Embodiments of the present invention relate to implantable systems, and method for use therein, that can detect T-wave alternans. When a projection pattern placed in the first plane and having a dark pattern and a light pattern around the dark pattern is projected onto the second plane, an average illuminance in a area where a projected image of the dark pattern is formed in the second plane is 8 or less, where an illuminance of an image of the light pattern around the dark pattern in the second plane is set to be 100. A square assembly includes a top wall and a bottom wall attached together in a spaced configuration with respect to each other.