Abstract
Background and Purpose—Hyperpermeability and iron deposition are 2 central pathophysiological phenomena in human cerebral cavernous malformation (CCM) disease. Here, we used 2 novel MRI techniques to establish a relationship between these phenomena.
Methods—Subjects with CCM disease (4 sporadic and 17 familial) underwent MRI imaging using the dynamic contrast-enhanced quantitative perfusion and quantitative susceptibility mapping techniques that measure hemodynamic factors of vessel leak and iron deposition, respectively, previously demonstrated in CCM disease. Regions of interest encompassing the CCM lesions were analyzed using these techniques.
Results—Susceptibility measured by quantitative susceptibility mapping was positively correlated with permeability of lesions measured using dynamic contrast-enhanced quantitative perfusion (r=0.49; P≤0.0001). The correlation was not affected by factors, including lesion volume, contrast agent, and the use of statin medication. Susceptibility was correlated with lesional blood volume (r=0.4; P=0.0001) but not with lesional blood flow.
Conclusions—The correlation between quantitative susceptibility mapping and dynamic contrast-enhanced quantitative perfusion suggests that the phenomena of permeability and iron deposition are related in CCM; hence, more leaky lesions also manifest a more cumulative iron burden. These techniques might be used as biomarkers to monitor the course of this disease and the effect of therapy.
- biomarkers
- cerebral cavernous malformations
- cerebrovascular disorders
- hemangioma, cavernous
- hemorrhagic stroke
- magnetic resonance imaging
- vascular permeability
Introduction
Cerebral cavernous malformations (CCM) are characterized by capillary dilatations with deficient blood–brain barrier and disrupted interendothelial tight junctions that may cause vessel hyperpermeability.1–3 This hyperpermeablity may cause chronic blood leakage with neurological sequelae, including epilepsy and focal deficits and hemorrhagic stroke. We previously showed that Rho kinase inhibition by fasudil decreases iron deposition and lesion burden in murine CCM models,4 and mice haploinsufficient in Ccm gene demonstrated increased vascular permeability to Evan’s blue dye that is reversible by Rho kinase inhibition.5
With the advent of Rho kinase inhibition as a potential therapy, a method for assessing its effect is needed. Recent advances in MRI have provided 2 potential methods to accomplish this. The first being dynamic contrast-enhanced quantitative perfusion (DCEQP) is used to measure hemodynamic parameters such as permeability. Applicability of DCEQP has been demonstrated by Larsson et al6 and has subsequently been validated by comparisons with histology7 and quantitative autoradiography.8 The second method, quantitative susceptibility mapping (QSM),9,10 measures the magnetic susceptibility of the brain tissue, an intrinsic biophysical property of the tissue that is directly proportional to the local iron content. Both methods may be applied to assess characteristics of CCM lesions in humans. Given the common underlying pathophysiological mechanisms of the disease and the potential to use these techniques as biomarkers for disease activity and response to treatment, we hypothesize that there is a positive correlation between permeability measured with DCEQP and iron burden measured by QSM in the same CCM lesions.
Materials and Methods
Subjects
After obtaining institutional review board approval and informed consent, 21 patients scheduled for routine clinical evaluation for CCM disease were recruited, including 1 case that included a second follow-up scan (characteristics in the Table).
Summary of Patient Clinical and Lesional Characteristics
Data Acquisition and Processing
All scans were obtained during regular clinical follow-up. Permeability was measured using a T1-weighted DCEQP protocol that included a precontrast T1 scan followed by a dynamic scan using gadolinium contrast (gadodiamide or gadobenate dimeglumine). Images were then processed in MATLAB using the Patlak mathematical model to calculate the permeability, cerebral blood flow, and volume (CBF and CBV) maps. Regions of interest were selected encompassing entire lesions as they appeared on T2-weighted images that were acquired simultaneously and were then superimposed on the maps.
A single 3-dimensional, multi-echo, T2*-weighted, spoiled gradient echo sequence was used for data collection for QSM reconstruction. The QSM images were reconstructed with customized software using a morphology-enabled dipole inversion algorithm.11,12 Regions of interest included the same lesions identified on T2 images used for permeability. More detailed information is available in the online-only Data Supplement.
Statistics
The Pearson correlation was used to examine the correlation between QSM susceptibility and permeability, CBF, and CBV. Multivariate linear regression was used to assess the effect of potential contributing factors on the above-mentioned correlations, with susceptibility as the dependent variable, including permeability, the factor, and their interaction in the model. Bland–Altman plots were constructed to evaluate intraobserver and interobserver consistency, and coefficients of variation were calculated to look at interpatient and intrapatient variability.
Results
Figure 1 shows an example of a T2 image used for regions of interest selection, as well as a permeability map and QSM image of the same lesion. Intraobserver and interobserver consistency was demonstrated with both techniques in a subcohort of cases (see Results in the online-only Data Supplement). The Table illustrates the patients’ salient clinical features and summarizes imaging results for each case with both techniques. A positive correlation was found between mean QSM susceptibility of lesions and mean permeability of the same lesions (r=0.49; P≤0.0001; Figure 2A). This correlation between susceptibility and permeability was present predominantly in familial cases and was independent of lesion volume, the contrast agent used, and whether the patient was receiving statin therapy (see Results in the online-only Data Supplement). The correlation persisted when the data from cases with multiple lesions were pooled and averaged in each patient (r=0.46; P=0.038; Figure 2B).
MR images used for assessing cerebral cavernous malformation (CCM) lesions. Left, An example of a quantitative susceptibility mapping (QSM) image of a sporadic CCM lesion shown as a bright area highlighted within a yellow box. A color-coded map of the lesion itself is shown to indicate the potential distribution of iron within the lesion in parts per million (ppm). Middle, An example of a permeability map of the same lesion generated by MATLAB with areas of high permeability and low permeability indicated according to the color scale to the right of the image with units in mL/100 g per minute. Right, An example of a T2 image with the same lesion highlighted within a yellow box.
Correlation of quantitative susceptibility mapping (QSM) and permeability in individual lesions and patients. A, Scatter plot showing a significantly positive correlation between QSM susceptibility and permeability for each lesion (r=0.49; P≤0.0001). B, Scatter plot showing a significantly positive correlation between QSM susceptibility and average permeability for each patient (r=0.46; P≤0.038).
There was a positive correlation between susceptibility and CBV in lesions (r=0.4; P≤0.0001) and no correlation between susceptibility and CBF in lesions (r=0.1; P=0.34; see Results in the online-only Data Supplement). Analyses using median values of susceptibility and permeability showed similar results (r=0.4; P=0.0001; data not shown).
Discussion
Vascular permeability is a hallmark of CCM disease, demonstrated as a result of loss of CCM gene expression in cultured endothelial cells and in the skin, lungs, and brain of CCM heterozygous mice.5,13 It has not been examined systematically in humans. Chronic iron deposition is also a cardinal feature of CCM lesions, demonstrated by imaging and histopathology. Here, we imaged human CCM lesions using 2 novel techniques examining dynamic permeability and the quantitative burden of iron deposit, respectively. The 2 techniques measure different and distinct features of the CCM lesion, yet these may be related biologically. We postulated and demonstrated that the more leaky CCM lesions also exhibited significantly greater mean susceptibility. This strong correlation was present, regardless of lesion volume, 2 different contrast agents, and whether the patient was using statin, a drug that may affect vascular permeability.14 The correlation was present mostly in familial cases, representing the vast majority of cases and lesions in our cohort. It will need to be examined in a larger group of sporadic cases. The correlation was present even when multiple lesions per case were averaged and analyzed by subject. Our results establish a proof of concept and help generate relevant hypotheses about the potential applicability of either technique to monitor CCM lesion behavior. The results may be interpreted using a transport model governed by mass conservation. Hence, permeability would reflect current ongoing rate of leaking, whereas susceptibility reflects the integral or historical accumulation of leaking. This and other hypotheses about QSM and DCEQP in CCM will need to be examined in prospective studies.
We noted a range of susceptibility and permeability (as well as CBF and CBV) among CCM lesions. The correlates of this variation will be examined in a larger cohort of cases, including the potential effect of age, lesion features, genotype, and prior clinical activity. Statin use and other therapies can potentially modulate permeability in CCM lesions, and their effects will need more systematic study. Future research will also address the potentially different implications of CBV, CBF, permeability, and susceptibility in lesions themselves and in background brain.
Conclusions
QSM and DCEQP are 2 unique imaging methods for quantitatively assessing CCM biological behavior. Strong correlation was observed between the 2 methods for measuring different end points of the same pathophysiological phenomena. This serves as proof of concept for these methods and the biological phenomena they measure. It also reflects their potential as biomarkers of CCM disease.
Sources of Funding
This work was supported, in part, by a Collaborative and Translational Studies Award through the Institute of Translational Medicine at the University of Chicago (UL1 TR000430) and by the Bill and Judy Davis Research Fund in Neurovascular Research.
Disclosures
None.
Footnotes
The online-only Data Supplement is available with this article at http://stroke.ahajournals.org/lookup/suppl/doi:10.1161/STROKEAHA.113.003548/-/DC1.
- Received September 16, 2013.
- Accepted October 10, 2013.
- © 2013 American Heart Association, Inc.
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