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Evaluate a Stepped TE Segmented EPI Sequence for 3D Dose Monitoring Using Gel Dosimetry

R Hira

R Hira*,

SU-E-J-155 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Problem:Gel based MR relaxometry provides accurate 3D dosimetric mapping. The routinely used multi spin echo (MSE) sequence has limitations. Our aim was to evaluate a stepped TE segmented EPI sequence for 3D dose monitoring at 1.5T and 3T.
Methods: The polymer gel (BANG KIT, MGS Research) was prepared under normoxic conditions and stored in glass vials. Calibration was performed under dose range of 3-18 Gy with irradiation using the True Beam. Segmented Echo Planar Imaging Sequence was evaluated for T2 relaxometry at 1.5T and 3T (GE medical systems). The parameters of the stepped TE segmented Echo Planar Imaging Sequence were kept identical at both 1.5 and 3T: TE=15.6,27,39,51,63,75,87,99 ms; TR= 4000ms; FOV=22*22;Slice thk/gap: 3/3; # Shots=14; Scan time=8 min. The MSE sequence parameters were set close to the EPI sequence on both scanners (scan time: 18 mins).
Results: The EPI image quality at 1.5 T was comparable to MSE sequence; at 3T ghosting artifacts could be seen in EPI images. The log linear plot of signal intensities of the segmented EPI sequence for irradiation in the range 0-20 Gy is shown at 1.5 and 3T (fig 1 & 2) .The T2 values agreed with that of corresponding MSE sequences at both 1.5 and 3Tand the fit was much better using the segmented EPI sequence than the MSE sequences. The dose calibration factor,a, was similar for all the sequences (fig. 3- 3T a: MSE=0.93, EPI=0.99 ; fig.4- 1.5T a: MSE=0.79, EPI=0.95)
Conclusions: The EPI image quality at 1.5T was comparable to MSE. The dose calibration factor evaluated with the EPI sequences was comparable at 1.5T and 3T and close to the MSE sequence. Acquisition time of EPI is less than half of MSE sequence with more flexibility in choice of TEs .The study establishes EPI as a novel sequence for dose mapping.

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