Experimental work in the field of nerve allotransplantation has dealt with the feasibility of nerve allografts reconstructing nerve gaps. In the majority of studies, the nerve gap studied has been short, and some degree of regeneration has been achieved, even in the untreated allograft. To better approximate clinical nerve-allograft reconstruction, a series of long-segment (8-cm) nerve allografts were performed in the ovine model. Twenty outbred ewes were randomized into two experimental groups with four experimental conditions. Animals received nerve allografts treated under one of the following conditions: fresh nerve autograft, fresh nerve allograft, cold-preserved nerve autograft, or cold-preserved nerve allograft. The nerve grafts were examined and compared at 6 and 10 months, using histological, morphometric, and electro-physiologic analyses. The results of the study demonstrated that, while excellent regeneration occurred across the nerve autograft, the long nerve allograft could not support axonal elongation. Similarly, cold nerve preservation did not enhance regeneration. The sheep animal model allows for investigation of the long nerve gap and may be beneficial in a better correlation of experimental nerve transplantation with clinical conditions.

Purpose: To investigate Gd-DTPA kinetics as predictors of histopathological changes following focused ultrasound (FUS) thermal ablation for improved planning and assessment. Materials and Methods:Twenty-nine FUS lesions were created in the thigh muscle of eight rabbits under MRguidance at 1.5 Tesla. Three rabbits were killed at four hours; and 11 lesions were analyzed with histopathology. Temperature-sensitive MRI using proton-resonant frequency-shift was used for time-dependent temperature measurements. Analysis of the uptake kinetics of Gd-DTPA was performed after Gd-DTPA injection, within 20 minutes after heating and again at two hours after heating. The resulting kinetic maps, permeability (K trans ) and leakage space (v e ), were correlated to peak temperatures, T 2 -weighted MR, and histopathology.Results: Images of K trans and v e reveal regions of histopathological change not visible on conventional posttherapy MR. At early times after heating, v e predicts the area of injury more accurately than T 2 (7 Ϯ 2% vs. 25 Ϯ 6% underestimation). A circular region of extensive structural/ vascular disruption is indicated only on K trans maps. The sharp decrease in K trans at the boundary of this region occurs at 47.5 Ϯ 0.5°C, and may be a better estimate of cell death than the conventional method of temperature threshold (55°C for coagulation) used in therapy planning. Conclusion:Our results suggest Gd-DTPA kinetics can predict different histopathological changes following FUS ablation and may be valuable for early prediction.

The purpose of this experiment was to use MRI to monitor microwave heating and thermal damage of brain tissue in vivo. Interstitial microwave antennas were implanted into the cerebral hemispheres of seven anesthetized rabbits. Variable power of 30 to 100 W was applied for periods of 5 to 15 minutes and tissue temperature was monitored continuously. MR images were obtained throughout the procedure at 20-second intervals, using a spoiled gradient-echo sequence, without significant artifact. Magnitude, phase, and complex difference images all demonstrated temperature-related signal changes during heating. The findings were better visualized on the phase and complex difference images. Phase difference image analysis revealed an approximately linear relationship between phase change and temperature. Post-treatment thermal lesions measured up to 2.0 cm in size on pathologic specimens and exhibited a zonal pattern on spin-echo MR images.

Purpose: To investigate Gd-DTPA kinetics as indicators of subacute and subchronic histopathological changes following focused ultrasound (FUS) thermal therapy for improved evaluation. Materials and Methods:A total of 18 FUS lesions were created in the thigh muscle of five rabbits under magnetic resonance (MR) guidance at 1.5 Tesla. The rabbits were killed at different times: 40 hours, three days, and seven days. All lesions were analyzed histologically. An analysis of the uptake kinetics of Gd-DTPA, injected within two hours postheating and before sacrifice, was performed. The resulting kinetic maps, permeability (K trans ) and leakage space (v e ), were correlated to T 2 -weighted MR and histology.Results: Images of K trans and v e better differentiate subacute and subchronic changes not visible on conventional MR in the days following therapy and are consistent with the histopathology observed. In particular, the border between nonviable and viable tissue is well demarcated. The extent of damage is best indicated on v e , whereas the borders of inflammation are shown on K trans . The total lesion extent is relatively stable over the 7 days posttherapy and can be predicted by v e or T 2 -weighted MR at early times after heating. Conclusion:Our results suggest that Gd-DTPA kinetics can complement conventional MR for improved evaluation of FUS thermal therapy by providing finer differentiation of necrotic states, inflammation, and repair processes.