DK2985_C000 1..28 - AlSharqia Echo Club

30 Transesophageal EchocardiographyIn order to enable the measurement of higher velocities,a modification called “high-PRF” has been implementedon most echo machines. In this system, the PRF is multipliedby 2, 3, or 4: a new burst of ultrasound waves issent before the electronic receiving gate is opened toreturning echoes. It, therefore, raises the number ofsampling sites but introduces a “range ambiguity”because the computer is unable to identify the origin ofthe echo (1). Fortunately, the gates are pictured on the2D images and the examiner can assume that the samplingvolume lies where the recorded flow velocity is expected.The actual PRF is determined by the most proximalsample volume but the most distal PRF is used forsampling flow in the zone of interest (3).An additional problem occurs with the PW technology.The bursts of ultrasound waves are produced at a certainrhythmic time period introducing an additional frequencyin the emission. The frequency of bursts of an ultrasoundwave ( f 0 ) is also Doppler-shifted by the moving blood.The resultant velocity profile is not as precise as CWDoppler and is affected by a significant spectral broadening(11).V. SPECTRAL DISPLAYTo display the Doppler information, the apparatus mustreproduce the spectrum of the frequency shifts. This spectrummust be updated regularly during a cardiac cycle. TheDoppler signal is a complex wave, containing informationabout the motion of all blood cells and tissue moving atdifferent velocities. The received signal is a wave, out ofphase with the original emitted signal. In the spectralmode, this shift is visually displayed as a power spectrumof frequencies against time. The ultrasound echoes gothrough a logarithmic amplifier which increases the amplitudeof the weaker signals more than the stronger signals,so that amplitudes become comparable. The signal is processedin segments of 1–5 ms duration by the computerand a mathematical calculation, called fast Fourier transform,is performed on each segment to resolve the Dopplersignal into its individual component frequencies. Thisspectrum represents the relative magnitude of eachfrequency component. The calculation of velocity(Doppler equation) is done automatically by the computerfrom these frequency shifts. Each segment of time isassigned a stack of vertical bins whose intensity is proportionalto the strength of the signal or to the numberof blood cells moving within the range of velocities representedby each bin (Fig. 2.8) (8). A trade-off existsbetween temporal and frequency resolution: the timeperiod represented by each time slice is correlated withthe ability to distinguish between two Doppler shifts.The spectral display of the Doppler trace has time onthe horizontal axis and flow velocity on the vertical axis.The gray scale is proportional to the number of bloodcells moving at a certain speed: the darker the trace, thegreater the number of blood cells. Usually, 16–32cm/secFigure 2.8 Spectral display. Construction of a spectral curve with vertical stacks of bins of varying intensity where the shade of gray isproportional to the quantity of blood cells moving at a corresponding velocity. The width of the curve corresponds to the spectrum of thedifferent velocities of blood cells.