Measurement of blood flow in rodents using radioactive microspheres

Aim: Regional blood flow represents an important physiological parameter in experimental animals of various disease models. The ‘gold standard’ in large animals is to inject microspheres into the left ventricle using heart catheters; the number of microspheres trapped in the tissue capillaries is directly proportional to blood flow. Injection of radioactive microspheres directly into the left ventricle allows easy assessment of relative blood flow using imaging technologies. Absolute blood flow quantification will be also possible after additional arterial catheterization (required for reference blood sampling). However, up to now this technique has been rarely used in small rodents. In this pilot study we evaluated the technique in small rodents (mouse, rats) using radioactive microspheres.

Methods: Human serum albumin microspheres (20 µm diameter) were modified with NOTA. NOTA-microspheres were radiolabeled with Ga-68, Cu-64 or with F-18-fluoro-benzyol-succinimide using non modified microspheres. The number of injected microspheres was varied between 50,000 and 500,000. The microspheres were directly injected into the left ventricle over a time period of 30 s in desflurane-anesthetized animals on a heating pad. The animals were sacrificed 60 s after the end of injection. Total body activity was measured using a dose calibrator followed by small animal PET scan. The PET images were coregistered with MRI or CT images for identification of the tissues of interest. Standard uptake values were calculated and used for determination of relative blood flow.

Results: Radiolabeling resulted in high radiochemical yields and purity. The microsphere distribution was derived from the radioactivity distribution in the organs. The microspheres were virtually completely trapped (>99%) in the capillary bed. The injection is the most critical step of the experimental procedure. The number of applied microspheres was the limiting factor for measurements in low flow areas, where the number of measured microspheres was below 400. The heart was not included because of potential injury of the coronary arteries. The highest perfusion was found in the kidneys and the cerebellum while the blood flow in the anesthetized animals was low in the cerebral cortex. In rats it was also possible to assess the high flow in the thyroid gland.

Conclusion: This pilot study shows that the microsphere technique allows imaging and assessment of relative organ blood flow. This work will be continued to enable absolute blood flow measurements as well.