System and method for direct digitization of NMR signals
A magnetic resonance imaging (MRI) system includes a transmitter that produces an RF excitation pulse that is applied to a subject positioned in the MRI system to induce emission of at least one of an NMR signal and an ESR signal therefrom, and that produces a reference signal indicative of the phase of the RF excitation pulse. A first analog-to-digital converter has an input for receiving the reference signal that is synchronous with the RF excitation pulse. One or more additional analog-to-digital converters/processors have inputs for receiving the at least one of NMR signals and ESR signals produced by a subject placed in the MRI system and produce one or more complex digital signals therefrom. A normalizer is connected to receive and normalize the digital reference signal and a mixer is connected to receive the normalized digital reference signal and the digital signal. Accordingly, the mixer is operable to multiply the normalized complex digital reference signal with the complex digital signal.
1. A method of processing an NMR signal received by a coil, the method comprising steps of:
(A) directly digitizing the NMR signal received by the coil without down-converting the NMR signal to a frequency below the Larmor frequency;
(B) receiving and digitizing a reference signal from an MRI system, the reference signal indicating a phase of an RF excitation pulse produced by the MR system to induce the NMR signal;
(C) normalizing the digitized reference signal; and
(D) mixing the normalized digital reference signal with the digitized NMR signal.
2. The method of claim 1 further comprising adjusting at least one of steps (A) through (D) to compensate for one of a plurality of MR imaging frequencies and reference signals specific to one of the plurality of MRI systems.
3. The method of claim 1 wherein step (D) includes mixing the digitized reference signal and the digitized NMR signal to generate a phase-accurate array of NMR data.
4. The method of claim 1 wherein steps (C) and (D) are performed by a processor executing a computer program stored on a computer readable storage medium.
5. The method of claim 1 wherein step (D) includes mixing a complex conjugate of the digitized reference signal with the digitized NMR signal to generate a phase-accurate array of NMR data that can be used to reconstruct one of a two-dimensional (2D) image and a three-dimensional (3D) image.
6. The method of claim 1 wherein step (A) further includes directly digitizing the NMR signal using a plurality of additional analog-to-digital converters and producing respective complex digital NMR signals therefrom and wherein step (D) further includes mixing the normalized digital reference signal with each digital NMR signal produced by the plurality of analog-to-digital converters.