Steven Yee^{1}, Michael Fadell^{2}, Mark S Brown^{2}, and Seonghwan Yee^{2}

^{1}SUNY Geneseo, Geneseo, NY, United States, ^{2}University of Colorado School of Medicine, Aurora, CO, United States

### Synopsis

Fast T1 mapping can be done by utilizing dual flip angles in
acquiring spoiled gradient echo signals. However, its accuracy may be
questionable even when the suggested optimal flip angle pair is used. Noting
that the faithful action of the prescribed flip angles is the key to the
accuracy, we present here a novel dual flip angle method by which the system-specific
RF-pulse fidelity of flip angles can be validated and, if necessary, calibrated
to improve the T1 accuracy in a wide in
vivo range. We tested this method on a few 1.5 or 3T MRI systems of major
vendors.

### Introduction

In the conventional dual flip angle (C-DFA) method, an
optimal flip angle (FA) pair is suggested for a specific target T1 value to
minimize the T1 uncertainty (1-3). However, the accuracy of T1 mapping is sometimes
questionable, particularly over a wide range of T1 values. The main reason for
this problem is the discrepancy between the prescribed and the actual FAs.
Although spatial B1 inhomogeneity may sometimes be responsible, the
system-specific FA calibration also plays a significant role to the
discrepancy. Here, we propose a novel dual flip angle (N-DFA) method which renders
us to reconsider the validity of conventionally suggested optimal FAs in order to
overcome practical limitations of the C-DFA method.### Methods

A T1 phantom was made of 12 vials (each of 50 ml
capacity) containing distilled water and different concentrations of gadolinium
contrast agent. MRI scans were performed using the phantom in 5
different MRI systems (1.5T or 3T, including all major vendors). At first, the phantom
was scanned by the inversion recovery-based sequence (TI= varied from 50ms to
2000ms, TR=4s, TE=8ms) to determine the T1 value of each vial, which was taken
as a reference T1 for each vial for the particular scanner at the time of the
experiment. The phantom was then repeatedly scanned by a spoiled gradient echo-based
sequence (SPGR, FLASH, or T1-FFE for GE, Siemens, or Philips) while the FA was
changed by a small step (2°) in the range from 1° to 43°. For the SPGR
sequences, a variation of the technique (3D or 2D) was also tested. The determined
T1 values by the C-DFA method with optimal FAs were made to the reference
values. A new strategy (the N-DFA method) was also developed to make the
signal ratio acquired with certain FA pair, S(θ1)/S(θ2), almost linear over the
wide T1 range. Fine calibration of FAs, if necessary, was performed by finding
the line equation in the N-DFA method, and the resulting T1 values were
compared to the reference values.### Results

SPGR signal
change curves over different FAs and their fit curves are shown in **Fig. 1**, where the discrepancy of the actual
data and the fit curve clearly suggests that the FA may not be accurate (therefore
C-DFA method may not work well) other than certain flip angles or angle ranges.
Simulation curves of SPGR signals divided by a certain reference FA signal are
shown in **Fig. 2**, where the signal with
FA=1°, 2° or 3°, when divided by the signal with FA=17°, demonstrates almost
linear relationship to the T1 values. The pair of FA=2° and 17° was chosen here
in the N-DFA method and, after acquiring actual data from the phantom, the linear
calibration parameters for the N-DFA method were obtained by the linear fit to
the actual curve (shown in **Fig. 3**),
and used successively to determine the T1 values with specific measured signal
ratios. The T1 values obtained by the C-DFA and N-DFA methods were compared to
the reference values and are shown in **Fig.
4**, where the T1 map obtained for the phantom, determined by the N-DFA
method, is also shown.### Discussion

Although optimal FA pairs are proposed in the C-DFA method (1-3)
to minimize the uncertainty of T1 determination, the underlying assumption is
the faithful action of the RF pulse for the prescribed FAs. However, system- and pulse sequence-specific FA
calibration cannot be precisely done in all FAs, and, as shown in our
experiments, that assumption may break. As a result, the accuracy of T1 mapping
by the C-DFA method can be compromised. Therefore, we have come up with a novel
strategy (the N-DFA method) to overcome this problem.

The need for tailored calibration for each technique is also
suggested in **Fig. 5**, where the
B1+rms (and SAR) for each prescribed FA is plotted for 2D or 3D technique. In **Fig. 5**, system FA calibration may be
different between 3D and 2D, and even for the same 3D, below and above the FA
around 19°. Therefore, we have chosen 17° in the N-DFA method, as well as the
low FA value of 2° to make the linear relationship as shown in **Figs. 2 & 3**.

### Conclusion

By utilizing the linear relationship of the signal ratio to
the T1 values when the specific FA pairs (e.g. 2° and 17°, not the
conventionally suggested optimal FAs) are used, the N-DFA method provides the possibility
of tailored FA calibration to specific systems or sequences and improves the
accuracy of T1 mapping in a wide in vivo
range of T1 values.### Acknowledgements

Dr. Ann Scherzinger, Ph.D., section chief, and Dr. Gerald Dodd, M.D., Chair of Department of Radiology for the other support

### References

1. Wang HZ, Riederer SJ, Lee JN. Optimizing the precision in
T1 relaxation estimation using limited flip angles. Magn Reson Med. 1987; 5:399‐416.

2. Deoni SCL, Rutt BK, Peters TM. Rapid combined T1 and T2 mapping
using gradient recalled acquisition in the steady state. Magn Reson Med. 2003;
49:515‐526.

3. Schabel MC, Morrell GR. Uncertainty in T1 mapping using
the variable flip angle method with two flip angles. Phys Med Biol. 2009;
54:N1-N8.