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Several investigations have been conducted to characterize MR system-related, gradient magnetic field-induced stimulation in human subjects ( Bourland et al., 1999 Schaefer et al., 2000 Smith et al., 2001). Shellock, in Neuromodulation, 2009 Gradient Magnetic Field-Induced Stimulation in Human Subjects The measurement of tissue pO 2 in the nonischemic contralateral brain can be used as internal control and can also provide valuable information on potential adaptive response to ischemic stroke.įrank G. The coordinates of the probes can be verified either by using 2,3,5-triphenyltetrazolium chloride (TTC) staining of the brain posteuthanasia, or noninvasively by MRI ( Bederson, Pitts, Germano, et al., 1986 Duong, 2013 Milidonis, Marshall, Macleod, & Sena, 2015 Wey, Desai, & Duong, 2013 Yang, Shuaib, & Li, 1998). It is important to ascertain that the probes are located in the regions of interest, i.e., infarct core and penumbra, to facilitate appropriate analysis of the data acquired. Additionally, the body temperature of the animals should be maintained at 37 ± 0.5 ☌, by using a heated water pad and/or warmth during surgery and EPR measurements. The animals should be maintained under physiological conditions to maintain blood pressure, heart rate, and blood gas during the experiments. It has been estimated that for implanted probes of rectangular shapes and 0.2 mm length, optimally resolved spectra with minimal (< 1%) distortion of Lorentzian line shapes are obtained if the distance between the implants exceeds 1.8 mm ( Smirnov et al., 1993). If the size of the implanted probe is sufficiently small compared with the distance between the implants, then the magnitude of magnetic field gradient should be minimal to resolve individual EPR spectra without undesirable increase of the line-width. The direction of the gradient should be chosen to maximize the separation of the spectra from multiple implants. However, precaution must be exercised to avoid any undesirable broadening of the line-width due to high-incident microwave power, high modulation amplitude, or high magnetic field gradients. In order to obtain well-resolved spectra from three to four sites simultaneously (multisite oximetry), a field gradient (1–3 G cm − 1) should be applied. Three to five scans each of 10 s scan time can be averaged to acquire a temporal profile of pO 2 at every minute during the experiments. The typical magnetic field center and modulation frequency of L-band EPR spectrometers are 410 G and 27 kHz respectively. Typical spectrometer settings that we have used for oximetry are incident microwave power of 10 mW or less, scan range of 1–2.5 G (depends on number of probes), and modulation amplitude of less than one-third of the EPR line-width. The parameters of the EPR spectrometer including magnetic field gradients should be optimized for cerebral pO 2 measurements in these experiments. Periannan Kuppusamy, in Methods in Enzymology, 2015 4.2.2 EPR Acquisition Parameters This is termed frequency and phase encoding, respectively. For spatial encoding within the selected slice, magnetic field gradients are applied to impose a unique combination of frequency and phase offsets. An RF pulse with a prescribed bandwidth excites only spins within a particular range of Larmor frequencies that correspond to the desired slice. To select a slice in 2-D imaging, a gradient in the magnetic field is produced in a direction that is normal to the slice giving the spins unique Larmor frequencies in this direction. Spatial encoding includes slice selection, frequency encoding, and phase encoding. A gradient that is applied parallel to B 0, for example, will cause the magnetic field to be stronger toward the subject’s head and weaker toward the feet or vice versa. The gradient system consists of a set of gradient coils, typically one for each axis ( x, y, and z), for creating (usually linear) variations of the magnetic field in a specified direction. These variations, superimposed on the main magnetic field, are created by the gradient system. The ability to create magnetic field gradients is essential in MRI, for example, for the purpose of spatial encoding. Lau, in Encyclopedia of Biomedical Engineering, 2019 Gradient System and Spatial EncodingĪ magnetic field gradient refers to a variation in the magnetic field across space.