Low drive field amplitude for improved image resolution in magnetic particle imaging

Abstract

Purpose: Magnetic particle imaging (MPI) is a new imaging technology that directly detects superparamagnetic iron oxide nanoparticles. The technique has potential medical applications in angiography, cell tracking, and cancer detection. In this paper, the authors explore how nanoparticle relaxation affects image resolution. Historically, researchers have analyzed nanoparticle behavior by studying the time constant of the nanoparticle physical rotation. In contrast, in this paper, the authors focus instead on how the time constant of nanoparticle rotation affects the final image resolution, and this reveals nonobvious conclusions for tailoring MPI imaging parameters for optimal spatial resolution. Methods: The authors first extend x-space systems theory to include nanoparticle relaxation. The authors then measure the spatial resolution and relative signal levels in an MPI relaxometer and a 3D MPI imager at multiple drive field amplitudes and frequencies. Finally, these image measurements are used to estimate relaxation times and nanoparticle phase lags.Results: The authors demonstrate that spatial resolution, as measured by full-width at half-maximum, improves at lower drive field amplitudes. The authors further determine that relaxation in MPI can be approximated as a frequency-independent phase lag. These results enable the authors to accurately predict MPI resolution and sensitivity across a wide range of drive field amplitudes and frequencies. Conclusions: To balance resolution, signal-to-noise ratio, specific absorption rate, and magnetostimulation requirements, the drive field can be a low amplitude and high frequency. Continued research into how the MPI drive field affects relaxation and its adverse effects will be crucial for developing new nanoparticles tailored to the unique physics of MPI. Moreover, this theory informs researchers how to design scanning sequences to minimize relaxation-induced blurring for better spatial resolution or to exploit relaxation-induced blurring for MPI with molecular contrast. C 2016 American Association of Physicists in Medicine. [http://dx

The relaxation wall: experimental limits to improving MPI spatial resolution by increasing nanoparticle core size

“…The spatial resolution minima of the curve is shifted to 27.4 nm at 0.4 kHz and there is significantly improved spatial resolution by almost 2-fold from the optimal resolution at 20 kHz curve. This can be attributed to the fact that the temporal blurring has a lower impact on the time-domain raw MPI signal shape when the drive waveform period is longer, which in turn results in less blurring during image reconstruction as shown by L. R. Croft et al 2016. Overall, while both approaches help mitigate magnetic relaxation and improve spatial resolution, the general trend of worsening spatial resolution beyond 27.4 nm still remains.…”

“…The exact synthesis and more characterization of the nanoparticles can be found in detail in the main article and supplementary information of (Vreeland et al 2015). The MPI performance of the SPIOs was also measured using a magnetic particle spectrometer/relaxometer described in (Tay, Patrick Goodwill, et al 2016) as well as the x-space MPI scanner described in (L. R. Croft et al 2016).…”

“…Other research groups (L. Croft et al 2012; L. R. Croft et al 2016; Dieckhoff et al 2016; Robert J Deissler et al 2014; R J Deissler et al 2015; Dhavalikar et al 2016) have also modeled how MPI spatial resolution varies with NP size, and with drive field frequency and amplitude.…”

Image reconstruction for Magnetic Particle Imaging using an Augmented Lagrangian Method

“…It was also assumed that a high receiver bandwidth could be used, which may not be easy to achieve in practice. One practical alternative is to divide the FOV into smaller FOVs to lower the drive field amplitude, which was shown to increase resolution with small number of received harmonics [15]. In fact, such an approach may be necessary to avoid nerve stimulation or prevent over-heating of the tissue for human imaging applications [16].…”

Seeing SPIOs Directly In Vivo with Magnetic Particle Imaging

“…For groups interested in developing MPI technology, challenges include (a) improving the spatial resolution of MPI, which is determined by SPIO tracer size and composition, applied magnetic fields, and SPIO relaxation times [47, 48]; (b) improving the detection sensitivity in MPI; (c) robust multi-color imaging [49, 50]; (d) theranostic MPI for guidance and real-time feedback on hyperthermia heating [51–53] (e) improving tracer circulation time and image contrast [21, 45, 54] ; (f) optimal combination of MPI with anatomic imaging modalities like CT or MRI for multi-modal imaging; (g) targeting SPIOs with great specificity to pathophysiology including cancer, cardiovascular disease, stroke; and (h) scaling up preclinical MPI hardware to human MPI, while observing FDA and EU biosafety restrictions, including peripheral nerve stimulation (PNS) and tissue heating Specific Absorption Rate (SAR) limits [27, 29] . These areas are the focus of the activities of the MPI interest group.…”