Scalable ultrasound calibration phantoms made from LEGO bricks

Scalable ultrasound calibration phantoms made from LEGO® bricks NM. Soehl, M. Holden, A. Lasso, R. C, G. Fichtinger Queen’s University, Kingston, Canada M. Holden is funded by the ISCAS student travel scholarship. G. Fichtinger is funded as a Cancer Care Ontario Research Chair. Introduction 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]. LEGO® brick N-wire phantoms offer several advantages over 3 D printed N-wire phantoms [2]: • High manufacturing tolerance (0. 05 mm) • Inexpensive and readily accessible • Models are reconfigurable and scalable • Build process is repeatable Poster 011: NM. Soehl, M. Holden, A. Lasso, R. C, G. Fichtinger, Scalable ultrasound calibration phantoms made from LEGO® bricks.

Objective 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. Methods 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 3 D-printed phantoms [1, 3]. One phantom was accommodated a shallow imaging depth (4. 5 cm), and the other accommodated a deeper imaging depth (18 cm). To test calibration precision, we used the PLUS Toolkit (www. plustoolkit. org) [3], with a Sonix. Tablet ultrasound scanner with Sonix. GPS electromagnetic tracking system (Ultrasonix Medical Corporation). With the shallow phantoms, we used a L 14 -5/38 Ultrasonix linear probe. With the deep phantoms, we used a C 5 -2/60 Ultrasonix convex probe. We performed all calibrations in water at room temperature. Poster 011: NM. Soehl, M. Holden, A. Lasso, R. C, G. Fichtinger, Scalable ultrasound calibration phantoms made from LEGO® bricks.

Results Five calibrations were performed with each phantom, and the 3 D reprojection error was measured. The 3 D 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 LEGO® brick shallow phantom 3 D printed shallow phantom LEGO® brick deep phantom 3 D printed deep phantom 3 D reprojection error average (mm) 0. 77 0. 80 1. 73 2. 54 3 D reprojection error standard deviation (mm) 0. 13 0. 11 0. 29 0. 52 The calibrations with the LEGO® brick phantom were significantly more precise than with the 3 Dprinted phantoms (p = 0. 048 for shallow phantoms, p = 0. 001 for deep phantoms, by unpaired t-test). Conclusion 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). [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. Poster 011: NM. Soehl, M. Holden, A. Lasso, R. C, G. Fichtinger, Scalable ultrasound calibration phantoms made from LEGO® bricks.