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Usg transducer and basic principles of ultrasound Doppler, this slide describe the basic physics of ultrasound transducer and Doppler , must know thing is given in this presentaion. Good review for radiology resident. Thanks.

Usg transducer and basic principles of ultrasound Doppler, this slide describe the basic physics of ultrasound transducer and Doppler , must know thing is given in this presentaion. Good review for radiology resident. Thanks.


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Ultrasound transducer doppler ppt pdf pk

  1. 1. Ultrasound transducer and interaction of ultrasound with tissues. Doppler USG Pradeep Kumar
  2. 2. Basic Component Basic components of US comprises of: • Transducer • Control panel • Pulse generator • Amplifier • Scan converter • Image memory and • Image display.
  3. 3. Transducer Convert an electrical energy to ultrasonic energy and vice versa. Constituents 1. Piezoelectric Crystal 2. Two electrodes 3. Backing block 4. Matching layer 5. Plastic Housing
  4. 4. Piezoelectric Crystal  It is the functional component of the transducer Characterized by well defined molecular arrangement of electrical dipoles An electrical dipole is molecular entity containing positive and negative electrical charge that has no net charge . Natural : Quartz Ceramics: PZT, PbTiO3, PLZTio3 Polymers: PVDF Latest: PMN-PT
  5. 5. An external voltage source applied to the element surfaces causes compression or expansion from equilibrium by realignment of the dipoles in response to the electrical attraction or repulsion force.
  6. 6. Matching layer The matching layer provides the interface between the transducer element and tissue and minimize the acoustic impedance difference b/w transducer and patient  made from epoxy resin ,perspex . It reduce the acoustic impedance difference between soft tissue and PZT so that more ultrasound is transmitted and less is reflected .  thickness 1/4th the wavelength of the ultrasound produced .
  7. 7. Backing block Aka damping material: layered on the back of the piezoelectric element , Damping material used is mixture of metal powder (tungsten or aluminum), plastic or epoxy resin It absorb the backward directed ultrasound energy and attenuates stray ultrasound signal from the housing .
  8. 8. Outer casing Made by a strong plastic ,and it prevent from passing into the housing Acts for housing for active material Insulate from electrical noise
  9. 9. Electrodes The surface of the piezoelectric crystal are placed with gold or silver electrodes The outside electrode is grounded to protect the patient from electrical shock and its outside is coated with a watertight electrical insulator The inside electrodes abuts against a thick backing layer .
  10. 10. Types of Transducer 1 Mechanical Scanner(Transducer is mechanically moved to form images in real time.) 2. Electronic Scanner
  11. 11. Electronic Array Transducer Current using technology Basically are of two types- - Linear array- produces rectangular scan - Phased/steer array- produces sector scans. Both are composed of many small rectangular transducer elements ( 2x10mm) arranged adjacent to each other. Unlike mechanical, the transducers in electronic array system do not move. US beam is moved by electronic controls.
  12. 12. Linear Array  Individual elements arranged in linear fashion  256 to 512 small rectangular elements each 2mmx10mm arranged in a line  By firing the elements in sequence either individual or in group a series of parallel pulse is generated  In linear array each time only a group of elements work together to transmit or receive  Size of FOV is equal in both the far field and near field USES: superficial structure such as vessels , neck , liver texture
  13. 13. Linear array transducer is operated by applying voltage pulses to groups of elements in succession . Each group of element acts as a larger transducer element . The width of the image is approx. equal to the length of the array
  14. 14. Curved Array Linear array that have been shaped into convex curves Gives sector display with relatively larger field of view. High frequency curved array transducers are often used in transvaginal and transrectal probes and for pediatric imaging. It is constructed as a curved line of elements rather than a straight line. Operation is similar to that of linear array Applications: Abdominal Obstetric Transabdominal pelvic scan
  15. 15. Focused array Focused transducer restrict beam width & improve lateral resolution but they are designed to focus at a specific depth so that it can pass through space between structures & permit them to appear as separate entities Focusing is done by- i)The lens material use to focus the beam are polystyrene or epoxy resin ii)By crystal with a concave face ii)By electronic focusing(steering)
  16. 16. Contd.. A 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.
  17. 17. Phased Array Also called electronic sector Transducer Generally 32 elements in the array Operates at frequency of 2-3 MHz Sector scan obtained Transducer does not move during imaging. Beam can be directed or steered at the desired angle by choosing appropriate time delay between stimulation of individual elements. In the similar way beam can be focused as well. Steering and focusing can be achieved at the same time.  it steer the beam by applying different time delay on each elements. Provide very broad imaging field at larger depth
  18. 18. Types of phased array • Convex phased array • Linear phased array • Focused phased array
  19. 19. Convex phased array It is operated by applying voltage pulse to all elements(not a small group) but with small time differences (phasing) so that the resulting sound pulse may be sent over a specific path If the same time difference are used, then the process is repeated and the sound pulse is produced from the same direction However the time differences ( phasing) are changed with each successive repetition so that the beam direction is continually changed. This can then result sweeping of the beam Used in spectral Doppler & color flow imaging
  20. 20. Linear phased array Phasing can be applied to the group in a linear array transducer to steer pulses in a direction other than perpendicular to the array or in various directions Also called vector array Image is similar to that of convex phased array except that the contact surface is smaller & the top of the display is flat More elements can be used at one time thus provide larger aperture compared to convex phased array
  21. 21. Focused phased array Phasing provides electronic control of the location or depth of the focus Multiple focuses can be employed. However a pulse can be focused at only one depth therefore multiple focuses require multiple pulses Use of multiple pulses per scan line takes more time & the frame rate is reduced thus decreasing temporal resolution but increasing detail resolution at the same time.
  22. 22. Annular Array A series of concentric elements nested within one another in a circular pieces of piezoelectric crystals Use of multiple concentric elements enables precise focusing with a cone shaped beam reducing the beam width in the scan plane & perpendicular to it Reduces section thickness artefact
  23. 23. Some other transducers Capacitive Micromachined US Transducers Wireless transducer Dual element transducers
  24. 24. Dual element transducers
  25. 25. TRANSDUCERS FOR SPECIFIC PURPOSE Trans-oesophageal Transducer Trans-rectal Transducer Trans-vaginal Transducer Endovascular Transducer Endoscopic Transducer Aspiration Transducer
  26. 26. Transducer used for special purpose
  27. 27. SELECTION OF TRANSDUCER For General purpose-convex with 3.5 MHz For obstetric purpose- convex or linear with 3.5 MHz For Superficial structures-linear with 5 MHz For pediatric or in thin people-5 MHz
  28. 28. A short-duration voltage spike causes the resonance piezoelectric element to vibrate at its natural frequency, which is determined by the thickness of the transducer equal to ½ lambda. Low-frequency oscillation is produced with a thicker piezoelectric element. The spatial pulse length (SPL) is a function of the operating frequency and the adjacent damping block.
  29. 29. Beam focusing For a single transducer or a linear array , the focal distance is the function of the transducer diameter ,the center operating frequency and presence of any acoustic lenses attached to the element surface.  Phase array and many linear array transducers allows a selectable focal distance by applying specific timing delays between transducer elements that cause the beam to converge at a specified distance A shallow focal zone (close to transducer surface ) is produced by firing outer element in array before the inner element in a symmetrical pattern Greater focal distances are achieved by reducing the delay time difference among the transducer elements , resulting in more distal beam convergence.
  30. 30. The transducer crystal are fired simultaneously with programmable delay time for transmit focusing
  31. 31. In the figure we can figure out variable timing circuitry that will differ the focal zone. Firing the crystal at different time changes the position of the focal plane.
  32. 32. A-mode Also called amplitude mode ,echoes are displayed as spike projecting from a base line It is the graphical display of measurement of tissue interface The base line identifies the central axis of the beam The spikes height is proportional to echo intensity Uses: ophthalmology, EEC, ECHO, midline shift in brain and together with B mode when accurate measurement is required.
  33. 33. B-mode B-mode (B for brightness) is the electronic conversion of the A-mode and A-line information into brightness-modulated dots on a display screen In general, the brightness of the dot is proportional to the echo signal amplitude The position of a dot along the time base is a measure of the distance of the associated reflector from the transducer. Used for M-mode and 2D gray-scale imaging
  34. 34. M-Mode M-mode (“motion” mode) or T-M mode (“time-motion” mode): displays time evolution vs. depth M-mode is valuable for studying rapid movement, such as mitral valve leaflets Sequential US pulse lines are displayed adjacent to each other, allowing visualization of interface motion The transducer is placed in one fixed position in relation to the moving structure. Returning echoes are displayed in the form of dots of varying intensity along a time base as in B mode.
  35. 35. Coupling Agent • The coupling agent prevents air pockets forming between the skin and the transducer. • Composition: Carbomer : 10.0 gm Propylene Glycol : 7.5 gm Theolamine : 12.5 gm EDTA(edetic acid) : 0.25 gm Distilled water : upto 500 ml
  36. 36. • One of the problem that must be considered is the attenuation of the ultrasonic beam with depth. • Equally reflective interface produce different signal levels depending on their relative distance from the transducer. • It is often advantageous to display reflectors of similar size, shape and equal reflection coefficient with equal signal strength or brightness levels. • Exponential amplification is used to correct the signal for attenuation known as Time Gain Compensation(TGC)
  37. 37. Time Gain Compensation It is a way to overcome ultrasound attenuation in which signal gain is increased as time passes from the emitted wave pulse. This correction makes equally echogenic tissue look the same even if they are located in different depths. Attenuation: decrease in intensity of sound as they pass through the tissues measured in db/cm. Gain: amplification of the reflected ultrasound waves by the ultrasound unit. Deeper tissues need more gain. Acoustic impedence: resistence offered by the tissues to the movement of particles caused by ultrasound waves. Density x velocity
  38. 38. Pulser Provides the electrical voltage for excitation of Piezoelectric element and controls the output transmit power by adjustment of applied voltage. Increase of transducer amplitude results in the increase of sound intensity so higher will be the SNR. Output power is usually indicated in terms of thermal index (TI) and mechanical index(MI).
  39. 39. Ultrasound waves when strike the medium, cause expansion and compression of the medium. Various types of interactions Reflection Scattering Refraction Absorption Attenuation.
  40. 40. • Reflection: when us is deflected towards the transducer. The major factors affecting the amount of reflection are : • Angle of incidence. • Acoustic impedence mismatch • Width of tissue boundry . If less than wavelength wont be reflected • Angle of tissue boundry.
  41. 41. • Scattering : occurs when the width or lateral dimension of the tissue boundary is less than one wavelength. RBC • Refraction: when ultrasound beam is deflected from a straight path and angle of deflection is away from the transducer. Enhanced image quality by using acoustic lenses. Can result in ultrasound double image artefact. • Attenuation: depends upon 1.transducer frequency low freq low attenuation. 2. Acoustic impedance 3.Acoustic impedance mismatch
  42. 42. • Absorption: 1. Dissipation of sound energy into the medium. 2. Energy transformed into other form (heat). 3. Medical application of therapeutic US.
  43. 43. Doppler ultrasound Christian Doppler (1803-1853)
  44. 44. Doppler Effect • Change in the perceived frequency of sound produced by a moving source. • Basis of Doppler Ultrasonography: Reflected/scattered ultrasonic waves from a moving interface will undergo a frequency shift
  45. 45. • In diagnostic ultrasound, the Doppler effect is used to measure blood flow velocity. • When emitted US beam strikes moving blood cells, the latter reflect the pulse with a specific Doppler Shift frequency that depends upon the velocity and direction of the blood flow.
  46. 46. Factors affecting Doppler signal • Blood velocity • Ultrasound frequency • Angle of insonation
  47. 47. • Choice of frequency is a compromise between better sensitivity to flow or better penetration; • Doppler ultrasound equipment : filters to cut out the high amplitude, low-frequency Doppler signals from tissue movement, (e. g. due to vessel wall motion). Filter frequency can usually be altered by the user, for example, to exclude frequencies below 50, 100 or 200 Hz (limits the minimum measureable flow velocities).
  48. 48. Doppler techniques 1. Continuous wave Doppler 2. Pulsed Doppler: Color, Power and Spectral Doppler
  49. 49. Continuous Doppler • Uses two crystals, one to send and one to receive. • Uses continuous transmission and reception of ultrasound. • Doppler signals are obtained from all the vessels in the path of US beam (until it gets sufficiently attenuated due to depth).
  50. 50. • Unable to determine the specific location of velocities within the beam and cannot be used to produce color flow images. • Used in adult cardiac scanners to investigate the high velocities in the aorta.
  51. 51. Pulsed Wave Doppler • Same transducer as transmitter & receiver. • The sensitive volume from which flow are sampled can be controlled in terms of shape, depth and position. • Transmitter mode → short duration bursts of pulses at precise rate of repetition (Pulse repetition frequency or PRF) → receiver turned on (gating on) for short period at specified time → returning signals (echoes). • Used in general and obstetric Doppler ultrasound scanner. Provides data for Doppler sonograms and color flow images.
  52. 52. Advantages: 1. Selection of site & sample volume for Doppler. 2. No overlap of information from other vessels/structures. Disadvantages: 1. Angle restriction if same transducer for imaging & Doppler. 2. Flow information from one site in image.
  53. 53. Flow imaging Modes • Color Flow • Power/energy/amplitude flow • Spectral Doppler
  54. 54. Color Doppler  to identify vessels for examination,  to identify the presence and direction of flow,  to highlight gross circulation anomalies, throughout the entire color flow image,  to provide beam/vessel angle correction for velocity measurements.
  55. 55. Advantage of DCFI 1. Technical efficiency → small vessel detection. 2. Global Doppler sampling. 3. Vascular evaluation: entire luminal evaluation, vascular vs non vascular structures, stenotic measurements, severe stenosis vs occlusion, flow direction (reversal /reflux), turbulence at high velocity gates. 4. Rapid acquisition of data.
  56. 56. Limitations 1. Qualitative flow information (mean velocity at each point). 2. Limited PRF → limited Doppler shift frequency → aliasing. 3. No accurate information of abnormal flow like range gated Doppler. 4. Angle dependent flow information → stenosis vs occlusion. 5. Obscured vascular pathology d/t flow artifacts. 6. Color out by other moving structures (peristalsis). 7. High cost.
  57. 57. Power Doppler (Energy Doppler/ Amplitude Doppler/ Doppler angiography) Features: a. The magnitude of the color flow output is displayed & not Doppler frequency signal. b. Does not display flow direction or different velocities (no directional information) c. Sensitive to low flows (used in conjunction with frame averaging to increase sensitivity to low flows and velocities). Hybrid color flow modes (incorporating power and velocity data )are also available from some manufacturers. These can also have improved sensitivity to low flow Complements other two modes
  58. 58. Duplex Scanning • Real time B-mode scanners with built in Doppler Capabilities. • B-mode: Outlines anatomic structures • Pulsed Doppler: Flow and movement patterns Triplex mode: When combined with spectral analysis
  59. 59. Spectral Doppler • Display in Graphic form as time varying plot of the frequency spectrum of the returning signal, Provides Quantitative information. • Fast Fourier transformation used to perform frequency analysis. • Gate: Used to determine the interval after emission when returning signals are received and therefore the depth from which sample is taken.
  60. 60. Doppler Spectrum Assessment • Presence of flow • Direction of flow • Amplitude • Window • Pulsatility
  61. 61. How to improve the sensitivity? • Increase power or gain • Decrease velocity scale • Decreasing reject or filter • Slowly increasing the SV size.
  62. 62. Direction of flow • Monophasic • Biphasic • Triphasic • Bidirectional
  63. 63. Amplitude • Displayed as the brightness of display • Determined by: • Echo intensity • Power • Gain • Dynamic range
  64. 64. Window • The received shift consists of a range of frequencies. • Narrow range of frequencies will result in a narrow display line. • The clear underneath the spectrum is called window.
  65. 65. Spectral Widening Loss of the spectral window Occurs: • As the blood decelerates in diastole • SV placed close to vessel wall • In small vessels (parabolic velocity profile) • Tortuous vessels • Low flow states • Excessive gain/power/dynamic range
  66. 66. Pulsatility • Measures the difference between maximum and minimum velocities within the cardiac cycle. • Unitless • Increase in pulsatility means increase in all the values. • No need of knowledge of Doppler angle.
  67. 67. Doppler Indices • PI = S-D/Vm (Gosling index) • RI= S-D/S (Pourcelot index) • S/D ratio • Acceleration time (AI) and Acceleration Index (AI). • Spectral broadening
  68. 68. Doppler artifacts Artifacts related to inappropriate settings Anatomically related artifacts Instrument and processor related artifacts 1. Doppler gain setting errors 2. Velocity settings errors and aliasing 3. Inappropriate wall filter settings 1. Mirror imaging 2. Vascular motion artifact 3. Color in neurovascular structures 1. Directional ambiguity 2. Grating or side lobe artifact 3. Spectral broadening
  69. 69. THANK YOU

Notas del editor

    Latest: Lead magnesium niobate- lead titanate( PMN-PT)

  • In front of the piezoelectric crystal is a thin layer of material on the surface of the probe called the matching layer.

  • It has the effect of decreasing ringing time of the PZT crystals, therefore shorten the spatial pulse length and improve spatial resolution.

    It has effect of widening the bandwidth and therefore decreasing the quality factor

  • Abuts-in contact
  • Transesophageal
    modification of standard gastroscope with optics removed and one or more phased array inserted at its tip. 3.5, 5 and 7.5 MHz. generally used. monoplane and biplane transducers used these days. Sooner, they will be replaced by multiplane ones.

    Are end firing. Axial and longitudinal scans obtained by rotating the probe. 5 and 7.5 MHz

    same probes as in transrectal.
  • Pediatric scaning ,small vessel and CBD
  • Slight large footprint 38 mm visualization of deeper structure like nerve(infraclavicular and popliteal, guide for needle placement.
  • Doppler shift depends upon the cosine of the angle between the sound beam and the direction of the motion.
    Optimal angle is 30-60 degrees.
  • Continuous wave Doppler:
    Two PZt elements in single head, one as transmitter & other as receiver. Maximum sensitivity → Inclined / focused elements → overlapping of beams → most sensitive region. (fig)
    Continuous sonic signal by transmitter (f) →2-10 MHz according to depth of vessel.
    Recording of returning echoes from blood cells by receiver (f1).
    Doppler shift ( f - f1) → audible range → amplification → Doppler signal.
  • Same transducer as transmitter & receiver.
    Depth information by cursor at any point along the axis of pulsed beam. Measurement of the depth (or range) of the flow site. Additionally, the size of the sample volume (or range gate) can be changed.
    Transmitter mode → short duration bursts of pulses at precise rate of repetition (Pulse repetition frequency or PRF) → receiver turned on (gating on) for short period at specified time → returning signals (echoes).
    4. Depth (range) controlled by time between pulse transmission & gating on. Axial length ( thickness) determined by time the gate is open. Width of sampling volume (gated Doppler acquisition area) → width of Doppler beam (pear shaped).
    5. 7 μs for ultrasound to travel 1 cm (average soft tissue) →
    if gate opened after 70 μs after transducer pulsed & closed 7 μs later → velocity sampling of 5 mm thick tissue at 5 cm depth.
    6. PRF determines the maxm. depth for Doppler shift. So ↑PRF → ↓depth. Doppler shift detection in pulsed Doppler = 1/2 of PRF. If Doppler shift frequency > detectable frequency in deep vessels→ aliasing (incorrect shift).

  • DCFI-Doppler color flow imaging
  • Doppler Spectrum:
    Envelope of the spectrum: Maximum frequencies present at any given point in time.
    Width of the spectrum: range of frequencies present at any time.