A method of imaging within an absorptive object comprising: placing transmit and receive antennas in close proximity to a surface of said object; transmitting electromagnetic pulses from the transmit antenna into the object; and receiving a receive signal at the receive antenna simultaneously with the transmit antenna transmitting said pulses; wherein the transmitting and receiving comprises the following steps: a) setting a threshold level for the receive signal strength; b) transmitting one or more pulses; c) comparing the receive signal for said one or more pulses with the threshold level; d) changing the threshold level; e) repeating steps b), c) and optionally d) one or more times. This arrangement can operate at extremely high speed due to the absence of any slow multi-bit ADCs.