Mribased Control Of Energy Delivery

With the introduction of magnetic resonance imaging (MRI) as a monitoring method for thermal therapies, a novel mechanism for controlling energy deposition became available (8). It was recognized that many MRI parameters are sensitive to temperature changes, which makes MRI suitable for monitoring thermal ablations by noninvasive means. Furthermore, one can take advantage of diffusion MRI, which detects changes in water mobility and compartmental-ization, to identify reversible as well as irreversible thermally induced tissue changes (9,10). It became obvious, however, that MRI monitoring of thermal ablation is only feasible if the imaging and therapy delivering systems are integrated (2).

Most endogenous MRI parameters are temperature sensitive. The most commonly used parameters for temperature monitoring have been T1 (8,11-13), the diffusion coefficient (14,15), and the water proton resonant frequency (PRF) (16,17).

Having the highest temperature sensitivity and being independent of tissue type, the PRF method has been the most promising (18). However, it has the disadvantage of being very sensitive to movement.

The role of MRI during thermal ablations is to monitor temperature levels, to restrict the thermal coagulation to the targeted tissue volume, and to avoid heating of normal tissue. MRI can also detect irreversible tissue necrosis and demonstrate permanent changes within the treated tissue. Physiologic effects such as perfusion or metabolic response to elevated temperature can also be used for monitoring the ablation. Both flow and tissue perfusion can affect the rate and extent of energy delivery and the size of the treated tissue volumes (3). Monitoring can optimize treatment protocols.

Thus far, three approaches have been proposed for using MRI monitoring of thermal therapies: temperature threshold, thermal dose control, and imaging control. In the temperature control approach, the temperature threshold necessary to induce tissue damage is empirically determined from animal experiments and clinical experience. In some studies, temperature thresholds measured by MRI were used as indicators of tissue necrosis during thermal therapies (19,20). Although there is a good correlation between temperature and tissue necrosis, experimental studies have shown that the exposure time also plays a significant role in achieving tissue damage.

In addition to temperature, the thermal dose approach also takes into account the exposure time. The thermal dose is a nonlinear function of the temperature and exposure time. This method is mainly indicated for monitoring thermal therapies that employ long heating times at constant temperatures (4,21).

Whereas the temperature threshold method is useful, especially when the heating profile has the same shape and size, the dosimetry method is thought to be independent of these parameters. Control systems based on temperature threshold and thermal dosimetry have been described in (22-24).

The third approach—imaging control—is based on T2- or T1- acquisitions with or without contrast agent during energy delivery (13,25). The evolving lesions appear to correlate well or slightly underestimate the final lesion size. However, the specificity of this method may be too low in some cases, and differentiating between thermal lesion and viable tumor tissue/peritumoral edema is not always possible (26).

Since thermal lesions may become visible on MRI with certain delays, this can result in underestimation of the extent of the thermal lesion.

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