Magnetic particle imaging (MPI) is an emerging imaging modality that is ideally suited for long-term in vivo tracking of adoptively-transferred cell therapies (ACTs) due to MPI’s properties of long-lived magnetic signal from the nanoparticle (MNP) labels that was shown to last longer than 90 days in vivo [1], and having no signal attenuation with tissue depth enabling fully quantitative 3D images even in dark-fur mice strains without shaving [2,3,4]. However, practical use of MPI for preclinical research and clinical monitoring to improve ACTs, especially for treatment of solid tumours, has been hindered by (1) the relatively high costs / complexity of current MPI imaging systems compared to BLI or FLI systems and (2) the difficulty of accumulating sufficient MNP label in quiescent and non-permissive T cells for sensitive and quantitative longitudinal MPI studies of ACT in preclinical disease models.  

Innovation in MPI instrumentation: A new design of a compact high-throughput MPI scanner system was developed with a similar workflow to the in vivo BLI/FLI systems familiar to preclinical imaging researchers. The new system is capable of a higher scan throughput of up to 5 mice in a single setup as opposed to existing MPI rodent scanners that scan 1 mouse at a time. It also occupies a much smaller space footprint than existing MPI systems, fitting within a 70cm x 70cm area. Power consumption is significantly reduced, requiring only a single 1kW power amplifier placed in the bottom of this portable trolley MPI system, in contrast to server racks of power amplifiers typical of existing MPI and MRI systems. Furthermore, a novel magnetic method of T cell labeling with MNPs achieving greater than 10-fold improved efficiency (net-uptake / time-taken) over conventional direct incubation methods [5,6] is utilized.

In vitro system validation: The performance of the new system was evaluated in vitro for signal linearity, sensitivity and spatial resolution. Results show that MPI signal scales linearly with ICP-measured MNP mass (R^2=0.99), with a limit-of-detection of 48 ng Fe (Synomag(TM), micromod Partikeltechnologie GmbH, Germany) equivalent to approximately 6,000 labeled murine CD8 cells. The 2.8T/m/μ0 field-free line (FFL) with a 14mT/μ0, 1 kHz drive field was able to just resolve a 2mm gap between wells, matching the expected 5mT/μ0 FWHM at 14mT shown by recent studies [7] characterizing the improved spatial resolution qualities of Synomag(TM) MNPs. 

In vivo system validation: To demonstrate the utility of the new system for MPI longitudinal tracking of adoptively-transferred immune cells for cancer therapy, timecourse imaging of ACT T cells migration within a murine xenograft model was performed. Each mouse served as its own control (OVA-expressing MC38 tumor on left flank, WT MC38 tumor on right flank) and the MPI-tracked ACT T cells were observed to selectively migrate towards the OVA+ tumor. The high throughput capability of the new system enables a much higher density of data (up to 20 mice per experiment, imaged at intervals of 1-2 days) than previous MPI studies scanning one mouse at a time.

Limitations: Due to the dependence on mechanical rastering to cover the 5-mouse field-of-view, the temporal resolution of this new system falls short of the 46 frames-per-second achieved by larger and more complex commercial scanners. However, immune cell migration occurs at slower timescales of hours to days, therefore the new system is still capable of capturing these dynamics even when using a slower scan speed (minutes). Although MPI does not attenuate with tissue depth, the single-sided design limits effective operating depth (up to medium-sized mice). 

Conclusions: The presented innovation in MPI instrumentation combined with high-efficiency labeling methods capable of working with hard-to-label quiescent T cells is anticipated to lower barriers-to-adoption, improve convenience-of-use, and overall contribute towards the practical long-term monitoring of cell therapy products in preclinical cancer immunotherapy research. 

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Innovation in MPI instrumentation featuring a compact, high-throughput mouse scanner tailored for longitudinal imaging studies of adoptive cell immunotherapy. (A) The presented scanner design is self-sufficient with power amplifier, support electronics, PC and animal stage all within a wheeled trolley occupying a 70cm x 70cm footprint that is much smaller than existing MPI preclinical scanners. When deployed, the animal imaging stage can accommodate up to a full cage of 5 mice for a single scan – 5x higher throughput than existing scanners. A new design of a permanent magnet array concentric to the transmit/receive generates a 2.8T/m field-free-line gradient projected into a shallow bed that accommodates small to medium-sized mice. (B) Characterization of instrument performance for signal linearity, and also detection sensitivity measured in both MNP mass and number of labeled murine T-cells. (C) Characterization of spatial resolution shows that the new scanner is able to resolve a gap of 2 mm with its 2.8T/m magnetic field gradient. (D) In vivo validation of instrument performance confirms the ability to longitudinally visualize and track labeled OVA-specific CD45.1+ OT-I T-cells as they migrate specifically towards the OVA+ MC38 tumor and not towards the wild-type MC38 tumor (each mouse serves as its own control with one tumor on each flank). This task-based validation that the labeled T-cells are still functional in vivo shows that the MNP-labeling and subsequent Magnetic Particle Imaging is a compatible and useful method for longitudinal imaging studies of adoptive cell immunotherapy. 

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