Apparent exchange rate mapping with diffusion MRI

Although diffusion MRI has shown promise for characterization of breast cancer, it has low specificity to malignant subtypes. Higher specificity might be achieved if the effects of cell morphology and molecular exchange across cell membranes are disentangled. Quantifying exchange might thus allow differentiation of different types of breast cancer cells. Based on differences in diffusion rates between the intra and extracellular compartments, filter exchange spectroscopy/imaging (FEXSY/FEXI) provides non-invasive quantification of the apparent exchange rate (AXR) of water between the two compartments. To test the feasibility of FEXSY for differentiating different breast cancer cells, we performed experiments on several breast epithelial cell lines in vitro. Furthermore, we performed the first in vivo FEXI measurement of water exchange in human breast. In cell suspensions, pulse gradient spin-echo (PGSE) experiments with large b-values and variable pulse duration allow characterizing the intracellular compartment while FEXSY provides a quantification of AXR. These experiments are very sensitive to the physiological state of cells and can be used to establish reliable protocols for culturing and harvesting of cells. Our results suggest that different breast cancer subtypes can be distinguished based on their AXR values in cell suspensions. Time-resolved measurements allow monitoring the physiological state of cells in suspensions over the time-scale of hours, and reveal an abrupt disintegration of the intracellular compartment. In vivo, exchange can be detected in a tumor, while in normal tissue the exchange rate is outside the range experimentally accessible for FEXI. At present, low SNR and limited scan time allows quantification of AXR only in an ROI of relatively large tumors.

The rates and activation parameters of water exchange at pH 3.0 have been determined using variable temperature 17O NMR spectroscopy for four CoII complexes and one MnII complex: [Co(bpy)(H2O)4]2+, [Co(bpy)2 (H2O)2]2+, [Co(phen)-(H2O)4]2+, [Co(phen)2 (H2O)2]2+, and [Mn(bpy)(H2O)4]2+ (bpy = 2,2'-bipyridyl and phen = 1,10-phenanthroline). Substitution of aquo ligands with 1,10-phenanthroline or 2,2'-bipyridyl leads to an increase in the rate of exchange in the manganese complexes, from k298 (1.8 ± 0.1) × 107 s-1 for [Mn(H2O)6]2+ to (7.2 ± 0.3) × 107 s-1 for [Mn(phen)2 (H2O)2]2+, whereas the trends are more complex for the cobalt complexes. We have used the new data in conjunction with literature data for similar complexes to analyse the effect of M–OH2 distance and degree of substitution.

Cell death plays a key role in cancer progression and treatment. After its onset, cell membrane integrity is often compromised. Here, it is shown in cells, animals and humans that the transmembrane water exchange rate, measured using magnetic resonance filter-exchange spectroscopy (FEXSY) or imaging (FEXI), and quantified by the apparent exchange rate (AXR), is an early biomarker of cell death. AXR measurements detect the onset of cell death in vivo earlier than diffusion-weighted MRI (DWI) and in vitro with similar sensitivity as flow cytometry. 13C magnetic resonance imaging (MRI) measurements of the malate/fumarate ratio (MFR), following injection of hyperpolarized [1,4-13C2]fumarate, in tumors displaying diffuse necrosis, are correlated with AXR. AXR detects early treatment response in colorectal and lymphoma tumor models while the apparent diffusion coefficient (ADC) measured by DWI remains unchanged. AXR measurements in human uterine fibroids show sensitivity to post-treatment changes soon after embolization, demonstrating clinical translatability of the method.

Measurements have been made of the rate of exchange of tritiated water in both intact and hypophysectomized Fundulus kansae in a variety of media. Hypophysectomy reduces the rate of exchange in fresh water. Low doses (30 mu) of ovine prolactin stimulate water turnover in hypophysectomized fish in fresh water. The rate of exchange declines in both intact and hypophysectomized animals with increasing salinity. Experiments with synthetic solutions show that the decline in the rate of exchange in sea water and in higher salinities is due mainly to the effects of calcium ions. Fishes maintained in synthetic sea water containing a low concentration of calcium have both a higher rate of exchange of tritiated water and a higher drinking rate than fish in normal sea water. Supported in part by National Science Foundation Grant GB 7200, and by Biomedical Support Grant Fr-07053 from the General Research Support Branch, Division of Research Resources, Bureau of Health Professional Education and Manpower Training, National Institutes of Health. Visiting Professor, Zoology Department, University of Missouri, Columbia. Professor Potts’s travel and research was supported by the Faculty Improvement Programme. Prolactin (NIH-P-S9) was generously supplied by the National Institute of Arthritis and Metabolic Diseases.

Cystathionine γ-synthase catalyzes replacement of the succinyl group of O-succinylhomoserine by cysteine to yield cystathionine, a step in methionine biosynthesis in Salmonella. The enzyme catalyzes rapid exchanges of both α and β hydrogen atoms in many amino acids that are not otherwise substrates. Stereospecificity was indicated by the exchangeability of all 4 hydrogen residues in alanine but of only 2 in α-aminobutyrate. Neither the replacement of a substituent on carbon 4 nor the exchange of a hydrogen on carbon 3 have been observed with any other pyridoxal phosphate enzyme. Cystathionine itself underwent exchange of 4 hydrogen residues, the α hydrogen and 1 β hydrogen in each chain. The rate of exchange in the 3-carbon chain was 6 times greater than either the exchange rate in the other chain or the Vmax for net cystathionine formation. In the 4-carbon chain, the apparent initial rate of β hydrogen exchange was twice that of α hydrogen exchange. Cystathionine was also found to undergo slow γ elimination and γ replacement reactions; the hydrogen exchange rates were about 103 times faster than the rates of cysteine exchange or elimination. The apparent Km for cystathionine was the same in all reactions. No hydrogen exchanges were catalyzed by apoenzyme (the Km for pyridoxal-P was also determined (4 x 10-8 m)). These results indicate that the γ elimination step catalyzed by cystathionine synthase is unconcerted and that the enzyme is efficiently designed for general base attack on an amino acid β hydrogen. The solvent hydrogen previously shown to be acquired at carbon 3 of α-ketobutyrate during the γ elimination reaction is introduced stereospecifically. This protonation is, therefore, catalyzed by the enzyme.

Blood–brain barrier (BBB) dysfunction occurs in many brain diseases, and there is increasing evidence to suggest that it is an early process in dementia which may be exacerbated by peripheral infection. Filter-exchange imaging (FEXI) is an MRI technique for measuring trans-membrane water exchange. FEXI data is typically analysed using the apparent exchange rate (AXR) model, yielding estimates of the AXR. Crusher gradients are commonly used to remove unwanted coherence pathways arising from longitudinal storage pulses during the mixing period. We first demonstrate that when using thin slices, as is needed for imaging the rodent brain, crusher gradients result in underestimation of the AXR. To address this, we propose an extended crusher-compensated exchange rate (CCXR) model to account for diffusion-weighting introduced by the crusher gradients, which is able to recover ground truth values of BBB water exchange (kin) in simulated data. When applied to the rat brain, kin estimates obtained using the CCXR model were 3.10 s−1 and 3.49 s−1 compared to AXR estimates of 1.24 s−1 and 0.49 s−1 for slice thicknesses of 4.0 mm and 2.5 mm respectively. We then validated our approach using a clinically relevant Streptococcus pneumoniae lung infection. We observed a significant 70 ± 10% increase in BBB water exchange in rats during active infection (kin = 3.78 ± 0.42 s−1) compared to before infection (kin = 2.72 ± 0.30 s−1; p = 0.02). The BBB water exchange rate during infection was associated with higher levels of plasma von Willebrand factor (VWF), a marker of acute vascular inflammation. We also observed 42% higher expression of perivascular aquaporin-4 (AQP4) in infected animals compared to non-infected controls, while levels of tight junction proteins remain consistent between groups. In summary, we propose a modelling approach for FEXI data which removes the bias in estimated water-exchange rates associated with the use of crusher gradients. Using this approach, we demonstrate the impact of peripheral infection on BBB water exchange, which appears to be mediated by endothelial dysfunction and associated with an increase in perivascular AQP4.

The white matter (WM) of the brain is damaged in multiple sclerosis (MS), even in areas that appear normal on standard MR imaging. The purpose of our study is to evaluate the damage of normal appearing white matter (NAWM) in patients with MS. In our study, 84 MS patients and 42 healthy adults underwent a routine brain MRI, including also diffusion tensor imaging (DTI). All studies were performed on a 3 T MRI scanner. Fractional anisotropy (FA) and apparent diffusion coefficient (ADC) values were obtained. The DTI parameters of NAWM were correlated with expanded disability status scales (EDSS) scores. Our results showed statistically significant differences in FA and ADC values between MS plaques and the symmetrical NAWM, as also between NAWM and the respective white matter in controls. The ADC values of the NAWM correlated with the EDSS scores. The present study demonstrated damage of the NAWM in MS patients, using DTI in 3.0 T. DTI may be used in the detection of subtle damage of the white matter.

Background: While it is known that a reduction of blood flow leads to a decrease in apparent diffusion coefficient (ADC), how ADC evolves as a function of time from ischemia onset is unclear. We evaluated ADC signal change in brain tissue with temporally stable hypoperfusion, examining the relationship between severity of hypoperfusion and the rate of ADC decline during acute ischemia. Methods: Ischemic stroke patients were sequentially imaged with diffusion-perfusion weighted imaging at <4.5hr (tp1) and at 6hr (tp2) after stroke onset, to directly assess the temporal evolution of ADC. One month FLAIR provided the final infarct region. Registration aligned all images for each patient. Dynamic susceptibility contrast method allowed calculation of mean transit time (MTT). Hypoperfusion was defined as tissue with MTT > 4sec + the contralateral median MTT. In tissue with stable hypoperfusion on tp1 and tp2, ADC regions of interest (ROIs) with normal or moderate reduction on tp1 were evaluated. To avoid capturing tp1 ADC voxels that had already maximally declined (floor effect, <30 ×10 -5 mm 2 /s), ADC values < 60 were excluded. To assess how ADC declined with varying degrees of hypoperfusion, tissue was divided into 6 MTT ranges: 3-6 (ROI1), 6-9 (ROI2), 9-12 (ROI3), 12-15 (ROI4), 15-18 (ROI5) and >18 (ROI6) seconds. Paired t-tests compared tp1 and tp2 ROIs for each patient. Infarct probability based on tp3 FLAIR was computed for each ROI. Results: 43 patients were imaged at 2.9 (tp1) and 6.4 hrs (tp2) after stroke onset, of which 37 returned for 1 month FLAIR. The 6 ROIs with increasing MTT severity were found in 41, 27, 19, 12, 6, and 10 patients for ROI’s 1-6, respectively. ADC values for all ROIs were similar at tp1 (near normal), despite different tp1 MTT. For ROI1-3, ADC values did not decline between tp1 and tp2 or decreased mildly (ROI3, P=0.079). For severe hypoperfusion, ADC decreased significantly: ROI4 (p=0.006), ROI5 (P=0.038), and ROI6 (P=0.0001). Using linear regression to approximate the ADC decline, the slopes were significantly different from zero with ADC decay rates of -1.87×10 -5 mm 2 /s/hr (R=0.58), -2.86×10 -5 mm 2 /s/hr (R=0.65) and -4.51×10 -5 mm 2 /s /hr (R=0.82) for ROI4-6, respectively ( Figure ). The median infarct probabilities for the 6 ROI’s increased with increasing MTT severity: 16%, 53%, 57%, 87%, 100%, and 90% for ROI1-6, respectively. Conclusions: ADC remained normal after ischemic stroke in regions with stable, moderate hypoperfusion. In contrast, with stable, severe hypoperfusion (MTT>12s), ADC decreased from 3-6 hours and the rate of decline was highly dependent on the degree of hypoperfusion.

Objective To analyze the characteristics of magnetic resonance diffusion tensor imaging(DTI)and diffusion tensor tractography (DTIT) in patients with cerebral infarction, and explore the diagnosis values and prognosis of diffusion weighted imaging (DWI) and diffusion tensor imaging (DTI) in patients with cerebral infarction in different stages. Methods 58 patients with cerebral infarction in different stages and 25 healthy volunteers were examined by magnetic resonance imaging (MRI), including conventional T1 and T2 weighted imaging, DWI and DTI. Fractional anisotropy (FA) images were reconstructed. The values of FA and apparent diffusion coefficient (ADC) were measured in the infarcted regions, corresponding contralateral normal regions and corresponding normal regions in normal control group. Results ①DWI and DTI showed size of infarction focus was more accurate and clearer than that of conventional MRI; ②The FA and ADC values of the infarcted regions during superacute stage, acute stage, subacute stage and chronic stage were (0.24±0.02, 0.31 ±0.11), (0.20±0.02, 0.32±0.12), (0.18±0.02, 0.34±0.11) and (0.16±0.02, 0.37±0.13), respectively, lower than those in the contralateral corresponding regions which were (0.40±0.03, 0.70±0.21), (0.37±0.03, 0.71±0.21), (0.39±0.03, 0.72±0.22) and (0.40:±0.03, 0.72±0.23), respectively. The differences were statistically significant (P＜0.05). The FA and ADC values had no significant differences between the uninjured sides in patients with cerebral infarction and the corresponding regions in the normal control group (P＞0.05); ③The FA and ADC values in brain tissues changed regularly with the time of infarction after cerebral infarction. The FA values in the affected sides had no consistent changes as compared with the contralateral sides in the superacute stage. They increased or decreased slightly, then (during acute stage, subacute stage and chronic stage) decreased irreversibly. The ADC values in the affected sides changed with time regularly, they decreased significantly, gradually returned to normal, and then increased again. Conclusion DTI and DTT examination contribute to the diagnosis of cerebral infarction. The combination of the FA and ADC values may more accurately conduct clinical staging and evaluate the time of the occurrence of cerebral infarction. Key words: Magnetic resonance; Cerebral infarction; Diffusion tensor imaging; Diffusion tensor tractography

When is flow re-entrainment important for the flushing time in coastal reef systems?

Objective To investigate the value of diffusion tensor imaging (DTI) at high b value for unilateral middle cerebral artery (MCA) occlusive disease in patients without obvious infarct lesions on conventional MR imaging.Methods DTI at high b value (2200 s/mm2 ) was performed using a 3.0 Tesla MR scanner in 34 patients with unilateral middle cerebral artery occlusion,who had no obvious infarct lesions on conventional MR imaging. Fractional anisotropy (FA),apparent diffusion coefficient (ADC),axial diffusivity (eigenvalue λ1) and radial diffusivity (eigenvalues λ2,λ3) were measured at the ipsilateral and contralateral corona radiata,anterior and posterior limbs of the internal capsule,cerebral peduncle and pons in all subjects.Mean ADC,FA,λ1,λ2 and λ3 values of corona radiata,anterior and posterior limbs of the internal capsule,cerebral peduncle and pons were compared between the ipsilateral and contralateral MCA territory by t test. Results Among the 34 patients,left MCA occlusion in M1 segment occurred in 16 patients and right MCA occlusion in Ml segment occurred in 18 patients.At the ipsilateral corona radiata,mean FA,ADC,λ1,λ2 and λ3 were 0.419 ±0.032,(5.975 ±0.272) × 10 3,(5.704 ±0.365) ×10-3,(6.412 ±0.368) × 10-3 and (6.605 ±0.343) × 10-3 mm2/s,respectively.At the contralateral corona radiata,mean FA,ADC,λ1,λ2 and λ3 were 0.443 ± 0.033,(5.804 ± 0.282) × 10 -3,(5.651 ±0.350) × 10-3,(6.099 ±0.353) × 10-3 and(6.372 ±0.355) × 10-3 mm2/s,respectively.At the ipsilateral corona radiata,mean FA was significantly decreased(t =11.614,P ＜0.01),and mean ADC (t=12.421,P＜0.01),λ1(t =7.447,P＜0.01),λ2(t=10.244,P＜0.01) and λ3(t=9.890,P＜0.01) were significantly increased.At the ipsilateral anterior and posterior limb of the internal capsule,mean FA were 0.609 ±0.026 and 0.674 ±0.033,λ1 were(5.330 ±0.462) × 10 -3 and(5.171 ±0.456) ×10-3 mm2/s,respectively.At the contralateral anterior and posterior limb of the internal capsule,FA were 0.622 ±0.026 and 0.694 ±0.034,λ1 were(5.064 ± 0.448) × 10 -3 and(4.924 ± 0.365) × 10 -3 mm2/s,respectively.Mean FA was significantly decreased (t =7.823,8.013,all P ＜ 0.01) and mean λ1 was significantly increased (t =7.811,8.800,all P ＜0.01) at the ipsilateral anterior and posterior limbs of the internal capsule.There was no significant difference in ADC,λ2 and λ3 value between the ipsilateral and contralateral sides.And all the DTI parameters,including mean ADC,FA,λ1,λ2 and λ3 values,showed no statistical difference between both sides of cerebral peduncle and pons.Conclusion DTI at high b valuc can provide useful information for visualizing ischemic white matter injury in patients without obvious infarct lesions on conventional MR imaging. Key words: Infarction, middle cerebral artery; Diffusion magnetic resonance imaging

Dystrophic muscles show a high variability of fibre sizes and altered sarcolemmal integrity, which are typically assessed by histology. Time-dependent diffusion MRI is sensitive to tissue microstructure and its investigation through age-related changes in dystrophic and healthy muscles may help the understanding of the onset and progression of Duchenne muscular dystrophy (DMD). We investigated the capability of time-dependent diffusion MRI to quantify age and disease-related changes in hind-limb muscle microstructure between dystrophic (mdx) and wild-type (WT) mice of three age groups (7.5, 22 and 44 weeks). Diffusion time-dependent apparent diffusion coefficients (ADCs) of the gastrocnemius and tibialis anterior muscles were determined versus age and diffusion-gradient orientation at six diffusion times (Δ; range: 25-350 ms). Mean muscle ADCs were compared between groups and ages, and correlated with T2 , using Student's t test, one-way analysis of variance and Pearson correlation, respectively. Muscle fibre sizes and sarcolemmal integrity were evaluated by histology and compared with diffusion measurements. Hind-limb muscle ADC showed characteristic restricted diffusion behaviour in both mdx and WT animals with decreasing ADC values at longer Δ. Significant differences in ADC were observed at long Δ values (≥ 250 ms; p < 0.05, comparison between groups; p < 0.01, comparison between ages) with ADC increased by 5-15% in dystrophic muscles, indicative of reduced diffusion restriction. No significant correlation was found between T2 and ADC. Additionally, muscle fibre size distributions showed higher variability and lower mean fibre size in mdx than WT animals (p < 0.001). The extensive Evans Blue Dye uptake shown in dystrophic muscles revealed substantial sarcolemmal damage, suggesting diffusion measurements as more consistent with altered permeability rather than changes in muscle fibre sizes. This study shows the potential of diffusion MRI to non-invasively discriminate between dystrophic and healthy muscles with enhanced sensitivity when using long Δ.

Disorders of consciousness, EEG background suppression, and epileptic seizures are associated with poor outcome after cardiac arrest. Our objective was to identify the distribution of diffusion MRI-measured anoxic brain injury after cardiac arrest and to define the regional correlates of disorders of consciousness, EEG background suppression, and seizures. We analyzed patients from a single-center database of unresponsive patients who underwent diffusion MRI after cardiac arrest (n = 204). We classified each patient according to recovery of consciousness (command following) before discharge, the most continuous EEG background (burst suppression vs continuous), and the presence or absence of seizures. Anoxic brain injury was measured with the apparent diffusion coefficient (ADC) signal. We identified ADC abnormalities relative to controls without cardiac arrest (n = 48) and used voxel lesion symptom mapping to identify regional associations with disorders of consciousness, EEG background suppression, and seizures. We then used a bootstrapped lasso regression procedure to identify robust, multivariate regional associations with each outcome variable. Last, using area under receiver operating characteristic curves, we then compared the classification ability of the strongest regional associations to that of brain-wide summary measures. Compared to controls, patients with cardiac arrest demonstrated ADC signal reduction that was most significant in the occipital lobes. Disorders of consciousness were associated with reduced ADC most prominently in the occipital lobes but also in deep structures. Regional injury more accurately classified patients with disorders of consciousness than whole-brain injury. Background suppression mapped to a similar set of brain regions, but regional injury could no better classify patients than whole-brain measures. Seizures were less common in patients with more severe anoxic injury, particularly in those with injury to the lateral temporal white matter. Anoxic brain injury was most prevalent in posterior cerebral regions, and this regional pattern of injury was a better predictor of disorders of consciousness than whole-brain injury measures. EEG background suppression lacked a specific regional association, but patients with injury to the temporal lobe were less likely to have seizures. Regional patterns of anoxic brain injury are relevant to the clinical and electrographic sequelae of cardiac arrest and may hold importance for prognosis. This study provides Class IV evidence that disorders of consciousness after cardiac arrest are associated with widely lower ADC values on diffusion MRI and are most strongly associated with reductions in occipital ADC.

Diffusion MRI: apparent diffusion coefficient (ADC) values in the normal brain and a classification of brain disorders based on ADC values

The measured signal response in contrast-enhanced myocardial perfusion imaging has been shown to be affected by the rate of water exchange between the intravascular and extravascular compartments, the effect being particularly significant when intravascular contrast agents are used. In the present study, the T(1) relaxation rates were measured in eight pigs in blood and myocardium using a Look-Locker sequence after repeated injections of the intravascular contrast agent NC100150. The selection of myocardial region of interest was automated based on a minimum chi-square method. The intra- and extravascular water exchange rates and the myocardial blood volume were calculated from the measured relaxation rates by applying a two-compartment water exchange limited model that accounts for biexponential longitudinal relaxation. The following (mean +/- SD) values were obtained for the exchange frequency (f), the extravascular residence time (tau(e)), the intravascular residence time (tau(i)) and blood volume (BV), respectively: f = 1.39 +/- 0.52 s(-1), tau(e) = 708 +/- 264 ms, tau(i) = 107 +/- 63 ms, and BV = 11.2 +/- 2.1 mL/100 g. The mean value of f was found to be about 15% higher if biexponential relaxation was not accounted for, supporting the hypothesis that significant biexponential relaxation in tissues with large blood volume can lead to an overestimation of water exchange rates unless corrected for.