It has been shown repeatedly that breathing oxygen shortens the $$$T_1$$$ relaxation time in ventilated regions of the lung$$$^\text{1}$$$. The most widespread methods to quantify $$$T_1$$$ in the lung is the inversion-recovery SNAPSHOT FLASH sequence$$$^\text{2}$$$. However, this method is gradient-echo based and therefore suffers from signal attenuation due to air tissue interfaces. Furthermore, an echo time (TE) dependency of $$$T_1$$$ has been demonstrated using an ultra-short TE sequence$$$^\text{3}$$$: Avoiding $$$T_2'$$$-decay, extra-vascular components contribute to the signal, influencing the average $$$T_1$$$, promising additional diagnostic information$$$^\text{3}$$$. Since UTE-sequences are time intensive, we propose an inversion-recovery spin echo SNAPSHOP FLASH sequence, employing self-refocusing excitation pulses in the regime of weak dephasing, which allow an echo time - measured from the end of the RF-pulse - that exceeds the pulse duration$$$^\text{4}$$$. It is shown that quantitative $$$T_1$$$ and proton density (PD) maps of the entire lung can be acquired within $$$5.5~s$$$. The highly undersampled data is addressed in an iterative reconstruction.

Data were acquired in a healthy volunteer during a breath-hold at full expiration. A 3T PRISMA scanner (Siemens, Erlangen, Germany) was used equipped with the manufacturer's spine- and body-array of which 30 channels were used. All experiments were performed with the subject breathing room air and were repeated under the consumption of oxygen. An inversion-recovery SNAPSHOT-FLASH sequence was modified to use a spin echo generating excitation pulse$$$^\text{4}$$$. The employed pulse has a duration of $$$150~\mu\text{s}$$$ and an echo time of $$$630~\mu\text{s}$$$ measured from the end of the RF-pulse. The pulse shape is depicted in Fig.1 along with the corresponding phase evolution of the excited spin isochromats. A flip angle of $$$1.2^\circ$$$ was used. For comparison, a $$$100~\mu\text{s}$$$ rectangular pulse was used.

The nominal spatial resolution was $$$5~\text{mm}\times5~\text{mm}\times10~\text{mm}$$$ with a $$$FOV=640~\text{mm}\times640~\text{mm}\times240~\text{mm}$$$ in coronal orientation. With $$$TR=1.6~\text{ms}$$$ the total acquisition time was $$$5.5~\text{s}$$$. Spatial encoding was performed with a stack-of-stars trajectory: While sweeping through $$$k_z$$$, an increment of $$$\alpha_\text{gold}\cdot{N_{shots}}/N_z$$$ was used, where $$$\alpha_\text{gold}\approx111^\circ$$$ is the golden angle, $$$N_{shots}$$$ is the total number of excitations and $$$N_z$$$ is the matrix size in z-direction. After acquiring one spoke for each $$$k_z$$$, the whole trajectory is repeated rotated by $$$\alpha_\text{gold}$$$, ensuring good k-space coverage both within each partition as well as in comparison to the neighboring partitions.

In total 3456 spokes were acquired, which corresponds approximately to 72% of Nyquist sampling. With this data, $$$T=144$$$ images were reconstructed, using one spoke per $$$k_z$$$-partition and minimizing the following cost function: $$\tilde{x}=\arg\min_{x~\in~\mathbb{C}^{N \times T}}\sum_{t=0}^{T-1}\left|\left|\mathbf{E}_{t,:,:}\cdot{x_{:,t}}–S_{:,t}\right|\right|_2^2+\lambda^2\sum_{n=0}^{N-1}\left|\left|x_{n,:}-\mathbf{P}_{\mathcal{A}}(x_{n,:})\right|\right|_2^2\;.$$The search variable $$$x$$$ contains all $$$N$$$ voxels of all $$$T$$$ time-frames. The first summand is the data consistency term with the forward operator $$$\mathbf{E}$$$ which incorporates a non-uniform FFT and ESPIRiT$$$^\text{5}$$$ coil sensitivities. The second term compares the time series of each voxel with its projection $$$\mathbf{P}$$$ onto the manifold $$$\mathcal{A}=\{\exp(i\phi)(a-b\exp(-t/c))\;|\;\phi,a,b,c\in\mathbb{R}\}$$$. Because this algorithm approaches the Bloch response recovery via iterated projection$$$^\text{6}$$$ algorithm (BLIP) when setting the regularization parameter to $$$\lambda\rightarrow\infty$$$ we refer to it as generalized BLIP (gBLIP). Given this projection, $$$T_1=c(b/a-1)$$$ reveals the desired relaxation time for Look-Locker sequences. For computational efficiency, the projection was performed with a precomputed dictionary.

Fig.2 shows one slice of the resulting PD-maps. One can observe an increase of the parenchyma signal when employing the proposed spin echo pulse (b) compared to the traditional gradient-echo IR-SNAPSHOT-FLASH (a). In a region of interest in the upper left lung, the signal increase was measured to be 38%. Furthermore, one can observe an increase of the fat signal, which is a side effect resulting from fat-water-shift in combination with the excitation profile of the RF-pulse$$$^\text{4}$$$. Fig.3 shows a slice of the $$$T_1$$$-maps under the consumption of oxygen. The spin echo image shows a similar reduction of the relaxation time compared to UTE-sequences$$$^\text{3}$$$. Smoothed maps of the $$$\Delta{T_1}=T_1^{\text{air}}-T_1^{\text{O}_2}$$$ are shown in Fig.4, overlayed onto PD-maps. $$$\Delta{T_1}$$$ is slightly reduced when employing the SE-pulses and the structure of $$$\Delta{T_1}$$$ is changed.

In this study, a similar increase of the parenchyma signal has been observed compared to previous studies$$$^\text{4}$$$. Furthermore, the reduction of $$$T_1$$$ due to signal contributions of the extra-vascular components is strongly comparable to the reduction observed with UTE-sequences$$$^\text{3}$$$. Thus, our experiment supports the hypothesis of $$$^\text{3}$$$ that the change in $$$T_1$$$ results from extra-vascular signal contributions with short $$$T_1$$$ that are less visible in gradient-echo images, since their neighborhood to air results in a fast $$$T_2'$$$-relaxation. Overall, the results demonstrate the feasibility of the proposed IR-SE-SNAPSHOT-FLASH sequence for quantitative lung imaging with additional diagnostic information. The changes in the $$$\Delta{T_1}$$$-maps are not yet fully understood and will be subject of future investigations.