The classic definition of the ischemic penumbra is a hypoperfused region in which metabolism is impaired, but still sufficient to maintain cellular polarization. Perfusion-and diffusion-weighted MRI (PWI, DWI) can identify regions of reduced perfusion and cellular depolarization, respectively, but it often remains unclear whether a PWI-DWI mismatch corresponds to benign oligemia or a true penumbra. We hypothesized that pH-weighted MRI (pHWI) can subdivide the PWI-DWI mismatch into these regions. Twenty-one rats underwent permanent middle cerebral artery occlusion and ischemic evolution over the first 3.5 h post-occlusion was studied using multiparametric MRI. End point was the stroke area defined by T 2 -hyperintensity at 24 h. In the acute phase, areas of reduced pH were always larger than or equal to DWI deficits and smaller than or equal to PWI deficits. Group analysis showed that pHWI deficits during this phase coincided with the resulting infarct area at endpoint. Final infarcts were smaller than PWI deficits (range 65% to 90%, depending on the severity of the occlusion) and much larger than acute DWI deficits. These data suggest that the outer boundary of the hypoperfused area showing a decrease in pH without DWI abnormality may correspond to the outer boundary of the ischemic penumbra, while the hypoperfused region at normal pH may correspond to benign oligemia. These first results show that pHWI can provide information complementary to PWI and DWI in the delineation of ischemic tissue.

Amide proton transfer (APT) imaging is a type of chemical exchange saturation transfer imaging in which the amide protons of cellular proteins and peptides are saturated and detected via the water resonance. To study this effect, conventional magnetization transfer and direct saturation effects in the frequency-dependent water saturation spectrum (z-spectrum) need to be removed by asymmetry analysis with respect to the water frequency offset. When using echo planar imaging, it was found that unequal pericranial fat saturation at equidistant higher and lower frequencies with respect to water leads to a lipid artifact in APT asymmetry images. The proton exchange process between solute and water protons depends on parameters such as solute concentration, the environmental pH, and temperature (1). The dependence on pH of the exchange rate of amide protons in proteins and peptides has recently gained interest for in vivo assessment (2-5) of pH. This has become more practical in view of the use of the chemical exchange saturation transfer (CEST) sensitivity enhancement approach (6 -10), which allows enhancements of signals by several orders of magnitude. When using biologic polymers, increases by factors as large as 10 7 have been reported and more is expected in the future (8,11). In amide proton transfer (APT) experiments (4,5,12), the magnetization transfer saturation pulse is applied to saturate signals from amide protons around 8.3 ppm (i.e., 3.5 ppm from water) and their chemical exchange leads to indirect saturation of the water signal. When the relationship between the exchange rate and pH value is known, the results can be used to reconstruct a pH map noninvasively. Because pH is closely related to cellular metabolism, such a physiologic monitoring capability is significant for studying pathologies such as acute ischemic stroke, especially in the ischemic penumbra, where the breakdown of oxidative metabolism may lead to breakdown of membrane function and spreading of the infarct.Both pulsed and continuous wave RF fields can be used to saturate the amide protons and to achieve the indirect saturation of water protons through chemical exchange. Such effects can be visualized using so-called z-spectra (13) (Fig. 1a), in which the water signal attenuation is displayed as a function of the RF saturation frequency offset. It is known that in addition to the proton exchange, the z-spectra have contributions from processes such as conventional MT effects and direct water proton saturation (14,15). Assuming that such effects are mainly symmetric around the water resonance, an asymmetry analysis with respect to the water offset may be used to suppress such effects (Fig. 1b), from which the pH effects can be deduced (4,5). To obtain pH-weighted images at reasonable spatial and temporal resolution in vivo, the saturation pulse can be combined with fast imaging methods such as fast spin echo or echo planar imaging (EPI). When performing such APT-EPI imaging during our stroke studies in vivo, we noticed that the usual smal...

Glioblastoma (GBM) is a highly aggressive primary brain tumor that is especially difficult to treat. The tumor's ability to withstand hypoxia leads to enhanced cancer cell survival and therapy resistance, but also yields a microenvironment that is in many aspects unique within the human body, thus offering potential therapeutic opportunities. The spore-forming anaerobic bacterium Clostridium novyi-NT(C. novyi-NT) has the ability to propagate in tumor-generated hypoxia, leading to oncolysis. Here, we show that intravenously injected spores of C. novyi-NT led to dramatic tumor destructions and significant survival increases in implanted, intracranial syngeneic F98 and human xenograft 060919 rat GBM models. C. novyi-NT germination was specific and confined to the neoplasm, with sparing of the normal brain parenchyma. All animals tolerated the bacteriolytic treatment, but edema and increased intracranial pressure could quickly be lethal if not monitored and medically managed with hydration and antibiotics. These results provide pre-clinical data supporting the development of this therapeutic approach for the treatment of patients with GBM.