Ultrasound. Introduction Ultrasound is a non-ionizing method which uses sound waves of frequencies (2 to 10 MHz) exceeding the range of human hearing.

Ultrasound Ultrasound

IntroductionIntroduction

Ultrasound is a non-ionizing method which uses sound waves of Ultrasound is a non-ionizing method which uses sound waves of frequencies (2 to 10 MHz) exceeding the range of human hearing for frequencies (2 to 10 MHz) exceeding the range of human hearing for imaging imaging Medical diagnostic ultrasound uses ultrasound energy and the acoustic Medical diagnostic ultrasound uses ultrasound energy and the acoustic properties of the body to produce an image from stationary and moving properties of the body to produce an image from stationary and moving tissues tissues Ultrasound is used in pulse-echo format, whereby pulses of ultrasound Ultrasound is used in pulse-echo format, whereby pulses of ultrasound produced over a very brief duration travel through various tissues and produced over a very brief duration travel through various tissues and are reflected at tissue boundaries back to the source are reflected at tissue boundaries back to the source

IntroductionIntroduction

Returning echoes carry the ultrasound information that is used to Returning echoes carry the ultrasound information that is used to create the sonogram or measure blood velocities with Doppler create the sonogram or measure blood velocities with Doppler frequency techniques frequency techniques Along a given beam path, the depth of an echo-producing structure is Along a given beam path, the depth of an echo-producing structure is determined from the time between the pulse-emission and the echo determined from the time between the pulse-emission and the echo return, and the amplitude of the echo is encoded as a gray-scale value return, and the amplitude of the echo is encoded as a gray-scale value In addition to 2D imaging, ultrasound provides anatomic distance and In addition to 2D imaging, ultrasound provides anatomic distance and volume measurements, motion studies, blood velocity measurements, volume measurements, motion studies, blood velocity measurements, and 3D imaging and 3D imaging

1. Characteristics of Sound1. Characteristics of SoundFrequencyFrequency

Frequency (f) Frequency (f) is the number of times the wave oscillates is the number of times the wave oscillates through a cycle each second (sec) (Hertz: Hz or cycles/sec) through a cycle each second (sec) (Hertz: Hz or cycles/sec)

Infra sound < 15 Hz Infra sound < 15 Hz Audible sound ~ 15 Hz - 20 kHz Audible sound ~ 15 Hz - 20 kHz Ultrasound > 20 kHz; for medical usage typically 2-10 MHz with Ultrasound > 20 kHz; for medical usage typically 2-10 MHz with specialized ultrasound applications up to 50 MHz specialized ultrasound applications up to 50 MHz

period (period () ) - the time duration of one wave cycle: - the time duration of one wave cycle: = 1/f = 1/f

1. Characteristics of Sound1. Characteristics of SoundSpeedSpeed

TheThe speed or velocity speed or velocity of sound is the distance traveled by the wave of sound is the distance traveled by the wave per unit time and is equal to the wavelength divided by the period per unit time and is equal to the wavelength divided by the period (1/f) (1/f)

speed = wavelength / period speed = wavelength / period speed = wavelength x frequency speed = wavelength x frequency c = c = f f c [m/sec] = c [m/sec] = [m] [m] ** f [1/sec] f [1/sec]

Speed of sound is dependent on the propagation medium and varies Speed of sound is dependent on the propagation medium and varies widely in different materials widely in different materials

1. 1. CharacteristicsCharacteristics of Sound of Sound Speed Speed

A highly compressible medium such as air, has a low speed of sound, A highly compressible medium such as air, has a low speed of sound, while a less compressible medium such as bone has a higher speed while a less compressible medium such as bone has a higher speed of sound of sound The difference in the speed of sound at tissue boundaries is a The difference in the speed of sound at tissue boundaries is a fundamental cause of contrast in an ultrasound image fundamental cause of contrast in an ultrasound image

c.f. Bushberg, et al. The Essential Physics of Medical c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., p. 472. ed., p. 472.

1. Characteristics of Sound1. Characteristics of Sound Wavelength, Frequency and Speed Wavelength, Frequency and Speed

The ultrasound frequency is The ultrasound frequency is unaffected by changes in sound unaffected by changes in sound speed as the acoustic beam speed as the acoustic beam propagates through various media propagates through various media Thus, the ultrasound wavelength is Thus, the ultrasound wavelength is dependent on the medium (c = dependent on the medium (c = f ) f ) A change in speed at an interface A change in speed at an interface between two media causes a between two media causes a change in wavelength change in wavelength Higher frequency sound has Higher frequency sound has shorter wavelength shorter wavelength



Ultrasound wavelength determines the Ultrasound wavelength determines the spatial resolution achievable along the spatial resolution achievable along the direction of the beam direction of the beam A high-frequency ultrasound beam A high-frequency ultrasound beam (small wavelength) provides superior (small wavelength) provides superior resolution and image detail than a low-resolution and image detail than a low-frequency beam frequency beam

However, the depth of beam However, the depth of beam penetration is reduced at high penetration is reduced at high frequency and increased at low frequency and increased at low frequenciesfrequencies



For thick body parts (abdomen), a lower For thick body parts (abdomen), a lower frequency ultrasound wave is used (3.5 frequency ultrasound wave is used (3.5 to 5 MHz) to image structures at to 5 MHz) to image structures at significant depth significant depth For small body parts or organs (thyroid, For small body parts or organs (thyroid, breast), a higher frequency is employed breast), a higher frequency is employed (7.5 to 10 MHz) (7.5 to 10 MHz)


D63. D63. The wavelength of a 2 MHz ultrasound beam is ________ mm. The wavelength of a 2 MHz ultrasound beam is ________ mm. A. 0.02 A. 0.02 B. 0.55 B. 0.55 C. 0.77 C. 0.77 D. 2.0 D. 2.0 E. 5.0 E. 5.0 CC Wavelength λWavelength λ = c / f = c / f The average velocity in tissue is 1540 m/secThe average velocity in tissue is 1540 m/sec.. Frequency = 2 x 10Frequency = 2 x 1066 /sec /sec = 1540 m/sec / 2 x 10= 1540 m/sec / 2 x 1066 /sec = 770 x 10 /sec = 770 x 10-6-6 m = 0.77 mm m = 0.77 mm

1. Characteristics of Sound1. Characteristics of SoundPressure, Intensity and the dB scalePressure, Intensity and the dB scale

The amplitude of a wave is the size of the wave displacement The amplitude of a wave is the size of the wave displacement Larger amplitudes of vibration produce denser compression bands and, Larger amplitudes of vibration produce denser compression bands and, hence, higher intensities of sound hence, higher intensities of sound Intensity of ultrasound Intensity of ultrasound

is the amount of power (energy per unit time) per unit area is the amount of power (energy per unit time) per unit area proportional to the square of the pressure amplitude, I proportional to the square of the pressure amplitude, I P P22 units of milliwatts/cmunits of milliwatts/cm22 or mW/cm or mW/cm2 2

is measured in decibels (dB) as a relative intensity is measured in decibels (dB) as a relative intensity dB = 10 logdB = 10 log1010 (I (I22/I/I11) or dB = 20 log) or dB = 20 log1010 (P (P22/P/P11) since I ) since I P P22

II11 and I and I22 are intensity values are intensity values

PP11 and P and P22 are pressure or amplitude variations are pressure or amplitude variations

1 B = 10 dB where B is bels 1 B = 10 dB where B is bels




Example: Calculate the remaining intensity of a 100-mW ultrasound Example: Calculate the remaining intensity of a 100-mW ultrasound pulse that loses 30 dB while traveling through tissue. pulse that loses 30 dB while traveling through tissue.

Relative Intensity (dB) Relative Intensity (dB)

mW1.0mW100x001.0mW 100

10

mW 100log3

mW 100log10 dB 30

log10

2

23

2

2

1

2

I

I

I

I

I

I

1. Characteristics of Sound – Key Points1. Characteristics of Sound – Key Points

Propagation of sound - Key Points Propagation of sound - Key Points Speed of sound is dependent on the medium Speed of sound is dependent on the medium Ultrasound frequency is independent of the medium, and does Ultrasound frequency is independent of the medium, and does not change not change Wavelength changes with the changes of speed Wavelength changes with the changes of speed c [m/sec] = c [m/sec] = [m] [m] ** f [1/sec] f [1/sec]

For most calculations, the average speed of sound in soft tissue, For most calculations, the average speed of sound in soft tissue, 1540 m/sec, should be assumed 1540 m/sec, should be assumed

In air = 330 m/sec and in fatty tissue = 1450 m/sec In air = 330 m/sec and in fatty tissue = 1450 m/sec dB = 10 logdB = 10 log1010 (I (I22/I/I11) )

2. Interactions of Ultrasound with Matter2. Interactions of Ultrasound with Matter

Ultrasound interactions are determined by the acoustic properties of Ultrasound interactions are determined by the acoustic properties of matter matter As ultrasound energy propagates through a medium, interactions that As ultrasound energy propagates through a medium, interactions that occur include occur include

reflection reflection refraction refraction scattering scattering Absorption (attenuation)Absorption (attenuation)

2. Interactions of Ultrasound with Matter2. Interactions of Ultrasound with MatterAcoustic ImpedanceAcoustic Impedance

Acoustic Impedance, Z Acoustic Impedance, Z is equal to density of the material times speed of sound in the is equal to density of the material times speed of sound in the material in which ultrasound travels, material in which ultrasound travels, Z = Z = c c

= density (kg/m= density (kg/m3)3) and c = speed of sound (m/sec) and c = speed of sound (m/sec) measured in rayl (kg/mmeasured in rayl (kg/m22sec) sec) Air and lung media have low values of Z, whereas bone and metal Air and lung media have low values of Z, whereas bone and metal have high values have high values Large differences in Z (air-filled lung and soft tissue) cause Large differences in Z (air-filled lung and soft tissue) cause reflection, small differences allow transmission of sound energy reflection, small differences allow transmission of sound energy The differences between acoustic impedance values at an The differences between acoustic impedance values at an interface determines the amount of energy reflected at the interfaceinterface determines the amount of energy reflected at the interface


2. Interactions of Ultrasound with Matter2. Interactions of Ultrasound with MatterReflectionReflection

A portion of the ultrasound beam is reflected at tissue interface A portion of the ultrasound beam is reflected at tissue interface The sound reflected back toward the source is called an echo and The sound reflected back toward the source is called an echo and is used to generate the ultrasound image is used to generate the ultrasound image The percentage of ultrasound intensity reflected depends in part The percentage of ultrasound intensity reflected depends in part on the angle of incidence of the beam on the angle of incidence of the beam As the angle of incidence increases, reflected sound is less likely As the angle of incidence increases, reflected sound is less likely to reach the transducer to reach the transducer


2. Interactions of Ultrasound with Matter2. Interactions of Ultrasound with MatterReflectionReflection

Sound reflection occurs at tissue boundaries with differences in Sound reflection occurs at tissue boundaries with differences in acoustic impedance acoustic impedance

The intensity reflection coefficient, R = IThe intensity reflection coefficient, R = Irr/I/Iii = ((Z = ((Z2 2 – Z– Z11)/(Z)/(Z2 2 + Z+ Z11))))2 2

The subscripts 1 and 2 represent tissues proximal and distal to The subscripts 1 and 2 represent tissues proximal and distal to the boundary. the boundary. Equation only applies to normal incidence Equation only applies to normal incidence The transmission coefficient = T = 1 – R The transmission coefficient = T = 1 – R T = (4ZT = (4Z11ZZ22)/(Z)/(Z11+Z+Z22))2 2


Diagnostic QuestionDiagnostic Question

D54.D54. Approximately what fraction of an ultrasound beam is reflected Approximately what fraction of an ultrasound beam is reflected from an interface between two media with Z values of 1.65 and 1.55? from an interface between two media with Z values of 1.65 and 1.55? A. 1/2 A. 1/2 B. 1/10 B. 1/10 C. 1/100 C. 1/100 D. 1/500 D. 1/500 E. 1/1000 E. 1/1000 R = R = ((Z((Z2 2 – Z– Z11)/(Z)/(Z2 2 + Z+ Z11))))2 2

= (1.65-1.55)= (1.65-1.55)22 / (1.65 + 1.55) / (1.65 + 1.55)22 = 1/1024 = 1/1024

D58.D58. Ultrasound moves with the highest velocity in: Ultrasound moves with the highest velocity in: medium medium ZZ

A. A. Fat Fat 1.38 1.38 B. B. Blood Blood 1.61 1.61 C. C. Muscle Muscle 1:70 1:70 D. D. Bone Bone 7.807.80

Diagnostic QuestionDiagnostic Question

2. Interactions of Ultrasound with Matter2. Interactions of Ultrasound with MatterTissue reflectionsTissue reflections

Air/tissue interfaces reflect virtually all of the incident ultrasound beam Air/tissue interfaces reflect virtually all of the incident ultrasound beam Gel is applied to displace the air and minimize large reflections Gel is applied to displace the air and minimize large reflections Bone/tissue interfaces also reflect substantial fractions of the incident Bone/tissue interfaces also reflect substantial fractions of the incident intensity intensity Imaging through air or bone is generally not possible Imaging through air or bone is generally not possible

The lack of transmissions beyond these interfaces results in an area The lack of transmissions beyond these interfaces results in an area void of echoes called shadowing void of echoes called shadowing

In imaging the abdomen, the strongest echoes are likely to arise from gas In imaging the abdomen, the strongest echoes are likely to arise from gas bubbles bubbles Organs such as kidney, pancreas, spleen and liver are comprised of Organs such as kidney, pancreas, spleen and liver are comprised of subregions that contain many scattering sites, which results in a speckled subregions that contain many scattering sites, which results in a speckled texture on images texture on images Organs with fluids such as bladder, cysts, and blood vessels have almost Organs with fluids such as bladder, cysts, and blood vessels have almost no echoes (appear black)no echoes (appear black)

2. Interactions of Ultrasound with Matter2. Interactions of Ultrasound with MatterRefractionRefraction

Refraction is the change in direction of an ultrasound beam when Refraction is the change in direction of an ultrasound beam when passing from one medium to another with a different acoustic passing from one medium to another with a different acoustic velocity velocity

Wavelength changes causing a change in propagation Wavelength changes causing a change in propagation direction (c = direction (c = f)f)sin(sin(tt) = sin() = sin(ii) ) ** (c (c22/c/c11), Snell’s law; ), Snell’s law; for small for small ≤ 15 ≤ 15oo: : tt = = ii ** (c (c22/c/c11) ) When When cc22 > c > c11, , tt > > ii , , When When cc11 > c > c22, , tt < < ii Ultrasound machines assume straight line propagation, and Ultrasound machines assume straight line propagation, and refraction effects give rise to artifacts refraction effects give rise to artifacts

c.f. Bushberg, et al. c.f. Bushberg, et al. The Essential Physics The Essential Physics of Medical Imaging, 2of Medical Imaging, 2ndnd ed., p. 478.ed., p. 478.

2. Interactions of Ultrasound with Matter2. Interactions of Ultrasound with MatterScatterScatter

Acoustic scattering arises from objects within a tissue that are about the Acoustic scattering arises from objects within a tissue that are about the size of the wavelength of the incident beam or smaller, and represent a size of the wavelength of the incident beam or smaller, and represent a rough or nonspecular reflector surface rough or nonspecular reflector surface

As frequency increases, the non-specular (diffuse scatter) As frequency increases, the non-specular (diffuse scatter) interactions increase, resulting in an increased attenuation and loss interactions increase, resulting in an increased attenuation and loss of echo intensity of echo intensity Scatter gives rise to the characteristic speckle patterns of various Scatter gives rise to the characteristic speckle patterns of various organs, and is important in contributing to the grayscale range in the organs, and is important in contributing to the grayscale range in the imageimage

c.f. Bushberg, et al. c.f. Bushberg, et al. The Essential Physics The Essential Physics of Medical Imaging, of Medical Imaging, 22ndnd ed., p. 480. ed., p. 480.

2. Interactions of Ultrasound with Matter2. Interactions of Ultrasound with MatterAttenuationAttenuation

Ultrasound attenuation, the loss of energy with distance traveled, is Ultrasound attenuation, the loss of energy with distance traveled, is caused chiefly by scattering and tissue absorption of the incident beam caused chiefly by scattering and tissue absorption of the incident beam (dB) (dB) The intensity loss per unit distance (dB/cm) is the attenuation The intensity loss per unit distance (dB/cm) is the attenuation coefficient coefficient

Rule of thumb: attenuation in soft tissue is approx. Rule of thumb: attenuation in soft tissue is approx. 1 dB/cm/MHz 1 dB/cm/MHz The attenuation coefficient is directly proportional to and increases The attenuation coefficient is directly proportional to and increases with frequency with frequency

Attenuation is medium dependentAttenuation is medium dependent

2. Interactions of Ultrasound with Matter – Key Points2. Interactions of Ultrasound with Matter – Key Points

Acoustic Impedance, Z Acoustic Impedance, Z is equal to density of the material times speed of sound in the is equal to density of the material times speed of sound in the material in which ultrasound travels, material in which ultrasound travels, Z = Z = c c

= density (kg/m= density (kg/m3)3) and c = speed of sound (m/sec) and c = speed of sound (m/sec) As ultrasound energy propagates through a medium, interactions that As ultrasound energy propagates through a medium, interactions that occur include occur include

reflection reflection refraction refraction scattering scattering Absorption (attenuation)Absorption (attenuation)

3. Transducers3. Transducers

A transducer is a device that can A transducer is a device that can convert one form of energy into convert one form of energy into another another Piezoelectric transducers convert Piezoelectric transducers convert electrical energy into ultrasonic electrical energy into ultrasonic energy and vice versa energy and vice versa

Piezoelectric means pressure Piezoelectric means pressure electricity electricity



High-frequency voltage High-frequency voltage oscillations are produced by a oscillations are produced by a pulse generator and are sent to pulse generator and are sent to the ultrasound transducer by a the ultrasound transducer by a transmitter transmitter The electrical energy causes the The electrical energy causes the piezoelectric crystal to piezoelectric crystal to momemtarily change shape momemtarily change shape (expand and contract depending (expand and contract depending on current direction) on current direction)



This change in shape of the crystal increases and decreases the This change in shape of the crystal increases and decreases the pressure in front of the transducer, thus producing ultrasound waves pressure in front of the transducer, thus producing ultrasound waves When the crystal is subjected to pressure changes by the returning When the crystal is subjected to pressure changes by the returning ultrasound echoes, the pressure changes are converted back into ultrasound echoes, the pressure changes are converted back into electrical energy signals electrical energy signals Return voltage signals are transferred from the receiver to a computer Return voltage signals are transferred from the receiver to a computer to create an ultrasound image to create an ultrasound image Transducer crystals do not conduct electricity but are coated with a thin Transducer crystals do not conduct electricity but are coated with a thin layer of silver which acts as an electrode layer of silver which acts as an electrode


The piezoelectric effect of a transducer is destroyed if heated above its The piezoelectric effect of a transducer is destroyed if heated above its curie temperature limit curie temperature limit Transducers are made of a synthetic ceramic (peizoceramic) such as Transducers are made of a synthetic ceramic (peizoceramic) such as lead-zirconate-titanate (PZT) or plastic polyvinylidence difluoride lead-zirconate-titanate (PZT) or plastic polyvinylidence difluoride (PVDF) or a composite (PVDF) or a composite A transducer may be used in either pulsed or continuous-wave mode A transducer may be used in either pulsed or continuous-wave mode A transducer can be used both as a transmitter and receiver of A transducer can be used both as a transmitter and receiver of ultrasonic waves ultrasonic waves


The thickness of a piezoelectric The thickness of a piezoelectric crystal determines the resonant crystal determines the resonant frequency of the transducer frequency of the transducer The operating resonant frequency The operating resonant frequency is determined by the thickness of is determined by the thickness of the crystal equal to ½ wavelength the crystal equal to ½ wavelength (t=(t=/2) of emitted sound in the /2) of emitted sound in the crystal compound crystal compound Resonance transducers transmit Resonance transducers transmit and receive preferentially at a and receive preferentially at a single “center frequency” single “center frequency”


3. Transducers 3. Transducers Damping BlockDamping Block

The damping block absorbs the backward directed ultrasound energy The damping block absorbs the backward directed ultrasound energy and attenuates stray ultrasound signals from the housing and attenuates stray ultrasound signals from the housing

It also dampens (ring-down) the transducer vibration to create an It also dampens (ring-down) the transducer vibration to create an ultrasound pulse with a short spatial pulse length, which is necessary ultrasound pulse with a short spatial pulse length, which is necessary to preserve detail along the beam axis (axial resolution) to preserve detail along the beam axis (axial resolution)

c.f. Bushberg, et al. The c.f. Bushberg, et al. The Essential Physics of Medical Essential Physics of Medical Imaging, 2Imaging, 2ndnd ed., p. 484. ed., p. 484.

3. Transducers 3. Transducers Q factorQ factor

The Q factor is related to the The Q factor is related to the frequency response of the crystal frequency response of the crystal

The Q factor determines the The Q factor determines the purity of the sound and length purity of the sound and length of time the sound persists, or of time the sound persists, or ring down time ring down time Q = operating frequency Q = operating frequency (MHz) / bandwidth (width of the (MHz) / bandwidth (width of the frequency distribution) frequency distribution) Q = fQ = f00/BW /BW

High-Q transducers produce a High-Q transducers produce a relatively pure frequency relatively pure frequency spectrum spectrum Low-Q transducers produce a Low-Q transducers produce a wider range of frequencies wider range of frequencies c.f. Bushberg, et al. c.f. Bushberg, et al.

The Essential Physics The Essential Physics of Medical Imaging, 2of Medical Imaging, 2ndnd ed., p. 487.ed., p. 487.

3. Transducers 3. Transducers Matching LayerMatching Layer

A matching layer of material is placed on the front surface of the A matching layer of material is placed on the front surface of the transducer to improve the efficiency of energy transmission into the transducer to improve the efficiency of energy transmission into the patient patient The material has acoustic properties intermediate to those of soft tissue The material has acoustic properties intermediate to those of soft tissue and the transducer material and the transducer material The matching layer thickness is equal to ¼ the wavelength of sound in The matching layer thickness is equal to ¼ the wavelength of sound in that material (quarter-wave matching) that material (quarter-wave matching) Acoustic coupling gel is used to eliminate air pockets that could Acoustic coupling gel is used to eliminate air pockets that could attenuate and reflect the ultrasound beam attenuate and reflect the ultrasound beam


3. Transducers 3. Transducers Nonresonance (Broad-Bandwidth) “Multifrequency” Nonresonance (Broad-Bandwidth) “Multifrequency” TransducersTransducers


3. Transducers 3. Transducers Transducer ArraysTransducer Arrays

Linear or curvilinear array transducers Linear or curvilinear array transducers 256 to 512 elements 256 to 512 elements Simultaneous firing of a small group of approx. 20 elements Simultaneous firing of a small group of approx. 20 elements produces the ultrasound beam produces the ultrasound beam Rectangular field of view produced for linear and trapezoidal for Rectangular field of view produced for linear and trapezoidal for curvilinear array transducerscurvilinear array transducers


3. Transducers 3. Transducers Transducer ArraysTransducer Arrays

Phased array transducers Phased array transducers 64 to 128 elements 64 to 128 elements All are activated simultaneously All are activated simultaneously Using time delays can steer and focus beam electronicallyUsing time delays can steer and focus beam electronically


3. Ultrasound Beam Properties3. Ultrasound Beam PropertiesNear Field and Far FieldNear Field and Far Field

Near (parallel) Field “Fresnel Near (parallel) Field “Fresnel zone” zone”

Is adjacent to the transducer Is adjacent to the transducer face and has a converging face and has a converging beam profile beam profile Convergence occurs because Convergence occurs because of multiple constructive and of multiple constructive and destructive interference destructive interference patterns of the ultrasound patterns of the ultrasound waves (pebble dropped in a waves (pebble dropped in a quiet pond) quiet pond) Near Zone length = dNear Zone length = d22/4/4 = = rr22// d=transducer diameter, d=transducer diameter, r=transducer radius r=transducer radius



Unfocused transducer, Near Zone Unfocused transducer, Near Zone length = dlength = d22/4/4 = r = r22//A focused single element transducer A focused single element transducer uses either a curved element or an uses either a curved element or an acoustic lens: acoustic lens:

Reduce beam diameter Reduce beam diameter All diagnostic transducers are All diagnostic transducers are focused focused Focal zone is the region over Focal zone is the region over which the beam is focused which the beam is focused A focal zone describes the region A focal zone describes the region of best lateral resolution of best lateral resolution



The far field or Fraunhofer The far field or Fraunhofer zone is where the beam zone is where the beam diverges diverges

Angle of divergence for Angle of divergence for non-focused transducer is non-focused transducer is given by given by sin(sin() = 1.22) = 1.22/d /d Less beam divergence Less beam divergence occurs with high-occurs with high-frequency, large-diameter frequency, large-diameter transducerstransducers


3. Ultrasound Beam Properties3. Ultrasound Beam Properties - Side Lobes - Side Lobes

Side lobes are unwanted emissions of ultrasound energy directed away Side lobes are unwanted emissions of ultrasound energy directed away from the main pulse from the main pulse Caused by the radial expansion and contraction of the transducer Caused by the radial expansion and contraction of the transducer element during thickness contraction and expansion element during thickness contraction and expansion Lobes get larger with transducer size Lobes get larger with transducer size Echoes received from side lobes are mapped into the main beam, Echoes received from side lobes are mapped into the main beam, causing artifactscausing artifacts


3. Ultrasound Beam Properties3. Ultrasound Beam Properties - Side Lobes - Side Lobes

For multielement arrays, side lobe For multielement arrays, side lobe emission occurs in a forward emission occurs in a forward direction along main beam direction along main beam Grating lobes result when ultrasound Grating lobes result when ultrasound energy is emitted far off-axis by energy is emitted far off-axis by multielement arrays, and are a multielement arrays, and are a consequence of the noncontinuous consequence of the noncontinuous transducer surface of the discrete transducer surface of the discrete elements elements

results in appearance of highly results in appearance of highly reflective, off-axis objects in the reflective, off-axis objects in the main beammain beam

c.f. Bushberg, et al. The Essential Physics of c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2Medical Imaging, 2ndnd ed., p. 496. ed., p. 496.

Image Data AcquisitionImage Data Acquisition


3. Pulse Echo Operation3. Pulse Echo OperationPulse Repetition Frequency (PRF)Pulse Repetition Frequency (PRF)

Diagnostic ultrasound utilizes a pulse-echo format using a single Diagnostic ultrasound utilizes a pulse-echo format using a single transducer to generate images transducer to generate images Most ultrasound beams are emitted in brief pulses (1-2 Most ultrasound beams are emitted in brief pulses (1-2 s duration) s duration) For soft tissue (c = 1540 m/s or 0.154 cm/For soft tissue (c = 1540 m/s or 0.154 cm/sec), the time delay between sec), the time delay between the transmission pulse and the detection of the echo is directly related the transmission pulse and the detection of the echo is directly related to the depth of the interface as to the depth of the interface as c = 2D / time c = 2D / time

Time (Time (sec) = 2D (cm) / c (cm/ sec) = 2D (cm) / c (cm/ sec) = 2D/0.154 sec) = 2D/0.154 Time (Time (sec) = 13 sec) = 13 sec x D (cm) sec x D (cm) Distance (cm) = [c (cm/ Distance (cm) = [c (cm/ sec) x Time (sec) x Time (sec)] / 2 sec)] / 2 Distance (cm) = 0.077 x Time (Distance (cm) = 0.077 x Time (sec)sec)

Key PointsKey Points

Piezoelectric transducers convert electrical energy into ultrasonic energy Piezoelectric transducers convert electrical energy into ultrasonic energy and vice versa The thickness of a piezoelectric crystal determines the and vice versa The thickness of a piezoelectric crystal determines the resonant frequency of the transducer The Q factor determines the purity resonant frequency of the transducer The Q factor determines the purity of the sound and length of time the sound persists, or ring down time of the sound and length of time the sound persists, or ring down time Linear or curvilinear array transducers, phased array transducers Linear or curvilinear array transducers, phased array transducers Near Zone length = dNear Zone length = d22/4/4 = r = r22//The far field or Fraunhofer zone is where the beam diverges, sin(The far field or Fraunhofer zone is where the beam diverges, sin() = ) = 1.221.22/d /d Time (Time (sec) = 13 sec) = 13 sec x D (cm) or sec x D (cm) or Distance (cm) = 0.077 x Time (Distance (cm) = 0.077 x Time (sec) sec) PRF = c/2D PRF = c/2D

4. Spatial Resolution4. Spatial Resolution

Spatial resolution has 3 distinct measures: axial, lateral and slice Spatial resolution has 3 distinct measures: axial, lateral and slice thickness (elevational)thickness (elevational)

c.f. Bushberg, et c.f. Bushberg, et al. The al. The Essential Essential Physics of Physics of Medical Medical Imaging, 2Imaging, 2ndnd ed., ed., p. 497.p. 497.

4. Spatial Resolution - Axial4. Spatial Resolution - Axial

Axial resolution (linear, range, Axial resolution (linear, range, longitudinal or depth resolution) is the longitudinal or depth resolution) is the ability to separate two objects lying ability to separate two objects lying along the axis of the beam along the axis of the beam Achieving good axial resolution Achieving good axial resolution requires that the returning echoes be requires that the returning echoes be distinct without overlap distinct without overlap The minimal required separation The minimal required separation distance between two boundaries is distance between two boundaries is ½ SPL (about ½ ½ SPL (about ½ )) to avoid overlap to avoid overlap of returning echoesof returning echoes

c.f. Bushberg, et al. The Essential Physics c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2of Medical Imaging, 2ndnd ed., p. 498. ed., p. 498.

4. Spatial Resolution - Lateral4. Spatial Resolution - Lateral

Lateral resolution - the ability to Lateral resolution - the ability to resolve adjacent objects resolve adjacent objects perpendicular to the beam perpendicular to the beam direction and is determined by the direction and is determined by the beam width and line density beam width and line density Typical lateral resolution Typical lateral resolution (unfocused) is 2 - 5 mm, and is (unfocused) is 2 - 5 mm, and is depth dependent depth dependent Single focused transducers Single focused transducers restrain the beam to within narrow restrain the beam to within narrow lateral dimensions at a specified lateral dimensions at a specified depth using lenses at the depth using lenses at the transducer facetransducer face


4. Spatial Resolution - Slice thickness (Elevational)4. Spatial Resolution - Slice thickness (Elevational)

Elevational resolution is dependent on the transducer element height Elevational resolution is dependent on the transducer element height Perpendicular to the image plane Perpendicular to the image plane Use of a fixed focal length lens across the entire surface of the array Use of a fixed focal length lens across the entire surface of the array provides improved elevational resolution at the focal distance, however provides improved elevational resolution at the focal distance, however partial volume effects before and after focal zonepartial volume effects before and after focal zone

c.f. Bushberg, c.f. Bushberg, et al. The et al. The Essential Essential Physics of Physics of Medical Medical Imaging, 2Imaging, 2ndnd ed., ed., p. 497.p. 497.

5. Display5. Display

Ultrasound scanners use time gain Ultrasound scanners use time gain compensation (TGC) to compensation (TGC) to compensate for increased compensate for increased attenuation with depth attenuation with depth

TGC is also known as depth TGC is also known as depth gain compensation, time varied gain compensation, time varied gain, and swept gain gain, and swept gain

Images are normally displayed on a Images are normally displayed on a video monitor or stored in a video monitor or stored in a computer computer Generally 512 x 512 matrix size Generally 512 x 512 matrix size images, 8 bits deep allowing 256 images, 8 bits deep allowing 256 gray levels to be displayed, 0.25 gray levels to be displayed, 0.25 MB data MB data


5. Display Modes: A-mode5. Display Modes: A-mode

A-mode “amplitude” mode: displays A-mode “amplitude” mode: displays echo amplitude vs. time (depth) echo amplitude vs. time (depth) One “A-line” of data per pulse One “A-line” of data per pulse repetition repetition A-mode used in ophthalmology or A-mode used in ophthalmology or when accurate distance when accurate distance measurements are required measurements are required

c.f. Bushberg, et al. The Essential Physics c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2of Medical Imaging, 2ndnd ed., p. 507. ed., p. 507.

5. Display Modes: B-mode5. Display Modes: B-mode

B-mode (B for brightness) is the B-mode (B for brightness) is the electronic conversion of the A-electronic conversion of the A-mode and A-line information into mode and A-line information into brightness-modulated dots on a brightness-modulated dots on a display screen display screen In general, the brightness of the In general, the brightness of the dot is proportional to the echo dot is proportional to the echo signal amplitude signal amplitude Used for M-mode ad 2D gray-Used for M-mode ad 2D gray-scale imagingscale imaging


5. Display Modes: M-mode5. Display Modes: M-mode

M-mode (“motion” mode) or T-M M-mode (“motion” mode) or T-M mode (“time-motion” mode): mode (“time-motion” mode): displays time evolution vs. depth displays time evolution vs. depth Sequential US pulse lines are Sequential US pulse lines are displayed adjacent to each other, displayed adjacent to each other, allowing visualization of interface allowing visualization of interface motion motion M-mode is valuable for studying M-mode is valuable for studying rapid movement, such as mitral rapid movement, such as mitral valve leafletsvalve leaflets


5. Scan Converter5. Scan Converter

The function of the scan converter is to create 2D images from echo The function of the scan converter is to create 2D images from echo formation received and to perform scan conversion to enable image formation received and to perform scan conversion to enable image data to be viewed on video display monitors data to be viewed on video display monitors Scan conversion is necessary because the image acquisition and Scan conversion is necessary because the image acquisition and display occur in different formats display occur in different formats Modern scan converters use digital methods for processing and storing Modern scan converters use digital methods for processing and storing data data For color display, the bit depth is often as much as 24 bits or 3 bytes of For color display, the bit depth is often as much as 24 bits or 3 bytes of primary colorprimary color

ΚΑΛΟ ΣΑΒΒΑΤΟΚΥΡΙΑΚΟ

Ultrasound. Introduction Ultrasound is a non-ionizing method which uses sound waves of frequencies (2 to 10 MHz) exceeding the range of human hearing.

Documents

ultrasound frequency

speed ultrasound wavelength

characteristics of sound

ultrasound slide

low speed of sound

higher speed of sound

sec speed of sound

highfrequency ultrasound