Radiofrequency induced heating around aneurysm clips using a generic birdcage head coil at 7 Tesla under consideration of the minimum distance to decouple multiple aneurysm clips.

PURPOSE: To evaluate radiofrequency (RF) induced tissue heating around aneurysm clips during a 7T head MR examination and to determine the decoupling distance between multiple implanted clips. METHODS: A total of 120 RF exposure scenarios of clinical relevance were studied using specific absorption rate and temperature simulations. Variations between scenarios included 2 clips (18.8 and 51.5 mm length), 2 MR-operating modes, 2 head models, and 3 thermoregulation models. Furthermore, a conservative approach was developed to allow for safe scans of patients with aneurysm clips even if detailed information on the implanted clip is unknown. A dedicated simulation-based approach was applied to determine the decoupling distance between multiple implanted clips. RESULTS: For all 60 clinical scenarios with the 18.8-mm-long clip, the absolute tissue temperature remained below regulatory limits. For 15 of 60 scenarios with the 51.5-mm-long clip, limits were slightly exceeded (less than 1°C). The conservative approach led to a maximum time-averaged input power of the RF coil of 3.3W. The corresponding B1+ is 1.32 µT. A decoupling distance of 35 mm allows the aneurysm clips to be treated as uncoupled from one other. CONCLUSION: Safe scanning conditions with respect to RF-induced heating can be applied for single or decoupled aneurysm clips in a 7T ultra-high field MRI setting. Multiple aneurysm clips separated by less than 35 mm need further investigations.

In vitro and in silico assessment of RF-induced heating around intracranial aneurysm clips at 7 Tesla.

PURPOSE: To examine radiofrequency-induced tissue heating around intracranial aneurysm clips during a 7 Tesla (T) head MR examination. METHODS: Radiofrequency (RF), temperature simulations, and RF measurements were employed to investigate the effects of polarization and clip length on the electric field (E-field) and temperature. Heating in body models was studied using both a conservative approach and realistic exposure scenarios. RESULTS: Worst-case orientation was found for clips aligned parallel to the E-field polarization. Absolute tissue temperature remained below International Electrotechnical Commission regulatory limits for 44 of 50 clinical scenarios. No significant effect on heating was determined for clip lengths below 18.8 mm, and worst-case heating was found for clip length 51.5 mm. The conservative approach led to a maximum permissible E-field of 72 V/m corresponding to B1+ of 1.2 µT, and an accepted power of 4.6 W for the considered RF head coil instead of 38.5 W without clip. CONCLUSION: Safe scanning conditions with respect to RF-induced heating can be applied depending on the information about the clip gained during screening interviews. However, force and torque measurements in the MR system shall be conducted to give a final statement on the MR safety of aneurysm clips at 7T. Magn Reson Med 79:568-581, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Deep Learning Detection of Aneurysm Clips for Magnetic Resonance Imaging Safety.

Flagging the presence of metal devices before a head MRI scan is essential to allow appropriate safety checks. There is an unmet need for an automated system which can flag aneurysm clips prior to MRI appointments. We assess the accuracy with which a machine learning model can classify the presence or absence of an aneurysm clip on CT images. A total of 280 CT head scans were collected, 140 with aneurysm clips visible and 140 without. The data were used to retrain a pre-trained image classification neural network to classify CT localizer images. Models were developed using fivefold cross-validation and then tested on a holdout test set. A mean sensitivity of 100% and a mean accuracy of 82% were achieved. Predictions were explained using SHapley Additive exPlanations (SHAP), which highlighted that appropriate regions of interest were informing the models. Models were also trained from scratch to classify three-dimensional CT head scans. These did not exceed the sensitivity of the localizer models. This work illustrates an application of computer vision image classification to enhance current processes and improve patient safety.

Tailoring fenestrated aneurysm clips intraoperatively: Instant solution for a difficult problem.

The anterior communicating artery (AcoA) aneurysms represent the most complex aneurysms of the anterior circulation. For years, surgical challenges including the intricate anatomy and narrow surgical corridor have been overcome using supplementary techniques including extended craniotomies, wide opening of the cisterns, gyrus rectus resection and special clips like fenestrated clips. However, imaginative solutions such as intraoperative clip modification may be inevitable in particular cases for safe clipping. We retrospectively analyzed clinical records of two patients who required clip modification intraoperatively. Case #1 underwent microsurgical clipping of a ruptured, 4-mm AcoA aneurysm. Unfortunately, given the short distance between the two A2s, it was not possible to clip the aneurysm without a compromise to the contralateral A2 with the available shortest 3mm-fenestrated clip. We then used the clip modification technique intraoperatively by shortening the clip tips with mesh-plaque cutter and smoothening the remaining sharp ends using cautery sanding. Eventually, the aneurysm was clipped successfully with the modified-fenestrated clip. Post-clipping imagings confirmed complete occlusion of the aneurysm and patency of parent arteries. Case 2# underwent microsurgical clipping for a ruptured, 1-mm AcoA aneurysm. Like Case 1#, the initial clipping attempt with the available shortest 4mm-fenestrated clip failed given the excessive length of the tips. The patient, thus, required clip modification as described above. The aneurysm was then clipped successfully using the modified-fenestrated clip, protecting bilateral A2s. Post-clipping imagings demonstrated patency of parent arteries with no residual aneurysm filling. Clip modification seems to be an effective option in clipping the AcoA aneurysms when available clips are too long to secure them safely.

An Improved Surgical Technique to Increase Neck Width of Elastase-Induced Aneurysm Model in Rabbits: A Prospective Study.

Background: Rabbit elastase-induced aneurysms have widely been used to test various endovascular materials over the past two decades. However, wide-necked aneurysms cannot be stably constructed. Objective: The purpose of the study was to increase the neck width of the elastase-induced aneurysm model in rabbits via an improved surgical technique with two temporary clips. Materials and Methods: Fifty-four elastase-induced aneurysms in rabbits were successfully created. Group 1 was (n = 34) composed of cases in which two temporary aneurysm clips were placed closely medially and laterally to the origin of the right common carotid artery (RCCA), respectively. Group 2 (n = 20) included cases in which a single temporary aneurysm clip was placed crossed the origin of RCCA. Digital subtraction angiography (DSA) was performed before and immediately after elastase incubation and 3 weeks later. The diameter of the origin of RCCA before and immediately after elastase incubation and aneurysm sizes of the two groups were measured and compared. Moreover, the correlation analysis was performed between the diameter of the origin of RCCA immediately after elastase incubation and aneurysm neck width. Results: The mean aneurysm neck and dome width of group 1 were both significantly larger than that of group 2 (p-value < 0.001 and p-value = 0.005, respectively). Moreover, the proportion of wide-necked aneurysms (neck width ≥4 mm) in group 1 was significantly larger than that in group 2 (p-value = 0.004) and the mean dome to neck ratio (D/N) of group 1 was smaller than that of group 2 (p-value = 0.008). Furthermore, there was a positive correlation between the diameter of the origin of RCCA immediately after elastase incubation and aneurysm neck width. Conclusion: The improved surgical technique with two temporary clips, focusing on the direct contact of elastase with the origin of RCCA, could increase the neck width of elastase-induced aneurysm models in rabbits.

Intraoperative Shortening of Aneurysm Clips: Revisiting an Old Technique in a New Era.

BACKGROUND: It can sometimes be challenging to find a suitable clip to treat an unusual aneurysm, or when the surrounding anatomy is unusual, especially in resource-limited environments. We describe a method to modify aneurysm clips based on the method originated by Sugita et al in 1985. Herein clip modification (Clip-Mod) is used to treat anatomically difficult anterior communicating artery aneurysms. METHODS: The Department of Neurological Surgery database was reviewed to find aneurysm patients treated using modified aneurysm clips. Clip-Mod was performed during surgery by shortening the tines of titanium aneurysm clips by abrasion applied from the side of a standard 3-mm surgical diamond drill bit under constant irrigation. Note that the thickness of the tines and the clip spring were not modified or contacted by the drill. RESULTS: Four cases used modified aneurysm clips, from 648 total clip-treated aneurysms (0.6%) by 2 surgeons over a 14-year period. Three patients presented with subarachnoid hemorrhages that were determined to be due to anterior communicating artery aneurysms. One patient presented with an incidental unruptured anterior communicating artery aneurysm. All 4 patients were treated with 3-mm titanium clips shortened intraoperatively to 1- to 2-mm lengths, to achieve aneurysm obliteration without stenosing parent or perforating vessels. CONCLUSIONS: All 4 patients have done well clinically with no reoccurrences after 2-6 years' follow-up, which included angiographic evaluation. The use of this "Clip-Mod" technique thus appears useful for anterior communicating artery aneurysms. Clip-Mod could also be considered for treating other aneurysms when the "perfect" length clip is not available.