Abstract
Off-focus radiation (OFR) has previously been described by many investigators [R. Thoraeus, Acta Radiologica 18, 753 (1937); L. Mallet and R. Maurin, Radiology 48, 628-632 (1947); J. F. Timmer, Medicamundi 19(2), 52-54 (1974); G. U. Rao, Appl. Radiol. 3(3), 45-49 (1974); R. Birch, Br. J. Radiol. 49, 951-955 (1976); W. W. Roeck, AAPM Symposium, 217-247 (1991)]. Off-focus radiation is frequently manifested on radiographs by the appearance of faint images of anatomical structures outside the collimated field of interest, i.e., soft tissues, ear lobes and hair, which we will refer to as "penumbral images." The observable small detail resolution within these penumbral images led to the assumption that minute sources of increased radiation intensity (secondary microfocal spots) must be present outside the area of the primary focal spot. The existence of these multiple secondary microfocal spots has been established and their locations within the area of the anode surface from which the general OFR originates was identified. The number, size, and distribution of the secondary focal spots vary over time and their magnitudes vary widely. The source of the well-focused electron beams creating the spots is attributed to the cold cathode emission principle.
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