Monitoring the formation of thermal lesions with heat-induced echo-strain imaging: a feasibility study.
We investigated the feasibility of using echo-strain images to visualize the extent of high-intensity ultrasound (US)-induced thermal lesions during their formation. Echo-strain, defined as the relative deformation of the backscattered ultrasonic signal, is due to tissue expansion and to changes in the speed of sound during heating. First, a theoretical framework was developed to predict the influence of these effects on the echo signal. Then, a simulation tool was developed to create simulated echo-strain images in thermal lesions. Finally, experimental echo-strain images were acquired in 10 porcine liver samples in vitro for various exposure durations and ultrasonic intensities (resulting in lesions that extended 3 to 8 mm deep from the surface). For this purpose, radiofrequency (RF) frames were acquired at 8 frames per s while heating. For each consecutive pair of RF frames, an echo-strain image was calculated using standard elastographic processing. The echo-strain images were cumulated and displayed. The experimental echo-strain images were compared with gross pathology. The (isoechoic) lesions were visible both in simulated and in experimental cumulated echo-strain images as apparent expansion areas (tensile echo-strain), whereas surrounding tissues exhibited apparent compression. The tensile echo-strain area underestimated the lesion in simulations, but was representative of the lesion in experiments. High correspondence was found between the lesion depth measured from experimental cumulative echo-strain images (y) and from gross pathology (x) (Pearson's correlation = 0.90, linear regression y = x-0.1 mm, residual error = 0 +/- 0.9 mm). We hypothesized that significant tissue expansion made the thermal lesions highly visible in the experimental echo-strain images. In two cases, the ultrasonic intensity was too low to induce a lesion, and the corresponding experimental echo-strain images showed no visible lesion. We conclude that cumulative echo-strain images have the potential to monitor the formation of high-intensity US-induced thermal lesions.
Souchon
Rémi
R
INSERM UMR 556, Lyon, France. Souchon@lyon.inserm.fr
Bouchoux
Guillaume
G
Maciejko
Eva
E
Lafon
Cyril
C
Cathignol
Dominique
D
Bertrand
Michel
M
Chapelon
Jean-Yves
JY
D000465:Algorithms; D000818:Animals; D002417:Cattle; D005240:Feasibility Studies; D006358:Hot Temperature; D008099:Liver / Q000000981:diagnostic imaging* / Q000473:pathology; D008107:Liver Diseases / Q000000981:diagnostic imaging; D013314:Stress, Mechanical; D013552:Swine; D014159:Transducers; D014464:Ultrasonic Therapy / Q000295:instrumentation / Q000379:methods*; D014465:Ultrasonics; D014463:Ultrasonography