Bio


B.S. Physics
M.S. Bioengineering
P.h.D Bioengineering

Stanford Advisors


All Publications


  • Evaluation of a Workflow to Define Low Specific Absorption Rate MRI Protocols for Patients With Active Implantable Medical Devices. Journal of magnetic resonance imaging : JMRI Martinez, J. A., Moulin, K., Yoo, B., Shi, Y., Kim, H. J., Villablanca, P. J., Ennis, D. B. 2020

    Abstract

    MRI exams for patients with MR-conditional active implantable medical devices (AIMDs) are contraindicated unless specific conditions are met. This limits the maximum specific absorption rate (SAR, W/kg). Currently, there is no general framework to guide meeting a lower SAR limit.To design and evaluate a workflow for modifying MRI protocols to whole-body SAR (WB-SAR ≤0.1 W/kg) and local-head SAR (LH-SAR ≤0.3 W/kg) limits while mitigating the impact on image quality and exam time.Prospective.Twenty healthy volunteers on head (n = 5), C-spine (n = 5), T-spine (n = 5), and L-spine (n = 5) with IRB consent.Vendor-provided head, C-spine, T-spine, and L-spine protocols (SARRT ) were modified to meet both low SAR targets (SARLOW ) using the proposed workflow. in vitro SNR and CNR were evaluated with a T1 -T2 phantom. in vivo image quality and clinical acceptability were scored using a 5-point Likert scale for two blinded readers.1.5T/spin-echoes, gradient-echoes.In vitro SNR and CNR values were evaluated with a repeated measures general linear model. in vivo image quality and clinical acceptability were evaluated using a generalized estimating equation analysis (GEE). The two reader's level of agreement was analyzed using Cohen's kappa statistical analysis.Using the workflow, SAR limits were met.0.12 ± 0.02 W/kg, median (SD) values for LH-SAR were 0.12 (0.02) W/kg and WB-SAR: 0.09 (0.01) W/kg. Examination time did not increase ≤2x the initial time. SARRT SNR values were higher and significantly different than SARLOW (P < 0.05). However, no significant difference was observed between the CNR values (value = 0.21). Median (IQR) CNR values were 14.2 (25.0) vs. 15.1 (9.2) for head, 12.1 (16.9) vs. 25.3 (14.2) for C-spine, 81.6 (70.1) vs. 71.0 (26.6) for T-spine, and 51.4 (52.6) vs. 37.7 (27.3) for L-spine. Image quality scores were not significantly different between SARRT and SARLOW (median [SD] scores were 4.0 [0.01] vs. 4.3 [0.2], P > 0.05).The proposed workflow provides guidance for modifying routine MRI exams to achieve low SAR limits. This can benefit patients referred for an MRI exam with low SAR MR-conditional AIMDs.1 Technical Efficacy Stage: 5 J. Magn. Reson. Imaging 2020.

    View details for DOI 10.1002/jmri.27044

    View details for PubMedID 31922311

  • MRI of Patients with Cardiac Implantable Electronic Devices CURRENT CARDIOVASCULAR IMAGING REPORTS Martinez, J. A., Ennis, D. B. 2019; 12 (7)
  • It's the little things: On the complexity of planar electrode heating in MRI. NeuroImage Erhardt, J. B., Lottner, T., Martinez, J., Ozen, A. C., Schuettler, M., Stieglitz, T., Ennis, D. B., Bock, M. 2019

    Abstract

    Neurological disorders are increasingly analysed and treated with implantable electrodes, and patients with such electrodes are studied with MRI despite the risk of radio-frequency (RF) induced heating during the MRI exam. Recent clinical research suggests that electrodes with smaller diameters of the electrical interface between implant and tissue are beneficial; however, the influence of this electrode contact diameter on RF-induced heating has not been investigated. In this work, electrode contact diameters between 0.3 and 4 mm of implantable electrodes appropriate for stimulation and electrocorticography were evaluated in a 1.5 T MRI system. In situ temperature measurements adapted from the ASTM standard test method were performed and complemented by simulations of the specific absorption rate (SAR) to assess local SAR values, temperature increase and the distribution of dissipated power. Measurements showed temperature changes between 0.8 K and 53 K for different electrode contact diameters, which is well above the legal limit of 1 K. Systematic errors in the temperature measurements are to be expected, as the temperature sensors may disturb the heating pattern near small electrodes. Compared to large electrodes, simulations suggest that small electrodes are subject to less dissipated power, but more localized power density. Thus, smaller electrodes might be classified as safe in current certification procedures but may be more likely to burn adjacent tissue. To assess these local heating phenomena, smaller temperature sensors or new non-invasive temperature sensing methods are needed.

    View details for PubMedID 30935911

  • Patient Orientation Affects Lead-Tip Heating of Cardiac Active Implantable Medical Devices during MRI. Radiology. Cardiothoracic imaging Martinez, J. A., Serano, P., Ennis, D. B. 2019; 1 (3): e190006

    Abstract

    To evaluate changes in patient orientation to mitigate radiofrequency-induced lead-tip heating (LTH) during MRI.LTH was evaluated for device type, lead path, and distance to the isocenter of a 1.5-T MRI system. LTH for 378 conditions in both head-first (HF) and feet-first (FF) orientations was measured for nine MRI-unsafe cardiac active implantable medical devices (AIMDs) placed along three (two anatomic, one planar) left-sided lead paths at nine landmark locations. The devices were exposed to 5 minutes of continuous radiofrequency energy at 4 W/kg whole-body specific absorption rate.LTH was greater in HF than in FF orientation for the planar and one anatomic lead path (P < .05). LTH was significantly affected by lead path, distance to isocenter, and patient orientation (all P < .05), but not by cardiac AIMD device type. Maximum LTH was observed in an HF orientation for the planar lead path when the lead tip was at isocenter (right ventricular [RV] lead: 32.0 °C ± 16.3 [standard deviation], right atrial [RA] lead: 16.1°C ± 9.3). In the FF orientation, LTH was significantly reduced (RV lead: 1.6°C ± 1.4; mean RA lead: 0.5°C ± 1.0; P = .008).LTH for supine FF patient orientations among patients with anterior left-sided cardiac AIMDs can be significantly lower than LTH for supine HF orientations. There was no scenario in which LTH was significantly worse in the FF position. Changing patient orientation is a simple method to reduce radiofrequency-induced LTH.© RSNA, 2019See also the commentary by Litt in this issue.

    View details for DOI 10.1148/ryct.2019190006

    View details for PubMedID 32076667

    View details for PubMedCentralID PMC6735361