Synchronized digital signal processor for MRI reception

• US 5,529,068 A
• Filed: 06/16/1994
• Issued: 06/25/1996
• Est. Priority Date: 06/16/1994
• Status: Expired due to Term

• Current Assignee: Regents of the University of California (University of California)
• Original Assignee: Regents of the University of California (University of California)
• Inventors: Jones, James W., Hoenninger, John C. III, Crooks, Lawrence E.
• Primary Examiner(s): ZELE, KRISTA MICHELLE
• Application Number:
  US08/260,789
• Time in Patent Office: 740 Days
• Field of Search: 128/653.2, 128/653.5, 324/307, 324/309, 324/318, 324/322
• US Class Current: 600/413
• CPC Class Codes: G01R 33/3621 NMR receivers or demodulators, e.g. preamplifiers, means for frequency modulation of the MR signal using a digital down converter, means for analog to digital conversion [ADC] or for filtering or processing of the MR signal such as bandpass filtering, resampling, decimation or interpolation

Abstract

A multi-rate sample MRI (magnetic resonance imaging) digital receiver maintains synchronization of digitally processed signals. This multi-rate sample data system is used to demodulate and filter a digitized MRI RF signal. A phase/timing relationship is established between the signal received and processed by the digital receiver, and the physical nuclear magnetic resonance (NMR) process the body being imaged is undergoing. Once established, the phase/timing relationship is maintained for the duration of the particular NMR experiment being performed. Special logic in the digital system ensures that the data output is synchronized with an external synchronous signal controlling data acquisition within the MRI system. Optimum signal processing is performed to minimize the time between the end of the acquisition control signal and the last output from the digital signal processing system.

Claims

1. A method of digitally receiving nuclear magnetic resonance (NMR) signals emanating from an object undergoing NMR phenomena during a magnetic resonance imaging (MRI) experiment, said method comprising the following steps:

(a) establishing and maintaining, throughout said experiment, a fixed phase relationship between periodic sample timing and said NMR phenomenon;

(b) digitally sampling NMR signals emanated by said object at said sample timing to provide a sequence of discrete digital samples; and

(c) generating a phase index throughout said MRI experiment based on said sample timing, said phase index indicating a phase relationship between each of said discrete digital samples and a reference phase.

2. A method as in claim 1 further including digitally filtering said sequence of digital samples based at least in part on said phase index.

3. A method as in claim 1 wherein said generating step (c) includes sequentially changing state each time said step b) samples said NMR signals.

4. A method as in claim 3 further including initially setting the state to a predetermined state.

5. A method as in claim 1 wherein said generating step (c) includes incrementing a counter each time said step (b) samples said NMR signals.

6. A method as in claim 1 further including indexing at least one digital filter input value with said phase index.

7. A method as in claim 1 wherein said generating step (c) includes continually processing said sequence of discrete samples throughout said MRI experiment.

8. A method as in claim 1 wherein said generating step (c) includes continually receiving and processing said sequence of discrete samples with a demodulating digital signal processor throughout said MRI experiment.

9. A method as in claim 1 further including continually starting and stopping a digital filtering operation throughout said MRI experiment.

10. A method as in claim 1 further including stopping and restarting a control sequencer throughout said MRI experiment at a time responsive to said phase index.

11. A method of generating and processing nuclear magnetic resonance (NMR) signals to produce a magnetic resonance image of an object comprising the following steps:

(a) providing a time base;

(b) generating radio frequency pulses having a phase that is synchronized with said time base;

(c) applying said generated radio frequency pulses to said object in a manner that causes said object to emanate an NMR signal;

(d) sampling said emanated NMR signal throughout an MRI experiment at a rate that is synchronous with said time base to provide a sequence of digital samples; and

(e) counting said digital samples to provide a phase index relative to each of said samples.

12. A method of generating and processing nuclear magnetic resonance (NMR) signals to produce a magnetic resonance image of an object, said method comprising the following steps:

(a) intermittently exciting said object with an RF signal to produce responsive NMR signals;

b) periodically generating digital samples throughout an MRI experiment at a rate that is phase locked to said RF signal, at least some of said digital samples being responsive to said NMR signals; and

(c) tracking the order of said generated digital samples throughout said MRI experiment.

13. A method as in claim 12 further including the step of digitally filtering said generated digital samples including a last digital sample to generate a sequence of filtered digital values, said digitally filtering step comprising:

(1) calculating a partial sum of products prior to said last digital sample becoming available, and

(2) completing said sum of products once said last digital sample becomes available.

14. A method as in claim 12 further including calculating partial convolution results before all of said digital samples corresponding to one of said intermittent excitations becomes available, said calculating step including calculating products based on each successive digital sample generated by said step (b) and accumulating said calculated products in partial sums.

15. A magnetic resonance imaging system including:

a time base producing periodic timing signals;

a RF transmitter that produces RF signals having a phase responsive to said periodic timing signals, said RF signals at least in part inducing an object to produce NMR signals;

RF receiver circuitry which receives said NMR signals;

sampling circuitry for sampling said NMR signal throughout an MRI experiment at timings based on said periodic timing signals to provide a sequence of digital samples; and

phase index means responsive to said digital samples for allowing the phase of each/any of said digital samples in said sequence to be determined relative to a reference phase.

16. A magnetic resonance imaging system including:

a time base producing periodic timing signals;

a RF transmitter that produces RF signals having a phase responsive to said periodic timing signals, said RF signals at least in part inducing an object to produce NMR signals;

RF receiver circuitry which receives said NMR signals;

sampling circuitry for sampling said NMR signal throughout an MRI experiment at timings based on said periodic timing signals to provide a sequence of digital samples; and

a phase index counter which counts said digital samples so as to allow the phase of each/any of said samples to be determined relative to a reference phase.

17. A method of digitally filtering an NMR input signal comprising:

(a) sampling an input signal to provide a sequence of digital input values;

(b) filtering said sequence of digital input values to generate a sequence of filtered digital values terminating with a last filtered digital value;

(c) prior to generating said last filtered digital value with step (b), calculating at least a part of further digital filter results in response to generation of at least some of said filtered digital values generated by said step (b); and

(d) once said step (b) generates said last digital value, completing said further digital filter results based on said last digital value.

18. A method of timely digitally filtering an NMR signal comprising:

(a) receiving successive digital NMR signal values over time; and

(b) digitally filtering said successive digital NMR signal values, including the following steps;

(1) as each of said successive digital values is received, calculating products based on said received value;

(2) accumulating said calculated products in partial sums; and

(3) completing and outputting said sums as a digitally filtered version of said digital NMR signal values when a last of said digital NMR signal values is received.

19. A method of maintaining phase synchronization in a magnetic resonance imaging system comprising:

(a) periodically sampling a nuclear magnetic resonance (NMR) signal to provide digital samples;

(b) in an inactive mode, changing the state of a phase index count in response to said samples; and

(c) in an active mode, quadrature demodulating said samples while tracking the phase of said samples relative to said phase index count.

20. A method as in claim 19 further including the following steps:

(d) changing from said active mode to said inactive mode; and

(e) updating said phase index count with said tracked phase at the time said changing step (d) is performed.

21. A method as in claim 19 wherein said step (c) comprises demodulating said samples with plural sequential demodulating stages, and said method further includes terminating said active mode, and updating said phase index count based on which of said plural sequential demodulating stages is being used at the time said active mode is terminated.

22. A magnetic resonance imaging system, comprising:

a master clock producing a periodic timing signal;

a RF transmitter producing a RF signal at least in part inducing an object to produce NMR signals;

an analog mixer, having an input to receive a first signal derived from and in-phase with the periodic timing signal, said analog mixer receiving the NMR signal and translating the NMR signal to an intermediate frequency NMR signal in response to the first signal;

a digital sampler, having an input to receive a sample signal derived from and in-phase with the periodic timing signal, said sampler producing digital samples of the intermediate frequency NMR signal in response to the sample signal; and

a phase indexer receiving the digital samples from the digital sampler and determining the phase of a predetermined set of digital samples relative to a reference phase.