BASICS OF ULTRASOUND

IntroductionIntroduction

In this section, a brief history and general overview of ultrasound diagnosis are described. Also the merits and demerits of medical ultrasound as well as a comparison with other techniques like CT and MR are outlined.

Ultrasound has been used as a navigational and detection aid by the bat for millions of years. It was not until the second world war, however, that man started extensive use of ultrasound for the same purpose. With the enormous potential of military research programs, ultrasound technology rapidly developed.

Although ultrasound had already been used in the therapy and was proposed by S.Y. Sokolov for diagnostic use in 1937, no successful attempt to apply the ultrasound echo-sounder principle to medical diagnosis was made until the early 1950s.

Most of the equipment used at that time were industrial-type ultrasound devices for detecting flows in metal, but soon ultrasonic devices generally known as ultrasonoscopes specifically intended for medical diagnostics were developed. The major advantages of these devices are the non-invasive and non-ionizing nature of the examination and their relatively low cost when compared to X-Ray, Magnetic Resonance (MR), CT and Isotopic Scanning techniques.

Over the last decade, the diagnostic usefulness of the equipment has been vastly improved, as better instruments were developed and more clinical experience gained, and in several diagnostic fields, ultrasound technique has shown to be superior to other methods.

History

In Medical Ultrasound, images representing human organs are formed by transmitting soundwaves into the body and receiving back and processing the resultant echoes from the tissues.To accomplish this, medical ultrasound uses a process very similar to an ocean-going vesselsdepth sounding equipment or oceanic survey equipment. All of these systems make use of sound waves and their reflections.

General Overview

Sea

Merits and Limitations

By comparison with CT, MR, X-Ray and other diagnostic methods, UltrasoundDiagnosis, especially for soft tissues and moving organ like heart and bloodflow, has shown great advantages as following: * Real Time Imaging (Except MR) * Non-invasive (Except MR) * Non-ionizing Radiation (Except MR) * Relatively Low Cost * Wide Applications * Mobility * Flexible Imaging * Biopsy

For the reason of physical and technological limitations, ultrasound method also suffers from restrictions in imaging and applications as does other technique.Apart from the geometric distortion of the image display, another important limitation is the Resolution. Higher frequency ultrasound gives better resolution, but attenuationin the tissue also increases with increased frequency. Therefore, a compromise has to be made between resolution and penetration depth.

Frequency Low High

Resolution Better

Penetration Better

Due to the nature of ultrasound propagation, strong reflection of ultrasound beam fromboundaries between tissue and air or boundaries between tissue and bone prohibitnormal scanning of the lungs and the intracranial soft structure in adults as well as tosome extent the intestines.

Generally, ultrasound diagnosis, being one of the diagnostic imaging, is playing amore and more important role in Medicine as technology is rapidly developing, andis selected for a screening diagnosis as a first and a finished Diagnostic Imagingin the world.

Physics and PrinciplesPhysics and Principles

In this section, some basic concepts are defined and explained as foundationalknowledge to introduce and understand ultrasound system.

* Properties of Propagation - Velocity and Frequency - Reflection - Refraction - Diffraction - Scattering - Attenuation etc..* Transducer and Impedance Matching* Doppler Effect* Pulse Ultrasound

- Mechanical vibration or wave

- With frequencies above the range of human ear which is greater than 20 kHz. For medical diagnosis, typically ranging from 1 to 30 MHz.

- Obeys the same physical laws as wave

The Nature of Ultrasound

Compressive Wave

Diagnostic Imaging

0 20 Hz 20 kHz 1 MHz 30 MHz

Infrared Audible NDTSound Sound Cleaning

Sound Spectra

Velocity- Dependent on the medium and temperature

- Roughly be considered as a constant 1540 m/s in human body.

- The relation between velocity and frequency is following the equation below:

Velocity = Frequency * Wavelength ( )

Table 1. Approximate velocities of sound in human medium

Medium Velocity (m/s)

Blood 1570

Brain 1540

Fat 1450

Kidney 1560

Muscle 1590

Distilled Water 1540

Specific Acoustic Impedance

- The specific acoustic impedance Z is defined as the product of the density of a medium and the velocity of the sound in that medium (c).

- Basic concept to understand ultrasound wave reflection.

Reflection

Medium 1 Medium 2

Transmitted waveIncident wave

Reflected wave

Reflection

- One of the basic principles of medical ultrasound diagnosis.- Occurs at areas of acoustic impedance mismatch.- Divided into several different types including: Specular Reflections, which occur at large change in impedance producing a large reflection, and also reducing the continuing wave amplitude.

Medium Reflections, which occur with dense tissues such as muscle.

Diffuse Reflections, which occur with soft tissues such as liver.

Refraction

Medium 1

Medium 2

incident wave Reflected wave

Transmitted wave

When a propagating ultrasound wave encounters a Specular interface at an oblique angle, it is Refracted in the same way that light is refracted througha lens. The portion of the wave that is not reflected continues into the second medium. It is dependent on the velocities of thetwo medium. If the velocities are equal,There would be no refraction occurredand the beam goes straight into the second medium. For the velocities of the different tissues in the human body are quite close, refraction's can be ignored.

Diffraction

DiffractingObject

If an ultrasound beam passes an obstacle within a distance of 1 or 2 wavelengths, its direction of propagation is deflected by diffraction as shown in the figure. The closer the beam is to the diffracting object, the greater the deflection is.

1 or 2 wavelengths

Deflecting beam

Scattering

Spherical Scatter-wave- Occurs when small particles absorb part of the ultrasound energy and re-radiate it in all directions as a spherical field. This means that the transducer can be positioned at any angle to the ultrasound beam and still receive echoes back. Scattering allows reflections from objects even smaller than the wavelength. Many biological interfaces have irregular surfaces, tending to give scatter-like reflection, which is quite useful, as it will give at least some echoes even though the beam is not directly perpendicular to the reflecting interface.

Backscatter

Backscatter or Rayleigh scattering occurs with structures smaller than the transmitted wavelength. Reflected energy is very low, but contributes to the texture of the image.

AttenuationAttenuation of ultrasound wave occurs when it is propagating through the medium. Loss of propagating energy will be in the form of heat absorbed by the tissue, approximately 1 dB/cm/MHz,or caused by wavefront dispersion or wave scattering.

Transducer

The transducer is the component which, when connected tothe ultrasound equipment, transmits the ultrasound and receives its reflections or echoes from tissues.

Transducer is one of the most important component of the ultrasound system. For more detail information, please referto System Components.

Matching Layer

TransducerCrystal

Tissue

Impedance Matching

TransducerCase

-To transmit as much power as possible from transducer to the tissue.

Doppler EffectIn ultrasound Imaging, echoes received from most tissues will be at the same frequency as the transmitted beam. However, if echoes received are from tissues or blood cells that are moving, the transmitted and received frequencies will not be the same. This shifted frequency can be used to determine the relative velocity and the direction of this moving tissues. This effect is known as the Doppler Principle. Essentially, the greater the frequency shift, the higher the velocity of the moving object. Additionally, movement toward the transducer results in a higher received frequency, and movement away in a lower received frequency.

Doppler Effect

TXM

RCV RCV

If the reflector is moving toward the transmitter, the received frequency will be higher than the transmitfrequency.

If the reflector is traveling awayfrom the transmitter, the received frequency will be lower than thetransmit frequency.

TXM

Pulse Ultrasound

For practical use, most modern ultrasound systems are designed based on the principle of pulse-echo technique, which means that transducer emits only a few cycles of pulses at a time into the human body. When encountering tissues interfaces, reflection and scattering will occur and produce pulse echoes, By detecting these echoes, tissue positioning and identification as well as diagnosis can be made.

Spectral Doppler

Spectral Doppler, of high value in ultrasound diagnosis, can be used for evaluation of blood flow, includes three kinds:

- Pulse Doppler(PW) - High Pulse Repetition Frequency Pulse Doppler (HPRF) - Continuous Wave Doppler (CW).

Pulse Doppler

Transducer

Pulsed Doppler Line

In Pulse Doppler, a single ultrasound lineis repeatedly fired. Echoes reflected from moving structure, including blood cells, experience a Doppler shift in frequency.Using the Doppler equation, the echo information obtained within the Sample Volume is analyzed for shifted frequency content and amplitude, rather than transmitfrequency amplitude. From this, the bloodvelocity can be determined.

SampleVolume

Pulse Doppler

Time

Velocity

In order to obtain enough data to calculate the frequency componentsof the sampled volume, many ultrasound lines must be fired.

The frequency data is converted to velocity, and displayed in a scrollingstrip format on the monitor.

The highest detectable velocity islimited by one half of the rate at which the ultrasound lines are fired, known as *Nyquist Limit .

Pulse Repetition Frequency T T T T

R R R R

Pulse RepetitionPeriod

* Pulse Repetition Frequency (PRF) is the number of times per second that transducer transmits a pulse.

* Pulse Repetition Frequency is dependent on transmit depth and propagation velocity. ( 1540 m/s )

Nyquist Limit

The maximum Doppler shift velocity measurable in Pulse Doppler is limited to onehalf the sampling rate defined by the PRF, which is mainly determined by the samplingdepth. For a given transducer and depth, this maximum measurable velocity, which isknown as the Nyquist Limit, can be calculated using the following equation:

PRF Nyquist Limit = 2

Aliasing

If the maximum velocity for that transducer and depth exceeds the Nyquist limit, aphenomenon known as Aliasing occurs. Aliasing results in the display of erroneousvelocity information.(Showing a wraparound effect.)

Velocity

2

0

-2

Spectral Display Showing Aliasing

HPRF Doppler

The Nyquist limit will decrease with depth of the sample volume. As this limit reaches 1/2 PRF, the system automatically increases the sample rate by increasing the PRF and the number of samples. This results in more than one wavefront propagating through the body simultaneously. Therefore, information obtained may be from more than one location.

Transducer

Angle of Incidence

When the motion of the object and the transmittedbeam are not parallel, it is necessary to correct for the angular difference. Motion that occurs at an angle to the beam axis will result in a decreasein the magnitude of the frequency shift and a lowercalculated velocity. Therefore, the transmitted beam needs to be parallel to the flow for the most accurate velocity. An equation is used to correct for the angle offset. The transducer receives only the componentparallel to the beam Vcos .

Ultrasound Beam

Blood FlowV

Continuous Wave Doppler

Continuous Wave Doppler, or CW Doppler, is a similar modality to Pulse Doppler in that frequencydata is gathered to determine blood velocity along the ultrasound line. With CW Doppler, the transmit and receive functions happen simultaneously. This overcomes the maximum velocity limit, but the exact point along the ultrasound line from which the velocity data originated can not be determined.(No range resolution).

CW Doppler is used primarily in diagnosing abnormalities in which range resolution is not importantor when the sonographer is interested in the quantification of high velocity jets.

CW PW

Range Resolution None Determined by Sample Volume Maximum Velocity Virtually Unlimited Limited by 1/2 PRF

Continuous Wave Doppler

Time

Velocity

Transducer

Monitor

Color Flow Mapping

Color Flow Mapping (CFM) combines B-mode image format and Pulsed Doppler to provide a two dimensional representation of blood flow in Real Time.

The Doppler ultrasound lines, like B-mode lines, are sequentially scanned through the frame. Multiple range gates are taken along the Doppler lines. The calculated velocity data is assigned a color to represent a certain velocity and direction, and then displayedcombining with the B-mode image at the original location.

+ =Blood Flow 2 - D CFM

MTI (Moving Target Indicator)

Blood Cell

Blood Cell

Ultrasound Line

Ultrasound Line

First

Second

Subduction Blood Cell Signal

CFM Display

Transducer

Doppler Ultrasound Line

Color Box

Monitor

Limitations of CFM

- Only give the average of the velocity across the beam, can not get the maximum velocity.- Sensitivity, a compromise to be made among the depth, velocity range, and PRF- Frame Rate, influenced by FOV , scan angle and control system of transmit and receive.

In General, a ultrasound imaging system consists of several components listed below:

* Transducer - Transmitting and receiving ultrasound * Beamfomer - Transmit and receive control including phase delay, focus, aperture, signal conversion, etc..

System Overview

Phased Array

In a phased array system, a series of elements are arranged into a array. Thetiming of the transmit drive pulses to each element and are arranged so that thewavefronts from all the transmitting elements arrive at a selected spatial pointat the same time. This is accomplished by introducing a curve into the timingdelays whose center is the desired focal point. This in effect is the same as using an acoustic lens, as a lens implements focus by delaying waves to a specific degree so that the same result is achieved. Using electronic instead of physical delay allows the transmit focal point to be changed simply by changingthe delay relationships.

Transducer

Phased Array

Time Delay

Focal Point

Wavefront from Elements

SummationWavefront

Linear Array

- Rectangular Scan Format- Large Aperture- Wide View at Near Field- Smaller Effect of Side Lobe

Convex Format

- Wide View at Near and Far Field

Sector Format

- Radial Pie-shaped Scan Format- Narrow Aperture- Wide View at Far Field

Expand or Vector Format

- Wide View at Near and Far Field

Beamformer

Conventional Beamformer

AnalogSummation

A-D DelayControl

Pre- AmplifierArray

Transmit/Receive

Analogor Digital

Digital Beamformer

DigitalSummation

Detector DigitalControl

Pre- AmplifierArray

A-D Converter

Digital Beamformer

Digital Transmit- Dynamic Focus Software Control

Sum DetectorDigital Tracking Lens- Dynamic Focus- Dynamic Aperture

MemoryCine

Advantages of the Digital Beamformer

- Optimize Frequency and Band Control- Multi Focal Zone at Any Depth- Dynamic Receiving Focusing- Dynamic Tracking Lens- Sidelobe Compression- Dynamic Aperture- Higher Doppler Sensitivity

Clinical Merits

* Better Image Quality - Assure Accurate Spatial Resolution - Improve Contrast Resolution - High Frame Rate - Decrease Artifacts

* More Reliable Diagnosis - More Accurate Detection and Analysis in CFM and Pulse Doppler - More Sensitivity

Image QualityImage Quality

Introduction

* Resolution - Spatial Resolution (Lateral and Axial Resolution) - Contrast Resolution* Uniformity* Effects on Image Quality* Phantoms

In this section, several parameters listed below, which are usually used to judge the image quality of ultrasound system are introduced and, for general purpose, some examples about the effects on the image quality are given.

Spatial Resolution

Spatial Resolution is defined as the ability to distinguish small structures with clarity.Generally speaking, it can be divided into Lateral and Axial Resolution, and it is dependent on the numbers of the channels of the system and the frequency used. The more channels , the better lateral resolution is. The higher frequency used, the better axial resolution is.

Lateral Resolution

Lateral

Lateral Resolution is the ability of the system to resolve structures that are very close to one another at the same depth.

Lateral Resolution is dependent on the beam width as determined by crystal geometry, depthand focusing.

Lateral Resolution

Beam width is narrow enough to be able to resolve these two structures separately.

Beam width is too wide to be able to resolve these two structures separately.

Axial Resolution

Axial Resolution is the ability of the system to resolve structure that arevery close to one another at different depth.

Axial Resolution is dependent on frequency and transmit pulse width. Axial

3.5 MHz 0.4 mm5.0 MHz 0.3 mm7.5 MHz 0.2 mm10 MHz 0.1 mm

Axial Resolution

Good Axial Resolution

Poor Axial Resolution

Beam

Contrast Resolution

Contrast Resolution, or gray scale resolution, is the ability to differentiate tissue types and to see subtle structures in the presence of very bright reflectors. It is one of the most important parameters to judge ultrasound image quality, and also this tissue-differentiating capability provides critical diagnostic information.

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