The determination of brain water content: microgravimetry versus drying-weighing method

The microgravimetric technique and the drying-weighing method for the determination of brain water content are analyzed and compared. A new method has been devised for the automatic production of the gradient column. For gravimetry, tissue samples weighing more than 30 mg have proven adequate for measurement. Specific gravity (SG) should be determined as early as 1 minute after tissue is inserted into the gradient column. Calculations of cerebral blood volume (CBV) from changes in SG of both brain tissue and intravascular perfusate have shown that the SG of brain tissue is considerably influenced by changes in CBV. This is because the SG of blood is higher than that of brain tissue, and may lead to a decrease of SG of about 0.002 in anemic cortex and of 0.001 in anemic white matter, which will simulate a false increase in tissue volume as water of 4% and 2%, respectively. This methodological error may be relevant when the early stages of ischemic brain edema development are studied. Water content of brain tissue can also be determined with acceptable accuracy by vacuum freeze-drying samples of brain tissue weighing about 100 mg. In contrast to cortex, white matter shows a wide range of individual and regional differences in water content. Thus, conclusions on the presence of brain edema drawn from tissue water determinations should always be subjected to cautious analysis and criticism.