The role of diffusion tensor imaging tractography for Gamma Knife thalamotomy planning

Conservation of pyramidal tract in radiosurgery for brain metastases of lung adenocarcinoma: Three-dimensional analysis of biologically effective dose

BACKGROUND

Gamma knife surgery (GKS) for brain metastases (BMs) adjacent to the pyramidal tract (PT) is still a challenge to conduct. PT visualization and biologically effective dose (BED) calculation on a voxel-by-voxel basis may provide data to establish clinically safe values. We aimed to assess the relationship of parameters extracted from the BED-volume histogram with outcomes of PT after GKS-treating target (adjacent BM of lung adenocarcinoma).

METHODS

We formed BED-volume histograms for 672 BMs in a retrospective cohort, using 3-dimensional (3D) coordinate values of PT, target, and each iso-centre to calculate the 3D BED distribution in a 200 × 200 × 200 matrix. PT conservation failure (PTCF) was judged clinically and radiologically and classified as lesion progression and radionecrosis. Cox proportional hazards models were used to analyse 3D BED parameters. Internal validation of models was performed by bootstrapping.

RESULTS

There were 116 (17.3 %) subjects with PTCF in the cohort, of which 74 (11.0 %) and 42 (6.3 %) were caused by lesion progression and radionecrosis, respectively. Multivariate analysis showed that D_{Lesion_min} BED and D_{Lesion_90%} BED significantly predicted lesion progression (P <.001). D_{PT_Max} BED and V_{PT_ BED40} significantly predicted radionecrosis (P <.001). The model predicting PTCF showed fair discrimination and calibration of D_{Lesion_min} BED + D_{Lesion_90%} BED and D_{PT_Max} BED + V_{PT_ BED40}.

CONCLUSIONS

The conservation of PT in GKS for BMs of lung adenocarcinoma depends on the combination of PT-tolerated BED and target effective control BED. Therefore, a BED-volume histogram with a 3D BED algorithm is proposed to assess plan quality.

Skull phantom-based methodology to validate MRI co-registration accuracy for Gamma Knife radiosurgery

PURPOSE

Target localization, for stereotactic radiosurgery (SRS) treatment with Gamma Knife, has become increasingly reliant on the co-registration between the planning MRI and the stereotactic cone-beam computed tomography (CBCT). Validating image registration between modalities would be particularly beneficial when considering the emergence of novel functional and metabolic MRI pulse sequences for target delineation. This study aimed to develop a phantom-based methodology to quantitatively compare the co-registration accuracy of the standard clinical imaging protocol to a representative MRI sequence that was likely to fail co-registration. The comparative methodology presented in this study may serve as a useful tool to evaluate the clinical translatability of novel MRI sequences.

METHODS

A realistic human skull phantom with fiducial marker columns was designed and manufactured to fit into a typical MRI head coil and the Gamma Knife patient positioning system. A series of "optimized" 3D MRI sequences-T₁ -weighted Dixon, T₁ -weighted fast field echo (FFE), and T₂ -weighted fluid-attenuated inversion recovery (FLAIR)-were acquired and co-registered to the CBCT. The same sequences were "compromised" by reconstructing without geometric distortion correction and re-collecting with lower signal-to-noise-ratio (SNR) to simulate a novel MRI sequence with poor co-registration accuracy. Image similarity metrics-structural similarity (SSIM) index, mean squared error (MSE), and peak SNR (PSNR)-were used to quantitatively compare the co-registration of the optimized and compromised MR images.

RESULTS

The ground truth fiducial positions were compared to positions measured from each optimized image volume revealing a maximum median geometric uncertainty of 0.39 mm (LR), 0.92 mm (AP), and 0.13 mm (SI) between the CT and CBCT, 0.60 mm (LR), 0.36 mm (AP), and 0.07 mm (SI) between the CT and T₁ -weighted Dixon, 0.42 mm (LR), 0.23 mm (AP), and 0.08 mm (SI) between the CT and T₁ -weighted FFE, and 0.45 mm (LR), 0.19 mm (AP), and 1.04 mm (SI) between the CT and T₂ -weighted FLAIR. Qualitatively, pairs of optimized and compromised image slices were compared using a fusion image where separable colors were used to differentiate between images. Quantitatively, MSE was the most predictive and SSIM the second most predictive metric for evaluating co-registration similarity. A clinically relevant threshold of MSE, SSIM, and/or PSNR may be defined beyond which point an MRI sequence should be rejected for target delineation based on its dissimilarity to an optimized sequence co-registration. All dissimilarity thresholds calculated using correlation coefficients with in-plane geometric uncertainty would need to be defined on a sequence-by-sequence basis and validated with patient data.

CONCLUSION

This study utilized a realistic skull phantom and image similarity metrics to develop a methodology capable of quantitatively assessing whether a modern research-based MRI sequence can be co-registered to the Gamma Knife CBCT with equal or less than equal accuracy when compared to a clinically accepted protocol.

Movement disorders

Over 30 years it has been demonstrated that Gamma Knife thalamotomy is a valuable method of treating the tremor of Parkinson's Disease and Essential Tremor. It is also effective against the tremor associated with Multiple Sclerosis. To date Gamma Knife pallidotomy has not received the acceptance of the profession.

Spinal alignment shift between supine and prone CT imaging occurs frequently and regardless of the anatomic region, risk factors, or pathology

Computer-assisted spine surgery based on preoperative CT imaging may be hampered by sagittal alignment shifts due to an intraoperative switch from supine to prone. In the present study, we systematically analyzed the occurrence and pattern of sagittal spinal alignment shift between corresponding preoperative (supine) and intraoperative (prone) CT imaging in patients that underwent navigated posterior instrumentation between 2014 and 2017. Sagittal alignment across the levels of instrumentation was determined according to the C2 fracture gap (C2-F) and C2 translation (C2-T) in odontoid type 2 fractures, next to the modified Cobb angle (CA), plumbline (PL), and translation (T) in subaxial pathologies. One-hundred and twenty-one patients (C1/C2: n = 17; C3-S1: n = 104) with degenerative (39/121; 32%), oncologic (35/121; 29%), traumatic (34/121; 28%), or infectious (13/121; 11%) pathologies were identified. In the subaxial spine, significant shift occurred in 104/104 (100%) cases (CA: *p = .044; T: *p = .021) compared to only 10/17 (59%) cases that exhibited shift at the C1/C2 level (C2-F: **p = .002; C2-T: *p < .016). The degree of shift was not affected by the anatomic region or pathology but significantly greater in cases with an instrumentation length > 5 segments (“∆PL > 5 segments”: 4.5 ± 1.8 mm; “∆PL ≤ 5 segments”: 2 ± 0.6 mm; *p = .013) or in revision surgery with pre-existing instrumentation (“∆PL presence”: 5 ± 2.6 mm; “∆PL absence”: 2.4 ± 0.7 mm; **p = .007). Interestingly, typical morphological instability risk factors did not influence the degree of shift. In conclusion, intraoperative spinal alignment shift due to a change in patient position should be considered as a cause for inaccuracy during computer-assisted spine surgery and when correcting spinal alignment according to parameters that were planned in other patient positions.

A preliminary study on the application of DTI in the treatment of brain tumors in motor function areas with gamma knife

OBJECTIVES

The treatment safety and efficiency as well as the life quality of patients are still main concerns in gamma knife radiosurgery. In this study, the feasibility of applying diffusion tensor imaging (DTI) in gamma knife radiosurgery for the treatment of brain tumor in motor function areas was investigated, which aims to provide protection on the pyramidal tract and preserve the motor function in patients.

PATIENTS AND METHODS

Total 74 patients with solid brain tumor were enrolled and divided into DTI group and control group. The tumor control rate was assessed at 3 months after surgery. The muscle strength of affected limb, KPS scores, ZEW scores and complications were evaluated at 3 and 6 months after gamma knife radiosurgery.

RESULTS

Our results indicated that the tumor control rate, complication rate, the muscle strength of affected limb and KPS scores were not significantly different between the two groups at 3 months after surgery. At 6 months after gamma knife radiosurgery, the complication rate (0% vs 50 %, P = 0.044), KPS scores (64.9 % vs 37.8 %, P = 0.036) and ZEW scores (78.4 % vs 54.1 %, P = 0.044) of DTI group were better than the control group. Furthermore, the stability of muscle strength in patients with limb dysfunction was significantly improved in DTI group (86.4 % vs 50 %, P = 0.028).

CONCLUSION

In summary, the application of DTI in gamma knife radiosurgery for the treatment of brain tumors in motor function areas can precisely define the tumor edge from pyramidal tract, which will support on designing individual treatment plan, reducing the incidence of complications, and improving long-term life quality in patients.

Tractography-based targeting of the ventral intermediate nucleus: accuracy and clinical utility in MRgFUS thalamotomy

OBJECTIVE

Tractography-based targeting of the thalamic ventral intermediate nucleus (T-VIM) is a novel method conferring patient-specific selection of VIM coordinates for tremor surgery; however, its accuracy and clinical utility in magnetic resonance imaging-guided focused ultrasound (MRgFUS) thalamotomy compared to conventional indirect targeting has not been specifically addressed. This retrospective study sought to compare the treatment locations and potential adverse effect profiles of T-VIM with indirect targeting in a large cohort of MRgFUS thalamotomy patients.

METHODS

T-VIM was performed using diffusion tractography outlining the pyramidal and medial lemniscus tracts in 43 MRgFUS thalamotomy patients. T-VIM coordinates were compared with the indirect treatment coordinates used in the procedure. Thalamotomy lesions were delineated on postoperative T1-weighted images and displaced ("translated") by the anteroposterior and mediolateral difference between T-VIM and treatment coordinates. Both translated and actual lesions were normalized to standard space and subsequently overlaid with areas previously reported to be associated with an increased risk of motor and sensory adverse effects when lesioned during MRgFUS thalamotomy.

RESULTS

T-VIM coordinates were 2.18 mm anterior and 1.82 mm medial to the "final" indirect treatment coordinates. Translated lesions lay more squarely within the boundaries of the VIM compared to nontranslated lesions and showed significantly less overlap with areas associated with sensory adverse effects. Translated lesions overlapped less with areas associated with motor adverse effects; however, this difference was not significant.

CONCLUSIONS

T-VIM leads to the selection of more anterior and medial coordinates than the conventional indirect methods. Lesions moved toward these anteromedial coordinates avoid areas associated with an increased risk of motor and sensory adverse effects, suggesting that T-VIM may improve clinical outcomes.

Targeting of the dentato-rubro-thalamic tract for MR-guided focused ultrasound treatment of essential tremor

BACKGROUND

Magnetic resonance-guided focused ultrasound ablation of the thalamic ventral intermediate nucleus is a safe and effective treatment for medically refractory essential tremor. However, indirect targeting of the ventral intermediate nucleus using stereotactic coordinates from normal neuroanatomy can be inefficient. We therefore evaluated the feasibility of supplementing this method with direct targeting of the dentato-rubro-thalamic tract.

METHODS

We retrospectively identified four patients undergoing magnetic resonance-guided focused ultrasound ablation for essential tremor in which preoperative diffusion tractography imaging of the dentato-rubro-thalamic tract was fused with T2 weighted-imaging and utilized for intra-procedural targeting. The size and location of the dentato-rubro-thalamic tract and 24-hour lesion, as well as the center of the stereotactic coordinates, was evaluated. Finally, the amount of overlap between the dentato-rubro-thalamic tract and the lesion was calculated.

RESULTS

The 24-hour lesion size was homogeneous in the cohort (mean 31.3 mm², range 30-32 mm²), while there was substantial variation in the dentato-rubro-thalamic tract area (mean 14.3 mm², range 3-24 mm²). The center of the stereotactic coordinates and dentato-rubro-thalamic tract diverged by more than 1 mm in mediolateral and anterposterior directions in all patients, while the dentato-rubro-thalamic tract and lesion centers were in close proximity (mean mediolateral separation 1 mm, range 0.1-2.2 mm; mean anteroposterior separation 0.75 mm, range 0.4-1.2 mm). There was greater than 50% coverage of the dentato-rubro-thalamic tract by the lesion in all patients (mean 82.9%, range 66.7-100%). All patients experienced durable tremor relief.

CONCLUSION

Direct targeting of the dentato-rubro-thalamic tract using diffusion tractography imaging fused to T2 weighted-imaging may be a useful strategy for focused ultrasound treatment of essential tremor. Further investigation of the technique is warranted.

Understanding gamma ventral capsulotomy: Potential implications of diffusion tensor image tractography on target selectivity

Background:

The role of tractography in gamma ventral capsulotomy (GVC) planning is still unclear. This paper aims to describe the spatial distribution of medial orbitofrontal cortex (OFC) and lateral OFC fibers passing through the anterior limb of the internal capsule (ALIC) and analyze quantitative tractography parameters that differentiate obsessive-compulsive disorder (OCD) individuals from other neurosurgery functional patients (morbid obesity and Parkinson’s disease [PD]).

Methods:

Twenty patients undergoing functional stereotactic procedures, between 2013 and 2016, were included in this study. OCD patients underwent GVC (single shot 150 Gy and 4-mm collimators). PD and morbid obesity patients were submitted to deep brain stimulation implants. Diffusion tensor image tractography was reconstructed using Brainlab Elements software (Brainlab AG, Munich, Germany).

Results:

Nine PD, six morbid obesity, and five OCD patients were included with a mean age of 65.4 ± 9.1, 41.0 ± 8.2, and 31.2 ± 5.5, respectively, which are statistically different from each other (P < 0.001). Fourteen patients (70%) were men. A total of 40 cerebral hemispheres were analyzed. Medial OFC fibers are localized more inferior in the ALIC than the lateral OFC fibers in all hemispheres, but the level of intersection and exact topography of fiber bundles are variable among individuals. Both medial and lateral OFC fiber tracts of PD and morbid obesity patients have lower volume than, respectively, medial and lateral counterparts of OCD patients (P < 0.001).

Conclusions:

Medial and lateral OFC tract fibers have a general standard distribution in the anterior internal capsule (lateral OFC higher than medial OFC fibers). There are differences between obesity, Parkinson, and OCD patients regarding fiber tract statistics.

Outcomes from stereotactic surgery for essential tremor

There are several different surgical procedures that are used to treat essential tremor (ET), including deep brain stimulation (DBS) and thalamotomy procedures with radiofrequency (RF), radiosurgery (RS) and most recently, focused ultrasound (FUS). Choosing a surgical treatment requires a careful presentation and discussion of the benefits and drawbacks of each. We conducted a literature review to compare the attributes and make an appraisal of these various procedures. DBS was the most commonly reported treatment for ET. One-year tremor reductions ranged from 53% to 63% with unilateral Vim DBS. Similar improvements were demonstrated with RF (range, 74%-90%), RS (range, 48%-63%) and FUS thalamotomy (range, 35%-75%). Overall, bilateral Vim DBS demonstrated more improvement in tremor reduction since both upper extremities were treated (range, 66%-78%). Several studies show continued beneficial effects from DBS up to five years. Long-term follow-up data also support RF and gamma knife radiosurgical thalamotomy treatments. Quality of life measures were similarly improved among patients who received all treatments. Paraesthesias, dysarthria and ataxia were commonly reported adverse effects in all treatment modalities and were more common with bilateral DBS surgery. Many of the neurological complications were transient and resolved after surgery. DBS surgery had the added benefit of programming adjustments to minimise stimulation-related complications. Permanent neurological complications were most commonly reported for RF thalamotomy. Thalamic DBS is an effective, safe treatment with a long history. For patients who are medically unfit or reluctant to undergo DBS, several thalamic lesioning methods have parallel benefits to unilateral DBS surgery. Each of these surgical modalities has its own nuance for treatment and patient selection. These factors should be carefully considered by both neurosurgeons and patients when selecting an appropriate treatment for ET.

Subfield-specific tractography of the hippocampus in epilepsy patients at 7 Tesla

PURPOSE

MRI-negative epilepsy patients could benefit from advanced imaging techniques such as high-resolution diffusion magnetic resonance imaging (dMRI). Our aim was to perform hippocampal subfield-specific tractography and quantify connectivity of the subfields in MRI-negative patients. Abnormal connectivity of the hippocampal subfields may help inform seizure focus hypothesis and provide information to guide surgical intervention.

METHODS

Hippocampal structural imaging and dMRI was acquired in 25 drug resistant MRI-negative patients and 25 healthy volunteers. The hippocampi of each subject was segmented on high-resolution structural images and dMRI-based probabilistic tractography was performed in each subfield. The degrees of connectivity and fiber densities of the hippocampal subfields were quantified and compared between epilepsy patients and healthy volunteers.

RESULTS

We were able to perform subfield-specific hippocampal tractography in each subject that participated in this study. These methods identified some hippocampal subfields that are abnormally connected in MRI-negative patients. In particular patients suspected of left temporal seizure focus exhibited increased connectivity of certain ipsilateral subfields, especially the subiculum, presubiculum, and parasubiculum, and reduced connectivity of some contralateral subfields, such as CA1 and subiculum.

CONCLUSIONS

Our data suggest that the hippocampal subfields are connected in distinct ways in different types of epilepsy. These results may provide important information that could help inform seizure focus hypothesis and in the surgical treatment of MRI-negative patients. These observations suggest that high-resolution dMRI-based tractography of the hippocampal subfields can detect subtle abnormalities in otherwise normal-appearing MRI-negative patients.

Quantification of the radiation dose to the pyramidal tract using tractography in treatment planning for stereotactic radiosurgery

In stereotactic radiosurgery for intracranial lesions, optimization of the dose to the at-risk organs is important to avoid neurological complications. We aimed to quantify the dose to the pyramidal tract (PT) and improve treatment planning for gamma knife radiosurgery by combining tractography. Pyramidal tractography images were depicted in 23 patients with lesions adjacent to the PT and fused with stereotactic magnetic resonance images. We regarded the PT as an at-risk organ and performed dose planning. To assess the efficacy of this process, we compared clinical parameters between plans with and without tractography. In the plans with tractography, the maximum PT dose was significantly reduced, although the irradiation time was prolonged by 3.5 min. There was no significant difference in the dose covering 95% of the lesion volume (D95). This result suggests that the PT dose can be reduced while maintaining the D95 with clinically acceptable prolongation of the irradiation time.