Doppler Ultrasound: Effective visualization of blood flow.
©Walter Rasmussen, R.D.C.S.
Doppler ultrasound relies up the shift in frequency and phase that occurs when pressure is exerted upon the ultrasound beam by moving blood and tissue. The ultrasound machine is equipped to measure the difference between the transmitted and returning frequencies, displaying the shift on a graph or as colorized areas superimposed over the ultrasound image.
In Doppler ultrasound, it is not enough that the ultrasound beam simply crosses paths with the area of interest. The ultrasound beam needs to be aligned as parallel as possible (within 20 degrees) to the direction of blood flow in order to record its velocity with sufficient accuracy. This is in direct contrast to the ideal alignment for imaging, where it is best to be directing the ultrasound beam perpendicular to the structure. Inadequate alignment with the blood flow can lead to under-estimation of the severity of pathological conditions such as valvular stenosis, insufficiency, or elevated intra-cardiac pressure.
Transducer frequency affects the sensitivity of Color Flow, CW and PW Doppler but is most critical with Color Flow Doppler because the jet area can be diminished. Higher frequencies interact with the tissue to a higher degree, producing a loss of energy and thus, signal strength. For Adult Echocardiography, the machine should be set at the lowest frequency for better penetration and signal return, usually 1.9 to 2.0 MHz. It is not uncommon to find a machine that is preset at an unnecessarily high frequency. On less expensive and more automated ultrasound machines, selecting color Doppler frequency is not an option and the resultant color map will have lower sensitivity when beam penetration is less than ideal.
Even though Doppler operates better at a lower frequency and penetrates better than 2-D Echo it is still necessary to choose the most unobstructed view of the heart for the best Doppler quality.