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Us hand book (1)

  1. 1. US HAND BOOK (1) Dr. Kamal Sayed / MSc US AAU Technology/freq & resol/atten/
  2. 2. • Ultrasound Technology • Ultrasound wave is produced when an electric current is applied to an array of piezoelectric crystals. • This causes distortion of the crystals, makes them vibrate and produce this acoustic mechanical wave which is transmitted into the body (The target reflector). • Mechanical sound waves are reflected back into the probe & PZT convert them into electric signal again which are analyzed by the US system into an image on the display screen • • •
  3. 3. • The ultrasound waves are produced in pulses. • Each pulse is 2-3 cycles of the same frequency. • The pulse length is the distance each pulse travels. • The pulse repetition frequency is the rate at which the transducer emits the pulses. • The pulses have to be spaced. • This allows enough time between pulses so the beam has enough time to reach the target and return to the transducer before the next pulse is generated.
  4. 4. • Ultrasound image is produced when the pulse wave that is generated travels through the body, reflects off the tissue interface (echo) and returns to the transducer. • When the wave is transmitted back to the transducer its amplitude is represented by its brightness or echogenicity. • It is represented as a dot. •
  5. 5. • The final image is produced by the combination of these dots. • Strong reflections produce bright dots (hyperechoic images) e.g. bone. • weaker reflections produce grey dots (hypoechoic images) e.g. solid organs. • No reflection produces anechoic images, e.g. blood vessels
  6. 6. • Frequency and Resolution • Ultrasound frequency is above 20,000 Hz or 20 KHz. • Medical ultrasound is in the range of 3 -15 MHz. • Average speed of sound through most soft human tissues is 1,540 meters per second. • This can be calculated multiplying the wavelength with frequency. •
  7. 7. • The best resolution is obtained at the focus. • Diagnostic US TXRs often have better axial resolution than lateral resolution, although the two may be comparable in the focal region of strongly focused. • Elevational (azimuthal) resolution represents the extent to which an US system is able to resolve objects within an axis perpendicular to the plane formed by the axial and lateral dimensions.
  8. 8. • The higher frequency wavelength will have shorter wavelength whereas lower frequency wavelength will have longer wavelength. • The wavelength for the 2.5 MHz is 0.77 mm whereas that for 15 MHz is 0.1 mm
  9. 9. • SPATIAL RESOLUTION is the ability of the US system to detect and display structures that are close together. • Since an US image displays depth into the patient and width across a section of anatomy it is therefore reasonable to consider two types of spatial resolution – Axial & Lateral. • In terms of digital images, spatial resolution refers to the number of pixels utilized in construction of the image. • Images having higher spatial resolution are composed with a greater number of pixels than those of lower spatial resolution. •
  10. 10. • Lateral resolution is the image generated when the two structures lying side by side are perpendicular to the beam. This is directly related to the width of the US beam. • Narrower the beam better is the resolution. • The width of the beam is inversely related to the frequency. Higher the frequency narrower is the beam. • If the beam is wide the echoes from the two adjacent structures will overlap and the image will appear as one. • Lateral resolution is roughly four times worse than axial resolution in ultrasound.
  11. 11. • Modes in US • a mode in US is a process that uses the reflection of high- frequency sound waves to make an image of structures deep within the body. • US modes are : • A-mode: A-mode is the simplest type of ultrasound. ... • B-mode: In B-mode ultrasound, a linear array of transducers simultaneously scans a plane through the body that can be viewed as a two-dimensional image on screen. • M-mode: M stands for motion. • Mode doppler
  12. 12. • D – mode • Definition. - Constant depth mode. – • A gate is used to only receive echoes from a specific depth. - Echoes received from a specific depth is imaged in the plane and is at a CONSTANT depth from the transducer and perpendicular to the beam. - A cross-sectional image is created.
  13. 13. Modes of US
  14. 14. • A-mode : is the simplest type of US. A single transducer scans a line through the body with the echoes plotted on screen as a function of depth. • Therapeutic US aimed at a specific tumor or calculus is also A- mode, to allow for pinpoint accurate focus of the destructive wave energy. • A-mode scans result in a waveform with spikes or peaks at the interface of two different tissues (e.g., where subcutaneous fat and muscle meet). • Both A-mode and B-mode ultrasound have been used to measure subcutaneous fat thickness.
  15. 15. • B-Mode is a two-dimensional ultrasound image display composed of bright dots representing the ultrasound echoes. The brightness of each dot is determined by the amplitude of the returned echo signal. • B-mode or 2D mode: • In B-mode (brightness mode) US, a linear array of transducers simultaneously scans a plane through the body that can be viewed as a two-dimensional image on screen. • More commonly known as 2D mode now.
  16. 16. • M-mode is defined as time motion display of the ultrasound wave along a chosen ultrasound line. It provides a monodimensional view of the heart. ... The advantage of the M-mode is its very high sampling rate, which results in a high time resolution so that even very rapid motions can be recorded, displayed, and measured. • M-mode, or motion mode, is used clinically for the assessment of valve motion, chamber sizes, aortic root size, wall thickness, and ventricular function •
  17. 17. {M- mode} tracing across the aortic sinus and the left atrium ([LT upper) tracing across the left ventricle (LT lower) tissue dopp M mode (RT upper) Color Doppler M-mode (RT lower)
  18. 18. • Highly dense tissues such as bone or kidney stones readily reflect echoes and, therefore, appear bright white on an ultrasound. Air, such as in the bowel, also readily reflects echoes. ... Remember, ultrasound does not detect tissue density
  19. 19. • The physics of US • The production of ultrasound waves is based on the so-called 'pulse-echo-principle'. • The source of the ultrasound wave is the piezoelectric crystal, which is placed in the transducer. • This crystal has the ability to transform an electrical current into mechanical pressure waves (ultrasound waves) and vice versa. •
  20. 20. • dynamic range (DR) • in medical US imaging, DR is defined as the difference between the maximum and minimum values of the displayed signal to display and it is one of the most essential parameters that determine its image quality. • Its effect on image is by changing the gray scale mapping.
  21. 21. • image too dark, first, increase receiver gain image too bright, first, reduce output power. •
  22. 22. Young's Modulus a mechanical property that measures the tensile stiffness of a solid material. • The Young's Modulus of a material is a fundamental property of every material that cannot be changed. It is dependent upon temperature and pressure however. • The Young's Modulus (or Elastic Modulus) is in essence the stiffness of a material. In other words, it is how easily it is bended or stretched. • ss
  23. 23. • To be more exact, the physics and numerical values are worked out like this: • Young's Modulus = Stress / Strain • where: • Stress = force / cross sectional area • Strain = change in length / original length •
  24. 24. • Acoustic impedance (Z) is a physical property of tissue. • It describes how much resistance an ultrasound beam encounters as it passes through a tissue. • Acoustic impedance depends on: • the density of the tissue (d, in kg/m3) • the speed of the sound wave (c, in m/s) • and they are related by: • Z = d x c (units in Rayels in kg/sq meter)
  25. 25. • Sequencing (to produce scan lines) is the process through which a sequence of ultrasound pulses are transmitted into the tissue and the RF data are collected. • Line density adjusts the number of scan lines in your ultrasound image. A higher level provides better resolution in the image (more scan lines), but reduces the frame rate. Use this to get the best possible image with the most acceptable frame rate.
  26. 26. • This Doppler-shifted ultrasonic image of a partially occluded artery uses color to indicate velocity. The highest velocities are in red, while the lowest are blue. The blood must move faster through the constriction to carry the same flow • (next slide). •
  27. 27. • Low frequency noises, below the frequency that human ears can usually detect, are used by elephants to communicate over long distances. The infrasound frequencies are good for long distance communication because they travel well through objects instead of being reflected • . People use this frequency range for monitoring earthquakes and volcanoes, charting rock and petroleum formations below the earth, and also in ballistocardiography and seismocardiography to study the mechanics of the heart. Infrasound is characterized by an ability to get around obstacles with little dissipation.
  28. 28. • Higher frequencies are absorbed more rapidly in the air. This effect reduces as the frequency reduces. Hence, infrasound travels further, but does weaken as it spreads out, just like any other wave • . There is no agreement about the biological activity of infrasound. Reported effects include those on the inner ear, vertigo, imbalance, etc.; intolerable sensations, incapacitation, disorientation, nausea, vomiting, and bowel spasm; and resonances in inner organs, such as the heart. •
  29. 29. • 7 hz • The most dangerous frequency is at the median alpha-rhythm frequencies of the brain: 7 hz. This is also the resonant frequency of the body's organs • That's due to the refraction of light: the way rays of light bend when they move from a medium like air to a medium like water. ... Because sound moves faster in warm air than colder air, the wave bends away from the warm air and back toward the ground. That's why sound is able to travel farther in chilly weather.
  30. 30. • The molecules in the medium, as they are forced to vibrate back and forth, generate heat. Consequently, a sound wave can only propagate through a limited distance. In general, low frequency waves travel further than high frequency waves because there is less energy transferred to the medium. • What is Infrasound? ... For example, some animals, such as whales, elephants and giraffes communicate using infrasound over long distances. Avalanches, volcanoes, earthquakes, ocean waves, water falls and meteors generate infrasonic waves. •
  31. 31. • vacuum • Sound cannot travel through a vacuum. • A vacuum is an area without any air, like space. • So sound cannot travel through space because there is no matter for the vibrations to work in.
  32. 32. • Why sound is louder at night ? • There is a phenomenon called refraction that affects the direction of sound propagation. • During the day, the sound bends away from the ground; during the night, it bends towards the ground. • Hence at night you have additional "sound" reaching you, making it louder.
  33. 33. • During attenuation the ultrasound wave stays on the same path and is not deflected.