Technique for Targeting Arteriovenous Malformations Using Frameless Image-Guided Robotic Radiosurgery

Frameless Co-Registration of Biplane 2D Digital Subtraction Angiography Whole Frames and 3D Rotational Angiography-Based Cone-Beam Computed Tomography Angiogram on Dedicated Software for Stereotactic Radiosurgery of Cranial Vascular Malformations

Purpose: Given its high spatial resolution and vasculature selectivity, the cone-beam computed tomography (CT) angiography (CBCTA) image acquired by selective 3D rotational angiography (3DRA) is the most suitable 3D image for the target definition of stereotactic radiosurgery (SRS) for intracranial arteriovenous malformations (AVMs) and dural arteriovenous fistulas (DAVFs). Furthermore, the relatively low temporal resolution of 3DRA-based CBCTA can be complemented by the stereotactic co-registration of orthogonally paired 2D dynamic digital subtraction angiography (2D-DSA). The integration of 2D-DSA, which is usually limited to one or a few frames for each projection, into CBCTA and/or planning CT can be achieved only by catheter-directed angiography on the day of SRS via a dedicated image localizer under rigid frame fixation to the skull, which imposes substantial burdens on patients. This study aimed to demonstrate a novel, convenient, and significantly less invasive method for the frameless co-registration of biplane 2D-DSA whole frames and CBCTA on commercially available dedicated software, namely, Brainlab^® Elements (Brainlab AG, Munich, Germany), and present its prerequisite for successful image fusion.

Technical Report: Elements have afforded the following functionality: A 3D vasculature image is automatically extracted as a floating image from any 3D image series containing vascular details and then subsequently co-registered manually and automatically to a selected frame pair of 2D-DSA with a six-degree-of-freedom rigid registration. As a preclinical feasibility study, two anonymous image datasets from patients harboring cerebral AVM and transverse-sigmoid (TS) DAVF were used to verify the accuracy and practicality of Elements for the frameless co-registration of 2D/3D vascular images, particularly on the assumption of clinical workflow for the target delineation of SRS planning. The use of ordinary unsubtracted CBCTA resulted in the insufficient extraction of abutting vessels or vessels that are in close proximity to bony structures, particularly in the case of TS-DAVF, where the fistulous pouch and the affected venous sinuses were adjacent to the cranial bone. By contrast, the amount and selectivity of vasculatures and the accuracy of subsequent image fusion were significantly improved from the subtracted CBCTA. The integration of CBCTA into dynamic 2D-DSA allowed the simultaneous review of both image information by sharing any concerning point and 2D or 3D structures under a common 3D coordinate.

Conclusions: Elements enable the clinically useful frameless co-registration of biplane 2D-DSA whole frames into CBCTA, for which the routine acquisition of both subtracted and unsubtracted CBCTA axial images for ordinary diagnostic purposes is an indispensable prerequisite for successful image fusion and further widespread application. This frameless integration of the 2D/3D angiogram would dramatically enhance both the frame-based and frameless SRS workflow and circumstances by allowing users to forward SRS planning well in advance before SRS, along with the omission of invasive angiography on the day of SRS, and would broaden the implementation of frameless SRS. Furthermore, the comprehensive alternating interactive review of the 2D/3D integrated angiogram leads to a more in-depth quasi-4D understanding of the affected angioarchitectures compared with the separate viewing of each image.

Segmentation techniques of brain arteriovenous malformations for 3D visualization: a systematic review

BACKGROUND

Visualization, analysis and characterization of the angioarchitecture of a brain arteriovenous malformation (bAVM) present crucial steps for understanding and management of these complex lesions. Three-dimensional (3D) segmentation and 3D visualization of bAVMs play hereby a significant role. We performed a systematic review regarding currently available 3D segmentation and visualization techniques for bAVMs.

METHODS

PubMed, Embase and Google Scholar were searched to identify studies reporting 3D segmentation techniques applied to bAVM characterization. Category of input scan, segmentation (automatic, semiautomatic, manual), time needed for segmentation and 3D visualization techniques were noted.

RESULTS

Thirty-three studies were included. Thirteen (39%) used MRI as baseline imaging modality, 9 used DSA (27%), and 7 used CT (21%). Segmentation through automatic algorithms was used in 20 (61%), semiautomatic segmentation in 6 (18%), and manual segmentation in 7 (21%) studies. Median automatic segmentation time was 10 min (IQR 33), semiautomatic 25 min (IQR 73). Manual segmentation time was reported in only one study, with the mean of 5-10 min. Thirty-two (97%) studies used screens to visualize the 3D segmentations outcomes and 1 (3%) study utilized a heads-up display (HUD). Integration with mixed reality was used in 4 studies (12%).

CONCLUSIONS

A golden standard for 3D visualization of bAVMs does not exist. This review describes a tendency over time to base segmentation on algorithms trained with machine learning. Unsupervised fuzzy-based algorithms thereby stand out as potential preferred strategy. Continued efforts will be necessary to improve algorithms, integrate complete hemodynamic assessment and find innovative tools for tridimensional visualization.

Analysis of potential time saving in brain arteriovenous malformation stereotactic radiosurgery planning using a new software platform

To evaluate the utility of integrating a 3D vessel tree co-registration software platform into the stereotactic radiosurgery (SRS) workflow and its time saving for brain arteriovenous malformation (bAVM) treatment in adults compared to the conventional stereotactic head frame workflow. Eight consecutive adult bAVM cases were selected and retrospectively reviewed. Total number of angiograms and SRS procedures were 8. The electronic medical records were analyzed by time stamps to determine the length of time for each component of the set-up, transport, and frame removal. Times were averaged and the start of sedation by anesthesia used as a surrogate for the start of the frame application process. Reductions in workflow times were then modeled assuming cerebral angiography as a separate procedure. There were 8 adult bAVM cases included. Six were female. All patients had a single treatment session. Average age was 51.5 years (Range: 36-71). All patients were treated under monitored anesthesia care. In 6 patients, the AVM was deeply located (basal ganglia, midbrain, brainstem); in 2 cases, the lesion was frontal. Spetzler-Martin grades were 4 (50%) Grade 2 and 4 (50%) Grade 3. The average prescription isodose volume (PIV) and 12 Gy volumes (V_12Gy) were 0.85 cc and 1.74 cc, respectively. The mean time from frame application to arrival in the angiography room was 111.5 minutes (range 40 to 171 min; median 107 min; SD 35.3 min); transport from angiography room to SRS was 47.5 minutes (range 15 to 107 min; median 36 min; SD 31.1 min), and frame removal after SRS was 20.5 minutes (range 10 to 47 min; median 16 min; SD 11.6 min). The average total additional time for the entire process of frame application, patient transportation, and frame removal was 132 minutes (range 87 to 181 min; median 127.5 min; SD 28.4 min). Therefore, assuming a non-frame based workflow and with angiography performed ahead of the actual radiosurgical treatment, the total time savings on the day of treatment was estimated at 132 minutes (range 87 to 181 min; median 127.5 min; SD 28.4 min). The ability to perform angiography, image fusion, and treatment planning for the actual day-of-delivery using 3-dimensional vessel tree co-registration could result in significant time savings over traditional workflow practices. Further experience with this system will evaluate its accuracy, reproducibility, and potential broader use in SRS workflow paradigms for the treatment of vascular pathologies. For bAVMs, the benefits of this time savings might allow for streamlined workflows on the day of SRS.

Magnetic Resonance Imaging-Based Robotic Radiosurgery of Arteriovenous Malformations

Objective

CyberKnife offers CT- and MRI-based treatment planning without the need for stereotactically acquired DSA. The literature on CyberKnife treatment of cerebral AVMs is sparse. Here, a large series focusing on cerebral AVMs treated by the frameless CyberKnife stereotactic radiosurgery (SRS) system was analyzed.

Methods

In this retrospective study, patients with cerebral AVMs treated by CyberKnife SRS between 2005 and 2019 were included. Planning was MRI- and CT-based. Conventional DSA was not coregistered to the MRI and CT scans used for treatment planning and was only used as an adjunct. Obliteration dynamics and clinical outcome were analyzed.

Results

215 patients were included. 53.0% received SRS as first treatment; the rest underwent previous surgery, embolization, SRS, or a combination. Most AVMs were classified as Spetzler-Martin grade I to III (54.9%). Hemorrhage before treatment occurred in 46.0%. Patients suffered from headache (28.8%), and seizures (14.0%) in the majority of cases. The median SRS dose was 18 Gy and the median target volume was 2.4 cm³. New neurological deficits occurred in 5.1% after SRS, with all but one patient recovering. The yearly post-SRS hemorrhage incidence was 1.3%. In 152 patients who were followed-up for at least three years, 47.4% showed complete AVM obliteration within this period. Cox regression analysis revealed Spetzler-Martin grade (P = 0.006) to be the only independent predictor of complete obliteration.

Conclusions

Although data on radiotherapy of AVMs is available, this is one of the largest series, focusing exclusively on CyberKnife treatment. Safety and efficacy compared favorably to frame-based systems. Non-invasive treatment planning, with a frameless SRS robotic system might provide higher patient comfort, a less invasive treatment option, and lower radiation exposure.

Evaluation of a new software prototype for frameless radiosurgery of arteriovenous malformations

Background

In order to locate an arteriovenous malformation, typically, a digital subtraction angiography (DSA) is carried out. To use the DSA for target definition an accurate image registration between CT and DSA is required. Carrying out a non-invasive, frameless procedure, registration of the 2D-DSA images with the CT is critical. A new software prototype is enabling this frameless procedure. The aim of this work was to evaluate the prototype in terms of targeting accuracy and reliability based on phantom measurements as well as with the aid of patient data. In addition, the user’s ability to recognize registration mismatches and quality was assessed.

Methods

Targeting accuracy was measured with a simple cubic, as well as with an anthropomorphic head phantom. Clearly defined academic targets within the phantoms were contoured on the CT. These reference structures were compared with the structures generated within the prototype. A similar approach was used with patient data, where the clinically contoured target served as the reference structure.

An important error source decreasing the target accuracy comes from registration errors between CT and 2D-DSA. For that reason, the tools in BC provided to the user to check these registrations are very important. In order to check if the user is able to recognize registration errors, a set of different registration errors was introduced to the correctly registered CT and 2D-DSA image data sets of three different patients. Each of six different users rated the whole set of registrations within the prototype.

Results

The target accuracy of the prototype was found to be below 0.04 cm for the cubic phantom and below 0.05 cm for the anthropomorphic head phantom. The mean target accuracy for the 15 patient cases was found to be below 0.3 cm.

In the registration verification part, almost all introduced registration errors above 1° or 0.1 cm were detected by the six users. Nevertheless, in order to quantify and categorize the possibility to detect mismatches in the registration process more data needs to be evaluated.

Conclusion

Our study shows, that the prototype is a useful tool that has the potential to fill the gap towards a frameless procedure when treating AVMs with the aid of 2D-DSA images in radiosurgery. The target accuracy of the prototype is similar to other systems already established in clinical routine.

Integrating 3D Rotational Angiography into Gamma Knife Planning

3D rotational angiography provides remarkable spatial resolution for cerebrovascular disorders; however, it cannot be integrated directly into gamma knife planning due to the discrepancy of DICOM "tag" information, and most physicians still cannot benefit from 3D rotational angiography. Here, we describe a simple and easy technique to enable the integration of 3D rotational angiography.

Skeletonization method for vessel delineation of arteriovenous malformation

Cerebral arteriovenous malformation (AVM) presents a great health threat due to its high probability of rupture that can cause severe brain damage. Image segmentation alone is not sufficient to support AVM embolization procedure. In order to successfully navigate the catheter and perform embolization, the segmented blood vessels need to be classified into feeding arteries, draining veins and the AVM nidus. For this reason we address here the AVM localization and vessel decomposition problem. We propose in this paper a novel AVM localization and vessel delineation method using ordered thinning-based skeletonization. The main focus of vessel delineation is the delineation of draining veins since it is essential for the embolization procedure. The main contribution is a graph-based method for exact extraction of draining veins which, in combination with our earlier work on AVM detection, allows the AVM decomposition into veins, arteries and the nidus (with an emphasis on the draining veins). We validate the proposed approach on blood vessel phantoms representing the veins and the AVM structure, as well as on cerebral 3D digital rotational angiography (3DRA) images before and after embolization, paired with digital subtraction angiography (DSA) images. Results on AVM delineation show high correspondence to the ground truth structures and indicate potentials for use in surgical planning.

Modern robot-assisted radiosurgery of cerebral angiomas-own experiences, system comparisons, and comprehensive literature overview

Cerebral arteriovenous malformations (AVMs) are rare vascular lesions potentially responsible for substantial neurological morbidity and mortality. Over the past four decades, radiosurgery has become a valid therapeutic option for many patients with small intracranial AVMs, but reports describing the use of robotic stereotactic radiosurgery (SRS) are rare. The purposes of this study are to describe the efficacy and toxicity of robotic SRS for AVMs and to review the literature. The reports of 48 consecutive patients treated with SRS were reviewed. A total dose of 18 Gy in a single fraction was prescribed to the 70% isodose line. Efficacy (i.e., total obliteration of the AVM) and toxicity were analyzed. Literature search was performed on Embase and PubMed for the terms "Radiosurgery and AVMs", "Cyberknife and AVMs" and "Radiation therapy and AVMs." The median follow-up was 41 months. The median AVM volume was 2.62 cm³. The incidence of obliteration was 59% at 3 years. Regarding toxicity, 92% of patients remained symptom-free, 66% developed radiogenic edema on MRI, and none developed radionecrosis. Forty-one patients (85%) had embolization prior to SRS. Our study was incorporated in an exhaustive review of 25 trials categorized by SRS technique. In this review, the median follow-up was 60 months. The median nidus volume was 2 cm³. The median overall obliteration rate for SRS was 68% (range 36 to 92). The median embolization rate prior to SRS was 31% (range 8.23 to 90). Compared to other studies, tolerability was excellent and the obliteration rate was acceptable but probably affected by the high embolization rate prior to radiosurgery. Our study suggests that a higher dose is feasible. A larger cohort with a longer follow-up period will be needed to confirm the safety and effectiveness, and subsequently validate different prognosis and predictive scores with this treatment modality to maximize the benefits of this technology for selected patients in the long term.

Analysis of the targeting uncertainty of a stereotactic frameless radiosurgery technique for arteriovenous malformation

In order to target arteriovenous malformations (AVM) in a frameless approach, registration of two-dimensional (2D) digital-subtracted-angiographs (DSA) with three-dimensional (3D) computed tomography (CT) is required. Targeting accuracy and delineation of a frameless 2D-DSA and 3D-CT image registration tool based on bony anatomy of the skull was evaluated. This frameless approach assures accurate target localization and can be used in a clinical setting.

Multi-staged robotic stereotactic radiosurgery for large cerebral arteriovenous malformations

PURPOSE

To investigate a multi-staged robotic stereotactic radiosurgery (SRS) delivery technique for the treatment of large cerebral arteriovenous malformations (AVMs). The treatment planning process and strategies to optimize both individual and composite dosimetry are discussed.

METHODS

Eleven patients with large (30.7 ± 19.2 cm(3)) AVMs were selected for this study. A fiducial system was designed for fusion of targets between planar angiograms and simulation CT scans. AVMs were contoured based on single contrast CT, MRI and orthogonal angiogram images. AVMs were divided into 3-8 sub-target volumes (3-7 cm(3)) for sequential treatment at 1-4 week intervals to a prescription dose of 16-20 Gy. Forward and inversely developed treatment plans were optimized for 95% coverage of the total AVM volume by dose summation from each sub-volume, while minimizing dose to surrounding tissues. Dose-volume analysis was used to evaluate the PTV coverage, dose conformality (CI), and R50 and V12 Gy parameters.

RESULTS

The treatment workflow was commissioned and able to localize within 1mm. Inverse optimization outperformed forward planning for most patients for each index considered. Dose conformality was shown comparable to staged Gamma Knife treatments.

CONCLUSION

The CyberKnife system is shown to be a practical delivery platform for multi-staged treatments of large AVMs using forward or inverse planning techniques.

Target delineation for radiosurgery of a small brain arteriovenous malformation using high-resolution contrast-enhanced cone beam CT

Three years following endovascular embolization of a 3 mm ruptured arteriovenous malformation (AVM) of the left superior colliculus in a 42-year-old man, digital subtraction angiography showed continuous regrowth of the lesion. Thin-slice MRI acquired for treatment planning did not show the AVM nidus. The patient was brought back to the angiography suite for high-resolution contrast-enhanced cone beam CT (VasoCT) acquired using an angiographic c-arm system. The lesion and nidus were visualized with VasoCT. MRI, CT and VasoCT data were transferred to radiation planning software and mutually co-registered. The nidus was annotated for radiation on VasoCT data by an experienced neurointerventional radiologist and a dose/treatment plan was completed. Due to image registration, the treatment area could be directly adopted into the MRI and CT data. The AVM was completely obliterated 10 months following completion of the radiosurgery treatment.

Target delineation for radiosurgery of a small brain arteriovenous malformation using high-resolution contrast-enhanced cone beam CT

Three years following endovascular embolization of a 3 mm ruptured arteriovenous malformation (AVM) of the left superior colliculus in a 42-year-old man, digital subtraction angiography showed continuous regrowth of the lesion. Thin-slice MRI acquired for treatment planning did not show the AVM nidus. The patient was brought back to the angiography suite for high-resolution contrast-enhanced cone beam CT (VasoCT) acquired using an angiographic c-arm system. The lesion and nidus were visualized with VasoCT. MRI, CT and VasoCT data were transferred to radiation planning software and mutually co-registered. The nidus was annotated for radiation on VasoCT data by an experienced neurointerventional radiologist and a dose/treatment plan was completed. Due to image registration, the treatment area could be directly adopted into the MRI and CT data. The AVM was completely obliterated 10 months following completion of the radiosurgery treatment.

Frameless angiogram-based stereotactic radiosurgery for treatment of arteriovenous malformations

PURPOSE

Stereotactic radiosurgery (SRS) is an effective alternative to microsurgical resection or embolization for definitive treatment of arteriovenous malformations (AVMs). Digital subtraction angiography (DSA) is the gold standard for pretreatment diagnosis and characterization of vascular anatomy, but requires rigid frame (skull) immobilization when used in combination with SRS. With the advent of advanced proton and image-guided photon delivery systems, SRS treatment is increasingly migrating to frameless platforms, which are incompatible with frame-based DSA. Without DSA as the primary image, target definition may be less than optimal, in some cases precluding the ability to treat with a frameless system. This article reports a novel solution.

METHODS AND MATERIALS

Fiducial markers are implanted into the patient's skull before angiography. Angiography is performed according to the standard clinical protocol, but, in contrast to the previous practice, without the rigid frame. Separate images of a specially designed localizer box are subsequently obtained. A target volume projected on DSA can be transferred to the localizer system in three dimensions, and in turn be transferred to multiple CT slices using the implanted fiducials. Combined with other imaging modalities, this "virtual frame" approach yields a highly precise treatment plan that can be delivered by frameless SRS technologies.

RESULTS

Phantom measurements for point and volume targets have been performed. The overall uncertainty of placing a point target to CT is 0.4 mm. For volume targets, deviation of the transformed contour from the target CT image is within 0.6 mm. The algorithm and software are robust. The method has been applied clinically, with reliable results.

CONCLUSIONS

A novel and reproducible method for frameless SRS of AVMs has been developed that enables the use of DSA without the requirement for rigid immobilization. Multiple pairs of DSA can be used for better conformality. Further improvement, including using nonimplanted fiducials, is potentially feasible.