SHER-I-KASHMIR INSTITUTE OF MEDICAL SCIENCESSOURA

DEPARTMENT OF RADIODIAGNOSIS & IMAGING

PRESENTATION BY

Dr SAMEER PEER

1ST YEAR PG STUDENT

CONTENTS

• Introduction• Basic principles of elastography• Methods and Types of elastography• Applications of elastography• Pitfalls and limitations• Recent Advances and future prospects• Conclusion• References and further reading

INTRODUCTION

Ever since Hippocrates, physicians have used palpation todetect differences in tissue stiffness as an aid to diagnosis.

Elastography is a novel imaging modality which depends onthe same differences in mechanical properties betweenhealthy and abnormal tissues but uses imaging to detectthese differences at depths not reachable by manualpalpation.

Elastography techniques have been developed for bothultrasound and MR imaging.

–Sono-Elasticity Lerner 1990

–Elastography Ophir 1991

–Shear Waves Elastography Sarvazyan1997

–Vibro-acoustography Fatemi 1998

–Transient Elastography Sandrin 1999

–Acoustic Radiation Force Imaging(ARFI) Nightingale 2001

–Supersonic Shear Imaging Elastography Bercoff 2004

– MR Elastography Muthupillai1995

TIMELINE REVIEW

BASIC PRINCIPLES OF ELASTOGRAPHY

Elastogarphy is a tissue elasticity imaging, hence it is importantto understand the general principles which govern thebehaviour of an elastic material when acted upon by an externalforce.

When external forces are applied to objects made up of elasticmaterials, they undergo a change in shape and size.

STRAIN is a relative change in shape or size of an object due toexternally applied forces.STRESS is the internal force ( per unit area) associated withstrainHOOK’S LAW states that strain is directly proportional to stressfor a linearly isotropic elastic material

e= (h-ho)/ho

Strain = Change in length/original length

e= / E

e = Strain (dimensionless)

= Stress (Pascal) = Force / Area

E = Young’s Modulus (Pascal)

Elastic Moduli

YOUNG’S MODULUS ( E ) : It is the ratio of stress along an axis to

the strain along that axis in the range of stress in which hooke’s lawholds.

BULK MODULUS (K or B) : It is the ratio of infinitesimal pressure

increase to the resultant relative volume decrease.

SHEAR MODULUS ( G or S or m ) : It is the ratio of shear stress toshear strain. It concerns with deformation of solids when two opposingsurfaces experience forces in opposing directions.

YOUNG’S MODULUS (E) BULK MODULUS ( K OR B)

SHEAR MODULUS ( G or m)

Conventional ultrasound

• Propagation of ultrasound depends on bulk modulus• Bulk modulus is homogeneous in different tissues• No information provided regarding tissue stiffness

Elastography

• Imaging of tissue stiffness• Estimation of strain in tissues under stress• Imaging of shear waves whose propagation is governed

by tissue stiffness or Young’s modulus(E)• Young’s modulus exhibits important differences in

tissues which is ideal for characterization of tissues withexcellent contrast.

• Young’s modulus is a quantitative estimation ofclinician’s palpation and has diagnostic value

ILLUSTRATIVE EXAMPLE

dA dB

tA= 2dA/ c tB= 2dB/ c

c = Speed of sound

Displacement ( d) = dB – dA = c ( t B-t A)/2

d = c/2 ( = t B – t A )

METHODS AND TYPES OF ELASTOGRAPHY

Two established methods of elastography

• ULTRASOUND ELASTOGRAPHY• MAGNETIC RESONANCE ELASTOGRAPHY

ULTRASOUND ELASTOGRAPHY

METHODS

• QUASI-STATIC • DYNAMIC

QUASI-STATIC METHODS

In the case of quasi-static elastography, a constant stress is applied to

the tissue. The displacement and the generated strain are then

estimated using two-dimensional correlation of ultrasound images. The

Young’s modulus is then given via the Hooke’s law , which links stress

and strain in a purely elastic medium.

In practice, since the applied stress is unknown, only the strain is

displayed, this strain map is sometimes called the elastogram.

This was the first elastography technique, developed by the Ophir Group

at the beginning of the 1990s. Today, the technique is being tested in

breast lesion characterization with interesting results.

The main limitations of this technique are

• the control of the stress applied, which remains operator

dependent,

• the absence of a specific quantification.

• the use of a stress applied by the operator limits the technique to

superficial organs, mainly the breast or the thyroid.

This technique is simple to implement and is widely spread in the

world of radiology, and has been mainly validated for breast lesionsclassification

DYNAMIC METHODS

In dynamic methods, a time-varying force is applied to the tissue, it can

either be a short transient mechanical force or an oscillatory force with a

fixed frequency.

A time-varying mechanical perturbation will propagate as mechanical

waves which in a solid body can be compressional waves or shear

waves.

The compressional waves propagate very quickly in the human body

(∼1500 m/s), and at high frequencies, these waves, also known as

ultrasounds, can be used to image the body.

Shear waves, which are only generated at low frequencies (10 Hz to

2000 Hz) due to absorption at higher frequencies, propagate more

slowly, and their speed (∼1—50 m/s) is directly related to the medium

shear modulus.

In biological tissues, which are almost incompressible, the Young’s

modulus can be approximated as three times the shear modulus. The

shear wave propagation speed can thus be used to map the Young’s

modulus quantitatively.

Dynamic elastography techniques, which rely on shear waves

propagation, can produce quantitative and higher resolution Young’s

modulus map compared to quasi-static methods.

The use of shear waves, however, requires a more complex system,

able to generate the shear wave (mechanical vibrator or ultrasound

radiation pressure) and to image the small displacements induced by theshear wave (ultrafast or stroboscopic ultrasound).

VIBRO-ACOUSTOGRAPHY

Vibro-acoustography is a dynamic elastography method based on

the ultrasound radiation pressure and developed by the American

Group of James Greenleaf

The radiation pressure is a volumic force created by transfer of

momentum within the medium This momentum is related to the

absorption of the ultrasound wave.

Vibro-accoustography uses two confocal ultrasound beams with

slightly different frequencies w and w0 + w. It results in beats at the

frequency w , which give rise to a modulated force at frequency w,

only at the focal spot. Therefore, everything looks like if

the target vibrates at the frequency w.

It is then sufficient to listen to the sound produced by this excitation

to deduce the mechanical properties of the target, particularly its stiffness.

For this, one can place a hydrophone that records the response of

the target at the frequency w . In order to create an image, the

entire area is swept by moving the focal spot and listening to the response for each point of the image.

However the sweeping of the entire image causes an important

deposit of energy within the medium and a long acquisition time,

which makes its implementation in real-time and in vivo difficult.

In particular, the measured parameter depends on the stiffness

of the zone, but also on the amplitude of the generated force and

the geometry of the vibrating object. Therefore images

correspond to a mixture of several physical parameters,

including elasticity.

Greenleaf’s team is now trying to combine this method with

mammography systems for the diagnosis of breast cancerand with conventional ultrasound machines

TRANSIENT ELASTOGRAPHY METHODS

Acoustic Radiation Force Impulse Imaging (ARFI) or ‘‘Acoustic Radiation

Force Imaging’’, is a method developed by the American team of Kathy

Nightingale.

This technique uses the acoustic radiation force but, unlike vibro -

acoustography, ARFI only uses one focalized ultrasound beam.

The radiation force slightly displaces the tissue at the focal spot according

to Hooke’s law. Then the transducer switches into imaging mode and

detect displacements of the focal spot by tracking of the ultrasound signal

(called ‘‘speckle’’). The ultrasound speckle-tracking , already used in static

elastography, allows to correlate the ultrasound signal window by window

in order to detect the displacement of a tissue with sensitivity of less than amicrometer. By window, I mean a piece of an ultrasound signal.

Today, Nightingale’s team is interested in the propagation of shear

waves generated by the radiation force and has recently proposed a

new ARFI model called ‘‘ARFI-SWS’’.

Based on this concept, it allows to quantitatively measure the Young’s

modulus in a small region of interest. This variation is currently being

evaluated for the liver staging and is available on commercial ultrasound systems, such as the Siemens Acuson S2000.

1D transient elastography: the 1D shear elasticity probe

The 1D transient elastography probe was first developed at the Institut

Langevin in 1995 by Catheline et al.

It consists of generating a transient impulse (little shock) on the medium

and recording the shear wave that propagates within the medium by using

an ultrasound transducer.

First, the front face of the transducer acting as a piston gives a slight

mechanical impulse on the surface of medium, which generates a

spherical compression wave as well as a spherical shear wave.

The displacement generated, which is a function of depth and of time, is

thus estimated by correlations of retro-diffused echoes (via ultrasound

speckle) recorded at a framerate higher than one thousand time per

second with a mono-dimensional ultrasound transducer (5 MHz).

This device was the first to use the principle of ultrafast imaging in one

dimension to visualize in a transient manner the propagation of shearwaves.

Finally, by measuring the phase for each depth, we extract the phase

speed of the shear wave at the central frequency, leading to an

estimation of Young’s modulus by considering the medium to be

homogeneous and nonviscous.

Since 2001, the company Echosens has been commercializing this

technique, 1D transient elastography, under the name FibroScan.

2D transient elastography

In 1997, at the ‘‘Institut Langevin’’, the 1D transient elastography

technique was extended to 2D, allowing the creation of elasticity maps of

biological tissues.

A programmable ultrasound electronic device used for time reversal

experiments (process of refocalizing acoustic waves) was modified to be

able to perform ultrafast imaging based on ultrasound plane wave

emission.

It allowed storing raw data acquired with a frame rate of more than 5000

images per second. A vibrator was fixed to the ultrasound imaging array,

which is then used as an impactor to generate a quasi-plane shear wave.

Once the movie of the propagating shear wave is reconstructed, the wave

equation is inverted to recover a map of Young’s modulus.

The first in vivo tests were carried out in 2003 with volunteers at Institut

Curie, the results were encouraging, but the device was bulky, heavy anddifficult to use in practice.

The study of shear wave by using ultrafast imaging

The Supersonic Shear Imaging technique is the outcome of researches at

the Institute Langevin in transient elastography.

The idea of combining radiation pressure or acoustic radiation force and the

study of shear waves generated comes from Armen Sarvazyan, which can

be considered as a precursor of elastography techniques based on

ultrasonic radiation pressure via its technique: Shear Wave Elasticity

Imaging.

In 2004, two fundamental ideas are developed to overcome the limitations

of 2D elastography technique:

• acoustic radiation force

• ultrasound ultrafast imaging.

These two concepts described below are at the heart of the SupersonicShear Imaging technique

• a Mach cone: ultrasound beams are successively focused at different

depths. The different spherical waves generated for each focal beam

interfere like a Mach cone in which the source propagates more quickly

than the generated shear wave and creates a quasi-plane wave front in

the imaging plane (cylindrical in three dimensions).

The use of constructive interfaces makes it possible to increase the

amplitude of the wave and thus the signal to noise ratio of the

displacement field.

The generated quasi-plane shear wave in the imaging plane allows also

simplifying the propagation hypotheses, which is of great interesting to

solve inverse problems.

Finally, only one Mach cone allows illuminating almost all of the medium

with one plane shear wave; shear wave being generated, the propagation

equation of the waves is inverted to rebuild a map of Young’s modulus;

• ultrafast imaging: complete acquisition all at once. Ultrafast imaging

allows scanning the entire imaging plane with very good temporal

resolution in one single acquisition, typically with a frame rate of 5000

images per second, and up to 30,000 images per second in the case

of tissues such as the peripheral arteries or the eye.

Therefore there is no need to repeat the acquisition several times by

stroboscopy to acquire the entire displacement field. This allows, not

only imaging in real-time, which makes the examination easier, but

also averaging the very rapidly acquired images to improve imagequality.

Therefore, the Supersonic Shear Imaging technique uses the technology of

the 2D transient elastography, but substitutes vibrator by the acoustic

radiation pressure.

The whole excitation-imaging method is then integrated onto one single

component: the ultrasound imaging transducer array. Amplified by the Mach

cone, the generated shear wave has amplitude of tens of microns. This latter

is detectable with a good signal to noise ratio by ultrasound speckletracking

algorithm and ultrafast imaging.

Thanks to ultrafast imaging, the acquisition of the shear wave propagation can

be carried out all at once in less than 30 milliseconds. The technique is

therefore slightly sensitive to patient movements (as an example breathing)

and can be displayed in real-time, like a conventional ultrasound image.

The Young’s modulus maps are then reconstructed by estimating the speed

of the shear wave between two points of the image, using a time of flight

algorithm.

The first in vivo experiences with this technique were conducted at the Institut

Curie where, in approximately 50 patients, the use of the technique todifferentiate benign tissue from malignant tissue was demonstrated

This technique was implemented on an ultrasound diagnostic imaging

device called, the Aixplorer (Supersonic Imagine, Aix-en-Provence,

France) and its diagnosis performance as well as its reproducibility were

demonstrated in several organs, and more particularly in the breast withthe addition of the elasticity parameters to the classical BIRADs criteria

Shear wave

excitation

Measurement

of shear wave

propagation

Shear modulus

reconstruction

Shear

wave

Force

Probe Probe

Tx Echo Rx

Displacement

of

reflectors

Tissue displacement map

Shear wave

speed

calculation

Shear modulus

Shear modulus

Shear wave speed

Density

Tx = transmission

Rx = receiver

Multiple M-mode or B-mode images: tissue displacement map

MAGNETIC RESONANCE ELASTOGRAPHY

Magnetic Resonance Elastography (MRE) is a rapidly developing technology

for quantitatively assessing the mechanical properties of tissue.

MRE obtains information about the stiffness of tissue by assessing the

propagation of mechanical waves through the tissue with a special magnetic

resonance imaging (MRI) technique. The technique essentially involves three

steps:

1. generating shear waves in the tissue,

2. acquiring MR images depicting the propagation of the induced shear waves

and

3. processing the images of the shear waves to generate quantitative maps of

tissue stiffness, called elastograms.

Generating Mechanical Waves in Tissue

MRE typically uses vibrations of a single frequency (within the audio frequency

range) generated by external driver devices. The electrical signal for these devices

is created by a signal generator triggered by and synchronized to the MR pulse

sequence and is amplified by an audio amplifier before being fed into the

mechanical driver

Over the years, several driving mechanisms have been developed, each with their

own advantages and limitations. Three of the most commonly used driver

systems are:

• an electromechanical driver that works via the Lorentz force and

utilizes the magnetic field of the main MRI magnet.

• A piezoelectric stack driver system where the motion created is based

on the piezoelectric property of certain materials. Focused-ultrasound-based

(FUSbased) radiation force has also been investigated as a means to create

mechanical motion for various elasticity imaging strategies including MRE,

where shear waves are created directly within tissue with externally placed

ultrasound transducers.

• pressure-activated driver another widely used method of creating the

required vibrations for MRE utilizes the motion of the voice coils used in

acoustic speaker systems. The required vibrations are again produced by the

Lorentz force, but the static magnetic field is from a devoted permanent magnet

present in the acoustic speaker .

Imaging the Propagating Waves

Measuring the tissue motion produced by a driver with MRE is based on an MR

imaging technique called phase-contrast MRI.

Muthupillai et al. developed the technique of dynamic phase-contrast MRE

where propagating shear waves in tissue are encoded into the phase of the MR

images with the help of motion encoding gradient (MEG) pair.

After continuous harmonic motion is induced in the tissue, a MEG oscillating at

the same frequency as the motion is applied and conventional MR imaging is

performed. The phase contribution to the MR image ϕ due to the motion and the

applied magnetic field gradient at a given position vector r⃗ and phase offset θ

between the motion and the MEG can be written as.

where γ is the gyromagnetic ratio of the tissue protons , N is the number of

gradient pairs used to sensitize the motion, T is the period of the motion

encoding gradient, G⃗ is its amplitude, ξ⃗0 is the peak amplitude of motion,

and k⃗ is the wave number. This equation states that the phase of

harmonically vibrating tissue is directly proportional to its displacement.

Figure shows a typical MRE pulse sequence using a gradient-recalled echo with

the conventional radiofrequency (RF) pulse waveform, slice-selection gradient,

phase-encoding gradient and frequency-encoding gradient.

The motion-encoding gradient (shown here only in the frequency-encoding

direction) is placed after the RF excitation of the sample and before the

measurement of the induced signal.

Motion occurring in any direction can be encoded into the phase of the MR

image by manipulating the axes on which the MEGs are placed. In this example,

only the motion occurring in the frequency-encoding direction will be sensitized

to and encoded into the image phase.

The motion-encoding capability of this technique is very sensitive and can detect

motion on the order of 100’s of nanometers.

An MR image thus obtained containing information about the propagating wave

in its phase is called a wave image. Typically two such wave images are collected

with opposite polarity of the MEG and a phase-difference image is calculated to

remove non-motion-related phase information.

Since the motion-encoding gradients necessary for MRE are inserted into

conventional MR pulse sequences, MRE can be implemented with many MR

imaging sequences, each with its own advantages and limitations. MRE pulse

sequences based on spin echo (SE), gradient-recalled echo (GRE), balanced

steady-state free precision (bSSFP) and echo planar imaging (EPI) techniques

exist.

Mechanical Parameter Estimation

From the wave images indicating the propagation of shear waves in the tissue,

mathematical inversion algorithms based on equations of motion, with

simplifying assumptions like isotropy, homogeneity, and incompressibility, allow

for the calculation of mechanical properties like the shear modulus to be used for

clinical interpretations.

The images of the mechanical properties of tissue calculated in MRE are often

referred to as elastograms. Depending on the technique used to derive the

elastograms from the original MR images, elastograms can theoretically (e.g.,

absent any noise) have half the resolution of the native MR images (which can

range from 50 μm to 10 mm depending on the application), however they more

typically have one-third to one-fifth of the MRI resolution.

Shear wave interference can cause artifacts in the stiffness calculations using

several of the above techniques, and a preprocessing technique called

directional filtering has been developed to reduce these artifacts

MRE of an inclusion phantom

(a) MR magnitude image of an inclusion phantom with soft and stiff inclusions

seen as the hyperintense and hypointense regions, respectively.

(b) A single wave image from the MRE acquisition performed at 100 Hz. The

difference in the wavelengths in the different regions is evident.

(c) An elastogram obtained from these data showing the stiff and soft regions.

APPLICATIONS OF ELASTOGRAPHY

ULTRASOUND ELASTOGRAPHY

Breast US Elastography

Compression elastography may have a role for further evaluating

abnormal findings on conventional breast US images and differentiating

benign from malignant breast lesions. Thus, the eventual goal of

incorporating US compression elastography into routine practice would

be to reduce the biopsy rate of benign lesions.

The two most important elastographic characteristics in evaluating breast

lesions are size and stiffness criteria

Normal Breast TissueBiomechanical testing has shown normal fibroglandular breast tissue to be

markedly stiffer than normal fatty breast tissue by as much as two orders of

magnitude . Therefore, at elastography, fatty tissue will appear bright with

respect to the adjacent glandular tissue, and normal fibrous parenchyma appears darker

(a) B-mode US image of the breast shows that a lobule of normal fatty tissue (f) is

hypoechoic with respect to the surrounding glandular tissue (g).

(b) US elastogram shows that the fatty tissue (f) surrounded by dense breast

tissue appears bright because it is appreciably “softer” than the surrounding

glandular tissue (g).

Simple cyst. (a) B-mode US image of the breast shows a classic

cyst (arrowheads). (b) US elastogram demonstrates a bull’s-eye

configuration within the lesion (arrowheads), which contains

bright compressible material (thin arrow), and a bright spot

behind the cyst (thick arrow).

Complex cyst mimicking a solid lesion at US. (a) B-mode US image

demonstrates a round hypoechoic breast lesion without associated

through transmission. (b) US elastogram shows that the lesion that

appeared “solid” on the B-mode image is a cyst. .

Fibrocystic change. (a) B-mode US image demonstrates an irregular

heterogeneous lesion in a patient who presented with a palpable mass

in the right breast. (b) On the US elastogram, the lesion appears soft

and smaller than on the B-mode image. The findings from a subsequent

biopsy showed fibrocystic change and periductal inflammation. Dotted

lines indicate measured lesion diameters: 1 = 7.16 mm; 2 = 5.55 mm; 3

= 5.02 mm; and 4 = 4.29 mm. Distance ratio was calculated as 5.02

7.16 = 0.70.

Fibroadenoma

Fibroadenoma. (a) B-mode US image of the left breast shows a

lobulated hypoechoic lesion that is taller than wide, with posterior

acoustic shadowing.

(b) US elastogram, however, shows the lesion to be smaller than on

the B-mode image. The findings from biopsy revealed a fibroadenoma.

Fibroadenoma with elasticity score of 1 in 51-year-old woman. US images were

obtained in transverse plane. Left: On conventional B-mode image, lesion was

classified as BI-RADS category 2.

Right: On elasticity image, the entire hypoechoic lesion was evenly shaded green,

as was the surrounding breast tissue.

Fibroadenoma with elasticity score of 2 in 39-year-old woman. US images were

obtained in transverse plane.

Left: On conventional B-mode image, lesion was classified as BI-RADS category 3.

Right: On elasticity image, hypoechoic lesion shows mosaic pattern of green and

blue.

Invasive Ductal Carcinoma

Invasive ductal carcinoma. The breast lesion measures smaller on the B-mode

US image (a) than on the US elastogram (b) in the transverse plane. The

lesion is too large to measure accurately in the anteroposterior plane; dotted

lines indicate measurements of the lesion diameter: 21.9 34.2 mm in a and

24.9 29.6 mm in b. In addition, the lesion is stiffer than the surrounding

tissues.

invasive ductal carcinoma typically is appreciably darker than normal

tissues or benign lesions and is substantially larger on the elastogramthan on B-mode US images

Nonscirrhous type invasive ductal carcinoma with elasticity score of 4 in 29-year

old woman. US images were obtained in transverse plane.

Left: On conventional B-mode image, lesion was classified as BI-RADS category

5.

Middle: On elasticity image, the entire hypoechoic lesion was blue.

Right: Pathologic section of lesion is shown. (Hematoxylin-eosin stain; original

magnification,1.)

Scirrhous type invasive ductal carcinoma with elasticity score of 5 in 55-year-old

woman. US images were obtained in sagittal plane. Left: On conventional B-mode

Image, lesion was classified as BIRADS category 5. Middle: On elasticity image,

both the entire hypoechoic lesion and its surrounding area were blue. Right:

Pathologic section of lesion is shown. (Hematoxylin-eosin stain; original

magnification X 1.),

Lobular carcinoma in situ with elasticity score of 3 in 46-year-old woman. US

images were obtained in transverse plane. Left: On conventional B-mode image,

lesion was classified as BI-RADS category 3. Right: On elasticity image, the

central part of the hypoechoic lesion was blue, and the peripheral part of the lesion

was green.

Lymph node with metastatic involvement. (a) B-mode US image demonstrates a

lymph node that appears benign on the basis of its reniform shape and

echogenicity. Dotted line indicates a lesion diameter of 20.4 mm. (b)

Corresponding US elastogram, however, shows a stiff area that turned out to be

a metastatic focus (arrows). Dotted line indicates a lesion diameter of 24.0 mm.

Hematoma. (a) B-mode US image of the breast demonstrates a superficial

heterogeneous mass. (b) US elastogram shows that the lesion is stiff but

measures smaller than on the B-mode image. Dotted lines indicate

measurements of the lesion diameter: 11.0 5.3 mm in a and 10.2 4.2 mm in b.

The distance ratios were calculated as 10.2 11.0 = 0.93 (for the long-axis

dimension) and as 4.2 5.3 = 0.79 (for the short-axis dimension). At physical

examination, skin discoloration was noted in the corresponding area, a finding

consistent with a hematoma.

Prostate US Elastography

Prostate Cancer

Endorectal US continues to be the most commonly used imaging modality for

assessing the prostate; however, it is relatively inaccurate for cancer

detection. Endorectal US has a role in guiding prostate biopsies. Magnetic

resonance imaging and computed tomography are mainly implemented for

staging.

At B-mode US, most prostate cancers are hypoechoic, but many are

isoechoic, and a few are hyperechoic. As a result, the overall sensitivity of

endorectal US for prostate cancer detection is about 50%. The added use of

Doppler imaging increases the detection rate by only 5%. In contrast,

endorectal real-time elastography enables the diagnosis of prostate cancer

with a reported accuracy of 76%. However, the major role of elastography is

to improve the results of image-directed biopsy and therapy, compared with

the use of endorectal US alone.

Prostate adenocarcinoma with Gleason score of 6/10. (a) Sagittal endorectal US

Image with the prostate gland outlined shows no discrete lesion (arrow). (b) Sagittal

US elastogram shows strong vibration in the normal prostate tissue and also an area

of no vibration (arrow) anteriorly from the midportion of the gland to the base. (c)

Photomicrograph (hematoxylineosin stain) of a transverse histologic section that

included the corresponding lesion (arrow) at the base shows good correlation, with

the deficit noted anteriorly on the elastogram through the selected plane of section

(dashed line in a and b) in the midportion of the gland. The smaller lesion outlined in

blue in c is outside the plane of the elastogram.

Prostate adenocarcinoma with Gleason score of 9/10, involving two of two core

samples and approximately 60% of the tissue. (a) Right parasagittal conventional

US image shows that the lesion (arrow) is mildly hypoechoic. (b) Corresponding

right parasagittal US elastogram better depicts the lesion (arrow) as a dark area of

low strain. (c) Photomicrograph (hematoxylin-eosin stain) of a transverse histologic

section (dashed line in a and b indicates the plane of section) shows that there is

good size correlation between the histologic extent of the lesion (arrow) and the

lesion extent delineated on the elastogram.

Benign Prostatic Hyperplasia

The appearance of benign prostatic hyperplasia at endorectal US is variable but

usually consists of a heterogeneous hypoechoic area or areas in the transitional

zone .

In general, foci of benign prostatic hyperplasia have elastic moduli (stiffness)

that are an order of magnitude greater than those of normal prostate tissues but

are less than those of prostate carcinomas. As a result,on elastograms, benign

prostatic hyperplasia will appear darker than normal prostate tissue.

However, the difference between benign prostatic hyperplasia and prostate

carcinoma can be difficult to discern because benign prostatic hyperplasia also

appears darker than the background tissues. Consequently, benign prostatic

hyperplasia can represent a false-positive finding for cancer .

Benign prostatic hyperplasia. (a) Transverse B-mode US image demonstrates a

mildly prominent isoechoic area (arrow) in the central zone of the prostate gland.

(b) Corresponding transverse elastogram shows that

the area of benign prostatic hyperplasia (arrow) is more conspicuous. The lesion

demonstrates greater stiffness than the surrounding normal prostate tissue.

(c) Photomicrograph (hematoxylin-eosin stain) shows that there is good

correlation between the extent of the lesion (arrow) on the histologic section

and the lesion extent on the elastogram.

Elastography of Thyroid Lesions

A thyroid lesion may have different levels of stiffness within it, depending

on the cellularity and composition of the nodule. Information from these

elastograms helps assess the relative stiffness of the lesion compared

with its surrounding tissues and within itself. However, this information

cannot be used to compare the stiffness of thyroid lesions from different

patients because strain changes with applied compression.

When pulsation of the carotid artery is used as the compression source,

strain near the carotid artery can indicate the amount of compression

applied by carotid arterial pulsation.

Because the thyroid gland is located between the trachea and the carotid

artery, lateral expansion of the carotid artery during systole compresses

the thyroid gland against the trachea. As a result, the thyroid gland

expands in the anteroposterior direction, that is, in the direction of the US

probe (beam axis).

Because US is sensitive to detecting motion along the beam axis,

deformation of the thyroid gland in this direction can be readily detected.

Because the thyroid stiffness index is a ratio between the carotid strain

and the strain in the thyroid nodule, the varying strain from change in the

blood pressure did not affect the ratio.

Example of selecting region of interest (ROI) to compute thyroid stiffness index

within a nodule.

(a) Transverse US image showing a nodule (arrows) within thyroid gland (Tg),

the trachea (Tr), and the carotid artery (C).

(b) Combined US image and elastogram of this nodule; average strain of

stiffest region inside a thyroid nodule (ROI #1) is used as denominator of

thyroid stiffness index.

(c) Combined US image and elastogram of this nodule with a different scale to

better visualize the areas of highest strain near the carotid artery. Average

strain of the highest-strain regions near the carotid artery (ROI #2, ROI #3) is

used as numerator of thyroid stiffness index. Range of the color map is

adjusted to better visualize ROIs.

(a) Left: Transverse US image in a 62-year-old man shows a predominantly solid

nodule (arrows) occupying entire thyroid gland. Right: Elastogram shows the

predominantly homogeneous appearance within nodule, with small areas of

decreased stiffness (blue indicates stiffer part; red indicates softer part). This was

diagnosed as a follicular lesion at FNA.

(b) Left: Transverse US image in a 60-year-old woman shows a predominantly

solid nodule with small cystic areas (arrows). Right: Elastogram shows the

heterogeneous appearance within nodule. This was diagnosed as a nodular goiter

at FNA.

(c) Left: Transverse US image in a 36-year-old woman shows a hypoechoic

nodule (arrows) with irregular borders and tiny punctate calcifications in it. Right:

Elastogram at same level shows stiff areas within lesion (blue region). This was

diagnosed as papillary carcinoma at FNA. Ccarotid artery; Trtrachea.

Ultrasound elastography for musculoskeletal applications

Disease in the musculoskeletal system results in alterations to its

biomechanical properties. Although EUS techniques have been

extensively employed for in vitro research of muscle and tendon

biomechanics since the early 1990s, the recent introduction of EUS

into commercially available ultrasound systems has driven research

activity towards potential clinical applications of this novel method in the musculoskeletal system

(a) (b)

(c) (d)

(a, b) Longitudinal and (c, d) transverse free-hand strain elastograms of the

middle third of asymptomatic Achilles tendons (T) showing the two distinct

ultrasound elastography patterns of normal tendons.

Type 1 (a, c) tendons appear homogeneously stiff, (green/blue) with no distinct

soft (red) areas.

Type 2 (b, d) tendons appear considerably inhomogeneous with soft (red) areas

(longitudinal bands or spots), which did not correspond to any changes in B-

mode ultrasound.

The retro- Achilles fat appears as a mosaic of green, red and blue. Note the red

areas at the lateral and medial sides of the tendon in the transverse plane (c, d),

corresponding to an artefact, secondary to difficulty in stabilising the hand-held

transducer.

(a) (b)

(c)

(a) Longitudinal colour Doppler ultrasound image and (b) free-hand strain

elastogram of a 23-year-old recreational runner with insertional Achilles

tendinopathy. There is hypoechogenicity and neovascularity at the tendinopathic

area [asterisk in (a)], which appears softer (red) compared with the stiff

(blue/green) normal-appearing remaining tendon (T). The retro-

Achilles fat appears as a mosaic of various levels of stiffness. (c) The

elastogram of a normal (asymptomatic) Achilles tendon calcaneal insertion is

presented for comparison. The retrocalcaneal bursa (asterisk) appears

considerably softer (red) compared with the stiffer tendon (green/yellow). Note

the presence of soft (red) foci beneath the calcaneal bone (C), which correspond

to artefacts. LT, left.

(a) (b)

(c) (d)

Longitudinal shear wave elastograms of a normal (a) Achilles and (c) patella

tendon, as well as (b, d) a case of distal patella tendinopathy in a 23-year-old

football player.

The elasticity qualitative and quantitative scale is presented at the upper right

corner of the images. Measurements (mean, minimum, maximum and

standard deviation) within the circular region of interest (ROI) are presented in

kilopascals ranging from 0 (dark blue) to 300 (dark red).

(a, c) The normal Achilles and patella tendons (T) appear as homogeneous

stiff (red) structures, as opposed to fat, which is homogeneously soft (blue).

(a) The mean stiffness of a representative area at the mid-portion of the

Achilles free tendon is 300 kPa. (d) In the case of distal patella

tendinopathy, the tendinopathic area appears hypoechoic with

neovascularity (asterisk).

(b) In the corresponding elastogram, the abnormal area appears softer (blue;

mean elasticity 40.94 kPa) compared with the stiffer normal tendon (red;

mean elasticity 261.16 kPa). The small amount of fluid in the deep infrapatella

bursa appears softer than the tendinopathic area (blue, mean elasticity 34.38

kPa).

(a) (b)

Axial free-hand strain elastograms of (a) normal relaxed calf muscles and (b) vastus

lateralis muscle.

The medial and lateral heads of gastrocnemius (M and L, respectively), as well as

the vastus lateralis (VL), appear as a mosaic of intermediate or increased stiffness

(green or blue colour, respectively), with scattered softer (red) areas near muscle

boundaries. The subcutaneous fat appears soft (red/yellow). The homogeneous stiff

appearance of soleus (S) in (a) is probably because of inadequate B-mode and

ultrasound elastography data due to increased depth.

ultrasound elastography for diffuse liver disease

Accurate assessment of the degree of liver fibrosis is important for

estimating prognosis and deciding on an appropriate course of

treatment for cases of chronic liver disease (CLD) with various

etiologies. Because of the inherent limitations of liver biopsy, there is a

great need for noninvasive and reliable tests that accurately estimate

the degree of liver fibrosis. Ultrasound (US) elastography is considered

a non-invasive, convenient, and precise technique to grade the degreeof liver fibrosis by measuring liver stiffness.

Transient elastography of a normal and a cirrhotic patient.

A

B

The displacement M-mode image located in the center of the monitor shows

axial displacement as a function of depth (y-axis) and time

(x-axis). In the healthy patient

(A), the shear wave is relatively slow and the liver stiffness is low. However,

in the patient with cirrhosis (B), the shear wave can propagate more rapidly

though a hard tissue, and the time-depth gradient is very steep. The

controlled attenuation parameter is displayed, which is known to correlate with the fatty liver severity

Acoustic radiation force impulse imaging of a normal and a cirrhotic

patient.

The cylindrical region of interest (ROI) in the middle of the

ultrasonogram was used as the sample volume for the measurement of

stiffness. Instead of Young modulus, the propagating velocity of the

shear wave is displayed. Although the grayscale ultrasonograms are

similar to each other, the propagating velocity is different: 1.2 m/sec in the healthy patient (A) and 1.6 m/sec in the patient with cirrhosis (B)

Supersonic shear-wave imaging of a normal and a cirrhotic patient.

A large trapezoidal color box display of the distribution of the elastic properties of

the liver. The severity of stiffness is depicted with the colored look-up table. Dark

blue color represents normal liver tissue (A), and bright blue-green color

represents increased liver stiffness such as liver cirrhosis (B). The round region

of interest (ROI) in the color box is the Q-Box and the mean Young modulus and

standard deviation in the ROI have been calculated

A B

C D

Acute exacerbation of hepatitis B virus-induced hepatitis.

Grayscale ultrasonography (A) shows mild parenchymal coarseness of

the liver; the liver stiffness is 10.3 kPa according to the shear wave

elastography (B). Six months later, the patients recovered from liver

function deterioration. The grayscale ultrasonographic feature (C) is

similar to the previous study, but the liver stiffness decreases to 6 kPa(D).

Elastography in the Differential Diagnosis of Enlarged Cervical Lymph Nodes

Drawings show typical diagrammatic appearance of five patterns of lymph nodes. Elastographic

patterns were determined on distribution and percentage of lymph node area having high elasticity

(hard): pattern 1, absent or small hard area; pattern 2, hard area < 45% of lymph node; pattern 3,

hard area ≥ 45%; pattern 4, peripheral hard and central soft areas; pattern 5, hard area occupying

entire lymph node. Increasing tissue hardness appears in ascending order as red, yellow, green,

and blue.

Transverse sonogram of level 1 lymph node in 30-year-old man with left submandibular

lymphadenopathy. Elastography image on left shows pattern 1, absent or small hard area. B-mode

sonographic image on right shows score of 5, reactive. Final diagnosis from clinical and serologic findings was reactive lymph node.

Longitudinal sonogram of level 2 lymph node in 33-year-old woman with left posterior

auricular lymphadenopathy. Elastography image on left shows pattern 2, hard area <

45% of node. B-mode sonographic image on right shows score 5, reactive. Final

diagnosis from clinical and serologic findings was reactive lymph node.

Transverse sonogram of level 2 lymph node in 79-year-old man with laryngeal carcinoma. Elastography image on left shows pattern 3, hard area ≥ 45%. B-mode sonographic image on right shows score 7, metastatic. Final diagnosis by histopathology was metastatic lymph node.

Longitudinal sonogram of level 5 lymph node in 52-year-old man with nasopharyngeal carcinoma. Elastography image on left shows pattern 4, peripheral hard and central soft areas. B-mode sonographic image on right shows score 7, metastatic. Final diagnosis by CT was metastatic lymph node.

MR ELASTOGRAPHY

Following are the some of the current applications of MRE which are being investigated :

Liver MR Elastography—Magnetic resonance elastography has been widely investigated for the diagnosis of hepatic diseases and is currently used in clinical practice for fibrosis and cirrhosis assessment where the stiffness of the diseased liver is significantly higher than normal liver tissue stiffness

Clinical hepatic MRE is performed at a frequency of 60 Hz using pneumatic-based pressure-activated drivers. Wave data are acquired with four phase offsets and a modified direct-inversion algorithm with multiscalecapabilities is used for the estimation of the liver stiffness.

MRE-derived stiffness of healthy liver tissue at 60 Hz compared to the stiffness of

liver tissue at various stages of fibrosis. The stiffness increases gradually with the

progression of the fibrosis. A cutoff of 2.93 kPa well differentiates the healthy and

fibrotic livers.

Hepatic MRE

Results are shown from clinical hepatic MRE exams of a patient with a normal

liver (top row) and a patient with a cirrhotic liver. (a,d)

Conventional abdominal MR magnitude images of the two patients, showing no

significant difference between the two livers. (b,e)

Wave images from the MRE acquisition at 60 Hz showing shear waves with a

shorter wavelength in the first patient, and a substantially longer wavelength in

the second patient. (c,f)

The corresponding elastograms indicating that the two livers were normal (1.7

kPa) and cirrhotic (18.83 kPa), respectively.

MRE is also being investigated as a means to characterize hepatic tumors and it

has been found that malignant liver tumors have significantly greater mean shear

stiffness than benign tumors and normal liver tissue and a cutoff value of 5 kPa

can differentiate malignant tumors from benign tumors and normal liver

parenchyma.

Breast MR Elastography—Another application of MRE that is being

investigated with great interest is for the assessment of breast cancer .

Breast tumors are known to be typically stiffer than benign lesions and normal

breast tissue . Manual palpation is a recommended part of routine screening for

breast cancer and helps in the detection of these hard masses.

Contrast-enhanced MR imaging (CE-MRI) has proven to have a very high

sensitivity for the detection of tumor nodules, but the specificity of the technique

can be a problem leading to numerous false positives and unnecessary biopsies.

MRE is being investigated as a complementary technique to CE-MRI to provide

additional information about these suspicious regions and the combined

technique has shown promise to increase diagnostic specificity

Breast MRE

(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.

Skeletal Muscle MR Elastography—MRE has also been extensively

investigated for studying the stiffness of skeletal muscle since it is well known

that the stiffness changes significantly depending upon the contractile state of the

muscle . Skeletal muscle MRE can be used for studying the physiological

response of diseased and damaged muscles. For instance, it has been found that

there is a difference in the stiffness of muscles with and without neuromuscular

disease

Skeletal muscle MRE

(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.

Brain MR Elastography—Assessment of the mechanical properties of

brain tissue with MRE is another area of significant research and clinical

interest due to the diagnostic potential of brain tissue stiffness information

as it may be related to diseases like Alzheimer’s disease, hydrocephalus, brain

cancer and multiple sclerosis. While it would be difficult to use ultrasound

based approaches to noninvasively assess brain mechanical properties, MRE is

well suited for this application.

Brain MRE

(a) Shown is an axial MR magnitude image of the brain showing white matter, gray

matter, and cerebrospinal fluid. (b) A single wave image from MRE performed

at 60 Hz. (c) The corresponding elastogram is shown and a good correlation

between the magnitude image and the stiffness estimate is evident.

MR Elastography for Visualizing Anatomy—The primary applications of

MRE that have been developed have focused on the study of the mechanical

properties of different tissues. However, the capability of MRE to measure the

displacement of tissues has also been investigated to address other clinical questions.

Such applications include a shearline imaging technique designed to investigate the

functionality of slip interfaces and a vibration imaging technique used to localize the

functional compartments of the forearm flexor muscles

As an example of these applications, the vibration imaging technique is introduced

here, the goal of which is to locate the finger-specific functional compartments of

the extrinsic forearm flexor muscles that partly control the flexion of the

interphalangeal joints of the index, middle, ring and little fingers.

Even though these flexor muscles (flexor digitorum superficialis, FDS and flexor

digitorum profundus, FDP) do not possess anatomically distinct compartments,

they can exert almost independent action on specific fingers through selective

activation of functional compartments specific for each finger.

The knowledge of the location and boundaries of these segments is useful for

localizing techniques like MR spectroscopy and biopsy, but is difficult to obtain

currently.

With the proposed vibration imaging technique based on MRE principles, this can

be achieved by vibrating a finger of interest independently of the others and

measuring the amount of motion within the forearm musculature that is transferred

through the structurally connected tendons.

Functional compartments of the flexor muscles

(a) MR magnitude image of the right forearm of a healthy volunteer.

(b) An example wave image obtained using MRE motion encoding of tissue

vibrations in the forearm induced by selectively vibrating the ring finger at

90 Hz.

(c) An amplitude map obtained from wave images indicating localized regions

of higher displacement corresponding to the compartments of the flexor and

extensor muscles (arrows).

(d) A phase map obtained from the wave images indicating that the

compartments of the flexor and extensor muscles of the activated finger are

moving out of phase with each other.

In vivo difficulties:

•Uncontrolled Motion–Respiratory

–Cardiac

–Patient

•Requirement–Fast Acquisition

RECENT ADVANCES AND FUTURE PROSPECTS

• Intracardiac Myocardial elastography• Acute Pancreatitis risk assessment on admission• Pancreatic Masses and chronic pancreatitis• Renal elastography• Obstretics & Gynaecology e.g. Cervical stiffness, Prediction of PTL,

Gynaecological cancer etc.• Vascular imaging, Non-Invasive Vascular Elastography• Optical Coherence Elastography• Organ transplantation......and many more……

CONCLUSION

Elastography is an important research field It has also been evaluated

in the clinical field for more than 10 years. It has provided a new

feature to assess tissue stiffness and has shown that tissue elasticity

is of great value for diagnosis.

Indeed the variation of the Young’s modulus in biological tissues offers

a contrast that is potentially more interesting than conventional

ultrasound.

If the elastography techniques developed in research laboratories

start making their commercial appearance, they are not all

quantitative or operator independent, the word ‘‘elastography’’ can

hide very different physical phenomena. It is therefore important to

know how to decrypt the underlying physics of each elastography

method in order to understand their advantages and physical

limitations.

Shear wave based techniques have strong advantages over quasi-

static techniques, as they are more reproducible, quantitative, rely on

automatic shear wave generation and provide good elasticity contrast.

The availability of true quantitative numerical data allows adjustment

of the dynamic range to optimize the visualization of structures. These

advantages should certainly lead to new applications of shear wave

elasticity imaging, not only for diagnosis but also for follow-up.

The real-time capability of some of these SW techniques also allows

the development of 3D elastography imaging that should facilitate the

clinical use for detection, therapy planning and monitoring in the

routine clinical practice.

Finally, the integration of elastography techniques in conventional

ultrasound systems open the door for routine application during

ultrasound examination and will allow information fusion with other imaging techniques to strengthen their diagnostic performance.

REFERENCES AND FURTHER READING

• Ultrasound elastography: Principles and techniques BY J.-L. Gennisson∗, T. Deffieux, M. Fink, M. Tanter : Diagnostic and Interventional Imaging (2013) 94, 487—495

• Tissue Elasticity Imaging BYJean Martial Mari Department of Medical Physics and Bioengineering,University College London

• Ultrasound elastography for musculoskeletal applications BY E E DRAKONAK, G M ALLEN and D J WILSON : The British Journal of Radiology, 85 (2012), 1435–1445

• MAGNETIC RESONANCE ELASTOGRAPHY: A REVIEW BY Yogesh K Mariappan, Kevin J Glaser, and Richard L EhmanClin Anat. 2010 July ; 23(5): 497–511

• Principles and clinical application of ultrasound elastography for diffuse liver disease BY Woo KyoungJeong, Hyo K. Lim, Hyoung-Ki Lee, Jae Moon Jo, YongsooKim : Ultrasonography 2014;33:149-160

• US Elastography ofBreast and Prostate Lesions BY Daniel T. Ginat, Stamatia V. Destounis, Richard G.Barr, Benjamin Castaneda, John G. Strang,Deborah J. Rubens : RadioGraphics2009; 29:2007–2016

• Differential Diagnosis of Thyroid Nodules with US ElastographyUsing Carotid Artery Pulsation BY Manjiri Dighe, Unmin Bae, Michael L. Richardson, Theodore J. Dubinsky Satoshi Minoshima, Yongmin Kim : Radiology: Volume 248: Number 2—August 2008

• Accuracy of Sonographic Elastography in the Differential Diagnosis of Enlarged Cervical Lymph Nodes: Comparison with Conventional B-Mode Sonography BY Farzana Alam, KumikoNaito, Jun Horiguchi, Hiroshi Fukuda, Toshihiro Tachikake, Katsuhide Ito : AJR 2008; 191:604–610

• RUMAC’s Diagnostic Ultrasound 4th Edition.

HOPE WE SEE ELASTOGRAPHY BEING DONE IN OUR DEPARTMENT SOON!!!!!