1. SMASH Contrast-enhanced 3D MR Angiography with Simultaneous Acquisition of Spatial Harmonics: a Pilot Study by Dr. Sodickson and Dr. McKenzie, et al Anthony C. Lanctot 2 nd yr Diagnostic Medical Physics lanctota@UTHSCSA.edulanctota@UTHSCSA.edu 567-6638

2. Radiology Journal The Pilot Study Published in Radiology Journal Oct, 2000 A partially parallel image acquisition technique—S.M.A.S.H. Simultaneous Acquisition of Spatial Harmonics Used to increase the spatial and/or temporal resolution in contrast material enhanced 3-D MR angiography of the abdominal aorta and renal arteries.

4. Pilot Study Eight healthy subjects Breath-hold duration halved at constant spatial resolution- Spatial Resolution doubled at fixed breath- hold duration- 30-55% reduction in SNR Preserved or improved image quality

5. With any given MR imaging system, the spatial and temporal resolution is generally limited by the gradients or gradient safety considerations. SMASH overcomes these limitations without the need for changes in the gradient system. Combinations of component coil signals in a radio-frequency coil array are used to substitute for omitted gradient steps. This allows some portion of the MR image data to be acquired in parallel.

6. Parallel Imaging A reduced data set in phase encoding direction(s) of k-space is acquired to shorten acquisition time. In principle, MRI imaging techniques can be applied to any imaging sequence and k-space trajectory. SENSE: sensitivity encoding /IPAT: integrated Parallel Acquisition Techniques/ GRAPPA: k- space technique/ also SPEEDER/ASSET/ and of course SMASH.

7. Imaging Protocol  Contrast-enhanced first-pass 3D MR angiography performed on 1.5 T Phillips  A custom six-element surface-coil array, each 8 x 18 cm  Initial low-resolution scout images obtained  Breath-hold non-enhanced images obtained  Provides invivo coil sensitivity references

8. Imaging Protocol  Administered 2-mL test bolus of Magnevist with a 30mL saline flush—this is to identify bolus timimg with the signal intensity measurements on dynamic traverse images with 2D T1W spoiled gradient-echo sequence.  12.3 msec repitition time/1.1 msec echo time/20° flip angle/350mm FOV/82x128 matrix/10mm section thickness/50mm saturation slabs/1 sec per dynamic image temporal resolution

9. Imaging Protocol  Two contrast material injections of about 18mL each with 30mL saline flush  These are seperated by about 15 minutes.  One injection is used for conventional angiography imaging.  The other is for SMASH imaging.

10. Contrast-Enhanced Studies  3D T1W radio-frequency spoiled gradient echo imaging sequence  7.0 msec rep time 1.5 msec echo time  30° flip angle 350 x 350 mm FOV  128x 256 matrix size  20 4mm thick sections interpolated to 40 2mm thick sections  Center of k-space coincides with the peak of the bolus material  No EKG gating Breath-hold duration 24 secs

11. Reduced-Breath-hold SMASH  FOV and the matrix size in the right to left phase encoding direction were reduced by a factor of two.  Only 64 phase encoding steps are used.  With twice the k-space interval.  Results in a 12 sec breath-hold time.

12. Inc Spatial Resoltion SMASH  FOV was halved  Number of Phase encoding steps stayed the same at 128.  Resulted in a doubled Intrinsic Spatial Resolution.  Breath-hold time the same as the reference hold time of 24 secs.

13. Image Reconstruction Reference images were reconstructed using a sum of squares combination of component coil images. Here L is the number of array elements, k y = 2 /(field of view), and R 0 (x,y) is an arbitrary function multiplying all the target spatial harmonics exp(im k y y). Final in-plane matrix 128x256 reduced breath-hold matrix and 256x256 increased spatial resolution SMASH images

14. The top row shows typical 0th harmonic target functions used in the original phased sum (left) and coil-by-coil (right) implementations of SMASH. (The solid lines are the real part of the complex target function, the dotted lines are the imaginary part.) The second row shows the matching 1st harmonic target functions. The bottom row shows images formed from simulated SMASH fits to the corresponding sets of target functions. Wiley-Liss, Inc

15. Copyright ©Radiological Society of North America, 2000 Sodickson, D. K. et al. Radiology 2000;217:284-289 Figure 1. Maximum intensity projections of 3D image sets demonstrating the use of SMASH for reduction of breath-hold duration Three different Subjects: A,B, and C. Key Point: Image Quality is preserved on the accelerated SMASH images despite the shorter breath- hold duration. A-C: Maximum Intensity Projections of Reference Images: breath- hold duration 24 seconds. D-F: Corresponding maximum intensity projections of SMASH images: breath- hold time 12 seconds.

16. Results The SMASH reconstructions showed negligible artifact throughout the 3D volume in all cases. A slight increase in edge enhancement artifact in the distal abdominal aorta due to the timing of the shorter acquisitions is seen in Figure 1. Otherwise, Image Quality is preserved on the accelerated SMASH images despite the shorter breath-hold time.

17. A-C: Maximum intensity projections of reference images (matrix: 128x256x20): breath-hold time 24 seconds.

18. Results In Figure 2: Increased Spatial Resolution at the edges of structures such as the aorta, which are oriented perpendicular to the right-to-left phase encoding directioncan be seen on the SMASH images. In the increased spatial resolution series as a whole the mean “sharpness” of the abdominal aorta was 0.74 +/- 0.05 compared with 0.66 +/- 0.05 on the reference images. This corresponds to a net 12% increase in vessel sharpness on the SMASH images.

19. Plot of the mean SNR relative to the reference image: Sodickson, D. K. et al. Radiology 2000;217:284-289

20. Contrast-to-Noise Ratio Results Reduced Breath-hold versus reference images: Sodickson, D. K. et al. Radiology 2000;217:284-289

21. Copyright ©Radiological Society of North America, 2000 Sodickson, D. K. et al. Radiology 2000;217:284-289 Figure 5. Maximum intensity projections of three successive time-resolved SMASH 3D image sets obtained during a single breath hold

22. Final Analysis and Discussion: The reduced breath-hold series has an approx. 40% reduction in the average Signal-to-noise and Contrast-to-noise ratios. The increased spatial resolution series resulted in an approx. 55% reduction in these. The measured SNR and CNR in this study approached the optimum theoretically predicted results for a SMASH acceleration factor of 2.

23. Final Analysis and Discussion: The SNR penalty associated with SMASH reconstruction may be offset by using faster contrast material injection rates as the temporal resolution is improved. Although these results are consistent with theoretical expectations, their statistical significance remains to be assessed in studies with larger number of subjects.

24. Final Analysis and Discussion: With SMASH ( Simultaneous Acquisition of Spatial Harmonics), temporal resolution can be increased without sacrificing spatial resolution, making SMASH a particularly promising approach for time-resolved MR angiography. Further studies will be undertaken to asses the value of SMASH for improving image quality and increasing the accuracy of stenosis quantification in clinical applications of contrast- enhanced 3D MR angiography.

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