Abstract
We describe with a theoretical and numerical analysis the use of small permanent magnets moving along prescribed helical paths for 3D spatial encoding and imaging without sample adjustment in ultra-low field magnetic resonance imaging (ULF-MRI). With our developed method the optimal magnet path and orientation for a given encoding magnet number and instrument architecture can be determined. As a proof-of-concept, we studied simple helical magnet paths and lengths for one and two encoding magnets to evaluate the imaging efficiency for a mechanically operated ULF-MRI instrument with permanent magnets. We demonstrate that a single encoding magnet moving around the sample in a single revolution suffices for the generation of a 3D image by back projection.
Highlights
The conventional setup of magnetic resonance imaging (MRI) or nuclear magnetic resonance (NMR) instruments comprises a static magnet field to magnetize the sample; a system of transmitter and receiver coils to generate and detect a sample signal; and a coil system to encode spatial information for image generation[1]
The last decade has seen the development of ultra-low magnetic field (ULF) NMR/MRI instruments with main magnetic fields below 10 mT4–12
We previously described the use of adjustable small permanent magnet arrays (SPMA’s) that exploit the advantages of Halbach arrays to generate and dynamically control the magnetic fields in ultralow field magnetic resonance imaging (ULF-MRI)/NMR4
Summary
The conventional setup of magnetic resonance imaging (MRI) or nuclear magnetic resonance (NMR) instruments comprises a static magnet field to magnetize the sample; a system of transmitter and receiver coils to generate and detect a sample signal; and a coil system to encode spatial information for image generation[1]. Image quality depends mainly on signal-to-noise ratio (SNR) which increases with the magnitude and homogeneity of the main magnetic field (commonly referred to as B0). This has been the primary motivation for increases in magnetic field strength in MRI and NMR instruments[2,3]. For 2D spatial encoding, the Halbach array was rotated about the sample and RF pulses were required for 3D imaging[6,16] Bluemler extended this approach, proposing nested Halbach arrays to generate 2D linear gradients by superposition of two quadrupole fields to avoid complex image reconstruction methods[17]. 3D spatial encoding can be achieved without additional RF pulses or relative sample motion to the instrument
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