R. M. Arthur, "Temperature Imaging Using Ultrasound", in Physics Of Thermal Therapy, Fundamentals and Clinical Applications, Editor: E. G. Moros, Series: Imaging In Medical Diagnosis And Therapy (William R. Hendee, Series Editor), Taylor & Francis, in press, 2012.
Ultrasound is an attractive modality for volumetric temperature imaging to monitor thermal therapies because it is non-ionizing, portable, convenient, inexpensive, and has relatively simple signal-processing requirements. This modality has proven useful for estimation of temperatures from the hyperthermia range (41-45oC) to border zones of regions of high-temperature ablation (> 60oC).
The most prominent methods for exploiting ultrasound as a non-invasive thermometer rely on either 1) echo shifts due to changes in tissue thermal expansion and speed of sound (SOS), 2) variation in the attenuation coefficient, or 3) change in backscattered energy from tissue inhomogeneities. Each method has its strengths in terms of temperature range for which it yields a useful thermal signal and how well it can handle tradeoffs between temperature accuracy and spatial resolution.
The use of echo shifts has received the most attention in the last decade. By tracking scattering volumes and measuring the time shift of received echoes, investigators have been able to estimate temperature with encouraging preliminary in vivo studies. Acoustic attenuation is dependent on temperature, but with significant changes occurring only at temperatures above 50oC. This property may lead to further development of its use in high-temperature thermal ablation therapy. Minimal change in attenuation, however, below this temperature range reduces its attractiveness for use in clinical hyperthermia.
The change in backscattered energy is scatterer dependent. Taking advantage of scatterer-dependent behavior enhances the thermal signal. This behavior has been matched with novel simulation methods for diverse scatterer populations and can be enhanced with stochastic signal processing methods. Monotonic thermal dependence of the change in backscattered energy has been measured to 60oC. Temperature maps with 1-2oC accuracy and 0.5 cm2 spatial resolution can be produced routinely during non-uniform heating in vitro.
All of the ultrasonic thermometry methods, just like temperature imaging from MRI, must be able to cope with motion of the image features on which temperature estimates are based. Echo shift methods track and exploit that motion. Motion must be compensated in attenuation and CBE thermometry. Motion tracking and compensation are usually the most computationally intensive components of ultrasonic temperature imaging and limit frame rates for temperature imaging.
Thermal therapies are poised for rapid development and advancement due in part to a shift to volumetric temperature imaging from sparse invasive thermometry, offering improved monitoring as well as feedback for improved therapy control. Noninvasive temperature imaging with ultrasound could better 1) monitor and guide both hyperthermia treatment and high-temperature ablation and 2) deepen our understanding of tissue changes during hyperthermia and in ablation border zones now performed blindly, with limited invasive thermometry, or more expensive fixed-installation MRI thermometry that may limit options for heating sources. A crucial step in identifying a viable ultrasonic approach to temperature estimation is its performance during in vivo tests. The potential for significant clinical impact of ultrasonic thermometry is imminent with minimal addition if any to the existing hardware of ultrasonic imaging systems.