Radiofrequency heating and magnetically induced displacement of dental magnetic attachments during 3.0 T MRI.

OBJECTIVE: The aim of this study was to estimate the risk of injury from dental magnetic attachments due to their radiofrequency (RF) heating and magnetically induced displacement during 3.0 T MRI. METHODS: To examine the magnetic attachments, we adopted the American Society for Testing and Materials F2182-02a and F2052-06 standards in two MRI systems (Achieva 3.0 T Nova Dual; Philips, Tokyo, Japan, and Signa HDxt 3.0 T; GE Healthcare, Milwaukee, WI). The temperature change was measured in a cylindrical keeper (GIGAUSS D600; GC, Tokyo, Japan) with coping of the casting alloy and a keeper with a dental implant at the maximum specific absorption rate (SAR) for 20 min. To measure the magnetically induced displacement force, three sizes of keepers (GIGAUSS D400, D600 and D1000) were used in deflection angle tests conducted at the point of the maximum magnetic field strength. RESULTS: Temperature elevations of both coping and implant were higher in the Signa system than in the Achieva system. The highest temperature changes in the keeper with implant and keeper with coping were 0.6 °C and 0.8 °C in the Signa system, respectively. The temperature increase did not exceed 1.0 °C at any location. The deflection angle (α) was not measurable because it exceeded 90°. GIGAUSS D400 required an extra 3.0 g load to constrain the deflection angle to less than 45°; GIGAUSS D600 and D1000 required 5.0 and 9.0 g loads, respectively. CONCLUSIONS: Dental magnetic attachments pose no risk due to RF heating and magnetically induced displacement at 3.0 T MRI. However, it is necessary to confirm that these keepers are securely attached to the prosthesis before imaging.

Dimensional change of laser-welded gold alloy induced by heat treatment.

PURPOSE: This study examined the effects of laser welding and heat treatment on the dimensional change of cast gold alloy frameworks. MATERIALS AND METHODS: Pairs of cast gold alloy plates were matched, fixed in a jig, and welded in a laser-welding machine at constant welding parameters. The specimens were welded unilaterally (on one surface) or bilaterally (on two surfaces) with five spots as follows: two ends fixed/unilaterally welded (A); two ends fixed/bilaterally welded (AA); one end fixed/unilaterally welded (B); two ends fixed/welded on one surface and then one end fixed/welded on the opposite surface (AB); or one end fixed/bilaterally welded (BB). The dimensional change was determined by measuring the gap between the jig base and one end of the specimen after each welding application. Dimensional change was also measured after two different heat treatments (softening and hardening). The results were analyzed using a two-way ANOVA and Duncan's test (p < 0.05). RESULTS: The dimensional change of the specimens fixed at only one end on either surface (AB, B, and BB) was higher compared with the two ends-fixed specimens (A and AA) after laser welding. The heat treatments also increased the dimensional change in all groups except for the B group. The dimensional change was similar for each fixing method between the two types of heat treatment. CONCLUSIONS: The method of fixing the specimens in the jig significantly affected the amount of dimensional change of the gold alloys. The heat treatments of the laser-welded specimens increased the dimensional change by releasing the residual stress.

Evaluation of a device for attenuation of electron release from dental restorations in a therapeutic radiation field.

STATEMENT OF PROBLEM: For some patients, radiation treatment is a part of tumor therapy in the head and neck area before and/or after surgery. The oral cavity and teeth are thereby frequently exposed to high doses of radiation. In this situation, electronic backscatter from dental materials may damage the surrounding soft tissue. PURPOSE: This study determined the degree of absorption and the backscatter effect of therapeutic radiation used in the presence of 4 different dental materials. The efficacy of a protective stent also was investigated. MATERIAL AND METHODS: The influence of 4 dental materials (a high-gold alloy, pure titanium, amalgam, and a synthetic material) on radiation dose distribution was tested on 2 test models that simulated the presence of teeth. An alanine dosimeter was used to make measurements with and without the presence of a protective stent. To verify the results, one of the test models was compared to a computer simulation. RESULTS: Backscatter effects on the surface of dental materials caused an increase of up to 170% of the radiation dose measured without the materials. The rate of overdose increased with the atomic number of the dental material. The extent of the backscatter effect was a maximum of 4 mm. CONCLUSION: The considerable overdose of 170% found in this study suggests that soft tissue surrounding dental restorations should be protected from radiation. The backscatter results indicate that soft tissue could be effectively shielded with a 3-mm synthetic stent.

In vivo dose increase in the presence of dental alloys during 60Co-gamma-ray therapy of the oral cavity.

During irradiation of the mouth cavity, dental metallic materials emit secondary electrons and thus increase the applied radiation dose in their vicinity. Therefore, local destruction of the mucous membrane contacting metallic dental crowns and fillings may be observed. Available data on this dose increase are based on measurements with beam arrangements perpendicular to the metallic surface. Since the dose modification depends on the beam direction in relation to specimen surface, a reliable prediction of dose modification in the close vicinity of dental caps on fillings under complex beam arrangements, as applied in the irradiation of head and neck region from the published data is not possible. Therefore, we measured dose increase in the immediate surrounding of metallic dental material using thermoluminescence dosimetry on the phantom and during routinely applied 60Co gamma ray therapy. Phantom measurements were carried out using several oblique irradiation angles and rotational therapy. In vivo measurements were carried out at alloy specimens containing gold, palladium, and amalgam in six patients and at permanently fixed golden teeth in five patients. In vivo, the following relative dose increase values according to a simultaneously measured reference value were obtained at the surface of different dental materials: 61% for fixed golden caps. 68% for the specimen containing gold, 33% for the specimen of palladium and 61% for the specimen of amalgam. The measured dose increases due to metallic dental material during routinely applied external 60Co beam irradiation are lower compared with those of perpendicular beam arrangements. Although, the extent of dose modification is less than expected, we still advocate protection of the oral mucosa to prevent painful lesion spots.

[The in-vivo determination of dosage intensification due to dental alloys in the therapeutic irradiation of the oral cavity]. / In-vivo-Bestimmung der Dosiserhöhung durch Dentallegierungen bei therapeutischer Bestrahlung der Mundhöhle.

BACKGROUND: In the course of therapeutic irradiation of regions in the mouth cavity dosage increase due to the emission of secondary electrons in the vicinity of dental materials can be observed. This could induce local destruction of the mucous membrane in contact with metallic dental crowns and fillings. As at now, only rough estimates of this dosage increase could be made with the help of measurements carried out with models. PATIENTS AND METHODS: The degree of dosage increase in the immediate surrounding of metallic dental material was measured in an in-vivo study during therapeutic irradiation with 60Co gamma rays in the area of mouth cavity of 11 patients. Measurements were carried out by thermoluminescent dosimetry at permanently fixed golden teeth and alloy specimens containing gold and palladium and amalgam. RESULTS: The following relative dosage values according to a simultaneously measured reference value were measured at the surface of the different dental materials: 161% near fixed golden caps, 168% near the specimen containing gold in a high percentage, 133% near the specimen of palladium and 161% near the specimen of amalgam. CONCLUSION: The in vivo measured dosage increases due to metallic dental prosthesis are less than values obtained using back scatter arrangements for irradiating phantoms. Despite this, they could be of clinical relevance. Thus the usage of a mucous membrane protection during irradiation with 60Co, as a means of preventing local lesions of the oral mucosa, due to dental alloys within the treatment volume remains inevitable.

Measurement of absorbed doses near metal and dental material interfaces irradiated by x- and gamma-ray therapy beams.

Soft-tissue damage adjacent to dental restorations is a deleterious side effect of radiation therapy which is associated with low-energy electron scatter from dental materials of high electron density. This study was designed to investigate the enhancement of dose to soft tissue (or water) close to high electron-density materials and to measure the detailed lateral and depth-dose profiles in soft-tissue-simulating polymer adjacent to planar interfaces of several higher atomic-number materials: 18-carat gold dental casting alloy; Ag-Hg dental amalgam alloy; Ni-Cr dental casting alloy; and natural human tooth structure. Interleaved stacks of calibrated thin radiochromic dosimeter films and tissue-simulating polymer were used for these measurements. Assemblies of these polymer-dosimeter stacks on both sides of the dental materials were irradiated in one fixed direction by collimated 60Co gamma-ray or 10 MV x-ray beams directed perpendicularly to the material interfaces. In another test, designed to simulate more closely therapeutic treatment conditions, a phantom constructed on both sides of a row of restored and unrestored whole teeth (restoration materials: gold alloy crown; Ni-Cr alloy crown; Ag-Hg mesial-occlusal-distal (MOD) amalgam filling; unrestored tooth) was irradiated in one fixed direction by the collimated photon beams. Results indicate that the dose-enhancement in 'tissue' is as great as a factor of 2 on the backscatter side adjacent to gold and a factor of 1.2 adjacent to tooth tissue, but is insignificant on the forward-scatter side because of the predominant effect of attenuation by the high-density, high atomic-number absorbing material.

The metal/tissue interface effect in irradiation of the oral cavity.

Metal dental prostheses or overlays may lead to increased mucosal reactions of adjacent tissue surfaces during intensive irradiation of the oral cavity. The dosimetry of this phenomenon was investigated by irradiating dental phantoms with 4-and 6-MeV photons. Gold and amalgam interfaces may produce local mucosal doses as high as 150-170% depending on the beam geometry, but doses of 111-126% for gold crowns and about 109-118% for amalgam fillings were found for opposed-beam configurations. 2-4 mm of tissue-equivalent absorber is sufficient to re-establish a homogeneous dose distribution and should be employed throughout therapy whenever dental extraction is unwarranted. The radiobiological exaggeration of the interface effect in treatment with opposed beams using one field per day is discussed.