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PRINCIPLES OF ULTRASONOGRAPHY

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PRINCIPLES OF ULTRASONOGRAPHY

  1. 1. PRINCIPLES OF ULTRASONOGRAPHY Jerome A
  2. 2. CATEGORIES OF SOUND •Infrasound (subsonic) below 20Hz •Audible sound 20-20,000Hz •Ultrasound above 20,000Hz •Non-diagnostic 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. ULTRASOUND PHYSICS • Characterized by sound waves of high frequency – Higher than the range of human hearing • Sound waves are measured in Hertz (Hz) – Diagnostic U/S = 1-20 MHz • Sound waves are produced by a transducer
  5. 5. • The ultrasound machine transmits high-frequency (1 to 12 megahertz) sound pulses into the body using a probe. • The sound waves travel into the body and hit a boundary between tissues (e.g. between fluid and soft tissue, soft tissue and bone). • 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. • The reflected waves are detected by the probe and relayed to the machine.
  6. 6. • Transducer (Probe) –Piezoelectric crystal • Emit sound after electric charge applied • Sound reflected from patient • Returning echo is converted to electric signal  grayscale image on monitor • Echo may be reflected, transmitted or refracted • Transmit 1% and receive 99% of the time
  7. 7. • 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). • The machine displays the distances and intensities of the echoes on the screen, forming a two dimensional image.
  8. 8. FREQUENCY AND RESOLUTION • As frequency increases, resolution improves • As frequency increases, depth of penetration decreases – Use higher frequency transducers to image more superficial structures Penetration Frequency
  9. 9. COMPONENTS
  10. 10. TRANSDUCERS/PROBES • Sector scanner – Fan-shaped beam – Small surface required for contact – Cardiac imaging • Linear scanner – Rectanglular beam – Large contact area required • Curvi-linear scanner – Smaller scan head – Wider field of view
  11. 11. MONITOR AND COMPUTER • Converts signal to an image/ archive • Tools for image manipulation – Gain – amplification of returning echoes – Time gain compensation (curve) – Freeze – Depth – Focal zone
  12. 12. • For diagnostic ultrasound, sound wave frequencies of between 2 and 10 MHz (1 MHz =1 million sound waves per second) are most commonly used. • High frequency sound waves provide greater detail, whereas lower frequency provides greater tissue penetration. • With low frequency transducer, a larger area is viewed, but with less Details.With a high frequency transducer, a smaller area is viewed but with more detail. The lower frequency transducers (<3.5 MHz) are suited for viewing larger structures at a greater distance from the transducer. • The higher frequency transducers (>5.0 MHz) are intended for detailed study of superficial structures close to the transducer (eg. evaluating the ovaries, uterus etc). • Most transducer currently available nowadays produces single as well as multiple frequency sound waves.
  13. 13. MODES OF DISPLAY
  14. 14. • Ultrasonography (A, B and M mode, 3D and 4D imaging) • Doppler flow measurement, including Duplex and Triplex methods (Duplex, Colour Doppler, Triplex, Power Doppler) • Tissue Doppler imaging • Ultrasound densitometry
  15. 15. • A mode – Spikes – where precise length and depth measurements are needed – ophtho • B mode (brightness) – used most often – 2 D reconstruction of the image slice • M mode – motion mode – Moving 1D image – cardiac mainly
  16. 16. 16 A-MODE – ONE-DIMENSIONAL Distances between reflecting interfaces and the probe are shown. Reflections from individual interfaces (boundaries of media with different acoustic impedances) are represented by vertical deflections of base line, i.e. the echoes. Echo amplitude is proportional to the intensity of reflected waves (Amplitude modulation) Distance between echoes shown on the screen is approx. proportional to real distance between tissue interfaces. Today used mainly in ophthalmology.
  17. 17. 17 A tomogram is depicted. Brightness of points on the screen represents intensity of reflected US waves (Brightness modulation). Static B-scan: a cross-section image of examined area in the plane given by the beam axis and direction of manual movement of the probe on body surface. The method was used in 50‘ and 60‘ of 20th century B-MODE – TWO-DIMENSIONAL
  18. 18. ULTRASOUND TERMINOLOGY • Anechoic – No returning echoes= black (acellular fluid) • Echogenic – Regarding fluid--some shade of grey d/t returning echoes • Relative terms – Comparison to normal echogenicity of the same organ or other structure – Hypoechoic, isoechoic, hyperechoic • Solid structures – acoustic shadow (caused by absorption and reflection of US) • Air bubbles and other strongly reflecting interfaces cause repeating reflections.
  19. 19. PATIENT POSITIONING AND PREPARATION • Dorsal recumbency • Lateral recumbency • Standing • Clip hair • Apply ultrasound gel
  20. 20. 20 BIOLOGICAL EFFECTS  Possible bioeffects: inactivation of enzymes, altered cell morphology, internal haemorrhage, free radical formation.  Mechanisms of bioeffects: – Mechanical effects – Elevated tissue temperatures (absorption of ultrasound and therefore increase in temperature high in lungs, less in bone, least in soft tissue)  All bioeffects are deterministic with a threshold (cavitation) or without it (heating).
  21. 21. PITFALLS OF ULTRASOUND • Ultrasound cannot penetrate air or bone – Size of organs is largely subjective – Unable to evaluate extra-abdominal structures – Cost – User dependent results
  22. 22. Thank you

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