UTE–ΔR2–ΔR2* combined MR whole‐brain angiogram using dual‐contrast superparamagnetic iron oxide nanoparticles

The ability to visualize whole‐brain vasculature is important for quantitative in vivo investigation of vascular malfunctions in cerebral small vessel diseases, including cancer, stroke and neurodegeneration. Transverse relaxation‐based ΔR₂ and ΔR₂* MR angiography (MRA) provides improved vessel–tissue contrast in animal deep brain with the aid of intravascular contrast agents; however, it is susceptible to orientation dependence, air–tissue interface artifacts and vessel size overestimation. Dual‐mode MRA acquisition with superparamagnetic iron oxide nanoparticles (SPION) provides a unique opportunity to systematically compare and synergistically combine both longitudinal (R₁) and transverse (ΔR₂ and ΔR₂*) relaxation‐based MRA. Through Monte Carlo (MC) simulation and MRA experiments in normal and tumor‐bearing animals with intravascular SPION, we show that ultrashort TE (UTE) MRA acquires well‐defined vascularization on the brain surface, minimizing air–tissue artifacts, and combined ΔR₂ and ΔR₂* MRA simultaneously improves the sensitivity to intracortical penetrating vessels and reduces vessel size overestimation. Consequently, UTE–ΔR₂–ΔR₂* combined MRA complements the shortcomings of individual angiograms and provides a strategy to synergistically merge longitudinal and transverse relaxation effects to generate more robust in vivo whole‐brain micro‐MRA. Copyright © 2016 John Wiley & Sons, Ltd.     

Simultaneous multi‐slice spin‐ and gradient‐echo dynamic susceptibility‐contrast perfusion‐weighted MRI of gliomas

Although combined spin‐ and gradient‐echo (SAGE) dynamic susceptibility‐contrast (DSC) MRI can provide perfusion quantification that is sensitive to both macrovessels and microvessels while correcting for T₁‐shortening effects, spatial coverage is often limited in order to maintain a high temporal resolution for DSC quantification. In this work, we combined a SAGE echo‐planar imaging (EPI) sequence with simultaneous multi‐slice (SMS) excitation and blipped controlled aliasing in parallel imaging (blipped CAIPI) at 3 T to achieve both high temporal resolution and whole brain coverage. Two protocols using this sequence with multi‐band (MB) acceleration factors of 2 and 3 were evaluated in 20 patients with treated gliomas to determine the optimal scan parameters for clinical use. ΔR₂*(t) and ΔR₂(t) curves were derived to calculate dynamic signal‐to‐noise ratio (dSNR), ΔR₂*‐ and ΔR₂‐based relative cerebral blood volume (rCBV), and mean vessel diameter (mVD) for each voxel. The resulting SAGE DSC images acquired using MB acceleration of 3 versus 2 appeared visually similar in terms of image distortion and contrast. The difference in the mean dSNR from normal‐appearing white matter (NAWM) and that in the mean dSNR between NAWM and normal‐appearing gray matter were not statistically significant between the two protocols. ΔR₂*‐ and ΔR₂‐rCBV maps and mVD maps provided unique contrast and spatial heterogeneity within tumors.     

Using microbubbles as an MRI contrast agent for the measurement of cerebral blood volume

The susceptibility differences at the gas–liquid interface of microbubbles (MBs) allow their use as an intravascular susceptibility contrast agent for in vivo MRI. However, the characteristics of MBs are very different from those of the standard gadolinium‐diethylenetriaminepentaacetic acid (Gd‐DPTA) contrast agent, including the size distribution and hemodynamic properties, which could influence MRI outcomes. Here, we investigate quantitatively the correlation between the relative cerebral blood volume (rCBV) derived from Gd‐DTPA (rCBV_Gd) and the MB‐induced susceptibility effect (ΔR₂*_MB) by conventional dynamic susceptibility contrast MRI (DSC‐MRI). Custom‐made MBs had a mean diameter of 0.92 µm and were capable of inducing 4.68 ± 3.02% of the maximum signal change (MSC). The MB‐associated ΔR₂*_MB was compared with rCBV_Gd in 16 rats on 4.7‐T MRI. We observed a significant effect of the time to peak (TTP) on the correlation between ΔR₂*_MB and rCBV_Gd, and also found a noticeable dependence between TTP and MSC. Our findings suggest that MBs with longer TTPs can be used for the estimation of rCBV by DSC‐MRI, and emphasize the critical effect of TTP on MB‐based contrast MRI. Copyright © 2013 John Wiley & Sons, Ltd.     

Technical considerations on the validity of blood oxygenation level‐dependent‐based MR assessment of vascular deoxygenation

A blood oxygenation level‐dependent (BOLD)‐based apparent relative oxygen extraction fraction (rOEF) as a semi‐quantitative marker of vascular deoxygenation has recently been introduced in clinical studies of patients with glioma and stroke, yielding promising results. These rOEF measurements are based on independent quantification of the transverse relaxation times T₂ and T₂* and relative cerebral blood volume (rCBV). Simulations demonstrate that small errors in any of the underlying measures may result in a large deviation of the calculated rOEF. Therefore, we investigated the validity of such measurements. For this, we evaluated the quantitative measurements of T₂ and T₂* at 3 T in a gel phantom, in healthy subjects and in healthy tissue of patients with brain tumors. We calculated rOEF maps covering large portions of the brain from T₂, T₂* and rCBV [routinely measured in patients using dynamic susceptibility contrast (DSC)], and obtained rOEF values of 0.63 ± 0.16 and 0.90 ± 0.21 in healthy‐appearing gray matter (GM) and white matter (WM), respectively; values of about 0.4 are usually reported. Quantitative T₂ mapping using the fast, clinically feasible, multi‐echo gradient spin echo (GRASE) approach yields significantly higher values than much slower multiple single spin echo (SE) experiments. Although T₂* mapping is reliable in magnetically homogeneous tissues, uncorrectable macroscopic background gradients and other effects (e.g. iron deposition) shorten T₂*. Cerebral blood volume (CBV) measurement using DSC and normalization to WM yields robust estimates of rCBV in healthy‐appearing brain tissue; absolute quantification of the venous fraction of CBV, however, is difficult to achieve. Our study demonstrates that quantitative measurements of rOEF are currently biased by inherent difficulties in T₂ and CBV quantification, but also by inadequacies of the underlying model. We argue, however, that standardized, reproducible measurements of apparent T₂, T₂* and rCBV may still allow the estimation of a meaningful apparent rOEF, which requires further validation in clinical studies. Copyright © 2014 John Wiley & Sons, Ltd.     

Dynamic oxygen challenge evaluated by NMR T1 and T2* – insights into tumor oxygenation

There is intense interest in developing non‐invasive prognostic biomarkers of tumor response to therapy, particularly with regard to hypoxia. It has been suggested that oxygen sensitive MRI, notably blood oxygen level‐dependent (BOLD) and tissue oxygen level‐dependent (TOLD) contrast, may provide relevant measurements. This study examined the feasibility of interleaved T₂*‐ and T₁‐weighted oxygen sensitive MRI, as well as R₂* and R₁ maps, of rat tumors to assess the relative sensitivity to changes in oxygenation. Investigations used cohorts of Dunning prostate R3327‐AT1 and R3327‐HI tumors, which are reported to exhibit distinct size‐dependent levels of hypoxia and response to hyperoxic gas breathing. Proton MRI R₁ and R₂* maps were obtained for tumors of anesthetized rats (isoflurane/air) at 4.7 T. Then, interleaved gradient echo T₂*‐ and T₁‐weighted images were acquired during air breathing and a 10 min challenge with carbogen (95% O₂–5% CO₂). Signals were stable during air breathing, and each type of tumor showed a distinct signal response to carbogen. T₂* (BOLD) response preceded T₁ (TOLD) responses, as expected. Smaller HI tumors (reported to be well oxygenated) showed the largest BOLD and TOLD responses. Larger AT1 tumors (reported to be hypoxic and resist modulation by gas breathing) showed the smallest response. There was a strong correlation between BOLD and TOLD signal responses, but ΔR₂^* and ΔR₁ were only correlated for the HI tumors. The magnitude of BOLD and TOLD signal responses to carbogen breathing reflected expected hypoxic fractions and oxygen dynamics, suggesting potential value of this test as a prognostic biomarker of tumor hypoxia. Copyright © 2015 John Wiley & Sons, Ltd.     

Measurements of cerebral blood volume using quantitative susceptibility mapping,R2* relaxometry,and ferumoxytol‐enhanced MRI

Ferumoxytol‐enhanced MRI holds potential for the non‐invasive assessment of vascular architecture using estimates of cerebral blood volume (CBV). Ferumoxytol specifically enables steady‐state imaging with extended acquisition times, for substantial improvements in resolution and contrast‐to‐noise ratio. With such data, quantitative susceptibility mapping (QSM) can be used to obtain images of local tissue magnetic susceptibility and hence estimate the increase in blood susceptibility after administration of a contrast agent, which in turn can be correlated to tissue CBV. Here, we explore the use of QSM for CBV estimation and compare it with R₂* (1/T₂*)‐based results. Institutional review board approval was obtained, and all subjects provided written informed consent. For this prospective study, MR images were acquired on a 3.0 T scanner in 19 healthy subjects using a multiple‐echo T₂*‐weighted sequence. Scanning was performed before and after the administration of two doses of ferumoxytol (1 mg FE/kg and 4 mg FE/kg). Different QSM approaches were tested on numerical phantom simulations. Results showed that the accuracy of magnetic susceptibility measurements improved with increasing image resolution and decreasing vascular density. In vivo changes in magnetic susceptibility were measured after the administration of ferumoxytol utilizing QSM, and significantly higher QSM‐based CBV was measured in gray matter compared with white matter. QSM‐ and R₂*‐based CBV estimates correlated well, with similar average values, but a larger variance was found in QSM‐based estimates.     

Changes in the MR relaxation rate R(2)* induced by respiratory challenges at 3.0 T: a comparison of two quantification methods

The consistent determination of changes in the transverse relaxation rate R₂* (ΔR₂*) is essential for the mapping of the effect of hyperoxic and hypercapnic respiratory challenges, which enables the noninvasive assessment of blood oxygenation changes and vasoreactivity by MRI. The purpose of this study was to compare the performance of two different methods of ΔR₂* quantification from dynamic multigradient‐echo data: (A) subtraction of R₂* values calculated from monoexponential decay functions; and (B) computation of ΔR₂* echo‐wise from signal intensity ratios. A group of healthy volunteers (n = 12) was investigated at 3.0 T, and the brain tissue response to carbogen and CO₂–air inhalation was registered using a dynamic multigradient‐echo sequence with high temporal and spatial resolution. Results of the ΔR₂* quantification obtained by the two methods were compared with respect to the quality of the voxel‐wise ΔR₂* response, the number of responding voxels and the behaviour of the ‘global’ response of all voxels with significant R₂* changes. For the two ΔR₂* quantification methods, we found no differences in the temporal variation of the voxel‐wise ΔR₂* responses or in the detection sensitivity. The maximum change in the ‘global’ response was slightly smaller when ΔR₂* was derived from signal intensity ratios. In conclusion, this first methodological comparison shows that both ΔR₂* quantifications, from monoexponential approximation as well as from signal intensity ratios, are applicable for the monitoring of R₂* changes during respiratory challenges. Copyright © 2010 John Wiley & Sons, Ltd.     

Difference in the intratumoral distributions of extracellular‐fluid and intravascular MR contrast agents in glioblastoma growth

Contrast enhancement by an extracellular‐fluid contrast agent (CA) (Gd‐DOTA) depends primarily on the blood–brain‐barrier permeability (bp ), and transverse‐relaxation change caused by intravascular T₂ CA (superparamagnetic iron oxide nanoparticles, SPIONs) is closely associated with the blood volume (BV). Pharmacokinetic (PK) vascular characterization based on single‐CA‐using dynamic contrast‐enhanced MRI (DCE‐MRI) has shown significant measurement variation according to the molecular size of the CA. Based on this recognition, this study used a dual injection of Gd‐DOTA and SPIONs for tracing the changes of bp and BV in C6 glioma growth (Days 1 and 7 after the tumor volume reached 2 mL). bp was quantified according to the non‐PK parameters of Gd‐DOTA‐using DCE‐MRI (wash‐in rate, maximum enhancement ratio and initial area under the enhancement curve (IAUC)). BV was estimated by SPION‐induced ΔR₂* and ΔR₂. With validated measurement reliability of all the parameters (coefficients of variation ≤10%), dual‐contrast MRI demonstrated a different region‐oriented distribution between Gd‐DOTA and SPIONs within a tumor as follows: (a) the BV increased stepwise from the tumor center to the periphery; (b) the tumor periphery maintained the augmented BV to support continuous tumor expansion from Day 1 to Day 7; (c) the internal tumor area underwent significant vascular shrinkage (i.e. decreased ΔR₂ and ΔR₂) as the tumor increased in size; (d) the tumor center showed greater bp ‐indicating parameters, i.e. wash‐in rate, maximum enhancement ratio and IAUC, than the periphery on both Days 1 and 7 and (e) the tumor center showed a greater increase of bp than the tumor periphery in tumor growth, as suggested to support tumor viability when there is insufficient blood supply. In the MRI–histologic correlation, a prominent BV increase in the tumor periphery seen in MRI was verified with increased fluorescein isothiocyanate–dextran signals and up‐regulated immunoreactivity of CD31–VEGFR. In conclusion, the spatiotemporal alterations of BV and bp in glioblastoma growth, i.e. augmented BV in the tumor periphery and increased bp in the center, can be sufficiently evaluated by MRI with dual injection of extracellular‐fluid Gd chelates and intravascular SPION.     

Investigating intracranial tumour growth patterns with multiparametric MRI incorporating Gd‐DTPA and USPIO‐enhanced imaging

High grade and metastatic brain tumours exhibit considerable spatial variations in proliferation, angiogenesis, invasion, necrosis and oedema. Vascular heterogeneity arising from vascular co‐option in regions of invasive growth (in which the blood–brain barrier remains intact) and neoangiogenesis is a major challenge faced in the assessment of brain tumours by conventional MRI. A multiparametric MRI approach, incorporating native measurements and both Gd‐DTPA (Magnevist) and ultrasmall superparamagnetic iron oxide (P904)‐enhanced imaging, was used in combination with histogram and unsupervised cluster analysis using a k‐means algorithm to examine the spatial distribution of vascular parameters, water diffusion characteristics and invasion in intracranially propagated rat RG2 gliomas and human MDA‐MB‐231 LM2–4 breast adenocarcinomas in mice. Both tumour models presented with higher ΔR₁ (the change in transverse relaxation rate R₁ induced by Gd‐DTPA), fractional blood volume (fBV) and apparent diffusion coefficient than uninvolved regions of the brain. MDA‐MB‐231 LM2–4 tumours were less densely cellular than RG2 tumours and exhibited substantial local invasion, associated with oedema, whereas invasion in RG2 tumours was minimal. These additional features were reflected in the more heterogeneous appearance of MDA‐MB‐231 LM2–4 tumours on T₂‐weighted images and maps of functional MRI parameters. Unsupervised cluster analysis separated subregions with distinct functional properties; areas with a low fBV and relatively impermeable blood vessels (low ΔR₁) were predominantly located at the tumour margins, regions of MDA‐MB‐231 LM2–4 tumours with relatively high levels of water diffusion and low vascular permeability and/or fBV corresponded to histologically identified regions of invasion and oedema, and areas of mismatch between vascular permeability and blood volume were identified. We demonstrate that dual contrast MRI and evaluation of tissue diffusion properties, coupled with cluster analysis, allows for the assessment of heterogeneity within invasive brain tumours and the designation of functionally diverse subregions that may provide more informative predictive biomarkers.     

Measurement of parenchymal extravascular R2* and tissue oxygen extraction fraction using multi‐echo vascular space occupancy MRI at 7 T

Parenchymal extravascular R₂* is an important parameter for quantitative blood oxygenation level‐dependent (BOLD) studies. Total and intravascular R₂* values and changes in R₂* values during functional stimulations have been reported in a number of studies. The purpose of this study was to measure absolute extravascular R₂* values in human visual cortex and to estimate the intra‐ and extravascular contributions to the BOLD effect at 7 T. Vascular space occupancy (VASO) MRI was employed to separate out the extravascular tissue signal. Multi‐echo VASO and BOLD functional MRI (fMRI) with visual stimulation were performed at 7 T for R₂* measurement at a spatial resolution of 2.5 × 2.5 × 2.5 mm³ in healthy volunteers (n = 6). The ratio of changes in extravascular and total R₂* (ΔR₂*) was used to estimate the extravascular fraction of the BOLD effect. Extravascular R₂* values were found to be 44.66 ± 1.55 and 43.38 ± 1.51 s^–1 (mean ± standard error of the mean, n = 6) at rest and activation, respectively, in human visual cortex at 7 T. The extravascular BOLD fraction was estimated to be 91 ± 3%. The parenchymal oxygen extraction fraction (OEF) during activation was estimated to be 0.24 ± 0.01 based on the R₂* measurements, indicating an approximately 37% decrease compared with OEF at rest. Copyright © 2014 John Wiley & Sons, Ltd.     

Simultaneous measurement of T1/B1 and pharmacokinetic model parameters using active contrast encoding (ACE)‐MRI

The aim of this study was to assess the feasibility of combining dynamic contrast enhanced‐magnetic resonance imaging (DCE‐MRI) with the measurement of the radiofrequency (RF) transmit field B ₁ and pre‐contrast longitudinal relaxation time T ₁₀. A novel approach has been proposed to simultaneously estimate B ₁ and T ₁₀ from a modified DCE‐MRI scan that actively encodes the washout phase of the curve with different amounts of T ₁ and B ₁ weighting using multiple flip angles and repetition times, hence referred to as active contrast encoding (ACE)‐MRI. ACE‐MRI aims to simultaneously measure B ₁ and T ₁₀, together with contrast kinetic parameters, such as the transfer constant K ^trans, interstitial space volume fraction v _e and vascular space volume fraction v _p. The proposed method was tested using numerical simulations and in vivo studies with mouse models of breast cancer implanted in the flank and mammary fat pad, and glioma in the brain. In the numerical simulation study with a signal‐to‐noise ratio of 10, both B ₁ and T ₁₀ were estimated accurately with errors of 5.1 ± 3.5% and 12.3 ± 8.8% and coefficients of variation (CV) of 14.9 ± 8.6% and 15.0 ± 5.0%, respectively. Using the same ACE‐MRI data, the kinetic parameters K ^trans, v _e and v _p were also estimated with errors of 14.2 ± 8.3% (CV = 13.5 ± 4.6%), 14.7 ± 9.9% (CV = 13.3 ± 4.5%) and 14.0 ± 9.3% (CV = 14.0 ± 4.5%), respectively. For the in vivo tumor data from 11 mice, voxel‐wise comparisons between ACE‐MRI and DCE‐MRI methods showed that the mean differences for the five parameters were as follows: ΔK ^trans = 0.006 (/min), Δv _e = 0.016, Δv _p = 0.000, ΔB ₁ = ?0.014 and ΔT ₁ = ?0.085 (s), which suggests a good agreement between the two methods. When compared with separately measured B ₁ and T ₁₀, and DCE‐MRI estimated kinetic parameters as a reference, the mean relative errors of ACE‐MRI estimation were B ₁ = ?0.3%, T ₁₀ = ?8.5%, K ^trans = 11.4%, v _e = 14.5% and v _p = 4.5%. This proof‐of‐concept study demonstrates that the proposed ACE‐MRI method can be used to estimate B ₁ and T ₁₀, together with contrast kinetic model parameters.     

Assessing manganese efflux using SEA0400 and cardiac T1‐mapping manganese‐enhanced MRI in a murine model

The sodium–calcium exchanger (NCX) is one of the transporters contributing to the control of intracellular calcium (Ca²⁺) concentration by normally mediating net Ca²⁺ efflux. However, the reverse mode of the NCX can cause intracellular Ca²⁺ concentration overload, which exacerbates the myocardial tissue injury resulting from ischemia. Although the NCX inhibitor SEA0400 has been shown to therapeutically reduce myocardial injury, no in vivo technique exists to monitor intracellular Ca²⁺ fluctuations produced by this drug. Cardiac manganese‐enhanced MRI (MEMRI) may indirectly assess Ca²⁺ efflux by estimating changes in manganese (Mn²⁺) content in vivo, since Mn²⁺ has been suggested as a surrogate marker for Ca²⁺. This study used the MEMRI technique to examine the temporal features of cardiac Mn²⁺ efflux by implementing a T₁‐mapping method and inhibiting the NCX with SEA0400. The change in ¹H₂O longitudinal relaxation rate, ΔR₁, in the left ventricular free wall, was calculated at different time points following infusion of 190 nmol/g manganese chloride (MnCl₂) in healthy adult male mice. The results showed 50% MEMRI signal attenuation at 3.4 ± 0.6 h post‐MnCl₂ infusion without drug intervention. Furthermore, treatment with 50 ± 0.2 mg/kg of SEA0400 significantly reduced the rate of decrease in ΔR₁. At 4.9–5.9 h post‐MnCl₂ infusion, the average ΔR₁ values for the two groups treated with SEA0400 were 2.46 ± 0.29 and 1.72 ± 0.24 s^?1 for 50 and 20 mg/kg doses, respectively, as compared to the value of 1.27 ± 0.28 s^?1 for the control group. When this in vivo data were compared to ex vivo absolute manganese content data, the MEMRI T₁‐mapping technique was shown to effectively quantify Mn²⁺ efflux rates in the myocardium. Therefore, combining an NCX inhibitor with MEMRI may be a useful technique for assessing Mn²⁺ transport mechanisms and rates in vivo, which may reflect changes in Ca²⁺ transport. Copyright © 2009 John Wiley & Sons, Ltd.     

Pseudo‐extravasation rate constant of dynamic susceptibility contrast‐MRI determined from pharmacokinetic first principles

Dynamic susceptibility contrast‐magnetic resonance imaging (DSC‐MRI) is widely used to obtain informative perfusion imaging biomarkers, such as the relative cerebral blood volume (rCBV). The related post‐processing software packages for DSC‐MRI are available from major MRI instrument manufacturers and third‐party vendors. One unique aspect of DSC‐MRI with low‐molecular‐weight gadolinium (Gd)‐based contrast reagent (CR) is that CR molecules leak into the interstitium space and therefore confound the DSC signal detected. Several approaches to correct this leakage effect have been proposed throughout the years. Amongst the most popular is the Boxerman–Schmainda–Weisskoff (BSW) K₂ leakage correction approach, in which the K₂ pseudo‐first‐order rate constant quantifies the leakage. In this work, we propose a new method for the BSW leakage correction approach. Based on the pharmacokinetic interpretation of the data, the commonly adopted R₂* expression accounting for contributions from both intravascular and extravasating CR components is transformed using a method mathematically similar to Gjedde–Patlak linearization. Then, the leakage rate constant (K_L) can be determined as the slope of the linear portion of a plot of the transformed data. Using the DSC data of high‐molecular‐weight (~750 kDa), iron‐based, intravascular Ferumoxytol (FeO), the pharmacokinetic interpretation of the new paradigm is empirically validated. The primary objective of this work is to empirically demonstrate that a linear portion often exists in the graph of the transformed data. This linear portion provides a clear definition of the Gd CR pseudo‐leakage rate constant, which equals the slope derived from the linear segment. A secondary objective is to demonstrate that transformed points from the initial transient period during the CR wash‐in often deviate from the linear trend of the linearized graph. The inclusion of these points will have a negative impact on the accuracy of the leakage rate constant, and even make it time dependent.     

Phase contrast MRI is an early marker of micrometastatic breast cancer development in the rat brain

The early growth of micrometastatic breast cancer in the brain often occurs through vessel co‐option and is independent of angiogenesis. Remodeling of the existing vasculature is an important step in the evolution of co‐opting micrometastases into angiogenesis‐dependent solid tumor masses. The purpose of this study was to determine whether phase contrast MRI, an intrinsic source of contrast exquisitely sensitive to the magnetic susceptibility properties of deoxygenated hemoglobin, could detect vascular changes occurring independent of angiogenesis in a rat model of breast cancer metastases to the brain. Twelve nude rats were administered 10⁶ MDA‐MB‐231BRL ‘brain‐seeking’ breast cancer cells through intracardiac injection. Serial, multiparametric MRI of the brain was performed weekly until metastatic disease was detected. The results demonstrated that images of the signal phase (area under the receiver operating characteristic curve, 0.97) were more sensitive than T₂* gradient echo magnitude images (area under the receiver operating characteristic curve, 0.73) to metastatic brain lesions. The difference between the two techniques was probably the result of the confounding effects of edema on the magnitude of the signal. A region of interest analysis revealed that vascular abnormalities detected with phase contrast MRI preceded tumor permeability measured with contrast‐enhanced MRI by 1–2 weeks. Tumor size was correlated with permeability (R² = 0.23, p < 0.01), but phase contrast was independent of tumor size (R² = 0.03). Histopathologic analysis demonstrated that capillary endothelial cells co‐opted by tumor cells were significantly enlarged, but less dense, relative to the normal brain vasculature. Although co‐opted vessels were vascular endothelial growth factor‐negative, vessels within larger tumor masses were vascular endothelial growth factor‐positive. In conclusion, phase contrast MRI is believed to be sensitive to vascular remodeling in co‐opting brain tumor metastases independent of sprouting angiogenesis, and may therefore aid in preclinical studies of angiogenic‐independent tumors or in the monitoring of continued tumor growth following anti‐angiogenic therapy. Published 2011. This article is a US Government work and is in the public domain in the USA.     

Influence of regional cerebral blood volume on voxel‐based morphometry

The investigation of structural brain alterations is one focus in research of brain diseases like depression. Voxel‐based morphometry (VBM) based on high‐resolution 3D MRI images is a widely used non‐invasive tool for such investigations. However, the result of VBM might be sensitive to local physiological parameters such as regional cerebral blood volume (rCBV) changes. In order to investigate whether rCBV changes may contribute to variation in VBM, we performed analyses in a study with the congenital learned helplessness (cLH) model for long‐term findings. The 3D structural and rCBV data were acquired with T₂‐weighted rapid acquisition with relaxation enhancement (RARE) pulse sequences. The group effects were determined by standard statistical parametric mapping (SPM) and biological parametric mapping (BPM) and examined further using atlas‐based regions. In our genetic animal model of depression, we found co‐occurrence of differences in gray matter volume and rCBV, while there was no evidence of significant interaction between both. However, the multimodal analysis showed similar gray matter differences compared with the standard VBM approach. Our data corroborate the idea that two group VBM differences might not be influenced by rCBV differences in genetically different strains. Copyright © 2016 John Wiley & Sons, Ltd.     

Quantification of α‐subunit isoforms of Na,K‐ATPase in rat resistance vessels

Aim: Rat mesenteric resistance vessels (RV) were characterized with respect to concentration of individual α‐subunit isoforms of Na,K‐ATPase. Methods: Total vessel homogenates were used to avoid any loss or subfractionation of membranes. They were applied to sodium dodecyl sulphate gels and, for calibration, in parallel lanes were run purified rat Na,K‐ATPase preparations with known isoform distribution and content. The capacity per mg protein for Na⁺‐dependent ³²P‐phosphorylation of Na,K‐ATPase isolated from rat kidney was used for α₁ calibration and that for high‐affinity (³H)ouabain binding of Na,K‐ATPase isolated from rat brain was used for (α₂ + α₃) calibration. Western blots containing homogenate proteins and reference enzyme were incubated with isoform‐specific antibodies and radiolabelled secondary antibodies. The signals from adjacent α spots were used for qualitative and quantitative characterization of rat vessels. Results: A concentration of 100.7 ± 14.4 pmol (n = 11) per g wet weight of the α₁‐isoform containing Na,K‐ATPase was found in RV from 12–14‐week rats. A much lower and more unreliable content of α₂‐ and α₃‐isoforms was found. These ouabain‐sensitive isoforms seem to represent a maximum of 5–10% each compared with the ouabain‐insensitive rat α₁‐isoform. Conclusions: The isoform pattern in RV, in which the isoform with high/intermediate Na⁺‐affinity is the absolutely dominating one representing nearly all sodium pumps in this tissue, is very different from that seen in rat skeletal muscles. Due to the high content of the ouabain‐insensitive α₁‐isoform in rat RV this species would seem a less relevant model in studies addressing a role of cardiac glycosides and putative endogenous ouabain‐like factors in hypertension.     

Manganese‐enhanced MRI of rat brain based on slow cerebral delivery of manganese(II) with silica‐encapsulated MnxFe1–xO nanoparticles

In this work, we report a monodisperse bifunctional nanoparticle system, MIO@SiO₂‐RITC, as an MRI contrast agent [core, manganese iron oxide (MIO); shell, amorphous silica conjugated with rhodamine B isothiocyanate (RITC)]. It was prepared by thermal decomposition and modified microemulsion methods. The nanoparticles with varying iron to manganese ratios displayed different saturated magnetizations and relaxivities. In vivo MRI of rats injected intravenously with MIO@SiO₂‐RITC nanoparticles exhibited enhancement of the T₁ contrast in brain tissue, in particular a time‐delayed enhancement in the hippocampus, pituitary gland, striatum and cerebellum. This is attributable to the gradual degradation of MIO@SiO₂‐RITC nanoparticles in the liver, resulting in the slow release of manganese(II) [Mn(II)] into the blood pool and, subsequently, accumulation in the brain tissue. Thus, T₁‐weighted contrast enhancement was clearly detected in the anatomic structure of the brain as time progressed. In addition, T₂*‐weighted images of the liver showed a gradual darkening effect. Here, we demonstrate the concept of the slow release of Mn(II) for neuroimaging. This new nanoparticle‐based manganese contrast agent allows one simple intravenous injection (rather than multiple infusions) of Mn(II) precursor, and results in delineation of the detailed anatomic neuroarchitecture in MRI; hence, this provides the advantage of the long‐term study of neural function. Copyright © 2013 John Wiley & Sons, Ltd.     

Relationship between blood and myocardium manganese levels during manganese‐enhanced MRI (MEMRI) with T1 mapping in rats

Manganese ions (Mn²⁺) enter viable myocardial cells via voltage‐gated calcium channels. Because of its shortening of T₁ and its relatively long half‐life in cells, Mn²⁺ can serve as an intracellular molecular contrast agent to study indirect calcium influx into the myocardium. One major concern in using Mn²⁺ is its sensitivity over a limited range of concentrations employing T₁‐weighted images for visualization, which limits its potential in quantitative techniques. Therefore, this study assessed the implementation of a T₁ mapping method for cardiac manganese‐enhanced MRI to enable a quantitative estimate of the influx of Mn²⁺ over a wide range of concentrations in male Sprague‐Dawley rats. This MRI method was used to compare the relationship between T₁ changes in the heart as a function of myocardium and blood Mn²⁺ levels. Results showed a biphasic relationship between ΔR₁ and the total Mn²⁺ infusion dose. Nonlinear relationships were observed between the total Mn²⁺ infusion dose versus blood levels and left ventricular free wall ΔR₁. At low blood levels of Mn²⁺, there was proportionally less cardiac enhancement seen than at higher levels of blood Mn²⁺. We hypothesize that Mn²⁺ blood levels increase as a result of rate‐limiting excretion by the liver and kidneys at these higher Mn²⁺ doses. Copyright © 2010 John Wiley & Sons, Ltd.     

Bi‐component T2* analysis of bound and pore bone water fractions fails at high field strengths

Osteoporosis involves the degradation of the bone's trabecular architecture, cortical thinning and enlargement of cortical pores. Increased cortical porosity is a major cause of the decreased strength of osteoporotic bone. The majority of cortical pores, however, are below the resolution limit of MRI. Recent work has shown that porosity can be evaluated by MRI‐based quantification of bone water. Bi‐exponential T₂* fitting and adiabatic inversion preparation are the two most common methods purported to distinguish bound and pore water in order to quantify matrix density and porosity. To assess the viability of T₂* bi‐component analysis as a method for the quantification of bound and pore water fractions, we applied this method to human cortical bone at 1.5, 3, 7 and 9.4 T, and validated the resulting pool fractions against micro‐computed tomography‐derived porosity and gravimetrically determined bone densities. We also investigated alternative methods: two‐dimensional T₁–T₂* bi‐component fitting by incorporation of saturation recovery, one‐ and two‐dimensional fitting of Carr–Purcell–Meiboom–Gill (CPMG) echo amplitudes, and deuterium inversion recovery. The short‐T₂* pool fraction was moderately correlated with porosity (R² = 0.70) and matrix density (R² = 0.63) at 1.5 T, but the strengths of these associations were found to diminish rapidly as the field strength increased, falling below R² = 0.5 at 3 T. The addition of the T₁ dimension to bi‐component analysis only slightly improved the strengths of these correlations. T₂*‐based bi‐component analysis should therefore be used with caution. The performance of deuterium inversion recovery at 9.4 T was also poor (R² = 0.50 vs porosity and R² = 0.46 vs matrix density). The CPMG‐derived short‐T₂ fraction at 9.4 T, however, was highly correlated with porosity (R² = 0.87) and matrix density (R² = 0.88), confirming the utility of this method for independent validation of bone water pools. Copyright © 2015 John Wiley & Sons, Ltd.     

Combined endogenous MR biomarkers to predict basal tumor oxygenation and response to hyperoxic challenge

Hypoxia is a common feature of solid tumors, which translates into increased angiogenesis, malignant phenotype cell selection, change in gene expression and greater resistance to radiotherapy and chemotherapy. Therefore, there is a need for markers of hypoxia to stratify patients, in order to personalize treatment to improve therapeutic outcome. However, no modality has yet been validated for the screening of hypoxia in routine clinical practice. Magnetic resonance imaging (MRI) R₁ and R₂* relaxation parameters are sensitive to tissue oxygenation: R₁ is sensitive to dissolved oxygen and R₂* is sensitive to intravascular deoxyhemoglobin content. Two rat tumor models with distinct levels of hypoxia, 9L–glioma and rhabdomyosarcoma, were imaged for R₁ and R₂* under air and carbogen (95% O₂ and 5% CO₂) breathing conditions. It was observed that the basal tumor oxygenation level had an impact on the amplitude of response to carbogen in the vascular compartment (R₂*), but not in the tissue compartment (R₁). In addition, the change in tissue oxygenation estimated by ΔR₁ correlated with the change in vascular oxygenation estimated by ΔR₂*, which is consistent with an increase in oxygen supply generating an elevated tumor pO₂. At the intra‐tumoral level, we identified four types of voxel to which a hypoxic feature was attributed (mild hypoxia, severe hypoxia, normoxia and vascular steal), depending on the carbogen‐induced change in R₁ and R₂* values for each voxel. The results showed that 9L–gliomas present more normoxic fractions, whereas rhabdomyosarcomas present more hypoxic fractions, which is in accordance with a previous study using ¹⁸F–fluoroazomycin arabinoside (¹⁸F–FAZA) and electron paramagnetic resonance (EPR) oximetry. The response of the combined endogenous MRI contrasts to carbogen challenge could be a useful tool to predict different tumor hypoxic fractions.