Basic physics of ultrasound.JH
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Ultrasound physics and image optimization1 (1)
- 1. ULTRASOUND PHYSICS AND IMAGE OPTIMIZATION DR PRAJWITH K J RAI
- 2. Overview Basic principles Instrumentation Image optimization Bioeffects of Ultrasound
- 3. What is ultrasound? Ultrasound or ultrasonography is a medical imaging technique that uses high frequency sound waves to obtain cross sectional images of the body. Also known as ‘pulse echo’ technique
- 4. Audible sound 15 – 20 000 Hz < 15 infrasound > 20 000 ultrasound Medical USG 1-20 MHz
- 5. Sound Sound is a mechanical energy that travels through matter, as a result of vibration of the particles of the medium through which the sound wave is moving. Vibration of particles Transport the energy through medium pressure wave Change in pressure sinusoidal waveform
- 6. compression compression compression compression rarefaction rarefaction rarefaction rarefaction compression rarefaction
- 7. Waves Transverse waves Longitudinal waves waves parallel to the direction of energy transfer Waves perpendicular to the direction of energy transfer
- 8. Sound waves longitudinal waves (air) Compression Space where density and pressure is elevated (positive) Rarefaction Space where density and pressure is depressed (negative)
- 9. Unit of acoustic frequency(number of cycles in a unit of time) - Hertz (Hz) 1 Hz = 1 cycle per second High frequencies expressed in kHz and MHz 1 kHz = 1000Hz 1 MHz = 1,000kHz
- 10. Speed of sound Determined by properties of medium i.e. Its resistance to compression C α-1 COMPRESSIBILITY • FASTER IN SOLID • SLOWER IN GAS Propagation velocity of sound (c) = f x λ
- 11. VELOCITY BONE AND METAL FASTER AVERAGE SOFT TISSUE 1540m/s LUNG AND AIR SLOWER
- 12. REFRACTIONREFLECTION ECHO ABSORPTION Interaction of ultrasound with matter
- 13. Reflection Reflection occurs at a boundary/interface between two adjacent tissues Z = ρc
- 14. Z1 Z2 The difference in acoustic impedance (z) between the two tissues causes reflection of the sound wave
- 15. angle of incidence α-1 amount of reflected sound
- 16. soft tissue - air interface reflects almost the entire beam, transducer must be directly coupled to the patient skin without an air gap bone - tissue interface Bone transmits about 70% of sound energy due to its impedance difference - hence not possible to image through it air/gas interface causes total reflection - hence they cast a shadow and structures underneath cannot be imaged E.g Bowel wall can be imaged and not its lumen
- 17. Specular reflectors are smooth so when incident beam strikes, the reflected beam leaves in the same angle as the incident beam (bright fibrous structures like) Diaphragm, Bladder wall Tendons Diffuse reflectors tend to be rough so when the incident beam strikes, echoes from the interfaces are scattered in all directions kidney, liver, small blood vessels walls of heart chambers Specular reflectors Diffuse reflectors
- 18. Interference If two sound waves of the same wavelength cross each other, the pressure waves combine. Constructive interference waves in step/phase, their amplitudes add up - Destructive interference waves out of phase, they tend to cancel out each other
- 19. Refraction When sound passes from a tissue with one acoustic propagation velocity to a tissue with a higher or lower sound velocity, there is a change in the direction of the sound wave Causes artefacts
- 20. Absorption As the ultrasound pulse moves through matter, it continuously loses energy - attenuation There is absorption of the ultrasound energy by the material and gets converted into HEAT. Scattering and refraction interactions also remove some of the energy from the pulse and contribute to its overall attenuation, but absorption is the most significant Reduction of intensity of beam as it traverses through matter.
- 21. Attenuation Attenuation of sound energy influences the depth in tissue, from which useful information can be obtained Affects transducer selection and operator controlled instrument settings The deeper the wave travels in the body, the weaker it becomes The amplitude/strength of the wave decreases with increasing depth Decibel
- 22. TISSUE absorption + reflection + scattering Attenuation
- 23. frequencies a attenuation Attenuation determines the efficiency with which ultrasound penetrates a specific tissue
- 24. Instrumentation 1. Transmitter - energize transducer 2. Transducer 3. Receiver and processor - detect and amplify backscattered energy 4. Display – presents the ultrasound image/data 5. Recording/Storage
- 25. Transmitter Pulsed ultrasound Produces precisely timed high amplitude voltage as pulses to energize the transducer Controls PRF Pulse repetition frequency – rate of pulses emitted by transducer
- 26. Pulse repetition frequency Determines time interval between pulses Determines depth PRF – 1 to 10 kHz for diagnostic imaging Pulse should travel to depth and return before next pulse is send Highest USG frequency permitting penetration to depth of interest should be selected
- 27. Pulse echo principle To determine the depth of the tissues. T = 2D / c ---- principle of pulse echo Generation of sound pulse Reflection of pulse from targets Detection of reflected echoes Calculation and display of range of targets
- 28. Transducer Device that converts one form of energy into another Ultrasonic transducers convert an electric signal ultrasonic energy reflected back from the tissues into electrical signal Transducer functions as both transmitter and receiver Used in both pulsed and continuous wave modes
- 29. Piezoelectric element located near the face of transducer. Front and back faces coated with thin conducting film Surfaces of crystal coated with gold or silver electrodes Outside electrode grounded to protect the patient and its surface is coated with water tight electric insulator Inside electrode rest against a thick backing block that absorbs sound waves transmitted back into transducer Housing – strong plastic An acoustic insulator of rubber or cork prevents the sound from passing into the housing
- 30. Piezoelectricity Discovered by Pierre and Jacques Curie in 1880 Greek word “piezein” - to press Piezoelectric effect – application of an electric field to certain materials causes a change in their physical dimensions Piezoelectric material - innumerable dipoles arranged in a geometric pattern Each dipole – positive charge at one end and negative charge at other
- 31. Generates ultrasound waves Crystal expands and contracts Alignment of dipoles Electric field Generation of voltage When crystal gets compressed
- 32. Serves as ultrasonic signal for display Voltage amplified Induces voltage between the electrodes Forces dipoles to change orientation Physical compression of crystal element Transmits their energy to transducer Echoes reflect back to transducer from each tissue interface Sound waves pass though the body
- 33. Compression force and associated voltage – piezoelectric effect In response to a compressive force on a piezoelectric slab, a net electric dipole moment detection of a voltage in response to this strain Reverse piezoelectricity - In response to the application of an electric field, a deformation of the piezoelectric slab detected.
- 34. Piezoelectric material Quartz – original transducer material Ferroelectrics ( artificial piezoelectric material ) – barium titanate, lead zirconate titanate To produce polarization the ceramic is heated at high temperature in a strong electric field At high temperature, the dipoles are free to move, and the electric field brings them into the desired alignment The crystal is then gradually cooled while subjected to a constant high voltage
- 35. Curie temperature It is the temperature above which this polarization is lost PZT - 365˚C Quartz - 573˚C
- 36. Resonant frequency Frequency at which the wavelength = 2 x thickness of piezoelectric disc Constructive interference / resonance of waves emitted from forwards and back face of disc occurs Transducer most efficient as transmitter and receiver of sound Depends on thickness of disc and material of disc (determines velocity)
- 37. • The thicker the piezoelectric element, the lower the resonant frequency • Conversely thinner the element higher the operating frequency • Range of frequencies produced by a transducer – bandwidth • Broad band technology produces medical transducers that contain more than one operating frequencies
- 38. 2.5 MHz deep abdomen, obstetric and gynaecological imaging 3.5MHz general abdomen, obstetric and gynaecological imaging 5.0MHz vascular, breast, pelvic imaging 7.5MHz breast, thyroid 10.0MHz breast, thyroid, superficial veins, superficial masses, musculoskeletal imaging. 15.0MHz superficial structures, musculoskeletal imaging.
- 39. Pulse duration/length Transducer continues to vibrate for a short time after it is stimulated ultrasound pulse will be several cycles long Damping block is used to stop the crystal vibration so that transducer is ready to receive the reflected waves from tissue interfaces within the body Damping materials used – tungsten and rubber powder suspended in epoxy resin(backing block)
- 40. Q factor Mechanical coefficient ( Q ) – mean frequency : bandwidth Greater Q – narrower bandwidth, lesser damping, longer ring down time/ pulse duration Lesser Q – broader bandwidth, heavier damping, smaller ring down time/ pulse duration
- 41. Coupling agents Commonly known as GEL Fluid medium needed to provide a link between the transducer and the patient Composition – Carbomer – 10gm Propylene glycol – 75gm Trolamine – 12.5gm EDTA – 0.25gm Distilled water – upto 550ml
- 42. Ultrasound beam characteristics Unfocused beam – An ultrasound beam leaving a flat crystal element has an initial cylindrical segment, followed by a diverging conical portion Frequency – Higher the frequency, longer will be the cylindrical segment or near field (Fresnel zone) and far field (Fraunhofer zone) becomes less divergent Lateral resolution – junction of near and far fields Depth resolution improves at higher frequencies As frequency is increased, greater absorption of sound energy weakens beam intensity
- 43. Near field / Fresnel zone Interference of pressure waves near the transducer results in great amplitude variation Far field / Fraunhofer zone Farther from the transducer, the waves diverge and amplitude decreases at steady state Higher frequency transducer, larger diameter - increases length of near zone and decreases divergence of far zone, hence beam becomes more directional Crystal diameter –Increase in crystal diameter increases the near zone length but worsens lateral and depth resolution
- 44. Focused beam Improves both lateral and depth resolution Concave crystal face – Greater the concavity, nearer the beam will focus relative to the crystal Acoustic lenses – Shorter the focal length, the nearer the lens will focus the beam relative to the lens face
- 45. Receiver Detects and amplifies weak signals Compensates the differences in echo strength, which results from attenuation by different tissue thickness Compression of wider range of amplitudes returning to transducer into a range that can be displayed to the user
- 46. Time gain compensation Amplitude of sound pulse diminishes as it travels through body due to attenuation and echo pulse similarly attenuated as it travels back to transducer Selective amplification of signals received from greater depth TGC is under operator control
- 47. Dynamic range Equivalent to number of shades of gray Ratio of the largest to the smallest echoes processed by components of ultrasound device Dynamic range decreases as signals pass through the imaging system as operations such as TGC eliminate small and large signals smaller change in echo amplitude that will effect the gray scale brightness image contrast is enhanced for smaller dynamic range Echoes returning from tissues can have dynamic ranges of 100 to 150dB Increasing dynamic range yields more shades of gray in ultrasound signals
- 48. Modes of display A mode B mode M mode
- 49. A mode ‘A’ - amplitude Earliest incarnation of ultrasound system Shows signals as spikes and height of the spike represents the amplitude of the reflected signal Only shows position of tissue interfaces Ophthalmology studies – to detect optic nerve
- 50. M mode ‘M’ – motion Detection of motion of a structure or structures over time along scan line of interest A B mode image is frozen on the screen and used to direct the beam from a stationary transducer along a line of interest Echoes are displayed as a line of moving bright dots Displays echo amplitude and shows position of moving reflectors Evaluation of rapid motion of cardiac valves and of cardiac chamber and vessel walls
- 51. B mode ‘B’ – brightness Involves the pulse echo sequence with the buildup of an image from innumerable echo blips Brightness corresponds to received echo amplitude Greatest intensity signal – white Absence of signal – black Signs of intermediate intensity – gray
- 52. Static B mode ultrasound - Image is compiled as sound beam is scanned across patient Larger field of view, restricted transducer movements and single-depth focusing • Real time ultrasound - Image is automatically scanned in a succession of frames sufficiently rapidly to demonstrate the motion of tissues Frame rate of 15-60/second Assessment of both anatomy and motion
- 53. Gray scale imaging Display great amplitudes of echoes arising from tissues as varying shades of gray on monitor Scan convertor – Improve grayscale images Enable freeze frame Manipulate data and archive if necessary Write, read and erase Analog and digital scan convertors
- 54. 3D Ultrasound Images obtained from a set of 2D scans 3D scanners used for fetal, gynaecological and cardiac scanning 4D ultrasound is the term used for real time 3D imaging
- 55. To produce real-time images Mechanical scanners Conventional /group of single element transducers is mechanically moved to form images in real time Oscillating Rotating electronic scanners Other uses an array of transducers, which do not move but are activated electronically – Linear Curvilinear Phased
- 56. Mechanical – Oscillating transducers With unenclosed crystal – single element caused to oscillate through an angle, which defines the field of view; Sector scan With enclosed crystal – transducer enclosed within a fluid filled container; use of an electromagnet and trapezoidal image produced
- 57. Mechanical – Rotating transducers Employs 3 or 4 crystals mounted symmetrically on a rotating wheel The wheel is driven by an electric motor to perform circular motion in one direction only The crystal elements are excited one at a time to provide the ultrasound beam Replaced by annular array transducers
- 58. Linear Array Array element is pulsed sequentially to produce a rectangular scan pattern At any time, only one element or subset of elements is in operation. The echoes are received before the next element is excited Used for small parts, vascular and obstetric applications
- 59. Curvilinear array One element or a subset of elements at a time scanned sequentially Produces trapezoid scan pattern
- 60. Phased array All elements are scanned at the same time Ultrasound beam is caused to sweep back and forth across the patient by electronically controlled steering and focusing Ideal for cardiac imaging
- 61. Transducer selection For general purpose – convex with 3.5MHz For obstetric purpose – convex or linear with 3.5MHz For superficial structures – linear with 5MHz For paediatric or thin people – 5MHz
- 62. Tissue harmonics imaging Based on nonlinear propagation of ultrasound wave propagating through tissue causing distortion and generating harmonics It decreases “clutter” noise It yields sharper overall images with improved contrast, axial and lateral resolution Used in obstetrics, cardiology and abdominal applications
- 63. Spatial Resolution Ability to distinguish between two closely situated structures from one another Axial or lateral resolution
- 64. Axial (depth) resolution Minimum reflector resolution along axis of ultrasound beam Determined by pulse length Higher frequencies lesser pulse length higher axial resolution
- 65. Lateral (horizontal) resolution Minimal reflector resolution perpendicular to beam and parallel to transducer Determined by beam width Beam width is a function of wavelength, piezoelectric transducer diameter, distance from transducer Can be controlled by focusing the beam
- 66. Image optimization Gain and TGC - Sensitivity controls Focus – Lateral resolution Frequency – Axial resoultion High good axial resolution but poor penetration Low less axial resolution but better penetration Depth – decrease in depth increase in frame rate (temporal resolution) Sector width increase in frame rate
- 67. TGC gain freeze depth focus
- 68. Gain – Overall brightness of the image Amplification of returning ultrasound signal received from all depths High gain high amplitude differentiation lost Adjust to minimum required to obtain a complete range from low (dark gray) to high (white) amplitude signals Time Gain compensation – Adjustment of image brightness at a selective depth Corrects varying depths of intensities Adjust TGC to decrease in near field and increase in far field (‘’ shape)
- 69. Focus – Allows focus of ultrasound beam to area of interest Optimizes ultrasound intensity in near and far field improved spatial resolution Depth – Adjustment of the depth of field of view Greater the depth, lesser resolution and frame rate Set the depth to minimum required to visualize all structures of interest Frequency – Increased frequency increase resolution Sector width – Wide sector frame rate and temporal resolution
- 70. • Dynamic range – • 100dB of ultrasound information available but monitor can display a much smaller range • Allows compression of wide spectrum of amplitudes • Compressed signals then displayed on monitor varying shades of gray
- 71. Linear array 18-5MHz Linear array 12-3MHz Curved array 5-1MHz Tightly curved array 10-3MHz
- 72. Bioeffects Thermal Mechanical
- 73. Thermal bioeffect Mechanism - with sound wave attenuation, energy is lost as heat Longer use of ultrasound on the tissue longer the ultrasound has attenuated in the tissue More the tissue attenuates ultrasound more the bioeffects More ultrasound attenuates at the surface and focal zone Higher the frequency/power/PRF/pulse duration , more the attenuation and thus the heating
- 74. Thermal indices NCRP guidelines – in ultrasound examinations, no temperature rise > 1°C TI – indicator of relative potential for increasing tissue temperature TIS – Thermal index for soft tissue TIB – Thermal index for bone, at the focus TIC – Thermal index for bone, at surface
- 75. Mechanical bioeffects Current USG systems can produce cavitation Determines potential for bubble formation in vivo Cavitation - Stable Inertial / Transient
- 76. Stable cavitation A bubble forms as the peak low pressure region of an ultrasound pulse passes through a nucleation site This bubble grows and shrinks with the high pressure and low pressure region of the ultrasound pulses This growing and shrinking of the bubble causes fluid to flow around the bubble - micro-streaming Causes mechanical damage, sometimes cell lysis
- 77. Inertial/transient cavitation Here, the bubbles formed in a similar way but under high pressure or intensities, they expand very quickly following which they collapse violently This collapse produces high temperature and pressure, which damages cells in the region of the collapse
- 78. Mechanical index It is an approximation of mechanical effects It is a calculated number based on ultrasound parameters and facts about the machine that the manufacturer built into the programming Mechanical index less than 1.9, no mechanical effects is seen
