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Real time shimming (RTS) for compensation of respiratory induced field changes

Real time shimming (RTS) for compensation of respiratory induced field changes. P van Gelderen, JA de Zwart, P Starewicz, RS Hinks, JH Duyn. Introduction

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Real time shimming (RTS) for compensation of respiratory induced field changes

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  1. Real time shimming (RTS) for compensation of respiratory induced field changes P van Gelderen, JA de Zwart, P Starewicz, RS Hinks, JH Duyn Introduction Respiration results in field changes in the head [1,2], proportional to the main field, which can cause distortion, ghosting, and signal fluctuation. The field changes are not uniform, meaning a navigator or a simple B0 correction maybe insufficient [3,4]. Here a solution is presented where the respiration induced field changes are compensated by adjusting both the B0 and the shimming in real time. The method does not require additional MR data during the compensation, so that it can be applied with any sequence. Phase changes in a single slice EPI data set @ 7T, with TE 30ms TR 100 ms, with respiratory amplitude (signal from bellows) indicated by the bar. The movie plays at 2.5 times the real speed. Amplitude of phase changes related to respiration. Based on phase changes in EPI data @ 7T, with TE 30ms TR 1s, 2 minutes average correlation with respiratory signal. Note the spatial heterogeneity, showing a simple navigator B0 correction would result in incomplete compensation. 1) P. van Gelderen, C.T.W. Moonen. ISMRM 1998, 1500. 2) van de Moortele PF, et.al. Magn Reson Med 2002 47:888-95. 3) Barry RL, Menon RS. Magn Reson Med 2005 54:411. 4) Pfeuffer J, et.al. Magn Reson Med 2002 47:344.5

  2. Correlation of phase fluctuation with respiration This plot shows the slice averaged phase difference (B0) over time, measured with a single slice EPI at 7T with TR 100ms and TE of 30ms (white line with +). The fluctuation is about 3Hz peak to peak. The yellow curve is the recorded respiratory signal, scaled in amplitude and shifted in time to fit the phase. The correlation coefficient was 0.93 without filtering, and 0.99 after band pass filtering between 0.11 and 0.49Hz. The difference between the phase and the respiratory signal is (orange with -0.4 offset), shows that the remaining fluctuation is unrelated to the respiration and partly explained by the cardiac cycle, extracted from the pulse oximeter data (purple).

  3. Measurement (Training) A time series of EPI is collected (for 2 minutes), the phase changes are filtered (band pass 0.11-0.49Hz) and correlated with the respiration for each voxel. The correlation maps are fitted with 2nd order 3D polynomial functions, the coefficients of which are translated to shim currents using the reference matrix, resulting in a set of required shim currents as function of respiratory signal amplitude. f0 x y zy z2 Correlate phase with respiration Real time compensation The respiration amplitude is converted to an offset in the shim currents in real time, in 100 ms intervals, using the correlations calculated from the training data. This step is independent of the normal scanner operation and can be applied with any sequence. Ishim Principle of operation Reference maps First a reference is created on a oil phantom by adding an offset current to each of the channels (f0, x, y, x, zx, zy, z2, x2y2 and xy) and measuring the corresponding change in phase. The phase changes are summarized by fitting with 2nd order 3D polynomial functions, resulting in a matrix relating the currents to the polynomial coefficients. The LS pseudo inverse of this matrix serves to calculate the required currents as function of desired field changes. Find phase map of all shim terms All measurements are EPI on a GE 7T, with a Nova Medical volume transmit and 8 or 24 channel surface receive array, TE 30ms, 12 slices of 2mm with 6mm gap and TR of 1s, or 1 slice of 2mm with TR 100ms. The respiratory trace is the digitally recorded signal from the scanner bellows, positioned around the chest of the volunteers at the height of the diaphragm.

  4. Gradients (x, y, z) to scanner Implementation Hardware modifications + GE electronics Shims (zx, zy, z2, xy, x2y2) + Linux PC with 9 DACs to scanner physiology signals (digital) FM on reference synthesizer existing hardware additional hardware Practical issues Two corrections appeared to be necessary in order to make the RTS work: distortion correction of the EPI and compensation of the delay times from the respiratory detection to the compensation in the magnet. Frequency changes result in changes in distortion, which can, in combination with the static inhomogeneity, result in extra phase differences. Therefore all EPI data has to be distortion corrected shot by shot before calculating the phase changes. Secondly, the detection has about a 50ms delay and the output from the PC to the shim coils another 150ms. This is corrected by predicting the respiratory signal 200ms into the future with a linear prediction based on the last 500ms of data. The prediction coefficients are continuously optimized based on the past 10s of data. Prediction (in green) of the respiratory signal (in white)

  5. Phase fluctuation with and without real time shimming Slice averaged phase of one EPI slice over time, with real time shimming off (top trace in white) and on (bottom trace in blue). The EPI was single slice, 96x72, at 7T, TR 100ms, TE 30ms. The yellow curves show the respiratory traces for both acquisitions. The phase stability over the whole brain did improve some (averaged over six volunteers SD 0.19 versus 0.15 with RTS). The reduction of the fluctuations in the respiratory band (taken as 0.11-0.49Hz) was more substantial (SD 0.14 versus 0.08). The higher frequency component of the phase modulation (white curve) is in general more deleterious to image quality than the low frequency drift remaining after compensation (blue curve). Over six subjects, the amplitude of the B0 compensation ranged from 2-3.8Hz, while the shimming terms added a spatial variation ranging from 1.7-3.7Hz.

  6. Improvement of anatomical images with real time shimming The images without compensation show ghosting, non-uniformity in white matter contrast and interference. The images on the bottom demonstrate the effectiveness of RTS for preventing these artifacts without any sequence modification. RT shim off RT shim on Scan parameters: gradient echo field 7T dimensions 512x384 resolution 0.43 mm slices: 1mm TR: 500 ms TE: 40 ms Flip angle: 30 imaging time: 3:18

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