Mr Thermometry Using Echo-shifted Sequence With Simultaneous Multi-slice Imaging

BackgroundHigh intensity focused ultrasound(HIFU)provides a non-invasive treatment tool that can precisely focus on the treatment of tumors noninvasively.A good image monitoring is crucial to ensure the safety and effectiveness of HIFU treatment during the therapy.Ultrasound(US)and Magnetic resonance imaging(MRI)are the common imaging monitoring modalities.Compared with US,MRI is the preferred imaging modality for monitoring HIFU treatment because of its good soft tissue contrast and unique real-time non-invasive temperature imaging capabilities.For real-time MR temperature imaging,the temporal resolution and spatial resolution should be increased as much as possible while maintaining good thermometry accuracy.GRE and EPI sequences are the most commonly used thermometry sequences.The traditional RF-spoiled GRE sequence has a relatively high signal-to-noise ratio,but has long acquisition time.With EPI readouts,the sequence has a higher temporal resolution,however the images are prone to distortions or artifacts,which in turn affects the accuracy of thermometry.Moreover,most temperature monitoring is only a single slice scan.However,it is reported that HIFU may cause unwanted heating in the tissue interface and far-field area,which easily damages normal tissues.In order to monitor HIFU treatment more safely and efficiently,the temporal resolution of the MR temperature imaging and the spatial coverage should be further improved while maintaining good temperature measurement accuracy.Therefore,we studied magnetic resonance temperature imaging based on echo-shifted RF-spoiled GRE with simultaneous multi-slice(SMS)technique in this thesis.Objective1.Study on the feasibility of echo-shifted RF-spoiled GRE sequence and the calibration of its temperature measurement accuracy.Comparing echo-shifted RF-spoiled GRE with conventional RF-spoiled GRE sequence to show the advantage of low temperature uncertainty of echo-shifted RF-spoiled GRE sequence.2.Implementation of SMS and study on combining echo-shifted RF-spoiled GRE with SMS in temperature monitoring application to prove that the proposed techniques can increase temporal resolution and expand spatial coverage while maintaining the accuracy of the thermometry.Methods1.Study on echo-shifted RF-spoiled GRE sequenceTo test whether the relationship between signal intensities and flip angles of echo-shifted RF-spoiled GRE sequence is in accordance with the theoretical formula,a reference phantom with T1/T2 known was scanned at different flip angles.A water bath phantom experiment was then performed to calibrate the temperature accuracy of the sequence.Phantoms were made using 1.2% agar(SIGMA-ALDRICH)and 0.5% sodium chloride and pure water,and the fluorescent thermometers(INDIGO PRECISION,FOTS-DINA-1000-S,with precision of ± 0.05°C)were inserted into the phantoms to calibrate the temperature accuracy of echo-shifted RF-spoiled GRE sequence.The optimal flip angles of echo-shifted RF-spoiled GRE and RF-spoiled GRE sequence were found by an ex vivo porcine muscle comparison experiment at room temperature.Comparing the temperature uncertainty of echo-shifted RF-spoiled GRE with conventional RF-spoiled GRE sequence at the same temporal resolution.2.Implementation of SMS and study on its application in temperature monitoringA multi-band pulse was designed to incorporate SMS technique with echo-shifted RF-spoiled GRE sequence.SENSE reconstruction was performed to unfold the aliased images.In addition,a phantom experiment was performed and using thermometers to calibrate the temperature measurement accuracy of two slices of echo-shifted RF-spoiled GREcombined with SMS technique.A HIFU heating ex vivo porcine liver experiment was performed to compare the temperature change between echo-shifted RF-spoiled GRE with and without SMS at two same position.The frequency of HIFU was 0.8 MHz,the input electric power was 25 W,using a 4-channel flex small coil.All experiments were performed on a Siemens 3 T magnetic resonance scanner(Siemens,MAGNETOM,Erlangen,Germany),and the data post-processing was done by MATLAB(Mathworks 2015 a,Natick,MA)and Origin(Origin 8.5,Origin Lab,USA).Results1.Study on echo-shifted RF-spoiled GRE sequenceThe results show that the relationship between the signal intensities and flip angles were basically in accordance with the theoretical values.The temperature measurement error of echo-shifted RF-spoiled GRE sequence calibrated with thermometers was-0.02 ± 0.10°C.Time of single slice scan was 1.35 s.The optimal flip angles of echo-shifted RF-spoiled GRE and RF-spoiled GRE sequence were both 10° in ex vivo porcine muscle by setting the same scanning protocols.Although the SNR of echo-shifted RF-spoiled GRE was less than that of RF-spoiled GRE sequence at the same temporal resolution,the temperature uncertainty of the former(1.04°C)was smaller than that of the later(2.75°C).2.Implementation of SMS and study on its application in temperaturemonitoringSMS was realized by SENSE reconstruction.In the phantom experiment of echo-shifted RF-spoiled GRE combining with SMS technique,the mean temperature errors of two slices calibrated by thermometers were-0.01 ± 0.06°C and 0.01 ± 0.06°C,respectively.In HIFU heating experiment,the temporal resolution of the sequence was 0.86 s.The temperature change results of two slices obtained by echo-shifted RF-spoiled GRE acombing with SMS were in accordance with the results in the same position obtained by the single slice scanning using echo-shifted RF-spoiled GRE sequence.ConclusionTo be concluded,this work showed that echo-shifted RF-spoiled GRE sequence has high temporal resolution and good temperature measurement accuracy.Compared to the traditional RF-spoiled GRE sequence,echo-shifted RF-spoiled GRE sequence shows lower temperature uncertainty.After using the SMS technique,the spatial coverage was increased without decreasing temporal resolution,while maintaining high temperature measurement accuracy.