Abstract
BackgroundCurrent clinical targets for transcranial magnetic resonance-guided focused ultrasound (tcMRgFUS) are all located close to the geometric center of the skull convexity, which minimizes challenges related to focusing the ultrasound through the skull bone. Non-central targets will have to be reached to treat a wider variety of neurological disorders and solid tumors. Treatment envelope studies utilizing two-dimensional (2D) magnetic resonance (MR) thermometry have previously been performed to determine the regions in which therapeutic levels of FUS can currently be delivered. Since 2D MR thermometry was used, very limited information about unintended heating in near-field tissue/bone interfaces could be deduced.MethodsIn this paper, we present a proof-of-concept treatment envelope study with three-dimensional (3D) MR thermometry monitoring of FUS heatings performed in a phantom and a lamb model. While the moderate-sized transducer used was not designed for transcranial geometries, the 3D temperature maps enable monitoring of the entire sonication field of view, including both the focal spot and near-field tissue/bone interfaces, for full characterization of all heating that may occur. 3D MR thermometry is achieved by a combination of k-space subsampling and a previously described temporally constrained reconstruction method.ResultsWe present two different types of treatment envelopes. The first is based only on the focal spot heating—the type that can be derived from 2D MR thermometry. The second type is based on the relative near-field heating and is calculated as the ratio between the focal spot heating and the near-field heating. This utilizes the full 3D MR thermometry data achieved in this study.ConclusionsIt is shown that 3D MR thermometry can be used to improve the safety assessment in treatment envelope evaluations. Using a non-optimal transducer, it is shown that some regions where therapeutic levels of FUS can be delivered, as suggested by the first type of envelope, are not necessarily safely treated due to the amount of unintended near-field heating occurring. The results presented in this study highlight the need for 3D MR thermometry in tcMRgFUS.
Highlights
Current clinical targets for transcranial magnetic resonance-guided focused ultrasound are all located close to the geometric center of the skull convexity, which minimizes challenges related to focusing the ultrasound through the skull bone
Non-invasive treatment monitoring has been made possible with the use of magnetic resonance imaging (MRI)
With 3D MR temperature imaging (MRTI) coupled with constrained reconstruction for improved spatial and temporal resolution, the fully insonified field of view (FOV) can simultaneously be monitored and characterized
Summary
Current clinical targets for transcranial magnetic resonance-guided focused ultrasound (tcMRgFUS) are all located close to the geometric center of the skull convexity, which minimizes challenges related to focusing the ultrasound through the skull bone. Focused ultrasound (FUS) is a promising treatment modality for a wide range of disorders due to its precise, noninvasive, and ionizing radiation-free nature. It is currently being performed under ultrasound [1,2,3,4] (US) and magnetic resonance (MR) guidance [5,6,7]. Challenges to transcranial FUS have included the defocusing of the US beam due to phase aberrations by the skull bone [36], and the difficulty of non-invasively monitoring the procedures. Typical MRTI methods cover only one or a few two-dimensional (2D) slices in order to get high enough spatio-temporal resolution for accurate temperature measurements [23,24]
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