Monitoring glycosaminoglycan replenishment in cartilage explants with gadolinium-enhanced magnetic resonance imaging

We previously devised a magnetic resonance imaging method that allows for the nondestructive and quantitative determination of glycosaminoglycan concentration in excised cartilage. The technique measures the concentration of the charged contrast agent Gd-DTPA2- (gadolinium diethylenetriamine-pentaacetic acid) equilibrated within cartilage, from which the tissue distribution of glycosaminoglycan can be calculated. The goals of our study were to determine the practicality of nondestructively monitoring glycosaminoglycan concentration in cartilage explants over a long-term culture period and to determine if glycosaminoglycan could be restored to glycosaminoglycan-depleted cartilage explants maintained in long-term culture. To meet our objectives, we harvested bovine cartilage explants, treated them initially with trypsin to reduce the glycosaminoglycan concentration, and cultured them for as long as 8 weeks. Images depicting glycosaminoglycan concentration were calculated from magnetic resonance images acquired at selected intervals during the trypsinization process and the subsequent culture period. The results indicate that gadolinium-enhanced magnetic resonance imaging can follow the reduction of glycosaminoglycan concentration over the course of enzymatic digestion and the replenishment of glycosaminoglycan over several weeks of culture and that cultured cartilage explants are capable of restoring glycosaminoglycan to 85% of its initial concentration. Of particular interest, samples cultured for 5 weeks indicated a depth dependence of glycosaminoglycan regeneration to values similar to the initial physiologic distribution. Thus, this magnetic resonance imaging method may be a very powerful means for exploring the spatial and temporal evolution of glycosaminoglycan in cartilage.

Nondestructive imaging of human cartilage glycosaminoglycan concentration by MRI

Despite the compelling need mandated by the prevalence and morbidity of degenerative cartilage diseases, it is extremely difficult to study disease progression and therapeutic efficacy, either in vitro or in vivo (clinically). This is partly because no techniques have been available for nondestructively visualizing the distribution of functionally important macromolecules in living cartilage. Here we describe and validate a technique to image the glycosaminoglycan concentration ([GAG]) of human cartilage nondestructively by magnetic resonance imaging (MRI). The technique is based on the premise that the negatively charged contrast agent gadolinium diethylene triamine pentaacetic acid (Gd(DTPA)2-) will distribute in cartilage in inverse relation to the negatively charged GAG concentration. Nuclear magnetic resonance spectroscopy studies of cartilage explants demonstrated that there was an approximately linear relationship between T1 (in the presence of Gd(DTPA)2-) and [GAG] over a large range of [GAG]. Furthermore, there was a strong agreement between the [GAG] calculated from [Gd(DTPA)2-] and the actual [GAG] determined from the validated methods of calculations from [Na+] and the biochemical DMMB assay. Spatial distributions of GAG were easily observed in T1-weighted and T1-calculated MRI studies of intact human joints, with good histological correlation. Furthermore, in vivo clinical images of T1 in the presence of Gd(DTPA)2- (i.e., GAG distribution) correlated well with the validated ex vivo results after total knee replacement surgery, showing that it is feasible to monitor GAG distribution in vivo. This approach gives us the opportunity to image directly the concentration of GAG, a major and critically important macromolecule in human cartilage.

In vivo microelectrode track reconstruction using magnetic resonance imaging

To obtain more precise anatomical information about cortical sites of microelectrode recording and microstimulation experiments in alert animals, we have developed a non-invasive, magnetic resonance imaging (MRI) technique for reconstructing microelectrode tracks. We made microelectrode penetrations in the brains of anesthetized rats and marked sites along them by depositing metal, presumably iron, with anodic monophasic or biphasic current from the tip of a stainless steel microelectrode. The metal deposits were clearly visible in the living animal as approximately 200 microm wide hypointense punctate marks using gradient echo sequences in a 4.7T MRI scanner. We confirmed the MRI findings by comparing them directly to the postmortem histology in which the iron in the deposits could be rendered visible with a Prussian blue reaction. MRI-visible marks could be created using currents as low as 1 microA (anodic) for 5 s, and they remained stable in the brains of living rats for up to nine months. We were able to make marks using either direct current or biphasic current pulses. Biphasic pulses caused less tissue damage and were similar to those used by many laboratories for functional microstimulation studies in the brains of alert monkeys.

MRI visualization of focal induced neocortical malformations of the rat

In an attempt to determine whether small focal malformations of the neocortex can be visualized in vivo, focal microgyria were induced in the neocortex of otherwise normal rats by freezing injury to the developing cortical plate, and in adulthood the malformation was visualized using MRI. Induced microgyria of varying size were successfully visualized with MRI, and the location and extent of the malformation was confirmed on subsequent histology. This work has potential implications for the field of experimental neuropathology by enhancing the ability to study the behavioral and connectional consequences of these malformations in animals. In addition, this work points toward future research for the in vivo visualization of these small, focal malformations in humans.

Glycosaminoglycan in articular cartilage: in vivo assessment with delayed Gd(DTPA)(2-)-enhanced MR imaging

PURPOSE

To investigate the feasibility of applying magnetic resonance (MR) imaging with use of an anionic compound, Gd(DTPA)2- (gadolinium diethylenetriamine-pentaacetic acid), for measuring glycosaminoglycan concentration in human cartilage in clinical studies.

MATERIALS AND METHODS

Penetration of Gd(DTPA)2- into cartilage was monitored through sequential T1-calculated images obtained after intraarticular (n = 2) or intravenous (n = 2) injection. T1-weighted and T1-calculated image series were then obtained in seven volunteers (nine knees) after penetration of Gd-(DTPA)2- into cartilage. If T1 was heterogeneous on Gd(DTPA)(2-)-enhanced images, images were also obtained after penetration of the cartilage with the nonionic contrast agent, gadoteridol.

RESULTS

Gd(DTPA)2- penetrated cartilage from the articular surface after intraarticular injection and from both the articular surface and the subchondral bone after intravenous injection. The latter resulted in shorter overall penetration time. T1 values on Gd(DTPA)(2-)-enhanced images were homogeneous in four knees, but in five knees T1 differences of up to 30% were observed. These T1 differences were not seen in the presence of gadoteridol. These variations in T1 reflected about 50% variations in glycosaminoglycan.

CONCLUSION

The data suggest that Gd(DTPA)(2-)-enhanced MR imaging has potential for monitoring glycosaminoglycan content of cartilage in vivo.

Multibolus stimulated echo imaging of coronary artery flow

One limitation of traditional bolus tagging techniques for MR angiography is the small amount of blood labeled by one tagging, resulting in a limited filling of the downstream vessels. We describe a multiple bolus technique using stimulated echoes (STE) for imaging coronary flow. A series of radiofrequency (RF) pairs are given with each pair selective at the region of tagging, thus tagging consecutive volumes of blood, and a final nonselective pulse is given to "read out" all of the tagged spins. In this way, multiple boluses of tagged blood are imaged at one time.

Water diffusion and exchange as they influence contrast enhancement

The contrast-enhanced magnetic resonance imaging (MRI) signal is rarely a direct measure of contrast concentration; rather it depends on the effect that the contrast agent has on the tissue water magnetization. To correctly interpret such studies, an understanding of the effects of water movement on the magnetic resonance (MR) signal is critical. In this review, we discuss how water diffusion within biological compartments and water exchange between these compartments affect MR signal enhancement and therefore our ability to extract physiologic information. The two primary ways by which contrast agents affect water magnetization are discussed: (1) direct relaxivity and (2) indirect susceptibility effects. For relaxivity agents, for which T1 effects usually dominate, the theory of relaxation enhancement is presented, along with a review of the relevant physiologic time constants for water movement affecting this relaxation enhancement. Experimental issues that impact accurate measurement of the relaxation enhancement are discussed. Finally, the impact of these effects on extracting physiologic information is presented. Susceptibility effects depend on the size and shape of the contrast agent, the size and shape of the compartment in which it resides, as well as the characteristics of the water movement through the resulting magnetic field inhomogeneity. Therefore, modeling of this effect is complex and is the subject of active study. However, since susceptibility effects can be much stronger than relaxivity effects in certain situations, they may be useful even without full quantitation.

Body cavity-based malignant lymphoma containing Kaposi sarcoma-associated herpesvirus in an HIV-negative man with previous Kaposi sarcoma

BACKGROUND

The role of Kaposi sarcoma-associated herpesvirus in the development of malignant lymphomas in patients negative for the human immunodeficiency virus (HIV) has not been established.

OBJECTIVE

To examine the possible role of Kaposi sarcoma-associated herpesvirus in a case of body cavity-based malignant lymphoma that occurred in an HIV-negative patient who had previously had Kaposi sarcoma.

DESIGN

Case study.

SETTING

Academic medical center.

PATIENT

A 94-year-old man with lymphomatous ascites.

MEASUREMENTS

Polymerase chain reaction (PCR) and Southern blot DNA analysis.

RESULTS

The body cavity-based lymphoma cells were positive for Kaposi sarcoma-associated herpesvirus by PCR and were negative for other herpesviruses, including Epstein-Barr virus, cytomegalovirus, and human herpesviruses 6 and 7. Southern blot analysis of lymphoma DNA showed high levels of Kaposi sarcoma-associated herpesvirus (> 40 to 80 genomes/cell). Clonal rearrangement of the immunoglobulin JH and JK genes was present, confirming the presence of a clonal B-cell proliferation.

CONCLUSIONS

Kaposi sarcoma-associated herpesvirus may be involved in the development of malignant lymphoma after Kaposi sarcoma in HIV-negative patients. This type of lymphoma, in contrast to body cavity-based lymphoma related to the acquired immunodeficiency syndrome, may have an indolent clinical course.

Gd-DTPA2- as a measure of cartilage degradation

Glycosaminoglycans (GAGs) are the main source of tissue fixed charge density (FCD) in cartilage, and are lost early in arthritic diseases. We tested the hypothesis that, like Na+, the charged contrast agent Gd-DTPA2- (and hence proton T1) could be used to measure tissue FCD and hence GAG concentration. NMR spectroscopy studies of cartilage explants demonstrated that there was a strong correlation (r > 0.96) between proton T1 in the presence of Gd-DTPA2- and tissue sodium and GAG concentrations. An ideal one-compartment electrochemical (Donnan) equilibrium model was examined as a means of quantifying FCD from Gd-DTPA2- concentration, yielding a value 50% less but linearly correlated with the validated method of quantifying FCD from Na+. These data could be used as the basis of an empirical model with which to quantify FCD from Gd-DTPA2- concentration, or a more sophisticated physical model could be developed. Spatial distributions of FCD were easily observed in T1-weighted MRI studies of trypsin and interleukin-1 induced cartilage degradation, with good histological correlation. Therefore, equilibration of the tissue in Gd-DTPA2- gives us the opportunity to directly image (through T1 weighting) the concentration of GAG, a major and critically important macromolecule in cartilage. Pilot clinical studies demonstrated Gd-DTPA2- penetration into cartilage, suggesting that this technique is clinically feasible.

Magnetization transfer in cartilage and its constituent macromolecules

The goal of this work was to investigate magnetization transfer (MT) in cartilage by measuring water proton signals Ms/Mo, as an indicator of MT, in (i) single-component systems of the tissue's constituent macromolecules and (ii) intact cartilage under control conditions and after two pathomimetic interventions. Ms/Mo was quantified with a 12-microT saturation pulse applied 6 kHz off resonance. Both glycosaminoglycans (GAG) and collagen exhibited concentration dependent effects on Ms/Mo, being approximately linear for GAG solutions (Ms/Mo = -0.0137[% GAG] + 1.02) and exponential for collagen suspensions (Ms/Mo = 0.80 x exp[-(%collagen)/6.66] + 0.20); the direct saturation of water could not account for the measured Ms/Mo. Although the effect of collagen on Ms/Mo is much stronger than for a corresponding concentration of GAG, Ms/Mo is not very sensitive to changes in collagen concentration in the physiological range. Tissue degradation with 25 mg/ml trypsin led to an increase in Ms/Mo from the baseline value of 0.2 (final/initial values = 1.15 +/- 0.13, n = 11, P < 0.001). In contrast, a 10-day treatment of cartilage with 100 ng/ml of interleukin-1 beta (IL-1 beta) caused a 19% decrease in Ms/Mo (final/initial values = 0.81 +/- 0.08, n = 3, P = 0.085). The changes in hydration and macromolecular content for the two treatments were comparable, suggesting that Ms/Mo is sensitive to macromolecular structure as well as concentration. In conclusion, whereas the baseline Ms/Mo value in cartilage may be primarily due to the tissue collagen concentration, changes in Ms/Mo may be due to physiological or pathophysiological changes in GAG concentration and tissue structure, and the measured Ms/Mo may differentiate between various pathomimetic degradative procedures.

Studies of Gd-DTPA relaxivity and proton exchange rates in tissue

The image intensity in many contrast agent perfusion studies is designed to be a function of bulk tissue T1, which is, in turn, a function of the compartmental (vascular, interstitial, and cellular) T1s, and the rate of proton exchange between the compartments. The goal of this study was to characterize the compartmental tissue Gd-DTPA relaxivities and to determine the proton exchange rate between the compartments. Expressing [Gd-DTPA] as mmol/liter tissue water, the relaxivities at 8.45 T and room temperature were: saline, 3.87 +/- 0.06 (mM.s)-1 (mean +/- SE; n = 29); plasma, 3.98 +/- 0.05 (mM.s)-1 (n = 6); and control cartilage (primarily an interstitium), 4.08 +/- 0.08 (mM.s)-1 (n = 17), none of which are significantly different. The relaxivity of cartilage did not change with compression, trypsinization, or equilibration in plasma, suggesting relaxivity is not influenced by interstitial solid matrix density, charge, or the presence of plasma proteins. T1 relaxation studies on isolated perfused hearts demonstrated that the cellular-interstitial water exchange rate is between 8 and 27 Hz, while the interstitial-vascular water exchange rate is less than 7 Hz. Thus, for Gd-DTPA concentrations, which would be used clinically, the T1 relaxation rate behavior of intact hearts can be modeled as being in the fast exchange regime for cellular-interstitial exchange but slow exchange for interstitial-vascular exchange. A measured relaxivity of 3.82 +/- 0.05 (mM.s)-1 (n = 8) for whole blood (red blood cells and plasma) and 4.16 +/- 0.02 (mM.s)-1 (n = 3) for frog heart tissue (cells and interstitium) (with T1 and Gd-DTPA concentration defined from the total tissue water volume) supports the conclusion of fast cellular-extracellular exchange. Knowledge of the Gd-DTPA relaxivity and maintaining Gd-DTPA concentration in the range so as to maintain fast cellular-interstitial exchange allows for calculation of bulk Gd-DTPA concentration from bulk tissue T1 within a calculable error due to slow vascular exchange.

MRI evaluation of myocardial perfusion without a contrast agent using magnetization transfer

We propose a new magnetic resonance imaging (MRI) technique that is sensitive to myocardial tissue perfusion that obviates the use of an extrinsic contrast agent. Significant advantages of such a technique are that it avoids accumulation of contrast agent when repeated studies are performed on the same subject and that it is completely noninvasive. The method makes use of a combination of magnetization transfer (MT) and T1sat (measured spin-lattice relaxation time in the presence of MT) weighting. In this Communication, we present observations from experiments with an isolated rat heart model that demonstrate increase of MT-weighted signal intensity and T1sat with flow. Also included are data showing that these effects can be made synergistic for enhancing the sensitivity to perfusion. We have observed about a 3% change in MT-weighted intensity and up to 10% change in MTT1sat-weighted intensity for a change of 1 ml/min in global flow rate.

Diffusion of small solutes in cartilage as measured by nuclear magnetic resonance (NMR) spectroscopy and imaging

The ability of water and solutes to move through the cartilage matrix is important to the normal function of cartilage and is presumed to be altered in degenerative diseases of cartilage such as osteoarthritis and rheumatoid arthritis. Nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) techniques were used to measure a self diffusion coefficient (D) for small solutes in samples of explanted cartilage for diffusion times ranging from 13 ms to 2 s. With a diffusion time of 13 ms, the intratissue diffusivity of several small solutes (water, Na+, Li+, and CF3CO2-) was found consistently to be about 60% of the diffusivity of the same species in free solution. Equilibration of the samples at low pH (which titrates the charge groups so that the net matrix charge of -300 mM at pH 8 becomes approximately -50 mM at pH 2) did not affect the diffusivity of water or Na+. These data, and the similarity between the D in cartilage relative to free solution for water, anions, and cations, are consistent with the view that charge is not an important determinant of the intratissue diffusivity of small solutes in cartilage. With 35% compression, the diffusivity of water and Li+ dropped by 19 and 39%, respectively. In contrast, the diffusivity of water increased by 20% after treatment with trypsin (to remove the proteoglycans and noncollagenous proteins). These data and the lack of an effect of charge on diffusivity are consistent with D being dependent on the composition and density of the solid tissue matrix. A series of diffusion-weighted proton images demonstrated that D could be measured on a localized basis and that changes in D associated with an enzymatically depleted matrix could be clearly observed. Finally, evidence of restriction to diffusion within the tissue was found with studies in which D was measured as a function of diffusion time. The measured D for water in cartilage decreased with diffusion times ranging from 25 ms to 2 s, at which point the measured D was roughly 40% of the diffusivity in free solution. Although changes in matrix density by compression or digestion with trypsin led to a decrease or increase, respectively, in the measured D, the functional change in measured diffusivity with diffusion time remained essentially unchanged. In a different type of study, in which bulk transport could be observed over long periods of time, cartilage was submerged in 99% D2O and MRI studies were performed to demonstrate the bulk movement of water out of the cartilage matrix.(ABSTRACT TRUNCATED AT 400 WORDS)

Characteristics of extracellular sodium relaxation in perfused hearts with pathologic interventions

Sodium spectroscopy and imaging sequences designed to emphasize fast T2 decay or the multiple quantum signal have previously demonstrated a high contrast between normal and pathologic tissue which may be due to changes in intracellular versus extracellular sodium distribution. Since alterations in the amount of signal with fast T2 decay have previously been shown to occur with changes in intracellular sodium content, this study investigated the fast T2 relaxation characteristics of extracellular sodium during pathologic interventions on nonsubmerged perfused rat hearts. T2 data on total sodium content were obtained while global ischemia (stopping all perfusate flow) and extracellular edema (due to long perfusion times) were induced in the heart. The data were fit to a biexponential, with Mf(T2f) the magnitude (time constant) of the fast component of decay. Mf increased significantly in both pathologies (to 319 +/- 26%, n = 3, of baseline for ischemia and to 527 +/- 284%, n = 3, of baseline for edema); the increase with edema was demonstrated to be due to extracellular sodium by intermittently perfusing the heart for a short period with shift reagent. When shift reagent was not used until the conclusion of the edema experiment, Mf increased to 169 +/- 35% of baseline, also due mainly to extracellular sodium. T2f did not exhibit any trends with these experiments, with values ranging from 1.7 to 5.5 ms. We believe that these results indicate that compartmental sodium content will most likely not be quantifiable in pathologic states in the heart with relaxation-based techniques. However, correlations between the pathologic state of the tissue and the sodium NMR signal obtained with pulse sequences or images that emphasize a particular aspect of relaxation may prove to be useful.

Determination of fixed charge density in cartilage using nuclear magnetic resonance

Many biomechanical and chemical properties of cartilage are dependent on the fixed charge density (FCD) of the extracellular matrix. In this study, nuclear magnetic resonance (NMR) spectroscopy was investigated as a nondestructive technique for determining FCD in cartilage. Sodium content was measured by NMR in cartilage explants and was compared with sodium content measured by inductively coupled plasma emission spectroscopy (ICP) in order to verify the total NMR visibility of sodium in cartilage. The ratio of NMR to ICP results was 1.02 +/- 0.04 (calf, mean +/- SD, n = 7) and 1.04 +/- 0.11 (adult bovine, n = 8). Sodium concentration as measured by NMR was then used with ideal Donnan theory to compute estimates of FCD. For calf articular cartilage (AC) near physiological conditions, calculated FCD was -0.28 +/- 0.03 M (n = 10). NMR measurements were then made for individual cartilage specimens sequentially equilibrated in baths of differing salt composition, pH, or ionic strength. For calf and adult AC, calculated FCD decreased dramatically between pH 3 and 2, with adult specimens becoming positively charged but calf tissue retaining a net negative charge. For calf AC equilibrated in 0.3-0.015 M NaCl, calculated FCD was observed to decrease slightly with decreasing bath ionic strength. For epiphyseal cartilage, FCD varied with the position of origin of the explant within the joint, ranging from -0.19 to -0.35 M in a manner that correlated with tissue glycosaminoglycan content. Preliminary NMR imaging experiments demonstrated similar variations of sodium concentration in intact ulnar epiphyseal cartilage. Collectively, these results demonstrate the ability of NMR to nondestructively follow FCD in cartilage. The technique is applicable to dynamic studies as well as to both in vitro and in vivo studies on living tissue.

Coronary arteries: breath-hold MR angiography

The authors describe a method for performance of ultrafast magnetic resonance (MR) angiography of coronary arteries with a standard clinical MR system and a body coil. Each image was obtained within a single breath hold by using an electrocardiography-gated, segmented, ultrafast, gradient-echo pulse sequence with an incremental excitation flip angle for the eight phase-encoding steps acquired per segment. By using overlapping 4-mm-thick sections, the coronary arteries were routinely depicted from the coronary ostia distally at MR in healthy subjects. Ultrafast MR angiography of the coronary arteries is feasible with use of a standard body coil. This technique offers considerable potential as an investigational tool and, with further development, may become a clinically useful imaging application.

Factors in myocardial "perfusion" imaging with ultrafast MRI and Gd-DTPA administration

Ultrafast magnetic resonance imaging (MRI) and first pass observation of an interstitial contrast agent are currently being used to study myocardial perfusion. Image intensity, however, is a function of several parameters, including the delivery of the contrast agent to the interstitium (coronary flow rate and diffusion into the interstitium) and the relaxation properties of the tissue (contrast agent concentration, proton exchange rates, and relative intra- and extracellular volume fractions). In this study, image intensity during gadopentetate dimeglumine (Gd-DTPA) administration with T1-weighted ultrafast MR imaging was assessed in an isolated heart preparation. With increasing Gd-DTPA concentration, the steady-state myocardial image intensity increased but the time to reach steady state remained unchanged, resulting in an increased slope of image intensity change. A range of physiologic perfusion pressures (and resulting coronary flow rates) had insignificant effects on kinetics of Gd-DTPA wash-in or steady-state image intensity, suggesting that diffusion of Gd-DTPA into the interstitium is the rate limiting step in image intensity change with this preparation. Following global ischemia and reperfusion, transmural differences in the slope of image intensity change were apparent. However, the altered steady-state image intensity (due to postischemic edema) makes interpretation of this finding difficult. The studies described here demonstrate that although Gd-DTPA administration combined with ultrafast imaging may be a sensitive indicator of perfusion abnormalities, factors other than perfusion will affect image intensity. Extensive studies will be required before image intensity with this protocol is fully understood.

MR imaging of coronary artery flow in isolated and in vivo hearts

Methods for imaging flow in coronary arteries with magnetic resonance (MR) imaging techniques are demonstrated in isolated heart preparations and live animal models. Coronary artery flow was first imaged with a flow-compensated gradient-echo pulse sequence in isovolumic and working perfused rat hearts and then in vivo. A bolus tracking technique was used to measure flow velocity in the coronary arteries. Ultrafast gradient-echo imaging techniques were then applied, with high resolution obtained by combining the information from several cardiac cycles. A stimulated-echo pulse sequence was demonstrated as a method for performing coronary angiography by flow tagging in isovolumic perfused hearts. This report describes the results of coronary flow MR imaging in isolated rat hearts and live mice and rats. The general approach has proved useful in evaluating new methods for coronary MR angiography and should permit well-controlled studies of pathologic conditions. This ability to image coronary flow in isolated hearts and in small animals should permit integrated MR studies of coronary flow, myocardial perfusion, myocardial metabolism, and cellular ionic status.

Nuclear magnetic resonance studies of sodium/calcium exchange in frog perfused, beating hearts

This study explores the effect of extracellular Ca2+ concentration ([Ca2+]o), on the intracellular Na+ concentration ([Na+]i), in frog intact hearts using nuclear magnetic resonance spectroscopy, which allows for the measurement of [Na+]i in perfused, beating hearts. Decreases in [Ca2+]o yielded marked increases in [Na+]i. A similar effect was seen during inhibition of the Na+/K+ pump and was fully reversible. This sensitivity of [Na+]i to [Ca2+]o, previously observed using microelectrodes, supports a crucial physiological role for Na+/Ca2+ exchange in frog intact, beating hearts.

Interstitial sodium nuclear magnetic resonance relaxation times in perfused hearts

Separation of intracellular and extracellular sodium nuclear magnetic resonance (NMR) signals would enable nondestructive monitoring of intracellular sodium. It has been proposed that differences between the relaxation times of intracellular and extracellular sodium be used either directly or indirectly to separate the signal from each compartment. However, whereas intracellular sodium relaxation times have been characterized for some systems, these times were unknown for interstitial sodium. In this study, the interstitial sodium NMR relaxation times have been measured in perfused frog and rat hearts under control conditions. This was achieved by eliminating the NMR signal from the extracardiac (perfusate) sodium, and then quantifying the remaining cardiac signal. The intracellular signal was measured to be 8% (frog) or 22% (rat) of the cardiac signal and its subtraction was found to have a negligible effect on the cardiac relaxation times. Therefore this cardiac signal is considered to provide a good estimate of interstitial relaxation behavior. For perfused frog (rat) hearts under control conditions, this signal was found to have a T1 of 31.6 +/- 3.0 ms (27.3 +/- 1.6 ms) and a biexponential T2 of 1.9 +/- 1.0 ms (2.1 +/- 0.3 ms) and 25.2 +/- 1.3 ms (26.3 +/- 3.2 ms). Due to the methods used to separate cardiac signal from perfusate signal, it is possible that this characterized only a part of the signal from the interstitium. The short T2 component attributable to the interstitial signal indicates that separation of the NMR signals from each compartment on the basis of relaxation times alone may be difficult.

Parvovirus B19-associated hemophagocytic syndrome

Parvovirus B19 is a recently described pathogen, associated with an increasing spectrum of clinical manifestations. We present the first reported case, to our knowledge, of parvovirus B19-associated hemophagocytic syndrome, in which the diagnosis of parvovirus infection was documented by the presence of B19-specific IgM and IgG antibodies. Pancytopenia resolved immediately following splenectomy and the patient recovered completely.

Measurement of left ventricular mass in rats using electrocardiogram-gated magnetic resonance imaging

To evaluate the ability of electrocardiogram (ECG)-gated magnetic resonance (MR) imaging to assess in vivo left ventricular (LV) mass in the rat, we studied 20 healthy adult Sprague-Dawley and Fischer 344 rats and 8 additional rats that underwent scanning after induction of volume overload by aortic leaflet disruption. ECG-gated spin-echo pulse sequences were used to acquire a series of 1-mm thick modified short-axis images of the left ventricle. The area enclosed by the endocardial and epicardial borders of the left ventricle was multiplied by the interslice distance and specific gravity of myocardium to obtain calculated slice mass. Total LV mass was obtained by summing the individual slices. The calculated value for LV mass was then compared with the LV mass as determined at postmortem examination. Linear regression analysis showed an excellent correlation of MR-estimated mass (x) with autopsy-measured LV mass (y) (y = 0.90x + 65, r = 0.98). For this method intraobserved and interobserver slice correlations were 0.97 and 0.96, respectively. There was no significant difference in LV mass as determined from a series of diastolic vs. systolic images in a subset of six animals. Over a mean of 6.5 wk of observation, LV mass increased by 40% in the animals subjected to aortic leaflet disruption. These results demonstrate that MR imaging is highly accurate for the non-invasive in vivo assessment of LV mass in the adult rat.

First-pass cardiac perfusion: evaluation with ultrafast MR imaging

The authors studied cardiac perfusion by administering gadolinium diethylenetriaminepentaacetic acid (DTPA) in conjunction with an ultrafast imaging technique that produces strongly T1-weighted images. The method consisted of a 180 degrees inversion pulse, followed by a gradient-echo acquisition with a very short repetition time (less than 4 msec). Each image was acquired throughout a small fraction of the cardiac cycle. The method was applied in an isolated perfused rat heart model (acquisition time = 116 msec) and in human subjects without known cardiac disease (acquisition time = 125 msec). Fast, high-resolution images (128 X 128 matrix) were created by combining sequentially acquired small matrixes. After bolus administration of Gd-DTPA in the perfused rat heart model, contrast was pronounced between the nonperfused myocardium and perfused normal myocardium. First-pass wash-in and washout phases of the contrast material were observed in the perfused rat heart model and in human subjects. Results demonstrated the clinical feasibility of first-pass perfusion studies of the heart. The studies can be performed on a conventional whole-body imaging system with standard hardware.

Prevention of diabetes in BB/Wor rat by single transfusion of spleen cells. Parameters that affect degree of protection

Previous studies have shown that multiple transfusions of spleen cells from histocompatible nondiabetic donors prevent autoimmune diabetes mellitus in diabetes-prone (DP) BioBreeding/Worcester (BB/Wor) rats. In this study, a single transfusion of greater than or equal to 50 x 10(6) cells from either diabetes-resistant (DR) BB/Wor or Wistar-Furth (WF) rats substantially reduced the incidence of diabetes when given to DP rats 27 or 46 days old but not 61 days old. Transfusion and protection were associated with the appearance of RT6+ donor lymph node cells in recipient rats. In vivo depletion of RT6+ T-lymphocytes in 150-day-old protected animals did not produce diabetes. DR BB/Wor and WF spleen cells were equally efficacious when given either intraperitoneally or intravenously. Mitogen-activated spleen cells were relatively less effective than untreated cells. We conclude that BB rat diabetes can be prevented by one transfusion of spleen cells from histocompatible DR and WF donors, and that the protective effect is dependent on recipient age and cell dose. The effect may be mediated by a population of RT6+ T-lymphocytes that, during a critical developmental period, regulate the expression of autoimmunity in these animals.