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(TM) 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