X-Ray Tubes Quality Control in the Optimization of Doses to Patients and Personnel in Conventional Radiology

The use of X-rays, for medical purposes contributes to the increase of human exposure to ionizing radiation. Maintaining human exposures in radiology, as low as reasonably achievable, taking social and economic factors into account, without altering the quality of the images is a challenge of radiation protection [1-5]. This desire to optimize doses to professionals and patients is dependent on human and material factors. The acquired physical facilities must allow the production of an image of sufficient quality to constantly show the appropriate diagnostic information at the lowest possible cost and with minimum irradiation of the patient and the staff. We report, through this work the failures of radiological facilities due to age and inadequate maintenance and their implications for the optimization of doses delivered in the radiology departments of the Abidjan University Hospitals.

We conducted a prospective cross-sectional descriptive study in the radiology departments of the three University hospitals in Abidjan, representing the highest reference level of the health pyramid in Ivory Coast. It took place over a period of eight weeks from 06/15/2011 to 08/15/2011. The survey covered all the conventional radiology equipment of the various University Hospitals. The devices in working condition used for the production of X-rays during the study period were selected. Facilities not used and those used for purposes other than conventional radiology at the time of the survey were not included in the study. We are interested in the quality control of the X-ray tubes of these different installations. The investigation concerned the periodicity of the checks, the evaluation of the technical characteristics of the X-ray tubes, and the quality of X-ray emitted. 

Measurements and controls were carried out by equipment supplied by the national public health laboratory. It was about RMI Beam alignment test tool model 162 A for alignment of the x-ray beam, RMI Collimator test tool model 161 B for the collimation of X-ray beams, DIAVOLT Universal type 43014 for linearity, accuracy and reproducibility of the X-ray tube, PTW-DensiX densitometer for homogeneity of the X-ray beam and Focal test tool model 162 A for focal spot size. 

Quality of X-ray beam includes the following elements:

Alignment of the X-ray beam is evaluated by the concordance between the light and the X-ray beam and by orthogonality of X-ray beam and the image receptor through the calculation of the angle between the central axis of the X-ray beam and the image receptor plane.

Automatic collimation of the X-ray beam measured by the deviation of the X-ray beam from the focus-image receptor distance, on any side of the image receiver.

Linearity of the X-ray tube evaluated by the coefficient of proportionality between the dose delivered by the X-ray radiation and the intensity of the current at the control console.

Accuracy of the X-ray tube appreciated, in the case of repeated measurements, by the deviation of the tube voltage and the actual exposure times from the displayed values.

Reproducibility of the X-ray tube evaluated by the constant tension of the tube, the dose of irradiation and the exposure time. Here, we measured the deviations of voltage, exposure time and irradiation dose from the mean for identical repeated exposures with a given filtration. The standard parameters used were: 80 kV, 250mAs, 50ms and 100cm focal-film distance.

Focal spot showed limits of detestability of high contrast anatomical structures. Using a magnifying glass, different images, obtained with standard parameters, were examined for groups and the number of bars per group. The parameters used were 60 kV, 3.6mAs, 60cm distance between focus and film and a light field of 34mm x 34mm.

Homogeneity of the X-ray beam evaluated by the blackening differences after measurement of the optical density at the four corners and at the center of the film.

The observations made during this work allowed us to identify the following particularities: Concerning the intrinsic performances of X-ray tubes, all posts were able to deliver a maximum voltage of 150 kV. The application of such high voltages leads to a reduction in the dose delivered to the patient. Indeed, the dose parameters to the patient, which are the dose at the entry and the dose-surface product, are reduced by 15 to 20% for a voltage increase of 115 to 140 kV [8-10]. Posts 1 and 2, due to insufficient additional filtration, each showed a total filtration of 1.5 mm aluminum. Manufacturers recommend a minimum total aluminum filtration of 2 mm or 2.5 mm copper for tube voltages of 60 to 120 kV. Posts 1 and 2 thus provided excessive irradiation to patients because optimal filtration allows a reduction of the dose at the patient's entrance and the dose-surface product by removal of the highly irradiating components of low energy [11,12]. All posts had small and large foci with dimensions of 0.6 to 1.2 mm, respectively. Their powers of resolution were satisfactory [9].

Machines surveyed in the course of our work carried the CE logotype certifying they had been subjected to the procedures for checking and verifying conformity of technical characteristics provided for in the directives of the European Union. Our investigation revealed an inadequate total filtration, insufficient for post 1 and 2 due to low additional filtration. It should be noted that at the time of the investigation, there was no formally identified state structure responsible for performing radiation protection, nuclear safety and security checks. Indeed, it was after the law of 10 October 2013, bringing safety and nuclear safety and radiation protection against ionizing radiation in Ivory Coast, that was created by decree, in June 2014, the authority for radiation protection, safety and nuclear safety authorities responsible for these nuclear regulatory and oversight missions [13]. 

Concerning the quality of the emitted radiation. The parameters studied were reproducibility, precision, alignment and linearity of the tube and homogeneity and collimation of the X-ray beam.

After 5 identical and successive exposures with the following acquisition parameters 80 kV, 250 mAs, 50 ms and 100 cm focal-film distance, we observed with posts over four years of age, an increase of 5.2 to 8.8% in measured voltages. Since the tolerance threshold set by the manufacturers was less than 5%, the reproducibility of the voltage of these tubes was not in conformity. An increase in scattered radiation, causing excessive irradiation, was responsible for this increase in voltage and the production of images whose contrast was deteriorated. This resulted in a significant number of reprints of pictures between 14 and 21%. A systematic check of the lead tube of the tubes was recommended. We noted also a non-significant dose variation with deviations of less than 1% between successive measured values. A difference of less than 5% being tolerated by the manufacturers, the reproducibility of doses delivered was acceptable [1,11,14]. Post 3 after 12 years of activity, showed a lack of accuracy of the voltage applied to the tube terminals with an average deviation of 5.3% of variation between the displayed tension and the measured voltage. The increase in scattered radiation was also the cause. This also led to deterioration in the contrast of the images generated, which resulted in the highest rate of image recovery. The other posts were considered accurate with variations less than 5%. Indeed, the 5% threshold between the value of the displayed and measured voltages is usually tolerated [9,10,12,14]. 

With a measured pause time, capped at 25 ms in spite of higher posted times, a variation of 75.4%, post 1 was considered imprecise. Indeed, the accuracy of the pause time is good, for manufacturers, when the variations between displayed and measured times remain less than 5% [1,6,7,9,10,12,14]. This observation, reported to the establishment managers, allowed for a successful maintenance mission. Indeed, the reprints of photographs were observed for the examinations requiring a long pause time.

In our series, post 3, the oldest, showed a lack of alignment between the light field and the radiation field. He was off-center. This finding is consistent with data from the literature. Indeed, there is a deaxation of the X-ray tubes over the years, due to a deposition of target material inside tube window and the rigidification of the tube [11,15,16]. On this post, excessive radiation was noted in relation to an increase in the surface dose product because a misalignment exposes to ionizing radiation a wider area than the region of interest [9].

We noted with, posts 2 and 4, aged over than four-year and with more than two years of activities, a linearity fault for intensity values between 160 and 250 mA. Indeed, the measured dose values were lower than the expected doses. There was no proportionality between current intensity and delivered dose [12,14,17]. This reduction in doses due to tube saturation by electronic flow limitation was responsible for poor quality images due to deterioration in resolution. The numerous induced recoveries eventually increased the dose of irradiation.

A uniformly distributed irradiation dose on the receiver defines a homogeneous beam. According to the manufacturers' recommendations, a variation in dose density between the 4 corners and the center of the film is tolerated when it remains less than 10% [12,14,18]. This in homogeneity responsible for uneven irradiation of the exposed surfaces, impacts the quality of X-ray images. We observed that post 2 with a dose variation of 10.2% did not meet standards. The use of locating cones has been proposed to solve this problem.

All posts had well collimated beams, indicating the diaphragms were functioning correctly. Adjusting the opening of the diaphragm to the anatomical limits of the region to be explored will eventually reduce the irradiated volume and hence the dose-surface product. Indeed, when passing from an irradiated surface of 36 × 43 Square centimeter (cm2) to 30 × 40 cm2, the dose-surface product decreases by 30% [8,9,12,14].

Totally, we observed through this work that the failures caused by the aging and insufficient or uncontrolled installations concerned voltage applied to the tubes in all cases. This is due to a lack of precision and reproducibility of tension responsible for a highly irradiating scattered radiation and negatively impacting the contrast of the rendered images. KV reproducibility fails beyond 2 years of activity and the failure increases with the duration of activity. The reprints representing 11 to 21% of the photographs. The quality of electrical installations and the recurring electricity supply cuts would be at stake.

Llinearity of the tube in two thirds of cases. There was a decrease in measured dose values compared to expected doses. This reduction in doses due to tube saturation was responsible for poor image quality due to deterioration in image resolution. The numerous induced recoveries eventually increased the dose of irradiation [12,14,17].

Alignment of the tube and the homogeneity of the X-ray beam, less frequently in one third of cases. Excessive irradiation was noted in relation to an exposed area wider than the region of interest and uneven irradiation of the exposed surfaces, further degrading the quality of the X-ray images and causing numerous occasions [9]. 

As for the post aged less than four years, the shortfalls concerned the pause time due to its cap at 25 ms. The focal spot was also damaged for the small focus with a partial limit of detectability of high contrast structures such as bone and lung.