Ultrasound uses sound waves to produce images of internal organs and tissues. Sound waves are transmitted into the body and the echoes produced by reflections from structures and tissues are detected. Three key points: 1) Ultrasound transducers convert electrical pulses into sound waves which penetrate the body and receive the echoes. Piezoelectric crystals in the transducer perform this function. 2) Reflected sound waves are displayed as images on screen to visualize internal structures. The brightness of each pixel depends on the strength of reflection. 3) Different transducer designs like linear arrays and curved arrays allow imaging of different body regions. Imaging modes like B-mode show anatomical structures while M-mode depicts motion.

1. Basic physics of ultrasoundSPEAKER: Dr. JAYAPRABHA NARAYAN 1st YEAR PGMODERATOR: Dr. ANAND S H ASSISTANT PROFESSOR DEPARTMENT OF RADIODIAGNOSIS

2. The basis of all diagnostic ultrasound applications are the detection and display of acoustic energy reflected from within the body.In Medical Ultrasound, images representing human organs are formed by transmitting sound waves into the body and receiving back and processing the resultant echoes from the tissues.To accomplish this, medical ultrasound uses a process very similar to an ocean-going vessels “depth sounding” equipment or oceanic survey equipment. All of these systems make use of sound waves and their reflections.Sea

3. BASIC ACOUSTICSSound is the result of mechanical energy travelling through matter as a wave Sound waves are transmitted as a series of alternating pressure waves with high pressure and low pressure pulses.The high pressure areas (compression) are where the particles have been squeezed together; the low pressure areas (rarefaction) are where the particles have been spread apart. Sound waves cannot travelin vaccum

4. Audible sound waves range in frequency from 20Hz to 20,000Hz.Ultrasound waves – frequency more than 20 KHz.For medical imaging – 2 to 20 MHzSound waves – Transverse waves and longitudinal waves20 MHzDiagnostic Imaging1 MHzUltrasonic 20 KHzAudible sound 20 HzInfrasonic0 Hz

5. Ultrasound waves – propagating through body tissues and fluids - longitudinal waves – producing compression and rarefaction of conducting medium.A zone of compression and an adjacent zone of rarefaction constitute one cycle of an ultrasound wave.

6. Wavelength :The distance covered by one cycle is the wavelength of the ultrasound wave, λ The time for the ultrasound wave to pass a given point is Period, T Frequency: The number of cycles per unit time (cps, or just sec−1) introduced into the medium each second is referred to as the frequency of the wave, expressed in units of Hertz, Kilohertz or Megahertz, where 1Hz equals 1 cps.The maximum height of the wave cycle is the amplitude of the ultrasound wave.

7. Propagation: in diagnostic ultrasound pressure waves are longitudinal waves and they propagate along the direction of particle movement.The product of the frequency (ν) and the wavelength (λ) is the propagationvelocity of the wave; that is, c = νλ.Propagation depends on the resistance of medium, resistance depends on density and its stiffness or elasticity of the medium .

8. More stiff/less compressible is the molecules or less dense, more is the velocity in that medium. i.e. if density is more, velocity will be decresed and vice versa.

9. On average the propagation velocity of sound in general body tissue is approximately 1540 m/sec

10. ReflectionCurrent diagnostic ultrasound scanners rely on the detection and display of reflected sound or echoes from interfaces between different tissues in the body. The fraction of the impinging energy reflected from an interface depends on the difference in acoustic impedance of the media on opposite sides of the interface.The acoustic impedance Z of a medium is the product of the density ρ of the medium and the velocity of ultrasound (c) in the medium:Z = ρcWith a large impedance mismatch at an interface, much of the energy of an ultrasound wave is reflected, and only a small amount is transmitted across the interface.

11. Incident waveTransmitted waveMedium 1Medium 2Reflected wave

12. Specular Reflections, which occur at large change in impedance producing a large reflection, and also reducing the continuing wave amplitude.Medium Reflections, which occur with dense tissues such as muscle.Diffuse Reflections, which occur with soft tissues such as liver.

13. Specular reflectors :DiaphragmWall of urine filled urinary bladderEndometrial stripeSpecular reflector Diffuse reflector

14. Different types of reflectionsScattering

15. Distance measurement

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

17. AbsorptionCause (a) Removal of energy from the ultrasound beam and (b) Eventual dissipation of this energy primarily as heat. Ultrasound is propagated by displacement of molecules of a medium into regions of compression and rarefactionTherefore, the energy of the ultrasound beam is gradually reduced as it passes through the medium.

18. Attenuation Loss of intensity of sound during propagation.

19. Due to mainly reflection, refraction and absorptionAttenuation of ultrasound wave occurs when it is propagating through the medium.

20. Loss of propagating energy will be in the form of heat absorbed by the tissue, approximately 1 dB/cm/MHz, or caused by wavefront dispersion or wave scattering.

21. ScatteringOccurs when small particles absorb part of the ultrasound energy and re-radiate it in all directions as a spherical field. This means that the transducer can be positioned at any angle to the ultrasound beam and still receive echoes back. Scattering allowsreflections from objects evensmaller than the wavelength.Many biological interfaceshave irregular surfaces,tending to give scatter-likereflection, which is quite Useful, as it will give atleast some echoes eventhough the beam is not directly perpendicularto the reflecting interface. Spherical Scatter-wave

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

23. INSTRUMENTATION

24. BASIC COMPONENTS TO PERFORM KEY FUNCTIONSA transmitter/pulser – energize the transducerThe TransducerA receiverProcessor – to detect and amplify the backscattered energy and to manipulate the reflected signals for displayA display – that presents ultrasound image or dataA method to record or store data or images.

25. Transducer A device – generates ultrasound waves for medical imaging.Two functions – Coverts electrical energy provided by transmitter into acoustic pulses directed into the patient. also serves as receiver of reflected echoes.Works on Piezoelectric Effect/piezoelectricity. it’s the unique property of certain natural or artificial crystals , Piezoelectric crystals – when subjected to electric voltage – crystal changes shape and vibrates with its resonant frequency(realignment and change in dimension) , produces sound beam which propagates into tissues( electrical energy converted to mechanical energy) and vice versa.

26. Piezoelectric CrystalsSome naturally piezoelectric occurring materials include Berlinite (structurally identical to quartz), cane sugar, quartz, Rochelle salt, topaz, tourmaline, and dry bone An example of man-made piezoelectric materials includes barium titanate and lead zirconate titanate (PZT).Most widely used

27. TRANSDUCER DESIGNA crystal exhibits its greatest response at the resonance frequencyThe resonance frequency is determined by the thickness of the crystal and propagation velocity in its materialMost efficient operation is achieved for a crystal with a thickness equal to half the wavelength of the desired ultrasound In most of the pulsed mode of operating ultrasound the output sound has frequency both above and below the resonance frequencyThe range of frequencies are called bandwidth. Generally shorter the pulse length of the transducer more is the bandwidth. Broad bandwidth helps to reduce speckle by frequency compounding

28. TRANSDUCER DESIGN contdSpatial pulse length or Pulse length is the number of cycles per pulseIn diagnostic ultrasound we need very small pulse length for better axial resolution.So suitable damping materials (backing) are kept behind the transducer material to shorten the pulse length and spacial pulse length by stopping reverberating pulses coming from the interface behind the transducer, thus gives improved axial resolution. also reduces ultrasound amplitude and decreases efficiency and sensitivity of the system.Damping materials used – mixture of metal powder(tungsten or aluminum) and a plastic or epoxy

29. A 1/4λ (quarter-wavelength) thickness matching layer or coating in front of the transducer (generally made up of aluminum powder and epoxy resin) maximizes energy transfer from the transducer to the patient( i.e. increases sound transmission)The filler material enables the transducer assembly to be flat.The elements with its associated case, damping and matching material is called Transducer Assembly Or Probe

30. Transducer schematic diagram

31. PULSE – ECHO PRINCIPLEWhen the crystal is electrically pulsed, it changes shape and vibrates produces sound beam that propagates through tissues and reflected echoes reach the transducer it vibrates and produces electric voltage comparable to returning echo.

32. The ultrasound pulses produced by the transducer results in a series of wave fronts that form a three dimensional beam of ultrasound. the features of beam are influenced by constructive and destructive interferences of the pressure waves.

33. ULTRASOUND BEAMSUltrasound from a point source creates spherical wave fronts. Ultrasound from a two-dimensional extended source creates planar wave fronts.These sources can be considered to be a collection of point sources, each radiating spherical wave fronts (termed wavelets) into the medium. In regions where compression zones for one wavelet intersect those of another, a condition of constructive interference is established & the wavelets reinforce each other. Reverse that is cancellation occurs in destructive interference

34. Interference of the waves

35. In this figure, the reinforcement and cancellation of individual wavelets are most noticeable in the region near the source of ultrasound. They are progressively less dramatic with increasing distance from the ultrasound sourceThe near field where pressure amplitude change is maximum that zone is called “Fresnel zone”Beyond the Fresnel zone, some of the energy escapes along the periphery of the beam to produce a gradual divergence of the ultrasound beam – called Fraunhofer ( or far) zone

37. For medical applications of ultrasound, beams with little lateral dispersion of energy (i.e., long Fresnel zones) are preferred. Rules for Transducer DesignFor a given transducer diameter, the near-field length increases with increasing frequency; beam divergence in the far field decreases with increasing frequencyFor a given transducer frequency the near-field length increases with increasing transducer diameter; beam divergence in the far field decreases with increasing transducer diameter.

38. Focused TransducersA focused ultrasound transducer produces a beam that is narrower at some distance from the transducer face than its dimension at the face of the transducer.In the region where the beam narrows (termed the focal zone of the transducer), the ultrasound intensity may be heightened by 100 times or more compared with the intensity outside of the focal zone. Because of this increased intensity, a much larger signal will be induced in a transducer from a reflector positioned in the focal zone.The distance between the location for maximum echo in the focal zone and the element responsible for focusing the ultrasound beam is termed the focal length of the transducerAn ultrasound beam also may be focused with mirrors and refracting lenses or phasing the linear array electronically

39. Focused Transducers

40. Receiver Returning echoes strike the transducer face (which is on quiescent mode electronically) and they produce minute voltages in the piezoelectric crystals.The receiver detects those signals and amplifies thoseThey also provide compensatory amplification of the weaker signals those are coming from the deeper tissues.This is calledtime gain compensation

41. Time gain compensation

42. Time gain compensationWe can also suppress increased amplitudes of the near field for better comparable image

43. Receiver Another function of the receiver is compression of the wide range of amplitudes returning to the transducer into a range that can be displayed to the user. The range is called dynamic rangeof the transducer.Compression and remapping of the data is required to adapt the dynamic range of backscattered signal intensity to the dynamic range of display

44. Receiver – dynamic range The widest dynamic range (60 dB) shown permits the best differentiation of subtle differences in echo intensity and preferred in most imaging applicationThe narrower ranges increase conspicuity of larger echo differences.

45. ARRAYS(TRANSDUCER TYPES)

46. Linear Array The linear array scanners produce sound waves parallel to each other and produces a rectangular image. The width of the image and number of scan lines are the same at all tissue levels. This has the advantage of good near field resolution. Often used with high frequencies i.E. 7mhz.Can be used for viewing surface texture of the liver. There disadvantage is artifacts when applied to a curved part of the body creating air gaps between skin and transducer.

47. Sector/Phased array ( freq 1 to 3MHz in adult, in pediatric sector probes upto 8 MHz) Produces a fan like image that is narrow near the transducer and increase in width with deeper penetration. It is useful when scanning between the ribs as it fits in the intercostals space.The disadvantage is poor near field resolution

48. Curved ArrayOften with frequencies of 2 - 5 MHz (to allow for a range of patients from obese to slender). It is a compromise of the Linear and Sector scanners. The density of the scan lines decreases with increasing distance from the transducer.Can be difficult to use in curved regions of the body eg. the spleen behind the left costal margin.For abdominal ultrasound curved type scanners are used as the best compromise of two other standard type probes the linear and the sector scanner.

49. Annular ArrayA series of concentric elements nested within one another in a circular piece of piezoelectric crystal to produce an annular arrayUse of multiple concentric elements enables precise focussing.

50. Linear & Phased arrayLinear array-individual elements or groups are fired in sequence resulting parallel US beamsPhasic array: produce a sector field of view by firing multiple transducer elements in precise sequence to generate interference of acoustic wave fronts

52. Transducer SelectionFor general purpose – convex with 3.5 MHzFor obstetric purpose – convex or linear with 3.5 MHzFor superficial structures – linear with 5MHzFor pediatric or in thin people – 5MHz

53. COUPLING AGENTSCommonly known as “GEL”Fluid medium needed to provide a link between the transducer and the patientComposition :Carbomer – 10.0 gm Propylene glycol – 75.0gm(72.4ml)Trolamine – 12.5gm(11.2ml) EDTA – 0.25gm Distilled water – upto 500gm or 500ml

54. IMAGE DISPLAY

55. A-Mode (amplitude mode)A mode display consists of a horizontal baseline. This baseline represents time and or distance with upward (vertical) deflections (spikes depicting the acoustic interfaces)In the A-mode presentation of ultrasound images, echoes returning from the body are displayed as signals on an oscilloscope.

56. A-Mode (amplitude mode)A-mode reveals the location of echo-producing structures only in the direction of the ultrasound beam. A-mode displays are not found on most imaging systems used today. The concept of A-mode is however, useful in explaining how pixels are obtained from scan lines in B-mode imaging.

57. B-mode (Brightness Mode)Real-time gray scale B-Mode displayA two-dimensional display of ultrasound. To generate a two dimensional image multiple ultrasound pulses are sent down as a series of successive scan lines, building a 2D representation of echoes arising from the area being scanned.When an ultrasound image js displayed on a black background, signals of greatest intensity appear as white, absence of signal is shown as black(anechoic) and signals of intermediate intensity appear as shades of grey.

59. B-mode, or brightness mode, ultrasound display.

60. B-mode images may be displayed as either “static” or “real-time” images. In static imaging the image is compiled as the sound beam is scanned across the patient, and the image presents a “snapshot” averaged over the time required to sweep the sound beam.

61. B mode real time imagingIn real-time imaging, the image is also built up as the sound beam scans across the patient, but the scanning is performed automatically and quickly, and one image follows another in quick succession.( as many as 30-60 complete images per second)Real-time B-mode images are useful in the display of moving structures such as heart valves.

62. M-mode Ultrasound (motion mode)Is the motion mode displaying moving structures along a single line in the ultrasound beamA single beam in an ultrasound scan can be used to produce an M-mode picture where movement of a structure such as a heart valve can be depicted in a wavelike manner. Because of its high sampling frequency (up to 1000 pulses per second) This is useful in assessing rates and motion and is still used extensively in cardiac and fetal cardiac imaging

63. IMAGE QUALITYSpacial ResolutionAxial : higher frequency – low wavelength  higher axial resolution Lateral resolution: excessive beam width and thickness limit the ability to delineate small features such as tiny cystic changes in atherosclerotic plaqueLateral resolution is controlled by focusing the beam by electronic phasingElevation or Azimuthal resolution is determined by construction of the transducer and can not be controlled by the user

64. IMAGE ARTIFACTS

65. Mirror Image Artifact • This is where a strong reflector at an angle to the probe causes structures that lie in front and to the side of it to appear as if they lie behind it, just as something viewed through a mirror appears to lie behind it.• e.g. diaphragm

66. Reverberation artifactWhen the ultrasound signal reflects repeatedly between highly reflective interfaces that are usually near the transducerReduced by changing the scan angle and placement of the transducer

67. Comet tail artifacts• This is the same process as reverberation, but occurs within a very small structure, with smooth highly reflective borders, e.g. metal fragment.• Tiny bright reverberations are seen deep to the structure slowly diminishing is size as if it had a tail

68. Refraction ArtifactThis can lead to subtle miss placement of structures and some degradation of image quality when the angle of incidence is particularly acute.Rectus abdominis acting as refracting lens

69. Range artifact

70. Side Lobe ArtifactsThe probe cannot produce a pulse that travels purely in one direction.Pulses also travel off at specific angles.Side lobes may interact with strong reflectors that lie outside the scan plane.These side lobes are relatively weak and so normally do little to degrade the image.Can be reduced by repositioning of the transducer ,or its focal zone or using a different transducer.GB

71. Acoustic shadowing Marked reduction in the intensity of ultrasound deep to a strong reflector or attenuator. ( other cause of loss of image information are improper system gain and TGC setting, poor scanning angles and poor resolution)

72. Acoustic enhancementDue to high penetration of sound through a structure and then being reflected from the tissue behind it.

73. Multipath artifactEchoes reflected (e.g. from diaphragm and wall of ovarian cyst) may create complex echo path that delay return of the reflected signal thus showing false deeper/abnormal position of the tissuesOvarian cyst --- false mural nodule

74. Speckle ArtifactUsual grainy appearance of the USG imageDue to random interference of the scatterer .Irregular interference pattern. This is generated by more scatterers somewhat randomly distributed. The speckle pattern is thus random too. Interference pattern. Here is simulated two wave sources or scatterers at the far field (white points).

75. Speckle Artifact

76. Spacial imaging modesHarmonic imaging: to improve spatial resolution and reduction of side lobesSpatial compounding: to reduce speckle using multiple scan angleSpatial compounding

77. “HARDWORK , SINCERITY AND PERSEVERENCE PAVES THE WAY TO SUCCESS”THANK YOU