Quantitative ultrasound assessment of ultrasound therapy in rodent mammary tumors: In vivo and ex vivo results

Abstract

Abstract-High-intensity focused ultrasound (HIFU) is a promising means of non-invasive therapy for the treatment of tumors. Monitoring and assessment challenges for HIFU therapy remain, however, and ultrasound is under investigation to accomplish non-invasive treatment feedback. Quantitative ultrasound (QUS) was investigated for acute HIFU therapy assessment as well as temperature monitoring in a rat model mammary tumor both in vivo and ex vivo. In the in vivo study, 40 rodent mammary tumors were exposed to focused ultrasound produced by a 1-MHz single-element transducer (f/1.1) at three spatial-peak temporal-average intensity levels (335, 360, and 502 W/cm 2 ). Ultrasound assessment scans were performed on each tumor before and again immediately after HIFU exposure using clinical (Ultrasonix L14/5, 3-8 MHz) and small-animal high-frequency (VisualSonics MS-200, 6-14 MHz) ultrasound systems.For comparison purposes, twelve additional tumors were excised and scanned using a 20-MHz single element transducer before and again immediately after treatment in a 60 ºC saline bath for 30 minutes to determine both scattering and attenuation changes with thermal insult. These treated tumors were again scanned at elevated temperature (40, 45, and 50 ºC). For the excised tumors, a bandwidth of 7 to 25 MHz was used for backscatter analysis.Backscatter coefficient (BSC) and integrated backscatter coefficient (IBSC) estimates were generated in vivo for each tumor, both before and after treatment. A statistically significant (p<0.05) difference was observed in the change in IBSC with treatment for the first exposure group (335 W/cm 2 ) compared to controls. In ex vivo tumor samples a statistically significant difference (increase) in BSC was observed between pre-treatment and post-treatment scan data. BSC was observed to decrease with increasing tumor temperature. These changes were not statistically significant, however. These studies demonstrated increases in the BSC in rodent mammary tumors with therapy in both ex vivo, water bath exposures and in vivo HIFU exposures.