Method and apparatus for single carrier wideband magnetic resonance imaging (MRI) data acquisition

Abstract

A method and apparatus for single carrier wideband magnetic resonance imaging (MRI) data acquisition are provided. The method includes the following steps: exciting a slice or slab with the use of RF pulse and a slice/slab selection gradient; applying a phase encoding gradient along a phase encoding direction and reducing a FOV along the phase encoding direction by a factor of W through k-space subsampling; applying a frequency encoding gradient along a frequency encoding direction and increasing a FOV along the frequency encoding direction by a factor of W.sub.f; and applying a separation gradient along the phase encoding direction during the frequency encoding duration and k-space data acquisition.

Claims

1. A method for single carrier wideband magnetic resonance imaging (MRI) data acquisition, using a sequence controller to execute the following steps, comprising: exciting a slice or slab with the use of RF pulse and a slice/slab selection gradient by a RF excitation module; applying a phase encoding gradient along a phase encoding direction and reducing a field of view (FOV) along the phase encoding direction by a factor of W, and through a k-space subsampling by a gradient output module; applying a frequency encoding gradient along a frequency encoding direction and increasing a FOV along the frequency encoding direction by a factor of W.sub.f and the gradient output module; applying a separation gradient which comprises a plurality of segments along the phase encoding direction during the frequency encoding duration and the k-space data acquisition by the gradient output module; and reconstructing an image through the k-space data by an image processing module, wherein, the ratio of the separation gradient satisfying the formula in isotropic voxel size: $.Math.G_{\mathrm{sep}}.Math.W\frac{{\mathrm{FOV}}_{\mathrm{fe}}}{{\mathrm{FOV}}_{\mathrm{pe}}},$ where FOV.sub.fe represented the FOV of the frequency encoding, FOV.sub.Pe represented the FOV of the phase encoding, and G.sub.sep represented the separation gradient.

2. The method of claim 1, wherein the factor of W comprises decimal fraction.

3. The method of claim 1, wherein the factor of W.sub.f comprises positive number.

4. The method of claim 1, wherein when the factor of W comprises a decimal, W.sub.f is a value of W rounding up.

5. The method of claim 1, wherein the plurality of segments comprise a plurality of slopes or a same slope.

6. The method of claim 1, wherein the frequency encoding gradient comprises a plurality of segments.

7. The method of claim 5, wherein the k-space data acquired have a trajectory in a zigzag form.

8. The method of claim 5, wherein the k-space data acquired have a trajectory in a V or inverted V than.

9. An apparatus for single carrier wideband magnetic resonance imaging (MRI) data acquisition, comprising: a sequence controller; an RF excitation module controlled by the sequence controller and generating an RF pulse to excite a slice or slab; and a gradient output module controlled by the sequence controller and outputting magnetic field gradients along a plurality of different directions, the magnetic field gradients comprising: a slice/slab selection gradient; a phase encoding gradient along a phase encoding direction and with a reduced field of view (FOV) along the phase encoding direction by a factor of W; a frequency encoding gradient along a frequency encoding direction and with an increased FOV along the frequency encoding direction by a factor of W.sub.f; a separation gradient which comprises a plurality of segments along the phase encoding direction during the frequency encoding duration and k-space data acquisition; and an image processing module connected with the gradient output module to acquire the k-space data and reconstruct an image, and wherein, the ratio of the separation gradient satisfying the formula in isotropic voxel size: $.Math.G_{\mathrm{sep}}.Math.W\frac{{\mathrm{FOV}}_{\mathrm{fe}}}{{\mathrm{FOV}}_{\mathrm{pe}}},$ where FOV.sub.fe represented the FOV of the frequency encoding, FOV.sub.Pe represented the FOV of the phase encoding, and G.sub.sep represented the separation gradient.

10. The apparatus of claim 9, wherein the factor of W comprises decimal.

11. The apparatus of claim 9, wherein the factor W.sub.f comprises a positive number.

12. The apparatus of claim 9, wherein when the factor of W comprises a decimal, W.sub.f is a value of W rounding up.

13. The apparatus of claim 9, wherein the plurality of segments comprise a plurality of slope or a same slope.

14. The apparatus of claim 9, wherein the frequency encoding gradient comprises a plurality of segments.

15. The apparatus of claim 13, wherein the k-space data acquired have a trajectory in a zigzag form.

16. The apparatus of claim 13, wherein the k-space data acquired have a trajectory in a V or inverted V form.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1a-1c give an example which demonstrates the imaging process and the effect of the single carrier wideband MRI technique. FIG. 1a is the original image (target); FIG. 1b is the actual image acquired by single carrier wideband MRI technique with a wideband factor of 2; FIG. 1c is the reconstructed image with the shear effect corrected.

(2) FIG. 2a illustrates the spatial encoding process of an ordinary 2D MRI.

(3) FIG. 2b illustrates the spatial encoding process of a 2D MRI with single carrier wideband MRI acceleration technique; a separation gradient is added along the phase encoding direction.

(4) FIG. 3 compares an ordinary Cartesian k-space trajectory with the k-space trajectory with single carrier wideband MRI acceleration applied to explain the k-space data loss and origin of the blur in the reconstructed image.

(5) FIGS. 4a-4g give examples of the blur mitigation technique. FIG. 4a gives the spatial encoding sequence without blur mitigation and the corresponding k-space data trajectory; FIGS. 4b, 4d and 4f gives the spatial encoding sequence with blur mitigation which resets the phase accumulated; FIGS. 4c, 4e and 4g gives the spatial encoding sequence with blur mitigation which reverses the direction of the phase accumulation.

(6) FIGS. 5a, 5b, and 5c give a value of k-space loss cause by the ratio between the Voxel Size.sub.phase and Voxel Size.sub.freq.

(7) FIG. 6 illustrates an apparatus for single carrier wideband magnetic resonance imaging (MRI) data acquisition.

(8) FIG. 7a, FIG. 7b, and FIG. 7c give an example which demonstrates the effect of the single carrier wideband MRI as well as the blur mitigation technique. FIG. 7a is the reconstructed image from standard gradient echo (without acceleration). FIG. 7b is the reconstructed image from single carrier wideband MRI with W=2 acceleration. FIG. 7c is the reconstructed image from single carrier wideband MRI with W=2 acceleration and blur mitigation applied.

(9) FIG. 8a, FIG. 8b, and FIG. 8c give the image contrast of the feature with 1 mm high resolution in FIG. 7a, FIG. 7b, and FIG. 7c respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(10) To demonstrate the single carrier wideband MRI and the blur mitigation technique further, three scan protocols are compared, including a) standard gradient echo, b) single carrier wideband MRI with W=2 acceleration, and c) Single carrier Wideband MRI with W=2 acceleration and blur mitigation applied. All the images are taken on a 3T Bruker MRI/MRS system with quadrature head coils. The gradient echo sequence scan covers a FOV of 25.625.6 cm; matrix size is 256256; resolution is 1 mm.sup.2; thickness is 4 mm; and TR/TE is 70 ms/10 ms. The scan time using wideband MRI technique is 8 s, which reduces the original scan time by a factor of 2. The contrast of features with 1 mm high resolution (as the region indicated by the box in FIG. 1a) is used to examine the blurring effect.

(11) FIG. 7a, FIG. 7b, and FIG. 7c show the images acquired by a) standard gradient echo, b) W=2 single carrier acceleration wideband MRI, and c) W=2 single carrier acceleration wideband MRI with blur mitigation. The scan time using wideband MRI technique is 8 s, which reduces the standard gradient echo scan time by a factor of 2. The image contrast of features with 1 mm high resolution from each protocol is used to examine the blurring effect, and the results are shown in FIG. 8a, FIG. 8b, and FIG. 8c. The left row demonstrates image of high resolution structure. The right row demonstrates the profile of the high resolution structure. The x axis of right row of FIGS. 8a, 8b, and 8c are the position in the image of high resolution structure, and the y axis of right row of FIG. 8a, FIG. 8b, and FIG. 8c is the signal intensity of the high resolution structure image. The contrast of high resolution structure is 66.4%, 8.5% and 64.2%. In standard gradient echo, SCWB without blur mitigation and SCWB with blur mitigation, respectively.

(12) The results from the standard gradient echo and the single carrier wideband MRI with blur mitigation have shown a peak-to-valley contrast about 64%66% (shown in FIG. 8a), while the single carrier wideband MRI without blur mitigation fails to display the high resolution features (shown in FIG. 8b), which gives a merely 8.5% peak-to-valley contrast. The results demonstrate the image characteristics of single carrier wideband MRI technique and the blur mitigation (shown in FIG. 8c), wherein the blur mitigation has improved the high resolution contrast significantly. To sum up, single carrier wideband MRI technique can reduce the scan time and speed up the data acquisition; however, a simple separation gradient sequence results k-space data loss; this loss depends on the actual aspect ratio of the imaging target and the wideband factor W and could blur the reconstructed image to some extent. With the blur mitigation technique broached, this artifact can be addressed. Consequently, single carrier wideband MRI with blur mitigation can provide fast and high spatial resolution magnetic resonance images with image quality comparable to standard sequences, which is valuable in clinical studies.