What is ‘tagging’?
What is tagging’?
Craig Hamilton, PhD, assistant professor of radiology at Wake Forest University Baptist Medical Center in Winston-Salem, NC, describes tagging this way: "Imagine drawing two parallel lines on your skin with a ballpoint pen. If you flex your hand, the lines will move. That’s the way tagging works." It superimposes lines or strips on the image of the heart muscle that originate as a signal from the heart. As the heart beats, the lines move with the muscle, and you can measure the distance between the lines and get a quantitative view of how the muscle is contracting. HARP is a way of extracting the measurement information quickly.
"Tagging has been available to us for some time, and we’ve done a good bit of it," explains Hamilton, "but the problem is, once you get those little lines on there, how do you measure them? You could sit there with the mouse and measure those links all day long. HARP will be a clinically useful tool because it semi-automates that process so measurement results can be available quickly and diagnoses can be made in a reasonable time."
Nael Osman, a doctoral candidate at Johns Hopkins University in Baltimore and co-inventor of HARP MRI, (see article, p. 135) says that the tagging process (in which a computer marks locations on heart muscles then tracks their movements) produces several regions of energy concentration in the spectrum of the MRI, called the k-space.
By using bandpass filters that are tuned to some of these spectral peaks, they produce a complex image. It has magnitude and three phase images, called the harmonic image, harmonic magnitude image, and harmonic phase image, or HARP. The HARP images have a simple relation to the underlying motion, and several methods have been developed to measure this motion.
"One of these methods measures the local strain by observing the difference in the HARP values between neighboring points," Osman explains. "This way, we can produce maps of the heart that show the cardiac function through a cross-section of the heart. What is significant about all these computations is that they take less than 10 seconds."
[Editor’s note: See http://iacl.ece.jhu.edu/ projects/HARP on the Internet and scan down to Figure 10, which shows a circumferential strain map. Depicted are 20 time frames of a mechanically activated canine heart in cross section. The circle in the middle is the left ventricle. The darker regions indicate circumferential shortening, while the yellowish color indicates stretching. Starting from the second time frame, you can observe darker spots at the position of the pacing lead, 5 o’clock, and stretching (yellow) at the 10 o’clock region.
Concerning this, Osman says, "The heart wall thickens, and there is shortening in the circumferential direction of the heart wall. Darkening means shortening; yellow means stretching, which is abnormal. The fifth column of circles has yellowing which indicates stretching, and that would be present in an adult heart that is abnormal. If there were infarction, for example, there would be a lighter region like this. That shows where the problem is."
The plots at that Web site show how contractions evolve with numbers — how strain is changing in the heart.]
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