MR thermometry for monitoring tumor ablation

Abstract

Local thermal therapies are increasingly used in the clinic for tissue ablation. During energy deposition, the actual tissue temperature is difficult to estimate since physiological processes may modify local heat conduction and energy absorption. Blood flow may increase during temperature increase and thus change heat conduction. In order to improve the therapeutic efficiency and the safety of the intervention, mapping of temperature and thermal dose appear to offer the best strategy to optimize such interventions and to provide therapy endpoints. MRI can be used to monitor local temperature changes during thermal therapies. On-line availability of dynamic temperature mapping allows prediction of tissue death during the intervention based on semi-empirical thermal dose calculations. Much progress has been made recently in MR thermometry research, and some applications are appearing in the clinic. In this paper, the principles of MRI temperature mapping are described with special emphasis on methods employing the temperature dependency of the water proton resonance frequency. Then, the prospects and requirements for widespread applications of MR thermometry in the clinic are evaluated.

Driving delivery vehicles with ultrasound

“…We and others are developing cross-modality imaging methods in which nuclear medicine, magnetic resonance imaging, and optical imaging probes are used to visualize the delivery vehicle or the model drug [51][52][53][54].The great strength of nuclear imaging techniques, particularly positron emission tomography, is the real-time and highly sensitive full-body pharmacokinetics currently possible in pre-clinical studies [54,55]. Magnetic resonance imaging (MRI) techniques can be used to visualize the biodistribution of drugs or vehicles [52] and the activation of delivery vehicles in limited cases [51], and MRI can also be used to monitor local temperature in pre-clinical and clinical studies [56]. Optical imaging can be broadly applied to monitor the activation of a delivery vehicle or release of a drug, although in vivo studies of ultrasound-based drug delivery do not yet incorporate real-time optical imaging.…”

Compensating for magnetic field inhomogeneity in multigradient-echo-based MR thermometry

“…Because the gradient strength is known, the real frequency can be retrieved using Eq. [1]. This calculated frequency results in an adjusted temperature map equal to the map without added background gradients.…”

“…At higher field strengths, the errors become more serious, as can be seen from Eq. [1], because larger frequency differences and more pronounced background gradients occur. This makes the compensation more valuable.…”

“…The series in which the readout direction is in the same direction as the background gradient will lead to frequency Df 0 , as seen in Eq. [1]. However, in the other series the equation becomes:…”

Prediction of cell necrosis with sequential temperature mapping after radiofrequency ablation

“…Different MR techniques have been proposed for temperature measurement (6 -10); the most frequently used is the proton resonance frequency shift (PRF) method (11)(12)(13)(14), which is based on a temperature-dependent phase shift allowing for the measurement of temperature changes relative to a baseline image. With frequent repetitive measurements during thermoablative therapy, this approach allows a precise estimation of the lethal thermal dose (15) based on the Arrhenius model (16,17) and could be used to predict the size and position of the coagulation zone in animal studies (3,5). Furthermore, real-time temperature monitoring could allow for visualizing a vessel-related cooling effect and thus prevent a survival of cancer cells due to an undetected heat-sink effect.…”