MRI: High-Precision DC Measurement

In an MRI system, the gradient amplifier unit refers to all circuit units related to the gradient magnetic field. Its function is to provide the system with a gradient field that meets linearity requirements and can switch rapidly, determining the position and thickness of imaging slices; the linearity of the gradient coil is the index for evaluating the stability of the gradient field. The better the linearity, the more accurate the gradient field, and the better the image quality. If the gradient field's linearity is poor, spatial and intensity distortions may occur at the edges of the image. Therefore, the gradient amplifier unit is one of the core components of MRI equipment.

The gradient magnetic field is generated by gradient currents along the axial direction. Its performance indicators mainly include effective resolution, linearity, gradient field strength, gradient field slew rate, and gradient field drift time. To generate a gradient signal along the axial direction, the system requires several pairs of current coils. Within each pair of coils, the current flows in opposite directions.

In fact, three pairs of gradient coils within the MRI device cavity produce three orthogonal magnetic fields. Therefore, the magnetic field can be adjusted at any position within the walls. The gradient amplifier operating in a closed servo loop drives the current through the gradient coils (as shown below). Hence, each MRI requires three such current control loops.

The feedback output from the current sensor allows real-time control of the precise phase of the gradient amplifier, which is the foundation of the entire system. As can be seen from the MRI principle outlined above, the quality, clarity, and resolution of the image are directly related to the magnetic field generated by the current through the gradient coils. Therefore, the accuracy of the current sensor is one of the most critical factors determining the current control loop.

Danisense, Hangzhi, and LEM’s DCCT high-precision fluxgate current sensors can very effectively handle high-precision measurement of gradient currents.