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Extrafocal radiation was investigated using four different rotating-anode radiographic units currently employed in a pediatric hospital. It was found to exhibit significant penetration, with an effective energy only slightly lower than that of the primary radiation. The percentage of extrafocal relative to focal radiation in pediatric procedures involving rare-earth screens ranged from 2 to 15%. The ratio of extrafocal to focal radiation tended to increase with increasing kVp, mA, and field size but was independent of focal spot size. Since extrafocal radiation may extend significantly outside of the primary collimated field, radiosensitive organs such as the gonads, thyroid, and eyes which are assumed to be shielded may actually receive as much as 25 mR per radiograph.

The authors show that the intrinsic ultrasonic attenuation of a gallstone is an important factor in determining the degree of shadowing. B scans in vivo and in vitro demonstrated that stones with higher attenuation also show the most shadowing. In addition, attenuation can be correlated with physical structure: more highly attenuating stones tend to have the largest percentage of crystalline material, a larger average crystal size, and a more rigid structure. The authors recommend that higher-frequency transducers be used where possible if nonshadowing structures are seen in the gallbladder.

In three patients with skeletal infectious disease, scintigrams with a Tc-99m bone-seeker and In-111-tagged leukocytes, made within 24 hr of each other, showed striking similarities. In two cases, the findings from the In-111 WBC images were ultimately determined to be artefacts due to Tc-99m crosstalk within the 173-keV photopeak of In-111. In the second case, the error was traced to failure to use a pure In-111 source for energy calibration: the camera had been peaked on the radiation from the patient himself, who had had an earlier Tc-99m bone scan.