Michihiro Takeuchi

Polymerization and dissolution of fibrin under homogeneous magnetic fields

We investigated whether or not a mutually compensating state of coagulation and fibrinolysis is changed by homogeneous magnetic fields. We used a superconducting magnet which produced magnetic fields of up to 14 T at its center. Fibrin polymerization over time, and the subsequent dissolution of the fibrin fiber network, were observed by measuring the optical absorbance of the mixture at 350 nm. A spectrophotometer with an external optical cell box in a superconducting magnet was used. We observed that the optical absorbance of the mixture at 350 nm increased during the fibrin-polymerization process, and decreased during the fibrinolytic processes. The optical absorbance was stable in the transient state between fibrin-polymerization and fibrinolytic processes. A magnetic field of 14 T increased the rate of the polymerization process by 55%–70% compared to the control group. On the other hand, the rate of the fibrinolytic process under a magnetic field at 14 T, increased by 27%–140% compared to the control. The results indicate that the magnetic orientation of fibrin fibers accelerated both the polymerization and the dissolution of fibrin fibers.

Aggregation of blood platelets in static magnetic fields

We investigated the effects of intense magnetic fields on the blood platelet aggregation process with and without static magnetic fields of up to 14 T. A rabbit plasma and collagen mixture was used as the model system for a wounded blood vessel. Platelet aggregation was activated by the stimulation of acid soluble collagen. The platelet aggregates in strong magnetic fields were larger than the aggregates in an ambient field. An optical transmission of blood plasma during platelet aggregation also indicated that strong magnetic fields enhanced blood platelet aggregation in plasma.

Multicomponent proton spin-spin relaxation of fibrin gels with magnetically oriented and randomly oriented fibrin fiber structures

We investigated the effect of structural differences in fibrin fibers on the T2 relaxation time. Fibrin fibers have the characteristic of orienting parallel to high magnetic fields during polymerization. Two fibrin gels were polymerized from a fibrinogen solution with and without a 7.05 T magnetic field. Water molecules in the fibrin gel that were polymerized in the high magnetic field exhibited only one relaxation time T2=0.35 s, whereas, water molecules in the fibrin gel that were not exposed to a magnetic field during polymerization had at least two exponential components in the T2 relaxation. The long component, T2=0.35 s, was the same order as the T2 of the fibrinogen solution (=0.41 s) and the fibrin gel polymerized in the high magnetic field. The short component was T2=0.07 s. This difference is attributed to a change in the magnetic dipole–dipole interactions between water molecules and fibrin fibers.

Spin-spin relaxation and apparent diffusion coefficient of magnetically oriented collagen gels

This paper describes a study on the effect of structural differences in collagen fibers on the spin-spin (T/sub 2/) relaxation and the apparent diffusion coefficient (ADC) of water molecules. Two collagen gels were polymerized from a type-I collagen solution with and without a 4.7-T magnetic field. The T/sub 2/ relaxation time was measured by the Carr-Purcell-Meiboom-Gill sequence. The ADCs were measured using a stimulated echo sequence with motion probing gradient (MPG) pulses. The temperature in the bore was 15/spl deg/C. The T/sub 2/ relaxation times of water molecules in the collagen gels with magnetically oriented fibers and randomly oriented fibers were T/sub 2/=0.52 s and T/sub 2/=1.32 s, respectively. The ADCs of water molecules measured with MPG directions parallel and perpendicular to the collagen fibers were ADC=2.08/spl times/10/sup -9/ m/sup 2//s and ADC=1.92/spl times/10/sup -9/ m/sup 2//s, respectively. These differences are attributed to a change of collagen fiber structures due to the magnetic orientation.

Subvoxel limits of magnetic resonance angiography: One-dimensional case

Although the pixel size of magnetic resonance angiography (MRA) defines the spatial resolution of measured images, MRA visualizes blood vessels whose diameters are comparable to or smaller than the pixel size. In the present study, we carried out simplified one-dimensional numerical simulations and two-dimensional imaging experiments to show that discretization errors significantly appear in the measurement of very thin samples, or samples having subpixel structures. Magnetic resonance signals were calculated for a numerical model of blood vessel. The signal intensity was significantly affected by the small displacements. The experimentally obtained signal intensities agreed well with numerical simulations. The signals were summed within a region-of-interest (ROI) covering several pixels. A decrease in the number of pixels included in the ROI led to a decrease in the fluctuation of signal intensity.

Measurements of the spin-spin relaxation time and the degree of orientation of magnetically oriented collagen gels

In this study, the spin-spin relaxation time of magnetically oriented collagen gels were measured . In addition, the degree of orientation of the collagen fibers was evaluated using electron micrographs of the gels. Water molecules in the collagen gel that were polymerized in the magnetic field had a relaxation time T/sub 2/ of 0.52 s, whereas water molecules in the collagen gel that were not exposed to a magnetic field during polymerization had the relaxation time T/sub 2/ = 1.32 s. The magnetically oriented fibers had a peak at approximately 180 degrees. The respective degrees of orientation of the magnetically oriented fibers and the randomly oriented fibers (polymerized without the magnetic field) were [cos/sup 2//spl theta/] = 0.53 and [cos/sup 2//spl theta/] = 0.40. The magnetically oriented fibers exhibited higher value compared to the randomly oriented fibers. The result was consistent with the structural difference between these fibers.