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ObjectiveTo evaluate the role of blood pressure as mediator of the effect of conscious sedation (CS) compared to local anesthesia (LA) on functional outcome after EVT.MethodsPatients treated in MR CLEAN Registry centers with CS or LA as preferred anesthetic approach during EVT for ischemic stroke were analyzed. First, we evaluated the effect of CS on area under the threshold (AUT), relative difference between baseline and lowest procedural mean arterial pressure (∆LMAP) and procedural blood pressure trend, compared to LA. Second, we assessed the association between blood pressure and functional outcome (modified Rankin Scale, mRS) with multivariable regression. Lastly, we evaluated whether blood pressure explained the effect of CS on mRS.ResultsIn 440 patients with available blood pressure data, patients treated under CS (n = 262) had larger AUTs (median 228 vs 23 mm Hg*min), larger ∆LMAP (median 16% vs 6%) and a more negative blood pressure trend (−0.22 vs −0.08 mm Hg/min) compared to LA (n = 178). Larger ∆LMAP and AUTs were associated with worse mRS (adjusted common OR (acOR) per 10%-drop 0.87, 95%CI 0.78–0.97, and acOR per 300 mm Hg*min 0.89, 95%CI 0.82–0.97). Patients treated under CS had worse mRS compared to LA (acOR 0.59, 95%CI 0.40–0.87) and this association remained when adjusting for ∆LMAP and AUT (acOR 0.62, 95%CI0.42–0.92).ConclusionsLarge blood pressure drops are associated with worse functional outcome. However, blood pressure drops do not explain the worse outcomes in the CS group.

Background and purpose: A wide variation of MRI systems is a challenge in multicenter imaging biomarker studies as it adds variation in quantitative MRI values. The aim of this study was to design and test a quality assurance (QA) framework based on phantom measurements, for the quantitative MRI protocols of a multicenter imaging biomarker trial of locally advanced cervical cancer.Materials and methods: Fifteen institutes participated (five 1.5 T and ten 3 T scanners). Each institute optimized protocols for T2, diffusion-weighted imaging, T1, and dynamic contrast-enhanced (DCE-)MRI according to system possibilities, institutional preferences and study-specific constraints. Calibration phantoms with known values were used for validation. Benchmark protocols, similar on all systems, were used to investigate whether differences resulted from variations in institutional protocols or from system variations. Bias, repeatability (%RC), and reproducibility (%RDC) were determined. Ratios were used for T2 and T1 values.Results: The institutional protocols showed a range in bias of 0.88-0.98 for T2 (median %RC = 1%; % RDC = 12%), -0.007 to 0.029 x 10(-3) mm(2)/s for the apparent diffusion coefficient (median %RC = 3%; % RDC = 18%), and 0.39-1.29 for T1 (median %RC = 1%; %RDC = 33%). For DCE a nonlinear vendor-specific relation was observed between measured and true concentrations with magnitude data, whereas the relation was linear when phase data was used.Conclusion: We designed a QA framework for quantitative MRI protocols and demonstrated for a multi-center trial for cervical cancer that measurement of consistent T2 and apparent diffusion coefficient values is feasible despite protocol differences. For DCE-MRI and T1 mapping with the variable flip angle method, this was more challenging. (C) 2020 The Authors. Published by Elsevier B.V.

Objective To assess the feasibility of the clinical use of 3 Tesla and 7 Tesla Magnetic Resonance Imaging for early (cT1) glottic carcinoma, including structural assessment of technical image quality and visibility of the tumor; and if feasible, to correlate MRI findings to routine diagnostics. Methods Prospective feasibility study. Twenty patients with primary clinical T1 glottic carcinoma underwent both routine clinical staging and CT. In addition, a 3 T and 7 T MRI protocol, developed for small laryngeal lesions, was performed in a 4-point immobilization mask, using dedicated surface coils. Afterwards, routine endoscopic direct suspension laryngoscopy under general anaesthesia was performed. Results Only 2 of 7 (29%) of 7 T MRI scans were rated as moderate to good technical image quality. After exclusion of three patients with only mild to moderate dysplasia at the time of MRI, 13 of 17 (76%) of 3 T MRIs were of adequate technical image quality. Tumor visualization was adequate in 8 of 13 (62%) of patients with invasive squamous cell carcinomas. With exclusion of the four MRIs with motion artefacts, the tumor and its boundaries could be adequately seen in 8 of 9 (89%) patients with squamous cell carcinoma versus only one in four (25%) of patients with carcinoma in situ lesions. Conclusions 7 Tesla MRI was considered not feasible. 3 Tesla MRI, with adequate patient selection, namely clinical exclusion of patients with a history of claustrophobia and inclusion of only histologically proven invasive squamous cell carcinoma, can be feasible. Especially with further improvement of MR image q

Hyperthermia treatment planning aims to calculate and optimize the thermal dose of hyperthermia treatments for individual patient cases. For this purpose extensive electromagnetic and thermal modelling techniques have been successfully developed over the last years. Unfortunately, means to monitor and verify a pre-calculated treatment plan during treatment are limited. MR imaging is an excellent imaging modality to visualize anatomy and physiology. As field strengths increase, problems with respect to greater RF field inhomogeneities and elevated RF power deposition pose technical challenges. This thesis demonstrates that hyperthermia treatment planning can be verified with MR imaging and that MR imaging can be optimized with hyperthermia treatment planning.

The ultimate potential of the MR-linac is real-time adaptive MR-guided radiotherapy (aMRgRT), i.e. adapt the radiation plan in real-time according to real-time 3D motion estimates. One of the major technical roadblocks towards achieving this goal is the real-time 3D motion estimation. This thesis presents two new approaches in this context. The main method is called MR-MOTUS and is the subject of Chapters 2-4. All methods in this thesis are motivated by the observation that internal body motion exhibits a high level of spatio-temporal correlation, and could be reconstructed from minimal MRI-data that can be acquired in real-time. Chapter 2 demonstrates the proof of concept. The MR-MOTUS signal model was derived that explicitly relates motion-fields and a reference image to k-space data, and the minimization problem was formulated to reconstruct these motion-fields from the data. The proof-of-concept was demonstrated by reconstructions of in vivo 3D rigid head motion and 3D non-rigid respiratory motion from retrospectively highly undersampled k-space data, and 2D non-rigid respiratory motion-field reconstruction on prospectively undersampled data. Chapter 3 introduces several improvements to tighten the gap towards clinical application, and extends the framework to 3D+t spatio-temporal motion-field reconstructions by introducing a low-rank motion model, which naturally separates motion-fields into two components: a spatial component, and a temporal component. This model reduced the number of unknowns for space-time motion-fields by two orders of magnitude, and thereby enabled 3D+t motion-field reconstruction with high temporal resolution on a desktop PC. However, just high temporal resolution is not sufficient; the reconstructions also need to be available in real-time during the treatments. In Chapter 4, the previous reconstructions were therefore extended to real-time reconstructions at 6.7 Hz using a two-step approach, which leverages the low-rank separation of motion-fields into spatial and temporal components. In the first phase, the spatial component is assumed to be fixed in time over several minutes and is reconstructed with an offline reconstruction. In the second phase, the temporal component that encodes the dynamics in the motion-field is reconstructed per dynamic in an online reconstruction. The main rationale behind this approach is that the temporal component has few degrees of freedom (

The MR-linac combines an MRI scanner with a radiotherapy treatment device and enables the visualization of a patient’s anatomy while the treatment is delivered (MRI-guided radiotherapy). However, MRI receive arrays, which pick up the MRI signals, are traditionally not compatible with MRI-guided radiotherapy, as the arrays block too much of the radiation beam. Therefore, special, radiation transparent receive arrays have been developed, which barely attenuate radiation by moving all electronics out of the beam’s path. Unfortunately, the MRI performance of these arrays is lower than that of the conventional arrays. In this thesis, multiple radiation transparent receive arrays have therefore been developed and manufactured to improve the image quality and acquisition speed with respect to the current clinical receive arrays, while barely attenuating radiation. The improved performance can be used to reduce treatment times or to make (real-time) treatment adaptations based on the MR images. Additionally, this thesis has shown that aforementioned receive arrays can be used for hybrid PET/MRI as well. Here, the radiation transparency is required to avoid PET signal absorption by the array and thereby disturbing the PET image reconstruction.

MRI is often considered to be a risk for patients with medical implants, like pacemakers and deep brain stimulators. The heating around the electrodes of these type of implants that are in contact with human tissue to deliver therapeutic doses is one of the most notorious challenges. Unintendedly, the implants can act as antennas and pick up energy from the radiofrequency field that is necessary to produce MR images and deposit it locally causing focused temperature hot spots. This process is described by the so-called transfer function of an implant. It is challenging to get an accurate estimate of the heating around the electrodes because it depends on many factors. The design of the implant, the anatomy of the patient, the type of RF coil, the relative positions of the implant, the patient and the RF coil all influence the heating. As shown in this thesis MRI can be used to measure the transfer function of an implant and hence get an accurate description of its interaction with the radio frequency field in more realistic scenarios. Nowadays transfer functions can only be measured in phantoms in straight trajectories in dedicated measurements setups. Translating these transfer functions to in-vivo situations introduce currently unknown uncertainties which can be probed using the MRI methods described in this thesis. This provides options for implant manufacturers to describe their products better and has the potential to improve care for an increasing patient population.

In this thesis novel transmitter designs for human magnetic resonance imaging for high static magnetic field strengths (7 Tesla) were developed. The theory for both dielectric resonators as well as plasma based resonators have been described in the introduction and outline. It was shown that by using empirically derived formulas and computer simulations dielectric resonators can be designed also by using water as a dielectric (chapter 2). In chapter 3 it was shown that alternative dielectric materials, especially high dielectric materials can be used instead of water. Chapter 4 shows the advanced usage of the technology developed in chapter 2 and chapter 3. Here an array for human cardiac magnetic resonance imaging at 7 Tesla field strength was designed and constructed. In chapter 5 a first practical design for a plasma based transmit coil for magnetic resonance imaging was demonstrated. It was shown that it is possible to guide a surface wave via the plasma sheath within the magnetic resonance imaging system. The thesis ends with chapter 6 a general discussion and an outlook to possible future developments.

RF transmit signals are an inherent part of MRI that is crucial for spin excitation. Current developments in MRI are moving towards higher magnetic field strengths and more transmit coils leading to more inhomogeneous RF distributions. This increases the possibility of SAR hotspots, creating higher risks of local heating. In order to cope with these developments and still perform reliable safety assessments the MRI safety community is discussing to revise the guidelines on RF induced tissue heating. Instead of restricting the scanners on a derived measure, i.e. SAR, the scanners should be limited by the guidelines that are direct measures for tissue damage such as absolute temperature. In this debate the translation from SAR to temperature estimations is crucial. In order to make this translation thermal modeling is required. In this thesis MR-based methods were developed in order to measure the influence of RF exposure in human subjects, both in temperature and perfusion. Those results could subsequently be used to test whether current thermal modeling was able to predict those effects for a given subject. In vivo temperature measurements in the calf resulted in precise representations of the heating that could be used as a reliable gold standard. Furthermore, perfusion increases over the complete leg due to this local temperature increase could be observed. Although the simulations matched with the experiments in the EM regime, the accuracy of the estimated temperature distributions was not sufficient for a safety assessment. This showed that more work on thermal modeling is required before MRI scanners can be restricted on measures based on temperature. Therefore it would be unwise to immediately discard all SAR related restrictions. Local SAR still proved to be a very useful restrictive measure since it can be quickly determined by simulations and can more easily be verified. Correct thermal modeling proved to rely on inputs that are not easily measured such as subject specific thermoregulation. Although the first steps in this direction were made in this thesis, obtaining a proper estimate of thermal behavior over the whole human body is a highly challenging pursuit.