Local Specific Absorption Rate (SAR) in computational models of blood vessels compared to ASTM phantom

Transcription

1 Local Specific Absorption Rate (SAR) in computational models of blood vessels compared to ASTM phantom Kyoko Fujimoto 1, Leonardo M. Angelone 1, Elena Lucano 2, Peter Serano 1, Sunder S. Rajan 1, Maria I. Iacono 1 1 U.S. Food and Drug Administration, Center for Devices and Radiological Health, Silver Spring, MD 2 Sapienza University of Rome, Department of Information Engineering, Electronics, Telecommunication, Italy

2 Magnetic Resonance Imaging (MRI) Used as a diagnostic tool Obtains images by using a strong static magnetic field, gradient fields, and a radiofrequency (RF) field Widely accepted as Safe Non-invasive Non-ionizing radiation Image from keckmedicine.adam.com Has some concerns such as RF energy absorption in tissues 2

3 RF energy absorption in tissues Energy dissipation in tissues can cause thermal injury It is measured as Specific Absorption Rate (SAR) RF safety is especially important for implanted devices such as stents RF absorption of stents is usually measured on a standard gel-based phantom such as ASTM phantom [ASTM F a] The ASTM result is commonly considered as a worst case scenario Image from britanica.com 3

4 Stents in Blood Vessels RF energy absorption might be higher in specific anatomical regions Stents are implanted in arteries or veins which have never been simulated due to a computational limitation GOAL: Run electromagnetic simulations with a high-resolution detailed numerical human model and compare with ASTM phantom [Image courtesy of Jake McCright and Maria Iacono] 4

5 Simulation Methodology Finite-difference time-domain (FDTD) method, Sim4Life [Zurich Med Tech] 1.5 T MRI (at 64 MHz) A standard body coil [Lucano 2016] 5

6 Simulation Methodology 2 models, 5 landmarks Brain Heart Hip joint ASTM phantom Knee AustinMan highresolution model with the 63 anatomical structures [Massey 2016] Refined grid in the vessel regions of the AustinMan body minimum resolution: 0.98mm, maximum resolution: 2mm 6

7 What is SAR? SAR = σ 2ρ E 2 σ = electrical conductivity ρ = tissue mass density E = electric field Unit is in W/kg It is often averaged over the surrounding mass (average 1g SAR 1g ) Electric fields depend on anatomical models, imaging landmarks, coils, etc. Simulation setting E SAR 7

8 Maximum SAR 1g : ASTM vs. Blood Vessels in AustinMan ASTM SAR 1g Map Heart Landmark SAR 1g Map 35 W/kg 35 W/kg Max: 61. Max: 8.6 W/kg 8

9 SAR 1g in Blood Vessels Maximum SAR 1g Values in Blood Vessels 92.2 W/kg 77.9 Wkg 73.8 W/kg 61. ASTM 8.6 W/kg Brain Heart Hip Joint Knee SAR 1g values are normalized to meet the MRI SAR limits [ IEC 2010] based on an averaged SAR values 9

10 Blood Vessel SAR with the Brain Landmark Brain Landmark 17.2 W/kg 10

11 lood vessels SAR 1g with the Brain Landmark slice Anatomical Map SAR 1g Map Max: 32 W/kg 5 SAR 1g in db scale 11

12 Blood Vessel SAR with the Heart Landmark Heart Landmark 17.2 W/kg 12

13 Blood vessels SAR 1g with the Heart Landmark slice Anatomical Map Max: 2 SAR 1g Map 5 SAR 1g in db scale 13

14 Blood Vessel SAR with the Hip Joint Landmark Hip Joint Landmark 17.2 W/kg 14

15 Femoral Artery SAR 1g with the Hip Joint Landmark Anatomical Map SAR 1g Map slice Max: 23 W/kg 5 SAR 1g in db scale 15

16 Blood Vessel SAR with the Knee Landmark Knee Landmark 17.2 W/kg 16

17 Popliteal Artery SAR 1g with the Knee Landmark Anatomical Map SAR 1g Map slice 5 SAR 1g in db scale Max: 4 17

18 Summary Simulations with refined resolution showed the SAR values in blood vessels The SAR values in blood vessels can be much higher than the SAR values of ASTM phantom The ASTM standard may need to be rescaled to reflect the in-vivo SAR values. 18

19 More simulations - Models - Landmarks - Field strengths - Coil models Future Work Blood vessels with Fat vs. Muscle Anatomical Map SAR Map 1g 2 19

20 Thank you for your attention! Acknowledgement: Brian Beard, Amir Razjouyan, David Soltysik, and Eriko Yoshimaru Disclaimer: This work was supported by the Research Participation Program at the Center for Devices and Radiological Health administered by the Oak Ridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy and the U.S. Food and Drug Administration (FDA). The mention of commercial products, their sources, or their use in connection with material reported herein is not to be construed as either an actual or implied endorsement of such products by the Department of Health and Human Services.