Intravascular and Intracellular Hepatic Relaxivities of Superparamagnetic Particles: An Isolated and Perfused Organ Pharmacokinetics Study

The relative contributions of intravascular and intracellular compartments to the proton transverse relaxation of the isolated and excised rat liver were determined during the phagocytosis of superparamagnetic particles. The evolution of the proton transverse magnetization of the organ perfused with increasing doses of starch-coated magnetic microspheres was followed up using a Carr–Purcell–Meiboom–Gill sequence with various echo times. From the multiexponential fit of the echo train, the amplitudes and the relaxation ratesR₂of the liver tissue were obtained. The results clearly indicate that shortly after contrast medium administration, an internalization takes place which can be followed by the rapid and biphasic evolution of the transverse relaxation rate of the water protons. A very fast decaying component looking like an initial loss of the magnetization is observed together with an increase of the relaxation rate of the remaining water tissue. This regime is strongly dependent on both the echo time and the iron concentration, a behavior characteristic of the agglomeration of magnetic particles. The examination of the liver tissues by electron microscopy shows that this clustering arises in cytoplasmic vacuoles.     

Efficiency of paramagnetic relaxation enhancement in off-resonance rotating frame

Transferring from laboratory frame to off-resonance rotating frame for the (1)H spin can compensate the relaxivity loss for paramagnetic agents at the magnetic field strength higher than 3 Tesla and enhance water relaxation rate constant significantly. A comprehensive theory for calculating the relaxation rate constants in the off-resonance rotating frame is described. This theory considers the contributions from both inner shell and outer shell water. The derived relaxation rate constants and relaxation enhancement efficiency as a function of the magnetic field strength and the effective field parameters are directly correlated to the structures, dynamics and environments of paramagnetic agents. To validate the theoretical predictions, we have measured the relaxation enhancement efficiency for a series of macromolecule conjugated gadolinium chelates at 9.4 Tesla. The experimental results confirmed the theoretical predictions. The theory also predicts the relaxation enhancement for T(2)-type paramagnetic agents at high magnetic fields. Promising fields of applications include situations where T(1)- or T(2)-type paramagnetic agents are used for labeling molecular/cellular events.     

Biodistribution of GdCl3 and Gd-DTPA and their influence on proton magnetic relaxation in rat tissues

The biodistribution and relative molar effectiveness of the ionic (GdCl3) and chelated (Gd-DTPA) forms of gadolinium (Gd) to enhance proton relaxation rates in rat kidney, liver and spleen were evaluated. Rats were given intravenous injections of either GdCl3 (100 mumol/kg) or Gd-DTPA (178 mumol/kg). Gd-DTPA was primarily contained in the vascular compartment and was quickly accumulated in the kidney after injection with a relaxivity of 4.3 sec-1 (mumol/g kidney)-1. It was eliminated quickly from the body with only 2% of the injected dose remaining after 120 min. After GdCl3 injection, Gd was found primarily in liver and spleen. It accumulated continuously reaching 72% of the injected does in these two tissues after 120 min. Despite this continuous increase in tissue Gd concentration, the relaxation rates showed saturation in liver and spleen. The results suggest that after GdCl3 was injected it distributed either in a protein bound form that was effective at causing relaxation or in a colloid form that was not effective. The biodistribution of GdCl3 was such that it was determined by the phagocytic action of the recticuloendothelial system on a colloid. The biodistribution and tissue relaxivity of Gd-DTPA suggest it will be a useful vascular MRI contrast agent. However, the usefulness of GdCl3 as an MRI contrast agent is limited not only by its acute toxicity but also by its saturable effect on tissue relaxation rates. Consequently, GdCl3 has only a modest influence on tissue relaxivity.     

Gadolinium oxide: a protoype agent for contrast enhanced imaging of the liver and spleen with magnetic resonance

Gd2O3 particles (less than 2 microns) in suspension were evaluated as a potential contrast agent for liver-spleen imaging with magnetic resonance. The agent was administered IV to rabbits in doses ranging from 10 to 120 mumol/kg and the tissues removed after sacrifice for in vitro T1 and T2 analysis. The temporal response was determined in liver and spleen samples of rabbits given a fixed dose (60 mumol/kg) and sacrificed at intervals from 15 min to 60 hr later. Documentation of the subanatomic location of Gd2O3 particles in tissue was accomplished by electron microscopy and x-ray dispersion microanalysis. T1 weighted images were obtained at 0.12T on a prototype resistive scanner. The liver, spleen, and lung relaxation times are very responsive to Gd2O3 IV and the effect is dose related. A peak effect is observed between 3-7 hr after injection and relaxation times may normalize by 60 hr. By electron microscopic and x-ray analysis, Gd2O3 is most prominently found in the hepatic and splenic sinusoids. The images show marked enhancement of liver and splenic tissues, aiding in the clear delineation of these tissues from neighboring structures.     

MR imaging of the liver and spleen: a comparison of the effects on signal intensity of two superparamagnetic iron oxide agents

We compared the effects of two superparamagnetic iron oxide (SPIO) contrast agents, ferumoxides and SHU-555A, in MR imaging of the liver and spleen. Thirty-six patients with known malignant lesions of the liver underwent T2W turbo spin-echo (TSE) and T1WGRE FLASH opposed-phase imaging before and after SPIO injection on a 1.0 T MR system. Post-ferumoxides images were obtained in 18 patients 90 min after infusion of 15 micrommol Fe/kg of the agent. In 18 other patients SHU-555A was administered as a rapid bolus at a dose of 7.0-12.9 micrommol Fe/kg. T1WGRE FLASH images were obtained immediately, 30 s and 480 s and T2WTSE images 10 min after injection. Signal intensity of the liver, spleen, and malignant liver lesions before and after SPIO was measured with operator-defined regions of interest. The effects of ferumoxides and SHU-555A were measured as the percentage signal intensity change (PSIC) and in the malignant liver lesions additionally as changes in lesion-to-liver contrast-to-noise ratio (deltaDCNR). On T2W TSE images, there was no significant difference between the two agents in signal loss of liver parenchyma (p > 0.05). The signal loss in the spleen produced by ferumoxides was greater than with SHU-555A (p < 0.05). Both SPIO agents produced a significant increase in the CNR of malignant liver lesions. Delta CNR was slightly greater with ferumoxides than with SHU-555A (p < 0.05). On T1WGRE FLASH images, a slight decrease of liver SI induced by both agents was found on late post-SPIO images. No significant difference of liver PSIC between the two SPIO agents was noted on T1W images. The SI of spleen was significantly increased with both agents on T1W images and no difference in PSIC of spleen was noted (p > 0.05). The T1 and T2 effects produced by ferumoxides and SHU-555A were comparable in the liver although ferumoxides produced a stronger T2 effect in the spleen.     

Structure activity relationship of magnetic particles as MR contrast agents

Structure activity relationship (SAR) of superparamagnetic MR contrast agents is discussed based on physicochemical properties and relaxivity data of 16 different particles. All the magnetic particles reduce both relaxation times, T1 and T2. The effect on T2 is stronger than the effect on T1. The relaxation efficacy varies over a wide range. Minor modifications in the preparation of the magnetic particles result in products with different susceptibility properties. The T2 relaxivity is dependent upon the magnetic susceptibility as well as particle size. Small particles reduce the relaxation times to a larger extent than the larger particles. No significant difference in relaxivity is observed between compact and porous particles. Magnetic particles coated with nonmagnet polymer are effective relaxation agents, while nonmagnetic monodisperse particles show no effect on the relaxivity.     

Corrected equations for susceptibility-induced T2-shortening

Local field gradients created by paramagnetic entities shorten water proton relaxation times, particularly T2. This "novel" relaxation mechanism, now known as susceptibility-induced relaxation, was described twenty years ago by Gueron, and later extended by others to superparamagnetic particles which have a much larger magnetization. Unfortunately, because of subtle but significant errors, those results are valid only in the strict zero-field limit. These errors are corrected in the present article, and new versions of the relaxation equations are presented. The correction is shown to be significant, not only for transverse and longitudinal relaxation in aqueous superparamagnetic colloids, where the "Gueron" effect is known to be important, but even in some cases for transverse paramagnetic relaxation.     

Determination of saturation transfer parameters of human tissues in vivo

In order to study the applicability of magnetization transfer contrast (MTC) to tissue differentiation, the determination of the magnetization transfer (MT) parameters of normal tissues is necessary for the evaluation of pathological conditions. The time-dependent saturation transfer technique was used to investigate the observed magnetization transfer parameters in several human tissues in vivo at 0.1 T. The length of the off-resonance saturation pulse varied from 0 to 750 ms. The magnetization transfer contrast (MTC) was 0.71 in striated muscle, 0.49 in liver, 0.49 in renal cortex, and 0.50 in spleen. The observed magnetization transfer rates (R_wm) were 5.5 s⁻¹ for muscle, 3.1 s⁻¹ for liver, and 1.5 s⁻¹ for both renal cortex and spleen. Our results indicate that measuring R_wm and possibly other relaxation parameters could be useful in tissue differentiation.     

Liver imaging at 1.5 tesla: pulse sequence optimization based on improved measurement of tissue relaxation times

In order to predict the most sensitive MR imaging sequence for detecting liver metastases at 1.5 T, in vivo measurements of T1 and T2 relaxation times and proton density were obtained using multipoint techniques. Based on these measurements, two-dimensional contrast contour plots were constructed demonstrating signal intensity contrast between hepatic lesions and surrounding liver parenchyma for different pulse sequences and pulse timing parameters. The data predict that inversion recovery spin echo (IRSE) imaging should yield the greatest contrast between liver metastases and liver parenchyma at 1.5 T, followed by short tau inversion recovery (STIR) and spin-echo (SE) pulse sequences. T2-weighted SE images provided greater liver/lesion contrast than T1-weighted SE pulse sequences. Calculated T1, T2, and proton density values of the spleen were similar to those of hepatic metastatic lesions, indicating that the signal intensity of the spleen may be used as an internal standard to predict the signal intensity of hepatic metastases on T1- and T2-weighted images at 1.5 T.     

T1rho dispersion of rat tissues in vitro.

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

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.     

T1rho Dispersion profile of rat tissues in vitro at very low locking fields

The purpose of this study was to show the T(1rho) dispersion profile in various rat tissues (liver, brain, spleen, kidney, heart and skeletal muscle) at low (0.1 T) B(0) field at very low locking field B1, starting from 10 microT. The T(1rho) dispersion profile showed a quite similar pattern in all tissues. The highest R(1rho) relaxation rates were seen in the liver and muscle followed by the heart, whereas the values for spleen, kidney and brain were rather similar. The greatest difference between R2 relaxation rate and R(1rho) relaxation rate at B1=10 microT was seen in the liver and muscle. The steepest slope for a dispersion curve was seen in the muscle. The value of T(1rho) approximately approached the value of T2 when the locking field B1 approached 0. Except for the liver, the calculated apparent relaxation rate R2' was slightly larger than the calculated one. The potential value of T(1rho) imaging is to combine high R1 contrast of low-field imaging with the high signal-to-noise ratio (SNR) of high static field imaging. T(1rho) relaxation and dispersion data presented in the current study help to optimize the rotating-frame MR imaging.     

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.     

Differentiation of hepatic cavernous hemangioma from metastases by rare sequence MR imaging.

In this study, in order to differentiate cavernous hemangioma and hepatic metastases, rapid acquisition relaxation enhanced (RARE) sequence was used. First, in vivo measurements of T1, T2 relaxation times and proton density were obtained using T1, T2 calculation protocol (TOMIKON S50, 0.5T) and multipoint techniques. These measurements were made from regions of interest placed over the liver, spleen (because of similarity of relaxation time values between hepatic metastases and spleen) and cavernous hemangioma (HCH). Based on these intrinsic parameters, T2 curves signal intensity of three different tissues were constructed. At TE = 500 ms, the signal intensity of the liver and spleen has been near zero whereas in HCH, the signal intensity remained. As RARE sequence is very similar to spin echo (SE), by replacing effective TE(ETE) = 500 ms in the RARE equation, two dimensional contrast-to-noise ratio (CNR) contour plots were constructed demonstrating signal intensity contrast between liver-spleen, liver-Hemangioma for two different scan times (3 min, 7.5 s) and pulse timing. Then, optimal RARE factor and inter echo times were obtained in order to have maximum CNR between liver-Hemangioma and minimum CNR between liver-spleen. These optimal parameters were performed on ten normal and five persons with known HCH. Images showed that in both scan times (3 min, 7.5 s); the liver and spleen were suppressed whereas the HCH was enhanced. The image quality in the scan time of 3 min was better than the scan time of 7.5 s. Moreover, in this study, two different sequences were compared: i) Multi-slice single echo (MSSE) for T1 weighted image ii) RARE (ETE = 80 ms) for T2-weighted image. This comparison was done to show maximum CNR between liver-spleen (metastases) and to choose a better sequence for detecting metastases. CNR in the RARE sequence was more than in the MSSE sequence.     

Investigation of microenvironmental factors influencing the longitudinal relaxation times of drugs and other compounds

The aim of this study was to investigate the microenvironmental factors likely to influence the longitudinal relaxation time of MR visible drugs or compounds in vivo at 1.5 T. The relative influence that viscosity, albumin and paramagnetic contrast agent concentrations have on the observed longitudinal relaxation times of three 19F MR detectable drugs and compounds have been investigated. Our data show that for 5-fluorouracil, flucloxacillin and tetrafluorosuccinic acid-containing phantoms, the presence of albumin at normal physiological concentrations will have relaxation effects of the same order of magnitude as that of a commonly clinically administered contrast agent, gadolinium diethylenetriamine pentaacetic acid. The contribution of viscosity is shown, in the examples studied here, to be of minor importance, contributing less than 6.5% to the observed relaxation effects. It is also demonstrated that in the presence of competitive binding of other ligands for common binding sites on albumin, the 19F longitudinal relaxation time of 5-fluorouracil can increase by up to 340% from its value in the absence of the competing ligand. The relevance of the findings to in vivo studies is discussed.     

Relaxation enhancement using liposomes carrying paramagnetic species

Studies were performed to investigate the effects upon the relaxation times of mouse organs of intravenously administered Mn-DTPA entrapped in multilamellar liposomes, Mn-DTPA, 0.9% NaCl entrapped in liposomes, 0.9% NaCl. Manganese concentrations in injectates and tissues were assessed with 54Mn and atomic absorption. T1 and T2 of freshly excised tissues were measured in an NMR spectrometer at 20 MHz and 37% C with IR and CPMG sequences. Entrapment of Mn-DTPA in liposomes increased 54Mn accumulation in liver by 207% and in spleen 1208% and reduced 54Mn in heart by 20% and in kidney by 24% relative to free Mn-DTPA. Statistically significant increases in relaxation rates were produced. However, the increase in relaxation rate per unit concentration of Mn in tissue is reduced by 31% in liver and 62% in spleen when Mn is delivered inside liposomes. These observations have implications for the design of NMR contrast agents.     

Study of interparticle interaction in conjugates of magnetic nanoparticles injected into mice

In this work the first fast stage of the biodegradation in vivo of magnetic ferrofluid was investigated. The appearance of a paramagnetic doublet was observed in M?ssbauer spectra of mouse liver within 2?h after intravenous injection of the ferrofluid. It was shown that nanosized superparamagnetic particles were combined into groups in the initial magnetic beads of the ferrofluid and were connected inside each group by magnetic dipole interaction. It was found that the appearance of a paramagnetic doublet in the spectrum of mouse liver is caused by the decrease of the magneto-dipole interaction between the superparamagnetic nanoparticles.     

Peliosis hepatis and neoplastic/dysplastic lesions in aged male Long-Evans Cinnamon rats: MR imaging with pathologic correlation

The Long-Evans Cinnamon (LEC) rat, an animal model of Wilson's disease, abnormally accumulates copper in the liver. There have been a lot of reports on preneoplastic and neoplastic hepatic tumors in LEC rats, but few studies have been focused on other lesions. The aim of this study was to describe the MR findings of the liver of LEC rats with pathologic correlation to characterize the hepatic lesions developed in them. We measured MR images of the liver of six aged (over the age of 70 weeks old) male LEC rats. Measurements of T(1), T(2)-weighted images, and the dynamic and delayed studies after i.v. gadolinium injection were performed. The rats were sacrificed immediately after the measurements, and the diagnosis was histologically made. We identified seven lesions of peliosis hepatis, three neoplastic/dysplastic lesions, three cysts and one cholangiofibrosis. Peliosis hepatis was characterized as showing a significantly long T(2) relaxation time of 57.9 +/- 13.3 ms (mean +/- standard deviation) compared with 41.3 +/- 1.7 ms in normal liver, and prolonged enhancement after a gadolinium injection. Neoplastic/dysplastic lesions tended to show prolonged T(2), and they showed isointensity on T(1)-weighted images. They were best characterized by early enhancement followed by a rapid wash-out after a gadolinium injection. In conclusions, the frequent occurrence of peliosis hepatis observed in the present study suggests this can be a characteristic lesion in aged LEC rats. The characteristic MR findings enable us to distinguish between peliosis hepatis and neoplastic/dysplastic lesions.     

Effects of paramagnetic cations on the nonexponential spin-lattice relaxation of rare spin nuclei in solids

Nonexponential spin-lattice relaxation is often observed for rare spin nuclei in the solid state. Deviation from single-component decay may be amplified by the coupling of rare spin nuclei to paramagnetic centers. Nonexponential spin-lattice relaxation was observed in derivatized silica gels resins. This phenomenon was localized and enhanced when paramagnetic transition metal cations were bound to surface functional groups. A stretched exponential analysis method was determined to be robust in fitting nonexponential relaxation curves for silica gels both with and without bound paramagnetic ions. Spin-lattice relaxation rates (T₁⁻¹) for functional group nuclei increased as a function of percent surface coverage with metal ion. The magnitude of the relaxation rate increase was dependent upon internuclear distances from the paramagnetic center. At low surface coverages, a semi-random distribution of paramagnetic centers increased the degree of stretching of spin-lattice relaxation decays, as measured by decreases in the calculated stretching parameter β. At higher surface coverages, calculated β values reached a limiting value, indicating that while the spin-diffusion mechanism in metal-ex-changed silica gels is restricted, it is not completely diminished.     

MR enteroclysis using iron oxide particles (ferristene) as an endoluminal contrast agent: an open phase III trial

To evaluate efficacy and safety of a superparamagnetic iron oxide contrast agent (ferristene) as an endoluminal contrast medium for magnetic resonance (MR) enteroclysis in a phase III trial. Twenty-three patients with history of known or suspected small bowel Crohn's disease underwent MR imaging of the abdomen at 0.5 T unit. The imaging protocol included two phases: the first one without administration of any contrast agent and the second one, where the small bowel was filled by enteroclysis with 800 ml of the luminal iron oxide contrast medium and Gd-DTPA (0.1 mmol/Kg) was administered intravenously. Axial Spin-Echo (SE) T1-weighted (T1w), proton-density and T2w images, sagittal and coronal SE T1w and Short TI Inversion Recovery (STIR) sequences were subsequently obtained. Three investigators blindly evaluated images to determine small bowel distribution of ferristene, presence of artifacts, delineation of bowel lesion/wall and the diagnostic value of ferristene combined with gadolinium. Pre- and postcontrast signal intensity measurements of bowel lesion/wall, bowel lumen and background noise were also calculated. Three patients withdrew before the procedure, therefore 20 patients were effectively included in the study. No significant difference between the three investigators' evaluations of the improvement of the diagnostic information was found (percentage of improvement of 90% with 95% confidence limits of 68% and 99%). A statistically significant difference between the first and third investigators was found for grading of quality of delineation of bowel lesion/wall. Signal intensity measures showed a significant increase of the bowel lesion/wall and background noise/lesion for the SE T1w images. No serious adverse event was reported in our series. MR enteroclysis using ferristene as an endoluminal contrast agent appears to be a safe and efficient procedure for the study of the small bowel.