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When determining the total body content of many elements by neutron activation analysis, irradiation and whole body counting by scanning along the length of the body has significant advantages. A neutron source of lower output and a whole body counter using smaller detectors and shielding (than otherwise needed) can provide a high, uniform response for many elements throughout the body. This was previously achieved by making the speed and direction of scanning identical in irradiation and counting (one-way scanning). A simple theoretical model for scanned irradiation and counting is described. The model is used to show that a scanning regime in which both scanning directions are used (two-way scanning) can provide acceptably uniform response for elements having induced activities of a wide range of half-life. This is supported by measurements made using an existing scanning facility for total body in vivo neutron activation analysis. The two-way scanning regime offers the advantages of increased patient comfort, simpler operation and more efficient use of the time available for counting induced activity.

Methods of assessing the uniformity of thermal and fast neutron fluence in relation to total body in vivo neutron activation analysis are described. Results are presented for 14 MeV neutrons emitted by sealed tube generators housed in a massive concrete shield, representing a substantial source of neutrons degraded in energy. Optimisation of conditions for patient irradiations is discussed and it is shown that acceptable uniformity of fluence can be achieved with little or no premoderation of the incident neutrons.

A system has been established for determining the total body contents of calcium, phosphorus, sodium, chlorine and nitrogen by neutron activation analysis using sealed-tube neutron generators and a shadow-shield whole-body counter. The system was calibrated using three anthropomorphic phantoms of different sizes, filled with various mixtures of the activatable elements in amounts similar to those in man. The reproducibility determined from replicate measurements, was on average +/- 2.9% (SD). The average uncertainty in absolute measurements was estimated to be +/- 4.2% (68% confidence interval).

This paper describes an in vivo method for measuring total thyroidal iodine stores by activation analysis, its evaluation and measurements in thyrotoxic patients. There was good correlation between measurements of solutions of iodine and post-mortem thyroids by activation analysis and chemical analysis. Measurements in thyrotoxic patients showed low levels in untreated and treated (antithyroid drugs) patients and a marked increase in patients studied whilst in clinical remission. The practical importance of this method of measurement of thyroidal iodine stores is that it is a reliable in vivo measurement obtained at a single visit and should enable the definition of the relationship of thyroidal iodine stores to pathophysiology and prognosis.

Factors influencing the precision of measuring changes in the calcium content of sections of bone, including vertebrae, using reactor neutrons for partial body in vivo activation analysis have been examined and quantified. These factors include (a) the choice of incidient neutron spectrum, (b) the uniformity of activation with depth in the body, (c) reproducibility, (d) interfering reactions, (e) sensitivity and (f) dosimetry. It is concluded that an overall precision of 2.8%-3.2% (coefficient of variation) can be attained with a neutron dose of 50 mrad or 3-4% with a dose as low as 20 mrad.

The development of a dual-detector shadow-shield whole-body counting system of high sensitivity is described. The sensitivity and performance of the counter are analysed, with specific references to total body in vivo neutron activation analysis, and shown to be at least comparable with those of many conventional shielded-room counters.