Synopsis

We present a radial stack-of-stars TSE pulse sequence with an efficient radial view ordering and optimized refocusing flip angles for 3D T2 mapping of the carotid artery. The technique provides excellent anatomical coverage within clinically acceptable times. The short acquisition time makes the technique less susceptible to motion. Performance of the technique is evaluated using phantoms and in vivo experiments.

INTRODUCTION

There has been increasing interest in the quantitative characterization of plaque components using T2 relaxation times^1-7. If a 2D pulse sequence is used, data can be acquired with adequate temporal resolution, e.g., 8 TE points^7-9. The acquisition of 3D data, however, limits the number of time points to 2-3 TEs in order to complete the scan within a reasonable time^2,4. The coarse sampling of the T2 signal can affect T2 estimation which is more pronounced when the signal model deviates from a single exponential, as it is the case when the refocusing RF pulses deviate from the ideal 180^o10.

Here, we present an efficient stack-of-stars variable flip angle TSE (SOS- vFATSE) pulse sequence for 3D T2 mapping of the carotid artery. We introduce (i) an efficient radial view ordering to enable reconstruction of echo images and T2 maps from highly undersampled data and (ii) a framework to design the refocusing flip angle to minimize T2 estimation error.

METHODS

Pulse Sequence: The SOS-vFATSE pulse sequence samples each kz partition using a radial trajectory. Cartesian phase encoding is applied along the kz dimension (Figure 1 (A)). A flip back pulse¹¹ is played at the end of the echo train to restore the longitudinal magnetization to enable the use of shorter TRs to improve scan efficiency. Phase cycled measurements are used to reduce spurious signals from FIDs.

Pseudo-random radial view ordering: Previous radial view ordering schemes such as bit-reverse view ordering¹² or golden angle ordering¹³ do not provide optimum coverage at very long echo train lengths (ETL) and high acceleration rates. To overcome this restriction, we propose the pseudo-random reordering scheme. It combines flexibility in the choice of ETL and uniform sampling of k-space for each TE. The angle acquired at TEj and the TRk is given by:

$$\phi_{(j,k)} = (mod(k+j,\rho)*\frac{\pi}{\rho})+(ETL-j)*(\frac{\pi}{N});j=1,3,…,ETL-1$$

$$\phi_{(j,k)} = (mod(k+j,\rho)*\frac{\pi}{\rho})+j*(\frac{\pi}{N});j=0,2,…,ETL-1$$

where, N is the number of radial views and $$$\rho= N/ETL$$$.

Flip Angle Design: The refocusing flip angle scheme was designed to: (i) optimize SNR by maximizing the area under the T2 decay curve, (ii) minimize the T2 estimation error, and (ii) minimize SAR. Flip angles were computed using a prospective extended phase graph (EPG) algorithm¹⁴, parametrized by four control angles $$$\vec{\alpha} = [\alpha_{min},\alpha_{cent},\alpha_{end},\alpha_{max} ]$$$. The optimized flip angle scheme for carotid imaging applications is shown in Figure 1(B) along with the corresponding signal evolution.

Data Reconstruction: The highly undersampled TE k-space data sets from the SOS-vFATSE pulse sequence were reconstructed using a subspace constrained algorithm¹⁵ with locally low rank regularization¹⁶. T2 maps were generated from a library of pre-computed T2 curves using the slice-resolved EPG model.

Imaging experiments: All imaging was carried out at 3T (Skyra, Siemens). T2 estimation accuracy was evaluated using agarose gel phantoms. SOS-vFATSE data were acquired with: FOV=120 cm, base resolution = 256, radial views = 384, ETL = 96, and 1 mm thick slices. Reference T2 estimates were obtained using a single-echo spin-echo pulse sequence. Subjects were imaged after obtaining informed consent. In vivo carotid vessel wall data were acquired using a dedicated carotid coil¹⁷. A TOF pulse sequence was acquired to locate the carotid arteries. SOS-vFATSE data were acquired with ETL=64, TR=1300ms, base resolution=256, radial views=256, in-plane resolution=0.53 x 0.53 mm2, slice thickness=2 mm. Data for 32 slices were acquired in 3.78 minutes. Motion sensitizing gradients were used to suppress signal from blood flow.

RESULTS

T2 estimates from SOS-vFATSE for the agarose phantoms are shown in Figure 2. Note that the estimated T2 values match well the reference values. A reproducibility study was conducted on 4 healthy volunteers by imaging the subjects in two different days. Figure 3(A) shows a significant correlation between the two sets of measurements (correlation coefficient = 0.952, p-val <1e-3). The Bland-Altman plot in Figure 3(B) show that the mean difference in T2 estimates between the measurements is 0.11 ms with a coefficient of variation of 4.44 ms.

Figure 4 shows dark blood T2w images at two of the 64 acquired TEs for a subject with carotid stenosis. The T2 maps calculated from the full TE data set are shown in color overlaid on the TE=45 ms images. The range of T2 value is in agreement with literature values^1,18. SOS-vFATSE data were acquired in less than 4 min covering efficiently the extracranial carotid artery.

CONCLUSION

Acknowledgements

References

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