Development and characterization of a bone 
\nphantom for MR-HIFU systems

In this study, a bone phantom was developed and characterised for quality control of MR
\nguided high-intensity focused ultrasound (MR-HIFU) treatments. 
\nHigh-intensity focused ultrasound (HIFU) is a promising treatment approach based on the 
\ncompletely non-invasive heating of tissue. MR imaging is used to guide and measure the 
\ntemperature of the ultrasound. The main applications of MR-HIFU are currently the treatment 
\nof uterine fibroids, adenomyosis, desmoid tumours and bone metastases. 
\nThe main advantage of MR-HIFU is the completely non-invasive approach. This makes it a 
\nvery patient-friendly and low-risk procedure. MR-HIFU treatment has already been approved 
\nfor many applications, but randomised controlled trials are currently lacking to make this type 
\nof treatment even more widespread. 
\nCurrently, there are several different MR-HIFU systems on the market which are mainly 
\nmanufactured by the companies Profound Inc. (Mississauga, Canada) and Insightec Corp. 
\n(Haifa, Israel). They differ regarding the transducers, software and MR-systems in use. This 
\ncan lead to challenges, for example in multicentre studies, as the comparability of the individual 
\ndevices is difficult to predict. For this reason, a bone phantom was developed in this study for 
\nthe quality control of MR-HIFU devices and treatment methods. 
\nThe developed bone phantom should be made of a durable material that can withstand 
\nrepeated treatments with focused ultrasound. In this thesis, a variety of materials was tested, 
\nwhereas the polymer polyurethane, which is also used to manufacture ultrasound absorbers, 
\nwas found to be the best candidate. This material was then embedded in a phantom gel 
\nconsisting of polyacrylamide and exposed to different ultrasound treatments. The heating on 
\nthe surface of the bone phantom during the ultrasound exposure was measured via MR- 
\nthermometry. The reproducibility of the temperature development of the HIFU experiments was 
\ntested by means of repeated standardised sonications and measurements over a period of 
\nseveral weeks. This showed a reproducible, continuous heat development. 
\nIn addition, the influence of different power levels on the heating at the phantom surface and 
\ntwo different approaches to sonication planning were investigated. The two approaches to 
\nsonication planning differ regarding the positioning of the focal point and regarding the applied 
\npower level and sonication time. A linear relationship between applied energy and heat 
\ndevelopment was found. The two approaches to sonication planning showed no significant 
\ndifference regarding the heated volume of the surrounding polyacrylamide gel. 
\nThe developed bone phantom can be used in the future to calibrate MR-HIFU devices prior to 
\ntreatments of bone diseases and is therefore an important tool in the implementation of future 
\nmulticentre studies. The results of the comparison of the treatment planning approaches and 
\nthe influence of the energy level have only limited significance due to the low sample size of 
\nthe experiments but can be taken as an indicator for future research. 
\nAn added focus of this project was the manufacturing of a more realistic shape of the bone 
\nphantom. Two different ways of manufacturing a spine phantom were developed.

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