ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 45, no. 6, november 1998 1437 Letters A Swept-Scanning Mode for Estimation of Blood Velocity in the Microvasculature Dustin E. Kruse, Ronald H. Silverman, Rafael J. Fornaris, D. Jackson Coleman, and Katherine W. Ferrara, Member, IEEE Abstract—In contrast to previous systems in which an ultrasonic pulse was repeatedly directed to a discrete line of sight, a new method has been developed to continuously scan over a region in order to rapidly assess blood veloci- ties in superficial small blood vessels. Using this technique, which we call swept-scan, a high frequency transducer can rapidly translate across a region of interest, and sensitive maps of blood velocity in small blood vessels can be con- structed. This system has been applied to flow mapping in the anterior segment of the eye, which is clinically signifi- cant in cases of trauma and glaucoma. No previous imaging technique has been capable of estimating blood velocities within this region in a clinically useful manner. With this new technique, each 2-D scan of the eye can be obtained in an interval on the order of 1 second, and blood flow through the iris and ciliary body can be detected in vessels as small as 40 microns. A major implication of this new technique is that a wall filter can be applied continuously to the return from all regions, thus eliminating the transient response that occurs along each line of sight in traditional Doppler systems. I. Introduction H igh frequency ultrasound has shown promise for use as a clinical tool, but practical limitations have restricted its use. A significant factor that limits the ap- plication of high frequency ultrasound is the acquisition delay associated with mechanically scanning a single el- ement, piston transducer. In typical high frequency sys- tems, the transducer is translated a distance on the order of the lateral beam resolution, and one or more pulses are directed to one line of sight (the single pulse permutation is known as a B-scan). This method is illustrated in Fig. 1 as the discrete line of sight method. To obtain blood flow information, typically 10 to 60 pulses are directed to a line of sight depending on the estimation strategy and desired signal-to-noise ratio. Because a low pulse repetition fre- quency is required for the estimation of low velocity flow, proper spatial sampling using this method can require 1 Manuscript received July 6, 1998; accepted August 31, 1998. This research is supported in part by NIH R01 EY 11468, the Research to Prevent Blindness, Inc., and the Dyson Foundation, New York, NY. D. Kruse, J. Fornaris, and K. Ferrara are with the Department of Biomedical Engineering, University of Virginia, Charlottesville, VA 22903 (e-mail: dek5y@virginia.edu). R. Silverman and D. J. Coleman are with the Department of Ophthalmology, Cornell University Medical College, New York, NY 10021. Fig. 1. Two possible scanning modes for a single element transducer system. The cylinders represent the piston transducer, and the cubes represent the focal volumes, one for each line of sight. It is important to recognize the difference between the discrete lines-of-sight mode and the swept-scan mode. In the discrete mode, numerous pulses (30 in this example) are directed to independent focal volumes. In contrast, adjacent pulses in the swept-scan mode are directed to over- lapping focal volumes. hour for 3-D scans. In conventional Doppler ultrasound systems, this limitation is not crucial because transducer arrays can electronically scan in real-time and interleave lines of sight; however, these devices are currently limited to frequencies below 15 MHz. Previous attempts to mea- sure blood flow within the eye using conventional color Doppler ultrasound methods were limited by insensitivity to very slow velocities (< 1.5 cm/s) [1], and the inability to resolve vessels smaller than 300 microns. Most stud- ies have demonstrated the ability to assess blood flow in the opthalmic artery and vein and in the short posterior ciliary artery; however, these vessels are generally larger and contain higher flow velocities compared to those found in the anterior segment [2], [3]. In contrast, studies using high frequency ultrasound clearly demonstrate the ability to resolve structures down to 40 microns in the anterior segment of the eye, and such B-scans of the eye are clini- cally useful in diagnosing diseases such as melanoma of the ciliary body and open angle glaucoma [4]–[6]. However, no 0885–3010/98$10.00 c  1998 IEEE