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Ultrasonography final

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Ultrasound &  properties
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Ultrasonography final

  1. 1. SOUND •Sound waves consist of mechanical vibrations containing condensations (compressions) & rarefactions (decompressions)that are transmitted through a medium. •Sound is mechanical. •Sound is not electromagnetic. •Matter must be present for sound to travel
  2. 2. CATEGORIES OF SOUND •Infrasound (subsonic) below 20Hz •Audible sound 20-20,000Hz •Ultrasound above 20,000Hz •Nondiagnostic medical applications <1MHz •Medical diagnostic ultrasound >1MHz
  3. 3. WHAT IS ULTRASOUND? • Ultrasound or ultrasonography is a medical imaging technique that uses high frequency sound waves and their echoes. • Known as a ‘pulse echo technique’ • The technique is similar to the echolocation used by bats, whales and dolphins, as well as SONAR used by submarines etc.
  4. 4. Bats navigate using ultrasound
  5. 5. Bats: Navigating with ultrasound • Bats make high-pitched chirps which are too high for humans to hear. This is called ultrasound • Like normal sound, ultrasound echoes off objects • The bat hears the echoes and works out what caused them • We can also use ultrasound to look inside the body… • Dolphins also navigate with ultrasound • Submarines use a similar method called sonar
  6. 6. Bats: Navigating with ultrasound • If a bat hears an echo 0.01 second after it makes a chirp, how far away is the object? • Clue 1: the speed of sound in air is 330 ms-1 • Clue 2: The speed of sound equals the distance travelled divided by the time taken • Answer: distance = speed x time • Put the numbers in: distance = 330 x 0.01 = 3.3 m • But this is the distance from the bat to the object and back again, so the distance to the object is 1.65 m.
  7. 7. 1. The ultrasound machine transmits high-frequency (1 to 12 megahertz) sound pulses into the body using a probe. 2. The sound waves travel into the body and hit a boundary between tissues (e.g. between fluid and soft tissue, soft tissue and bone). 3. Some of the sound waves reflect back to the probe, while some travel on further until they reach another boundary and then reflect back to the probe . 4. The reflected waves are detected by the probe and relayed to the machine. In ultrasound, the following events happen:
  8. 8. 5. The machine calculates the distance from the probe to the tissue or organ (boundaries) using the speed of sound in tissue (1540 m/s) and the time of the each echo's return (usually on the order of millionths of a second). 6. The machine displays the distances and intensities of the echoes on the screen, forming a two dimensional image.
  9. 9. DIAGNOSTIC AND THERAPEUTIC USE • Diagnostic ultrasound is applied for obtaining images of almost the entire range of internal organs in the abdomen. • Include kidney, liver, spleen, pancreas, bladder, major blood vessels and the foetus during pregnancy.it is based on echo aspect and doppler shift aspect.
  10. 10. • Therapeutic ultrasound are based on the thermal effects and cavitation effects developed during the irradiation of ultrasound on the body. • Ultrasonography – ultrasonic energy is used to detect the state of internal body organs.
  11. 11. PHYSICS OF ULTRASONIC WAVES • Like other forms of sonic energy,it exists as a sequence of alternate compressions and rarefractions of a suitable medium. • It is propogated through the medium at some velocity. • Also depends on frequency of the sonic energy and the density of the medium through which it travels.
  12. 12. Characteristic impedance • Characteristic or specific acoustic impedance of a medium is defined as the product of density of the medium with the velocity of the sound in the same medium. Z=ƿV It determines the degree of reflection and refraction at the interface between two media.
  13. 13. • The percent of incident wave energy which is reflected is given by [z1-z2/ z1-z2]2×100% Z1 = acoustic impedance of medium 1 Z2 = acoustic impedance of medium 2 provided the ultrasonic beam strikes the interface at a right angle.
  14. 14. • Greater the difference in acoustic impedance, greater the amount of reflected energy. • For eg., the acoustic impedance of air and tissue are 42.8g/cm2 and 1.6x105g/cm2 . • This difference is so large that most of the ultrasonic energy tends to be reflected at the interface.
  15. 15. Wavelength & frequency • Ultrasonics follow the wavelength & frequency relationship as V=nʎ V = propagation velocity of sound n = frequency ʎ = wavelength
  16. 16. Propagation of velocity and absorption • It is determined by the density of the medium it is travelling through and the stiffness of the medium. • Depth of penetration = velocity of sound in the medium x time / 2 • The reduction of amplitude of ultrasonic beam while passing through a medium can be due to its absorption by the medium and its deviation from the parallel beam by reflection, refraction, scattering and diffraction etc.,
  17. 17. Beam width • Ultrasonic waves are projected in a medium as a beam • The beam enters the medium and two regions (near & far field) are defined. • In near field,ultrasound radiated from different parts of the element travels as spherical waves that interfere constructively and destructively. • In far field, the ultrasound diverges and appears to be coming from a point source located at the centre of the transducer.
  18. 18. Resolution • Resolution of an ultrasound system is the system’s ability to distinguish between closely related structures. • Axial resolution – is the minimal axial distance, parallel to the beam axis. • It is determined by the wavelength of the transmitted pulse. • Smaller the wavelenth, higher the frequency and better the axial resolution. • Lateral resolution – is the lateral distance, perpendicular to the beam axis • It is determined by the shape/divergence of the ultrasound beam, produced by the probe.
  19. 19. GENERATION & DETECTION OF ULTRASOUND • The physical mechanism normally use to generate & detect ultrasonic waves is the piezo- electric effect • Which have the property to develop electric potentials on definite crystal surfaces when subjected to mechanical strain and vice versa. • The effect is demonstrated by crystals of materials like quartz,tourmaline and rochelle salt. • Quartz is replaced by barium nitrate & lead zirconate titanate. • They can be moulded to any shape to obtain better focusing action for producing high intensity ultrasonic waves.
  20. 20. • Depend upon its applications, materials with high Q factor are suitable as transmitters whereas with low Q & high sensitivity are preferred as receivers. • PZT are much better than quartz crystals upto 15 MHz,because of its high electro- mechanical conversion efficiency and low intrinsic losses. • At frequencies higher than this,quartz is normally used because of its better mechanical properties. • PVDF is another ferro- electric polymer used effectively in high frequency transducers • There are three parameters that are important in optimizing transducers for various types of applications. • These are frequency,active element diameter & focusing
  21. 21. MEDICAL ULTRASOUND • Ultrasound in the medical field used in both diagnostic and therapeutic applications. • The therapeutic equipment is designed to agitate the tissue to the level where thermal heating occurs in the tissue and found to be quite sucessful in the treatment of muscular ailments. • In this, the system operate at several W/cm2 • In diagnostic equipment considerably lower ultrasonic power levels are employed (100mW/cm2). • It is used either as continuous waves or in pulsed wave mode. • Applications making use of continuous waves depend on doppler’s effect. Eg.,used as foetal heart detecter and blood flow measuring instruments. • The majority of modern diagnostic ultrasound is based on pulse- technique. • It is used for the detection & location of defects or abnormalities in the structures at various depths of the body.
  22. 22. BASIC MODES OF TRANSMISSION • Pulsed ultrasound • Ultrasound is transmitted in short bursts at arepitition rate ranging from 1 – 12 kHz. • Returning echoes are displayed as a function of time which is proportional to the distance from the source to the interface. • Burst duration is generally about 1µsec. • It is used in most imaging applications. • Continuous doppler • Continuous ultrasonic signal is transmitted while returning echoes are picked up by a separate receiving transducer

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