Imaging nanoscopic features with Scanning Electron Microscopes (SEMs) requires rapid specimen scanning with a low-energy electron beam. The electron detector is highly pixelated. Each pixel is interfaced with a high-precision, widebandwidth, low-noise readout integrated circuit (ROIC), to enable single-electron counting operation. This paper introduces an innovative power-efficient pixel readout frontend architecture, achieving a time resolution of 2.5 ns. The fabricated prototype in 40 nm CMOS process demonstrates better than 6 ppm electron detection precision. It consumes only 200 μ W, with an area of 150 μ m × 100 μ m.