MRI Gradient Amplifier Dead Time Compensation
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Solution Overview
Problem
The existing magnetic resonance imaging (MRI) systems face image quality degradation due to asymmetry in current waveforms generated by gradient magnetic field power supplies, caused by dead times in switching cycles, leading to deviations in MR signal timing and symmetry.
Innovation Solution
Incorporating an adjusting circuitry that modulates the pulse width of driving signals and adjusts the gain of the amplifier based on dead time ratios within switching cycles, ensuring symmetric current waveforms between rising and falling periods.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If dead time is introduced for switching semiconductor switches in the amplification amplifier, then the amplifier can operate at high speed with large voltage output, but the current waveform symmetry deteriorates between rising and falling periods
Solution Approach 1:
The patent applies preliminary action by pre-calculating the dead time compensation amount based on the switching frequency and voltage amplitude, and storing it in a lookup table. This allows the control system to proactively compensate for waveform asymmetry before it occurs, rather than reacting to it afterward. The compensation value is determined in advance and applied to the pulse width modulation signals to counteract the expected distortion from dead time.
Solution Approach 2:
The patent changes the pulse width parameter of the driving signals based on the dead time compensation amount. By dynamically adjusting the pulse width according to the calculated compensation value (which depends on switching frequency and voltage amplitude), the system maintains symmetric current waveforms despite the fixed dead time in the switching circuitry. This parameter adjustment ensures that the effective on-time and off-time remain balanced.
2Device complexity
If dead time compensation is not applied, then the device complexity remains low, but image quality degrades due to asymmetric current waveforms
Solution Approach 1:
The patent introduces an intermediary compensation mechanism that acts as a mediator between the simple switching circuitry and the quality requirements. The dead time compensation amount calculation unit and lookup table serve as intermediaries that translate the fixed hardware dead time into corrective pulse width adjustments. This intermediary layer adds minimal complexity while effectively eliminating waveform asymmetry and its impact on image quality.
Solution Approach 2:
The patent implements a feedback mechanism where the switching frequency and voltage amplitude are continuously monitored, and the dead time compensation amount is adjusted accordingly. The control system uses the actual operating conditions to determine the appropriate compensation value from the lookup table, creating a closed-loop system that maintains optimal waveform symmetry across varying operating points without requiring complex real-time calculations.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enhances image quality by maintaining symmetry in current waveforms, thereby improving the temporal consistency of gradient magnetic field power supplies and reducing image distortion.
Implementation Method 1
a gradient coil (inductive load)
Implementation Method 2
pulse width modulation circuitry configured to modulate a pulse width of a driving signal, which is input to a plurality of switching elements
Data Source
AI summary
According to one embodiment, a magnetic resonance imaging apparatus includes an amplifier, a gradient coil, and adjusting circuitry. The amplifier includes pulse width modulation circuitry modulating a pulse width of a driving signal, which is input to switching elements, in accordance with an input of a control signal corresponding to a waveform of a gradient magnetic field. The gradient coil generates the gradient magnetic field by an electric current supplied in accordance with an output voltage which is output from the amplifier. The adjusting circuitry executes adjustment of a gain of the amplifier, which is included in the control signal, or adjustment of the pulse width of the driving signal, in accordance with a dead time included in a switching cycle of the switching elements.