Ultrasound – Physics & Advances

Rakesh C A

• Sound beam is similar to x ray beam as both waves transmit energy

Sound

DIAGNOSTIC ULTRASOUND

X RAYS

Wave type Longitudinal waves Electromagnetic waves

transmission Elastic medium No medium

generation Stressing the medium

Accelerating electric charges

velocity Depends on the medium

constant

Similar waves Seismic, acoustic Radio, light

Longitudinal Waves• Ultrasound pulses are

transmitted, as longitudinal waves, ie; the motion of particles in the medium is parallel to direction of wave propagation.

• They produce bands of compression and rarefaction

• Wave length is distance between two bands of compression or rarefaction

• Audible range is 20 to 20,000 cycles per second • Ultrasound has frequency greater than 20,000

cycles per second

“Ultra”…….sound?

• It is independent of frequency ,• but depends on material through which it

propagates

• Depends on– Compressibility– Density

Velocity of Sound

Velocity of Sound

1. Compressibility– velocity is inversely proportional– liquids and solids propagate sound more rapidly than

gases (easily compressed)

2. Density– denser materials have greater inertia - so, decreased

velocity

ULTRASOUND CHARACTERISTICS

• Frequency• Velocity• Wavelength• Amplitude• Intensity and Power

Frequency• The frequency of sound is

determined by the source.(2-20MHz)

v = f λ

• In ultrasonic frequency range, the velocity of sound is constant in any particular medium.

f

• If the frequency increases then the wave length must decrease as they are inversely proportional to each other.

Velocity

• The significance of ultrasound velocity is that it is used to determine the depth location of structures in the body

• The period (T) is the time required for one vibration cycle. It is the reciprocal of the frequency

Amplitude

• The amplitude of an ultrasound pulse is the range of pressure excursions , related to the energy content.

• In diagnostic applications, it is usually necessary to know only the relative amplitude of ultrasound pulses.

• Units of decibels (dB).

• Also called loudness • Determined by amplitude of oscillation

• As amplitude is increased , intensity also increases

Intensity

Intensity

• Intensity is the rate at which ultrasound energy is applied to a specific tissue location within the patient's body.

• It is the quantity that must be considered with respect to producing biological effects and safety

• The intensity of most diagnostic ultrasound beams at the transducer surface is on the order of a few milliwatts per square centimeter.

• First successful application – SONAR in world war 2 (SOund Navigation And Ranging)

History

Successful medical application – 1940s

Uses of ultrasonic energy in the 1940s. Left, in gastric ulcers. Right, in arthritis

Ultrasonic therapy generator, the "Medi-Sonar" in the 1950s.

A British ultrasonic apparatus for the treatment of Meniere's disease in the late 1950s

History

Denier's Ultrasonoscopic apparatus with ultrasound generator, emitter transducer and oscilloscope. This can be adapted for both therapeutic and diagnostic purposes The first hand-held imaging instrument was

developed by John Wild and John Reid in the early 1950's

The Present

1. Transmitter2. Transducer 3. CPU4. Display5. Key board / cursor6. Disc storage device.7. Printer

Components

Transducer

• Instrument which converts one form of energy to other

• The conversion of electrical pulses to mechanical vibrations and the conversion of returned mechanical vibrations back into electrical energy.

Electrical Energy Mechanical Energy

SELECTION OF TRANSDUCER

• Superficial vessels and organs within 1 to 3cms depth and intra operative imaging –

• 7.5 to 15 Hz

• Deeper structures in abdomen and pelvis within 12 to 15cms –

• 2.25 to 3.5Hz

Transducer - Parts

• A simple single-element, plane-piston source transducer has major components including the – Piezoelectric material, – Sensor electrodes,– Insulated layer, – Backing block, – Acoustic insulator– Insulating cover, and – Transducer housing.

Piezoelectric Element• The active element is basically a piece of polarized

material - a piezoelectric ceramic sandwiched between electrodes

• The piezoelectric element converts electrical signals into mechanical vibrations (transmit mode) and mechanical vibrations into electrical signals (receive mode).

• Natural – Quartz • Artificial– most of USG materials– ferroelectrics

• barium titanate • PZT (lead zirconate titanate)

ADVANTAGE – they can be formed into different shapes

Piezoelectric materials

Ceramic Piezoelectric Crystal?

• The piezoelectric attributes are attained after a process of

- Molecular synthesis, (dipoles)- Heating, (Curie Temperature)- Orientation of internal dipole structures with an

applied external voltage, (Poling)- Cooling to permanently maintain the dipole

orientation, and - Cutting into a specific shape.

Ceramic Piezoelectric Crystal?• Once the material has cooled, the dipoles

retain their alignment. • Heating the crystal above this temp reduces

its usefulness. So, transducers should not be autoclaved.

Piezoelectric crystal• At equilibrium, there is no net

charge on ceramic surfaces. • When compressed, an

imbalance of charge produces a voltage between the surfaces - piezoelectric effect

• Similarly, when a voltage is applied between electrodes attached to both surfaces, mechanical deformation occurs - electrostriction

Piezoelectric Effect

Electrostriction

Piezoelectric crystal – how thick?

• The thickness of the active element is determined by the desired frequency of the transducer

• Piezoelectric crystals are cut to a thickness that is 1/2 the desired radiated wavelength

Backing/Damping Block

• The rear face of the piezoelectric crystal material is usually supported by a backing material which is tungsten loaded araldite, so that the vibrations in the piezoelectric material are rapidly damped after the initial excitation.

Backing/Damping Block

• This component also dampens the transducer vibration to create an ultrasound pulse with a short spatial pulse length, which is necessary to preserve detail along the beam axis (axial resolution).

Couplant

• Material (usually liquid) that facilitates the transmission of ultrasonic energy from the transducer into the test specimen.

• Necessary to overcome the acoustic impedance mismatch between air and solids.

Modes of Vibration

2 TYPES:

1. thickness mode • most common • Used in medical

crystals

2. radial mode

• Natural frequency to which the transducer is sensitive

• Resonant frequency determined by thickness of crystal

• Thick crystal – low frequency sound • Natural frequency – one that produces internal

wavelengths that are twice the thickness of crystal• Frequency corresponding to half the wavelength is-

fundamental resonant frequency

Resonant Frequency

Resonance Transducers Non Resonance Transducers

Transducers

Resonance Transducers

• They are manufactured to operate in a “resonance” mode, whereby a voItage (commonly 150 V) of very short duration (a voltage spike of 1 msec) is applied, causing the piezoelectric material to initially contract, and subsequently vibrate at a natural resonance frequency.

• The operating frequency is determined from– the speed of sound in, and – the thickness of, the piezoelectric material.

Resonance Transducers

• Higher frequencies are achieved with thinner elements, and lower frequencies with thicker elements.

Nonresonance (Broad-Bandwidth) “Multifrequency” Transducers

• Modern transducer design coupled with digital signal processing enables “multifrequency or “multihertz” transducer operation, whereby the center frequency can be adjusted in the transmit mode.

Unlike the resonance transducer design, the piezoelectric element is intricately machined into a large number of small “rods,” and then filled with an epoxy resin to create a smooth surface.

Nonresonance (Broad-Bandwidth) “Multifrequency” Transducers

• Excitation of the multifrequency transducer is accomplished with a short square wave burst of 150 V with one to three cycles, unlike the voltage spike used for resonance transducers.

• This allows the center frequency to be selected within the limits of the transducer bandwidth.

Nonresonance (Broad-Bandwidth) “Multifrequency” Transducers

• Likewise, the broad bandwidth response permits the reception of echoes within a wide range of frequencies.

• For instance, ultrasound pulses can be produced at a low frequency, and the echoes received at higher frequency.

• “Harmonic imaging” is a recently introduced technique that uses this ability.

• Lower frequency ultrasound is transmitted into the patient, and the higher frequency harmonics (e.g., two times the transmitted center frequency) created from the interaction with contrast agents and tissues, are received as echoes.

Spatial Pulse Length

• The length of the sonic pulse.• The number of waves multiplied by their wavelengths• The backing block is incorporated to quench the

vibrations and to shorten the sonic pulse.

Refers to two characteristics of crystal• purity of their sound and • length of time the sound persists

Transducer Q Factor (Q = Quality)

Transducer Q Factor

• The “Q factor” describes the bandwidth of the sound emanating from a transducer as:

• where fo = center frequency

bandwidth = width of the frequency distribution.

Bandwidth

fQ o

• The interval between initiation of the wave and complete cessation of vibrations is called the “ring down time”.

• Dampening of the vibration lessens the purity of the resonance frequency and introduces a broadband frequency spectrum.

• A “high Q” transducer has a narrow bandwidth (i.e., very little damping) and a corresponding long spatial pulse length – organ imaging

• A “low Q” transducer has a wide bandwidth and short spatial pulse length – doppler

𝑄=𝑓 0

h𝐵𝑎𝑛𝑑𝑤𝑖𝑑𝑡∝𝑆𝑝𝑎𝑡𝑖𝑎𝑙 𝑃𝑢𝑙𝑠𝑒 h𝐿𝑒𝑛𝑔𝑡

• A single vibrating point sets out waves in all directions

• Waves move away as concentric circles

Characteristics of Ultrasound Beam

When two sound waves interact , they cancel each other or reinforce each other

Beam Properties• The ultrasound beam propagates as a longitudinal

wave from the transducer surface into the propagation medium, and exhibits two distinct beam patterns: – a slightly converging beam out to a distance specified

by the geometry and frequency of the transducer (the near field), and

– a diverging beam beyond that point (the far field).

The Near Field• The near field, also known as the Fresnel zone,

is adjacent to the transducer face and has a converging beam profile.

• Beam convergence in the near field occurs because of multiple constructive and destructive interference patterns of the ultrasound waves from the transducer surface.

• Huygen’s principle describes a large transducer surface as an infinite number of point sources of sound energy where each point is characterized as a radial emitter.

• By analogy, a pebble dropped in a quiet pond creates a radial wave pattern.

Near Field Length……….• The near field length for an unfocused, single-element

transducer is dependent on the transducer frequency and diameter:Near Field Length = =

• For multiple transducer element arrays, an “effective” transducer diameter is determined by the excitation of a group of’ transducer elements.

• Because of the interactions of each of the individual beams and the ability to focus and steer the overall beam, the formulas for a single-element, unfocused transducer are not directly applicable.

=

• Lateral resolution (the ability of the system to resolve objects in a direction perpendicular to the beam direction) is dependent on the beam diameter and is best at the end of the near field for a single-element transducer.

• Lateral resolution is worst in areas close to and far from the transducer surface.

• Pressure amplitude characteristics in the near field are very complex, caused by the constructive and destructive interference wave patterns of the ultrasound beam.

• Peak ultrasound pressure occurs at the end of the near field, corresponding to the minimum beam diameter for a single-element transducer.

• Pressures vary rapidly from peak compression to peak rarefaction several times during transit through the near field. – Only when the far field is reached do the

ultrasound pressure variations decrease continuously.

Far Field

• The far field is also known as the Fraunhofer zone, and is where the beam diverges.

• For a large-area single-element transducer, the angle of ultrasound beam divergence, , for the far field is given by

• where d is the effective diameter of the transducer and l is the wavelength; both must have the same units of distance.

d

22.1sin

sin 𝜃=1.22𝑑

=1.22

𝑣𝑓𝑑

=𝑣

𝑑 . 𝑓

• Less beam divergence occurs with:– High - frequency transducers– Large - diameter transducers

• High frequency beams – – fresnel zone is longer – depth resolution is superior

• Disadvantage : – Tissue absorption is more, leading to deterioration of side

to side resolution

=

Solution: Focused Transducer

Focused Transducers

• Single-element transducers are focused by using – a curved piezoelectric element or – a curved acoustic lens

to reduce the beam profile.

Focal Distance

• The focal distance, the length from the transducer to the narrowest beam width, is shorter than the focal length of a non-focused transducer and is fixed.

Focal Zone

• The focal zone is defined as the region over which the width of the beam is less than two times the width at the focal distance; – Thus, the transducer frequency and dimensions

should be chosen to match the depth requirements of the clinical situation.

Interactions B/N Ultrasound and Matter

1. Reflection

2. Refraction

3. Absorption

Reflection • Diagnostic images are produced by reflected

portion of the beam.

• Percentage of beam reflected at tissue interfaces depends on – Tissue’s acoustic impedance– Beam’s angle of incidence.

Acoustic Impedance (Z)

• The most important tissue property in imaging.• This quantity is more properly called the specific

acoustic impedance of the medium• A simplified definition is

Z = d vd = density of the tissue (g/cm3)

v = velocity of sound (cm/sec)

• Z – Rayl (g/cm2 x 10-5)

• At most interfaces within the body, only a portion of the ultrasound pulse is reflected

• The pulse is divided into two pulses - one pulse, the echo, is reflected back toward the transducer and the other penetrates into the other material.

• The brightness of a structure in an ultrasound image depends on the strength of the reflection, or echo.

• This in turn depends on the difference in acoustic impedance of the two materials.

Angle of incidence

• Angle b/n sound waves and reflecting surfaces

• The more the angle , the less the reflection

• In medical USG, reflected sound is not detected when angle is greater than 3o

Reflection

• Two distinct patterns of reflection give rise to the echoes that make up an ultrasound image– specular reflection and – scattering

Specular reflection• When an incident ultrasound pulse encounters a large,

smooth interface between two types of tissue with different acoustic impedance values , the result is a partially reflected echo that travels back toward the transducer and a partially transmitted pulse that travels deeper into the patient.

• Responsible for the bright appearance of fibrous structures such as tendons and of boundaries between different tissues.

Scattering / Diffuse Reflection• If the ultrasound pulse encounters reflectors whose

dimensions (d) are smaller than the ultrasound wavelength (ie, d << λ), scattering occurs.

• Scattering gives rise to the characteristic texture (echo texture) of the image seen within soft tissue.

• Bending of waves as they pass from one medium to other

• The change in wavelength and direction of propagation of sound occurs, but frequency remains constant

• Artifacts due to refraction are – Loss of resolution of image– Spatial distortion

Refraction

𝒔𝒊𝒏𝜽 𝒊

𝒔𝒊𝒏𝜽𝒕

=𝒗𝟏

𝒗𝟐

Snell’s Law

• Refers to conversion of ultrasound energy to heat energy

Absorption / Attenuation

Absorption / Attenuation

• Depends On– frequency of sound (increase absorption)– viscosity of medium (increase absorption)– relaxation time of medium– temperature (varies with tissues)

• The transmission of sound waves without much energy loss can be done by– using mineral oil b/n transducer and patient skin– mechanical impedance matching

• Any process that lessens the reflection. it is done by a coupler who has intermediate density b/n transducer and tissue• The thickness of this matching layer must be equal to

one fourth the wavelength of sound in the matching layer (Quarter-wave matching)

IMAGE DISPLAY

• Electronic representation of data generated from returning signals and displayed on TV monitor– A MODE– M MODE– Real time B MODE

AMPLITUDE MODE

• Echoes are displayed in the form of spikes on CRO traced along time base

• Amplitude of spike measures the echo size

• Information about the depth of the structures and the amplitude of the returning echo

AMPLITUDE MODE

• Used in – Ophthalmology– Echoencephalography– Echocardiography.

• Disadv:– 1D information, – takes lot of space in CRO

M-MODE & TM-MODE• Detects motion of

structures- cardiac valves and of cardiac chambers , vessels

• Echoes are displayed as dots of varying intensity

• M-Mode does not have Time factor.

• It also provides 1D information

• TM-Mode has Time factor

BRIGHTNESS MODE

• Echo signals as – Line of dots. • Intensity of dot gives relative size of echo

• It provides depth of information and variations in direction of beam

• System is fast enough to allow movements to be followed.

• At least 16 frames / sec

REAL TIME B MODE

• Display variation of amplitudes of echoes arising from tissues as varying shades of gray.

GRAY SCALE IMAGING

Controls

Controls

• Adjustment of image to obtain Optimal Gray scale image done using several controls:– TGC– Coarse Gain– Intensity governs the amplitude– Reject– Delay– Near Gain - diminish near echoes– Far Gain - enhance distant echoes

TGC• Time Gain Compensator• Compensates differences in echo

strength by adjusting the variations in degree of amplification

• The slope of the TGC adjusts the degree of amplification

• The delay control regulates the depth at which the TGC begins to augment weaker signals

Pulse Rate

• The number of separate little packets of sound that are sent out each second

• It determines the total number of echoes returning to the transducer in a unit of time.

• High pulse rate is desirable.• But then the receiving time decreases.• So, the pulse rate must be set to accommodate

the thickest part that might be examined.

Spatial Resolution

• In ultrasound, the major factor that limits the spatial resolution and visibility of detail is the volume of the acoustic pulse.

• The axial, lateral, and elevational (slice thickness) dimensions determine the minimal volume element.

• Each dimension has an effect on the resolvability of objects in the image.

Axial Resolution

• Also known as linear, range, longitudinal, or depth resolution

• Refers to the ability to discern two closely spaced objects in the direction of the beam.

• Achieving good axial resolution requires that the returning echoes be distinct without overlap.

• The minimal required separation distance between two reflectors is one-half of the spatial pulse length (SPL) to avoid the overlap of returning echoes, as the distance traveled between two reflectors is twice the separation distance.

Lateral Resolution

• Lateral resolution, also known as azimuthal resolution

• Refers to the ability to discern as separate two closely spaced objects perpendicular to the beam direction.

• For both single element transducers and multielement array transducers, the beam diameter determines the lateral resolution.

• Since the beam diameter varies with the distance from the transducer in the near and far field, the lateral resolution is depth dependent.

• The best lateral resolution occurs at the near field—far field face.

• At this depth, the effective beam diameter is approximately equal to half the transducer diameter.

• In the far field, the beam diverges and substantially reduces the lateral resolution.

• The typical lateral resolution for an unfocused transducer is approximately 2 to 5 mm.

• A focused transducer uses an acoustic lens (a curved acoustic material analogous to an optical lens) to decrease the beam diameter at a specified distance from the transducer.

Elevational Resolution

• The elevational or slice-thickness dimension of the ultrasound beam is perpendicular to the image plane.

• Slice thickness plays a significant part in image resolution, particularly with respect to volume averaging of acoustic details in the regions close to the transducer and in the far field beyond the focal zone.

• Elevational resolution is dependent on the transducer element height in much the same way that the lateral resolution is dependent on the transducer element width.

Scanning the Ultrasound Beam

Types:

• Mechanical Scanners• Electronic Array Scanners

Mechanical scanners• Early US systems relied on

the operator to manually change the position and orientation of the transducer and scan the ultrasound beam through a plane in the patient to obtain the echo data necessary for each image.

• 3 types:– Oscillating Transducer (Unenclosed type)– Oscillating Transducer (Enclosed type)– Rotating Wheel Transducer

Mechanical scanners

• This was a time-consuming process but produced images that covered large FOVs

• Image artefacts occurred if the patient moved during the acquisition

Mechanical scanners

Electronic array scanning

• Most modern US imagers automatically scan the ultrasound beam using transducers consisting of arrays of many narrow piezoelectric elements.

• The array may consist of as many as 128–196 elements

• Two modes of activation are used to produce a beam. – These are the “linear” (sequential) and “phased” activation/receive

modes.

Linear• The linear array scanners produce sound

waves parallel to each other and produces a rectangular image.

• The width of the image and number of scan lines are the same at all tissue levels.

• Often used with high frequencies ie 7MHz. • Advantage -good near field resolution.• Disadvantage is artifacts when applied to

a curved part of the body creating air gaps between skin and transducer.

Sector/Vector

• Produces a fan like image that is narrow near the transducer and increase in width with deeper penetration.

• The disadvantge is poor near field resolution.

Curved

• Often with frequencies of 2 - 5 MHz (to allow for a range of patients from obese to slender).

• The array of elements is arranged across a convex arc (instead of a straight line), which rapidly scans a larger FOV

ADVANCES

Endoscopic Ultrasound• Dedicated linear-array and radial

echo-endoscopes for structural evaluation of the luminal wall adjacent tissues.

• Flexible shaft with a central wire that drives rotation of a mechanical transducer at the tip.

• The transducer is surrounded by oil that serves as an acoustic interface with tissue,

providing 360-degree imaging

• With the introduction of the curved linear array echoendoscope , the indications for EUS have expanded.

• Allows the endoscopist to perform a whole range of interventional applications ranging from fine needle aspiration (FNA) of lesions surrounding the gastrointestinal tract to celiac plexus block and drainage of pancreatic pseudocyst

• Ultrasound imaging of vascular and non-vascular lumina employing miniaturized high-frequency (20-30 MHz) transducers inserted into catheters as small as 2 mm in external diameter

Recent Innovations in B-mode US

• Tissue Harmonic Imaging• Spatial Compound Imaging• Extended FOV Imaging• Coded Pulse Excitation• 3D and 4D imaging• Elastography.• Ultrasound contrast media.

Tissue Harmonic Imaging

• A musical note has three characteristics.– Pitch (or frequency)– Loudness– Quality (or tone).

• Quality is the audible difference heard between two musical notes of the same pitch and loudness.

• Hence, a piano based C-note does not sound the same as a C-note played on a guitar, due to the existence of harmonic frequencies.

• The same note from various instruments has different qualities because the sounds are not pure notes i.e. of one frequency and these integral multiples of the fundamental frequency (overtones) give an instrument its characteristic sound.

• Harmonics are frequencies that occur at multiples of the fundamental or transmitted sonographic frequency.

• In conventional gray-scale sonography, the same frequency spectrum that is transmitted into the patient is subsequently received to produce the sonographic image.

• In THI sonography, higher harmonic frequencies generated by propagation of the ultrasound beam through tissue are used to produce the sonogram.

Tissue Harmonic Imaging

Why harmonics?• The ultrasonic pulse gets altered with

time as it traverses the tissues with non-linear motion.

• The peaks within the pulse waveform move faster than the troughs because the propagation speed is higher in compressed regions of tissue than in the areas which are expanded by the passing pressure wave.

• The degree of such acoustic signal distortion in tissue depends on the amplitude of the emitted pulse and the distance it has travelled in the tissue.

Drawing of undistorted pressure wave (top) and of pressure wave after undergoing nonlinear propagation (bottom). Nonlinear effects cause high-pressure regions of sound wave to travel faster than low-pressure regions and result in progressive distortion of transmitted wave with generation of sound at higher harmonics of transmit frequency

• As much of the fundamental sonographic signal as possible must be removed to make these theoretical harmonic improvements a clinical reality.

• This can be done by either frequency- based or phase inversion methods.

• There are 2 basic harmonic imaging methods. – conventional frequency-based second-harmonic

imaging– pulse inversion or phase inversion harmonic imaging

Conventional Frequency-based Second-harmonic Imaging

• A narrowband pulse is emitted, and then high-pass or narrow-bandpass filtering is applied to the received echoes to filter out the fundamental echo components.

• This results in reduction of both spatial and contrast resolution.

Phase Inversion Harmonic Imaging• In wideband harmonic imaging

(also called pulse inversion or phase inversion harmonic imaging)

• A train of 2 pulses is emitted, with the phase of the second inverted relative to the phase of the first.

• When the echoes from the transmitted pulses are added, the linear components of the echoes cancel each other, whereas the nonlinear components are amplified.

• This results in superior contrast and spatial resolution.

Two phase-inverted but otherwise identical sonographic pulses are transmitted. Summing the returning echoes in a buffer cancels most of the fundamental and odd harmonic echoes and effectively amplifies the second harmonic.

1. Improve lateral resolution

2. Reduce side lobe artifacts

3. Improve signal to noise ratio

4. Improved near field and far field image quality

ADVANTAGES OF THI

ADVANTAGES OF THI

5. Lesions are clearer & better defined.6. Use of higher frequencies improves resolution.7. Helps differentiate cysts from hypoechoic solid

masses. 8. Better clarity of contents.9. It is superior to conventional USG in

visualization of lesions containing highly reflective tissues like fat, calcium & air.

10.Harmonic imaging is generally considered to be most useful for “technically difficult” patients with thick and complicated body wall

ADVANTAGES OF THI

• Better defining the borders of pancreas• Better visualization of lower pole of left kidney• Better visualization of fatty livers• Small renal cysts can be easily seen• Presence or absence of sludge in GB• Metastatic lesions in liver easily seen• Minimal fluid present between liver and kidney• Provides better images of aortic wall, IVC, portal vein

and renal arteries

THI is useful in

Spatial Compound Imaging• Electronic steering of ultrasound beams from

an array transducer is used to image the same tissue multiple times by using parallel beams oriented along different directions.

• The echoes from these multiple acquisitions (upto 9 sets) are then averaged together into a single composite image.

Disadvantage

• More time is required for data acquisition • The compound imaging frame rate is reduced

compared with that of conventional B-mode imaging.

Advantage

• Spatial compound images often show reduced levels of speckle, noise, clutter, and refractive shadows and improved contrast and margin definition

• Enhancement and shadowing artifacts may also be reduced, which may be an advantage or potential drawback, depending on the imaging situation.

Conventional image: acoustic clutter produces spurious echoes within a simple cyst (inset), which can be difficult to eliminate regardless of incident angle or adjustment of system parameters.

Compound image: compound Imaging results in significant clutter suppression in the cyst. Also note the good depth of field in this single focus compound image, comparable to the multi-focus conventional image.

Clutter Suppression

Extended FOV Imaging

• A benefit of early static B-mode scanners that was lost with the introduction of mechanical and electronic automatic scanning was large imaging FOVs.

• Extended FOV imaging, has sought to restore this capability. • The transducer is slowly translated laterally across the large

anatomic region of interest. During this motion, multiple images are acquired from many transducer positions.

• The registered image data are accumulated in a large image buffer and then combined to form the complete large FOV image

• Extended FOV images are not limited just to B-mode acquisitions.

• Restores the capability of visualizing large anatomic regions in a single image and simplifies measurements made over these large regions.

• It is useful in evaluating aneurysms, detecting small ligament and tissue damage, and to image and calibrate long sections of blood vessels, to locate anatomical landmarks for location of disease.

Coded Pulse Excitation

• A fundamental trade-off in US is that between imaging depth and spatial resolution.

• Coded ultrasound pulses help overcome this limitation, providing good penetration at the higher frequencies necessary for high spatial resolution.

• In this imaging approach, long ultrasound pulses are used instead of the very short pulses. These long pulses carry greater ultrasonic energy, increasing the energy of echoes that return from large depths in the patient.

The coded pulses are produced with a very specific, characteristic shape, and the resulting echoes will have a similar shape.

Conventional B-mode (a) and coded pulse (b) US images of the liver show the benefits of coded pulse imaging. The spatial resolution of the coded pulse image (b) is very comparable with that of the 13-MHz conventional image (a). However, the useful imaging depth is about 7.5 cm for the coded pulse image (b) compared with only about 2.8 cm for the conventional image (a).

The end result is an image with good echo signal and good spatial resolution at large depths.

ELASTOGRAPHY

• Dynamic technique that uses ultrasound to provide information on tissue stiffness by measuring the degree of distortion under the application of an external force - viscoelastic properties of tissue

• Surrogate for that obtained with manual palpation.• Principle: malignant tissues have more stiffness

and are harder to distort. On tissue compression strain (displacement) within harder tissue is less than in softer tissue.

Layers in Jello show soft and hard materials after light compression

Important Quantities

• Young’s modulus (E) describes longitudinal deformation in terms of strain

• The shear modulus (G) relates to transverse strain to transverse stress

• The bulk modulus (K) of elasticity describes the change in volume

• Poisson ratio (v) which is a ratio of transverse contraction per unit breadth divided by longitudinal extension per unit length

Shear and Young’s moduli, are the most suitable elasticity parameters to measure.

The tissue is insonified a) before and b) after a small uniform compression. In the harder tissues (e.g. the circular lesion depicted) the echoes will be less distorted than in the surrounding tissues, denoting thus smaller strain

Elastography - methods

• Three methods—a) spatial correlation method,b) the phase-shift tracking method, and c) the combined autocorrelation method (CAM)—

• have been introduced for measuring tissue strain at elastography.

Elastography - Uses

• In the liver it is a very useful method in depicting liver hardness and prediction and prognostication of liver cirrhosis.

• Breast imaging, cervix imaging, musculoskeletal imaging (especially supraspinatus and tendoachilles), thyroid lesions assessment and assessment of cervical nodes.

3D and 4D ultrasound

• 3D ultrasound - data set that contains a large number of 2D planes (B-mode images).

• This volume data can be manipulated in different planes by rotation.

• Dissected in any plane, to get multiplanar images (similar to multislice CT).

• Special probes and software are necessary in order to perform 3D and 4D imaging.

• 4D ultrasound is also known as "Real-time 3D Ultrasound". The 3D datasets with their multiplanar reformations and renderings in real time, give us photographic quality images.

Techniques of 3D US

• Data acquisition• Image reconstruction and • Image display

Data Acquisition Techniques

a) Tracked freehand systems, b) Untracked freehand systems, c) Mechanical assemblies, and d) 2D arrays

• Regardless of which method is used, one must know the relative position and angulation of each 2D image and must acquire the images rapidly or with gating to avoid motion artifacts. If these two criteria are not met, the 3D images may be inaccurate.

(a) Tracked Freehand Systems

• The operator holds an assembly composed of the transducer with an attachment and manipulates it over the anatomic area being evaluated

• Ensure that there are no significant imaging gaps.

• Acoustically tracked 3D scanning• Articulated-arm-tracked 3D scanning• Magnetic field-tracked 3D scanning

(b) Untracked Freehand

• 2D images are digitized as the operator moves the transducer with a smooth, steady motion

• Most convenient for the operator• Image quality is variable and depends largely on

how smoothly and steadily the operator moves the transducer

• Geometric measurements may be inaccurate because there is no direct information regarding the relative position of the digitized images.

• Mechanical movement of the transducer across the skin.

• Tilted about a fixed point on the skin surface.• Rotated about its own axis.

(c) Mechanical Assemblies

• The transducer is propelled or rotated mechanically, and 2D images are digitized at predetermined spatial or angular intervals .

• Cumbersome for the user• Improve the geometric accuracy • To date, their greatest utility has been in

intracavitary and intraluminal examinations, in which the area of interest is relatively small and motion artifact is less of a problem.

• Three different scanning techniques can be used: – linear scanning, – tilt scanning, and – rotational scanning.

(d) Two-dimensional Arrays

• With the first three types of data acquisition systems, mechanical motion is used to obtain 3D images.

• An alternative is to keep the transducer stationary and use electronic scanning with a 2D transducer array generates pyramidal or conical US pulses to generate 3D information in real time

Image Reconstruction Techniques

• Two types:– 3D surface model – Voxel-based volume model.

Three-dimensional Surface Model:

• Outline the boundaries of the areas of interest on the 2D images manually or with a computer algorithm

• Reduces the amount of 3D data needed• Shorter 3D reconstruction times and greater

efficiency• Identification of boundaries can be tedious

and time-consuming.

Voxel-based Volume Model

• The computer builds a 3D voxel-based volume (3D grid)

• This process preserves the original information

• Allows a variety of rendering techniques • Generates very large data files, which slows

processing and requires large amounts of computer memory.

Image Display Techniques

• Surface Rendering: operator identifies the boundaries of pertinent structures either manually or with an algorithm

• Multiplanar Reformatting: three orthogonal planes or texture mapping – image rotated to obtain the desired image orientation

• Combined Surface Rendering and Multiplanar Reformatting.

• Volume Rendering

Artefacts

• Respiratory motion artifact• Artifact caused by incorrect calibration

Advantages of 3D US

• Many benefits in obstetrical and gynaecological scanning where it is important to analyze anatomy. It has shown promising results in diagnosing cleft lip/palate, spina bifida, polydactyly, club foot, facial dysmorphism, low set ears ,fetal cardiac imaging.

• Excellent tool for demonstrating and accurately diagnosing congenital uterine anomalies, pelvic floor muscle and sphincters.

• Considerable interobserver variability in 2D; exact relationship between anatomic structures is accurately recorded in the 3D image

Advantages of 3D US

• Unrestricted access to an infinite number of viewing planes.

• Suited for monitoring the effects of therapy over a long period of time

• More accurate quantitative volume estimates

Limitations of 3D US

• More cumbersome , requiring more user input• Data archiving and communication more challenging• Waiting for the 3D image to appear can be frustrating

to users • Slow the image interpretation process, especially if

inexperienced, as the right algorithm needs to be chosen.

• The ability to obtain a good 3-D picture is very much dependent on operator skill, the amount of liquor, fetus position and the degree of maternal obesity.

Applications for 3D US

• Fetal Imaging

Applications for 3D US

• Gynecologic Imaging volume data sets to be acquired with both

transvaginal and transabdominal probes

Applications for 3D US

• Three-dimensional Power Doppler Imaging 3D US angiography • Prostate Imaging

Applications for 3D US

• Breast Imaging: demonstrate lesion margins and topography

• Biopsy-related Imaging: needle localization and guidance

ULTRASOUND CONTRAST MEDIA

• Microbubbles of air or other gases which act as echo enhancers

• Microbubbles are less than 10 µm in diameter, and

• Contrary to most other contrast media which are rapidly distributed to the extravascular, extracellular space, most microbubbles are confined to the vascular space.

• Microbubbles may produce up to 25 dB (more than 300-fold) increase in echo strength.

• Stability is increased by – external bubble encapsulation (galactose,

phospholipids, denatured albumin or poly-butyl-cyanoacrylate) with or without surfactants

– using gases with a low diffusion coefficient (perfluorocarbons) or

– a combination of both

• The gas components of the microbubbles are normally eliminated via the lungs.

• Stabilizing components are eliminated via the hepato-renal route.

Contrast Microbubbles

• The ideal diameter - 2 µm to 8 µm

• Enhancement life-time of the microbubble, often several minutes (8–10 min)

Microbubbles – Structure

• The coatings include albumin, gelatin, galactose microspheres, polyglutaminic acid, lipophilic monolayer surfactants, and lipid bilayers (liposomes).

• The gas inside the shell may be either air or various perfluorocarbons, which are liquids at room temperature but gas at body temperature.

The ideal USCA

• Non-toxic, • Injectable intravenously, • Capable of crossing the pulmonary capillary

bed after a peripheral injection, and • Stable enough to achieve enhancement for

the duration of the examination.

Microbubbles - Generations

• Several "generations" of gas microbubble contrast media have evolved;

• The "1st generation" products do not pass the pulmonary vascular bed, and are therefore limited to the venous system and the right heart cavities after injection.

• The "2nd generation" contrast media are both sufficiently small and stable to pass into the systemic circulation, and these contrast media enhance the doppler signal in various arteries after injection. They are short-lived, however, the effect is over in a few minutes.

Microbubbles - Generations

• The "3rd generation" gas microbubble contrast media are even more echogenic and stable, and are able to enhance the echogenicity of parenchyma on B-mode images. They may thus show perfusion, even in such a difficult region as the myocardium.

• The various gas microbubble contrast media are generally safe with low toxicity in humans.

Ultrasound Contrast Agents - Types

• Different types of ultrasound contrast agents: – tissue contrast agents; – contrast agents for vascular enhancement; – agents for targeted contrast imaging.

PRINCIPLES• The main mechanisms for signal enhancement are – backscattering, – bubble resonance and – bubble rupture.

• These mechanisms are highly dependent on the acoustic power of the transmitted ultrasound, which is reflected by the mechanical index (MI).

• very high reflectivity ( gas content makes them very reflective)

Acoustic Excitation of Ultrasound Contrast Agents

• The behavior of these agents under acoustic excitation fall into three classes (Frinking et al., 1999), depending on the structure of the microbubble and the level of the insonifying pressure amplitude and frequency:

– stable linear (low MI), – stable nonlinear scattering (medium MI)– transient nonlinear scattering (high MI).

Back Scattering

• At low acoustic power (MI < 0.1), gas microbubbles may be regarded as point scatterers, and the mechanism of ultrasound reflection is that of Rayleigh Tyndall scattering.

• The scattering strength of a point scatterer is proportional to the sixth power of the particle radius and to the fourth power of the ultrasound frequency; the echogenicity of such contrast media is therefore highly dependent upon particle size and transmit frequency.

• The backscattered intensity of a group of point scatterers is furthermore directly proportional to the total number of scatterers in the insonified volume; the concentration of the contrast medium is therefore also of importance.

MI- 0.1-0.3

Reflect and scatter

Bubble Resonance and Harmonics

• At intermediate acoustic power (0.1 < MI < 0.5) gas microbubbles may show strong oscillatory motion provided the frequency of the incident ultrasound is close to the resonant (fundamental) frequency of the microbubbles.

MICROBUBBLE AS LINEAR RESONATOR• By virtue of their compressibility, microbubbles display unique

properties in an ultrasound beam, which sets them into resonance when there is a match between their diameter and the ultrasonic wavelength which for microbubbles 2-7microns range at ultrasound frequency 2-10 Mhz.

MI- 0.5

Resonates

MICROBUBBLE AS NONLINEAR RESONATOR

• nonlinear behaviour (compression and expansion phases) produces echoes that contain frequencies not present in the transmitted pulses.

• An elegant way to extract these nonlinear signals is to send a series of pulses down each line, varying their phase and amplitude; the returning signals are combined to cancel the linear signals from tissue and the remaining bubble-specific signals are used to form an image that can be presented as a colour overlay on the B-mode image or shown on a side-by-side display, all in real-time.

MI- 0.7-0.8

Oscillates in non harmonic pattern

Bubble rupture• At high acoustic power (MI > 0.5), ultrasound at the microbubble

resonance frequency will cause the bubbles to rupture.• The result is a transient high-amplitude, broadband signal

containing all frequencies, not only the harmonics.• It will create a transient, strong signal in B-mode, or a short-lasting

multicoloured, mosaic-like effect in colour Doppler sonography.

• Several terms for the strong, transient signal have been proposed: induced or stimulated acoustic emission, loss of correlation imaging and sono-scintigraphy.

MI- =/>1

Disrupt and Burst

Applications

• Give angiographic capacity to ultrasound ie help in demonstrating tissue perfusion, bleeding points,vascularity of lesions etc.

• Widely used in imaging of solid organs, particularly the liver• Improvement in detection of colour Doppler signal from

large vessels• Microbubble contrast has also found a niche outside the

vascular compartment in the setting of vesico-ureteric reflux in children where a high sensitivity and specificity compared with conventional micturating cystourography (MCUG) has been demonstrated

Applications

• Gene therapy and targeted delivery of drugs

Conclusion

• Modern US equipment is based on many of the same fundamental principles employed in the initial devices used for human imaging over 50 years ago.

• US has the characteristics of being relatively inexpensive, portable, safe, and real-time in nature, all of which make it one of the most widely used imaging modalities in medicine.

• In short, US science, technology, and applications are expanding at a brisk pace and are far from mature.

• Even more exciting developments are on the horizon.