A Research On Electromagnetic Compatibility For MRI Guided Focused Ultrasound

High intensity focused ultrasound(HIFU)is a safe and noninvasive treatment for tumor.The ultrasound power could be delivered via intact skin and tissue into the target region without any incision on the skin which is necessary in a routine surgery.Taking advantage of the focusing feature,the acoustic energy is confined in the targeted region,where the acoustic intensity is above 1000 W/cm~2.Therefore,the tissue in the targeted region can be fully ablated,remaining the surrounding tissue healthy.Moreover,HIFU therapy can be applied multiple times to a patient as it does not have obvious toxic effects as radiotherapy does.Magnetic Resonance Imaging(MRI)guided HIFU has gained a great amount of interest in recent years.Taking advantage of imaging and temperature mapping of the tissue in real time,doctors can modify the treatment plan during the MRI guided HIFU surgery according whether the thermodose in the targeted region has reached the predetermined level.This makes the treatment procedure controllable and safe.Nevertheless,the electromagnetic compatibility(EMC)of HIFU and MRI is always a technical difficulty.This is because there are several mutual interferences between HIFU device and MRI scanner.(1)Magnetic field affects HIFU device.The magnetic field generated by MRI scanner may affect the ferromagnetic components in the HIFU device,especially the mechanical components;(2)HIFU device affect the magnetic field.When ferrite components of HIFU are placed in the imaging zone of MRI,the magnetic field will be distorted.The experiment results showed that the magnetic field will be affected by even foreign paramagnetic materials in the imaging zone because of the eddy current.(3)Radio frequency(RF)noise.The MR image quality will be severely affected by RF noise emitted from the foreign device no matter how far the device is placed from the scanner in the MR scanner room.(4)Foreign RF noise affects the gain amplifier in the receiving coil of MR scanner.The gain amplifier would be detuned because of the RF noise from the HIFU device.To solve these EMC problems,this dissertation developed a full bridge amplifier with harmonic cancellation to reduce the interference source on the one hand,and shielded the HIFU probe with a wire mesh screen to cut off the interference emission way on the other hand.This dissertation emphatically analyzed the effects of the screen on the acoustic field of HIFU and the shielding effectiveness(SE)of the screen.The main contributions of this dissertation are as follow:(1)Considering the manufacture cost,energy density and transmission efficiency,we developed a full bridge amplifier(Class D)with harmonic cancellation to reduce the interference source.The principle of harmonic cancellation technique is to change the waveform of the output by shifting the phase signals for the four bridge hand,such that the energy is redistributed on the spectrum.The foundamental harmonic can gain the most energy against other harmonics by applying proper phase shift value.The simulated and measured results showed that this approach could effectively reduce the harmonic components of the driving signals of the transducers.Especially,the third harmonic could be eliminated theoretically.One technical problem is that one amplifier requires four channels of control signals.Even using invertors to reduce the required channels of signals,the signal generator need to double its output channels than routine amplifiers.This dissertation developed an FPAG signal generator for the full bridge amplifier to meet its requirements of channel number and phase precision.(2)Scattering occurs when a wire mesh screen is installed in the propagation path of the acoustic wave,because the radius ware is comparable with the wavelength of the ultrasound in water.Considering that the scattering field near the wire mesh is complex and the propagation field between the mesh and the observation point in the target region is simple,this dissertation developed a hybrid simulation approach composed of k-space method and distributed point source method(DPSM).This approach can significantly reduce the simulation time under high accuracy.One assumption of this approach was“rigid scattering”.The results of rigid and elastic models were almost identical which verified that the“rigid scattering assumption”was rational.This hybrid simulation approach could also be used to solve other rigid scattering problems where the shape of the scatterers was complex and the simulation scope is large.(3)The wire mesh screen may affect the acoustic field.Simulations of the acoustic field of one transducer piston showed that although complex interference patterns occurred near the wire mesh,they dimmed in the target region near the geometric focus because of the divergence feature of scattering waves.Therefore,the normalized acoustic pressure distribution in the focal region was not affected by the wire mesh.The main influence of the wire mesh was the loss of energy and reduction in the focal acoustic intensity.This dissertation analyzed the effects of the screen parameters,HIFU working frequency,installation height of the screen and focus position on the focal acoustic intensity.Experiments were performed to verify the simulation results.These results are helpful when optimizing the screen parameters.The effect of nonlinear acoustic propagation was also analyzed.Because of the low strength of acoustic source and the short acoustic propagation path,the nonlinear effect was small and could be ignored.The simulation and ex-vivo experiment showed that the effect of nonlinearity on the acoustic focal intensity attenuation caused by the screen was small.(4)A sheet-impedance model was introduced to calculated SE of the wire mesh screen.The shielding effectivenesses of three screens were compared by measuring the radiation emission and capturing MR images.Compared with values without a screen,the signal-to-noise(SNR)values of MR images using three screens were greatly improved.Nevertheless,the increased value using a relatively coarse screen was obviously smaller than the values using the other two screens.Along with the effects of the screens on the acoustic field,researchers could optimize the screen parameters according to their HIFU devices and MR scanners.