Brain tumors tend to concentrate certain ions (charged atoms or molecules). When these ions are gamma-ray emitters, it is possible to take advantage of the penetrating power of their gamma rays to locate the tumor with a scanning device located outside the skull. Arsenic-74 and copper-64 are isotopes emitting _positrons_,[8] which have one peculiar property. Immediately after a positron is emitted from a nucleus it decays, producing two gamma rays that travel in exactly opposite directions. The scanning device has two detectors called scintillation counters, one mounted on each side of the patient’s head. The electrical circuitry in the scanner is such that only those gamma rays are counted that impinge simultaneously on both counters. This procedure eliminates most of the “noise”, or scattered and background radiation. Chromium-51 Because chromium, in the molecule sodium chromate, attaches itself to red blood cells, it is useful in several kinds of tests. The procedures are slightly complicated, but yield useful information. In one, a sample of the patient’s blood is withdrawn, stabilized with heparin (to prevent clotting) and incubated with a tracer of radioactive sodium chromate. Excess chromate that is not taken up by the cells is reduced and washed away. Then the radioactivity of the cells is measured, just before injection into the patient. After a suitable time to permit thorough mixing of the added material throughout the blood stream, a new blood sample is taken and its radioactivity is measured. The total volume of red blood cells then can be calculated by dividing the total radioactivity of the injected sample by the activity per milliliter of the second sample. [Illustration: _Spleen scans made with red blood cells, which had been altered by heat treatment and tagged with chromium-51. Such damaged cells are selectively removed by the spleen. A is a normal spleen. B shows an abscess in the spleen. Note dark ring of radioactivity surrounding the lighter area of decreased activity at the central portion of spleen._] In certain types of anemia the patient’s red blood cells die before completing the usual red-cell lifetime of about 120 days. To diagnose this, red cells are tagged with chromium-51 (⁵¹Cr) in the manner just described. Then some of them are injected back into the patient and an identical sample is injected into a compatible normal individual. If the tracer shows that the cells’ survival time is too short in both recipients to the same degree, the conclusion is that the red cells themselves must be abnormal. On the other hand, if the cell-survival time is normal in the normal individual and too short in the patient, the diagnosis is that the patient’s blood contains some substance that destroys the red cells.