title: SQUID-sensor-based ultra-low-field MRI calibration with phantom images: towards quantitative imaging.
abstract: In ultra-low-field magnetic resonance imaging (ULF MRI), measured resonance signals oscillate at Larmor frequencies around 1 kHz compared to even above 100 MHz in high-field MRI. Thus, detection by induction coils in ULF MRI is not feasible, whereas superconducting quantum interference device (SQUID) sensors can measure these femtotesla-level signals. The signal-to-noise ratio is enhanced by prepolarization in a field that is typically 100-1000 times higher than the field during acquisition. Based on both measurements and simulations, a procedure for calibrating a SQUID-sensor-based MRI system with MR images is presented in this article. Magnetoencephalography (MEG) can be integrated with ULF MRI, and may also benefit from such a calibration procedure. Conventionally, electromagnet probe signals have been used for the SQUID-sensor calibration in MEG; the presented ULF-MRI-based approach using an imaging phantom could replace this procedure in hybrid MEG-MRI or ULF MRI alone. The necessary theory is provided here with experimental verification. The calibration procedure opens the possibility of performing quantitative ULF MRI without sample-specific reference scans.
authors.affiliation: Department of Biomedical Engineering and Computational Science, Aalto University School of Science, Aalto, Finland. juhani.dabek@aalto.fi
authors.forename: Juhani
authors.initials: J
authors.lastname: Dabek
authors.forename: Panu T
authors.initials: PT
authors.lastname: Vesanen
authors.forename: Koos C J
authors.initials: KC
authors.lastname: Zevenhoven
authors.forename: Jaakko O
authors.initials: JO
authors.lastname: Nieminen
authors.forename: Raimo
authors.initials: R
authors.lastname: Sepponen
authors.forename: Risto J
authors.initials: RJ
authors.lastname: Ilmoniemi
mesh_terms: D002138:Calibration; D004867:Equipment Design / Q000592:standards; D019544:Equipment Failure Analysis / Q000592:standards; D038622:Internationality; D008279:Magnetic Resonance Imaging / Q000295:instrumentation* / Q000592:standards*; D019047:Phantoms, Imaging / Q000592:standards*; D014159:Transducers / Q000592:standards*