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SEMINAR ONULTRASOUND Tharanath PP India Ultrasound
Ultrasound Basic Idea – Send waves into body which are reflected at the interfaces between tissue – Return time of the waves tells us of the depth of the reflecting surface History – First practical application, 1912 unsuccessful search for Titanic – WW II brought massive military research - SONAR (SOund Navigation And Ranging) – Mid-century used for non-destructive testing of materials – First used as diagnostic tool in 1942 for localizing brain tumors – 1950’s 2D gray scale images – 1965 or so real-time imaging
Sound waves • Sound wave propagate by longitudinal motion(compression/expansion), but not transverse motion(side-to-side) • Can be modelled as weights connected by springs
Specular - echoes originating from relatively large, regularly shaped objects with smooth surfaces. These echoes are relatively intense and angle dependent. (i.e.valves) - Reflection from large surfaces Scattered - echoes originating from relatively small, weakly reflective, irregularly shaped objects are less angle dependant and less intense. (i.e.. blood cells) -Reflection from small surfaces
Along each line we transmit a pulse and plot thereflections that come back vs time
Propagation of ultrasound waves intissue Propagation of ultrasound waves in tissue • Ultrasound imaging systems commonly operate at 3.5 MHz, which corresponds to a wavelength of 0.44 mm when c = 1540 m/s. Refraction • When a wave passes from one medium to another the frequency is constant, and since c changes then so must the wavelength
Propagation of ultrasound waves in tissue Bending of waves from onemedium to another is refraction• Follows Snell’s Law sin theta/c1 since λ2 < λ1 we have c2 <c1 and θ2 < θ1
Definition/ Terminology Cycle Frequency: cycles per second Wave length Period Amplitude Compression - area of high density Rarefication - area of low density velocity = l x ƒ= constant for a given medium
A, Initial observation.B, After a short time, regions of high & low density are displaced tothe right
Ultrasound Production Piezoelectric effect: – Crystals vibrate at given frequency when an alternating current is applied – Crystal acts as speaker and microphone Continuous mode: – continuous-wave Doppler (CW) Pulsed-echo mode:
there is no information about the time interval from the signal to the reflection, and, hence, no information about the depth of the received signal; the signal may come from any depth. The continuous Doppler has no Nykvist limit, and can measure maximal velocities. It is used for measuring high velocities.
Pulsed Echo Signal generation = only ~1% of the entire pulse cycle On times; off times Time of signal return proportional to distance travelled
if the PRF = 5 kHz and the time between pulses is 0.2 msec, it will take 0.1 msec to reach the target and 0.1 msec to return to the transducer. This means the pulse will travel 15.4 cm before the next pulse is emitted (1,540 m/sec x 0.1 msec = 0.154 m in 0.1 msec = 15.4 cm).
Pulsed Echo PRF: pulse repetition frequency – Very important in Color Doppler SPL: spatial pulse length = wavelength x no. of cycles Maximal resolution = 0.5 SPL – The smallest distance between 2 points that USG can delineate
SPL with maximal resolution. Two objects(vertical lines) areseparated by 0.5 SPL. The echo from eachinterface is shownby dashed lines. The objects are just resolvable.
Pulse length shortened by increasing thefrequency.A ........The four-cycle pulse from a low-frequencytransducer includes both objects within the SPL.B .........The four-cycle pulse from a high-frequency transducer has a shorter spatial pulselength and can resolve objects located more closelytogether.
the main principle is that blood has high velocity(Typically above 50 cm/s, although also all velocitiesdown to zero), but low density, resulting in lowintensity (amplitude) reflected signals.Tissue has high density, resulting in high intensitysignals, but low velocity (typically below 20 cm/s).
The pulsed modus results in a practical limit on the maximum velocity that can be measured. In order to measure velocity at a certain depth, the next pulse cannot be sent out before the signal is returned. The Doppler shift is thus sampled once for every pulse that is transmitted, and the sampling frequency is thus equal to the pulse repetition frequency (PRF). Frequency aliasing occurs at a Doppler shift that is equal to half of the PRF. fD = ½ * PRF
Sampling from increasing depth will increase the time for the pulse returning, thus increasing the sampling interval and decrease the sampling frequency. The Nykvist limit thus decreases with depth. This means that pulsed Doppler has depth resolution, but this leads to a limit to the velocities that can be measured.
Ultrasound Transmission 1540 m/s– Velocity assumed the same for all tissue in calculation (which is not totally true) Acoustic impedance Attenuation Reflection Refraction Scatter: objects irregular or smaller than the ultrasound beam
Resolution Ability to delineate between two different objects Axial resolution: – high frequency = shorter SPL = better axial resolution but lower penetration
Resolution Lateral Resolution – Sound beam: width of the crystal – Near field – Focal zone: best lateral resolution – Far field – Dead zone: distance between the transducer face and the first identifiable echo
Resolution Temporal Resolution – Frame per second – Multiple focal zones • decreases frame rate • decrease temporal resolution
Modes A mode: amplitude B mode: brightness Real time (frames/sec) M mode: motion
A-Scan Presentation. The A-scan presentation displays the amount of received ultrasonic energy as a function of time. The relative amount of received energy is plotted along the vertical axis and the elapsed time (which may be related to the sound energy travel time within the material) is displayed along the horizontal axis
B-Scan Presentation. The B-scan presentation is a profile (cross-sectional) view of the test specimen. In the B-scan, the time-of-flight (travel time) of the sound energy is displayed along the vertical axis and the linear position of the transducer is displayed along the horizontal axis
Doppler Ultrasound Continuous wave (CW) – Seldom used, not a/v in most AED machines Pulsed wave (PW) Color Doppler Duplex Doppler:– Putting Color Doppler on top of Grey-scale Bmode Power Doppler
Transducers Formats – linear - rectangular field of view – sector - pie-shaped field of view
In sector scanning, theresolution becomes poorerat increasing depth.
Transducers Mechanical Probe: seldom used now Electronic Probe: – Linear array transducers • piezoelectric elements linearly arranged • sequentially activated to produce an image – Phased array transducers • smaller scanning surface (foot print) • good for echocardiography • more expensive • elements are activated with phase differences to allow steering of the ultrasound signal
The Logiq 100 PROs five main probes: 5.0 MHz Convex-Array probe, 40 mm radius, 68° field of view 3.5 MHz Convex-Array probe, 50 mm radius, 68° field of view 7.5 MHz Linear-Array probe, 60 mm field of view 3.5 MHz Micro convex Cardiac probe, 13 mm radius, 82° field of view 6.5 MHz Endocavity probe, 10 mm radius, 120° field of view