High Dynamic Range Processing for Magnetic Resonance Imaging

Purpose

To minimize feature loss in T₁- and T₂-weighted MRI by merging multiple MR images acquired at different T_R and T_E to generate an image with increased dynamic range.

Materials and Methods

High Dynamic Range (HDR) processing techniques from the field of photography were applied to a series of acquired MR images. Specifically, a method to parameterize the algorithm for MRI data was developed and tested. T₁- and T₂-weighted images of a number of contrast agent phantoms and a live mouse were acquired with varying T_R and T_E parameters. The images were computationally merged to produce HDR-MR images. All acquisitions were performed on a 7.05 T Bruker PharmaScan with a multi-echo spin echo pulse sequence.

Results

HDR-MRI delineated bright and dark features that were either saturated or indistinguishable from background in standard T₁- and T₂-weighted MRI. The increased dynamic range preserved intensity gradation over a larger range of T₁ and T₂ in phantoms and revealed more anatomical features in vivo.

Conclusions

We have developed and tested a method to apply HDR processing to MR images. The increased dynamic range of HDR-MR images as compared to standard T₁- and T₂-weighted images minimizes feature loss caused by magnetization recovery or low SNR.     

PETER PHAN: An MRI phantom for the optimisation of radiomic studies of the female pelvis

PurposeTo develop a phantom for methodological radiomic investigation on Magnetic Resonance (MR) images of female patients affected by pelvic cancer.MethodsA pelvis-shaped container was filled with a MnCl₂ solution reproducing the relaxation times (T₁, T₂) of muscle surrounding pelvic malignancies. Inserts simulating multi-textured lesions were embedded in the phantom. The relaxation times of muscle and tumour were measured on an MR scanner on healthy volunteers and patients; T₁ and T₂ of MnCl₂ solutions were evaluated with a relaxometer to find the concentrations providing a match to in vivo relaxation times. Radiomic features were extracted from the phantom inserts and the patients’ lesions. Their repeatability was assessed by multiple measurements.ResultsMuscle T₁ and T₂ were 1128 (806–1378) and 51 (40–65) ms, respectively. The phantom reproduced in vivo values within 13% (T₁) and 12% (T₂). T₁ and T₂ of tumour tissue were 1637 (1396–2121) and 94 (79–101) ms, respectively. The phantom insert best mimicking the tumour agreed within 7% (T₁) and 24% (T₂) with in vivo values. Out of 1034 features, 75% (95%) had interclass correlation coefficient greater than 0.9 on T₁ (T₂)-weighted images, reducing to 33% (25%) if the phantom was repositioned. The most repeatable features on phantom showed values in agreement with the features extracted from patients’ lesions.ConclusionsWe developed an MR phantom with inserts mimicking both relaxation times and texture of pelvic tumours. As exemplified with repeatability assessment, such phantom is useful to investigate features robustness and optimise the radiomic workflow on pelvic MR images.     

High Spatial Resolution and Temporally Resolved T2 * Mapping of Normal Human Myocardium at 7.0 Tesla: An Ultrahigh Field Magnetic Resonance Feasibility Study

Myocardial tissue characterization using T₂ ^* relaxation mapping techniques is an emerging application of (pre)clinical cardiovascular magnetic resonance imaging. The increase in microscopic susceptibility at higher magnetic field strengths renders myocardial T₂ ^* mapping at ultrahigh magnetic fields conceptually appealing. This work demonstrates the feasibility of myocardial T₂ ^* imaging at 7.0 T and examines the applicability of temporally-resolved and high spatial resolution myocardial T₂ ^* mapping. In phantom experiments single cardiac phase and dynamic (CINE) gradient echo imaging techniques provided similar T₂ ^* maps. In vivo studies showed that the peak-to-peak B₀ difference following volume selective shimming was reduced to approximately 80 Hz for the four chamber view and mid-ventricular short axis view of the heart and to 65 Hz for the left ventricle. No severe susceptibility artifacts were detected in the septum and in the lateral wall for T₂ ^* weighting ranging from TE = 2.04 ms to TE = 10.2 ms. For TE >7 ms, a susceptibility weighting induced signal void was observed within the anterior and inferior myocardial segments. The longest T₂ ^* values were found for anterior (T₂ ^* = 14.0 ms), anteroseptal (T₂ ^* = 17.2 ms) and inferoseptal (T₂ ^* = 16.5 ms) myocardial segments. Shorter T₂ ^* values were observed for inferior (T₂ ^* = 10.6 ms) and inferolateral (T₂ ^* = 11.4 ms) segments. A significant difference (p = 0.002) in T₂ ^* values was observed between end-diastole and end-systole with T₂ ^* changes of up to approximately 27% over the cardiac cycle which were pronounced in the septum. To conclude, these results underscore the challenges of myocardial T₂ ^* mapping at 7.0 T but demonstrate that these issues can be offset by using tailored shimming techniques and dedicated acquisition schemes.     

Acute changes in knee cartilage transverse relaxation time after running and bicycling

PurposeTo compare the acute effect of running and bicycling of an equivalent cumulative load on knee cartilage composition and morphometry in healthy young men. A secondary analysis investigated the relationship between activity history and the change in cartilage composition after activity.MethodsIn fifteen men (25.8±4.2 years), the vertical ground reaction force was measured to determine the cumulative load exposure of a 15-min run. The vertical pedal reaction force was recorded during bicycling to define the bicycling duration of an equivalent cumulative load. On separate visits that were spaced on average 17 days apart, participants completed these running and bicycling bouts. Mean cartilage transverse relaxation times (T₂) were determined for cartilage on the tibia and weight-bearing femur before and after each exercise. T₂ was measured using a multi-echo spin-echo sequence and 3T MRI. Cartilage of the weight bearing femur and tibia was segmented using a highly-automated segmentation algorithm. Activity history was captured using the International Physical Activity Questionnaire.ResultsThe response of T₂ to bicycling and running was different (p=0.019; mean T₂: pre-running=34.27 ms, pre-bicycling=32.93 ms, post-running=31.82 ms, post-bicycling=32.36 ms). While bicycling produced no change (−1.7%, p=0.300), running shortened T₂ (−7.1%, p<0.001). Greater activity history predicted smaller changes in tibial, but not femoral, T₂.ConclusionsChanges in knee cartilage vary based on activity type, independent of total load exposure, in healthy young men. Smaller changes in T₂ were observed after bicycling relative to running. Activity history was inversely related to tibial T₂, suggesting cartilage conditioning.     

In vivo High Angular Resolution Diffusion-Weighted Imaging of Mouse Brain at 16.4 Tesla

Magnetic Resonance Imaging (MRI) of the rodent brain at ultra-high magnetic fields (> 9.4 Tesla) offers a higher signal-to-noise ratio that can be exploited to reduce image acquisition time or provide higher spatial resolution. However, significant challenges are presented due to a combination of longer T ₁ and shorter T ₂/T₂* relaxation times and increased sensitivity to magnetic susceptibility resulting in severe local-field inhomogeneity artefacts from air pockets and bone/brain interfaces. The Stejskal-Tanner spin echo diffusion-weighted imaging (DWI) sequence is often used in high-field rodent brain MRI due to its immunity to these artefacts. To accurately determine diffusion-tensor or fibre-orientation distribution, high angular resolution diffusion imaging (HARDI) with strong diffusion weighting (b >3000 s/mm²) and at least 30 diffusion-encoding directions are required. However, this results in long image acquisition times unsuitable for live animal imaging. In this study, we describe the optimization of HARDI acquisition parameters at 16.4T using a Stejskal-Tanner sequence with echo-planar imaging (EPI) readout. EPI segmentation and partial Fourier encoding acceleration were applied to reduce the echo time (TE), thereby minimizing signal decay and distortion artefacts while maintaining a reasonably short acquisition time. The final HARDI acquisition protocol was achieved with the following parameters: 4 shot EPI, b = 3000 s/mm², 64 diffusion-encoding directions, 125×150 μm² in-plane resolution, 0.6 mm slice thickness, and 2h acquisition time. This protocol was used to image a cohort of adult C57BL/6 male mice, whereby the quality of the acquired data was assessed and diffusion tensor imaging (DTI) derived parameters were measured. High-quality images with high spatial and angular resolution, low distortion and low variability in DTI-derived parameters were obtained, indicating that EPI-DWI is feasible at 16.4T to study animal models of white matter (WM) diseases.     

Multiparametric MRI of Epiphyseal Cartilage Necrosis (Osteochondrosis) with Histological Validation in a Goat Model

Purpose

To evaluate multiple MRI parameters in a surgical model of osteochondrosis (OC) in goats.

Methods

Focal ischemic lesions of two different sizes were induced in the epiphyseal cartilage of the medial femoral condyles of goats at 4 days of age by surgical transection of cartilage canal blood vessels. Goats were euthanized and specimens harvested 3, 4, 5, 6, 9 and 10 weeks post-op. Ex vivo MRI scans were conducted at 9.4 Tesla for mapping the T₁, T₂, T_1ρ, adiabatic T_1ρ and T_RAFF relaxation times of articular cartilage, unaffected epiphyseal cartilage, and epiphyseal cartilage within the area of the induced lesion. After MRI scans, safranin O staining was conducted to validate areas of ischemic necrosis induced in the medial femoral condyles of six goats, and to allow comparison of MRI findings with the semi-quantitative proteoglycan assessment in corresponding safranin O-stained histological sections.

Results

All relaxation time constants differentiated normal epiphyseal cartilage from lesions of ischemic cartilage necrosis, and the histological staining results confirmed the proteoglycan (PG) loss in the areas of ischemia. In the scanned specimens, all of the measured relaxation time constants were higher in the articular than in the normal epiphyseal cartilage, consistently allowing differentiation between these two tissues.

Conclusions

Multiparametric MRI provided a sensitive approach to discriminate between necrotic and viable epiphyseal cartilage and between articular and epiphyseal cartilage, which may be useful for diagnosing and monitoring OC lesions and, potentially, for assessing effectiveness of treatment interventions.     

Oxygen Mapping within Healthy and Acutely Infarcted Brain Tissue in Humans Using the NMR Relaxation of Lipids: A Proof-Of-Concept Translational Study

The clinical applicability of brain oxygenation mapping using the MOBILE (Mapping of Oxygen By Imaging Lipids relaxation Enhancement) magnetic resonance (MR) technique was assessed in the clinical setting of normal brain and of acute cerebral ischemia as a founding proof-of-concept translational study. Changes in the oxygenation level within healthy brain tissue can be detected by analyzing the spin-lattice proton relaxation (‘Global T ₁ ’ combining water and lipid protons) because of the paramagnetic properties of molecular oxygen. It was hypothesized that selective measurement of the relaxation of the lipid protons (‘Lipids T ₁ ’) would result in enhanced sensitivity of pO₂ mapping because of higher solubility of oxygen in lipids than in water, and this was demonstrated in pre-clinical models using the MOBILE technique. In the present study, 12 healthy volunteers and eight patients with acute (48–72 hours) brain infarction were examined with the same clinical 3T MR system. Both Lipids R₁ (R₁ = 1/T₁) and Global R₁ were significantly different in the infarcted area and the contralateral unaffected brain tissue, with a higher statistical significance for Lipids R₁ (median difference: 0.408 s^-1; p<0.0001) than for Global R₁ (median difference: 0.154 s^-1; p = 0.027). Both Lipids R₁ and Global R₁ values in the unaffected contralateral brain tissue of stroke patients were not significantly different from the R₁ values calculated in the brain tissue of healthy volunteers. The main limitations of the present prototypic version of the MOBILE sequence are the long acquisition time (4 min), hampering robustness of data in uncooperative patients, and a 2 mm slice thickness precluding accurate measurements in small infarcts because of partial volume averaging effects.     

High Temporal Resolution Parametric MRI Monitoring of the Initial Ischemia/Reperfusion Phase in Experimental Acute Kidney Injury

Ischemia/reperfusion (I/R) injury, a consequence of kidney hypoperfusion or temporary interruption of blood flow is a common cause of acute kidney injury (AKI). There is an unmet need to better understand the mechanisms operative during the initial phase of ischemic AKI. Non-invasive in vivo parametric magnetic resonance imaging (MRI) may elucidate spatio-temporal pathophysiological changes in the kidney by monitoring the MR relaxation parameters T₂* and T₂, which are known to be sensitive to blood oxygenation. The aim of our study was to establish the technical feasibility of fast continuous T₂*/T₂ mapping throughout renal I/R. MRI was combined with a remotely controlled I/R model and a segmentation model based semi-automated quantitative analysis. This technique enabled the detailed assessment of in vivo changes in all kidney regions during ischemia and early reperfusion. Significant changes in T₂* and T₂ were observed shortly after induction of renal ischemia and during the initial reperfusion phase. Our study demonstrated for the first time that continuous and high temporal resolution parametric MRI is feasible for in-vivo monitoring and characterization of I/R induced AKI in rats. This technique may help in the identification of the timeline of key events responsible for development of renal damage in hypoperfusion-induced AKI.     

31P nuclear magnetic resonance studies of the association of basic proteins with multilayers of diacyl phosphatidylserine

Lysozyme, cytochrome c, poly(l-lysine), myelin basic protein and ribonuclease were used to form multilayer dispersions containing about 50% protein (by weight) with bovine brain diacyl phosphatidylserine (PS). ³¹P nuclear magnetic resonance shift anisotropies, spin-spin (T₂) and spin-lattice (T₁) relaxation times for the lipid headgroup phosphorus were measured at 36.44 MHz. At pH 7.5, lysozyme, cytochrome c, poly(l-lysine) and ribonuclease were shown to increase the chemical shift anisotropy of PS by between 12–20%. Myelin basic protein altered the shape of the phosphate resonance, suggesting the presence of two lipid components, one of which had a modified headgroup conformation. The presence of cytochrome c led to the formation of a narrow spike at the isotropic shift position of the spectrum. Of the various proteins or peptides we have studied, only poly(l-lysine) and cytochrome c had any effect on the T₁ of PS (1050 ms). Both caused a 20–30% decrease in T₁ of the lamellar-phase phosphate peak. The narrow peak in the presence of cytochrome c had a very short T₁ of 156 ms. The possibility is considered that the cytochrome Fe³⁺ contributes to the phosphate relaxation in this case. The effect of all proteins on the T₂ of the phosphorus resonance was to cause an increase from the value for pure PS (1.6 ms) to between 2 and 5 ms. The results obtained with proteins are compared with the effects of small ions and intrinsic membrane proteins on the order and motion of the headgroups of lipids in bilayers.     

In Vivo MRI Assessment of Hepatic and Splenic Disease in a Murine Model of Schistosmiasis

BackgroundSchistosomiasis (or bilharzia), a major parasitic disease, affects more than 260 million people worldwide. In chronic cases of intestinal schistosomiasis caused by trematodes of the Schistosoma genus, hepatic fibrosis develops as a host immune response to the helminth eggs, followed by potentially lethal portal hypertension. In this study, we characterized hepatic and splenic features of a murine model of intestinal schistosomiasis using in vivo magnetic resonance imaging (MRI) and evaluated the transverse relaxation time T₂ as a non-invasive imaging biomarker for monitoring hepatic fibrogenesis.Conclusions/SignificanceOur multiparametric MRI approach confirms that this murine model replicates hepatic and splenic manifestations of human intestinal schistosomiasis. Quantitative T₂ mapping proved sensitive to assess liver fibrogenesis non-invasively and may therefore constitute an objective imaging biomarker for treatment monitoring in diseases involving hepatic fibrosis.     

T1 Relaxation Time in Lungs of Asymptomatic Smokers

Purpose

Interest in using T₁ as a potential MRI biomarker of chronic obstructive pulmonary disease (COPD) has recently increased. Since tobacco smoking is the major risk factor for development of COPD, the aim for this study was to examine whether tobacco smoking, pack-years (PY), influenced T₁ of the lung parenchyma in asymptomatic current smokers.

Materials and Methods

Lung T₁ measurements from 35 subjects, 23 never smokers and 12 current smokers were retrospectively analyzed from an institutional review board approved study. All 35 subjects underwent pulmonary function test (PFT) measurements and lung T_1, with similar T₁ measurement protocols. A backward linear model of T₁ as a function of FEV₁, FVC, weight, height, age and PY was tested.

Results

A significant correlation between lung T₁ and PY was found with a negative slope of -3.2 ms/year (95% confidence interval [CI] [-5.8, -0.6], p = 0.02), when adjusted for age and height. Lung T₁ shortens with ageing among all subjects, -4.0 ms/year (95%CI [-6.3, -1.7], p = 0.001), and among the never smokers, -3.7 ms/year (95%CI [-6.0, -1.3], p = 0.003).

Conclusions

A correlation between lung T₁ and PY when adjusted for both age and height was found, and T₁ of the lung shortens with ageing. Accordingly, PY and age can be significant confounding factors when T₁ is used as a biomarker in lung MRI studies that must be taken into account to detect underlying patterns of disease.     

Long-lived spin state of a tripeptide in stretched hydrogel

The longitudinal (T ₁), transverse (T ₂), and singlet state (T _s) relaxation times of the geminal backbone protons (CH₂) of l-Leu-Gly-Gly were studied by NMR spectroscopy at 9.4 T in a bovine hide gelatin gel composed in D₂O at 25 °C. Gelatin granules were dissolved in a hot solution of the tripeptide and then the solution was allowed to gel inside a flexible silicone tubing. With increases in gelatin content, the T ₂ and T _s of the CH₂ protons correspondingly decreased (T _s/T ₂ ~ constant), while the change in T ₁ was relatively small. The largest observed T _s/T ₁ value was 3.3 at 46 % w/v gelatin that was the lowest gelatin content examined. Stretching the tubing, and hence the gel, brought about anisotropic alignment of the constituents resulting in residual quadrupolar splitting of the resonance from D₂O in ²H NMR spectra, and residual dipolar splitting of the CH₂ resonance in ¹H NMR spectra. WALTZ-16 decoupling during the relaxation intervals extended the singlet state relaxation time, but the efficacy diminished as the gels were stretched. Theoretically predicted T ₁, T ₂, and T _s values, assuming intramolecular dipolar coupling as the only source of relaxation, were within the same order of magnitude as the experimentally observed values. Overall we showed that it is possible to observe a long-lived spin state in an anisotropic medium when T ₂ is shorter than T ₁ in the presence of non-zero residual dipolar couplings.     

Lithium nuclear magnetic resonance measurements in halotolerant bacterium Ba1

Measurements of relaxation times T₁ and T₂, were carried out on a high-salt- and low-salt-grown bacterial pellets of halotolerant bacterium B_a1. In our measurements, T₁ ? T₂ and both were frequency-independent. In the high-salt-grown pellet the relaxation time values were much shorter than in the case of low-salt growth medium. Intensity measurements show that only 55% of the lithium in the high-salt pellet is detected; for the low-salt pellet almost all the lithium is detected. Growth measurements were carried out on the B_a1. It is suggested that there is some form of adaptation of the bacteria to the growth medium. The adaptation is reflected in the lithium NMR results.     

Relaxation Along a Fictitious Field (RAFF) and Z-spectroscopy using Alternating-Phase Irradiation (ZAPI) in Permanent Focal Cerebral Ischemia in Rat

Cerebral ischemia alters the molecular dynamics and content of water in brain tissue, which is reflected in NMR relaxation, diffusion and magnetization transfer (MT) parameters. In this study, the behavior of two new MRI contrasts, Relaxation Along a Fictitious Field (RAFF) and Z-spectroscopy using Alternating-Phase Irradiation (ZAPI), were quantified together with conventional relaxation parameters (T₁, T₂ and T_1ρ) and MT ratios in acute cerebral ischemia in rat. The right middle cerebral artery was permanently occluded and quantitative MRI data was acquired sequentially for the above parameters for up to 6 hours. The following conclusions were drawn: 1) Time-dependent changes in RAFF and T_1ρ relaxation are not coupled to those in MT. 2) RAFF relaxation evolves more like transverse, rather than longitudinal relaxation. 3) MT measured with ZAPI is less sensitive to ischemia than conventional MT. 4) ZAPI data suggest alterations in the T₂ distribution of macromolecules in acute cerebral ischemia. It was shown that both RAFF and ZAPI provide complementary MRI information from acute ischemic brain tissue. The presented multiparametric MRI data may aid in the assessment of brain tissue status early in ischemic stroke.     

P relaxation responses associated with n(2)/o(2) diffusion in soybean nodule cortical cells and excised cortical tissue

N₂-fixing Bradyrhizobium japonicum nodules and cortical tissue derived from these nodules were examined in vivo by ³¹P nuclear magnetic resonance (NMR) spectroscopy. Perfusion of the viable nodules and excised cortical tissue with O₂ followed by N₂ or Ar caused a loss of orthophosphate (Pi) resonance magnetization associated with the major portion of acidic Pi (δ 0.9 ppm, pH 5.5) residing in the cortical cells. Resumption of O₂ perfusion restored approximately 80% of the intensity of this peak. Detailed examination of the nuclear relaxation processes, spin-lattice relaxation time (T₁), and spin-spin relaxation time (T₂), under perfusion with N₂ or Ar as opposed to O₂, indicated that loss of signal was due to T₁ saturation of the acidic Pi signal under the rapid-pulsed NMR recycling conditions. In excised cortical tissue, Pi T₁, values derived from biexponential relaxation processes under perfusing O₂ were 59% 3.72 ± 0.93 s and 41% 0.2 ± 0.08 s, whereas under N₂ these values were 85% 7.07 ± 1.36 s and 15% 0.39 ± 0.07 s. The T₁ relaxation behavior of whole nodule vacuolar Pi showed the same trend, but the overall values were somewhat shorter. T₂ values for cortical tissue were also biexponential but were essentially the same under O₂ (38% 0.066 ± 0.01 s and 63% 0.41 ± 0.08 s) and N₂ (39% 0.07 ± 0.01 s and 61% 0.37 ± 0.01 s) perfusion. Soybean (Glycine max) root tissue as well as Pi solutions exhibited single exponential T₁ decay values that were not altered by changes in the perfusing gas. These data indicate that oxygen induces a change in the physical environment of phosphate in the cortical cell tissue. Although under certain conditions oxygen has been observed to act as a paramagnetic relaxation agent, model T₁ experiments demonstrate that O₂ does not significantly influence Pi relaxation in this manner. Alternatively, we suggest that an increase in solution viscosity brought on by the production of an occlusion glycoprotein (under O₂ perfusion) is responsible for the observed relaxation changes.     

Assessment of Clinical Signs of Liver Cirrhosis Using T1 Mapping on Gd-EOB-DTPA-Enhanced 3T MRI

Objectives

To assess the differences between normal and cirrhotic livers by means of T1 mapping of liver parenchyma on gadoxetic acid (Gd-EOB-DTPA)-enhanced 3 Tesla (3T) MR imaging (MRI).

Methods

162 patients with normal (n = 96) and cirrhotic livers (n = 66; Child-Pugh class A, n = 30; B, n = 28; C, n = 8) underwent Gd-EOB-DTPA-enhanced 3T MRI. To obtain T1 maps, two TurboFLASH sequences (TI = 400 ms and 1000 ms) before and 20 min after Gd-EOB-DTPA administration were acquired. T1 relaxation times of the liver and the reduction rate between pre- and post-contrast enhancement images were measured.

Results

The T1 relaxation times for Gd-EOB-DTPA-enhanced MRI showed significant differences between patients with normal liver function and patients with Child-Pugh class A, B, and C (p < 0.001). The T1 relaxation times showed a constant significant increase from Child-Pugh class A up to class C (Child-Pugh class A, 335 ms ± 80 ms; B, 431 ms ± 75 ms; C, 557 ms ± 99 ms; Child-Pugh A to B, p < 0.001; Child-Pugh A to C, p < 0.001; Child-Pugh B to C, p < 0.001) and a constant decrease of the reduction rate of T1 relaxation times (Child-Pugh class A, 57.1% ± 8.8%; B, 44.3% ± 10.2%, C, 29.9% ± 6.9%; Child-Pugh A to B, p < 0.001; Child-Pugh A to C,p < 0.001; Child-Pugh B to C, p < 0.001).

Conclusion

Gd-EOB-DTPA-enhanced T1 mapping of the liver parenchyma may present a useful method for determining severity of liver cirrhosis.     

Probing water compartments and membrane permeability in plant cells by 1H NMR relaxation measurements

¹H NMR relaxation times (T₁ and T₂) in parenchyma tissue of apple can identify three populations of water with different relaxation characteristics. By following the uptake of Mn²⁺ ions in the tissue it is shown that the observed relaxation times originate from particular water compartments: the vacuole, the cytoplasm, and the cell wall/extracellular space.

Proton exchange between these compartments is controlled by the plasmalemma and tonoplast membranes. During the Mn²⁺ penetration experiment, conditions occur that cause the relaxation times of protons of cytoplasmic water to be much shorter than their residence time in the cytoplasm. Then the tonoplast permeability coefficient P_d for water can be calculated from the vacuolar T₁ and T₂ values to be 2.44 10^-5 m·s^-1.

     

Effects of temperature and field strength on the NMR relaxation times of 23Na in frog striated muscle

Pulsed NMR techniques have been applied to the study of the relaxation parameters characterizing ²³Na within frog striated muscle. Experiments were performed at 3°C, 22–24°C and 39°C at a Larmor frequency of 15.7 MHz; at 22–24°C, measurements were obtained both at 15.7 MHz and at 7.85 MHz.As previously reported, only a single spine-lattice relaxation time (T₁) was observed, but both slow (T₂)_I and fast (T₂)_II components of the spin-spin relaxation time were measured. The effect of temperature (θ) upon (1/T₁) was qualitatively similar to that reported for ²³Na in free solution; (θ) did not significantly affect (1/T₂) over the range of temperatures studied. (1/T₂)_I, and to a lesser degreee, (1/T₁) exhibited a modest inverse dependence of doubtful significance on the Larmor frequency.The data are examined within the framework of a simple specific model; a conservative values in assumed for the quadrupolar coupling constant characterizing immobilized intracellular Na⁺. Within this framework, the results suggest that the fraction of bound ions whose molecular tumbling is severely restricted does not exceed some few percent of the total sodium population.     

Techniques for in utero,longitudinal MRI of fetal brain development in baboons at 3 T

Magnetic Resonance Imaging (MRI) is a promising tool for the noninvasive, longitudinal study of developing primate brains. We developed a protocol to scan pregnant baboons serially at 3 T for up to 3 h per session. This protocol includes procedures for animal preparation, anesthesia, MRI scanning, and post-scan animal care. We applied this protocol to scan 5 baboons multiple times across the latter 70% of gestation—from as early as 56 days post-conceptional age to as late as 185 days (term ～180 days). We successfully acquired high-resolution anatomical images and maps of relaxation times (T₁ and T₂) of the fetal brains at multiple time points across gestation. These images and maps demonstrated the convergence of gray and white matter contrast near term, and furthermore demonstrated that the convergence of contrast is a consequence of the continuous change in relaxation times during fetal brain development. We estimated the rates of decrease of T₁ and T₂ in white matter and gray matter, respectively. In addition, we measured the volumes of fetal brain at different gestational ages and calculated the growth rates of whole brain (0.91 ± 0.08 cm³/day) and cortical gray matter (0.40 ± 0.04 cm³/day). We also measured the mean diffusivity in white matter and deep gray matter using diffusion tensor imaging. In conclusion, in utero MRI of fetal baboon brains greatly enhances the use of nonhuman primate models to study fetal brain development longitudinally.