Designing feedback-based contrast enhancement for in vivo imaging

Multi-chromatic magnetic resonance imaging using frequency lock-in suppression

This study developed a multi-chromatic MR contrast using the frequency lock-in technique. An electronic feedback device that generates a specific narrow-frequency-bandwidth RF field is presented. The effects of this RF field on MR images are assessed both theoretically and experimentally. Spectroscopy and imaging experiments were performed. Frequency tuning allowed the selected spectral peak to be suppressed. Phantom tests using methanol, ethanol, and water showed different contrasts using different feedback RF field frequencies. The frequency lock-in was also found to help differentiate among the small structural variations in biological tissues. The contrast achieved in in vivo mouse brain imaging using the lock-in suppressed technique indicated a better spatial discrimination when compared with that achieved using conventional imaging methods, especially in the hippocampus region. Selective lock-in suppressed imaging is a new approach to provide frequency information in MRI; rather than determining the evolution of image contrast over time, this approach allows small susceptibility variations to be distinguished by tuning the frequency of the narrow-bandwidth lock-in RF field. A new and enhanced contrast can be achieved using this technique.

Sensitive imaging of magnetic nanoparticles for cancer detection by active feedback MR

PURPOSE

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.

METHODS

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 ¹ H 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.

RESULTS

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.

CONCLUSION

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.

Novel MRI contrast development by lock-in suppression

PURPOSE

The goal of this study is to develop novel MR contrast by frequency lock-in technique.

METHODS

An electronic feedback device that can control the frequency and bandwidth of the feedback RF field is presented. In this study, the effects of lock-in suppressed imaging are discussed both theoretically and experimentally.

RESULTS

Two important imaging experiments were performed. The first experiment used magnetizations with the same central frequency but different frequency distributions and was compared with MR images obtained with T2 contrast agents. Lock-in suppressed images showed an improvement in contrast relative to the conventional imaging method. The second experiment used magnetizations with small shifts in frequency and a broad frequency distribution. This is helpful for differentiating between small structural variations in biological tissues. The contrast achieved in in vivo tumor imaging using the lock-in suppressed technique provide higher spatial resolutions and discriminate the regimes of necrosis and activation consistent with pathologic results.

CONCLUSION

Lock-in suppressed imaging introduces a conceptually new approach to MRI. Heightened sensitivity to underlying susceptibility variations and their relative contribution to total magnetization may thus be achieved to yield new and enhanced contrast.