Table 1.

Average displacements between automatically matched feature points and manual landmarks without brain shift compensation and with either spline-based or FEM-based brain shift compensation. Results are reported for two time intervals during surgery: 1) for deformation that occurred between iUS images acquired immediately prior to and immediately after opening the dura (Dural opening) and 2) for deformation that occurred between iUS images acquired immediately prior to opening the dura and after significant tumor resection (Subtotal resection). In all cases, the FEM method provided better results for manual landmarks than the spline-based method. We note that because very small residuals for feature points are expected after applying the thin plate spline (since they were used to train the spline), feature point residuals for the spline-based and FEM method cannot be compared to each other in a meaningful way. They are included here for completeness.

	Without compensation		After thin-plane spline		After FEM
	Manual landmarks	Automatic features	Manual landmarks	Automatic features	Manual landmarks	Automatic features
Case 1
Dural opening	1.96 ± 1.22	1.34 ± 0.66	1.72 ± 0.75	3.5e−3 ± 2.8e−2	1.64 ± 0.86	0.99 ± 0.60
Subtotal resection	3.25 ± 0.99	2.60 ± 1.39	2.10 ± 1.20	6.4e−7 ± 2.3e−7	1.27 ± 0.62	1.33 ± 1.04
Case 2
Dural opening	0.77 ± 0.40	0.81 ± 0.63	0.84 ± 0.50	6.6e−7 ± 2.5e−7	0.82 ± 0.50	0.80 ± 0.64
Subtotal resection	2.33 ± 1.11	2.01 ± 0.16	1.54 ± 1.37	9.7e−7 ± 2.6e−7	1.46 ± 1.06	0.77 ± 0.25