Almost invariably , a new baby’s photo album begins with a grainy black-and-white picture taken months before birth--a prenatal ultrasound image, which is often detailed enough to inspire comments about the child’s resemblance to various members of the family. But jokes about balding uncles notwithstanding, such scans serve a serious purpose and can prove immensely important, as when they allow doctors to diagnose and sometimes even repair a congenital malformation while the baby is still in the womb.
When seeing such an image for the first time, most people are awestruck. How can mere sound waves provide such remarkably clear views? Engineers may well ask something more: How can we give doctors even better ultrasound images? That question has engaged the three of us, along with other members of our Stanford acoustics group, for much of the last decade.
Whereas the signal-processing and image-reconstruction techniques used in medical ultrasonography have made huge advances since this type of imaging became commonplace three decades ago, the business end of the apparatus--the transducer, which converts electrical impulses to sound waves and vice versa--has remained largely unchanged. So we found fertile ground when we began digging for ways to improve those transducers using tools from the microelectronics industry. You will soon find the fruits of those efforts at your local hospital. Indeed, this strategy promises to revolutionize ultrasound imaging within the next few years.
How ultrasound imaging works is easy enough to describe, at least in broad strokes. High-frequency (1- to 50-megahertz) sound waves transmitted into the body create reflections when they encounter a change in tissue density or stiffness. These faint echoes are picked up with the same set of transducers used to generate the sound. Or the imager may use just a single transducer moved over the body--usually with the aid of much slimy goo, to ensure good acoustic coupling. The resulting electrical signals are then amplified, combined, and displayed as images.
Ultrasonography is valuable for several reasons. For one, it’s inexpensive--at least compared with CT (computed tomography) and PET (positron-emission tomography) scanning, or with MRI (magnetic resonance imaging). Also, the low-amplitude ultrasound waves used for imaging do not involve ionizing radiation and are thus harmless to the patient, so repeated scans can be made without worry. And with this technique it is not difficult to get real-time imagery, which doctors may want for such things as guiding a biopsy needle. These virtues make the market for medical ultrasound equipment huge--more than US $5 billion annually, a figure that’s only expected to swell in coming years with growing sales of these systems in China and India.
An ultrasound imager has four main parts: the transducer probe, the analog front-end electronics, the digital signal-processing hardware, and the display. Advances in electronics over the years have brought an extraordinary level of refinement to all but the transducer, which means that most of the remaining opportunities for improving system performance lie in the design of this one critical component. In particular, researchers have lately been seeking reliable ways to fashion many individual transducers into compact arrays.
Having a series of transducers laid out in a line--a one-dimensional array--is the simplest example of this strategy. Such transducer arrays are now employed routinely for most forms of ultrasound imaging. Like multielement radio antennas, such arrays can be steered so as to send energy in a narrow, directed beam. Steering an array also works in reverse, allowing it to detect acoustic echoes that come from one particular direction. While a one-dimensional transducer array can be steered and focused within a single plane to make a two-dimensional image, a 2-D array can be steered and focused throughout a volume to make a three-dimensional image--and this can be done in real time.
With this capability, physicians can, for example, follow heart motions in great detail if they want to assess a patient’s cardiac functioning. In the not-so-distant future, such ultrasound imaging may even allow robotic surgeons to operate on a beating heart so that patients need not run the risk of having to depend on a heart-lung machine.
In the nearer term, doctors are keen to use small 2-D arrays of tiny ultrasonic transducers to obtain forward-looking images as they probe an artery with a catheter. That would permit them to examine obstructions and map the composition of plaque deposits on vessel walls in three dimensions. What’s more, sufficiently small transducers can be arranged in a ring on the end of a catheter, leaving space at the center for an excision device. Such an instrument would allow for simultaneous ultrasound imaging and surgical therapy.
Two-dimensional arrays of ultrasonic transducers would certainly help physicians perform minimally invasive treatments in this way. But making such tiny arrays using traditional transducers is frustratingly difficult. Fortunately, the precise fabrication required can readily be carried out using methods developed by the microelectronics industry, methods that are now routinely used to produce various sorts of microelectromechanical systems, or MEMS.
MEMS fabrication techniques have enabled us to construct something we call a capacitive micromachined ultrasonic transducer. This name, we admit, is an ungainly mouthful, and the acronym we use in our scholarly papers, CMUT, is a bit cryptic to all but a few specialists. Perhaps this is why some of our colleagues in industry refer to this new technology by the more pleasing phrase ”silicon ultrasound,” which tells you right away what stuff these new transducers are for the most part made of.