Reproducibility of geometric distortion in magnetic resonance imaging based on phantom studies

Background and purpose: Image distortion is one of the major drawbacks of m agnetic resonance (MR) imaging for use in radiotherapy treatment planning ( RTTP). In this study, the reproducibility of MR imaging distortion was eval uated by repeated phantom measurements. Materials and methods: A grid-pattern acrylic phantom was scanned with a 0. 2-Tesla permanent magnetic unit. We repeated a series of scans three times to evaluate the reproducibility of the distortion. In each series, co-ordin ates at 432 intersections of the grid were measured for both T1- and T2-wei ghted spin-echo (SE) pulse sequences. Positional displacements and their va riations at the intersections were calculated. Results: Averages of the displacements were distributed between 1.58 and 1. 74 mm, and maximum values (MAX) between 12.6 and 15.0 mm. Within 120 mm of the image center, the average values ranged from 0.73 to 0.80 mm, and from 3.4 to 5.0 mm for MAX. The absolute values of the positional variations amo ng three series were distributed between 0.41 and 0.88 mm for average value s, and between 1.4 and 4.5 mm for MAX. Conclusions: The positional variations were mostly within 3 pixels, and mos t of the positional displacements within the radius of 120 mm of the image center were 2 mm or less. Therefore, it will be possible to use this MR sys tem in RTTP under limited situations, although careful applications are req uired for RTTP of the body. The development of a computer program to correc t image distortion is expected. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.