advanced imaging techniques · 06.12.2018 1 advanced imaging techniques elastography prof. dr....

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Advanced Imaging TechniquesElastography

Prof. Dr. Frank G. ZöllnerComputer Assisted Clinical MedicineMedical Faculty Mannheim Heidelberg University

Theodor-Kutzer-Ufer 1-3D-68167 Mannheim, Germany

[email protected]/inst/cbtm/ckm

Name I Slide 2 I 12/6/2018

Learning Goals

� introduction to advanced imaging techniques in MR, CT and CBCT

� basic MRI principles -> Physics of Imaging Techniques

� Goals:

1. How does the technique work ?

2. What kind of images do we receive?

3. Where is this applied to ?

� Literature is given in the respective lectures

� Slides of the lectures at https://www.umm.uni-heidelberg.de/inst/cbtm/ckm/lehre/index.html

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Readings� Rosa M.S. Sigrist,1 Joy Liau,1 Ahmed El Kaffas,1 Maria Cristina Chammas,2 and

Juergen K. Willm. Ultrasound Elastography: Review of Techniques and Clinical Applications. Theranostics. 2017; 7(5): 1303–1329.

� Mariappan et al., Magnetic resonance elastography: A review. Clinical Anatomy, 2010.23(5)p.397-511

� Tse, H. Janssen, A. Hamed, M. Ristic, I. Young, and M. Lamperth, Magneticresonance elastography hardware design: a survey. Proc Inst Mech Eng H, vol. 223, pp. 497-514, May 2009.

� Lonbani, Zohreh & Wall, David & Paulsen, K & Weaver, J & Watts, R & Van Houten, Elijah. (2010). Magnetic resonance elastography artifacts due to actuationsystems. International Conference on Bioinformatics, Computational Biology, Genomics and Chemoinformatics 2010, BCBGC 2010. 61-69.

� Tomokazu Numano, Yoshihiko Kawabata Kazuyuki Mizuhara,Toshikatsu Washio, Naotaka Nitta, Kazuhiro Homm. Magnetic resonance elastography using an airball-actuator. Magnetic Resonance Imaging, 2013, 31(6):939-946

� Hwang SI, Lee HJ. The future perspectives in transrectal prostate ultrasound guided biopsy. Prostate Int. 2014 Dec;2(4):153-60

� Franiel, T., Asbach, P., Teichgräber, U., Hamm, B., & Foller, S. (2015). ProstateImaging--An Update. RoFo : Fortschritte auf dem Gebiete der Rontgenstrahlenund der Nuklearmedizin, 187 9, 751-9.


Overview

� Basics of Elastography

� Ultrasound Elastography (USE)

� MR Elastography (MRE)

� Applications

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Introduction to stress, strain and elasticity

Elasticity : ability of a body to resist stress and to return to its original size and shape when the stress is removed

Strain : amount of deformation

Stress : ratio of the force to the cross-sectional area

Young’s modulus E : slope of stress-strain curve in the elastic deformation region. � A stiffer material has a higher elastic modulus

★ shear stress : component of stress coplanar to material cross section

� ��

2�1 � �

Background


Introduction to stress, strain and elasticity

Sigrist et al. Theranostics 2017

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Elasticity, Young’s modulus E, shear modulus G

Viscosity : measure of its resistance to gradual deformation by stress

Viscoelasticity : elements with both properties will exhibit time-dependent strain

�∗ � ��

BackgroundIntroduction to stress, strain and elasticity


Measurement techniques - Overview

Mariappan et al, Clin Anat 2010

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US Elastography

� three types of elastic moduli Γ defined by the method of deformation:

� Young's modulus (E),

� shear modulus (G)

� bulk modulus (K)

� elastic modulus Γ also characterizes the propagation speed of waves

� two types of wave propagation in ultrasound:

� longitudinal waves

� shear waves


US Elastography

� Longitudinal waves

� particle motion parallel to the direction of wave propagation

� are defined using the bulk modulus K as

� Longitudinal waves used in B mode imaging -> not suitable for USE

� shear waves

� particle motion perpendicular to the direction of wave propagation

� are defined using the shear modulus G as

� shear wave speed (cS) is approximately 1-10 m/s in soft tissues

� allows for high differences in G

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US Elastography



US Elastography Techniques


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US Elastography Techniques – Strain imaging

� Excitation methods:� the operator exerts manual compression on the tissue with the

ultrasound transducerworks fairly well for superficial organs (breast and thyroid) is challenging for assessing elasticity in deeper located organs

(liver) � the ultrasound transducer is held steady, and tissue displacement

is generated by internal physiologic motion (e.g. cardiovascular, respiratory)

not dependent on superficially applied compression, it may beused to assess deeper located organs

� induced tissue displacement measured in the same direction as theapplied stress� a number of different methods dependent on the manufacturer,

including radiofrequency (RF) echo correlation-based tracking, Doppler processing, or a combination of the two methods


US Elastography Techniques – ARFI Strain imaging

� Acoustic radiation force impulse (ARFI) strain imaging

� a short-duration (0.1-0.5 ms) high-intensity acoustic “pushing pulse” (acoustic radiation force) is used to displace tissue

� spatial peak pulse average = 1400 W/cm2, spatial peak temporal average = 0.7 W/cm2

� displacement of ~ 10-20 µm in the normal direction, i.e. perpendicular to the surface

� displacement within a specified ROI is subsequently measured by thesame methods as in strain elastography

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US Elastography Techniques – Shear Wave Imaging

� Shear wave imaging (SWI) employs a dynamic stress to generateshear waves in the parallel or perpendicular dimensions

� shear wave speed results in qualitative and quantitative estimates oftissue elasticity

� three technical approaches for SWI:

� 1 dimensional transient elastography (1D-TE)

� point shear wave elastography (pSWE)

� 2 dimensional shear wave elastography (2D-SWE)


US Elastography Techniques –Shear Wave Imaging


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Technical Limitations

� general sonography limitations such as

� shadowing, reverberation, and clutter artifacts

� operator-dependent nature of free-hand ultrasound systems

� tissue attenuation decreases ultrasound signal as a function of depth, limiting accurate assessment of deeper tissue or organs

� Fluid or subcutaneous fat also attenuates propagation of the externalstimulus applied at the skin surface which can invalidatemeasurements in the setting of obesity or abdominal ascites

� system settings and parameters (i.e. ultrasound frequency, samplingrate, gains, etc.) can also produce biased results if not standardizedacross patient groups and time points in longitudinal applications

� lack of uniformity of commercial system design and settings makescomparing measurements from one manufacturer system to another a difficult task


• Quantification of tissue stiffness during MR examinations

Magnetic Resonance ElastographyIntroduction

Generate of sinusoidal shear

waves by external source

Encode wave in MR Phase by

motion-encoding gradient

Reconstruct wave propagation into

viscoelastic maps

��

��

E ∝ ��

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At present, solely one FDA-approved setup for liver fibrosis grading only

MRE – Impact on Diagnostic AreasIntroduction

0

1

2

3

4

5

Ord

ers

of M

agni

tude

Transducer-related issues:• Preserved transducer amplitude at

higher ��• No induction of image artefacts

• Accuracy of ��• Modularity of the setup

Goal: Establish MR elastography asnon-invasive diagnostic parameter

Atte

nuat

ion

coef

ficie

nt (c

m"�)

T1

Rel

axat

ion

(ms)

Bul

k m

odul

us (Pa)

She

ar m

odul

us (Pa)

Adapted from Mariappan et al., Magnetic resonance elastography: A review.Clinical Anatomy, 2010.23(5)p.397-511


At present, solely one FDA-approved setup for liver fibrosis grading only

MRE – Impact on Diagnostic AreasIntroduction

Transducer-related issues:• Preserved transducer amplitude at

higher ��• No induction of image artefacts

• Accuracy of ��• Modularity of the setup

Goal: Establish MR elastography asnon-invasive diagnostic parameter

She

ar m

odul

us (Pa)

0

5000

10000

15000

20000

25000

30000

Liver Fat Muscle

She

ar m

odul

us (

Pa)

40 Hz 60 Hz 80 Hz 30 to 90 Hz average

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MR Elastography


MR Elastography workflow

Phase encoding sequence

ImagingMechanical excitation

(Amp. & Freq.)

Signal Processing

Phase & Amplitude maps

Scanner

Construction of Elastogram

3D linear elastic tissue model

Shear modulus/ Elastogram

Inversion Technique

Mechanical Synchronization

DriverSynchronous

Trigger Pulses

Synchronous Motion Sensitizing

Gradient

Actuator

Z. T. Tse, H. Janssen, A. Hamed, M. Ristic, I. Young, and M. Lamperth, "Magnetic resonance elastography hardware design: a survey," Proc Inst Mech Eng H, vol. 223, pp. 497-514, May 2009.

Introduction

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Wave Induction in MRE� Acoustic driving systems

� most commonly used systems in earlier MRE studies are acousticdriving systems, also called remote pneumatic actuators

� Electromechanical driving systems

� actuator generally consists of a coil

� the actuator oscillates due to the magnetic induction (Lorentz force) in the coil about a fixed axis of rotation within the scanner

� Piezoelectric driving systems

� composed of a stack of piezoelectric crystals, placed between a spring and rigid housing wall

� expands when a voltage is applied, a precise longitudinal vibrationis generated

� Gravitational driving systems

� waves poduced a rotational eccentric mass

� driven by steper motor or pneumatic turbine


Wave Induction in MRE


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MRE Actuator Examples

Lonbani et al. BCBGC 2010, Numano et al, MRI 2013, Neumann et al., Plos One 2018, http://mriquestions.com/mr-elastography.html


MRE Actuator Examples

W. Neumann F. Zöllner . EP3384844A1, filed:5.Apr.2017, (2018).

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MRE Imaging – Encoding the induced vibration

Neumann, PhD Thesis 2018, Heidelberg University


Motion Encoding Gradients

� in presence of a gradient field, a moving spin accumulates a phase

� assuming a sinusoidal motion in MRE, the displacement is given by

� sensitivity to small shear wave amplitudes can be achieved byaccumulating phase shifts over multiple cycle of mechanical excitation

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� accumulated phase shift is proportional to the dot product of

� the gradient vector and the displacement vector,

� number of gradient cycles

� periods of the gradient wave form

with ϕ the initial phase offset, N the number of gradient cycles, T the period of mechanical excitation, &0 the displacementamplitude, and k the wave vector




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MRE - Sequence


MRE Post-processing

� Shear modulus� Vs is the wave speed� ρ is the density of the material (typically assumed to be around 1000 kg/m3 for

tissue in MRE)� wave speed can be written as a product of the operating frequency and the spatial

wavelength� local frequency estimation (LFE)� phase gradient (PG)� direct inversion (DI)

µ = ρVs2

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ApplicationsTissue

Shear stiffness(kPa)

Frequency ofoperation

(Hz)References

Ocular Vitreous Humor 0.01 10 (Litwiller., 2010b)

Lung 0.95 40 (Goss et al., 2006)

Liver:HealthyCirrhotic 2.2

8.9

60 (Yin et al., 2007)

Prostate:CentralPeripheral 2.2

3.3

65 (Kemper et al., 2004)

Breast:Adipose tissueFibroglandular tissueTumor

3.37.525

100 (McKnight et al., 2002)

Brain:Gray matterWhite matter 5.2

13.6

100 (Kruse et al., 2008)

Muscle:HealthyNeuromuscular

disease 16.638.4

150 (Basford et al., 2002)

Cartilage 2000 5000 (Lopez et al., 2008)

Bone 0.8 × 106

1500 (Chen et al., 2009)Mariappan et al, Clin Anat 2010


Applications - Liver

� Left: liver stiffness

� Right: top row: healthy volunteer, bottom row: patient with cirrhoticliver


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Applications - Breast

� (a) An axial MR magnitude image of the right breast of a patient volunteer is shown. A large adenocarcinoma is shown as the outlined, mildly hyperintense region on the lateral side of the breast.

� (b) A single wave image from MRE performed at 100 Hz is shown along with the corresponding elastogram (c).

� (d) An overlay image of the elastogram and the magnitude image shows good correlation between the tumor and the stiff region detected by MRE. Mariappan et al, Clin Anat 2010


Applications - Muscleskeleton

� (a) A sagittal MR image of the calf soleus muscle with the location of the driver indicated by the arrow is shown.

� 100-Hz MRE wave images of the muscle are shown while exerting 0 (b), 5 (c) and 10 N/m (d) of force.

� The increase in the wavelength (and thus stiffness) with the increase in muscle force is easily visible and is indicated by the double sided arrows


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Applications - Prostate

� Contrast enhanced transrectalultrasound (TRUS) findings of prostate cancer in a 62-year-old man.

� Contrast enhanced TRUS image shows increase vascularity and contrast agent signals from left peripheral zone suggesting increased vascularity (arrows).

� Note that the focal lesion shows low echogenicity in gray-scale TRUS, which is one of common findings of prostate cancer.

� This lesion was confirmed as prostate cancer after TRUS guided targeted biopsy.

Hwang SI, Lee HJ. Prostate Int. 2014, Franiel et al. Rofo 2015


Summary

� Elastography sensitive modality to detect tissuestiffness

� Allows for functionalcharacterisation of tissuestructure

� can be measured by US and MR

� needs actuator thatinduces wave

� several different USE imaging technqiues

� MRE uses motionencoding gradientssynchornised to the waveinduction

advanced imaging techniques · 06.12.2018 1 advanced imaging techniques elastography prof. dr....

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