2012 Proceedings - International Tissue Elasticity Conference

074 PERFORMANCE ASSESSMENT AND OPTIMIZATION OF PULSE WAVE IMAGING FOR PULSE
WAVE ANALYSIS IN EX VIVO CANINE AORTAS AND IN VIVO NORMAL HUMAN AORTAS.
Ronny X. Li 1 , Danial Shahmirzadi 1 , William W. Qaqish 1 , Elisa E. Konofagou 1 .
1 Columbia University, 622 W 168 th Street, Vanderbilt Clinic, VC 12–232, New York, NY, USA.
Background: Pulse Wave Imaging (PWI) is an ultrasound–based method [1–3] to noninvasively visualize and
map the pulse wave–induced arterial wall motion in normal and pathological (e.g., hypertensive) human
arteries [2,3]. Because PWI is capable of acquiring multiple waveforms along an imaged segment over a single
cardiac cycle in vivo, the regional pulse wave velocity (PWV) can be quantified, and the local morphological
changes in the pulse wave can be assessed. Tradeoffs exist between the spatial resolution (i.e. beam density),
temporal resolution (i.e. frame rate), and fundamental upper limit on the PWV estimate obtained with PWI.
Aims: The aim of this study was to assess the effects of PWI image acquisition variables (beam
density/frame rate and scanning orientation) and signal processing methods (beam sweep compensation
and waveform feature tracking) on the PWV measurements in order to validate the optimal parameters.
Methods: A peristaltic pump (Manostat Varistaltic, Barrington, IL) was used to generate pulsatile flow through an ex
vivo canine aorta embedded in saline. Radiofrequency (RF) signals were acquired using a 10MHz linear array
transducer (SonixTouch, Ultrasonix Medical Corp., Burnaby, Canada) at a constant 25x38mm field of view while
varying the beam density from 24–128 for both forward (i.e. beam sweeping in the same direction as the fluid flow)
and reverse scan orientations. The upper limit on the PWV estimate was derived with respect to the frame rate
associated with each beam density, and the inter–frame axial wall displacements were estimated using a 1D cross
correlation–based motion estimation method [4] with a 3.5mm window size and 80% overlap. The PWV was
estimated, both with and without a previously described scheme to compensate for the beam sweeping–induced time
delays [2,3], by tracking the foot (i.e., the beginning of the upstroke) of the consecutive displacement waveforms for all
combinations of beam density and transducer orientation. PWI was also performed in six normal human abdominal
aortas in vivo, and the precisions of the PWVs obtained by tracking different features of the waveform were assessed.
Figure 1: Effects of beam density, scan orientation and beam
sweep compensation on the ex vivo PWV
measurements. The black line represents the upper
limit of the measurable PWV given the frame rate
associated with each beam density. The shaded areas
represent the range of compensated PWVs measured
using PWI in normal and hypertensive human aortas
obtained from a previous in vivo feasibility study [3].
Results: As the beam density decreased (i.e., frame rate increased), the uncompensated forward and reverse scan
PWV estimates converged towards a common value, resulting in smaller discrepancies between the uncompensated
forward and reverse scan estimates (Figure 1). We can expect the true (i.e. post–beam sweep compensated) PWV to
fall in between the uncompensated forward and reverse scan estimates. As expected, the beam sweep compensation
adjusted the PWV estimates to a consistent value which fell between the forward and reverse scan estimates. The
PWVs obtained using the reverse scan orientation exhibited higher precision and lower deviation from the
post–compensated PWVs. For the in vivo waveforms, the highest precision PWV measurements were obtained by
tracking the 50% upstroke (i.e., the region in each waveform midway between its peak and its foot).
Conclusions: Increasing the frame rate caused the PWVs obtained using the two different scan orientations
to converge towards the post–compensated PWV values, despite the reduction in beam density. Thus, the
ex vivo results indicated that the PWI temporal resolution is more important for accurate PWV estimation
than the PWI spatial resolution. Also, the reverse scan orientation is preferable due to its higher precision
and underestimation of the PWV relative to the forward scan (Figure 1). If the typical ranges of the PWV
measured using PWI in normal and hypertensive aortas [3] (Figure 1) are considered, a beam density of 32
(i.e. frame rate of 642 fps) would be required to reliably measure the PWV in both normal and hypertensive
aortas (assuming a 25x38mm field of view). Finally, the in vivo results suggest that for clinical PWI on
human aortas, tracking the 50% upstroke will yield the most consistent PWV estimates.
Acknowledgements: This work was supported in part by NIH grant R01HL098830.
References:
[1] K Fujikura et al.: Ultrasonic Imaging, 29(3), pp. 137–54, 2007.
[2] J Luo, R Li, E Konofagou: IEEE Trans UFFC, 59(1), pp. 174–181, 2012.
[3] R Li et al.: Conf Proc. IEEE Eng Med Biol Soc, pp. 567–70, 2011.
[4] J Luo, E Konofagou: IEEE Trans UFFC, 57(6), pp. 1347–57, 2010.
112
indicates Presenter