[4] D. A. Christopher, B. G. Starkoski, P. N. Burns, F. S. Foster,. "A High Frequency ... [6] K. W. Ferrara, "Blood Flow Measurement Using Ultra sound," in J. D. ...
1997 IEEE International Ultrasonics Symposium, Toronto
Application of High Frequency Broadband Ultrasound for High Resolution Blood Flow Measurement M. Vogt and H. Ermert Dept. of Electrical Engineering, Ruhr University, Bochum, Germany
Abstract - In this paper flow estimation approaches using high frequency ultrasound (20-150 MHz) are compared in conjunction with broadband transmit signals. Simulations show that estimation strategies matched to broadband transmit signals, known as wideband techniques, lead to a high resolution and a robust flow estimation. We implemented a high frequency Pulsed Wave Doppler system (center frequency 50 MHz, bandwidth 40 MHz). Using this setup the simulation results are confirmed and a reliable velocity estimation is obtained.
element transducers are available. Typically, strongly focused transducers are used to obtain high sensitivity and an optimal lateral resolution. Therefore, the B/D-scan-concept (‘brightness/depth’) as described in [1, 2] is used to account for the resulting short focal zone and to get homogeneous images of the flow scene over a sufficient depth range. In the following section flow estimation approaches are compared to their resolution, sensitivity and properties in conjunction with broadband transmit signals. FLOW ESTIMATION
INTRODUCTION
With recent advances in the development of flow estimation techniques the application of ultrasound for the qualitative and quantitative detection of blood flow in small vessels is of growing interest. In dermatology especially the microcirculatory system of the skin as well as the imaging of its morphology and functionality is of diagnostic interest. With extremely small vessels and low velocities within this system sufficiently high spatial and velocity resolution have to be achieved. In order to obtain high spatial resolution, high frequency and broadband ultrasound has to be utilized. This, in conjunction with strongly frequency dependent attenuation of tissue, makes flow estimation challenging. Consequently, robust and high sensitive estimation strategies have to be applied to get estimations for flow velocity and flow signal power with low variance. Presently, due to technological limitations in the high frequency range (20-150 MHz) only single
With the pulsed wave Doppler approach a train of successive pulses is transmitted to observe moving scatterers within the sound beam. In most common flow estimation strategies the resulting echo sequence is analyzed with respect to its spectral content. This corresponds to the ‘classical’ Doppler spectrum analysis. The discussed techiques are as follows: 1. In the Fast Fourier Transform approach (FFT) successive demodulated echoes are sampled at constant time delays, i.e. at points of constant depth. Velocity distribution is obtained by Fourier transform of these samples, as moving scatterers cause a Doppler shift [3, 4]. 2. The Autocorrelation approach (AC) [5] is a fast method to compute the mean velocity from time domain data. But the axial movement of successive echoes is not considered by the above described way of sampling. Especially with broadband transmit signals, flow estimations with high variance are obtained in conjunction with these
techniques which were developed for narrowband signals [6]. To account for the axial movement and wideband signals a modified sampling procedure is required. 3. In principle, Wideband maximum likelihood estimation (WMLE) [7, 8] provides a bank of optimal filters matched to the expected demodulated echo signals which correspond to various velocities. The maximum likelihood velocity is then given by the filter with the largest output. 4. A specialization of WMLE is given by the Butterfly Search Technique on quadrature components [9]. Here, axial movement is considered by sampling along trajectories of constant velocities. A disadvantage of this specialization is given by the fact that the estimation procedure does not match to the shape of the complex envelope of the demodulated echo signal. SIMULATION RESULTS
L ( vax,z ) Autocorrelation 1
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To compare the above cited flow estimation approaches, the echo sequence of an ensemble of six discrete moving point scatterers within the sound beam was calculated for a single transducer position. The bandwidth B was 80 % of the center frequency f0. Hence, very broadband signals were considered (Fig. 1). pulse sequence
In both, FFT approach and Butterfly Search, the likelihood L(vax,z) of existence of a point scatterer with axial velocity vax in depth z is estimated. The velocity distribution versus the depth resulting from the simulation data are shown in Fig. 2 (FFT approach) and in Fig. 3 (Butterfly Search).
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vax / vaxmax Figure 2: Estimation from FFT approach, relative bandwidth B/f0=80%
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Figure 1: Simulated echo sequence (left) for group of discrete point scatterers (right)
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The axial movements of several scatterers can directly be observed from the axial movement of local echo maximums. On the other hand interferences of echo pulses from different scatterers occur as well.
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Figure 3: Estimation from Butterfly Search, relative bandwidth B/f0=80%
Using the FFT approach, the spatial resolution ∆z and the velocity resolution ∆vax depend on the velocity of the scatterers. The resolutions become especially poor in case of higher velocities. This is caused by the weighting of the sampled Doppler shifted carrier with the pulsed shape broadband envelope if the sampling procedure of the FFT approach is applied. In contrast, with the Butterfly Search the resolutions reach their optima and they are independent of the scatterer velocity. The finite observation time given by the length M of the pulse train leads to a broadening of the Doppler spectrum. This affects the velocity resolution ∆vax. In the Butterfly Search approach the broadening is reciprocal to M. The spatial resolution ∆z only depends on the pulse length, i.e. on the bandwidth B. In case of the FFT approach ∆vax as well as ∆z are determined also by M and by the pulse shape. The resulting calculated resolutions are shown in Fig. 4 (velocity resolution) and Fig. 5 (spatial resolution). The best resolutions are determined by the Butterfly Search approach. Consequently, in case of low scatterer velocities vax the velocity resolution ∆vax is limited by the flow sequence length M, the spatial resolution ∆z is limited by the relative bandwidth B/f0.
biased, its variance is much higher and the flow detection is more dependent on the thresholds. ∆vax / vaxmax ( -6 dB ) 0.9
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Measurements performed with a vessel phantom, consisting of a plastic tube were used to calibrate our high frequency Pulsed Wave Doppler System in vitro. A transducer with a center frequency of 50 MHz and a bandwidth of about 40 MHz was used to image the phantom, that was fed by a syringe pump with a solution of silica gel in water. The syringe pump, which was driven by a stepper motor, generated a mean spatial axial velocity of about 4.8 mm/s in the tube. An upper and a lower color flow decision threshold (TH and TL, respectively) for the flow signal power with respect to its maximum were applied. The results are presented in Fig. 6 (Butterfly Search) and Fig. 7 (AC approach). Flow within the plastic tube (inner diameter 0.5 mm) was detected properly. The Butterfly Search provided a proper estimation of the velocity distribution. In contrast, with the AC approach, the estimation of the velocity is significantly
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Figure 4: Velocity resolution (-6 dB) normalized to the limit vax max=c0 fPRF/(4f0), FFT vs. Butterfly, B/f0: relative bandwidth, M: flow sequence length ∆ z / λ0 ( -6 dB ) 4.5
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PHANTOM MEASUREMENTS
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Figure 5: Spatial resolution (-6 dB) normalized to the center wavelength λ0=c/f0, FFT vs. Butterfly
CONCLUSIONS
Simulations and measurements prove that wideband estimation strategies provide a high resolution and a robust flow estimation. In vivo flow imaging using the calibrated system will be a subject of our future work.
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Figure 6: Phantom Measurement, Butterfly Search: f0=50MHz, B=40MHz, vaxmean=4.8mm/s
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Figure 7: Phantom Measurement, AC: f0=50MHz, B=40MHz, vaxmean=4.8mm/s
ACKNOWLEDGMENTS
This work was made possible by cooperation with S. el Gammal, K. Kaspar, K. Hoffmann, M. Stücker and Prof. P. Altmeyer from the Dermatologic Clinic of the Ruhr-University, Bochum. REFERENCES [1] C. Passmann, H. Ermert, "A 100-MHz Ultrasound Imaging System for Dermatologic and Ophthalmologic Diagnosis," IEEE Trans. Ultrason. Ferroelec. Freq. Contr., Vol. 43, No. 4, July 1996, pp. 545-552 [2] H. Ermert, M. Vogt, C. Passmann, S. el Gammal, K. Kaspar, J. Hoffmann, P. Altmeyer, "High-Frequency Ultrasound (50-150 MHz) in Dermatology," in P. Altmeyer, K. Hoffmann, M. Stücker (Eds.), "Skin Cancer and UV Radiation," Springer, 1997, pp. 1023-1051 [3] J. A. Jensen, "Estimation of Blood Velocities Using Ultrasound," Cambridge University Press, 1996 [4] D. A. Christopher, B. G. Starkoski, P. N. Burns, F. S. Foster, "A High Frequency Pulsed-Wave Doppler Ultrasound
System for Detecting and Imaging Blood Flow in the Microcirculation," 1996 IEEE Ultrasonics Symposium, San Antonio, Texas [5] C. Kasai, K. Namekawa, A. Koyano, R. Omoto, "Real-Time Two-Dimensional Blood Flow Imaging Using an Autocorrelation Technique," IEEE Trans. Ultrason. Ferroelec. Freq. Contr., Vol. SU-32, pp. 458-464, 1985 [6] K. W. Ferrara, "Blood Flow Measurement Using Ultra sound," in J. D. Bronzion (Ed.), "Biomedical Engineering Handbook," CRC Press, IEEE Press, pp. 1099-1118, 1995 [7] K. W. Ferrara, V. R. Algazi, "A New Wideband Spread Target Maximum Likelihood Estimator for Blood Velocity Estimation-Part I: Theory," IEEE Trans. Ultrason. Ferroelec. Freq. Contr., Vol. 38, pp. 1-16, 1991 [8] K. W. Ferrara, V. R. Algazi, "A New Wideband Spread Target Maximum Likelihood Estimator for Blood Velocity Estimation-Part II: Evaluation of Estimators with Experimental Data," IEEE Trans. Ultrason. Ferroelec. Freq. Contr., Vol. 38, pp. 17-26, 1991 [9] S. K. Alam, K. J. Parker, "The Butterfly Search Technique for Estimation of Blood Velocity," Ultrasound in Med. & Biol., Vol. 21, pp. 657-670, 1995
