The quantitative 19F-imaging of albumin at 1.5 T: a potential in-vivo tool

19F-MR-imaging has been used to quantitate albumin concentration in a phantom at 1.5 T. The experimentally derived relationship between albumin concentration and the T1 relaxation time of a fluorinated marker, tetrafluorosuccinic acid (TFSA) was used to calculate the albumin concentration from a quantitative 19F T1 map acquired using a gradient echo sequence. There was close correlation between calculated and actual BSA concentrations (r = 0.99, SE = 0.15). The potentially interfering effect of paramagnetic species on T1 relaxation times was also investigated. Relaxivity data show that albumin concentration measurements should be performed prior to any contrast agent administration.     

19F Magnetic resonance imaging of perfluorooctanoic acid encapsulated in liposome for biodistribution measurement

The tissue distribution of perfluorooctanoic acid (PFOA), which is known to show unique biological responses, has been visualized in female mice by (19)F magnetic resonance imaging (MRI) incorporated with the recent advances in microimaging technique. The chemical shift selected fast spin-echo method was applied to acquire in vivo (19)F MR images of PFOA. The in vivo T(1) and T(2) relaxation times of PFOA were proven to be extremely short, which were 140 (+/- 20) ms and 6.3 (+/- 2.2) ms, respectively. To acquire the in vivo (19)F MR images of PFOA, it was necessary to optimize the parameters of signal selection and echo train length. The chemical shift selection was effectively performed by using the (19)F NMR signal of CF(3) group of PFOA without the signal overlapping because the chemical shift difference between the CF(3) and neighbor signals reaches to 14 kHz. The most optimal echo train length to obtain (19)F images efficiently was determined so that the maximum echo time (TE) value in the fast spin-echo sequence was comparable to the in vivo T(2) value. By optimizing these parameters, the in vivo (19)F MR image of PFOA was enabled to obtain efficiently in 12 minutes. As a result, the time course of the accumulation of PFOA into the mouse liver was clearly pursued in the (19)F MR images. Thus, it was concluded that the (19)F MRI becomes the effective method toward the future pharmacological and toxicological studies of perfluorocarboxilic acids.     

Study of biodistribution of enflurane in rats with in vivo 19F MRI.

In vivo 19F magnetic resonance imaging (MRI) of anesthetized rats enabled us to visualize the biodistribution of fluorinated anesthetics and to document the changes in MR signals in the body during the induction and the elimination phase of anesthesia. The authors examined in vivo 19F MRI in rats anesthetized with concentrations of 1.75-2.0% enflurane and demonstrated its in vivo distribution with concomitant 1H and 13C MRI to verify the anatomical correlation. Distinct 19F MR signals were acquired predominantly from the systemic adipose tissue and the liver. Additionally, the temporal changes in the tissue during and after anesthesia were characterized with in vivo 19F MRI in 6.4 min of the acquisition time. The 19F MR signals increased with time after anesthesia; however, the signals from the adipose tissue were apparently stronger than those from the liver. Following the discontinuation of inhalation, the MR signals in the liver decreased far more rapidly than those from the adipose tissue. When the animal woke up and began to move, the MR signals were still visible in the adipose tissue. These results confirmed the fact that enflurane dissolves preferentially in the adipose tissue and remains when the anesthetic effect disappears. Additionally, 19F MR signals of the liver during the elimination phase might reflect the concentration of enflurane in the blood.     

Immunospecific NMR contrast agents

Most NMR contrast agents suggested to date have been paramagnetic. These agents, which include the transition and lanthanide metal ions as well as stable organic free radicals, do not provide effective contrast at concentrations much below 1 mM. However, the use of macromolecular ferromagnetic and superparamagnetic particles provides, for the first time, an NMR relaxation agent that is effective at subnanomolar concentrations. Two different sized superparamagnetic particles have been coupled to monoclonal antibodies with high affinity for a neuroblastoma-specific cell surface antigen. The specific binding of these particles, both in vivo and in vitro is demonstrated and the consequences for immunospecific NMR contrast are discussed.     

Selective F MR imaging of 5-fluorouracil and α-fluoro-β-alanine

A ¹⁹F MR chemical shift imaging (CSI) technique is presented which enables selective imaging of the antineoplastic drug 5-fluorouracil (5-FU) and its major catabolite α-fluoro-β-alanine (FBAL). The CSI sequence employs a chemical shift selective (CHESS) saturation pulse to suppress either the 5-FU or the FBAL resonance before the other component of the two-line ¹⁹F MR spectrum is measured. Because the transmitter frequency can always be set to the Larmor frequency of the ¹⁹F resonance to be imaged, this approach yields 5-FU and FBAL MR images free of chemical shift artifacts in read-out and slice-selection direction. In phantom experiments, selective 5-FU and FBAL images with a spatial resolution of 15 × 15 × 20 mm³ (4.5 ml) were obtained in 30 min from a model solution, whose drug and catabolite concentrations were similar to those estimated in the liver of tumor patients undergoing IV chemotherapy with 5-FU. The drug-specific MR imaging technique developed is, therefore, well-suited for the direct and noninvasive monitoring of the up-take and trapping of 5-FU in liver tumors in vivo.     

Isointense model for the evaluation of tumor-specific MRI contrast agents

An isointense model has been developed to evaluate the applicability of putative tumor-specific MRI contrast agents. Data for tissue relaxation measurements in the presence of Mn(III)TPPS4 are used to illustrate the model. The concentration of contrast agent in tumor tissue required for a tumor/normal tissue signal difference-to-noise ratio of 5 (delta SNR = 5) is determined for a T1 weighted pulse sequence and several hypothetical tumor/normal tissue pairs. The impact of various contrast agent characteristics including initial tumor/normal tissue relaxation values, differential uptake of contrast agent, and in vivo relaxivity are considered. Isointense tumor/normal tissue with longer initial relaxation times are shown to be more affected by the presence of contrast agent. In addition those with initially longer relaxation times have less rigorous requirements for tumor specificity. Typically, a normal tissue/tumor uptake ratio of 1:2 increases the concentration required for delta SNR = 5 by a factor of two compared to that of exclusive uptake in tumor. For the T1 weighted pulse sequence employed, the concentration required for delta SNR = 5 is shown to be linear with the inverse of in vivo relaxivity for the hypothetical tissues considered. The isointense model is also extended to predict the field dependence of tumor-specific contrast enhancement by Mn(III)TPPS4.     

Water diffusion-exchange effect on the paramagnetic relaxation enhancement in off-resonance rotating frame

The off-resonance rotating frame technique based on the spin relaxation properties of off-resonance T1rho can significantly increase the sensitivity of detecting paramagnetic labeling at high magnetic fields by MRI. However, the in vivo detectable dimension for labeled cell clusters/tissues in T1rho-weighted images is limited by the water diffusion-exchange between mesoscopic scale compartments. An experimental investigation of the effect of water diffusion-exchange between compartments on the paramagnetic relaxation enhancement of paramagnetic agent compartment is presented for in vitro/in vivo models. In these models, the size of paramagnetic agent compartment is comparable to the mean diffusion displacement of water molecules during the long RF pulses that are used to generate the off-resonance rotating frame. The three main objectives of this study were: (1) to qualitatively correlate the effect of water diffusion-exchange with the RF parameters of the long pulse and the rates of water diffusion, (2) to explore the effect of water diffusion-exchange on the paramagnetic relaxation enhancement in vitro, and (3) to demonstrate the paramagnetic relaxation enhancement in vivo. The in vitro models include the water permeable dialysis tubes or water permeable hollow fibers embedded in cross-linked proteins gels. The MWCO of the dialysis tubes was chosen from 0.1 to 15 kDa to control the water diffusion rate. Thin hollow fibers were chosen to provide sub-millimeter scale compartments for the paramagnetic agents. The in vivo model utilized the rat cerebral vasculatures as a paramagnetic agent compartment, and intravascular agents (Gd-DTPA)30-BSA were administrated into the compartment via bolus injections. Both in vitro and in vivo results demonstrate that the paramagnetic relaxation enhancement is predominant in the T1rho-weighted image in the presence of water diffusion-exchange. The T1rho contrast has substantially higher sensitivity than the conventional T1 contrast in detecting paramagnetic agents, especially at low paramagnetic agent volumetric fractions, low paramagnetic agent concentrations, and low RF amplitudes. Short pulse duration, short pulse recycle delay and efficient paramagnetic relaxation can reduce the influence of water diffusion-exchange on the paramagnetic enhancement. This study paves the way for the design of off-resonance rotating experiments to detect labeled cell clusters/tissue compartments in vivo at a sub-millimeter scale.     

Assessment of scanner performance and normalization of estimated relaxation rate values

The purpose of this study was to develop and test a method for the assessment of Magnetic Resonance (MR) scanner performance suitable for routine brain MR studies and for normalization of calculated relaxation times. We hypothesized that regular monitoring of machine performance changes could provide a helpful normalization tool for calculating tissue MR parameters, thus contributing to support their use for longitudinal and comparative studies of both normal and diseased tissues.The method is based on the acquisition of phantom images during routine brain studies with standard spin-echo sequences. MR phantom and brain tissue parameters were used to assess the influence of machine related changes on relaxation parameter estimates. Experimental results showed that scanner performance may affect relaxation rate estimates. Phantom and in vivo results indicate that the correction method yields a reduction in variability of estimated phantom R1 values up to 29% and of R1 for different brain structures up to 17%. These findings support the validity of using brain coil phantoms for routine system monitoring and correction of tissue relaxation rates.     

An evaluation of gadolinium polyoxometalates as possible MRI contrast agent

Two gadolinium polyoxometalates, K(9)GdW(10)O(36) and K(11)[Gd(PW(11)O(39))(2)], have been evaluated both in vivo and in vitro as candidates for tissue-specific MRI contrast agents. T(1)-relaxivities of 6.89 mM(-1). s(-1) for K(9)GdW(10)O(36) and 5.27 mM(-1). s(-1) for K(11)[Gd(PW(11)O(39))(2)] are slightly higher than that of the commercial MRI contrast agent (Gd-DTPA). Both compounds bind with bovine serum albumin and human serum transferrin and favorable liver-specific contrast enhancement in in vivo MRI with Sprague-Dawley rats after i.v. administration has been demonstrated. Imaging studies demonstrate that the two agents have a long residence time, showing MR signal enhancement in the liver for more than 40 min, longer than commercially available contrast agents. In vivo and in vitro assays showed that GdW(10) and Gd(PW(11))(2) are promising liver-specific MRI contrast agents and GdW(10) may be used in the diagnosis of the pathological state. However, with the higher acute toxicity, the two gadolinium polyoxometalates need to be modified and studied further before clinical use.     

Monte Carlo simulation and theory of proton NMR transverse relaxation induced by aggregation of magnetic particles used as MRI contrast agents

Superparamagnetic particles are widely used in MRI as R2 contrast agents. In this last decade, different studies have focused on aggregation of superparamagnetic particles for important applications such as multimodal agents. A complete study--via simulations--of the influence of aggregation on the MR efficiency of these particles at high magnetic field is presented here. First, an empirical expression is proposed for R2 in the presence of uniformly distributed nanoparticles, taking into account two regimes at once (motional averaging and slow motion regimes). Three cluster shapes are simulated: Sphere, shell and line. An analytical model is proposed to understand water transverse relaxation induced by spherical and shell aggregates. Simulations lead to the conclusion that, in the motional averaging regime, the most efficient aggregate contrast agent is the densest sphere or shell.     

Fluorine-19 NMR spectroscopic studies of the metabolism of 5-fluorouracil in the liver of patients undergoing chemotherapy

Fluorine-19 nuclear magnetic resonance allows direct observation of fluorinated drugs and their metabolites in the human body without background signal from the tissue. A well-known fluorinated chemotherapeutic drug, 5-fluorouracil, and its metabolites were observed noninvasively in the liver of three patients undergoing cancer chemotherapy. Spectra were obtained at 1.5 T with a surface coil centered over the right lobe of the patient's liver. Administration of 1.5 gm of 5-fluorouracil was done after positioning in the magnet. Serial spectra, collected over a 2-h period, revealed both the nature of the metabolites present in the liver, and the time course of each patient's metabolism. These observations represent the first noninvasive NMR study of drugs in human patients and show the feasibility of using in vivo F-19 NMR spectroscopy for human studies of fluorinated compounds.     

A functionalized superparamagnetic iron oxide colloid as a receptor directed MR contrast agent.

We have synthesized a surface functionalized superparamagnetic iron oxide colloid whose clearance from the vascular compartment was inhibited by asialofetuin but not fetuin. Unlike other particulate or colloidal magnetic resonance (MR) contrast agents, the agent of the current communication is not withdrawn from the vascular compartment by cells of the macrophage-monocyte phagocytic system, as indicated by its selective increase in hepatic relaxation rates. Because of this we refer to this colloid as a hepatic selective (HS) MR contrast agent. At 20 mumol Fe/kg the HS MR agent darkened MR images of liver. The HS MR agent exhibited no acute toxicity when injected into rats at 1800 mumol Fe/kg. Based on these observations, surface functionalized superparamagnetic iron oxide colloids may be the basis of MR contrast agents internalized by receptor mediated endocytosis generally, and by the asialoglycoprotein receptor in particular.     

Perfluorocarbon imaging in vivo: a 19F MRI study in tumor-bearing mice

Multiresonance perfluorocarbon emulsions (Oxypherol and Fluosol-DA) were imaged in tumor-bearing mice using 19F spin-echo magnetic resonance imaging in vivo. Multiple thin-slice fluorine images free of chemical shift artifacts were obtained in 13 minutes and these were correlated with proton images obtained during the same experiment to delineate the anatomic distribution of perfluorocarbons. Sequential images were used to determine the time course of the distribution and the retention of the compounds in tumors and organs. 19F MR spectroscopy was used ex vivo to determine with high sensitivity the relative concentration of perfluorocarbons in different tissues and organs and to confirm the results obtained from imaging experiments. The fluorine images visually demonstrated the preferential localization of the perfluorocarbons in the liver and spleen; shortly after injection, the images also revealed the highly vascularized tumor-chest wall interface. Imaging and spectroscopy together showed that the perfluorocarbons were removed from the blood pool within hours and remained sequestered in tissues at later times; the highest concentrations were found in the spleen and liver, where the agents were retained without spectral changes for the duration of these studies. The perfluorocarbons accumulated within tumors at dose-dependent concentrations, one to two orders of magnitude smaller than those observed in the spleen and liver.     

MnDPDP-enhanced magnetization transfer MR imaging: implications for effective liver imaging

The benefit of combining magnetization transfer (MT) MR imaging technique with liver-specific contrast agent manganese dipyridoxyldiphosphate (MnDPDP) was assessed in our experimental investigation. The study was accomplished by imaging a phantom containing serial concentrations of MnDPDP in cross-linked bovine serum albumin (BSA) with various protein concentrations. A 0.1T clinical MR imager with different parameters for MT and conventional MR sequences were used. The combination of an offset frequency of 8 kHz and an amplitude of 25 microT produced nearly maximal MT effect for all protein samples either without MnDPDP or with different MnDPDP concentrations. With long TRs (TR > 200 ms) MT dramatically improved CNR in conjunction with MnDPDP. With short TRs, the gain in CNR with MT was negligible. However, long TRs with increased number of images are beneficial in liver imaging. We conclude that MT like preparation pulse is useful when paramagnetic contrast agents such as MnDPDP are employed.     

Analysis of longitudinal relaxation rate constants from magnetization transfer MR images of human tissues at 0.1 T.

The human calf muscle was examined by using the magnetization transfer MR imaging technique. The time-dependent saturation transfer (TDST) method was applied at low magnetic field 0.1 T in order to measure the mobile water relaxation time T1w, the magnetization transfer rate Rwm from water to solid macromolecules, and the magnetization transfer contrast (MTC) of the human tissue. The magnetization transfer contrast of 0.67 was attained. The transfer rate Rwm was 4.5 sec-1 (+/- 0.3 sec-1) for the anterior tibial muscle and 5.0 sec-1 (+/- 0.4 sec-1) for the gastrocnemius muscles. The values of Rwm are considerably larger than the values of corresponding relaxation rates measured at high fields. The relaxation rate measurements of human tissues in vivo was shown to be possible at 0.1 T even within the framework of normal routine MR imaging. Magnetization transfer MR imaging is a very promising and practical method in order to assess the relaxation processes in heterogeneous human tissues in vivo, and it can improve the tissue characterization possibilities of MR imaging techniques.     

Diffusion-weighted spatial information from1H relaxation in restricted geometries

Summary NMR relaxation of water¹H confined in restricted geometries, whatever is the nature of the system (porous media saturated by water as well as biological tissues), exhibits common characteristics. Artificial microporous media saturated by water have been chosen as model systems to study the longitudinal and transverse relaxation of¹H magnetization of water molecules diffusing in restricted geometries. These systems are very stable, easy to prepare, with well-characterized pore size distribution and connections, and with highly homogeneous surface properties. The response was compared with that from more complex natural porous media. Scanning Electron Microscopy techniques demonstrated spatial characteristics and surface properties of the samples. The information content of longitudinal relaxation curves associated with spatial structure and due to restricted diffusion is shown in these samples. The effect on transverse relaxation of self-diffusion in the presence of spatially varying magnetic fields due to susceptibility differences is shown. A simple linear relationship has been found in all samples between the transverse relaxation rate and the interpulse delay in CPMG experiments, in spite of the variety of pore shapes and sizes. In general, one can say that relaxation curves beardiffusion-weighted information on the pore space framework. The role of the investigated relaxation mechanisms is important also in the response of biological tissues, including in the presence of MR Imaging contrast agents inducing microscopic magnetic-field gradients. Work partially supported by CNR and MURST Grants.     

Bayesian Modeling of NMR Data: Quantifying Longitudinal Relaxation in Vivo,and in Vitro with a Tissue-Water-Relaxation Mimic (Crosslinked Bovine Serum Albumin)

Recently, a number of magnetic resonance imaging protocols have been reported that seek to exploit the effect of dissolved oxygen (O₂, paramagnetic) on the longitudinal ¹H relaxation of tissue water, thus providing image contrast related to tissue oxygen content. However, tissue water relaxation is dependent on a number of mechanisms and this raises the issue of how best to model the relaxation data. This problem, the model selection problem, occurs in many branches of science and is optimally addressed by Bayesian probability theory. High signal-to-noise, densely sampled, longitudinal ¹H relaxation data were acquired from rat brain in vivo and from a cross-linked bovine serum albumin (xBSA) phantom, a sample that recapitulates the relaxation characteristics of tissue water in vivo. Bayesian-based model selection was applied to a cohort of five competing relaxation models: (1) monoexponential, (2) stretched-exponential, (3) biexponential, (4) Gaussian (normal) R ₁-distribution, and (5) gamma R ₁-distribution. Bayesian joint analysis of multiple replicate datasets revealed that water relaxation of both the xBSA phantom and in vivo rat brain was best described by a biexponential model, while xBSA relaxation datasets truncated to remove evidence of the fast relaxation component were best modeled as a stretched exponential. In all cases, estimated model parameters were compared to the commonly used monoexponential model. Reducing the sampling density of the relaxation data and adding Gaussian-distributed noise served to simulate cases in which the data are acquisition-time or signal-to-noise restricted, respectively. As expected, reducing either the number of data points or the signal-to-noise increases the uncertainty in estimated parameters and, ultimately, reduces support for more complex relaxation models.     

An NMR study of the origin of dioxygen-induced spin-lattice relaxation enhancement and chemical shift perturbation

Due to its depth-dependent solubility, oxygen exerts paramagnetic effects which become progressively greater toward the hydrophobic interior of micelles, and lipid bilayer membranes. This paramagnetic gradient, which is manifested as contact shift perturbations (19F and 13C NMR) and spin-lattice relaxation enhancement (19F and 1H NMR), has been shown to be useful for precisely determining immersion depth, membrane protein secondary structure, and overall topology of membrane proteins. We have investigated the influence of oxygen on 19F and 13C NMR spectra and spin-lattice relaxation rates of a semiperfluorinated detergent, (8,8,8)-trifluoro (3,3,4,4,5,5,6,6,7,7)-difluoro octylmaltoside (TFOM) in a model membrane system, to determine the dominant paramagnetic spin-lattice relaxation and shift-perturbation mechanism. Based on the ratio of paramagnetic spin-lattice relaxation rates of 19F and directly bonded 13C nuclei, we conclude that the dominant relaxation mechanism must be dipolar. Furthermore, the temperature dependence of oxygen-induced chemical shift perturbations in 9F NMR spectra suggests a contact interaction is the dominant shift mechanism. The respective hyperfine coupling constants for 19F and 13C nuclei can then be estimated from the contact shifts <(deltav/v0)19F> and <(deltav/v0)13C>, allowing us to estimate the relative contribution of scalar and dipolar relaxation to 19F and 13C nuclei. We conclude that the contribution to spin-lattice relaxation from the oxygen induced paramagnetic scalar mechanism is negligible.     

On the effect of the correlation in phonon motion on the viscosity of impure dielectrics

The effect of the correlation in phonon motion on the viscosity of impure dielectrics is studied. It is shown that for perfect dielectrics the relaxation times entering the expressions for the absorption of longitudinal and transverse acoustic waves are different due to phonon correlations. The influence of impurities on the viscosity of dielectrics is discussed. Special emphasis is given to the problem of longitudinal-acoustic-wave absorption in impure dielectrics. It is shown that the effect of impurities on phonon correlations can lead to a substantial change in absorption of longitudinal waves.     

Flourine-19 NMR spectroscopy and imaging investigations of myocardial perfusion and cardiac function

Fluorine-19 nuclear magnetic resonance images of rabbit hearts have been obtained with the administration of a fluorinated contrast compound. The use of fluorine-labeled compounds allows differences in tissue perfusion to be demonstrated between hearts with and without coronary artery ligation. In postischemic reperfusion studies, regional hypoperfusion is also demonstrated by ¹⁹F imaging. The results of in vivo rabbit studies show that ¹⁹F NMR can be used to monitor the rate of blood pool and extravascular space dilution of a single bolus injection of either a water soluble or emulsified fluorine-labeled agent.