Abstract:
A magnetic resonance imaging apparatus has a magnet with at least one patient receptacle and at least one support plate, as well as a predetermined number of transmit and/or receive coils. In at least two predetermined exposure positions, at least one exposure respectively takes place using predetermined adjustment parameters. High-contrast exposures can be obtained in a short time, by the required adjustment parameters being determined in a preceding adjustment process, and the exposures are carried out in a subsequent exposure process.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a magnetic resonance imaging apparatus of the type having a magnet with at least one patient receptacle and at least one support plate, as well as a predetermined number of transmit and/or receive coils, and which allows exposures with the subject in at least two predetermined exposure positions, with at least one exposure taking place using predetermined adjustment parameters. 
     2. Description of the Prior Art 
     In known magnetic resonance imaging apparatuses, for different exposure positions the associated exposure parameters must in general be modified at least partially. The exposure parameters that may have to be modified include e.g. the position of the support plate and the connection or disconnection of transmit and/or receive coils. As a rule, a modification of the exposure parameters requires a readjustment of the magnetic resonance imaging apparatus. 
     The adjustment essentially serves to optimize the RF chain (transmit and receive coils and associated amplifiers) and to optimize the homogeneity of the main magnetic field produced by the magnet (also called the basic magnetic field or B 0  field) in a volume under examination (imaging volume) located inside the patient receptacle. Due to the necessary homogeneity of the examination volume, this volume is also called the homogeneity volume. 
     The exposure parameters are also patient-dependent, since the patient represents an attenuation or damping for the transmit and/or receive coils. A precise adjustment thus also serves for patient safety with respect to the RF exposure. 
     In general, known magnetic resonance imaging apparatuses recognize automatically whether the. exposure parameters belonging. to particular exposure positions must be modified, and carry out a readjustment if warranted. 
     This adjustment normally requires 10 to 90 seconds per exposure position. Given certain examination procedures, this time is not available. This includes e.g. the tracking of doses of contrast agent over a larger body region that exceeds the available homogeneity volume of the nuclear spin resonance apparatus. In such cases, the patient must be guided by displacement of the support plate in a manner corresponding to the flow of contrast agent. If a smaller viewing field is not acceptable, the readjustment that is thereby required per imaging measurement (exposure) requires a multiple dosage of contrast agent, which is not desirable for the patient. 
     Alternatively to a smaller viewing field or to multiple injections of contrast agent, it is possible after the first adjustment to omit the further adjustments (readjustments) inherently required for high-contrast exposures. However, this leads to a considerable worsening of the image quality. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide magnetic resonance imaging apparatus of the type described above that provides high-contrast exposures in a short time, even given an examination of larger body segments. 
     This object is achieved in accordance with the principles of the present invention in a magnetic resonance imaging apparatus having a magnet and at least one patient receptacle and at least one support plate, as well as a predetermined number of transmit and/or receive coils. At least in two predetermined exposure positions, an exposure respectively takes place using predetermined adjustment parameters. The required adjustment parameters are inventively determined in a preceding adjustment process, and the exposures are executed in a subsequent exposure process. 
     For example, the exposure parameters can be modified by means of a spatial modification of position (longitudinal displacement, transverse displacement, rotation) of the support plate within the patient receptacle. Alternatively, or in addition, a modification of the adjustment parameters can take place by connection and/or disconnection of the transmit coils and/or the receive coils. 
     In the inventive magnetic resonance imaging apparatus, the required adjustment parameters are not determined immediately before each individual exposure, as is conventional. Rather, the required adjustment parameters are determined in an adjustment process that precedes the exposure process. Only after the determination of the required adjustment parameters are the exposures carried out, in a separate imaging exposure process. 
     The adjustment parameters are of course stored at least until the conclusion of the examination. The adjustment parameters thus can be used again, when identical or suitably similar exposure parameters (position of the support plate and configuration of the transmit and/or receive coils) are again reached in the context of the same examination. 
     In examinations with the inventive apparatus, high-contrast exposures are thus obtained, since it is not necessary to omit an adjustment. Due to the fact that the adjustment is carried out in a separate adjustment process, and the adjustment parameters are stored until the conclusion of the examination, the transmit and receive coils, or their coil elements, can be switched quickly during the examination, so that, in addition, reduced examination times result. 
     The inventive solution is suitable for a large number of different forms of magnetic resonance imaging apparatuses. Thus, for example, the magnet can be fashioned as a cylindrical magnet (solenoid) or as a horseshoe magnet (C-arm apparatus). Given cylindrically shaped magnets, the patient receptacle is fashioned as a patient tube. 
    
    
     DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic block diagram of a magnetic resonance imaging apparatus constructed and operating in accordance with principles of the present invention. 
     FIG. 2 schematically illustrates an embodiment of an apparatus in accordance with the invention, wherein the magnet which generates the basic magnetic field is a horseshoe magnet. 
     FIG. 3 schematically illustrates an embodiment of an apparatus in accordance with the invention, wherein the magnet which generates the basic magnetic field is a cylindrical magnet. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The apparatus in FIG. 1 has a support plate  1  that is arranged in longitudinally placeable fashion inside an examination volume of a magnet. Within the scope of invention, the magnet can for example a cylindrical magnet  18  as shown in FIG. 3 (solenoid) or a horseshoe magnet (C-arm)  19  as shown in FIG.  2 . Given cylindrically shaped magnets, the patient receptacle is fashioned as a patient tube as own in FIG.  3 . 
     The longitudinal displaceability of the support plate  1  is indicated with a double arrow  2 . Due to the longitudinal displaceability of the support plate  1 , larger body sections of a patient  3  lying on the support plate  1  can be examined. 
     The nuclear spin resonance apparatus shown in the drawing additionally has a predetermined number of transmit coils  4  and a predetermined number of receive coils  5 . 
     The transmit coils  4  can be connected, in a desired configuration, to a generator  7  by means of a transmit coil changeover switch  6 . The generator  7  supplies the transmit coils  4  with current via a-transmit amplifier  8  and via a matching element  9 . 
     The receive coils  5  can be connected, in a desired configuration, to a receiver  11  by means of a receive coil changeover switch  10 . The signals of the connected receive coils  5  are given to the receiver  11  via. a matching element  12  and via a receive amplifier  13 . 
     The configurations of the transmit coils  4  and the receive coils  5 , defined by the transmit coils changeover switch  6  and by the receive coils changeover switch  10 , are supplied to an adjustment unit  14  as inputs. 
     As a further input, the position of the support plate  1 , which is determined by a position sensor  15 , is supplied to the adjustment unit  14 . 
     The adjustment unit  14  processes the inputs that it has received from the transmit coils changeover switch  6 , from the receive coils changeover switch  10 , and from the position sensor  15 , and at its output supplies corresponding control signals to the generator  7 , to the transmit amplifier  8 , to the matching elements  9  and  12 , as well as to the receive amplifier  13  and to the receiver  11 . 
     In addition, the adjustment unit  14  supplies a control signal to a shim coil system  16 . 
     The inputs and the control signals (outputs) are stored, as adjustment parameters, in a memory  17  until the conclusion of the examination. 
     With the embodiment shown in the drawing of the inventive apparatus, larger bodily segments of the patient  3  can be examined. Such examinations are, for example, the tracking of doses of contrast agent over a larger body region, as carried out for example in subtraction angiography or in physiologically controlled imaging. 
     In the context of the preparation for measurement, which in the case of a peripheral angiography at the leg, includes slice positioning along the vascular tree, several measurements are already made without contrast agent. Due to the homogeneity volume of the magnet being too small, in these measurements the support plate  1  must be displaced, and so must be newly adjusted. The associated adjustment parameters for each position of the support plate  1  are stored in the memory  17 . As additional adjustment parameters, the connected configuration of the transmit coils  4 , as well as the connected configuration of the receive coils  5 , are stored in the memory  17 . In addition, the adjustment parameters include the corresponding control signals for the generator  7 , for the transmit amplifier  8 , for the matching elements  9  and  12 , as well as for the receive amplifier  13 , for the receiver  11  and for the shim coil system  16 . 
     After the conclusion of the measurement preparation, which includes the determination of the adjustment parameters, the support plate  1  is guided back into the initial position, and the contrast agent is administered. In the imaging measurement that now takes place, each of the positions of the support plate  1  used in the measurement preparation is newly set in succession, and the transmit coils  4  and the receive coils  5  are connected as in the measurement preparation. Subsequently, an imaging measurement (exposure). is immediately carried out with the known adjustment parameters stored in the memory  17 , i.e. without a new adjustment. 
     In the inventive apparatus, the required adjustment parameters are thus not determined immediately before each individual imaging measurement; rather, the required adjustment parameters are completely determined in a preceding adjustment process, in the context of the measurement preparation. According to the invention, the adjustment process thus precedes the exposure process. Only after the determination of the required adjustment parameters are the exposures (imaging measurement) carried out, in a separate exposure process. In examinations with the inventive apparatus, high-contrast exposures. are thereby obtained, since it is not necessary to omit an adjustment. In addition, due to the fact that the adjustment parameters are stored in a memory  17  until the conclusion of the examination, reduced examination times result. Due to the short examination times, in subtraction angiography the course of the contrast agent can thus be tracked without chronological gaps. 
     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.