1. M. Holden is funded by the ISCAS student travel scholarship. G. Fichtinger is funded as a Cancer Care Ontario Research Chair. Scalable ultrasound calibration phantoms made from LEGO® bricks NM. Soehl, M. Holden, A. Lasso, R.C, G. Fichtinger Queen’s University, Kingston, Canada Poster 011: NM. Soehl, M. Holden, A. Lasso, R.C, G. Fichtinger, Scalable ultrasound calibration phantoms made from LEGO® bricks. In tracked ultrasound-guided systems, spatial calibration between the image and tracker coordinate space is a prerequisite, and is commonly performed using a phantom with N-wire configuration [1]. Introduction LEGO® brick N-wire phantoms offer several advantages over 3D printed N-wire phantoms [2]: High manufacturing tolerance (0.05mm) Inexpensive and readily accessible Models are reconfigurable and scalable Build process is repeatable

2. The objective of this paper is to examine the viability of calibration phantoms made from LEGO® bricks, customized and scaled for different ultrasound transducers used in different clinical settings. We assessed the precision of ultrasound calibrations performed with two different ultrasound transducers and differently sized N-Wire phantoms made from LEGO® bricks, through comparison to corresponding standard 3D-printed phantoms [1, 3]. One phantom was accommodated a shallow imaging depth (4.5cm), and the other accommodated a deeper imaging depth (18cm). To test calibration precision, we used the PLUS Toolkit (www.plustoolkit.org) [3], with a SonixTablet ultrasound scanner with SonixGPS electromagnetic tracking system (Ultrasonix Medical Corporation). With the shallow phantoms, we used a L14-5/38 Ultrasonix linear probe. With the deep phantoms, we used a C5-2/60 Ultrasonix convex probe. We performed all calibrations in water at room temperature.www.plustoolkit.org Objective Poster 011: NM. Soehl, M. Holden, A. Lasso, R.C, G. Fichtinger, Scalable ultrasound calibration phantoms made from LEGO® bricks. Methods

3. Five calibrations were performed with each phantom, and the 3D reprojection error was measured. The 3D reprojection error was defined as the root mean square difference between calculated ultrasound probe-to-image transformations at different probe positions (over a total of 50 positions). Phantom 3D reprojection error average (mm) 3D reprojection error standard deviation (mm) LEGO® brick shallow phantom0.770.13 3D printed shallow phantom0.800.11 LEGO® brick deep phantom1.730.29 3D printed deep phantom2.540.52 Results LEGO® bricks are a viable material for constructing ultrasound calibration phantoms of different sizes. A detailed methodology, design and build instructions for creating custom phantoms made from LEGO® bricks can be found online as part of the open source PLUS Toolkit (www.plustoolkit.org).www.plustoolkit.org Poster 011: NM. Soehl, M. Holden, A. Lasso, R.C, G. Fichtinger, Scalable ultrasound calibration phantoms made from LEGO® bricks. The calibrations with the LEGO® brick phantom were significantly more precise than with the 3D- printed phantoms (p = 0.048 for shallow phantoms, p = 0.001 for deep phantoms, by unpaired t-test). Conclusion [1] Carbajal G, et al., Improving N-Wire Phantom-based Freehand Ultrasound Calibration, IJCARS 2013. [2] Walsh R, et al., Design of a tracked ultrasound calibration phantom made of LEGO® bricks, SPIE 2014. [3] Lasso A, et al., PLUS: open-source toolkit for ultrasound-guided intervention systems, IEEE TBME 2014.