Function of Heart's Helical Band Visualized in New MR Imaging Technology

Scientists have created images of the heart's muscular layer that show, for the first time, the connection between the arrangement of those muscles and the manner in which the human heart contracts. More precisely, they showed that the muscular band, which wraps around the inner chambers of the heart in a helix, is a kind of twisting thoroughfare along which each contraction of the heart travels.

Medical specialists have long have known that the heart's beat is not a straightforward in-and-out movement--that it has more than a little bit of a twist to it. "The heart twists to push blood out the same way you twist a wet towel to wring water out of it,” explained Dr. Morteza Gharib, the lead investigator on the study, and professor of aeronautics and bioengineering in the division of engineering and applied science at the California Institute of Technology (Caltech; Pasadena, USA).

Using a technique pioneered by Dr. Han Wen and his team at the U.S. National Institutes of Health (Bethesda, MD, USA), Dr. Gharib and his colleague Abbas Nasiraei Moghaddam, a Caltech graduate and visitor in bioengineering, were able to create some of the first dynamic images of normal myocardium in action at the tissue level. "We tagged and traced small tissue elements in the heart, and looked at them in space, so we could see how they moved when the heart contracts,” Dr. Gharib explained. "In this way, we were able to see where the maximum physical contraction occurs in the heart and when--and to show that it follows this intriguing helical loop.”

Characterization of local and global contractile activities in the myocardium is needed for a better determination of cardiac form and function, according to the investigators. The spatial distribution of regions that contribute the most to cardiac function play an important role in defining the myocardium's pumping parameters, ejection fraction and dynamic aspects such as twisting and untwisting. In general myocardium shortening tangent to the wall, and ventricular wall thickening are important parameters that characterize the regional contribution within the myocardium to the global function of the heart.

The researchers have calculated these parameters utilizing myocardium displacement fields that were gathered through displacement encoding with stimulated echoes-magnetic resonance imaging (DENSE-MRI) technique in three volunteers. High spatial resolution of the acquired data revealed transmural changes of thickening and tangential shortening with high fidelity in beating hearts. By filtering myocardium areas that demonstrated a tangential shortening index of less than 0.23, the investigators were able to identify the complete or portion of a macrostructure composed of connected regions in the form of a helical bundle within the left ventricle mass.

With each beat of the heart, a wave of contraction starts at the heart's apex--which, despite its name, is actually at the very bottom of the heart--and then travels up through the myocardium. "The only time the whole helix shows up in the images is at the end of systole, which is when the heart is contracting,” noted Dr. Gharib. "This simple band structure is akin to an engine behind the heart pumping action.”

In addition to going a long way toward resolving the decades-long structure/function dispute surrounding Torrent-Guasp's work, this finding also has major implications for the surgical treatment of heart disease, according to Dr. Gharib. "It's going to change the way we repair the heart,” he explained. Knowing that the contractile wave travels along the helical pathway--instead of occurring throughout the heart all at once--has implications for which parts of the heart will be most vulnerable to a surgeon's scalpel, for instance. Seventy-five percent of the function of the heart depends on this muscle. Surgeons now know what to cut and what not to cut. This will help them to come up with new and more effective surgical procedures.”

The study was published in the December 2008 issue of the journal AJP Heart and Circulatory Physiology.

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