Deconvolution of compartmental water diffusion coefficients in yeast-cell suspensions using combined T(1) and diffusion measurements

An NMR method is presented for measuring compartment-specific water diffusion coefficient (D) values. It uses relaxography, employing an extracellular contrast reagent (CR) to distinguish intracellular (IC) and extracellular (EC) (1)H(2)O signals by differences in their respective longitudinal (T(1)) relaxation times. A diffusion-weighted inversion-recovery spin-echo (DW-IRSE) pulse sequence was used to acquire IR data sets with systematically and independently varying inversion time (TI) and diffusion-attenuation gradient amplitude (g) values. Implementation of the DW-IRSE technique was demonstrated and validated using yeast cells suspended in 3 mM Gd-DTPA(2-) with a wet/dry mass ratio of 3.25:1.0. Two-dimensional (2D) NMR data were acquired at 2.0 T and analyzed using numerical inverse Laplace transformation (2D- and sequential 1D-ILT) and sequential exponential fitting to yield T(1) and water D values. All three methods gave substantial agreement. Exponential fitting, deemed the most accurate and time efficient, yielded T(1):D (relative contribution) values of 304 ms:0.023x10(-5) cm(2)/s (47%) and 65 ms:1.24x10(-5) cm(2)/s (53%) for the IC and EC components, respectively. The compartment-specific D values derived from direct biexponential fitting of diffusion-attenuation data were also in good agreement. Extension of the DW-IRSE method to in vivo models should provide valuable insights into compartment-specific water D changes in response to injury or disease. (c) 2002 Elsevier Science (USA).     

Cyclic variation of T1 in the myocardium at 3 T

Standard methods of longitudinal relaxation (T1) measurements in the heart produce only one T1 map of the myocardium, usually at the end diastole (ED). In this article, we investigated the feasibility of using a dual flip angle fast gradient echo technique in the steady state to generate a movie of T1 maps in the myocardium during the cardiac cycle. The effects of nonideal slice profile and transient steady state on the T1 measurements were evaluated by Bloch simulations. Based on these results, we introduce a linear correction to the measured T1 values, which was validated by phantom experiments. In vivo T1 cine maps in healthy volunteers show 70+/-7% drop in T1 from the ED to the end systole in the septum and a 43+/-13% drop in the left ventricular lateral wall. With further improvements, this technique could be used to assess the myocardial blood volume changes during the cardiac cycle.     

基于动态有机钆纳米颗粒的T1-T2双模态MRI造影剂

本文设计、合成并测试了一种新型的基于有机钆纳米颗粒的磁共振成像（MRI）造影剂.以1, 2-氨基硫醇与氰基的缩合反应为基础,成功合成了粒径在8~23 nm范围内的有机钆纳米颗粒.该有机钆纳米颗粒作为磁共振造影剂时,随着时间的推移,其纵向弛豫率逐渐减弱,横向弛豫率先增强后逐渐减弱,这与钆纳米颗粒粒径增大有关.有机钆纳米颗粒同时存在随时间变化的纵向弛豫和横向弛豫,表明它有望成为一种先进的T₁-T₂双模态MRI造影剂.     

Changes of relaxation times T1 and T2 in rat tissues after biopsy and fixation

NMR spectroscopical measurements of relaxation times were conducted on muscle, intestine, fatty tissue and cerebral cortex and white matter of the rat at various time intervals following removal of the tissue. It appeared that most tissues can be stored at 4 degrees C up to 24 hours without noticeable effects on NMR relaxation parameters. Exceptions are the T2 of muscle and the T1 and T2 of intestine, which tended to change in the first hour after biopsy. Relaxation parameters change considerably after fixation of the tissues. Therefore the effects of fixation have to be taken into account when carrying out NMR measurements on fixed tissues.     

Determination of T1- and T2-relaxation times in the spleen of patients with splenomegaly

Twenty-nine patients with known splenomegaly and seven healthy volunteers were examined. The T1 and T2 relaxation times were read out from a region of interest centrally in the spleen. Even though different mean T1 and T2 relaxation times were found between the groups, the great scatter and the considerable overlap between the groups makes the contribution of relaxation time measurements to the differential diagnosis of splenomegaly of limited value.     

Simultaneous measurement of total water content and myelin water fraction in brain at 3 T using a T2 relaxation based method

PurposeThis work demonstrates the in vivo application of a T₂ relaxation based total water content (TWC) measurement technique at 3 T in healthy human brain, and evaluates accuracy using simulations that model brain tissue. The benefit of using T₂ relaxation is that it provides simultaneous measurements of myelin water fraction, which correlates to myelin content.MethodsT₂ relaxation data was collected from 10 healthy human subjects with a gradient and spin echo (GRASE) sequence, along with inversion recovery for T₁ mapping. Voxel-wise T₂ distributions were calculated by fitting the T₂ relaxation data with a non-negative least squares algorithm incorporating B₁⁺ inhomogeneity corrections. TWC was the sum of the signals in the T₂ distribution, corrected for T₁ relaxation and receiver coil inhomogeneity, relative to either an external water standard or cerebrospinal fluid (CSF). Simulations were performed to determine theoretical errors in TWC.ResultsTWC values measured in healthy human brain relative to both external and CSF standards agreed with literature values. Simulations demonstrated that TWC could be measured to within 3–4% accuracy.ConclusionIn vivo TWC measurement using T₂ relaxation at 3 T works well and provides a valuable tool for studying neurological diseases with both myelin and water changes.     

Colorectal carcinoma: Ex vivo evaluation using 3-T high-spatial-resolution quantitative T2 mapping and its correlation with histopathologic findings

PurposeIn this study, we aimed to evaluate the feasibility of determining the mural invasion depths of colorectal carcinomas using high-spatial-resolution (HSR) quantitative T2 mapping on a 3-T magnetic resonance (MR) scanner.Materials and methodsTwenty colorectal specimens containing adenocarcinomas were imaged on a 3-T MR system equipped with a 4-channel phased-array surface coil. HSR quantitative T2 maps were acquired using a spin-echo sequence with a repetition time/echo time of 7650/22.6–361.6 ms (16 echoes), 87 × 43.5-mm field of view, 2-mm section thickness, 448 × 224 matrix, and average of 1. HSR fast-spin-echo T2-weighted images were also acquired. Differences between the T2 values (ms) of the tumor tissue, colorectal wall layers, and fibrosis were measured, and the MR images and histopathologic findings were compared.ResultsIn all specimens (20/20, 100%), the HSR quantitative T2 maps clearly depicted an 8-layer normal colorectal wall in which the T2 values of each layer differed from those of the adjacent layer(s) (P < 0.001). Using this technique, fibrosis (73.6 ± 9.4 ms) and tumor tissue (104.2 ± 6.4 ms) could also be clearly differentiated (P < 0.001). In 19 samples (95%), the HSR quantitative T2 maps and histopathologic data yielded the same findings regarding the tumor invasion depth.ConclusionsOur results indicate that 3-T HSR quantitative T2 mapping is useful for distinguishing colorectal wall layers and differentiating tumor and fibrotic tissues. Accordingly, this technique could be used to determine mural invasion by colorectal carcinomas with a high level of accuracy.     

Hepatic lipid composition analysis using 3.0-T MR spectroscopy in a steatotic rat model

Purpose

To investigate the feasibility of in vivo assessment of hepatic lipid composition using 3.0-T proton magnetic resonance spectroscopy (¹H-MRS) in a steatotic rat model and compare it to histopathological and biochemical assessment.

Materials and Methods

Hepatic steatosis was induced by feeding rats with a methionine/choline-deficient (MCD) diet for 1, 2, 3, 5 or 7 weeks (n=5 per group). At the end of the diet period, ¹H-MRS of the liver was performed, and rats were sacrificed for histopathological and biochemical assessment of the liver. Spectra were acquired in a single voxel (1.2 cc) using a point-resolved spectroscopic sequence with TE/TR=35/2000 ms and 64 signal acquisitions. From the MR spectra, peak area ratios were calculated to estimate hepatic lipid composition.

Results

During MCD diet periods, hepatic steatosis significantly increased on histopathology (P<.001). The ¹H-MRS measurements of total hepatic fat content [1.3/(1.3+4.65) ppm] correlated strongly with histological macrovesicular hepatic steatosis (r=0.93, P<.001) and with the biochemical total hepatic fatty acids (r=0.94, P<.001). Total unsaturated fatty acids [TUFA, 5.4/(1.3+4.65) ppm] estimated with ¹H-MRS strongly correlated with the biochemical unsaturated fatty acids (r=0.90, P<.001). Polyunsaturated fatty acids [PUFA, 2.8/(1.3+4.65) ppm] estimated with ¹H-MRS strongly correlated with biochemical PUFA (r=0.91, P<.001). The proportion of total unsaturated fatty acids relative to the amount of total fatty acids (rTUFA, 5.4/1.3ppm) measured with ¹H-MRS strongly correlated with the biochemical amount of unsaturated relative to total hepatic fatty acids (r=0.81, P<.001). The proportion of PUFA relative to the amount of total fatty acids (rPUFA, 2.8/1.3 ppm) measured with ¹H-MRS correlated with the biochemical amount of PUFA relative to total fatty acids (r=0.59, P=0.005,) and with the biochemical amount of omega-6 PUFA relative to total fatty acids (r=0.73, P<.001).PUFA at ¹H-MRS correlated with the histopathologically assessed degree of lobular inflammation in the liver (r=0.57, P=.001).

Conclusion

3.0T ¹H-MRS is able to measure poly- and unsaturated hepatic fatty acids and this strongly correlates with biochemical assessment. This study provides evidence that 3.0-T ¹H-MRS is a noninvasive technique to assess hepatic lipid composition.     

Twists in Hopf algebras andRT 1 T 2=T 2 T 1 relations

LetR be a matrix unitary quasi-classical solution of the Yang-Baxter equation. Considering an associative algebra defined by the relationRT ₁ T ₂=T ₂ T ₁ we find a universal twistF such thatR is the image ofR=F ₂₁ F ⁻¹ in the vector representation. Presented at the 11th Colloquium “Quantum Groups and Integrable Systems”, Prague, 20–22 June 2002.     

Osmotic and aging effects in caviar oocytes throughout water and lipid changes assessed by 1H NMR T1 and T2 relaxation and MRI

By combining NMR relaxation spectroscopy and magnetic resonance imaging techniques, unsalted (us) and salted (s) caviar (Acipenser transmontanus) oocytes were characterized over a storage period of up to 90 days. The aging and the salting effects on the two major cell constituents, water and lipids, were separately assessed. T1 and T2 decays were interpreted by assuming a two-site exchange model. At Day 0, two water compartments that were not in fast exchange were identified by the T1 relaxation measurements on the us oocytes. In the s samples, T1 decay was monoexponential. During the time of storage, an increment of the free water amount was found for the us oocytes, ascribed to an increased metabolism. T1 and T2 of the s oocytes shortened as a consequence of the osmotic stress produced by salting. Selective images showed the presence of water endowed with different regional mobility that severely changed during the storage. Lipid T1 relaxation decays collected on us and s samples were found to be biexponential, and the T1 values lengthened during storage. In us and s oocytes, the increased lipid mobility with the storage was ascribed to lipolysis. Selective images of us samples showed lipids that were confined to the cytoplasm for up to 60 days of storage.     

Diffusion generated T1 and T2 contrast

In MR images of porous organic samples (such as roots or wood) in water media, the sample is often surrounded by a bright ring, with a corresponding decreased T1 value in T1 maps. When the medium is removed, or contrast agents are added, the ring disappears, indicating that the signal does not originate in the outer layers of the sample, but from the medium itself. It can be shown that this "bright ring effect" is only observed when the medium experiences a reduction in T1 when permeating the sample. In order to investigate this effect, a computer model was used to simulate the diffusion of magnetisation between regions that exhibit different relaxation constants. Using this model, the origin of the signal increase was found to be an inflow effect, as diffusion transports relaxed magnetisation from the boundary regions of the sample into the surrounding medium. In the case of the "bright ring" around the plants described above, a mixing of short T1 values from within the sample and long T1 values within the medium occurs, yielding a "transition region" between the two values. There, a signal increase can be observed at T1 weighted images, compared to the signal from the medium beyond this transition region. The width of the transition region is on the order of magnitude of the diffusion displacement that is calculated from the T1 value as diffusion time. In addition to causing the bright ring around the plant samples, this diffusion effect also limits the resolution of the relaxation time maps. This effect is not limited to T1 relaxation but also applies to T2 relaxation. However, at high B0 field strengths such as those used in this study (11.7 T), a T2 effect is not usually observed due to the considerably shorter T2 times in plants (about 50 ms, compared to T1 times of higher than 1 s). Because the diffusion length during this T2 relaxation is short with respect to the resolution of the imaging experiments, no T2 ring effect is seen.     

Proton T1 and T2 relaxivities of serum proteins

In the present study, T(1) and T(2) of phantoms containing serum sets with varying amounts of proteins, serum samples with certain amounts of proteins, serum diluted by distilled water, and serum treated with iron were measured. In addition, T(1) and T(2) of phantoms containing normal serum, diluted serum, and albumin-doped serum were also measured. Relaxation rates were plotted versus protein concentrations. The slope of relation was taken as relaxivity. The T(1) relaxivities of proteins were ranged from 0.035 to 0.080 s(-1)(g/dl)(-1), whereas T(2) relaxivities were ranged from 0.24 to 0.68 s(-1)(g/dl)(-1). The T(1) and T(2) relaxivities of transferrin iron were 2.40 and 2.60 mM(-1)s(-1), respectively. The contributions of diamagnetic proteins and transferrin iron to the relaxation rate of serum were also calculated for each of diluted serum, normal and albumin-doped serum. The contributions and the average TP relaxivities(calculated by using individual relaxivities and the ratios of protein fractions in TP) were used for TP calculations. The agreement between the calculated TP and TP by autoanalyzer and also the agreement between average TP relaxivities and the TP relaxivities determined from dilution experiments show that the data of relaxivities are reliable. The results suggest that individual protein relaxivities explain the influence of serum TP composition on T(1) and T(2) relaxation times.     

Leakage and water exchange characterization of gadofosveset in the myocardium

Purpose

To determine the compartmentalization of the blood pool agent gadofosveset and the effect of its transient binding to albumin on the quantification of steady-state fractional myocardial blood volume (fMBV).

Methods

Myocardial vascular fraction measurements were simulated assuming the limiting cases (slow or fast) of two-compartment water exchange for different contrast agent injection concentrations, binding fractions, bound and free relaxivities, and true cardiac vascular fractions.fMBV was measured in five healthy volunteers (4 males, 1 female, average age 33) at 1.5 T after administration of five injections of gadofosveset. The measurements in the volunteers were retrospectively compared to measurements of fMBV after three serial injections of the ultra-small, paramagnetic iron oxide (USPIO) blood pool agent ferumoxytol in an experimental animal. The true fMBV and exchange rate of water protons in both human and animal data sets was determined by chi square minimization.

Results

Simulations showed an error in the measurement of fMBV due to partial binding of gadofosveset of less than 30%. Measured fMBV values over-estimate simulation predictions, and approach cardiac extracellular volume (22%), which suggests that the intravascular assumption may not be appropriate for the myocardium, although it may apply to more distal perfusion beds. In comparison, fMBV measured with ferumoxytol (5%, with slow water proton exchange across vascular wall) agree with published values of myocardial vascular fraction. Further comparison between myocardium relaxation rates induced by gadofosveset and by other extracellular and intravascular contrast agents showed that gadofosveset behaves like an extracellular contrast agent.

Conclusions

The distribution of the volunteer data indicates that a three-compartment model, with slow water exchange of gadofosveset and water protons between the vascular and interstitial compartments, and fast water exchange between the interstitium and the myocytes, is appropriate. The ferumoxytol measurements indicate that this USPIO is an intravascular contrast agent that can be used to quantify myocardial blood volume, with the appropriate correction for water exchange using a two-compartment water exchange model.     

T1rho dispersion of rat tissues in vitro.

The purpose of this study was to demonstrate T1rho dispersion in different rat tissues (liver, brain, spleen, kidney, heart, and skeletal muscle), and to compare the 1/T1rho data to previous 1/T1 data and magnetization transfer of rat tissues at low (0.1 T) B0 field. The 1/T1rho dispersion showed a fairly similar pattern in all tissues. The highest 1/T1rho relaxation rates were seen in liver and muscle followed by heart, whereas the values for spleen, kidney, and brain were quite similar. Compared to 1/T2 relaxation rate, the greatest difference was seen in liver and muscle. The rank order 1/T1rho value at each locking field B1 was the same as the transfer rate of magnetization from the water to the macromolecular pool (Rwm) for liver, muscle, heart, and brain. The potential value T1rho imaging is to combine high T1 contrast of low field imaging with the high signal to noise ratio of high static field imaging. When the T1rho value for a given tissue is known, the contrast between different tissues can be optimized by adjusting the locking time TL. Further studies are encouraged to fully exploit this. Targets for more detailed research include brain infarct, brain and liver tumors.     

Comparison of T2 and LASER T2dagger image contrast in a rat model of acute cerebral ischemia

Previous studies have shown that T2(dagger)-weighted magnetic resonance images acquired using localization by adiabatic selective refocusing (LASER) can provide early tissue contrast following ischemia, possibly due to alterations in microscopic susceptibility within the tissue. The purpose of this study was to make a direct in vivo comparison of T2-, T2(dagger)- and diffusion-weighted image contrast during acute ischemia. Acute middle cerebral artery (MCA) occlusion was attempted in 14 rats using a modified Tamura approach incorporating electrocoagulation of the left MCA. T2(dagger)-weighted LASER images (Echo Time [TE]=108 ms), T2-weighted Carr-Purcell-Meiboom-Gill (CPMG) images (TE=110 ms) and diffusion-weighted images (b value=105 s/mm(2)) were acquired at 4 T within 1.5 h of ischemia onset. Tissue contrast in the MCA territory was quantified for histologically verified ischemic tissue (n=6) and in sham controls (n=4). T2(dagger)-weighted LASER images demonstrated greater contrast compared to the T2-weighted CPMG images, and more focal contrast compared to the diffusion-weighted images, suggesting different contrast mechanisms were involved.