This simple procedure has been used widely. One difficulty in using it is that its success is dependent upon the time interval between injection and measurement. An overactive gland both concentrates iodine rapidly and also discharges it back to the blood stream as PBI more rapidly than normal. Modifications of the test have been made to compare the amount of iodine-131 that was administered with the amount circulating in the blood as PBI. The system acquires chemical separation of the two forms of iodine from a sample of blood removed from a vein, followed by separate counting. This computation of the “conversion ratio” of radioactive plasma PBI to plasma-total ¹³¹I gives results that are less subject to misinterpretation. To determine local activity in small portions of the thyroid, an automatic scanner is used. A collimator[9] shields the detector (a Geiger-Müller tube or scintillating crystal) so that only those impulses originating within a very small area are accepted by the instrument. The detector is then moved back and forth slowly over the entire area and the radiation is automatically recorded at definite intervals, creating a “map” of the active area. In cases where lumps, or nodules, have been discovered in the thyroid, the map is quite helpful in distinguishing between cancerous and benign nodules. The former are almost always less radioactive than surrounding tissues. [Illustration: _Seven serial scans made with the whole body scanner were put together to provide a whole body scan of this patient with thyroid cancer that had spread to the lung. One millicurie of iodine-131 was administered and the scan made 72 hours later. Note the uptake in the lung. This patient was successfully treated with large doses of iodine-131._] Fragments of cancerous thyroid tissue may migrate to other parts of the body and grow there. These new cancers are known as metastatic cancers and are a signal of an advanced state of disease. In such a situation even complete surgical removal of the original cancer may not save the patient. If these metastases are capable of concentrating iodine (less than 10% of them are), they can be located by scanning the whole body in the manner that was just described. When a thyroid cancer is discovered, therefore, a doctor may look for metastases before deciding to operate. Human blood serum albumin labeled with ¹³¹I is used for measurement of the volume of circulating plasma. The procedure is quite similar to that used with radioactive chromium. Iodinated human serum albumin labeled with ¹³¹I is injected into a vein. Then, after allowing time for complete mixing of the sample with the blood, a second sample is counted using a scintillation counter.