Device for determining liquid volumes by measurement of the dilution of a radio-active solution

摘要

A liquid volume e. g. the blood volume of a human being, is measured by determining the dilution of a radio-active solution using a photodetector to measure the activities and thereby concentrations of samples before and after the dilution. In the determination of blood volume, the concentration of a radio-active marker in a solution is measured, the solution injected intravenously, and after a certain time, a blood sample taken and its concentration determined and compared with that of the original injection solution. Since the concentration of radio-active material is directly proportional to the volume it is contained in, the total volume of the vascular system into which the injection was made, may be determined. The invention seeks to provide for the great difference between the concentrations, and thus activities, of the blood sample and the original injection solution, but to avoid having to dilute the injection solution before determining its activity and thereby avoid errors due to radio-active decay before the injection can be carried out. The apparatus used comprises a movable tripod 1, carrying a housing 4 containing the detector system, a photo-multiplier 7 and a hollow, thallium-activated sodium iodide scintillation crystal 6. Lead shielding 10 is formed with a pedestal 14 in which a threaded column 12 turns freely on rotation by a wheel 15, causing a threaded nut 17 to move up or down the column. The nut is integral with a support plate 18 for a syringe 20, allowing the latter to be moved to the required distance from the detector system. The distance of the syringe support from the detector is adjusted so that the activity, measured by the detector, of a syringe full of the injection solution, is of the same order as that which will be recorded when a test tube containing a blood sample taken a certain time after the injection, is placed in the bore of the scintillator crystal 6. To obtain this distance a test tube of solution diluted to a known dilution, say 1 in a 1, 000, is placed in the crystal bore and the activity recorded. The same amount of the concentrated solution in a syringe is then positioned in the support plate 18 and the position changed until the same activity is recorded. A series of measurements may then be carried out without dilution of the injection solution beforehand, a first measurement of the injection solution being carried out with the syringe in the support plate and a second of a blood sample carried in a test tube in the crystal bore. The known ratio is taken into account when the activities are compared to calculate the blood volume.