The appearance of non-blood cells circulating in human peripheral bloodstream indicates an abnormal condition. One important category of these cells is circulating endothelial cells (CECs) shed by compromised blood vessels. Clinical applications that measure the blood level of CECs are hindered due to a lack of standardized instruments. The major challenge in detecting circulating non-blood cells is their extreme scarcity; 1 in to . Described here is a new method for detection of rare cells in blood samples deposited on the adhesive microscopic slides and immunostained with distinct fluorescent markers. The key novelty of the proposed approach is an intelligent search principle and a dual-mode scanner to implement this principle. To begin, a fast scanning that uses a single beam is performed in the spectral channel where only rare cells produce florescence. Once a target cell is registered, the scanner switches on the imaging mode, auto-focuses and then records images in multiple spectral channels at the selected area. The instrument runs in repetitive cycles until the entire slide is scanned. The technology has been validated via detection of human umbilical vein endothelial cells spiked into human blood samples. In addition, the operational principle can be adapted for detection of other types of rare cells in blood.