Method for assessing structural spatial frequencies using hybrid sampling with non-zero gradient for enhancement of selective sampling
The disclosed embodiments provide a method for acquiring MR data at resolutions down to tens of microns for application in in-vivo diagnosis and monitoring of pathology for which changes in fine tissue textures can be used as markers of disease onset and progression. Bone diseases, tumors, neurologic diseases, and diseases involving fibrotic growth and/or destruction are all target pathologies. Further the technique can be used in any biologic or physical system for which very high-resolution characterization of fine scale morphology is needed. The method provides rapid acquisition of selected values in k-space, with multiple successive acquisitions of individual k-values taken on a time scale on the order of microseconds, within a defined tissue volume, and subsequent combination of the multiple measurements in such a way as to maximize SNR. The reduced acquisition volume, and acquisition of only select values in k-space along selected directions, enables much higher in-vivo resolution than is obtainable with current MRI techniques.
1. A method for selective sampling to assess texture using magnetic resonance (MR) comprising:
transmitting a first RF pulse with a first gradient chosen for first slice selection;
transmitting a second RF pulse with application of a second gradient chosen for slice selective refocusing in a region defined by an intersection of the first slice and a second slice;
encoding a specific k-value with a selected gradient pulse;
transmitting a third RF pulse with a third gradient activated, said third gradient adapted for slice selective refocusing, defining a region defined by the intersection of the first and second slices and a third slice selection to define a volume of interest (VOI);
turning off all gradients;
recording multiple samples of an RF signal encoded with the specific k-value;
applying a non-zero magnitude gradient as a time-dependent phase-encode determining a trajectory through k-space while recording samples at a sequence of k-values across a neighborhood of k-values defined by height and pulse width of the non-zero magnitude gradient, the sequence of k-values being a subset of k-values required to make an image;
post processing samples at a combination of sequential k values, recorded within a time span while the non-zero magnitude gradient is applied.
2. The method as defined in claim 1 wherein the step of encoding comprises:
applying an encoding gradient pulse to induce phase wrap to create a spatial encode for the specific k-value and orientation, the specific k-value selected based on texture within the VOI.
3. The method as defined in claim 2 wherein the specific k value is in a range of 0.2 mm −1 to 100 mm −1 and the neighborhood of k-values induced by the non-zero magnitude gradient are a subset of k-values required to make an image.
4. The method as defined in claim 2 further comprising:
applying a second non-zero magnitude gradient as a time-dependent phase-encode determining a second trajectory through k-space while recording samples at a second set of sequential k-values in a second neighborhood of the specific k-value, with all recording in a single TR.
5. The method as defined in claim 4 further comprising:
transmitting a second refocusing RF pulse prior to applying the second non-zero magnitude gradient.
6. The method as defined in claim 4 wherein the second non-zero magnitude gradient comprises a gradient inducing samples in k-space which are a subset of the k-values required to make an image.
7. The method as defined in claim 4 wherein the specific k value is in a range of 0.2 mm −1 to 100 mm −1 and the second non-zero magnitude gradient comprises a gradient inducing samples in k-space outside the neighborhood of k-values induced by the non-zero magnitude gradient, the k-values in the second neighborhood induced by the second non-zero gradient remain a subset of k-values required to make an image.
8. The method as defined in claim 2 further comprising:
following the recording of multiple samples of an RF signal encoded with the specific k-value, issuing a k-value selection pulse on a selected vector combination gradient to determine a second k-value;
turning off the vector combination gradient; and,
recording multiple samples of the RF signal at the second k-value in a single TR.
9. The method as defined in claim 8 further comprising:
applying additional k-value selection pulses for a predetermined plurality of pulses on a selected vector combination gradient, each k value selection pulse determining a next k-value;
turning off the vector combination gradient after each pulse; and,
recording multiple samples of the RF signal at the next k-value determined by each k value selection pulse in the TR.
10. The method as defined in claim 1 further comprising:
transmitting a refocusing RF pulse prior to applying the non-zero magnitude gradient.
11. The method as defined in claim 1 wherein the non-zero magnitude gradient is determined based on a windowing function and is defined as
G
≤
3
!
(
1
-
η
min
)
π
X
·
N
·
Δ
t
wherein η min is a lower correlation bound, X is a constant determining the windowing function and N is the number of samples.
12. The method as defined in claim 1 further comprising correcting for change in k-values across the neighborhood in k-space selected by the non-zero magnitude gradient by inducing a set of k-value measurements with prescribed k-value encodes.
13. A method for selective sampling to assess texture using magnetic resonance (MR) comprising:
transmitting RF pulses and associated gradients to define a volume of interest (VOI);
applying an encoding gradient pulse to induce phase wrap to create a spatial encode for a specific k-value and orientation determined based on texture within the VOI as an initial value;
applying a non-zero magnitude gradient as a time-dependent phase-encode determining a trajectory through k-space while recording samples at a sequence of k-values across a neighborhood proximate the specific k-value defined by height and pulse width of the non-zero magnitude gradient, the sequence of k-values being a subset of k-values required to make an image; and,
post processing the samples at the sequence of k values, recorded within a time span while the non-zero magnitude gradient is applied.
14. The method as defined in claim 13 wherein recording samples of a set at a sequence of k-values comprises sampling in overlapping blocks of k-values.
15. The method as defined in claim 14 wherein samples at the sequence of k-values in the blocks are combined as data points for an improvement in SNR via the combination and further comprising:
characterizing a power distribution across a range of the combined data points to measure underlying texture within the VOI.
16. The method as defined in claim 13 wherein the specific k value is in a range of 0.2 mm −1 to 100 mm −1 and the sequence of k-values in the neighborhood are a subset of the range.
17. The method as defined in claim 13 wherein the subset of k-values is less than 30% of k-values required to make an image.
18. A method for selective sampling to assess texture using magnetic resonance (MR) comprising:
exciting and internal volume of interest (VOI);
applying an encoding gradient pulse to induce phase wrap to create a spatial encode for a specific k-value and orientation, the specific k-value selected based on expected texture within the VOI;
applying a time varying gradient determining a trajectory through k space, from the specific k-value, of a sequence of k-values in a neighborhood of the specific k-value; and,
recording a set of signal measurements at the sequence of k-values in the neighborhood to measure texture within the VOI.