Measurement of kinetic parameters in skeletal muscle by magnetic resonance imaging with an intravascular agent

Sciatic nerve microvascular permeability in type 2 diabetes decreased in patients with neuropathy

Objectives

Clinical and histological studies have found evidence that nerve ischemia is a major contributor to diabetic neuropathy (DN) in type 2 diabetes (T2D). The aim of this study was to investigate peripheral nerve microvascular permeability using dynamic contrast enhanced (DCE) magnetic resonance neurography (MRN) to analyze potential correlations with clinical, electrophysiological, and demographic data.

Methods

Sixty‐five patients (35/30 with/without DN) and 10 controls matched for age and body mass index (BMI) underwent DCE MRN of the distal sciatic nerve with an axial T1‐weighted sequence. Microvascular permeability (K^trans), plasma volume fraction (v_p), and extravascular extracellular volume fraction (v_e) were determined with the extended Tofts model, and subsequently correlated with clinical data.

Results

K^trans and v_e were lower in T2D patients with DN compared to patients without DN (0.037 min⁻¹ ± 0.010 vs. 0.046 min⁻¹ ± 0.014; p = 0.011, and 2.35% ± 3.87 vs. 5.11% ± 5.53; p = 0.003, respectively). In individuals with T2D, K^trans correlated positively with tibial, peroneal, and sural NCVs (r = 0.42; 95%CI = 0.18 to 0.61, 0.50; 95%CI = 0.29 to 0.67, and 0.44; 95%CI = 0.19 to 0.63, respectively), with tibial and peroneal CMAPs (r = 0.27; 95%CI = 0.01 to 0.49 and r = 0.32; 95%CI = 0.07 to 0.53), and with the BMI (r = 0.47; 95%CI = 0.25 to 0.64). Negative correlations were found with the neuropathy deficit score (r = −0.40; 95%CI = −0.60 to −0.16) and age (r = −0.51; 95%CI = −0.67 to −0.31). No such correlations were found for v_p.

Conclusion

This study is the first to find associations of MR nerve perfusion parameters with clinical and electrophysiological parameters related to DN in T2D. The results indicate that a decrease in microvascular permeability but not plasma volume may result in nerve ischemia that subsequently causes demyelination.

Troponin T Is Negatively Associated With 3 Tesla Magnetic Resonance Peripheral Nerve Perfusion in Type 2 Diabetes

OBJECTIVE

The pathogenesis of diabetic polyneuropathy (DN) is poorly understood and given the increasing prevalence of DN, there is a need for clinical or imaging biomarkers that quantify structural and functional nerve damage. While clinical studies have found evidence of an association between elevated levels of troponin T (hsTNT) and N-terminal pro brain natriuretic peptide (proBNP) with microvascular compromise in type 2 diabetes (T2D), their implication in mirroring DN nerve perfusion changes remains unclear. The objective of this study was, therefore, to investigate whether hsTNT and proBNP assays are associated with MRI nerve perfusion in T2D.

METHODS

In this prospective cross-sectional single-center case-control study, 56 participants (44 with T2D, 12 healthy control subjects) consented to undergo magnetic resonance neurography (MRN) including dynamic contrast-enhanced (DCE) perfusion imaging of the right leg. Using the extended Tofts model, primary outcome parameters that were quantified are the sciatic nerve's microvascular permeability (K^trans), the extravascular extracellular volume fraction (v_e), and the plasma volume fraction (v_p), as well as hsTNT and proBNP values from serological workup. Further secondary outcomes were clinical, serological, and electrophysiological findings.

RESULTS

In T2D patients, hsTNT was negatively correlated with K^trans (r=-0.38; p=0.012) and v_e (r=-0.30; p=0.048) but not with v_p (r=-0.16; p=0.294). HsTNT, K^trans, and v_e were correlated with peroneal nerve conduction velocities (NCVs; r=-0.44; p=0.006, r=0.42; p=0.008, r=0.39; p=0.014), and tibial NCVs (r=-0.38;p=0.022, r=0.33; p=0.048, r=0.37; p=0.025). No such correlations were found for proBNP.

CONCLUSIONS

This study is the first to find that hsTNT is correlated with a decrease of microvascular permeability and a reduced extravascular extracellular volume fraction of nerves in patients with T2D. The results indicate that hsTNT may serve as a potential marker for the assessment of nerve perfusion in future studies on DN.

Test-Retest Performance of a 1-Hour Multiparametric MR Image Acquisition Pipeline With Orthotopic Triple-Negative Breast Cancer Patient-Derived Tumor Xenografts

Preclinical imaging is critical in the development of translational strategies to detect diseases and monitor response to therapy. The National Cancer Institute Co-Clinical Imaging Resource Program was launched, in part, to develop best practices in preclinical imaging. In this context, the objective of this work was to develop a 1-hour, multiparametric magnetic resonance image-acquisition pipeline with triple-negative breast cancer patient-derived xenografts (PDXs). The 1-hour, image-acquisition pipeline includes T1- and T2-weighted scans, quantitative T1, T2, and apparent diffusion coefficient (ADC) parameter maps, and dynamic contrast-enhanced (DCE) time-course images. Quality-control measures used phantoms. The triple-negative breast cancer PDXs used for this study averaged 174 ± 73 μL in volume, with region of interest–averaged T1, T2, and ADC values of 1.9 ± 0.2 seconds, 62 ± 3 milliseconds, and 0.71 ± 0.06 μm²/ms (mean ± SD), respectively. Specific focus was on assessing the within-subject test–retest coefficient-of-variation (CV_WS) for each of the magnetic resonance imaging metrics. Determination of PDX volume via manually drawn regions of interest is highly robust, with ∼1% CV_WS. Determination of T2 is also robust with a ∼3% CV_WS. Measurements of T1 and ADC are less robust with CV_WS values in the 6%–11% range. Preliminary DCE test–retest time-course determinations, as quantified by area under the curve and K^trans from 2-compartment exchange (extended Tofts) modeling, suggest that DCE is the least robust protocol, with ∼30%–40% CV_WS.

Dynamic contrast-enhanced magnetic resonance imaging in denervated skeletal muscle: Experimental study in rabbits

Purpose

To investigate the value of dynamic contrast-enhanced (DCE) magnetic resonance imaging (MRI) for evaluating denervated skeletal muscle in rabbits.

Materials and methods

24 male rabbits were randomly divided into an irreversible neurotmesis group and a control group. In the experimental group, the sciatic nerves of rabbits were transected for irreversible neurotmesis model. A sham operation was performed in the control group. MRI of rabbit lower legs was performed before nerve surgery and 1 day, 3 days, 5 days, 1 week, 2 weeks, 3 weeks, 4 weeks, 6 weeks, 8 weeks, 10 weeks, and 12 weeks after surgery.

Results

Signal intensity changes were seen in the left gastrocnemius muscle on the T2-weighted images. DCE-MRI derived parameters (K^trans, K_ep, and V_p) were measured in vivo. In the irreversible neurotmesis group, T2-weighted images showed increased signal intensity in the left gastrocnemius muscle. K^trans, V_p values changes occur as early as 1 day after denervation, and increased gradually until 4 weeks after surgery. There are significant increases in both K^trans and V_p values compared with those in the control group after surgery (P < 0.05). K_ep values show no significant difference between the irreversible neurotmesis group and the control group.

Conclusion

DCE-MRI hold the promise of an early and sensitive diagnosis of denervated skeletal muscle.

Validation of Blood Volume Fraction Quantification with 3D Gradient Echo Dynamic Contrast-Enhanced Magnetic Resonance Imaging in Porcine Skeletal Muscle

The purpose of this study was to assess the accuracy of fractional blood volume (v_b) estimates in low-perfused and low-vascularized tissue using dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI). The results of different MRI methods were compared with histology to evaluate the accuracy of these methods under clinical conditions. v_b was estimated by DCE-MRI using a 3D gradient echo sequence with k-space undersampling in five muscle groups in the hind leg of 9 female pigs. Two gadolinium-based contrast agents (CA) were used: a rapidly extravasating, extracellular, gadolinium-based, low-molecular-weight contrast agent (LMCA, gadoterate meglumine) and an extracellular, gadolinium-based, albumin-binding, slowly extravasating blood pool contrast agent (BPCA, gadofosveset trisodium). LMCA data were evaluated using the extended Tofts model (ETM) and the two-compartment exchange model (2CXM). The images acquired with administration of the BPCA were used to evaluate the accuracy of v_b estimation with a bolus deconvolution technique (BD) and a method we call equilibrium MRI (EqMRI). The latter calculates the ratio of the magnitude of the relaxation rate change in the tissue curve at an approximate equilibrium state to the height of the same area of the arterial input function (AIF). Immunohistochemical staining with isolectin was used to label endothelium. A light microscope was used to estimate the fractional vascular area by relating the vascular region to the total tissue region (immunohistochemical vessel staining, IHVS). In addition, the percentage fraction of vascular volume was determined by multiplying the microvascular density (MVD) with the average estimated capillary lumen, π(d2)2, where d = 8μm is the assumed capillary diameter (microvascular density estimation, MVDE). Except for ETM values, highly significant correlations were found between most of the MRI methods investigated. In the cranial thigh, for example, the v_b medians (interquartile range, IQRs) of IHVS, MVDE, BD, EqMRI, 2CXM and ETM were v_b = 0.7(0.3)%, 1.1(0.4)%, 1.1(0.4)%, 1.4(0.3)%, 1.2(1.8)% and 0.1(0.2)%, respectively. Variances, expressed by the difference between third and first quartiles (IQR) were highest for the 2CXM for all muscle groups. High correlations between the values in four muscle groups—medial, cranial, lateral thigh and lower leg - estimated with MRI and histology were found between BD and EqMRI, MVDE and 2CXM and IHVS and ETM. Except for the ETM, no significant differences between the v_b medians of all MRI methods were revealed with the Wilcoxon rank sum test. The same holds for all muscle regions using the 2CXM and MVDE. Except for cranial thigh muscle, no significant difference was found between EqMRI and MVDE. And except for the cranial thigh and the lower leg muscle, there was also no significant difference between the v_b medians of BD and MVDE. Overall, there was good v_b agreement between histology and the BPCA MRI methods and the 2CXM LMCA approach with the exception of the ETM method. Although LMCA models have the advantage of providing excellent curve fits and can in principle determine more physiological parameters than BPCA methods, they yield more inaccurate results.

Flow versus permeability weighting in estimating the forward volumetric transfer constant (Ktrans) obtained by DCE-MRI with contrast agents of differing molecular sizes

PURPOSE

To quantify the differential plasma flow- (F_p-) and permeability surface area product per unit mass of tissue- (PS-) weighting in forward volumetric transfer constant (K^trans) estimates by using a low molecular (Gd-DTPA) versus high molecular (Gadomer) weight contrast agent in dynamic contrast enhanced (DCE) MRI.

MATERIALS AND METHODS

DCE MRI was performed using a 7T animal scanner in 14 C57BL/6J mice syngeneic for TRAMP tumors, by administering Gd-DTPA (0.9kD) in eight mice and Gadomer (35kD) in the remainder. The acquisition time was 10min with a sampling rate of one image every 2s. Pharmacokinetic modeling was performed to obtain K^trans by using Extended Tofts model (ETM). In addition, the adiabatic approximation to the tissue homogeneity (AATH) model was employed to obtain the relative contributions of F_p and PS.

RESULTS

The K^trans values derived from DCE-MRI with Gd-DTPA showed significant correlations with both PS (r²=0.64, p=0.009) and F_p (r²=0.57, p=0.016), whereas those with Gadomer were found only significantly correlated with PS (r²=0.96, p=0.0003) but not with F_p (r²=0.34, p=0.111). A voxel-based analysis showed that K^trans approximated PS (<30% difference) in 78.3% of perfused tumor volume for Gadomer, but only 37.3% for Gd-DTPA.

CONCLUSIONS

The differential contributions of F_p and PS in estimating K^trans values vary with the molecular weight of the contrast agent used. The macromolecular contrast agent resulted in K^trans values that were much less dependent on flow. These findings support the use of macromolecular contrast agents for estimating tumor vessel permeability with DCE-MRI.

Fitting the two-compartment model in DCE-MRI by linear inversion

PURPOSE

Model fitting of dynamic contrast-enhanced-magnetic resonance imaging-MRI data with nonlinear least squares (NLLS) methods is slow and may be biased by the choice of initial values. The aim of this study was to develop and evaluate a linear least squares (LLS) method to fit the two-compartment exchange and -filtration models.

METHODS

A second-order linear differential equation for the measured concentrations was derived where model parameters act as coefficients. Simulations of normal and pathological data were performed to determine calculation time, accuracy and precision under different noise levels and temporal resolutions. Performance of the LLS was evaluated by comparison against the NLLS.

RESULTS

The LLS method is about 200 times faster, which reduces the calculation times for a 256 × 256 MR slice from 9 min to 3 s. For ideal data with low noise and high temporal resolution the LLS and NLLS were equally accurate and precise. The LLS was more accurate and precise than the NLLS at low temporal resolution, but less accurate at high noise levels.

CONCLUSION

The data show that the LLS leads to a significant reduction in calculation times, and more reliable results at low noise levels. At higher noise levels the LLS becomes exceedingly inaccurate compared to the NLLS, but this may be improved using a suitable weighting strategy. Magn Reson Med 76:998-1006, 2016. © 2015 Wiley Periodicals, Inc.

Dynamic contrast-enhanced magnetic resonance imaging: fundamentals and application to the evaluation of the peripheral perfusion

INTRODUCTION

The ability to ascertain information pertaining to peripheral perfusion through the analysis of tissues' temporal reaction to the inflow of contrast agent (CA) was first recognized in the early 1990's. Similar to other functional magnetic resonance imaging (MRI) techniques such as arterial spin labeling (ASL) and blood oxygen level-dependent (BOLD) MRI, dynamic contrast-enhanced MRI (DCE-MRI) was at first restricted to studies of the brain. Over the last two decades the spectrum of ailments, which have been studied with DCE-MRI, has been extensively broadened and has come to include pathologies of the heart notably infarction, stroke and further cerebral afflictions, a wide range of neoplasms with an emphasis on antiangiogenic treatment and early detection, as well as investigations of the peripheral vascular and musculoskeletal systems.

APPLICATIONS TO PERIPHERAL PERFUSION

DCE-MRI possesses an unparalleled capacity to quantitatively measure not only perfusion but also other diverse microvascular parameters such as vessel permeability and fluid volume fractions. More over the method is capable of not only assessing blood flowing through an organ, but in contrast to other noninvasive methods, the actual tissue perfusion. These unique features have recently found growing application in the study of the peripheral vascular system and most notably in the diagnosis and treatment of peripheral arterial occlusive disease (PAOD).

REVIEW OUTLINE

The first part of this review will elucidate the fundamentals of data acquisition and interpretation of DCE-MRI, two areas that often remain baffling to the clinical and investigating physician because of their complexity. The second part will discuss developments and exciting perspectives of DCE-MRI regarding the assessment of perfusion in the extremities. Emerging clinical applications of DCE-MRI will be reviewed with a special focus on investigation of physiology and pathophysiology of the microvascular and vascular systems of the extremities.

Multi-parametric MRI characterization of inflammation in murine skeletal muscle

Myopathies often display a common set of complex pathologies that include muscle weakness, inflammation, compromised membrane integrity, fat deposition, and fibrosis. Multi-parametric, quantitative, non-invasive imaging approaches may be able to resolve these individual pathological components. The goal of this study was to use multi-parametric MRI to investigate inflammation as an isolated pathological feature. Proton relaxation, diffusion tensor imaging (DTI), quantitative magnetization transfer (qMT-MRI), and dynamic contrast enhanced (DCE-MRI) parameters were calculated from data acquired in a single imaging session conducted 6-8 hours following the injection of λ-carrageenan, a local inflammatory agent. T2 increased in the inflamed muscle and transitioned to bi-exponential behavior. In diffusion measurements, all three eigenvalues and the apparent diffusion coefficient increased, but λ3 had the largest relative change. Analysis of the qMT data revealed that the T1 of the free pool and the observed T1 both increased in the inflamed tissue, while the ratio of exchanging spins in the solid pool to those in the free water pool (the pool size ratio) significantly decreased. DCE-MRI data also supported observations of an increase in extracellular volume. These findings enriched the understanding of the relation between multiple quantitative MRI parameters and an isolated inflammatory pathology, and may potentially be employed for other single or complex myopathy models.

Optimization of the reference region method for dual pharmacokinetic modeling using Gd-DTPA/MRI and (18) F-FDG/PET

PURPOSE

The combination of MRI and positron emission tomography (PET) offers new possibilities for the development of novel methodologies. In pharmacokinetic image analysis, the blood concentration of the imaging compound as a function of time, [i.e., the arterial input function (AIF)] is required for MRI and PET. In this study, we tested whether an AIF extracted from a reference region (RR) in MRI can be used as a surrogate for the manually sampled (18) F-FDG AIF for pharmacokinetic modeling.

METHODS

An MRI contrast agent, gadolinium-diethylenetriaminepentaacetic acid (Gd-DTPA) and a radiotracer, (18) F-fluorodeoxyglucose ((18) F-FDG), were simultaneously injected in a F98 glioblastoma rat model. A correction to the RR AIF for Gd-DTPA is proposed to adequately represent the manually sampled AIF. A previously published conversion method was applied to convert this AIF into a (18) F-FDG AIF.

RESULTS

The tumor metabolic rate of glucose (TMRGlc) calculated with the manually sampled (18) F-FDG AIF, the (18) F-FDG AIF converted from the RR AIF and the (18) F-FDG AIF converted from the corrected RR AIF were found not statistically different (P>0.05).

CONCLUSION

An AIF derived from an RR in MRI can be accurately converted into a (18) F-FDG AIF and used in PET pharmacokinetic modeling.

Practical dynamic contrast enhanced MRI in small animal models of cancer: data acquisition, data analysis, and interpretation

Dynamic contrast enhanced magnetic resonance imaging (DCE-MRI) consists of the continuous acquisition of images before, during, and after the injection of a contrast agent. DCE-MRI allows for noninvasive evaluation of tumor parameters related to vascular perfusion and permeability and tissue volume fractions, and is frequently employed in both preclinical and clinical investigations. However, the experimental and analytical subtleties of the technique are not frequently discussed in the literature, nor are its relationships to other commonly used quantitative imaging techniques. This review aims to provide practical information on the development, implementation, and validation of a DCE-MRI study in the context of a preclinical study (though we do frequently refer to clinical studies that are related to these topics).

Markers of cochlear inflammation using MRI

PURPOSE

To quantify spatial and temporal inflammation-induced changes in vascular permeability and macrophage infiltration in guinea-pig (GP) cochlea using MRI.

MATERIALS AND METHODS

GPs were injected with lipopolysaccharide (LPS) to induce cochlear inflammation. One group was injected with a gadolinium based contrast agent (GBCA) and dynamic contrast enhanced (DCE)-MRI was performed at 4, 7, and 10 days after LPS treatment. A two-compartment pharmacokinetic model was used to determine the apparent rate constant of GBCA extravasation (K(trans) ). A second group was injected with ultrasmall superparamagnetic iron oxide particles (USPIOs) and studied at 2, 3, and 7 days after LPS treatment to detect tissue USPIO uptake and correlate with histology. For both groups, control GPs were scanned similarly.

RESULTS

The signal enhancement increased substantially and more rapidly at day 4 in LPS-treated than in control cochlea shortly following GBCA injection. K(trans) of LPS-treated cochlea was maximum on day 4 at 0.0218 ± 0.0032 min(-1) and then decreased to control level at 0.0036 ± 0.0004 min(-1) by day 10. In the second group, the relative signal intensity and T2 in cochlear perilymphatic spaces on day 2 decreased, on average, by 54% and 45%, respectively, compared with baseline and then remained under control levels by day 7. This suggests the infiltration of inflammatory cells, although unconfirmed by histology.

CONCLUSION

This provides the first measurement of cochlear vascular permeability using MRI and a quantitative evaluation of the development of cochlear inflammation. MRI holds considerable potential for the assessment of disease processes such as clinical diagnosis of conditions such as labyrinthitis.

Simultaneous magnetic resonance angiography and perfusion (MRAP) measurement: initial application in lower extremity skeletal muscle

PURPOSE

To obtain a simultaneous 3D magnetic resonance angiography and perfusion (MRAP) using a single acquisition and to demonstrate MRAP in the lower extremities. A time-resolved contrast-enhanced exam was used in MRAP to simultaneously acquire a contrast-enhanced MR angiography (MRA) and dynamic contrast-enhanced (DCE) perfusion, which currently requires separate acquisitions and thus two contrast doses. MRAP can be used to assess large and small vessels in vascular pathologies such as peripheral arterial disease.

MATERIALS AND METHODS

MRAP was performed on 10 volunteers following unilateral plantar flexion exercise (one leg exercised and one rested) on two separate days. Data were acquired after administration of a single dose of contrast agent using an optimized sampling strategy, parallel imaging, and partial-Fourier acquisition to obtain a high spatial resolution, 3D-MRAP frame every 4 seconds. Two radiologists assessed MRAs for image quality, a signal-to-noise ratio (SNR) analysis was performed, and pharmacokinetic modeling yielded perfusion (K(trans) ).

RESULTS

MRA images had high SNR and radiologist-assessed diagnostic quality. Mean K(trans) ± standard error were 0.136 ± 0.009, 0.146 ± 0.012, and 0.191 ± 0.012 min(-1) in the resting tibialis anterior, gastrocnemius, and soleus, respectively, which significantly increased with exercise to 0.291 ± 0.018, 0.270 ± 0.019, and 0.338 ± 0.022 min(-1) . Bland-Altman analysis showed good repeatability.

CONCLUSION

MRAP provides simultaneous high-resolution MRA and quantitative DCE exams to assess large and small vessels with a single contrast dose. Application in skeletal muscle shows quantitative, repeatable perfusion measurements, and the ability to measure physiological differences.

A dedicated automated injection system for dynamic contrast-enhanced MRI experiments in mice

PURPOSE

To develop a reproducible small-animal dynamic contrast-enhanced (DCE) MRI set-up for mice through which volumes <100 μL can be accurately and safely injected and to test this set-up by means of DCE measurements in resting muscle and tumor tissue.

MATERIALS AND METHODS

The contrast agent (CA) injection system comprised 2 MR-compatible syringe pumps placed 50 cm from the 7T magnet bore where the fringe field is approximately 40 mT. Microbore tubing and T-connector, close to the injection site, minimized dead volume (<10 μL). For DCE-MRI measurements in 8 CB-17 SCID mice with 1500-2500 mm(3) large orthotopic neuroblastoma, a bolus of 10-fold-diluted Gd-DTPA CA solution (0.1 mmol/kg) was delivered (5 μL/s), followed by a 50-μL saline flush. Retro-orbital injections were given instead of tail vein injections, because the peripheral vasculature was reduced because of large tumor burden.

RESULTS

The CA injection was successful in 19 of 24 experiments. Optical assessment showed minimal dispersion of ink-colored CA bolus. Mean (± SD) pharmacokinetic parameters retrieved from DCE-MRI examinations in resting muscle (K(trans) = 0.038 ± 0.025 min(-1), k(ep) = 0.66 ± 0.48 min(-1), v(e) = 0.060 ± 0.014, v(p) = 0.033 ± 0.021) and tumor (K(trans) = 0.082 ± 0.071 min(-1), k(ep) = 0.82 ± 0.80 min(-1), v(e) = 0.121 ± 0.075, v(p) = 0.093 ± 0.051) agreed with those reported previously.

CONCLUSION

We successfully designed and implemented a DCE-MRI set-up system with short injection lines and low dead volume. The system can be used at any field strength with the syringe pumps placed at a sufficiently low fringe field (<40 mT).

Diffusion-weighted MRI, dynamic susceptibility contrast MRI and ultrasound perfusion quantification of denervated muscle in rabbits

OBJECTIVE

The purpose of this study was to assess denervated muscle perfusion using dynamic susceptibility contrast MRI (DSCMRI) and contrast-enhanced ultrasound (CEUS), and to measure denervated muscle apparent diffusion coefficient (ADC) on b1000 diffusion-weighted MRI (DWMRI) at 3 T in order to clarify whether muscle denervation leads to an increase in the extracellular extravascular space, or an increase in blood flow-or both.

MATERIALS AND METHODS

Axotomy of the right sciatic nerve of six white rabbits was performed at day 0. At day 9, hind limb muscles MRI and CEUS were performed to assess the consequences of denervation and both semimembranosus muscles of each rabbit were explanted for histological studies. Signal intensity on T2- and T1-weighted MRI, ADC on DWMRI, maximum signal drop (MSD) on DSCMRI and the area under the curve (AUC) on CEUS were measured over circular regions of interest (ROI), in both semimembranosus muscles. Non-parametric Wilcoxon matched-pairs tests were used to assess the mean differences between denervated and normal muscles.

RESULTS

T2 fat-saturated (FS) MRI studies showed a strong signal in the right semimembranosus muscles compared with the left side, and gadolinium enhancement was observed on T1 FS MRI. Denervated muscles show a significant increase in ADC on DWMRI (p < 0.01) and a significant signal enhancement on DSCMR imaging (p < 0.05) and on first-pass CEUS (p < 0.05).

CONCLUSION

The results of this study--based on perfusion- and diffusion-weighted images--suggest that, after denervation, both increased blood flow through muscle tissue and expansion of the extracellular water volume are present.

Use of dynamic contrast-enhanced magnetic resonance imaging to evaluate the microcirculation of lower extremity muscles in patients with Type 2 diabetes

AIMS

To quantify the capillary permeability of the lower extremity muscles using dynamic contrast-enhanced magnetic resonance imaging in healthy control subjects without diabetes and patients with diabetes before and after percutaneous angioplasty.

METHODS

Fifty-five patients (30 without diabetes, 25 with Type 2 diabetes with occluded vasculature of the lower extremities) were examined by dynamic contrast-enhanced magnetic resonance imaging. The transfer constant (K-trans) of the tibialis anterior muscle was determined before (for all patients) and after (for patients with diabetes only) percutaneous angioplasty of the intrapopliteal artery. Clinical assessment was also recorded. Statistical significant differences were defined at the level of P < 0.05.

RESULTS

Before percutaneous angioplasty, patients with diabetes displayed significantly lower K-trans values in the tibialis anterior muscles than control subjects. Although percutaneous angioplasty increased the K-trans of these patients, the final values were still less than those of the control group. Differences were also observed between the groups in the Fontaine classification and ankle-brachial index, which reflect ischaemia in the lower extremities. After percutaneous angioplasty, these values were increased in the group with diabetes.

CONCLUSION

K-trans can be used to quantify changes in the capillary permeability of the lower extremity muscles, reflecting the microcirculation of the lower extremities.

Optimized pharmacokinetic modeling for the detection of perfusion differences in skeletal muscle with DCE-MRI: effect of contrast agent size

PURPOSE

The goal of this study was to optimize dynamic contrast-enhanced (DCE)-MRI analysis for differently sized contrast agents and to evaluate the sensitivity for microvascular differences in skeletal muscle.

METHODS

In rabbits, pathophysiological perfusion differences between hind limbs were induced by unilateral femoral artery ligation. On days 14 and 21, DCE-MRI was performed using a medium-sized contrast agent (MCA) (Gadomer) or a small contrast agent (SCA) (Gd-DTPA). Acquisition protocols were adapted to the pharmacokinetic properties of the contrast agent. Model-based data analysis was optimized by selecting the optimal model, considering fit error, estimation uncertainty, and parameter interdependency from three two-compartment pharmacokinetic models (normal and extended generalized kinetic models and Patlak model). Model-based parameters were compared to the model-free parameter area-under-curve (AUC). Finally, the sensitivity of transfer constant Krans and AUC for physiological and pathophysiological microvascular differences was evaluated.

RESULTS

For the MCA, the optimal model included Ktrans and plasma fraction nu(p). For the SCA, Ktrans and interstitial fraction nu(e) should be incorporated. For the MCA, Ktrans were (4.8 +/- 0.2) x 10(-3) min(-1) (mean standard error) and (3.6 +/- 0.1) x 10(-3) min(-1) for the red soleus and white tibialis muscle, respectively, p < 0.01. With the SCA, Ktrans were (81 +/- 5) x 10(-3) min(-1) (soleus) and (66 +/- 5) x 10(-3) min(-1) (tibialis) p < 0.01. In the ischemic limb, Ktrans was significantly decreased relative to the control limb (soleus: 15%-20%; tibialis: 5%-10%). Similar differences in AUC were found for both contrast agents.

CONCLUSIONS

For optimal estimation of microvascular parameters, both model-based and model-free analysis should be adapted to the pharmacokinetic properties of the contrast agent. The detection of microvascular differences based on both Ktrans and AUC was most sensitive when the analysis strategy was tailored to the contrast agent used. The MCA was equally sensitive for microvascular differences as the SCA, with the advantage of improved spatial resolution.

Reliability of pharmacokinetic parameters: small vs. medium-sized contrast agents

Current clinical applications of dynamic contrast-enhanced MRI (DCE-MRI) are based on the extravasation of relatively small contrast agents (SCAs). SCAs are considered disadvantageous, as they require high image sampling rates. Medium-sized contrast agents (MCAs) leak more slowly into tissue and allow longer dynamic acquisition times, enabling improved image quality. The influence of molecular size on the reliability of pharmacokinetic parameters, including the transfer constant K(trans), was investigated. Computer simulations were performed, with in vivo measured arterial input functions (AIFs), to determine the bias and variance of pharmacokinetic parameters as a function of contrast agent size, sampling frequency, noise level, and acquisition time. Better reliability of all parameters was obtained for the MCA compared to the SCA. To obtain similar variance (10%) in K(trans), the sampling frequency for the SCA (28 min(-1)) had to be 20 times faster than for the MCA (1.3 min(-1)). Optimal reliability in parameter estimation required longer acquisition times for MCAs (13 min for the fraction of the extravascular extracellular space into which the contrast agent distributes (v(e)) and 5 min for K(trans)) than for SCAs (1.7 min for K(trans) and v(e)). Reliable estimation of the fractional blood plasma volume (v(p)) was only achieved with MCAs. In conclusion, MCAs provided superior reliability for pharmacokinetic parameter estimation compared to SCAs.

Magnetic resonance imaging defines cervicovaginal anatomy, cancer, and VEGF trap antiangiogenic efficacy in estrogen-treated K14-HPV16 transgenic mice

Noninvasive detection of dysplasia provides a potential platform for monitoring the efficacy of chemopreventive therapy of premalignancy, imaging the tissue compartments comprising dysplasia: epithelium, microvasculature, and stromal inflammatory cells. Here, using respiratory-gated magnetic resonance imaging (MRI), the anatomy of premalignant and malignant stages of cervical carcinogenesis in estrogen-treated K14-HPV16 transgenic mice was noninvasively defined. Dynamic contrast enhanced (DCE)-MRI was used to quantify leakage across premalignant dysplastic microvasculature. Vascular permeability as measured by DCE-MRI, K(trans), was similar in transgenic (0.053 +/- 0.020 min(-1); n = 32 mice) and nontransgenic (0.056 +/- 0.029 min(-1); n = 17 mice) animals despite a 2-fold increase in microvascular area in the former compared with the latter. DCE-MRI did detect a significant decrease in vascular permeability accompanying diminution of dysplastic microvasculature by the antiangiogenic agent, vascular endothelial growth factor Trap (K(trans) = 0.052 +/- 0.013 min(-1) pretreatment; n = 6 mice versus K(trans) = 0.019 +/- 0.008 min(-1) post-treatment; n = 5 mice). Thus, we determined that the threshold of microvessel leakage associated with cervical dysplasia was <17 kDa and highlighted the potential of DCE-MRI to noninvasively monitor the efficacy of antiangiogenic drugs or chemoprevention regimens targeting the vasculature in premalignant cervical dysplasia.

On the measurement of absolute cerebral blood volume (CBV) using vascular-space-occupancy (VASO) MRI

Recently, a vascular-space-occupancy (VASO) MRI technique was developed for quantitative assessment of cerebral blood volume (CBV). This method uses the T(1)-shortening effect of gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA) with imaging parameters chosen that null the precontrast blood magnetization but allow the postcontrast blood magnetization to recover to equilibrium. A key advantage of VASO CBV estimation is that it provides a straightforward procedure for converting MR signals to absolute physiologic values. However, as with other T(1)-based steady-state approaches, several important factors need to be considered that influence the accuracy of CBV values obtained with VASO MRI. Here, the transverse relaxation (T(2)/T(2) (*)) effect in VASO MRI was investigated using multiecho spin-echo and gradient-echo experiments, resulting in underestimation of CBV by 14.9% +/- 1.1% and 16.0% +/- 2.5% for spin echo (TE = 10 ms) and gradient echo (TE = 6 ms), respectively. In addition, the influence of contrast agent clearance was studied by acquiring multiple postcontrast VASO images at 2.2-min intervals, which showed that the concentration of Gd-DTPA in the first 14 min (single dose) was sufficient for the blood magnetization to fully recover to equilibrium. Finally, the effect of vascular Gd-DTPA leakage was assessed for scalp tissue, and signal extrapolation as a function of postinjection time was demonstrated to be useful in minimizing the associated errors. Specific recommendations for VASO MRI acquisition and processing strategies are provided.

Imaging hemodynamics

Microvascular permeability is a pharmacologic indicator of tumor response to therapy, and it is expected that this biomarker will evolve into a clinical surrogate endpoint and be integrated into protocols for determining patient response to antiangiogenic or antivascular therapies. This review discusses the physiological context of vessel permeability in an imaging setting, how it is affected by active and passive transport mechanisms, and how it is described mathematically for both theoretical and complex dynamic microvessel membranes. Many research groups have established dynamic-enhanced imaging protocols for estimating this important parameter. This review discusses those imaging modalities, the advantages and disadvantages of each, and how they compare in terms of their ability to deliver information about therapy-associated changes in microvessel permeability in humans. Finally, this review discusses future directions and improvements needed in these areas.

Dysferlin-deficient muscular dystrophy: gadofluorine M suitability at MR imaging in a mouse model

PURPOSE

To compare the usefulness of gadofluorine M with that of Gadomer in assessment of dysferlin-deficient muscular dystrophy at 7.0-T magnetic resonance (MR) imaging.

MATERIALS AND METHODS

All experiments were approved by local review boards. SJL/J mice (n = 24) with dysferlin-deficient muscular dystrophy and C57BL/6 control mice (n = 24) were imaged at 12-15 weeks (young) or older than 30 weeks (old) by using dynamic contrast material-enhanced imaging with inversion-prepared steady-state free-precession sequence before, during, and after administration of gadofluorine M at 2 micromol or Gadomer at 4 micromol intravenously. After imaging, regions of interest were determined from the upper extremity and left ventricular chamber; fractional extravascular extracellular volume, v(e), and permeability surface tissue density product, PS rho, were measured by using a two-compartment pharmacokinetic model. The natural history of muscular dystrophy was assessed histologically in 70 mice (seven five-mouse groups each of SJL/J mice and of control mice) at 4-week intervals from 8 to 32 weeks. In addition, three SJL/J mice and three control mice at age 33 weeks were sacrificed, and fluorescence microscopy was performed for visualization of intravenously administered carbocyanine-labeled gadofluorine M in muscle cells. Statistical analysis was performed by using the t test.

RESULTS

Gadofluorine M enhancement was significantly greater in skeletal muscle of 30-week-old mice with dysferlin-deficient muscular dystrophy, compared with control mice. Gadofluorine M demonstrated both increased rate of enhancement (PS rho sec(-1) +/- standard error of the mean: 0.004 e(-)(4) +/- 3 vs 0.002 e(-)(4) +/- 3; P < .05) and increased level of enhancement (v(e) +/- standard error of the mean: 0.035 +/- 0.004 vs 0.019 +/- 0.004; P < .05). Gadomer showed no differential enhancement in the two mouse groups. Histologic examination confirmed the presence of labeled gadofluorine M in muscle cells.

CONCLUSION

Gadofluorine M-enhanced MR imaging may be of value in monitoring dysferlin-deficient muscular dystrophy disease progression in this animal model and could prove to be a useful tool in following the course of chronic muscle diseases in humans.

MR imaging of adventitial vasa vasorum in carotid atherosclerosis

Vasa vasorum in the adventitia of atherosclerotic arteries may play a role in plaque progression. In this investigation, a method for characterizing vasa vasorum in the carotid artery is proposed, in which the perfusion properties of the adventitia are probed via dynamic contrast-enhanced (DCE) MRI. A parametric "vasa vasorum image" is automatically generated that depicts the plasma volume (vp) and transfer constant (K trans). The average K trans within the adventitia is proposed as a quantitative measurement related to the extent of the vasa vasorum. In 25 subjects with lesions meeting the requirements for carotid endarterectomy (CEA) significantly higher adventitial K trans of 0.155 +/- 0.045 min(-1) was observed, compared to 0.122 +/- 0.029 min(-1) in the remaining 20 subjects with moderate disease (P < 0.01). In the 25 subjects with endarterectomy specimens, histological evaluation showed that adventitial K trans was significantly correlated with the amount of neovasculature (R = 0.41; P = 0.04) and macrophages (R = 0.49; P = 0.01) in the excised plaque. In the remaining 20 subjects without histology, elevated adventitial K trans was significantly correlated with the log of C-reactive protein (CRP) levels (R = 0.57; P = 0.01) and was elevated in active smokers compared to nonsmokers (0.141 +/- 0.036 vs. 0.111 +/- 0.017 min(-1); P = 0.02). Because these factors are all associated with higher risk of atherosclerotic complications, these results suggest that adventitial K(trans) may be a marker of risk as well.

Incorporating a vascular term into a reference region model for the analysis of DCE-MRI data: a simulation study

A vascular term was incorporated into a reference region (RR) model analysis of DCE-MRI data, and its effect on the accuracy of the model in estimating tissue kinetic parameters in a tissue of interest (TOI) was systematically investigated through computer simulations. Errors in the TOI volume transfer constant (K(trans,TOI)) and TOI extravascular extracellular volume (v(e,TOI)) that result when the fractional plasma volume (v(p)) was included in (1) neither region, (2) TOI only (3) both regions were investigated. For nominal values of tumor kinetic parameters (v(e,TOI) = 0.40 and K(trans,TOI) = 0.25 min(-1)), if the vascular term was included in neither region or the TOI only, K(trans,TOI) error was within 20% for 0.03 < v(p,TOI) < 0.10, and v(e,TOI) error was within 20% for the range of v(p,TOI) studied (0.01-0.10). The effects of temporal resolution were shown to be complex, and in some cases errors increased with increasing temporal resolution.

Dynamic contrast-enhanced MRI of muscle perfusion combined with MR angiography of collateral artery growth in a femoral artery ligation model

To assess the use of MRI for evaluating changes in muscle blood flow and number of collateral arteries, serial dynamic contrast-enhanced MRI (DCE-MRI) was combined with high-spatial-resolution contrast-enhanced MR angiography (MRA) in a peripheral ischemia model. The combined MRI (DCE-MRI and MRA) protocol was performed serially in 15 male rabbits at 2 h (day 0(+)), 7 days, and 21 days after femoral artery ligation. In the anterior tibial and soleus muscle, changes in resting muscle blood flow determined as the endothelial transfer coefficient (K(trans)) and arterial inflow delay from DCE-MRI and changes in the number of sub-millimeter sized collateral arteries as scored with MRA were measured. Directly after ligation, K(trans) in the anterior tibial muscle was reduced to 23% of that in the control limb, then recovered to 81% on day 7, and to 85 % on day 21. K(trans) in the soleus muscle recovered from a reduction to 63% on day 0(+), to 85% on day 7, and to 90% on day 21. The number of collaterals around the ligated femoral artery increased from 1.1 on day 0(+) to 4.2 on day 7, and 6.0 on day 21 in the ligated limb only. Combined DCE-MRI and MRA allows non-invasive serial monitoring of changes in muscle blood flow and growth of sub-millimeter sized collateral arteries in a rabbit femoral artery ligation model.

T1 measurement of flowing blood and arterial input function determination for quantitative 3D T1-weighted DCE-MRI

PURPOSE

To propose a simple, accurate method for measuring T(1) in flowing blood and the arterial input function (AIF), and to evaluate the impact on dynamic contrast-enhanced MRI (DCE-MRI) quantification of pharmacokinetic parameters.

MATERIALS AND METHODS

A total of 10 rabbits were scanned at 1.5 Tesla and administered a bolus of Gadomer. Preinjection T(1) and AIF measurements were acquired in the iliac arteries using a rapid three-dimensional (3D) spoiled gradient recalled echo (SPGR) approach. Correction was made for imperfect B(1) fields, in-flow, and partial volume effects. DCE-MRI parameters blood volume (v(b)) and endothelial transfer constant (K(trans)) in resting skeletal muscle were estimated from pharmacokinetic analysis using individually measured AIFs. Literature comparisons were made to assess accuracy.

RESULTS

Blood T(1) was more accurate and precise after correction for B(1) and partial volume errors (1267 +/- 72 msec). Measured AIFs followed reported biexponential decay characteristics for Gadomer clearance in rabbits. Parameters v(b) (2.47 +/- 0.65%) and K(trans) (3.6 +/- 1.0 x 10(-3) minute(-1)) derived from AIFs based on corrected blood T(1)s were more reproducible and in better agreement with literature values.

CONCLUSION

The proposed method enables accurate in vivo blood T(1) and AIF measurements and can be easily implemented in a range of DCE-MRI applications to improve both the accuracy and reproducibility of pharmacokinetic parameters.

Pharmacokinetic modeling of dynamic contrast-enhanced MRI of the hand and wrist in rheumatoid arthritis and the response to anti-tumor necrosis factor-α therapy

Dynamic contrast-enhanced MRI (DCE-MRI) of the hand and wrist was performed in 11 patients with rheumatoid arthritis twice before and once 2 weeks after treatment with anti-tumor necrosis factor (TNF)-alpha therapy. A rapid, T1-weighted 3D spoiled gradient echo (SPGR) sequence was used for the dynamic imaging. T1 estimation was performed using similar images obtained at different flip angles. The relative radiofrequency field was estimated from the known T1 of the periarticular fatty marrow. The arterial input function (AIF) was measured at each examination, and normalized to the expected plasma concentration to reduce partial volume effects. Synovial enhancement was modeled to yield values for Ktrans, ve, and vp. Ktrans and ve showed good reproducibility. There was a significant decrease of about 20% in Ktrans after 2 weeks of treatment. This study demonstrates the potential of DCE-MRI and pharmacokinetic modeling to study early changes in inflammatory activity in rheumatoid arthritis following treatment.

Estimation of contrast agent concentration in intra- and extra-vascular spaces of brain tissue

This article presents a new method for estimating the leakage of a contrast agent out of a vessel. The proposed method is developed based on tissue homogeneity (TH) model, modified Patlak model, and Monte Carlo simulation. The analytical methods published in the literature estimate the contrast agent leakage by solving the coupled differential equations associated with the TH model under adiabatic conditions. These methods employ unrealistic simplifying assumptions and become intractable in their applications to the vessels that have a non-uniform permeability. Without making any unrealistic assumptions, our approach simply tracks the passage of the contrast agent through the capillary and its crossing of the vessel walls based on the blood flow in the vessel, the vessel's permeability, and the condition of the blood-brain barrier (BBB). These are treated as statistical processes that can be modeled reasonably well using the Monte Carlo method. In the proposed approach, the intra- and extra-vascular spaces are divided into multiple compartments, similar to the Patlak model. A real, measured arterial input function (AIF) is used as the capillary input and the concentration of the contrast agent is found as a function of time and distance, inside and outside of the capillary. This is done for normal and abnormal capillaries with uniform and non-uniform permeability. The proposed method generates concentration curves similar to those of the analytical method for simple AIF models. It also generates reasonable concentration curves for a real AIF. The proposed method does not fit a mathematical function to the measured AIF and does not make unrealistic simplifying assumptions. It is not therefore prone to the fitting errors and generates more realistic and more accurate results than the analytical methods.