Doppler Spectral Analysis IN THE NAME OF GOD Doppler Spectral Analysis

Spectral Analysis Red blood cells at various radii from the center are moving at different velocities. Resulting in a Doppler signal that is combination of all of the frequency shifts. Doppler signal and the relative importance of each is called Spectral analysis.

FAST FOURIER TRANSFORM Fast Fourier transform (FFT) is the process a of separating waveform into a series of single- frequency since-wave components. Doppler shift is largest for region 3 because RBCs in this region are moving at the greatest velocity. The across sectional view of a vessel lumen

POWER SPECTRUM One way to display the spectral analysis is in the form of a power spectrum in which the magnitude of individual frequency components is plotted against the frequency. It is an extremely useful analysis technique. If the number of RBCs are the same in different region then the height for each frequency is the same

Beat frequency for region 1,2,3 Beat frequency for region 1,2,3 .Region 1 contains twice as many red blood cells as the other regions and in the complex Doppler signal from doubling the number of RBCs in that region.

Cont.

Effect of beam shape on the laminar flow power spectrum

TIME DISPLAY OF THE POWER SPECTRUM In vessel the velocity distribution is not constant with time. High –velocity group of RBCs produces twice the signal as the middle-velocity group. To point of varying brightness along a straight line.

A useful presentation is to convert the frequency distribution to a velocity distribution using Doppler shift equation with values for transmitted frequency, acoustic velocity in tissue and Doppler angle

Aliasing is characterized by wraparound, whereby the high velocity components above the Nyquist limit appear below the baseline It can be eliminated by: Increasing velocity scale (PRF) Adjust baseline Increase angle of information Decrease depth of sample volume Switch to lower transmitted frequency

SPECTRAL MIRROR ARTIFACT Improper presentation of the power spectra called mirror image artifact. Occurring when weak Doppler signals are detected whit high gain settings. The large amount of clutter from stationary scatterers prevents the receiver from processing all incoming Doppler signals . The quadrature phase detector becomes saturated, resulting in a loss of directional discrimination .forward and spectra emulate each other.

SPECTRAL BROADENING A single frequency Doppler shift for a reflector moving at constant velocity is obtained only for a very large –plane target insonated by a large acoustic field. The fluctuation of the signal as RBCs move in and out of the sampling volume cause the beat frequency to vary in amplitude.

The broadening of the spectrum is called transit time broadening and creates difficulties in spectral interpretation. The spectrum produced by scatters moving at different velocities. The overall effect of transit time broadening is to smear the magnitude of a frequency component over a wider range of frequencies . Transit time broadening that is 7% of the Doppler shift is considered acceptable but can be much higher(33%).

POWER SPECTRUM DECIPTION Maximum frequency ,mean frequency median frequency and mode frequency are descriptors of the power spectrum that help characterize vascular Doppler signals.

Maximum frequency The maximum Doppler shift corresponds to the fasten-moving RBCs within the sample volume at the time of measurement. Each FFT segment is analyzed for the maximum frequency shift. Converted to velocity using transducer frequency and Doppler angle.

Mean, Median and , Mode Frequencies The mean frequency is calculated from the weighted sum of all frequencies in the power spectrum. Estimation of volume flow rate is commonly based on measurement of the average velocity. The preferential absorption of high frequency components by tissue causes a downward shift of the mean frequency. The application of a filter can also alter frequency spectrum by removing low-frequency components.

TIME-COMPRESS SPECTRAL ANALYSIS In time compression the complex Doppler signal is sampled for a short time (8ms). Stored in digital memory . It can be played back at an accelerated rate. The time-compressed analyzed during the time that next Doppler signal is being collected for analysis. A filter hit a bandwidth of150Hz is swept through a wide frequency rage.

ZERO CROSSING DETECTORS Zero crossing detectors are designed to monitor how rapidly the complex Doppler signal is oscillating between maximum and minimum. To reduce the influence of electronic noise, signal must exceed a threshold value above zero for a trigger pulse to be produced . The number of pulses per second gives rise to the zero crossing frequency . The zero crossing frequency is similarly dependent on the velocity components of the flowing blood.

PULSATLITY INDEX AND RESISTIVITY INDEX Pulsatility index (PI) is used to quantify impedance or resistance to flow. This index is calculated from the maximum velocity (vmax) and the minimum velocity (vmin ). PI=( vmax vmin)/ vmax Sometime PI is defined in terms of sytolic velocity (S) and diastolic velocity (D) PI=( S-D)/ vmean Two other index clinically commonly used are: RI=(Vmax-Vmin)/Vmax Systolic-to-diastolic ratio=S/D

VOLUME FLOW RATE MEASUREMENTS Volume flow rate delineates the amount of blood flowing through the vessel per unit time and it is: Q(cm3/s)=Mean velocity (cm/s)XArea(cm2) Several methods have been developed to quantify volume flow rate: -1Velocity Profile Method. 2-Even Insonation Method. 3-Assumed Velocity Profile Method. 4-Time Domain Velocity Profile Method.

Velocity Profile method A PD Doppler system with small sample volume is used to evaluate the mean velocity at multiple location across the vessel lumen. At each measurement point the mean velocity is followed throughout the cardiac cycle to obtain a time average A profile of velocities across the lumen is thus generated The product of the velocity and the corresponding semicircular area at each point in the profile provide the total flow

Even Insonation Method The mean velocity is determined for the entire vessel, which is insonated with a uniform beam This is multiplied by the cross sectional area of the lumen to yield the instantaneous volume flow rate Averaging the instantaneous value over the cardiac cycle results in the time averaged flow rate

Assumed Velocity Profile Method The velocity profile is determined at one segment of a vessel and this is assumed to exist at different location in the vessel

Time Domain Velocity Profile Method The time domain Doppler detection technique measures displacement of acoustic speckle pattern associated with group of RBCs. Since the time interval between the measurements are known, the velocity can be calculated

Clinical exmaples Carotid arteries: Peripheral arteries Abdominal and pelvic vascular sonography

THNKS FOR ATTENTION The end