In the previous sections MR temperature measurements based on proton density, T1 and T2 relaxations times, magnetization transfer, diffusion, proton resonance frequency, and thermosensitive contrast agents have been introduced. With these different temperature-sensitive parameters, MRI has been shown to be an excellent modality to noninvasively monitor thermal therapy to ensure efficacy and safety of the treatment. Temperature monitoring has been successfully demonstrated in vivo and is regularly used in a number of clinical applications mainly in combination with FUS heating. However, a number of challenges remain for MR thermometry to be widely accepted for monitoring thermal procedures. Because the target of many ablation procedures lies in the abdomen, reliable and robust motion-insensitive acquisition techniques and reconstruction algorithms are indispensable. To date, only T1, diffusion and PRF have been used for in vivo temperature monitoring during thermal therapy.
Numerous studies have attempted to compare different methods (35
), but because the usefulness of any method depends strongly on the application, the imaged body part, field strength, and other parameters, it is difficult to draw general conclusions. And although the proportionality constant of PRF with temperature appears low compared to T1
and D, PRF-based methods have resulted in higher precision (103
), suggesting that temperature monitoring with the PRF is the most sensitive among endogenous MRI parameters in detecting small temperature changes (104
). When field inhomogeneity is poor, e.g. due to an inserted needle or applicator, the PRF method may not be as accurate as diffusion or T1
relaxation which can be acquired with spin echo methods. In addition, at very low field strength the PRF method may be less sensitive than diffusion or T1
relaxation because of its linear dependence on field strength.
Generally, to avoid errors from fat, lipid suppression is necessary when T1, D, or PRF methods are employed in tissues that contain fat. All three methods require very good registration to correct for displacements between scans. In addition, the diffusion method is particularly sensitive to motion artifacts during the scan.
A major need is a reliable method to measure absolute temperature with good spatial and temporal resolution. Even if the temporal resolution is still too low for real-time monitoring, a fast PRF phase mapping sequence could be interspersed with absolute temperature measurements. This would allow taking new baseline images at any time during the procedure when the absolute temperature is measured. This would greatly alleviate the requirement for motion insensitivity and could avoid having to terminate the procedure in case of tissue motion. In cases where cooling is applied to protect sensitive structures near the heated area, absolute temperature measurement would be helpful to prevent measurement errors resulting from baseline images not taken at body temperature.
The fact that PRF phase mapping cannot be used to measure temperature in lipids poses significant problems for treatment of organs that contain large amounts of fat, (e.g. breast and skin). Skin burns can be a problem in FUS procedures, because the skin interface lies in the near field of the ultrasound beam. Without the ability to monitor temperature in the fatty skin layer, skin burns can occur.
Because PRF phase mapping gives a linear relationship to temperature and is not influenced by tissue changes, this methods provides accurate temperature measurements in the temperature range of interest for thermal ablation. This linearity is usually considered an advantage when different MR temperature methods are compared. However, to be able to control the treatment outcome, it is not only necessary to accurately measure the temperature during treatment, but also to be able to relate treatment temperature to actual thermal tissue damage. The nonlinearity of measurements with T1
, diffusion, and MT might provide a more direct measure of tissue changes as a response to the thermal treatment. MR elastography has also been used to detect changes in tissue stiffness caused by ablation (105
). Because tissue microstructure undergoes major changes during thermal coagulation, it is hypothesized that these MR parameters provide a more direct estimate of cell death. A quantitative interpretation of the changes of these MR parameters is complicated because the term thermal coagulation encompasses multiple different responses of tissue to heating at different temperatures. These effects include enzyme deactivation and reversible cell injury, cell shrinkage and hyperchromasia, cell death and denaturation of proteins (2
), but it is likely that MR parameters are only sensitive to a subset of these tissue changes (33