Electromagnetic (EM) tracking systems are often used for real time navigation of medical tools in an Image
Guided Therapy (IGT) system. They are specifically advantageous when the medical device requires tracking
within the body of a patient where line of sight constraints prevent the use of conventional optical tracking. EM
tracking systems are however very sensitive to electromagnetic field distortions. These distortions, arising from
changes in the electromagnetic environment due to the presence of conductive ferromagnetic surgical tools or
other medical equipment, limit the accuracy of EM tracking, in some cases potentially rendering tracking data
unusable. We present a mapping method for the operating region over which EM tracking sensors are used,
allowing for characterization of measurement errors, in turn providing physicians with visual feedback about
measurement confidence or reliability of localization estimates.
In this instance, we employ a calibration phantom to assess distortion within the operating field of the
EM tracker and to display in real time the distribution of measurement errors, as well as the location and
extent of the field associated with minimal spatial distortion. The accuracy is assessed relative to successive
measurements. Error is computed for a reference point and consecutive measurement errors are displayed relative
to the reference in order to characterize the accuracy in near-real-time. In an initial set-up phase, the phantom
geometry is calibrated by registering the data from a multitude of EM sensors in a non-ferromagnetic ("clean")
EM environment. The registration results in the locations of sensors with respect to each other and defines
the geometry of the sensors in the phantom. In a measurement phase, the position and orientation data from
all sensors are compared with the known geometry of the sensor spacing, and localization errors (displacement
and orientation) are computed. Based on error thresholds provided by the operator, the spatial distribution of
localization errors are clustered and dynamically displayed as separate confidence zones within the operating
region of the EM tracker space.
Estimation of volume change of structures in response to treatment or growth during breast screening exams is a challenge primarily because of ill-defined boundary. Some treatment procedures alter the lesion completely out of its original shape. In this paper, we present an overview of our recent work on identifying a technique based on Image Volume Based Registration (IVBaR) for estimation of volume. We propose that as long as a region of interest around the lesion can be identified, the exact boundary information would not be necessary. Here, we assume that the surrounding tissue remains nearly unaffected by the treatment procedure, an assumption that is valid in many cases. It is the motion of this tissue in response to changes in the central tumor that would be tracked and used to estimate the change in tumor volume.
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