Vücuda takılabilen tıbbi elektronik üreteçlerin MR görüntülenmesinin güvenlik indeksi kullanılarak radyo frekans modellendirilmesi

Magnetic Resonance Imaging (MRI) is known as a safe imagingmodality that can be hazardous for patients with active implantable medicaldevices, such as a pacemakers or deep brain stimulators. The primary reason forthat is the radio frequency (RF) heating at the tips of the implant leads. In thepast, this problem has been analyzed with phantom, animal and humanexperiments. The amount of temperature rise at the lead tip of these implants,however, has not been theoretically analyzed. In this thesis, a simpleapproximate formula for the safety index of implants, which is the temperatureincrease at the implant lead tip per unit deposited power in the tissue without theimplant in place, was derived.For that purpose, an analytical quadrature birdcage coil model wasdeveloped and the longitudinal incident electric field distribution inside the bodywas formularized as follows:E z ( R) = − ω µ H − Rin which ω is the angular frequency, µ is the magnetic permeability of the tissue,H- is the left hand rotating component of the RF magnetic field and R is theradial distance from the center of the body. This formula was examined bysimulations and phantom experiments. The analytical, simulation andexperimental results of that model are in good agreement.iiiThen, depending on the quadrature birdcage coil model safety index (SI)formula for active implants with short leads was derived as shown below:∆Tmax 1 2Rl + Ae jθSI = = f ( Dv)SARpeak 2α ct Rb2where ∆Tmax is the maximum temperature increase in the tissue, SARpeak is themaximum deposited power in the body when there is no implant in the body, αis the diffusivity of the tissue, ct is the heat capacity of the tissue, Rb is radius ofthe body, R is the radial distance from the center of the body, l is the length ofthe implant lead, A is the area of the curvature of the lead, θ is the angle thatcurvature of the implant makes with the radial axis, and f(Dv) is the perfusioncorrection factor, which is function of the diameter of the electrode andperfusion. The safety index formula was tested by simulations. Simulationresults showed that the theoretical safety index formula approximates andidentifies the RF heating problem of active implants with short leads accurately.The safety index formula derived in this thesis is valid for only shortwires. However, the formulation for long wires is currently under investigation.Despite the fact that the results obtained for short leads can not be generalizedfor the safety of patients with active implants, it is believed that this study is thefirst step towards safety of these patients. Using safety index as a measure ofsafety is very beneficial to ensure the safety of patients with active implants.Because, it uses the MR scanner-estimated deposited power that does not takethe existence of the implant in the patient body into account. This formulation isthe first study illustrating the advantage of the safety index metric for RFheating studies of active implants.Keywords: MRI, RF heating, Active Implants, RF Safety, Safety Index,Quadrature Birdcage Coiliv