Why are values for systolic function different between LV long axis and LV short axis M-mode images? Which image should I use to calculate LV systolic function?

​Long and short axis formulas make different geometric assumptions about the heart that can be reflected in your data. They will also represent different directional contributions to that parameter. For example, Fractional Shortening (FS) derived from a short axis M-Mode image will reflect the radial contribution to FS, whereas, the same parameter derived from long axis B-Mode will reflect the longitudinal contribution to FS. 

We recommend choosing one method and being consistent with your data analysis. ASE guidelines recommend the B-mode long axis view, but many publications use the M-mode short axis for systolic calculations. Depending on your animal and disease model, some methods (and some geometric assumptions) will be more appropriate than others. We recommend looking into what others in your field have published as a guide.