The clinical utility of endoscopic systems is now being augmented in several procedures through the addition of navigation technology and registration using pre-procedure volumetric models segmented from CT or MRI data [25]. For the development of navigation systems, accurate segmentations of the retroperitoneal organs have been required. While many tools exist for the other organs of the torso, such as those reviewed for the liver by Heimann et al. [26] and for the heart by Pfeifer et al. [27], the retroperitoneal organs have been, comparatively, neglected. A primary approach has been to use conventional semiautomated segmentation techniques, an example is the developing of models for the Image registered gastroscopic ultrasound (IRGUS) system [6, 9, 11, 28] using the open-source image analysis software 3D Slicer [29], and ensure that the process was overseen by an abdominal radiologist with extensive experience in cancer of the pancreas and nearby organs. The development of more rapid methods which require minimal supervision, perhaps using atlas-based techniques, is an interesting area for future study. Additionally, current advancement in accelerated computer graphic hardware has enabled volume rendering of full clinical volumes allowing for quick volumetric representation of anatomical structures in CT and MR without explicit model generation [30, 31]. Patient-specific model generations are still required for target organs where a geometric description may be necessary to assess the boundaries and volumes.

There are four common technologies for tracking medical instruments: EM, optical, mechanical arm, and acoustic [32]. EM or optical are most commonly used technologies for tracking in medical applications. Optical systems have a high accuracy, but require a free line of sight between the sensors/markers and the cameras. This can sometimes be challenging to achieve since procedure rooms are generally cluttered and the freedom of movement is limited. In minimally invasive surgery, independence from line of sight is important to facilitate the tracking of flexible instruments inside the human body, such as flexible video laparoscopes, endovascular catheters, and LUS probes. For this reason, EM tracking systems are most suitable as they are unaffected by sensor occlusion. However, distortions may occur from metallic objects in the working space and the instrument that is being tracked that induce perturbations of the EM field that reduce the accuracy of the EM techniques.

We have explored several approaches [33] to implement model-registered electromagnetic tracking, assisted by commercial developments and collaborations [34] that now provide both good registration of 3D models to the anatomy in situ and the instrument locations. Key elements of the technical capabilities include very small (~1.0 mm) electromagnetic sensors attached to instruments, and “flat plate” electromagnetic transmitter systems [Ascension Technologies Corp, Burlington, VT, USA] which have a large working volume (sufficient to cover the upper abdomen) and excellent immunity to electromagnetic interference.

A calibration procedure must be performed to determine the location of the endoscopic and laparoscopic ultrasound plane in space in relation to the tracking sensor attached to the probe [35]. A precise calibration can be best obtained by scanning a phantom with a known geometry. The features are identified in the ultrasound image of the phantom and these features are also located in physical space. The spatial relationship between the two data sets is computed in the calibration process. Some of the commonly used phantoms for US probe calibration are listed below:

Single point target and cross–wire phantoms: The single point target phantom generally consists of a bead or a pinhead. The cross-wire phantom is based on imaging the intersection of two cross wires. The point target or wire crossing is aligned in the image plane and is imaged from multiple viewing angles. A method referred to as plane mapping requires no phantom. This method simply images the tip of a tracked pointer, which can freely move around within a coupling medium while the probe is fixed.

Multiple point targets and cross–wire phantoms: Multiple cross-wire techniques are derived from the single cross-wire phantom. These phantoms are composed of more than one wire crossing that again requires the alignment of one or more of the crossings in the image. The ultrasound images of these phantoms consist of points (cross wire) and/or lines.