From 1.5t To 3t Mr Image In Gamma Knife Radiosurgery, Is There Clinical Improvement?

Keywords: Imaging, magnetic resonance imaging, gamma knife, brain tumor, technique

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     This gamma knife radiosurgery (RS) center began treating patients in 1995 with a 0.5T  magnetic resonance (MR); in 2001 we changed to a 1.5T MR and since 2007 a 3T MR images had been used for LGK ELEKTA® RS treatment planning; we haven’t had significant clinical differences in treatment dose prescription per pathology as a result. 3T MR is known to have major magnetic and bigger gradient field inhomogeneities but a superior anatomical contrast and resolution than 0.5T or 1.5T.
     In this center we started the use of 3T MR for RS procedures after validation and quality control tests of the MR were performed.
      A Philips® Achieva 3T MR was installed 26 months prior to this report, after the execution and development of image distortion and quality control tests were done, in which we realize that none of the original sequences fulfilled the characteristics for use in RS. After performing work on amending the technical parameters of the sequences, only 12 of them meet the requirements for gamma knife radiosurgery. So far 158 RS treatments had been performed using these images with GammaPlan® V5.34 for B model. Patients with Leksell® frame where set to the TR quadrature head coil antenna and a combination of sequences were acquired (T1 3D, T2 MS, FLAIR) from 0.95, 1.00 and 1.5 mm images in axial and coronal views.
     Axial image presents interference that comes from the stereotactic frame which manifests as a signal loss and with distortion increment in areas near the frame and extend up to 25 mm the superior rim. Coronal images also show low signal in the inferior rim of the antenna which amplifies by electric susceptibility effect in the frame; this gives poor fiducial image and definition. T1W3D sequences have a greater distortion approaching the frame, while T2 MS and T1 FLAIR images have a better stability in areas close to the frame. In some cases, the patient could have metallic implants in the head or mouth increasing the TR head coil load. Maximal deviations were 1.2 mm and minimal average was 0.2 mm, axial images were better than coronal and T2 and FLAIR were better in both.
     This is a retrospective study.
     3T MR images should be used in GKRS matched with multislice CT and DSA if needed, they provide additional clinical information that can be used in functional RS where precision must be warranted in absence of morphological target. Sequence combination is required in different scenarios, with metallic implants or without them -sequences were modified for the cases of metal or non metal conditions-also in fast, and in high resolution sequences combination is needed.
     We also confirm that radiofrequency interference depends on the axial or coronal orientation views, these are mainly associated with the induced electrical current, dielectric RF stimulation in the frame and metallic or not metallic post and the inferior part of the head coil which in the 3T MR is shorter than our previous 1.5T MR. Care should be considered in regions near or close to the upper and inferior rim of the frame. 3T MR was not considered a heavy weight factor in deciding dose prescription in this RS center.


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