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Photopumping measurements for the dilute thin films of the representative proton-transfer laser dye, 2-͑2-hydroxyphenyl͒benzothiazole ͑HBT͒ revealed that it exhibited better stimulated-emission performance, especially at the high doping level of 26 wt %. This suggests that HBT has an ability to form high gain media, in addition to an inherent potential for widely tuning gain wavelength. Both are advantageous for overcoming the polaron-absorption problem that is a major obstacle to making organic laser diodes. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1868885͔Since the reports that the possibility of organic laser diodes ͑OLDs͒ was referred, 1,2 a considerable number of attempts have been made to date. [3][4][5][6] Although photopumped lasing has been observed in the slab waveguides made of small molecules or conjugated polymers, there has yet been no report on electrically pumped lasing from organic lightemitting diode ͑OLED͒ structures. The primary requirement for the achievement of laser radiation is that the optical gain of emissive medium must be predominant over the sum of all the losses, such as scattering and absorption; namely, the net gain must become positive. Within the OLED devices, however, inherent low carrier mobility of organics brings about the situation that the density of charge carriers ͑i.e., polarons͒ is much higher than that of emissive excitons, thereby causing a large absorption loss of stimulated emission by polarons. 7,8 Actually, in a thin film of tris-͑8-hydroxyquinoline͒ aluminum ͑Alq͒-doped with a DCM2 laser dye, which is a host-guest type of emitting layer enabling an efficient Förster energy transfer, it is shown that the optical gain of DCM2 cannot be obtained due to the absorption loss by large amounts of anionic Alq hosts. 7 The drastic raising of mobility enough to change such a situation could hardly be expected under the carrier-transport mechanism of organics described by hopping, so that the polaronabsorption problem is a serious obstacle to the realization of OLDs.One of the strategies for circumventing this problem is to design the device such that the gain spectrum of a laser dye has little overlap with the absorption spectra of charge carriers. 5 For this purpose, the use of the cascade Förster energy-transfer system that is formed by simultaneous doping of two or more guests into a host matrix 9 might be an effective way, since the gain wavelength can be widely changed by the combination of guests suitably matched in their absorption and emission spectra. Thus, for example, a further addition of adequate laser dyes into the Alq:DCM2 system might enable one to shift the gain wavelength to the region where absorption of Alq anions is relatively weak; however, it also might require a high technique for vapor codeposition of several materials with their concentration ratio exactly controlled.On the other hand, as another gain-wavelength shifting method, we have been focusing on the use of proton-transfer ͑PT͒ laser dyes. A characteristic property of PT dyes ...

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Septal penetration of high-energy photons affects quantitative results in imaging of 123 I-labeled tracers. We investigated acquisition protocols (collimator choice and energy window setting) and correction methods for estimating the heart-to-mediastinum (H/M) ratio in cardiac 123 I-metaiodobenzylguanidine (MIBG) imaging. Methods: Four hours after 123 I-MIBG injection, 40 patients successively underwent planar anterior chest imaging with the medium-energy (ME) (ME method) and low-energy high-resolution (LEHR) (LEHR method) collimators. A 20% energy window was used for both collimators. Another 40 patients were imaged successively with the ME collimator and a 20% window (ME method), the low-medium-energy (LME) collimator and a 20% window (LME20 method), and the LME collimator and a 15% window (LME15 method). The H/M ratios obtained by the LEHR, LME20, and LME15 methods were corrected using their correlations with the H/M ratio obtained by the ME method (empiric correction). The 123 I-dual-window (IDW) correction was also applied to remove the influence of high-energy photons. Results: Without correction, severe underestimation of the H/M ratio was shown for the LEHR method using the ME method as a standard, and this underestimation increased with increasing H/M ratios. Underestimation substantially decreased using the LME20 method and further using the LME15 method. Empiric correction reduced the error in the H/M ratio by the LEHR method, but the error was still evident. After empiric correction, the H/M ratios with the LME collimator were comparable to those with the ME collimator. The IDW correction only partially reduced underestimation by the LEHR method and caused a small overestimation for the LME15 method. Conclusion: The use of an LME collimator appears to be acceptable for cardiac 123 I-MIBG imaging as an alternative to an ME collimator, and the application of a 15% energy window is recommended when an LME collimator is used. Empiric correction is also expected to improve exchangeability between H/M ratios calculated with ME and LME collimators. Neither the use of an LEHR collimator nor the use of IDW correction is recommended.