Atsuko Matsumoto

High-Resolution Mapping of Tumor Redox Status by Magnetic Resonance Imaging Using Nitroxides as Redox-Sensitive Contrast Agents

Purpose: There is considerable research directed toward the identification and development of functional contrast agents for medical imaging that superimpose tissue biochemical/molecular information with anatomical structures. Nitroxide radicals were identified as in vivo radioprotectors. Being paramagnetic, they can provide image contrast in magnetic resonance imaging (MRI) and electron paramagnetic resonance imaging (EPRI). The present study sought to determine the efficacy of nitroxide radioprotectors as functional image contrast agents. Experimental Design: Nitroxide radioprotectors, which act as contrast agents, were tested by EPRI and MRI to provide tissue redox status information noninvasively. Results: Phantom studies showed that the nitroxide, 3-carbamoyl-PROXYL (3CP), undergoes time-dependent reduction to the corresponding diamagnetic hydroxylamine only in the presence of reducing agents. The reduction rates of 3CP obtained by EPRI and MRI were in agreement suggesting the feasibility of using MRI to monitor nitroxide levels in tissues. The levels of 3CP were examined by EPRI and MRI for differences in reduction between muscle and tumor (squamous cell carcinoma) implanted in the hind leg of C3H mice simultaneously. In vivo experiments showed a T1-dependent image intensity enhancement afforded by 3CP which decreased in a time-dependent manner. Reduction of 3CP was found to be the dominant mechanism of contrast loss. The tumor regions exhibited a faster decay rate of the nitroxide compared to muscle (0.097 min-1 versus 0.067 min-1, respectively). Conclusions: This study shows that MRI can be successfully used to co-register tissue redox status along with anatomic images, thus providing potentially valuable biochemical information from the region of interest.

Absolute oxygen tension (pO2) in murine fatty and muscle tissue as determined by EPR

The absolute partial pressure of oxygen (pO2) in the mammary gland pad and femoral muscle of female mice was measured using EPR oximetry at 700 MHz. A small quantity of lithium phthalocyanine (LiPc) crystals was implanted in both mammary and femoral muscle tissue of female C3H mice. Subsequent EPR measurements were carried out 1–30 days after implantation with or without control of core body temperature. The pO2 values in the tissue became stable 2 weeks after implantation of LiPc crystals. The pO2 level was found to be higher in the femoral muscle than in the mammary tissue. However, the pO2 values showed a strong dependence on the core body temperature of the mice. The pO2 values were responsive to carbogen (95% O2, 5% CO2) breathing even 44–58 days after the implantation of LiPc. The LiPc linewidth was also sensitive to changes in the blood supply even 60 days after implantation of the crystals. This study further validates the use of LiPc crystals and EPR oximetry for long‐term non‐invasive assessment of pO2 levels in tissues, underscores the importance of maintaining normal body core temperature during the measurements, and demonstrates that mammary tissue functions at a lower pO2 level than muscle in female C3H mice. Magn Reson Med, 2005. Published 2005 Wiley‐Liss, Inc.

Axonogenesis in the medaka embryonic brain.

In order to know the general pattern of axonogenesis in vertebrates, we examined axonogenesis in the embryonic brain of a teleost fish, medaka (Oryzias latipes), and the results were compared with previous studies in zebrafish and mouse. The axons and somata were stained immunocytochemically using antibodies to a cell surface marker (HNK‐1) and acetylated tubulin and visualized by retrograde and anterograde labeling with a lipophilic dye. The fiber systems developed correlating with the organization of the longitudinal and transverse subdivisions of the embryonic brain. The first axons extended from the synencephalic tegmentum, forming the first fiber tract (fasciculus longitudinalis medialis) in the ventral longitudinal zone of the neural rod, 38 hours after fertilization. In the neural tube, throughout the entire brain two pairs of longitudinal fiber systems, one ventral series and one dorsal or intermediate series, and four pairs of transverse fiber tracts in the rostral brain were formed sequentially during the first 16 hours of axon production. In one of the dorsal longitudinal tracts, its branch retracted and disappeared at later stages. One of the transverse tracts was found to course in the telencephalon and hypothalamus. The overall pattern of the longitudinal fiber systems in medaka brain is similar to that in mouse, but apparently different from that in zebrafish. We propose that a ventral tract reported in zebrafish partially belongs to the dorsal fiber system, and that the longitudinal fiber systems in all vertebrate brains pass through a common layout defined by conserved genetic and developmental programs. J. Comp. Neurol. 476:240–253, 2004.

Influence of proton T1 on oxymetry using Overhauser enhanced magnetic resonance imaging.

In Overhauser enhanced magnetic resonance imaging (OMRI) for in vivo measurement of oxygen partial pressure (pO2), a paramagnetic contrast agent is introduced to enhance the proton signal through dynamic nuclear polarization. A uniform proton T1 is generally assumed for the entire region of interest for the computation of pO2 using OMRI. It is demonstrated here, by both phantom and in vivo (mice) imaging, that such an assumption may cause erroneous estimate of pO2. A direct estimate of pixel‐wise T1 is hampered by the poor native MR intensities, owing to the very low Zeeman field (15–20 mT) in OMRI. To circumvent this problem, a simple method for the pixel‐wise mapping of proton T1 using the OMRI scanner is described. A proton T1 image of a slice through the center of an SCC tumor in a mouse clearly shows a range of T1 distribution (0.2∼1.6 s). Computation of pO2 images using pixel‐wise T1 values promises oximetry with minimal artifacts by OMRI. Magn Reson Med 54:213–217, 2005. Published 2005 Wiley‐Liss, Inc.

Effect of amifostine, a radiation-protecting drug, on oxygen concentration in tissue measured by EPR oximetry and imaging

Effect of amifostine, a radiation-protecting drug, on muscle tissue partial pressure of oxygen was investigated by electron paramagnetic resonance spectroscopy and imaging. When amifostine was administered intraperitoneally or intravenously to mice, the linewidth of the electron paramagnetic resonance spectra of the lithium octa-n-butoxy-substituted naphthalocyanine implanted in the mouse leg muscle decreased. Electron paramagnetic resonance oximetry using a lithium octa-n-butoxy-substituted naphthalocyanine probe and electron paramagnetic resonance oxygen mapping using a triarylmethyl radical paramagnetic probe was useful to quantify pressure of oxygen in the tissues of living mice. The result of electron paramagnetic resonance oximetric imaging showed that administration of amifostine could decrease pressure of oxygen in the muscle and also tumor tissues. This finding suggests that lowering pressure of oxygen in tissues might contribute in part to the radioprotection of amifostine.