Trans-cranial Magnetic Resonance-guided Focused UltraSound (tcMRgFUS) system integrated with a 1.5T: an SNR study on DQA phantoms

Background: In recent years transcranial Magnetic Resonance-guided Focused Ultrasound Surgery (tcMRgFUS) treatments have been performed only using 3T units. Since following some internal analysis, planning images obtained using a 1.5T MRI’s body RF coil (as usually done with 3T systems) showed generally reasonable quality in terms of anatomy visualization and SNR but thermal images noise was above acceptable standard for treatments, a dedicated 2-channels head coil was developed (InSightec Ltd.) to ensure an adequate signal-to-noise ratio (SNR). Hence, we present the imaging protocol and the technological methods successfully used with the world-first installation of a certified tcMRgFUS system (ExAblate 4000, InSightec Ltd.) integrated with a 1.5T scanner (Signa HDxt, GE Medical Systems). Materials and Methods: We used the NEMA (2008) test methods for measuring the SNR of both MRI’s body RF coil and the dedicated coil. T2w-FRFSE images were obtained from a dedicated tcMRgFUS daily quality assurance (DQA) phantom using the same MRI protocol subsequently used for real treatments planning. Results: Compared to that achieved using MRI’s body RF coil, the dedicated coil resulted in significantly larger values of the SNR on all planes. In particular, for the axial plane, using the body RF coil we measured a SNR equal to 2.5, 5.1 and 6.0 at 1, 4 and 6 NEXs respectively whereas it was equal to 27.5, 51.5 and 67.3 when using the dedicated 2-ch head coil: therefore, an increase of more than 10 times was achieved. For the coronal and the sagittal planes, we measured a gain of about 6 and 3 times respectively. Conclusion: Even though the SNR linearly increases with magnetic field strength, it should be underlined that on a 1.5T scanner the use of a dedicated coil enabled us to obtain images with a SNR roughly 5 times larger than those acquired on 3T scanners using MRI’s body RF coil. Acknowledgements: The installation of the tcMRgFUS equipment used in this work was funded by the Italian Ministry of Education, University and Research (MIUR) within the project “Programma Operativo Nazionale 2007-3013” (PONa3_00011; Project Leader: Prof. Carlo Catalano).