Print this Article 
Send to a Friend
Introduction
Pulmonary nodules and masses are common reasons for referring patients to the pulmonologist, radiologist, and thoracic surgeon for evaluation. The burgeoning use of chest computed tomography for screening, to rule out pulmonary embolism, and for other indications may lead to a significant increase in patients with newly discovered lesions. [1] Some of these lesions will be malignant while many are benign. Since lung cancer is a common disease, the main concern has to be to identify those patients with malignancies early, especially since the only known cure for lung cancer is surgical resection at an early stage.
If the majority of those nodules and masses were cancerous, surgical resection of all those lesions would be the logical first choice. Historically, this was the case based on older technology. A small percentage of benign nodule resections was considered a necessary byproduct of appropriate therapy. Currently, most large cohort studies demonstrate that most nodules are benign. [2] Therefore, surgery, with its associated morbidity and mortality, is not indicated for the majority of
 |
| Figure 1. Shown is an image of a CT fluoroscopy guided transbronchial biopsy of a lesion in the left upper lobe. The forceps is clearly visible in the lesion periphery. |
Bronchoscopy presents a less invasive option for the diagnostic work up of these lesions. Unfortunately, the historical yield of endoscopic lung biopsies is poor. [4] This is due to suboptimal guidance of the forceps and other instruments with the help of conventional fluoroscopy. The yield is especially low for lesions less than 2cm in size. Additionally, the instruments cannot be steered per se, but can only be indirectly manipulated by pulling and pushing the forceps into different segments and subsegments.
Visualization can be improved by using CT fluoroscopy (Figure 1). This does not solve the problems with instrument steerability, is associated with significant radiation exposure to patient and operator, has limited availability, and is often logistically difficult to accomplish.
Technique
Electromagnetic (EM) guidance systems present a promising way out of this dilemma. Examples from one such system (superDimension, Inc., Minneapolis, MN) are shown below. Derived from military technology, the necessary components have been miniaturized and can now be used with endoscopy. During the procedure, the patient is placed in a low frequency electromagnetic field that is generated by a board located below the mattress of the endoscopy table (Figure 2). Sensors on his torso help locate him in the field and compensate for movement (Figure 3). A sensor mounted on a probe provides feedback about the position (x, y and z axis) and movement (yaw, pitch and roll) of the instrument or extended working channel within the electromagnetic field. A important feature of this system is the steerability of the device itself, facilitated by a handle on the proximal end. This vastly improves the ability of the operator to maneuver the instruments into the desired direction and location (Video 1). The feedback is displayed in real time on a monitor and, coupled with a previously acquired CT, creates a virtual environment of the chest of the patient. Currently, the sensor is located on a probe that is attached to a sheath catheter. When the target is reached, the probe is removed and the sheath left in place, allowing for instruments to be passed through to the target lesion.
 |
 |
| Figure 2. Shown is the board creating the electromagnetic field. It is thin and can be easily placed below the mattress of the stretcher. | Figure 3. Shown is the miniaturized sensor that allows for tracking within a electromagnetic field. |
In the pre-procedure phase, recent CT data on CD is fed into the system. The quality of the chest CT determines the quality of the virtual environment. Whenever possible, a multidetector CT scanner should be used with finer detail leading to better resolution. The proprietary software analyzes the digitized CT and a virtual environment is created with an endoscopic view as well as a sagittal, axial and coronal views. These views can be individualized to the operator’s preference.
Once the CT has been analyzed, the next step is the planning phase. The operator identifies the target lesion(s) and the airways leading to them, marks points on the track as well as at least four reference points at carinal surfaces. Marking easily recognizable reference points is important, as the actual procedure happens within the patient, not in the virtual environment. When starting the endoscopy, the prior established reference points will again be identified and touched in the patient with the EM sensor (Video 2). This allows for the virtual and actual environments to be properly synchronized.
 |
| Figure 4. Shown is a screen snapshot of the approached lesion in question. The radiological views confirm entry into the lesion and so does the “Bull’s eye image” in the lower right hand corner. |
Once synchronization has been achieved with an acceptable margin of error, the sheath with the guiding probe is advanced and the progress is observed on the virtual screen. The paths can be tracked, observed in multiple planes, and the instruments actively steered towards the target. An additional helpful view is the “forward view” simulating the view straight from the bronchoscopic tip which is similar to a weapon’s view in a fighter plane. It gives the actual distance to the target and allows it to be centered when reached. The arrows also indicate in which direction to move the locatable guide and when the direction is wrong (Video 3). Once the target is reached (Figure 4), the probe is removed and biopsy forceps, brushes or needles can be introduced and specimens obtained.
Discussion
In earlier publications,[5,6] biopsy yields could be improved to 75% with this technology. In our experience this should be even higher, since several improvements have been made to the system since then. It also allows for the exact localization and biopsies of even very small abnormalities down to a diameter of about 9 or 10 mm.
This new generation of endoscopic technology seems highly effective and promising. Additional improvements are already on the way that should make it even more precise. For example, new CT technology allows for precise delineation of airways leading the lesions, therefore potentially allowing for the creation of specific electronic tracks, similar to a GPS system in a car. Additionally, the sensors, when smaller and cheaper, could be directly mounted to the biopsy instruments. This would decrease the potential error associated with the manipulation of the sheath once the probe is removed.
An intriguing peek into the future is the potential to create multipurpose instruments: once the lesion is reached, an optical coherence tomography (OCT) catheter could be passed for instant optical biopsy of the lesions, followed by a biopsy procedure. If a malignancy is seen by OCT, the lesion could be ablated in appropriate patients in the same setting with for example radiofrequency or other local ablative therapy. Other types of local therapies could be precisely delivered to specific benign and malignant targets.
Conclusion
The introduction of electromagnetic guidance systems presents one of many new exiting new technologies, helping to propel flexible bronchoscopic procedures and its capabilities into the new century. The technology combines the relatively low risk of bronchoscopy with an increased yield, lower radiation exposure, and potential for targeted therapy with the increasingly commonly found pulmonary nodule. This allows surgery to be applied more specifically, hopefully leading to a higher percentage of malignancies resected at an early stage while avoiding the resection of more benign lesions.