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Basic of ultrasound

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Basic of ultrasound

  1. 1. VASCULAR ULTRASOUND BASICS, COLOUR & POWER DOPPLER DR. ASAD MOOSA
  2. 2. VASCULAR ULTRASOUND IMAGING Vascular ultrasound imaging has become an integral and essential part of the provision of a Vascular Surgical Service Duplex ultrasound scanners combine real time B-mode imaging with pulsed wave Doppler to display anatomy and blood velocity data simultaneously Ultrasound is defined as a sound wave which has a frequency greater than 20kHz Sound waves travel through a medium by causing a local displacement of particles within the medium
  3. 3. BASICS
  4. 4. GENERATION OF ULTRASOUND WAVES The piezoelectric effect is the method by which ultrasound is generated An ultrasound transducer, consisting of an array of piezoelectric crystals, is used to generate and detect ultrasound waves An ultrasound transducer converts electrical energy to a mechanical vibration and vice versa Since ultrasound is a mechanical wave in a longitudinal direction, it is transmitted in a straight line and it can be focused. These waves obey laws of reflection and refraction.
  5. 5. Since small objects in the human body will reflect ultrasound, it is possible to collect the reflected data and compose a picture of these objects to further characterize them. Major drawback of ultrasound is the fact that it cannot be transmitted through a gaseous medium (like air or lung tissue). As ultrasound transverses tissue, its energy decreases. This is called attenuation and is more pronounced in tissue with less density (like lung). There are seven parameters that describe ultrasound waves. • Period • Amplitude • Power • Wavelength • Propagation of Speed • Pulse Repetition Frequency
  6. 6. Period Period of an ultrasound wave is the time that is required to capture one cycle, i.e., the time from the beginning of one cycle till the beginning of the next cycle. Period of ultrasound is determined by the source and cannot be changed by the sonographer. Frequency is the inverse of the period and is defined by a number of events that occur per unit time. The units of frequency is 1/sec or Hertz (Hz). Since f = 1/P, it is also determined by the source and cannot be changed.
  7. 7. Amplitude Amplitude is an important parameter and is concerned with the strength of the ultrasound beam. It is defined as the difference between the peak value and the average value of the waveform. It is expressed in decibels or dB, which is a logarithmic scale. It can be changed by a sonographer. Amplitude decreases as the ultrasound moves through tissue, this is called attenuation. Amplitude decreases usually by 1 dB per 1 MHz per 1 centimeter traveled.
  8. 8. Power Power of ultrasound is defined as the rate of energy transfer and is measured in Watts. It is determined by the sound source and it decreases as the beam propagated through the body. Intensity of the ultrasound beam is defined as the concentration of energy in the beam. Intensity = Power / beam area = amplitude2 / beam area, thus it is measured in Watts per cm2 . It is the key variable in ultrasound safety. Intensity also decreases as the ultrasound propagates through tissue.
  9. 9. Wavelength Wavelength is defined as the length of a single cycle. It is measured in the units of length. It is determined by both the source and the medium. Wavelength cannot be changed by the sonographer. It influences the longitudinal image resolution and thus effect image quality. Typical values of wavelength are 0.1 – 0.8 mm. Wavelength (mm) = Propagation speed in tissue (mm/microsecond) / frequency (MHz). High frequency means short wavelength and vice versa.
  10. 10. Propagation of Speed Propagation speed in human soft tissue is on average 1540 m/s. It is defines as to how fast the ultrasound can travel through that tissue. It is determined by the medium only and is related to the density and the stiffness of the tissue in question. Density of the medium is related to its weight and the stiffness of the medium is related to its “squishability”. As the medium becomes more dense, the slower is speed of ultrasound in that medium (inverse relationship). The stiffer the tissue, the faster will the ultrasound travel in that medium.
  11. 11. PULSE REPETITION FREQUENCY & TIME For imaging purposes, a transducer is required to generate short pulses of ultrasound rather than continuous waves This is achieved by applying very short voltage pulses across the transducer The pulse repetition frequency is the number of pulses transmitted per unit time (Hz) PRF is the number of pulses that occur in 1 second. PRF can be altered by changing the depth of imaging. It is measured in Hertz (Hz). PRF = 77,000 / depth of view (cm). As evident from the equation, as the location of the target gets further away, the PRF decreases. PRF is related to frame rate or sampling rate of the ultrasound.
  12. 12. INTERACTION OF ULTRASOUND WAVES •The energy of ultrasound decreases (attenuation) as it travels through tissue. •The stronger the initial intensity or amplitude of the beam, the faster it attenuates. •Attenuation of ultrasound in soft tissue depends on the initial frequency of the ultrasound and the distance it has to travel. •As we saw in the example above, in soft tissue the greater the frequency the higher is the attenuation. •So we can image deeper with lower frequency transducer. •The further into the tissue the ultrasound travels, the higher the attenuation is, so it is ultimately the limiting factor as to how deep we can image clinically relevant structures •Creation of an ultrasound image depends on the way in which an ultrasound wave interacts with tissue as it passes through the body – Reflection – Refraction – Scattering – Attenuation
  13. 13. IMAGE RESOLUTION Axial Resoluion “The ability to distinguish and display the minimum reflector spacing along the axis of the ultrasound wave”. Axial resolution is determined by the Pulse duration (the time is takes for a single pulse to occur). PD=No of cycles in a pulse/frequency. Therefore, by increasing the transducer frequency (and decreasing wavelength) better image resolution is achieved.
  14. 14. Lateral Resolution Lateral resolution is the minimum distance that can be imaged between two objects that are located side to side or perpendicular to the beam axis. Again, the smaller the number the more accurate is the image. Since the beam diameter varies with depth, the lateral resolution will vary with depth as well. The lateral resolution is best at the beam focus (near zone length). Lateral resolution is usually worse than axial resolution because the pulse length is usually smaller compared to the pulse width.
  15. 15. PROPAGATION OF ULTRASOUND Acoustic Impedance and Reflection • Perpendicular Interface • Non-Perpendicular Interface Refraction Scattering Attenuation Absorption
  16. 16. ACOUSTIC IMPEDANCE Interfaces with a large miss-match of acoustic impedance reflect more of the incident beam than those of similar qualities. For example, at the muscle-air interface in the lungs, the majority of the incident beam is reflected back to the transducer. This is why ultrasound is not a good imaging modality for air filled spaces/tissues and why coupling gel must be used to transfer ultrasound waves from the probe into the tissue Ζ=ρc At an interface between two different tissues (i.e. blood vessel/muscle interface) the strength of the reflected echoes is determined by the acoustic impedance of the material. Acoustic impedance (Ζ) is determined by the tissue density (ρ) and propagation speed (known as 1540m/s in soft tissue).
  17. 17. REFLECTION-PERPENDICULAR INTERFACES Upon arrival at an tissue boundary/interface, some of the echoes will be reflected back to the transducer, some will propagate through the tissue beyond the boundary. Tissue acoustic impedance effects the amplitude of reflected sound waves. This is calculated by the Amplitude Reflection Coefficient (R) R=Z2-Z1 Z1=Acoustic impedance prior to tissue boundary. Z2=Acoustic impedance across the tissue boundary
  18. 18. REFRACTION-NON-PERPENDICULAR INTERFACE If a sound wave encounters an interface at a non-perpendicular angle, the reflected angle is the same at the incident angle. Refraction of the transmitted beam beyond the tissue boundary/interface may also occur. Two conditions must be met: 1) Non perpendicular beam incidence. 2) Differing speed of sounds either side of the interface. The amount of Refraction is calculated using SNELL’S LAW. sinθt = c2 sinθi = c1
  19. 19. ATTENUATION Attenuation is the weakening of sound waves as they propagate through tissue. The main culprit for attenuation is depth. Attenuation (dB)= α(dB/cm) x d (cm) α=attenuation coefficient d= Depth Another factor affecting attenuation is Frequency. Attenuation Increases with Increasing Frequency
  20. 20. B- MODE
  21. 21. A B-Mode image is formed by transmitting pulses of ultrasound waves into the body, where they encounter interfaces/tissue boundaries. These surfaces produce echoes which are returned back to the transducer for converting and displaying on the ultrasound monitor. In order to display the echoes within their corresponding anatomical position, the depth of the returning echo must be calculated using the RANGE EQUATION. T=2D/c T=Time taken for the transmitted echo to return. D= Reflector depth from the transducer. c=Average speed of sound in soft tissue = 1540m/s
  22. 22. B-MODE ARTIFACTS Propagation Artifacts Reverberation Mirror Image Speed error and Refraction Attenuation Artifacts Shadowing-picture of calcified plaque Enhancement-picture of seroma
  23. 23. REVERBERATION Occurs when a sound reverberates back and forth between the two strong parallel reflectors. This artifact can be improved by changing the angle of isonation.
  24. 24. MIRROR IMAGE This type of artefact is closely associated with reverberation. Mirror images form when echoes are falsely displayed beyond a strong reflector. Transmission of sound waves through structure to a strongly reflecting surface such as bone or air. These waves are then reflected back up towards the transducer, however they are reflected by the initial structure and returned back down throughout the tissue. After numerous rounds of reverberation, the echoes eventually arrive back to the transducer but because they arrive after the echoes from the strongly reflecting surface, the ultrasound machine falsely displays a second ‘mirror image’ beyond the strong reflector surface.
  25. 25. • The average speed of sound in soft tissue is 1540m/s. This value is assumed when calculating depth of reflector echoes. • If the propagation speed deviates from this value, the reflector will be displayed either to near or too far from the transducer. • Speed error therefore displaces structures within the axial plane. • Can be prevented by changing the angle of the beam. Speed Error
  26. 26. SHADOWING Shadowing occurs when echoes beyond a strongly reflective or attenuating structure are reduced in strength (decreased amplitude). The reduced amplitudes are displayed as much darker echoes. Examples are calcific plaque along a vessels. This results in shadowing, sometimes obscuring the vessel segment
  27. 27. ENHANCEMENT In contrast to shadowing, some weakly attenuating structures facilitate strengthening of echo amplitudes. These high amplitude signals are displayed as bright echoes compared to surrounding tissue. Fluid filled collections such as a seroma often display enhancement beyond the structure. Enhancement can help to distinguish fluid filled seromas (weakly attenuating) from resolving haematomas (less enhancement compared to seromas).
  28. 28. COLOR DOPPLER
  29. 29. DOPPLER EFFECT The Doppler effect is used in vascular ultrasound imaging and is defined as “the change in frequency of a detected wave when the source or the detector are moving” The Doppler equation is: fd = 2 ft v cosθ c • fd is the Doppler shift frequency • ft is the frequency emitted by the transducer • θ is the angle between the Doppler beam & the direction of flow • c is the velocity of sound in the medium • v is the speed of the blood cells
  30. 30. ANGLE OF INSONATION θ The angle between the ultrasound beam & the direction of blood flow θ is of fundamental importance in clinical practice As θ increases, cos θ decreases and the Doppler shift frequency fd decreases At θ =90°, there is no Doppler shift and no signal will be heard
  31. 31. VELOCITY PROFILE & DOPPLER SPECTRUM The velocity profile is the range of velocities across the vessel Each of the velocities within the blood flow velocity profile will produce a different Doppler shift frequency fd The Doppler spectrum is the combination of the different velocities reflected from across the lumen of a blood vessel
  32. 32. GENERATION OF A COLOUR DOPPLER IMAGE Using the Doppler principle, images of tissue motion or blood flow are produced Blood cells are displayed as a colour coded image, which is superimposed onto the B-mode image The mean Doppler frequency in each blood cell along a scan line is represented by an associated shade of colour
  33. 33. GENERATION OF A COLOUR DOPPLER IMAGE Low mean Doppler frequencies are displayed in darker shades and high Doppler frequencies in lighter shades The display is arranged to show both flow toward and away from the direction of the incident beam of ultrasound e.g. shades of red for one direction and shades of blue for the opposite direction Red - blood flow travelling towards transducer Blue - blood flow travelling away from transducer
  34. 34. BLOOD FLOW VELOCITY PROFILE – LAMINAR FLOW Normal laminar flow demonstrates slow flow close to the vessel walls, due to frictional forces and faster flow centre stream
  35. 35. ALIASING Colour Doppler information is calculated from a sampled signal, therefore is subject to the Nyquist limit.(NL=one half of PRF) Hence, colour aliasing can occur in the presence of high mean Doppler frequencies (Exceeding Nyquist limit) In aliasing, displayed colours “wrap around” the colour scale & the colours change from the maximum colour in one direction, to the maximum colour in the opposite direction
  36. 36. MAIN COLOUR DOPPLER CONTROLS The following controls on the ultrasound scanner are adjusted continuously throughout the scan to optimise the colour Doppler images • Colour box steering • Colour direction assignment • Colour scale/pulse repetition frequency • Colour display baseline • Colour wall filter • Colour box size • Colour gain • Focal zones
  37. 37. COLOUR DOPPLER IMAGES Indicate patency or absence of flow Flow through a stenosis Highlight areas of flow disturbance Pulsatile flow Indicate flow direction, helping to define tortuosity or steal phenomena Indicate flow in vessels too small to be resolved in B-mode e.g. arteriovenous malformations
  38. 38. GENERATION OF A POWER DOPPLER IMAGE Power Doppler displays the amplitude of the Doppler signals A single colour scale is used and different shades represent different signal strengths Bright shades represent strong signals Dimmer shades represent weaker signals
  39. 39. GENERATION OF A POWER DOPPLER IMAGE The colour and brightness of signals are related to the number of blood cells producing the Doppler shift Advantages over Colour Doppler • More sensitive than colour Doppler • Not direction dependent • No aliasing
  40. 40. POWER DOPPLER IMAGES Indicate patency or absence of flow Flow through a stenosis Pulsatile flow Indicate flow in vessels too small to be resolved in B-mode e.g. arteriovenous malformations
  41. 41. ARTEFACTS IN COLOUR & POWER DOPPLER Flash Artefact • High-pass filters are used to suppress signals arising from stationary or near stationary tissue • When the transducer is moved or there is movement of tissue e.g. patient movement from respiration, flashes of colour are seen across the field of view Mirror Artefact • Vessels overlying strong reflectors e.g. bone/air may also be seen as reflections in the image lying beyond the reflector surface
  42. 42. APPLICATIONS OF COLOUR & POWER DOPPLER Determine the presence, location and degree of disease in the peripheral arteries and veins Evaluate blood flow in the lower limb arteries in patients with intermittent claudication or critical ischemia. Evaluate blood flow in the upper limb arteries in patients with ischemia or symptoms of TOS. Evaluate blood flow in the carotid arteries after a TIA or stroke Evaluate blood flow in the lower limb veins in patients with varicose veins or suspected DVT
  43. 43. APPLICATIONS OF COLOUR & POWER DOPPLER Map veins and check patency to determine suitability for use for bypass grafts or fistula formation Monitor the flow of blood following vascular surgery e.g. Lower limb bypass surveillance, EVAR surveillance or fistula surveillance Guide treatment of varicose veins such as laser, radio frequency ablation or foam sclerotherapy Determine patency or vascularisation in collections e.g. false aneurysms, carotid body tumours
  44. 44. THANK YOU!

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