body diffusion mri: basics and beyondmri/seminars/slides/spring 2016/bodydiff_xzhong...physics pulse...

Handout 1

Answers for life.© Siemens GmbH 2016 All rights reserved.

Body Diffusion MRI:Basics and BeyondXiaodong Zhong, PhDSenior R&D Expert, MR R&D Collaborations, Siemens HealthcareAdjunct Assistant Professor, Radiology and Imaging Science, Emory UniversityAtlanta, GA

© Siemens GmbH 2016. All rights reserved.

Body Diffusion MRI: Basics and BeyondOutline

█ 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




optimization



Background PhysicsWhat is Diffusion?

The process of Brownian motion

Source: http://en.wikipedia.org/wiki/Brownian_motion


Background PhysicsWhat is Diffusion?

Diffusion in Isotropic

Medium e.g. water

Diffusion in

Oriented Tissue

E1E2 E3

E1

E2

E3


Background PhysicsWhat is Diffusion Imaging?

`

time

No differentiation between bound and free proton

Signal difference

more mobility

restricted mobility

90o

180o

Phase dispersion

Handout 2


Body DiffusionDiffusion Sequence Card Parameters


Bipolar diffusion encoding scheme.

Benefits:

• better eddy current compensation

=> less spatial distortions

• better for WB-DWI

Monopolar (Stejskal-Tanner)diffusion encoding scheme.

Benefits:

• shorter TE’s possible

• higher SNR => less averages needed

• better for DWI in breast, abdomen, pelvis

RFADC

Gdiff

Gr

Gp

RFADC

Gdiff

Gr

Gp

Background PhysicsDiffusion Encoding Schemes and Sequence Designs


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

Background PhysicsBasic Ideas (1)

time

diffusion gradient diffusion gradient

G G


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*

* S. Warach et al., J. Cereb. Blood Flow Metab. 16 (1996)

b = 0, not DW (T2 weighted image)

b = 500 s/mm2 b = 1000 s/mm2



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)

B = 1000 s/mm2ADC map


Compare


Diffusion Weighted Imaging (DWI)Fundamental Equation

S(0)

b

S(b)

∙

Handout 3


Diffusion Weighted Imaging (DWI)Fitting the ADC

Generally, more b-values give better fitting

2-pt: / -> partly due to the perfusion effect3-pt: /

6-pt: 0.7 /


ADCb-value filter for ADC maps calculation

Selectable b-value range for ADC calculation

• eliminate perfusion effects by excluding lower b-values

(organ specific e.g. < 150 s/mm2… ) from ADC calculation

ADC calc. from b 0,100,600

ADC calc. from b 100, 600


Diffusion Weighted Imaging (DWI)What is the optimal b-value?

Siemens recommended b-values

Anatomy b-value1 b-value 2 b-value 3

Liver 50 400 800

Prostate 50 400 800

Cervix 50 400 800

Kidney 50 400 800

Pancreas 50 400 800

Whole Body 50 800


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

: Isotropic DW image (geometric averaging), or trace-weighted image (TraceW) Averaged ADC map Reflect diffusion weight independent of diffusion orientation.



Background PhysicsDiffusion Mode Options

Used in most applications and is the standard for all diffusion sequences

Can be used to replace 3-Scan Trace. Will add SNR to images but will also increase time

Used in Whole body diffusion. Reduces TE and overall scan time. Allows for larger amounts of averaging. Note: No Trace weighted Images


b=50 s/mm2 b=400 s/mm2 b=800 s/mm2

Averaged ADC

DWI ExamplesFree breathing EP2D-DIFF 3-Scan Trace

Handout 4


Multiple measurements/repetitions can be used to improve SNR

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



Background Physicsb-value Averaging


DWI Examples3-Scan Trace vs 4-Scan Trace vs Increased Averages

3 scan trace

B 50(1avg), 400(2avg), 800

(4avg)

TA: 3:15

4 scan trace

B 50(1avg), 400(2avg), 800

(4avg)

TA 4:34

3 scan trace

B 50(1avg), 400(2avg), 800

(7avg)

4:15

• Increase in SNR from 3 scan trace to 4 scan trace

• Time penalty isn’t that severe in order to improve overall SNR

• Adding averages to the later b-value improves SNR to an already signal deficient image


DWIb-value Image Extrapolation

Calculated b-value

S(b) = image signal at b-value b

b = b - b0

∆ ∙


Courtesy University of Homburg/Saar, Germany

DWI Examplesb-value Image Extrapolation

b = 0 s/mm2 b = 750 s/mm2 b = 2000 s/mm2

• Extrapolating images to b = 2000 s/mm2 reveals the presence of water restriction

within a peri-tumoral margin in liver metastases (red arrow).

• Furthermore, it is observed that the contrast between metastases and background tissue

is improved (green arrows).

Courtesy Royal Marsden Hospital / UK

Example liver DWI


Background Physicsb-value Image Extrapolation & Inverted Grayscale

Handout 5


DWI Examplesb-value Image Extrapolation & Inverted Grayscale

acquired b = 900 s/mm2 computed b = 1500 s/mm2

patient with multiple metastases along thespine from a primary prostate tumour

The contrast in the computed image isimproved compared to the acquired imageas signal from tissues such as the kidneys,testes and salivary glands (red, green and

blue arrows respectively) has been reduced.

Example WB-DWI

Courtesy Royal Marsden Hospital / UK


Courtesy University of Homburg/Saar, Germany

4-years-old female patient with suspicion of Wilms tumorMIP images based on high b-value (800 s/mm2)5 steps composed

Whole Body DWIPediatric tumor staging


Courtesy: Anwar Padhani, Mount Vernon Cancer Centre, UK

pre …. and … post 8 days after chemo cycle

Courtesy: Mount Vernon Cancer Centre, UK

Whole Body DWITherapy Control of a patient with Hodgkin’s Disease



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



Typical Artifacts in Diffusion (1)Chemical Shift Artifacts in EPI

Solution: Fat suppression.



Typical Artifacts in Diffusion (2)N/2 ghosting in EPI

Eddy currents and other imperfections cause phase differences between even and odd lines

Shifted by N/2 from main image

Phase correction not always perfect

N/2

After phase correctionBefore phase correction

Handout 6



Typical Artifacts in Diffusion (2)N/2 ghosting in EPI

Possible reasons and solutions for N/2 EPI ghosts

Mechanical resonance of scanner components – Use appropriate echo spacing (Tesp), for example, avoid 0.6-0.79 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

Tesp

b=50



Typical Artifacts in Diffusion (3)Long Echo Spacing and TE

• Leads to longer TE

• More distortion

• More ghosting

• Decreased SNR

TE 115Echo Spacing 1.3

TE 60Echo Spacing 0.5

Possible solutions: • Shorten echo spacing and TE as possible.


Typical Artifacts in Diffusion (4)Off-Center Patient Positioning

• Critical to position the patient in the center of the magnet to ensure consistent image quality!!



Typical Artifacts in Diffusion (5)Free Breathing vs. Navigator

Navigator Triggered

Free Breathing

TA: 5:00

TA: 3:43

• 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


Typical Artifacts in Diffusion (6)ADC Thresholding

Thresholding the ADC can lead to removal of anatomy

Noise Level = 10

Noise Level = 60


Typical Artifacts in Diffusion (7)Image Resolution

Stair-step artifact, likely from low spatial resolution

Interpolation ON

May need to switch ON interpolation when 128 matrix is used

Handout 7


Basics of diffusion MRI and body diffusionKey Take Aways

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




optimization


Body DiffusionNew Sequences in Diffusion Imaging

syngo RESOLVE syngo ZOOMit

Three Body Diffusion SequencesPros and Cons

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

RESOLVE 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 availableon pTX systems (Parallel Transmit)

• Not useful for whole body diffusion

RESOLVEPrinciple

Porter DA & Heidermann RM (2009). High resolution diffusion weighted imaging using readout-segmented echo-planar imaging, parallel imaging and a two-dimensional navigator-based reacquisition. Magn Reson Med, 62(2), 468-475.

• Readout-segmented, multi-shot diffusion-weighted EPI

• High-quality, high-resolution DWI and DTI

• Reduced susceptibility and blurring artefacts due to reduced TE and echo spacing

• Insensitivity to motion-induced phase errors

• Reduced SAR in comparison to TSE-based methods

Conventional single shot epi

k-space trajectory

Single shot

Long TE

Long Echo Spacing

RESOLVE

k-space trajectory

Readout direction (kx)

Ph

as

e-e

nc

od

e d

ire

cti

on

(k

y)

1st shot 2nd shot 3rd shot 4th shot 5th shot

RESOLVEBetter delineation of tumor boundaries for prostate carcinoma

T2 TSE

T2 TSE

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 high-resolution DWI.

National University Hospital, Singapore

Conventional

b0

Conventional

b800

Conventional

ADC

RESOLVE

b0

RESOLVE

b800

RESOLVE

ADC

Thian YL. (2014). Readout-segmented echo-planar imaging for diffusion-weighted imaging in the pelvis at 3T – A feasibility study. Acad Radiol, 21, 531-537.

•*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 8

RESOLVEExcellent correspondence to anatomy for rectal carcinoma

T1 3D VIBE FatSat, post contrast

T2 TSE fused with RESOLVE b1000

RESOLVE

b1000 ADC mapb0

Conventional DWI

National University Hospital, Singapore

b1000b0 ADC map

RESOLVESuperior delineation of breast tumor boundaries

Conventional DWI, b750 and ADC map, matrix 192, SL 4 mm, TA 2:47 min

Seoul St. Mary s Hospital, Seoul, South Korea

RESOLVE, b750 and ADC map, matrix 192, SL 4 mm, TA 4:10 min

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.

•*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.

Kim, HJ, Kim SH et al. (2014) . Readout-segmented echo-planar imagingin diffusion-weighted MR imaging in breastcancer: Comparison w ith single-shot echo-planarimaging in image quality. Korean J Radiol, 15(4), 403-410.

Syngo ZoomItNew Sequence

syngo ZOOMit

Shapeyour

image

ZoomItPrinciple

• Utilizes 2 orthogonal pulses for slice selection

• No need for oversampling

• Reduced distortion

• Utilized in Prostate, Pelvis, Kidney, Pancreas

• Only available currently on pTX systems (Skyra, Prisma)

https://www.healthcare.si emens.com/magnetic-resonance-imaging/options-and-upgrades/clinical-applicati ons /syngo-zoomit/features

ZoomIt in ProstateLess distortions, better tumor delineation

ZOOMit DWI, matrix 58 x 98, FoV 71 x 120, SL 3 mm, TA 4:21

b100 b400 b800 b1200 ADC

Conventional DWI, matrix 96 x 128, FoV 190 x 190, SL 3 mm, TA 4:28Kantonsspital Aarau, Aarau, Switzerland

ZoomIt in KidneyHigher resolution, better differentiation of cortex & medulla

Conventional DWI2.1 mm x 2.1 mm

ZOOMit DWI1.4 mm x 1.4 mm

Handout 9

ZoomIt in RectumBetter delineation of rectal carcinoma with less distortions

University Hospital IKRN, Mannheim, Germany

Conventional DWI b=50, ADC

ZOOMit

3D T2 SPACE - morphology

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 tomotion

Improvements Correction for the left

lobe signal loss Correction for the

misregistration of diffusion weighted images for ADC calculation

* Siemens research w ork-in-progress development.

Original Moco



b=50, 6 acqs b=400, 9 acqs b=800, 15 acqs

Original

Moco



b=50, 6 acqs b=400, 9 acqs b=800, 15 acqs

Original

Moco



Original

Moco

ADC~ 610

ADC~ 90

Emerging New Improvements in Body Diffusion (2)Simultaneous Multi-Slice*

Simultaneous excitation of multiple slices with blipped CAIPIRINHA1

* Siemens research w ork-in-progress development. 1 Setsompop, K. (2012). Blipped-controlled aliasing in parallel imaging for simultaneous multislice echo planar imaging

w ith reduced f-factorpenalty. Magn Reson Med, 67, 1210-1224.

Handout 10

Emerging New Improvements in Body Diffusion (2)About 40% time reduction without compromise2


NYU School of Medicine, MAGNETOM Skyra, Head/Neck 20

Conventional

TA 2:20

SMS 2TA 1:211

(42% reduction in scan time)

1 Note that scan time reduction may not be an exact factor of slice acceleration as scan times depends on the TR value specif ied 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, 2014.

b0 b1000 ADC


Advanced topicsKey Take Aways

Understand three diffusion sequences and when to utilize each pulse sequence Single-shot EPI RESOLVE ZoomIt

Emerging new improvements Motion correction / registration Advanced acceleration techniques: Simultaneous multi-slice

acquisition (Multiband)


Body Diffusion MRI: Basics and BeyondAcknowledgement

• Peter Kollasch, PhD• Marcel Dominik Nickel, PhD• Elisabeth Weiland, PhD• Vibhas Deshpande, PhD• Brian Dale, PhD

© Siemens GmbH 2016 All rights reserved.

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