With the development of interventional MRI, heating of biological tissues along the metallic wires in the MRI scanner has become an important issue. To assess thermal response to RF exposure during MRI, we studied the temperature elevation near nonmagnetic metallic wires. All tests were performed on a 1.5 T clinical scanner. Four experiments were conducted to investigate the effects of the wire diameter, the excitation flip angle, the temperature distribution along the wire, and the wire length. Electromagnetic simulations of the experimental setup were made with the use of commercial method of moments In the last few years, interest in interventional MRI has grown considerably (1,2). Since the use of metallic guidewires or catheters is necessary in interventional MRI, the security aspects of such devices are of great importance. The use of endoluminal coils also raises specific safety issues. Such coils are generally made of metallic parts both at the top (loop as sensor) and along the catheter (coaxial wires for transmitting RF signal) (3). Since metallic wires are located in the MRI tunnel, heating hazards can occur inside the patient's body. Thus, along with the development of endoluminal sensors, patient safety must be ensured with respect to international standards. Previous attempts to characterize patient safety showed that heating hazards are difficult to predict. For this reason, it is important to focus first on the simple model of wires inside an MRI tunnel before studying the endoluminal sensor.The major issue is to ensure patient safety against potential heating of tissues located in the vicinity of the metallic wire (4 -6). During RF excitation, in addition to the magnetic field B 1 , an electric field E induces currents (at the same frequency) in the metallic wire placed in the MR scanner. Furthermore, the metallic wire concentrates the RF electric field, and may lead to a significant temperature increase.Beyond the well-known case of metallic implants (even nonferromagnetic ones) that can cause heating hazards (7,8), several authors have investigated the specific case of metallic wires during interventional MRI (9 -12). Most studies have shown that the specific absorption rate (SAR) measured in the presence of a metallic wire can overcome the SAR limitation of 2 W/kg (1,13-15). This SAR elevation induces an increase of temperature in the near vicinity of the wire. The heating close to the wire follows the tissue bio-heat law established by Pennes in 1948 (16). According to this law, local heating can be expressed as the algebraic sum of the following energies: 1) the metabolic activity of the cells forming the surrounding tissues, 2) the energy evacuated by the blood flow irrigating the tissues, and 3) the energy concentrated locally by the SAR. Without RF stimulation, the first two terms equilibrate the temperature of the region at a steady level equal to the blood flow temperature. During a clinical MRI examination, the third term adds local heating by SAR. Indeed, since the metallic wire conc...

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PurposeNovel irradiation techniques are continuously introduced in radiotherapy to optimize the accuracy, the security and the clinical outcome of treatments. These changes could raise the question of discontinuity in dosimetric presentation and the subsequent need for practice adjustments in case of significant modifications. This study proposes a comprehensive approach to compare different techniques and tests whether their respective dose calculation algorithms give rise to statistically significant differences in the treatment doses for the patient.MethodsStatistical investigation principles are presented in the framework of a clinical example based on 62 fields of radiotherapy for lung cancer. The delivered doses in monitor units were calculated using three different dose calculation methods: the reference method accounts the dose without tissues density corrections using Pencil Beam Convolution (PBC) algorithm, whereas new methods calculate the dose with tissues density correction for 1D and 3D using Modified Batho (MB) method and Equivalent Tissue air ratio (ETAR) method, respectively. The normality of the data and the homogeneity of variance between groups were tested using Shapiro-Wilks and Levene test, respectively, then non-parametric statistical tests were performed. Specifically, the dose means estimated by the different calculation methods were compared using Friedman’s test and Wilcoxon signed-rank test. In addition, the correlation between the doses calculated by the three methods was assessed using Spearman’s rank and Kendall’s rank tests.ResultsThe Friedman’s test showed a significant effect on the calculation method for the delivered dose of lung cancer patients (p <0.001). The density correction methods yielded to lower doses as compared to PBC by on average (−5 ± 4.4 SD) for MB and (−4.7 ± 5 SD) for ETAR. Post-hoc Wilcoxon signed-rank test of paired comparisons indicated that the delivered dose was significantly reduced using density-corrected methods as compared to the reference method. Spearman’s and Kendall’s rank tests indicated a positive correlation between the doses calculated with the different methods.ConclusionThis paper illustrates and justifies the use of statistical tests and graphical representations for dosimetric comparisons in radiotherapy. The statistical analysis shows the significance of dose differences resulting from two or more techniques in radiotherapy.