099 Generating synthetic ultrasound imaging artifacts for the purpose of computerized training of cryosurgery

The objective of this study is to develop a method to simulate ultrasound imaging artifacts during a virtual cryoprocedure for the purpose of training. Key artifacts include opacity and shadowing of the frozen region, and fuzziness and brightening at the freezing front. Cryosurgery has been demonstrated as an effective minimally invasive image-guided treatment for prostate cancer. During the procedure an array of cryoprobes are inserted into the prostate in order to freeze and destroy the cancerous tissue. A trans-rectal ultrasound (TRUS) transducer is most commonly used to guide the placement of the cryoprobes as well as to track the formation of the frozen region. Key imaging artifacts introduced by freezing include, opacity and shadowing of the frozen region, and lightening at the freezing front due to a sudden change of the tissue’s acoustic impedance. Visualizing these artifacts on a virtual cryoprocedure platform will help training new surgeons, especially when the same platform also integrates bioheat simulations, computerized planning of cryosurgery, and commonly accepted rules of clinical practice – the subject matter of parallel studies by the current research team. The proposed method is inspired by concepts from spectral beam radiation in a participating medium. By analogy, the expression of the acoustic signal on the medical imaging screen is simulated by a temperature-dependent attenuation coefficient, which is the sum of absorption and reflection coefficients. The absorption coefficient is responsible for darkening the image in the frozen region and at large distance from the ultrasound transducer, while the reflection coefficient is responsible for whitening the image at the freezing front and other solid walls – such as a cryoprobe surface. The proposed method is based on an available 3D ultrasound image of the area to be treated and follows five key steps: (i) A bioheat-transfer simulation is executed for the cryosurgery procedure, generating a time-dependent temperature field in the target area. (ii) At any instant of interest, the coefficients of absorptivity and reflectivity are calculated based on temperature and proximity to solid structures. (iii) Simulated attenuation of the acoustic signal is calculated by integration along a beam extending from the ultrasound transducer. (iv) Simulated reflection of the acoustic signal is calculated based on proximity to solid walls. (v) Calculated attenuation and reflection fields are converted to numerical values, proportionally shifting the gray-scale at each pixel of the ultrasound image accordingly. Shift values are selected experimentally to match available ultrasound artifacts from documented ultrasound images, from real cryoprocedures. The current study employs GPU computing for simultaneous real-time bioheat-transfer simulations and synthetic image processing. This implementation allowed simulated images to be rendered at 30 fps, fast enough to give a realistic and smooth animation of the procedure on a mid-range desktop computer. The significance of the proposed method, in combination with real-time computation, is in the potential for creating a surgical simulation system that displays realistic changes to the TRUS images during virtual cryosurgery. Source of funding: This study was supported by Award Number R01CA134261 from the National Cancer Institute. Conflict of interest: None declared. rabin@cmu.edu