System and Method for Direct Digitization of NMR Signals

Abstract

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 signal 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.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an MRI system for use with the present invention;

FIG. 2 is a schematic representation of a transceiver system for use with the MRI system of FIG. 1 and incorporating a receiver system in accordance with the present invention;

FIG. 3 is a block diagram showing the components of a receiver system in accordance with one embodiment of the present invention; and

FIG. 4 is a flow chart setting forth the steps of a technique for receiving and processing MRI information in accordance with the present invention.

Claims

1. A receiver for a magnetic resonance (MRI) system comprising: a first analog-to-digital converter having an input for receiving a reference signal from a transmitter and producing a complex digital reference signal therefrom;a second analog-to-digital converter having an input for receiving at least one of an NMR signal and an ESR signal produced by a subject placed in the MRI system and producing a complex digital signal therefrom;a normalizer connected to receive and normalize the digital reference signal; anda mixer connected to receive the normalized digital reference signal and the digital NMR signal, the mixer being operable to mix the normalized digital reference signal with the digital signal.

2. The system of claim 1 wherein the second analog-to-digital converter detects the at least one of the NMR signal and the ESR signal at the Larmor frequency.

3. The system of claim 1 wherein the first and second analog-to-digital converters directly digitize the reference signal and the at least one of the NMR signal and the ESR signal about the Larmor frequency.

4. The system of claim 1 wherein the second analog-to-digital converter detects real and imaginary parts of the at least one of the NMR signal and the ESR signal.

5. The system of claim 1 wherein the first and second analog-to-digital converters include at least one of a single-channel receiver chip and multi-channel receiver chip configured to digitize the reference signal and the at least one of the NMR signal and the ESR signal.

6. The system of claim 5 wherein the at least one of the single-channel receiver chip and the multi-channel receiver chip includes a digital down-converter (DDC) for decimation and filtering.

7. The system of claim 6 wherein the DDC is a Graychip 4016.

8. The system of claim 6 wherein the DDC is programmed to reduce a bandwidth and increase a dynamic range of the first and second analog-to-digital converters.

9. The system of claim 1 further comprising a clock that produces a clock signal, and the first and second analog-to-digital converters digitize the reference signal and the at least one of the NMR signal and the ESR signal at a common conversion frequency determined by the clock signal.

10. The system of claim 1 wherein the first and second analog-to-digital converters directly digitize the reference signal and the at least one of the NMR signal and the ESR signal about a proton magnetic resonance frequency at a selectable static magnetic field strength.

11. The system of claim 1 further comprising a plurality of additional analog-to-digital converters having inputs for receiving the at least one of the NMR signal and the ESR signal produced by the subject placed in the MRI system and producing respective complex digital signals therefrom and wherein the mixer is connected to receive the normalized digital reference signal and the respective digital signals and mix the normalized digital reference signal with each digital signal from the plurality of additional analog-to-digital converters.

12. 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 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.

13. The method of claim 12 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.

14. The method of claim 12 wherein step (D) includes mixing the digitized reference signal and the digitized NMR signal to generate a phase-accurate array of NMR data.

15. The method of claim 12 wherein steps (C) and (D) are performed by a processor executing a computer program stored on a computer readable storage medium.

16. The method of claim 12 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.

17. The method of claim 12 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.

18. A kit configured to retrofit an RF transceiver system of an MRI system comprising: an RF receiver circuit having a plurality of analog-to-digital conversion channels, wherein at least one of the channels is configured to digitize a reference phase signal generated by the MRI system and arrange the digitized reference phase signal as a reference phase array, and wherein at least one of the channels is configured to digitize at least one NMR signal received from a subject in the MRI system and arrange the digitized at least one NMR signal as at least one data array; anda processor configured to normalize the reference phase array to create a complex conjugated array and mix the complex conjugated array with the NMR data array to generate phase-accurate NMR data.

19. The kit of claim 18 wherein the plurality of analog-to-digital conversion channels are configured to digitize the at least one NMR signal at a Larmor frequency of the at least one NMR signal.

20. The kit of claim 18 wherein the plurality of analog-to-digital conversion channels detect real and imaginary parts of the at least one NMR signal.

21. The kit of claim 18 wherein the at least one of the channels is further configured to digitize at least one ESR signal received from the subject in the MRI system and arrange the digitized at least one ESR signal as at least one data array.