Acceptance procedure for the linear accelerator component of the 1.5 T MRI-linac.

PURPOSE: To develop and implement an acceptance procedure for the new Elekta Unity 1.5 T MRI-linac. METHODS: Tests were adopted and, where necessary adapted, from AAPM TG106 and TG142, IEC 60976 and NCS 9 and NCS 22 guidelines. Adaptations were necessary because of the atypical maximum field size (57.4 × 22 cm), FFF beam, the non-rotating collimator, the absence of a light field, the presence of the 1.5 T magnetic field, restricted access to equipment within the bore, fixed vertical and lateral table position, and the need for MR image to MV treatment alignment. The performance specifications were set for stereotactic body radiotherapy (SBRT). RESULTS: The new procedure was performed similarly to that of a conventional kilovoltage x-ray (kV) image guided radiation therapy (IGRT) linac. Results were acquired for the first Unity system. CONCLUSIONS: A comprehensive set of tests was developed, described and implemented for the MRI-linac. The MRI-linac met safety requirements for patients and operators. The system delivered radiation very accurately with, for example a gantry rotation locus of isocenter of radius 0.38 mm and an average MLC absolute positional error of 0.29 mm, consistent with use for SBRT. Specifications for clinical introduction were met.

Dielectric waveguides for ultrahigh field magnetic resonance imaging.

PURPOSE: The design of RF coils for MRI transmit becomes increasingly challenging at high frequencies required for MRI at 7T and above. Our goal is to show a proof of principle of a new type of transmit coil for higher field strengths. METHOD: We demonstrate an alternative transmit coil design based on dielectric waveguide principles which transfers energy via evanescent wave coupling. The operating principles and conditions are explored by simulations. The waveguide is applied for in vivo imaging at 7T. RESULTS: The waveguide can be an efficient transmit coil when four conditions are fulfilled: (1) the waveguide should be operated just above the cutoff frequency of the lowest order transverse electric mode, (2) the waveguide should not operate at a frequency where the wavelength fits an integer number of times in the waveguide length and standing wave patterns become very prominent, (3) for homogeneous excitation, the waveguide should be bent around the object, and (4) there should be an air gap between the waveguide and the object. CONCLUSIONS: By choosing the dielectric and the dimensions adequately, the dielectric waveguide couples the magnetic field efficiently into the body. The waveguide can be redesigned for higher frequencies by simple adaptations and may be a promising transmit alternative. Magn Reson Med 76:1314-1324, 2016. © 2015 Wiley Periodicals, Inc.

MRI of the carotid artery at 7 Tesla: quantitative comparison with 3 Tesla.

PURPOSE: To evaluate the 7 Tesla (T) MRI of the carotid arteries, as quantitatively compared with 3T. MATERIALS AND METHODS: The 7T MRI of the carotid arteries was performed in six healthy subjects and in two patients with carotid stenosis. The healthy group was scanned at 3T and at 7T, using current coil setups at both field strengths. T1 and T2 values of the normal carotid vessel wall were assessed at both field strengths. B1 (+) maps and signal to noise ratio (SNR) maps were obtained, as well as T1 weighted images with a resolution as high as 0.4 × 0.4 × 1.5 mm(3) . RESULTS: The T1 of the normal carotid vessel wall was found to be 1227 ± 47 ms at 3T and 1628 ± 130 ms at 7T, while a T2 of 55 ± 11 ms at 3T and 46 ± 4 ms at 7T was found. A twofold average gain in SNR at the carotid arteries was found with 7T. T1 weighted images showed an increased SNR at 7T for all subjects. CONCLUSION: Evaluation between 3T and 7T carotid MRI with optimized setups at both field strengths showed improved SNR at 7T, an increase in vessel wall T1 and a decrease in vessel wall T2.

Clinical value of (dedicated) 3 Tesla and 7 Tesla MRI for cT1 glottic carcinoma: A feasibility study.

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 quality. LEVEL OF EVIDENCE: 2B, prospective diagnostic study.

Systematic review of the diagnostic value of magnetic resonance imaging for early glottic carcinoma.

Objective: In early glottic cancer, accurate assessment of tumor extension, including depth infiltration, is of great importance for both staging, therapeutic approach and systematic comparison of data. Our goal was to assess the diagnostic value of MRI in pre-therapeutic staging of primary early stage (T1 and T2) glottic carcinoma. Study design: Systematic review of literature. Methods: We conducted a systematic search in Pubmed, Embase, and Scopus up to September 23, 2016. Included studies were selected and critically appraised for relevance and validity. Results: Seven out of 938 unique articles were selected, including 64 cases. MRI over-staged 6% and under-staged 13% of cT1 and cT2 tumors. However, available data is heterogeneous, very limited and mainly based on subanalysis of a small amount of patients. Reported MRI protocols appear to be suboptimal for small laryngeal lesions. Diagnostic value of MRI for subtle depth infiltration or laryngeal anatomical subsites (eg, laryngeal ventricle, vocal cord, etc.) could not be assessed. Conclusions: More studies are needed to assess the diagnostic value of MRI for small glottic tumors.