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Several institutes are currently working on the development of a radiotherapy treatment system with online MR imaging (MRI) modality. The main difference between their designs is the magnetic field strength of the MRI system. While we have chosen a 1.5 Tesla (T) magnetic field strength, the Cross Cancer Institute in Edmonton will be using a 0.2 T MRI scanner and the company Viewray aims to use 0.3 T. The magnetic field strength will affect the severity of magnetic field dose effects, such as the electron return effect (ERE): considerable dose increase at tissue air boundaries due to returning electrons. This paper has investigated how the ERE dose increase depends on the magnetic field strength. Therefore, four situations where the ERE occurs have been simulated: ERE at the distal side of the beam, the lateral ERE, ERE in cylindrical air cavities and ERE in the lungs. The magnetic field comparison values were 0.2, 0.75, 1.5 and 3 T. Results show that, in general, magnetic field dose effects are reduced at lower magnetic field strengths. At the distal side, the ERE dose increase is largest for B = 0.75 T and depends on the irradiation field size for B = 0.2 T. The lateral ERE is strongest for B = 3 T but shows no effect for B = 0.2 T. Around cylindrical air cavities, dose inhomogeneities disappear if the radius of the cavity becomes small relative to the in-air radius of the secondary electron trajectories. At larger cavities (r > 1 cm), dose inhomogeneities exist for all magnetic field strengths. In water-lung-water phantoms, the ERE dose increase takes place at the water-lung transition and the dose decreases at the lung-water transition, but these effects are minimal for B = 0.2 T. These results will contribute to evaluating the trade-off between magnetic field dose effects and image quality of MR-guided radiotherapy systems. 相似文献
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19 F MRSI of capecitabine in the liver at 7 T using broadband transmit–receive antennas and dual‐band RF pulses
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Jetse S. van Gorp Peter R. Seevinck Anna Andreychenko Alexander J. E. Raaijmakers Peter R. Luijten Max A. Viergever Miriam Koopman Vincent O. Boer Dennis W. J. Klomp 《NMR in biomedicine》2015,28(11):1433-1442
Capecitabine (Cap) is an often prescribed chemotherapeutic agent, successfully used to cure some patients from cancer or reduce tumor burden for palliative care. However, the efficacy of the drug is limited, it is not known in advance who will respond to the drug and it can come with severe toxicity. 19 F Magnetic Resonance Spectroscopy (MRS) and Magnetic Resonance Spectroscopic Imaging (MRSI) have been used to non‐invasively study Cap metabolism in vivo to find a marker for personalized treatment. In vivo detection, however, is hampered by low concentrations and the use of radiofrequency (RF) surface coils limiting spatial coverage. In this work, the use of a 7T MR system with radiative multi‐channel transmit–receive antennas was investigated with the aim of maximizing the sensitivity and spatial coverage of 19 F detection protocols. The antennas were broadband optimized to facilitate both the 1H (298 MHz) and 19 F (280 MHz) frequencies for accurate shimming, imaging and signal combination. B1+ simulations, phantom and noise measurements showed that more than 90% of the theoretical maximum sensitivity could be obtained when using B1+ and B1? information provided at the 1H frequency for the optimization of B1+ and B1? at the 19 F frequency. Furthermore, to overcome the limits in maximum available RF power, whilst ensuring simultaneous excitation of all detectable conversion products of Cap, a dual‐band RF pulse was designed and evaluated. Finally, 19 F MRS(I) measurements were performed to detect 19 F metabolites in vitro and in vivo. In two patients, at 10 h (patient 1) and 1 h (patient 2) after Cap intake, 19 F metabolites were detected in the liver and the surrounding organs, illustrating the potential of the set‐up for in vivo detection of metabolic rates and drug distribution in the body. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
44.
Similar network activated by young and old adults during the acquisition of a motor sequence 总被引:8,自引:0,他引:8
Daselaar SM Rombouts SA Veltman DJ Raaijmakers JG Jonker C 《Neurobiology of aging》2003,24(7):1013-1019
In this functional MRI (fMRI) study, we investigated ageing effects on motor skill learning. We applied an adapted version of the serial reaction time (SRT) task to extensive groups of young (N=26) and elderly (N=40) subjects. Since indications have been provided for age-related shrinkage of brain regions assumed to be critical to motor skill learning, we tested the hypothesis that age effects on implicit sequence learning are larger on a neurofunctional level than on a behavioural level. The SRT task consisted of two identical scan sessions, in which subjects had to manually trail an asterisk appearing serially in one of four spatial positions by means of button-pressing. Reliable response time reductions were already found in the first session for both the young and the elderly groups, when comparing a fixed sequence condition to a random sequence, but the learning effect was greater for the young subjects. In the second session, though, both groups showed a similar degree of learning. This indicates that implicit sequence learning is still intact in elderly adults, but that the rate of learning is somewhat slower. Reliable learning-related changes in brain activity were also observed. A similar network of brain regions was recruited by both groups during the fixed compared to the random sequence, involving several regions that have been previously associated with implicit sequence learning, including bilateral parietal, and frontal regions, the supplementary motor area (SMA), cerebellum and the basal ganglia. The direct group comparison did not reveal any differences in brain activity. In addition, we did not observe any significant differences in activity when comparing the different sessions either, neither for the young nor for the elderly subjects. Hence, we did not find indications for an age-related functional reorganisation of neural networks involved in motor sequence learning. In view of earlier reports of pronounced ageing effects on the performance on declarative memory tasks, our finding of age-related sparing of processes that sustain motor skill learning, provides further support for the proposition of different memory systems relying on different brain substrates. 相似文献
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Antonetta C. Houweling Cornelis A.T. van den Berg Judith M. Roesink Chris H.J. Terhaard Cornelis P.J. Raaijmakers 《Radiotherapy and oncology》2010,97(2):239-243
Purpose
Besides sparing the parotid gland, sparing the submandibular gland is considered to be important in preventing xerostomia in head-and-neck cancer patients. Delineation of the submandibular gland at CT, and even on T1- and T2-weighted MR images, is difficult, due to low contrast with the surrounding tissues. MR sialography might be used for delineation.Methods and materials
Sixteen oropharyngeal cancer patients received a CT and MRI exam as part of the standard treatment imaging protocol. Patients were scanned in their five-point RT immobilization mask. The MRI exam included T1- and T2-weighted MRI scans and an MR sialography scan. Thirty submandibular glands were delineated on only CT, on the combined CT and T1- and T2-weighted MRI scans and on all MR images. A Wilcoxon signed-rank test was performed to test if the delineated volumes were significantly different.Results
The delineated volume of the submandibular gland was 7.3 mL in the CT-delineation, 7.1 mL in the CT/MRI-delineation and 8.1 mL in the MRI-delineation. The MRI-delineation was significantly larger than the other delineations (p < 0.001). The differences were mainly located in the cranial direction.Conclusion
The delineation of the submandibular gland was improved in the cranial direction by using T1- and T2-weighted MRI and MR sialography, compared to the other delineations. 相似文献49.
50.
Marieke H. Verhappen Philip M.P. Poortmans Esther Raaijmakers John M.M. Raemaekers 《Radiotherapy and oncology》2013