Measurements of extracellular volume fraction and capillary permeability in tissues using dynamic spin-lattice relaxometry: studies in rabbit muscles

Assessment of Extracellular Volume Fraction in Becker Muscular Dystrophy by Using MR Fingerprinting

Background Quantitative MRI is increasingly proposed in clinical trials related to dystrophinopathies, including Becker muscular dystrophy (BMD). Purpose To establish the sensitivity of extracellular volume fraction (ECV) quantification using an MR fingerprinting sequence with water and fat separation as a quantitative imaging biomarker of skeletal muscle tissue alterations in BMD compared with fat fraction (FF) and water relaxation time quantification. Materials and Methods In this prospective study, study participants with BMD and healthy volunteers were included from April 2018 until October 2022 (ClinicalTrials.gov identifier NCT02020954). The MRI examination comprised FF mapping with the three-point Dixon method, water T2 mapping, and water T1 mapping before and after an intravenous injection of a gadolinium-based contrast agent by using MR fingerprinting, from which ECV was calculated. Functional status was measured with use of the Walton and Gardner-Medwin scale. This clinical evaluation tool stratifies disease severity from grade 0 (preclinical; elevated creatine phosphokinase; all activities normal) to grade 9 (unable to eat, drink, or sit without assistance). Mann-Whitney U tests, Kruskal-Wallis tests, and Spearman rank correlation tests were performed. Results Twenty-eight participants with BMD (median age, 42 years [IQR, 34-52 years]; 28 male) and 19 healthy volunteers (median age, 39 years [IQR, 33-55 years]; 19 male) were evaluated. ECV was higher in participants with dystrophy than in controls (median, 0.21 [IQR, 0.16-0.28] vs 0.07 [IQR, 0.07-0.08]; P < .001). In muscles of participants with BMD with normal FF, ECV was also higher than in muscles of healthy controls (median, 0.11 [IQR, 0.10-0.15] vs 0.07 [IQR, 0.07-0.08]; P = .02). ECV was correlated with FF (ρ = 0.56, P = .003), Walton and Gardner-Medwin scale score (ρ = 0.52, P = .006), and serum cardiac troponin T level (ρ = 0.60, P < .001). Conclusion Quantitative MR relaxometry with water and fat separation indicates a significant increase of skeletal muscle extracellular volume fraction in study participants with Becker muscular dystrophy. Clinical trial registration no. NCT02020954 Published under a CC BY 4.0 license. Supplemental material is available for this article.

Feasibility of MRI based extracellular volume fraction and partition coefficient measurements in thigh muscle

OBJECTIVE

This study aimed to assess the feasibility of extracellular volume-fraction (ECV) measurement, and time to achieve contrast equilibrium (CE), in healthy muscles, and to determine whether in-flow and partial-volume errors in the femoral artery affect measurements, and if there are differences in the partition coefficient (λ) between muscles.

METHODS

T1 was measured in the biceps femoris, vastus intermedius, femoral artery and aorta of 10 healthy participants. This was repeated alternately between the thigh and aorta for ≥25 min following a bolus of gadoterate meglumine. λ was calculated for each muscle/blood measurement. Time to CE was assessed semi-quantitatively.

RESULTS

8/10 participants achieved CE. Time to CE = 19±2 min (mean ± 95% confidence interval). Measured λ: biceps femoris/aorta = 0.210±0.034, vastus intermedius/aorta = 0.165±0.015, biceps femoris/femoral artery = 0.265±0.054, vastus intermedius/femoral artery = 0.211±0.026. There were significant differences in λ between the muscles when using the same vessel (p < 0.05), and between λ calculated in the same muscle when using different vessels (p < 0.05).

CONCLUSION

ECV measurements in the thigh are clinically feasible. The use of the femoral artery for the blood measurement is associated with small but significant differences in λ. ECV measurements are sensitive to differences between muscles within the healthy thigh.

ADVANCES IN KNOWLEDGE

This paper determines the time to contrast equilibrium in the healthy thigh and describes a method for measuring accurately ECV in skeletal muscle. This can aid in the diagnosis and understanding of inflammatory auto-immune diseases.

Assessment of passive muscle elongation using Diffusion Tensor MRI: Correlation between fiber length and diffusion coefficients

In this study we investigated the changes in fiber length and diffusion parameters as a consequence of passive lengthening and stretching of the calf muscles. We hypothesized that changes in radial diffusivity (RD) are caused by changes in the muscle fiber cross sectional area (CSA) as a consequence of lengthening and shortening of the muscle. Diffusion Tensor MRI (DT-MRI) measurements were made twice in five healthy volunteers, with the foot in three different positions (30° plantarflexion, neutral position and 15° dorsiflexion). The muscles of the calf were manually segmented on co-registered high resolution anatomical scans, and maps of RD and axial diffusivity (AD) were reconstructed from the DT-MRI data. Fiber tractography was performed and mean fiber length was calculated for each muscle group. Significant negative correlations were found between the changes in RD and changes in fiber length in the dorsiflexed and plantarflexed positions, compared with the neutral foot position. Changes in AD did not correlate with changes in fiber length. Assuming a simple cylindrical model with constant volume for the muscle fiber, the changes in the muscle fiber CSA were calculated from the changes in fiber length. In line with our hypothesis, we observed a significant positive correlation of the CSA with the measured changes in RD. In conclusion, we showed that changes in diffusion coefficients induced by passive muscle stretching and lengthening can be explained by changes in muscle CSA, advancing the physiological interpretation of parameters derived from skeletal muscle DT-MRI.

MEMRI and tumors: a method for the evaluation of the contribution of Mn(II) ions in the extracellular compartment

The purpose of the work was to set-up a simple method to evaluate the contribution of Mn(2+) ions in the intra- and extracellular tumor compartments in a MEMRI experiment. This task has been tackled by "silencing" the relaxation enhancement arising from Mn(2+) ions in the extracellular space. In vitro relaxometric measurements allowed assessment of the sequestering activity of DO2A (1,4,7,10-tetraazacyclododecane-1,7-diacetic acid) towards Mn(2+) ions, as the addition of Ca-DO2A to a solution of MnCl2 causes a drop of relaxivity upon the formation of the highly stable and low-relaxivity Mn-DO2A. It has been proved that the sequestering ability of DO2A towards Mn(2+) ions is also fully effective in the presence of serum albumin. Moreover, it has been shown that Mn-DO2A does not enter cell membranes, nor does the presence of Ca-DO2A in the extracellular space prompt migration of Mn ions from the intracellular compartment. On this basis the in vivo, instantaneous, drop in SE% (percent signal enhancement) in T1 -weighted images is taken as evidence of the sequestration of extracellular Mn(2+) ions upon addition of Ca-DO2A. By applying the method to B16F10 tumor bearing mice, T1 decrease is readily detected in the tumor region, whereas a negligible change in SE% is observed in kidneys, liver and muscle. The relaxometric MRI results have been validated by ICP-MS measurements.

Can dynamic contrast-enhanced magnetic resonance imaging combined with texture analysis differentiate malignant glioneuronal tumors from other glioblastoma?

An interesting approach has been proposed to differentiate malignant glioneuronal tumors (MGNTs) as a subclass of the WHO grade III and IV malignant gliomas. MGNT histologically resemble any WHO grade III or IV glioma but have a different biological behavior, presenting a survival twice longer as WHO glioblastomas and a lower occurrence of metastases. However, neurofilament protein immunostaining was required for identification of MGNT. Using two complementary methods, dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) and texture analysis (MRI-TA) from the same acquisition process, the challenge is to in vivo identify MGNT and demonstrate that MRI postprocessing could contribute to a better typing and grading of glioblastoma. Results are obtained on a preliminary group of 19 patients a posteriori selected for a blind investigation of DCE T1-weighted and TA at 1.5 T. The optimal classification (0/11 misclassified MGNT) is obtained by combining the two methods, DCE-MRI and MRI-TA.

Imaging the extracellular pH of tumors by MRI after injection of a single cocktail of T1 and T2 contrast agents

The extracellular pH (pH(e) ) of solid tumors is acidic, and there is evidence that an acidic pH(e) is related to invasiveness. Herein, we describe an MRI single-infusion method to measure pH(e) in gliomas using a cocktail of contrast agents (CAs). The cocktail contained gadolinium-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraaminophosphonate (GdDOTA-4AmP) and dysprosium-1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetrakis(methylenephosphonic acid) (DyDOTP), whose effects on relaxation are sensitive and insensitive to pH, respectively. The Gd-CA dominated the spin-lattice relaxivity ΔR(1) , whereas the Dy-CA dominated the spin-spin relaxivity ΔR(2)*. The ΔR(2)* effects were used to determine the pixel-wise concentration of [Dy] which, in turn, was used to calculate a value for [Gd] concentration. This value was used to convert ΔR(1) values to the molar relaxivity Δr(1) and, hence, pH(e) maps. The development of the method involved in vivo calibration and measurements in a rat brain glioma model. The calibration phase consisted of determining a quantitative relationship between ΔR(1) and ΔR(2)* induced by the two pH-independent CAs, gadolinium-diethylenetriaminepentaacetic acid (GdDTPA) and DyDOTP, using echo planar spectroscopic imaging (EPSI) and T(1) -weighted images. The intensities and linewidths of the water peaks in EPSI images were affected by CA and were used to follow the pharmacokinetics. These data showed a linear relationship between inner- and outer-sphere relaxation rate constants that were used for CA concentration determination. Nonlinearity in the slope of the relationship was observed and ascribed to variations in vascular permeability. In the pH(e) measurement phase, GdDOTA-4AmP was infused instead of GdDTPA, and relaxivities were obtained through the combination of interleaved T(1) -weighted images (R(1) ) and EPSI for ΔR(2)*. The resulting r(1) values yielded pH(e) maps with high spatial resolution.

Dual-temporal resolution dynamic contrast-enhanced MRI protocol for blood-brain barrier permeability measurement in enhancing multiple sclerosis lesions

PURPOSE

To design a more accurate and reproducible technique for the measurement of blood-brain barrier (BBB) permeability in gadolinium-enhancing multiple sclerosis (MS) lesions.

MATERIALS AND METHODS

Four MS patients were scanned using a new dynamic contrast-enhanced (DCE)-magnetic resonance imaging (MRI) protocol based on an uninterrupted two-part acquisition consisting of an initial part at high temporal and low spatial resolutions and a second part at low temporal and high spatial resolutions. The method preserves both the high spatial resolution needed for the often small size of lesions and the high temporal resolution required during the first minute after injection to sufficiently sample the first-pass bolus. Simulations compared the performance of this new protocol with the conventional one at low temporal and high spatial resolutions throughout.

RESULTS

The BBB permeability estimates changed by up to 33% between the two protocols. The new protocol led to simulated error on K(trans) of 7%-10%, versus 7%-30% with the conventional protocol, and was more robust with respect to offsets between acquisition and injection start times, differences in shape of the first-pass peak, and permeability values.

CONCLUSION

The dual-temporal resolution protocol produces improved BBB permeability estimates and provides a more complete view of active inflammatory MS lesion pathology.

Myofiber ellipticity as an explanation for transverse asymmetry of skeletal muscle diffusion MRI in vivo signal

Due to its unique non-invasive microstructure probing capabilities, diffusion tensor imaging (DTI) constitutes a valuable tool in the study of fiber orientation in skeletal muscles. By implementing a DTI sequence with judiciously chosen directional encoding to quantify in vivo the microarchitectural properties in the calf muscles of three healthy volunteers at rest, we report that the secondary eigenvalue is significantly higher than the tertiary eigenvalue, a phenomenon corroborated by prior DTI findings. Toward a physics-based explanation of this phenomenon, we propose a composite medium model that accounts for water diffusion in the space within the muscle fiber and the extracellular space. The muscle fibers are abstracted as cylinders of infinite length with an elliptical cross section, the latter closely approximating microstructural features well documented in prior histological studies of excised muscle. The range of values of fiber ellipticity predicted by our model agrees with these studies, and the spatial orientation of the cross-sectional ellipses is consistent with local muscle strain fields and the putative direction of lateral transmission of stress between fibers in certain regions in three antigravity muscles (Tibialis Anterior, Soleus, and Gastrocnemius), as well as independent measurements of deformation in active calf muscles. As a metric, fiber cross-sectional ellipticity may be useful for quantifying morphological changes in skeletal muscle fibers with aging, hypertrophy, or sarcopenia.

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.

Extracellular space volume measured by two-color pulsed dye infusion with microfiberoptic fluorescence photodetection

The extracellular space (ECS) is the aqueous matrix surrounding cells in solid tissues. The only method to measure ECS volume fraction (alpha) in vivo has been tetramethylammonium iontophoresis, a technically challenging method developed more than 25 years ago. We report a simple, quantitative method to measure alpha by microfiberoptic fluorescence detection of a self-quenched green dye, calcein, and a reference red dye, sulforhodamine 101, after pulsed iontophoretic infusion. The idea is that the maximum increase in calcein fluorescence after iontophoresis is proportional to the aqueous volume into which the dye is deposited. We validated the method theoretically, and experimentally, using cell-embedded gels with specified alpha and ECS viscosity. Measurements in living mice gave alpha of 0.20 +/- 0.01 in brain, 0.13 +/- 0.02 in kidney and 0.074 +/- 0.01 in skeletal muscle. The technical simplicity of the "pulsed-infusion microfiberoptic photodetection" method developed here should allow elucidation of the relatively understudied biological roles of the ECS.

Quantitative estimation of dynamic contrast enhanced MRI parameters in rat brain gliomas using a dual surface coil system

RATIONALE AND OBJECTIVES

The study of vascular targeted cancer therapies is critically dependent on the development of noninvasive methods for assessing changes in vascular permeability and blood flow in small-animal tumor models.

MATERIALS AND METHODS

A multicoil apparatus consisting of two receive-only surface coils for observation of the rat brain and chest, a whole-body transmit-only volume coil, and the switching circuitry necessary for sharing a single receiver channel between the two surface coils was constructed for the parallel observation of left ventricular arterial input function and magnetic resonance imaging (MRI) signal intensity kinetics in the rat brain. Dynamic contrast-enhanced MRI was performed on four Fischer rats bearing intracranial 9L gliomas, and the dynamic data were evaluated using the bolus-enhanced relaxation overview (BOLERO) model yielding maps of K(trans), v(e), and tau(i) values from the tumor.

RESULTS

The use of the multicoil apparatus resulted in images with high signal-to-noise ratios from the rat brain and chest in parallel, with no detectable crosstalk between the surface coils. The BOLERO method accurately fit the observed data to within experimental error. Mean values of the parameters generated by the BOLERO analysis for the tumor were K(trans) = 0.023 +/- 0.014 s(-1), tau(i) = 1.3 +/- 0.6 seconds, and v(e) = 0.51. K(trans) and tau(i) values were slightly elevated in the tumor periphery, whereas v(e) was elevated in the tumor core.

CONCLUSION

These results demonstrate the feasibility of measuring quantitative dynamic contrast-enhanced MRI parameters in a rat brain tumor model using a multicoil apparatus. These methods might play an important role in determining the efficacy of antiangiogenic therapies in small-animal models.

Recent advances in breast MRI and MRS

Breast MRI is an area of intense research and is fast becoming an important tool for the diagnosis of breast cancer. This review covers recent advances in breast MRI, MRS, and image post-processing and analysis. Several studies have explored a multi-parametric approach to breast imaging that combines analysis of traditional contrast enhancement patterns and lesion architecture with novel methods such as diffusion, perfusion, and spectroscopy to increase the specificity of breast MRI studies. Diffusion-weighted MRI shows some potential for increasing the specificity of breast lesion diagnosis and is even more promise for monitoring early response to therapy. MRS also has great potential for increasing specificity and for therapeutic monitoring. A limited number of studies have evaluated perfusion imaging based on first-pass contrast bolus tracking, and these clearly identify that vascular indices have great potential to increase specificity. The review also covers the relatively new acquisition technique of MR elastography for breast lesion characterization. A brief survey of image processing algorithms tailored for breast MR, including registration of serial dynamic images, segmentation and extraction of morphological features of breast lesions, and contrast uptake modeling, is also included. Recent advances in MRI, MRS, and automated image analysis have increased the utility of breast MR in diagnosis, screening, management, and therapy monitoring of breast cancer.

Capillary permeability and extracellular volume fraction in uterine cervical cancer as patient outcome predictors: measurements by using dynamic MRI spin-lattice relaxometry

PURPOSE

To improve the outcome prediction of uterine cervical carcinoma by measuring the vascular permeability (k(ep)) and the extracellular volume fraction (v(e)) of the tumor from Dynamic T(1)- IRM Relaxometry.

MATERIALS AND METHODS

Twenty-six patients with proven cervical carcinoma were divided into good outcome and poor outcome groups. Classic tumor prognostic factors, the longest diameter L and the volume V of the tumor, were measured from morphologic MR images. The tumor parameters k(ep) and v(e) were determined from the relaxometry time-curve acquired during the contrast uptake after a bolus intravenous injection of an extracellular contrast agent.

RESULTS

All "small" tumors (L<35 mm or V<11 cm(3)) were good outcome with 100% sensitivity but a rather low specificity (36% and 43% for L and V, respectively). With regard to the physiopathological parameter k(ep), "large" tumors (L >or= 35 mm) can also be classified as good outcome on the condition that k(ep) >or= 2.2 min(-1) with 100% sensitivity and 89% specificity. Regarding the extracellular volume fraction (v(e)), no significant difference was observed between the two groups.

CONCLUSION

Measurement of the tumor vascular permeability might be useful to predict prognostic, to evaluate the treatment efficacy, and to adapt a proper therapy schedule.

In vivo study of cross-sectional skeletal muscle fiber asymmetry with diffusion-weighted MRI

Skeletal muscles are highly organized hierarchical structures characterized by an anisotropic arrangement of muscle fibers (myocytes) in fascicles. Due to its unique non-invasive microstructure probing capabilities, diffusion-weighted Magnetic Resonance Imaging (DW-MRI) constitutes a valuable non-invasive tool in the study of such fibrous biological tissues. We have implemented a DW-MRI sequence with highly sensitive directional encoding to quantify the microarchitectural properties of human calf muscles at rest. We have specifically focused on a composite model-based analysis of diffusion tensor MRI measurements to quantify in vivo the cross-sectional asymmetry of muscle fiber geometry, which is a microstructural feature well documented in prior histological studies.

Magnetic resonance imaging measurements of vascular permeability and extracellular volume fraction of breast tumors by dynamic Gd-DTPA-enhanced relaxometry

Vascular permeability (k(ep), min(-1)) and extracellular volume fraction (v(e)) are tissue parameters of great interest to characterize malignant tumor lesions. Indeed, it is well known that tumors with high blood supply better respond to therapy than poorly vascularized tumors, and tumors with large extracellular volume tend to be more malignant than tumors showing lower extracellular volume. Furthermore, the transport of therapeutic agents depends on both extracellular volume fraction and vessel permeability. Thus, before treatment, these tissue parameters may prove useful to evaluate tumor aggressiveness and to predict responsiveness to therapy and variations during cytotoxic therapies could allow to assess treatment efficacy and early modified therapy schedules in case of poor responsiveness. As a consequence, there is a need to develop methods that could be routinely used to determine these tissue parameters. In this work, blood-tissue permeability and extracellular volume fraction information were derived from magnetic resonance imaging dynamic longitudinal relaxation rate (R(1)) mapping obtained after an intravenous bolus injection of Gd-DTPA in a group of 92 female patients with breast lesions, 68 of these being histologically proven to be with carcinoma. For the sake of comparison, 24 benign lesions were studied. The measurement protocol based on two-dimensional gradient echo sequences and a monoexponential plasma kinetic model was that validated in the occasion of previous animal experiments. As a consequence of neoangiogenesis, results showed a higher permeability in malignant than in benign lesions, whereas the extracellular volume fraction value did not allow any discrimination between benign and malignant lesions. The method, which can be easily implemented whatever the imaging system used, could advantageously be used to quantify lesion parameters (k(ep) and v(e)) in routine clinical imaging. Because of its large reproducibility, the method could be useful for intersite comparisons and follow-up studies.

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

The purpose of this work was to investigate the use of an intravascular contrast agent to determine perfusion kinetics in skeletal muscle. A two-compartment kinetic model was used to represent the flux of contrast agent between the intravascular space and extravascular extracellular space (EES). The relationship between the image signal-to-noise ratio (SNR) and errors in estimating permeability surface area product (Ktrans), interstitial volume (ve), and plasma volume (vp) for linear and nonlinear curve-fitting methods was estimated from Monte Carlo simulations. Similar results were obtained for both methods. For an image SNR of 60, the estimated errors in these parameters were 10%, 22%, and 17%, respectively. In vivo experiments were conducted in rabbits to examine physiological differences between these parameters in the soleus (SOL) and tibialis anterior (TA) muscles in the hind limb. Values for Ktrans were significantly higher in the SOL (3.2+/-0.9 vs. 2.0+/-0.5x10(-3) min-1), as were values for vp (3.4+/-0.8 vs. 2.1+/-0.7%). Differences in ve for the two muscles (8.7+/-2.2 vs. 8.5+/-1.6%) were not found to be significant. These results demonstrate that relevant physiological metrics can be calculated in skeletal muscle using MRI with an intravascular contrast agent.

Measurement of skeletal muscle perfusion during postischemic reactive hyperemia using contrast-enhanced MRI with a step-input function

The regional distribution of skeletal muscle blood flow was measured during postischemic reactive hyperemia using Gd-DTPA contrast-enhanced (CE) MRI. The release of an occlusive thigh cuff was used to deliver a step-input of contrast concentration that was coincident with the onset of reactive hyperemia. A first-order tracer kinetic equation was used to estimate the unidirectional influx constant, Ki (ml/100 g/min), and the distribution volume of Gd-DTPA in the tissue, v(e), from T1-weighted images acquired with saturation recovery (SR) steady-state free precession (SSFP) and spoiled gradient-echo (SPGR) protocols. The capillary permeability surface (PS) area increased significantly during reactive hyperemia, which facilitated rapid extraction of Gd-DTPA during the first pass. Regional muscle group studies from 11 normal volunteers yielded blood flow (Ki) values of 108.3 +/- 34.1 ml/100 g/min in the gastrocnemius, 184.3 +/- 41.3 ml/100 g/min in the soleus, and 122.4 +/- 34.4 ml/100 g/min in the tibialis anterior. The distribution volumes (v(e)) in the corresponding muscle groups were respectively 8.3% +/- 2.1%, 9.3% +/- 1.9%, and 7.9% +/- 1.8% from the kinetic model, and 8.8% +/- 2.4%, 9.1% +/- 1.9%, and 7.2% +/- 1.4% from tissue relaxometry studies. Bulk blood flow studies in the same volunteers using phase-contrast velocimetry (popliteal artery) yielded significantly lower flow values, but with a correlation coefficient R2 = 0.62 and P = 0.004.

Magnetic resonance imaging contrast-enhanced relaxometry of breast tumors: an MRI multicenter investigation concerning 100 patients

Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) using extracellular contrast agents has proved to be useful for the characterization of breast tumors. DCE-MRI has demonstrated a high sensitivity (around 95%) but a rather poor and controversial specificity, varying, according to the different studies, from 45% to 90%. In order to increase (a) the specificity and (b) the robustness of this quantitative approach in multicenter evaluation (five MRI units), a quantitative approach called dynamic relaxometry has been developed. According to the proposed method, the time-dependent longitudinal relaxation rate measured on region of interest of the lesion was calculated during the contrast uptake, after intravenous bolus injection of contrast agent. A specifically developed method was used for fast R(1) measurements. Relaxometry time curves are fitted to the Tofts model allowing the measurement of the parameters describing the enhancement curve (maximum relation rate enhancement, initial, 30-s and 60-s slopes) and the tissue parameters [transfer constant (K(trans) min(-1)) and extracellular extravascular space fraction (v(e))]. Correspondence factorial analysis followed by hierarchical ascendant classification are then performed on the different parameters. Higher K(trans) values were observed in infiltrative ductal carcinomas than in infiltrative lobular carcinomas, in agreement with data published by other groups. Specificity of DCE-MRI has been increased up to 85%, with a sensitivity of 95% with K(trans)/v(e) and enhancement index I (ratio of initial slope by maximum relaxation rate enhancement). A multiparametric data analysis of the calculated parameters opens the way to include quantitative image-based information in new nosologic approaches to breast tumors.

Phosphate uptake in rat skeletal muscle is reduced during isometric contractions

During contractions, there is a net efflux of phosphate from skeletal muscle, likely because of an elevated intracellular inorganic phosphate (P(i)) concentration. Over time, contracting muscle could incur a substantial phosphate deficit unless P(i) uptake rates were increased during contractions. We used the perfused rat hindquarter preparation to assess [(32)P]P(i) uptake rates in muscles at rest or over a range of energy expenditures during contractions at 0.5, 3, or 5 Hz for 30 min. P(i) uptake rates were reduced during contractions in a pattern that was dependent on contraction frequency and fiber type. In soleus and red gastrocnemius, [(32)P]P(i) uptake rates declined by approximately 25% at 0.5 Hz and 50-60% at 3 and 5 Hz. Uptake rates in white gastrocnemius decreased by 65-75% at all three stimulation frequencies. These reductions in P(i) uptake are not likely confounded by changes in precursor [(32)P]P(i) specific activity in the interstitium. In soleus and red gastrocnemius, declines in P(i) uptake rates were related to energy expenditure over the contraction duration. These data imply that P(i) uptake in skeletal muscle is acutely modulated during contractions and that decreases in P(i) uptake rates, in combination with expected increases in P(i) efflux, exacerbate the net loss of phosphate from the cell. Enhanced uptake of P(i) must subsequently occur because skeletal muscle typically maintains a relatively constant total phosphate pool.