15. Effects of Dental Alloys and a Magnetic Keeper on MRI. Part 1 Analysis from Dicom Data

H. Mizutani, D. Destine, M. Hideshima, M. Sato, J. Wadachi, A. Nishiyama, K. Nakamura, S. Ishikawa, D. Okusa, H. Sasaki, S. Matsuzaki, Y. Igarashi

Section of Removable Proshodontics Graduate School, Tokyo Medical and Dental University

Introduction

Magnetic attachments have various characteristics that conventional mechanical attachments cannot achieve, however�@these materials contains ferromagnetic substance that can distort the image taken from magnetic resonance imaging.

MRI is an important examination tool for the head and neck in dentistry. It gives excellent images of anatomical structures differing in proton density and other tissues characteristics. The magnetism of the metals and alloys, commonly used in dentistry, differs within each other according to their magnetic characteristics depending on whether they are diamagnetic, ferromagnetic or paramagnetic substances. The artifacts generated in MRI caused by the magnetic attachment (keepers) and dental alloys have not yet been analyzed in detail. In our preliminary study, we centered this study on the Signal Intensity (SI) and Coefficient of variation (CV) in order to evaluate the artifacts.

Objective

Aims of this paper is to present a format of the conference to authors.

Materials and Methods

1.Samples

One commercially available keeper and 3 casted keepers of different composition alloy were selected. (Table 1. Fig 1.)

2.Phantom

In order to analyze the samples and control, a cubic shape phantom without cover was made, similarly to a human adult head�fs dimension. We fill the phantom with agar-agar by melting it in a SPM Colloid bath only until half in order to insert the keeper in the middle of the phantom, vertically simulating its position as in a clinical situation. After the first layer of agar-agar, a second one was filled in the phantom.(Fig. 3.)

3. MRI apparatus

Keeper shape samples had been placed on the positioning post on the table of the MRI apparatus (1.5 T, Magnetom Vision, Siemens, Germany) in a head coil which surrounding the patient�es head and can either generate a magnetic field or just receive it. The laser beam of the MR system had been centered on the sample surrounding by the Volume coils.(Fig. 4)

4. MRI sequence (T1 weighted)

Pulse sequence, Repetition time (TR) 550.0 msec, time to echo 14.0m/sec, Acquisition Time (TA) 04�f45�h, with a flip angle FA= 900. All the pictures were taken with a number of acquisitions Ac: 2. Slice thickness, 5 mm. Field of view (FOV): 230 mm. Matrix size: 256*256. (Fig. 5.)

In the result, only the axial and sagittal data will be used.

Results

R-1: Images from MRI are shown for the 4 simples and the agar-agar control with the use of T1 weighted images axial and sagittal. Artifacts appear as signal intensifications, loss of signals, and signal deformation surrounded by bright line. In Au and Pd images it appeared as a black regular area pointed by white spot artifacts. No deformation was observed.

R-2: Around the central of the phantom for the axial images, we can observe the wide distortion of the images follow by a dark gap surrounding by bright line in some area.

R-3: In SuS and CoCr, A conical curvature image (wide distortion of the images follow by a dark gap surrounding by bright line in some areas).

As for Au and Pd, A single white spot images for both sagittal and axial.

R-4: Axial and sagittal signal intensities are shown in these histograms.

R-5: This slide shows that the comparison among samples of coefficient variances of signal intensity in ROI. In both sagittal and axial planes SUS and CoCr are significantly higher than control. While in Pd and Au, there are no significant differences compared to control.

Discussions

Depending on the magnetic field, three major types of alloys can be identified: Ferromagnetic, Paramagnetic, and Diamagnetic. Hence, in the oral cavity (head and neck section) selection of the dental alloys has to be made according to the compositions and the susceptible of the dental alloys.

The signal generated by those alloys can have lower and higher value depend on the areas and the coordinate plane of the image. Where there have signal void the number of pixel decrease producing no signal response. The artifacts images of Au and Pd alloys did not show any disturbance on the phantom MR images and the SI generated by them had no significant difference with Control.

The control of the artifact area proved that the metal size is not depending upon the artifact size. SUS was the smaller in volume and weight but its artifact created was considerable. Artifacts can be projected far away from the sample to create distortion of the images.

Au, CoCr, SUS and Pd are often used in prosthodontics dentistry for their properties as higher life expectancy, maintenance of good condition. However, in radiology field, CoCr does generate artifacts. In addition, magnetic attachment had been proved to be a concrete solution to get the better of retention in aged patients who have disabled and where dislodgement of the denture is easy. To avoid the artifacts area, the dental keeper can also be removed from the oral cavity. As it is situated in the mouth, it is not a disturbance in the other part of the body who need to be exposed to MRI.

Conclusions

1. Coefficient of variation helped us to evaluate the variation of the signal intensity among metal alloys in MRI.

2. Pd alloys shows less distortion than both CoCr and SuS.

References

1. Shafiei F, Honda E, Takahashi H, Sasaki T. Artifacts from Dental Casting Alloys in Magnetic Resonance Imaging. J Dent Res 82: 602-606, 2006.

2. Shellock FG, Kanal E. Aneurysm Clips: Evaluation of MR Imaging Artifacts at 1.5T. Radiology 209: 563-566, 1998.

Discussion Board