An investigation into ultrasonic CW Doppler beams for measuring cardiac output via the suprasternal notch
Measurement of maximum aortic velocity in the aortic arch by employing CW Doppler ultrasonic probes via the suprasternal notch is a simple, accurate and reproducible method for the assessment of cardiac output in man. The performance of such systems is strongly dependent on the spatial properties of the ultrasonic beams used. We propose to use the Interrogation Field (IF) as the basis for describing the spatial characteristics of the combined transmitter and receiver. Firstly we determine the spatial extent of the Required Interrogation Field (RIF) for the measurement of blood flow in the line of sight from the suprasternal notch to the arch. This is done by performing measurements on appropriate MRI images of the thorax. Then, in order to determine the IF of CW probes used for this purpose, we constructed a 3D scanning hydrophone system by which, the 3-dimensional pressure distributions of both elements of the probe are obtained. These are then combined to provide the 3D distribution of the IF of the probe. 3D visualisation of this distribution provides essential information about the probe, but also the RMS Error can be used to express the degree by which the actual IF approximates the RIF. We tested two commercially available probes currently used is such application and we found that they provide inadequate coverage of the RIF. Thus we proceed to propose a method for synthesis of the probe elements directly from the RIF, by use of a semi-automatic iterative algorithm. Although this algorithm can be significantly improved, it nevertheless provided a probe design with much improved IF characteristics over the currently used probes, as our simulations showed. It is concluded that there is potential for improvement of the spatial characteristics of CW ultrasonic probes to provide enhanced performance in the estimation of cardiac output.