Alterations of the Blood-Brain Barrier and Regional Perfusion in Tumor Development: MRI Insights from a Rat C6 Glioma Model

Abstract

Objectives: Angiogenesis and anti-angiogenetic medications play an important role in progression and therapy of glioblastoma. In this context, in vivo characterization of the blood-brain-barrier and tumor vascularization may be important for individual prognosis and therapy optimization.

Methods: We analyzed perfusion and capillary permeability of C6-gliomas in rats at different stages of tumor-growth by contrast enhanced MRI and dynamic susceptibility contrast (DSC) MRI at 7 Tesla. The analyses included maps of relative cerebral blood volume (CBV) and signal recovery derived from DSC data over a time period of up to 35 days after tumor cell injections.

Results: In all rats tumor progression was accompanied by temporal and spatial changes in CBV and capillary permeability. A leakage of the blood-brain barrier (slow contrast enhancement) was observed as soon as the tumor became detectable on T2-weighted images. Interestingly, areas of strong capillary permeability (fast signal enhancement) were predominantly localized in the center of the tumor. In contrast, the tumor rim was dominated by an increased CBV and showed the highest vessel density compared to the tumor center and the contralateral hemisphere as confirmed by histology.

Conclusion: Substantial regional differences in the tumor highlight the importance of parameter maps in contrast or in addition to region-of-interest analyses. The data vividly illustrate how MRI including contrast-enhanced and DSC-MRI may contribute to a better understanding of tumor development.

Fig 1. Simulated signal-intensity time curve after administration of the contrast agent.

Left: schematic illustration of signal recovery (SR) and percentage of signal recovery (PSR). SR is defined as the difference between the signal intensity immediately after the first pass of the contrast agent (S_post at t_post, in humans usually 60 s after bolus arrival) and the pre-contrast (S_pre) signal intensity, while PSR is given by the difference of the signal intensity at t_post to the minimum of the signal intensity-curve (S_min) divided by the difference between pre-contrast (S_pre) and minimum (S_min) signal intensity. Right: Influence of TR and TE on the signal-intensity time curve. The stronger the T1-weighting (reduction of TR) and the weaker the T2*- weighting (reduction of TE) the higher S_min and S_post for the identical time curve of the contrast agent concentration (solid line: TR/TE = 1500/50 ms, dashed line: TR/TE = 1200/50 ms, dotted line: TR/TE = 1500/40 ms assuming T1/T2 = 1000/100 ms and r1/r2 = 4/5 l mmol^-1 s^-1).

Fig 2. Time course of tumor development after intracerebral injection of 10,000 C6-glioma cells.

T2-weighted images (T2w) revealed an increase in tumor mass over time which was accompanied by an increasing area of contrast enhancement (G1-T1w). At day 9 maps of percentage of signal recovery (PSR) and signal recovery (SR) indicated a higher capillary permeability in the tumor center (white ring) extending over time. Higher PSR and SR values were also seen in the region of the choroid plexus (open arrow). Higher cerebral blood volume (CBV) was mainly found at the tumor rim (white arrow and ring).

Fig 3. Spatial heterogeneity of cerebral blood volume (CBV) and signal recovery (SR).

Maps of SR (purple) were overlaid on maps of CBV (green) obtained on day 21 after injection of 100,000 C6 cells. Areas of increased CBV were mostly found at the tumor rim whereas signal recovery exceeding the baseline was mainly seen in the tumor center excluding regions which were most likely necrotic (dark on T2 weighted images). Coronally and axially oriented T2-weighted images (T2w) are shown as reference.

Fig 4. Cerebral blood volume (CBV) and vessel density.

The highest CBV was found at the rim of the tumor which also showed the highest vessel density (bar graph) as revealed by immunohistochemistry for von Willebrand factor (vWF, lower row): (left) overview showing the position of the magnified view of the tumor rim (orange box) and tumor center (green box), upper row, left: the corresponding axially oriented T2-weighted image.

Fig 5. Maps of signal recovery (SR), percentage of signal recovery (PSR) and cerebral blood volume (CBV) in comparison.

Recovery of signal intensity at baseline level corresponds to a value of 0% on the SR map and 100% on the PSR map (black arrow). For better comparison the maps were scaled in a way that these two values marked the end of the first third of the entire value range of the respective map. Thus, with the used color coding, regions with a signal increase above baseline appeared yellow to red while those in which the signal intensity did not recover to baseline appeared blue on both maps. The signal-intensity time curve of selected regions of interest (ROI) is shown on the right, lower row. PSR and SR revealed a similar spatial distribution of regions with an increased capillary permeability, with highest level in the tumor center (blue ROI). Low PSR and SR were mostly accompanied by high CBV values (red ROI), whereas the signal intensity on the contralateral side went back to baseline (yellow ROI) within the observation time.

Fig 6. Robustness of signal recovery (SR) and percentage of signal recovery (PSR) maps.

In case of alteration of the bolus peak PSR maps became unusable, while SR still provided exploitable results. On the right the corresponding axially oriented T2-weighted image.