Figure 1. — Diagnostic X-ray machine used in 

 meniiaden radiography. 



interference is held to a mininiun by virtue 

 of the penetrating qualities of hard radia- 

 tion vjhich requires shorter time exposures 

 than are necessary when using soft-rays to 

 make radiographs of fish of conq^arable 

 sizes. Prolonged exposures at high kilo- 

 voltages would increase the likelihood of 

 interference from reflected radiation by 

 objects extraneous to the X-ray tube as 

 most materials will absorb and re-emit 

 radiation to some extent. Ordinarily, such 

 prolonged exposures are not necessary in 

 fish radiography, therefore, the effects of 

 interference are not of serious consequence. 

 Limitations of diagnostic units are similar 

 to those inherent in soft-ray machines, 

 viz., the vertebrae of small specimens are 

 difficult to resolve satisfactorily. For 

 large specimens the diagnostic unit is pre- 

 ferable since less time is required to 

 complete the filming process. 



The diagnostic machine assembled for 

 use by the !?enJiaden Investigations was 

 installed in a plywood cabinet, 2iij" X 59" 

 X lila", with controls situated at one end. 

 The X-ray tube was suspended 28g" from the 



top of the cabinet by means of metal frajne- 

 work welded to a l|" diameter pipe. A 2I4" 

 X 28" X 3/16" steel plate was fastened to 

 the top of the cabinet beneath the tube to 

 absorb most of the direct radiation. In 

 addition, a second steel plate, 2U" X 31^" 

 X 3/16", was mounted between the tube and 

 the controls to protect the operator from 

 scattered radiation. High voltage wires 

 leading from the transformer, inside the 

 cabinet, to the X-ray tube were enclosed in 

 l/U" conduit pipes as a safeguard against 

 arcing when the machine is in operation 

 (fig. 1). 



Initially, the machine was equipped 

 vjith a preset electronic timing device, 

 calibrated from one-half second to three 

 minutes, for precise exposure, but atmos- 

 pheric corrosion caused frequent failui^es 

 of the timer. It subsequently vjas replaced 

 by an open-type electric push button con- 

 trol. Exposure time was determined by 

 means of an electric exposure timer. 



X-RAY FIL'IS 



In general, standard fine grain X-ray 

 film, either 10" X 12" or lU" X 17", has 

 been found to be satisfactory for radio- 

 graphing menhaden specimens larger than 

 approximately 35 mm. In specimens which 

 had not undergone metamorphosis, grain size 

 of standard film was found to be too course 

 and overly sensitive to radiation to show 

 good contrast between incompletely ossified 

 vertebrae and comparatively dense surround- 

 ing body tissues. In metamorphosed speci- 

 mens, where ossification iiad been completed, 

 density differences were sufficiently great 

 for successful resolution. 



Kenliaden specimens smaller than 35 mm. 

 have been X-rayed successfully for vertebral 

 determinations by means of fine grain photo- 

 graphic film designed to produce sharply 

 contrasting black and wiiite prints. Expo- 

 sures made with Kcdalith film, for example, 

 indicate that acceptable radiographs may be 

 obtained of specimens ranging down to 21; mm. 

 fork length, providing metamorjjhosis is at 

 least partially com.pleted. Results of some 

 of these e:cposures are shown in figure 2. 

 The relation of e:cposure ti.Te to fish length 

 using Kodalith film is shown in figui^e 3. 



As mentioned earlier, the density of 

 vertebrae apparently varies greatly between 



