This paper presents a new power efficient asynchronous multiplexer(MUX) for application in analog front-endelectronics(AFE) used in X-ray medical imaging systems. Contrary to typica lsynchronous MUXes that have to be controlled by a clock, this circuit features a simple structure, as the clock is not required. The circuit dissipates power only while detecting the active signals and then automatically turns back to the power down mode. Medica limaging systems usually consist of several dozen to even several hundreds of channels that operate asynchronously. The proposed MUX enables anunambiguous choice of the active channel. In case of two or more channels that become active at the same time the MUX serializes the reading out data from particular channels. This characteristic leads to 100% effectiveness in data processing and no impulses’ loss. The proposed MUX together with an experimental readout ASIC has been implemented in the CMOS0.18 mm process and occupies 1100 mm2/channel area. It works properly in a wide range of the voltage supply in between 0.8 and 1.8 V. Energy consumed during the detection of one active channel is below 1pJ, while the detection time is about 1ns.