Neurite orientation dispersion and density imaging can detect presymptomatic axonal degeneration in the spinal cord of ALS mice.

Neurite orientation dispersion and density imaging (NODDI), a MRI multi-shell diffusion technique, has offered new insights for the study of microstructural changes in neurodegenerative diseases. Mainly, the present study aimed to determine the connection between NODDI-derived parameters and changes in white matter (WM) abnormalities at early stages of amyotrophic lateral sclerosis (ALS). Spinal cords from ALS mice (G93A-SOD1 mice) were scanned in a Bruker Avance III HD 17.6T magnet. Fluorescent axonal-tagged mice (YFP, G93A-SOD1 mice) were used for quantitative histological analysis. NODDI showed a decrease in intra-cellular volume fraction (-24%) and increases in orientation dispersion index (+35%) and isotropic volume fraction (+33%). In addition, histoathological results demonstrated a reductions in axonal area (-11%) and myelin content (-29%). A histological decrease in WM intra-axonal space (-71%) and an increase in the extra-axonal compartment (+22%) were also detected. Our studies demonstrate that NODDI may be a suitable technique for detecting presymptomatic spinal cord WM microstructural degeneration in ALS.

Vibratory coherence as an alternative to radiography in assessing bone healing after osteodistraction.

Distraction osteogenesis is used in orthopedics to lengthen bones by cutting or breaking the bone and gradually separating the two pieces as new bone fills the intervening space. There is a need for early assessment of the degree of bone healing that allows for normal functioning without unwanted side effects. This study compared different techniques used to evaluate the degree of bone healing during mandibular osteodistraction in 21 rabbits. For each rabbit, the mandible was cut in a surgical procedure and then 72 h later distraction began at a rate of 3 mm per day. Bone formation at the distraction site was assessed by in vivo photodensitometry on head radiographs, an in vivo (nondestructive) vibratory coherence test across the distraction site, a postmortem, ex vivo (destructive) three-point bending mechanical test, and by postmortem, ex vivo (destructive) histological examination. Statistical analyses included analysis of variance and correlation coefficient tests. The findings revealed that the results of bone photodensity and the mechanical three-point test are highly and positively correlated with the results of the vibration test. The use of the vibration test may provide a substitute for or augment the routine use of radiography for in vivo evaluation and monitoring of bone healing.

Fluorescein dermofluorometry for the assessment of diabetic microvascular disease.

BACKGROUND/AIMS: Fluorescein dermofluorometry can be used to relate the uptake of fluorescein in the skin to blood flow. We have characterized the uptake of the dye by a wash-in time constant that is inversely proportional to the local blood flow. The purpose of this study was to explore the use of dermofluorometry in the assessment of patients with diabetic microvascular disease. METHODS: Fluorescein dermofluorometry was performed in four groups of patients: non-diabetic control patients, diabetic control patients, diabetic patients with chronic foot ulcers, and diabetic patients with acute foot ulcers. The outcomes of the patients with foot ulcers were documented 4-14 months after participation. Following an intravenous injection of sodium fluorescein, the change in the fluorescein signal with time was continuously measured at the plantar surface of the foot. Both the initial slope of the signal and the wash-in time constant were calculated in each subject. RESULTS: Significant differences in the wash-in time constant were found between diabetic and non-diabetic subjects and between diabetic subjects with and without foot ulcers. Of the eight patients with foot ulcers, two of them did not display an early wash-out in the dermofluorometer signal and later both required amputations. CONCLUSION: The fluorescein wash-in time constant demonstrated better correlation with the presence of diabetic microvascular disease than did the initial slope of the signal. Differences in the wash-in time constants of non-diabetic and diabetic subjects support the hemodynamic hypothesis for the development of microvascular disease. The indication of early wash-out of the fluorescein signal may also be useful in the prediction of ulcer healing.

Biological effects of long-duration, high-field (4 T) MRI on growth and development in the mouse.

The effects of long-duration, high-field magnetic resonance imaging (MRI) on fetal growth and postnatal development in mice were studied. Seven experimental groups of pregnant ICR mice were exposed for 9 hours on day 9 and/or day 12 post coitus (pc) to magnetic fields (4 T static, 5 T/sec switched gradient, and 0.2 W/kg radiofrequency at 170 MHz) associated with MRI conditions. Two experimental groups (sham and exposure groups) were exposed to a combination of ultrasound (day 9 pc, 3.25 MHz, focused) and MRI-associated fields (day 12 pc). No statistically significant changes in fetal growth were observed in the animals exposed to only MRI or ultrasound fields. However, in the combined ultrasound and MRI-exposed group, the fetal weight and crown-rump length were reduced compared with the sham and cage controls. These results suggest that MRI and ultrasound exposure well in excess of current clinical conditions can exert biological effects if applied at sensitive stages of fetal development.

Integrating microfabricated fluidic systems and NMR spectroscopy.

The philosophy of miniature total analysis systems (mu-TAS) hinges on the integration of multiple chemical processing steps and the means of analyzing their results on the same miniaturized system. We have constructed chip-based capillary electrophoresis (CE) devices equipped with an integrated planar radio-frequency detector coil used for nuclear magnetic resonance spectroscopy (NMR). Separations were accomplished in the devices, but satisfactory NMR spectra could only be obtained from samples of high concentration. The relative sensitivity is explained and the scaling law dichotomy of CE and NMR explored.

Ultrasound interaction with large unilamellar vesicles at the phospholipid phase transition: perturbation by phospholipid side chain substitution with deuterium.

The ultrasonic absorption, alpha lambda, as a function of temperature and frequency was determined in large unilamellar vesicles (LUVs) in which specific phospholipid side chains were deuterated. Deuteration significantly altered the temperature and frequency dependence of alpha lambda. The frequency change was especially marked, with decreased frequency and broadening of the ultrasound relaxation, even with only minor changes in the phase transition temperature. Deuteration decreased the Tm and enthalpy of the lipid phase transition, as shown by differential scanning calorimetry, whereas electron spin resonance showed that at and above the lipid phase transition, no differences in the mobility as a function of temperature were observed. These results show that the observed increase in ultrasonic absorption in LUVs at the phospholipid phase transition arises from the interaction of ultrasound with the hydrophobic side chains, probably coupling with structural reorganization of small domains of molecules, a process which is maximized at the phase transition temperature.

Nanoliter volume, high-resolution NMR microspectroscopy using a 60-micron planar microcoil.

Previous studies demonstrated the feasibility of using 100-microns inner diameter planar spiral inductors (microcoils) as detectors in 1H nuclear magnetic resonance (NMR) microspectroscopy. However, high-resolution NMR applications were not possible due to poor spectral resolution and low signal-to-noise ratio (SNR). These limitations in performance have now been largely overcome by using a nonconductive liquid fluorocarbon (FC-43) to minimize the effects of susceptibility mismatch between materials, and by carefully optimizing the microcoil geometry for maximum SNR. In this study, liquid samples were loaded into a fused silica capillary (75-microns inner diameter, 147-microns outer diameter). The capillary was positioned 50 microns above a 3.5-turn microcoil so that approximately 1 nL of the sample was present in the sensitive region of the microcoil. The microcoil was fabricated on a gallium arsenide substrate with an inner diameter of 60 microns, an outer diameter of 200 microns, trace width of 10 microns, trace spacing of 10 microns, and trace height of 3 microns. At 5.9 T (250 MHz) in 1H-NMR microspectroscopy experiments using a spectral width of 1 kHz, 4096 sampled data points, and a recovery delay of 1 s, a SNR of 25 (per acquisition) and a spectral linewidth of less than 2 Hz were obtained from a sample of water. These results demonstrate that planar microcoils can be used for high-resolution NMR microspectroscopy. Such coils may also be suitable for localized NMR studies at the cellular level and as detectors in capillary electrophoresis or microbore liquid chromatography.

A comparison of RIGR and SVD dynamic imaging methods.

Several constrained imaging methods have recently been proposed for dynamic imaging applications. This paper compares two of these methods: the Reduced-encoding Imaging by Generalized-series Reconstruction (RIGR) and Singular Value Decomposition (SVD) methods. RIGR utilizes a priori data for optimal image reconstruction whereas the SVD method seeks to optimize data acquisition. However, this study shows that the existing SVD encoding method tends to bias the data acquisition scheme toward reproducing the known features in the reference image. This characteristic of the SVD encoding method reduces its capability to capture new image features and makes it less suitable than RIGR for dynamic imaging applications.

Measurement of sodium fluorescein wash-in time constants in subjects with peripheral vascular disease.

The authors developed a noninvasive two-channel dynamic dermofluorometer that can quantitatively follow the rapid skin wash-in kinetics of a fluorescent dye to provide an assessment of local skin perfusion. The dermofluorometer was tested in normal subjects and diabetic patients with and without peripheral vascular disease. After an intravenous injection of 1-2 mL of a 10% solution of sodium fluorescein (1.1-2.8 mg/kg), the fluorescent signal was monitored from two sites on the skin surfaces of the forearm and foot. A 3.2-mm-diameter glass fiberoptic bundle was used both to transmit the excitation light (489 nm) and to receive the fluorescent emission (517 nm). Dermofluorometer readings were recorded approximately every second for 10-15 minutes following the injection. The time course of the fluorescein signal intensity was fit to a single exponential curve characterized by a wash-in time constant. There was no significant difference in arm wash-in time constants. Foot wash-in time constants were increased in diabetic patients who had past histories of foot ulcers relative to diabetic patients without a history of foot ulcers (3.2 vs 1.6 min., p < 0.05). Foot wash-in time constants were decreased in diabetic patients who had active infected foot ulcers. This study demonstrates the ability of the dynamic dermofluorometer to measure wash-in constants that reflect the local skin perfusion in less than 15 minutes after a low intravenous dose of sodium fluorescein.

Computation of electromagnetic fields for high-frequency magnetic resonance imaging applications.

A numerical method is presented to compute electromagnetic fields inside a 2 mm high resolution, anatomically detailed model of a human head for high-frequency magnetic resonance imaging (MRI) applications. The method uses the biconjugate gradient algorithm in combination with the fast Fourier transform to solve a matrix equation resulting from the discretization of an integrodifferential equation representing the original physical problem. Given the current distribution in an MRI coil, the method can compute both the electric field (thus the specific energy absorption rate (SAR)) and the magnetic field, also known as the B1 field. Results for the SAR and B1 field distribution, excited by a linear and a quadrature birdcage coil, are calculated and presented at 64 MHz, 128 MHz and 256 MHz, corresponding to the operating frequencies of the 1.5 T, 3 T and 6 T MRI systems. It is shown that compared with that at 64 MHz, the SAR at 128 MHz is increased by a factor over 5 and the SAR at 256 MHz is increased by a factor over 10, assuming the same current strength in the coil. Furthermore, compared with the linear excitation, the average SAR for the quadrature excitation is reduced by a factor over 2 and the maximum SAR is reduced by a factor over 3. It is also shown that the B1 field at high frequencies exhibits a strong inhomogeneity, which is attributed to dielectric resonance.

Effects of in utero exposure to 4.7 T MR imaging conditions on fetal growth and testicular development in the mouse.

The effects of exposure in utero to high field (4.7 T) magnetic resonance imaging (MRI) conditions on fetal growth, adult growth, and testicular development in the mouse were investigated. Exposures of 8-h duration were made on day 9 and/or day 12 of gestation. Also investigated were effects of a combined treatment regime using continuous wave, unfocused 1 MHz ultrasound on day 9 and MRI on day 12. Treatment groups consisted of 8 to 10 pregnant dams. Fetal data were taken on day 18 of gestation and data from adult animals that were exposed in utero were taken on day 50 postpartum. Significant differences in sensitive biological end points following in utero exposure to complex MRI field conditions were found. Fetal weights of the exposed groups ranged from 11% (day 9 MRI) to 17% (day 9 ultrasound/day 12 MRI) lower (p < or = .05) than those of the cage control and sham-exposed fetuses and crown-rump length of exposed animals was reduced by 8% (p < or = .05). The postpartum death rate was as high as 38% (significant at p < or = .05 level) following MRI exposure on day 9 and day 12 of gestation. Daily sperm production was reduced by as much as 44% (p < or = .05) in adults exposed to MRI on day 12 of gestation. A reduction in testis weight and volume in the adult males was also seen, but the difference was not statistically significant at the p < or = .05 level. No effect was seen on embryonic deaths, sex ration, body weight at day 50, spleen weight, or seminal vesicle weight. The mechanism responsible for these results could involve one or more of the applied fields interacting with a sensitive stage of fetal development. Further studies are planned to isolate the individual field components and to elucidate biochemical and/or hormonal changes that may follow in utero exposure.

In-vivo NMR thermometry with liposomes containing 59Co complexes.

The ability to make localized temperature measurements in tissue during hyperthermia treatment of cancer is an essential factor in optimizing its efficacy. To this end we have developed and evaluated the complex tris(ethylenediamine) cobalt(III) trichloride as a temperature sensor by determining the temperature dependence of it 59Co nuclear magnetic resonance chemical shift. Encapsulating this complex within liposomes targets the agent to the reticuloendothelial system. Temperature changes of the order of 0.1 degrees C have been measured in vivo on rats, and the half-life of the complex within the body determined by plasma emission spectroscopy.

Monitoring pH of otitis media effusion in chinchillas using fluorescence spectroscopy.

We have developed a fiber optic fluorometer to measure fluorescent signal intensities across an epithelium barrier. As a medically relevant example, we have measured the pH of the effusion formed during Hemophilus influenzae induced otitis media infection in the chinchilla, the classical animal model for human middle ear disease. Because the choice of antibiotic used in clinical therapy is dependent on the pH of the effusion, a noninvasive method of measuring pH is highly desirable. Using the fluorescent pH probe carboxy-seminapthorhodafluor, we were able to detect pH changes of 0.15 units in the pH range around 7.0. The development and resolution of the otitis media was followed with magnetic resonance imaging to confirm the presence of the effusion formed during the infection.

Design and analysis of microcoils for NMR microscopy.

The signal-to-noise ratio achieved in a nuclear magnetic-resonance microscopy experiment is directly related to the performance of the radiofrequency coil. An accurate determination of coil performance requires that the resistance of the coil be well characterized. Traditional high-frequency electric-circuit models used to describe larger NMR coils are inadequate when the diameter of the conductor is reduced to the dimensions of the electrical skin depth (delta) at the frequency of operation. A more extensive model based on a scaling parameter that includes delta is presented. This model complements other existing circuit models that represent sample losses, ground-loop and parasitic losses, and the signal induced in the RF coil. Experimental verification is accomplished using a series of solenoidal microcoils in 1H NMR microspectroscopy experiments at 4.7 T (200 MHz). This study demonstrates for the first time that a predictable performance enhancement is achieved using microcoils as small as 50 microns in diameter.

A simple method to incorporate the effects of an RF shield into RF resonator analysis for MRI applications.

A simple but effective method is proposed to incorporate the effects of a radio-frequency (RF) shield into the analysis of RF resonators for magnetic resonance imaging (MRI) applications. It is shown that the method can predict the resonant frequencies of RF resonators within 5% of the measured values.

NMR microspectroscopy using 100 microns planar RF coils fabricated on gallium arsenide substrates.

A family of planar gold RF microcoils were fabricated using microlithography on a gallium arsenide substrate. The microcoils were used in 1H nuclear magnetic resonance (NMR) spectroscopy experiments at 300 MHz (7.05 T). These new microcoils are a key component in the design of integrated MR coils and amplifiers for NMR microspectroscopy.

Dendrimer-based metal chelates: a new class of magnetic resonance imaging contrast agents.

We have developed a new class of magnetic resonance imaging contrast agents with large proton relaxation enhancements and high molecular relaxivities. The reagents are built from the polyamidoamine form of Starburst dendrimers in which free amines have been conjugated to the chelator 2-(4-isothiocyanatobenzyl)-6-methyl-diethylenetriaminepentaacetic acid. The dendrimer gadolinium poly-chelates have enhancement factors, i.e., the ratio of the relaxivity per Gd(III) ion to that of Gd(III)-diethylenetriaminepentaacetic acid, of up to 6. These factors are more than twice those observed for analogous metal-chelate conjugates formed with serum albumins, polylysine, or dextran. One of the dendrimer-metal chelate conjugates has 170 gadolinium ions bound, which greatly exceeds the number bound to other macromolecular agents reported in the literature, and has a molecular relaxivity of 5,800 (mM.s)-1, at 25 MHz, 20 degrees C, and pH of 7.4. We observed that these dendrimer-based agents enhance conventional MR images and 3D time of flight MR angiograms, and that those with molecular weights of 8,508 and 139,000 g/mole have enhancement half lives of 40 +/- 10 and 200 +/- 100 min, much longer than the 24 +/- 4 min measured for Gd(III)-diethylenetriaminepentaacetic acid. Our results suggest that this new and powerful class of contrast agents have the potential for diverse and extensive application in MR imaging.

Applications of reduced-encoding MR imaging with generalized-series reconstruction (RIGR).

The RIGR (reduced-encoding imaging by generalized-series reconstruction) technique for magnetic resonance imaging uses a high-resolution reference image as the basis set for the reconstruction of subsequent images acquired with a reduced number of phase-encoding steps. The technique allows increased temporal resolution in applications requiring repeated acquisitions, such as the dynamic imaging of contrast agent biodistribution, and in intrinsically time-consuming protocols such as the acquisition of a series of T2-weighted images. Several examples are presented to demonstrate that a four- to eightfold improvement in spatial or temporal resolution can be achieved with this technique.

Three-dimensional NMR microscopy of rat spleen and liver.

Three-dimensional microscopic NMR images of spleen and liver specimens from rats injected with dextran magnetite particles and from controls were obtained at 4.7 T, using a specially designed probe in conjunction with a 3D filtered back projection reconstruction algorithm. All of the images were reconstructed as 64(3) arrays with (25 microns) 3 isotropic voxels. With the aid of the MR contrast agent, the red pulp and marginal zone of the spleen and the portal triad of the liver could be distinguished from the surrounding tissue in T2-weighted images. For mature rat spleen, natural contrast in T2-weighted images was found to distinguish the same features. Histological examinations of the tissues with and without contrast agent were also performed using an optical microscope. Microscopic NMR images, despite their lower resolution, clearly revealed many features seen in the optical images.

Selective MR imaging of labeled human peripheral blood mononuclear cells by liposome mediated incorporation of dextran-magnetite particles.

Human peripheral blood mononuclear cells (PBMCs) were incubated with large unilamellar vesicles (LUV) containing encapsulated dextran-magnetite particles (DMP). This resulted in an efficient incorporation of DMP. Electron microscopy revealed the presence of DMP in cells mainly in phagosomes and secondary lysosomes. DMP-labeled PBMCs showed a strong increase of the transverse relaxation rate (up to 16.6 s-1 for 5 x 10(7) cells/ml) and, accordingly, a great loss of signal intensity in MR imaging. The fraction of DMP containing PBMCs could be enriched by magnetic cell separation. The major population of the DMP containing cells proved to be monocytes. When PBMCs depleted of monocytes were used for labeling, DMP uptake was observed also in the peripheral blood lymphocytes. The labeling of PBMCs presented here may be used in future studies of selective MR imaging of in vivo cell migration in a variety of immunologically compromised tissue states, e.g., tumors, transplantations, and abscesses.