Cerebrovascular reactivity measured with quantitative susceptibility mapping and TRUST MRI under hypercapnia
Jean-Christophe Brisset1, Olga Marshall1, Louise E Pape1, Hanzhang Lu2, and Yulin Ge1

1Radiology, New York University School of Medicine, New York, NY, United States, 2Radiology, Johns Hopkins University, Baltimore, MD, United States

Synopsis

Cerebrovascular reactivity (CVR) measures the capacity of regulation of blood flow in response to vasoactive stimuli (e.g., CO2) via changes in cerebral arterial resistance. CVR is responsible for maintaining optimal blood flow to meet the energy demand by neurovascular coupling during neuronal tasks. The defected CVR can cause transient states of inefficient blood delivery during neural activity leading to subsequent neurodysfunction and degeneration. CVR is commonly measured with mild hypercapnia (5%CO2) ASL or BOLD MRI. The purpose of this study was to test the feasibility to measure CVR using hypercapnia MRI with quantitative susceptibility mapping (QSM) and T2-relaxation-under-spin-tagging (TRUST) techniques.

Target Audience

Scientists and clinician who are interested in MS imaging research

Purpose

Cerebrovascular reactivity (CVR) is a robust measure of vascular function – the capacity of regulation of blood flow in response to vasoactive stimuli (e.g., CO2) via changes in cerebral arterial resistance. In healthy condition, CVR serves to modulate and maintain the optimal blood flow to meet the energy demand by neurovascular coupling during neuronal tasks. The defected CVR in a disease state can cause transient states of inefficient blood delivery during neural activity leading to subsequent neurodysfunction and degeneration. CVR is commonly measured with mild hypercapnia (breathing 5% CO2) perfusion (ie. arterial spin labeling or ASL) (1) or BOLD (2) MRI. It is well known that the changes of blood CO2 level can also alter the venous oxygenation level by increasing flow and oxygen, therefore, the purpose of this study was to test the feasibility to measure CVR using hypercapnia MRI with quantitative susceptibility mapping (QSM) and T2-relaxation-under-spin-tagging (TRUST) (3) techniques.

Methods

Seven subjects underwent MRI scans at 3T under room air and hypercapnia (breathing a gas mixture of 5% CO2, 21%O2, 74%N2) conditions to assess whole brain CVR measured by QSM and TRUST MRI. QSM was generated using a susceptibility-weighted imaging (SWI) acquired with parameters of TR/TE/FA: 32/20/15 (2’37”). TRUST MRI is a well-established quantitative technique (3) for cerebral venous oxygenation (Yv). It starts with a spin labeling module with labeling slab placed above the imaging slice to tag the superior saggital sinus (SSS) pure venous blood and measures T2-relaxation time using a series of non-slice-selective T2-preparation pulses in order to quantify blood T2. The SSS blood T2 was then converted to Yv using a calibration plot. The details of imaging sequences and image processing can be found in our previous studies. Image processing of QSM used the algorithm described previously, first, the phase images were unwrapped using SWIM algorithm with Laplacian unwrapping and SHARP background removal. Images at normocapnia were threshold at 70 ppb and area that not belongs to veins subtract from the analysis (ie basal ganglia or artifact) to have pure venous structures segmented. The breathing apparatus is composed of gas delivery through mouthpiece, switching from room air to CO2, and endtidal CO2 recording, please find details in the previous study (2). The QSM-based CVR and TRUST-based CVR are calculated as normocapnia to hypercapnia percent QSM changes (from whole segmented venous blood voxels) and percent Yv changes (from SSS) per EtCO2 change (mmHg), respectively.

Results

The EtCO2 values increased significantly between the normocapnic and hypercapnic QSM or TRUST scans (37.7±7.0 to 48.6±5.5, p=0.0006). There was no significant difference between normocapnic and hypercapnic EtCO2 levels during the QSM and the TRUST runs. There was a significantly decreased QSM from venous blood voxels (p=0.0007) and increased Yv (p=0.006) during hypercapnia breathing as compared to normocapnia breathing. CVR values based on QSM are 0.033±0.006 %QSM/mmHg EtCO2 and based on TRUST are 0.025±0.011 %QSM/mmHg EtCO2, which are not significantly different (p=0.24). As shown in Figure 1, Figure 2 showed the comparison of average QSM-based CVR and TRUST-based CVR.

Discussion

The current study confirms that CVR calculated based on QSM and TRUST are highly correlated, and their average CVR value is very with the previous reported valued based hypercapnia perfusion MRI (1). Since CO2 is considered a potent vasodilator and has minimal effect on neuronal activity, the blood oxygenation changes on QSM and TRUST are principally reflect vascular modulation or CVR. However, QSM can provide much higher spatial resolution and TRUST has much quicker scan times as opposed to arterial ASL perfusion or BOLD MRI for CVR quantification, which are all critical in clinical settings.

Acknowledgements

This work was supported in part by NIH Grants (NS029029-20S1 and NS076588) and National Multiple Sclerosis Society (NMSS) research grant (RG 4707A), this study was also performed under the rubric of the Center for Advanced Imaging Innovation and Research (CAI2R, www.cai2r.net), a NIBIB Biomedical Technology Resource Center (NIH P41 EB017183).

References

1.Marshall O, Lu H, Brisset JC, Xu F, Liu P, Herbert J, Grossman RI, Ge Y. Impaired cerebrovascular reactivity in multiple sclerosis. JAMA neurology 2014;71(10):1275-1281.

2.Lu H, Zhao C, Ge Y, Lewis-Amezcua K. Baseline blood oxygenation modulates response amplitude: Physiologic basis for intersubject variations in functional MRI signals. Magn Reson Med 2008;60(2):364-372.

3.Lu H, Liu P, Yezhuvath U, Cheng Y, Marshall O, Ge Y. MRI mapping of cerebrovascular reactivity via gas inhalation challenges. Journal of visualized experiments : JoVE 2014(94).

Figures

Figure 1. Voxel-based QSM map was generated and segmentation of venous blood was performed with thresholding (>70ppb) to remove non-venous blood background including artifact and deep gray matter. There is decreased susceptibility values in most venous blood on hypercapnia versus normocpania map. Percent change QSM of global venous blood susceptibility changes was used to calculated QSM-based CVR.

Figure 2. Comparison between the average CVR based using QSM-based method and TRUST Yv-based method. The CVR values from both methods are comparable to the reported values using perfusion (i.e., ASL) or BOLD hypercapnia MRI.

Figure. The TRUST MRI sequence (Lu H, Ge Y, MRM 2008) (A) starts with spin labeling slab of venous blood (green box) placed above imaging slice (yellow line) to isolate the pure venous blood. A series of non-slice-selective T2-preparation pulses were acquired at the same time for T2 relaxation measurement at the imaging slice through the lower superior sagittal venous sinus. The blood T2 is then converted to blood oxygenation (Yv) using a calibration plot (B).



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