MRI USING RF GRADIENTS FOR SPATIAL ENCODING

• US 20150253403A1
• Filed: 03/10/2015
• Published: 09/10/2015
• Est. Priority Date: 03/10/2014
• Status: Active Grant

• Current Assignee: Vanderbilt University
• Original Assignee: Vanderbilt University
• Inventors: Grissom, William, Does, Mark, Cao, Zhipeng
• Granted Patent: US 9,995,808 B2
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: G01R 33/4616 using specific RF pulses or specific modulation schemes, e.g. stochastic excitation, adiabatic RF pulses, composite pulses, binomial pulses, Shinnar-le-Roux pulses, spectrally selective pulses not being used for spatial selection
• CPC Class Codes: G01R 33/4831 using B1 gradients, e.g. rotating frame techniques, use of surface coils

Abstract

Embodiments of the invention concern systems and methods for performing MRI using RF gradients for spatial encoding. One aspect of the invention involves providing |B₁⁺|-selective pulses designed using the Shinnar-Le Roux algorithm. Another aspect of the invention involves RF encoding based on the Bloch-Siegert (BS) shift. Together, these techniques can be used to support MRI based on RF gradient encoding instead of the conventional B₀ encoding.

Claims

1. A method of magnetic resonance imaging comprising:

generating an magnetic resonance (MR) signal for a volume of interest, the generating comprising;

providing an excitation pulse for a volume of interest, the excitation pulse comprising a B₁⁺-selective radio frequency (RF) pulse, and providing spatial encoding based on a frequency encoding of a RF gradient field;

detecting a MR signal; and

reconstructing an image for the volume of interest based on the MR signal.

2. The method of claim 1, wherein the providing of the spatial encoding comprises using Bloch-Siegert shift encoding sequence to provide the frequency encoding for the RF gradient field.

3. The method of claim 1, wherein the providing of the excitation pulse comprises designing the B₁⁺-selective radio frequency (RF) pulse using a Shinnar-Le Roux algorithm.

4. The method of claim 3, wherein the algorithm comprises:

calculating a duration of a half-pulse and the number of samples in the half-pulse;

calculating inputs for a finite impulse response (FIR) β

filter;

running an FIR filter design tool to design the FIR β

filter;

sine-modulating coefficients for the FIR β

filter to a target passband center;

scaling to the desired tip angle and divide by the dwell time to get a sampled waveform; and

building a normalized waveform based on the sampled waveform.

5. The method of claim 4, further comprising splitting and reflecting the modulated filter.

6. The method of claim 4, wherein the inputs comprise normalized band edges, an amplitude of frequency response at the normalized band edges, and band error weights.

7. The method of claim 1, wherein the RF gradient field is generated via modulation of at least one of an amplitude or a frequency of an RF electromagnetic field.

8. A computer-readable storage medium having stored thereon a computer program for controlling magnetic resonance imaging (MRI) system, the computer program comprising instruction for causing the MRI system to perform a method comprising:

generating an magnetic resonance (MR) signal for a volume of interest, the generating comprising;

providing an excitation pulse for a volume of interest, the excitation pulse comprising a B₁⁺-selective radio frequency (RF) pulse, and providing spatial encoding based on a frequency encoding of a RF gradient field;

detecting a MR signal; and

reconstructing an image for the volume of interest based on the MR signal.

9. The computer-readable storage medium of claim 8, wherein the providing of the spatial encoding comprises using Bloch-Siegert shift encoding sequence to provide the frequency encoding for the RF gradient field.

10. The computer-readable storage medium of claim 8, wherein the providing of the excitation pulse comprises designing the B₁⁺-selective radio frequency (RF) pulse using a Shinnar-Le Roux algorithm.

11. The computer-readable storage medium of claim 10, wherein the algorithm comprises:

calculating a duration of a half-pulse and the number of samples in the half-pulse;

calculating inputs for a finite impulse response (FIR) β

filter;

running an FIR filter design tool to design the FIR β

filter;

sine-modulating coefficients for the FIR β

filter to a target passband center;

scaling to the desired tip angle and divide by the dwell time to get a sampled waveform; and

building a normalized waveform based on the sampled waveform.

12. The computer-readable storage medium of claim 11, further comprising splitting and reflecting the modulated filter.

13. The computer-readable storage medium of claim 11, wherein the inputs comprise normalized band edges, an amplitude of frequency response at the normalized band edges, and band error weights.

14. The computer-readable storage medium of claim 8, wherein the RF gradient field is generated via modulation of at least one of an amplitude or a frequency of an RF electromagnetic field.

15. A magnetic resonance imaging system comprising:

a plurality of signal generating elements configured for generating radio frequency (RF) fields in a volume of interest;

at least one RF receiver configured for detecting a magnetic resonance (MR) signal from the volume of interest;

a processor communicatively coupled to the plurality of signal generating elements and the at least one RF receiver; and

a computer-readable medium having stored thereon a plurality of instructions for causing the processor to perform steps comprising;

configuring the plurality of single generating elements to generating a magnetic resonance (MR) signal for the volume of interest, the generating comprising;

providing an excitation pulse for a volume of interest, the excitation pulse comprising a B₁⁺-selective radio frequency (RF) pulse, and providing spatial encoding based on a frequency encoding of a RF gradient field;

receiving signals from the at least one RF receiver corresponding to the MR signal; and

reconstructing an image for the volume of interest based on the MR signal.

16. The system of claim 15, wherein the providing of the spatial encoding comprises using Bloch-Siegert shift encoding sequence to provide the frequency encoding for the RF gradient field.

17. The system of claim 15, wherein the providing of the excitation pulse comprises designing the B₁⁺-selective radio frequency (RF) pulse using a Shinnar-Le Roux algorithm.

18. The system of claim 17, wherein the algorithm comprises:

calculating a duration of a half-pulse and the number of samples in the half-pulse;

calculating inputs for a finite impulse response (FIR) β

filter;

running an FIR filter design tool to design the FIR β

filter;

sine-modulating coefficients for the FIR β

filter to a target passband center;

scaling to the desired tip angle and divide by the dwell time to get a sampled waveform; and

building a normalized waveform based on the sampled waveform.

19. The system of claim 18, further comprising splitting and reflecting the modulated filter.

20. The system of claim 15, wherein the RF gradient field is generated via modulation of at least one of an amplitude or a frequency of an RF electromagnetic field.