No abstract

In this work we present a device that is capable of wireless synchronisation to the MRI pulse sequence time frame with sub-microsecond precision. This is achieved by detecting radio frequency pulses in the parent pulse sequence using a small resonant circuit. The device incorporates a 3-axis pickup coil, constructed using conventional printed circuit board (PCB) manufacturing techniques, to measure the rate of change of the gradient waveforms with respect to time. Using Maxwell's equations, assuming negligible rates of change of curl and divergence, a model of the expected gradient derivative (slew) vector field is presented. A 3-axis Hall effect magnetometer allows for the measurement of the direction of the static magnetic field in the device coordinate frame. By combining the magnetometer measurement with the pickup coil voltages and slew vector field model, orientation and position can be determined to within a precision of 0.1 degrees and 0.1 mm, respectively, using a pulse series lasting 880 μs. The gradient pulses are designed to be sinusoidal enabling the detection of a phase shift between the time frame of the pickup coil digitisation circuit and the gradient amplifiers. Signal processing is performed by a low power microcontroller on the device and the results are transmitted out of the scanner bore using a low latency 2.4 GHz radio link. The device identified an unexpected 40 kHz oscillation relating to the pulse width modulation (PWM) frequency of the gradient amplifiers that is predominantly in the direction of the static magnetic field. The proposed Wireless Radio frequency triggered Acquisition Device (WRAD) enables users to probe the scanner gradient slew vector field with minimal hardware setup and shows promise for future developments in prospective motion correction.

Purpose The efficacy of a Wireless Radio frequency triggered Acquisition Device (WRAD) is evaluated for high frequency (50 Hz) prospective motion correction in a 3‐dimensional spoiled gradient echo pulse sequence. Methods The device measures the rate of change in the gradient vector fields (slew) using a 3‐dimensional assembly of Printed Circuit Board (PCB) inductors and the direction of the static magnetic field using a 3‐axis Hall effect magnetometer. The slew vector encoding is highly efficient, because the Maxwell‐term position encoding is observable, allowing overconstrained pose measurement using 3 sinusoidal gradient pulses lasting 880 μs. Since small offsets in the magnetometer can introduce bias into the pose estimates, sensor/system biases are tracked using a lightweight Kalman filter. The only calibration required is determining a geometric scaling factor for the pickup coils, which is specific to the device and will therefore be valid in any scanner. Results The device was used to perform prospective motion correction in 3 subjects, resulting in an increase in Average Edge Strength (AES) for involuntary and deliberate motion. Conclusions The WRAD is simple to set up and use, with well‐defined measurement variance. This could enable “plug and play” prospective motion correction if pulse sequence independence is achieved.

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Purpose The purpose of this work is to describe a T1‐weighted fluid‐attenuated inversion recovery (FLAIR) sequence that is able to produce sharp magnetic resonance images even if the subject is moving their head throughout the acquisition. Methods The robustness to motion artifacts and retrospective motion correction capabilities of the PROPELLER (periodically rotated overlapping parallel lines with enhanced reconstruction) trajectory were combined with prospective motion correction. The prospective correction was done using an intelligent marker attached to the subject. This marker wirelessly synchronizes to the pulse sequence to measure the directionality and magnitude of the magnetic fields present in the MRI machine during a short navigator, thus enabling it to determine its position and orientation in the scanner coordinate frame. Three approaches to incorporating the marker‐navigator into the PROPELLER sequence were evaluated. The specific absorption rate, and subsequent scan time, of the T1‐weighted FLAIR PROPELLER sequence, was reduced using a variable refocusing flip‐angle scheme. Evaluations of motion correction performance were done with 4 volunteers and 3 types of head motion. Results During minimal out‐of‐plane movement, retrospective PROPELLER correction performed similarly to the prospective correction. However, the prospective clearly outperformed the retrospective correction when there was out‐of‐plane motion. Finally, the combination of retrospective and prospective correction produced the sharpest images even during large continuous motion. Conclusion Prospective motion correction of a PROPELLER sequence makes it possible to handle continuous, large, and high‐speed head motions with only minor reductions in image quality.