The National Radiological Protection Board (NRPB) has recommended the introduction of dose-width product (DWP) for the measurement of patient dose in panoramic dental radiology and has proposed a reference level of 65 mGy mm for adult exposures. This paper describes a method for measuring DWP and dose-area product (DAP) using thermoluminescent dosemeters (TLDs). The technique was used on 16 sets with a range of exposure settings. The mean value of DWP was 14% higher than the mean value reported from a survey by the NRPB. This difference is most likely to be caused by systematic variations due to measurement method. The average DAP for a standard adult examination was shown to be 11.3 cGy cm2. Data are presented so that the DAP can be derived from the exposure factors (tube current and operating potential) and beam area. Based on published data for effective dose, it is estimated that the DAP to effective dose conversion factor is approximately 0.06 mSv(Gy cm2)-1. The average DAP value (11.3 cGy cm2) can be compared with the average value for intraoral radiography (9.3 cGy cm2) based on the NRPB survey of entrance surface doses assuming 6 cm circular collimation.

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Rat thyroidectomy resulted in changes in a number of parameters used to characterise the mechanical and histochemical status of skeletal muscle. Thus thyroidectomy resulted in a prolongation of soleus slow-twitch muscle isometric contraction time and half-relaxation time with a reduced maximum velocity of shortening and maximum rate of development of tetanic tension but no significant change in twitch: tetanus ratio i.e. the ratio of twitch force/unit area to tetanic force/unit area. In addition the percentage of IIA fibres was reduced and the percentage of type I fibres increased. Triiodothyronine, administered to hypothyroid rats, brought about a speeding of these parameters again with no change in twitch: tetanus ratio. There was an increase in the percentage of IIA fibres with a concomitant reduction in the percentage of type I fibres. These changes were induced over 18 days and resulted in isotonic and isometric properties almost identical to those of soleus muscles from chronically hyperthyroid rats; speeding could be detected as early as 2 days after triiodothyronine had been given. Consideration is given to the possibility that changes in myosin isoforms and/or the kinetics of changes in intracellular calcium concentration in activation and relaxation could account for the time course of the observed changes.

SUMMARY1. Prolonged inactivity of soleus (slow twitch) and medial head of flexor digitorum longus (fast twitch: previously commonly known as flexor hallucis longus and hereafter referred to as FDL) muscles of the cat was produced by sodium pentobarbitone anaesthesia or spinal isolation. Isometric contraction characteristics were examined after 4-22 days and 8-49 days respectively.2. Sleep of up to 3 weeks' duration was associated with progressive changes in the weight, maximum tetanic tension, speed of contraction and absolute refractory period of fast twitch muscle. The slow muscle showed corresponding changes which were not so pronounced. Cord isolation resulted in similar changes which tended to be reversed after longer periods of disuse.3. Fast muscle frequently exhibited an after-contraction with corresponding electromyographic activity following two weeks or more of disuse.4. The time to peak of FDL was prolonged, becoming approximately one and a half times normal. In contrast to other reports of disuse, that of soleus was normal or slightly prolonged.5. The ratio of twitch to tetanic tension of FDL increased to twice the normal value or greater and remained high. The soleus muscle showed a smaller increase with evidence of a return towards normal after longer periods of inactivity.6. Absolute refractory period of FDL was considerably prolonged (more than 50 %) following barbiturate sleep and this also occurred, but to a lesser degree, with soleus after 4-7 weeks of cord isolation.

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