We introduce an efficient microwave imaging system for medical applications, especially suited for breast cancer detection. The imaging system consists of a fixed source for transmit on one end, and a scanning, receiving sensor on the other, with the target placed in between. The image of the internal organ/limb tissue under examination is then obtained across a three-dimensional region, based upon the data collected by the scanning sensor, by use of our field mapping algorithm (FMA).Application of our FMA provides a full threedimensional image of tissue structure. Six test measurement cases are given by way of demonstrating FMA performance, with promising results evident throughout.