Intraoperative CT – Better Views for Neurosurgeons
Philipp Grätzel von Grätz | 28-09-2015
SOMATOM Sessions talked to three pioneers of intraoperative CT imaging in brain surgery, spinal surgery, and neurovascular surgery. They argue that intraoperative CT can increase patient safety, results in a smoother neuronavigation workflow, and has a reasonable cost-benefit ratio.
Tumor surgery: Where exactly is the brain?
The specific benefits of intraoperative CT are best illustrated with typical patient scenarios. Professor Jörg-Christian Tonn, MD, Department of Neurosurgery, University Hospital Grosshadern, Munich uses a neuronavigation system to resect brain tumors with the goal of preventing further brain damage. But a neuronavigation system is only as reliable as the images that it can draw upon: “We usually have preoperative MRI images. However, during surgery, many things change. When we resect tumors of the brain tissue, the brain might shift to one side or the other. And in meningioma patients, the surgical field changes due to the drilling that needs to be done to remove this tumor, which can involve the adjacent bony region.” Using intraoperative CT allows the dataset for the neuronavigation system to be updated whenever necessary. “The result is that intraoperative CT imaging with the SOMATOM Definition AS becomes easier to preserve delicate structures, and at the same time we can be more certain that we have removed the tumor as completely as possible,” says Tonn.
Spinal fusion: More accurate screw placement, fewer complications
Spinal fusion surgery is another area where intraoperative CT comes into its own. In spinal fusion, adjacent vertebral bodies are fused mechanically with the help of screws and rods in order to reduce spinal instabilities. According to Professor Stefan Zausinger, MD, Department of Neurosurgery, University Hospital Grosshadern, Munich, these spinal instabilities mostly affect patients with degenerative diseases of the spine: “But we also see patients with traumatic spinal injuries and with spinal instabilities as a result of malignant tumors.”
Intraoperative CT-imaging after navigated screw placement in L4: All screws are placed correctly and completely within the pedicles and vertebral bodies without damage to surrounding structures. Please note the dorsally positioned radiotranslucent retractor, which can be left in place during the scanning procedure without causing artifacts to the images (Fig. 1A). Postoperative X-ray image of the lumbar spine confirms correct position of transpedicularly placed screws in vertebral L4 and L5 (Fig. 1B).
In times of shifting demographics, spinal fusion surgery looks set to become more common. In the U.S., there were more than 465,000 fusion surgeries in 2011, according to the analysts at GlobalData. The challenge with spinal fusion is that the screws need to be placed as accurately as possible for two reasons, as Zausinger points out: “Malpositioning of the implants can cause damage to the nerve roots or to the spinal cord, or to some vessels adjacent to the spinal cord. And it can also lead to an insufficient stabilization procedure with persistent and often painful instability.”
In order to minimize the risks that are associated with malpositioning, Zausinger routinely uses intraoperative CT for spinal fusion surgery. The first examination is performed right before the screws are to be placed. The dataset is fed directly into the neuronavigation system, so that navigation is as accurate as possible. Once all the screws are in place, a second intraoperative CT is performed in order to check their positions.
Zausinger says there are two major advantages to this setup: “First, the images that are necessary for the navigation-aided stabilization procedure are generated in the final surgical position of the patient. This results in more accurate images and thus improved navigation. The second advantage is that we can check the position of the implants immediately. We can see during surgery whether the screws or the rods are compressing any structure, and we also see hematomas right away.” Should a problem be identified, it can be corrected immediately. This is not only efficient, it can also spare the patient from a potential re-operation.
The benefits of intraoperative CT in spinal fusion come without additional radiation exposure: A CT examination before surgery has to be carried out anyway. And after surgery, it is necessary to perform a control CT as well. “The only difference is that, in our setup, the two CTs are taken intraoperatively,” says Zausinger. In fact, with intraoperative CT there are even workflow benefits: Thanks to a deep integration with the neuronavigation system, the coregistration procedure with the CT images takes place automatically. All that has to be done is to place a reference star at a spinous process.
Clipping of aneurysms: What about perfusion?
In aneurysm surgery, there is no navigation. Nevertheless, intraoperative CT can make a big difference. Associate Professor Christian Schichor, MD, Department of Neurosurgery, University Hospital Grosshadern, Munich started using intraoperative CT in patients in need of aneurysm clipping surgery four years ago. Three to four such surgeries are performed in Grosshadern per week, and many of them take advantage of intraoperative CT. “Patients with complicated vessel lesion, for example large aneurysms, or aneurysms that are partially thrombosed, benefit most from intraoperative CT,” says Schichor. When using intraoperative CT, he looks specifically at CT angiography and CT perfusion scans: “In CT angiography, we see whether the vessel patency is preserved. And with CT perfusion scans, we can have a look at the distant perfusion in order to evaluate whether the clipping has some detrimental effect on other brain regions.”
Dynamic CT imaging allows them obtain functional information, beyond tissue morphology. With CT perfusion imaging it is possible to obtain information about blood flow, blood volume, and various other perfusion metrics as permeability, time to drain, time to peak and mean transit time. With 4D-CT angiography one can see movies of blood flow from arterial to venous phase to assess the hemodynamic status of the tissue.
Schichor says that no other imaging modality is capable of providing this kind of information intraoperatively: “With MRI and ultrasound, you are always dealing with the problem of visual artifacts produced by the clips. And conventional angiography tells us nothing about distant brain perfusion.” Looking back, Schichor recalls several patients in whom the surgeon who performed the clipping was convinced that he had occluded the aneurysm. Only intraoperative CT revealed that this was not the case. “We also had one patient in whom a large aneurysm was pressing the clip downwards toward the vessel. Only in the perfusion CT did we see an elative perfusion deficit in distant regions of the brain. We corrected the clipping, and perfusion was back to normal.”
Intraoperative CT: Advantages in terms of safety, usability, and costs
It is these kinds of patient that best illustrate the benefits of intraoperative CT in neurosurgery. Tonn, Schichor, Zausinger, and their colleagues have gathered data from several prospective patient series in order to better analyze these benefits scientifically. In one of these studies, the experts compared intraoperative CT-assisted neuronavigation with conventional neuronavigation based on preoperative imaging in patients with spinal fusion surgery. Zausinger: “We found that repositioning was necessary in about seven percent of screws. This percentage is very similar to the percentage of patients who had to be reoperated in the era before intraoperative CT was available. Since the introduction of intraoperative CT, there has not been a single patient who had to be reoperated due to repositioning of the screws. Screw position can now be proved by intraoperative CT images during the procedure and can be corrected immediately if necessary.
Another patient series was about meningioma patients. Surgeons at the University used intraoperative CT to visualize tumor tissue for resection that might otherwise have been missed without having images from intraoperative CT available for assessing the tumor resection success during surgery.
Reducing postoperative complications is also a concern for surgeons; therefore, having access to reliable images during surgery is important. Tonn explains that by reducing complications, the likelihood of a re-admission decreases, which means that the hospital saves money. “Intraoperative CT therefore can actually support cost efficiency for a hospital,” he concludes.
One gantry and two sliding doors
It is no surprise, then, that Tonn was able to convince the hospital administration at University Hospital Grosshadern to buy a second intraoperative CT to be installed in the hospital’s new surgery building, which opened its doors in summer 2014.
The new “Operationszentrum” (OPZ) directly adjacent to the hospital’s main building is an eye-catcher, architecturally. But above all it is designed as an answer to the demands of the increasingly interdisciplinary nature of surgery. The building, which required an investment of 196 million euros, features 32 brand-new operating theaters that are equipped with state-of-the-art medical technology.
The OPZ’s intraoperative CT is a modern SOMATOM Definition AS+ with 128 slices. It is a dual-room solution that is mounted on rails in-between two operating theaters, separated by two sliding doors, so that the CT can be used by surgeons in both rooms.
Tonn expects the second intraoperative CT to increase interest in the technology considerably: “It will be easier for other surgical specialties to use intraoperative CT, since the technology will be available in two rooms. The previous system is already being used occasionally by trauma surgeons to screw pelvic or other complex fractures. ENT surgeons are also very interested, and in the new building we will share a floor. I am convinced that adoption of intraoperative CT will increase, and this is certainly in the interests not only of the patients but also of our hospital administrators.”
About the Author
Philipp Grätzel von Grätz, MD is a medical doctor turned freelance writer and book author based in Berlin, Germany. His focus is on biomedicine, medical technology, health IT, and health policy.