To develop a phantom that can evaluate the image distortion along the body axis objectively, and elucidate the relationship between the occurrence and geometrical position of image distortion in Magnetic Resonance Angiography (MRA). Furthermore, we validated the characteristics and utility of distortion correction filters.

We used a MAGNETOM Verio 3T (Siemens, Erlangen, Germany) as the MR device and the Body matrix coil and Spine matrix coil together for the receiver coil. The phantom for evaluating image distortion was an acrylic container 40cm in width, 50cm in depth, and 6cm in height. Seven acrylic pipes (with an inner diameter of 11mm and an outer diameter of 15mm) were arranged in equal intervals of 50mm inside the container. In this experiment, we enclosed a 1% aqueous solution of gadolinium inside the acrylic pipes and filled distilled water around it (Fig.1).

First, we placed the phantom in such a way that it coincided with the acrylic pipe placed right at the center of the magnetic field of the MR device. While doing so, we adjusted the height of the phantom using polystyrene foam. We performed coronal plane imaging around the acrylic pipe in the center, and performed sagittal plane imaging individually at three acrylic pipes - one at the center, one shifted to the left by 50mm, and one shifted by 100mm respectively. Next, we used polystyrene foam of 50mm thickness and carried out imaging at three locations similar to the above mentioned method: one at 50mm above the center of magnetic field, one at 50mm below the center of magnetic field and one at 100mm below the center of the magnetic field (Fig.2).

The MR images acquired in the coronal planes and sagittal planes were reconstructed in the transverse plane, and the central coordinates of the acrylic pipe were obtained from the measurements at seven places. Then we evaluated the image distortion from the front view and lateral view.

When the acrylic pipe was placed in the center of the magnetic field, image distortion did not occur at all (Fig.3). When the acrylic pipe was shifted up or down and left or right, the foot and the head of the acrylic pipes became distorted towards the center of the magnetic field. The distortion ratio was at its maximum of 5% at a location 10cm away from the center of the magnetic field in the direction of the body axis, and it was at a maximum of 10% at a location 15 cm away. As the distance from the center of the magnetic field increased, the image distortion ratio also increased. Regardless of whether the imaging plane was set to coronal or sagittal, no significant difference was observed in the degree of distortion.

The two-dimensional distortion correction filter was able to correct only the in-plane image distortion that exists in the slice plane and it had no effect on the image distortion that occurs across the slices (Fig.4). The three-dimensional distortion correction filter was able to correct the image distortion across the slices as well (Fig.5).

The equipment used in this experiment has built-in technology to make the magnetic field cylindrically uniform based on the human body. ¹This technology has enabled the imaging of areas that were not possible to date using the spherical magnetic fields. However, the image distortion cannot be reduced if the correction filters are not used. This would not be a problem in the imaging of cerebrovascular and neck blood vessels, where the imaging range along the direction of the body axis is not longer than 10 cm. However, since the imaging range is longer in MRA imaging of the body trunk, much image distortion occurs at locations that are away from the center of the magnetic field.^2-4 Since the image distortion that occurs in MR devices may hinder medical diagnosis, the image distortion should be proactively reduced by varying the imaging position and through the use of filters.The phantom that we have developed has made it possible to evaluate image distortion objectively over a wide range of the trunk.^5,6 When separated from the center of the magnetic field in vertical or horizontal directions, image distortion became larger with the increase in distance. In the case of a two-dimensional distortion correction filter, no significant effect can be obtained unless an optimal plane is not determined in accordance with the distortion occurrence conditions. However, in the case of a three-dimensional distortion filter, it was possible to correct the distortion under any condition.