Performance of PROPELLER FSE T2WI in Reducing Metal Artifacts for Patients with Various Material Porcelain Fused To Metal Crown: A Preliminary Study

Wenjin Li Second Hospital of Shanxi Medical University Jing Shi Second Hospital of Tianjin Medical University Wenjin Bian Shanxi Medical University Jianting Li Second Hospital of Shanxi Medical University Xiaoqing Chen The Sixed Hospital of Wuhan Juan Feng The First Hospital of Yulin Jiali Yu The First A liated Hospital of Sun Yat-Sen University Jun Wang Second Hospital of Shanxi Medical University Jinliang Niu (  sxlscjy@163.com ) Second Hospital of Shanxi Medical University


Introduction
The porcelain fused to metal (PFM) is the common traditional method for xed dentures in patients with dental defects or dentition defects and is especially used for single tooth restoration. The PFM are valued for their visual appeal (they can match the color of the surrounding teeth and have similar visual properties to natural teeth), are extremely durable, and affordable. Also, these materials are not expensive and offer superior mechanical properties, and inertness compared with all other ceramic crowns 1,2 . At present, cobalt-chromium (Co-Cr) alloy, pure titanium (Ti) and gold-palladium (Au-Pd) alloy are the most common materials used for PFM crowns 1 .
Magnetic resonance imaging (MRI) is the most common head-neck clinical imaging technique used in clinical work mainly due to its non-ionizing radiation nature and superior soft-tissue image contrast 3,4 .
However, MR image quality of the oral cavity and maxillofacial is often impaired by metallic dental restorations and implant-supported prostheses. For example, the metal crown of PFM causes artifacts including signal-loss, signal-pileup, geometric distortion 5,6 , which can affect the visibilities of the anatomic structures near the PFM such as a tooth, periodontal space, tongue. The artifacts of metal implants depend on many factors, including the MRI hardware and room shielding, MRI software, sequence parameters, amount, shape, and material characteristics of used abutments and metal crowns.
Though several studies have described the effect of material characteristics on MRI interpretation as the most signi cant among these factors 7,8 , few have addressed these problems in clinical situations. Therefore, identifying preferable material compositions of PFM crown and investigating methods to reduce or avoid metal artifacts in patients with PFM may improve individualized treatment and MRI scanning regimens.
To address the decreased image quality due to metal implants, some optimized MRI scanning protocols were proposed to minimize the metal artifacts, such as using spin-echo or fast spin-echo sequences with long echo train lengths, a high bandwidth, thin section selection, and an increased matrix 9 . Recently, several MRI sequences were developed to reduce susceptibility artifacts, including view angle tilting (VAT), slice-encoding for metal artifact correction (SEMAC), multi-acquisition with variable resonance image combination (MAVRIC), ultrashort time-to-echo (UTE) and combinations of these techniques 10-12 . Furthermore, various deep learning-based approaches were developed to reduce metal artifacts, improve image quality, and even predict the missing information/regions in MR images affected by metal artifacts [13][14][15] . However, the clinical uses of these methods were restricted due to safety and quality control issues, as well as complex principles and higher demand for MRI equipment in hardware and software 16 .
Periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) combines a fast-spin echo (FSE) has been shown to be effective in decreasing motion artifact and suppressing ow artifacts after applying of contrast agent in the whole body [17][18][19][20] . The recent study con rmed that the PROPELLER sequence could decreases metallic artifacts apart from motion artifacts. The sequence was used to minimize the metallic artifacts and distortion near a metallic prosthesis in patients with hip metal work 21 . So, we hypothesize that the PROPELLER sequences would probably reduce metal artifacts of PFM crowns. Therefore, we aimed to compare MR imaging quality in common FSE T 2 WI with PROPELLER FSE T 2 WI for patients with PFM of different metal crowns and investigate the value of the PROPELLER technique in reducing metal artifacts.

Study population
This prospective study was approved by the local ethics committee (Second Hospital of Shanxi Medical University, Taiyuan, China). Written informed consent was obtained from all participants. All methods were performed in accordance with the relevant guidelines and regulations. Between July 2020 and March 2021, participants with single unit PFM crowns of three different metal materials scheduled to undergo a clinically indicated 1.5T head MRI for known or suspected head pathology (cerebrovascular disease, tumor, infectious lesion) were enrolled in this study from the Department of Stomatology (Second Hospital of Shanxi Medical University) consecutively. Inclusion criteria included participants who had no metal llings, implants, titanium plates or other metal materials except for PFM, had good compliance and were eligible for MRI. Exclusion criteria included space-occupying lesion in the oral cavity and maxillofacial regions or motion artifacts of MRI images. The participant characteristics (e g. age, sex, and material of metal crown) were collected. The metal crown materials and their components used in this study were: Co-Cr (Co 60.2%, Cr 25%, Mo 4.8%, W 6.2%, Si 1.0%), Au-Pd (Au 65%, Pd 25%), Ti (Ti 99.9%).

Image acquisition
All MRI data were acquired on a 1.5T MR scanner (Signa; GE Healthcare, Waukesha, Wis) with an 8channel head and neck coil. Each participant was placed in the supine position, and underwent head MRI scan. The MRI sequences consisted of axial FSE T 2 WI, axial PROPELLER FSE T 2 WI, axial FSE T1WI, axial diffusion weighted imaging (DWI) and sagittal FSE T1WI. Parameters of axial FSE T 2 WI were as follows: repetition time (TR)/echo time (TE), 3000/113ms; Field of View (FOV), 240×240mm 2 ; slice thickness/gap, 6/1 mm; the number of excitations (NEX) , 2; Echo-train length(ETL), 19; matrix size , 352 × 352; Axial PROPELLER was optimized by selecting a bandwidth to minimize TE with the following parameters: TR/TE, 3000/112ms; NEX, 2; ETL, 32; FOV, matrix size, slice thickness and orientation were matched to T 2 WI. All sequences used 2D acquisition.

Image analysis
The image quality of common axial FSE T 2 WI and axial PROPELLER FSE T 2 WI were evaluated quantitatively and qualitatively using our institution´s picture archiving and communication system (PACS) workstation (Advantage Windows Workstation 4.6; GE Healthcare). The images were independently reviewed and scored by two radiologists (X.X. and X. X.), with 5 and 10 years of experience in head and neck MRI, respectively. All images were deidenti ed and evaluated in a blinded and randomized fashion with respect to the method of image acquisition.

Qualitative image analysis
Visualizations of the anatomic structures around PFM were evaluated by two radiologists using a 5-point scale in all image sets as described before 11 . The visibilities of four anatomic structures near the PFM including visualization of the periodontal space, the tooth, the tongue, and bone (maxilla or mandible) in the MR images were graded as follows: grade 1 indicated the worst quality for interpretation where the anatomic structures around PFM were barely delineated; grade 2 indicated that 25% of the above structures were visible; grade 3, visualization of 50% of the above structures; grade 4, visualization of 75% of the above structures; and grade 5, none of the four anatomical structures around the PFM were affected by artifacts.

Quantitative image analysis
The artifact was de ned as areas of signal void pileup or geometric distortion. First, the plane with the maximal artifact were determined, then the maximum areas of metal artifacts were outlined and measured by two radiologists jointly using Advantage Workstation. The artifact area reduction rate of PROPELLER FSE T 2 WI image were calculated as the difference between PROPELLER FSE T 2 WI image artifact area and common FSE T 2 WI image artifact area divided by common FSE T 2 WI image artifact area. Next, in the plane with the maximal artifact, circular ROIs were drawn in the white artifact and the background of the same level respectively. Positioning and sizing of these ROIs were identical in common FSE T 2 WI and PROPELLER FSE T 2 WI images to minimize individual variations for sequence comparison.
The SNR were then calculated as the mean signal intensity within an artifact ROI divided by the standard deviation of signal intensity in the back ground.

Statistical analysis
Statistical analysis was performed using SPSS 25.0 software (SPSS, Chicago, IL, USA). Interrater agreement for qualitative scores was assessed by Cohen's weighted kappa (κ) and was interpreted as follows: Poor correlation (κ < 0.20); Fair correlation (κ= 0.21-0.40); Moderate correlation (κ= 0.41-0.60); Good correlation (κ= 0.61-0.80); and excellent correlation (κ= 0.81-1.00). The image quality scores and artifact areas of different metal crowns were compared using post-hoc analysis's Friedman test. The image quality scores, artifact areas and SNR of all common FSE T 2 WI and PROPELLER FSE T 2 WI were compared using a two-sample Wilcoxon test. Results were provided as mean ±standard deviation (SD). A P value < 0.05 was considered to be statistically signi cant.

Patient data
A total of 64 participants with single unit PFM crowns underwent MRI in the head. One participant with maxillofacial space-occupying lesion, two participants with tumor of tongue and two participants with motion artifacts were excluded. The nal study sample consisted of 59 participants (24 males and 35 females; mean age ± SD, 63 ± 5.2 years; age range,35-70 years old). There were 21 participants with the metal crown of Co-Cr alloy in PFM, 20 with the metal crown of Au-Pd alloy in PFM, and 18 participants with the metal crown of Ti in PFM.

Inter-reader variability
There were good agreements between readers for scoring image quality of Co-Cr (κ= 0.80) and Au-Pd (κ= 0.73) in common FSE T 2 WI, with the excellent agreement (κ= 0.84) for the image quality of Ti.
The common FSE T 2 WI sequence and PROPELLER FSE T 2 WI sequence differed signi cantly in image quality score and artifact area for Co-Cr and Ti, but not for the Au-Pd. PROPELLER FSE T 2 WI exhibited signi cantly better image quality than common FSE T 2 WI for Co-Cr (reader 1 (score ± SD): 1.4±0.1 VS.  Table 2 and Table 3.

Discussion
PFM restorations are increasingly used in prosthetic dentistry. Dental MRI offers radiation-free and highresolution in vivo imaging of the teeth, jaw and adjacent soft tissue. However, image assessment may be distorted by artifacts due to metallic dental restorations. Prior studies have demonstrated that the PROPELLER sequence could reduce metal artifacts in patients with a metal implant for orthopedic and neurosurgical applications 18,21 . Our study has further shown that PROPELLER FSE T 2 WI may signi cantly improve imaging quality and reduce artifact areas compared to the common FSE T 2 WI sequence in dental MR imaging especially in the PFM crown of Co-Cr alloy.
There is an increasing demand for PFM in patients with dental defects or dentition defects. In the future, radiologists will be required to select appropriate MRI sequences and parameters which could reduce metal artifacts caused by metal crowns in PFM, because high quality MR images was bene t of clinicians for diagnosis of diseases in head and oromaxillo-facial region. Therefore, the understanding about causes of artifacts related to metal implants on MR images would be bene cial to dentists making individualized regimens 5 . Our data suggested that the PFM crown of Co-Cr alloy produces the more metal artifacts compared to the pure Ti and the gold-palladium alloy in the common FSE T 2 WI and PROPELLER FSE T 2 WI sequence. The reasons is most likely due to the speci c ferromagnetic compositions of these alloys. Cobalt and chromium are ferromagnetic metals, they distort local magnetic elds, causing large artifacts that make image interpretation impossible. Titanium itself has ferromagnetic properties but has a lower magnetic susceptibility. Although gold is a diamagnetic substance, gold alloys contain traces of other ferromagnetic metals could also explain the ability to degrade MRI images 7,22 . Nevertheless, some studies showed that high gold-content alloys and pure Ti materials produce more artifacts 23,24 . The probable reasons were 1) The materials used probably come from different manufacturers with varied standards for material processing. 2) MRI scanners, imaging parameters and experimental methods used in different studies are different.
3) The research objects were not uni ed, including dental implants, orthodontic devices, embedded phantoms etc 25 . Therefore, it is necessary to formulate uni ed experimental criteria for accurately evaluating the effects of different dental materials on MRI metallic artifacts.
The previous studies showed that the spin-echo (SE) sequence signi cantly reduces the susceptibility artifact compared with the gradient-echo (GRE) sequence, yet, this still did not meet the expected standards. Furthermore, advances in MR sequence (e.g., VAT, SEMAC, MAVRIC, UTE) and serious deep learn-based methods now allow signi cantly improved image quality in the presence of ferromagnetic materials [10][11][12][13][14][15] . The PROPELLER sequence has the advantages of mature technology, imaging easily and high signal-to-noise ratio, and has been widely applied in clinical MR imaging to reduce motion artifacts [17][18][19][20] . The recent study con rms that the PROPELLER could decreases both artifact and distortion in patients with hip metalwork 21 . Our qualitative and quantitative studies con rmed that PROPELLER FSE T 2 WI signi cantly improves imaging quality and reduces artifact areas and SNR compared to the common FSE T 2 WI sequence in patients with PFM. Meanwhile PROPELLER FSE T 2 WI had the best e ciency on reducing metal artifacts of cobalt-chromium alloy compared with the other two kinds of materials in our study. These ndings can be attributed to 1) PROPELLER's unique radial k-space acquisition sequencing, combined with a fast-spin echo (FSE) technique, which diminishes artifact in the phase-encoding direction 24 . 2) Susceptibility effects primarily affect T 2 *signal decay, by inducing local distortions in the static magnetic eld. Therefore, the PROPELLER sequence refocuses T 2 using a spinecho pulse prior to each readout and removes the distorted T 2 * signal component from the NMR signal to reduce the subsequent image distortion caused by magnetic susceptibility 21 .
Our study has several limitations. First, all MRI examinations were performed at 1.5T, and we did not obtain sagittal and coronal images. Because the level of metallic susceptibility artifact in output images is directly related to eld strength, we anticipate that there might be larger artifact when scanning at 3T. For the sake of clinical imaging diagnosis, further research is necessary to combine axial, sagittal and coronal images to study the ability of PROPELLER to reduce metal artifacts in patients with PFM. Second, although our study selected adult participants whose PFM was closed to same size, the shape of PFM and the porcelain in PFM may be varied, which may lead to bias. Third, we only studied the effect of PROPELLER technology in reducing artifacts of metal copying in the single unit PFM crowns, the applications of PROPELLER technology to metal artifact reduction in multiple unit PFM crowns and other oral elds such as implant prosthesis and orthodontics will be performed in the next study. Finally, the sample size was small. Increasing the sample size would have increased the statistical power of the study.
In conclusion, the different PFM crown generates varying degrees of metal artifact areas in MR imaging. The PROPELLER sequence can effectively reduce metal artifacts in dental MR imaging especially in the PFM crown of Co-Cr alloy. These ndings could add value to the clinical management and MRI examination planning in patients with PFM.