting and receiving device ( 5 . 6 ) is disposed within the housing, that of the Radarsend- and -empfangseinrichtung ( 5 . 6 ) radiated ultra-wideband radar signals in the form of short radar pulses on the part to be examined ( 8th . 9 ) of the body ( 3 ) and that in the radar transmitting and receiving device ( 5 . 6 ) an evaluation device generates pulses in that the emitted radar pulses are transmitted by a reflecting tissue transition in the part to be examined ( 8th . 9 ) of the body ( 3 ) are compared by an autocorrelation function or cross-correlation function and that an average transit time (τ ₁ , τ ₂ ) is set for the reflected pulse so that the mean transit time (τ ₁ , τ ₂ ) always on an edge of the in the evaluation device formed pulse, whereby a measured amplitude at a time (τ ₁ , τ ₂ ) serves as a measurement for the determination of the position in order to only in a same position of the reflective tissue transition in the part to be examined ( 8th . 9 ) of the body ( 3 ) to utilize acquired magnetic resonance images. Magnetic resonance apparatus according to claim 1, characterized in that the radar transmitting and receiving device ( 5 . 6 ) has a transmission stage and a separate reception stage. Magnetic resonance apparatus according to claim 1 or 2, characterized in that the magnetic resonance imaging is controllable by the measured position. Magnetic resonance apparatus according to claim 1 or 2, characterized in that the data obtained in the magnetic resonance image acquisition can be corrected by the measured position. Method for operating a magnetic resonance apparatus, with which a magnetic resonance imaging of a part to be examined ( 8th . 9 ) of the body ( 3 ) during a magnetic resonance imaging with a radar transmitting and receiving device ( 5 . 6 ) the movement in a part to be examined ( 8th . 9 ) of the body ( 3 ), characterized in that the radar transmitting and receiving device ( 5 . 6 ) is arranged within the housing of the magnetic resonance apparatus, that of the Radarsend- and -empfangseinrichtung ( 5 . 6 ) radiated ultra-wideband radar signals in the form of radar pulses on the part to be examined ( 8th . 9 ) of the body ( 3 ) and that in the radar transmitting and receiving equipment ( 5 . 6 ) an evaluation device generates pulses in that the emitted radar pulses are transmitted by a reflecting tissue transition in the part to be examined ( 8th . 9 ) of the body ( 3 ) are compared by an autocorrelation function or cross-correlation function and that an average transit time (τ ₁ , τ ₂ ) is set for the reflected pulse so that the mean transit time (τ ₁ , τ ₂ ) always on an edge of the in the evaluation device formed pulse, whereby a measured amplitude at a time (τ ₁ , τ ₂ ) serves as a measurement for the determination of the position in order to only in a same position of the reflective tissue transition in the part to be examined ( 8th . 9 ) of the body ( 3 ) to utilize acquired magnetic resonance images. A method according to claim 5, characterized in that the magnetic resonance image acquisition is triggered only at the same detected positions. A method according to claim 5, characterized in that the magnetic resonance image data are corrected with information about a detected changed position.

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