In vitro measurement of ambient pressure changes using a realistic clinical setup
Klaus Scheldrup Andersen and Jørgen Arendt Jensen
Motivation and objective:
Many attempts to find a non-invasive procedure to measure the local blood pressure have been made. In the last decade independent experiments have indicated that the amplitude of the subharmonic response from contrast agents is sensitive to the ambient pressure.
This paper presents results from a new experimental setup for measuring the subharmonic response of a contrast agent when subjected to ambient over pressure. The setup is very flexible offering completely arbitrary excitation and data acquisition, fast and accurate ambient pressure control, and precise timing. More importantly, it resembles a realistic clinical setup using a single array transducer for transmit and receive. The standard signal processing steps usually seen for these experiments are moreover accompanied by steps to reduce dependence on factors as bubble concentration and time.
Up to now ambient pressure measurements have been done using two single element transducers. The measurement setup for this experiment consisted of a single B-K Medical (Herlev, Denmark) phased array transducer with 64 elements and a -6 dB bandwidth between 2 and 4 MHz. The transducer was sealed to an airtight phantom with inlets for injection of Sonovue (Bracco, Milano, Italy) and regulation of the ambient pressure, which was automatically adjusted from Matlab using a custom made pressure controller (Alicat Scientific, Tucson, AZ). Data was acquired using the experimental scanner RASMUS. 100 focused lines were acquired with a pulse repetition frequency of 50 Hz at 6 different ambient pressures in the interval 0 to 25 kPa. To ensure subharmonic generation, a 32 cycles cosine tapered pulse with a center frequency of 4 MHz and an acoustic pressure of 485 kPa was used for excitation. 64 elements were used in receive and the RF data was filtered and beamformed before further processing. To compensate for variations in bubble response and to make the estimates more robust, each spectrum was normalized according to the fundamental before averaging and the energy of the respective components was chosen as measure over the peak amplitude.
The measurements showed that the energy of the subharmonic component decreased by 10.3 dB when the over pressure was increased from 0 to 25 kPa. In the same interval, the fundamental changed by less than 1 dB. Fitting a line to the six measuring points shows a linear correlation of 0.78 for the subharmonic reduction yielding a pressure sensitivity of 0.41 dB/kPa.
Discussion and Conclusion:
The experiment has for the first time shown it is possible to detect ambient pressure changes using a single array transducer setup as obligatory in a clinical situation. Moreover, suggestions for further signal processing was presented to increase the robustness of the estimates. The amount of subharmonic reduction obtained is in good correspondence with results by Shi et al 1999 and Adam et al 2005 who both used a receiver separated from the emitting single element transducer.
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