Christine Walczak

Passive staining: A novel ex vivo MRI protocol to detect amyloid deposits in mouse models of Alzheimer's disease

Amyloid plaques are one of the hallmarks of Alzheimers disease (AD). This study evaluated a novel μMRI strategy based on “passive staining” of brain samples by gadoteric acid. The protocol was tested at 4.7T on control animals and APP/PS1 mice modeling AD lesions. T1 was strongly decreased in passively stained brains. On high‐resolution 3D gradient echo images, the contrast between the cortex and subcortical structures was highly improved due to a T2* effect. The brains of APP/PS1 mice revealed plaques as hypointense spots. They appeared larger in long compared to short TE images. This suggests that, after passive staining, plaques caused a susceptibility effect. This easily performed protocol is a complementary method to classic histology to detect the 3D location of plaques. It may also be used for the validation of in vivo MRI protocols for plaque detection by facilitating registration with histology via post mortem MRI. Magn Reson Med, 2006.

Bolus‐tracking MRI with a simultaneous T1‐ and T 2*‐measurement

The aim of this study was to propose and evaluate a methodology to analyze simultaneously acquired T  2* ‐weighted dynamic susceptibility contrast (DSC) MRI and T1‐weighted dynamic contrast enhanced (DCE) MRI data. Two generalized models of T  2* ‐relaxation are proposed to account for tracer leakage, and a two‐compartment exchange model is used to separate tracer in intra‐ and extravascular spaces. The methods are evaluated using data extracted from ROIs in three mice with subcutaneously implanted human colorectal tumors. Comparing plasma flow values obtained from DCE‐MRI and DSC‐MRI data defines a practical experimental paradigm to measure T  2* ‐relaxivities, and reveals a factor of 15 between values in tissue and blood. Comparing mean transit time values obtained from DCE‐MRI and DSC‐MRI without leakage correction, indicates a significant reduction of susceptibility weighting in DSC‐MRI during tracer leakage. A one‐parameter gradient correction model provides a good approximation for this susceptibility loss, but redundancy of the parameter limits the practical potential of this model for DSC‐MRI. Susceptibility loss is modeled more accurately with a variable T  2* ‐relaxivity, which allows to extract new parameters that cannot be derived from DSC‐MRI or DCE‐MRI alone. They reflect the cellular and vessel geometry, and thus may lead to a more complete characterization of tissue structure. Magn Reson Med, 2009.

Morphological and carbogen-based functional MRI of a chemically induced liver tumor model in mice

A multifocal mouse liver tumor model chemically induced with 5,9‐dimethyl‐7H‐dibenzo[c,g]carbazole was investigated by respiratory‐triggered morphological and functional MRI (fMRI) at 4.7 Tesla. The model is characterized by the presence of two tumor types: hypovascular cholangioma and vascularized hepatocellular carcinoma (HCC). Growth curves measured by 3D‐MRI showed limited growth of cholangiomas and rapid growth of HCCs after a latency of about 25 weeks. Functional imaging based on T  2* ‐weighted fast gradient‐echo MRI and carbogen breathing was optimized for liver imaging in mice. A response to carbogen was observed in HCCs but not in cholangiomas. Transversal analysis (50 HCCs) of signal change upon carbogen revealed four different types of response patterns: 1) signal increase upon carbogen administration (74%); 2) small or insignificant signal change (10%), 3) transient signal decrease and delayed increase (8%), and 4) signal decrease (8%). Longitudinal follow‐up of a subgroup (N = 17) showed that an initially observed type 1 response, attesting to the presence of a functional vasculature, remained stable for at least 3 weeks in 14 HCCs. A switch from a type 1 response to another response type may be useful for demonstrating, in a noninvasive manner, a disturbance of tumor vasculature induced by anti‐vascular or anti‐angiogenic therapy. Magn Reson Med 50:522–530, 2003.

Relationship between tumour growth rate and carbogen-based functional MRI for a chemically induced HCC in mice

We previously performed MRI studies of HCC (hepatocellular carcinomas) in mice showing the feasibility of measuring a carbogen effect. In the present study carbogen response of the whole tumour was compared with growth characteristics during longitudinal follow-up. HCC were chemically induced. The imaging protocol at 4.7 T comprised a fast spin-echo sequence for high-resolution screening and measurement of growth curves, and a fast gradient echo sequence allowing an entire T2*w image acquisition per respiratory cycle to perform fMRI under carbogen breathing. A new parameter, T+, the fraction of tumour voxels with increased intensity under carbogen was measured on manually defined ROIs. Twenty-two HCC were followed for 3–10 weeks. Tumours were divided into two groups, “regularly” and “irregularly” growing tumours. A linear correlation between T+ and tumour growth rate was observed only for “regularly” growing HCC. These results suggest a link between tumour growth rates and tumour fractions exhibiting signal increase upon carbogen breathing. They are compatible with observations by others that rapidly growing tumours are more hypoxic than slowly growing ones. Combined measurement of T+ and tumour growth may become a useful noninvasive follow-up approach for assessment and/or management of therapies involving vasculature-targeting and anti-proliferative drugs.

2D and 3D radial multi‐gradient‐echo DCE MRI in murine tumor models with dynamic R*2‐corrected R1 mapping

Dynamic contrast‐enhanced MRI is extensively studied to define and evaluate biomarkers for early assessment of vasculature‐targeting therapies. In this study, two‐dimensional and three‐dimensional radial multi‐gradient‐echo techniques for dynamic R*2‐corrected R1 mapping based on the spoiled gradient recalled signal equation were implemented and validated at 4.7 T. The techniques were evaluated on phantoms and on a respiratory motion animated tumor model. R1 measurements were validated with respect to a standard inversion‐recovery spin‐echo sequence in a four‐compartment phantom covering a range of relaxation rates typically found in tumor tissue. In the range of [0.4, 3] sec−1, R1 differences were less than 10% for both two‐dimensional and three‐dimensional experiments. A dynamic contrast‐enhanced MRI pilot study was performed on a colorectal tumor model subcutaneously implanted in mice at the abdominal level. Low motion sensitivity of radial acquisition allowed image recording without respiratory triggering. Three‐dimensional Ktrans maps and significantly different mean Ktrans values were obtained for two contrast agents with different molecular weights. The radial multi‐gradient‐echo approach should be most useful for preclinical experimental conditions where the tissue of interest experiences physiologic motion, like spontaneous extracerebral tumors developed by transgenic mice, and where dynamic contrast‐enhanced MRI is performed with high‐relaxivity contrast agents. Magn Reson Med, 2010.

T2*-relaxivity contrast imaging: First results

In this study, T2*‐ relaxivity contrast imaging (RCI) is proposed for new contrast generation in MRI. The method produces images of relaxivities r*2,vasc and r*2,EES caused by susceptibility gradients across the vessel walls and cell membranes, respectively. The sensitivity to noise was assessed with a simulation study, and initial results are presented for five colorectal tumor xenografts in nude mice. Simulations show that the new relaxivity parameters are at least as accurate and precise as standard parameters such as plasma volume and interstitial volume. Mean values of both relaxivities were significantly different (r*2,vasc = 10.9 ± 2.9 mM−1 s−1 and r*2,EES = 15.6 ± 2.6 mM−1 s−1). r*2,vasc (r = 0.67) and r*2,EES (r = 0.52) were weakly correlated with plasma volume and interstitial volume, respectively. Images of r*2,vasc and r*2,EES reveal a different tumor structure than plasma volume and interstitial volume maps. These results suggest that relaxivity contrast imaging is practically feasible and might offer supplementary information compared to dynamic contrast‐enhanced‐MRI. Magn Reson Med, 2013.

Radial multigradient-echo DCE-MRI for 3D Ktrans mapping with individual arterial input function measurement in mouse tumor models

The purpose of this study was to provide proof of concept for a new three‐dimensional (3D) radial dynamic contrast enhanced MRI acquisition technique, called “Radial Entire Tumor with Individual Arterial input function dynamic contrast‐enhanced MRI” (RETIA dynamic contrast‐enhanced MRI), which allows for the simultaneous measurement of an arterial input function in the mouse heart at 2 s temporal resolution and coverage of the whole tumor. Alternating 2D and 3D projections contribute to the 2D heart image or 3D tumor data with a 3‐cm field of view. Sixty‐four 2D images of the heart are obtained during acquisition of each 3D tumor dataset. In a pilot study, global Ktrans and ve values were measured in four mice, in a respiratory motion‐animated subcutaneously implanted breast tumor model. This technique is expected to be most useful for the characterization of microvasculature in motion‐animated orthotopic tumors. Magn Reson Med 70:823–828, 2013.

NRL and CRX Define Photoreceptor Identity and Reveal Subgroup-Specific Dependencies in Medulloblastoma

Cancer cells often express differentiation programs unrelated to their tissue of origin, although the contribution of these aberrant phenotypes to malignancy is poorly understood. An aggressive subgroup of medulloblastoma, a malignant pediatric brain tumor of the cerebellum, expresses a photoreceptor differentiation program normally expressed in the retina. We establish that two photoreceptor-specific transcription factors, NRL and CRX, are master regulators of this program and are required for tumor maintenance in this subgroup. Beyond photoreceptor lineage genes, we identify BCL-XL as a key transcriptional target of NRL and provide evidence substantiating anti-BCL therapy as a rational treatment opportunity for select MB patients. Our results highlight the utility of studying aberrant differentiation programs in cancer and their potential as selective therapeutic vulnerabilities.

Tumor tissue analysis by self organizing maps from combined DCE-/DSC-MRI data

Self organizing maps are utilized to analyze tumor tissue from simultaneously acquired dynamic T1 and T2* magnetic resonance imaging measurements of five tumor-bearing mice. The method allowed for tumor segmention obtaining regions characterized by distinct perfusion patterns (i.e. T1 and T2* perfusion time curves). Compared to histopathological analysis, these regions comprise typical tumor areas like pre-necrotic or vascularized tissue. Furthermore, the detected regions showed differences in physiological parameters (Ktrans, ve) extracted by a pharmacokinetic model. In summary, tumor tissue analysis by self organizing maps is feasible and seems to be a valuable tool in model-free assessment of tumor physiology.

Amyloid deposits imaging by passive staining in mouse models of Alzheimer’s disease

A 1-42). Conclusions: Compact neuritic A plaques were abundant in the frontal cortex and absent in the cerebellum. 6-CN-BTA-1 bound to blood vessels and A 1-16/tau dual-labeled compact neuritic plaques in the frontal cortex; binding was not detected in the cerebellum. 6-CN-BTA-1 was more readily detected in A 1-40-immunoreactive (ir) plaques than in A 1-42-ir plaques. Diffuse non-neuritic A deposits were devoid of 6-CN-BTA-1 staining. Pretreatment of tissue sections with formic acid completely abolished 6-CN-BTA-1 staining. Our results demonstrate that 6-CN-BTA-1 labels preferentially compact neuritic plaques in AD brain, suggesting its affinity for binding to aggregated A . These data aid in interpreting the binding properties of the parent compound of 6-CN-BTA-1, PIB, which is currently under evaluation as an in vivo diagnostic marker of diseasespecific A plaque pathology progression in AD brains.