An alternative approach in the skull density ratio (SDR) computation used in MRgFUS treatments: A preliminary study.

Magnetic Resonance guided Focused Ultrasound (MRgFUS) is a hybrid technique used for surgery lesioning treatment for brain disorders. Morphological information is obtained from MR images, while the ablative process is performed with US. The main limit of the US beam is represented by the skull because bone can absorb and reflect the US energy. The skull bone is made up of two surfaces of compact bone separated by a layer of spongy bone (also called trabecular or diploe). The factor that takes into account the skull bone is called Skull Density Ratio (SDR) defined as the median ratio between spongy bone and compact bone. The SDR estimates the skull transparency to the US, and it is obtained from Computed Tomography images. In this study, we propose an alternative approach in SDR computation, always starting from CT images but only considering the spongy bone and not the compact tables. This approach is based on the different Hounsfield Units (HU) existing between compact bone and diploe, which could influence SDR computation and consequently the setting of physical parameters used for MRgFUS treatment. We compared the HU of this new approach with several physical parameters such as the n° of sonication; the maximum power delivered MPD (watt); the maximum energy delivered MED (J); the Highest average temperature (HAT) and the accumulated thermic dose (ATD). In addition, a comparison between the used SDR and the proposed SDR was performed. The obtained results showed a negative correlation between the HU and the US power and a significant trend between the HU and the Energy of the US. In the first case, a trabecular section with low HU reduces the transmission of US and to reach the temperature required to obtain ablation an increase in power is needed. The results from the SDR comparison showed that for high SDR values, there is no difference between Used SDR and Proposed SDR, while for low SDR values, we have noticed a great difference between used SDR and proposed SDR. The above information would be expected to be helpful for MRgFUS patients' selection and also in optimizing treatment results. [ABSTRACT FROM AUTHOR]