Rapid Rotating-Frame Imaging Using an RF Pulse Train (RIPT)

The rotating-frame imaging (RFI) method has proven useful for measuring the spatial distributions of NMR-visible nuclides without the aid of static magnetic field gradients. However, traditional RFI has several disadvantages, including relatively poor time efficiency and high RF power deposition in the sample. A B1-gradient imaging technique, rapid rotating-frame imaging using an RF pulse train (RIPT), is discussed which can reduce both the image data-acquisition time and the RF power deposition by several orders of magnitude compared to RFI. The method is most readily applied to samples having only one resonance line, but a chemical-shift-selective version is also described which permits imaging of arbitrary lines in multipeak spectra. It is shown that RIPT images can be produced even when sample relaxation times are only a few milliseconds. Furthermore, by adding a suitable preparation period to the sequence, the RIPT signal intensity can be weighted by the relaxation time constant. Finally, postprocessing techniques are described which correct some of the image distortions common to most B1-gradient imaging techniques.