Magnetic particle imaging
Magnetic particle imaging (MPI) is an emerging non-invasive tomographic technique that directly detects superparamagnetic nanoparticle tracers. The technology has potential applications in diagnostic imaging and material science. Currently, it is used in medical research to measure the 3-D location and concentration of nanoparticles. Imaging does not use ionizing radiation and can produce a signal at any depth within the body. MPI was first conceived in 2001 by scientists working at the Royal Philips Research lab in Hamburg. The first system was established and reported in 2005. Since then, the technology has been advanced by academic researchers at several universities around the world. The first commercial MPI scanners have recently become available from Magnetic Insight and Bruker Biospin.
The hardware used for MPI is very different from MRI. MPI systems use changing magnetic fields to generate a signal from superparamagnetic iron oxide (SPIO) nanoparticles. These fields are specifically designed to produce a single magnetic field free region. A signal is only generated in this region. An image is generated by moving this region across a sample. Since there is no natural SPIO in tissue, a signal is only detected from the administered tracer. This provides images without background. MPI is often used in combination with anatomical imaging techniques (such as CT or MRI) providing information on the location of the tracer.
Magnetic particle imaging combines high tracer sensitivity with submillimeter resolution. Imaging is performed in a range of milliseconds to seconds. The iron oxide tracer used with MPI are cleared naturally by the body through the mononuclear phagocyte system. The iron oxide nanoparticles are broken down in the liver, where the iron is stored and used to produce hemoglobin. SPIOs have previously been used in humans for iron supplementation and liver imaging.
Blood pool imaging
MPI has numerous applications to the field of oncology research. Accumulation of a tracer within solid tumors can occur through the enhanced permeability and retention effect. This has been successfully used to detect tumor sites within rats. The high sensitivity of the technique means it may also be possible to image micro-metastasis through the development of nanoparticles targeted to cancer cells. MPI is being investigated as a clinical alternative screening technique to nuclear medicine in order to reduce radiation exposure in at-risk populations.
By tagging therapeutic cells with iron oxide nanoparticles, MPI allows them to tracked throughout the body. This has applications in regenerative medicine and cancer immunotherapy. Imaging can be used to improve the success of stem cell therapy by following the movement of these cells in the body. The tracer is stable while tagged to a cell and remains detectable past 87 days.
The SPIO tracer used in magnetic particle imaging is detectable within biological fluids, such as the blood. This fluid is very responsive to even weak magnetic fields, and all of the magnetic moments will line up in the direction of an induced magnetic field. These particles can be used because the human body does not contain anything which will create magnetic interference in imaging.
- High resolution (~0.4 mm)
- Fast image results (~20 ms)
- No radiation
- No iodine
- No background noise (high contrast)
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- First in vivo magnetic particle imaging of lung perfusion in rats. Zhou XY, Jeffris K, Yu E, Zheng B, Goodwill P, Nahid P, Conolly S. Phys Med Biol. 2017 Feb 20.
- Tracking short-term biodistribution and long-term clearance of SPIO tracers in magnetic particle imaging. Keselman P, Yu E, Zhou X, Goodwill P, Chandrasekharan P, Ferguson RM, Khandhar A, Kemp S, Krishnan K, Zheng B, Conolly S. Phys Med Biol. 2017 Feb 8
- Evaluation of PEG-coated iron oxide nanoparticles as blood pool tracers for preclinical magnetic particle imaging. Khandhar AP, Keselman P, Kemp SJ, Ferguson RM, Goodwill PW, Conolly SM, Krishnan KM. Nanoscale. 2017 Jan 19;9(3):1299-1306.
- Combining magnetic particle imaging and magnetic fluid hyperthermia in a theranostic platform. Hensley DW, Tay ZW, Dhavalikar R, Zheng B, Goodwill P, Rinaldi C, Conolly S. Phys Med Biol. 2016 Dec 29.
- Finite magnetic relaxation in x-space magnetic particle imaging: Comparison of measurements and ferrohydrodynamic models. Dhavalikar R, Hensley D, Maldonado-Camargo L, Croft LR, Ceron S, Goodwill PW, Conolly SM, Rinaldi C. J Phys D Appl Phys. 2016 Aug 3;49(30)
- A High-Throughput, Arbitrary-Waveform, MPI Spectrometer and Relaxometer for Comprehensive Magnetic Particle Optimization and Characterization. Tay ZW, Goodwill PW, Hensley DW, Taylor LA, Zheng B, Conolly SM. Sci Rep. 2016 Sep 30;6:34180.
- Eddy current-shielded x-space relaxometer for sensitive magnetic nanoparticle characterization. Bauer LM, Hensley DW, Zheng B, Tay ZW, Goodwill PW, Griswold MA, Conolly SM. Rev Sci Instrum. 2016 May;87(5):055109.
- Low drive field amplitude for improved image resolution in magnetic particle imaging. Croft LR, Goodwill PW, Konkle JJ, Arami H, Price DA, Li AX, Saritas EU, Conolly SM. Med Phys. 2016 Jan;43(1):424. doi: 10.1118/1.4938097.
- A Convex Formulation for Magnetic Particle Imaging X-Space Reconstruction. Konkle JJ, Goodwill PW, Hensley DW, Orendorff RD, Lustig M, Conolly SM. PLoS One. 2015 Oct 23;10(10):e0140137. doi: 10.1371/journal.pone.0140137.
- Effects of pulse duration on magnetostimulation thresholds.Saritas EU, Goodwill PW, Conolly SM. Med Phys. 2015 Jun;42(6):3005-12. doi: 10.1118/1.4921209.
- In vivo multimodal magnetic particle imaging (MPI) with tailored magneto/optical contrast agents. Arami H, Khandhar AP, Tomitaka A, Yu E, Goodwill PW, Conolly SM, Krishnan KM. Biomaterials. 2015 Jun;52:251-61. doi: 10.1016/j.biomaterials.2015.02.040.
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- Twenty-fold acceleration of 3D projection reconstruction MPI. Konkle JJ, Goodwill PW, Saritas EU, Zheng B, Lu K, Conolly SM. Biomed Tech (Berl). 2013 Dec;58(6):565-76. doi: 10.1515/bmt-2012-0062.
- Magnetostimulation limits in magnetic particle imaging. Saritas EU, Goodwill PW, Zhang GZ, Conolly SM. IEEE Trans Med Imaging. 2013 Sep;32(9):1600-10. doi: 10.1109/TMI.2013.2260764..
- Linearity and shift invariance for quantitative magnetic particle imaging. Lu K, Goodwill PW, Saritas EU, Zheng B, Conolly SM. IEEE Trans Med Imaging. 2013 Sep;32(9):1565-75. doi: 10.1109/TMI.2013.2257177.
- Magnetic particle imaging (MPI) for NMR and MRI researchers. Saritas EU, Goodwill PW, Croft LR, Konkle JJ, Lu K, Zheng B, Conolly SM. J Magn Reson. 2013 Apr;229:116-26. doi: 10.1016/j.jmr.2012.11.029. Review.
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- Magnetic Insight, Inc. - Commercializing MPI technology originally developed at the University of California, Berkeley 11/2014
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