Magnetic resonance imaging assessment of a convective therapy delivery paradigm in a canine prostate model

PURPOSE: To quantitatively investigate the feasibility of MRI as a tool for assessing the spatial distribution of a convectively delivered agent using a canine prostate model. MATERIALS AND METHODS: Canine prostates (ex vivo, n = 3; in vivo, n = 12) were injected under several injection paradigms with a solution of gadolinium-DTPA for MR contrast and methylene blue as a grossly visible surrogate drug marker. Ex vivo and in vivo distributions were assessed at 1.5T and quantitatively compared. RESULTS: Measured distributions using MRI and methylene blue pathology photographs were analyzed using a Bland-Altman method. The fractional percentage volume covered (V frac) compared the measurements grossly: Pearson's correlation coefficients were R = 0.99 for ex vivo and R = 0.77 for in vivo (P < 0.05). The fractional percentage of area covered (A frac) demonstrated the high degree of spatial correlation between individual slices: R = 0.93 for ex vivo and R = 0.98 for in vivo (P < 0.05). There was no statistically observable bias in scale or offset between the measurements. CONCLUSION: Measured distributions using MRI and pathology were highly correlated and unbiased, indicating the potential of MRI as a tool for quantitative assessment of interstitial delivery of injected therapies in vivo.     

Serial in vivo positive contrast MRI of iron oxide-labeled embryonic stem cell-derived cardiac precursor cells in a mouse model of myocardial infarction

Myocardial regeneration with stem-cell transplantation is a possible treatment option to reverse deleterious effects that occur after myocardial infarction. Since little is known about stem cell survival after transplantation, developing techniques for "tracking" cells would be desirable. Iron-oxide-labeled stem cells have been used for in vivo tracking using MRI but produce negative contrast images that are difficult to interpret. The aim of the current study was to test a positive contrast MR technique using reduced z-gradient rephasing (GRASP) to aid in dynamically tracking stem cells in an in vivo model of mouse myocardial infraction. Ferumoxides and protamine sulfate were complexed and used to magnetically label embryonic stem cell-derived cardiac-precursor-cells (ES-CPCs). A total of 500,000 ES-CPCs were injected in the border zone of infarcted mice and MR imaging was performed on a 9.4T scanner using T(2)*-GRE sequences (negative contrast) and positive contrast GRASP technique before, 24 hours, and 1 week after ES-CPC implantation. Following imaging, mice were sacrificed for histology and Perl's staining was used to confirm iron within myocardium. Good correlation was observed between signal loss seen on conventional T(2)* images, bright areas on GRASP, and the presence of iron on histology. This demonstrated the feasibility of in vivo stem cell imaging with positive contrast MRI.     

Quantitative magnetic resonance imaging of estradiol-induced pituitary hyperplasia in rats

Magnetic resonance imaging (MRI) has been used for the determination in vivo of rat pituitary size. In midsagittal T2-weighted sections the pituitary, having a lower T2 value than the surrounding tissue, was visible with pronounced contrast. The size has been estimated by pixel counting. A close correlation (r = 0.96) with the pituitary weights determined postmortem has been obtained, demonstrating the reliability of the in vivo method. Using MRI the effects of Sandostatin, a somatostatin analog, on the pituitary size have been monitored in a rat model of prolactinoma (estradiol-induced hyperplasia of the pituitary). Treatment with Sandostatin over 4 weeks resulted in a 40% reduction of the hyperplastic pituitaries. These results have been confirmed by determination of pituitary weights postmortem. However, due to a large interindividual variation in size of hyperplastic pituitaries, more animals are required to reach statistical significance when only endpoints of treatment can be measured. In contrast, MRI allows one to monitor individually the drug effects over a long period of time, eliminating interindividual variations.     

Magnetic resonance imaging of trabecular and cortical bone in mice: comparison of high resolution in vivo and ex vivo MR images with corresponding histology

Measurements of bone morphometry and remodeling have been shown to reflect bone strength and can be used to diagnose degenerative bone disease. In this study, in vivo and ex vivo magnetic resonance imaging (MRI) techniques to assess trabecular and cortical bone properties have been compared to each other and to histology as a novel means for the quantification of bone. Femurs of C57Bl/6 mice were examined both in vivo and ex vivo on an 11.7 T MRI scanner, followed by histologic processing and morphometry. A thresholding analysis technique was applied to the MRI images to generate contour lines and to delineate the boundaries between bone and marrow. Using MRI, an optimal correlation with histology was obtained with an in vivo longitudinal sectioned short echo time gradient-echo versus an in vivo long echo time spin-echo sequence or an ex vivo pulse sequence. Gradient-echo images were acquired with a maximum in-plane resolution of 35 microm. Our results demonstrated that in both the in vivo and ex vivo data sets, the percent area of marrow increases and percent area of trabecular bone and cortical bone thickness decreases moving from the epiphyseal growth plate to the diaphysis. These changes, observed with MRI, correlate with the histological data. Investigations using in vivo MRI gradient-echo sequences consistently gave the best correlation with histology. Our quantitative evaluation using both ex vivo and in vivo MRI was found to be an effective means to visualize non-invasively the normal variation in trabecular and cortical bone as compared to a histological "gold standard" The experiments validated in vivo MRI as a potential high resolution technique for investigating both soft tissue, such as marrow, and bone without radiation exposure.     

Investigation of relationships between transverse relaxation rate,diffusion coefficient,and labeled cell concentration in ischemic rat brain using MRI

MRI has been used to evaluate labeled cell migration and distribution. However, quantitative determination of labeled cell concentration using MRI has not been systematically investigated. In the current study, we investigated the relationships between labeled cell concentration and MRI parameters of transverse relaxation rate, R₂, and apparent diffusion coefficient (ADC), in vitro in phantoms and in vivo in rats after stroke. Significant correlations were detected between iron concentration or labeled cell concentration and MRI measurements of R₂, ADC, and ADC×R₂ in vitro. In contrast, in vivo labeled cell concentration did not significantly correlate with R₂, ADC, and ADC×R₂. A major factor for the absence of a significant correlation between labeled cell concentration and MRI measurements in vivo may be attributed to background effects of ischemic tissue. By correcting the background effects caused by ischemic damage, ΔR₂ (difference in R₂ values in the ischemic tissue with and without labeled cells) exhibited a significant correlation to labeled cell concentration. Our study suggests that MRI parameters have the potential to quantitatively determine labeled cell concentration in vivo. Magn Reson Med, 2009. © 2008 Wiley‐Liss, Inc.     

Functional contrast based on intermolecular double-quantum coherences: influence of the correlation distance.

A functional MRI (fMRI) study with visual stimulation of healthy subjects was performed at 3T exploiting intermolecular double-quantum coherences. The correlation distance, d(c), was varied between 60 and 300 microm for different evolution times, tau. Robust activation was obtained in all experiments with average signal changes (DeltaS(2)) = 8.4 +/- 0.7% and 9.4 +/- 0.8% for tau = 15 and 20 ms, respectively) exceeding those normally associated with conventional blood oxygen level-dependent (BOLD) fMRI. Relaxation-rate changes (DeltaR(2) = 0.33 +/- 0.36 s(-1) and DeltaR(2)* = 0.77 +/- 0.54 s(-1)) were similar to those commonly obtained for the extravascular BOLD effect. The number of activated voxels increased with increasing d(c) until a plateau was reached at approximately equal 120 microm. Similar trends were observed for the activation-induced percent signal change and for the maximal Z-scores. These effects were quantitatively explained by a reduced sensitivity at short d(c) due to increasing signal attenuation related to diffusion and an increasing amount of signal fluctuations in the fMRI time series due to imperfect suppression of unwanted coherence pathways. Consistent indications of a preferential selection of susceptibility changes in blood vessels of a particular size were not obtained.     

Evaluation of multicontrast MRI including fat suppression and inversion recovery spin echo for identification of intra-plaque hemorrhage and lipid core in human carotid plaque using the mahalanobis distance measure

Intra-plaque hemorrhage (IPH) and lipid core, characteristics of rupture prone carotid plaques, are often visualized in vivo with MRI using T1 weighted gradient and spin echo, respectively. Increasing magnetic field strength may help to identify IPH and lipid core better. As a proof of concept, automatic segmentation of plaque components was performed with the Mahalanobis distance (MD) measure derived from image contrast from multicontrast MR images including inversion recovery spin echo and T1 weighted gradient echo with fat suppression. After MRI of nine formaldehyde-fixated autopsy specimens, the MDs and Euclidean Distances between plaque component intensities were calculated for each MR weighting. The distances from the carotid bifurcation and the size and shape of calcification spots were used as landmarks for coregistration of MRI and histology. MD between collagen/cell-rich area and IPH was largest with inversion recovery spin echo (4.2/9.3, respectively), between collagen/cell-rich area/foam cells and lipid core with T1 weighted gradient echo with fat suppression (26.9/38.2/4.6, respectively). The accuracy of detection of IPH, cell-rich area, and collagen increased when the MD classifier was used compared with the Euclidean Distance classifier. The enhanced conspicuity of lipid core and IPH in human carotid artery plaque, using ex vivo T1 weighted gradient echo with fat suppression and inversion recovery spin echo MRI and MD classifiers, demands further in vivo evaluation in patients.     

Accuracy and uniqueness of three in vivo measurements of atherosclerotic carotid plaque morphology with black blood MRI.

High-resolution MRI provides unique information about morphology of atherosclerotic carotid plaque. In this study, the accuracy and precision of measurements of carotid plaque burden and lumen narrowing were determined for in vivo black blood MRI assessment with respect to ex vivo MRI in a group of 37 atherosclerosis patients who underwent carotid endarterectomy (CEA). Three different plaque measures were compared between paired in vivo and ex vivo MR images: maximum wall area (MWA), minimum lumen area (mLA), and wall volume (WV). MWA and WV are measures of plaque burden, while mLA is a measure of lumen narrowing. The matched in vivo and ex vivo measurements showed good agreement (the correlation coefficients for in/ex vivo WV, MWA, and mLA were 0.92, 0.91, 0.90, respectively) with predictable bias. This study indicates that in vivo black blood MRI can be used to directly estimate the morphology of the plaque. Comparison of the three plaque measures showed that mLA and MWA or WV provide different information regarding the atherosclerotic lesions (the correlation coefficients between mLA and MWA or WV were less than 0.3). Black blood MRI technique is a potentially powerful clinical tool to characterize the severity of atherosclerotic plaque. It can provide accurate measurements on different aspects of the plaque, from plaque burden to lumen narrowing.     

Does loss of gray- and white-matter contrast in injured spinal cord signify secondary injury? In vivo longitudinal MRI studies.

Longitudinal in vivo magnetic resonance imaging (MRI) of moderately injured rat spinal cord was performed to investigate the transient loss in gray matter (GM) and white matter (WM) contrast. In addition, neurobehavioral scores (based on open field walking, inclined plane test, and grid walking) and tissue cholesterol concentrations at pre-determined time points were measured. On MRI, a loss in contrast between GM and WM in sections 2-4 mm caudal to the site of injury was consistently observed 2 days after injury. A substantial recovery in the contrast was observed within 1 week after injury. A strong correlation between the loss of GM-WM contrast and the cholesterol concentration was also observed. The neurobehavioral scores improved with the return of the GM-WM contrast. These serial MRI studies indicate that the loss of contrast in the cord is mainly due to edema accumulated in the WM. The resolution of edema seems to occur around the same time as the restoration of cholesterol in the cord tissue. It is speculated that the compromised membrane stability due to the loss of cholesterol may be one of the factors leading to the accumulation of edema in WM.     

Magnetic resonance imaging-guided diskography and diagnostic lumbar 0.23T MRI: an assessment study

Purpose: To describe the in vivo appearance of magnetic resonance imaging (MRI) diskograms of normal and degenerated lumbar intervertebral disks, and to evaluate the differences in imaging findings between sequential diagnostic MRI and MRI diskography.

Material and Methods: Nine consecutive patients underwent MRI-guided diskography in order to determine possible pain provocation during puncture and contrast medium injection. All patients had preceding clinical suspicion of lumbar diskogenic pain and findings of lumbar disk degeneration in diagnostic (MRI, computed tomography (CT), plain radiography). A 0.23T open MRI scanner with interventional tools was used for imaging and instrument guidance. On all patients, a complementary diagnostic MRI study of the lumbar spine before and after the MRI-guided disk injection was performed, and subsequent axial MRI diskograms were obtained.

Results: A total of 25 disk punctures were initialized, and 25 MRI diskograms were obtained and their expression described. There was a correlation between the degenerative disk findings visualized by diagnostic MRI and MRI diskograms.

Conclusion: The use of gadolinium contrast media in MRI-guided diskography enables the evaluation of MRI diskograms. Our results suggest that MRI-guided diskography can be used to substitute conventional diskography or CT-diskography and as an augmenting method to assess diagnostic information upon degenerative processes of the lumbar spine.     

Microcoil-based MR phase imaging and manganese enhanced microscopy of glial tumor neurospheres with direct optical correlation

Susceptibility differences among tissues were recently used for highlighting complementary contrast in MRI different from the conventional T(1), T(2), or spin density contrasts. This method, based on the signal phase, previously showed improved image contrast of human or rodent neuroarchitecture in vivo, although direct MR phase imaging of cellular architecture was not available until recently. In this study, we present for the first time the ability of microcoil-based phase MRI to resolve the structure of human glioma neurospheres at significantly improved resolutions (10 × 10 μm(2)) with direct optical image correlation. The manganese chloride property to function as a T(1) contrast agent enabled a closer examination of cell physiology with MRI. Specifically the temporal changes of manganese chloride uptake, retention and release time within and from individual clusters were assessed. The optimal manganese chloride concentration for improved MR signal enhancement was determined while keeping the cellular viability unaffected. The presented results demonstrate the possibilities to reveal structural and functional observation of living glioblastoma human-derived cells. This was achieved through the combination of highly sensitive microcoils, high magnetic field, and methods designed to maximize contrast to noise ratio. The presented approach may provide a powerful multimodal tool that merges structural and functional information of submilimeter biological samples.     

Statistical framework and noise sensitivity of the amplitude radial correlation contrast method.

A statistical framework for the amplitude radial correlation contrast (RCC) method, which integrates a conventional pixel threshold approach with cluster-size statistics, is presented. The RCC method uses functional MRI (fMRI) data to group neighboring voxels in terms of their degree of temporal cross correlation and compares coherences in different brain states (e.g., stimulation OFF vs. ON). By defining the RCC correlation map as the difference between two RCC images, the map distribution of two OFF states is shown to be normal, enabling the definition of the pixel cutoff. The empirical cluster-size null distribution obtained after the application of the pixel cutoff is used to define a cluster-size cutoff that allows 5% false positives. Assuming that the fMRI signal equals the task-induced response plus noise, an analytical expression of amplitude-RCC dependency on noise is obtained and used to define the pixel threshold. In vivo and ex vivo data obtained during rat forepaw electric stimulation are used to fine-tune this threshold. Calculating the spatial coherences within in vivo and ex vivo images shows enhanced coherence in the in vivo data, but no dependency on the anesthesia method, magnetic field strength, or depth of anesthesia, strengthening the generality of the proposed cutoffs.     

Detection and volume determination of colonic tumors in Min mice by magnetic resonance micro-imaging.

We applied MRI to the in vivo detection of spontaneous colorectal tumors in a unique mouse model, the Fox Chase Cancer Center (FCCC) ApcMIN mouse. Unlike other Min (multiple intestinal neoplasia) strains, FCCC ApcMIN animals develop an appreciable number of tumors in the large intestine, which makes them an appropriate mouse model for colon cancer in humans. We describe a method for marking the colon on MRI data sets that involves a bowel-cleansing procedure and the insertion of a polyurethane tube (filled with an MRI contrast agent) fully into the colon. We found that tumors as small as 1.5 mm in diameter can be consistently identified from MRI datasets with a voxel size of 0.1 mm x 0.133 mm x 0.133 mm. Tumor volumes were determined from the MRM data sets with the use of a novel approach to planimetry in 3D data sets. We observed a correlation between tumor volume (as measured from the MRI datasets) and tumor weight of 0.942, and a P-value of 0.008, based on Spearman's test. These data show that MRI can be used to accurately monitor tumor growth in mouse models of colorectal carcinogenesis.     

Differential uptake of ferumoxtran-10 and ferumoxytol, ultrasmall superparamagnetic iron oxide contrast agents in rabbit: critical determinants of atherosclerotic plaque labeling

PURPOSE: To compare atherosclerotic plaque uptake of a first (ferumoxtran-10) and second generation (ferumoxytol) ultrasmall superparamagnetic iron oxide (USPIO) contrast agent with different pharmacokinetic/pharmacodynamic properties. MATERIALS AND METHODS: New Zealand White rabbits maintained on a high cholesterol/fat diet were subjected to balloon injury to the abdominal aorta. Ferumoxtran-10 or ferumoxytol (500 micromol/kg) was administered at 2, 4, and 8 weeks following injury. In vivo magnetic resonance imaging (MRI) was performed immediately prior to, immediately after, and 6 days post-contrast administration. Ex vivo MRI, histologic, and inductively coupled plasma-mass spectrometry (ICP-MS) iron analyses were performed on the excised vessels. RESULTS: The blood pool clearance of ferumoxytol (t(1/2) < or = 6 hours) was more rapid than that of ferumoxtran-10 (t(1/2) < or = 48 hours). Decreased in vivo MRI signal intensity in the abdominal aorta was observed at 2, 4, and 8 weeks following injury with ferumoxtran-10, but not with ferumoxytol. Consistent with these observations, ex vivo MRI signal intensity was decreased in the ferumoxtran-10 vessels, and to a lesser degree in the ferumoxytol vs. control vessels (- contrast agent). In contrast, in vitro macrophage phagocytosis of USPIO was four to six fold greater with ferumoxytol than with ferumoxtran-10. Additionally, the absolute iron content correlated with ex vivo MRI signal intensity in all vessels (r = -0.86, P < 0.0001). CONCLUSIONS: These data suggest that the exposure period of atherosclerotic plaque to USPIO rather than the kinetics of the USPIO uptake by plaque alone is a critical criterion for experimental design of in vivo studies.     

Labeling fibroblasts with biotin-BSA-GdDTPA-FAM for tracking of tumor-associated stroma by fluorescence and MR imaging.

Fibroblasts at the tumor-host interface can differentiate into myofibroblasts and pericytes, and contribute to the guidance and stabilization of endothelial sprouts. After intravenous administration of biotin-BSA-GdDTPA-FAM in mice with subcutaneous MLS human ovarian carcinoma tumors, the distribution of the macromolecular MRI/optical contrast material was confined to blood vessels in normal tissues, while it co-registered with alphaSMA-positive stroma tracks within the tumor. These alphaSMA-positive tumor-associated myofibroblasts and pericytes showed uptake of the contrast material into intracellular granules. We evaluated the use of this contrast material for in vitro labeling of tumor fibroblasts as an approach for tracking their involvement in angiogenesis. Fluorescence microscopy demonstrated internalization of the contrast material, and MRI revealed a significant increase in the R(1) relaxation rate of labeled fibroblasts. R(1) not only remained elevated for 2 weeks in culture, it also increased with cell proliferation, indicating prolonged retention of the contrast material and subsequent intracellular processing and redistribution of the material, and thereby enhancing MR contrast. Moreover, cells that were labeled ex vivo with MR contrast material and co-inoculated with tumor cells in mice were detected in vivo by MRI. Uptake of the contrast material was suppressed by nystatin, suggesting internalization by caveolae-mediated endocytosis. This study shows that labeling of fibroblasts with biotin-BSA-GdDTPA-FAM is feasible and would allow noninvasive in vivo tracking of fibroblasts during tumor angiogenesis and vessel maturation.     

In vivo detection of single cells by MRI.

The use of high-relaxivity, intracellular contrast agents has enabled MRI monitoring of cell migration through and homing to various tissues, such as brain, spinal cord, heart, and muscle. Here it is shown that MRI can detect single cells in vivo, homing to tissue, following cell labeling and transplantation. Primary mouse hepatocytes were double-labeled with green fluorescent 1.63-microm iron oxide particles and red fluorescent endosomal labeling dye, and injected into the spleens of recipient mice. This is a common hepatocyte transplantation paradigm in rodents whereby hepatocytes migrate from the spleen to the liver as single cells. One month later the animals underwent in vivo MRI and punctuated, dark contrast regions were detected scattered through the livers. MRI of perfused, fixed samples and labeled hepatocyte phantoms in combination with histological evaluation confirmed the presence of dispersed single hepatocytes grafted into the livers. Appropriate controls were used to determine whether the observed contrast could have been due to dead cells or free particles, and the results confirmed that the contrast was due to disperse, single cells. Detecting single cells in vivo opens the door to a number of experiments, such as monitoring rare cellular events, assessing the kinetics of stem cell homing, and achieving early detection of metastases.     

Fast multislice pH‐weighted chemical exchange saturation transfer (CEST) MRI with Unevenly segmented RF irradiation

Chemical exchange saturation transfer (CEST) MRI is a versatile imaging technique for measuring microenvironment properties via dilute CEST labile groups. Conventionally, CEST MRI is implemented with a long radiofrequency irradiation module, followed by fast image acquisition to obtain the steady state CEST contrast. Nevertheless, the sensitivity, scan time, and spatial coverage of the conventional CEST MRI method may not be optimal. Our study proposed a segmented radiofrequency labeling scheme that includes a long primary radiofrequency irradiation module to generate the steady state CEST contrast and repetitive short secondary radiofrequency irradiation module immediately after the image acquisition so as to maintain the steady state CEST contrast for multislice acquisition and signal averaging. The proposed CEST MRI method was validated experimentally with a tissue‐like pH phantom and optimized for the maximal contrast‐to‐noise ratio. In addition, the proposed sequence was evaluated for imaging ischemic acidosis via pH‐weighted endogenous amide proton transfer MRI, which showed similar contrast as conventional amide proton transfer MRI. In sum, a fast multislice relaxation self‐compensated CEST MRI sequence was developed, with significantly improved sensitivity and suitable for in vivo applications. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

Preliminary in vivo validation of a dedicated breast MRI and sonographic coregistration imaging system

OBJECTIVE: Sonographic correlation of breast MRI findings is often challenging. We present a preliminary in vivo feasibility study evaluating the degree of error of a new MRI-sonography coregistration system for showing MRI and sonographically visible breast lesions. CONCLUSION: In 10 patients with 13 lesions, the system was found to be an accurate means for targeting sonography to MRI of the same breast lesions.     

脑星形细胞瘤的MRI征象与MMP-9表达相关性的研究

目的 探讨星形细胞瘤MRI征象与基质金属蛋白酶-9（matrix metalloproteinase,MMP-9）表达的相关性。资料与方法 采用免疫组织化学（S-P）法,用抗MMP-9抗体对40例星形细胞瘤组织标本进行标记分析,并测定MRI图像上肿瘤信号的均匀度、水肿程度及强化程度。结果 星形细胞瘤MRI征象与MMP-9的表达明显相关（P〈0．01）,而且随着肿瘤恶性程度的增加,MMP-9的表达也增高（P〈0．01）。结论 星形细胞瘤的MRI征象与MMP-9的表达密切相关。可以间接地反映星形细胞瘤的某些分子生物学特性。     

Measurements of T(1) -relaxation in ex vivo prostate tissue at 132 μT

The proton T(1) was measured at 132 μT in ex vivo prostate tissue specimens from radical prostatectomies of 35 patients with prostate cancer. Each patient provided two specimens. The NMR and MRI measurements involved proton repolarization, a field of typically 150 mT and detection of the 5.6-kHz signal with a superconducting quantum interference device. Values of T(1) varied from 41 to 86 ms. Subsequently, the percentages of tissue types were determined histologically. The theoretical image contrast is quantified for each case by δ = [1 - T(1) (more cancer)/T(1) (less cancer)]. A linear fit of δ versus difference in percentage cancer yields T(1) (100% cancer)/T(1) (0% cancer) = 0.70 ± 0.05 with correlation coefficient R(2) = 0.30. Two-dimensional T(1) maps for four specimens demonstrate variation within a single specimen. These results suggest that MR images with T(1) contrast established at ultra-low fields may discriminate prostate cancer from normal prostate tissue in vivo without a contrast agent.