01 basic principles of ultrasound & basic term


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01 basic principles of ultrasound & basic term

  1. 1. Basic Principles of Ultrasound & Basic term Produced by International Clinical Application Team
  2. 2. PHYSICAL PROPERTIES Defines Ultrasound (US) as : sound with frequency greater than 20,000 cycles per second or 20kHz. Advantages : US can direct as a beam. It obeys the laws of reflection and refraction. It is reflected by objects of small size. Disadvantages : It propagates poorly through a gaseous medium. The amount of US reflected depends on the acoustic mismatch. Reflection and Propagation : • Effect of propagation through gaseous zones - poor propagation, inadequate imaging. • Effect of propagation through dense zones - nearly all of the US is reflected. Structures below dense zones are poorly imaged Examples of dense materials - bone, calcium, metal.
  3. 3. <Definitions> • Cycle - the combination of one rarefaction and one compression equals one cycle. • Wavelength the distance between the onset of peak compression or cycle to the next. • Velocity - the velocity is the speed at which sound waves travel through a particular medium. Velocity is equal to the frequency x wavelength. The velocity of US through human soft tissue is 1540 meters per second. λ
  4. 4. • Frequency - the number of cycles per unit of time. Frequency and wavelength are inversely related. The higher the frequency the smaller the wavelength. • Acoustic Impedance - simply put, acoustic impedance is dependent on the density of the material in which sound is propagated through. The greater the impedance the more dense the material. • Reflection The portion of a sound that is returned from the boundary of a medium. (echo) • Refraction The change of sound direction on passing from one medium to another. • Acoustic Mismatch The boundary between two different media where reflection and refraction occurs • Attenuation The decrease in amplitude and intensity as a sound wave travels through a medium
  5. 5. <Types of Echoes> • Specular : echoes originating from relatively large, regularly shaped objects with smooth surfaces. These echoes are relatively intense and angle dependent. (i.e. IVS, valves) • Scattered : echoes originating from relatively small, weakly reflective, irregularly shaped objects are less angle dependant and less intense.(i.e.. blood cells) • Frequencies: Frequencies for adult imaging - 2.0mHz to 3.0mHz. Frequencies for pediatric imaging - 5.0mHz to 7.5mHz and 10mHz. Effect of higher frequencies on penetration - the higher the frequency the less penetration, the lower the frequency the greater the penetration.
  6. 6. • Artifacts: Acoustic Shadowing - the loss of information below an object because the greater portion of the sound energy was reflected back by the object. This occurs in objects like prosthetic valves. Enhancement - the increase in amplitude from objects that lie behind a weakly attenuating structure. Enhancement may occur in structures below a cyst. Reverberation - produced from the multiple reflections from an object as the sound energy bounces back and forth between the object and the transducers face or dense structure.
  7. 7. <Resolution> • Lateral resolution the ability to resolve objects side by side. Lateral resolution is proportionally affected by the frequency, the higher the frequency the greater the lateral resolution. Higher frequency transducers are used in fetal and pediatric echocardiography because the lateral resolution displays the smaller structures in those patients and there is less need for depth penetration. Lower frequencies are used for adults where structures are larger and the need for greater depth penetration is important. • Axial Resolution Axial resolution is the ability to resolve objects that lie one above the other. Axial resolution is inversely proportional to the frequency of the transducer depending on the size of the patient. The higher the frequency the lower the axial resolution is in large patients. This state results from the rapid absorption of the ultrasound energy with lower penetration. Lower frequencies are utilized to increase depth of penetration. • Depth of Penetration Higher frequencies are attenuated by tissue more than lower frequencies. This means that the higher the frequency the greater the resolution but the lower the depth of penetration. User lower frequencies for adults and higher frequencies for children. The advent of harmonic imaging allows the use of a lower frequency pulse to be picked up and sampled at a higher frequency (the second harmonic) where the low frequency allows greater penetration and high frequency provides better resolution. 2.0mHz 5.0mHz Axial Decrease Increase Lateral Decrease Increase Penetration Increase Decrease
  8. 8. <Basic Components of the Imaging System> • Transducer the probe housing the elements, backing material, electrodes, matching layer and protective face that both sends and receives the sound waves. • Transmitter - the component that creates the impulses sent to the transducer to generate sound energy. Also called the pulser. • Receiver - the component that receives the current generated in the transducer from the returning sound energy. • Amplifier - the component that amplifies the returning signals and prepares them to be displayed on the CRT.
  9. 9. <Instrumentation> • Depth : Adjusts the depth from which the returning ultrasound signals will be displayed. Most ultrasound systems have a maximum depth of 24cm. The deeper the depth the slower the frame rate of the image. A shallow depth results in a higher frame rate. • Output gain (power) : Adjusts the amount of power used to send ultrasound into the body. The higher the output gain the stronger the returning signal and the better The signal to noise ratio. • Receiver gain : Adjusts the amount of power used to amplify the returning ultrasound signal. Higher receiver gains result in higher levels of signal and noise in the image. • TGC : Depth gain compensation adjusts the amount of power used to amplify the returning ultrasound signals at a specific depth. Higher TGC levels result in higher levels of signal and noise at a given depth. The TGC sliders are used to ensure a uniform display of the gray scale intensity throughout the display. Strong signals can be decreased and weak signals can be increased at specific depths • Focal zone : Modern phased array transducers can be dynamically focused. As the focal zone control is adjusted up and down a marker can be seen moving up and down a marker can be seen moving up and down pointing at the current zone of focus. • Transmit frequency : Most of today's cardiac ultrasound systems have phased array transducers which use wide band width technology. These wide band width Transducers can be set to selectively receive anywhere from 2 to 4 frequencies within a given range.
  10. 10. • Reject : Filters out low-level signals so that only the stronger returning signals are displayed. Reject can be used to decrease the amount of noise in the image if the noise has a significantly lower intensity than the signals returning from anatomic structure. • EE (Edge Enhancement) : The boundaries between strong and week returning signals are enhanced by increasing the strong signal and decreasing the weak signal. • DR (Dynamic range) : Determines the number of gray shades used to map the gray scale image on the display. Higher compression results in more shades of gray (a softer looking image). Lower compression results in a more bi-stable (black and white) image with fewer shades of gray. • Post processing : Sometimes referred to as gamma curves. Post processing of the gray levels allows the user to determine the gray scale intensity that will be assigned to the different returning signals. • Persistence (Frame Average) : Normally a frame is built by memory and then immediately sent to the display. Real image data can be enhanced and noise can be averaged out by performing frame averaging.
  11. 11. • Sweep speed : The display rate for the M-mode or Doppler data. Common display rates are 12.5, 25, 50 and 100 mm/sec • Sample size : The length of the pulsed Doppler sample gate can be adjusted from 1 to 10 mm on most ultrasound systems. Increasing the sample gate, increases the amount of time that the system samples (listens for) returning signals. • Sample position : The pulsed and continuous wave Doppler cursor position can be adjusted from side to side across the field of view. The pulsed Doppler sample volume can be adjusted up and down within the field of view but is limited to the range of the selected PRF. • PRF : The pulse repetition frequency is the number of times that a pulse (pulsed wave Doppler) is repeatedly sent during one second. The PRF determines the scale of the pulsed Doppler display and is labeled ‘scale’ on most ultrasound systems. • Filter : The low velocity component of the pulsed or continuous wave Doppler signal can be rejected by adjusting the Doppler filters. • Baseline shift : Flow towards the transducer is displayed above the spectral Doppler baseline. Flow away from the transducer is displayed below the spectral Doppler baseline. The position of the baseline can be adjusted up and down within the spectral Doppler display.
  12. 12. Pulsed wave Doppler (PW) : Pulsed wave Doppler uses a single transducer element which sends out short bursts of ultrasound. Range resolution (the ability to localize and analyze blood flow at specific depths) can be achieved by sampling the returning bursts of ultrasound at various time intervals. The major limitation of standard PW Doppler Is its inability to measure high velocities. Advantage Is range specific thus allowing for the measurement of various flow characteristics in small selected regions of interest. Disadvantage Spectral wrap around (aliasing) at relatively low velocities Continuous wave Doppler (CW) : Continuous wave Doppler uses two side by side transducers elements. One element continuously sends (transmits) while the other element continuously receives. This method will allow maximum velocities to be measured but has no range resolution CW Doppler samples all blood flow in its path. A spectrum of Doppler frequencies are observed. Only the highest velocity in the Doppler path can be clearly measured. It is possible however to distinguish between flow patterns which have different time and intensity relationships. Advantage Can display high velocities without aliasing. Disadvantage Displays all velocities in its path. The non-imaging probe requires a High degree of skill to operate.