Ultrasound Physics and Instrumentation (MRD535) - PDF

Ultrasound Physics and Instrumentation MRD535 Imaging Application By Dr Leong Sook Sam Learning objectives Describe the principle, physics, instrumentations, accessories and image recording in ultrasonography (PLO1, C2) Explain the principles of ultrasonography including ultrasound...

Ultrasound Physics and Instrumentation MRD535 Imaging Application By Dr Leong Sook Sam Learning objectives Describe the principle, physics, instrumentations, accessories and image recording in ultrasonography (PLO1, C2) Explain the principles of ultrasonography including ultrasound physics Contents 1. Describe the display modes A-mode B-mode M-mode Real time mode Doppler mode  Amplitude mode (A-mode) One dimension presentation of reflected sound wave. The signals from returning echoes are displayed in the form of spikes. On one axis (vertical axis) the amplitude of the signal (magnitude of the voltage pulse) is displayed, and on the other axis (horizontal), the position of the echo delay (depth). Amplitude mode (A-mode) (cont) The position of a spike along the time base is a measure of the distance of the associated reflecting boundary from the transducer. Amplitude mode (A-mode) (cont) The limitation of displaying only 1-D information, representing the echoes lying along the beam path. The information does not constitute an image. Brightness Mode (B-mode) In the brightness mode, signals from returning echoes are displayed as dots of varying intensities. The intensity of a dot (the brightness) is a relative measure of echo size, with large echoes appearing as very bright dots, while at the other extreme non-reflectors appear totally dark. Brightness Mode (B-mode) (cont) The position of a dot along the time base is a measure of the distance of the associated reflector from the transducer. For any given position of the beam direction (scan line), a line of dots is displayed, corresponding to the 1- D information of reflectors lying along the scan line. When the beam is swept across a selected section of the subject (the process of scanning), different dot lines are created for each scan line. Brightness Mode (B-mode) (cont) The combined information from different scan lines provides a 2-D image of the cross-section through which the beam sweeps Motion Mode (M-mode) The motion mode is used to generate an electronic trace of a moving object lying along the path of the ultrasound beam. The transducer is placed in one fixed position in relation to the moving structure. Returning echoes are displayed in the form of dots of varying intensity along a time base as in B-mode.  To capture the time variation of moving structures graphically, dot lines obtained at different moments are recorded at different lateral positions. The M-mode provides I-D information along the beam path. It should be noted that for a moving structure to be Depth detected, it must lie along the ultrasound beam path. The M-mode is Time particularly useful in examining cardiac motion. Real Time Mode Real-time imaging is rapid B-mode scanning to generate images of a selected cross-section within the subject repetitively at a rate high enough to create the motion picture impression. Although each image in the series represents an independent static image, the effect of rapid acquisition and viewing at rates exceeding about 25 image frames per second creates the impression of continuity in time. Doppler Effect A bug in the centre of a circular water puddle. The bug shakes its legs and produce disturbances that travel through the water. The disturbance originates at a point and would travel in all direction. Since the disturbance is traveling in the same medium, they would all travel in every direction at the same speed. These circles would reach the edges of the water puddle at the same frequency. An observer at point A (the left edge of the puddle) would observe the disturbances to strike the puddle's edge at the same frequency that would be observed by an observer at point B (at the right edge of the puddle). Doppler Effect (cont) The bug is moving to the right across the puddle of water and producing disturbances at frequency 2 disturbances per second. Since the bug is moving towards the right, each consecutive disturbance originates from a position that is closer to observer B and farther from observer A. Each consecutive disturbance has a shorter distance to travel before reaching observer B and thus takes less time to reach observer B. Thus, observer B observes that the frequency of arrival of the disturbances is higher than the frequency at which disturbances are produced. Each consecutive disturbance has a further distance to travel before reaching observer A. Observer A observes a frequency of arrival that is less than the frequency at which the disturbances are produced. Doppler Effect (cont) The Doppler effect is associated with motion between the source of sound and the receiver, resulting in an apparent difference in frequency between that emitted by the source and that perceived by the receiver. It is important to note that the effect does not result because of an actual change in the frequency of the source. The Doppler effect can be observed for any type of wave - water wave, sound wave, light wave, etc.  The effect is only observed because the distance between observer B and the bug is decreasing and the distance between observer A and the bug is increasing.  A- the source is moving towards the observer as it transmits the sound wave This cause the wavefront travelling towards the observer to be more closely packed, observer witnesses a higher freq than that emitted B- the source moving away from the observer, wavefront spread out, observer witnesses a lower freq than that emitted Change in the observe freq from the transmitted is Doppler shift A B Doppler effect (cont) The apparent difference in frequency is called the Doppler shift. For a stationary source, the wave fronts are neither compressed nor stretched, hence no shift of frequency is observed. The Doppler shift can be measured and used to: detect motion determine the direction of motion determine the velocity of a moving structure. Doppler mode The Doppler mode is used in studies of blood flow and cardiac movements. When a beam of ultrasound emitted by a transducer at constant frequency interacts with a moving acoustic boundary (red blood cell), the echoes are send back to the transducer. The transducer serves as the observer. Because the boundary is moving, the transducer will detect the echoes with a Doppler shift in frequency, being of higher frequency if the interface is approaching, or of lower frequency if the interface is moving away.  The shift of frequency is related to the velocity of the moving reflector, and to the direction of motion. The greater the Doppler shift, the higher the velocity of the moving structure, and a higher detected frequency implies relative motion towards the transducer, while a lower detected frequency implies a receding reflector.  Color used to indicate relative velocity direction or flow BART-Blue Away, Red Toward Doppler Shift Formula Important: angle between the u/s beam and the direction of flow fd = fr - ft = 2(ft)(v)(cosθ) c fd: Doppler shift ft: Transmitted frequency fr: Received frequency v: source velocity c: speed of sound cos θ: angle between the path of u/s beam and blood flow direction Angle Cos θ 0 1 30 0.87 45 0.7 60 0.5 80 0.17 90 0 Exercise Calculate the Doppler shift for a 5MHz transducer with 45 Doppler angle and blood velocity 120cm/s Angle Cos θ fd = fr - ft = 2ftvcosθ 0 1 30 0.87 c 45 0.7 60 0.5 Answer 80 0.17 Fd = 2 x (5MHz) x( 1.2 m/s) x 0.7 90 0 1540 m/s = 8.4MHz/ 1540 = 0.0054MHz Types of Doppler Continuous wave doppler Pulse wave doppler Color flow doppler Power Doppler Continuous Wave Doppler Continuous-wave (CW) ultrasound = repeat indefinitely Ultrasound wave are continuously emitted from the transducer and the echoes wave analyzed continuously. It requires transducer with separate elements One element transmits continuously & one element receives continuously CW unable to determine where, along the Doppler line, the velocities are recorded. Continuous Wave Doppler (cont) The CW produces a filled spectral curve, which explained by the fact that all velocities (from zero to max) are recorded along the Doppler line. CW cannot determine the location of the max velocity. Pulse Wave Doppler (PW Doppler) PW employs elements of the transducer that send as well as receive signals. Ultrasound is emitted in "pulses" between these pulses. As every emitted pulse is paired with a corresponding return signal, it is possible to determine where the reflection has occurred and calculate the distance of the "reflector". Pulse Wave Doppler (PW Doppler) (cont) PW Doppler has a major drawback: it cannot correctly depict higher velocities Color Flow Doppler Display color - determined the presence, direction and relative flow velocity, superimposed on B-mode Kidney colour Doppler (www.mindray.com) Same limitation with PW Doppler: high velocity will cause aliasing Other colors = turbulence or aliasing artefacts Aliasing in lumen reduction (iame.com)  Power Doppler Power Doppler – capable detecting small blood flow No display of direction Contains only one-color scale, produces homogenous color appearance overlying B-mode image Thank you