Navigation systems have been developed to increase target accuracy, reduce procedure time, reduce procedure cost and, if, possible to reduce radiation exposure, writes Miltiadis Krokidis, Cambridge, UK.
The overall concept behind navigation systems is to localise an object and the exact relationship that the object has with its surrounding structures. Navigation systems in medicine were initially introduced in neurosurgery and orthopaedics and their use has been considered a significant milestone, comparable to the use of general anaesthesia or the use of microscopes as they offered surgeons the possibility to reach targets that were otherwise not visible, to avoid adjacent organs and to be oriented in the absence of anatomical landmarks.
In interventional radiology, imaging offers all that. Tissue targets are visible, adjacent organs can (mostly) be avoided and there is always perception of needle position in relationship with the surrounding anatomical organs. So what could be the added value of navigation systems and why have they been developed for interventional radiology applications?
The quintessence of interventional radiology is accurate device manipulation under image guidance to target specific tissue. Percutaneous image-guided interventions offer a minimally invasive solution for a variety of conditions and their number has exponentially increased in the last decades. Image-guided biopsies, drainages and tumour ablation have an established role in the management of complex patients. Accurate needle placement is of paramount importance to achieve treatment and to avoid complications. In the past, tissue targeting depended exclusively on the talent, the stereotactic perception and the experience of the operator. As the complexity of percutaneous interventions increased the necessity for accuracy and reproducibility has also increased. Navigation systems have been therefore developed aiming to increase target accuracy, to reduce procedure time, to reduce procedure cost and if possible to reduce radiation exposure.
The main system categories that have been developed for interventional radiology navigation are computed tomography (cone-beam CT [CBCT]) systems, electromagnetic tracking systems and optical navigation devices.
CBCT systems may be used alone or in a hybrid setup with fluoroscopy. In the latter, the needle trajectory is planned after CT pictures are obtained and the path is fused onto real time fluoroscopy pictures. Such systems appear to increase technical success rates of needle placement procedures. The main drawback is the radiation exposure that is required for real time needle placement and the fact that patient respiratory motion may impair registration and decrease accuracy. Significant reduction of radiation may be achieved with the use of electromagnetic systems. These are based on the use of sensor coils with differential magnetic fields and offer a significant advantage in ablation procedures and lung biopsy.
Optical navigation devices use a stereoscopic camera that emits infrared light and can determine a 3D position of reflective marker spheres. This allows for real-time tracking of the position of the marker spheres. A free line of sight is required between the camera and the markers. Systems that use fusion imaging combined with optical navigation have been also developed. The CAS-One navigation system (Cascination) is based on this principle. The system uses four to six reflective spheres that are positioned in a predefined array in the line of sight of the infrared camera. The patient is positioned in a vacuum fixation system in order to limit potential movement. A CT scan of the target region with 1mm slice reconstruction is obtained and the images are fused with the registration information obtained from the optical markers. The position of the markers is confirmed on fusion imaging. Three further optical markers are placed in a mechanical arm that is used for needle placement. When the needle trajectory is defined, the mechanical arm is locked and the needle can be potentially inserted without image guidance, significantly reducing radiation exposure.
The system was recently introduced in the UK and the first two cases were performed in Cambridge University Hospitals for the ablation of two kidney tumours with excellent results.
Navigation systems inevitably come with an added initial cost that is not negligible. Nevertheless, in the long term, the standardisation of procedures, the predictability of the result, the reduction of procedure time and the lower morbidity/ mortality are expected to offer a reduction of the overall cost. More precise needle placement will lead to more accurate tissue sampling for biopsies; increase the range of lesions that may be considered as suitable for drainage; and permit the placement of multiple ablation electrodes with specific distance and array configuration. Furthermore, with the use of fusion imaging, lesions that are not visible on standard imaging modalities may be detected and targeted, therefore, increasing the range of lesions that may be treated percutaneously.
As technology evolves, navigation systems will be further integrated into interventional radiology procedures. The use of optical navigation systems with integrated fusion imaging and no requirement for continuous screening appears to open new perspectives for percutaneous interventions.
Miltiadis Krokidis is a consultant vascular and interventional radiologist, Cambridge University Hospitals, Cambridge, UK. He has reported no disclosures pertaining to this article.
