MRI using RF gradients for spatial encoding
Systems and methods for performing MRI include using a RF gradient field for spatial encoding. In particular implementations, |B + i |-selective pulses designed using the Shinnar-Le Roux algorithm can be provided as the excitation pulse for the RF gradient field. Further, frequency encoding for the RF gradient field can be 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 Bo encoding.
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 B 1 + -selective radio frequency (RF) pulse, and providing spatial encoding for the volume of interest via an RF gradient field, the RF gradient field configured to provide frequency encoding of spatial location information for spatial locations in the volume of interest;
detecting the 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 B 1 + -selective radio frequency (RF) pulse using a Shinnar-Le Roux algorithm.
4. The method of claim 3 , wherein the Shinnar-Le Roux 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 non-transitory 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 B 1 + -selective radio frequency (RF) pulse, and providing spatial encoding for the volume of interest via an RF gradient field, the RF gradient field configured to provide frequency encoding of spatial location information for spatial locations in the volume of interest;
detecting a MR signal; and
reconstructing an image for the volume of interest based on the MR signal.
9. The non-transitory 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 non-transitory computer-readable storage medium of claim 8 , wherein the providing of the excitation pulse comprises designing the B 1 + -selective radio frequency (RF) pulse using a Shinnar-Le Roux algorithm.
11. The non-transitory computer-readable storage medium of claim 10 , wherein the Shinnar-Le Roux 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 non-transitory computer-readable storage medium of claim 11 , further comprising splitting and reflecting the modulated filter.
13. The non-transitory 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 non-transitory 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 non-transitory 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 B 1 + -selective radio frequency (RF) pulse, and providing spatial encoding for the volume of interest via an RF gradient field, the RF gradient field configured to provide frequency encoding of spatial location information for spatial locations in the volume of interest;
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 B 1 + -selective radio frequency (RF) pulse using a Shinnar-Le Roux algorithm.
18. The system of claim 17 , wherein the Shinnar-Le Roux 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.