Simultaneous perfusion and permeability assessments using multi-band multi-echo EPI (M2-EPI)
Deqiang Qiu1, Junjie Wu1, Seena Dehkharghani1, and Amit Saindane1

1Department of Radiology and Imaging Sciences, Emory University, Atlanta, GA, United States

Synopsis

We proposed a novel multi-band multi-echo DSC perfusion imaging method to estimate leakage-corrected perfusion parameters and additional vascular permeability parameters. Simulations were performed and showed that higher temporal resolution provided by the novel sequence improves the accuracy in the calculation of perfusion parameters.

Introduction

Dynamic susceptibility contrast (DSC) MR perfusion imaging can provide valuable perfusion metrics, including cerebral blood flow (CBF), cerebral blood volume (CBV) and mean transit time (MTT). DSC is typically performed with a single-band single-echo EPI sequence, which has limited spatial and temporal resolution, and is prone to measurement errors due to contrast agent (CA) extravasation in cases of blood brain barrier (BBB) breakdown such as in brain tumors1. In the present study, we proposed a novel multi-band multi-echo EPI (M2-EPI) DSC method for perfusion imaging with leakage correction, which can also provide additional vascular permeability parameters Ktran and ve. Simulations were performed to study the effects of improved temporal resolution afforded by multi-band acquisition on the accuracy of perfusion parameter estimation.

Methods

DSC was performed with a Siemens Tim Trio 3T scanner equipped with a 32 channel head coil using a novel M2-EPI pulse sequence2: TR = 800 ms, TE = 18 / 41 /65 ms, FOV = 240 x 240 mm2, matrix = 100 x 100, slice thickness/gap = 4/1 mm, 24 slices multiband-factor = 3, GRAPPA factor = 3. Gadobenate Dimeglumine (MultiHance, Bracco, Milan, Italy) was injected at 4 ml/s 10 seconds following initiation of the M2-EPI sequence. T1-weighted MPRAGE imaging was also acquired after the contrast agent injection. Three patients with known brain tumors underwent MRI scans with the above sequences. The temporal signal following contrast injection can be expressed as the following equation:$$S(t,TE)=S_{0}(t)e^{-TE\cdot R_2^*(t)}+\sigma$$ $$S_{0}(t)=M_{0}\frac{1-e^{-TR\cdot R_{1}(t)}}{1-e^{-TR\cdot R_{1}(t)}\cos\theta}\sin\theta$$

where TE denotes the echo time, TR the repetition time, θ the flip angle, M0 the equilibrium longitudinal magnetization, σ the noise, and R1(t) the longitudinal relaxivity which is a function of CA leaked into the extravascular-extracellular space (ESS), R2*(t) the transverse relaxivity which is approximately a linear function of CA concentration in the intravascular space. CA leakage primarily causes a change in S0(t). The acquisition of multi-echo images using M2-EPI allows the isolation of this effect by estimating S0(t) and R2*(t) at each time point. The obtained R2*(t) dynamic volumes was subject to DSC modeling using a regularized deconvolution technique following automatic identification of the arterial input function to produce the CBF, CBV, MTT and Tmax images3. DSC processing was also performed on the second echo without leakage correction for comparison. The volume transfer constant Ktrans and the EES volume fraction ve were estimated from S0(t) using the adiabatic approximation to the tissue homogeneity model1. Simulations were performed to study the accuracy of permeability and perfusion estimations by varying temporal resolution (TR) and perfusion/permeability parameters. Gaussian noise was added based on SNR measurement of experimental data. Simulations were repeated 100 times to calculate the mean values and SD of the perfusion parameters. TR was varied from 400 to 3200 ms, CBF from 20 to 100 ml/100g/min, Ktrans from 0.00 to 0.12 min-1. MTT was set to 5 second and ve to 0.25.

Results

Figure 1 shows signal time courses of the DSC acquisition as well as calculated S0(t) from voxels in healthy tissue and tumor. Increased S0(t) following contrast injection can be observed in the tumor region due to extravasation of the contrast agent into the ESS caused by BBB breakdown. Figure 2 shows CBV and MTT maps calculated from the second echo without leakage correction, as well as from all three echoes with leakage correction in a patient with a brain tumor. CBV map without leakage correction underestimates the blood volume in the tumor, while leakage-corrected CBV map showed high blood volume in the tumor. Additional vascular permeability measures including the volume transfer constant Ktrans and the EES volume fraction ve maps were also obtained. Elevated permeability parameters Ktrans and ve in tumor suggest substantial leakage of contrast agent in this region. Multi-echo based leakage correction were successfully performed in all three patients with brain tumors and provided improved stimulation of the CBV in regions of brain tumor (Figure 3). Simulation showed that shorter TR is associated with more accurate estimation of CBF and MTT while differences in TR have minimal effects on the estimation of Ktrans, ve, and CBV (Figure 4).

Discussion and Conclusion

Perfusion and vascular permeability parameters can be simultaneously assessed through the multi-band multi-echo DSC perfusion imaging method. Multi-echo data acquisitions improve perfusion measurements by reducing or eliminating T1-shortening effects due to CA extravasation, with additional permeability determination. Multi-band data acquisitions provide high temporal resolution, allowing more accurate perfusion quantification. In conclusion, M2-EPI DSC could facilitate accurate perfusion/permeability evaluation in brain tumor and thus can be used as a valuable diagnosis tool.

Acknowledgements

No acknowledgement found.

References

1. Schmiedeskamp, H., et al., Simultaneous perfusion and permeability measurements using combined spin- and gradient-echo MRI. J Cereb Blood Flow Metab, 2013. 33(5): p. 732-743.

2. Qiu, D., et al. Multi-band Multi-echo EPI for Dynamic Susceptibility Contrast Perfusion Imaging: A feasibility Study. in ISMRM. 2015.

3. Straka, M., G.W. Albers, and R. Bammer, Real-time diffusion-perfusion mismatch analysis in acute stroke. Journal of Magnetic Resonance Imaging, 2010. 32(5): p. 1024-1037.

Figures

Figure 1. (A) Post-contrast T1-weighted image in a patient with a brain tumor. (B) Signal time courses of the second echo in voxels indicated on (A). (C) Estimated S0(t) of voxels indicated on (A). Compared to normal tissue, S0(t) in tumor increased following contrast agent injection due to CA extravasation.

Figure 2. (A) Perfusion and (B) permeability maps of a brain-tumor patient. Underestimation of CBV in the tumor was found without leakage correction. M2-EPI sequence with leakage correction found high CBV in the tumor along with the measurement of high permeability parameters Ktran and ve.

Figure 3. CBV maps from the second echo without leakage correction and CBV maps with leakage correction as well as Ktrans and contrast enhanced T1-weighted images (post-Gad T1w) for all three patients.

Figure 4. (A) Simulation results showed that shorter TR is associated with improved estimation accuracy of CBF. Similar results were found for MTT. (B) Simulation showed minimal effects of different TR has minimal effects on the estimation accuracy of Ktrans. Similar results were found for ve and CBV.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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