Magnetic Resonance Relaxation Times T1 Research Articles

The dynamic viscoelastic characterisation and magnetic resonance imaging of poly(vinyl alcohol) cryogel: Identifying new attributes and opportunities

Poly(vinyl alcohol) (PVA) cryogel is a biocompatible, synthetic hydrogel, compatible with magnetic resonance (MR) imaging. It is widely used as a biomaterial in tissue scaffolds and mimics to test various diagnostic techniques. The aim of this study is to characterise the effect of varying PVA concentration, molecular weight (MW) and manufacturing protocol on the viscoelastic mechanical properties and MR T2 relaxation time. Further to this MR imaging (MRI) was investigated as a method to quantify material homogeneity. Cylindrical samples of PVA, of varying MW, concentration and number of freeze thaw cycles (FTCs), were manufactured. Dynamic mechanical analysis was performed to evaluate the storage and loss moduli between frequencies of 0.5 and 10 Hz. MR T2 relaxation maps were imaged using a 7 T MRI instrument. Storage and loss moduli were shown to increase with MW, concentration, or the number of FTCs; with storage modulus ranging from 55 kPa to 912 kPa and loss modulus ranging from 6 kPa to 103 kPa. MR T2 relaxation time was shown to increase linearly with PVA concentration. The qualitative and quantitative heterogeneity of the PVA sample were identified through MR T2 relaxation time maps. Excitingly, PVA demonstrated a composition-dependent casual correlation between the viscoelastic mechanical properties and MR T2 relaxation time. In conclusion, this research thoroughly characterised the viscoelastic mechanical properties of PVA to support its extensive use as a biomaterial, and demonstrated the use of MRI to non-invasively identify sample heterogeneity and to predict the composition-dependent viscoelastic properties of PVA.

Understanding the role of d‐Allulose and soy protein addition in pectin gels

Abstractd‐Allulose (a monosaccharide and C3 epimer of fructose), one of the common rare sugars is getting attention due to its low caloric values. In this study, d‐Allulose was used as a replacement of sucrose at different ratios (d‐Allulose/Sucrose: 35/0, 20/15, 10/25, 0/35) to formulate pectin‐based soft confectionery gels. Soy protein isolate was also added to increase the protein content. Physical properties, such as hardness, moisture content, pH, and color, were measured for the gels. Higher hardness values were obtained for the soy protein containing gels due to pectin–soy protein interaction (p < 0.05). Also, higher moisture content was observed in soy protein containing gels (p < 0.05). In addition, nuclear magnetic resonance T2 relaxation times were measured at low field (~0.5 T) to determine how the water distribution in the samples changed and to observe how d‐Allulose affected the polymer–water interactions. The study also showed that the presence of d‐Allulose increased the crystallization tendency (% crystallinity of 7.9) of the pectin gels. X‐ray diffraction results showed the d‐allulose peaks at 33.76 and 48.68oθ. Morphologies of the gels were also examined by scanning electron microscope. Sugar type and soy protein isolate addition were found to have significant impact on the gel formulations.

Magnetic resonance imaging T1- and T2-mapping to assess renal structure and function: a systematic review and statement paper.

This systematic review, initiated by the European Cooperation in Science and Technology Action Magnetic Resonance Imaging Biomarkers for Chronic Kidney Disease (PARENCHIMA), focuses on potential clinical applications of magnetic resonance imaging in renal non-tumour disease using magnetic resonance relaxometry (MRR), specifically, the measurement of the independent quantitative magnetic resonance relaxation times T1 and T2 at 1.5 and 3Tesla (T), respectively. Healthy subjects show a distinguishable cortico-medullary differentiation (CMD) in T1 and a slight CMD in T2. Increased cortical T1 values, that is, reduced T1 CMD, were reported in acute allograft rejection (AAR) and diminished T1 CMD in chronic allograft rejection. However, ambiguous findings were reported and AAR could not be sufficiently differentiated from acute tubular necrosis and cyclosporine nephrotoxicity. Despite this, one recent quantitative study showed in renal transplants a direct correlation between fibrosis and T1 CMD. Additionally, various renal diseases, including renal transplants, showed a moderate to strong correlation between T1 CMD and renal function. Recent T2 studies observed increased values in renal transplants compared with healthy subjects and in early-stage autosomal dominant polycystic kidney disease (ADPKD), which could improve diagnosis and progression assessment compared with total kidney volume alone in early-stage ADPKD. Renal MRR is suggested to be sensitive to renal perfusion, ischaemia/oxygenation, oedema, fibrosis, hydration and comorbidities, which reduce specificity. Due to the lack of standardization in patient preparation, acquisition protocols and adequate patient selection, no widely accepted reference values are currently available. Therefore this review encourages efforts to optimize and standardize (multi-parametric) protocols to increase specificity and to tap the full potential of renal MRR in future research.

Synthesis of a cell penetrating peptide modified superparamagnetic iron oxide and MRI detection of bladder cancer.

Bladder cancer is the most common malignancy of the urinary tract for which the accurate measurement of minimal residual disease is critical to treatment and determining prognosis. Although cystoscope examination and voided urine cytology remain the current standard of care for detecting residual disease, these approaches are limited by mechanical trauma and lack sensitivity. To develop a new accurate noninvasive method, we developed a novel contrast agent where the surface of superparamagnetic iron oxide (SPIO) nanoparticles is functionalized with a bladder cancer-specific fluorescein isothiocyanate (FITC) labeled cell penetrating peptide (CPP)-polyarginine peptides (R11) for active targeting and imaging. The stable nanoparticles have an average hydrodynamic diameter of 51 nm, surface charge of -21 mV and MRI r2 relaxivity 135 mM−1s−1. In vitro cell studies demonstrated that the R11-conjugated SPIO (SPIO-R11) nanoparticles were taken up by bladder cancer cells (T24) in a dose-dependent manner, which was higher than unconjugated SPIO. TEM showed that SPIO-R11 was mainly concentrated on cell vesicle and lysosome, not in cell nucleus, and no obvious damage was seen on cell ultrastructure. Moreover, uptake of the nanoparticles showed significantly more SPIO-R11 accumulation in bladder cancer cells than in immortalized bladder epithelial cells unlike control SPIO. Further, SPIO-R11 was compatible with immortalized bladder epithelial cells at all tested concentrations up to 200 μg/mL after 72 h incubation. Moreover, SPIO-R11 decreased the magnetic resonance T2 relaxation time by 73% in tumors cells in vitro compared to 12% with SPIO. These results indicate great potential of SPIO-R11 as contrast agent to target bladder cancer for diagnostic and therapeutic applications.

Effect of Modified Wheat Gluten on Boiling Resistance Capacity of Pork Meatballs.

The effect of the modified wheat gluten (MWG) extender, prepared by alcalase-based hydrolysis and transglutaminase cross-linking, on meatballs was analyzed in this study. Here, we studied the effect of MWG addition on the boiling resistance capacity of pork meatballs (MB-MWG) at high temperature (100 °C) and increasing cooking time; meatballs with added soy protein isolates (MB-SPI) and raw wheat gluten (MB-WG) were used as references. The cooking loss, water-holding capacity (WHC), and textural properties of meatballs were investigated. The results revealed that MB-MWG showed lower cooking loss, which decreased by 49.16% compared to meatballs without added extenders when treated for 30 min. The WHC of MB-MWG significantly increased from 80.68% to 95.42%. The hardness, springiness, and chewiness (textural properties) of MB-MWG were also significantly increased by 97.05%, 6.68%, and 121.96%, respectively. The addition of MWG increased the cross-linking in meatballs during the cooking process, as indicated by the higher G'. SDS-PAGE indicated an obvious decrease in myosin heavy chain in MB-MWG cooked for 30 min at 100 °C, which was attributed to the interaction of myofibrillar proteins in pork meat with MWG. The nuclear magnetic resonance T2 relaxation time patterns indicated that MWG addition caused an increase in the bound water content, and decrease in the free water content, of meatballs. An analysis of the microstructures revealed that the MB-MWG formed the most regular and compact network. Therefore, MWG could be used as an ingredient to facilitate the processing of meat products.

Magnetic resonance T2 relaxation time at 7 Tesla associated with amyloid β pathology and age in a double-transgenic mouse model of Alzheimer’s disease

The aim of this study was to better understand the effect of amyloid-β plaques on magnetic resonance T2 relaxation time. We investigated these changes associated with age in an APP/PS1 mouse model of AD at 7 Tesla, combined with histology. Ten double-transgenic AD and ten wild type (WT) female mice (aged 12–20 months) were used in a cross-sectional study. Mean T2 values and standard deviations were calculated for each Regions of interest (ROIs) on T2 map. Immunohistochemistry for amyloid plaques and fluorescence staining with thioflavine S were performed of brain sections after imaging. The results showed that mean T2 values of the hippocampus, cortex, corpus callosum, and thalamus of older mice were significantly lower than of the younger. Compared to WT mice, the T2 values of the hippocampus, corpus callosum, and thalamus in younger AD mice were significantly greater, while the T2 values of the hippocampus and cortex in older AD mice were significantly less. Aβ-40 immunohistochemistry and thioflavine S stainging were positive in the matched region both for younger and older AD mice, while neither Aβ-40 nor thioflavine S were observed in WT mice. These findings suggest that regional T2 values of AD mice may decrease with age, and changes in T2 values in AD mice may be influenced by many factors besides amyloid-β plaque accumulation. Furthermore, they support that the standard deviation of the mean T2 value should be considered as well as the mean.

Normalization of T2 relaxation time and apparent diffusion coefficient in relation to the inflammatory changes in the substantia nigra of rats with focal cerebral ischemia.

Focal cerebral ischemia results in delayed neurodegeneration in remote brain regions, such as the substantia nigra. To date, a reasonable explanation is still lacking regarding the correlation of magnetic resonance (MR) signal pseudo-normalization following a transient abnormal change and subsequent progressive pathological damage. To characterize the substantia nigra following middle cerebral artery occlusion and to evaluate the potential pathophysiological changes associated with the pseudo-normalization of MR signals in the substantia nigra at the subacute stage after stroke onset. Sprague-Dawley rats were subjected to transient middle cerebral artery occlusion. During the occlusion of single middle cerebral artery, computed tomography (CT) perfusion was acquired to observe the blood flow perfusion in the primary ischemic striatum and ipsilateral substantia nigra. Next, the MR T2 relaxation time and apparent diffusion coefficient changes within the substantia nigra were determined on days 1, 3, 7, and 14 after stroke onset, and compared with immunohistochemistry for microglia activation and astrogliosis. Twenty-four rats with strong hypoperfusion in the primary ischemic territory and no alterations of the perfusion in the ipsilateral substantia nigra detected both visually and measurably during the middle cerebral artery occlusion were further studied. All animals showed MR pseudo-normalization with T2 relaxation time and apparent diffusion coefficient recovered in the ipsilateral substantia nigra at the subacute phase following focal cerebral ischemia. Normalization of the MR signals corresponded well with the spatio-temporal occurrence of microglia activation and astrogliosis. The pseudo-normalization of T2 relaxation time and apparent diffusion coefficient reflects the neuroinflammatory changes that accompany activation of microglia and astrocytes in the ipsilateral substantia nigra following middle cerebral artery occlusion.

Effects of hydrolyzed wheat gluten on the properties of high-temperature (≥100 °C) treated surimi gels

Previous studies have found that the gel strength of the surimi decreased significantly with treating temperature increasing from 100 °C to 120 °C, which severely reduced the water-holding capacity (WHC), destroyed the network structure and thereby affected the texture of the surimi products. In this study, hydrolyzed wheat gluten (HWG) was added into Alaska Pollock surimi, and gels were obtained by maintaining their central temperature at 100 ± 1 °C, 105 ± 1 °C, 110 ± 1 °C, 115 ± 1 °C and 120 ± 1 °C for 10 min respectively under a certain pressure (0.12 MPa), to explain how HWG affected the thermal stability of the high-temperature treated surimi gels. With temperature increasing, adding HWG significantly increased the texture characteristic of the surimi gels, and the WHC of the plural gels took on a linear upward trend. The Nuclear Magnetic Resonance T2 relaxation time showed that adding HWG significantly decreased the relaxation time of T21. The sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed the same results, the content of myosin heavy chain decreased entirely and the content of actin decreased significantly. Scanning electron microscopy results showed that when treating with 120 °C, the network frames of the surimi gels became fragile, while the surimi-HWG gels kept almost the same density and uniformity. Transmission electron microscopy results showed that when treating with 120 °C, the structure of the surimi gels became loose and inhomogeneous, and the protein flocculated and aggregated seriously, while the myofibrillar protein did not aggregate and the HWG dispersed in the network evenly.

Using magnetic resonance imaging to study enzymatic hydrogelation.

Herein, we report, for the first time, the use of MRI methods to study enzymatic hydrogelation. Supramolecular hydrogels have been exploited as biomaterials for many applications. However, behaviors of the water molecules encapsulated in hydrogels have not been fully understood. In this work, we designed a precursor 1 which could self-assemble into nanofibers and form hydrogel I (gel I) upon the catalysis of phosphatase. The differences of mechanic property, pore size, water diffusion rate, and magnetic resonance relaxation times T1 and T2 of gel I containing different concentrations of 1 were systematically studied and analyzed. T1, T2, and diffusion-weighted (1)H MR images from gel I phantoms were obtained at 9.4 T. Analyses of the MRI data uncovered how the density of the nanofiber networks affects the relaxation behaviors of the water protons encapsulated in such hydrogels. Rheological analyses and cryo-TEM observations showed increased gel elasticities with increased concentrations of 1 while the pore sizes of gel I decreased. This also resulted in an increase in the proton relaxation rate (i.e., shortened T1, T2, and apparent diffusion coefficient (ADC)) for the water encapsulated in the hydrogel. With MRI, our study provides a new in vitro method to potentially mimic and study in vivo diseases that involve fibrous aggregates.

Human Aortic Endothelial Cell Labeling with Positive Contrast Gadolinium Oxide Nanoparticles for Cellular Magnetic Resonance Imaging at 7 Tesla

Positive T₁ contrast using gadolinium (Gd) contrast agents can potentially improve detection of labeled cells on magnetic resonance imaging (MRI). Recently, gadolinium oxide (Gd₂O₃) nanoparticles have shown promise as a sensitive T₁ agent for cell labeling at clinical field strengths compared to conventional Gd chelates. The objective of this study was to investigate Gado CELLTrack, a commercially available Gd₂O₃ nanoparticle, for cell labeling and MRI at 7 T. Relaxivity measurements yielded r1 = 4.7 s⁻¹ mM⁻¹ and r₂/r₁ = 6.2. Human aortic endothelial cells were labeled with Gd₂O₃ at various concentrations and underwent MRI from 1 to 7 days postlabeling. The magnetic resonance relaxation times T₁ and T₂ of labeled cell pellets were measured. Cellular contrast agent uptake was quantified by inductively coupled plasma-atomic emission spectroscopy, which showed very high uptake compared to conventional Gd compounds. MRI demonstrated significant positive T₁ contrast and stable labeling on cells. Enhancement was optimal at low Gd concentrations, attained in the 0.02 to 0.1 mM incubation concentration range (corresponding cell uptake was 7.26 to 34.1 pg Gd/cell). Cell viability and proliferation were unaffected at the concentrations tested and up to at least 3 days postlabeling. Gd₂O₃ is a promising sensitive and stable positive contrast agent for cellular MRI at 7 T.

Determining Cervical Ripeness and Labor Outcome

To determine whether the magnetic resonance assessment of cervical water content using the T2 relaxation time correlated with cervical ripening, as evidenced by the time to onset of spontaneous labor, need for induction, and the incidence of cesarean delivery in women whose pregnancy reached 41 weeks of gestation. The cervical T2 relaxation time was calculated from magnetic resonance data obtained in a previous study of magnetic resonance pelvimetry. After consent was obtained, the patients underwent a magnetic resonance imaging (MRI) protocol consisting of a dual fast spin echo T2-weighted scan. From images of a single slice, the cervical T2 relaxation time was calculated from two different regions of interest (anterior and posterior) on the cervix. The average cervical T2 relaxation time was then correlated to obstetric outcomes linked with cervical ripening. A total of 119 patients gave their consent for the study. Of these patients, 93 had optimal imaging of the cervical stroma and were included in the analysis. There was no significant correlation between the cervical T2 relaxation time and any individual component of the Bishop score or the total score. The cervical T2 relaxation time did not predict whether labor was spontaneous or induced and whether or not a woman underwent cesarean delivery. Cervical magnetic resonance T2 relaxation times did not correlate with the clinical Bishop score or predict labor outcome in our series of women whose pregnancies reached 41 weeks of gestation. Quantifying the magnetic resonance T2 relaxation time does not appear to be useful in the assessment of cervical ripening. III.

Evaluation of Magnetic Resonance for Detection of Bacterial Contamination in Low-Acid, Shelf-Stable Packaged Soymilk

This study evaluated magnetic resonance (MR) as a nondestructive method for detection of bacterial contamination in shelf-stable soymilk and cheese sauce. To accomplish this, individual 355-ml polymeric trays filled with soymilk and inoculated with Bacillus stearothermophilus and Bacillus subtilis (103 CFU) were incubated for up to 28 h at 55°C and 62 h at 37°C, respectively. MR relaxation times (T2) of these samples were then correlated with the bacterial growth as well as viscosity and pH changes caused by the bacteria in the packaged soymilk. In addition, this study investigated the ability of MR to differentiate between regularly processed cheese sauce and cheese sauce that was modified with α-amylase as a spoilage simulation. Results showed increased MR T2 relaxation times after the bacterial populations reached 108 CFU/ml (after 18 h) and 107 CFU/ml (after 44 h) for B. stearothermophilus and B. subtilis, respectively. B. subtilis had an undetectable influence on viscosity but a profound influence on pH. B. stearothermophilus, in comparison, significantly lowered the pH and increased the viscosity of the soymilk. MR was able to distinguish between regularly processed 85-g pouches of cheese sauce and other pouches with sauce that were modified with 0.5 ml of 1% α-amylase solution. These results showed that MR has the potential to be used for nondestructive detection of physical changes in soymilk and cheese sauce induced by bacterial growth and enzymatic activities, respectively.

Spatial Quantification of Mn2+and Mn3+Concentrations in the Mn-Catalyzed 1,4-Cyclohexanedione/Acid/Bromate Reaction Using Magnetic Resonance Imaging

This paper presents a magnetic resonance imaging (MRI) method for quantifying the spatial concentrations of the paramagnetic species Mn2+ and Mn3+, during pattern formation, in the manganese-catalyzed 1,4-cyclohexanedione-acid-bromate reaction. The relaxivity of each species was measured, which enabled their concentration to be calculated from nuclear magnetic resonance T2 relaxation times. Images were acquired using the fast spin-echo imaging method RARE. By determining the relation, in these experiments, between signal intensity and T2 relaxation time, it was possible to produce T2 maps from the images. These T2 maps were then used to produce concentration maps for the oxidized (Mn3+) and reduced (Mn2+) states of the catalyst.

Nuclear magnetic resonance investigation of Li+-ion dynamics in the perovskitefast-ion conductor Li3xLa2/3-x1/3-2xTiO3

7Li nuclear magnetic resonance relaxation times T1,T1ρ and T2 versus temperature are reported in the150-900 K temperature range for the lithium lanthanum titanateLi3xLa2/3-x□1/3-2xTiO3 perovskite-typefast-ionic conductors. The presence of Li+ ions of two kindswith slightly differing environments is displayed in theseexperiments. These ions exhibit two different motions: a fast onewith a characteristic frequency around 100 MHz at 350 K and a slowone whose frequency is around 60 kHz at 280 K. These two differentLi+ species cannot be differentiated by means of the fastmotion (only one T1 is observed from the experiments), but onlyby means of the slow ones (two T1ρ and two T2 areobserved). These motions are respectively attributed to Li+motion inside the A-cage of the perovskite structure formed by theoxygen ions and to Li+ hops between the cages. T1- andT1ρ-studies also performed on the 6Li nucleus clearlyshow that just dipolar nuclear interaction is responsible forLi+ relaxation. This result is at variance with what has beenpreviously put forward for the relaxation process in thesecompounds.

Infections, inflammations, and immune-mediated orbital disease

In recent months, many authors have described distinct surgical approaches to orbital decompression in thyroid ophthalmopathy. Comparison of outcomes data should allow the surgeon to decide on the best approach to orbital decompression in an individual patient. Patients with thyroid-associated lid retraction may also benefit from approaches that allow for postoperative suture adjustment, because alignment of the lids can be particularly challenging in these patients. The clinical evaluation of patients with thyroid disease has evolved with the recognition that magnetic resonance T2 relaxation time prolongation may correlate with an inflammatory disease phase and, perhaps, with improved response to medical management. Medical management of orbital inflammation and infection should be tailored to the specific disease process and, in many cases of infection, to the age of the patient.

Nuclear magnetic resonance T2 relaxation times in multiple sclerosis

An original method was used to carry out the mathematical analysis of T2 transverse magnetization decay curves and the measure of T2 relaxation times on multiple sclerosis (MS) patients. The presumably normal white matter (WM) of these patients presented higher T2 relaxation times (98.6 msec), in comparison with that found in a population sample (88 msec). In this case, magnetization decay curves remain mostly monoexponential and are characterized by a single T2. On the other hand, areas of increased signal (AIS) curves are always better fitted by a biexponential function characterized by a short (82 msec) and a long (> 200 msec) T2. The spreading out of long T2 varies from one AIS to another in the same patient and among different patients; values of long T2 also vary with time, but without any correlation with the clinical state. In fact, no correlation was been established between relaxation times and clinical parameters. Quantitative MRI therefore enables a different approach to interpret MRI images; results suggest that several histobiochemical parameters play a role in the pathogenesis of an AIS and that MS is a dynamic and constantly evolving disease.

Tumours of the central nervous system. Proton magnetic resonance relaxation times T1 and T2 and histopathologic correlates.

Proton MR relaxation times T1 and T2 were determined in vitro in 136 small specimens of astrocytomas grades I-IV, of oligodendrogliomas, metastases of adenocarcinomas, meningiomas and acoustic neuromas. In addition, 7 samples of peritumoural white matter were analysed. The analysed specimens were studied microscopically in their entirety regarding tumour type and occurrence of necrosis and non-tumour tissue admixture, such as fibrosis and haemorrhage. Most of the gliomas had longer relaxation times than normal white matter and T2 was significantly longer than in the other three tumour groups. The metastases had longer T1 than normal white matter, while T2 varied. The astrocytomas tended to show shorter relaxation times with increasing degree of malignancy, and shortening of T1 and T2 correlating with the proportion of tissue necrosis. Similarly, the metastases with tissue necrosis had shorter T1 and T2 than non-necrotic samples. The meningiomas had T1 values comparable with normal cortex, while the T2 values varied. Tumours containing a large proportion of fibrous tissue had shorter relaxation times than the others. Acoustic neuromas had only slightly longer T1 than normal white matter, while T2 was not prolonged. Both T1 and T2 were significantly shorter than in all other tumours studied. Peritumoural white matter had prolonged relaxation times compared with normal white matter, correlating to increased water content. These in vitro differences regarding relaxation times in various types of tumours of the central nervous system, dependent on various types of tissue alterations, should be of interest for the interpretation of in vivo images.

Regional Differences in the Proton Magnetic Resonance Relaxation Times T1 and T2 within the Normal Human Brain

The proton magnetic resonance (MR) relaxation times T1 and T2 were determined in autopsy specimens from 13 different regions of normal human brains. One hundred and seventy-four tissue samples from 25 brains were examined in a pulsed MR analyzer of 0.25 T and were then also studied histologically. There were regional differences in T1 and T2 within the cerebral gray matter but not within the white matter. These regional differences might reflect the different composition and cytoarchitectonic structure of the cortical regions and should be taken into consideration in the interpretation of cortical lesions on MR images.

Proton magnetic resonance relaxation times T1 and T2 related to postmortem interval. An investigation on porcine brain tissue.

In order to establish the validity of in vitro determination of the proton magnetic resonance (MR) relaxation times T1 and T2 in brain tissue at increasing time delay after death or operative excision, 81 brain tissue samples from 23 pigs were analyzed repeatedly. These samples, representing cortex, caudate nucleus and white matter, were studied microscopically after MR measurements. The T1 values exhibited no time dependence and the T2 values decreased slightly during the interval 2 to 90 hours after death. The samples were stored at +8 degrees C between the measurements. These results indicate that reliable in vitro measurements can be obtained in autopsy or surgical brain tissue specimens within 90 hours after death or excision, if handled properly.

Nuclear magnetic resonance characteristics of fresh and fixed tissue: the effect of elapsed time.

The nuclear magnetic resonance relaxation times T1 and T2 of liver and spleen from freshly euthanized rats were measured over 48 hours for samples treated with normal saline, fixed with 70% ethanol, neutral buffered formalin, or 10% formalin, or left untreated. The initial values of T1 and T2 for both tissues agreed with previous determinations. For untreated tissues of both liver and spleen, T1 was stable for 6 hours and T2 was stable for 48 hours. For fixed tissues, T1 was stable for 0.5 hours, but T2 changed in 0.5 hours.