Dynamical structure factors of S=1/2S=1/2 two-leg spin ladder systems

We investigate dynamical properties of $S=1/2$ two-leg spin ladder systems. In a strong coupling region, an isolated mode appears in the lowest excited states, while in a weak coupling region, an isolated mode is reduced and the lowest excited states become a lower bound of the excitation continuum. We find in the system with equal intrachain and interchain couplings that due to a cyclic four-spin interaction, the distribution of the weights for the dynamical structure factor and characteristics of the lowest excited states are strongly influenced. The dynamical properties of two systems proposed for ${\rm SrCu_2O_3}$ are also discussed.Comment: 5 pages, 6 figure

Optical absorption spectra in SrCu_2O_3 two-leg spin ladder

We calculate the phonon-assisted optical-absorption spectra in SrCu_2O_3 two-leg spin-ladder systems. The results for two models proposed for SrCu_2O_3 are compared. In the model including the effects of a cyclic four-spin interaction, the shoulder structure appears at 978 cm^{-1} and the peak appears at 1975 cm^{-1} in the spectrum for polarization of the electric field parallel to the legs. In the other model which describes a pure two-leg ladder, the peak appears around the lower edge of the spectrum at 1344 cm^{-1}. The feature can be effective in determining the proper model for SrCu_2O_3.Comment: 5 pages, 5 figures, to appear in PRB vol. 67 (2003

Dynamical structure factors of the magnetization-plateau state in the S=1/2S=1/2 bond-alternating spin chain with a next-nearest-neighbor interaction

We calculate the dynamical structure factors of the magnetization-plateau state in the $S=1/2$ bond-alternating spin chain with a next-nearest-neighbor interaction. The results show characteristic behaviors depending on the next-nearest-neighbor interaction $\alpha$ and the bond-alternation $\delta$. We discuss the lower excited states in comparison with the exact excitation spectrums of an effective Hamiltonian. From the finite size effects, characteristics of the lowest excited states are investigated. The dispersionless mode of the lowest excitation appears in adequate sets of $\alpha$ and $\delta$, indicating that the lowest excitation is localized spatially and forms an isolated mode below the excitation continuum. We further calculate the static structure factors. The largest intensity is located at $q=\pi$ for small $\delta$ in fixed $\alpha$. With increasing $\delta$, the wavenumber of the largest intensity shifts towards $q=\pi/2$, taking the incommensurate value.Comment: to appear in Phys. Rev. B (2001

Intractable epilepsy associated with angiographic obliteration of a medial temporal arteriovenous malformation following stereotactic radiosurgery

We present a case with recurrence of intractable complex partial epilepsy, despite angiographic obliteration of a cerebral arteriovenous malformation in the left medial temporal lobe following gamma knife radiosurgery. Preoperative memory function was normal, with no evidence of hippocampal atrophy by magnetic resonance imaging. Intraoperative electrocorticography demonstrated two independent foci of paroxysmal activity in the lateral and medial temporal lobes. As preoperative magnetic resonance imaging demonstrated subcortical hyperintensity, the lateral temporal lobe was resected. Histologically, there was evidence of subcortical myelin pallor, likely resulting from ischemic damage associated with hemodynamic changes following radiosurgery. As there was a decrease in the frequency of paroxysmal discharges in the hippocampus following arteriovenous malformation nidus removal, the hippocampus was preserved. Histological examination of the nidus revealed that some of the vessels had patent lumina, while ischemia affecting the neighboring hippocampus was considered to show improvement. A good seizure outcome was obtained. In summary, perioperative multimodal examinations, including intraoperative electrocorticography, revealed various epileptogenic mechanisms, probably related to myelin pallor in the lateral temporal lobe and focal ischemia secondary to patent AVM in the medial temporal lobe, of this patient. Keywords: Intraoperative electrocorticography, Myelin pallor, Hippocampus, Patent arteriovenous malformatio

Hemodynamic state of periictal hyperperfusion revealed by arterial spin-labeling perfusion MR images with dual postlabeling delay

Background: Magnetic resonance imaging (MRI), including perfusion MRI with arterial spin labeling (ASL) and diffusion-weighted imaging (DWI), are applied in the periictal detection of circulatory and metabolic consequences associated with epilepsy. Although previous report revealed that prolonged ictal hyperperfusion on ASL can be firstly detected and cortical hyperintensity of cytotoxic edema on DWI secondarily obtained from an epileptically activated cortex, the hemodynamic state of the periictal hyperperfusion has not been fully demonstrated. Methods: study-1: We retrospectively analyzed the relationship between seizure manifestations and the development of periictal MRI findings, in Case 1 with symptomatic partial epilepsy, who underwent repeated periictal ASL/DWI examination for three epileptic ictuses (one examination for each ictus). Study-2: We evaluated the hemodynamic state of periictal hyperperfusion with the ASL technique using a dual postlabeling delay (PLD) of 1.5 and 2.5 s in nine patients, according to the presence or absence of the localized epileptogenic lesion (EL) on conventional 3 T-MRI, who were divided into Group EL+ (six patients) and Group EL− (three patients). Results: Study-1 confirmed that the stratified representation of the periictal MRI findings depends on the time interval between the ictal cessation and MRI examination in addition to the magnitude and duration of the epileptic activity. In Study-2, two types of periictal hyperperfusion were noted. In all six Group EL+ patients, periictal ASL findings showed “fast flow type”. Markedly increased ASL signals were noted at the epileptically activated cortex, having a tight topographical relationship with EL, on ASL with a PLD of 1.5 s, which is decreased on ASL with a PLD of 2.5 s. In all three Group EL− patients, periictal ASL findings showed “gradual flow type”, which is characterized by gradual signal increase of the epileptically activated cortex on ASL with a PLD of 1.5 and 2.5 s. Conclusion: We confirmed that ASL hyperperfusion is superior to DWI in the periictal detection of epileptic events. ASL with dual PLD offers the ability to document two types of hemodynamics of periictal hyperperfusion. Keywords: Arterial spin labeling, Cytotoxic edema, Diffusion-weighted image, Ictal hyperperfusio