Introduction

Dynamic Contrast Enhanced (DCE) and Dynamic Susceptibility Contrast (DSC) MRI are two non-invasive perfusion imaging techniques widely applied to detect the vessel permeability and blood flow respectively. The combination of these two modalities gives more benefit diagnosis results than either alone. However, the potential nephrotoxity of full dose gadolinium-based contrast agents is well recognized and the growing evidence that gadolinium deposition may occur in the brain following repeated exposures. ^1,2,3 Hence, the single-dose perfusion imaging combined DCE and DSC MRI is necessary. In addition, conventional DCE-MRI is implemented with FLASH and performs high spatial resolution but the temporal resolution is low. ⁴ While conventional DSC-MRI is implemented with single-shot GRE-EPI and performs high temporal resolution but the spatial resolution is low.⁵ Both high spatial and temporal resolutions are important to the diagnosis accuracy in DCE- and DSC-MRI.⁶ Herein, we propose an improved Spiral-Out-In (iSOI) sequence to implement DCE and DSC MRI for high spatial-temporal resolution brain imaging at a clinical 1.5 Tesla (T) MRI with a single-dose administration of contrast agents. Interleaved spirals were used to realize the high spatial resolution and reduce the off-resonance effect. A model-based methods were used to reconstruct the images for the high temporal resolution DCE- and DSC-MRI.

Methods

A study was performed on five healthy beagle canines with intravenous 25% mannitol (25 ml/kg) to open the blood-brain barrier (BBB) that underwent perfusion analysis. All studies were performed on an iSPACE 1.5 T MRI scanner (Wandong Medical Inc, Beijing, China) equipped with 3.3 G/cm gradients and 110 mT/m/ms slew rate. 8-channel phased array coil was used. Pre-contrast T₁, M₀ mapping and perfusion weighted imaged were performed using a multi-shot, dual gradient-echo, spiral-out and -in sequence with the following parameters: FOV=15×15 cm², matrix=192×192, slice thickness=5 mm, skip=1.5 mm, TE1=4 msec, TE2=34.2 msec, TR=260 msec, number of slices=4, number of interleaved arms=16. Pre-contrast T₁, M₀ mapping used flip angle=10°, 15°, 20°, 25°, 35°. Perfusion weighted imaging used flip angle=60°, number of samples=90. A standard dose of Gadopentetate Dimeglumine Injection (0.2 mmol/kg, magnevist) was injected at the 30^th time point using a power injector. The spiral trajectory is customized for the spiral-out and -in part to two contrast echo signals. The high frequency part in k-space can be shared to reconstruction. Thus the acquisition window width can be controlled. A combined strategy of parallel imaging, compressed sensing, and manifold learning was used for spiral image reconstruction and perfusion calculations were performed offline by using Matlab.

Results

Figure 1 displays the multi-shot dual-echo spiral sequence diagram. TE1 is 1^st echo time and TE2 is 2^nd echo time of this spiral sequence. The first spiral-out echo is used to collect DCE data, and the second spiral-in echo is used to collect DSC data. Figure 2(a) and (e) display the reconstructed spiral images from 1^st and 2^nd of the perfusion-weighted images for the brain of beagle canines with intravenous mannitol, respectively. The in-plane resolution is 0.78mm and the temporal resolution is 0.52 second. The structure of the canine brain is clearly shown. The representative signal intensity time courses displayed in Figure 2(b) shows the signal intensity time courses of the two echoes, respectively. Estimates of Ktrans and Vp obtained from Patlak model fitting show the BBB open after mannitol injection in Figure 2(c) and (d). Figure 2(f) displays R₁ changes and R₂^* changes of brain from DCE signals (red curve) and DSC signals (blue curve) respectively. Estimates of CBF and CBV are displayed in Figure 2(g) and (h). These results suggest that both DSC and DCE parameters can be obtained simultaneously using a multi-shot iSOI acquisition method with a standard dose of contrast agent.

Discussion

This sequence can realize DCE and DSC in a single-dose scan. The repetition time (TR) is crucial to the contrast type. Hence, an optimum TR should be considered. Ultra-short TR which enables fast scan is not suitable with this sequence. After mannitol injection, the BBB will open and the contrast agent will perfuse into the brain of canines. Therefore, signal intensity-time curves of both echoes show perfusion-like. The relaxivity changes can be used to calculate the arterial input function, which is important for the DCE.⁷ Noteworthy, the R₂^* is robust to noise which this sequence can obtained. As a result, the DSC data can be used to optimum calculations of DCE perfusion parameters. Estimations of Ktrans, Vp, CBV and CBF show the brain perfusions.

Conclusion

We have presented the theory and feasibility of a dual-echo spiral-based perfusion imaging method by which DSC-and DCE-MRI perfusion imaging data can be derived simultaneously, with high temporal and spatial resolution using only a single dose of contrast agent. This approach has several distinct advantages over the more common approach of obtaining DSC and DCE data separately and with different imaging sequences. An important advantage of the approach is that all data can be obtained using only a single-dose contrast agent. This advantage is of particular importance given the recent restrictions implemented by the Food and Drug Administration on the use of Gd-based agents because of the small but real risk of nephrogenic systemic fibrosis and more recent concerns regarding Gd deposition in brain.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (NSFC) (No. 61971151)

References

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