Impact of focal spot size on radiologic image quality: A visual grading analysis

Abstract 

Fine and broad focal spot sizes are available on general X-ray tubes. Excessive use of fine focus can impact on tube life and whilst it is established that fine focal spot size reduces geometric unsharpness, the extent of this benefit on clinical image quality is unclear.

The current cadaver-based work compares images produced with effective focal sizes of 0.8mm and 1.8mm. Four projection types were included, lateral ankle, antero–posterior (AP) knee, AP thoracic spine and horizontal beam lateral (HBL) lumbar spine, and a visual grading analysis was used to assess visibility of anatomical criteria. Five clinicians scored each image using a 1–4 scoring scale, a reference image was employed for standardization and a Mann–Whitney U statistical test compared results derived from each focus. Radiation doses were monitored using a dose area product (DAP) meter.

Statistical analyses demonstrated no significant differences between images produced at each focus, although a relationship between body part thickness and number of criteria with a higher (non-significant) score for the fine focus compared with the broad focal spot size was demonstrated. Choice of focus had no radiation dose implications.

Fine foci X-ray sources are used predominantly for extremity imaging to enhance visualization of fine detail such as trabecular patterns, yet there is no evidence to support this practice. The argument for regular employment of fine foci, particularly for the type of acquisition and display devices used in this study, needs to be revisited.

Keywords: Geometric unsharpness, Cadaver, X-ray projection