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Heart imaging gets faster, clearer

Heart imaging gets faster, clearer

For decades, heart disease was the leading cause of death in the U.S. By some statistics, it accounted for more deaths among persons 25 and older than most other natural causes of death combined, according to the Center for Disease Control and Prevention, National Center for Health Statistics. For the elderly, it remains a potent killer.

While cutting down on the donuts and increasing regular exercise have definitely helped reduce heart disease, advances in imaging technology help doctors non-invasively diagnose many types of heart disease early. Early diagnosis often prolongs lives that would otherwise be cut short.

Ultrasound is valuable in the diagnosis of cardiovascular disease by using acoustical energy for producing anatomical two-dimensional images of the heart and blood flow dynamics in heart chambers.

The latest ultrasonic and computed tomography (CT) technologies described below detect a wide range of heart disease symptoms, including pericardial effusion, valve problems, cardiomyopathy, and plaque build-up. Benefits of the ultrasound advances begin in the exam room, where doctors now see signs of heart disease sooner with portable ultrasound devices enabled by application-specific integrated circuits (ASIC). Once signs are detected and a patient advances to a more thorough examination, new CT machines take multi-slice scans and reconstruct them, providing better imaging quality that helps heart specialists definitively diagnose heart problems.

Seeing heart disease sooner. Agilent Technologies (Andover, MA) is introducing a point-of-care ultrasound (PCU) device this year called OptiGo for use in settings where physicians directly interact with patients. Doctors use these images for assessing the heart's anatomy and blood flow and determining if further study is required.

"OptiGo gives us the ability to get precise information on the function of the heart almost immediately," says David Liang of the Stanford University Medical Center. He explains that he uses it to include diagnoses that would not have been explored because the tests were too expensive or inconvenient. "This should reduce the number of times that some of the rare diagnoses are missed."

Designed specifically for imaging the heart, the Optigo has a true phased array transducer, consisting of piezoelectric elements that transmit and receive acoustical energy for producing anatomical two-dimensional images of the heart as well as color images showing blood flow dynamics in heart chambers. True phased array uses one processing channel for each element of the transducer. "This permits better image quality than using fewer processing channels," says Agilent Design Engineer Steve Leavitt.

There were a number of design challenges early on, explains Leavitt. "As a portable product, it had to have a small size and low weight. The user interface had to be simple and intuitive. And it needed ac or battery operation," says Leavitt. "But first and foremost, it had to exhibit excellent image quality."

One of Leavitt's primary concerns was the number of channels the device would have because it directly impacts image quality, size, and cost. A phased array imaging system starts with a phased array transducer made up of a single crystal cut into many elements. Each element behaves like a single crystal and feeds a dedicated processing channel within the front-end electronics of the system. The individual channels are formed into a single signal for further processing by the back-end electronics prior to display.

The major electrical design with OptiGo involves three integrated ASICs: one analog front-end ASIC and two unique back-end ASICs. They perform amplification, filtration, beam formation, detection, scan conversion, memory management, and video display functions. In the past, these functions were performed by separate board-level designs. Optigo relies on only one system board. Leavitt says using a common memory was also important to the design's success. "Instead of having each functional area of the design manage its own localized memory, a common memory was used for all functions and managed by one memory controller."

For the transducer, which plays an important role in meeting cost goals, Agilent engineers developed a low-cost connector with an integral printed circuit assembly. "The transducer's connector allows it to be repaired or exchanged separately from the system, thereby reducing future support costs," explains Leavitt.

All energy transmitted into and received from the patient is centered around the 2.5 MHz frequency, which is within the limited range to which the human body is receptive according to Marienne Sanders, a transducer project engineer with Agilent. The transducer converts mechanical vibrations received into electrical signals. It also converts electrical stimulation into mechanical vibrations for the transmit path. "It is coupled to the system via microaxial cables and a removable connector based on a laptop docking station," says Sanders.

Images are obtained in real time. A 300 MHz power PC is the main processor.

Cleaner images of heart. Another weapon in the arsenal of technology for fighting heart disease is CT. Like ultrasound, it's non-invasive. Although CT has been around for 20 years, it hasn't been used for diagnosing heart disease until now, according to David He, Manager of Global CT Cardiology for GE Medical Systems (Waukesha, WI).

In the past, CT's limitation was that it did not provide the resolution required for detecting diseases of the heart. Most adult coronary arteries measure between 2 and 5 mm in diameter, but CT scans were acquired in 5- to 10-mm slices. "Our challenge was getting image quality down to 2 mm," says He. "That's the level at which clinicians see coronary artery calcification," he says. Calcified plaques show up as bright spots in CT images. Non calcified plagues are darker.

"Unlike other organs, the heart is always in motion and it's more difficult imaging objects that are in motion," notes He. Typically, a CT detector on a tube assembly is rotated 360 degrees around the patient to get a 3-D image, one slice at a time. GE Medical Systems engineers changed that concept by using a new digital Lightspeed multi-slice detector technology that simultaneously acquires four continuous slices of the heart at a time. The result: imaging detail in 1- to 2.5-mm slices. The enabler for the detector is a polycrystalline ceramic from GE that molds into a uniform 1.25- x 1.00-mm matrix design.

Single-slice CT systems collimate the x-ray beam to the desired slice thickness and focus on the center of the detector arc. With the LightSpeed matrix detector, the x-ray beam is collimated to a wider thickness and focussed on the central 4, central 8, central 12, or all 16 rows of detectors.

"Image slice thickness is a reflection of z-axis spatial resolution," says He. The number of detector rows employed depends on the image slice thickness selected. For each imaging rotarion four 2.5-mm slices are used.

Single-slice CT can image continuously in helical scan modes, but it takes longer than multi-slice technology. "Due to limitations in scan coverage time, it was impractical to scan patients with high z-axis resolution to cover most of the anatomy of interest," He says. "With multi-slice CT, that limitation has been removed."

With each gantry rotation, multi-slice helical scanning acquires four interweaving helices. Then, reconstruction algorithms arrange the helices into the "slices" that the radiologist uses for evaluating the patient.

The scanner's 0.5 sec scan time provides the quick 250 msec temporal resolution needed for imaging dynamic cardiac anatomy and coronary arteries.

"What's driving future designs is the ability to quantify plaques and determine which ones are unstable," says He.

Deaths attributed to heart disease*
Year 70 80 85 90 94 95 96 97
All causes 714 586 549 520 507 504 491 479
Diseases of the heart 254 202 181 152 140 138 135 130
Ischemic heart disease 150 126 103 91 90 87 83
*Deaths per 100,000 resident population, all persons since 1970
Source: Centers for Disease Control and Prevention, US Department of Health and Human Services
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