![Introduction
Methods
Results
Discussion
Problems
2
[4] Vannesjo S, Neuroimage 167:191-202
T2*-w @7T: ghosting from ΔB0
dynamic ΔB0
(e.g. respiration &
swallowing [3,4])
[1] Stroman P, Neuroimage 84:1070-81
[2] Saritas E , Chapter 2.3 of Quantitative MRI of the Spinal Cord
[3] Verma T, Magn Res Med 72:1629-36
distortions
[2]
static ΔB0
(e.g vertebrae, airways)
inspired
expired
phase encoding
(AP)
[3]
10 mm
“The greatest challenge for acquiring MR images in the spinal cord is [ΔB0] the
inhomogeneous magnetic field in this region.” [1]](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
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Integrated RF and Shim coils for MRI
- 1. Integrated ΔB0/Rx coil array for improved spinal cord imaging at 3T ISMRM 26th Annual Meeting June 20, 2018, Paris, France Topfer R1, Foias A1, Lopez Rios N1,2, Chauffray A1,3, Germain G1, Arango N4, Wald LL5,6, Stockmann JP5,6, Cohen-Adad J1,7 1. NeuroPoly Lab, Institute of Biomedical Engineering, Polytechnique Montreal, Montreal, QC, Canada, 2. Centro de Biofisica Medica, Universidad de Oriente, Santiago de Cuba, Cuba, 3. École polytechnique fédérale de Lausanne, Lausanne, Switzerland, 4. Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, United States, 5. A. A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, MA, United States, 6. Harvard Medical School, Boston, MA, United States, 7. Functional Neuroimaging Unit, CRIUGM, Université de Montréal, Montreal, QC, Canada
- 2. Introduction Methods Results Discussion Problems 2 [4] Vannesjo S, Neuroimage 167:191-202 T2*-w @7T: ghosting from ΔB0 dynamic ΔB0 (e.g. respiration & swallowing [3,4]) [1] Stroman P, Neuroimage 84:1070-81 [2] Saritas E , Chapter 2.3 of Quantitative MRI of the Spinal Cord [3] Verma T, Magn Res Med 72:1629-36 distortions [2] static ΔB0 (e.g vertebrae, airways) inspired expired phase encoding (AP) [3] 10 mm “The greatest challenge for acquiring MR images in the spinal cord is [ΔB0] the inhomogeneous magnetic field in this region.” [1]
- 3. Introduction Methods Results Discussion Solutions How to address these issues? Design principle Rx benefit Shim benefit Positioning coils near body ↑sensitivity, ↑SNR ↑efficiency (Hz/A) Using many coils enhances parallel imaging enables higher order ΔB0 correction [3] [1] Stockmann JP, Magn Res Med 75:441-51 [2] Truong TK, Neuroimage 103, 235–40 [3] Juchem C, J Mag Reson 236: 95-104 3 Parallel imaging + high-order dynamic shimming Solving the “real-estate problem” [1,2]
- 4. Introduction Methods Results Discussion 4 Conceptualization [1] Germain G, ISMRM 2016. Abstract #3490. Optimal coil arrangement from simulations 3. ΔB0 + Rx SNR performance assessed for various coil arrangements 1. Sagittal ΔB0 maps acquired (n=5) +75 Hz -75 Hz 𝛾𝛥B0Avg.ΔB0 Inspired Expired } } 2. Auto-segmentation of cervical spinal cord ROI using the Spinal Cord Toolbox 𝛾𝛥B0 +30 Hz -30 Hz single coil optimal
- 5. Introduction Methods Results Discussion 5 Construction RF only 5.5 cm [1] Stockmann JP, Magn Res Med 75:441-51 RF + shim
- 6. Introduction Methods Results Discussion 6 Construction DC twisted pairs routed away from RF outputs Preamps atop plastic supports, away from coils TODO: periodic cable traps + chokes along DC cables [1] [1] Stockmann JP, Magn Res Med 75:441-51
- 7. Introduction Methods Results Discussion +75 Hz/A-75 Hz/A ch1 ch5 ch2 ch3 ch6 ch7 ch4 ch8 ΔB0 reference maps used in shim optimization 7 Coil performance Coupling: min = 2 % max = 48 % avg = 18% 0 % 100% 50 % Noise correlation (Tx = 0V) Max shift per ch. avg = ±25 Hz/A over SC Rx measurements
- 8. Introduction Methods Results Discussion Computer in control room shim update via USB +fibre optic • images transferred over local network • runs MATLAB + Spinal Cord Toolbox 8 Experimental setup [1] Arango N, ISMRM 2016. Abstract #1157. RF common mode chokes Power supply Equipment room Current driver [1] + heatsink Microcontroller +DAC pressure sensor + microcontroller 3T Prisma MRI room Pressure(t) respiratory bellows shim DC [2] Topfer R, Magn Res Med 80:935-46
- 9. Introduction Methods Results Discussion 9 In vivo results Expired GRE mag +100 Hz-100 Hz 𝛾𝛥B0 Original shim VOI |Pred. − Actual| 25 Hz0 Hz 𝛾𝛥B0 Inspired σ 𝛥B0 =54 Hz σ 𝛥B0 =31 Hz σ 𝛥B0 =32 Hz σ 𝛥B0 =53 Hz Predicted Shimmed σ 𝛥B0 =29 Hz Actual σ 𝛥B0 =29 Hz 46% improvement σ 𝛥B0 40% improvement σ 𝛥B0
- 10. Introduction Methods Results Discussion 10 In vivo results 14 mm-14 mm Original Shift EPI InspiredExpired mean |shift| ± st.dev 2.7±2.1mm mean |shift| ± st.dev 2.5±2.1mm Shimmed Shift EPI 10 cm mean |shift| ± st.dev 1.4±1.3mm mean |shift| ± st.dev 1.8±1.2mm max |shift| = 13 mm max |shift| = 8 mm
- 11. Introduction Methods Results Discussion 11 Future directions 1. Real-time shimming 2. Making the most of the low inductance coils —> Potential for slicewise-dynamic shimming +100 Hz-100 Hz 𝛾𝛥B0 Inspired Expired Pressure(t) Inputs static shim respiration- correction shim(t) +20 Hz -20 Hz 𝛾𝛥B0 static ΔB0 respiration- induced ΔB0(t) +100 Hz -100 Hz 𝛾𝛥B0 Static Dynamic Outputs real-time shimmed real-time shimmed + = + = 3. Clinical app.: T2* for quantitative spinal-cord assessment healthy cord = healthy WM/GM contrast [1] Martin AR, Am J Neuroradiol 38:1266–73
- 12. Introduction Methods Results Discussion Acknowledgements Agah Karakuzu Aldo Zaimi Ariane Saliani Atef Badji Axel Guittonneau Benjamin De Leener Charley Gros Christian S. Perone Gabriel Mangeat Jennifer Campbell Stephanie Alley Tommy Boshkovski Alexandru Foias Nibardo Lopez-Rios Grégoire Germain Angel Chauffray Julien Doyon Carollyn Hurst André Cyr Jason Stockmann Larry Wald Nick Arango Robert Barry Thomas Witzel Special thanks to collaborators + co-authors! See you at ISMRM 2019 in Montreal — Creatively addressing the “real-estate problem” since 1967
- 13. Introduction Methods Results Discussion References Topfer R et al., 2018, ’Integrated ΔB0/Rx coil array for improved spinal cord imaging at 3T’, in ISMRM 26th Annual Meeting, Paris, France link : http://cds.ismrm.org/protected/18MPresentations/abstracts/0834.html
Editor's Notes
- How to address these issues? Well, to reduce distortion in EPI from extended readouts and echo trains, we want to use parallel imaging. At the same time, to eliminate susceptibility artifacts at the source, we want a high-order shim system capable of correcting both the small-scale static and dynamic changes to B0. The efficiency of both RF receive and shim subsystems improves with the proximity of the coils to the tissue of interest. So, to solve what Stockmann et. al dubbed "the real-estate problem", a natural choice is to integrate these 2 functionalities -- receive and B0 shim -- into the same set of coils.