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Ultrasound – Chapter 16 Bushberg Diagnostic Imaging Physics Course 11 – 28 April 2005 Introduction Ultrasound – Chapter 16 Kalpana Kanal, Ph.D., DABR Lecturer, Diagnostic Physics Dept of Radiology UW Medicine a copy of this lecture may be found at: http://courses.washington.edu/radxphys/PhysicsCourse04http://courses.washington.edu/radxphys/PhysicsCourse04-05.html ™ Ultrasound is a nonnon-ionizing method which uses sound waves of frequencies (2 to 10 MHz) exceeding the range of human hearing for for imaging ™ Medical diagnostic ultrasound uses ultrasound energy and the acoustic properties of the body to produce an image from stationary stationary and moving tissues ™ Ultrasound is used in pulsepulse-echo format, whereby pulses of ultrasound produced over a very brief duration travel through various various tissues and are reflected at tissue boundaries back to the source source Kanal Introduction ™ Returning echoes carry the ultrasound information that is used to to create the sonogram or measure blood velocities with Doppler frequency techniques ™ Along a given beam path, the depth of an echoecho-producing structure is determined from the time between the pulsepulse-emission and the echo return, and the amplitude of the echo is encoded as a graygrayscale value ™ Characteristics of Sound Propagation of Sound ™ Sound is a mechanical energy that propagates through a medium by the compression and rarefaction of particles that compose it (example: spring) ™ Energy propagation is shown as a function of time, resulting in areas of compression and rarefaction with corresponding variations in positive and negative pressure amplitude In addition to 2D imaging, ultrasound provides anatomic distance and volume measurements, motion studies, blood velocity measurements, and 3D imaging c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 471 Kanal Kalpana M Kanal, Ph.D Kanal Ultrasound – Chapter 16 Bushberg Diagnostic Imaging Physics Course 11 – 28 April 2005 Characteristics of Sound Propagation of Sound Characteristics of Sound Wavelength ™ ™ Sound energy can also be produced in short bursts, such that a small pulse travels by itself through the medium ™ Energy travels as longitudinal waves parallel to the direction of the transducer ™ Waves create bands of compression and rarefaction in the medium, the magnitude of which is proportional to the compressibility Wavelength (λ (λ) is distance between two adjacent bands of compression or rarefaction (meters, m) c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 471 Kanal Kanal Characteristics of Sound Frequency ™ ™ Characteristics of Sound Speed Frequency (f) (f) is the number of times the wave oscillates through a cycle each second (sec) (Hertz: Hz or cycles/sec) ™ Infra sound < 15 Hz ™ Audible sound ~ 15 Hz - 20 kHz ™ Ultrasound > 20 kHz; for medical usage typically 22-10 MHz with specialized ultrasound applications up to 50 MHz ™ The speed or velocity of sound is the distance traveled by the wave per unit time and is equal to the wavelength divided by the period (1/f) ™ speed = wavelength / period ™ speed = wavelength x frequency ™ c = λf ™ c [m/sec] = λ [m] * f [1/sec] ™ Speed of sound is dependent on the propagation medium and varies widely in different materials period (τ (τ) - the time duration of one wave cycle: τ = 1/f Kanal Kalpana M Kanal, Ph.D Kanal Ultrasound – Chapter 16 Bushberg Diagnostic Imaging Physics Course 11 – 28 April 2005 Characteristics of Sound Speed ← ™ ™ The ultrasound frequency is unaffected by changes in sound speed as the acoustic beam propagates through various media ™ Thus, the ultrasound wavelength is dependent on the medium (c (c = λf ) ™ A change in speed at an interface between two media causes a change in wavelength ™ Higher frequency sound has shorter wavelength ← ← ← ← ™ Characteristics of Sound Wavelength, Frequency and Speed ← 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 of sound The difference in the speed of sound at tissue boundaries is a fundamental cause of contrast in an ultrasound image c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 472 Kanal Characteristics of Sound Wavelength, Frequency and Speed ™ ™ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 472 Kanal Characteristics of Sound Wavelength, Frequency and Speed Ultrasound wavelength determines the spatial resolution achievable along the direction of the beam ™ A highhigh-frequency ultrasound beam (small wavelength) provides superior resolution and image detail than a lowlow-frequency beam For thick body parts (abdomen), a lower frequency ultrasound wave is used (3.5 to MHz) to image structures at significant depth ™ For small body parts or organs (thyroid, breast), a higher frequency is employed (7.5 to 10 MHz) ™ However, the depth of beam penetration is reduced at high frequency and increased at low frequencies c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 473 Kanal Kalpana M Kanal, Ph.D 10 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 473 11 Kanal 12 Ultrasound – Chapter 16 Bushberg Diagnostic Imaging Physics Course 11 – 28 April 2005 Characteristics of Sound Pressure, Intensity and the dB scale Raphex 2003 Diagnostic Question ™ ™ D63 The wavelength of a MHz ultrasound beam is mm ™ ™ A 0.02 B 0.55 C 0.77 D 2.0 E 5.0 ™ ™ ™ ™ ™ ™ ™ ™ C Wavelength λ = c / f The average velocity in tissue is 1540 m/sec m/sec Frequency = x 106 /sec = 1540 m/sec / x 106 /sec = 770 x 10-6 m = 0.77 mm Kanal ™ 13 The amplitude of a wave is the size of the wave displacement Larger amplitudes of vibration produce denser compression bands and, hence, higher intensities of sound Intensity of ultrasound ™ is the amount of power (energy per unit time) per unit area ™ proportional to the square of the pressure amplitude, I ∝ P2 ™ units of milliwatts/cm2 or mW/cm2 ™ is measured in decibels (dB) as a relative intensity ™ dB = 10 log10 (I2/I1) or dB = 20 log10 (P2/P1) since I ∝ P2 ™ I1 and I2 are intensity values ™ P1 and P2 are pressure or amplitude variations B = 10 dB where B is bels Kanal 14 Characteristics of Sound Pressure, Intensity and the dB scale Characteristics of Sound Pressure, Intensity and the dB scale ™ Example: Calculate the remaining intensity of a 100100-mW ultrasound pulse that loses 30 dB while traveling through tissue ™ Relative Intensity (dB) = 10 log I2 I1 − 30 dB = 10 log I2 100 mW I2 100 mW I2 10 −3 = 100 mW I = 0.001 x 100 mW = 0.1 mW − = log c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 476 Kanal Kalpana M Kanal, Ph.D 15 Kanal 16 Ultrasound – Chapter 16 Bushberg Diagnostic Imaging Physics Course 11 – 28 April 2005 Interactions of Ultrasound with Matter Characteristics of Sound – Key Points ™ ™ ™ Propagation of sound - Key Points ™ Speed of sound is dependent on the medium ™ Ultrasound frequency is independent of the medium, and does not change ™ Wavelength changes with the changes of speed ™ c [m/sec] = λ [m] * f [1/sec] ™ ™ For most calculations, the average speed of sound in soft tissue, tissue, 1540 m/sec, should be assumed ™ In air = 330 m/sec and in fatty tissue = 1450 m/sec dB = 10 log10 (I2/I1) Kanal 17 Kanal 18 Interactions of Ultrasound with Matter Reflection Interactions of Ultrasound with Matter Acoustic Impedance ™ Acoustic Impedance, Z ™ is equal to density of the material times speed of sound in the material in which ultrasound travels, Z = ρ c ™ ρ = density (kg/m3) and c = speed of sound (m/sec) ™ measured in rayl (kg/m2sec) ™ Air and lung media have low values of Z, whereas bone and metal have high values ™ Large differences in Z (air(air-filled lung and soft tissue) cause reflection, small differences allow transmission of sound energy ™ The differences between acoustic impedance values at an interface determines the amount of energy reflected at the interface ™ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 477 Kanal Kalpana M Kanal, Ph.D Ultrasound interactions are determined by the acoustic properties properties of matter As ultrasound energy propagates through a medium, interactions that occur include ™ reflection ™ refraction ™ scattering ™ Absorption (attenuation) 19 A portion of the ultrasound beam is reflected at tissue interface interface ™ The sound reflected back toward the source is called an echo and is used to generate the ultrasound image ™ The percentage of ultrasound intensity reflected depends in part on the angle of incidence of the beam ™ As the angle of incidence increases, reflected sound is less likely to reach the transducer c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 478 Kanal 20 Ultrasound – Chapter 16 Bushberg Diagnostic Imaging Physics Course 11 – 28 April 2005 Interactions of Ultrasound with Matter Reflection ™ Raphex 2001 Diagnostic Question Sound reflection occurs at tissue boundaries with differences in acoustic impedance ™ The intensity reflection coefficient, R = Ir/Ii = ((Z2 – Z1)/(Z2 + Z1))2 ™ The subscripts and represent tissues proximal and distal to the boundary ™ Equation only applies to normal incidence ™ The transmission coefficient = T = – R ™ T = (4Z1Z2)/(Z1+Z2)2 ™ D54 Approximately what fraction of an ultrasound beam is reflected from an interface between two media with Z values of 1.65 and 1.55? ™ A 1/2 B 1/10 C 1/100 D 1/500 E 1/1000 ™ ™ ™ ™ ™ ™ R = ((Z2 – Z1)/(Z2 + Z1))2 = (1.65(1.65-1.55)2 / (1.65 + 1.55)2 = 1/1024 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 479 Kanal 21 Kanal Interactions of Ultrasound with Matter Tissue reflections Raphex 2002 Diagnostic Question ™ ™ ™ D58 Ultrasound moves with the highest velocity in: ™ ™ ™ ™ ™ A B C D medium Fat Blood Muscle Bone ™ ™ Z 1.38 1.61 1:70 7.80 ™ ™ ™ Kanal Kalpana M Kanal, Ph.D 22 23 Kanal Air/tissue interfaces reflect virtually all of the incident ultrasound ultrasound beam Gel is applied to displace the air and minimize large reflections reflections Bone/tissue interfaces also reflect substantial fractions of the incident intensity Imaging through air or bone is generally not possible ™ The lack of transmissions beyond these interfaces results in an area void of echoes called shadowing In imaging the abdomen, the strongest echoes are likely to arise from gas bubbles Organs such as kidney, pancreas, spleen and liver are comprised of subregions that contain many scattering sites, which results in a speckled texture on images Organs with fluids such as bladder, cysts, and blood vessels have have almost no echoes (appear black) 24 Ultrasound – Chapter 16 Bushberg Diagnostic Imaging Physics Course 11 – 28 April 2005 Interactions of Ultrasound with Matter Refraction ™ Interactions of Ultrasound with Matter Scatter ™ Refraction is the change in direction of an ultrasound beam when passing from one medium to another with a different acoustic velocity ™ Wavelength changes causing a change in propagation direction (c = λf) ™ sin(θ sin(θt) = sin(θ sin(θi) * (c2/c1), Snell’ Snell’s law; o ™ for small θ ≤ 15 : θt = θi * (c2/c1) ™ When c2 > c1, θt > θi , When c1 > c2, θt < θi ™ Ultrasound machines assume straight line propagation, and refraction effects give rise to artifacts c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 480 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 478 Kanal 25 Interactions of Ultrasound with Matter Attenuation ™ ™ ™ Kanal Kalpana M Kanal, Ph.D 26 Interactions of Ultrasound with Matter – Key Points Ultrasound attenuation, the loss of energy with distance traveled, traveled, is caused chiefly by scattering and tissue absorption of the incident incident beam (dB) The intensity loss per unit distance (dB/cm) is the attenuation coefficient ™ Rule of thumb: attenuation in soft tissue is approx dB/cm/MHz ™ The attenuation coefficient is directly proportional to and increases with frequency Attenuation is medium dependent Kanal 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 rough or nonspecular reflector surface ™ As frequency increases, the nonnon-specular (diffuse scatter) interactions increase, resulting in an increased attenuation and loss of echo intensity ™ Scatter gives rise to the characteristic speckle patterns of various various organs, and is important in contributing to the grayscale range in the image ™ ™ 27 Kanal Acoustic Impedance, Z ™ is equal to density of the material times speed of sound in the material in which ultrasound travels, Z = ρ c ™ ρ = density (kg/m3) and c = speed of sound (m/sec) As ultrasound energy propagates through a medium, interactions that occur include ™ reflection ™ refraction ™ scattering ™ Absorption (attenuation) 28 Ultrasound – Chapter 16 Bushberg Diagnostic Imaging Physics Course 11 – 28 April 2005 Transducers ™ A transducer is a device that can convert one form of energy into another ™ Piezoelectric transducers convert electrical energy into ultrasonic energy and vice versa ™ Piezoelectric means pressure electricity Transducers ™ HighHigh-frequency voltage oscillations are produced by a pulse generator and are sent to the ultrasound transducer by a transmitter ™ The electrical energy causes the piezoelectric crystal to momemtarily change shape (expand and contract depending on current direction) c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 484 Kanal c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 484 29 Kanal 30 Transducers ™ ™ ™ ™ Transducers This change in shape of the crystal increases and decreases the pressure in front of the transducer, thus producing ultrasound waves waves When the crystal is subjected to pressure changes by the returning returning ultrasound echoes, the pressure changes are converted back into electrical energy signals Return voltage signals are transferred from the receiver to a computer to create an ultrasound image Transducer crystals not conduct electricity but are coated with with a thin layer of silver which acts as an electrode Kanal Kalpana M Kanal, Ph.D ™ ™ ™ ™ 31 Kanal The piezoelectric effect of a transducer is destroyed if heated above its curie temperature limit Transducers are made of a synthetic ceramic (peizoceramic) such as leadlead-zirconatezirconate-titanate (PZT) or plastic polyvinylidence difluoride (PVDF) or a composite A transducer may be used in either pulsed or continuouscontinuous-wave mode A transducer can be used both as a transmitter and receiver of ultrasonic waves 32 Ultrasound – Chapter 16 Bushberg Diagnostic Imaging Physics Course 11 – 28 April 2005 Transducers Damping Block Transducers ™ ™ ™ The thickness of a piezoelectric crystal determines the resonant frequency of the transducer The operating resonant frequency is determined by the thickness of the crystal equal to ½ wavelength (t=λ (t=λ/2) of emitted sound in the crystal compound Resonance transducers transmit and receive preferentially at a single “center frequency” ™ ™ The damping block absorbs the backward directed ultrasound energy and attenuates stray ultrasound signals from the housing It also dampens (ring(ring-down) 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) resolution) c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 484 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 486 Kanal 33 Kanal 34 Transducers Q factor ™ The Q factor is related to the frequency response of the crystal ™ The Q factor determines the purity of the sound and length of time the sound persists, or ring down time ™ Q = operating frequency (MHz) / bandwidth (width of the frequency distribution) ™ Q = f0/BW ™ HighHigh-Q transducers produce a relatively pure frequency spectrum ™ LowLow-Q transducers produce a wider range of frequencies Kanal Kalpana M Kanal, Ph.D Transducers Matching Layer ™ ™ ™ ™ A matching layer of material is placed on the front surface of the the transducer to improve the efficiency of energy transmission into the patient The material has acoustic properties intermediate to those of soft soft tissue and the transducer material The matching layer thickness is equal to ¼ the wavelength of sound sound in that material (quarter(quarter-wave matching) Acoustic coupling gel is used to eliminate air pockets that could could attenuate and reflect the ultrasound beam c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 487 35 Kanal c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 484 36 Ultrasound – Chapter 16 Bushberg Diagnostic Imaging Physics Course 11 – 28 April 2005 Transducers Nonresonance (Broad(Broad-Bandwidth) “Multifrequency” Transducers Transducers Transducer Arrays c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 489 ™ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 488 Kanal 37 Linear or curvilinear array transducers ™ 256 to 512 elements ™ Simultaneous firing of a small group of approx 20 elements produces the ultrasound beam ™ Rectangular field of view produced for linear and trapezoidal for for curvilinear array transducers Kanal 38 Transducers Transducer Arrays Davis Question 14 A 10 MHz U.S beam travels through cm of soft tissue with an attenuation of dB/cm/MHz The original intensity is reduced by by a factor of _ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 489 ™ A B C D E Phased array transducers ™ 64 to 128 elements ™ All are activated simultaneously ™ Using time delays can steer and focus beam electronically Kanal Kalpana M Kanal, Ph.D 30 100 300 1000 E: For a 10 MHz transducer, the attenuation is 10 dB/cm x cm = 30 dB dB = 10log10[I1/I2]; 30 = 10log10[I1/I2]; = log10[I1/I2]; 103 = I1/I2; I1 = 1000 I2 39 Kanal 40 10 Ultrasound – Chapter 16 Bushberg Diagnostic Imaging Physics Course 11 – 28 April 2005 Spatial Resolution - Axial Spatial Resolution - Lateral ™ ™ ™ ™ Typical wavelength is 0.3 mm and three waves per pulse, the axial resolution is 0.5 mm (0.3x3=0.9, ½ x 0.9 = 0.45 or 0.5 mm) Higher frequencies reduce SPL, improving axial resolution however, increases attenuation Axial resolution remains constant with depth ™ ™ Lateral resolution - the ability to resolve adjacent objects perpendicular to the beam direction and is determined by the beam width and line density Typical lateral resolution (unfocused) is - mm, and is depth dependent Single focused transducers restrain the beam to within narrow lateral dimensions at a specified depth using lenses at the transducer face c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 498 Kanal c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 499 53 Kanal 54 Spatial Resolution - Slice thickness (Elevational) ™ ™ ™ Davis Question Elevational resolution is dependent on the transducer element height Perpendicular to the image plane Use of a fixed focal length lens across the entire surface of the the array provides improved elevational resolution at the focal distance, however partial volume effects before and after focal zone 23 In real time phased array transducer systems, axial resolution resolution depends mainly on: A B C D E c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 497 Kanal Kalpana M Kanal, Ph.D 55 Kanal frequency of operation imaging depth transducer diameter pulse length length of Fraunhofer zone 56 14 Ultrasound – Chapter 16 Bushberg Diagnostic Imaging Physics Course 11 – 28 April 2005 Display Modes: AA-mode Display ™ ™ ™ Ultrasound scanners use time gain compensation (TGC) to compensate for increased attenuation with depth ™ TGC is also known as depth gain compensation, time varied gain, and swept gain Images are normally displayed on a video monitor or stored in a computer Generally 512 x 512 matrix size images, bits deep allowing 256 gray levels to be displayed, 0.25 MB data ™ ™ ™ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 507 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 507 Kanal 57 Kanal Display Modes: BB-mode ™ ™ ™ In general, the brightness of the dot is proportional to the echo signal amplitude ™ Used for MM-mode ad 2D graygrayscale imaging ™ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 509 Kanal Kalpana M Kanal, Ph.D 58 Display Modes: MM-mode B-mode (B for brightness) is the electronic conversion of the A-mode and AA-line information into brightnessbrightness-modulated dots on a display screen ™ A-mode “amplitude” mode: displays echo amplitude vs time (depth) One “A“A-line” of data per pulse repetition A-mode used in ophthalmology or when accurate distance measurements are required M-mode (“motion” mode) or TTM mode (“time(“time-motion” mode): displays time evolution vs depth Sequential US pulse lines are displayed adjacent to each other, allowing visualization of interface motion M-mode is valuable for studying rapid movement, such as mitral valve leaflets c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 509 59 Kanal 60 15 Ultrasound – Chapter 16 Bushberg Diagnostic Imaging Physics Course 11 – 28 April 2005 2D Image Display - Dynamic (real(real-time) BB-mode Scan Converter ™ Mechanical Scanning and RealReal-Time Display ™ ™ The function of the scan converter is to create 2D images from echo echo formation received and to perform scan conversion to enable image image data to be viewed on video display monitors Scan conversion is necessary because the image acquisition and display occur in different formats Modern scan converters use digital methods for processing and storing data For color display, the bit depth is often as much as 24 bits or bytes of primary color ™ ™ ™ ™ Kanal c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 511 61 Kanal 2D Image Display - Dynamic (real(real-time) BB-mode ™ ™ ™ ™ Sequential linear array: 256 to 512 individual transducer elements elements producing a rectangular or convex curvilinear array Sequential scanning uses transducers pulsed in groups, where each each group sends and receives before next group is pulsed Number of AA-lines is approx equal to the number of transducer elements Kalpana M Kanal, Ph.D Electronic Scanning and RealReal-Time Display ™ ™ ™ Electronic phased array transducer Sector or linear images; 64,128 or 256 elements Beam sweep is accomplished through pulsing the individual transducers with small timing delays between transducer elements c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 513 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 512 Kanal 62 2D Image Display - Dynamic (real(real-time) BB-mode Electronic Scanning and RealReal-Time Display ™ Mechanically driven transducers sweep out sections of tissue repeatedly, so many times per second Single wobbling or continuously rotated multimulti-element transducers are used 63 Kanal 64 16 Ultrasound – Chapter 16 Bushberg Diagnostic Imaging Physics Course 11 – 28 April 2005 Image Frame Rate Image Frame Rate ™ ™ ™ A 2D image (a single frame) is created from a number of AA-lines, N (typically 100 or more), acquired across the FOV Line density is the number of vertical lines per FOV The frame rates (1/acquisition time per frame) for real time imaging imaging are typically 1515-40 frames/second, which permits motion to be followed Kanal 65 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 514 Kanal Key Points ™ ™ ™ ™ ™ ™ Key Points Spatial resolution has distinct measures: axial, lateral and slice slice thickness (elevational) Axial resolution is the ability to separate two objects lying along along the axis of the beam Axial - The minimal required separation distance between two boundaries is ½ SPL (about ½ λ) to avoid overlap of returning echoes Higher frequencies reduce SPL, improving axial resolution however, however, increases attenuation Lateral resolution - the ability to resolve adjacent objects perpendicular to the beam direction and is determined by the beam beam width and line density Elevational or slice thickness resolution is dependent on the transducer element height Kanal Kalpana M Kanal, Ph.D 66 ™ ™ ™ ™ ™ 67 Kanal Display is generally 512 x 512 matrix size images, bits deep allowing 256 gray levels to be displayed A-mode, BB-mode and MM-mode are different display modes The function of the scan converter is to create 2D images from echo echo formation received and to perform scan conversion to enable image image data to be viewed on video display monitors Dynamic BB-mode scanning used A 2D image (a single frame) is created from a number of AA-lines, N (typically 100 or more), acquired across the FOV 68 17 Ultrasound – Chapter 16 Bushberg Diagnostic Imaging Physics Course 11 – 28 April 2005 Clinical Transducers ™ ™ ™ ™ ™ ™ ™ ™ ™ Low frequency transducers have better tissue penetration Transducers used for abdominal imaging are generally in the 2.5 to MHz range Specialized highhigh-resolution and shallowshallow-penetration probes (8 to 20 MHz) have been developed for studying the eye In infants, 3.5 to MHz transducers are used for echoencephalography echoencephalography Modern transducers can operate at different frequencies Endovaginal transducers – pelvic region and fetus Endorectal transducers – prostrate Transesophageal transducers – heart Intravascular transducers – blood vessels Ultrasound Artifacts ™ ™ ™ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 523 Artifacts arise from the incorrect display of anatomy or noise during imaging Refraction causes misplaced anatomic position in the image Shadowing and Enhancement ™ Shadowing is the reduced echo intensity behind a highly attenuating or reflecting object, such as a stone creating a “shadow” ™ Enhancement is the increased echo intensity behind a minimally attenuating object such as a fluid filled cyst c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 527 Kanal 69 Kanal 70 Raphex 2003 Diagnostic Question ™ ™ ™ ™ ™ ™ Ultrasound Artifacts D68 A shadowing artifact in ultrasound may be due to the reduction of reflected intensity behind all of the following except: ™ A A strong attenuator B A highly reflective interface C Gas or air D Water D A shadowing artifact is caused by a lack of reflection from an area This can be caused by an incident beam being highly attenuated, or if the beam is strongly reflected from an overlying overlying interface, such as between air or gas and tissue Water has a low low absorption coefficient, and acts as a window, generally producing producing no shadowing Reverberation artifacts commonly occurs between two strong reflectors, such as an air pocket and the transducer array at the skin surface ™ The echoes bounce back and forth between the two boundaries and produce equally spaced signals of diminishing amplitude in the image ™ This is often called a “comet“comet-tail” artifact c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 528 Kanal Kalpana M Kanal, Ph.D 71 Kanal 72 18 Ultrasound – Chapter 16 Bushberg Diagnostic Imaging Physics Course 11 – 28 April 2005 Ultrasound Artifacts ™ Raphex 2002 Diagnostic Question Speed displacement artifacts are caused by the variability of the speed of sound in various tissues ™ In the case of fatty tissues, the slower speed of sound in fat (1,450 m/sec) results in a displacement of the returning echoes from distal anatomy by about 6% of the distance traveled through the mass ™ D64 Reverberation artifacts in ultrasound are caused by: ™ A Scattering from small objects B Decreased signal intensity in air C Multiple reflections from two adjacent interfaces D Random signals produced in the electronics of the transducer E Patient movement ™ ™ ™ ™ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 528 Kanal 73 Kanal Ultrasound Artifacts 74 Ultrasound Artifacts c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 528 ™ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 528 Side Lobes and Grating Lobes ™ Side lobe energy emissions in transducer arrays can cause anatomy outside the main beam to be mapped into the main beam ™ For a curved boundary, such as the gallbladder, side lobe interactions can be remapped and produce findings such as “pseudo“pseudosludge” that is not apparent with other scanning angles Kanal Kalpana M Kanal, Ph.D ™ 75 Kanal Side Lobes and Grating Lobes ™ Grating lobes occur with multielement array transducers and result from the division of a smooth transducer surface unto a large number of small elements ™ Create ghost images of offoff-axis highhigh-contrast objects 76 19 Ultrasound – Chapter 16 Bushberg Diagnostic Imaging Physics Course 11 – 28 April 2005 Harmonic Imaging Overview Ultrasound Artifacts ™ A mirror image artifact arises from multiple beam reflections between a mass and a strong reflector, such as a diaphragm ™ Multiple echoes result in the creation of a mirror image beyond the diaphragm of the mass ™ Speckle is a textured appearance that results from small, closelyclosely-spaced structures that are too small to resolve as seen on images of solid organs c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 529 Kanal c.f Ultrasound Technology Update, GE Medical Systems Document, p 77 Kanal 78 Harmonic Imaging How Are Harmonics Generated? ™ ™ ™ Harmonic Imaging How Are Harmonics Generated? The harmonics are not generated by the ultrasound scanner itself These signals are generated in the body as a result of interactions with the tissue or contrast agents Interactions with contrast agents ™ Patient injected with contrast agents containing very small bubbles ™ A conventional ultrasound pulse is sent into the body ™ ™ c.f Ultrasound Technology Update, GE Medical Systems Document, p Kanal Kalpana M Kanal, Ph.D When the pulse encounters the bubble, it generates two kinds of responses First, echo returns from the bubble as in conventional ultrasound and second, the bubble vibrates in response to the shock from the pulse (bell) ™ Vibration generates a second harmonic at twice the frequency of the original ultrasound pulse c.f Ultrasound Technology Update, GE Medical Systems Document, p 79 Kanal 80 20 Ultrasound – Chapter 16 Bushberg Diagnostic Imaging Physics Course 11 – 28 April 2005 Harmonic Imaging How Are Harmonics Generated? Harmonic Imaging How Are Harmonics Generated? This kind of imaging benefits from the fact that the only the strong signal returning from the body at twice the fundamental frequency will be the signal that comes back from places where the bubbles are By listening only for the ring of the bell, the harmonic signal, the ultrasound system can generate very high contrast ultrasound images that are relatively free from the kind of interference that makes conventional ultrasound imaging difficult ™ ™ ™ Interactions with Tissue ™ When the sound wave passes through the tissue, it compresses and and expands the tissue ™ When the tissue is compressed, the speed of sound is higher and when it is expanded, the speed of sound is lower c.f Ultrasound Technology Update, GE Medical Systems Document, p Kanal 81 c.f Ultrasound Technology Update, GE Medical Systems Document, p Kanal Harmonic Imaging How Are Harmonics Generated? ™ 82 Harmonic Imaging How Are Harmonics Generated? Interactions with Tissue ™ Because the speed of sound is higher when the pressure is higher, higher, the top of the waveform gets pulled forward as the wave passes through tissue ™ This distortion of the tissue causes harmonics to be generated ™ Different tissues distort the wave in different ways (fat distorts distorts more then muscle, liver or kidney tissue) ™ The resultant waveform contains both the fundamental frequency plus the harmonic frequencies caused by the distortion ™ Interactions with Tissue ™ This ability to create harmonics in tissue is an effect that is seen in varying degrees through out the ultrasound field of view ™ The harmonic imaging effect without contrast agents is most pronounced in the mid field (middle of the ultrasound image) c.f Ultrasound Technology Update, GE Medical Systems Document, p Kanal Kalpana M Kanal, Ph.D c.f Ultrasound Technology Update, GE Medical Systems Document, p 83 Kanal 84 21 Ultrasound – Chapter 16 Bushberg Diagnostic Imaging Physics Course 11 – 28 April 2005 Harmonic Imaging Harmonic Imaging c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 519 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 518 Kanal 85 Kanal 86 Potential Advantages of the Harmonic Signal Potential Advantages of the Harmonic Signal ™ ™ ™ Harmonic beams are narrower than their conventional counterparts Side lobes are lower as well Kanal Kalpana M Kanal, Ph.D ™ c.f Ultrasound Technology Update, GE Medical Systems Document, p 87 Kanal The result is improved lateral spatial resolution and better contrast resolution, removal of multiple reverberation artifacts caused by anatomy adjacent to the transducer Furthermore, since the harmonics are generated inside the body, they only have to pass through the fat layer once (on receive), not twice (transmit and receive) c.f Ultrasound Technology Update, GE Medical Systems Document, p 88 22 Ultrasound – Chapter 16 Bushberg Diagnostic Imaging Physics Course 11 – 28 April 2005 Harmonic Imaging Harmonic Imaging c.f Ultrasound Technology Update, GE Medical Systems Document, p c.f Ultrasound Technology Update, GE Medical Systems Document, p Kanal 89 ™ ™ ™ Improves sensitivity to microbubble contrast agents Reduces signal from surrounding soft tissues Disadvantage include motion artifacts from moving tissues that occur between pulses and frame rate penalty (at least times slower than a standard scan) c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 521 Kanal Kalpana M Kanal, Ph.D 90 Doppler Ultrasound Pulse Inversion Harmonic Imaging Contrast Agents ™ Kanal 91 Kanal The Doppler ultrasound is based on the shift in frequency in an ultrasound wave caused by a moving reflector (siren on a fire truck) ™ Objects moving toward the observer (transducer) appear to have a higher frequency and shorter wavelength ™ Objects moving away from the observer (transducer) appear to have a lower frequency and longer wavelength ™ If object moving perpendicular to the observer (transducer), no change in the observed frequency or wavelength c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 532 92 23 Ultrasound – Chapter 16 Bushberg Diagnostic Imaging Physics Course 11 – 28 April 2005 Doppler Frequency Shift Doppler Frequency Shift ™ ™ ™ ™ ™ ™ ™ ™ The Doppler shift is the difference between the incident frequency and reflected frequency fd = Doppler frequency shift fi = transducer frequency fr = reflected frequency v = blood velocity ct = speed of sound in tissue As the angle of incidence increases with respect to the long axis of the blood vessel, the Doppler shift decreases according to the dependence on the cosine of the angle Cos = 1, cos 30 = 0.87, cos 45 = 0.707, cos 60 = 0.5, cos 90 = ™ ™ ™ ™ v cos (θ ) f d = fi − f r = fi ct v= Kanal ContinuousContinuous-Wave Doppler Operation ™ ™ Kalpana M Kanal, Ph.D c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 533 94 Pulsed Doppler Operation Continuous Wave Doppler: one transducer continuously transmits and one transducer continuously receives The frequency of the two signals are subtracted to give the Doppler Doppler shift ContinuousContinuous-wave Doppler is inexpensive, does not suffer from aliasing but lacks depth resolution and provides little spatial information ™ Samples everything along the Doppler line ™ Cannot position the Doppler to listen at a specific area along it’s it’s path Good for measuring fast flow and assessing deep lying vessels Kanal v cos (θ ) fi ct c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 532 93 ™ f d = fi − f r = f d ct fi cos (θ ) Kanal ™ Frequency shifts are in the audible range ™ fi = MHz, v = 35 cm/sec, θ = 45o ™ fd = (35 cm/sec)(0.707)(5 MHz)/(154,000 cm/sec) = 1.6 kHz ™ Human audible spectrum: 15 Hz – 20 kHz Preferred Doppler angle is from 3030-60 degrees At >60 deg, minor errors in angle accuracy can result in large errors in velocity At