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Principles of Doppler Ultrasound Imaging
A relative movement between transmitter and receiver of a sound wave , produces a change in the frequency of the sound wave. This physical phenomenon is called the Doppler effect. The Doppler effect also occurs when a ultrasound wave is reflected by a moving interface like the blood stream. The size of the frequency change depends on the used transmission frequency f, the speed of the moving interface v, the speed of sound c and the angle α between the moving interface and sound wave:
The size of the frequency shift in Doppler ultrasound lies between 50 Hz and 15 KHz. For exact speed calculation of the bloodstream, the measurement of the angle betweeen blood stream and ultrasound wave is necessary. Due to the cosine function, the Doppler effect is poorly detectable when the ultrasound wave hits vertically (cos 90°=0), an angle below 60° should be soughed.
Doppler ultrasound modes:
The frequency shift of the Doppler effect may be displayed differently:
Continuous wave Doppler. Two crystals allow a continuous transmission and reception of ultrasound waves. The disadvantage of CW Doppler is a lack of depth selectivity, the advantage is the accurate measurement of blood stream velocity.
Pulsed wave doppler. The ultrasound transmission and reception works analogous to the pulse echo principle in 2D real time ultrasound, this allows a depth assignment of the flow information. A disadvantage is the inaccurate measurement of high blood stream velocities. In color Doppler sonography, the PW Doppler is combined with the B-mode image: typically, blood flow to the transducer is shown in red color and blood flow away from the transducer is shown in blue.
Power-Doppler measures the amplitude of the frequency change, which are assigned to a color and superimposed on the B-mode image. Power Doppler cannot discriminate blood flow direction or correct velocity, but it is very sensitive to detect blood stream in small vessels and it is not angle dependent.
Resistive index (RI):
The resistive index is calculated from the peak systolic velocity VPSV and the end-diastolic velocity VEDV according to following equation:
A normal RI in a parenchymal organ such as the kidney is around 0.5–0.7. An RI near 1 indicates poor organ perfusion because only in systole a bloodstream is detectable and the end-diastolic bloodstream drops to zero. An RI near 0 corresponds to a continuous flow of blood as it is found in veins. RI is used in urology in the following situations: renal artery stenosis, after kidney transplantation, suspected testicular torsion, diagnosis of vascular erectile dysfunction.
Contrast enhanced ultrasound (CEUS):
The contrast agent is an aqueous dispersion of small gas bubbles which, administered intravenously, produces the contrast enhancement of perfused tissues. Urological applications: differential diagnosis of renal masses, suspected renal infarction. Another application CEUS is micturition sonography in children: vesicoureteral reflux into the kidney can be diagnosed after intravesical administration of the contrast agent.
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ReferencesSinger u.a. 2006 SINGER, Eric A. ; GOLIJANIN, Dragan J. ; DAVIS, Robert S. ; DOGRA, Vikram: What’s new in urologic ultrasound?
In: Urol Clin North Am
33 (2006), Aug, Nr. 3, S. 279–286
Deutsche Version: Grundlagen der Doppler Ultraschall Untersuchung