Development and optimization of hardware for delta relaxation enhanced MRI

Abstract

With active compensation, the dreMR imaging system is capable of 0.22T field shifts within a clinical 1.5T MRI with no significant residual eddy-current fields.

Nuclear magnetic relaxation dispersion of murine tissue for development of T1 (R1) dispersion contrast imaging

“…FFC‐MRI can be implemented with a variety of pulse sequences, where there is a specific time allotted during the sequence to allow the appropriate magnetic field shift. However, this obviously increases image acquisition time and design of the field‐cycling system must be optimized to mitigate potential eddy currents, which could cause imaging artefacts and reduced SNR …”

“…Three healthy adult Crl:NU‐Foxn1 nu female mice (27 ± 2 g) (Charles River Laboratories, Saint‐Constant, QC, Canada), and one MDA‐MB‐231 tumour‐implanted female mouse (27 g, 28 days after cell implantation) were imaged at 1.5 T (CVMR System, General Electric Healthcare, Waukesha, WI, USA) using an insertable, FFC electromagnet to dynamically control B 0 prior to image acquisition. The actively shielded electromagnet hardware, construction, and integration with the clinical scanner (including eddy‐current compensation) have previously been described by Harris et al The mice were anaesthetized with 2% Forane® (Baxter Corporation, Mississauga, ON, Canada) and murine body temperature was maintained using an MR‐compatible small rodent heating system (SA Instruments, Stony Brook, NY, USA). The temperatures of incoming heated air from the heater module entering the bore of the electromagnet and outgoing air from the exit of the electromagnet were confirmed with thermocouple measurements.…”

“…Whole‐body coronal image slices at five different field‐cycling magnetic fields ( B R = 1.26, 1.39, 1.50, 1.61, and 1.74 T) using a modified fast spin‐echo (FSE) single inversion recovery pulse sequence, where B 0 was modulated to B R for a duration τ k ( τ k = 50.0, 89.7, 161.1, 289.1, 518.9, 931.3, 1671.5, 3000.0, 5000.0, and 8000.0 ms) prior to imaging at the clinical field strength. A schematic representation of the magnetic field modulations and timings, including the specific pulse sequence diagram used for FFC‐MRI, is depicted in Figure C; however, a thorough review can be found in references by Harris et al and Ross et al for echo train lengths of 2. For each repetition time ( T R ), an initial 90° RF hard pulse with a duration of 1024 μs was applied followed by a fixed 2 s period for sample magnetization at 1.5 T. The initial 90° RF hard pulse ensured that each line of acquisition of k space started by manipulating the longitudinal magnetization arising from relaxation at 1.5 T and eliminating any potential remnant magnetization from 1.5 T ± Δ B 0 .…”

“…FFC‐MRI requires modulation of the magnetic field, B 0 , prior to image acquisition . The basis of field‐cycling MR is the ability to rapidly switch between magnetic field strengths for varying durations of relaxation prior to signal acquisition without causing field‐cycling‐induced image artefacts . Implementation of FFC‐MRI has several technical challenges.…”

“…Since the enhancement of MR contrast is non‐negligible for inactive agents, it is highly desirable to suppress signal from tissues that are enhanced by nearby unbound contrast agent as well as signal arising simply from unenhanced tissue. This can be accomplished using magnetic field‐cycling methods with MRI . This increased target specificity is particularly useful for situations where pre‐injection images are not available for subtraction from those acquired post‐injection.…”