Enhancing Sensing Depth and Measurement Sensitivity of Microwave Tissue Thermometry Using Near-Field Active Array Probe

Microwave radiometer operating above 2.5 GHz is less susceptible to electromagnetic interference (EMI) from wireless communication devices but has limited sensing depth due to high-tissue attenuation properties. We overcome this limitation using a near-field active antenna array probe and demonstrate its performance using a 2.9-GHz switch-circulator Dicke radiometer. The near-field probe consists of 2 × 2 arrays of receive antennas with high-gain low-noise amplifiers (LNAs) and a power combiner integrated into a multilayer printed circuit board (PCB). The probe parameters and array configuration were finetuned for return loss > 30 dB, 200-MHz bandwidth, and 40-mm sensing depth at 2.9 GHz. The performance of passive and active probe prototypes was assessed using hot source of 12-16-mm diameters immersed at varying depths in 37 ^{\circ }\text{C} tissue phantom and maintained at a differential temperature of 1 ^{\circ }\text{C} - 5 ^{\circ }\text{C} above 37 ^{\circ }\text{C} . The measurement resolution of the active array probe is 0.18 °C compared to 0.52 °C and 0.22 °C observed for the passive and single active probes, respectively. A 14-mm hot source with 3 °C differential temperature at 45-mm depth that was invisible to the passive and single active probes was visible to the active array probe. Radiometer measurements demonstrate enhanced sensing depth and measurement sensitivity at 2.9 GHz for the near-field active array probe.