JMBE-Journal of Medical and Biological Engineering

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: kkshung@usc.edu
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
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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).
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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
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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.
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Figure 7. Image of the anterior segment of an excised eye obtained using
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Figure 8. Photograph of a 256-element 30-MHz linear array showing the
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4. Conclusion
Ultrasound has come a long way since the early 1950s. It is
one of the most important tools in diagnostic medicine today.
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