Accelerating Clinical MRI in the 21 st Century

Transcription

1 Accelerating Clinical MRI in the 21 st Century Val M. Runge, MD Universitätsinstitut für Diagnostische, Interventionelle und Pädiatrische Radiologie University Hospital Bern Editor-in-Chief, Investigative Radiology Some of the problems today Need for standardization Imaging planes Acquired scans Proliferation of imaging techniques and increasing scanner complexity Time pressures and decrease in reimbursement Need to improve scanning throughput Need to read faster yet see more Do more for less (with no compromise) 1

2 1984 An example of how things have changed Then Now Innovations for accelerating MRI Simultaneous Multi-Slice (SMS) Simultaneous excitation and readout of multiple imaging slices Potential reductions in scan time with slice acceleration factor Significantly accelerates Diffusion and BOLD imaging Future outlook: SMS RESOLVE and SMS TSE 2

3 Simultaneous Multi-Slice with blipped CAIPIRINHA Multiple slices excited simultaneously Blipped CAIPIRINHA applied during echo train Minimization of g-factor related SNR loss Slice GRAPPA based unaliasing Inplane GRAPPA based unaliasing Setsompop K, et al. Blipped-controlled aliasing in parallel imaging for simultaneous multislice echo planar imaging with reduced f-factor penalty. Magn Reson Med 2012; 67: Setsompop K, et al. Advancing diffusion MRI using simultaneous multi-slice echo planar imaging. MAGNETOM Flash 2015; 63 (Special SMS Supplement): What can we do with this speed? Reduce scan times Acquire more slices Have thinner slices Use more b-values Increase temporal resolution (for BOLD) Potentially no SNR loss due to undersampling; better performance with high channel coils 3

4 To achieve these important ends Increased productivity Better imaging within shorter time slots Easier and better visualization of pathologies and functional anatomy Better patient care Reduction in scan time for DWI with no compromise to image quality Conventional, b1000, voxel 0.6x0.6x2.0mm, TA 2:38 SMS 2, b1000, voxel 0.6x0.6x2.0mm, TA 1:31 4

5 Increased slice resolution and whole brain coverage with SMS Conventional b1000 SL 0.8 mm TA 5:03 56 slices Conventional, b1000 SL 0.8 mm, 56 sl TA 5:03 SMS 2, b1000 SL 0.8 mm, 112 sl TA 5:15 SMS 2 b1000 SL 0.8 mm TA 5: slices Do we have good depiction of this medullary infarct? No! Not nearly as well as with 2 mm sections 5

6 Whole brain tractography in clinically-acceptable times Conventional, TA 12:11 min MDDW 64 directions SMS 3, TA 4:27 min Increased temporal sampling for higher sensitivity to BOLD Conventional, TR 3000 ms 2 SMS 4 1, TR 1500 ms 2 90 measurements 270 measurements Finger tapping, TE 30 ms, 2 mm iso, 48 slices, TA 4:30 min Higher tsnr and better noise modeling with SMS 6

7 Acceleration of DTI opens up new possibilities for visualizing what is currently unseen Anterior Ventral Anterior Ventral Lateral Dorsal Medial Ventral Posterior Lateral Posterior Pulvinar MDDW 256 directions, b0, b3000, p2, voxel 2.0x2.0x2.0 mm, SMS 3: TA 11:07 mins (without SMS TA 29:48 mins) Shepherd TM, Chung S, Glielmi C, Mogilner AY, Boada F, Kondziolka D. 3-Tesla magnetic resonance imaging track density imaging to identify thalamic nuclei for functional neurosurgery. Paper presented at: CNS Proceedings of the 63 rd Annual Congress of Neurological Surgeons; 2014 Oct 18-22; Boston, USA. SMS can be applied to accelerate diffusion of abdomen Conventional, TA 3:29 min SMS 2, TA 1:46 min (49% reduction in scan time) GRAPPA 2, b50, b400, b800, ADC, voxel 1.4 x 1.4 x 5.0 mm SMS [ ] has allowed us to achieve near two-fold acceleration in free-breathing DWI acquisitions. These time savings can be used to either improve volumetric coverage or resolution of the DWI or shorten the overall exam time. Obele C, et al. Simultaneous Multi-Slice accelerated free-breathing, diffusion-weighted imaging in abdomen and pelvis. MAGNETOM Flash 2015; 63 (Special SMS Supplement):

8 Conventional SMS can be applied to accelerate diffusion of abdomen b50 b400 b800 Monopolar diffusion encoding scheme SMS 2 Overall image quality at b50: SMS 3 c-dwi: MP/BP: 4.8±0.5 and 4.6±0.6 sms2-dwi: MP/BP: 4.6±0.5 and 4.2±0.5 Image set consisting of diffusion-weighted images obtained with the conventional sequence (c-dwi) and the corresponding sms-dwi with an acceleration factor of 2 (sms2- DIW) and 3 (sms3-diw) with b=50, b=400 and b=800 (in s/ mm²) with a monopolar (MP) gradient preparation. Images courtesy of J. Taron, University Hospital Tuebingen, Dept of Diagnostic and Interventional Radiology SMS for DTI of muscles in shorter acquisition times Conventional SMS 2 SMS 3 Conv SMS 2 SMS 3 SMS acquisition of diffusion tensor data is feasible and yields similar results as conventional DTI at notably shorter acquisition times. With the parameters used in this study, an acceleration factor of two showed to be the best compromise between total acquisition time, image quality and quantification accuracy. Filli L, et al. Simultaneous multislice echo planar imaging with blipped CAIPIRINHA: A promising technique for accelerated diffusion tensor imaging of skeletal muscle. Invest Radiol 2015; 50: Filli L, et al. Accelerated diffusion tensor imaging of skeletal muscle using Simultaneous Multi- Slice acquisition. MAGNETOM Flash 2015; 63 (Special SMS Supplement):

9 SMS for diffusion tensor imaging of peripheral nerves MD map Conventional SMS 2 SMS 3 ROI placement FA map FA map (color) Future DTI studies on peripheral nerves may strongly profit from twofold slice acceleration. The slightly decreased SNR is more than compensated for by the increase in SNR per time unit and the thereby reduced likelihood of motion artefacts. Filli L, et al. Accelerated MR DTI of the median nerve using SMS EPI imaging with blipped CAIPIRINHA. Eur Radiol 2015; Sep 15 [Epub ahead of print]. Future outlook: SMS RESOLVE RESOLVE: Readout-segmented, multi-shot EPI sequence for high-resolution, low-distortion diffusion imaging Can be applied to brain, spine, head/neck and body regions such as the prostate and breast Main drawback: Lengthened acquisition times relative to single-shot EPI SMS brings strong clinical benefits to RESOLVE: faster scan times (high-resolution EPI with no time penalty) thinner slices more slices more diffusion resolution (more b values, more directions) Porter D, Heidemann RM. High resolution diffusion-weighted imaging using readoutsegmented echo-planar imaging, parallel imaging and a 2D navigator-based reacquisition. Mag Reson Med 2009; 62:

10 SMS RESOLVE: Better visualization of small infarcts with thinner slices Non-accelerated SMS 2 SMS 3 By moving to a 2 mm slice (not possible without slice acceleration due to the long scan time), some small pinpoint infarcts are better seen (black arrows), and some revealed for the first time (white arrow). Runge VM, et al. Simultaneous Multi-Slice (slice accelerated) diffusion EPI: Early experience for brain ischemia and cervical lymphadenopathy. MAGNETOM Flash 2015; 63 (Special SMS Supplement): SMS RESOLVE: Advantages of a thinner section Non-accelerated SMS 2 SMS 3 AND, in certain instances small pinpoint lesions can be visualized only on the thinner sections, such as this small cortical infarct (arrow) in the left middle frontal gyrus. Runge VM, et al. Simultaneous Multi-Slice (slice accelerated) diffusion EPI: Early experience for brain ischemia and cervical lymphadenopathy. MAGNETOM Flash 2015; 63 (Special SMS Supplement):

11 Peripheral nerve imaging with SMS RESOLVE Functional MR neurography of the inferior alveolar nerve Manoliu A, et al. MR neurographic orthopantomogram: Ultrashort echo-time imaging of mandibular bone and teeth complemented with high-resolution morphological and functional MR neurography. J Magn Reson Imaging Feb 8. doi: /jmri [Epub ahead of print] SMS RESOLVE: Low distortion DWI in the same time as conventional diffusion Distortions in SS-EPI leading to projection of lymph node (arrow) that is anterior to the submandibular gland on the left being projected over the gland Non-accelerated SMS 2 Depiction of the multiple scattered, high signal intensity, normal lymph nodes and SNR are equivalent for the two scans, with slice acceleration reducing scan time by nearly a factor of 2 (from 3:44 to 2:07 min:sec). Images were acquired with the Head/Neck 64-channel coil. Non-accelerated SMS 2 Runge VM, et al. Simultaneous Multi-Slice (slice accelerated) diffusion EPI: Early experience for brain ischemia and cervical lymphadenopathy. MAGNETOM Flash 2015; 63 (Special SMS Supplement):

12 SMS RESOLVE for breast imaging b = 800 ADC Noise subtraction method conventional SMS 2 SMS 3 Δ ADC SMS Conventional [10-3mm 2 /sec] In conclusion, we have applied SMS rs-epi to achieve high-quality DWI of the breast at greatly reduced acquisition time. SMS acquisition has proven capable of eliminating the main shortcoming of rs-epi, i.e., the longer acquisition time compared to ss-epi. Two-fold slice acceleration seemed the best compromise between image quality and scan time. Filli L, et al. Simultaneous multislice readout-segmented echo planar imaging for accelerated diffusion-weighted imaging of the breast. Eur J Radiol 2016; 85: Future outlook: SMS TSE Turbo spin echo (TSE) is the most commonly applied sequence for diagnostic MR imaging Benefits of SMS: faster scan times thinner slices more slices 12

13 11-year-old with periventricular nodular heterotopia (PVNH) for further evaluation of the PVNH and to demonstrate any right occipital focal cortical dysplasia. SMS TSE: Accelerating pediatric epilepsy imaging T2 TSE TA 4:31 The images demonstrate a complex malformation of cortical development with clear delineation of multiple areas of PVNH and occipital polymicrogyria. SMS 2 T2 TSE TA 2:41 SMS TSE with SMS factor of 2 in conjunction ipat 2 is very encouraging, producing scans of a similarly high diagnostic quality when compared to our gold standard TSE (with ipat 2) in much shorter scan times. Modifications to the protocol have enabled our team to incorporate slice accelerated TSE into protocols developed for our pediatric epilepsy program. Kean M, et al. Epilepsy imaging with Simultaneous Multi-Slice Turbo Spin Echo. MAGNETOM Flash 2015; 63 (Special SMS Supplement): SMS TSE: Accelerating routine clinical MSK imaging SMS 2 2: 32 2:48 2:38 SMS 2 2: 32 2:48 Conv 5:11 5:41 4:24 Conv 5:11 5:41 T1 PD FS PD FS T1 PD FS SMS TSE reduced the scanning time significantly without compromise to diagnostic image quality in routine MSK imaging. This especially benefits patients with larger tumors where full coverage may be difficult to achieve using conventional TSE and also for patients with small injuries to ligaments and cartilage who will require higher spatial resolution imaging for an accurate diagnosis. Li XN, et al. Case report: Evaluation of Simultaneous Multi-Slice Accelerated TSE for knee joint MR imaging. MAGNETOM Flash 2015; 63 (Special SMS Supplement):

14 SMS TSE: Improving visualization of femoroacetabular impingement cartilage damage Conventional T2, 3mm SMS enables identification of cartilage delamination in a clinically feasible acquisition time. (b) With 3 mm slice thickness the cartilage delamination is not well visualized and cannot be confidently diagnosed. Conventional T2, 2mm SMS T2, 2mm (c) With 2 mm slice thickness, the cartilage delamination can be clearly seen but at the cost of increased acquisition time (4:30 mins to 6:30 mins). (d) SMS enables slice thickness at 2mm with reasonable acquisition time (4:34 mins) with comparable image quality to (c) despite being 30% faster. Multiband SMS accelerated TSE sequence can provide 30% higher spatial resolution within a given acquisition time to improve diagnostic accuracy of cartilage delamination and labral tears, or alternatively over 30% time savings for a given spatial resolution while maintaining diagnostic image quality. Johnson CP, et al. Improved visualization of femoroacetabular impingement cartilage damage with multi-band simultaneous multi-slice acceleration. MAGNETOM Flash 2015; 63 (Special SMS Supplement): om/magnetic-resonanceimaging/magnetom-world/ publications/magnetomflash 14

15 Conclusions Simultaneous multi-slice accelerated imaging offers a marked reduction in the time required for data acquisition (scan time). Using this approach, thin section imaging can also be acquired in a scan time and with image quality equivalent to a thicker scan acquisition. The technique has been developed for DWI, BOLD and TSE (FSE) imaging. The new era of MR imaging SMS (Simultaneous multi-slice) imaging offers: Shorter acquisition time Better resolution Better fmri data quality It will expand to different sequences and different clinical fields soon. 15