Faster, more powerful processing. A better handle on nonlinear wave propagation. Higher-performance transducers. The development of specialized contrasting agents. Advances in image and real-time signal processing. All of these factors are contributing to a renaissance in ultrasonic medical imaging. Healthcare providers already have an array of impressive tools at their disposal. But ultrasound manufacturers aren't sitting tight, as they pioneer innovative machines for applications like cardiac imaging and 4D neonatal imaging.
HOW IT ALL WORKS
Ultrasound imaging instruments are sophisticated systems that resemble sonar. Consequently, the technology also is called sonography. A single transducer or a transducer array transmits a soundwave and then listens for the reflected signal (Fig. 2). Signal processing combines the reflected signals, and by performing the process over a wide scan area, constructs an image to profile the scanned area.
Ultrasound frequencies range from 1 to 10 MHz. Signals penetrate several centimeters into the human body. The tissue type being penetrated (soft or hard) and the depth of penetration determines the sonograph's response time and resolution.
Ultrasound can be applied either in the pulsed mode or continuous-wave (CW) mode. Pulse mode is mainly used for diagnostics. In addition to diagnostics, CW mode tackles therapeutics. And through ablation, CW ultrasound can effectively treat all types of tumors.
Ultrasound signals will appear as gray-scale images, Doppler presentations, or venous and arterial displays. Gray-scale images are black and white. Colorized Doppler images detect the velocity of blood and other bodily fluids. Venous and ultrasound images use both the gray scale and Doppler methods.
For over 35 years, obstetricians have relied on 2D ultrasound. It works by viewing sound waves in a single X-Y plane, with waves propagating back and forth to form an image—much like slicing a loaf of bread.
Surface rendering, which is the main ingredient in 3D ultrasound creation, uses the Z as well as the X and Y axes. The same waves used in 2D scanning also are used in 3D scanning, though the waves are used at different angles. Sophisticated software interprets the waves and creates a 3D image, providing greater clarity than a 2D image.
4D ultrasound provides images in real time. It adds the time element to 3D imaging for real-time viewing. It's used for CW Doppler imaging of the direction and velocity of blood flow.
Ultrasound is a non-ionizing form of energy. Unlike X-rays, it can be used for imaging without any known side effects (see "Is Medical Ultrasound Imaging Safe?" p. 50).
ANALOG VS. DIGITAL
There are two types of ultrasound systems: analog beam-forming and digital beam-forming (Fig. 3). Digital beam-forming is the standard for ultrasound imaging systems. Yet doctors will continue to use analog beam-forming, particularly in high-end ultrasound systems, because of the wide diagnostic range of CW Doppler.
Analog beam-forming systems use a variable-gain amplifier to compensate for attenuation in the medium that's penetrated by the beam. A time-delay element maximizes the signal-tonoise ratio (SNR) of the reflected signal from the focal zone. Corresponding points on the time-delayed signals for each channel are summed, compressed, and amplitude-detected. Analog-to-digital converters (ADCs) next process image, audio, and Doppler information.
A digital beam-forming system processes data using a fieldprogrammable gate array (FPGA), fixed-function off-the-shelf digital components, and DSPs. Because digital processing provides real-time capability, digital processing requires signal optimization. The digitized data, in polar coordinates, is processed and mapped into rectangular coordinates, stored in memory, and sent to the video and audio encoders.
BIG SYSTEMS GET BETTER
Siemens Medical Solutions, GE Healthcare Systems, and Philips Medical Systems are three of the largest ultrasound system manufacturers. Siemens recently introduced its Acuson Anteres System, Premium Edition, for obstetrics/gynecology applications. GE Healthcare's Volusion 730 system targets fetal echocardiography. Philips' relatively compact HD11 XE ultrasound system suits cardiology.
Siemens' Acuson system relies on the company's Cadence Contrast Pulse Sequencing Technology (CPS). CPS can track cancer and metastases via ultrasound diagnostics as well. It is designed to improve sensitivity and specificity of contrast agents by combining the non-linear fundamental and higher-order harmonic contrast signals. This allows simultaneous detection of signatures that are unique to contrast agents and unique to the tissue being scanned. Also, the Acuson system's High Resolution Color Flow (HRCF) greatly improves ultrasound spatial resolution and sensitivity through chirp-coded excitation applied to the Doppler signals.
The GE Volusion 730 system is a 4D ultrasound system with advanced signal processing for high levels of image quality. GE's exclusive 4D technology provides a live update of scanned images. It "represents the difference between video and a still photograph" compared with 3D imaging, according to the company.
While the Siemens, GE, and Phillips ultrasound systems represent the state of the art in sonography, they're large systems with hefty price tags. Many such systems can cost upward of $200,000. And because of their large size, they're limited in mobility and flexibility of use. Many of these systems are located in a hospital's radiology room and cannot be moved to the patient's bedside. Instead, the patient must be brought to the machine.
PORTABLES ARE ALL THE RAGE
Recognizing the need for low cost, mobility, and flexibility, some companies have responded with high-performance portable machines that resemble laptop computers and can be purchased for as little as $40,000.
GE's Logiq Book XP is a 10.3-lb laptop-like ultrasound machine used for cardiovascular, abdominal, and obstetrics/ gynecology applications (weight includes the battery). Key to Logiq Book XP's performance is the incorporation of advanced transducers, including an intraoperative vascular small-parts transducer.
GE's portable ultrasound is strikingly similar to a laptop. The system has a keyboard that is common to GE's LOGIQ family of products, a trackball for easy navigation, and a color display. It has 60 Gbytes of memory, 30 Gbytes of which are dedicated to holding more than 30,000 ultrasounds. Images can be exported via the CD-R/W drive, USB device, or by wireless Digital Imaging and Communication in Medicine (DICOM) protocol.
Getting such performance into a portable form factor proved to be a technical challenge that led to the elimination of a lot of hardware.The colorflow doppler hardware, for example, used to be on separate, fairly large boards. Now it's all done with software.
Today, cardiologists routinely use portable ultrasound imagers to capture images of the heart. Obstetricians use portables to view fetuses, and nephrologists monitor blood and urine flow in kidneys.
Pulmonary specialists can use imagers to spot fluid buildups around the lungs or heart. Vascular specialists use portables to detect thickening of the arteries. And, emergency room physicians can examine trauma victims for injuries to the chest, stomach, and internal organs.
Many analysts believe the portable ultrasound market will grow substantially. In fact, Oppenheimer & Company expects the handheld ultrasound device market to surpass $500 million by 2010.
Sonosite often is credited with spawning the portable ultrasound imaging market, thanks to its system-on-a-chip (SoC) imaging technology. The SoC includes digital signal processing and multiple ultrasound functions on the same chip.
Sonosite's technology-had its roots in sonar research at the University of Washington's Applied Physics Laboratory. Its latest product is the 8-lb MicroMaxx, which is used to diagnose the leading cause of death in the U.S., cardiovascular diseases (Fig. 4).
Zonare produces the portable z.one system for general imaging, obstetrics and gynecology, cardiology, vascular, and other emerging applications. It's based on the company's proprietary Zone Sonography and Zone Speed Technology (ZST), which focuses on "zones" of imaged bodies to gather large amounts of data quickly (Fig. 5). Images can be acquired in 5.2 ms using Zone Sonography versus 52 ms for conventional ultrasound.
A key part of the z.one's performance is the use of a trio of Texas Instruments C6455 DSPs ICs. This results in increased performance of video processing and imaging functions.
The instrument can be used on a portable cart that holds a 5.5-lb scan engine. The engine can be removed from the cart when the need for portability arises. The company dubs its system the Convertible Ultrasound machine since it can be used quickly and easily in many different locations and settings for different applications.
ENDOSCOPIC ULTRASOUND GROWS
Endoscopic ultrasound (EUS) is one of the newest and fastest-growing applications in the medical community. With EUS, a slender tube several feet long is inserted into a patient's digestive tract either downward into the esophagus or upward through the rectum.
EUS allows the doctor to microscopically examine tissue not only within the digestive tract, but also surrounding it. The endoscopic tube works like a periscope. It allows the doctor to examine internal organs and photograph and videotape the findings. EUS is as comfortable as regular endoscopy, although it takes longer because it is more precise, and because there are more details for the doctor to examine and interpret.
A more advanced EUS technique, fine-needle aspiration, has been successfully performed on many patients by Kenneth Chang, MD, director of the Interventional Endoscopy Center at the University of California at Irvine (UCI), and Dr. Phuong Nguyen. It involves the insertion of a biopsy needle at the end of the endoscope. Controlled and monitored via a TV screen, the needle lets doctors examine and determine whether or not cancerous tumor growths are growing or shrinking (as a result of therapy). Without it, patients would need to undergo many exploratory surgeries to examine cancerous tissues.
Needle aspiration also can be used to explore organs, including the heart, liver, intestines, and pancreas. Pancreatic tumors can be seen even when they're as small as 6 mm in diameter. CT scans, which previously had been the best method for viewing tumors, can only find tumors no smaller than 2 cm.
One study found that 33% of esophogal cancer cases and 75% of pancreatic cancer cases have been found inoperable. This means that the surgical procedures that had been performed for exploration were unnecessary.
Chang estimates that EUS is used in about 150 hospitals across the U.S. “About half of those are equipped to perform fine-needle aspiration, and less than half of them are actually successfully doing needle biopsies on patients,” he says. “They have the biotechnology, but not the expertise. Right now, the bottleneck is the training.”
Making sense of the ultrasound images of EUS is a complicated task that requires training, practice, and mental agility. That’s why both Chang and Nguyen teach a course in ultrasound endoscopy at the UCI Medical Center three times a year. In fact, Chang has been a guest lecturer at the Mayo Clinic and other medical centers.
EUS is such a hot topic, medical device makers like Pentax are preparing EUS probes. Better known for its cameras, Pentax has performed clinical trials on third-generation all-electronic radialarray endoscopes with 360 viewing. Smaller linear-array endoscopes also under development. According to Pentax, new piezoelectric materials, better manufacturing efficiencies, and effective wiring and shielding materials all have contributed to Pentax’s EUS advances.
Another important advance comes from Duke University. Researchers there have developed ultrasound devices that combine 3D imaging with therapeutic heating, a development that bodes well for treating heart arrhythmias.
“No one else has developed a way for ultrasound to combine therapy and imaging in a catheter, let alone 3D imaging,” says Stephen Smith, the biomedical engineering professor who heads the team working on this project. He feels that this development may improve on the most widely used method by doctors for destroying or ablating aberrant heart tissue that makes the heart beat irregularly.