Authors
1
Associate Professor, Physics and Biomedical Engineering, Tehran University of Medical Sciences, Tehran, Iran. Research Center for Science & Technology in Medicine, Imam Khomeini Hospital, Tehran. Iran.
2
M.Sc. in Research Center for Science & Technology in Medicine, Imam Khomeini Hospital, Tehran. Iran.
3
B. Sc. in Medical Imaging Center, Imam Khomeini Hospital, Tehran University of Medical Sciences, Tehran, Iran.
4
Associate Professor, Radiology Dept., Medical Imaging Center, Imam Khomeini Hospital, Tehran University of Medical Sciences, Tehran, Iran.
Abstract
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.
Keywords
Main Subjects
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