MRI gradient trajectory mapping
Magnetic field temporal variations in magnetic resonance imaging (MRI) volume are determined based on the slope of a phase difference Δφ between spin responses in plural slices at a given temporal sampling time. Representations of the determined temporal magnetic field variations are stored for subsequent use, e.g., to achieve more accurate re-gridding of acquired k-space date before reconstruction of images in the spatial domain.
1. A magnetic resonance imaging (MRI) system comprising:
an assembly of gantry components including static and gradient magnetic field generators and at least one radio frequency (RF) coil defining an imaging volume;
an MRI control system, connected to control said gantry components, including at least one RF transmitter, at least one RF receiver and computer control circuits configured to effect specified MRI data acquisition sequences of RF and gradient magnetic pulses which acquire, from an object located within said imaging volume, RF nuclear magnetic resonance (NMR) spin responses emanating from different spatially located volumes of NMR nuclei respectively corresponding to different positions in k-space as a function of a magnetic field experienced by the nuclei;
said computer control circuits being configured to effect:
(A) for each of a series of slices j, a data acquisition sequence using (a) RF and slice select magnetic gradient pulses followed by (b) at least one magnetic gradient pulse to be analyzed, said magnetic gradient pulse to be analyzed having a same magnitude for each slice and being in a same direction as said slice select magnetic gradient pulse, located at least partly within an analog-to-digital converter (ADC) window providing successive temporal samples i of NMR phase responses φ i,j , wherein all magnetic gradient pulses to be analyzed in the data acquisition sequence have the same magnitude,
(B) determining magnetic field temporal variations at each sample point i based on the slope of the phase difference Δφ between plural slices j at temporal sampling time i, by comparing phase measurements across plural slices and across plural temporal sampling times where said same magnitude magnetic gradient pulse to be analyzed causes detected phase differences, and
(C) storing representations of the determined temporal magnetic field variations for subsequent use in MRI system operations.
2. The MRI system as in claim 1 , wherein said computer control circuits are further configured to:
(D) establish an open ADC window for a further period after said at least one magnetic gradient pulse to be analyzed and thereby acquire further NMR phase responses φ i,j related to magnetic fields caused by eddy currents,
(E) determine eddy current magnetic field temporal variations at each sample point i based on the slope of the phase difference Δφ between plural slices j at temporal time i during said further period, and
(F) store the determined temporal eddy current magnetic field variations for subsequent use in MRI system operations.
3. The MRI system as in claim 1 , wherein said computer control circuits are further configured to effect measurement of MRI system background magnetic fields by:
for each of a series of slices m, effecting a data acquisition sequence using RF and slice select magnetic gradient pulses followed by an ADC window providing successive background temporal samples n of NMR phase responses φ n,m , but without occurrence of a magnetic gradient pulse during temporal sample acquisition;
determining background magnetic field temporal variations at each background sample point n based on the slope of the phase difference between plural slices m at background temporal sampling time n; and
storing the determined background magnetic field temporal variations for subsequent use in MRI system operations.
4. The MRI system as in claim 1 , wherein said computer control circuits are configured to:
(D) store said determined temporal magnetic field variations for each of plural predetermined magnetic gradient pulse conditions associated with respectively corresponding MRI diagnostic data acquisition sequences in an MRI library of possible MRI diagnostic data acquisition sequences.
5. The MRI system as in claim 1 , wherein said computer control circuits are configured to:
(D) re-grid acquired k-space diagnostic MRI data, acquired while using said magnetic gradient pulse to be analyzed, in accordance with said determined temporal magnetic field variations for the magnetic gradient pulse to be analyzed, rather than in accordance with theoretical assumed temporal magnetic field variations of the magnetic gradient pulse to be analyzed, and
(E) re-construct the resulting re-gridded k-space diagnostic MRI data into a diagnostic image in the spatial domain.
6. The MRI system as in claim 1 , wherein said computer control circuits are configured to determine said temporal magnetic field variations with a temporal resolution equal to the sampling rate of the ADC temporal samples i.
7. The MRI system as in claim 1 , wherein said computer control circuits are configured to determine said temporal magnetic field variations without making any assumption as to the shape of expected temporal magnetic field variations to be determined.
8. The MRI system as in claim 1 , wherein said data acquisition sequence is performed while a phantom object is located at a predetermined position within said image volume.
9. The MRI system as in claim 1 , wherein said magnetic gradient pulse to be analyzed is subjected to at least one of the following conditions: a specified peak power gradient amplifier output level, a specified gradient amplifier slew rate, a specified gradient amplifier duty cycle over a first time period, a specified gradient amplifier duty cycle over a first and a second time period, and a specified gradient amplifier temperature.
10. The MRI system as in claim 1 , wherein said computer control circuits are configured to create a model of MRI system operation based on said determined temporal magnetic field variations as a function of varied operating parameters.
11. A magnetic resonance imaging (MRI) method comprising:
controlling an assembly of gantry components including static and gradient magnetic field generators and at least one radio frequency (RF) coil defining an imaging volume, at least one RF transmitter, at least one RF receiver and computer control circuits configured to effect specified MRI data acquisition sequences of RF and gradient magnetic pulses which acquire, from an object located within said imaging volume, RF nuclear magnetic resonance (NMR) spin responses emanating from different spatially located volumes of NMR nuclei respectively corresponding to different positions in k-space, as a function of a magnetic field experienced by the nuclei, to effect:
(A) for each of a series of slices j, a data acquisition sequence using (a) RF and slice select magnetic gradient pulses followed by (b) at least one magnetic gradient pulse to be analyzed, said magnetic gradient pulse to be analyzed having a same magnitude for each slice j and being in a same direction as said slice select magnetic gradient pulse, located at least partly within an analog-to-digital converter (ADC) window providing successive temporal samples i of NMR phase responses φ i,j , wherein all magnetic gradient pulses to be analyzed in the data acquisition sequence have the same magnitude,
(B) determining magnetic field temporal variations at each sample point i based on the slope of the phase difference Δφ between plural slices j at temporal sampling time i, by comparing phase measurements across plural slices and across plural temporal sampling times where said same magnitude magnetic gradient pulse to be analyzed causes detected phase differences, and
(C) storing representations of the determined temporal magnetic field variations for subsequent use in MRI system operations.
12. The MRI method as in claim 11 , further comprising:
(D) establishing an open ADC window for a further period after said at least one magnetic gradient pulse to be analyzed and thereby acquire further NMR phase responses φ i,j related to magnetic fields caused by eddy currents,
(E) determining eddy current magnetic field temporal variations at each sample point i based on the slope of the phase difference Δφ between plural slices j at temporal time i during said further period, and
(F) storing the determined temporal eddy current magnetic field variations for subsequent use in MRI system operations.
13. The MRI method as in claim 11 further comprising measurement of MRI system background magnetic fields by:
for each of a series of slices m, effecting a data acquisition sequence using RF and slice select magnetic gradient pulses followed by an ADC window providing successive background temporal samples n of NMR phase responses φ n,m , but without occurrence of a magnetic gradient pulse during temporal sample acquisition;
determining background magnetic field temporal variations at each background sample point n based on the slope of the phase difference between plural slices m at background temporal sampling time n; and
storing the determined background magnetic field temporal variations for subsequent use in MRI system operations.
14. The MRI method as in claim 11 , further comprising:
(D) storing said determined temporal magnetic field variations for each of plural predetermined magnetic gradient pulse conditions associated with respectively corresponding MRI diagnostic data acquisition sequences in an MRI library of possible MRI diagnostic data acquisition sequences.
15. The MRI method as in claim 11 , further comprising:
(D) re-gridding acquired k-space diagnostic MRI data, acquired while using said magnetic gradient pulse to be analyzed, in accordance with said determined temporal magnetic field variations for the magnetic gradient pulse to be analyzed, rather than in accordance with theoretical assumed temporal magnetic field variations of the magnetic gradient pulse to be analyzed, and
(E) re-constructing the resulting re-gridded k-space diagnostic MRI data into a diagnostic image in the spatial domain.
16. The MRI method as in claim 11 , wherein said temporal magnetic field variations are determined with a temporal resolution equal to the sampling rate of the ADC temporal samples i.
17. The MRI method as in claim 11 , wherein said temporal magnetic field variations are determined without making any assumption as to the shape of expected temporal magnetic field variations to be determined.
18. The MRI method as in claim 11 , wherein said data acquisition sequence is performed while a phantom object is located at a predetermined position within said image volume.
19. The MRI method as in claim 11 , wherein said magnetic gradient pulse to be analyzed is subjected to at least one of the following conditions: a specified peak power gradient amplifier output level, a specified gradient amplifier slew rate, a specified gradient amplifier duty cycle over a first time period, a specified gradient amplifier duty cycle over a first and a second time period, and a specified gradient amplifier temperature.
20. The MRI method as in claim 11 , further comprising creating a model of MRI system operation based on said determined temporal magnetic field variations as a function of varied operating parameters.
21. The MRI system as in claim 1 wherein said computer control circuits are configured to characterize performance of the gradient magnetic field generator for said same magnitude of magnetic gradient pulse output from the gradient magnetic field generator.
22. The MRI method as in claim 11 further comprising characterizing performance of the gradient magnetic field generator for said same magnitude of magnetic gradient pulse output from the gradient magnetic field generator.