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production of ultrasound and physical characteristics-

  2. 2. ULTRASONOGRAPHY • Application of medical with ultrasound-based imaging diagnostic technique used to visualize internal organs, their size, structure and their pathological lesions.
  3. 3. ULTRASOUND PHYSICS • INTRODUCTION • Sound waves are mechanical pressure waves which propagate through a medium with the acoustic energy causing the particles of the medium to oscillate backward and forward as alternating bands of compression and expansion (rarefaction) of the material.
  4. 4. Introduction  It results in cyclic change in the resting pressure of the medium.  The cyclic variations of the pressure can be plotted graphically as a sine wave.  Produced when a vibrator, such as a piezoelectric crystal in an ultrasound transducer, transmits its back and forth oscillation into a medium.
  5. 5. Cont…… • It is not capable of transmitting its energy through a vacuum thus it requires a medium through which it propagates. • It is a longitudinal wave in which the particles of the medium are made to oscillate in a direction parallel to the direction that the wave moves. • It travels in a straight line
  6. 6. Cont…
  7. 7. SOUND FREQUENCY SPECTRUM • Infrasound- below 20Hz (cant be head by humans) • Audible sound (20Hz-20KHz)- range detectable by the human ear. • Ultrasound > 20KHz • Medical Ultrasound (2- 20MHz) • Note: Medical imaging – 3-10MHz
  8. 8. PROPERTIES OF SOUND WAVES • INCLUDE Amplitude Wavelength (λ) Frequency (f) Velocity (C)
  9. 9. Cont…
  10. 10. Amplitude • Is the peak pressure of the wave. • When applied to ordinary sound, this term correlates with the loudness of the sound wave. • When applied to ultrasound images, it correlates with the intensity of the returning echo. • The amplitude of the pulse as it leaves the transducer is generally determined by how hard the crystal is "struck" by the electrical pulse.
  11. 11. Cont… • Relative amplitude- measure of how much the amplitude ( A) of a pulse decreases as it passes through a given thickness of tissue. • Relative amplitude (dB) = 20 log A2/A1
  12. 12. Velocity • The speed of the wave. Cal as (V= f λ ) • It is constant in a given medium and is calculated to be 1,540m/s in soft tissue • Using this principle, an ultrasound machine can calculate the distance /depth of a structure by measuring the time it takes for an emitted ultrasound beam to be reflected back to the source.
  13. 13. Wavelength • Is the distance the wave travels in a single cycle. • It is inversely related to frequency. • Therefore, high frequency decrease wavelength (and thus penetration), and lower frequency increases wavelength (and thus penetration) λ = c / f
  14. 14. Frequency • Is the number of times per second the wave is repeated. • One Hertz is equal to one wave cycle per second • Frequency and period are inversely related thus f = 1/ T f = c / λ
  15. 15. ULTRASOUND PULSE • A typical ultrasound pulse consists of several wavelengths or vibration cycles. • The number of cycles within a pulse is determined by the damping characteristics of the transducer. • Damping is what keeps the transducer element from continuing to vibrate and produce a long pulse.
  16. 16. Cont…. • Pulse length = wavelength X number of cycles within the pulse pulse length = λ f f is the number of cycles within the pulse
  17. 17. SPEED OF SOUND • Speed of sound is determined by Stiffness and density of matter. • Speed of sound is significantly higher in very stiff materials like bone, gallstones. • Stiffness- the speed of sound is directly proportional to the stiffness or resistance to compression • Density- speed is inversely proportional to the density of the material. • Thus the stiffer the medium the higher the propagating speed while the dense the medium the slower the speed.
  18. 18. SOUND VELOCITIES • Material Meters/sec • Vacuum 0 • Air 330 • Pure water 1430 • Fat 1450 • Soft Tissue 1540 • Muscle 1585 • Bone 4080
  19. 19. To calculate the speed of sound in a medium
  21. 21. Cont….
  22. 22. Cont… • The time (t) is the time between transmission of the pulse and reception of the echoes known as the (pulse-echo time, time of flight, go-return time) • Deep structures have long go return times • Superficial structures have short go-return times • Pulse –echo time is usually 13µs per cm
  23. 23. TISSUE INTERACTIONS • Processes include • Reflection • Scatter • Refraction • Absorption
  24. 24. Reflection • A reflection occurs at the boundary between two materials provided that a certain property of the materials known as the acoustic impedance (Z) is different. • A reflection of the beam is called an echo and the production and detection of echoes forms the basis of ultrasound. • Occurs at the interface, or boundary, between two dissimilar materials the different physical characteristic is known as acoustic impedance Z • Classified into two • Specular • Scatter
  25. 25. cont • Specular Reflection: • These originates from relatively large , strongly reflective, regularly shaped objects with smooth surfaces. • Echoes are reflected as a mirror reflects light. Since ultrasound scanners only detect reflections that return to the transducer, the display of specular interfaces depends heavily on the angle of intonation. • Specular reflectors (e.g. the diaphragm, urine filled bladder) only return echoes to the transducer if the sound beam is perpendicular to the interface. If not, the sound beam will be transmitted away from the transducer, and the echo will not be detected.
  26. 26. Formula (reflection) • R = Ir/Ii = (Z2 – Z1/Z1 + Z2) squared • And • T = It/Ii = 4Z1 – Z2/ (Z1 + Z2) Squared • Where  R = reflection intensity coefficient  T = transmission intensity coefficient  Ir = reflected U/S intensity  It = Transmitted U/S intensity  Z1 = Acoustic impedance of first medium  Z2 = Acoustic impedance of second medium
  27. 27. Cont… • . Diffuse reflection (Scattering)- • These echoes comes from smaller interfaces within solid organs with structures much smaller than the wavelength of the incident sound. • Echoes are scattered in all directions. The direction does not follow the laws of reflection rather depends on relative size of scattering objects and U/S beam diameter
  28. 28. Acoustic Impedance • The product of medium density and ultrasound velocity in the medium • It is the measure of the resistance of the particles of the medium to mechanical vibrations. • This resistance increases in proportion to the density of the medium, and the velocity of ultrasound in the medium.
  29. 29. Acoustic Impedance
  30. 30. Acoustic boundaries • Acoustic Boundaries • Positions within tissue where the values of acoustic impedance change are very important in U/S interactions. • Positions = acoustic boundaries or tissue interfaces • EX-urine (bladder) has Z value different from bladder wall- common interface constitutes an acoustic boundary
  31. 31. Refraction • The bending or change in direction of the sound wave when it passes from adjacent tissues having different acoustic propagation velocities and is governed by Snell’s Law.
  32. 32. Absorption • A process by which ultrasound energy is converted to heat in the medium. It is responsible for tissue heating. • Attenuation and absorption is often expressed in terms of decibels.
  33. 33. Attenuation • As the U/S wave propagates through a medium, the intensity reduces with distance travelled.
  34. 34. ULTRASOUND FIELD • Divided into two regions- near and far fields • Close to the transmitter the radiation field is very complex, changing relatively rapidly both in amplitude and phase position. • Close to the transmitter, these phase differences are relatively large, since the differences in path length are comparable to the shortest path between transmitter surface and field point. Small changes in the position of the field point can make large differences to the overall sum
  35. 35. Cont… • In the far field, differences between path lengths are small in comparison with the shortest distance from transmitter to field point thus all contributions arrive with nearly the same phase. • Small changes make little difference to the overall sum • Amplitude stays relatively constant with position along or across the sound beam
  36. 36. Piezoelectric Effect • Ultrasound waves are generated by piezoelectric crystals (Quartz crystals) • When an electric current is applied to a quartz crystal, its shape changes with polarity. • This causes expansion and contraction that in turn leads to the production of compression and rarefaction of sound waves
  37. 37. Cont… • The reverse is also true and an electrical current is generated on exposure to returning echoes that are processed to generate display • Hence the crystals are both transmitters and receiver.
  39. 39. Single crystal transducer (Probe)
  40. 40. Types of transducers 1. Mechanical swept Probe: seldom used now 2. Electronically steered Probe: i. Linear array transducers ii. Phased /sector array transducers iii. Convex /curved iv. Annular array
  41. 41. Types of transducers cont… • The choice is a compromise between Spatial resolution and Imaging depth (Tissue penetration) • Rule of thumb is use the highest freq that will penetrate to the depth of interest.
  42. 42. TRANSDUCERS • Linear Gives rectangular image Generally has higher frequency Good for looking at a smaller area and for gauging depth Gives more of a one dimensional view Sometimes referred to as the vascular probe
  43. 43. Cont…. • Curvilinear Uses same linear orientation but arranged on a curved surface Generally lower frequency Gives a wider angle of view
  44. 44. Types of probes
  45. 45. Transducers • The higher the frequency, the better the resolution • The better the resolution, the better you can distinguish objects from each other
  46. 46. Knobology • Power Controls the strength or intensity of the sound wave Use ALARA principle As low as reasonably achievable
  47. 47. Cont…. • Gain Degree of amplification of the returning sound Increasing the gain, increases the strength of the returning echoes and results in a lighter image Decreasing the gain, does the opposite
  48. 48. Cont… • Time gain compensation Used to equalize the stronger echoes in the near field with the weaker echoes in the far field Should be a gentle curve Zoom Can place zoom box on a portion of a frozen image to enlarge that portion of the image May lose some resolution because pixels are enlarged
  49. 49. Cont… • Focal Zone Where the narrowest portion of the beam is Gives the optimal resolution Depth Each frequency has a range of depth of penetration Decrease the depth to visualize superficial structures May need to increase the depth of penetration to visualize larger organs
  50. 50. Medical challenges it addresses • Capitalized on the deficiencies of conventional radiography in imaging anatomical structures of body organs except lungs and bones. • Supersedes the invasive conventional procedures to that of non-invasive • Offers a cheaper route to quick diagnosis compared to CT and MRI • Assures safety - obstetric imaging
  51. 51. Strength of ultrasound imaging Images muscle and soft tissue very well and is particularly useful for delineating the interfaces between solid and fluid-filled spaces. Renders “live” images, where the operator can dynamically select the most useful section for diagnosing and documenting changes, often enabling rapid diagnoses.
  52. 52. Cont… Demonstrates structures as well as some aspects of the function of organs. No undesirable side effects Equipment is widely available and comparatively flexible; exam can be performed at the bedside
  53. 53. Weaknesses of ultrasound imaging • Ultrasound can not penetrate bone and performs poorly when there is air between the scanner and the organ of interest. • Even in the absence of bone and air, the depth penetration of ultrasound is limited, making it difficult to image structures that are far removed from the body surface. • Operator- dependant
  54. 54. U/S Image: Generation & Display • An U/S transducer, T, sends a beam of U/S into the subject over a selected area of interest • At an acoustic boundary such as B within the tissue, some of the U/S energy is reflected, either specularly or by scattering. • Under favourable conditions, some of the reflected U/S will go back towards T.
  55. 55. Cont…. • At the transducer, the returning echo will interact with the piezoelectric crystal and generate an electric signal. • This signal will be electronically processed and measured. The location of its origin at B will be determined.
  56. 56. Representation
  57. 57. Cont…. • Transducer- The transducer is the component of the ultrasound system that is placed in direct contact with the patient's body. • Pulse Generator- The pulse generator produces the electrical pulses that are applied to the transducer. For conventional ultrasound imaging the pulses are produced at a rate of approximately 1,000 pulses per second.
  58. 58. Cont… • Amplification- increase the size of the electrical pulses coming from the transducer after an echo is received.. The amount of amplification is determined by the gain setting. • Scan Generator- controls the scanning of the ultrasound beam over the body section being imaged. This is usually done by controlling the sequence in which the electrical pulses are applied to the piezoelectric elements within the transducer.
  59. 59. Cont.. • Scan Converter- converts from the format of the scanning ultrasound beam into a digital image matrix format for processing and display. • Image Processor- The digital image is processed to produce the desired characteristics for display. This includes giving it specific contrast characteristics and reformatting the image if necessary • Display –monitor
  60. 60. IMAGE Formation • By measuring the time between when the sound was sent and received, the amplitude of the sound and the pitch of the sound, a computer can produce images, calculate depths and calculate speeds. • The strength or amplitude (brightness) of each reflected wave is represented by a dot. • The position of the dot represents the depth from which the returning echo was received. • These dots are combined to form an image.
  61. 61. Cont.. • Modern transducers use multiple small elements to generate the ultrasound wave. • If multiple small elements fire simultaneously however the individual curved wave fronts combine to form a linear wave front moving perpendicularly away from the transducer face. • This system, that is multiple small elements fired individually, is termed phased array
  62. 62. Cont…
  63. 63. Image Display • Once the diagnostic information has been acquired and electronically processed, it has to be displayed for viewing and recording. • Different methods are used to display the information acquired in modes (A-mode, B- mode, M-mode, Real-Time mode and Doppler mode).
  64. 64. Defining modes • A-mode- the original display mode of U/S measurements, in which the amplitude of the returned echoes along a single line is displayed on an oscilloscope. • B-mode (2D)- the current display mode of choice. This is produced by sweeping the transducer from side to side and displaying the strength of the returned echoes as bright sports in their geometrically correct direction and distance
  65. 65. Cont… • M-mode- followed A mode by recording the strength of the echoes as dark spots on moving light sensitive paper. Object that move, such as the heart cause standard patters of motion to be displayed. • And a lot of diagnostic information such as valve closures rates, whether valves opened and closed completely and wall thickness could be obtained from this mode.
  66. 66. Doppler mode • This mode makes use of the Doppler effect in measuring and visualizing blood flow • Color Doppler: Velocity information is presented as a color-coded overlay on top of a B-mode image • Continuous Doppler: Doppler information is sampled along a line through the body, and all velocities detected at each time point are presented (on a time line)
  67. 67. Cont… • Pulsed wave (PW) Doppler: Doppler information is sampled from only a small sample volume (defined in 2D image), and presented on a timeline • Duplex: a common name for the simultaneous presentation of 2D and (usually) PW Doppler information. (Using modern ultrasound machines, color Doppler is almost always also used; hence the alternative name Triplex.)
  68. 68. Artefacts • Artifacts are errors in images. • They are normally caused by physical processes that affect the ultrasound beam and that in some way alter the basic assumptions the operator makes about the beam
  69. 69. Cont….. • Reverberation artefact This occurs when ultrasound is repeatedly reflected between two highly reflective surfaces. • Ring Down Ring-down artifacts are produced when small crystals such as cholesterol or air bubbles resonate at the ultrasound frequency and emit sound. Because the sound is emitted after the transducer receives the initial reflection, the system thinks the emitted sound is coming from structures deeper in the body
  70. 70. Cont…. • Mirror Images Sound can bounce off a strong, smooth reflector such as the diaphragm. The surface acts as mirror and reflects the pulse to another tissue interface.  The ultrasound system believes the second interface is beyond the first surface, and this is where it appears on the scan.
  71. 71. Reflection artefact • similar to the mirror image but has a very different appearance and is caused by multiple reflections. • Sound can bounce off a strong, smooth reflector, such as the posterior bladder wall, and be reflected back to the transducer, giving the appearance of the structure deep to the bladder wall as would be seen with fluid collection.
  72. 72. Enhancement artefact • Enhancement is seen as an abnormally high brightness. • This occurs when sound travels through a medium with an attenuation rate lower than surrounding tissue. • Reflectors at depths greater than the weak attenuation are abnormally bright in comparison with neighboring tissues
  73. 73. Attenuation artefact • Tissues deeper than strongly attenuating objects, such as calcification, appear darker because the intensity of the transmitted beam is lower
  74. 74. Clinical Application • Ultrasonography is widely utilized in medicine, primarily in gastroenterology, cardiology, gynecology and obstetrics, urology and endocrinology. • It is possible to perform diagnosis or therapeutic procedures with the guidance of ultrasonography (for instance biopsies or drainage of fluid collections).
  75. 75. Other applications • Description Anaesthesiology- used by anaesthesiologists to guide injecting needles when placing local anaesthetics solutions near nerves • Neonatology
  76. 76. Image Quality • Resolution The ability to distinguish echoes in terms of space, time or strength and good resolution is thus critical to the production of high quality images. • Spatial resolution The ability of the ultrasound system to detect and display structures that are close together.
  77. 77. Cont… • Axial resolution- ability to display small targets along the path of the beam as separate entities. • Contrast resolution- ability of an ultrasound system to demonstrate differentiation between tissues having different characteristics e.g. liver/spleen. • Temporal resolution- ability of an ultrasound system to accurately show changes in the underlying anatomy over time, this is particularly important in echocardiography.
  78. 78. 1. Axial Resolution • Factors affecting axial resolution include Spatial Pulse Length (SPL) and frequency. • It is improved by higher frequency (shorter wavelength) transducers but at the expense of penetration. • Higher frequencies therefore are used to image structures close to the transducer.
  79. 79. 2. Lateral Resolution • Factors affecting lateral resolution are width of the beam, distance from the transducer, frequency, side and grating lobe levels. • To optimize lateral resolution therefore: Use the highest frequency transducer (reduced penetration) Optimize the focal zone Use the minimum necessary gain
  80. 80. 3. Temporal Resolution • The number of frames generated per second (frame rate) determines temporal resolution.
  81. 81. Recent Advances • Superb Micro-Vascular Imaging Technology expands the range of visible blood flow and provides visualization of low velocity microvascular flow never before seen with ultrasound. SMI's level of vascular visualization, combined with high frame rates, advances diagnostic confidence when evaluating lesions, cysts and tumors, improving patient outcomes and experience.
  82. 82. Fly Thru Imaging • a 3D volume rendering technique that allows you to soar through cavities, ducts and vessels from the inside and in 3D. • Similar to virtual endoscopy, Fly Thru is a revolutionary tool for exploring lesions and masses and planning interventional procedures. This technology function is available both during or post exam. • Fly Thru displays structures similar to the way they would appear using a virtual endoscope.
  83. 83. Smart Fusion • offers the best of both worlds by synchronizing ultrasound with CT or MR images for locating hard-to-find lesions and improving confidence during ultrasound-guided biopsies. • Smart Fusion reads 3D DICOM data sets from CT or MR systems then, using position sensors located on the ultrasound transducer, displays the corresponding images, side-by-side with the live ultrasound image.
  84. 84. Wireless Transducers • In 2012, Siemens Healthcare introduced the world’s first ultrasound system with wireless transducers, the Acuson Freestyle. • Completely untethered from the console, the Acuson’s wireless transducer can be used to image from up to 10 feet away
  85. 85. 3-D Ultrasound • uses a dataset that contains a large number of B-mode 2D planes. • Once the volume data is obtained it is possible to optimize the ultrasound image of the area of interest by rotating, reconstructing and rendering, allowing viewing in different planes and angles without further exposure of the patient to ultrasound
  86. 86. Contrast agents • Reflection of sound depends on the acoustic impedance which are defined by its density and the velocity of sound in the medium. • Acoustic impedances differences are very small between soft tissues. • Echopharmaceuticals have been proposed to increase acoustic impedance differences at tissue interfaces & increase echo intensities
  87. 87. Cont…. • The most effective principle by far that has emerged is the diffraction of ultrasonic • waves on gas bubbles (microbubble containing solutions ) and colloidal, sometimes • temperature dependent diphasic systems.
  88. 88. 4 Dimensional • also known as ‘real-time 3D ultrasound’. • the ultrasound equipment can acquire and display the 3D datasets with their multiplanar reformations and renderings in real time. • 3D or 4D can only build on the 2D B-mode images, therefore the limitations and artefacts that affect B-mode imaging, such as presence of gas and overlying structures, will also affect the quality of the 3D and 4D imaging
  89. 89. SAFETY • Ultrasound is energy and is absorbed by tissue, causing heating • 2D ultrasound has been used to image the foetus for about 50 years. It is thought to be completely safe and does not cause significant heating • 4D ultrasound is new, requires more energy and therefore generates more heating. We think it is safe.

Notas del editor

  • Respective echo intensities