Abstract:
A measuring device for measuring the intermediate circuit voltage of a gradient amplifier of a magnetic resonance imaging apparatus has a measuring circuit with a potential-dividing connection to a voltage indicator, the measuring circuit being connected in series with a discharging resistor of the intermediate circuit capacitance, and being driven from the discharging current via the discharging resistor.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed to a measuring device for measuring the intermediate circuit voltage of a gradient amplifier of a magnetic resonance imaging (MRI) apparatus. 
     2. Description of the Prior Art 
     In a typical magnetic resonance imaging apparatus, a number of gradient coils (usually three gradient coils for the x, y and z axes, respectively) are employed to spatially encode the magnetic resonance signals arising from an examination subject after excitation of nuclear spins in the subject. Each gradient coil is operated by a gradient amplifier, and each gradient amplifier typically includes a number of intermediate amplifier circuits or stages. Ideally, it would be desirable to measure or monitor the intermediate circuit voltage of each of these intermediate circuits, however, due to their large number this is impractical for reasons of cost and complexity. An additional problem in measuring the intermediate circuit voltage under such circumstances is that this voltage is usually a floating voltage, i.e., it is not always referenced to a reference voltage or ground. 
     For these reasons, generally only monitoring of one voltage limiting value or two voltage limiting values takes place whereby a threshold detector is driven by a power Zener diode. If more information is needed, measuring circuits in the intermediate circuit connected parallel to the discharging resistor of the intermediate circuit capacitor are utilized. Such measuring circuits, however, are not only considerably more expensive and require considerably more outlay in terms of circuitry, but also still require an auxiliary power supply. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a measuring device for measuring the intermediate circuit voltage of a gradient amplifier, which is simply and inexpensively constructed and which does not need its own auxiliary power supply. 
     This object is achieved in accordance with the invention in a measuring device having a measuring circuit with a potential-dividing connection to a voltage indicating device, the measuring circuit being connected in series with the discharging resistor of the intermediate circuit capacitor and being driven from the discharging current via the discharging resistor. 
     Such a circuit is simply constructed and operates without an auxiliary energy supply. In a further embodiment of the invention a driver that is preferably fashioned as a transistor is driven via an astable flip-flop, this driver is connected to the low impedance path of a light-emitting diode that emits light at a frequency that is proportional to the intermediate circuit voltage. The intermediate circuit voltage is identified in an evaluating unit that contains a photoreceiver that detects the light pulses. 
     In a particulary simple inexpensive embodiment the astable flip-flop contains a thyristor tetrode that is driven via a voltage divider having a Zener diode lying connected in parallel with a capacitor that is connected in series with the discharging resistor. The thyristor tetrode simply fashioned by two interconnected transistors. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically shows the arrangement of an inventive measuring circuit in the circuit path between a gradient amplifier (not shown) and a gradient coil (not shown) driven by the gradient amplifier. 
     FIG. 2 is a circuit diagram of the measuring circuit utilized in the arrangement according to FIG.  1 . 
     FIG. 3 shows the voltage-response characteristic of the optical coupler used in the circuit of FIG.  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows an intermediate circuit capacitance or C Z , at the output of a power supply unit, usually formed by multiple capacitors and usually followed by a discharging resistor R e . The discharging resistor R e  is designed such that it can effect a discharge of the intermediate circuit capacitance C z  within a few minutes in order to avoid danger to service personnel, as would exist if the intermediate circuit capacitance C z  would remain charged for days with excessively high voltage, for example. 
     Inventively, this discharging resistor R e  is connected in series with the actual measuring circuit  2 . The measuring circuit  2  is shown in detail in FIG.  2 . This measuring circuit  2  contains an astable flip-flop  1  for operating a driver that is fashioned as a transistor T 3 , for an optical coupler containing a light-emitting diode LD in the exemplary embodiment. The light pulses emitted by the light-emitting diode LD are detected by a photodiode P and are displayed as voltage value at a voltage indicator M. The indicator M “translates” the current pulses, produced by the photodetector P from the detected light pulses, into a voltage vale dependent on a characteristic, which has been previously empirically detected, representing the relation between the frequency of the light pulses and the intermediate circuit voltage U Z . 
     When the intermediate circuit voltage is close to zero, practically no voltage drop occurs at a capacitor C 1  connected in series with R e , and thus a Zener diode D 1  connected in parallel with C 1  remains inhibited. Transistors T 1 , T 2  and T 3  then are non-conducting and the light-emitting diode LD remains dark. When the intermediate circuit voltage then exceeds the Zener voltage of the Zener diode D 1 , the transistor T 1  initially becomes conductive and drives the transistor T 2 , which, together, form a thyristor tetrode, this thyristor tetrode forming a flip-flop circuit in combination with the driving voltage divider of across the capacitor C 1 . This driving voltage divider is formed by resistors (in the exemplary embodiment, 10K and 47K resistors) connected in series and in parallel with the Zener diode D 1 , and resistors (in the exemplary embodiment, two 47K resistors) connected at the base of transistor T 2 . The transistor T 3  that serves as driver for the light-emitting diode LD thereby conducts, and the light-emitting diode LD emits light. The capacitor C 1  discharges via the low-resistance path of the light-emitting diode circuit, causing the thyristor that is formed by the transistors T 1 , T 2 , quench and the transistor T 3  ceases to conduct, and the light-emitting diode LD ceases to emit light, so that a light pulse has been produced. The capacitor C 1  is slowly recharged by the discharging resistor R e  and the previously described procedure starts again. A saw-tooth-shaped voltage thereby arises at the capacitor C 1  and the optical coupler with the light-emitting diode LD blinks with a frequency that is almost linearly proportional to the intermediate circuit voltage. If the intermediate circuit voltage finally exceeds a maximum value, the thyristor no longer quenches and the light-emitting diode changes to steady emission light. An upper transgression of the measuring range can be immediately recognized in this way. 
     FIG. 3 shows the voltage-response characteristic with C 1 =100 nF and R e =33 kOhm. The almost linear relation between the voltage U z  and the frequency f at which the light-emitting diode LD flashes. Given commercially obtainable components, a precision of approximately 10% is obtained, which is sufficient for most applications. 
     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventor to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of his contribution to the art.