Fig 1.
Yellow L Landmark Porus acusticus externus, blue L Landmark Nasion, T Target.
Fig 2.
Assessment of the distance between the Landmark L and the target T, shown for nasion as landmark. For step-by-step explanations see paragraph Calculating part and the video guide. TS target slice, LS landmark slice.
Fig 3.
The distance from nasion to the target and the distance from porus to the target are marked onto the head with the help of a caliper.
The individual in this manuscript has given written informed consent (as outlined in PLOS consent form) to publish these case details.
Fig 4.
Example of the localization of one of the 20 targets on the saw bone model.
The intersection of the two circles meets at the target.
Fig 5.
Box plot of data of the saw bone model, for caliper navigation and computer navigation.
Whiskers: Max and min. Box: 25 and 75 percentiles and median.
Fig 6.
Box plot of data of the clinical validation, for caliper navigation, referenced to computer navigation as ground truth.
Whiskers: Max and min. Box: 25 and 75 percentiles and median.
Fig 7.
The demands to a caliper to facilitate the marking part of this method have been addressed in this model designed by the first author: The contact point of the center leg is smooth and can be inserted to the porus and placed on nasion. The length of legs of the caliper can be adjusted and together with the fix angle chosen for the mounting rings, this length adjustment allows to keep the angle of the center leg and the marker nearly vertical to the skin at all different convexity locations, which reduces the error. The mounting rings allow to insert different diameters of markers. The tool can be folded up and fits in a pocket.
Fig 8.
As long as the cursor position on the screen is not changed, the line a “travelled” by the cursor from landmark slice LS to target slice TS is vertical to the slice plane orientation.
Hence, it is also vertical to the line b. The distance between L and T can therefore be calculated using Pythagoras’ theorem.
Fig 9.
In order to be able to apply Pythagoras’ theorem, the line “travelled” by the cursor through the slices must be vertical to the slice orientation.
This is only the case if the slices are “piled aligned” in a cuboid dataset (right). If the gantry angle is modified during data acquisition, the resulting dataset is a non-cuboid parallelepiped, and the calculation cannot be applied. In that case, a cuboid dataset can be reconstructed at the CT workstation. Whether the regarded slices are applicable for the caliper navigation technique or not can be seen at a glance in the corresponding scout.
Fig 10.
Top. A common error occurs when the target on the skin surface is chosen from a single plan, here axial orientation. This approach leads to a wrong center of the craniotomy for the respective lesion. Bottom. A corrective approach is to move and scroll with respect to the slice thickness to obtain an oblique trajectory.
Fig 11.
Left, blue Landmark L, nasion: The slice that contains the center of both eyeballs, shown by presence of the lenses.
Right, yellow landmark L, porus: The slice containing the outer part of the external auditory canal is chosen and the point is placed medial to the tragus. This is the position where the blunt leg of the caliper will come to rest.