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.

Hemodynamic Principles.- Physiological Blood Flow.- Principles of Ultrasound Doppler Sonography.- Doppler Formula.- Continuous Wave and Pulsed Doppler Instruments.- Insonation Angle.- Frequency Spectrum Analysis.- Vascular Resistance.- Ultrasound Doppler Sonography and Blood Flow Velocity.- Transcranial Doppler Device.- Ultrasound and the Skull.- Technical Data.- Transcranial Doppler Investigation Technique.- Anatomic Considerations.- Insonation Angle, Measured Depths, and Flow Direction.- Compression Tests.- Normal Values.- Blood Flow Velocity Changes Under Physiological Deviations.- Age.- Blood Flow Velocity End-tidal CO2 Partial Pressure.- Orthostasis and TCD.- Body Acceleration.- Valsalva Test and TCD.- Spontaneous Subarachnoid Hemorrhage and Disturbed Intracranial Hemodynamics.- Hemodynamic Considerations in Stenosis and Vasospasm.- Pathomorphology of the Cerebral Arteries After SAH and Vasospasm.- Timing of Aneurysm Operation.- Natural Time Course of Vasospasm.- Patients.- Method.- Results.- Vasospasm and Aneurysm Surgery Within 72 Hours After Subarachnoid Hemorrhage.- Patients.- Vasospasm Within 72 Hours After the Last SAH.- Vasospasm Between Day 4 and Day 31 After SAH.- Clinical Outcome.- Discussion.- Correlation of Angiographically Confirmed Vasospasm and Stenosis with Transcranial Doppler.- Vasospasm and Delayed Aneurysm Surgery.- Intra-aneurysmal Flow Pattern.- Special Case: Vein of Galen-AVM.- Extracranial-Intracranial Bypass.- Method.- Patients.- Results.- Results: STA Compression Tests and TCD.- Results: Special Cases.- Results: Comparison of CBF Measurements with Transcranial Doppler Sonography.- Discussion.- Arteriovenous Malformations.- Feeding Arteries and Steal Effect.- SAH and AVM.- Vascular Resistance Following Surgical Exclusion.- Superselective Embolization and TCD Recordings.- Discussion.- Monitoring of Frequency Spectra in the MCA During Angiography.- Vascular Hemodynamic Response to Meningitis.- Case Reports.- Discussion.- Brain Death and TCD Recordings.- References.

Neurosurgical Applications of Transcranial Doppler Sonography

The value of transesophageal pulsed Doppler echocardiography (PDE) was studied in six patients with competent mitral valve and in 12 patients with angiographically proved mild to moderately severe mitral regurgitation. The results were compared with those from the standard transthoracic method of investigation. The advantages of the esophageal over the transthoracic approach in the detection of mitral regurgitation are (1) absence of anatomic obstacles between the ultrasound transducer and the heart; (2) nearly parallel alignment of the ultrasound beam with the blood flow direction; (3) the use of high pulse repetition frequencies; and (4) detection of localized regurgitant jets by left atrial scanning. This results in a superior recording quality and greater sensitivity. Based on a specific systolic flow pattern in the time-interval histogram, mitral regurgitation was detected in all patients by the transesophageal technique (100% sensitivity), but in only 58% by the precordial approach. Thus, transesophageal pulsed Doppler echocardiography can accurately detect the presence of mitral regurgitation, particularly in cases of mild or moderate severity.

Assessment of transesophageal pulsed Doppler echocardiography in the detection of mitral regurgitation.

Colour flow imaging (CFI) is an ultrasound imaging technique whereby colour-coded maps of tissue velocity are superimposed on grey-scale pulse-echo images of tissue anatomy. The most widespread use of the method is to image the movement of blood through arteries and veins, but it may also be used to image the motion of solid tissue. The production of velocity information is technically more demanding than the production of the anatomical information, partly because the target of interest is often blood, which backscatters significantly less power than solid tissues, and partly because several transmit-receive cycles are necessary for each velocity estimate. This review first describes the various components of basic CFI systems necessary to generate the velocity information and to combine it with anatomical information. It then describes a number of variations on the basic autocorrelation technique, including cross-correlation-based techniques, power Doppler, Doppler tissue imaging, and three-dimensional (3D) Doppler imaging. Finally, a number of limitations of current techniques and some potential solutions are reviewed.

Colour flow and motion imaging

Conventional pulsed wave (PW) Doppler ultrasound systems measure blood velocities unambiguously as long as the velocities are less than the Nyquist velocity. Here, a new PW Doppler ultrasound system is described, which can measure velocities up to 4.5 times higher than the Nyquist velocity. The new PW system presented here utilizes two ultrasound carrier frequencies, such that an appropriate processing of the Doppler frequencies results in an extended mean-velocity range. With this extended mean-velocity information, the complex Doppler signal is interpolated to reconstruct aliased Doppler spectra. The interpolation algorithm has been tested by computer simulations demonstrating the capabilities and limits. The integration into a commercial Doppler scanner allowed the evaluation of the new PW system in real time. In vitro and in vivo tests demonstrate the effective reconstruction of highly aliased Doppler spectra. >

A new pulsed wave Doppler ultrasound system to measure blood velocities beyond the Nyquist limit

Existing pulsed Dopplers have a fundamental limitation which makes them unable to measure a velocity v at a range R greater than that given by the limit Rv = c 2 /8 φ 0 where c is the sound velocity and f 0 the transmitted ultrasound frequency. This limit makes it impossible for conventional pulsed Dopplers to measure blood velocity transcutaneously in such locations as stenosed or regurgitant heart valves in adults. This paper described a principle using periodic changes of the repetition frequency of the transmitted signal. This increases the range limit of pulsed Doppler systems by the ratio of the Doppler spectrum enter frequency to its bandwidth. This technique which is easily applicable to existing pulsed Dopplers, should extend the clinical applicability of these systems to long range or high velocity flows whose Doppler spectra are sufficiently narrow

P. Lecong

Papers

On increasing the range of pulsed Doppler systems for blood flow measurement.