Application of Spatial Modulation of Magnetization (SPAMM) to Children: The Effect of Image Resolution on Tagging Pattern

Abstract

Spatial modulation of magnetization (SPAMM) is a valuable magnetic resonance imaging technique for studying ventricular biomechanics. In order to track the intersection points of the stripes to calculate regional wall motion and strain, the stripe spacing should be at most half the wall thickness, yet sufficiently larger than the image pixel size in order that the stripes be well resolved. These conflicting requirements, that the grid spacing be much smaller than the wall thickness yet much larger than the pixel size, are relatively easy to meet in adult subjects but are difficult in children because of their small size. The purpose of this study was to delineate the effect of pixel size relative to SPAMM grid spacing on the stripe pattern produced by SPAMM with application towards its use in children and to present a new approach to the analysis of these images. We performed SPAMM imaging on a 1.5 Telsa Siemen's Vision MR system on a phantom, using an artificial ECG (R-R interval = 450 ms for triggering), holding the pixel size constant and varying the degree of stripe spacing. We used both square (1 mm) and rectangular (1 mm by 2 mm) pixels. We express the ratio of grid to pixel size as the ratio of the center-center spacing of the grid lines to the horizontal pixel size. We retrospectively reviewed the SPAMM images on 10 patients with a ratio of grid to pixel size approximately 4:1 and 10 with a ratio of grid to pixel size approximately 8:1. We further performed SPAMM imaging in four patients with different grid to pixel size ratios in the same patient. Finally, we tested a new algorithm to track the signal intense regions rather than the signal poor intersection points of the grid lines, which were compared on three ventricles with SPAMM tagging. In a phantom, the effect of decreasing the separation between stripes while keeping the resolution of the image constant changed the stripe pattern from a series of two parallel lines perpendicular to each other to a "checkerboard" pattern. With a relative grid:pixel ratio of 8:1 as used with adult studies, the dark bands and the crossing points are well defined. As the ratio decreases from 8:1 to 6:1, the black band is less well resolved. When the resolution is reduced further to a grid:pixel ratio of 4:1, the image appears to be a checkerboard of white and dark squares. This occurred with both square and rectangular pixels. The effect in vivo is similar. When the ratio is approximately 8:1, all patients demonstrated a stripe pattern as a set of parallel lines perpendicular to each other. When the ratio was approximately 4:1, all patients demonstrated the stripe pattern as checkerboard. This was found to be the case in the same patient, whether varying the pixel size by changing the field of view or matrix or by changing the grid spacing. We also found that tracking the signal intense regions was equivalent to tracking the signal poor intersection points of the grid, and this approach was much easier to implement. With decreasing ratio of grid spacing to pixel size, SPAMM stripe patterns change from a set of parallel lines perpendicular to each other to a "checkerboard" pattern. This effect has implications for tracking techniques to determine strain and wall motion. At smaller ratios, as is needed sometimes in children, it is easier to track the signal intense regions rather than the "intersection" points of the stripes. Both these approaches to tracking are equivalent.