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SAR measurement and distribution in defined body volumes through the development of MRI phantoms

The development of a measurement methodology for the SAR evaluation in a patient, undergoing MRI examination, is presented. A suitable phantom model has been created to provide information about both the absorbed SAR values in the interested irradiated volumes and the SAR distribution therein. At the same time, the method should be able to validate the SAR values, provided by the MR apparatus for each individual exam, through its own mathematical algorithm, in relation to the used sequences and the patient's characteristics. Since this algorithm is generally unknown, tests were performed using tomographs from two different manufacturers, in order to collect the greatest possible amount of data. Moreover, the creation of a phantom, suitable to measure the SAR for different sequences of an MRI device, can provide also “real-time” information about the status of the MR equipment, regarding the quality assurance program, requested by national legislation. Furthermore, “a priori” and accurate knowledge, as regards the chosen sequence and the energy rate released in a given quantity of material (i.e. SAR), would guarantee a safer use of MRI also for patients implanted with last-generation of pacemakers, electromagnetically compatible with MR devices with a flux density of the static magnetic field up to 1.5 tesla. The measurements were carried out utilizing the calorimetric method. This method evaluates the energy rate in unit time, if the specific heat of a compound and the difference between the temperatures before and after the sequence are known. For the optimization of the phantom sizes, five systems of different shape, dimension and geometry were created. The temperatures were measured by a reference thermometer with a sensibility of 0.01 °C, used for the calibration of the elements. Two filling materials were chosen for the phantoms: a saline solution of NaCl (0.06M) for the SAR measurements and an ECG commercial gel, for the evaluation of the SAR distribution.