Calibration of non-invasive electronic kVp meters using an intrinsic germanium detector

Full text: The peak kilo-voltage (kVp) of a diagnostic x-ray generator is an important parameter that determines radiation quality and exposure to patients. Because this parameter has the potential to increase radiation exposure to the population, it is regularly monitored within the quality assurance programs. Non-invasive electronic kVp meters (like the NERO 6000 series) are extensively used to measure the peak voltage of equipment. Compliance testing protocols call for these instruments to be calibrated regularly. We have employed a spectroscopic technique using an intrinsic germanium detector to accurately determine x-ray spectral distribution. Energy resolution of 300 eV or better can be achieved at the diagnostic range. The method allows direct non-invasive measurement of the x-ray spectrum under real operating conditions of the x-ray generator. The end point of the spectral distribution corresponds to the peak kilovoltage. The detector system resolution introduces a small high energy tail to the measured spectrum. This tail does not introduce any error in the measured kVp greater than that of the system resolution at FWHM. The tail shape is a function of the wave form. The instantaneous spectrum taken at any point in the wave form cycle has a different tail shape, but the end point represents a mean kVp value. This kVp value is determined by extrapolating the high energy linear region down to the background level. This is one of the values the modern electronic kVp meters determine. The experimental set-up consists of a co-axial arrangement of an x-ray generator and a germanium detector with a series of collimators and Thoraeus filters (Trout ED et al, Amer J Roentgenol 85:933, 1961) in between. One of the collimators has a pin-hole aperture of a few tens of microns in diameter. This pin-hole aperture has to be accurately aligned with the focal spot of the generator. The aim of the extensive collimation is to reduce the photon flux by five to six orders of magnitude thus avoiding serious pulse pile-up in the detector system. The electronic kVp meter is suspended in air between the x-ray tube exit aperture and the first set of filters. The distance is determined from the meter specification. The chosen filter thickness is varied according to the energies at which the photons are generated. The filters also determine the signal to noise ratio of the spectral distribution. The germanium detector is coupled to standard nucleonics equipment and an MCA. The system is calibrated before any measurements are made. The technique is an extremely accurate, reliable, efficient and cost-effective way to calibrate the electronic kVp meters that are currently being used in Australia for quality assurance and compliance testing of diagnostic x-ray equipment