ABSTRACT
The 6-hour half-life of technetium is ideal in that it is short enough to minimize retention of radioactivity but long enough to conduct clinical studies. A starting lung activity of 5-10 MBq is sufficient to acquire good quality images for several hours. This level of activity equates to an effective dose of less than 0.1 mSv. IMAGING MUCOCILIARY CLEARANCE USING A GAMMA CAMERA
This method views the lungs two-dimensionally via the use of static gamma camera^). An anterior or posterior view can be used but a geometric mean of both views is preferred so as to reduce the effect of attenuation by body tissues on the count rate. All images need to be corrected for background activity and decay over time. A typical acquisition time of 30 seconds would be used for the initial image.Once an image such as the one in Figure 1 has been obtained, the lungs can be identified and the activity within each lung noted. The lung activity on subsequent images can then be expressed as a percentage of the baseline image.As decay progresses it may be necessary to lengthen the acquisition period to acquire a good quality image. Some studies will require clearance to be measured over 24 hours. For these studies, data will need to be acquired after several half-lives have passed. Data can then be collected by placing the subject between two gamma cameras, or in a whole body counter, and activity in the thorax counted over a longer acquisition period with no image obtained.The quantitative data obtained will be in the form of a percentage of activity remaining in the lungs at different time points. Clearance over time can then be plotted and analyzed by describing the area under the curve or other indices, such as the time taken to clear 50% of activity or the percentage of activity remaining at a certain time point such as 24 hours (6,9,10). TOTAL VERSUS REGIONAL DEPOSITION AND CLEARANCE
For many objectives the measurement of total lung clearance is all that is required. However, when assessing the effects of inhaled active agents or disease on mucociliary clearance, it may be considered an advantage to have information on regional clearance as well as total. Currently there are two ways of achieving this. The first is by ascribing regions of interest to a gamma camera image to represent “ central” and “ peripheral” airways. The second is by the measurement of 24-hour retention/clearance of the radioaerosol and extrapolating that data to clearance from the ciliated airways as opposed to the alveolar compartment.
