Radioaerosol Administration System

As the spinning disc (G) rotated, at the required speed [driving air pressure (H) monitored by a mercury manometer (C) was adjusted accordingly] to produce the desired size of particles, the radioaerosol solution was fed (I) on to the disc top for a period of three minutes. This spinning time achieved the maximum concentration of polystyrene particles in the tank (T). Increasing the spinning time would have increased the concentration of the particles but the tank would have become saturated with the solvent vapor preventing more particles from getting dry. Furthermore the loss of particles would increase by settling to the floor of the tank with a settling velocity of 0.7 mm/sec (Hatch & Gross, 1964; Lin & Goodwin, 1976).

Table 2.1.

Aerosol diameter as a function of air pressure driving spinning disc.

Driving Air Pressure (m m H g) DiametentSD ( p m ) 160 5.2 ± 0.4 163 5.1 ± 0 .5 165 5.0 ± 0.2 168 4.8 ± 0.4

[The size o f polystyrene particles was checked every 2 to 3 months by using a microscopic glass slide which was placed inside the tank. Each slide was divided into three columns and seven rows and five particles were assessed in each section. Therefore the size o f 105 particles were assessed and the coefficient o f variation was calculated (range between 3-12%)].

F ig . 2.2. Polystyrene particles of 5 |im in diameter as viewed by electron microscope.

Figures 2.3 and 2.4 show respectively a schematic diagram and an overall view of the radioaerosol generation and administration apparatus. The tank, which was placed in a fume cupboard to eliminate any escaped radiolabelled particles and solvent vapor, was made of aluminium and was approximately 56 cm in height, 46 cm in width and 46 cm in depth. A removable perspex window (L; 30 cm x 20 cm) in the front of the tank allowed access to the spinning disc generator for cleaning and maintenance. The window was kept in position by ten screws and a rubber seal round its edge. The operator was able to observe the performance of the disc and the formation of the particles through this window. The tank could be made airtight after producing the aerosol particles by closing the satellite exhaust (E). A water manometer (W) was connected to it to monitor the air tightness.

In order to administer the aerosols under strictly controlled manner a system of solenoid valves was connected to the tank (Thomson & Short, 1969). With valve B shut and valve A open radioaersosol particles could be inhaled through a mouthpiece (M); with A shut and B open fresh air could be inhaled via one-way valves N and O. The one-way valve (O) was connected to a Martindale-MP6 filter (F) to remove the expired aerosols. The dead space between the mouthpiece and the tank was of the order of 50 ml.

The volume of inhaled aerosol was controlled by a Krogh water spirometer (K) connected in series with the tank. This connection was provided with a tap (Q) to either connect the spirometer to the tank while inhaling the aerosols or room air for resetting the spirometer whilst the subject breathed fresh air. The spirometer was fitted with a pre-set microswitch to cut off inhalation through solenoid valve A at the required volume. This was followed by a breath-holding pause which was measured automatically by a timing mechanism which opened solenoid valve B three seconds after the closure of valve A. The purpose of this was to permit deposition by sedimentation of airborne particles particularly in the smaller peripheral airways.

E =0= W R -

n

MF

e

Fi g. 2.3. Schematic diagram of apparatus for generation and administration of radioaerosol particles. A and B = solenoid valves, C = mercury manometer, D = delay unit, E = satellite exhaust, F = filter, G = spinning disc, H = high pressure air supply, I = constant rate syringe injector, J = micromanometer, K = Krogh spirometer, L = perspex window, M = mouthpiece, N and O = one way valves, P = pneumotachygraph, Q = tap, R = ultraviolet light recorder, T = tank, W = water manometer.

F i g . 2 .4 . General view of radioaerosol generation and administration

apparatus.

The flow rate of the inhaled volume of radioaerosol particles was measured by means of a pneumotachygraph (P) interposed between the spirometer and the tank. The pneumotachygraph was composed of two conical shaped brass tubes joined at their widest end with a mesh resistance in the centre. As air flowed through the mesh a pressure difference was produced across it and transformed to an electrical signal by a micromanometer - Hilger I.R.D. (J) which was displayed together with volume on an ultraviolet light recorder - 1706 Visicorder (R).

Prior to the start of each test the system was checked and calibrated for the required volume of inhalation (450 ml). Air was drawn via the inhalation tube of the tank by means of a calibrated syringe and the necessary adjustment was made to the spirometer so that valve A closed at the required volume.

It was possible in every inhalation procedure, by using this radioaerosol administration system, to control and/or record almost all the factors that can affect the site of particle deposition within the human lungs as discussed in chapter one (1.2) and thus achieve similar deposition, at least within the same subject, on different study days.