This is the first study of the use of intraoperative contrast-enhanced 3-D fluoroscopy instead of a traditional CT-Scan for frameless stereotactic procedures, to the best of our knowledge. In a restricted number of patients, we report the first experiences with this technique. The examination protocol was developed during this study to generate images that display the cerebral vessels consistently via intravenous contrast administration. We rely on the experience of past studies, which were able to demonstrate that the intraoperative use of 3-D fluoroscopy for the imaging of aneurysm is possible [7]. In the field of functional and stereotactical neurosurgery, exact planning of the trajectory is crucial. By many groups, a contrast-enhanced CT scan fused with a preoperative MRI is considered to be the standard imaging in order to increase the accuracy of the anatomy depicted [2].
For frame-based systems, the patient's head must first be clamped in the ring before CT diagnostics with contrast medium injection can be performed under general anaesthesia. This requires an in-hospital transport, followed by a further delay of the point by fusion of the imaging at the planning station. In-hospital transports under analgosedation and mechanical ventilation have a higher risk of complications and delay the surgical procedure [3, 4]. Frameless systems, as used in this study, have the advantage that the patient does not need CT diagnostics under anaesthesia. The required CT with contrast agent injection can be performed the day before the operation and the images can be fused with the MRI data the day before as well. The time required for a stereotactic biopsy is thus reduced.
A disadvantage of both procedures is that intraoperative complications, in particular acute bleeding, can only be detected by CT scans after the operation. Thus, there will be a further in-hospital transport causing a delay of therapy. At times, surgeons may notice an ejection of blood through the puncture canal or a hemorrhagic biopsy. This observation, however, is only little sensitive and specific so that some kind of imaging has to be performed. Traditionally, this is done by CT meaning an interruption of surgery for a transport to the CT scanner unless a CT scanner is located in the OR. In case of a hemorrhage, valuable time is lost Intraoperative 3-D fluoroscopy might close this gap in the time line of this acute emergency situation. Figure 4a, b demonstrate that intracerebral hemorrhages can be visualized during the procedure at sufficient quality [2].
The benefits of the 3-D fluoroscopy for intraoperative position control of electrodes and for target and trajectory control have been described [8, 9]. The use of the intraoperative 3-D fluoroscopy is consistently being further developed and applied to other indications and other fields of cranial surgery as well as in the field of (neuro-) radiology to evaluate stent placements and occlusion during endovascular procedures [10].
Fluoroscopy scanners are mobile devices that can be placed wherever and whenever appropriate. Although the O-arm® device is quite large and requires a large operating room, it can be positioned to allow all necessary personnel and equipment to be placed normally [6, 7].
Surgery requires training and practice with the O-arm®. Our surgical team has 10 years of experience with the system, both in complex spinal procedures and in tumor navigation, including pituitary adenomas, in the skull base region. Our assessment is that the acquisition and reconstruction of the images is fast and can be fully integrated into the standard preoperative workflow without any delay if the necessary experience is given.
Since the result of the time analysis is clear and demonstrates a reduced operation time as another advantage of frameless procedures in combination with the intraoperative 3 D rotational fluoroscopy instead the CT scan.
Limitations
We demonstrate a potential new application field with this project. This study is based on our preliminary work and is intended to give an outlook on the potential this technology may develop. Of course we will conduct the follow-up studies with a correspondingly larger number of observers and patients.
3-D fluoroscopy is susceptible to metal artifacts (Fig. 5). Thus, special radiolucent Mayfield-Pins and a Carbon-Clamp were used. Since the use of equipment from the frameless stereotactic biopsy of the Medtronic Stealth Station® has not yet been combined with intaoperative O-arm® imaging, we developed a modification of the attachment of the guide-arm for the biopsy cannula and the navigation star (Fig. 6a, b). Contrast flow into the cerebral vessels depends on factors such as arterial blood pressure, injection speed, caliber and location of the venous line, and the presence of carotid stenosis. We injected the contrast agent manually via a 14- or 16-gauge central venous line and tolerated systolic arterial blood pressure values between 100 and 130 mm Hg. The factors mentioned above could delay the arrival of contrast and cause inadequate images. The use of an automated injector, tighter arterial blood pressure limits, a slight increase in contrast agent volume, and an algorithm that considers the height of the patient can optimize our present image acquisition protocol.
Radiation dose
As the system is normally used for spinal neurosurgery, there are no prefabricated profiles available for cranial use. A comparison with other radiological procedures is difficult because the doses used depend very much on the experience of the investigator and the constitution of the patient. The radiation dose of 25 mGy after one image series is similar to doses of native CT or CTA.
Future prospects
Any department that has experience with 3 d fluoroscopy can easily adopt this technique. The study was conceived to evaluate the effectiveness and feasibility of this technique in combination with a frameless stereotactical system. However, the present series is a pilot study and is too small to reliably validate whether this technique can increase the accuracy of biopsy or positioning of probes and if it may even be able to replace contrast CT for these purposes. The prerequisite for this would certainly be a further improvement and optimization of the radiation dose and contrast-injection protocol. We propose that intraoperative 3-D fluoroscopic angiography could also be a valuable diagnostic tool in emergency cases, e.g. hemorrhages under or following stereotactical procedures. In combination with a target and trajectory verification, which is also proven as a feasible method [8], it may add another useful application field driven by one device.
To reduce artifacts and to avoid damages at the carbon patch-clam, the production of carbon devices instead of metal guiding arms und navigation stars are desirable.
3-D fluoroscopy and image postprocessing are evolving techniques. Therefore, improved discrimination of soft-tissue structures, enhanced vascular and dynamic imaging, and improvements in image processing may be expected in the near future.