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Echo physics and instrumentation

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Echo physics and instrumentation

  2. 2. sound wave - area of compression alternating with an area of rarefaction
  3. 3.  sum of one compression and one rarefaction represents one cycle  distance between two similar points - wavelength  wavelength - 0.15 to 1.5 mm  frequency - number of wavelengths per unit of time  wavelength and frequency are inversely related
  4. 4.  Velocity through a given medium depends on the density and elastic properties or stiffness of that medium  Velocity is directly related to stiffness and inversely related to density.  Ultrasound travels faster through a stiff medium, such as bone.  Within soft tissue, velocity of sound is fairly constant -1,540 m/sec
  5. 5.  amount of reflection, refraction, and attenuation depends on acoustic properties  Tissues composed of solid material interfaced with gas (such as the lung) will reflect most of the ultrasound energy - poor penetration.  Very dense media - reflect a high percentage of the ultrasound energy.  Soft tissues and blood-allow more ultrasound energy ,increasing penetration and improving diagnostic utility.  Bone reflects most ultrasound energy
  6. 6.  Amplitude= strength of the sound wave  measured in decibels  increase of 6 dB is equal to a doubling of signal amplitude  60 dB represents a 1,000-fold change in amplitude or loudness
  7. 7.  Power- rate of energy transfer to the medium, measured in watts  represented over a given area (beam area)  intensity (watts per centimeter squared or W/cm2).  diminishes rapidly with propagation distance
  8. 8. Attenuation  loss of ultrasound as it propagates through a medium – attenuation  absorption, scattering, and reflection  Attenuation - increases with depth ; affected by frequency and the type of tissue  “half-value layer” or the “half-power distance,”  distance that ultrasound travels before its amplitude is attenuated to one half its original value.  attenuation = 0.5 and 1.0 dB/cm/MHz.  3-MHz transducer at a depth of 12 cm (24-cm round trip) - attenuated as much as 72 dB (or nearly 4,000-fold).  higher the frequency - smaller the structures that can be accurately resolved; less penetration
  9. 9. Attenuation  greater in soft tissue compared with blood and is even greater in muscle, lung, and bone.  Acoustic impedance (Z, measured in rayls) - product of velocity (in meters per second) and physical density (in kilograms per cubic meter).  When the beam crosses a boundary between two tissues, a portion of the energy is reflected, a portion is refracted, and a portion continues .  acoustic mismatch-magnitude of the difference in acoustic impedance.  Without the gel, the air-tissue interface at the skin surface results in 99% of the ultrasonic energy being reflected due to the very low acoustic impedance of air.  gel - increases the percentage of energy that is transmitted.
  10. 10. • Either decreasing the wavelength (increasing the frequency) or increasing the size of the transducer will lengthen the near field
  11. 11. • transducer size is limited by the size of the intercostal spaces
  12. 12. Don’t fear the gain control • a simple twist can make a world of difference in image quality. • amplification of the returning US signal. • noise -also amplified. • Minimal gain should be used to provide an optimal image with good quality without dropout or blooming of signals • High gain • image appears bright • linear structures, such as the mitral valve, appear thickened • LV cavity acquires a speckled appearance • entire LV takes on a whitened appearance, and the ability to differentiate structures is lost.
  13. 13. Optimum gain fluid and blood -black myocardium -medium grey pericardium and calcification- bright white color
  14. 14. Transmit power • Amplitude of the transmitted signal, at the probe. • measured in deciBels. • T mechanical index • amount of energy that is absorbed by the patient • dependent on the power & focussing of the beam • highest where the beam is focussed • decreases with depth
  15. 15. TGC: Time Gain Compensation controls • 5-10 slide controls grouped together. • adjust gain in specific areas of the image (near-, mid-, and far-field). • ove to right-of-center as image quality decreases deeper in the image. • have lower gain in the nearfield, and higher gain deeper in the image where image quality is weaker. • toggles amplify the weak returning signal proportionately to the time delay and increase the gain for that particular depth • ensure signals of similar magnitude at different depth .
  16. 16. Axial resolution • short ultrasound pulse needs to be transmitted • typical value for a 5-MHz transducer -can generate/receive frequencies in the range of 3– 7 MHz • absolute transducer bandwidth is proportional to the mean transmission frequency • A higher frequency transducer -produces shorter ultrasound pulses • higher frequencies are attenuated more by soft tissue and are impacted by depth • pediatric and neonatal -higher frequency transducers that increase image spatial resolution • infants 10–12-MHz transducers - axial resolution of the order of 250 μm • Most systems allow changing the transmit frequency of the ultrasound pulse within the bandwidth of the transducer • 5-MHz transducer can be used to transmit a 3.5-MHz pulse which can be practical when penetration is not sufficient at 5 MHz
  17. 17. Lateral resolution • determined by the width of the ultrasound beam (i.e., the width of the main lobe). • The narrower the ultrasound beam, the better the lateral resolution. • focusing-introducing time delays between the firing of individual array elements (similarto whatis done for beam steering)in order to assure that the transmitted wavelets of all individual array elements arrive at the same position at the same time and will thus constructively interfere • Similarly, time delaying the reflections of the individual crystals in the array will make sure that reflections coming from a particular point in front of the transducer will sum in phase and therefore create a strong echo signal • transmit focus should always be positioned close to the structure/region of interest • Most ultrasound systems allow selecting multiple transmit focal points. • The easiest way to improve the focus performance of a transducer is by increasing its size (i.e., aperture). • footprint needs to fit between the patient’s ribs
  18. 18. • 8-MHz pediatric transducer= • 0.3 mm at 2 cm going up to 1.2 mm at 7 cm depth.
  19. 19. Temporal resolution • About 33 images can be produced per second, which is sufficient to look at motion (e.g., standard television displays only 25 frames per second). • With parallel beam forming, higher frame rates can be obtained (70–80 Hz). • to increase frame rate 1. Reduce field of view 2. or number of lines per frame (i.e.,the line density) can be reduced. • The latter comes at the cost of spatial resolution, as image lines will be further apart. • Most systems have a “frame rate” button nowadays that allow changing the frame rate although this always comes at the expense of spatial resolution •
  20. 20. Higher frame rates are important
  21. 21. • beat-to-beat variations during the respiratory cycle=low sweep speed • high sweep speed is =flow characteristics within a single cardiac cycle. • Temporal resolution = M-mode
  22. 22. Tissue Harmonic Imaging  fundamental, frequency →interactions in tissue - generation of frequencies not present in the original signal  integer multiples of the original frequency = harmonics.  returning signal = fundamental and harmonic frequencies.  contrast echo (microbubbles produces vibrations that occur at multiple (harmonic) frequencies
  23. 23. reduces the artifact -weak signals that cause many artifacts are disproportionately suppressed reduces near field clutter . signal-to-noise ratio is improved and image quality is enhanced improved endocardial border definition. strong specular echoes, such as those arising from valves, appear “thicker” in the far field
  24. 24.  strength of harmonic frequency increases as the wave penetrates the body.  fundamental frequency -attenuates constantly during propagation  Close to the chest -little harmonic signal  harmonic frequency -avoids many of the near field artifacts  At depths of 4 to 8 cm- trength of the harmonic signal is near its maximum, whereas the fundamental frequency has diminished considerably.
  25. 25. 2 D Always use the highest possible transducer frequency to optimize spatial resolution infants =(8–12 MHz) different transducers have to be used for different parts of the exam subcostal imaging in a newborn- 5- or 8-MHz apical and parasternal windows - 10–12-MHz larger children and young adults= 5-MHz and rarely 2.5–3.5-MHz probes parasternal windows – higher frequency probes
  26. 26. in smaller children harmonic imaging does not necessarily result in better image quality due to its intrinsically lower axial resolution Harmonic imaging =useful in larger children and adults TGCs are used to make the images as homogeneous as possible at different depths. For optimizing temporal resolution the narrowest sector possible is to be used. Depth settings are minimized to include the region of interest
  27. 27. Optimization of CW Doppler 1. Align with the direction of the measured velocity 2. gain controls should be manipulated to produce a clean uniform profile without any “blooming.” 3. gain controls should be turned up to overemphasize the image and then adjusted down. This will prevent any loss of information due to too little gain. 4. reject button eliminates the smaller amplitude signals that are below a certain threshold level-cleaner image 5. use filter to reduce the noise from walls
  28. 28. Optimizing PW Doppler signals  align with the direction of the velocity  gain control, compress, filter settings are similar compared to CW  shift of the baseline allows the whole display to be used for either forward or reverse flow, which is useful if the flow is only in one direction  Nyquist limit should always be optimized and be set no higher or lower than necessary  Sample volume: increase in sample volume increases the strength of the signal and more velocity information at the expense of a lower spatial resolution  Smallest sample volume that results in adequate signal-to-noise ratio should be used
  29. 29. Optimizing color Doppler imaging • Use the smallest color Doppler sector as necessary. • Large sector color Doppler has lower temporal resolution. • Gain settings should be adjusted until background noise is detected in the color image and then reducing it so that the background noise disappears • Nyquist limit (scale): should be adapted depending on the velocities of the flows measured • high velocity flows- high Nyquist limit is chosen • low velocities (coronary flow, venous flows) - lower the sclae
  30. 30. • iSCAN =optimization quickly and automatically adjusts system parameters in both 2D and Doppler modes • iFOCUS -automatically computes beam characteristics for a selected region of interest, and then provides the best detail resolution and tissue uniformity. • iOPTIMIZE -adjusts system performance for different patient sizes, flow states, and clinical requirements.
  31. 31. • iRotate -electronically achieve the best view within the acoustical window between ribs
  32. 32. Compression • alters the difference between the highest and lowest echo amplitudes by compressing the wide spectrum of amplitudes and fitting them in a grey-scale range. • Increasing compression provides a smoother image with more shades of grey but may increase unwanted signals i.e. "noise • A mid-level compression is usually adequate for optimal imaging
  33. 33. • Reject The reject control is an adjustable control that eliminates low- level interference caused by refracted aberrant ultrasound and electronic "noize". Care must be taken not to eliminate low- intensity echoes from fresh intracardiac thrombi by setting the reject threshold too high. [24]
  34. 34. • Sampling and depth — Color sector size and placement changes width, length, and position of the color box • narrow box / close to the transducer-prevents aliasing, lower frame rates, and 'swimmy images' • Gain Excessive 2-D gain -obscure the flow. ;↑ noise and impair the image quality and interpretation • low gain -attenuate sensitivity and make the image appear smaller than the flow jet. Velocity scale When the blood-flow velocity exceeds the velocity scale of the color legend, the machine will continue to represent the velocity of the blood flow, but in the opposite direction • once the velocity of blood flow in one direction exceeds the brightest red, the color will instantaneously change to the brightest blue, then gradually darken. • velocity scale must be set to around 50 - 60 cm/second • Lower scales increase sensitivity, but promote aliasing •
  35. 35. • most transducers are broadband- adjusting the frequency will typically display a frequency range, not a single frequency. • Some machines will show the actual frequency range (represented by MHz), • others : “Res”, “Gen”, or “Pen” • “Res” -highest frequency band ;superficial imaging. • “Gen” =mid-range frequencies ;default setting. • “Pen” -transducer’s lowest range of frequencies ;deeper tissue or difficult-to- image patients • High frequencies provide the best resolution, but you lose penetration. Low frequencies provide the best penetration at the expense of image resolution
  36. 36. TEE • handle - two control wheels and array rotation buttons • tip of the probe should be aligned in the neutral position during movement, to prevent tissue damage • Each wheel has friction brakes that hold the tip position without locking it.
  37. 37. large wheel -anteflexion and retroflexion. small wheel - lateral flexion . probe can be advanced or withdrawn in a vertical fashion into the esophagus and stomach Can be turned to the left or right . rotation buttons turn the mutiplane angle from 0 to 180°.