Journal of Medical and Biological Engineering, 31(6): 371-374 371 Diagnostic Ultrasound: Past, Present, and Future K. Kirk Shung* Department of Biomedical Engineering, University of Southern California, Los Angeles, CA 90089-1111, USA Received 16 Nov 2010; Accepted 25 Jan 2011; doi: 10.5405/jmbe.871 Abstract Ultrasound has been used as a diagnostic tool for more than 40 years. Many medical applications have adopted ultrasound, mostly notably in obstetrics and cardiology. Started by a few scientists and clinicians in different parts of the world in the early 1950s, it did not become an established diagnostic tool until the early 1970s when grayscale ultrasonography was introduced. Modern ultrasound scanners are capable of producing images of anatomical structures in great detail in grayscale and of blood flow in color in real-time. State-of-the-art four-dimensional scanners that yield three-dimensional volumetric images in real-time are pushing the present technical capability to its limit. Ultrasound is currently the second-most used clinical imaging modality after conventional X-ray radiography. Although ultrasound is considered to be a mature technology, technical advances are constantly being made. The most significant achievements in ultrasound recently have been the developments of approaches capable of the quantitative measurement of tissue elastic properties, namely ultrasound elastography and radiation-force imaging, high-frequency imaging yielding improved spatial resolution, and therapeutic applications in drug delivery and high-intensity focused ultrasound surgery. The miniaturization of scanners has become a trend. In this paper, the history and current state of medical ultrasound are reviewed and future developments are discussed. Keywords: Ultrasound, Ultrasonic imaging, Color Doppler, Elastography, Radiation-force imaging 1. Past The potential of ultrasound as an imaging modality was realized as early as the late 1940’s when several groups of investigators around the world utilizing sonar and radar technology developed during World War II started exploring the diagnostic capabilities of ultrasound [1]. John Wild, a clinician and John Reid, an engineer, at the University of Minnesota Medical School, USA, developed a prototype 15-MHz B-mode ultrasonic mechanical scanner with components borrowed from a nearby naval laboratory. They were able to demonstrate the capability of ultrasound for imaging and characterization of cancerous tissues. John Wild’s pioneering effort and accomplishment were recognized with the Japan Prize in 1991. At the same time, Douglas Howry and Joseph Holms at the University of Colorado at Denver, apparently unaware of the effort made by Wild and Reid, also built an ultrasonic imaging device with which they produced cross-sectional images of the arm and leg. In Japan, starting in the late 1940’s, medical applications of ultrasound were explored by Kenji Tanaka and Toshio Wagai. Two Japanese investigators, Shigeo Satomura and Yasuhara Nimura, were credited for the earliest development of ultrasonic Doppler devices for monitoring tissue motion and blood flow in 1955. * Corresponding author: K. Kirk Shung Tel: +1-213-821-2653; Fax: +1-213-821-3897 E-mail: [email protected] At around the same time, Inge Edler and Hellmuth Hertz at the University of Lund in Sweden worked on echocardiography, an ultrasound imaging technique for imaging cardiac structures and monitoring cardiac functions. In parallel with these developments for diagnosis, William Fry and his colleagues at the University of Illinois at Urbana worked on applying high-intensity ultrasound beams to treat neurological disorders in the brain. Figure 1 shows an early ultrasonic scanner and an image of a fetus obtained by such a scanner. Figure 1. An early scanner and an image obtained using the scanner (http://www.ob-ultrasound.net/). There are many modes of ultrasonic imaging [2-4]. The primary form of ultrasonic imaging has been that of a pulse-echo mode. The principle is very similar to that of sonar and radar. In essence, following an ultrasonic pulse transmission, echoes from the medium being examined are detected and used to form an image. Many terminologies used in ultrasound have been imported from the field of sonar and radar. Although pulse-echo ultrasound had been used to 372 J. Med. Biol. Eng., Vol. 31 No. 6 2011 diagnose a variety of medical problems since the 1950’s, it did not become a widely accepted diagnostic tool until the early 1970’s when grayscale ultrasound, in which non-linear echo amplitude is mapped to gray levels, was introduced [2-5]. Continuous wave (CW) and pulsed Doppler (PW) ultrasound devices for measuring blood flow also became available during that time. Duplex ultrasound scanners that combined both functions, allowing the imaging of anatomy and the measurement of blood flow with a single instrument, soon followed. In 1985, a color Doppler flow-mapping system that combined Doppler flow imaging in color with B-mode imaging in grayscale was introduced by Aloka in Japan [6]. In these early scanners, an image was formed either by mechanically scanning a single-element piezoelectric transducer that converted the electrical energy into acoustic energy and vice versa or by electronic scanning via a linear array consisting of 64 or more rectangular-shaped piezoelectric elements. Analog components were used for amplification, demodulation, and other signal processing functions. 2. Present All high-end ultrasonic scanners are currently capable of real-time color Doppler flow imaging and performing CW and PW flow measurements. A color Doppler image is shown in Fig. 2 where the blood flow information is depicted in color. A blue away, red toward (BART) system is typically used to depict blood flow direction. Red indicates movement toward the transducer whereas blue indicates movement away from the transducer. Shades of a color indicate the magnitude of flow velocity. The anatomical information or B-mode image is displayed in grayscale. A variation called color Doppler power mode displays the power contained in the Doppler signal rather than the Doppler frequency shift [2-4]. This approach makes the image very similar to an X-ray angiogram, which is easy to interpret. In addition, it avoids the aliasing problem of conventional color Doppler. Figure 3 shows a color Doppler power mode image of a carotid artery where the shade of the color indicates the power contained in the Doppler signal. Color Doppler has been applied to many clinical applications, including the quick diagnosis of arterial atherosclerotic plaques, cardiac shunts, and tumor angiogenesis. Figure 2. Color Doppler image of a fetus and an umbilical cord (courtesy of Philips). ECA Figure 3. (top) Color Doppler power mode image of a carotid artery and (bottom) spectral Doppler of selected region of interest in the middle of the blood stream (courtesy of Siemens). Ultrasound propagation in tissues had been assumed to be linear for many years. Non-linear interaction between ultrasound waves and tissues was ignored. Given the high instantaneous peak pressure levels used in diagnostic ultrasound instruments, non-linear effects are bound to occur. Higher harmonics converted from the fundamental frequency are generated as the ultrasound beam propagates deeper into the body [7,8]. A wideband probe can be designed to transmit at the fundamental frequency and receive at the harmonic frequency. Novel approaches such as pulse inversion imaging have been developed to accommodate this need [2-4,9]. Native-tissue harmonic imaging has evolved over the years to become a major imaging option in diagnostic ultrasound due to its greater penetration depth. It also offers other advantages over conventional ultrasound, including less near field reverberation [2,8]. The development of non-toxic contrast agents, primarily encapsulated gas bubbles, has led to new forms of ultrasonic imaging, such as harmonic imaging and perfusion imaging [2-4,9-11]. Gaseous bubbles or contrast agents resonate at various frequencies, determined primarily by the size of the agent. For example, the resonance frequency of a free air bubble with a 3-μm radius is 1.1 MHz at 1 atomspheric pressure in water [2], yielding an increased signal-to-noise ratio if imaging is performed at this frequency. Tissue displacement imaging enables the assessment of the elastic properties of tissues and the delineation of lesions that do not appear in standard B-mode images [2-4,12-14]. Multi-dimensional imaging that utilizes multi-dimensional arrays improves image contrast due to better control of slice thickness and three-dimensional volumetric imaging in real-time or four-dimensional imaging [2-4,15,16]. Figures 4 and 5 respectively show a three-dimensional image of a fetus in utero and an image obtained using ultrasound elastography, where a volume of tissue is disturbed and the elastic properties of the tissue are estimated by measuring tissue displacement by correlating the speckle patterns before and after the mechanical disturbance. Diagnostic Ultrasound: Past, Present, and Future Figure 4. A three-dimensional image of a 10-week-old fetus in utero (courtesy of Philips). 373 The many advantages of ultrasound have allowed it to become a valuable diagnostic tool in medical disciplines such as cardiology, obstetrics, gynecology, surgery, pediatrics, radiology, and neurology. The relationship among ultrasound and other imaging modalities is a dynamic one. Ultrasound is the tool of choice in obstetrics primarily due to its non-invasive nature, cost-effectiveness, and real-time imaging capability. This role will not change in the foreseeable future. Ultrasound is also commonly used in cardiology; echocardiography is required training for a cardiologist. The future of ultrasound in cardiology is not guaranteed as other competing imaging modalities such as multi-slice spiral computed tomography (CT) and magnetic resonance (MR) are improving their image acquisition rate and image quality. Ultrasound may lose ground in certain areas but it may gain in others. For example, ultrasound mammography has gradually gained importance in breast cancer imaging. With heightened public concern over health care costs, the cost-effectiveness of an imaging tool is a crucial factor in planning diagnostic strategies. Diagnostic ultrasound is particularly attractive in this respect. 3. Future Figure 5. (left) B-mode image and (right) ultrasound elastogram of a breast cancer tumor (courtesy of Siemens). Ultrasound is currently the second-most utilized diagnostic imaging modality in medicine after conventional X-ray, and is a critically important diagnostic tool. Ultrasound not only complements more traditional approaches such as X-ray but also has unique characteristics. More specifically, (1) ultrasound is a form of non-ionizing radiation that is considered safe, (2) it is less expensive than imaging modalities of similar capabilities, (3) it produces images in real-time, unattainable at the present time by any other methods, (4) it has a resolution in the millimeter range for frequencies in clinical use, which can be improved if the frequency is increased, (5) it can yield blood flow information by applying the Doppler principle, and (6) it is portable. Ultrasound also has several drawbacks. Chief among them are that (1) organs containing gases and bony structures cannot be adequately imaged without introducing specialized procedures, (2) only a limited window is available for ultrasonic examination of certain organs such as the heart and neonatal brain, (3) interpretation of images is operator-skill-dependent, and (4) it is sometimes impossible to obtain good images from certain types of patient, such as obese patients. Technical advances in ultrasound are constantly being made. Developments include portable scanners, miniature pocket-size scanners, and high-frequency scanners. A reduction in physical size has been made possible by incorporating application-specific integrated circuits into the imaging system. A couple of pocket-sized scanners have been introduced into the market recently. Figure 6 shows an Ipod-sized scanner developed by GE which weighs only 390 g and has a 3.5-inch (8.9 cm) display and a 3.0-MHz phased array capable of color Doppler imaging. Figure 6. GE Vscan pocket ultrasound scanner (courtesy of GE). High-frequency (above 20 MHz) scanners have been developed for eye, skin, small-animal, and intravascular imaging. They have improved spatial resolution at the expense of penetration depth [2]. There are more than half a dozen eye scanner manufacturers around the world. The basic design includes a mechanical sector scanner in which a high-frequency single-element transducer is mechanically rotated to form an image. The electronics are relatively simple, consisting of a 374 J. Med. Biol. Eng., Vol. 31 No. 6 2011 single radio-frequency channel. The novelty of these imaging devices lies in the probe and transducer design [17-20]. Figure 7 shows an image of the anterior segment of the eye obtained at 40 MHz. Detailed anatomy can be clearly seen. Such a system has also been used to image small animals and skin lesions. Mechanical scanners suffer from a low frame rate and non-uniform image quality. As a result, high-frequency linear arrays and scanners have been developed [2,19,20]. Figure 8 shows a 256-element 30-MHz linear arrays made from 2-2 composites with a bandwidth of over 50%. 40 MHz US Image of excised eye 40-MHz US image of excised eye Although it is now considered a mature technology, technical advances are constantly being made. It is conceivable that in the not too distant future every physician’s office may have one. References [1] [2] [3] [4] [5] [6] [7] [8] Figure 7. Image of the anterior segment of an excised eye obtained using a mechanical high-frequency scanner. [9] [10] [11] [12] [13] [14] [15] Figure 8. Photograph of a 256-element 30-MHz linear array showing the aperture in the front and the flex circuit for interconnection. 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