FACTORS AFFECTING X-RAY-SPECTRA

The x-ray spectrum is defined as the energy distribution of the radiat ion produced in an x-ray exposure. The x-ray spectrum has a major impa ct on image quality and radiation dose delivered to patients. The auth ors explored the effects of three key factors on x-ray spectra: genera tor type, peak tube potential, and filtration. Different generator typ es are characterized by the amount of ripple in the kilovoltage wavefo rm. Those with high (100%) ripple such as single-phase units produce l ess penetrating radiation than units with low (4%) ripple such as thre e-phase, 12-pulse generators. As peak tube potential increases, the ha lf-value layer increases nearly linearly; radiation output increases b y approximately the square of the tube potential. Filtration materials with atomic numbers less than 42, such as aluminum, titanium, copper, and niobium, produce similar spectra, with only slight variations in efficiency. Although aluminum has the lowest efficiency, this may be c ompensated for by increasing milliampere seconds. Filtration in additi on to the inherent filtration provided by the tube reduces both skin s urface dose and average depth-dose, with the optimal amount being appr oximately 2-3 mm or less of aluminum-equivalent material.