Band-limited gradient waveforms

• US 6,566,877 B1
• Filed: 12/26/2000
• Issued: 05/20/2003
• Est. Priority Date: 12/26/2000
• Status: Expired due to Term

• Current Assignee: Marconi Medical Systems, Inc. (Koninklijke Philips N.V.)
• Original Assignee: Koninklijke Philips Electronics N.V. (Koninklijke Philips N.V.)
• Inventors: Anand, Christopher K., Thompson, Michael R.
• Primary Examiner(s): Lefkowitz, Edward
• Assistant Examiner(s): Vargas, Dixomara
• Application Number:
  US09/748,499
• Time in Patent Office: 875 Days
• Field of Search: 324/314, 324/307, 324/309
• US Class Current: 324/314
• CPC Class Codes: G01R 33/385 using gradient magnetic field coils

Abstract

A subject is disposed in an imaging region (10) of a magnetic resonance imaging apparatus. An operator submits a series of user preferences to the apparatus. A gradient optimizer (82) generates a gradient waveform that is optimal for the imaging procedure based on the user submitted specifications and the apparatus hardware specifications. The optimizer (82) accesses a memory that stores ideal gradient waveform models. The model that best fits the user specifications is selected and digitized (84). The digitized waveform is then convolved (86) with a band-limited kernel (88) that represents a frequency spectrum (89) of a gradient amplifier (28), producing a gradient waveform (90) that is smooth and does not exceed the capabilities of the amplifier. This optimized waveform is used in an imaging process including a collected data reconstruction portion of the process.

Claims

1. A magnetic resonance imaging apparatus comprising:

a main magnet assembly that produces a main magnetic field in an imaging region, wherein is disposed a subject;

an RF coil assembly that excites and manipulates magnetic resonance of selected dipoles in the imaging region;

a gradient coil assembly that spatially encodes the magnetic resonance;

gradient amplifiers that have a limited bandwidth;

a gradient means for optimizing gradient waveforms for compatibility with the bandwidth of the gradient amplifiers and other frequency limiting requirements; and

,

a reconstruction processor that reconstructs received resonance into an image representation.

2. The magnetic resonance imaging apparatus as set forth in claim 1, wherein the gradient optimizing means includes:

a model archive which stores models of ideal gradient waveforms.

3. The magnetic resonance imaging apparatus as set forth in claim 2, wherein the gradient optimizing means includes:

a digitizer that samples the ideal gradient waveform at a hardware clock speed to produce a digitized ideal gradient waveform.

4. The magnetic resonance imaging apparatus as set forth in claim 3, wherein the gradient optimizing means includes:

a spectrum memory which stores a model of a frequency response spectrum of the gradient amplifiers.

5. A magnetic resonance imaging apparatus comprising:

a main magnet assembly that produces a main magnetic field in an imaging region, wherein is disposed a subject;

an RF coil assembly that excites and manipulates magnetic resonance of selected dipoles in the imaging region;

a gradient coil assembly that spatially encodes the magnetic resonance;

gradient amplifiers;

a gradient optimizer that optimizes gradient waveforms based on user input parameters, ideal waveform models, and hardware specifications, the gradient optimizer including;

a model archive wherein are stored models of ideal gradient waveforms;

a digitizer that samples the ideal gradient waveform at a hardware clock speed to produce a digitized ideal gradient waveform;

a spectrum memory wherein is contained a model of a frequency response spectrum of the gradient amplifiers;

a gradient synthesizing circuit that;

selects one of the ideal gradient waveform models based on user input parameters;

convolves the digitized ideal gradient waveform model with the frequency response spectrum model of the gradient amplifiers to produce an optimal gradient waveform; and

,

a reconstruction processor that reconstructs received resonance into an image representation.

6. The magnetic resonance imaging apparatus as set forth in claim 5, wherein:

the ideal waveform models each include a series of digital values along the ideal waveform; and

,

the frequency response spectrum model includes a kernel corresponding to the frequency response spectrum of the gradient amplifiers.

7. The magnetic resonance imaging apparatus as set forth in claim 5, wherein the reconstruction processor includes:

a gradient waveform sub-memory that stores the optimal gradient waveform.

8. The magnetic resonance imaging apparatus as set forth in claim 7, wherein the reconstruction processor includes an adjustment circuit that:

compares the optimal gradient waveform with the ideal gradient waveform;

determines a deviance of the optimal gradient waveform from the ideal gradient waveform; and

,

adjusts received resonance signals according to the deviance.

9. The magnetic resonance imaging apparatus as set forth in claim 8, wherein the reconstruction processor includes a reconstruction algorithm that reconstructs the adjusted resonance signals into an image representation.

10. The method as set forth in claim 9, wherein the step of band limiting the gradient waveform includes:

selecting an imaging sequence which includes at least a radio frequency magnetic field pulse, a read gradient pulse, and at least one other gradient pulse;

sampling a selected read gradient at intervals to generate a series of gradient values;

convolving the selected gradient values with a preselected band-limited kernel which represents a frequency response spectrum of the gradient amplifier that applies the read gradient to generate a modified read gradient waveform.

11. The method as set forth in claim 10, further including:

selecting a plurality of imaging sequences;

sampling the read gradients from each of the sequences to generate a series of gradient values;

convolving the gradient values with the preselected band-limited kernel to generate a plurality of modified read gradient waveforms;

storing the modified read gradient waveforms;

accessing the stored read gradient waveforms when applying a selected imaging sequence.

14. The method as set forth in claim 10, further including:

sampling the other gradient pulses; and

,

convolving the sampled other gradient pulses with the band-limited kernel.

12. A method of diagnostic imaging comprising:

disposing a subject in a main magnetic field of a magnetic resonance apparatus;

constructing an optimum gradient waveform by;

selecting an imaging sequence which includes at least a radio frequency magnetic field pulse, a read gradient pulse, and at least one other gradient pulse;

sampling a selected read gradient at intervals to generate a series of gradient values;

convolving the selected gradient values with a preselected band-limited kernel which represents a frequency response spectrum of a gradient amplifier that applies the read gradient to generate a modified read gradient waveform;

exciting magnetic resonance and receiving magnetic resonance signals;

applying the optimum gradient waveform with a gradient amplifier during a resonance signal read portion of an imaging sequence by amplifying the optimum gradient waveform with gradient amplifiers that have a frequency response spectrum that is not greater than the frequency response spectrum represented by the kernel;

reconstructing the read magnetic resonance signals and converting them to an image representation of an interior of the subject.

13. The method as set forth in claim 12, further including:

replacing the gradient amplifiers with gradient amplifiers with a second frequency response spectrum;

repeating the convolving step using a kernel corresponding to the second frequency response spectrum.

15. A method of diagnostic imaging comprising:

disposing a subject in a main magnetic field of a magnetic resonance apparatus which includes gradient coils connected with bandwidth limited gradient amplifiers;

band limiting gradient waveforms such that the compressed gradient waveforms are compatible with bandwidths of the bandwidth limited amplifiers;

exciting magnetic resonance and receiving magnetic resonance signals;

supplying the compressed gradient waveforms to the gradient amplifiers during a resonance signal read portion of an imaging sequence;

reconstructing the read magnetic resonance signals and converting them to an image representation of an interior of the subject.

16. The method as set forth in claim 15, wherein the step of band limiting the gradient waveform includes:

submitting scan preferences to the apparatus from a user;

choosing an ideal gradient waveform based on the scan preferences;

digitizing the ideal waveform; and

,

convolving the digitized ideal waveform with a band-limited kernel representing a frequency spectrum of a gradient amplifier.

17. The method as set forth in claim 15, wherein the step of reconstruction includes:

adjusting the read resonance signals based on the band limited gradient waveform;

applying a reconstruction sequence to convert the resonance data into an image representation.

23. The method as set forth in claim 15, wherein the step of band limiting the gradient waveform includes:

storing digital values of a model gradient waveform in a model waveform archive;

modeling a frequency response spectrum to the gradient amplifier with a kernel generator to generate a kernel representing a frequency response spectrum of the gradient amplifier;

storing the gradient amplifier spectrum kernel in a spectrum kernel memory;

convolving the digitized gradient waveform with the band-limited kernel to generate an optimized gradient waveform.

18. A magnetic resonance imaging apparatus comprising:

a means for receiving a subject in a main magnetic field of a magnetic resonance apparatus;

a means for selecting an ideal gradient waveform;

a means for converting the ideal gradient waveform into an optimum gradient waveform tailored to a specific gradient amplifier;

a means for exciting magnetic resonance and receiving magnetic resonance signals;

a means for applying the optimum gradient waveform with the gradient amplifier during a resonance signal read portion of an imaging sequence in which resonance signals are read;

a means for determining a difference between the ideal and optimum gradient waveforms;

a means for adjusting the read resonance signals in accordance with the difference between the ideal and optimum gradient waveforms;

a means for reconstructing the adjusted magnetic resonance signals and converting them to an image representation of an interior of the subject.

20. The apparatus as set forth in claim 18, wherein the means for converting the ideal gradient waveform into the optimum gradient waveform includes:

a means for selecting an imaging sequence which includes at least a radio frequency magnetic field pulse, an ideal read gradient pulse, and at least one other gradient pulse;

a means for sampling a selected read gradient at intervals to generate a series of gradient values;

a means for convolving the selected gradient values with a preselected band-limited kernel which represents a frequency response spectrum of a gradient amplifier that applies the read gradient to generate a modified optimal read gradient waveform.

21. The apparatus as set forth in claim 20, further including:

a means for selecting a plurality of imaging sequences;

a means for sampling the read gradients from each of the sequences to generate a series of gradient values;

a means for convolving the gradient values with the preselected band-limited kernel to generate a plurality of modified read gradient waveforms;

a means for storing the modified read gradient waveforms;

a means for accessing the stored read gradient waveforms when applying a selected imaging sequence.

22. The apparatus as set forth in claim 18, wherein the means for converting the ideal gradient waveform into the optimum gradient waveform includes:

a means for storing digital values of a model ideal gradient waveform in a model waveform archive;

a means for modeling a frequency response spectrum to a gradient amplifier with a kernel generator to generate a kernel representing the frequency response spectrum of the gradient amplifier;

a means for storing the gradient amplifier spectrum kernel in a spectrum kernel memory;

a means for convolving the digitized gradient waveform with the band limited kernel to generate an optimized gradient waveform.

19. A magnetic resonance imaging apparatus comprising:

a means for receiving a subject in a main magnetic field of a magnetic resonance apparatus;

a means for constructing an optimum gradient waveform including;

a means for submitting scan preferences to the apparatus from a user;

a means for choosing an ideal gradient waveform based on the scan preferences;

a means for digitizing the ideal waveform; and

,

a means for convolving the digitized ideal waveform with a band-limited kernel representing a frequency spectrum of a gradient amplifier;

a means for exciting magnetic resonance and receiving magnetic resonance signals;

a means for applying the optimum gradient waveform with a gradient amplifier during a resonance signal read portion of an imaging sequence;

a means for reconstructing the read magnetic resonance signals and converting them to an image representation of an interior of the subject.

24. A magnetic resonance method comprising:

storing digital values of a model gradient waveform in a model waveform archive;

modeling a frequency response spectrum to a gradient amplifier with a kernel generator to generate a kernel representing a frequency response spectrum of the gradient amplifier;

storing the gradient amplifier spectrum kernel in a spectrum kernel memory;

convolving the digitized gradient waveform with the band-limited kernel to generate an optimized gradient waveform.