Real Time Volumetric Ultrasound lmaging System Olaf T. von Ramm and Stephen W. Smith A real time volumetric ultrasound imaging system and extensive operator interaction that preclude has been developed for medical diagnosis. The scan- imaging of dynamic structures without sophisti- ner produces images analogous to ah optical camera cated gating techniques. and supplies more information than conventional sonograms. Potential medical applications include In the field of diagnostic ultrasound, 3D imag- improved anatomic visualization, tumor tocalization, ings systems have been proposed in the past. and better assessment of cardiac function. The However, these have been too slow for clinical system uses pulsa-echo phased array principias to applications 4'5 or have operated primarily in a steer a two-dimensional erray trensducer of 289 through-transmission mode requiring a complex elements in a pyramidal sean format. Parallel process- ing in the receive moda produces 4992 scan lines eta water-path apparatus. 6'70ur objective was to rata of approximately 8 frames/second. Echo data develop ah ultrasound system, which uses a for the scanned volume is presented es projection hand-held transducer, capable of scanning a images with depth perspectiva, steraoscopic pairs, volume rapidly and enabling the 3-D visualiza- multiple tomographic images, or C-moda scans. tion of dynamic structures in real time. 9 1990 by W.B. Saunders Company. In order to extend cross-sectional ultrasound KEY WORDS: diagnostic ultrasound, phased array. imaging to volumetric imaging, three advances three-dimensional, volumetric, diagnostic imaging, were necessary: (1) a hand-held transducer was real time. needed that could be steered rapidly and focused at ~.ny desired orientation in the volume; (2) the A HIGH-SPEED volumetric ultrasound imag- data acquisition rate had to be increased signifi- ing system that uses pulse-echo principles cantly to obtain image data from many planes in analogous to sonar and radar has been developed real time; (3) a display method was required, for medical application. Conventional ultrasound which allowed the visualization of all or part of cross-sectional imaging has become a valuable the echo data obtained from the scanned tissue diagnostic modality in virtually every medical volume. speciality. ] In routine clinical practice, ultra- In order to demonstrate expeditiously the fea- sound pulse-echo images of soft tissue are ob- sibility of high-speed volumetric ultrasound imag- tained with high spatial resolution (1 to 2 mm), ing, the extant Duke University phased array at real time, vŸ frame rates (30/sec) using research scanner 8 was appropriately modified. portable systems. Currently, the most sophisti- Figure 1 illustrates the concept of a volumetric cated medical ultrasound scanners use a phased phased array ultrasound imaging system that linear array of approximately 100 piezoelectric uses a two-dimensional (2D) array of transducer transducer elements to steer and focus the ultra- r to produce a pyramidal scan composed sonic beam along approximately 150 directions of eight steering directions in the azimuth dimen- in a sector sean format. Beam steering and sion, O, by 16 steering directions in the elevation focusing is accomplished by properly delaying dimension, ~. the transmit pulses and the echoes returning to the transducer array from tissue structures within the body. ~ From the Department of Biomedical Engineering, Duke Over the years, there has been interest in the University, Durham, NC; the Centerfor Devices and Radio- medical imaging community in constructing logical Health, Food & Drug Administration, Rockville. three-dimensional (3D) rendcrings from serial MD; and the Department of Radiology, Duke University, cross-sectional images obtained via magnetic Durham, NC. Supported in part by Health and Human Sciences Grant resonance imaging, x-ray computed tomography No. NCI CA37586, National Science Foundation/Engineer- and diagnostic ultrasound. 3 The advantages of ing Research Center Grant No. CDR-86222, and by the Food such renderings include more realistic anatomic and Drug Administration. visualization and improved localization and siz- Address reprint requests to Olaf T. von Ramm, Depart- ment of Biomedical Engineering, Duke University, Durham, ing of mass lesions for surgical procedures. Cur- NC 27706. rent techniques are limited by lengthy data 9 1990 by W.B. SaundersCompany. acquisition times, computer processing times, 0897-t 889/90/0304-0002503.00/0 Journa/ofDigita//maging, Vol 3, No 4 (November), 1990: pp 261=266 261 262 VON RAMM AND SMITH Fig 1. Pyramidal sean for- mar of 3D ultresound imeging system showing 2D phesed ar- rey trensducer, and the trans- mit and receive elements within the arrey. Trensmit directions sre indicated by dotted lines. receive directions by dashes. Dots st end of scan volume ate "/~_~\.~~~/,.: :: .': .~/ image points used to produce the perspective display of Fig 3. Adjustable C-scan plane is indi- cated by darkened region, In order to obtain best image quality, aU of the by the velocity of sound in tissue, ~-1540 m/sec. 17 x 17 elements of the array should be used. The data acquisition was increased through par- However, the Duke system is limited to 32 allel signal processing in the receive ruede in independent .transmit channels and 32 indepen- which eight steering directions were obtained dent receive channels. In this study, the Mills simultaneously for each transmitted ultrasound cross antenna design of radio astronomy9 was pulse. chosen for the transmit aperture (shown by the This parallel processing technique, which we solid cross in Fig 1) and ah independent Mills have described previously, I~ is illustrated in Fig 1 cross for the receive transducer aperture (shown for a few representative lines. In the 9direction, by the shaded diamonds of Fig 1) to provide the orientation of the two transmit pulses T 1 and adequate sampling of the 2D array. The two T 2 (produced by the transmit array) are shown crosses are positioned at 450 with respect to each by the dotted lines. By using the smaller transmit other and neither one utilizes the center element. cross array, the transmit beato is broadened to a This design enabted the imaging system to be width of =6 o. On each side of each transmit steered and focused to any desired angle with a beam, four receive mode scan lines spaced at 0.5 ~ small number of phased array channels while increments (shown by the dashed lines) are maintaining adequate spatial resolution for high- obtained in parallel by the receive mode array. contrast targets. The tradeoff associated with The steering directions are shown on an exagger- this transducer design increases the undesired ated scale in the figure. off-axis energy from the transducer. The receive mode parallel processing is accom- The 2D array of 289 transducer elements plished by the addition of smail independent illustrated in Fig 1 was fabricated in our labora- tapped delay lines to the principle electronic tory with independent transmit and receive crosses delay lines used for steering and focusing. These of 32 elements each. ]'he array operates at 1.7 small delays add the desired steering angle incre- MHz with inter-element spacing of 0.64 mm ment in the receive mode of the phased array yielding a spatial resolution of =4 o. The phased system. The eight to one parailel processing array time delays were calculated using the increases the data acquisition rate of the echo Pythagorean theorem for each transducer array signals by a factor of eight. Thus, in the volumet- etement at each transmit and receive focal point ric scanner 12 transmitted pulses steered in the for each line in the pyramidal sean. elevation (,I,) direction over a 65 ~ sector produce Conventional pulse-echo data acquisition is 96 steer… directions. In the azimuth direction limited by the round trip propagation time of ah (0), no parailel processing is currently imple- ultrasound pulse into the body that is determined mented, so that 52 transmit pulses yield 52 scan REAL TIME VOLUMETRIC ULTRASOUND IMAGING SYSTEM 263 lines steered over a 65 ~ sector. Thus, in each image. Since the echo data is written on the pyramidal sean, 624 transmit pulses produced screen continuously, the viewer's eye integrates 4992 scan lines per image at a frame rate of 8 the data to a single brightness value. This tech- images/second. As a trade-off, the broadened nique is illustrated in Fig 1 in which the 8 • 16 transmit beam, necessary for the increased data scan directions are represented by a matrix of acquisition rate of parallel processing, increases 8 x 16 dots. In this way, the ultrasound scanner undesired off-axis energy from the transducer produces a projection image and functions as the and degrades the image contrast by =13 dB in direct analogue of a normal optical camera. The the resulting 3D images. (x, y) position of each point in the display is Figure 2 is a block diagram of the high-speed proportional to the tangent of the steering angle volumetric ultrasound phased array scanner. The of the corresponding sean line (tan O, tan ~). In figure shows the PDP-11 computer that serves as this display, a more distant object that subtends a the imaging system controller and provides the smaller angle within the pyramidal sean is dis- phased array delay data to the rest of the played as a smaller image. Thus depth is encoded imaging system. Also shown are the 32 transmit in the display as in the human eye ora conven- pulsers that ¡ the transmit transducer array tional optical camera. and the 32 preampli¡ that amplify the ultra- Figure 3 shows preliminary results of our 3D sonic echoes from the receive mode array. The imaging system in water tank evaluations of the figure also shows the electronic delay lines that projection display. Figure 3 A is a 4992-1ine provide the receive mode delays for steering and pyramidal sean of a 14 mm (9/16 in) wrench in focusing as well as providing the 8 to 1 receive water positioned parallel to the transducer face mode parallel processing.