Doppler ultrasound has now developed to the point where the rate of flow of blood in a given vessel can be measured with appropriate instrumentation. The theoretical basis of Doppler flow measurement is reviewed in this paper, with particular emphasis on the potential and actual sources of error. Three distinct approaches are identified, and the strengths and weaknesses of each discussed. The separate errors involved in estimating the vessel cross-sectional area, the angle of approach, and the Doppler shift are analyzed, together with the question of the uniformity of scattering from the blood. In vivo and in vitro tests of the accuracy obtained using a number of Doppler flow measuring instruments are then reviewed. It is concluded that the Doppler methods are capable of good absolute accuracy when suitably designed equipment is used in appropriate situations, with systematic errors of 6% of less. There are, however, considerable random errors, attributable primarily to errors in measuring the cross-sectional area and the angle of approach. Repeating the measurement of flow several times and averaging the results can reduce these random errors to an acceptable level.

Measurement of blood flow by ultrasound: accuracy and sources of error.

For ultrasound color flow images with high quality, it is important to suppress the clutter signals originating from stationary and slowly moving tissue sufficiently. Without sufficient clutter rejection, low velocity blood flow cannot be measured, and estimates of higher velocities will have a large bias. The small number of samples available (8 to 16) makes clutter filtering in color flow imaging a challenging problem. In this paper, we review and analyze three classes of filters: finite impulse response (FIR), infinite impulse response (IIR), and regression filters. The quality of the filters was assessed based on the frequency response, as well as on the bias and variance of a mean blood velocity estimator using an autocorrelation technique. For FIR filters, the frequency response was improved by allowing a non-linear phase response. By estimating the mean blood flow velocity from two vectors filtered in the forward and backward direction, respectively, the standard deviation was significantly lower with a minimum phase filter than with a linear phase filter. For IIR filters applied to short signals, the transient part of the output signal is important. We analyzed zero, step, and projection initialization, and found that projection initialization gave the best filters. For regression filters, polynomial basis functions provide effective clutter suppression. The best filters from each of the three classes gave comparable bias and variance of the mean blood velocity estimates. However, polynomial regression filters and projection-initialized IIR filters had a slightly better frequency response than could be obtained with FIR filters.

Clutter filter design for ultrasound color flow imaging

The acoustic Doppler velocimeter (ADV) measures three-dimensional velocities in a small, remote sampling volume at high frequencies, however, these measurements incorporate errors that are intrinsic to the measurement technique. This paper demonstrates a new method for calculating the total measurement errors, including sampling errors, Doppler noise and errors due to velocity shear in the sampling volume associated with single-point ADV measurements. This procedure incorporates both the effects of instrument configuration and the distribution of errors between velocity components for any probe orientation. It is shown that the ADV can characterize turbulent velocity fluctuations at frequencies up to the maximum sampling rate and that Reynolds shear stress errors are very small. Copyright © 2000 John Wiley & Sons, Ltd.

A new method for evaluating errors in high‐frequency ADV measurements

A theoretical treatment of the scattering of ultrasound from blood is given, assuming that the blood behaves essentially as a continuum. The scattering then arises from fluctuations in the mass density and compressibility of the blood, which is caused by a fluctuation in the red cell concentration. An expression for the received signal in ultrasonic blood velocity measurements is given. The stochastic properties of the signal are discussed with reference to the information content about the velocity field of the blood. Since the signal is Gaussian, all available information is contained in the power spectrum, which is a blurred approximation to the velocity distribution in the region of observation.

A Theoretical Study of the Scattering of Ultrasound from Blood

The extension of ultrasound (US) color flow imaging (CFI) techniques to high frequencies (> 20 MHz) has the potential to provide valuable noninvasive tools for scientific and clinical investigations of blood flow in the microcirculation We describe the development of a slow-scan CFI system operating in the 20–100-MHz range that has been optimized to image the microcirculation The apparatus has incorporated elements of a previously reported pulsed-wave Doppler system and is capable of operating in either CFI or pulsed-wave mode The performance of the CFI system was evaluated at a center frequency of 50 MHz using two PVDF transducers with −6-dB beam widths of 43 and 60 μm The −6 dB-axial resolutions were estimated to be 66 and 72 μm, respectively In vivo validation experiments conducted using the murine ear model demonstrated the detection of flow in vessels down to 15–20 μm in diameter with flow velocities on the order of mm per s Further experiments examining experimental murine tumors confirmed the successful detection of flow in the tumor microcirculation

High-frequency color flow imaging of the microcirculation

A relation is derived between the transmitted spectral density and the output spectral density of broad bandwidth, random signal, Doppler flow measurement systems operating under conditions where the fluid transit time is limited by the transmitted signal bandwidth and not by beam geometry. The fact that this result is already known to hold for pulsed radio frequency (RF) Doppler proves that random or pseudorandom Dopplers will give the same output spectrum as pulsed RF Dopplers, provided that the transmitted spectral density of the RF system has the same envelope as that of the noise system.

Analysis of Transit Time Effects on Doppler Flow Measurement

The bandwidth of the output spectrum of blood flow meters may prove of value in estimating flow parameters such as the degree of turbulence. It is therefore important to determine the various factors which affect this bandwidth. It is shown that scatterers moving in the near field of the sound beam generated by a single transducer ultrasound Doppler system produce a spectrum whose breadth is primarily determined by the range of angles over which backscattered ultrasound is received by the transducer. An empirical method of calculating the bandwidth of the output spectrum is described, and the results are compared with direct Doppler measurements. Since the spectrum broadening depends on the angle between the target path and the ultrasound beam, it is suggested that the phenomenon may be usable to provide an independent measure of this angle.

Geometrical Spectrum Broadening in Ultrasonic Doppler Systems

A doppler effect flow measurement device using ultrasonic stationary random noise whereby errors and inaccuracies common in existing doppler flow measurement systems are overcome.

Ultrasonic random signal doppler flow measurement system

A multiple repetition rate for pulsed Doppler systems is disclosed which permits Doppler frequency identification of flow rates, for example, blood flow, at deep interior points in a mass, such as a human subject.

Double repetition rate doppler flow measurement system

A random signal flaw detection system is disclosed that provides enhanced system sensitivity and/or increased range capabilities as well as being capable of improved resolution where necessary or desired. The system includes a generator which produces random electrical signals which are converted at a transducer into ultrasound for transmission into a test object. The reflected signal is received at a transducer and converted to an electrical signal which is then correlated with a delayed sample of the transmitted signal with the output being indicative of the sensed flaws in the test object.

Vernon L. Newhouse

Papers

Analysis of Transit Time Effects on Doppler Flow Measurement

Geometrical Spectrum Broadening in Ultrasonic Doppler Systems

Ultrasonic random signal doppler flow measurement system

Double repetition rate doppler flow measurement system

Random signal flaw detector system