From the criteria in table 2, 64X-ray machines were selected of which 36 had a fixed orientation (fixed X-ray machines) and 28 had a mobile orientation. Out of the 64 machines, 26 machines failed the HVL test while 38 machines failed the normalized dose output test. A detailed analysis of each classification and discussion is in the next subsections.
3.1. Machines that failed the HVL test
Figure 1 shows the X-ray machines that failed the HVL test at 80 kV with variation of dose output. Out of the 26 machines, 14 have a mobile orientation while 12 were fixed. About 54% of the 26 machines produced dose output within an optimal range of 0.025 mGy/mAs – 0.080 mGy/mAs. An implication of failure of the HVL test implies that the HVL of the machine at any selected kV was below the recommended regulatory limit indicating inadequate filtration of the damaging soft X-rays. Since the patient receives an optimal dose even at HVLs below the recommended limit, complicates the appropriate action to be recommended for such machines. With a little modification in Edmond’s Eq. (13), there exists an inverse relationship between HVL and dose as shown in Eq. 2. Noting already that the kV accuracy test passed, the facility may consider adding additional filters of either aluminum or copper material to normalize the HVL. But this may lower the patient dose and contrast which affects the planned purpose of the imaging. For most machines, the variation of X-ray tube potential results in variation of the filtration automatically (14).
\(\:Dose=\:\frac{{kV}^{n}*mAs}{FFD}\left(\frac{1}{HVL}+0.114\right)\) [2]
A total of 7 X-ray machines (~ 31%) of the 26 machines produced doses below the lower limit of 0.025 mGy/mAs. This is rather more complicated because we would expect a high dose the fact that there is less filtration of the X-ray beam. In this case, the patients are likely to receive very low dose and hence the purpose of the diagnosis may not be achieved. This may result in cases of repeated exposures, and or misdiagnosis and hence patients may receive unjustified dose. As already discussed, the HVL reduces beam intensity and increase the average beam energy which is controlled by the tube potential/kVp variation (1). This has no effect on the number of photons in the spectrum that contribute to dose which is controlled by the mA/tube current modulation as shown in Eq. 2. Therefore, while HVL may affect the dose output, the dose has no effect on HVL.
Three X-ray machines (GG, RR & ZZ) produced doses greater than the upper limit of the optimal dose range. Figure 2 shows the variation of the HVLs at different set kVs for the 3 X-ray machines. The X-ray machine RR had the lowest HVL at each kV compared to other machines, though all the machines had HVLs less than the regulatory limit at each kV. The HVL dose relationship for the 3 machines does not comply with Eq. 2, otherwise RR should have the greatest dose. This is because other factors contributing to dose specific to generator type, manufacturer, and age. For example studies have shown that 1- and 2-pulse waveforms yield softer X ray spectra associated with increased dose to the patient (8). Note for this case, the patients are subject to high doses greater than the permissible limit, therefore, we recommend an immediate action on the machines not to be used for exposures till the appropriate action is done.
Figure 1: Abar graph showing the variation of HVL (unshaded bars) with dose output (shaded bars) for different X-ray machines that failed the HVL test. Note that this is considered at only 80 kV as in table 1.
For all the above three cases, the immediate recommendation is to add additional filter plates of aluminum or copper to increase the HVL thickness. However, this should be after an adequate investigation that checks on the accuracy of the tube current and timer for the case of machines with different mA and timer consoles. We note from Eq. 2, that the tube current has a direct impact on dose output. Other than the accuracy of the tube output, other tests for example reproducibility and power quality also define the quality of kVp (15, 16). In this study, all the 26 machines had a good kV reproducibility shown by a coefficient of variation of less than 10% at each kV setting. However, we could not assess the power quality of each machine which was a limitation of the study. Therefore, as we conclude on this case, the type of orientation of the X-ray machine may not necessarily affect the performance standard of HVL. However, from the general observation during routine inspections, the failure rate of HVL test was more in mobile than fixed X-ray machines. This might be due to;
-
Old aged mobile X-ray machines manufactured below 2012 with low HVL compared to our standard (3, 6, 16).
-
Mobile X-ray machines are meant be used for emergencies, but they are used for general radiography practices which results into overloading and hence faster deterioration of their performance (17, 18).
Most of such type of machines were imported into the country even before the establishment of regulatory standards. We also note that the current trend of machines being imported into the country generally have high HVL standards for adequate filtration.
Figure 2: The bar chart showing the variation of HVLs compared to the regulatory limit at different kVs.
3.2. Machines that passed the HVL test but failed the dose output test.
From table 2, its noted that all machines that had HVL above 2.9 mmAl at 80 kV passed the HVL test. The number of the X-ray machines in each class were 5, 19, 10, and 4 for moderate, high, very high and extreme high respectively. Figure 3 shows the X-ray machines for the different classifications above the regulatory limit and their variation of measured dose output.
Figure 3: A subplot of bar graphs showing the variation of dose for X-ray machines in different HVL classifications above the recommended 2.9 mmAl at 80 kV.
Generally, all the machines exhibited a phenomenon in Eq. 2, except X-ray machines AB in the moderate class and QQ in the high class. It’s clearly observable that as the HVL values increased from moderate to extreme high, the dose continuously lowered below the lower limit of the optimal dose range of 0.025 mGy/mAs. The high HVL values imply that there is excessive filtration of the X-ray spectrum which may lower the dose to the patient. This is important for dose minimization to the patient; however, this may affect the optimal dose required to achieve the purpose of imaging for proper diagnosis. It’s noted during the inspections that some machines with HVL values above the recommended limit, similar to this case produced doses within the recommended optimal range. The excessive filtration greatly reduces the low energy photons which significantly reduces the contrast. At low photon energies, the absorption properties of body tissues vary (19, 20). Therefore, the reduction in contrast may only be perceived at very high values of HVLs greater than the recommended regulatory values (13).
For these cases, we recommend checking for removable additional filters to minimize the filtration. However, this should be after an adequate investigation including checking the geometry set up, and the accuracy of the tube current output (3). For the modern trend of X-ray machines, the inverse relationship between dose and HVL may not directly apply. We can observe this from the recent modifications in the traditional Edmond’s equation, refer to (21, 22) for details. This is because of the improved technologies of image processing that ensure a better quality image even at minimal doses like digital radiography systems (23–25).
In a follow up exercise on the previous inspection findings for the machines AB and QQ, we established the following: (1) the fixed X-ray machine AB manufactured in 2013 had no past inspection history and (2) the mobile X-ray machine QQ manufactured in 1991 had been inspected about four times. Table 3 shows a summary of its performance results at set parameters of 80 kV, 20 mAs, and FFD of 100 cm.
We noticed the variation of the results of performance parameters during different inspection visits despite no actions of machine repair or calibration were recorded. We established that the facility had resorted to operating the machine using a generator since there was an unstable power supply. Previous studies (26, 27) have noted that the low stability of the power supply can affect the performance of the X-ray machine. Therefore, it's recommended to maintain a high power supply for any operating X-ray equipment to ensure the production of stable and constant X-rays with enough penetrating power. The cases of power instability have been reported to affect mostly mobile X-ray machines which use battery charging systems for power supply. Therefore, we always recommend assessing performance parameters for such machines when the batteries are fully charged.
3.3. The general trend of measured HVL.
Figure 4 shows the trend of HVLs at set kVs from 50 to 90 kV for each X-ray machine under study. Despite the discrepancies in the HVLs discussed above in sections 4.1 and 4.2, the trend follows a normal variation of the linear relationship of HVL increasing with an increase in tube potential (13, 28–30). During our routine inspections, we found out that most machines that failed the kV accuracy test always failed the HVL test. And consequently, the calibration of kV automatically affected the HVL values. Also (28) noted that the linear relationship of kV and HVL tends to diminish at high kV values. This was not observed in our study due to a limited range of kV values that were selected. But this is also attributed to the fact that most facilities in Uganda use a range of kVs from 50–90 kV for exposures, hence the study intended to investigate the performance at the commonly used machine settings for patient exposure.
Figure 4: The figure showing the variation of HVL at different set kVs for different X-ray machines.