Assessment of the Characteristics of MRI Coils in Terms of RF Non-Homogeneity Using Routine Spin Echo Sequences

Introduction: One of the major causes of image non-uniformity in MRI is due to the existence of  non-homogeneity in RF receive and transmit. This can be the most effective source of error in  quantitative  studies  in  MRI  imaging.  Part  of  this  non-homogeneity  demonstrates  the  characteristics of RF coil and part of it is due to the interaction of RF field with the material being  imaged. In this study, RF field non-homogeneity of surface and volume coils is measured using an  oil phantom. The method employed in this work is based on a routine Spin Echo based sequence  as proposed by this group previously.  Materials and Methods: For the determination of RF non-uniformity, a method based on Spin  Echo  sequence  (θ-180)  was  used  as  reported  previously  by  the  same  author.  In  this  method,  several images were obtained from one slice using different flip angles while keeping all other  imaging  parameters  constant.  Then,  signal  intensity  at  a  ROI  from  all  of  these  images  were  measured  and  fitted  to  the  MRI  defined  mathematical  model.  Since  this  mathematical  model  describes the relation between signal intensity and flip angle in a (θ-180) Spin Echo sequence, it is  possible  to  obtain  the  variation  in  receive  and  transmit  sensitivity  in  terms  of  the  variation  of  signal  intensity  from  the  actual  expected  values.  Since  surface  coils  are  functioning  as  only  receiver (RF transmission is done by Body coil), first the results of receive coil homogeneity is  measured, then characteristic of transmit coil (for the body coil) is evaluated  Results: The coefficient of variation (C.V.) found for T(r) value obtained from images using head  coils was in the order of 0.6%. Since the head coil is functioning as both transmitter and receiver,  any non-uniformity in either transmit or receive stage can lead to non-homogeneity in RF field. A  part from the surface coils, the amount of non-homogeneity due to receive coil was less than that  of  the  transmit  coil.  In  the  case  of  the  surface  coils  the  variation  in  receive  homogeneity  measurement was high due to inherent sensitivity of the coil in different locations, which was as  expected.  Discussion and Conclusion: Since the RF interaction of RF field is very low in oil phantom, the  variation shown in the T(r) is due to B1 non-homogeneity of coil system. Since the head coil is  functioning as both transmitter and receiver, any non-uniformity in either transmit or receive stage  could lead to non-homogeneity of RF field.