Time and temperature dependence of MR parameters during thermal coagulation of ex vivo rabbit muscle

Abstract

Detailed measurements of the T1-weighted, T2-weighted, and MT-weighted signal were performed for ex vivo muscle samples heated to various temperatures for different times. Consistent, monotonic increases in signal intensity were observed with progressive thermal coagulation, corresponding to an increase in T2 relaxation time and an increase in MT-weighted signal for temperatures above 60 degrees C. The relationship for T1 relaxation was more complex, showing a decrease in T1 relaxation from 40 to 60 degrees C and an increase above 60 degrees C. These techniques provide a more direct measure of tissue thermal coagulation than that provided by MR thermometry and suggest MR imaging strategies for the optimization and monitoring of thermal coagulation therapy protocols that create thermal damage in target tissues.

Simultaneous temperature and magnetization transfer (MT) monitoring during high‐intensity focused ultrasound (HIFU) treatment: Preliminary investigation on ex vivo porcine muscle

“…This is particularly important for in vivo HIFU treatment, where subject motion could invalidate the TD calculation. One important issue that remains unanswered by this study is the observed trend of MTR increase in muscular tissues following HIFU treatment, which is opposite to the elegant results shown in the articles by Graham et al (11,34). To the best of our knowledge, the only investigations documenting postheating evolution of MT characteristics have been conflicting.…”

“…To the best of our knowledge, the only investigations documenting postheating evolution of MT characteristics have been conflicting. Whereas Graham et al (11,34) used a water-bath heating method and found a decrease of the MT effect on a comprehensive set of tissues, the data from Carraso et al (21) by means of temperature control via the built-in NMR facility (in vitro) and MR-guided HIFU (in vivo) indicated an enhancement in the exchange rate and hence might suggest increased MTR in porcine muscle and rat muscle. Whether these results could be extrapolated to the data obtained from our HIFU experiments is currently unclear due to the fundamental difference in the heating procedure.…”

“…Whether these results could be extrapolated to the data obtained from our HIFU experiments is currently unclear due to the fundamental difference in the heating procedure. In particular on T2-weighted images alone, muscular tissues following HIFU ablation are known to exhibit rather heterogeneous distribution of the signal intensity (36), sometimes demonstrating a relative hypointense core corresponding to coagulation necrosis surrounded by a hyperintense rim of disruption vacuoles (35), unlike the findings of consistently increased intensity on T2-weighted images paralleling that on MT-weighted images after water-bath heating as reported by Graham et al (11,34). The MR signal behaviors are further complicated by strong dependence on the use of continuous wave vs. pulsed HIFU treatment (37), as well as by the dependence of histological changes on the time elapsed after the HIFU procedure (38).…”

“…Even immediately after HIFU ablation, Rouviere et al (39) described the acute signal response for prostate tissue on T2-weighted images as being "generally hypointense, but can also show hyperintense or mixed intensity." It thus becomes evident that acute thermal and nonthermal alterations in tissues undergoing HIFU ablation seem to be extremely complex, and are therefore unlikely to be satisfactorily explained by the relatively simplified model of water-bathing experiments, which might involve changes in the water content and which stabilize relatively quickly (11,34). Clearly, further investigations at a larger scale to systematically explore the MTR characteristics with rigorous control of the HIFU heating procedure are warranted for a better understanding.…”

“…Several MR parameters, such as the equilibrium magnetization (M 0 ), T1, T2, and the diffusion coefficient of water molecules, are known to exhibit temperature dependence (4 -10). The temperature sensitivity of these parameters, however, is tissue-dependent, nonlinear, or varies with tissue conditions such as thermal coagulation or denaturation (5,11,12). Currently, the MR parameter that is sensitive to temperature and stable under various tissue conditions is the water proton resonant frequency (PRF) shift (5).…”