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Echo physics and instrumentation

  2. PHYSICS AND INSTRUMENTATION • Sound waves are mechanical vibrations that induce alternate reductions and compressions on their passage through any physical medium. • Sound waves are described by the following terms: • Frequency (f): number of cycles per second (Hz) • Wavelength (λ): distance between cycles (mm) • Amplitude: extension of cycles, „loudness“ (decibels) • Propagation velocity (c): depending on the medium in which the sound waves travels(m/sec)
  3. • Ultrasound is defined as sound with frequencies above the for humans audible range between 20 Hz and 20.000 Hz. • Diagnostic medical ultrasound uses frequencies between 1.000.000 and 20.000.000 Hz = 1 to 20 megahertz (MHz). • By selecting transducers with different frequencies and by adjusting the frequency on the machine display one can select the emitted wave length. • According to the wave equation: c = λ •f, a change in frequency results in a reciprocal change in wave length. • As the propagation velocity in the heart is 1540 m⋅s-1, the emitted wavelength can be calculated as λ = c / f or λ (mm) = 1,54 / f (f in MHz).
  4. • Importance of wavelength for the ultrasound diagnostic: • Image resolution: maximal 1 -2 wavelengths (approx. 1 mm). The shorter the wavelength, the higher the image resolution will be. • Depth of penetration: proportional to the wavelength, inversely related to thefrequency: short waves travel short distances, long waves travel long distances. • Knobology: Select transducer and adjust frequency to match the required penetration depth while allowing for optimal image resolution.
  5. • Interaction of ultrasound waves with tissue • Reflection: a part of the ultrasound wave is thrown back towards the transducer by an object. • Scattering: a part of the ultrasound wave is diffused into all directions by an object. • Refraction: the direction of the ultrasound wave is deflected from the straight path by an object. • Attenuation: the energy of the ultrasound wave is absorbed by conversion into heat.
  6. An ultrasound transducer consists of many small, carefully arranged piezoelectric elements that are interconnected electronically. The frequency of the transducer is determined by the thickness of these elements. Each element is coupled to electrodes, which transmit current to the crystals, and then record the voltage generated by the returning signals. An important component of transducer design is the dampening (or backing) material, which shortens the ringing response of the piezoelectric material after the brief excitation pulse
  7. The principles of piezoelectricity • A piezoelectric crystal will vibrate when an electric current is applied, resulting in the generation and transmission of ultrasound energy. • Conversely, when reflected energy encounters a piezoelectric crystal, the crystal will change shape in response to this interaction and produce an electrical impulse.
  8. RESOLUTION Resolution is the ability to distinguish between two objects in close proximity. It has at least two components: spatial and temporal Spatial resolution is defined as the smallest distance that two targets must be separated by for the system to distinguish between them. It, too, has two components: Axial resolution refers to the ability to differentiate two structures lying along the axis of the ultrasound beam (i.e., one behind the other), and lateral resolution refers to the ability to distinguish two reflectors that lie side by side relative to the beam .
  9. • A third component of resolution is called contrast resolution. Contrast resolution refers to the ability to distinguish and to display different shades of gray within the image. This is important both for the accurate identification of borders and for the ability to display texture or detail within the tissues.
  10. • Temporal resolution, or frame rate, refers to the ability of the system to accurately track moving targets over time. It is dependent on the amount of time required to complete a scan, which in turn is related to the speed of ultrasound and the depth of the image as well as the number of lines of information within the image. Generally, the greater the number of frames per unit of time, the smoother and more aesthetically pleasing the realtime image. Factors that reduce frame rate, such as increasing depth of field, will diminish temporal resolution. This is particularly important for structures with relatively high velocity, such as valves. Temporal resolution is the main reason that M-mode echocardiography is still a useful clinical tool
  11. Pre processing versus post processing controls • Preprocessing controls adjust transmission and acquisition and formatting of ultrasound signals to convert into electrical signals. • changes in these affect the information to the scanner on the basis of which image is created. • post processing settings affect the manner in which preprocessed information is displayed on the monitor
  12. • Monitor Calibration • As for ordinary PC monitors, echo monitors can be calibrated only in the room in which the scanning will be performed. A bright room will have different calibration results than a dark room. It is important because when it’s not calibrated bright enough you may lose low-intensity signals and if it’s too bright signals from strong reflectors may be over- represented. With correct calibration you can use all the brightness range the monitor can display .
  13. Gain • GAIN amplitude of electrical signals • Generated by returning usg signals. • Increase in gain increases the amount of visible noise.
  14. • 2D GAIN • Increasing the 2D GAIN, which will increase the amplitude of the returning ultrasound signal,compensates for signal attenuation. The cost of increasing the 2D gain is a decrease in spatial resolution secondary to increased noise. • Adjust to the minimum required to obtain an adequate image. There should be a complete range from low (dark gray) to high (white) amplitude signals
  15. Proper Gain Image Overgain Image Undergain
  16. Excessive gain settings • Linear structures like MV septum appear thick • And speckled losing the differentiation
  17. Too low gain settings • Only bright signals like pericardium are visible. • Very low amplitude signals like SEC are missed. • Grey scale bar graph on right side of monitor image guides to adjust low amplitude dark gray to high aplitudes white signals. • Clinical pearl :eliminate the effects of bright ambient room lights
  18. • TIME GAIN CONTROL (TGC) • Also called depth compensation, these levers controls gain for individual sectors of the display in a vertical plane. • TGC compensates for changes associated with variation of US penetration at increasing depth; thus ensuring that all signals will be of similar intensity regardless of distance traveled. • The TGC allows amplification of the weaker signals returning from the far field more than the signals returning from shallower depths. • Usually set so that controls for near field are lower and far field higher - a “” shape. Some ultrasound units automatically compensate for the attenuation of the far field and therefore require a lower setting in both the far field and near field—set these units in a “bell-shape
  19. • LATERAL GAIN COMPENSATION (LGC) • The LGC control, which is not found on all ultrasound machines, involves the horizontal levers. • LGC allows for selectively modulating the gain at the lateral aspect of the image where there may be more attenuation of signal due to increasing scatter of the ultrasound waves
  20. • POWER • This control adjusts the amount of acoustic power transmitted by the US transducer. Since acoustic power equals acoustic energy/time it becomes evident that US can produce heat. Adjust the power control to highest power level within thermal limits (mechanical index of approximately 0.3).
  21. • DEPTH • DEPTH controls the depth of the image displayed in one-centimeter gradation. • The greater the depth,the less the resolution. At a higher depth, the transducer needs to cover a longer distance, therefore the frame rate and the resolution are both lower. • Set the depth at the minimum required to visualize all structures of interest.
  22. Higher Depth (10 cm) Lower Depth (8 cm)
  23. • DYNAMIC RANGE (DR) COMPRESSION • The DR is the range of useful US signals expressed as the ratio between largest and smallest signals. • Usually about 100db of ultrasound information is available, but the monitor can only display a much smaller range, on the order of ~ 30 db. • Therefore, in order to display the range of ultrasound signals detected by the transducer the dynamic range control allows for compression of the wide spectrum of amplitudes. These compressed signals are then displayed on the monitor as varying shades of gray. • Awide system DR is necessary to display very weak signals such as the endocardium and very strong signals such as calcified valves. • Logarithmic DR compression is a method to record all ultrasound signals, i.e. a method to increase the DR of the US system.
  24. • Increasing the DR yields a higher number of gray scale levels (increased spatial resolution by increased contrast levels) and increased image detail and smoother images. Decreasing the dynamic range increases the contrast of the image, with more black and white areas than shades of gray. Set so that blood filled cavities appear dark. Decreased dynamic range Increased dynamic range
  25. • FOCUS • The transmission FOCUS optimizes the ultrasound intensity in near and far field resulting in improved resolution. • Enables to focus the USG beam at aselected distance from the transducer by altering the sequences of electrical impulses sent to transducer elements typified by phased array transducers. • Goal as thinner beam improves lateral resolution so beam should be narrowest at SOI
  26. • CLINICAL PEARL • Basically tells which depth we like to have the best image resolution When evaluating for PFO focus should be placed at this level of atrial septum as structures distal to it appear fuzzy or abnormally thick
  27. FREQUENCY • Denotes transmitted usg frequency which can be adjusted according to proximity of structures to the transducer esp in TEE “Res” translates to the highest frequency band available on the transducer. These settings are used for superficial imaging. “Gen” represents mid-range frequencies that are often the default setting. “Pen” represents the transducer’s lowest range of frequencies and is for deeper tissue or difficult-to-image patients • For shallow structures use higher frequencies and for deeper structures use lower frequencies. • In an obese patient, the ultrasound waves have farther to travel and are attenuated along the way. The lower end of the frequency range should be used in obese patients, which can compromise detail but allows for better penetration
  28. Tissue harmonics When sound waves of a given frequency pass through tissue, harmonics are produced at multiples of the initial frequency. The tissue harmonics setting interprets one of these harmonics, filtering out reverberation echoes allowing for a cleaner image with better contrast and less artifact. Certain applications, such as M mode LV measurements in these instances tissue harmonics should be turned off. Harmonics imaging allows the ultrasound to identify body tissue and reduce artifact in the image. It does this by sending and receiving signals at two different ultrasound frequencies. For example, with harmonics on, a probe would emit a frequency of 2MHz, but it would only “listen” for a 4MHz frequency. This improves image quality because body tissue reflects sound at twice the frequency that was initially sent, which results in a cleaner image that better displays body tissue without extra artifacts
  29. COMPRESSION • Post processing tool works in conjunction with preprocessing dynamic range settings to alter the range of displayed grey scale. • Reducing compression gives largest dynamic range of grey shades like “ matt finish” • Increasing compression eliminates the display of shades of grey at each spectrum resulting in softer smoother image like “glossy finish”
  30. Ultrasound Gray Maps • Adjusting gray maps on your image has a similar effect on an ultrasound image as changing the dynamic range., but they are different. While Dynamic Range adjusts the overall number of shades of gray, a gray map determines how dark or light you prefer to show each level of white/gray/black based upon the strength of the ultrasound signal
  31. REJECT • This control enables elimination of greater number of low intensity signals termed as acoustic noise coming from refraction signals from within the body and electric noise from equipment itself or ventilators. • Clinical pearl:increase reject control to eliminate random echoes from low intensity areas but care should be taken not to miss pathology like thrombus in formation
  32. • PERSISTENCE • Adjusts the updating and averaging of consecutive frames on the screen to reduce noise and speckling. Increased persistence will smooth out the images but sacrifices crispness of moving structures. Decreased persistence will give a grainier image. Increased Persistence Decreased Persistence Higher levels of persistence are more desirable for slow moving structures and lower levels for rapidly moving structures
  33. SECTOR SIZE • It controls the angle of the sector displayed on the monitor. • Wide angles >90⁰ are used to survey broad array of cardiac structures in a single frame • But wider the sector size lower the frame rate and thus reduced temporal resoltion • For fast moving structures sector width should be kept narrow so as to allow higher frame rates for better resolution. Line Density • 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.
  34. COLOR CONTROLS ROI • ROI is the area that defines where color will be displayed. • Depth and width of ROI is inversely proportional to frame rate and therefore temporal resolution of color Doppler image.
  35. COLOR GAIN • Similar to 2D gain as an amplifying tool but probably more imp. Clinically because a low set gain may miss flows of small leaks paravalvular or across small pfo.high gain settings lead to increased size of color signals such as of regurgitant jets leading to overestimation. • Clinical pearl adjust color gain until specles of color lie outside ROI and then reduce one otr two settings until the speckles go away also increase gain if heart rate is increased >110bpm
  36. COLOR SCALE • Displays the range of color velocities. • It should be adjusted acc to expected velocities at ROI if ROI is pulmonary veins then low velocity blood flow is expected so decrease color scale • For PISA measurements baseline of color scale nyquist limit is adjusted to get hemisphere measurements
  37. VARIANCE • This setting is given so as to distinguish between laminar blood flow (which appears as standard shades of blue or red depending on its direction of flow in relation to transducer)from turbulent flow jets by giving it mosaic colors in shades of yellow and green.
  38. • EDGE • The Edge control varies the degree of sharpness of borders and edge enhancements within an image. • Lower edge levels produce smoother delineation with less noise. Higher edge levels produce sharper images
  39. • Zoom • The zoom function allows magnifying region of interest on the screen. It is important to distinguish between ordinary zoom when the image appears larger, but the resolution of the magnified area does not change, and high resolution zoom when the resolution in the region of interest is increased. This function is important when performing precise measures such as left ventricular outflow tract or PISA radius.
  40. • CALIPER • Calipers are available for taking measurements in each of the major operating modes. • TRACE • This control enables tracing of cardiac structures (2D) and velocity envelopes (spectral Doppler).
  41. Speckle Reduction Imaging • Speckle Reduction Imaging (SRI,XRES,) uses an algorithm to identify strong and weak ultrasound signals. By evaluating the image on a pixel-by-pixel basis, it attempts to identify tissue and eliminate “speckle.” Weak signals that seem to be astray are eliminated, while strong signals are enhanced/brightented. It provides a smoother, cleaner image
  42. Compound Imaging • Compound Images combines three or more images from different steering angles into a single image. Traditionally, transducers send ultrasound signals in a single “line of sight.” This means it sends a sound signal perpendicular to the probe head, then listens for the echo. With compound imaging, the ultrasound sends signals at multiple angles, allowing it to “see” tissue from multiple angles and eliminate artifact
  43. Optimal settings • Depth 15-17 • Frame rate near 100 /sec • Sector width narrow • Compression low • Optimal 2d and color gain (50-60) • Zoom for smaller measurements • Decrease 2d gain for zoomed images • Increase sweep speed for time measurements • Use sweeps and decrease near field TGC to eliminate artifacts • Use focus for smaller structures evaluation • Power settings adjustment for contrast studies.