Proc. SPIE. 7629, Medical Imaging 2010: Ultrasonic Imaging, Tomography, and Therapy
KEYWORDS: Independent component analysis, Doppler effect, Ultrasonography, Linear filtering, Arteries, Turbulence, In vivo imaging, Electronic filtering, Doppler tomography, Simulation of CCA and DLA aggregates
The most widely performed test for patients suspected of having carotid atherosclerosis is Doppler ultrasound (DUS).
Unfortunately, limitations in sensitivity and specificity prevent DUS from being the sole diagnostic tool. Novel DUS
velocity-derived parameters, such as turbulence intensity (TI), may provide enhanced hemodynamic information within
the carotid artery, increasing diagnostic accuracy. In this study, we evaluate a new technique for recording, storing and
analyzing DUS in a clinical environment, and determine the correlation between TI and conventional DUS
measurements. We have recruited 32 patients with a mean age of 69±11 yrs. An MP3 recorder was used to digitally
record Doppler audio signals three times at three sites: the common carotid artery, peak stenosis and region of maximum
turbulence. A Fourier-based technique was used to calculate TI, facilitating clinical application without additional ECGgating
data. TI was calculated as the standard deviation of Fourier-filtered mean velocity data. We found that TI and
clinical PSV were linearly dependent (P<0.001) within the region of maximum turbulence and the precision of all TI
measurements was found to be 14%. We have demonstrated the ability to record Doppler waveform data during a
conventional carotid exam, and apply off-line custom analysis to Doppler velocity data to produce measurements of TI.
Doppler ultrasound velocity measurements are commonly used to diagnose atherosclerotic carotid artery
disease. However, current Doppler techniques exhibit limitations with respect to sensitivity and specificity.
We believe that advanced spectral analysis - including quantification of turbulence - could increase the
diagnostic accuracy of duplex Doppler ultrasound. Routine application of advanced spectral analysis requires
a practical technique to acquire and analyze the Doppler signal, which is compatible with clinical ultrasound
machines. We describe the implementation of a technique for offline Doppler waveform analysis of carotid
artery blood flow, using a portable MP3 recorder and custom analysis software. Forward and reverse audio
signals were recorded with compression at 128 bps at prescribed points throughout the carotid bifurcation of
human volunteers. Each data set was digitized at 44.1kHz and analyzed to produce velocity spectra at 12 ms
intervals. From these instantaneous spectra, advanced Doppler indices of mean velocity and Fourier-based
turbulence intensity (TI) were calculated. We found that MP3 compression had a negligible effect on the
calculation of mean velocity data (0.17%) and TI (0.5%). We also found that Fourier-based TI was
comparable to TI calculated by ensemble average. Finally, we were successful in applying this technique in vivo and demonstrated that long acquisitions and repeated measurements were possible in human volunteers.
Our study demonstrates that it is feasible to acquire Doppler audio data using an MP3 recording device for
off-line analysis, while only adding a short time to a conventional carotid exam.
Doppler ultrasound (DUS) is widely used to diagnose and plan treatments for vascular diseases, but the relationship between complex blood flow dynamics and the observed DUS signal is not completely understood. In this paper, we demonstrate that Doppler ultrasound can be realistically simulated in a real-time manner via the coupling of a known, previously computed velocity field with a simple model of the ultrasound physics. In the present case a 3D computational fluid dynamics (CFD) model of physiologically pulsatile flow a stenosed carotid bifurcation was interrogated using a sample volume of known geometry and power distribution. Velocity vectors at points within the sample volume were interpolated using a fast geometric search algorithm and, using the specified US probe characteristics and orientation, converted into Doppler shifts for subsequent display as a Doppler spectrogram or color DUS image. The important effect of the intrinsic spectral broadening was simulated by convolving the velocity at each point within the sample volume by a triangle function whose width was proportional to velocity. A spherical sample volume with a Gaussian power distribution was found to be adequate for producing realistic Doppler spectrogram in regions of uniform, jet, and recirculation flow. Fewer than 1000 points seeded uniformly within a radius comprising more than 99% of the total power were required, allowing spectra to be generated from high resolution CFD data at 100Hz frame rates on an inexpensive desktop workstation.
Turbulence is ubiquitous to many systems in nature, except the human vasculature. Development of turbulence in the human vasculature is an indication of abnormalities and disease. A severely stenosed vessel is one such example. In vitro modeling of common vascular diseases, such as a stenosis, is necessary to develop a better understanding of the fluid dynamics for a characteristic geometry. Doppler ultrasound (DUS) is the only available non-invasive technique for in vivo applications. Using Doppler velocity-derived data, turbulence intensity (TI) can be calculated. We investigate a realistic 70% stenosed bifurcation model in pulsatile flow and the performance of this model for turbulent flow. Blood-mimicking fluid (BMF) was pumped through the model using a flow simulator, which generated pulsatile flow with a mean flow rate of 6 ml/s. Twenty-five cycles of gated DUS data were acquired within regions of laminar and turbulent flow. The data was digitized at 44.1 kHz and analyzed at 79 time-points/cardiac cycle with a 1024-point FFT, producing a 1.33 cm/s velocity resolution. We found BMF to exhibit DUS characteristics similar to blood. We demonstrated the capabilities to generate velocities comparable to that found in the human carotid artery and calculated TI in the case of repetitive pulsatile flow.