Characterization of a digital microwave radiometry system for noninvasive thermometry using a temperature-controlled homogeneous test load

Abstract

Microwave radiometry has been proposed as a viable noninvasive thermometry approach for monitoring subsurface tissue temperatures and potentially controlling power levels of multielement heat applicators during clinical hyperthermia treatments. With the evolution of technology, several analog microwave radiometry devices have been developed for biomedical applications. In this paper, we describe a digital microwave radiometer with built-in electronics for signal processing and automatic self-calibration. The performance of the radiometer with an Archimedean spiral receive antenna is evaluated over a bandwidth of 3.7-4.2 GHz in homogeneous and layered water test loads. Controlled laboratory experiments over the range of 30-50 degrees C characterize measurement accuracy, stability, repeatability and penetration depth sensitivity. The ability to sense load temperature through an intervening water coupling bolus of 6 mm thickness is also investigated. To assess the clinical utility and sensitivity to electromagnetic interference (EMI), experiments are conducted inside standard clinical hyperthermia treatment rooms with no EM shielding. The digital radiometer provided repeatable measurements with 0.075 degrees C resolution and standard deviation of 0.217 degrees C for homogeneous and layered tissue loads at temperatures between 32-45 degrees C. Within the 3.7-4.2 GHz band, EM noise rejection was good other than some interference from overhead fluorescent lights in the same room as the radiometer. The system response obtained for ideal water loads suggests that this digital radiometer should be useful for estimating subcutaneous tissue temperatures under a 6 mm waterbolus used during clinical hyperthermia treatments. The accuracy and stability data obtained in water test loads of several configurations support our expectation that single band radiometry should be sufficient for sub-surface temperature monitoring and power control of large multielement array superficial hyperthermia applicators.

Figure 1

Illustration of a single DCC element with radiometric receive antenna used in the hyperthermia treatment of superficial tissue disease.

Figure 2

Basic block diagram of MMTC microwave radiometer with microcontroller and inbuilt digital electronics.

Figure 3

Experimental setup of the receive antenna mounted above the dual chamber waterbath with the test load maintained at stable uniform temperature inside the individual chambers.

Figure 4

Radiometer receive antenna inside the vertically stacked dual chamber waterbath with the load maintained at known uniform temperature.

Figure 5

Radiometer measurements of the homogeneous dual chamber test load with steep temperature gradient across the Mylar film separation.

Figure 6

Radiometer measurements as a function of homogeneous water load temperature over 3.7–4.2 GHz frequency band.

Figure 7

Radiometer brightness temperature measurements taken inside the 42.6°C upper waterbath as a function of distance from the bottom test load at a) 45°C and b) 37°C steady state temperatures.

Figure 8

Radiometer brightness temperature measurements of the homogeneous test load with the spiral receive antenna sensing through a 6mm thick water bolus layer maintained at 43°C.

Figure 9

Radiometer brightness temperature measurements across the dual chamber waterbath load following a single system calibration at the beginning of the measurement series.

Figure 10

Radiometer sensitivity to EMI over 3.7–4.2 GHz frequency band for a step change in temperature across the test load taken inside an EM shield room illuminated by fluorescent (-x-) and incandescent (-o-) lights

Figure 11

Radiometer measurements of the test load inside unshielded clinical hyperthermia treatment rooms with fluorescent lights off.

Figure 12

Probability denisty functions of the radiometer measurement error (e) for homogeneous test load obtained using Eqn (4); a) p(μ_e); b) p(σ_e).