Parallel Imaging. Marcin.

Transcription

1 Parallel Imaging Marcin

2 Parallel Imaging initial thoughts Over the last 15 years, great progress in the development of pmri methods has taken place, thereby producing a multitude of different and somewhat related parallel imaging reconstruction techniques and strategies. Currently, the most well known are: SMASH, SENSE, and GRAPPA. However, various other techniques, such as AUTO-SMASH,VD- AUTO-SMASH, GENERALIZED SMASH, MSENSE, PILS, and SPACE RIP have also been proposed. All these techniques require additional coil sensitivity information to eliminate the effect of undersampling the k-space. This sensitivity information can be derived either once during the patient setup by means of a prescan or by means of a few additionally acquired k-space lines for every subsequent pmri experiment (autocalibration), or some combination of the two. The present pmri reconstruction methods can roughly be classified into two groups: Those in which the reconstruction takes place in image space (eg, SENSE, PILS) consist of an unfolding or in- verse procedure those in which the reconstruction procedure is done in k-space (eg, SMASH, GRAPPA), consist of a calculation of missing k- space data. However, hybrid techniques like SPACE RIP are also conceivable. Blaimer M, Breuer F, Mueller M, Heidemann RM, Griswold MA, Jakob PM. SMASH, SENSE, PILS, GRAPPA: how to choose the optimal method. Top Magn Reson Imaging Aug;15(4):

3 Standard T2-weighted imaging, a comparison between (a) conventional spin-echo (SE) acquisition and (b) a half-fourier turbo spin-echo (HASTE) acquisition in combination with pmri acceleration factor of three. Both images showthe same contrastand comparable resolution.

4 Parallel imaging (Prof. Karla Miller teaching materials) (SENSE, SMASH, GRAPPA, ipat, etc) Surface coils Object in 8-channel array Single coil sensitivity Multi-channel coils: Array of RF receive coils Each coil is sensitive to a subset of the object Coil sensitivity to encode additional information Can leave out large parts of k-space Similar to partial k-space (faster imaging, reduced distortion, etc), but can go farther

5 The influence of the sampling scheme on the field of view. (a) Conventionally acquired MR image of the brain. (b, left) Depiction of the corresponding k-space sampling scheme. (b, right) For a parallel MRI (pmri) acquisition, the number of applied phase-encoding steps is reduced, resulting in a sparsely sampled k-space. c The periodic undersampling of k-space leads to fold-over artifacts in the spatial domain in phase-encoding direction

6 Parallel Imaging simplistic overview Coil sensitivity maps Partial k-space data from 2 coils Reconstructed image + =

7 Undersampled k-space = aliased image Full k-space Undersampled k-space Reconstructed image

8 Multiple coils = information to fix aliasing Full k-space Undersampled k-space Coils with complementary sensitivities

9 Parallel Imaging SENSE ACQUIRE RECONSTRUCT COMBINE Coil sensitivity, partial k-space Images from each coil Images from all coils, using coil sensitivity maps

10 Parallel Imaging SMASH ACQUIRE RECONSTRUCT COMBINE Coil sensitivity, partial k-space Missing lines of k-space, using coil sensitivity maps Images from all coils

11 Parallel imaging terminology Methods ipat Siemens name for all of its parallel imaging implementations SENSE SMASH GRAPPA auto-calibrating SMASH-like technique Parameters Reduction factor: integer describing k-space undersampling g: describes degradation of reconstructed image due to lack of independence between coil sensitivities (limits useful reduction factor) High field advantage of course

12 Parallel Imaging Clinical Applications Clinical applications The benefits of pmri have the potential to impact essentially all clinical applications. In particular, clinical MRI based on single-shot sequences, such as EPI or TSE Imaging, benefit from parallel imaging techniques due to reduced relaxation effects on the signal and therefore reduced blurring and other artifacts. The greatest impact on medical MRI with parallel imaging techniques has been thus far in head, thoracic, and cardiac imaging, and on magnetic resonance angiography (MRA). 1. The pmri techniques do not represent new imaging sequences. 2. The pmri approach can serve to accelerate any existing imaging sequence (e.g., gradient echo, spin echo, echo planar) in clinical use presently. 3. The pmri does not necessarily affect the contrast behavior of the underlying imaging sequence. 4. The pmri techniques can be used (a) to purely speed up the image acquisition, (b) to increase the spatial resolution in a given experimental time (c) a combination of both.

13 Parallel Imaging more examples Cine studies with pmri acceleration factor of two. a d Four of 42 phases showing the left ventricular output tract, imaged in 19 heartbeats. The mitral valve is indicated by an arrow in a. e h Four of 21 phases showing a short-axis view, imaged in only nine heartbeats Short-axis cardiac steady-state free precession cine images (in end diastole) obtained from apex to base by using SENSE (factor 2). Three sections were acquired per breath hold. Allowing the total acquisition time to be considerably shortened Parallel MR Imaging: A User s Guide James F. Glockner, Houchun H. Hu, David W. Stanley, Lisa Angelos, and Kevin King RadioGraphics :5,

14 Inversion recovery HASTE images in the lung of a healthy volunteer acquired with an eight-element cardiac array. a Conventional full-time acquisition with a matrix size of 128x256 in 220 ms. b In the same acquisition time, a HASTE image with pmri acceleration factor of two and a matrix size of 256x256 was obtained. c with pmri acceleration factor of three an image with matrix size of 256x256 was acquired in 161 ms. Enlarged sections of the corresponding images are shown in the bottom row Parallel MR Imaging: A User s Guide James F. Glockner, Houchun H. Hu, David W. Stanley, Lisa Angelos, and Kevin King RadioGraphics :5,

15 Parallel Imaging Artifacts Reduction of distortions in single-shot echo-planar image (EPI) by the use of pmri a Conventional EPI with 128x256 matrix size and an minimal interecho spacing of 1.4 ms. b In the same acquisition time, with pmri using an acceleration factor of two, resolution is doubled to 256x256 and the effective interecho spacing is reduced down to 0.7 ms. c Same as b but with a pmri acceleration factor of three, resulting in an effective interecho spacing of 0.37 ms. The pmri images show reduced distortions, due to the reduced effective interecho spacing

16 Parallel Imaging Artifacts Influence of pmri on image quality of diffusionweighted images (DWI). a Diffusion weighted imaging without parallel imaging. b Diffusion-weighted imaging with pmri acceleration factor of two c with an acceleration factor of three. The areas of hyperintensesignal due to the distortions (indicated by arrows) are reduced

17 Parallel Imaging Artifacts Axial contrast-enhanced 2D spoiled gradient-echo (SPGR) image obtained with SENSE (acceleration factor 2), applied in the anteroposterior phaseencoding direction) shows extensive artifact in the center of the image due to uncorrected aliasing. This artifact could be reduced by increasing the FOV in the anteroposterior direction.