Saudi Board of Radiology: Physics Refresher Course Kostas Chantziantoniou, MSc 2, DABR Head, Imaging Physics Section King Faisal Specialist Hospital & Research Centre Biomedical Physics Department Riyadh, Kingdom of Saudi Arabia Doppler Ultrasound

Introduction The Doppler Effect refers to the change in frequency that results when either the detector/observer or the sound source is moving with respect to each other. when sound source is moving towards the detector/observer (R), the sound appears to have a higher frequency (shorter wavelength) when sound source is moving away from the detector/observer (L), the sound appears to have a lower frequency (longer wavelength) if the sound source is moving perpendicular (90  ) to the detector/observer, there is no change in frequency or in wavelength and thus no Doppler Effect is observed Both source and detector Source moving with are stationary respect to detector

Doppler ultrasound is used primarily to identify and evaluate blood flow in vessels velocity and waveform information can be used to evaluate stenoses, resistance and vessel patency Doppler Frequency Shift (f D ) where f 0 is the original frequency and f is the frequency of the returning echo (from RBC) f0f0 f (RBC) maximum shift is obtained when  = 0° minimum shift is obtained when  = 90° (recall, imaging of strongest echoes) the shift is comparatively small and typically is between kHz (audible sound) the shift is positive when the RBC is moving towards the transducer and is negative when the RBC is moving away from the transducer doppler frequency shift (f D ) = change in frequency = (f 0 - f)

Flow Speed (v) f0f0 f (RBC) It can be shown, that f D = (f 0 - f) = 2 v f 0 cos  c and re-arranging the above equation we have v = c f D 2 f 0 cos  where f 0 is the original frequency, f is the frequency of the returning echo (from RBC), v is the speed of the interface (RBC), c is the speed of sound in soft tissue (1540 m/s), f D is the Doppler shift and  is the angle between transducer and RBC flow direction The velocity equation can be simplified to v (cm/s) = 77 f D (kHz) 2 f 0 (MHz) cos  ASIDE The factor 77 is only valid when v, f 0 and f D are given in the units shown above.

the values of v calculated from the observed f D are only as accurate as the estimated Doppler angle  one reason why sonography is combined with Doppler techniques is to estimate the Doppler angle with a cross-sectional image of the vessel walls, the sonographer can adjust a flow direction indicator which, when combined with an indication of the beam direction, can yield the Doppler angle the instrument uses this value of angle to convert Doppler shift to flow speed reflector speed  doppler shift  operating frequency (f 0 )  doppler shift  doppler angle  doppler shift  NOTE The moving reflector can be a tissue boundary (blood vessel wall or heart wall) or blood cells in circulation

high Q transducers are desirable for Doppler because they produce a narrow range of ultrasound frequencies (f 0 ) Doppler systems tend to run at lower frequencies than B-mode systems because resolution is not as important and there is a need to minimize attenuation (blood is a weak scatterer) the range and frequency distribution of flow velocities can be displayed as a spectrum Spectral broadening is the result of a mixture of velocities in the sample and produces a shaded area below the peak velocity value Flow Velocity Waveform (Spectrum) Operational Techniques

Continuous Doppler in continuous Doppler, one transducer (high Q) continually transmits and another transducer (low Q) continuously receives the frequency of the two signals are subtracted to give the Doppler shift, which is in the audio range continuous wave Doppler is inexpensive and does not suffer from aliasing artifacts but lacks depth resolution and provides little spatial information continuous wave Doppler is good for measuring fast flow and deep lying vessels depth gain compensation is not used in continuous wave Doppler

Pulsed Doppler pulsed Doppler allows both velocity and depth information (ranging) to be obtained the Doppler information is only provided for a specific area pulsed Doppler uses a longer pulse length than B-mode, typically up to 15 mm long Doppler information is displayed audibly and graphically as a waveform (spectrum) aliasing artifacts result in errors in estimating velocity the use of lower frequencies allow higher velocities to be measured without aliasing Dublex scanning involves displaying Doppler data on real time (B-mode) images and allows velocity and position information to be obtained simultaneously most of the time is spent in Doppler mode, the B-mode image being updated once a second

Continuous Doppler Pulsed Doppler

Color Flow Doppler Color Doppler is a hybrid that combines anatomic information obtained using B-mode system with flow information obtained using pulsed Doppler analysis pulse length in color Doppler is typically 2 mm colors (blue and red) are assigned dependent on motion (toward or away) from the transducer turbulence (i.e.: variations in flow direction) can vary between green and yellow the depth of each color varies with the velocity of flow, stationary tissues appear gray information is provided over a large area and superimposed on a gray scale image color Doppler can detect flow in vessels too small to see by imaging alone spectral analysis may also be obtained using commercial color Doppler systems modern instruments incorporate both color Doppler and spectral Doppler

Power Doppler Power Doppler is a signal processing method that relies on the total strength of the Doppler signals (amplitude) and ignores directional (phase) information the power (also known as energy) mode of signal acquisition is dependent on the amplitude of all Doppler signals, regardless of the frequency shift this dramatically improves the sensitivity to motion (i.e.: slow blood flow) at the expense of directional and quantitative flow information compared to conventional color flow imaging, power Doppler produces images that have more sensitivity to motion and are not affected by the Doppler angle and aliasing is not a problem as only strength of the frequency shifted signals are analyzed greater sensitivity allows detection and interpretation of very subtle and slow blood flow frames rates though tend to be slower and a significant amount of “flash artifacts” occur, which are related to color signals arising from moving tissues, patient motion or transducer motion Power Doppler does not mean more power to the patient, but in fact the power level are typically the same as those in a standard color flow procedure

Circle of Willis Submucosus fibroid, note small vessels inside the tumor