Techniques

Intra-arterial injection

When the tracer is given as a single rapid bolus injection (via the carotid artery), which is effectively instantaneous, there is a rapid maximum uptake on the first pass through the brain because of the large concentration gradient between the blood and the brain. Recirculation is minimal since a substantial proportion (95 per cent) of the radioactivity administered is excreted into the alveoli on its first passage through the lungs. The requirement for intracarotid puncture has severely limited the applicability of this technique and has led to the development of other means by which the isotope can be presented to the brain.

Intravenous administration

Xenon-133 (in physiological saline) is administered as a rapid intravenous injection. However, since 80 to 90 per cent of the administered activity is lost on its first passage through the lungs, a significantly greater initial dose is required. Exhaled air must be collected, either into activated charcoal or vented to the atmosphere, and the activity in the exhaled gas (as well as the clearance from the brain) must be measured (the end-expired activity is assumed to be in equilibrium with the arterial input to the cerebral circulation). This estimate of recirculating activity is important since, with this technique (as with the inhalation technique described below), the isotope is distributed not only to the brain but throughout the body. The cranial clearance is subjected to a computerized correction, which takes account of the recirculation of the xenon, and a single washout curve is constructed. As well as the potential for statistical noise in the convolution process, activity in the airways during the early part of the measurement can be greater than that in the brain and results in substantial artifact.

Inhalation technique

The subject breathes, or is ventilated, via a circle breathing system (into which xenon-133 (20 mBq/l) has been introduced) for 2 min. An adequate supply of oxygen is provided, and carbon dioxide is eliminated via soda lime. The circuit is then converted into an open breathing system and the exhaled gas is treated as described above. To minimize error due to the initially high activity in the airways, the first minute of the washout curve is rejected, or a decrease of 20 per cent from the initial maximum count is accepted, before the actual flow is estimated.

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