Assessment of Reproducibility of Geometric Distortion in MRI using Phantom Measurements

Introduction:  Image  distortion  is  one  of  the  major  problems  of  magnetic  resonance  imaging  (MRI) for use in 3DMRI, velocity MRI, FMRI and radiotherapy treatment planning (RTTP). It is  widely known that the most obvious effect of the inhomogenity of the magnetic fields and the  nonlinearity  of  the  gradient  is  the  Geometric  Distortion  of  MR  tomograms.  In  this  study,  the  accuracy of MR images was considered by the phantom study and the reproducibility of images  was evaluated by repeating the phantom measurements.  Materials and Methods: MRI scans of the phantom with grid pattern inside it were performed  using head coil in two 1.5 Tesla MRI systems (Picker VISTA TM HPQ, USA & GE Signa Echo  Speed, USA). For the Geometric Distortion reproducibility evaluation the T1W, T2W and PDW  SE  protocols  were  repeated  three  times.  The  Geometric  Distortion  was  measured  by  an  edge  detection program in MATLAB.   Results: The geometric distortion in the peripheral area of the images tends to be generally larger  than it in the central area in all images. The average displacement in Picker MRI was 3 pixels in  the y and 1 pixel in the x direction and in GE MRI it was 1 pixel in both x and y direction for 24  cm FOV (pixel = 0.9 mm).  Discussion and Conclusions: Since the positional variation was within 1 to 3 pixels for the head  coil, it will be possible to use this MR system in 3DMRI, velocity MRI, FMRI and RTTP. To  decrease the geometric distortion the use of suitable coil and protocol are expected.