Metal-Semiconductor-Metal (MSM) photodetectors are attractive as sensors due to their ease of fabrication and compatibility with thin film transistor fabrication process primarily because there is no P+ doped layer. We report an a- Si:H MSM lateral structure with low dark current, high dynamic range and comparable sensitivity to conventional p-i-n photodiodes. These improvements are achieved by the introduction of a thin polymer layer as a blocking contact. The fabricated amorphous silicon based MSM detector exhibits a photo-response of more than 3 orders of magnitude to a green light source (λ = 525nm) with intensity of 73μW/cm2. In comparison to vertical p-i-n structures, the reported MSM lateral devices show gains in terms of dynamic range, ease of fabrication (no p+ layer), and faster speed at the cost of slightly reduced responsivity. The experimental results of dark and photocurrent measurements as well as the responsivity for two in-house fabricated MSM structures at different bias voltages and light intensity are presented. This results are promising and encourage the development of a-Si:H lateral MSM devices for indirect conversion large area medical imaging applications and especially low cost flat panel computed tomography.