Tissue pulsatility imaging of cerebral vasoreactivity during hyperventilation

Abstract

Tissue pulsatility imaging (TPI) is an ultrasonic technique that is being developed at the University of Washington to measure tissue displacement or strain as a result of blood flow over the cardiac and respiratory cycles. This technique is based in principle on plethysmography, an older nonultrasound technology for measuring expansion of a whole limb or body part due to perfusion. TPI adapts tissue Doppler signal processing methods to measure the "plethysmographic" signal from hundreds or thousands of sample volumes in an ultrasound image plane. This paper presents a feasibility study to determine if TPI can be used to assess cerebral vasoreactivity. Ultrasound data were collected transcranially through the temporal acoustic window from four subjects before, during and after voluntary hyperventilation. In each subject, decreases in tissue pulsatility during hyperventilation were observed that were statistically correlated with the subject's end-tidal CO2 measurements. (

Fig. 1

Data collection hardware schematic.

Fig. 2

Data analysis flow diagram.

Fig. 3

(a) Displacement waveform from one data set from a single sample volume from subject 4 prior to band-pass filtering. (b) Displacement waveform after filtering. (c) Mean displacement waveform calculated by averaging cardiac cycles from (b). Vertical dotted lines indicate the beginning of each cardiac cycle.

Fig. 4

(a) End-tidal CO₂ from subject 3 along with the pulse amplitude measurements from a single sample volume. (b) Pulse amplitude versus end-tidal CO2 from the same sample volume along with the best-fit line with first-order linear regression. “pa” in the inset equation is pulse amplitude and “CO₂” is the end-tidal CO₂ measurement.

Fig. 5

BMode image and pulse amplitude images from subject 2. (a) Transverse BMode image of the brain and skull. (b) Pulse amplitude image prior to hyperventilation with an end-tidal CO₂ of 41.7 mmHg. (c) Pulse amplitude image during hyperventilation with an end-tidal CO2 of 20.7 mmHg. Positive pulse amplitude indicates displacement towards the US transducer during systole. Negative pulse amplitude indicates displacement away from the US transducer during systole.

Fig. 6

BMode image (left column) from the four subjects along with the expected percent change in pulse amplitude for a decrease in end-tidal CO₂ from 40 mmHg to 20 mmHg (right column). Pulse amplitude percent change is only shown for sample volumes where the linear regression p-value of pulse amplitude onto end-tidal CO₂ was less than 0.01.

Fig. 7

Percent changes in pulse amplitude for a decrease in end-tidal CO₂ from 40 mmHg to 20 mmHg for ultrasound sample volumes with p-values less than 0.01 for the linear regression of pulse amplitude onto end-tidal CO₂.