Ultrasound

• what are sound waves?

  mechanical pressure waves which propagate through a medium causing the particles of the medium to oscillate backward and forward.
• what is wave propagation?

  as an US pressure wave propagates through the medium, molecules in regions of high pressure will be pushed together (compression) and molecule in regions of low pressure will be pulled apart (rarefaction).
• describe amplitude?

  maximum displacement from the undisturbed portion of the medium to the top of a crest
• define frequency?

  how many waves are made per time interval i.e. cycles per second. frequency = 1/period
• define wavelength?

  distance between any 2 adjacent corresponding locations on a wave train
• define speed?

  speed at which a sound wave travels is determined by the medium. speed of sound in tissue = 1450m/s
• define ultrasound?

  sound waves of such a high frequency they are inaudible to humans. frequency above 20kHz
• what is range of normal human hearing?

  20Hz-20kHz
• what is the US frequencies used for imaging?

  2-15MHz
• define attenuation?

  energy lost as sound propagates through tissue and intensity decreases
• define absorption?

  some US energy converted to heat, reflection and scattering
• define refraction?

  occurs when beam crosses from one medium/tissue to another. divergence/diffraction
• how is sound produced?

  • vibrations produce soundpiezoelectric crystal has all molecules aligned in a certain directiondipolar molecule acts like little magnets: +ve end and -ve endvoltage in one direction aligns all molecules in one directionreverse voltage in another direction will align molecules in another directionmaterial compresses or expands in response to voltage: piezoelectric effectapplying alternating voltage to crystal will cause it to expand and contract at high frequencyproduces continuous sound wavesconverting electrical energy into mechanical energy (sound)
• how is sound recieved?

  • transducer can respond to sound wave which causes piezoelectric crystal to expand and contractproduces small voltage signals from the sound waves therefore converting mechanical energy (sound) into electrical energy
• how does transducer detect and recieve sound?

  • sound waves produced by vibrations in airtransmission: causing electrical energy to be converted to mechanical energy (ultrasonic pulses)detection: causing mechanical energy (ultrasonic echo pulses) to be converted to electrical signals
• how is sound pulse produced?

  • sound is produced as a pulse, not continuous waveshort, high voltage appliedproduces sound as special frequency = resonant frequencyfrequency depends on thickness go crystalsound wavelength is twice that of thickness of crystalat this frequency, oscillations produced at maximum amplitude
• pulse echo in tissue?

  •  US pulse is launched into first tissueat tissue interface a portion of US signal is transmitted into second tissue and portion reflected within first tissueecho signal detected by transducer
• what is pulse echo principle?

  • the transmit pulse travels along the US beam into tissues. echoes generated at interfaces return to transducerthe time of arrival of echoes after transmission increases with the depth of the reflector
• how is digital US image produced?

  • electrical signals are processed and grey-scale image is formedUS digital image is composed of pixelsdepth of echo is calculated from arrival timestrong echo: display is white. Medium: grey. No echo: black
• how does B-mode image form?

  • pulse-echo cycles are initiated at a series of adjacent beam positions along the transducer facethe line of echoes produced by each cycle is used to form a B-mode line in the image
• reflection and transmission?

  • reflection is most basic interaction in US. usually occurs at interfacesonly a portion of sound is reflected, rest is transmittedreflection depends on acoustic impedance
• what is acoustic impedance?

  • measure of how the tissue appears to the ultrasound2 tissues with similar acoustic impedance values will appear similar to USdifferent acoustic impedances will appear differentif particles are densely packed, it takes a lot go pressure for them to vibrate at certain speed: high acoustic impedanceif particles loosely packed then its much lower excess pressure for them to vibrate at certain speed: low acoustic impedance
• what is large diff in acoustic impedance?

  total reflection, strong echo.could be soft tissue and air interface.seen as bright linear structure in image
• what is no diff in acoustic impedance?

  total transmission of US energy.no reflection, no echointerface not seen in US image
• what is partial diff in acoustic impendance?

  partial reflection, partial transmission.could be liver tissue-fat interfaceinterface seen in image (some grey level) but not as strong as total reflection case.
• what is intensity reflection coefficient?

  for US to work, requires wave propagating through materials with similar acoustic impedances - small amount reflected and rest transmitted.DEF: the proportion of the sound energy reflected compared to that transmitted.1 = total reflection0 = total transmissionfor a lot of tissue interfaces, percentage of energy reflected is <1%
• what is doppler?

  • US emitted by probe and is reflectedfrequency of echoes is slightly altered due to movement of blood. known as doppler shiftmachine detects and displays doppler shift - detects alteration in doppler frequency as a result of movement3 types: colour doppler, power doppler, spectral doppler - pulsed wave + continuous wave
• what is colour doppler?

  a mode where doppler shift information is acquired for part (or all) of the image and displayed as a colour overlay on grey-scale image.red = towards transducer, blue = away from transducer. differentiate between venous and arterial vessels.
• what is spectral doppler pulsed?

  a mode where a single relatively small sample volume is placed by the user at the point of interest and doppler signals are acquired.sample volume placed in vessel. detailed info about blood flow obtained from sample volume.indicates direction and velocity of flow.
• what is spectral doppler - continous wave?

  a mode where US is transmitted and echoes are received continuously.important tool in cardiac US. extremely high velocities can occur in presence of disease. pulsed and colour doppler have limitation when measuring high velocities so continuous wave is only way to measure these velocities.indicated direction and velocity of flow.
• what is the doppler equation?

  fD = fE - ffD = doppler shiftfE = frequency of echof = transmitted frequency