Use of metal-based contrast agents for in vivo MR and CT imaging of phagocytic cells in neurological pathologies.

The activation of phagocytic cells is a hallmark of many neurological diseases. Imaging them in their 3-dimensional cerebral environment over time is crucial to better understand their role in disease pathogenesis and to monitor their potential therapeutic effects. Phagocytic cells have the ability to internalize metal-based contrast agents both in vitro and in vivo and can thus be tracked by magnetic resonance imaging (MRI) or computed tomography (CT). In this review article, we summarize the different labelling strategies, contrast agents, and in vivo imaging modalities that can be used to monitor cells with phagocytic activity in the central nervous system using MRI and CT, with a focus on clinical applications. Metal-based nanoparticle contrast agents such as gadolinium, gold and iron are ideal candidates for these applications as they have favourable magnetic and/or radiopaque properties and can be fine-tuned for optimal uptake by phagocytic cells. However, they also come with downsides due to their potential toxicity, especially in the brain where they might accumulate. We therefore conclude our review by discussing the pitfalls, safety and potential for clinical translation of these metal-based neuroimaging techniques. Early results in patients with neuropathologies such as multiple sclerosis, stroke, trauma, cerebral aneurysm and glioblastoma are promising. If the challenges represented by safety issues are overcome, phagocytic cells imaging will be a very valuable tool for studying and understanding the inflammatory response and evaluating treatments that aim at mitigating this response in patients with neurological diseases.

Neurofunctional and neuroimaging readouts for designing a preclinical stem-cell therapy trial in experimental stroke.

With the aim of designing a preclinical study evaluating an intracerebral cell-based therapy for stroke, an observational study was performed in the rat suture model of ischemic stroke. Objectives were threefold: (i) to characterize neurofunctional and imaging readouts in the first weeks following transient ischemic stroke, according to lesion subtype (hypothalamic, striatal, corticostriatal); (ii) to confirm that intracerebral administration does not negatively impact these readouts; and (iii) to calculate sample sizes for a future therapeutic trial using these readouts as endpoints. Our results suggested that the most relevant endpoints were side bias (staircase test) and axial diffusivity (AD) (diffusion tensor imaging). Hypothalamic-only lesions did not affect those parameters, which were close to normal. Side bias in striatal lesions reached near-normal levels within 2 weeks, while rats with corticostriatal lesions remained impaired until week 14. AD values were decreased at 4 days and increased at 5 weeks post-surgery, with a subtype gradient: hypothalamic < striatal < corticostriatal. Intracerebral administration did not impact these readouts. After sample size calculation (18-147 rats per group according to the endpoint considered), we conclude that a therapeutic trial based on both readouts would be feasible only in the framework of a multicenter trial.

MRI coupled with clinically-applicable iron oxide nanoparticles reveals choroid plexus involvement in a murine model of neuroinflammation.

Choroid plexus (ChPs) are involved in the early inflammatory response that occurs in many brain disorders. However, the activation of immune cells within the ChPs in response to neuroinflammation is still largely unexplored in-vivo. There is therefore a crucial need for developing imaging tool that would allow the non-invasive monitoring of ChP involvement in these diseases. Magnetic resonance imaging (MRI) coupled with superparamagnetic particles of iron oxide (SPIO) is a minimally invasive technique allowing to track phagocytic cells in inflammatory diseases. Our aim was to investigate the potential of ultrasmall SPIO (USPIO)-enhanced MRI to monitor ChP involvement in-vivo in a mouse model of neuroinflammation obtained by intraperitoneal administration of lipopolysaccharide. Using high resolution MRI, we identified marked USPIO-related signal drops in the ChPs of animals with neuroinflammation compared to controls. We confirmed these results quantitatively using a 4-points grading system. Ex-vivo analysis confirmed USPIO accumulation within the ChP stroma and their uptake by immune cells. We validated the translational potential of our approach using the clinically-applicable USPIO Ferumoxytol. MR imaging of USPIO accumulation within the ChPs may serve as an imaging biomarker to study ChP involvement in neuroinflammatory disorders that could be applied in a straightforward way in clinical practice.

The 10th Biennial Hatter Cardiovascular Institute workshop: cellular protection-evaluating new directions in the setting of myocardial infarction, ischaemic stroke, and cardio-oncology.

Due to its poor capacity for regeneration, the heart is particularly sensitive to the loss of contractile cardiomyocytes. The onslaught of damage caused by ischaemia and reperfusion, occurring during an acute myocardial infarction and the subsequent reperfusion therapy, can wipe out upwards of a billion cardiomyocytes. A similar program of cell death can cause the irreversible loss of neurons in ischaemic stroke. Similar pathways of lethal cell injury can contribute to other pathologies such as left ventricular dysfunction and heart failure caused by cancer therapy. Consequently, strategies designed to protect the heart from lethal cell injury have the potential to be applicable across all three pathologies. The investigators meeting at the 10th Hatter Cardiovascular Institute workshop examined the parallels between ST-segment elevation myocardial infarction (STEMI), ischaemic stroke, and other pathologies that cause the loss of cardiomyocytes including cancer therapeutic cardiotoxicity. They examined the prospects for protection by remote ischaemic conditioning (RIC) in each scenario, and evaluated impasses and novel opportunities for cellular protection, with the future landscape for RIC in the clinical setting to be determined by the outcome of the large ERIC-PPCI/CONDI2 study. It was agreed that the way forward must include measures to improve experimental methodologies, such that they better reflect the clinical scenario and to judiciously select combinations of therapies targeting specific pathways of cellular death and injury.

Effects of a TAFI-inhibitor combined with a suboptimal dose of rtPA in a murine thromboembolic model of stroke.

BACKGROUND: Since thrombolysis is the only approved intervention for ischemic stroke, improving its efficacy and safety is a therapeutic aim of considerable interest. The activated form of thrombin activatable fibrinolysis inhibitor (TAFI) has antifibrinolytic effects, and inhibition of TAFI might thus favor recanalization. The present study compared efficacy between TAFI inhibition alone and TAFI inhibition in combination with rtPA at a suboptimal dose, in a murine model of thromboembolic stroke. METHODS: Focal ischemia was induced in mice by thrombin injection in the middle cerebral artery. Animals were placed within the magnet immediately after surgery for baseline MRI (H0). MRI examination comprised diffusion-weighted imaging (DWI), perfusion-weighted imaging (PWI), and T2-weighted imaging (T2-WI). Animals were randomly assigned to 1 of 5 treatment groups: saline, rtPA 5 mg/kg (tPA(5): suboptimal or low dose), rtPA 10 mg/kg (tPA(10): standard dose), TAFI-I 100 mg/kg (TAFI-I), and rtPA 5 mg/kg + TAFI-I 100 mg/kg (tPA(5) + TAFI-I). Treatments were administered inside the magnet, via a catheter placed in the tail vein, using a power injector, as 10% bolus and 90% infusion over a period of 20 min. MRI examination was repeated at 3 h (H3) and 24 h (H24) after surgery. Therapeutic benefit was evaluated by: (1) improvement of reperfusion and (2) reduction in final lesion size. Microhemorrhages were assessed as black spots on T2-WI at H24. Animals were sacrificed after the last MR examination. The surgeon and all investigators were blinded to treatment allocation. RESULTS: A total of 104 mice were operated on. Forty four of these were excluded from the study and 27 from the analysis, according to a priori defined criteria (no lesion or no mismatch), leading to the following distribution: saline (n = 6), tPA(5) (n = 8), tPA(10) (n = 7), TAFI-I (n = 7), and TAFI-I + tPA(5) (n = 5). Standard-dose rtPA treatment (tPA(10)) significantly improved lesion regression between H0 and H24 compared to saline (-57 ± 18% vs. -36 ± 21%, p = 0.03), which treatment with rtPA(5) or TAFI-I alone did not. On the other hand, combined treatment with tPA(5) + TAFI-I showed only a trend toward lesion regression (-49 ± 26%), similarly to treatment with tPA(10), but not significantly different from saline (p = 0.46). Nine animals showed microhemorrhage on T2-WI at H24. These animals were evenly distributed between groups. CONCLUSIONS: The present study showed that the combination of TAFI-I with a suboptimal dose of rtPA is not as effective as the standard dose of rtPA, while TAFI inhibition alone is not effective at all. The thromboembolic model is of particular interest in assessing rtPA association to improve thrombolysis, especially when coupled with longitudinal MRI assessment.

Information-based analysis of X-ray in-line phase tomography with application to the detection of iron oxide nanoparticles in the brain.

The study analyzes noise in X-ray in-line phase tomography in a biomedical context. The impact of noise on detection of iron oxide nanoparticles in mouse brain is assessed. The part of the noise due to the imaging system and the part due to biology are quantitatively expressed in a Neyman Pearson detection strategy with two models of noise. This represents a practical extension of previous work on noise in phase-contrast X-ray imaging which focused on the theoretical expression of the signal-to-noise ratio in mono-dimensional phantoms, taking account of the statistical noise of the imaging system only. We also report the impact of the phase retrieval step on detection performance. Taken together, this constitutes a general methodology of practical interest for quantitative extraction of information from X-ray in-line phase tomography, and is also relevant to assessment of contrast agents with a blob-like signature in high resolution imaging.

In vitro and in vivo models of cerebral ischemia show discrepancy in therapeutic effects of M2 macrophages.

THE INFLAMMATORY RESPONSE FOLLOWING ISCHEMIC STROKE IS DOMINATED BY INNATE IMMUNE CELLS: resident microglia and blood-derived macrophages. The ambivalent role of these cells in stroke outcome might be explained in part by the acquisition of distinct functional phenotypes: classically (M1) and alternatively activated (M2) macrophages. To shed light on the crosstalk between hypoxic neurons and macrophages, an in vitro model was set up in which bone marrow-derived macrophages were co-cultured with hippocampal slices subjected to oxygen and glucose deprivation. The results showed that macrophages provided potent protection against neuron cell loss through a paracrine mechanism, and that they expressed M2-type alternative polarization. These findings raised the possibility of using bone marrow-derived M2 macrophages in cellular therapy for stroke. Therefore, 2 million M2 macrophages (or vehicle) were intravenously administered during the subacute stage of ischemia (D4) in a model of transient middle cerebral artery occlusion. Functional neuroscores and magnetic resonance imaging endpoints (infarct volumes, blood-brain barrier integrity, phagocytic activity assessed by iron oxide uptake) were longitudinally monitored for 2 weeks. This cell-based treatment did not significantly improve any outcome measure compared with vehicle, suggesting that this strategy is not relevant to stroke therapy.

Monitoring therapeutic effects in experimental stroke by serial USPIO-enhanced MRI.

OBJECTIVES: This study sought to evaluate whether the therapeutic effects of an anti-inflammatory drug such as minocycline could be monitored by serial ultrasmall superparamagnetic particles of iron oxide (USPIO)-enhanced MRI in experimental stroke. METHODS: Mice received a three-dose minocycline treatment (n = 12) or vehicle (n = 12) after permanent middle cerebral artery occlusion. USPIOs were administered 5 h post-surgery. MRI was performed before, 24 h and 48 h post-USPIO administration. MRI endpoints were the extent of signal abnormalities on R2 maps (=1/T2) and quantitative R2 changes over time (∆R2). Post-mortem brains were prepared either for immunohistology (n = 16) or for iron dosage (n = 8). RESULTS: As expected, treatment with minocycline significantly reduced infarct size, blood-brain barrier permeability and F4/80 immunostaining for microglia/macrophages. Areas of R2 maps > 35 ms(-1) also appeared significantly decreased in minocycline-treated mice (ANOVA for repeated measures, P = 0.017). There was a fair correlation between these areas and the amount of iron in the brain (R(2) = 0.69, P = 0.010), but no significant difference in ∆R2 was found between the two groups. CONCLUSIONS: This study showed that the extent of signal abnormalities on R2 maps can be used as a surrogate marker to detect minocycline effects in a murine experimental model of stroke.

MRI assessment of the intra-carotid route for macrophage delivery after transient cerebral ischemia.

The broad aim underlying the present research was to investigate the distribution and homing of bone marrow-derived macrophages in a rodent model of transient middle cerebral artery occlusion using MRI and ultrasmall superparamagnetic iron oxide (USPIO) to magnetically label bone marrow-derived macrophages. The specific aim was to assess the intra-carotid infusion route for bone marrow-derived macrophage delivery at reperfusion. Fifteen Sprague-Dawley rats sustained 1 h of middle cerebral artery occlusion. USPIO-labeled bone marrow-derived macrophages were slowly injected for 5 min immediately after reperfusion in ischemic animals (n=7), 1 h after the end of surgery in sham animals (n=5) and very shortly after anesthesia in healthy animals (n=3). Multiparametric MRI was performed at day 0, just after cell administration, and repeated at day 1. Immunohistological analysis included Prussian blue for iron detection and rat endothelial cell antigen-1 for endothelium visualization. Intra-carotid cell delivery brought a large number of cells to the ipsilateral hemisphere of the brain, as seen on both MRI and immunohistology. However, it was associated with high mortality (50%). The study of sham animals demonstrated that intra-carotid cell delivery could induce ischemic lesions and may thus favor additional brain damage. The present study highlights severe drawbacks to the intra-carotid delivery of macrophages at the time of reperfusion in this rodent model of transient cerebral ischemia. Multiparametric MRI appears to be a method of choice to monitor longitudinally the effects of cell infusion, allowing the assessment of both cell fate with the help of magnetic labeling and of potential tissue damage.

Pre- and post-treatment with cyclosporine A in a rat model of transient focal cerebral ischaemia with multimodal MRI screening.

BACKGROUND: Irreversible damage may occur at reperfusion after sustained cerebral ischaemia. AIMS: We investigated the value of cyclosporine A for reducing the infarct size in a model of transient middle cerebral artery occlusion. METHODS: Twenty-seven Sprague-Dawley rats sustained a middle cerebral artery occlusion of one-hour. Acute multimodal Magnetic Resonance Imaging (MRI) was used during occlusion to confirm the success of surgery and measure baseline lesion size. Animals were randomly treated by: (i) intracarotid cyclosporine A (10 mg/kg) 20 mins before middle cerebral artery occlusion (pretreatment group); (ii) intracarotid cyclosporine A (10 mg/kg) immediately after reperfusion (post-treatment group); and (iii) intracarotid saline immediately after reperfusion. RESULTS: Histopathological measurements on day 1 showed a significant reduction of infarct size in the pretreatment group compared to the post-treatment (percentage values of ipsilateral hemispheres: 16 ± 5% vs. 29 ± 11%, P = 0·004) and saline groups (16 ± 5% vs. 42 ± 12%, P = 0·015). No significant difference was observed between the post-treatment and saline groups (P = 0·065). Behavioural examinations on day 1 showed no significant difference between groups. Immunohistochemistry showed a statistically significant reduction of microglial cell count in the pretreatment group compared to either saline or cyclosporine A post-treatment groups. CONCLUSIONS: We conclude that intracarotid cyclosporine A is effective in reducing infarct size when given prior to ischaemia, but not when administered at reperfusion.

Does acute behavioral testing reflect successful ischemia in rats with transient middle cerebral artery occlusion?

BACKGROUND: Models of intraluminal middle cerebral artery occlusion present an intrinsic variability in infarct size. Behavioral evaluation is frequently performed during arterial occlusion to confirm success of surgery. AIMS AND/OR HYPOTHESIS: We compared the value of behavioral testing and multimodal magnetic resonance imaging performed during arterial occlusion for identifying successfully operated animals. METHODS: Rats were tested with behavioral assessment (using three scoring scales and the adhesive removal test) and multimodal magnetic resonance imaging (including magnetic resonance angiography, diffusion-weighted and perfusion-weighted imaging), both performed during the two-hours of middle cerebral artery occlusion using the intraluminal suture model. Behavioral assessment was repeated 24 h after reperfusion, followed by sacrifice. RESULTS: Acute apparent diffusion coefficient lesion volume was correlated with both 2,3,5-triphenyl tetrazolium chloride infarct size (r = 0·75, P = 0·02) and behavioral status (r = 0·66, P = 0·05) on day one. Conversely, no correlation was found between acute behavioral examination and day one outcomes (2,3,5-triphenyl tetrazolium chloride infarct volume, r = 0·40, P = 0·28; behavioral examination, r = 0·39, P = 0·30). Day zero apparent diffusion coefficient volumes (P = 0·04), but not behavioral assessment (P = 0·60), discriminated animals with day one corticostriatal infarcts from these with subcortical infarcts. CONCLUSIONS: Acute behavioral testing performed during arterial occlusion fails to identify successfully operated animals. Acute diffusion magnetic resonance imaging may be more appropriate to assess and reduce infarct size variability in this model.

Quantification of iron-labeled cells with positive contrast in mouse brains.

PURPOSE: To quantify small amounts of iron-labeled cells in mouse brains with magnetic resonance imaging (MRI). PROCEDURES: Iron-labeled cells (from 500 to 7,500) were stereotaxically transplanted into the brain of living mice that were subsequently imaged with MRI at 4.7 T. We compared four quantitative methods: (1) T2 relaxometry, (2) T2* relaxometry, (3) the volume of the cloverleaf hypointense artifact generated on T2*-weighted images, and (4) the volume of the cloverleaf hyperintense artifact generated on positive contrast images. RESULTS: The methods based on relaxometry, whether T2 or T2*, did not correlate with the number of injected cells. By contrast, those based on measurement of cloverleaf artifact volume, whether using negative or positive enhancement, showed a significant linear relationship for the given range of cells (R [0.92-0.95], p < 0.05). CONCLUSIONS: T2* artifact volume imaging (negative or positive) appears promising for the quantification of magnetically labeled cells following focal injection in the brain.

In vivo MRI assessment of permanent middle cerebral artery occlusion by electrocoagulation: pitfalls of procedure.

Permanent middle cerebral artery (MCA) occlusion (pMCAO) by electrocoagulation is a commonly used model but with potential traumatic lesions. Early MRI monitoring may assess pMCAO for non-specific brain damage. The surgical steps of pMCAO were evaluated for traumatic cerebral injury in 22 Swiss mice using diffusion and T2-weighted MRI (7T) performed within 1 h and 24 h after surgery. Temporal muscle cauterization without MCA occlusion produced an early T2 hyperintensity mimicking an infarct. No lesion was visible after temporal muscle incision or craniotomy. Early MRI monitoring is useful to identify non-specific brain injury that could hamper neuroprotective drugs assessment.

Early-stage investigations of ultrasmall superparamagnetic iron oxide-induced signal change after permanent middle cerebral artery occlusion in mice.

BACKGROUND AND PURPOSE: MR signal changes after intravenous ultrasmall superparamagnetic iron oxide (USPIO) injection are related to inflammatory cells at the subacute stages after focal cerebral injury. However, at the early stages, the interpretation of USPIO-related MR signal alterations remains controversial. Here, we compared MR signal changes after intravenous USPIO injection with the histological iron and macrophage distribution during the first 24 hours in a rodent model of acute stroke. METHODS: Multiparametric MRI at 7T and histological USPIO distribution were confronted from 6 to 24 hours after permanent middle cerebral artery occlusion in mice. Blood-brain barrier disruption was assessed using gadolinium MRI and immunoglobulin staining. Prussian blue staining was performed to depict the USPIO brain distribution. USPIO uptake by phagocytes was assessed by immunochemistry on brain tissue, peripheral blood cells, and monocyte cells derived from bone marrow culture. RESULTS: After USPIO injection, 4 areas of early signal change were observed on every MRI. In all these areas, iron particles were mostly free whether detected in the vascular and cerebrospinal fluid compartments or in the interstitium. Within the first 24 hours, USPIO-loaded cells were not detected in the blood of injured mice or in cultured monocytic cells incubated with USPIO at plasmatic concentration. CONCLUSIONS: These results suggest that, in this model, early reproducible USPIO-related MR signal changes are mainly caused by passive diffusion of free USPIO after blood-brain barrier leakage and by intravascular trapping rather than by peripheral phagocyte infiltration.

Characterization of hepatocellular carcinoma and colorectal liver metastasis by means of perfusion MRI.

PURPOSE: To characterize and compare hepatocellular carcinoma and liver metastases of colorectal metastatic cancer (CMC) by means of quantitative liver perfusion MRI. MATERIALS AND METHODS: Liver perfusion was assessed in 26 HCC and CMC patients (50 nodules) by means of contrast-enhanced MRI. Six perfusion parameters-hepatic perfusion index (HPI), mean transit time (MTT), distribution volume (DV), total blood flow (F(T)), arterial blood flow (F(A)), and portal blood flow (F(P))-were calculated in tumor nodules and the adjacent hepatic parenchyma. RESULTS: The values of F(T), F(A), F(P), and DV were significantly higher in the HCC than in the CMC group, whereas MTT was significantly higher in the CMC group. There was no significant difference in HPI. Arterial blood flow was higher than portal blood flow in the CMC group, while portal blood flow was slightly higher than arterial blood flow in the HCC group. CONCLUSION: The present work describes the use of dynamic MRI to quantitatively assess liver perfusion, which in the future may help studying liver cancers on the basis of their microvascular characteristics.

Magnetic resonance imaging (MRI) of inflammation in stroke.

Magnetic resonance imaging (MRI) of inflammation is based on the in vivo magnetic labelling of macrophages, the most abundant cells involved in the post-ischemic inflammatory response, by nanoparticles of iron oxides. Such approach has been successfully applied to study experimental rodent models of focal cerebral ischemia and has proved feasible in pioneer clinical studies. Despite current limitations, MRI of inflammation may become an important tool for the investigation of novel ischemic stroke therapeutics targeted at inflammation.

Influence of perfusion on high-intensity focused ultrasound prostate ablation: a first-pass MRI study.

Our aim was to evaluate the influence of regional prostate blood flow (rPBF) on high-intensity focused ultrasound (HIFU) treatment outcome. A total of 48 patients with clinically localized prostate cancer were examined by dynamic contrast-enhanced (DCE)-MRI prior to HIFU therapy. A prostate-specific antigen (PSA) nadir threshold of 0.2 ng/ml was used to define the populations of responders and nonresponders. A dedicated tracer kinetic model, namely "monoexponential plus constant" (MPC) deconvolution, was implemented to provide quantitative estimates of rPBF. The results were compared with those obtained by semiquantitative (steepest slope, mean gradient) and quantitative (Fermi deconvolution) approaches. Of the four methods studied, quantitative rPBF obtained by MPC deconvolution proved the most sensitive to the perfusion changes encountered in this study. Furthermore, blood-flow values obtained with MPC deconvolution in the prostate and muscle (12 +/- 8 and 5 +/- 3 ml/min/100 g, respectively) were in good agreement with literature data. The mean pretreatment rPBF obtained with MPC deconvolution was significantly higher in nonresponders compared to responders (16 +/- 9 vs. 10 +/- 6 ml/min/100 g), suggesting a correlation between baseline perfusion and treatment outcome. The present work describes and validates the use of dynamic MRI to estimate rPBF in patients, which in the future may help to refine the conduct of HIFU therapy.

MRI monitoring of neuroinflammation in mouse focal ischemia.

BACKGROUND AND PURPOSE: A growing body of evidence suggests that inflammatory processes are involved in the pathophysiology of stroke. Phagocyte cells, involving resident microglia and infiltrating macrophages, secrete both protective and toxic molecules and thus represent a potential therapeutic target. The aim of the present study was to monitor phagocytic activity after focal cerebral ischemia in mice. METHODS: Ultrasmall superparamagnetic particles of iron oxide (USPIO) were intravenously injected after permanent middle cerebral artery occlusion and monitored by high resolution MRI for 72 hours. RESULTS: We here present the first MRI data showing in vivo phagocyte-labeling obtained in mice with focal cerebral ischemia. USPIO-enhanced MRI kinetic analysis disclosed an inflammatory response surrounding the ischemic lesion and in the contralateral hemisphere via the corpus callosum. The imaging data collected during the first 36 hours postinjury suggested a spread of USPIO-related signal from ipsi- to contralateral hemisphere. Imaging data correlated with histochemical analysis showing inflammation remote from the lesion and ingestion of nanoparticles by microglia/macrophages. CONCLUSIONS: The present study shows that MR-tracking of phagocyte cells is feasible in mice, which may have critical therapeutic implications given the potential neurotoxicity of activated microglia/macrophages in central nervous system disorders.

Magnetic resonance imaging detects early changes in microvascular permeability in xenograft tumors after treatment with the matrix metalloprotease inhibitor Prinomastat.

Macromolecular contrast medium-enhanced magnetic resonance imaging was applied to monitor the effect of matrix metalloprotease (MMP) inhibition on microvascular characteristics of human breast cancers implanted in athymic rats. Twice-daily intraperitoneal administration of Prinomastat over 1.5 days induced significant declines in MRI-assayed microvascular permeabilities (p<0.05); but this leak suppression effect had extinguished by the 10(th) day of MMP treatment using the same dose and time schedule. Results demonstrate that Prinomastat produces a rapid but transient decrease in tumor vascular permeability. Contrast-enhanced MRI using macromolecular contrast medium may prove useful as a biomarker for the dynamic MMP biological effect in cancers.