The promising minimally invasive laser thermal therapy technique may be improved if thermal lesions induced into the tissue can be carefully monitored in extension and morphology during the treatment. According to results obtained in several recent experimentations, solutions that avoid tissue carbonization during the treatment have been proposed, in order to allow deeper and longer lasting light penetration in treated tissue and to reduce failures of the applicator tip and fiber optic, dangerous for patients. In the work the advantages in using a cooled fiber are shown, in order not only to induce efficient lesions but also in performing an accurate monitoring by ultrasound. Indeed, one important limit of the ultrasound control is caused by the gas bubbles generation, which represent an acoustic barrier that invalidate the ultrasonic image representation of the treated tissue. Ultrasonic radiofrequency signals were acquired from the same bovine liver ex vivo sample by using both bare and cooled fiber and processed to produce B-mode and spectral parametric images by implementing TUV (Thermotherapy Ultrasonic View) algorithm. Radiofrequency signals, B-mode and TUV images were analysed and compared in order to evaluate the different tissue heating processes during ablation and the different lesion extensions induced into the tissue after the treatment. Cooled fiber avoided carbonization and strongly reduced gas bubbles generation inducing a larger lesion and allowing a more effective ultrasound monitoring. Moreover by correlating optical images of the lesions and the corresponding Integral TUV images, by using Dice and Jaccard coefficients, it was proven that TUV algorithm is able to characterize the tissue portions differently modified by ablation exhibiting better performances in the case of cooled fiber and revealing to be a potential tool capable to improve the laser delivery settings control.