Article

An x-space magnetic particle imaging scanner

Department of Bioengineering, University of California, Berkeley, California 94720-1762, USA.
The Review of scientific instruments (Impact Factor: 1.61). 03/2012; 83(3):033708. DOI: 10.1063/1.3694534
Source: PubMed

ABSTRACT

Magnetic particle imaging (MPI) is an imaging modality with great promise for high-contrast, high-sensitivity imaging of iron oxide tracers in animals and humans. In this paper, we present the first x-space MPI hardware and reconstruction software; show experimentally measured signals; detail our reconstruction technique; and present images of resolution and "angiography" phantoms.

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    • "In comparison to other scanner topologies [1], [4], [5], [6], [7], [8], [9], the object size is not limited for a single-sided MPI scanner. However, the penetration depth is limited, as the field strength and the gradient decrease with increasing distance to the scanner surface (Fig. 3). "
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    ABSTRACT: Magnetic Particle Imaging (MPI) is a recently invented tomographic imaging method that quantitatively measures the spatial distribution of a tracer based on magnetic nanoparticles. The new modality promises a high sensitivity and high spatial as well as temporal resolution. There is a high potential of MPI to improve interventional and image-guided surgical procedures because, today, established medical imaging modalities typically excel in only one or two of these important imaging properties. MPI makes use of the non-linear magnetization characteristics of the magnetic nanoparticles. For this purpose, two magnetic fields are created and superimposed, a static selection field and an oscillatory drive field. If superparamagnetic iron-oxide nanoparticles (SPIOs) are subjected to the oscillatory magnetic field, the particles will react with a non-linear magnetization response, which can be measured with an appropriate pick-up coil arrangement. Due to the non-linearity of the particle magnetization, the received signal consists of the fundamental excitation frequency as well as of harmonics. After separation of the fundamental signal, the nanoparticle concentration can be reconstructed quantitatively based on the harmonics. The spatial coding is realized with the static selection field that produces a field-free point, which is moved through the field of view by the drive fields. This article focuses on the frequency-based image reconstruction approach and the corresponding imaging devices while alternative concepts like x-space MPI and field-free line imaging are described as well. The status quo in hardware realization is summarized in an overview of MPI scanners.
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    ABSTRACT: Magnetic particle imaging (MPI) is a new imaging modality that non-invasively images the spatial distribution of superparamagnetic iron oxide nanoparticles (SPIOs). MPI has demonstrated high contrast and zero attenuation with depth, and MPI promises superior safety compared to current angiography methods, X-ray, CT, and MRI angiography. Nanoparticle relaxation can delay the SPIO magnetization, and in this work we investigate the open problem of the role relaxation plays in MPI scanning and its effect on the image. We begin by amending the x-space theory of MPI to include nanoparticle relaxation effects. We then validate the amended theory with experiments from a Berkeley x-space relaxometer and a Berkeley x-space projection MPI scanner. Our theory and experimental data indicate that relaxation reduces SNR and asymmetrically blurs the image in the scanning direction. While relaxation effects can have deleterious effects on the MPI scan, we show theoretically and experimentally that x-space reconstruction remains robust in the presence of relaxation. Furthermore, the role of relaxation in x-space theory provides guidance as we develop methods to minimize relaxation-induced blurring. This will be an important future area of research for the MPI community.
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