Developing ultrasonic temperature imaging to aid cancer treatment
Hyperthermia is a cancer treatment in which tumors are elevated to temperatures toxic to cells (41–45° C).3 It has been used to augment the effects of chemotherapy and radiation. Unfortunately, lack of detailed temperature information to guide thermal therapy limits its broader use.1 A noninvasive, clinically useful method is needed to measure 3D temperature distributions to within 0.5° C in 1cm3 volumes.
Noninvasive temperature-estimation methods based on magnetic resonance imaging (MRI) and ultrasonic images have received the most attention.2,7 The required accuracy and spatial resolution can probably be achieved with MRI, but it is expensive and may be difficult to use in conjunction with some thermal therapies. On the other hand, ultrasound is inexpensive, nonionizing, portable, and convenient. It also has relatively simple signal-processing requirements. These qualities make it an attractive candidate for temperature estimation. Ultrasonic tissue properties affected by temperature include speed of sound, attenuation, and backscattered energy.
In addition, to measure changes in backscattered energy in the hyperthermia treatment range, conventional phased-array images were taken as a function of temperature.6,8 Tissue was heated in an insulated tank filled with deionized, degassed water. The temperature ranged from 37 to 45° C for measurements on nude mice in vivo and up to 50° C for specimens of bovine liver, turkey breast, and pork muscle in vitrio. The nude mouse image in Figure 1 highlights a region of interest including the leg, which had been implanted with an HT29 human colon cancer tumor.
Figure 1. Ultrasound image of a nude mouse in vivo at 37° C produced with a Terason 2000 system with a 128-element 7MHz array.
After motion compensation, CBE was calculated over the measured temperature range. Envelopes of motion-compensated image regions were found with the Hilbert transform, then smoothed with a 3×3 running average filter. Values were squared to determine the backscattered energy at each pixel.
Figure 2. Change in backscattered energy (CBE) in ultrasound images of bovine liver from 37 to 50° C after compensation for apparent motion. All images were referred, pixel-by-pixel, to the energy in the reference image at 37° C. Each colorbar is in dB.
R. Martin Arthur is the Newton R. and Louisa G. Wilson Professor of Engineering at Washington University. His current research interests include synthetic-focus methods for image improvement and tissue characterization using medical ultrasound
Jason W. Trobaugh is a research instructor in the School of Medicine and a research associate in the School of Engineering and Applied Science at Washington University. His research interests are in the fields of model-based image analysis, probabilistic image models, and treatment guidance applications for ultrasonic imaging.
William L. Straube is a research associate professor at the Washington University School of Medicine. His interests include developing and applying new thermal therapies.
Eduardo G. Moros is director of the Division of Radiation Physics and Informatics at the University of Arkansas. His main interest is the development of imaging/therapy technology to advance the treatment of cancer with heat and ionizing radiation.