Quantitative evaluation of the reduction of distortion and metallic artifacts in magnetic resonance images using the multiacquisition variable‑resonance image combination selective sequence

Abstract

Magnetic resonance imaging (MRI) is superior to computed tomography (CT) in determining changes in tissue structure, such as those observed following inflammation and infection. However, when metal implants or other metal objects are present, MRI exhibits more distortion and artifacts compared with CT, which hinders the accurate measurement of the implants. A limited number of reports have examined whether the novel MRI sequence, multiacquisition variable-resonance image combination selective (MAVRIC SL), can accurately measure metal implants without distortion. Therefore, the present study aimed to demonstrate whether MAVRIC SL could accurately measure metal implants without distortion and whether the area around the metal implants could be well delineated without artifacts. An agar phantom containing a titanium alloy lumbar implant was used for the present study and was imaged using a 3.0 T MRI machine. A total of three imaging sequences, namely MAVRIC SL, CUBE and magnetic image compilation (MAGiC), were applied and the results were compared. Distortion was evaluated by measuring the screw diameter and distance between the screws multiple times in the phase and frequency directions by two different investigators. The artifact region around the implant was examined using a quantitative method following standardization of the phantom signal values. It was revealed that MAVRIC SL was a superior sequence compared with CUBE and MAGiC, as there was significantly less distortion, a lack of bias between the two different investigators and significantly reduced artifact regions. These results suggested the possibility of utilizing MAVRIC SL for follow-up to observe metal implant insertions.

Keywords: MAVRIC SL; MRI; implant; metal artifacts; phantom.

Figure 1

Measurement of the screw diameter and the distance between the screws. (A) Overall view of a titanium implant-embedded agar phantom. (B) Selection of five slices for the measurement of the CT and MRI images. (C) The direction of the five columns of the measurements in each slice. (D) Example of the measurement of the screw diameter and the distance between the screws using the FWHM value for the profile curve of column d in (C). CT, computed tomography; MRI, magnetic resonance imaging; FWHM, full width at half maximum.

Figure 2

The process of quantitative evaluation of artifacts in MR images. Cross symbols indicate artifact regions. The white ‘phantom ROIs’ were used to create the standardized image and to define the artifact region by q1 and q3. The white ‘evaluation ROI’ indicates the phantom evaluation ROI, which was used to create the evaluation image. ROI, region of interest; MR, magnetic resonance; q1, lower quartile; q3, upper quartile.

Figure 3

ROI settings and evaluation images. (A) MR image of CUBE. (B) Phantom ROI. The range of the phantom ROI is shown in light gray for the MR image of (A). (C) Evaluation of ROI. The range of the evaluation of ROI is shown in light gray for the MR image of (A). The regions of the screws (white) and the implant support (black) are not included in the evaluation of the ROI. (D) Evaluation image of MAGiC. (E) Evaluation image of CUBE. (F) Evaluation image of MAVRIC SL. The scale bars at the bottom of the images indicate the range of signal values of each image. ROI, region of interest; MR, magnetic resonance; MAGiC, magnetic image compilation.

Figure 4

Histogram of the evaluation and artifact images. (A) Histogram of the evaluation image. (a) MAGiC, (b) CUBE, and (c) MAVRIC SL. The light gray color indicates the signal values of the pixels in the phantom ROI and the dark gray color indicates the signal values of the pixels used in the evaluation of ROI. (B) Artifact images. (a) MAGiC, (b) CUBE, and (c) MAVRIC SL. The white color indicates the defined artifact regions, whereas the gray color indicates the phantom regions and the black color indicates the regions of the screws, the implant support, and the background. MAGiC, magnetic image compilation; MAVRIC SL, multiacquisition variable-resonance image combination selective.