Background/Objective: Nowadays duplex ultrasound is used for both screening and preoperative assess-ment of carotid arteries, and has in many institutions completely replaced carotid angiography for the lat-ter. This study aimed to identify resistivity index ab-normalities in common carotid arteries in patients with occluded or severely stenosed internal carotid artery. Patients and Methods: A total of 16 patients with se-vere internal carotid artery stenosis (stenosis>60%) or occlusion, were studied by Duplex ultrasonography at the Imam Khomeini Hospital of Tabriz, from Novem-ber 2005 to September 2006. Data were expressed as mean (SD), and as percentages. Statistical analysis was performed by SPSS ver. 13. Results: The mean age of patients were 73.3± 7.67 years. Resistivity index in common carotid artery was 0.85± 0.03 on stenotic side compared with 0.74±0.04 on contralateral side (p<0.05). Peak systolic velocity in common carotid artery was 41.5±17.00 cm/s on stenotic side compared with 59.5±22.03 cm/s on con-tralateral side, and there was no side difference re-garding flow velocities. Conclusion: In patients with occluded or severely stenosed internal carotid arteries, compensatory va-sodilatation occurs in same side in cerebral vessels and this is significant that result in decrease of resis-tivity index on the contralateral common carotid ar-tery.

Electrical resistivity techniques are well-established and applicable to a wide range of geophysical problems. 2D resistivity measurements can give information about both the lateral and vertical variations of the subsurface resistivity and can be used in a qualitative fashion for the identification of the structure and depth of masses. The resistivity inverse problem involves constructing an estimate of a subsurface resistivity distribution, which is consistent with the experimental data. This is a fully non-linear problem and its treatment involves iterative full matrix inversion algorithms, which can give good quality results. The back-projection resistivity technique (BPRT) can be applied to a set of apparent resistivity measures to quickly obtain an approximate image of the resistivity distribution of the investigated volume. This technique is based on the consideration that a resistivity perturbation in a point element (voxel) of a bounded region produces a change in voltage thus an apparent resistivity anomaly at the surface of the region, according to a sensitivity coefficient. The value of the coefficient is dependent on the position of the voxel considered in respect of both the current and the voltage dipoles, in agreement with the sensitivity theorem of Geselowitz. This consideration suggests that it is possible to correlate all the measured resistivity values, weighted by the appropriate sensitivity coefficients to each voxel of the investigated volume and to estimate the resistivity value of each cell of the model using a weighted summation of the apparent resistivity measurements. The BPRT considering a two-step approach. Initially, a damped least squares solution is obtained after a full matrix inversion of the linearized geoelectrical problem. Furthermore, on the basis of the results, a subsequent filtering algorithm is applied to the Jacobian matrix, aiming at reducing smoothness, and the linearized damped least square inversion is repeated to get the final result. This fast imaging technique aims at increasing the resistivity contrasts, and practically, since it does not require a parameter set optimization, it can be used to easily obtain fast and preliminary results. The procedure proposed in this work consists of four steps: (1) Evaluation of sensitivity matrix B, (2) Inversion of matrix B using a damped LSQR solution, (3) Recalculation of a filtered Jacobian matrix B‘ obtained by means of a correlation filter, (4) Inversion of the filtered sensitivity matrix. The proposed technique is tested on resistivity synthetic data from the Schlumberger, Wenner, Dipole-dipole and Pole-pole arrays, the objective of which is to find the optimal parameter set. The synthetic tests carried out with 2D data suggested that a good compromise for 2D inversions is to choose 𝜆 for the Schlumberger, Wenner, Dipole-dipole and Pole-pole arrays, 0. 1, 20, 1 and 0. 5, respectively. Furthermore, all the synthetic tests carried out with 2D data suggested that a good compromise for 2D inversions is to choose 𝜒 ≈ 5. The approximate images using the BPRT inverse modeling for all synthetic data, with and without random noise, is compared with the least square inversion by RES2DINV software. Finally, a field case is discussed, and the comparison between the back-projection and inversion is shown.

The resistivity (r) data for RFeAsO compounds (R=Ce, Pr, Nd, Sm), in the temperature (T) range 35–315 K have been analyzed to identify the dominant scattering mechanisms. Close to the room temperature, the system appears to be a metal with low electron density, and the electron–phonon scattering is the dominant one. At lower temperatures, electron–electron scattering plays an important role. In an intermediate temperature region, unlike metallic system, dr/dT is negative; and r-1 varies as lnT as in a state of weak localization. We look into the origin of negative dr/dT. The analysis of r (T) data below the SDW transition temperature shows the presence of electron–electron interaction in addition to a SDW energy gap, and also gives an estimate of the SDW energy gap.