Ingerid Reinertsen

Ultrasound imaging in neurosurgery: approaches to minimize surgically induced image artefacts for improved resection control

BackgroundIntraoperative ultrasound imaging is used in brain tumor surgery to identify tumor remnants. The ultrasound images may in some cases be more difficult to interpret in the later stages of the operation than in the beginning of the operation. The aim of this paper is to explain the causes of surgically induced ultrasound artefacts and how they can be recognized and reduced.MethodsThe theoretical reasons for artefacts are addressed and the impact of surgery is discussed. Different setups for ultrasound acquisition and different acoustic coupling fluids to fill up the resection cavity are evaluated with respect to improved image quality.ResultsThe enhancement artefact caused by differences in attenuation of the resection cavity fluid and the surrounding brain is the most dominating surgically induced ultrasound artefact. The influence of the artefact may be reduced by inserting ultrasound probes with small footprint into the resection cavity for a close-up view of the areas with suspected tumor remnants. A novel acoustic coupling fluid developed for use during ultrasound imaging in brain tumor surgery has the potential to reduce surgically induced ultrasound artefacts to a minimum.ConclusionsSurgeons should be aware of artefacts in ultrasound images that may occur during brain tumor surgery. Techniques to identify and reduce image artefacts are useful and should be known to users of ultrasound in brain tumor surgery.

Intra-operative correction of brain-shift

BackgroundBrain-shift is a major source of error in neuronavigation systems based on pre-operative images. In this paper, we present intra-operative correction of brain-shift using 3D ultrasound.MethodsThe method is based on image registration of vessels extracted from pre-operative MRA and intra-operative power Doppler-based ultrasound and is fully integrated in the neuronavigation software.ResultsWe have performed correction of brain-shift in the operating room during surgery and provided the surgeon with updated information. Here, we present data from seven clinical cases with qualitative and quantitative error measures.ConclusionThe registration algorithm is fast enough to provide the surgeon with updated information within minutes and accounts for large portions of the experienced shift. Correction of brain-shift can make pre-operative data like fMRI and DTI reliable for a longer period of time and increase the usefulness of the MR data as a supplement to intra-operative 3D ultrasound in terms of overview and interpretation.

Three-dimensional ultrasound-guided placement of ventricular catheters.

OBJECTIVE Many studies demonstrate that the accuracy of freehand catheter placement for cerebrospinal fluid drainage is suboptimal. The aim of placement should be a single pass with a free-floating catheter tip in the intended position. The objective of this study was to achieve an accurate and user-friendly system for three-dimensional (3D) ultrasound-navigated catheter placement through a regular burr hole. METHODS A new phased-array ultrasound burr hole probe (4-10 MHz, 8 mm×9 mm footprint) was especially developed and optimized for navigated 3D ultrasound with the SonoWand Invite system. A catheter holder for optical tracking was also developed. Head immobilization was achieved with a vacuum cushion. With the described setup, 4 patients underwent surgery. RESULTS Ultrasound image quality and visualization of the ventricles was good in all cases. Optimal placement of the catheter was achieved in a single pass in all patients. One of the trajectories was slightly more medial on postoperative computed tomography than anticipated from the neuronavigation system. None of the patients experienced any adverse event related to the procedure. CONCLUSIONS 3D ultrasound with the described setup is a promising technique for accurate, fast, and user-friendly navigated placement of catheters for cerebrospinal fluid diversion.

A new system for 3D ultrasound-guided placement of cerebral ventricle catheters

PurposeWe present a new system for 3D ultrasound-guided placement of cerebral ventricle catheters. The system has been developed with the aim to provide accurate ultrasound-based guidance with only minimal changes to the current surgical technique and workflow.MethodsThe system consists of a pre-calibrated navigation adapter for the catheter and a reference frame attached to a standard surgical retractor in addition to an ultrasound-based navigation system with a probe that fits on top of a standard burr hole.ResultsThe accuracy of the pre-calibrated system has been evaluated, and our measurements indicate that the accuracy of the pre-calibrated system is better than 3 mm. We also present a clinical case.ConclusionsThe navigation accuracy is considered sufficient for clinical use, and initial clinical tests are promising. Further testing will be necessary to fully evaluate the performance of the system in a clinical setting.

Intra-rater variability in low-grade glioma segmentation

Assessment of size and growth are key radiological factors in low-grade gliomas (LGGs), both for prognostication and treatment evaluation, but the reliability of LGG-segmentation is scarcely studied. With a diffuse and invasive growth pattern, usually without contrast enhancement, these tumors can be difficult to delineate. The aim of this study was to investigate the intra-observer variability in LGG-segmentation for a radiologist without prior segmentation experience. Pre-operative 3D FLAIR images of 23 LGGs were segmented three times in the software 3D Slicer. Tumor volumes were calculated, together with the absolute and relative difference between the segmentations. To quantify the intra-rater variability, we used the Jaccard coefficient comparing both two (J2) and three (J3) segmentations as well as the Hausdorff Distance (HD). The variability measured with J2 improved significantly between the two last segmentations compared to the two first, going from 0.87 to 0.90 (p = 0.04). Between the last two segmentations, larger tumors showed a tendency towards smaller relative volume difference (p = 0.07), while tumors with well-defined borders had significantly less variability measured with both J2 (p = 0.04) and HD (p < 0.01). We found no significant relationship between variability and histological sub-types or Apparent Diffusion Coefficients (ADC). We found that the intra-rater variability can be considerable in serial LGG-segmentation, but the variability seems to decrease with experience and higher grade of border conspicuity. Our findings highlight that some criteria defining tumor borders and progression in 3D volumetric segmentation is needed, if moving from 2D to 3D assessment of size and growth of LGGs.

Glioblastoma Segmentation: Comparison of Three Different Software Packages

To facilitate a more widespread use of volumetric tumor segmentation in clinical studies, there is an urgent need for reliable, user-friendly segmentation software. The aim of this study was therefore to compare three different software packages for semi-automatic brain tumor segmentation of glioblastoma; namely BrainVoyagerTM QX, ITK-Snap and 3D Slicer, and to make data available for future reference. Pre-operative, contrast enhanced T1-weighted 1.5 or 3 Tesla Magnetic Resonance Imaging (MRI) scans were obtained in 20 consecutive patients who underwent surgery for glioblastoma. MRI scans were segmented twice in each software package by two investigators. Intra-rater, inter-rater and between-software agreement was compared by using differences of means with 95% limits of agreement (LoA), Dice’s similarity coefficients (DSC) and Hausdorff distance (HD). Time expenditure of segmentations was measured using a stopwatch. Eighteen tumors were included in the analyses. Inter-rater agreement was highest for BrainVoyager with difference of means of 0.19 mL and 95% LoA from -2.42 mL to 2.81 mL. Between-software agreement and 95% LoA were very similar for the different software packages. Intra-rater, inter-rater and between-software DSC were ≥ 0.93 in all analyses. Time expenditure was approximately 41 min per segmentation in BrainVoyager, and 18 min per segmentation in both 3D Slicer and ITK-Snap. Our main findings were that there is a high agreement within and between the software packages in terms of small intra-rater, inter-rater and between-software differences of means and high Dice’s similarity coefficients. Time expenditure was highest for BrainVoyager, but all software packages were relatively time-consuming, which may limit usability in an everyday clinical setting.

Liver deformation in an animal model due to pneumoperitoneum assessed by a vessel-based deformable registration

Abstract Purpose: Surgical navigation based on preoperative images partly overcomes some of the drawbacks of minimally invasive interventions - reduction of free sight, lack of dexterity and tactile feedback. The usefulness of preoperative images is limited in laparoscopic liver surgery, as the liver shifts due to respiration, induction of pneumoperitoneum and surgical manipulation. In this study, we evaluated the shift and deformation in an animal liver caused by respiration and pneumopertioneum using intraoperative cone beam CT. Material and methods: 3D cone beam CT scans were acquired with arterial contrast. The centerlines of the segmented vessels were extracted from the images taken at different respiration and pressure settings. A non-rigid registration method was used to measure the shift and deformation. The mean Euclidean distance between the annotated landmarks was used for evaluation. Results: A shift and deformation of 44.6 mm on average was introduced due to the combined effect of respiration and pneumoperitoneum. On average 91% of the deformations caused by the respiration and pneumoperitoneum were recovered. Conclusion: The results can contribute to the use of intraoperative imaging to correct for anatomic shift so that preoperative data can be used with greater confidence and accuracy during guidance of laparoscopic liver procedures.

Quantitative texture analysis in the prediction of IDH status in low-grade gliomas

OBJECTIVES Molecular markers provide valuable information about treatment response and prognosis in patients with low-grade gliomas (LGG). In order to make this important information available prior to surgery the aim of this study was to explore if molecular status in LGG can be discriminated by preoperative magnetic resonance imaging (MRI). PATIENTS AND METHODS All patients with histopathologically confirmed LGG with available molecular status who had undergone a preoperative standard clinical MRI protocol using a 3T Siemens Skyra scanner during 2008-2015 were retrospectively identified. Based on Haralick texture parameters and the segmented LGG FLAIR volume we explored if it was possible to predict molecular status. RESULTS In total 25 patients (nine women, average age 44) fulfilled the inclusion parameters. The textural parameter homogeneity could discriminate between LGG patients with IDH mutation (0.12, IQR 0.10-0.15) and IDH wild type (0.07, IQR 0.06-0.09, p=0.005). None of the other four analyzed texture parameters (energy, entropy, correlation and inertia) were associated with molecular status. Using ROC curves, the area under curve for predicting IDH mutation was 0.905 for homogeneity, 0.840 for tumor volume and 0.940 for the combined parameters of tumor volume and homogeneity. We could not predict molecular status using the four other chosen texture parameters (energy, entropy, correlation and inertia). Further, we could not separate LGG with IDH mutation with or without 1p19q codeletion. CONCLUSIONS In this preliminary study using Haralick texture parameters based on preoperative clinical FLAIR sequence, the homogeneity parameter could separate IDH mutated LGG from IDH wild type LGG. Combined with tumor volume, these diagnostic properties seem promising.

Brain-shift compensation using intraoperative ultrasound and constraint-based biomechanical simulation

HighlightsA constraint‐based biomechanical simulation method is proposed to compensate for brain‐shift.Intraoperatively, a single ultrasound acquisition is used to account for the vessels and cortical deformations.Quantitative validation over synthetic data and five clinical cases is provided.Improvements over one of the closest existing methods are shown.This method is fully compatible with a surgical process. Graphical abstract Figure. No caption available. Purpose. During brain tumor surgery, planning and guidance are based on preoperative images which do not account for brain‐shift. However, this deformation is a major source of error in image‐guided neurosurgery and affects the accuracy of the procedure. In this paper, we present a constraint‐based biomechanical simulation method to compensate for craniotomy‐induced brain‐shift that integrates the deformations of the blood vessels and cortical surface, using a single intraoperative ultrasound acquisition. Methods. Prior to surgery, a patient‐specific biomechanical model is built from preoperative images, accounting for the vascular tree in the tumor region and brain soft tissues. Intraoperatively, a navigated ultrasound acquisition is performed directly in contact with the organ. Doppler and B‐mode images are recorded simultaneously, enabling the extraction of the blood vessels and probe footprint, respectively. A constraint‐based simulation is then executed to register the pre‐ and intraoperative vascular trees as well as the cortical surface with the probe footprint. Finally, preoperative images are updated to provide the surgeon with images corresponding to the current brain shape for navigation. Results. The robustness of our method is first assessed using sparse and noisy synthetic data. In addition, quantitative results for five clinical cases are provided, first using landmarks set on blood vessels, then based on anatomical structures delineated in medical images. The average distances between paired vessels landmarks ranged from 3.51 to 7.32 (in mm) before compensation. With our method, on average 67% of the brain‐shift is corrected (range [1.26; 2.33]) against 57% using one of the closest existing works (range [1.71; 2.84]). Finally, our method is proven to be fully compatible with a surgical workflow in terms of execution times and user interactions. Conclusion. In this paper, a new constraint‐based biomechanical simulation method is proposed to compensate for craniotomy‐induced brain‐shift. While being efficient to correct this deformation, the method is fully integrable in a clinical process.

Model-guided placement of cerebral ventricular catheters

Purpose: Freehand placement of external ventricular drainage is not sufficiently accurate and precise. In the absence of high quality pre-operative 3D images, we propose the use of an average model for guidance of ventricular catheters. Methods: The model was segmented to extract the ventricles and registered to five normal volunteers using a combination of landmark based and surface based registration. The proposed method was validated by comparing the use of the average model to the use of volunteer-specific images. Results: The position and orientation of the ventricles were compared and the distances between the target points at the left and right foramen of Monroe were computed (Mean±std: 5.65±1.60mm and 6.05±1.34mm for the left and right side respectively). Conclusions: Although an average model for guidance of a surgical procedure has a number of limitations, our initial experiments show that the use of a model might provide sufficient guidance for determination of the angle of insertion. Future work will include further clinical testing and possible refinement of the model.