Comparison of fast field-cycling magnetic resonance imaging methods and future perspectives

Abstract

Fast field-cycling (FFC) nuclear magnetic resonance relaxometry is a well-established method to determine the relaxation rates as a function of magnetic field strength. This so-called nuclear magnetic relaxation dispersion gives insight into the underlying molecular dynamics of a wide range of complex systems and has gained interest especially in the characterisation of biological tissues and diseases. The combination of FFC techniques with magnetic resonance imaging (MRI) offers a high potential for new types of image contrast more specific to pathological molecular dynamics. This article reviews the progress in FFC-MRI over the last decade and gives an overview of the hardware systems currently in operation. We discuss limitations and error correction strategies specific to FFC-MRI such as field stability and homogeneity, signal-to-noise ratio, eddy currents and acquisition time. We also report potential applications with impact in biology and medicine. Finally, we discuss the challenges and future applications in transferring the underlying molecular dynamics into novel types of image contrast by exploiting the dispersive properties of biological tissue or MRI contrast agents. ARTICLE HISTORY KEYWORDS Field-cycling; FFC-MRI; delta relaxation enhanced MR; dispersion; NMRDCONTACT Markus Bödenler m.boedenler@tugraz.at * These authors have equally contributed to this work. of the scanner. Differences in the underlying relaxation behaviour at B 0 , and therefore changes in image contrast, can be used to distinguish between healthy and pathological tissues for many diseases [1]. However, contrast mechanisms may change dramatically with the applied magnetic field strength, and these changes can be exploited to obtain new information for medical diagnosis. One way to access field-dependant information is by using Fast Field-Cycling (FFC) methods.FFC Nuclear Magnetic Resonance (NMR) relaxometry is an established method to measure the changes

A whole-body Fast Field-Cycling scanner for clinical molecular imaging studies

“…Detection at such low fields allowed the use of relatively inexpensive equipment but posed specific challenges related to NMR signal detection at ultra-low frequencies and, even though the results were impressive, research efforts appear to have stalled before reaching human whole-body size. An alternative approach is Delta Relaxation Enhanced MRI (dreMR), which seeks to detect exogenous contrast agents via their T 1 -dispersion at high magnetic field, typically ±0.2 T centred on 1.5 T or 3.0 T [21][22][23] . This approach is very different from ours as it operates at high magnetic fields only, relies on the use of exogenous contrast agents and also requires inserting a field-offset magnet into a pre-existing MRI scanner, which limits the volume of the objects that can be scanned.…”

High‐Field Detection of Biomarkers with Fast Field‐Cycling MRI: The Example of Zinc Sensing

“…The contrast agent concentration used in this proof‐of‐principle study lies in the clinically relevant region (the approved dose ranges from 0.1 to 0.3 mmol kg −1 ); and the estimated Zn 2+ detection limit matches physiological concentrations which attain 100 μ m to 1 m m in pancreatic regions or neurons . It should be noted that FFC‐MRI requires an insert coil to vary the B 0 field which, with respect to classical MRI, implies technical constraints on image acquisition, such as duty cycle considerations for electromagnetic operation, prolonged scan times and limited imaging region . Despite these constraints, pre‐clinical FFC‐MRI studies have been promising and show the feasibility of in vivo visualization of MRI probes with a detection limit of ≈40 μ m .…”

In vivo 3D brain and extremity MRI at 50 mT using a permanent magnet Halbach array

“…[2][3][4] Representing a slight increase in hardware complexity, combinations of prepolarizing electromagnets and lower field either electro-based or permanent magnet-based readout systems have been used for imaging human extremities. 5,6 More sophisticated field-cycling systems [7][8][9][10][11] have also been used to obtain data over a wide range of field strengths, most recently in vivo data at 50 μT.…”