Measuring water content using T₂ relaxation at 3 T: Phantom validations and simulations

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• 作者：Sandra M. Meyers^a ; ^{smeyers@ualberta.ca" class="auth_mail" title="E-mail the corresponding author} ; Shannon H. Kolind^b ; ^{shannon.kolind@ubc.ca" class="auth_mail" title="E-mail the corresponding author} ; Cornelia Laule^c ; ^d ; ^e ; ^{corree@physics.ubc.ca" class="auth_mail" title="E-mail the corresponding author} ; Alex L. MacKay^a ; ^c ; ^{mackay@physics.ubc.ca" class="auth_mail" title="E-mail the corresponding author}
• 关键词：CSF ; cerebrospinal fluid ; EPG ; extended phase graph ; GRASE ; gradient echo spin echo ; MWF ; myelin water fraction ; NNLS ; non-negative least squares ; RF ; radiofrequency ; SD ; standard deviation ; SNR ; signal to noise ratio ; TWC ; total water content
• 刊名：Magnetic Resonance Imaging
• 出版年：2016
• 出版时间：April 2016
• 年：2016
• 卷：34
• 期：3
• 页码：246-251
• 全文大小：637 K

文摘

In vivo measurement of water content would be very useful for evaluating microstructural tissue changes, such as edema, that occur in neurological diseases. Careful assessment of the T₂ relaxation decay curve can provide simultaneous measurements of total water content (TWC) and myelin water fraction, a marker for myelin which is also relevant in brain pathology. This work validates a T₂ relaxation based method for TWC measurement at 3 T using phantoms and simulations.

Methods

A phantom consisting of tubes with known water concentrations was scanned using 3 T MRI. T₂ relaxation data was collected with both gradient echo spin-echo (GRASE) and spin echo sequences, while an inversion recovery experiment provided T₁ relaxation data. Voxel-wise T₂ distributions were calculated by fitting the T₂ relaxation data with a non-negative least squares algorithm that incorporated a correction for errors in flip angle due to B₁⁺ inhomogeneity. TWC was calculated as the sum of the signal in the T₂ distribution, corrected for T₁ relaxation, relative to that of a tube containing 100% water. TWC from GRASE was compared to that of spin echo in order to test if the accuracy of the TWC measurement was impacted by using additional gradient echoes to fill k-space. Simulations were performed to determine theoretical errors in TWC.

Results

Measured TWC strongly correlated to actual TWC (R = 0.997, p = 9 × 10^− 8, mean discrepancy = 1.8%). Accuracy of GRASE and spin echo TWC measurements did not significantly differ. Simulations indicated a mean systematic TWC error of 0.07% and random error of 0.8%, and revealed that the technique performs well in the presence of B₁⁺ inhomogeneity.

Conclusion

This work demonstrates that, using the T₂ relaxation decay curve, TWC can be measured to within 3% accuracy at 3 T. Given that T₂ relaxation can provide accurate estimates of both TWC and myelin water fraction, multi-echo T₂ measurement should be considered a multifaceted approach for assessing pathology and evaluating therapy of central nervous system diseases.