Dynamic range improvement in NMR imaging using phase scrambling

Abstract

Data collection efficiency in NMR imaging is impaired if the dynamic range of the receiver system is limited in comparison with that of the observed signal. This situation may occur in high-resolution proton imaging of large objects at high magnetic field strengths. The efficiency with which information is received can be increased by reducing the peak amplitude of the spin response by varying the phase distribution of the excited spins. This phase scrambling technique may be implemented using tailored RF excitation or by dephasing using nonlinear magnetic field gradients and can be applied in all dimensions of an acquired data set, providing a significant reduction in the dynamic range requirements of the detection electronics. Experimental results using 2D Fourier imaging have obtained up to 25 dB reduction in peak signal intensities. Image signal-to-noise ratios improved up to a factor of 6, with actual values dependent on experimental conditions. Simulation studies show that computational noise introduced during Fourier transformation is significantly reduced when phase scrambling is employed.