AUTHOR=Lee Yoojin , Callaghan Martina F. , Nagy Zoltan 

TITLE=Analysis of the Precision of Variable Flip Angle T1 Mapping with Emphasis on the Noise Propagated from RF Transmit Field Maps

JOURNAL=Frontiers in Neuroscience

VOLUME=11

YEAR=2017

URL=https://www.frontiersin.org/journals/neuroscience/articles/10.3389/fnins.2017.00106

DOI=10.3389/fnins.2017.00106

ISSN=1662-453X

ABSTRACT=<p>In magnetic resonance imaging, precise measurements of longitudinal relaxation time (<italic>T</italic><sub>1</sub>) is crucial to acquire useful information that is applicable to numerous clinical and neuroscience applications. In this work, we investigated the precision of <italic>T</italic><sub>1</sub> relaxation time as measured using the variable flip angle method with emphasis on the noise propagated from radiofrequency transmit field (<inline-formula><mml:math id="M1" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mi>B</mml:mi></mml:mrow><mml:mrow><mml:mn>1</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msubsup></mml:math></inline-formula>) measurements. The analytical solution for <italic>T</italic><sub>1</sub> precision was derived by standard error propagation methods incorporating the noise from the three input sources: two spoiled gradient echo (SPGR) images and a <inline-formula><mml:math id="M2" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mi>B</mml:mi></mml:mrow><mml:mrow><mml:mn>1</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msubsup></mml:math></inline-formula> map. Repeated <italic>in vivo</italic> experiments were performed to estimate the total variance in <italic>T</italic><sub>1</sub> maps and we compared these experimentally obtained values with the theoretical predictions to validate the established theoretical framework. Both the analytical and experimental results showed that variance in the <inline-formula><mml:math id="M3" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mi>B</mml:mi></mml:mrow><mml:mrow><mml:mn>1</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msubsup></mml:math></inline-formula> map propagated comparable noise levels into the <italic>T</italic><sub>1</sub> maps as either of the two SPGR images. Improving precision of the <inline-formula><mml:math id="M4" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mi>B</mml:mi></mml:mrow><mml:mrow><mml:mn>1</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msubsup></mml:math></inline-formula> measurements significantly reduced the variance in the estimated <italic>T</italic><sub>1</sub> map. The variance estimated from the repeatedly measured <italic>in vivo</italic><italic>T</italic><sub>1</sub> maps agreed well with the theoretically-calculated variance in <italic>T</italic><sub>1</sub> estimates, thus validating the analytical framework for realistic <italic>in vivo</italic> experiments. We concluded that for <italic>T</italic><sub>1</sub> mapping experiments, the error propagated from the <inline-formula><mml:math id="M5" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mi>B</mml:mi></mml:mrow><mml:mrow><mml:mn>1</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msubsup></mml:math></inline-formula> map must be considered. Optimizing the SPGR signals while neglecting to improve the precision of the <inline-formula><mml:math id="M6" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mi>B</mml:mi></mml:mrow><mml:mrow><mml:mn>1</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msubsup></mml:math></inline-formula> map may result in grossly overestimating the precision of the estimated <italic>T</italic><sub>1</sub> values.</p>