Rationale and Objectives
Blood perfusion of peripheral nerves plays an important role in regeneration after nerve injury. Functional recovery after a peripheral nerve injury depends not only on the survival of the affected neurons but also on the recovered blood perfusion. Previous studies have shown that it is possible to quantitatively assess blood perfusion of tissue using contrast-enhanced ultrasound (CEUS). The aim of this study was to evaluate the usefulness of CEUS for the quantitative evaluation of blood perfusion of the sciatic nerves with crush injury.
Materials and Methods
Crush injuries were created in the left sciatic nerve of 30 New Zealand white rabbits. CEUS of the bilateral sciatic nerves was performed in six experimental rabbits at 3 days, 1 week, 2 weeks, 4 weeks, and 8 weeks after injury. Pulse-inversion harmonic imaging was used for real-time CEUS. The other six rabbits were used as a control group. Serial laser Doppler measurements of blood flow and quantitative histologic evaluation were performed parallel to CEUS on all animals.
Results
Quantitative analysis of CEUS showed that the perfusion index of the crushed sciatic nerves was increased at 3 days after injury, with a peak at 1 week after injury ( P = .000). The area under the curve for the crushed sites was increased at 3 days after injury, with a peak at 2 weeks after injury ( P = .000). The mean transit time and maximum intensity of the crushed site of the left sciatic nerves were not significantly changed during the 2 months after injury ( P = .335 and P = .157 respectively). The perfusion indices measured by CEUS correlated well with those measured by laser Doppler ( r = 0.791, P = .000). Marked Wallerian degeneration was found at the crushed site of sciatic nerves at 3 days after injury. The percentage of degenerated myelinated axons was increased during the first 2 weeks after injury and then decreased during the following period. Regenerated axons with small diameter and thin myelin sheaths were found at 2 weeks after injury and during the following period.
Conclusions
CEUS may provide a new imaging method to quantitatively analyze blood perfusion of injured peripheral nerves.
The assessment of blood perfusion of peripheral nerves is of great value in clinical practice because of its close relationship with axonal regeneration. Functional recovery after a peripheral nerve injury depends not only on the survival of the affected neurons but also on the recovered blood perfusion. Nerve ischemia may occur because of thrombosis, endothelial swelling, endoneurial edema, granulocyte plugging of the vasa nervorum, or actual physical interruption of microvessels at the site of injury . With regard to nerve grafts, vascularized nerve grafts have been proved superior to nonvascularized nerve grafts with respect to healing, especially when used in the hypovascular and scarred recipient bed . Because of the presence of sufficient and uninterrupted blood supply to the nerve graft, Schwann cell viability could be high, which would result in successful nerve healing. Thus, therapies that could promote the angiogenesis of peripheral nerves, such as cytokines, genes, or increasing blood flow, have been tried, and the results have shown that an increase in posttraumatic nerve blood flow can significantly improve the function of injured axons .
The current techniques that have been used for evaluating peripheral nerve blood perfusion include radioactive microspheres, microelectrode hydrogen clearance polarography, laser Doppler flowmetry, and others . All these methods are unsuitable for the noninvasive assessment of blood perfusion in peripheral nerves in clinical practice. Contrast-enhanced ultrasound (CEUS), which is based on the use of gas-filled microbubbles, can display blood perfusion at the capillary level . Because these microbubbles have a microvascular rheology similar to that of red blood cells and remain entirely within the vascular space, they reflect blood volume more accurately than computed tomographic and magnetic resonance imaging contrast agents, which extravasate into the interstitial space . Despite these promising results of CEUS in evaluating tissue perfusion, few studies have been performed using CEUS to evaluate blood perfusion of the peripheral nerves. Our previous study showed that it was possible to quantitatively evaluate blood flow of the sciatic nerve in normal New Zealand white rabbits . Thus, we hypothesized that CEUS could be used to noninvasively assess blood perfusion of injured peripheral nerves. To test our hypothesis, CEUS of sciatic nerves with a crush injury model in New Zealand white rabbits was performed and compared to data on blood perfusion measured using laser Doppler and to axonal changes on histopathology.
Materials and methods
Animals and Surgical Procedure
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Sonographic Contrast Agent
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Sonographic Equipment and CEUS Examination
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Offline Analysis of CEUS
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Perfusion Measurement with Laser Doppler
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Histopathologic Examination
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Statistical Analysis
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Results
CEUS
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Laser Doppler Blood Perfusion Measurement
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Histopathologic Examination
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Discussion
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Conclusions
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