Every day, heart attacks kill or incapacitate people around us. Not long
ago, technology offered physicians few means of visualizing the damage done to a
heart by a heart attack. Today's ultrasonic imaging systems significantly
increase the amount of information available to physicians, without exposing the
patient to ionizing radiation. High-speed MRI, still an experimental means of
viewing the heart, may offer cardiologists another way to evaluate the damage
done by a heart attack, and plan treatment.
Real-time viewing. Suppose you have chest pains, and decide to consult a cardiologist. Ultrasonic imaging gives your doctor a fast, effective means of looking at your heart and seeing, in real time, how well or badly it's functioning.
Fifteen years ago, ultrasonic images were created using a mechanically steered ultrasonic beam. Images were of lower quality than those created today, and only anatomical information was visible. Today's equipment, such as the SONOS 2500™ imaging system from Hewlett Packard's Medical Products Group in Andover, MA, can display the motion of your heart's wall and give a doctor vital information on blood flow through your heart. Images on the 2500's monitor are refreshed at 30 Hz, so the cardiologist sees a true real-time view of his or her patient's heart as it pumps blood. Depending on the application, HP's ultrasonic systems can vary the screen refresh from 60 Hz to 8 Hz.
Philip Drew, who holds a Ph.D. in electrical engineering, is a medical equipment consultant with Concord Consulting Group, Concord, MA. "Cardiac ultrasound is used to detect wall motion anomalies. You can observe the valves and recognize valve deficiencies, and with Doppler you can observe the characteristics of blood flow. About 1/3 of the ultrasound business is for cardiac ultrasound, and they're used principally by cardiologists in hospitals and in their offices," says Drew.
The physician will examine you with a handheld transducer built around a phased array of piezoelectric elements that operate at frequencies ranging from 2.0 to 7.5 MHz. There are typically 128 elements in a transducer. Each element's signal amplitude and timing is separately controlled. By controlling the timing, the system can steer or focus the ultrasonic waves emitted by the transducer head. That steering procedure remains transparent to the cardiologist, who operates the unit using a rather simple control panel.
During operation, 128 transmitters excite the transducer elements which, in turn, feed 128 receivers and delay lines. Adjusting the delay time allows the system to change its focus in real time. As of now, says Paul Magnin, R&D manager for Imaging Systems at H-P in Andover, the company's systems use analog circuitry for beam formation. Speed and cost drove H-P's decision to use analog circuitry; digital circuitry will come on line when it becomes cost-effective.
"We need very, very wide dynamic range A/Ds," says Magnin, "because we might be listening to echoes from red blood cells." Data from the delay lines are summed, and a Doppler signal is extracted to tell the doctor the direction and velocity of flow in your heart. Using signal amplitude data, the system forms a black and white image of your heart. Blood flow information is presented to your cardiologist in color.
Proprietary software and hardware convert data produced by the fan-shaped ultrasonic beam into a two-dimensional image for display on a monitor, a printout or delivery to a built-in VCR. The image examined by your doctor can present information from a single slice taken at any angle through your heart.
Computing blood flow. In addition, the system can distinguish between your heart's wall and the blood within it. If your physician so desires, the equipment can measure the size of the blood pool in your heart-in real time-and compute the volume of blood it's pumping. Another set of images can point out abnormal heart wall motion. If a section of your heart is stunned or dead, it won't move in the same fashion as healthy tissue. A damaged valve's operation, and the nature and strength of your heart's contractions, can also be studied.
Your cardiologist could use a linear array to image, for example, your carotid artery, if he or she suspected a blockage. There can be as many as 288 elements in a linear transducer; data produced by it are handled in much the same way as data from an array that produces a fan-shaped beam.
To locate borders between heart muscle and blood, engineers had to differentiate between echoes from blood cells and echoes from tissue. The signatures of these different echoes were determined by phenomenology studies. Given that information, the controlling computer can recognize the moving wall of your heart and define it in color on the system's monitor.
"We have the ability to construct a flow image 200 lines by 300 pixels deep. We have the ability to detect the flow in every position in this image in 1/10 of a second. From a signal processing point of view, that means I've got to be able to calculate Doppler shift on every pixel in that matrix in 1/3 of a second" says Magnin. "The workstation guys aren't even close to the kind of processing power that we have in this system."
Microprocessors-68000-class units-control data collection, but dedicated circuitry does the necessary data reduction at a very high rate. Capturing data to form images for the SONOS 2500™ requires 750,000 lines of front-end code, 108,000 lines on the system's back end, and an overall total of 1,045,000 lines of code. The front end executes at a rate of 60 GIPS, the back end at more than 584 MIPS.
"This business is very unlike H-P's traditional test and measurement equipment business," Magnin says. In that business, equipment is designed by engineers for other engineers. "We call it the next bench phenomenon." In the medical arena, engineers must work hard to gain an intuitive understanding of the needs of medical personnel.
Although it does not produce exceptionally high-resolution images, modern ultrasonic imaging is fast, safe, relatively inexpensive, and portable. If a doctor needs to make a quick examination of your heart, ultrasonic imaging can produce important information in real time.
New views. After your physician makes his or her initial examination of your heart, further diagnosis and treatment may depend on obtaining more detailed images. Until recently, MRI-which can acquire very sharp images of tissue-did not provide good cardiac images. Research now underway may change that situation.
Conventional MRI builds up an image slice by gathering data one line at a time during a single scan-repetition interval. This type of imaging requires repeated excitations of the RF transmit antenna. Gradient coils establish magnetic gradients within the system. The coils are arranged orthogonally, and the gradients result in slightly different resonant frequencies for water in each part of the slice. After a Fourier transform, the frequency information yields spatial encoding that allows construction of an image.
Creating one conventional image of your heart requires 256 scan repetition times to collect data. "Conventional imaging requires on the order of five to ten minutes to acquire all the data," says Denise Angwin, manager of clinical science and compliance at Advanced NMR Systems, Inc., Wilmington, MA. "If the patient moves, you get blurring-motion artifacts-in the image." Intuitively, imaging a beating heart with this sort of system presents problems.
Advanced NMR designs and builds equipment that upgrades conventional MRI systems made by General Electric Corp. Called the InstaScan™, their equipment utilizes a technique called echo-planar imaging. It can collect all the data required to build an image in one repetition time. This approach allows capture of an image in an interval as short as 20 mseconds and performance of multiple-image studies in as little as 20 seconds.
The key to doing this type of high-speed scan on your damaged heart lies in the use of fast-switching gradient coil control electronics. (Engineers at ANMR design the coils, electronics and control software used in the system.) Gradient amplitudes in the InstaScan™ system range from 1.0 to 2.5 Gauss/cm.
"Changing the gradients is what provides you the spatial information required for an image," says Angwin. "In conventional imaging, you set the gradients up once to acquire a specific line, then repeat the process with a change in the gradients to acquire more data. The echo-planar hardware oscillates the gradients every 500 æseconds, and we can capture all the data to get one image in 20 to 64 mseconds. We can acquire 16 images/second." A SPARC-based system receives the data and processes it, and an embedded computer controls the coil gradients and controls data acquisition.
Shim coils are used to ensure that the 1.5T magnetic field in the MRI system remains uniform within 5 to 10 parts/million. Gradient coils and the shim coils reside in an annulus between two thick-walled fiberglass tubes. During the examination of your heart, you will lie inside the inner, 65-cm-diameter tube. The coils, all proprietary designs by Advanced NMR, are potted into the annular space. By using epoxy to fix their locations, design engineers prevent the coils from shifting during high-speed gradient changes. Overall, the coil assembly weighs from 2,500 to 3,000 lbs.
"Advanced NMR Systems has pioneered this echo-planar technique," says Philip Drew. "You can make pretty good stop-action images of the heart using echo-planar MRI. The question arises whether the MRI image is a significant improvement over the ultrasound image. There are reasons to be skeptical, but not reasons to be totally negative."
Next step. In the future, your cardiologist may use equipment like Advanced NMR's echo-planar MRI system to measure flow in coronary arteries and to watch your heart beat. He or she may be able to see damaged areas in your heart wall, with an image resolution ranging from less than 1 mm to 1.5 mm. That kind of information might prove vital when planning your surgery or other treatment.
"If the MRI is able to recognize ischaemic (blood-deprived) and infarcted (dead) tissue, so that you're seeing more than wall motion anomalies, then MRI might have a role in cardiac diagnosis, says Drew. "As it is now, the two methods commonly used for cardiac imaging are ultrasound and nuclear medicine. Nuclear medicine reveals zones of ischaemia and infarction, and does it pretty reliably."
Nuclear medicine also allows a good measure of the heart's ability to pump blood, Drew explains. "You inject a slug of radioactive material, you see the heart light up, and then you see how much it's dimmed on the next beat, and that gives you a direct measurement of the amount of blood ejected."
On the other hand, given the attitude of the public toward everything with the label "radioactive" on it, injecting radioactive substances into the body may present certain problems to physicians in the future. And handling radioactive materials requires special training for cardiologists and hospital personnel.
"My personal opinion is that the day will come when MRI and ultrasound are the important surviving modalities (techniques) for cardiac diagnoses, and probably for a lot of other diagnoses also. But that day's a long way off," Drew observes.
InstaScan™ systems cost $300,000 to $500,000. They are still thin on the ground-about 25 are in use in the United States. If you ever have the bad luck to need cardiac imaging with this type of MRI machine, you'll probably encounter the equipment in large cities and at teaching hospitals.
Here's a toast to your heart! May it have a long, happy string of beats ahead of it. But if your luck fails, here's another toast to the imaging technologies that will help physicians diagnose and treat your problem.