Sharon L Giles, Ian Rivens, Georgios Imseeh, Matthew RD Brown, Alexandra Taylor, Gail R ter Haar and Nandita M deSouza
Objectives: To investigate temperature changes at the focus with various pre-focal fat/muscle tissues in both an experimental set-up and in patients treated with Magnetic Resonance guided High Intensity Focused Ultrasound (MRgHIFU).
Materials and Methods: Focal and pre-focal heating (Philips/Profound 3T Achieva MR/ Sonalleve HIFU system) were measured in an experimental set-up at 4, 6 or 8 cm depths with varying clinically-encountered tissue distributions. Six patients with recurrent gynaecological malignancy treated within a larger trial (NCT02714621) had intra-procedural temperature and thermal dose volume recorded.
Results: In the experimental set-up, focal heating of a target behind fat caused pre-focal heating. Achievable focal thermal dose volumes depended on pre-focal fat thickness: 8 cm required 9 kJ, 6 cm required 4.6 kJ and 4 cm required 2.7 kJ to achieve a 1.6 ± 0.5 ml ablated V30EM volume. In comparison, a muscle only mimic of 8 cm thickness required 5.7 kJ. Layered fat/muscle mimic distributions caused greater temperature rises in the immediate, compared to the superficial, pre-focal region. In 2 patients with only prefocal fat (1.4 cm and 5.9 cm thick respectively), 66°C at the focus with a measurable thermal dose volume (V240EM=4.5 ml) was achieved in the former and 55°C without a measurable V240EM in the latter. In 4 cases with tumours at 6.9-8.7 cm depth (median 7.9 cm) and an asymmetrical fat/ muscle distribution, ablative temperatures (58°C) were only achieved in the shallowest tumour.
Conclusions: Increasing pre-focal fat thickness increases the power required to reach an ablative thermal dose. In tumours >7 cm deep to fat, ablation is limited. Symmetric or asymmetric layering of fat and muscle increases pre-focal heating.