Effect of NC100150 injection on the (1)H NMR linewidth of human whole blood ex vivo: dependency on blood oxygen tension

The linewidth of the (1)H NMR signal (7.05 T) of human whole blood titrated with a superparamagnetic contrast agent (NC100150 injection) was evaluated at different blood oxygen tensions. In deoxygenated blood and low contrast agent concentrations, NC100150 injection caused a decrease in linewidth. After reaching a minimum, the linewidth increased as the concentration of NC100150 injection increased. At the concentration corresponding to the minimum linewidth, the magnetization of the extracellular space containing the NC100150 injection was equal to that of the paramagnetic (deoxygenated hemoglobin) intracellular space. The minimum linewidth is therefore consistent with a complete elimination of the local microscopic susceptibility effect, the major cause of linebroadening. Additionally, phantom studies were performed at 1.5 T, confirming that the contrast enhancement of NC100150 injection in blood is dependent on oxygen tension. The data suggest that NC100150 injection may be useful in differentiating vessels with varying relative oxygen tensions.

NC100150 Injection, a preparation of optimized iron oxide nanoparticles for positive-contrast MR angiography

A preparation of monocrystalline iron oxide nanoparticles with an oxidized starch coating, currently in clinical trials (NC100150 Injection; CLARISCAN), was characterized by magnetization measurements, relaxometry, and photon correlation spectroscopy. By combining the results with a measure of iron content, one can obtain the size and magnetic attributes of the iron cores, including the relevant correlation times for outer sphere relaxation (tau(SO) and tau(D)), and information about the interaction of the organic coating with both core and solvent. The results are 6.43 nm for the iron oxide core diameter, a magnetic moment of 4.38x10(-17) erg/G, and a water-penetrable coating region of oxidized oligomeric starch fragments and entrained water molecules. The latter extends the hydrodynamic diameter to 11.9 nm and lowers the average diffusivity of solvent about 64% (which increases tau(D) accordingly). The nanoparticles show little size-polydispersity, evidenced by the lowest value of r(2)/r(1) at 20 MHz reported to date, an asset for magnetic resonance angiography.

Prediction of q-values and conformations of gadolinium chelates for magnetic resonance imaging

For gadolinium chelates, we determined that there is a linear correlation between calculated solvent-accessible surface area and q-value, the number of rapidly exchanging water molecules directly bound to the gadolinium ion. A calibration curve was developed to predict q-value based on the solvent-accessible surface area of gadolinium. This predictive method was validated with the following gadolinium crystal structures: (ethylenediaminetetraacetic acid)-gadolinium(III) [Gd(EDTA)] [Templeton, L. K., Templeton, D. H., Zalkin, A., and Ruben, H. W. (1982) Anomalous Scattering by Praseodymium, Samarium, and Gadolinium and Structures of their Thylenediaminetetraacetate (EDTA) Salts. Acta Crystallogr., Sect. B 38, 2155], (1,4,7,10-tetraazacyclododecane-N,N',N' ',N' "-tetraacetic acid)-gadolinium(III) [Gd(DOTA)] [Dubost, J.-P., Leger, J.-M., Langlois, M.-H., Meyer, D., and Schaefer, M. (1991) Structure of a Magnetic Resonance Imaging Agent - The Gadolinium-DOTA Complex C(16)H(24)N(4)O(8)NaGd, 5H(2)O. C. R. Acad. Sci., Ser. 2 312, 349], (diethylenetriaminepentaacetic acid)-gadolinium(III) [Gd(DTPA)] [Stezowski, J. J., and Hoard, J. L. (1984) Heavy Metal Ionophores - Correlations Among Structural Parameters of Complexed Nonpeptide Polyamino Acids. Isr. J. Chem. 24, 323], (diethylenepenta-acetato)-gadolinium(III) [Gd(DTPA-BEA)] [Smith, P. H., Brainard, J. R., Morris, D. E., Jarvinen, G. D., and Ryan, R. R. (1989) Solution and Solid-State Characterization of Europium and Gadolinium Schiff-Base Complexes and Assessment of their Potential as Contrast Agents in Magnetic Resonance Imaging. J. Am. Chem. Soc. 111, 7437], and (1,7,13-triaza-4,10, 16-trioxacyclo-octadecane-N,N',N' '-triacetato)-gadolinium(III) [Gd(TTTA)] [Chen, D., Squattrito, P. J., Martell, A. E., and Clearfield, A. (1990) Synthesis and Crystal Structure of a 9-Coordinate Gadolinium(III) Complex of 1,7,13-Triaza-4,10, 16-Trioxacyclooctadecane-N,N',N' '-Tri-Acetic Acid. Inorg. Chem. 29, 4366]. Predicted q-values were in complete agreement with experimentally determined q-values. A genetic algorithm-based conformational search method was developed to generate valid 3D models for gadolinium chelates. The method was successfully tested on the following gadolinium chelates: Gd(EDTA) (Templeton et al., 1982), Gd(DOTA) (Dubost et al., 1991), Gd(DTPA-BEA) (Smith et al., 1989), Gd(TTTA) (Chen et al., 1990), Gd(triethylene glycol) [Rogers, R. D., Voss, E. J., and Etzenhouser, R. D. (1988) F-Element Crown Ether Complexes. 17. Synthetic and Structural Survey of Lanthanide Chloride Tiethylene Glycol Complexes. Inorg. Chem. 27, 533], and Gd(tetraethylene glycol) [Rogers, R. D., Etzenhouser, R. D., Murdoch, J. S., and Reyes, E. (1991) Macrocycle Complexation Chemistry. 35. Survey of the Complexation of the Open-Chain 15-Crown-5 Analogue tetraethylene Glycol with the Lanthanide Chlorides. Inorg. Chem. 30, 1445].

'NC100150', a preparation of iron oxide nanoparticles ideal for positive-contrast MR angiography

A laboratory-scale synthesis of NC100150 (iron oxide particles with an oxidized starch coating) was characterized by magnetization measurements (vibrating sample magnetometry, VSM), relaxometry (1/T1 NMRD profiles and 1/T2 at 10 and 20 MHz), and dynamic light scattering (photon correlation spectroscopy, PCS). The results were related to give a self-consistent physical description of the particles: a water-impenetrable part making up 12% of the total particle volume, 82% of this volume consisting of an iron oxide core and the remaining 18% consisting of an oxidized starch rind; and, a water-penetrable part making up 88% of the total particle volume, consisting of oxidized starch polymers and entrained water molecules. Relating the magnetization to the relaxometry results required that the oxidized starch coating slows the diffusivity of solvent water molecules in the vicinity of the iron oxide cores. The effect of the organic coating on water diffusivity, not previously considered in the application of relaxation theory to iron oxide nanoparticles, is supported by the much greater (factor of about 2) diameter obtained from the dynamic light scattering measurements in comparison to that obtained from the magnetization measurements. The present work shows that three physical techniques--VSM, relaxometry, and PCS--are needed for properly assessing iron oxide nanoparticles for use as contrast agents for magnetic resonance angiography (MRA). It is also shown that NC100150 has a narrow range of diameters and the smallest value of r2/r1 reported to date, an asset for MRA.

NMRD investigation of DyDTPA- and GdDTPA-labeled starch particles. Selection of a suitable suspension medium and influence of the starch matrix on relaxivity

RATIONALE AND OBJECTIVES

The primary aim was to investigate the influence of the starch matrix on the T1 relaxivities of starch particles labeled with gadolinium and dysprosium diethylenetriamine pentaacetic acid (GdDTPA-SP and DyDTPA-SP). Achieving this required the selection of a medium that was suitable for suspending the particles and that had field-independent T1 relaxation rates, thereby eliminating errors in relaxivity determinations resulting from a field-dependent background.

METHODS

GdDTPA-SP with low and high gadolinium content, DyDTPA-SP, and empty DTPA-SP were suspended in an aqueous medium containing 5% (w/w) of a polyethylene glycol-based block copolymer. 1/T1 NMRD profiles were obtained in the temperature range of 5 degrees to 35 degrees C.

RESULTS

Using the block copolymer, particles did not settle, and samples could be prepared at a low temperature to avoid particle degradation, the intrinsic T1 relaxation rate of the suspension medium was field-independent and identical to that of water from 25 degrees to 35 degrees C. The T1 relaxivities of DyDTPA-SP were higher than those of dysprosium diethylenetriamine pentaacetate-bis(methylamide) (DyDTPA-BMA) and decreased with increasing magnetic field strength. The T1 relaxivity of GdDTPA-SP was higher than that of GdDTPA at all fields, and decreased with decreasing temperature and increasing gadolinium content.

CONCLUSIONS

The GdDTPA-SP results showed that the particulate starch matrix served a dual role, with opposing influences on relaxivity. It provided a means for increasing the rotational correlation time (tau R), which resulted in higher relaxivities. However, it also retarded radial diffusion of water molecules within the particle interior, which significantly counteracted the enhancing effect of tau R. For DyDTPA-SP, the starch matrix provided an additional diamagnetic contribution, resulting in relaxivities higher than those of DyDTPA-BMA. The block copolymer was suitable as a suspension medium for DyDTPA-SP and GdDTPA-SP and should also be applicable for other particulates.

Investigation of lanthanide-based starch particles as a model system for liver contrast agents

Gadolinium and dysprosium diethylenetriamine pentaacetic acid-labeled starch microparticles (Gd-DTPA-SP and Dy-DTPA-SP) were investigated as model liver contrast agents. The liver contrast efficacy of particles with low and high metal contents was compared in two imaging models: in vivo rat liver and ex vivo perfused rat liver. The biodistribution of intravenously injected particles was also assessed by ex vivo relaxometry and inductively coupled plasma atomic emission spectrophotometry of tissues. All particles reduced the liver signal intensity on T2-weighted spin-echo and gradient-recalled echo images as a result of susceptibility effects. Because of their higher magnetic susceptibility, the Dy-DTPA-SP were more effective negative contrast enhancers than the Gd-DTPA-SP. On T1-weighted spin-echo images, only the Gd-DTPA-SP with low metal content significantly increased the liver signal intensity. In addition, these low-loading Gd-DTPA-SP markedly reduced the blood T1. The two latter observations were not consistent with the anticipated blood circulation time of microparticles, but were a result of the lower stability of these particles in blood compared with Gd-DTPA-SP, which has a high metal content. Regardless of stability or imaging conditions, the paramagnetic starch particles investigated showed potential as negative liver contrast enhancers. However, the observed accumulation of particles in the lungs represented a biological limitation for their use as contrast agents.

Magnetic field dependence of solvent proton relaxation by solute dysprosium (III) complexes

RATIONALE AND OBJECTIVES

Many magnetic resonance imaging (MRI) agents are Gd(III)-based; its half-filled f-shell has an S-ground state and hence a long electronic relaxation time, leading to comparably large effects on 1/T1 and 1/T2 of water protons with no shift in the water-proton resonance frequency. 1/T1 and 1/T2 nuclear magnetic relaxation dispersion (NMRD) profiles of the Dy(III) aquo ion and its chelates have been reported recently. Dy(III) ions differ magnetically from Gd(III); the large spin-orbit interaction of its non-S-ground state reduces the electronic relaxation time 100-fold, and can have a large effect on proton 1/T2 and resonance frequency. Relaxation theory is well-developed and applicable to both ions but, for Dy(III), the phenomena are more wide-ranging. Recent interpretations have suggested that the data are anomolous, requiring a new mechanism for their explanation. The authors explain published Dy(III) data in terms of known theory, guided by experience with Gd(III) agents.

METHODS

For fields below 1 T, the authors incorporate the shortened electronic relaxation time into the usual low-field theory for magnetic dipolar interactions between water protons and Dy(III) magnetic moments. Both inner- and outer-sphere relaxations are included. At higher fields (and unusual for small single-ion agents) one must include dipolar interactions of protons with the magnetization of the Dy(III) moments. This "Curie magnetization" causes a quadratic dependence of 1/T1 on field, and--through dipolar-induced shifts--an even greater quadratic dependence of 1/T2.

RESULTS

All published data can be explained by magnetic dipolar interactions. For Dy(III), the Curie term has a longer correlation time than the low-field term, namely, the rotation of solute for 1/T1 and the even longer water exchange lifetime tau M for 1/T2. This exchange modulates the shift, producing phenomena not seen with Gd(III).

CONCLUSIONS

Relaxation by Dy(III) chelates can be explained by the same well-established theory of dipolar interactions used for their Gd(III) analogs. Interestingly, for MRI applications, tau M should be long for Dy(III)-based agents and short for Gd(III)-based agents.

Black blood magnetic resonance angiography with Dy-DTPA polymer: effect on arterial intraluminal signal intensity, lumen diameter, and wall thickness

Four rabbits in which atherosclerotic disease was induced by diet and balloon angioplasty underwent conventional angiography and MR angiography (MRA) using a black blood pulse sequence before and 10 minutes after the i.v. injection of a macromolecular contrast agent, NC 100283 (1.0 mmol/kg), a dysprosium diethylenetriaminepentaacetic acid hexamethylenediamine copolymer (Dy-DTPA polymer). Intraluminal signal intensity, apparent wall thickness, and lumen size measurements of the aorta and proximal common iliac arteries on precontrast MRA images were compared with postcontrast images. Aortic lumen diameter measurements on the precontrast and postcontrast MRA studies were compared with lumen diameters from conventional angiograms. Intraluminal signal intensity decreased on postcontrast MRA images compared with precontrast images, with an average loss of signal equal to 29% (P < .05). Apparent wall thickness decreased by 24% (P < .05). Lumen diameter and area were generally larger (average of 15% and 33%, respectively) on postcontrast MRA images than on precontrast images. Aortic lumen diameter measurements from postcontrast MRA agreed closely (95% confidence interval of the mean difference was -.2 to .3 mm), and precontrast MRA images tended to underestimate aortic lumen diameter (95% confidence interval of the mean difference was .3 to .8 mm) compared with conventional angiography. Postcontrast MRA with NC 100283, a macromolecular Dy-DTPA contrast agent, provides more accurate assessment of aortic lumen diameter than precontrast MRA, using conventional angiography as the standard reference.

Phantom standards with temperature- and field-independent relaxation rates for magnetic resonance imaging

RATIONALE AND OBJECTIVES

For use as magnetic resonance imaging reference standards, the optimal set of phantoms should cover a wide range of T1 values, with each phantom having a T1 that is stable over time and is independent of temperature and magnetic field strength. To date, no set of phantoms fulfilling these four requirements has been prepared. In the current work, the construction of such optimal phantom standards is attempted.

METHODS

Two linear gadolinium DTPA polymers are used: the first with relaxivities that are independent of temperature and magnetic field strength, and the second with relaxivities that are independent of magnetic field strength, but have an opposite temperature dependence from that of the 1/T1 of the diamagnetic matrix (agarose gel or water). Depending on the desired T1, either one or a combination of the two agents is used.

RESULTS

Phantom standards were constructed with 1/T1 values that are independent of temperature (20-35 degrees C) and magnetic field strength (0.47-1.2 T) over a wide range of 1/T1 values (> or = 0.95 s-1). Phantom standards prepared in water were found to be stable with respect to 1/T1 for at least 18 months.

CONCLUSIONS

Stable phantoms standards have been constructed covering a wide range of T1 values, where the T1 of any particular phantom is independent of temperature and magnetic field strength for magnetic resonance imaging conditions.

High relaxivity linear Gd(DTPA)-polymer conjugates: the role of hydrophobic interactions

A series of linear copolymers of DTPA-class Gd3+ conjugates, linked by alpha, omega-alkyldiamides with a varying number (n) of methylenes separating the amide function, were synthesized. Surprisingly, their relaxivities at all fields increased with increasing n. At MRI fields and 35 degrees C, the relaxivities of the n = 10 and n = 12 polymers were unexpectedly high, similar to those of rigid dendrimer-based Gd3+ chelates. The magnetic field dependence of solvent proton 1/T1 was measured for aqueous urea-free and urea-containing polymer solutions. The results for urea-free solutions imply an increase of rigidity (required for high relaxivities) with increasing n, arising from hydrophobic interactions of the methylene groups with solvent. This hypothesis is supported by a large decrease in the relaxivities upon addition of urea, which is known to weaken hydrophobic interactions. The relaxivities are also independent of polymer concentration, indicating that the hydrophobic interactions are intramolecular.

Relaxation of solvent protons by solute Gd3+-chelates revisited

The magnetic field dependence (NMRD profile) of 1/T1 of solvent protons in an aqueous solution of Gd(DTPA)2- was remeasured at 5, 15, 25, 30, and 35 degrees C. The data were reanalyzed with the usual low-field theory, using recently published values for tauM, the residence lifetime of the single inner-coordinated waters of solute Gd(DTPA)2-. (These tauM values are significantly longer than earlier estimates). Values were obtained for three dynamic parameters: tauR, the rotational relaxation time of solute ions, and tauSo and tauV, the low-field relaxation time of the Gd3+ magnetic moment and the related correlation time. These Gd(DTPA)2- values, together with recent results for tauM for Gd(DTPA-BMA)--a nonionic structural analog of Gd(DTPA)2- with an unusually long tauM--were used to calculate NMRD profiles at 5 and 35 degrees C. These profiles agree very well with new data given here for a solution of Gd(DTPA-BMA). This reaffirms the importance of knowing the temperature-dependent values of tauM a priori in order to obtain unambiguous quantitative theoretical analyses of NMRD profiles of chelates of known structure. Additionally, the theory of inner sphere relaxation is extended to high fields, at which the magnetic energy of a solute moment is greater than its thermal energy.

Low-molecular weight lanthanide contrast agents: evaluation of susceptibility and dipolar effects in red blood cell suspensions

Red blood cell (RBC) suspensions, containing low-molecular weight (LMW) dysprosium (Dy) and gadolinium (Gd) chelates, were selected as a two-compartment system for the evaluation of the magnetic dipolar and susceptibility contributions to the transverse (T2) relaxation of solvent water protons. The influence of RBC geometry and degree of metal chelate compartmentalization on T2 was investigated by variation of the osmolality and hematocrit (HC), respectively. The T2-relaxation ability of Dy-chelates was markedly improved in RBC suspensions, in comparison to aqueous solutions, due to the presence of susceptibility effects that more than compensated for the low dipolar relaxation efficacy. Despite a smaller susceptibility effect, the Gd-chelates were still the most efficacious in shortening T2 due to their comparatively larger dipolar relaxation contribution. The results obtained with the Dy-chelates allowed the evaluation of the relative contributions of susceptibility and dipolar mediated relaxation for the Gd-chelates. The RBC geometry and degree of compartmentalization influenced strongly the T2 relaxation efficacy of Dy-chelates, as opposed to the Gd-chelates. Hemolysis eliminated the susceptibility effect, essentially removing the T2 relaxation ability of Dy-chelates. The T2 relaxation efficacy of Gd-chelates was improved by hemolysis due to enhancement of the dipolar relaxation. As a conclusion, RBC suspensions have clearly been shown to be a suitable ex vivo model with which to distinguish the different contrast mechanisms of LMW Dy- and Gd-based MRI contrast agents.

Theory of 1/T1 and 1/T2 NMRD profiles of solutions of magnetic nanoparticles

Organically coated iron oxide crystallites with diameters of 5-50 nm ("nanoparticles") are potential magnetic resonance imaging contrast agents. 1/T1 and 1/T2 of solvent water protons are increased dramatically by magnetic interactions in the "outer sphere" environment of the nanoparticles; subsequent diffusive mixing distributes this relaxation throughout the solvent. Published theory, valid for the solute magnetic energy small compared with thermal energy, is applicable to small magnetic solutes (e.g., gadolinium and manganese diethylenetriaminopentaacetic acid, and nitroxide free radicals) at generally accessible fields (< or = 50 T). It fails for nanoparticles at fields above approximately 0.05 T, i.e., at most imaging fields. The authors have reformulated outer sphere relaxation theory to incorporate progressive magnetic saturation of solute nanoparticles and, in addition, indicate how to use empirical magnetization data for realistic particles when their magnetic properties are not ideal. It is important to handle the effects of rapid thermally induced reorientation of the magnetization of the nanoparticles (their "superparamagnetism") effectively, including their sensitivity to particle size. The theoretical results are presented as the magnetic field dependence (NMRD profiles) of 1/T1 and 1/T2, normalized to Fe content, for three sizes of particles, and then compared with the limited data extant for well-characterized material.

Dynamics of tumor imaging with Gd-DTPA-polyethylene glycol polymers: dependence on molecular weight

Macromolecular contrast media offer potential advantages over freely diffusible agents in magnetic resonance (MR) imaging outside the central nervous system. To identify an optimum molecular weight for macromolecular contrast media, the authors studied a novel macromolecular contrast agent, gadolinium diethylenetriaminepentaacetic acid polyethylene glycol (DTPA-PEG), synthesized in seven polymer (average) molecular weights ranging from 10 to 83 kd. Twenty-eight rabbits bearing V2 carcinoma in thighs underwent T1-weighted spin-echo imaging before injection and 5-60 minutes and 24 hours after injection of the Gd-DTPA-PEG polymers or Gd-DTPA at a gadolinium dose of 0.1 mmol/kg. Tumor region-of-interest measurements were obtained at each time point to determine contrast enhancement dynamics. Blood-pool enhancement dynamics were observed for the Gd-DTPA-PEG polymers larger than 20 kd. Polymers smaller than 20 kd displayed dynamics similar to those of the freely diffusible agent Gd-DTPA. Above the 20 kd threshold, tumor enhancement was more rapid for smaller polymers. The authors conclude that the 21.9-kd Gd-DTPA-PEG polymer is best suited for clinical MR imaging.

Determination of the relative amounts of free and complexed manganese ions in aqueous solution by nuclear magnetic resonance

The ratio of the transverse relaxation rate to the longitudinal relaxation rate of water protons (T1/T2) at 90 MHz can be used to determine the relative amounts of free Mn2+ ions (the hexaquo ion) and chelated manganese ions when both species are present in the same aqueous solution. This technique is demonstrated for simple aqueous solutions containing known concentrations of both the free Mn2+ ion and a manganese(III) porphyrin. The accuracy of the method is found to be +/- 7.3% for the solutions considered in this report. The determinations of the relative amounts of the free Mn2+ ion and the manganese(III) porphyrin by this T1/T2 method are shown to agree well with those determined by nuclear magnetic resonance dispersion profiles. The theoretical basis of this ratio method as well as the scope and limitations in its application to aqueous solutions containing both free Mn2+ ions and chelated manganese species other than manganese(III) porphyrins are also discussed. This work demonstrates a unique use of NMR as a means for the quantitative analysis of manganese in which the effects of the metal ion on the solvent, and not the metal itself, are observed.