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A differential multi-detector developed at Brookhaven National Laboratory locates brain tumors with positron-emitting isotopes. By using many pairs of detection crystals, the device shortens the scanning time and increases accuracy. (See cover for another type of positron scanner.)

Arsenic-74

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 In one, a sample of the patient’s blood is withdrawn, stabilized with heparin (to prevent clotting) and incubated with a tracer of radioactive sodium chromate.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.

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.

When chromium trichloride, CrCl₃, is used as the tagging agent, the chromium is bound almost exclusively to plasma proteins, rather than the red cells. Chromium-51 may thus be used for estimating the volume of plasma circulating in the heart and blood vessels. The same type of computation is carried on for red cells (after correction for a small amount of chromium taken up by the red blood cells). This procedure is easy to carry out because the radioactive chromium chloride is injected directly into a vein.

An ingenious automatic device has been devised for computing a patient’s total blood volume using the ⁵¹Cr measurement of the red blood cell volume as its basis. This determination of total blood volume is of course necessary in deciding whether blood or plasma transfusions are needed in cases involving bleeding, burns, or surgical shock. This ⁵¹Cr procedure was used during the Korean War to determine how much blood had been lost by wounded patients, and helped to save many, many lives.

For several years, iodine-131 has been used as a tracer in determining cardiac output, which is the rate of blood flow from the heart. It has appeared recently that red blood cells tagged with ⁵¹Cr are more satisfactory for this measurement than iodine-labeled albumin in the blood serum. It is obvious that the blood-flow rate is an extremely important physiological quantity, and a doctor must know it to treat either heart ailments or circulatory disturbances.

In contrast to the iodine-131 procedure, which requires that an artery be punctured and blood samples be removed regularly for measurement, chromium labeling merely requires that a radiation counter be mounted on the outside of the chest over the aorta (main artery leaving the heart). A sample of labeled red blood cells is introduced into a vein, and the recording device counts the radioactivity appearing in the aorta as a function of time. Eventually, of course, the counting rate (the number of radioactive disintegrations per second) levels off when the indicator sample has become mixed uniformly in the blood stream. From the shape of the curve on which the data are recorded during the measurements taken before that time, the operator calculates the heart output per second.

In this cardiac output study a probe is positioned over the heart and the passage of iodine-131 labeled human serum albumin through this area is recorded.

Obstetricians caring for expectant mothers use red cells tagged with ⁵¹Cr to find the exact location of the placenta. For example, in the condition known as placenta previa, the placenta—the organ within the uterus by which nourishment is transferred from the mother’s blood to that of the unborn child—may be placed in such a position that fatal bleeding can occur. A radiation-counting instrument placed over the lower abdomen gives information about the exact location of the placenta. If an abnormal situation exists, the attending physician is then alert and ready to cope with it. The advantages of chromium over iodine-131, which has also been used, are that smaller doses are required, and that there is no transfer of radioactivity to the fetal circulation.

Still another common measurement using ⁵¹Cr-labeled red blood cells is the determination of the amount and location of bleeding from the gastrointestinal tract (the stomach and bowels). The amount is found by simple measurement of chromium in the blood that appears in the stools. To find the location is slightly more complicated. The intestinal contents are sampled at different levels through an inserted tube, and the radiation of the samples determined separately.

Finally, gastrointestinal loss of protein can be measured with the aid of ⁵¹Cr-labeled blood serum. The serum is treated with CrCl₃ and then injected into a vein. In several very serious