HARP MRI provides clear pictures of heart problems
A shortcut to processing tagged’ images
Currently, it is too costly and time-consuming to use magnetic resonance imaging (MRI) coupled with cutting-edge tracing or "tagging" technology to examine the heart during a typical 20-minute cardiac stress test, says Nael Osman, a doctoral candidate at Johns Hopkins University in Baltimore. (For a definition of tagging, see box, p. 136.)
That’s unfortunate, he says, because when images are tagged, they can provide highly detailed heart data. But the additional image processing can take several hours — too long for a screening test — so doctors rely instead on ultrasound or straight MRI and must infer from the movement of its overall shape if the heart is beating normally. As a result, isolated abnormalities occasionally escape detection.
To eliminate the delay, Osman and his advisor, biomedical engineer Jerry L. Prince, PhD, also at Hopkins, developed HARP (HARmonic-Phase) MRI that, the inventors say, will provide physicians with an effective tool to determine whether a heart disorder exists. The two have found an impressive shortcut for detecting abnormalities inside the muscle of the heart — a technique that could make routine cardiac stress tests more accurate and definitive.
"At this point, HARP is a method, not a system," explains Osman. "It is a work in progress, but we expect the technology to have a profound impact on diagnostic cardiology."
What the two engineers have done is devise a method for analyzing detailed tagged images of a diseased heart within minutes rather than hours. The images show the condition of heart muscles and where heart damage has occurred. Prince, a professor in the Whiting School’s department of electrical and computer engineering, and Osman, his student, discussed their research at a recent meeting of the International Society for Magnetic Resonance in Medicine in Philadelphia and have applied for a patent. They believe HARP MRI’s most promising use will be in cardiac stress testing.
Because it is so time-consuming, doctors do not use MRI tagging routinely during cardiac stress tests. Rather, they use straight MRI or ultrasound, if possible, to monitor the heart during the test because both methods provide immediate feedback. "Ultrasound assesses heart function quickly, but it has problems. MRI provides better images and higher resolution, so we can see global function — how the heart is doing as a whole, such as ejection fraction — and muscular function," explains Osman. "For example, some regions of the heart could not be working well due to infarction and ischemia." But neither method is as precise as clinicians would have them, and it becomes up to them to judge function. There are often discrepancies between the different opinions of doctors. With MRI, you get numbers that tell you exactly what’s going on, but getting the numbers involves tagging.
HARP presents a quantitative measure rather than just a visual assessment, but it still has a way to go before it reaches clinical usefulness, says Craig Hamilton, PhD, assistant professor of radiology at Wake Forest University Baptist Medical Center in Winston-Salem, NC. He says that besides HARP, ejection fraction already is available to give a measurement of heart performance, but it doesn’t give the full picture. "But the key factor to identify is, which part of the heart muscle is having trouble. Then you know which coronary artery is potentially blocked. The visual assessment goes to which part of the muscle is having trouble."
HARP vs. standard techniques
Initial tests have shown that HARP quickly produces images equivalent in quality to those that take six to eight hours to process with older technology. And the researchers say they believe the images that the system now provides in a few minutes will be available in real time in the near future. The current HARP MRI images depict two-dimensional slices of heart muscle; the engineers are working to create three-dimensional views. The method now focuses on muscle activity only in the heart’s left ventricle; eventually, the system may be used to evaluate other portions of the heart.
HARP MRI is being tested at Johns Hopkins Hospital, and preliminary results are encouraging. "Basically," explains Osman, "it employs an algorithm that overcomes the drawbacks of other existing techniques that prevented cardiac MR tagging from being used clinically. It is a new way of looking at tagging that would enable us in the near future to monitor regional cardiac function in real time." Tagging, he says, puts features inside the heart while it is being imaged that move with the heart — they indicate how the muscle is doing in each region. "The tags are used to quantify function — how much contraction there is." The process has been available for 10 years, but mainly in the research arena. "That is because it took a long time to do the mathematics to measure the function."
Currently, the technique does the computations within minutes from the time the images are acquired — before the patient leaves the scanner. With continued research, the team hopes to capture real-time heart function data while the patient is still inside the scanner and breathing. "It’s a problem for patients to hold their breath while imaging," says Osman, "and this faster method will help that. We will be able to image while the patient is breathing in real time." The inventors are currently working on modifying the MRI scanner to achieve higher speed of image acquisition and predict that they would achieve some of this real-time imaging in the very near future.
Computerized guidance
Part of the goal is better patient diagnosis, but another thrust is toward computer-integrated surgery and minimally invasive operations, where more insightful imaging is required. "Imaging should not be qualitative, but quantitative," Osman says, "so computers can digest the situations and plan or guide the surgeries."
HARP MRI depicts the internal workings of the heart muscle to reveal trouble spots that might otherwise go unnoticed. Its developers arrived at their invention after discovering a particular signal produced by the beating heart in MRI videos. To make a heartbeat video, a sequence of MRI images is tagged with markers that, when played in sequence, show the expanding and contracting muscle in such detail that it is possible to determine the mechanical strain in the muscle wall. Processing these tagged images can take several hours, but Prince and Osman found a shortcut. They discovered that the motion of the tagged heart itself produces a signal, and this signal can be decoded to render the same high resolution images as those made through conventional image processing, only much faster.
HARP MRI produces a view of the left ventricle from which doctors can clearly see if parts of the heart muscle are not flexing normally. "No other modality gives you a picture like this except MRI," says Osman. When coupled with a stress test, the new method can quickly indicate whether to prescribe a modified lifestyle regimen or surgery.
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