Magneto-Acoustic Imaging
INTRODUCTION
Nanoparticles are attracting considerable interest as contrast agents in many different imaging modalities. However, ultrasound imaging can not visualize the nano-structures directly due to the limited spatial resolution and contrast. Magneto-acoustic (MA) imaging is a new technique, capable of imaging magnetic nanoparticles indirectly. It utilizes a focused high strength pulsed or continuous (harmonic) magnetic field to induce motion within the magnetically labeled tissue and then ultrasound to detect the internal tissue motion. Since nanoparticles are used as contrast agents, magneto-acoustic imaging is capable of imaging events at cellular and molecular level.
Block diagram of the magneto-acoustic imaging system
Magnetic field generation: The excitation
magnetic field can be either continuous wave (harmonic) or pulsed. Pulsed excitation
has several advantages such as shorter operation time, less-severe thermal management
constrains, and thus the possibility to increase magnetic flux density in order
to image deeper tissue structures. A harmonic system creates motion at predetermined
frequency and can be used to identify and filter out the other sources of tissue
motion.
Ultrasound imaging/Motion detection: The ultrasound
imaging system either can be based on mechanically scanned single element focused
ultrasound transducers or can utilize the linear array of ultrasound transducers.
Many different quantitative and qualitative techniques including Doppler ultrasound,
color/power Doppler, and speckle tracking could be used to detect the induced
motion.
Nano-scale contrast agents: Magneto-acoustic imaging
uses magnetic nanoparticles as contrast agent. Several types of commercial nanoparticles
are available – for example, super paramagnetic iron-oxide (SPIO) nanoparticles
were approved by the FDA in 1996 as tissue-specific MRI contrast agents. These
magnetic nanoparticles have been used in various clinical applications with
no cytotoxicity. Alternatively, new magnetic nanoparticles optimized for magneto-acoustic
imaging can be designed and produced.
PRELIMINARY RESULTS
To initially test and demonstrate the capabilities of the magneto-acoustic (MA) imaging system, several experiments were performed using tissue-mimicking phantoms (PVA) with different concentrations of magnetic nanoparticles. These studies, as expected, demonstrated that inclusions with higher concentrations of magnetic nanoparticles exhibited larger displacement.
Amplitude of the displacement detected in tissue mimicking phantoms with different concentration of magnetic nanoparticles
Furthermore, living macrophage cells (J774A.1 cell line) with internalized custom-designed magnetic nanoparticles were embedded into gelatin to form 3-D cell phantoms. The phantoms were then imaged using a custom-built MA imaging system. Although the grayscale B-scan ultrasound cannot differentiate between control and iron-laden cells, MA imaging can clearly demonstrate the contrast.
Optical dark-field images of a) non-labeled microphages and b) macrophages labeled with 20-30 nm MIONs. Grayscale ultrasound images of c) non-labeled and d) magnetically labeled macrophages embedded in 8% gelatin background. Color Power Doppler images of e) non-labeled and f) labeled microphages in the presence of magnetic field
The experimental results presented above suggest that magneto-acoustic imaging can be extended to become a cellular imaging tool capable of visualizing deep-lying structures. In addition, with the development of biocompatible nanoparticles that have the desired molecular targeting moiety, Magneto-Acoustic imaging can become a molecular imaging technique capable of detecting pathology at early stages of development.
Further reading:
J. Oh, M.D. Feldman, J. Kim, C. Condit, S. Emelianov, T.E. Milner, “Detection
of magnetic nanoparticles is tissue using magneto-motive ultrasound,”
Nanotechnology, 17, 4183-4190 (2006) PDF
M. Mehrmohammadi, J. Oh, L. Ma, E. Yantsen, T. Larson, S. Mallidi, S. Park, K. P. Johnston, K. Sokolov, T. Milner, and S. Emelianov, “Imaging of iron oxide nanoparticles using magneto-motive ultrasound,” Proceedings of the 2007 IEEE Ultrasonics Symposium, 652-655 (2007) PDF
M. Mehrmohammadi, J. Oh, L. Ma, S. Ryoo, E. Yantsen, S. Mallidi, K.P. Johnston, K. Sokolov, T.E. Milner, S.Y. Emelianov, "Pulsed magneto-motive ultrasound imaging", Abstract of the 25th Annual Houston Conference on Biomedical Engineering Research, The Houston Society for Engineering in Medicine and Biology, 7-8 February 2008, Houston, TX, p. 75 (2008) PDF
M. Mehrmohammadi, S.R. Aglyamov, A.B. Karpiouk, J. Oh, S.Y. Emelianov, “Pulsed magneto-motive ultrasound to assess viscoelastic properties of soft tissues,” in Proceedings of the Seventh International Conference on the Ultrasonic Measurement and Imaging of Tissue Elasticity, October 27-30, p.106 (2008) PDF
