Intraoperative Imaging Techniques
Indocyanine Green–Based Video Angiography
Indocyanine green–based video angiography provides real-time information on the residual filling of aneurysms and on the patency of blood vessels of any size in the surgical field, including perforators, based on high-quality images. First introduced into the neurosurgical operating room in 2003, The ICG-VA technique was found to be a simple and safe way to assess blood flow intraoperatively. Sequential studies compared the findings of ICG-VA with iDSA or postoperative DSA and reported them to be comparable in 90% of cases. Subsequently, the use of DSA for intraoperative evaluation of aneurysms has decreased. In the setting of ICH surgery with an underlying vascular pathological entity such as cerebral aneurysm or AVM, ICG is a powerful tool (Fig. 1). The ICG-VA technique is now widely used in multiple settings beyond aneurysms. It is routinely used for intracranial and spinal AVMs and arteriovenous fistulas. Unquestionably, ICG-VA has become an important adjuvant in vascular neurosurgery.
(Enlarge Image)
Figure 1.
A right MCA aneurysm (asterisks) is demonstrated on preoperative 3D reconstructions of CTA (A) and on preoperative DSA reconstructions (B). An attempted endovascular closure of the aneurysm ended in aneurysm rupture and significant ICH (C; artifacts are caused by coils). The patient was urgently transported to the operating room and a right pterional approach was implemented. After evacuation of the ICH, an optical mode photograph (D) demonstrating the right M1 branch (arrow) and the 2 right M2 branches (arrowheads) was obtained. The area of the coil-treated dome portion of the aneurysm is depicted by the dashed line. Intraoperative ICG-VA photographs showing patent circulation in the right M1 branch (E; arrow) and in the 2 right M2 branches (F; arrowheads). Postoperative CT scans (G) showing a satisfactory evacuation of the ICH.
However, ICG is not suited for all applications. Its use is limited to the microscope's field of view and only to exposed blood vessels, so that not all vasculature can be assessed. In addition, there are various factors that may limit the accuracy of ICG fluorescence signals, such as thick-walled or partially thrombosed aneurysms or cases of deep-seated or giant aneurysms. The utility of this imaging modality in the setting of AVM surgery is still unclear because of these visualization issues. Because only the parts of the AVM that are already visible in the surgical field will be visible to ICG-VA, standard intraoperative DSA remains the gold standard imaging modality in the assessment of AVM resection.
We use ICG-VA on a routine basis during vascular surgeries, including ICH evacuations, when the need arises (Fig. 2). It helps us assess residual filling of vascular lesions such as aneurysms and the patency of surrounding vessels after clipping. Sometimes multiple injections are performed after complex clip reconstructions. Use of ICG-VA has reduced our need for intraoperative DSA, which is now reserved for more difficult cases or those in which there is doubt following the ICG-VA.
(Enlarge Image)
Figure 2.
Imaging studies obtained in a 20-year-old man under treatment for subacute bacterial endocarditis who suddenly developed headache and left hemiparesis and subsequently lost consciousness. A: Preoperative CT scan showing right temporooccipital ICH involving the ventricular system. B: Preoperative CTA scan demonstrating a right distal sylvian fissure mycotic aneurysm (see inset, arrowhead). C: Preoperative neuronavigation images obtained using the Stealth S7 system for craniotomy planning. D: Intraoperative ICG-VA photograph locating the mycotic aneurysm (asterisk) and differentiating it from the parent artery.
Intraoperative DSA
This is a powerful diagnostic tool in the surgical management of cerebral aneurysms and AVMs and is still considered the gold standard procedure when assessing precise aneurysm clipping and vessel patency. It allows the neurosurgeon to confirm complete occlusion of the aneurysm and to demonstrate patency of all surrounding vasculature.
Several groups have evaluated iDSA efficiency and have revealed that it is both effective and accurate. In the largest single-center study, of 1093 vascular cases in which patients underwent routine iDSA, a greater than 8% revision of clip placement due to abnormalities found on iDSA was reported. In 9 (8.9%) of 101 patients, iDSA demonstrated residual AVM requiring additional resection. Two of these patients (22.2%) required a second surgical revision, and successful excision of the residual AVM was confirmed by repeat iDSA in all 9 patients. With minimal risk of morbidity (0.99%) and neurological complications occurring in only 0.09% of cases, the authors reaffirmed iDSA safety and utility in the surgical treatment of aneurysms and vascular malformations.
As always, the value of iDSA must be weighed against the risk of complications and technical difficulties. The reported complication rate is as high as 3%. Various technical challenges include the need for skilled neuroradiology staff, high costs, the need for a DSA C-arm, a radiolucent operating table head holder, and angiography equipment. At times the additional equipment can make the operating room particularly cramped (Fig. 3). In the setting of ICH treatment, iDSA may be too cumbersome to perform during an urgent operation due to availability of staff or equipment.
(Enlarge Image)
Figure 3.
Photograph showing the operating room setup for microsurgical clipping of an MCA aneurysm with iDSA guidance. Although this is still considered the gold standard procedure when assessing precise aneurysm clipping and vessel patency, one should consider the various technical difficulties involved.
The drawbacks of iDSA may lead some to conclude that it is not a practical tool for all vascular cases. It is therefore best applied selectively, for complex vascular malformations and lesions in challenging anatomical locations.
Hybrid Operating Room: Applications in ICH Surgery
The concept of a hybrid operating room combining endovascular and microsurgical strategies within the same surgical session is applicable, cost-effective, and safe. Full biplane neuroangiography in a fully equipped neurosurgical operating room provides a seamless transition between the operation and iDSA, which can be performed without repositioning the patient or moving in a portable C-arm. The result is higher-quality angiography in 2 planes, which provides immediate intraoperative assessment of vessel patency and occlusion rate of different vascular pathological entities.
An example for this kind of setup is the addition of an iMRI unit to the hybrid operating room. Examples are the Advanced Multimodality Image-Guided Operating (A.M.I.G.O.) Suite at the Brigham and Women's Hospital and Harvard Medical School and the implementation of so-called IMRIS suites (IMRIS, Inc.) at various locations internationally. These systems are able to integrate a 1.5- to 3-T iMRI unit into a fully operational neurosurgical operating room and a full biplane angiography suite.
Intraoperative Microdoppler Techniques
Microdoppler imaging is a safe, noninvasive, and reliable technique for evaluation of the hemodynamics of vessels in the surgical field and of vascular malformations. It is low cost, and the time to result is 1–5 minutes in most cases. Additionally, it shows good correlation with postoperative angiography and is therefore widely used. It allows for the measurement of blood flow velocities in the malformation itself and in the surrounding vasculature. It is a valuable tool for providing immediate feedback to the surgeon, thereby improving the chances for an optimal surgical outcome because intraoperative complications such as vessel occlusion can be diagnosed and dealt with immediately. Several groups compared iMD findings to those in iDSA or postoperative DSA. They found a high rate of concordance between the iMD and angiographic findings regarding proper clip placement and complete occlusion of an intracranial aneurysm and associated clip-induced adjacent-vessel stenosis.
There are several iMD techniques available on the market today. Some are simple and cost-efficient, whereas others, such as the ultrasonic perivascular flow probe, are more advanced but are expensive and more cumbersome. The iMD modality carries the advantage of speed and ease of use and is therefore very attractive during surgery for ICH. The primary disadvantage of this technique is its inability to identify residual aneurysm remnants in cases of thrombosed or low-flow aneurysms—which are readily identified by intraoperative angiography and ICG-VA. That is to say, iMD provides qualitative and not quantitative results to the surgeon. Additionally, it is only useful for high-caliber vessels observed under the operating microscope.
Intraoperative MRI
Intraoperative MRI has been incorporated into modern neurosurgical operating rooms for more than a decade as a guide for neurosurgical interventions. This technology has proved to be a useful modality in vascular neurosurgery, especially for cavernous angiomas and AVMs. It provides a highly accurate and precise navigation tool, with excellent resolution, which is a prerequisite for localizing and targeting vascular lesions. It also addresses the problem of the ever-changing organization of intracranial structures during surgery by providing near-real-time, high-quality images.
One of the drawbacks of intraoperative MRI with regard to resection of vascular lesions is delineating the extent of resection. Clearly determining the edge of the lesion can be challenging due to the deposition of hemosiderin from hemorrhage. There are several types of MRI units used today. In our experience the use of iMRI in ICH surgery is limited to cavernoma surgery, and in cases in which MRI may help in delineating the margins of deep lesions surrounded with hematoma, in those cases we consider the low-field portable systems as the most useful. An example of a low-field iMRI unit is the OdinPoleStar system, which was first introduced in 2001. This compact MR scanner can be installed in a standard operating theater without major modification (Fig. 4). It functions with both an integrated optical system and an MRI tracking system and is operated by the neurosurgeon from an in-room computer workstation.
(Enlarge Image)
Figure 4.
Photograph showing the compact and mobile Medtronic PoleStar N30 surgical MRI system, consisting of a 0.15-T MRI unit integrated with a Medtronic StealthStation navigation system (A). Photographs showing the system being used for craniotomy approach for an ICH surgery related to cerebral cavernous angiomas (B), and for a transsphenoidal approach in a case of pituitary apoplexy (C). Reprinted with permission from Medtronic, Inc.
Intraoperative CT
Intraoperative CT imaging is a standard tool in the successful planning and execution of a diverse range of neurosurgical procedures. In the treatment of ICH, iCT systems such as the CereTom mobile scanner are used to assist in the confirmation of intraoperative positioning of catheters, the extent of hematoma evacuation, and the extent of vascular anomaly resection. The iCT images can be obtained and merged with those obtained using other modalities such as MRI if desired. Limitations of iCT include lower resolution than the large CT scanners, technician availability, and intraoperative imaging of the skull base. In the setting of ICH surgery, iCT has the advantage of very high sensitivity to acute blood.
Ultrasound-guided Evacuation of ICH
The iUS technique has undergone extensive development in recent years, and the quality of ultrasound imaging has improved considerably. Still, many neurosurgeons have been reluctant to try advanced versions of iUS due to negative past experiences. The quality of iUS imaging has improved significantly, such that the surgeon can readily assess ICH dimensions and identify structures within the brain in real time. This modality supports the concept of minimally invasive surgery by allowing the surgeon to identify which part of the clot presents closer to the cortical surface, thereby minimizing disruption to the surrounding brain because the shortest transcortical trajectory is taken to enter the clot. Orientation problems of iUS were improved by integrating navigation technologies. As a result, neurosurgeons find it relatively easy to understand and use the iUS navigation systems found in the market today.
The integration of 3D iUS and neuronavigation technology has created a real-time imaging method that can accurately demonstrate ICH. The 3D iUS systems such as the SonoWand have made this technology an efficient tool that can combine the real-time feedback from iUS with the detailed preoperative imaging from CT or MRI studies. During ICH evacuation procedures, iUS facilitates directing an instrument such as a Cavitron ultrasonic surgical aspirator or an endoscope to a target point in real time. During various vascular operations the iUS helps localize lesions and characterize their internal structure, and distance from the surface to the target can be calculated. The relation of various lesions to the surrounding brain can be appreciated before, during, and after excision of vascular pathological entities.