Stanford Advisors

All Publications

  • Thermo-acoustic ultrasound for noninvasive temperature monitoring at lead tips during MRI. Magnetic resonance in medicine Dixit, N., Pauly, J. M., Scott, G. C. 2019


    We explore the use of thermo-acoustic ultrasound (TAUS) to monitor temperature at the tips of conductive device leads during MRI.In TAUS, rapid radiofrequency (RF) power deposition excites an acoustic signal via thermoelastic expansion. Coupling of the MRI RF transmit to device leads causes SAR amplification at lead tips, allowing MRI RF transmitters to excite significant lead tip TAUS signals. Because the amplitude of the TAUS signal depends on temperature, it becomes feasible to monitor the lead tip temperature during MRI by tracking the TAUS amplitude.The TAUS temperature dependence was characterized in a phantom and in tissue. To perform TAUS acquisitions in an MRI scanner, amplitude modulated RF chirps were transmitted by the body coil, and the lead tip TAUS signal was detected by an ultrasonic transducer. The TAUS signal level was correlated with the RF current induced on the lead and the associated B 1 artifacts in MRI. TAUS signals acquired during RF-induced heating were used to estimate the lead tip temperature.The TAUS signal exhibited strong dependence on temperature, increasing over 30% with 10 ∘ C of heating both in the phantom and in tissue. A lead tip TAUS signal was observed for a 100 mA rms current induced on a lead. During RF-induced heating, the TAUS signal appeared to accurately approximate the peak lead tip temperature.TAUS allows for noninvasive monitoring of lead tip temperature in an MRI environment. With further development, TAUS opens new avenues to improve RF device safety during MRI scans.

    View details for DOI 10.1002/mrm.28152

    View details for PubMedID 31883207

  • Thermo-Acoustic Ultrasound for Detection of RF-Induced Device Lead Heating in MRI IEEE TRANSACTIONS ON MEDICAL IMAGING Dixit, N., Stang, P. P., Pauly, J. M., Scott, G. C. 2018; 37 (2): 536–46


    Patients who have implanted medical devices with long conductive leads are often restricted from receiving MRI scans due to the danger of RF-induced heating near the lead tips. Phantom studies have shown that this heating varies significantly on a case-by-case basis, indicating that many patients with implanted devices can receive clinically useful MRI scans without harm. However, the difficulty of predicting RF-induced lead tip heating prior to scanning prevents numerous implant recipients from being scanned. Here, we demonstrate that thermo-acoustic ultrasound (TAUS) has the potential to be utilized for a pre-scan procedure assessing the risk of RF-induced lead tip heating in MRI. A system was developed to detect TAUS signals by four different TAUS acquisition methods. We then integrated this system with an MRI scanner and detected a peak in RF power absorption near the tip of a model lead when transmitting from the scanner's body coil. We also developed and experimentally validated simulations to characterize the thermo-acoustic signal generated near lead tips. These results indicate that TAUS is a promising method for assessing RF implant safety, and with further development, a TAUS pre-scan could allow many more patients to have access to MRI scans of significant clinical value.

    View details for DOI 10.1109/TMI.2017.2764425

    View details for Web of Science ID 000424467000019

    View details for PubMedID 29053449

    View details for PubMedCentralID PMC5942199

  • Optimal Broadband Noise Matching to Inductive Sensors: Application to Magnetic Particle Imaging IEEE TRANSACTIONS ON BIOMEDICAL CIRCUITS AND SYSTEMS Zheng, B., Goodwill, P. W., Dixit, N., Xiao, D., Zhang, W., Gunel, B., Lu, K., Scott, G. C., Conolly, S. M. 2017; 11 (5): 1041–52


    Inductive sensor-based measurement techniques are useful for a wide range of biomedical applications. However, optimizing the noise performance of these sensors is challenging at broadband frequencies, owing to the frequency-dependent reactance of the sensor. In this work, we describe the fundamental limits of noise performance and bandwidth for these sensors in combination with a low-noise amplifier. We also present three equivalent methods of noise matching to inductive sensors using transformer-like network topologies. Finally, we apply these techniques to improve the noise performance in magnetic particle imaging, a new molecular imaging modality with excellent detection sensitivity. Using a custom noise-matched amplifier, we experimentally demonstrate an 11-fold improvement in noise performance in a small animal magnetic particle imaging scanner.

    View details for DOI 10.1109/TBCAS.2017.2712566

    View details for Web of Science ID 000419308500007

    View details for PubMedID 28742047

    View details for PubMedCentralID PMC5741315