METHOD FOR RADIOFREQUENCY MAPPING IN MAGNETIC RESONANCE IMAGING

• US 20110025327A1
• Filed: 09/26/2007
• Published: 02/03/2011
• Est. Priority Date: 09/29/2006
• Status: Abandoned Application

• Current Assignee: King's College London
• Original Assignee: King's College London
• Inventors: Deoni, Sean C.L., Williams, Steven
• Application Number:
  US12/443,673
• Publication Number:
  US 20110025327A1
• Time in Patent Office: Days
• US Class Current: 324/309
• CPC Class Codes: G01R 33/50 based on the determination of relaxation times , e.g. T1 measurement by IR sequences; T2 measurement by multiple-echo sequences
• CPC Class Codes: G01R 33/5602 by filtering or weighting based on different relaxation times within the sample, e.g. T1 weighting using an inversion pulse
• CPC Class Codes: G01R 33/5613 Generating steady state signals, e.g. low flip angle sequences [FLASH]
• CPC Class Codes: G01R 33/5614 using a fully balanced steady-state free precession [bSSFP] pulse sequence, e.g. trueFISP
• CPC Class Codes: G01R 33/5618 using both RF and gradient refocusing, e.g. GRASE
• CPC Class Codes: G01R 33/583 Calibration of signal excitation or detection systems, e.g. for optimal RF excitation power or frequency G01R33/246 takes precedence

Abstract

A method of mapping a radio frequency magnetic field transmitted to a magnetic resonance imaging specimen. The method comprises the steps of: applying a first radio frequency pulse having a first excitation angle to the specimen and at a first time period after applying the first pulse applying one or more second radio frequency pulses each having a second excitation angle to the specimen, with a second time period between second pulses, to obtain a first data set defining a first sample of an image space; applying one or more third radio frequency pulses each having a third excitation angle to the specimen, with a third time period between third pulses, to obtain a second data set defining a second sample of the image space; applying one or more fourth radio frequency pulses each having a fourth excitation angle to the specimen, with a fourth time period between fourth pulses, to obtain a third data set defining a third sample of the image space; wherein the fourth excitation angle is different to the third excitation angle and/or the fourth time period is different to the third time period; calculating a magnetic field map data from at the three data sets; and outputting the magnetic field map data.

Claims

1. A method of mapping a radiofrequency (RF) magnetic field (B₁⁺) transmitted to a magnetic resonance imaging (MRI) specimen, the method comprising:

applying a first RF pulse having a first excitation angle to the specimen and at a first time period after applying the first pulse applying one or more second RF pulses each having a second excitation angle to the specimen, with a second time period between second pulses, to obtain a first data set defining a first sample of an image space;

applying one or more third RF pulses each having a third excitation angle to the specimen, with a third time period between third pulses, to obtain a second data set defining a second sample of the image space;

applying one or more fourth RF pulses each having a fourth excitation angle to the specimen, with a fourth time period between fourth pulses, to obtain a third data set defining a third sample of the image space, wherein the fourth excitation angle is different to the third excitation angle and/or the fourth time period is different to the third time period;

calculating B₁⁺ field map data from at least the three data sets; and

outputting the B₁⁺ field map data.

2. A method according to claim 1, wherein the image space is k-space.

3. A method according to claim 1, wherein the samples of the image space are one-dimensional, two-dimensional or three-dimensional.

4. A method according to claim 1, wherein the first RF pulse is an inversion pulse.

5. A method according to claim 4, wherein the inversion pulse has an excitation angle of 180 degrees.

6. A method according to claim 1, further comprising spoiling residual transverse magnetisation resulting from at least one of the steps of applying first, second, third or fourth pulses.

7. A method according to claim 6, wherein spoiling residual transverse magnetization comprises at least one of applying a gradient magnetic field subsequent to the application of the RF pulse resulting in the residual transverse magnetisation and varying the phase of the RF pulse applied subsequent to the RF pulse resulting in the residual transverse magnetisation, relative to the phase of the RF pulse resulting in the residual transverse magnetisation.

8. A method according to claim 6, wherein applying a first pulse and one or more second pulses and spoiling residual transverse magnetisation are comprised by obtaining an inversion recovery spoiled gradient recalled (IR-SPGR) signal.

9. A method according to claim 6, wherein the steps of applying one or more third pulses and spoiling residual transverse magnetisation are comprised by the stop of obtaining a first SPGR signal.

10. A method according to claim 6, wherein applying one or more fourth RF pulses is comprised by obtaining a second SPGR signal.

11. A method according to claim 1, wherein applying one or more third pulses and spoiling residual transverse magnetization are comprised by obtaining a first SPGR signal, applying one or more fourth RF pulses is comprised by obtaining a second SPGR signal, and obtaining a first and second SPGR signal are comprised by obtaining DESPOT1 data.

12. A method according to claim 1, wherein calculating B₁⁺ field map data from at least the three data sets comprises the steps of:

obtaining predicted data for the first data set from the second and third data sets; and

comparing the predicted data with the first data set.

13. A method according to claim 12, wherein obtaining predicted data comprises:

calculating T₁ (longitudinal relaxation time) and ρ

(a factor proportional to the equilibrium longitudinal magnetization including at least a factor of electronic amplifier gain or receive coil sensitivity effects by inserting the combined second and third data set into the following equation;

14. A method according to claim 12, wherein comparing the predicted data with the first data set comprises calculating residuals for the predicted data and the first data set as a function of κ

.

15. A method according to claim 1, wherein calculating B₁⁺ field map data from at least three data sets comprises performing a multi-parameter fit for a plurality of samples from the at least three data sets.

16. A method according to claim 1, further comprising obtaining at least one further first, second or third data set.

17. A method according to claim 16, comprising obtaining at least one further first data set with at least a different first time period, first excitation angle, second time period or second excitation angle.

18. A method according to claim 16, comprising obtaining at least one further second data set with at least a different third time period or third excitation angle.

19. A method according to claim 16, comprising obtaining at least one further third data set with at least a different fourth time period or fourth excitation angle.

20. A method according to claim 1, further comprising correcting for a B₁⁻

field.

21. A method according to claim 1, the method further comprising:

calculating T₁ (longitudinal relaxation time) map data from the three data sets; and

outputting the T₁ map data.

22. A method according to claim 1, further comprising using the output B₁⁺ field map data to dynamically generate a further RF pulse that minimises variation in B₁⁺ field.

23. A method according to claim 1, the method further comprising applying the output B₁⁺ field map data to an MRI image to produce a corrected image.

24. A method of correcting, in an MRI image, for inhomogeneities in a magnetic field (B₁⁺) transmitted to a MRI specimen, the method comprising:

acquiring B₁⁺ field map data by;

applying a first RF pulse having a first excitation angle to the specimen and at a first time period after applying the first pulse applying one or more second RF pulses each having a second excitation angle to the specimen, with a second time period between second pulses, to obtain a first data set defining a first sample of an image space,

applying one or more third RF pulses each having a third excitation angle to the specimen, with a third time period between third pulses, to obtain a second data set defining a second sample of the image space,

applying one or more fourth RF pulses each having a fourth excitation angle to the specimen, with a fourth time period between fourth pulses, to obtain a third data set defining a third sample of the image space, wherein the fourth excitation angle is different to the third excitation angle and/or the fourth time period is different to the third time period,

calculating B₁⁺ field map data from at least the three data sets, and

outputting the B₁⁺ field map data, and

applying B₁⁺ field map data to MRI image data to produce a corrected image.