| dc.description.abstract |
<p>Ultrasonic flow detection is a widely used technique to detect vessel, measure
blood flow velocities, and monitor perfusion. Conventional techniques include color
Doppler imaging and power Doppler (PD) imaging. These methods depend on either the
measurement of phase change or the detection of the power of backscattered echoes
from blood. Both techniques are susceptible to noise. Common noise sources include
thermal noise and clutter. The noise significantly deteriorates the performance of
color Doppler imaging, because color Doppler imaging estimates the axial blood velocity
from temporal changes in the echo phase, and phase change measurement is sensitive
to noise. Power Doppler imaging measures the power of the temporal differences in
backscattered echoes, and can provide higher sensitivity with small vessel and slow
flow detection than color Doppler imaging at the expense of direction and velocity
information. However, it requires a large ensemble length, limiting the frame rate
to a few frames per second. The limitations of color Doppler imaging and power Doppler
imaging are more severe in deep body vessel imaging due to depth dependent attenuation
of the ultrasound waves. Therefore, for deep body vessel imaging, including liver
vessel imaging and placental spiral artery imaging, better vessel detection techniques
are desirable.</p><p>Coherent flow power Doppler (CFPD) imaging was proposed as a
sensitive flow detection and imaging technique for slow flow and small vessels. In
this work, we present the study on CFPD from principles to clinical evaluation. </p><p>The
CFPD imaging technique detects blood flow from the spatial coherence of the blood
signal. The short-lag spatial coherence (SLSC) beamformer is used for the measurement
of spatial coherence. Because blood signals and common noise sources, including thermal
noise reverberation clutter, have different spatial coherence properties, CFPD can
suppress the noise. </p><p>The performance of CFPD in flow detection was evaluated
with simulations and flow phantom experiments under various imaging conditions, and
compared with the performance of PD. It is found that CFPD provides an improvement
of Doppler signal-to-noise ratio (SNR) of 7.5-12.5 dB over PD in slow flow and small
vessel imaging. The improvement in SNR translates to higher Doppler image contrast,
faster frame rate, or lower limit-of-detection (LOD). In similar imaging conditions
of slow flow, CFPD may detect up to 50% slower flow than PD. </p><p>The CFPD imaging
technique was also implemented with novel pulse sequences, including plane-wave synthetic
transmit aperture imaging, and diverging-wave synthetic transmit aperture imaging.
For plane-wave synthetic transmit aperture imaging, the angular coherence theory was
proposed to describe the coherence of backscattered waves corresponding to plane wave
transmits at different steering angles. In addition, we also propose the coherent
Kasai and Loupas estimators, which utilizes the coherence information of flow signals
to provide velocity estimates with reduced uncertainty. </p><p>To demonstrate the
clinical relevance of CFPD, we built a real-time CFPD imaging system and conducted
a pilot clinical study with it. In the system, the CFPD technique was implemented
on a Verasonics Vantage 256 research scanner. The software beamformer and CFPD processing
were implemented on the graphics processing unit (GPU). The Doppler frame rate of
the system is 10 frames per second for a field-of-view (FOV) of 10 cm axially and
4 cm laterally. </p><p>In the pilot clinical study, the liver vasculatures of 15 healthy
human volunteers were imaged by a trained sonographer using the real-time CFPD system.
The raw data corresponding to a 132 Doppler videos were captured and processed offline.
The SNR of the vessels in the CFPD and PD images were measured and analyzed. In all
of the 132 data sets, CFPD provides higher SNR than PD. The average improvement in
SNR is 8.6 dB. From the visual analysis of the images, it can be seen that the improvement
in SNR leads to more sensitive detection of small vessels in deeper parts of the liver.</p>
|
|
