De weg, de waarheid en het leven : masterscriptie van een receptie-historisch onderzoek over de exegesegeschiedenis van Psalm 119

Masterthesis Oude Testamen

Macroscopic analysis and modelling of multi-class, flexible-lane traffic

An excessive demand of vehicles to a motorway bottleneck leads to traffic jams. Motorbikes are narrow and can drive next to each other in a lane, or in-between lanes in low speeds. This paper analyses the resulting traffic characteristics and presents numerical scheme for a macroscopic traffic flow model for these two classes. The behavior included is as follows. If there are two motorbikes behind each other, they can travel next to each other in one lane, occupying the space of one car. Also, at low speeds of car traffic, they can go in between the main lanes, creating a so-called filtering lane. The paper numerically derives functions of class-specific speeds as function of the density of both classes, incorporating flexible lane usage dependent on the speed. The roadway capacity as function of the motorbike fraction is derived, which interesting can be in different types of phases (with motorbikes at higher speeds or not). We also present a numerical scheme to analyse the dynamics of this multi-class system. We apply the model to an example case, revealing the properties of the traffic stream , queue dynamics and class specific travel times. The model can help in showing the relative advantage in travel time of switching to a motorbike

Optimized flip angle schemes for the split acquisition of fast spin-echo signals (SPLICE) sequence and application to diffusion-weighted imaging

Purpose: The diffusion-weighted SPLICE (split acquisition of fast spin-echo signals) sequence employs split-echo rapid acquisition with relaxation enhancement (RARE) readout to provide images almost free of geometric distortions. However, due to the varying T (Formula presented.) -weighting during k-space traversal, SPLICE suffers from blurring. This work extends a method for controlling the spatial point spread function (PSF) while optimizing the signal-to-noise ratio (SNR) achieved by adjusting the flip angles in the refocusing pulse train of SPLICE. Methods: An algorithm based on extended phase graph (EPG) simulations optimizes the flip angles by maximizing SNR for a flexibly chosen predefined target PSF that describes the desired k-space density weighting and spatial resolution. An optimized flip angle scheme and a corresponding post-processing correction filter which together achieve the target PSF was tested by healthy subject brain imaging using a clinical 1.5 T scanner. Results: Brain images showed a clear and consistent improvement over those obtained with a standard constant flip angle scheme. SNR was increased and apparent diffusion coefficient estimates were more accurate. For a modified Hann k-space weighting example, considerable benefits resulted from acquisition weighting by flip angle control. Conclusion: The presented flexible method for optimizing SPLICE flip angle schemes offers improved MR image quality of geometrically accurate diffusion-weighted images that makes the sequence a strong candidate for radiotherapy planning or stereotactic surgery

Pushing functional MRI spatial and temporal resolution further: High-density receive arrays combined with shot-selective 2D CAIPIRINHA for 3D echo-planar imaging at 7 T

To be able to examine dynamic and detailed brain functions, the spatial and temporal resolution of 7 T MRI needs to improve. In this study, it was investigated whether submillimeter multishot 3D EPI fMRI scans, acquired with high-density receive arrays, can benefit from a 2D CAIPIRINHA sampling pattern, in terms of noise amplification (g-factor), temporal SNR and fMRI sensitivity. High-density receive arrays were combined with a shot-selective 2D CAIPIRINHA implementation for multishot 3D EPI sequences at 7 T. In this implementation, in contrast to conventional inclusion of extra kz gradient blips, specific EPI shots are left out to create a CAIPIRINHA shift and reduction of scan time. First, the implementation of the CAIPIRINHA sequence was evaluated with a standard receive setup by acquiring submillimeter whole brain T2 *-weighted anatomy images. Second, the CAIPIRINHA sequence was combined with high-density receive arrays to push the temporal resolution of submillimeter 3D EPI fMRI scans of the visual cortex. Results show that the shot-selective 2D CAIPIRINHA sequence enables a reduction in scan time for 0.5 mm isotropic 3D EPI T2 *-weighted anatomy scans by a factor of 4 compared with earlier reports. The use of the 2D CAIPIRINHA implementation in combination with high-density receive arrays, enhances the image quality of submillimeter 3D EPI scans of the visual cortex at high acceleration as compared to conventional SENSE. Both the g-factor and temporal SNR improved, resulting in a method that is more sensitive to the fMRI signal. Using this method, it is possible to acquire submillimeter single volume 3D EPI scans of the visual cortex in a subsecond timeframe. Overall, high-density receive arrays in combination with shot-selective 2D CAIPIRINHA for 3D EPI scans prove to be valuable for reducing the scan time of submillimeter MRI acquisitions

Feasibility of cardiac-synchronized quantitative T1 and T2 mapping on a hybrid 1.5 Tesla magnetic resonance imaging and linear accelerator system

Background and Purpose: The heart is important in radiotherapy either as target or organ at risk. Quantitative T1 and T2 cardiac magnetic resonance imaging (qMRI) may aid in target definition for cardiac radioablation, and imaging biomarker for cardiotoxicity assessment. Hybrid MR-linac devices could facilitate daily cardiac qMRI of the heart in radiotherapy. The aim of this work was therefore to enable cardiac-synchronized T1 and T2 mapping on a 1.5 T MR-linac and test the reproducibility of these sequences on phantoms and in vivo between the MR-linac and a diagnostic 1.5 T MRI scanner. Materials and methods: Cardiac-synchronized MRI was performed on the MR-linac using a wireless peripheral pulse-oximeter unit. Diagnostically used T1 and T2 mapping sequences were acquired twice on the MR-linac and on a 1.5 T MR-simulator for a gel phantom and 5 healthy volunteers in breath-hold. Phantom T1 and T2 values were compared to gold-standard measurements and percentage errors (PE) were computed, where negative/positive PE indicate underestimations/overestimations. Manually selected regions-of-interest were used for in vivo intra/inter scanner evaluation. Results: Cardiac-synchronized T1 and T2 qMRI was enabled after successful hardware installation on the MR-linac. From the phantom experiments, the measured T1/T2 relaxation times had a maximum percentage error (PE) of -4.4%/-8.8% on the MR-simulator and a maximum PE of -3.2%/+8.6% on the MR-linac. Mean T1/T2 of the myocardium were 1012 ± 34/51 ± 2 ms on the MR-simulator and 1034 ± 42/51 ± 1 ms on the MR-linac. Conclusions: Accurate cardiac-synchronized T1 and T2 mapping is feasible on a 1.5 T MR-linac and might enable novel plan adaptation workflows and cardiotoxicity assessments

Improved delineation with diffusion weighted imaging for laryngeal and hypopharyngeal tumors validated with pathology

OBJECTIVE: This study aims to determine the added value of a geometrically accurate diffusion-weighted (DW-) MRI sequence on the accuracy of gross tumor volume (GTV) delineations, using pathological tumor delineations as a ground truth. METHODS: Sixteen patients with laryngeal or hypopharyngeal carcinoma were included. After total laryngectomy, the specimen was cut into slices. Photographs of these slices were stacked to create a 3D digital specimen reconstruction, which was registered to the in vivo imaging. The pathological tumor (tumor HE) was delineated on the specimen reconstruction. Six observers delineated all tumors twice: once with only anatomical MR imaging, and once (a few weeks later) when DW sequences were also provided. The majority voting delineation of session one (GTV MRI) and session two (GTV DW-MRI), as well as the clinical target volumes (CTVs), were compared to the tumor HE. RESULTS: The mean tumor HE volume was 11.1 cm 3, compared to a mean GTV MRI volume of 18.5 cm 3 and a mean GTV DW-MRI volume of 15.7 cm 3. The median sensitivity (tumor coverage) was comparable between sessions: 0.93 (range: 0.61-0.99) for the GTV MRI and 0.91 (range: 0.53-1.00) for the GTV DW-MRI. The CTV volume also decreased when DWI was available, with a mean CTV MR of 47.1 cm 3 and a mean CTV DW-MRI of 41.4 cm 3. Complete tumor coverage was achieved in 15 and 14 tumors, respectively. CONCLUSION: GTV delineations based on anatomical MR imaging tend to overestimate the tumor volume. The availability of the geometrically accurate DW sequence reduces the GTV overestimation and thereby CTV volumes, while maintaining acceptable tumor coverage

Diffusion weighted MRI for tumor delineation in head and neck radiotherapy

In radiotherapy treatments of head and neck cancer, ionizing radiation is used to destroy malignant tumor tissue, while sparing the surrounding healthy tissue. For successful delivery of radiotherapy treatments, an accurate definition of the target is essential. This target is defined using threedimensional imaging techniques such as computed tomography (CT), positron emission tomography (PET) and magnetic resonance imaging (MRI). MRI can visualize, amongst others, local differences in the diffusion of water: diffusion weighted MRI (DW-MRI). Tumors have different diffusion characteristics than normal tissue. With DW-MRI these differences can be visualized and used in target volume delineation. In order to use DW-MRI in a radiotherapy setting, geometric accuracy is vital. The most common method to acquire DW-MRI, DW-EPI, is known for its sensitivity to geometric distortions, especially in areas with large magnetic field inhomogeneities. The geometric accuracy of DW-EPI was evaluated in patients with a retrospective analysis of magnetic field maps. It was found that severe geometric distortions are present when using DW-EPI in the head and neck region. Therefore, the use of an alternative acquisition method, DW-SPLICE, was proposed. This method is based on a turbo spin echo sequence, which is used for standard anatomical imaging and has comparable, high geometric accuracy. The method was implemented and demonstrated in patients. Using DW-SPLICE, diffusion weighted images with excellent geometric accuracy were acquired in head and neck cancer patients. Additionally, the DW-SPLICE technique was extended in order to improve fat suppression. Additional images were acquired in order to yield a dataset which was suitable for water fat separation. The results from the water fat separation were applied in order to provide a more robust and homogeneous fat suppression. The diffusion weighted images acquired with DW-SPLICE were used to generate target volume delineations. Using an intensity threshold, an initial volume was segmented on diffusion weighted images. Subsequently these were manually adjusted based on the diffusion coefficients. These delineations were compared with those based on PET. Pathology validation of PET delineations has shown these to be quite accurate. The (semi-)automatic target volume delineations on DW-SPLICE show good correspondence with target volume delineations based on PET, indicating that, for a large part, both techniques indicate the same target for treatment. Using surgical specimens, obtained after total laryngectomy, pathological validation of DW-MRI can be performed. Patients received an MRI exam with DW-MRI prior to surgery. The surgical specimen of the larynx was processed, resulting in a digitally reconstructed threedimensional volume which was matched to the imaging prior to surgery. The initial results from the first patient show that the target volume delineation based on DW-MRI has very good agreement with the tumor defined on pathology. When diffusion weighted images are acquired with good geometrical accuracy, these images can be a valuable addition for target volume delineation in head and neck radiotherapy

Diffusion weighted MRI with minimal distortion in head-and-neck radiotherapy using a turbo spin echo acquisition method

PURPOSE: Diffusion weighted (DW) MRI, showing high contrast between tumor and background tissue, is a promising technique in radiotherapy for tumor delineation. However, its use for head-and-neck patients is hampered by poor geometric accuracy in conventional echo planar imaging (EPI) DW-MRI. An alternative turbo spin echo sequence, DW-SPLICE, is implemented and demonstrated in patients. METHODS: The DW-SPLICE sequence was implemented on a 3.0T system and evaluated in 10 patients. The patients were scanned in treatment position, using a customized head support and immobilization mask. Image distortions were quantified at the gross tumor volume (GTV) using field map analysis. The apparent diffusion coefficient (ADC) was evaluated using an ice water phantom. RESULTS: The DW images acquired by DW-SPLICE showed no image distortions. Field map analysis at the gross tumor volumes resulted in a median distortion of 0.2 mm for DW-SPLICE, whereas for the conventional method this was 7.2 mm. ADC values, measured using an ice water phantom were in accordance with literature values. CONCLUSIONS: The implementation of DW-SPLICE allows for diffusion weighted imaging of patients in treatment position with excellent geometrical accuracy. The images can be used to facilitate target volume delineation in RT treatment planning. This article is protected by copyright. All rights reserved

Optimized flip angle schemes for the split acquisition of fast spin-echo signals (SPLICE) sequence and application to diffusion-weighted imaging

Purpose: The diffusion-weighted SPLICE (split acquisition of fast spin-echo signals) sequence employs split-echo rapid acquisition with relaxation enhancement (RARE) readout to provide images almost free of geometric distortions. However, due to the varying T (Formula presented.) -weighting during k-space traversal, SPLICE suffers from blurring. This work extends a method for controlling the spatial point spread function (PSF) while optimizing the signal-to-noise ratio (SNR) achieved by adjusting the flip angles in the refocusing pulse train of SPLICE. Methods: An algorithm based on extended phase graph (EPG) simulations optimizes the flip angles by maximizing SNR for a flexibly chosen predefined target PSF that describes the desired k-space density weighting and spatial resolution. An optimized flip angle scheme and a corresponding post-processing correction filter which together achieve the target PSF was tested by healthy subject brain imaging using a clinical 1.5 T scanner. Results: Brain images showed a clear and consistent improvement over those obtained with a standard constant flip angle scheme. SNR was increased and apparent diffusion coefficient estimates were more accurate. For a modified Hann k-space weighting example, considerable benefits resulted from acquisition weighting by flip angle control. Conclusion: The presented flexible method for optimizing SPLICE flip angle schemes offers improved MR image quality of geometrically accurate diffusion-weighted images that makes the sequence a strong candidate for radiotherapy planning or stereotactic surgery.</p