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Ultrasound Imaging (Basics)

Ultrasound Imaging (Basics). Why Ultrasound?. Over half a century old technique! Arguably the most widely used imaging technologies in medicine. Portable, free of radiation risk, and relatively inexpensive compared to MRI, CT and PET

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Ultrasound Imaging (Basics)

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  1. Ultrasound Imaging(Basics)

  2. Why Ultrasound? Over half a century old technique! Arguably the most widely used imaging technologies in medicine. Portable, free of radiation risk, and relatively inexpensive compared to MRI, CT and PET Tomographic, i.e., offering a “cross-sectional” view of anatomical structures. “Real time,”- providing visual guidance for interventional procedures

  3. Do you expect any similarities?

  4. Most amazing is that sound can actually help us to see what is hidden, just like the way bats 'see'. Bats always have the night shift. They go hunting for things to eat at night where food isn't well lit. Fortunately, bats are gifted with a system of locating things with sound. First they emit sound.

  5. The human ear cannot hear below 20 Hz. • Elephants can use infra sound. • The human ear cannot hear above 20,000 Hz. • Bats use ultrasound to locate food. • Dolphins use it to communicate. • Ultrasound used in medical imaging operate at frequencies way above human hearing: about 2 million Hz - 20 million Hz (2-20 MHz).

  6. Sound travels in waves. Ultrasound physics has to do with the higher frequencies of sound. Human hearing is from about 20 cycles per second or 20HZ (a low hum) to about 20,000 cycles per second or 20KHZ. A grasshopper sends out sound waves at 40KHZ. A dog can hear at about 30KHZ and bats send chirps and listens for the echoes at 100KHZ. 

  7. Properties of Sound Waves • Frequency • Velocity • Wavelength • Amplitude • Units to describe frequency: • Hertz= 1 cycle in one sec • kHz= 1000 Hz= 1000 cycles per sec • MHz= 1000000 Hertz • US imaging frequency range: 2-12 MHz High Frequency Wave Amplitude Crest Time Pressure Trough Period Period wavelength Low Frequency Wave Time Pressure • The number of cycles occurring in one sec of time (cycles per sec) • The high frequency wave sounds higher than the low freq wave http://www.genesis-ultrasound.com/Ultrasound-physics-2.html

  8. Wavelength • Length of space over which one cycle occurs (distance) wavelength wavelength Distance Distance • Given a constant velocity, as frequency increases wavelength decreases (V=  x f) • Common US frequencies and wavelengths • -2.25MHz = 0.6 microns • -5.0 MHz = 0.31 microns • -10.0 MHz = 0.15 microns

  9. Ultrasound Wavelength and Frequency • High frequency US waves High axial resolution  More attenuation  Superficial structure • Low frequency US waves Lower resolution Less degree attenuation  Deeper penetration • High frequency transducers (10-15 MHz) to image superficial structures (e.g. stellate ganglion blocks) • Low frequency transducers (2-5 MHz) to image the lumbar neuraxial structure Higher frequency waves are more highly attenuated than lower frequency waves at a given distance

  10. Velocity • Average speed of US in the human body is 1540 m/sec • Directly related to the stiffness of media • Inversely related to the density of media • Slowest in air/gasses • fastest in solids Medium Velocity (m/sec) -------------------------------------------- Air 330 Fat 1450 Water 1480 Soft tissue 1540 Blood 1570 Muscle 1580 Bone 4080 c =  × f  = c / f

  11. Amplitude • The strength/intensity of the sound wave at any given • point in time • Represented by the height of the wave • Amplitude/intensity decreases with increasing depth • Magnitude of the pressure changes along the sound • wave • Power: rate at which energy is transferred from a sound beam- • proportional to the amplitude squared • Intensity (Watts/cm2) is the concentration of energy in a sound beam

  12. Attenuation Coefficient 8 MHz 10MHz 12MHz The ultrasound amplitude decreases in certain media as a function of ultrasound frequency (attenuation coefficient) ScN-Sciatic nerve, PA - Popliteal artery. Practical consequence of attenuation: the penetration decreases as frequency increases

  13. 8 MHz 10MHz 12MHz A 0.5-mm-diameter object • Ultrasound frequency affects the resolution of the imaged object. • Resolution can be improved by increasing frequency and reducing the beam width by focusing. For a constant acoustic velocity, higher frequency US can detect smaller objects and provide a better resolution image.

  14. Spatial Resolution Axial and Lateral. Axial resolution is the minimum separation of above-below planes along the beam axis. It is determined by spatial pulse length, which is equal to the product of wavelength and the number of cycles within a pulse. Axial resolution = wavelength (λ) × number of cycle per pulse (n) ÷ 2

  15. Common Frequencies for Clinical US Dystrophic calcification of the choroids Portal Vein Ultrasound Color Doppler imaging shows a thrombus in upper PV moderately dilated (14.5 mm) with splenomegaly: Cirrhosis with PV thrombosis. Ablative therapy  MRI of a large tumor in the left kidney (L) and 12 days following HIFU treatment (R). 

  16. Dr. Karl Theo Dussik, an Austrian neurologist, was the first to apply US to image the brain. T1: ultrasonic generator, Q1: transmitter, Q2: receiver, T2: converter amplifier, W: water bath, L: light, P: photographic/ heat-sensitive paper Ultrasound in Med. & Biol., Vol. 30, No. 12, pp. 1565 - 1644, 2004

  17. Interaction Between Ultrasound and Tissue • Attenuation • Reflection • Refraction • Scattering Tissue absorbs the ultrasound energy, making the waves disappear. These waves don't return to the probe and are therefore "wasted". The more the body tissues that the ultrasound waves have to cross, the more attenuation the waves suffer. That is one reason why it is more difficult to image deeper structures. True reflection r=i

  18. Reflection Reflection occurs at the boundary/interface between two adjacent tissues The difference in acoustic impedance (z) between two tissues causes reflection of the sound wave z= density x velocity Reflection from a smooth tissue interface (specular) causes the soundwave to return to the scan head US image is formed from the reflected echoes

  19. Scattering Redirection of the sound wave in several directions Caused by interaction with a very small reflector or a very rough interface Only a portion of the sound wave returns to the scan head

  20. Transmission True reflection r=i Not all of the sound wave is reflected, therefore some of the wave continues deeper into the body These waves will reflect from deeper tissue structures

  21. Transducer Basics G E L Propylene glycol (propane-1,2-diol) conductive medium

  22. A Piezoelectric Material A piezoelectric disk generates a voltage when deformed (change in shape is greatly exaggerated) Tetragonal unit cell of lead titanate • Transducer (AKA: 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

  23. When a voltage is applied to an piezo electric crystal (shown in red below), it expands. When the voltage is removed, it contracts back into its original thickness. If the voltage is rapidly applied and removed repeatedly, the piezo electric crystal rapidly expands and relaxes, creating ultrasound waves.

  24. Piezoelectric crystal is compressed to generate a voltage Listen Striking

  25. Attenuation • Absorption = energy is captured by the tissue then converted to heat • Reflection = occurs at interfaces between tissues of different acoustic properties • Scattering = beam hits irregular interface – beam gets scattered

  26. Acoustic Impedance • The product of the tissue’s density and the sound velocity within the tissue • Amplitude of returning echo is proportional to the difference in acoustic impedance between the two tissues • Velocities: • Soft tissues = 1400-1600m/sec • Bone = 4080 • Air = 330 • Thus, when an ultrasound beam encounters two regions of very different acoustic impedances, the beam is reflected or absorbed • Cannot penetrate • Example: soft tissue – bone interface

  27. Frequency and Resolution • As frequency increases, resolution improves • As frequency increases, depth of penetration decreases • Use higher frequency transducers to image more superficial structures • Ex: Equine Tendons Frequency Penetration

  28. Modes of Display • 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

  29. Ultrasound Terminology • Never use dense, opaque, lucent • 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 • Spleen should be hyperechoic to liver • Liver is hyperechoic to kidneys

  30. Applications of US in Biomedicine Diagram illustrating development stage of microbubbles, nanobubbles, and nanodroplets for diagnostic and therapeutic purposes. HIFU = high-intensity focused ultrasound; KDR = kinase domain receptor.

  31. Ideal Characteristics of an Ultrasound Probe • High echogenicity • Low attenuation • Low blood solubility • Low diffusivity • Ability to traverse pulmonary system • Lack of biological effects in repeat exposures

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