| Objective: To observe the image and the influence on adjacent tissue of fixed prosthesis made of Ni-Cr alloy for different teeth and different kinds in magnetic resonance imaging(MRI)testing. Differences in degree of the influence of shape and weight of Ni-Cr alloy and different field strength on MRI artifact was compared. To provide experimental evidence whether to remove Ni-Cr alloy prosthesis and how to select field strength when MRI testing for head and neck was needed.Methods: 1. Preparation of specimens: An edentulous volunteer was selected and three upper full dentures and one lower full denture were made for him. Seven base crowns from 7 to 1 and four cast crowns from 7 to 4 were made of Ni-Cr alloy on the upper one. Two PFM bridges of 2-1 and 7-5 and one cast bridge of 5-7were made on the second upper one .One PFM bridges of 7-4 and one cast bridge of 4-7 were made on the third upper one. Total of the Ni-Cr alloy specimens upper full dentures were sixteen. Seven base crown-shaped alloy plates from 7 to 1and seven alloy plates with the same weight as the former ones were also made. Seven base crowns from 7 to 1 were marked Group A, Seven base crown-shaped alloy plates from 7 to 1 were marked Group B, and seven alloy plates were marked Group C. 2. MR scanning:(1)Experiment in mouth: First the full denture with no prosthesis wearing in the volunteer's mouth was scanned as the control. And then MRI scanner with 1.5T following the sagittal surface with the layer of 5mm, after sixteen specimens were mounted on the full denture in turn, scanned the full denture in the volunteer's mouth.(2)Experiment in water phantom: First the full denture with no prosthesis wearing mounted on the seat of water phantom was scanned as the control. And then sixteen specimens, Seven base crown-shaped alloy plates and seven alloy plates were scanned with the field strength of 0.23T in the water phantom in turn, following the sagittal surface with the layer of 10mm. 3. Observation and measurement: Artifact of sixteen specimens and the scale involved in the volunteer's mouth were observed in the image of MRI with 1.5T. Maximum vertical length and maximum horizontal length of artifact were measured and layers involved on the left and right side were recorded in the image of MRI with 1.5T. Maximum vertical length and maximum horizontal length of artifact of all the specimens in the water phantom were measured and layers involved on the left and right side were recorded in the image of MRI with 0.23T. 4. Statistics: Maximum length of artifact of sixteen specimens in the image of MRI with 1.5T and 0.23T was test with paired T-test by using software SPSS; Weight between Group A and B and between Group A and B was test with two-group T-test; and Maximum length of artifact between Group A and B and between Group A and B was test with two-group T-test. Results: 1. Size of artifact of prosthesis wearing in the volunteer's mouth and scale involved: the change of MR image was happened at the site of the alloy prosthesis and adjacent tissue. The change was characterized that the impacted area was circle-like and irregular low-signal area, and the adjacent tissue was taken on halo at high-signal rim. Image of the corresponding area was oblivious and vanishing, and tissue and structure were deformed. The size and scale involved were different with the sites of teeth and kinds of prosthesis. Artifact of 6 was the biggest among 7-1 , whose maximum vertical length was 26.4mm and maximum horizontal length was 34.6mm, and four continuous layers(20mm)near of the image were involved by artifact; artifact of 2 was the smallest, whose maximum vertical length was 16.5mm and maximum horizontal length was 13.2mm, two layers(10mm)of the image were involved. Artifact of 6 was the biggest among the cast crown of 4-7, whose maximum vertical length was 30.5mm and maximum horizontal length was 41.5mm, six layers(30mm)of the image were involved; artifact of 4 was the smallest, whose maximum vertical length was 28.4m...
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