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Sensitive imaging of superparamagnetic nanoparticles or aggregates is of great importance in MR molecular imaging and medical diagnosis. For this purpose, a conceptually new approach, termed active feedback magnetic resonance, was developed.
In the presence of the Zeeman field, a dipolar field is induced by the superparamagnetic nanoparticles or aggregates. Such dipolar field creates spatial and temporal (due to water diffusion) variations to the precession frequency of the nearby water 1H magnetization. Sensitive imaging of magnetic nanoparticles or aggregates can be achieved by manipulating the intrinsic spin dynamics by selective self-excitation and fixed-point dynamics under active feedback fields.
Phantom experiments of superparamagnetic nanoparticles; in vitro experiments of brain tissue with blood clots; and in vivo mouse images of colon cancers, with and without labeling by magnetic nanoparticles, suggest that this new approach provides enhanced, robust, and positive contrast in imaging magnetic nanoparticles or aggregates for cancer detection.
The spin dynamics originated from selective self-excitation and fixed-point dynamics under active feedback fields have been shown to be sensitive to dipolar fields generated by magnetic nanoparticles. Magn Reson Med 74:33–41, 2015. © 2014 Wiley Periodicals, Inc.
Sensitive imaging of superparamagnetic nanoparticles or aggregates is of great importance in MR molecular imaging and medical diagnosis. For this purpose, a conceptually new approach, termed active feedback magnetic resonance, was developed.
In the presence of the Zeeman field, a dipolar field is induced by the superparamagnetic nanoparticles or aggregates. Such dipolar field creates spatial and temporal (due to water diffusion) variations to the precession frequency of the nearby water 1H magnetization. Sensitive imaging of magnetic nanoparticles or aggregates can be achieved by manipulating the intrinsic spin dynamics by selective self-excitation and fixed-point dynamics under active feedback fields.
Phantom experiments of superparamagnetic nanoparticles; in vitro experiments of brain tissue with blood clots; and in vivo mouse images of colon cancers, with and without labeling by magnetic nanoparticles, suggest that this new approach provides enhanced, robust, and positive contrast in imaging magnetic nanoparticles or aggregates for cancer detection.
The spin dynamics originated from selective self-excitation and fixed-point dynamics under active feedback fields have been shown to be sensitive to dipolar fields generated by magnetic nanoparticles. Magn Reson Med 74:33–41, 2015. © 2014 Wiley Periodicals, Inc.
Nonlinear feedback interactions have been shown to amplify contrast due to small differences in resonance frequency arising from microscopic susceptibility variations. Determining whether the selectivity of feedback-based contrast enhancement for small resonance frequency variations remains valid even in the presence of macroscopic field inhomogeneity is important for transitioning this new methodology into in vivo applications in imaging systems with lower field strengths and poorer homogeneity. This work shows that contrast enhancement under the radiation damping (RD) feedback field is sensitive to microscopic intravoxel frequency variations. Feedback-enhanced contrast provides superior signal differentiation from voxels with distinct microscopic frequency distributions compared with T-weighted imaging, while remaining robust to macroscopic field gradients, which frequently give rise to artifacts by other frequency-sensitive methods. Applying multiple RF pulses during evolution under RD and actively adjusting the phase and amplitude of the feedback field are shown to further improve signal differentiation. Experimental results reveal that feedback-enhanced contrast can generate positive contrast, reflecting microscopic field variations induced by strong local dipole fields, such as those created by blood vessels and superparamagnetic iron oxide nanoparticles. Extensions to in vivo imaging at lower field strengths are discussed in the context of amplifying the RD field via active electronic feedback. Magn Reson Med, 2009. © 2009 Wiley-Liss, Inc.
A new method for enhancing MRI contrast between gray matter (GM) and white matter (WM) in epilepsy surgery patients with symptomatic lesions is presented. This method uses the radiation damping feedback interaction in high-field MRI to amplify contrast due to small differences in resonance frequency in GM and WM corresponding to variations in tissue susceptibility. High-resolution radiation damping-enhanced (RD) images of in vitro brain tissue from five patients were acquired at 14 T and compared with corresponding conventional T1-, T-, and proton density (PD)-weighted images. The RD images yielded a six times better contrast-to-noise ratio (CNR = 44.8) on average than the best optimized T1-weighted (CNR = 7.92), T-weighted (CNR = 4.20), and PD-weighted images (CNR = 2.52). Regional analysis of the signal as a function of evolution time and initial pulse flip angle, and comparison with numerical simulations confirmed that radiation damping was responsible for the observed signal growth. The time evolution of the signal in different tissue regions was also used to identify subtle changes in tissue composition that were not revealed in conventional MR images. RD contrast is compared with conventional MR methods for separating different tissue types, and its value and limitations are discussed. Magn Reson Med, 2006. © 2006 Wiley-Liss, Inc.
