Eric R. Muir^{1} and Shengwen Deng^{2,3}

Fast measurement of T1 and T2 can be made using inversion-recovery Look-Locker (LL) bSSFP methods. However, the LL-bSSFP signal is dependent on the off-resonance frequency which can affect calculated T1 and T2. In this study we develop and test methods to correct for effects of off-resonance on T1 and T2 calculation using multiple phase-cycled LL-bSSFP. The phase-cycled LL-bSSFP data could be combined with a maximum-T1* projection method to improve T1 and T2 accuracy in the case of off-resonance.

The
transient bSSFP signal evolves with effective relaxation rates, which when
properly prepared and for a wide range of off-resonance frequencies is
approximately monoexponential with only small oscillations.^{3} Even if
image artifacts from oscillations are avoided, the values of T1 and T2 calculated
from LL-bSSFP as in^{2} will still have inaccuracies at off resonance.^{4}
Two approaches to combine phase-cycled LL-bSSFP were investigated. For both
approached, three parameter fits for the apparent relaxation time T1*
were performed as in^{2} for data with 4 different RF phase increments.
The phase-cycle dataset with largest residuals from the fit was discarded
pixel-by-pixel. 1) Maximum T1* projection (MTP): T1* as a function of
off-resonance frequency has a maximum on-resonance and minimum at 180^{o}
off-resonance, so the single phase-cycle data with maximum T1* was
assumed to be nearly on-resonance and used for calculating T1 and T2
as in.^{2} 2) Non-linear averaging (NLA): the remaining three phase-cycled
data were averaged then refit for T1* from the NLA data and calculating T1 and
T2 as in.^{2}

LL-bSSFP Bloch simulations were run over
a range of off-resonance frequencies with multiple T1/T2
and flip angles, phase-cycling of 0, 90, 180, and 270^{o}, TR/TE=5/2.5ms,
instantaneous RF pulses, 180^{o} inversion, and linear ramp of 7
preparation pulses. The number of excitations was varied (from 640-2740) to
allow substantial recovery steady state for each T1/T2/flip angle.
T1 and T2 were calculated as given above.

Calculated
T1 for simulated phase-cycled LL-bSSFP, showing data calculated using only the
180^{o} phase-cycled (PC) data, the maximum T1* projection (MTP), and
the non-linear averaging (NLA) methods. A) Using 50^{o} flip angle and
B) 20^{o}. The NLA method did not perform well. At low flip angle there
is relatively little error until far off-resonance even with the single phase-cycled
data.

Calculated T2 for
simulated phase-cycled LL-bSSFP, showing data calculated using only the 180^{o}
phase-cycled (PC) data, the maximum T1* projection (MTP), and the non-linear
averaging (NLA) methods. A) Using 50^{o} flip angle and B) 20^{o}.
The NLA method did not perform well.