  Electromagnetic tracking (EMT) shows great potential for automating implant
reconstruction in brachytherapy. One of the challenges of this technology is
that it does not intrinsically share the same reference frame as the patient's
medical imaging. The purpose of this work is to register the reference frames
of an EMT-enabled afterloader and a CT scan image of a rigid phantom for
quality control of EMT-based implant reconstruction. The geometry of twelve 6F
catheters and a tandem and ring gynecological applicator were reconstructed
using an EMT sensor attached to an afterloader's check cable. All EMT
reconstructions were done in three different environments: disturbance free,
CT-on-rails brachytherapy suite and MRI brachytherapy suite. Implants were
reconstructed using two acquisition methods: step-and-record and continuous
motion. A CT scan of the phantom was obtained and manually reconstructed in the
treatment planning system, with the help of the solid model of the applicator.
Rigid registration algorithms were used to register EMT and CT scan reference
frames using corresponding points from the EMT and CT based reconstructions of
the phantom, following three approaches: one gynecological applicator, four
catheters inside four calibration plates having and S-shaped path, and four
ceramic marbles within the calibration plates. The absolute median deviation
from the expected value for EMT measurements in all three environments
significantly lie below the sensor's expected accuracy of 0.70 mm (p < 0.001),
suggesting that the environment did not have a significant impact on the
measurements, given that care is taken in the immediate surroundings. In all
three environments, the two acquisition and three registration approaches have
a mean and median registration errors (perpendicular distance) that lie at or
below 1 mm, which is lower than the clinical acceptable threshold of 2 mm.
