Robert Claeser^{1}, Markus Zimmermann^{1}, and Nadim Joni Shah^{1,2}

TAPIR is a highly accurate, precise and efficient method for T1 mapping of the brain. It combines an efficient slice-interleaving Look-Locker read-out to sample T1 relaxation by acquiring multiple k-space lines in one shot. However, mapping rapidly relaxing tissue requires the number of lines read in one shot to be small, thus increasing total measurement time. In this work we show how incorporating an interleaved spiral read-out into TAPIR enhances its T1 fitting abilities for rapidly relaxing tissue such as white matter myelin. Scanning time can be decreased by factors of up to 3.3 in comparison to classical Cartesian TAPIR.

**Introduction**

**Materials and Methods**

**Results**

A cylindrical phantom with 9 inserts containing doped water with T1 relaxation times of 266ms, 421ms, 621ms, 704ms, 925ms, 1309ms, 1436ms, 764ms and 3178ms for ROIs 1 to 9 as measured via spectroscopy was used to determine the fitting accuracy for all sequences.

Fig. 1 depicts the results for the 8th slice in the measured group. ROI1 with the smallest T1 value is located on the left, and ROI number increases counter-clockwise.

Fig. 2 shows the evolution of mean T1 values over slices 1 to 8.

Fig. 3 shows the mean T1 values per ROI over all slices.

Fig.
4 shows T1 maps for the in vivo scans. The mean and standard
deviation of T1 in WM and GM as shown in Fig. 5 were computed based
on each sequences’ class map, which were generated using FSL FAST ^{6}.

**Discussion**

1. N.J. Shah, M. Zaitsev, S. Steinhoff, et al. A New Method for Fast Multislice T1 Mapping. NeuroImage. 2001;14:1175-1185.

2. M. Zaitsev, S. Steinhoff, N.J. Shah. Error Reduction and Parameter Optimization of the TAPIR Method for Fast T 1 Mapping. Magn. Res. Med. 2003;49:1121-1132.

3. G.H. Glover. Simple Analytic Spiral K-Space Algorithm. Magn. Res. Med. 1999;42:412-415.

4. R. K. Robison, A. Devaraj, J.G. Pipe. Fast, Simple Gradient Delay Estimation for Spiral MRI. Magn. Res. Med. 2010;63:1683-1690.

5. S. Boyd, N. Parikh, E. Chu, et al. Distributed Optimization and Statistical Learning via the Alternating Direction Method of Multipliers. Foundations and Trends in Machine Learning. 2010;3:1-122.

6. M. Jenkinson, C. F. Beckmann, T.E.J. Behrens, et al. FSL. NeuroImage. 2012;62:782-790

Fig. 1: T1 fitting results for 8th
slice of the phantom measurement windowed from 0 to 3000ms relaxation
time. Upper row showing EPI5 (left) and EPI7 (right), lower row
showing EPI 9 (left) and spiral fully sampled (right). No Cartesian
trajectory allows fitting ROI 1, while the spiral trajectory data is
fitted successfully for almost all voxels in this ROI. EPI7 and EPI9
trajectories also display errors in fitting the bulk of the phantom
and ROI 2. The impact of field inhomogeneities is most apparent for
EPI9, but also on some of the borders of the ROIs for spiral
trajectories.

Fig. 2: Mean fitted values for ROI 1. The Cartesian sampling
trajectories with TR of 16.6ms and 24.4ms show significant decline in mean T1
as more and more voxels cannot be fitted with increasing slice number and are
consequently set to zero. The spiral trajectory with TR of 14.1ms shows no
change in mean T1 up to the 8th slice. Voxels were evaluated in a
square region of 17 by 17 voxels centered in the middle of the ROI.

Fig. 3: Mean fitted values over all slices for all ROIs. While all Cartesian
sampling trajectories underestimate T1 for ROI 1 and EPI9 also slightly for ROI
2, the spiral trajectory leads to an overestimation of T1 in ROI 7 to 9. Voxels
were evaluated in a square region of 17 by 17 voxels centered in the middle of
the ROI.

Fig. 4: T1 maps for slice 1 (top row) and slice 8 (bottom row).
From left to right: EPI9 Cartesian trajectory, fully sampled spiral trajectory
and 50% under-sampled spiral trajectory. All images are windowed to T1
relaxation times of 0 to 3000ms.

Fig. 5: Mean and standard deviation for T1 in WM and GM over all 8
slices scanned. There is no significant difference in mean T1 observable
between the Cartesian measurement with an EPI factor of 9 and both fully sampled (FS) and
under-sampled (US) spiral measurements. Standard deviation is slightly lower
for spiral sampling in white matter and slightly higher in grey matter.