Body Diffusion MRI: Basics and Beyond

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1 What is Diffusion? The process of Brownian motion Body Diffusion MRI: Basics and Beyond Xiaodong Zhong, PhD Senior R&D Expert, MR R&D Collaborations, Siemens Healthcare Adjunct Assistant Professor, Radiologyand Imaging Science, Emory University Atlanta, GA Siemens GmbH 2016 All rights reserved. Answers for life. Source: Body Diffusion MRI: Basics and Beyond Outline What is Diffusion? Basics of diffusion MRI and body diffusion Physics Pulse sequence Apparent diffusion coefficient (ADC) Common artifacts and parameter optimization Diffusion in Isotropic Medium e.g. water E1 E2 E3 Advanced topics New diffusion sequence techniques Motion correction / registration Advanced acceleration technique Diffusion in Oriented Tissue E3 E2 E1 Body Diffusion MRI: Basics and Beyond Outline What is Diffusion Imaging? 180 o Basics of diffusion MRI and body diffusion Physics Pulse sequence Apparent diffusion coefficient (ADC) Common artifacts and parameter optimization more mobility 90 o Phase dispersion time Advanced topics New diffusion sequence techniques Motion correction / registration Advanced acceleration technique restricted mobility ` Signal difference No differentiation between bound and free proton Handout 1

2 Body Diffusion Diffusion Sequence Card Parameters Basic Ideas (2) DW images (b-value images) Reflect the estimate of the water diffusion rate at that pixel The greater the b-value, the stronger the diffusion weighting, and the higher the contrast (hyperintense) in pathogenic regions (reduced diffusion) Primary application for early detection of cerebral ischemic stroke, more sensitive to early changes after a stroke than T1 or T2 weighted images* b = 0, not DW (T2 weighted image) b = 500 s/mm 2 b = 1000 s/mm 2 * S. Warach et al., J. Cereb. Blood Flow Metab. 16 (1996) Diffusion Encoding Schemes and Sequence Designs Basic Ideas (3) RF ADC Gdiff Gr Gp Bipolar diffusion encoding scheme. Benefits: better eddy current compensation => less spatial distortions better for WB-DWI Apparent diffusion coefficient (ADC) A measure of the strength (velocity) of diffusion in tissue Free of the influence of T1 and T2 effects The stronger the diffusion, the greater the diffusion coefficient Exhibits darker contrast (hypointense) in pathogenic regions (reduced diffusion) RF ADC Gdiff Monopolar (Stejskal-Tanner) diffusion encoding scheme. Compare G r Benefits: shorter TE s possible ADC map B = 1000 s/mm 2 Gp higher SNR => less averages needed better for DWI in breast, abdomen, pelvis Basic Ideas (1) Diffusion Weighted Imaging (DWI) Fundamental Equation Diffusion weighting factor, b-value Sets the measurement sensitivity to diffusion Determined by strength of diffusion gradient (G), duration of gradient ( ) and duration between the two gradients ( ) Direction of sensitivity can be altered by changing diffusion gradient direction diffusion gradient G diffusion gradient G S(b) S(0) time b Handout 2

3 Diffusion Weighted Imaging (DWI) Fitting the ADC Generally, more b-values give better fitting Basic Ideas (3) Anisotropy: In tissue water diffusion is limited by tissue boundaries Diffusion encoding can be applied to any directions, but typically three main orthogonal axes If the same b-value is used 2-pt: / -> partly due to the perfusion effect 3-pt: / 6-pt: 0.7 / : Isotropic DW image (geometric averaging), or trace-weighted image (TraceW) Averaged ADC map Reflect diffusion weight independent of diffusion orientation. ADC b-value filter for ADC maps calculation Diffusion Mode Options Selectable b-value range for ADC calculation eliminate perfusion effects by excluding lower b-values (organ specific e.g. < 150 s/mm 2 ) from ADC calculation ADC calc. from b 0,100,600 ADC calc. from b 100, 600 Can be used to replace 3- Scan Trace. Will add SNR Used to images in most but applications will also and increase is the time standard for all diffusion sequences Used in Whole body diffusion. Reduces TE and overall scan time. Allows for larger amounts of averaging. Note: No Trace weighted Images Diffusion Weighted Imaging (DWI) What is the optimal b-value? DWI Examples Free breathing EP2D-DIFF 3-Scan Trace b=50 s/mm 2 b=400 s/mm 2 b=800 s/mm 2 Siemens recommended b-values Anatomy b-value1 b-value 2 b-value 3 Liver Prostate Cervix Kidney Pancreas Whole Body Averaged ADC Handout 3

4 Basic Ideas (4) DWI b-value Image Extrapolation Multiple measurements/repetitions can be used to improve SNR Calculated b-value Multiple b-value images can be acquired to improve the ADC calculation Different averages can be set to different b-values to efficiently use the scan time The DW images can be inverted to mimic a PET-like appearance S(b) = image signal at b-value b b = b - b 0 b-value Averaging DWI Examples b-value Image Extrapolation Example liver DWI b = 0 s/mm 2 b = 750 s/mm 2 b = 2000 s/mm 2 Extrapolating images to b = 2000 s/mm 2 reveals the presence of water restriction within a peri-tumoralmargin in liver metastases (red arrow). Furthermore, it is observed that the contrast between metastases and background tissue is improved (green arrows). Courtesy Courtesy University Royal of Marsden Homburg/Saar, Hospital Germany / UK DWI Examples 3-Scan Trace vs 4-Scan Trace vs Increased Averages b-value Image Extrapolation & Inverted Grayscale Increase in SNR from 3 scan trace to 4 scan trace Time penalty isn t that severe in order to improve overall SNR 3 scan trace B 50(1avg), 400(2avg), 800 (4avg) 4 scan trace B 50(1avg), 400(2avg), 800 (4avg) 3 scan trace B 50(1avg), 400(2avg), 800 (7avg) Adding averages to the later b-value improves SNR to an already signal deficient image TA: 3:15 TA 4:34 4:15 Handout 4

5 DWI Examples b-value Image Extrapolation & Inverted Grayscale Example WB-DWI patient with multiple metastases along the spine from a primary prostatetumour The contrastin the computed image is improved compared to the acquired image as signalfrom tissues such as the kidneys, testes and salivary glands (red,green and blue arrows respectively)has been reduced. Typical Artifacts in Diffusion (1) Chemical Shift Artifacts in EPI Single shot: chemical shift differences have more time to evolve Low bandwidth along phase encode chemical shift of > 30 pixels kphase kread Courtesy Royal Marsden Hospital / UK acquired b = 900 s/mm 2 computed b = 1500 s/mm 2 Whole Body DWI Pediatric tumor staging Typical Artifacts in Diffusion (1) Chemical Shift Artifacts in EPI 4-years-old female patient with suspicion of Wilms tumor MIP images based on high b-value (800 s/mm 2 ) 5 steps composed Courtesy University of Homburg/Saar, Germany Solution: Fat suppression. Whole Body DWI Therapy Control of a patient with Hodgkin s Disease Typical Artifacts in Diffusion (2) N/2 ghosting in EPI Eddy currents and other imperfections cause phase differences between even and odd lines N/2 Shifted by N/2 from main image Phase correction not always perfect pre. and post 8 days after chemo cycle Before phase correction After phase correction Courtesy: Anwar Padhani, Mount Vernon Cancer Centre, UK Handout 5

6 Typical Artifacts in Diffusion (2) N/2 ghosting in EPI Typical Artifacts in Diffusion (5) Free Breathing vs. Navigator Possible reasons and solutions for N/2 EPI ghosts Mechanical resonance of scanner components Use appropriate echo spacing (T esp ), for example, avoid ms for Tim Trio ipat reconstruction artifacts Increase the number of reference lines (36-42), at the cost of recon time but not acquisition time b=50 T esp Note the improved sharpness in the sequence Navigator Navigator is longer but uses less averages to reduce scan time Navigator will produce more consistent image quality Free breathing is a great option for most routine exams Free Breathing TA: 3:43 Navigator Triggered TA: 5:00 Typical Artifacts in Diffusion (3) Long Echo Spacing and TE Typical Artifacts in Diffusion (6) ADC Thresholding Leads to longer TE More distortion More ghosting Decreased SNR TE 115 Echo Spacing 1.3 Noise Level = 10 TE 60 Echo Spacing 0.5 Possible solutions: Shorten echo spacing and TE as possible. Thresholding the ADC can lead to removal of anatomy Noise Level = 60 Typical Artifacts in Diffusion (4) Off-Center Patient Positioning Typical Artifacts in Diffusion (7) Image Resolution Interpolation ON Critical to position the patient in the center of the magnet to ensure consistent image quality!! Stair-step artifact, likely from low spatial resolution May need to switch ON interpolation when 128 matrix is used Handout 6

7 Basics of diffusion MRI and body diffusion Key Take Aways Three Body Diffusion Sequences Pros and Cons Basic diffusion concepts and principles b-value Diffusion weighted images (b-value images) ADC Very wide applications Liver, prostate, rectum, breast, etc Whole-body Therapy monitoring, tumor staging, etc Common artifacts in diffusion and important parameters for optimization Ghosting, failed fat suppression, etc Echo Spacing, TE, ipat, etc Sequence Applications Advantages Disadvantages Single-shot EPI Liver, Prostate, Whole Body, Prostate, Pelvis, Lung, Breast, Rectal CA, Pancreas, Kidney Most Commonly used diffusion in the field Can be used in all body applications More distortion than some techniques Prostate, Pelvis, Breast Reduced distortion Longer scan times Unable to use for free breathing applications ZOOMit Prostate, Rectal CA, Pancreas, Kidney, Breast Reduced distortion Zoomed Field of Views Only available on ptx systems (Parallel Transmit) Not useful for whole body diffusion Body Diffusion MRI: Basics and Beyond Outline Principle Basics of diffusion MRI and body diffusion Physics Pulse sequence Apparent diffusion coefficient (ADC) Common artifacts and parameter optimization Advanced topics New diffusion sequence techniques Motion correction / registration Advanced acceleration technique Readout-segmented, multi-shot diffusionweighted EPI High-quality, highresolution DWI and DTI Reduced susceptibility and blurring artefacts due to reduced TE and echo spacing Insensitivity to motioninduced phase errors Reduced SAR in comparison to TSE-based methods Phase-encode direction (ky) 1st shot 2nd shot 3rd shot 4th shot 5th shot Readout direction (kx) k-space trajectory Conventional single shot epi Single shot k-space trajectory Long TE Long Echo Spacing Porter DA & Heidermann RM (2009). High resolution diffusion weighted imaging using readout-segmented echo-planar imaging, parallel imaging and a twodimensional navigator-based reacquisition. Magn Reson Med, 62(2), Body Diffusion New Sequences in Diffusion Imaging Better delineation of tumor boundaries for prostate carcinoma RS-EPI DW images showed improved image quality compared to SS-EPI technique at 3T and is a feasible technique in the pelvis for producing highresolution DWI. Conventional b0 Conventional b800 Conventional ADC syngo syngo ZOOMit Thian YL. (2014). Readout-segmented echoplanar imaging for diffusion-weighted imaging in the pelvis at 3T A feasibility study. Acad Radiol, T2 TSE 21, T2 TSE b0 b800 ADC National University Hospital, Singapore *The statements by Siemens' customers described herein are based on results that were achieved in the customer's unique setting. Since there is no "typical hospital and many variables exist (e.g., hospital size, case mix, level of IT adoption) there can be no guarantee that other customers will achieve the same results. Handout 7

8 Excellent correspondence to anatomy for rectal carcinoma ZoomIt Principle Utilizes 2 orthogonal pulses for slice selection Conventional DWI No need for oversampling Reduced distortion T1 3D VIBE FatSat, post contrast b0 b1000 ADC map Utilized in Prostate, Pelvis, Kidney, Pancreas T2 TSE fused with b1000 b0 b1000 ADC map Only available currently on ptx systems (Skyra, Prisma) National University Hospital, Singapore are.si emens.com/magnetic-res onanc e-imaging/options-and-upgrades/clinic al-applicati ons /s yngo-z oomit/features Superior delineation of breast tumor boundaries ZoomIt in Prostate Less distortions, better tumor delineation Rs-EPI was superior to ss-epi [ ] for anatomical structure distinction, ghosting artifact and overall image quality [ ]. Rs-EPI was superior to ss-epi in SNR and CNR. Conventional DWI, b750 and ADC map, matrix 192, SL 4 mm, TA 2:47 min ZOOMit DWI, matrix 58 x 98, FoV 71 x 120, SL 3 mm, TA 4:21 b100 b400 b800 b1200 ADC, b750 and ADC map, matrix 192, SL 4 mm, TA 4:10 min Kim, HJ, Kim SH et al. (2014). Readoutsegmented echo-planar imaging in diffusion-weighted MR imaging in breast cancer: Comparison w ithsingle-shot echo-planar imaging in image quality. Korean J Radiol, 15(4), Seoul St. Mary`s Hospital, Seoul, South Korea Conventional DWI, matrix 96 x 128, FoV 190 x 190, SL 3 mm, TA 4:28 Kantonsspital Aarau, Aarau, Switzerland *The statements by Siemens' customers described herein are based on results that were achieved in the customer's unique setting. Since there is no "typical hospital and many variables exist (e.g., hospital size, case mix, level of IT adoption) there can be no guarantee that other customers will achieve the same results. Syngo ZoomIt New Sequence ZoomIt in Kidney Higher resolution, better differentiation of cortex & medulla syngo ZOOMit Shape your image Conventional DWI 2.1 mm x 2.1 mm ZOOMit DWI 1.4 mm x 1.4 mm Handout 8

9 ZoomIt in Rectum Better delineation of rectal carcinoma with less distortions Emerging New Improvements in Body Diffusion (1) Motion-compensation* b=50, 6 acqs b=400, 9 acqs b=800, 15 acqs Original Conventional DWI b=50, ADC 3D T2 SPACE - morphology ZOOMit Moco University Hospital IKRN, Mannheim, Germany Emerging New Improvements in Body Diffusion (1) Motion-compensation* Emerging New Improvements in Body Diffusion (1) Motion-compensation* A series of correction/refinement algorithms can be applied on b-value images Inplane-registration of images Filtering of images Denoising of images Rescaling of images to compensate signal loss due to motion Original Moco Improvements Correction for the left lobe signal loss Correction for the misregistration of diffusion weighted images for ADC calculation Original Moco ADC~ 610 ADC~ 90 Emerging New Improvements in Body Diffusion (1) Motion-compensation* b=50, 6 acqs b=400, 9 acqs b=800, 15 acqs Emerging New Improvements in Body Diffusion (2) Simultaneous Multi-Slice* Simultaneous excitation of multiple slices with blipped CAIPIRINHA 1 Original Moco 1 Setsompop, K. (2012). Blipped-controlled aliasing in parallel imaging for simultaneous multislice echo planar imaging w ith reduced f-factor penalty. Magn Reson Med, 67, Handout 9

10 Emerging New Improvements in Body Diffusion (2) About 40% time reduction without compromise 2 b0 b1000 ADC Conventional TA 2:20 SMS 2 TA 1:21 1 Thank you for your attention! (42% reduction in scan time) NYU School of Medicine, MAGNETOM Skyra, Head/Neck 20 1 Note that scan time reduction may not be an exact factor of slice acceleration as scan times depends on the TR value specified and also because of a fast reference scan required for slice separation. 2 Young, MG, Shepherd TM et al. Multiband sequence reduces scan time for diffusion MRI and tractography in clinical patients. RSNA, Siemens GmbH 2016 All rights reserved. Page 58 Answers for life. Magnetic Resonance Advanced topics Key Take Aways Understand three diffusion sequences and when to utilize each pulse sequence Single-shot EPI ZoomIt Emerging new improvements Motion correction / registration Advanced acceleration techniques: Simultaneous multi-slice acquisition (Multiband) Body Diffusion MRI: Basics and Beyond Acknowledgement Peter Kollasch, PhD Marcel Dominik Nickel, PhD Elisabeth Weiland, PhD Vibhas Deshpande, PhD Brian Dale, PhD Handout 10