Introduction
Strain encoding is the third type of encoding that can be done with MRI. In this case, the strain can be immediately encoded into the acquired images without measuring the displacement or velocity first. The technique does not measure the strain in arbitrary direction, but in a specific direction relative to the imaging plane. Similar to the previous two encoding, we can encode the regional strain in an image so that regional function can be assessed. There is one technique that can be used to do the strain encoding, which is called SENC. The encoding with SENC differs from the previous displacement and velocity encodings in that strain is not encoded in the complex phase of an image.
Strain encoding share the same essence of MR tagging. SENC encodes the tissue with patterns similar to tagging but produces images whose intensity varies according to regional contractility of the myocardium. The exact relation between the intensity and regional strain can be controlled by changing some of the imaging parameters. For example, it is possible to make reduce the signal intensity at some region as a result of contraction and vice versa: make it brighter with contraction. Because of these changes in intensity, regional dysfunction can be easily viewed (see figure) on the SENC images. The raw SENC images can be further processed to produce anatomical images with coloring that indicate the strain distribution. In Figure ?, the red coloring indicates the contraction in the longitudinal direction of the heart. As can be seen, the myocardial infarct (MI) of the patient stays white, indicating dysfunction. The MI was verified by the delayed contrast enhancement image. SENC has a number of limitations versus MR tagging (HARP) and DENSE. First, it does not low the tracking of the motion of the heart. Because it only encodes the strain (contractility), the exact motion of the tissue cannot be followed. Second, SENC provides strain measurements only in the through-plane direction. Therefore, in case of short-axis images, only the longitudinal compression of the myocardium from base to apex is measured. On the other hand, circumferential shortening of the myocardium can be measured in the long-axis views of the heart (such as the four-chamber view). The advantages of SENC include its simplicity, as images of strain can be directly produced real-time. Also, SENC has higher in-plane resolution of strain than MR tagging, which can be very useful in assessing regional function of the right ventricle (RV) free wall.
Read more about this topic: Strain Encoding MRI
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