MRE

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Magnetic Resonance

Elastography (MRE)

By Farukh Saeed. Candidate

Graduate School of Biomedical Engineering,

McMaster University

Supervisor: Dr. Gerald Moran

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Outline

• Elasticity.

• Magnetic Resonance elastography (MRE).

• Vibration Actuator.

• Modification of Gradient-recalled echoes (GRE) Sequence.

• Reconstruction Software.

• Agar Gel Phantom.

• Equipment and Experimental setup.

• Results.

• Conclusion Challenges and Future Plans.

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Elasticity

• Elasticity is the Physical Property of material.

• Hooke’s law described the basic property of elasticity of a spring as: F = kx. Where x is the stretch of spring and k is spring constant.

• Consider cube instead of spring then we have four elastic constants:

• E = Young’s Modulus

• G = Shear Modulus.

• K = Bulk Modulus

• σ = Poisson’s ratio

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Elasticity: Relationship between the

Young’s modulus and the normal strain.

(Source: Magnetic Resonance Elastography : in vivo Measurements of Elasticity for Human Tissue

Takenori OIDA, Akira AMANO and Tetsuya MATSUDA)

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Elasticity: Relationship between

the shear modulus and the shear strain.

(Source: Magnetic Resonance Elastography : in vivo Measurements of Elasticity for Human Tissue

Takenori OIDA, Akira AMANO and Tetsuya MATSUDA)

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Elasticity: Bulk modulus K and the

Poisson’s ratio σ

EG

EGK

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• Bulk Modulus measures the substances resistance to

uniform compression

• Poisson’s ratio is the ratio of the stretching of a material

in one direction to the contraction of the material in the

other direction.

• Measuring E and G in case of MRE, the K and σ can

be calculated from these formulas:

G

GE

2

2

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Magnetic Resonance Elastography

• Physicians routinely use palpation to detect the elastic properties of tissues.

• Differences in elasticity can help facilitate the diagnosis of tumors.

• Difficult to obtain such properties in vivo by using conventional methods.

• MRE has been developed that provides noninvasive in vivo measurement of elasticity for Human tissue.

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MRE: Method

• MRE is a method which calculates Elastic

modulus by measuring the waves caused by

oscillating the material externally.

• Wavelength of the propagating waves can be

easily acquired from MRI phase Images.

• In hard material waves travel fast and have longer

wavelength compare to soft materials where

waves travel slow and have shorter wavelength.

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MRE: Theory

• Mostly actuators produce

transverse waves and shear

modulus can be calculated

using equations below:

• Young's modulus can be

calculated with the application

of longitudinal waves

2G f

(Images Source: Yin M, Rouvière O, Glaser KJ, Ehman RL. Diffraction-biased shear wave fields generated

with longitudinal magnetic resonance elastography drivers. Magn Reson Imag. 26(6):770-80, Jul. 2008.)

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MRE: Required components

• Hardware device to oscillate the scanned region at known frequency called actuator.

• MRI Pulse sequence with motion encoding gradients to encode motion.

• Software to estimate the Wavelength from the acquired Phase images and reconstruct elasticity map.

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MRE: Block Diagram of acquisition Unit

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MRE: Vibration Actuator

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MRE: GRE Pulse Sequence

( Source: Thesis, Magnetic Resonance Elastography at 3 Tesla: Implementation, Validation and Application to

a degenerative Disc Model by Timothy Christian Dunn. 1995)

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MRE: Elasticity EstimationMathematical techniques are used to convert the phase difference

image into elasticity map.

Figure 1:Magnitude Image Figure 2: Phase Difference Image

Figure 3: Interpolated Wave Propagation Figure 4:Stiffness map

(Source: http://mayoresearch.mayo.edu/ehman_lab/mrw-wave.cfm)

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MRE: Clinical Applications

• Abdomen: Liver fibrosis.

• Brain: Alzheimer’s disease.

• Breast: Cancer.

• Cardiac: Shear modulus variations in

the LV Wall.

• Muscles: Healthy and pathological.

• Prostate: elastic properties.

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MRE Study

• Design and construction of vibration actuator.

• Modification of GRE sequence.

• Selection of the reconstruction software.

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Vibration Actuator: Drawing

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Vibration Actuator: Coil

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Vibration Actuator: Hardware

Block Diagram

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Modified GRE Sequence

• Siemens GRE sequence is modified using

Siemens Integrated Development

Environment for Applications (IDEA).

• Create parameters in the sequence special

card related to MRE.

• Programmed bipolar motion encoding

gradients (MEGs) in the sequence. MEGs

can be applied in any one of the three axis.

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MRE Parameters in sequence

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Modified GRE sequence timing diagram

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Modified GRE sequence Event Block

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Reconstruction Software

• Use the free software available on Mayo Clinic Website Called MRE/Wave. Link is http://mayoresearch.mayo.edu/ehman_lab/mrw-wave.cfm

• This software use Local Frequency Estimation (LFE) algorithm to reconstruct elasticity maps from phase data.

• LFE uses the output of different filters to calculate the local spatial frequency of the shear wave propagation.

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Local frequency estimation (LFE)

(a) Test image with two regions of 4 and 8 cycles per image. (b) Local frequency estimate for Test Image.

Source: (Manduca, A., Muthupillai, R., Rossman, P.J., Greenleaf, J.F. and Ehman, R.L., 1996.

Image processing for magnetic resonance elastography. SPIE Med Img 2710, 616-623)

(Vertical Profile)

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Agar Gel Phantom

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Equipment Setup

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Experimental procedure

• Sequence Parameters selected:

TR = 120ms / TE = 31.3ms

No of slices = 1, Slice thickness = 5mm

FOV = 20cm X 20cm, Resolution = 256 X 256.

Spine coil Coil used was and no filters selected.

• MRE parameters:

Frequency = 250 Hz and 125 Hz,

MEG amplitude 10mT/m,MEG Cycles = 4

and pre-MEG STIM = 2.

RO MEG ON and other two axes off.

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Results: Phase difference Images at 250 Hz

(Image at 0 degree phase delay) (Image at 45 degree phase delay)


1% Gel Cylinder

3% Gel Cylinder

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Results: Wave Propagation and Elasticity

map at 250Hz

(Wave propagation. Scale bar is in µm (micron)) (Elasticity Map. Scale bar is in kilopascals( KPa))

1% Gel Cylinder 3% Gel Cylinder1% Gel Cylinder 3% Gel Cylinder

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Results: Phase Difference Images at 125 Hz



1% Gel Cylinder

3% Gel Cylinder

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Results: Wave Propagation and Elasticity

map at 125Hz

(Wave propagation. Scale bar is in µm (micron)) (Elasticity Map. Scale bar is in kilopascals( KPa))

1% Gel Cylinder 3% Gel Cylinder1% Gel Cylinder 3% Gel Cylinder

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Results: Elasticity Values

measuredGel

Area

Literature

Values (KPa)

at 400 Hz

Elasticity at

250 Hz (KPa)

Elasticity at

125 Hz (KPa)

1% Gel

Cylinder

6.0 10+6 5+2

2% Gel

Filling

22.0 28+6 25+2

3% Gel

Cylinder

80.0 49+9 37+3

(Errors are Standard Deviation)

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Conclusion

• Elasticity values measured are comparable to the values available in literature.

• Electromagnetic coil has a tendency to produce artifacts.

• Phantom boundary seems to be less elastic because of air and water on the sides.

• Assumption of local homogeneity in reconstruction creates inaccurate results near boundaries and place limit on resolution.

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Challenges

• Spatial resolution of MRE can be improved by

increasing the frequency, however the high

frequency waves will be attenuated more

rapidly compared to lower frequency waves.

• Sample size needs to accommodate half a

shear wave in the MRE image in order to

measure the stiffness.

• Water and air exists between layers in

phantom damped and distorted the waves.

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Future Plans: Actuator

• Size of actuator.

• Weight of coil.

• Pivot point needs better design.

• Vertically adjustable actuator head.

• Other types of actuators.

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Future Plans: Sequence

• Eight different phase offsets in

one motion cycle.

• Modification of Image calculation

part of the sequence.

• Programming of other sequences

like SE for MRE.

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Future Plans: Reconstruction

Software

• Development of reconstruction

software.

• Use other algorithms like Algebraic

Inversion of the differential equation

or Overlapping subzone technique.

• Addition of new filtering techniques.

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Remote Vibration Generator

(Ingolf Sack, Bernd Beierbach, Uwe Hamhaber, Dieter Klatt and Jurgen Braun. Non-invasive measurement of brain

viscoelasticity using magnetic resonance elastography. NMR Biomed. 2008 Mar; 21(3):265-71)

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Coil Actuator

(Takenori Oida, Akira Amano and Tetsuya Matsuda. Magnetic Resonance Elastography: in vivo Measurements

of Elasticity for Human Tissue. Proceedings of the 12th International Conference on Informatics Research for

Development of Knowledge Society Infrastructure (ICKS’04) 0-7695-2150-9/04, 2004 IEEE.)

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MRE GRE Pulse Sequence

(Muthupillai R, Rossman PJ, Lomas DJ, Greenleaf JF, Riederer SJ, Ehman RL. Magnetic

resonance imaging of transverse acoustic strain waves. Magn Reson Med. 1996 Aug; 36(2):266-74.)

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MRE Parameters

Image Number Motion Encoding Gradient MEG PHASE MEG Delay

1 RO MEG ON Pos 0

2 RO MEG ON Neg 0

3 RO MEG ON Pos 45

4 RO MEG ON Neg 45

5 RO MEG ON Pos 90

6 RO MEG ON Neg 90

7 RO MEG ON Pos 135

8 RO MEG ON Neg 135

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MRE SE Pulse Sequence

(Thesis : Magnetic Resonance Elastography at 3 Tesla: Implementation, Validation and Application to

a degenerative Disc Model by Timothy Christian Dunn. 1995)

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Wave Image and shear stiffness Map

in four hepatic fibrosis patients

(Meng Yin, PhD, Jun Chen, PhD, Kevin J. Glaser, PhD, Jayant A. Talwalkar, MD, and Richard L. Ehman, MD.

Abdominal Magnetic Resonance Elastography. Top Magn Reson Imaging. 2009 Apr; 20(2):79-87.

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Coil in Main Magnetic Field

Direction of Main Magnetic field

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Phantom Magnitude Image

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MRE