1. Field of the Invention
The invention is directed to a method for operating a magnetic resonance apparatus comprising a magnetic stimulation device, and relates to a magnetic resonance device for implementing the method.
2. Description of the Related Art
The magnetic resonance technique is a known technique for generating images of the inside of a body of an examination subject. For this purpose, rapidly switched gradient fields are-superimposed onto a static basic magnetic field in a magnetic resonance apparatus. In order to initiate magnetic resonance signals, high-frequency signals are irradiated into the examination subject and the initiated magnetic resonance signals are accepted by a receiver, by which image datasets and magnetic resonance images are prepared on the basis of these magnetic resonance signals.
In medicine, all methods using repeated scanning of a structure of organs and tissues for imaging chronologically changing processes, such as physiological functions or pathological processes, are referred to as functional imaging. More concretely, with respect to the magnetic resonance technique, these are measuring methods which make it possible to identify and image areals in the nervous system, particularly cerebral areals of a patient stimulated by sensory stimuli and/or by a motor task, sensory task or cognitive task. For example, acoustic or visual stimuli represent sensory stimuli. In the simplest case, a defined movement, such as of the hand or a finger, represent one of the motor tasks.
The functional magnetic resonance imaging is based on the BOLD effect (Blood Oxygen Level Dependent). The BOLD effect is based on different magnetic properties of oxygenated and deoxygenated hemoglobin in the blood. An intensified neuronal activity in the brain is locally associated with an increased supply of oxygenated blood in which the intensity is correspondingly increased at a corresponding location given a magnetic resonance image that is generated by a gradient echo sequence. The BOLD effect, regarding an event initiating the neuronal activity, occurs with a time delay of a few seconds.
In the functional magnetic resonance imaging, three-dimensional image datasets of the brain are acquired (for example, every two to four seconds) based on, for example, an echo planar method. Echo planar methods have the advantage that the acquisition of image datasets are extremely fast—less than 100 ms is required for an individual three-dimensional image dataset. Image datasets with or without a specific neuronal activity are acquired at different points in time. In order to create the functional image, the images that are generated with neuronal activity are differentially compared to the ones without neuronal activity for identifying active cerebral areas. The image datasets acquired with neuronal activity and without neuronal activity are averaged during the comparison, for example, as a result of the comparatively weak BOLD effect of the magnetic resonance technique, so that the functional image contains a secured functional information. The exemplary article of U. Klose et al. “Funktionelle Bildgebung mit der Magnetresonanztomographie”, electromedica 67 (1999) book 1, pages 27 to 36 describes the magnetic resonance imaging in greater detail.
German Patent No. 199 14 762 A1 describes a high-frequency head antenna for a magnetic resonance apparatus, in which this high-frequency head antenna forms a structural unit together with a coil arrangement for the transcranial magnetic stimulation. As a result of the structural unit, a high signal-to-noise ratio can be obtained during the acquisition of magnetic resonance signals by the high-frequency head antenna, for example in the framework of a functional magnetic resonance imaging of the brain, and magnetic fields stimulate a selectable location at the surface of the brain. The coil arrangement is fashioned such that it can be separated from its current supply, for example, so that a damaging current induction does not occur in the coil arrangement during an operating state of the magnetic resonance apparatus in which high-frequency signals are transmitted.
European Patent No. 0 958 844 A2 describes a magnetic stimulation device comprising at least one stimulation coil which is connected, with its terminals, to the output of at least one controllable power converter and which has inductivity, current and voltage carrying capacity, so that the stimulation pulses generated by it intersperse at least a volume in the size of extremities, head or torso of a patient with a prescribable magnetic field intensity. The controllable power converter has at least one power semiconductor switch that can be turned on and off and that has short switching times; the power converter, with its input, is switched to a voltage intermediate circuit and the voltage intermediate circuit and the controllable power converter are designed for high output voltages and output currents so that action potentials can also be initiated in neuromuscular tissue of a patient that is situated deeper.
International patent document WO 99/59674 describes a magnetic stimulation device for initiating action potentials, particularly in deeply situated neuromuscular tissue of a patient. For this purpose, the magnetic stimulation device has at least one stimulation coil which is connected, with its terminals, to the output of a current generation unit. Since the current generation unit provides current pulses for the stimulation coil, by which these current pulses are generated in a non-resonant manner, the current pulse shapes can be selected more freely with respect to the magnetic stimulation device. All art references cited above are incorporated herein by reference.