1 Introduction

Computed Tomography (CT) was first discovered in 1972 by Sir Godfrey Hounsfield [1]. Since then, CT has developed into an extensively utilised diagnostic imaging technique. The majority of CT examination are sought for “routine” head and body applications, but the tests that are less common and need more technical expertise are the ones that garner the greatest attention for CT technology [2, 3]. Computed Tomography offers multi-slice helical imaging of a large volume of body tissue and, it is used for screening, therapeutic and a variety of diagnostic purposes [4]. It is widely acknowledged as a valuable diagnostic imaging modality that produces high-quality, detailed, cross-sectional images [5]. Computed Tomography examinations account for an estimated 44% of the global increase in radiation dose [5, 6]. While CT imaging offers valuable benefits, there is a growing concern that it is associated with an increased risk of biological effects and cancer due to increased radiation exposure. The CT Scanner manufacturers are therefore compelled by concerns about radiation risks to lower radiation doses [7].

The International Commission on Radiological Protection (ICRP) is an organisation that publishes rules and recommendations for the use of radiation globally. These recommendations have always been based on the latest scientific data and incorporates additional value judgments based on societal, economic, ethical and application experience. One such recommendation is that occupational exposure for ionising radiation should not result in an effective dose greater than 20 mSv annually, averaged over five year intervals (100 mSv in 5 years), and 50 mSv in any one year [8,9,10]. Patients are also considered for the risk of radiation. In 1905 gonadal shielding was introduced for patients and in 1907 shielding of testes was recommended for therapeutic and diagnostic X-ray exposure [11]. The general practice for all radiographic procedures was the use of Lead (Pb) shielding to reduce radiation dose to patients [11, 12]. However, these general practices changed when a positional statement was released by the American Association of Physicists in Medicine (AAPM). This statement indicated that the benefit of using gonad and foetal shielding are minimal to non-existent. When gonad and foetal shielding are positioned in the area of interest, they have the potential to obscure pathological findings or essential anatomical structures. In addition, if such shielding falls within the ionising chamber field during the use of automatic exposure controls, it may increase radiation exposure for the patient [13]. There is a scarcity of research that is devoted to investigating internal scatter and the use of Pb Shielding in CT, especially in the case of developing countries. Internal scatter as referred to in this paper is X-rays that propagates through the patient without interaction and/or X-rays that scatter within the patient through the Compton effect [4]. Therefore, investigating the impact of Pb Shielding on internal scatter is an important endeavour. The current study did so by evaluating internal scatter with and without Pb Shielding in an anthropomorphic phantom during CT of the brain, chest, abdomen, and pelvis.

2 Methods

This study used an anthropomorphic adult RANDO phantom [14, 15] and thermoluminescence detectors (TLDs), demonstrated in Figs. 1 and 2, to evaluate the internal scatter in the brain, chest, abdomen, and pelvis with Pb and without Pb Shielding. The RANDO phantom is made up of slabs/slices labelled 1 to 36. Selected slices were chosen for the brain, chest, abdomen, and pelvis in this study. The dimensions of the TLD chips were 4.5 mm x 0.24 mm and they were made of Lithium Fluoride doped with Magnesium and Titanium (LiF: Mg, Ti). Three TLDs were used for each measurement, with Pb and without Pb Shielding.

Fig. 1
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Anthropomorphic phantom with slabs/slices labelled for the study

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Fig. 2
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Brain TLD slots

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The CT scanner was a 128-channel multidetector-row General Electric Optima CT 660 and the Pb shield used was 0.35 mm Pb equivalent. Quarterly quality control tests as recommended by the South African Health Products Regulatory Authority (SAHPRA), which follow the Institute of Physics and Engineering in Medicine recommendations for routine performance testing of diagnostic X-ray imaging systems [16], were performed on both the CT scanner and Pb apron to ensure the integrity of both. Appendix A demonstrates the tests performed with the results. The scanning parameters used on the CT were pre-selected to ensure uniform exposures with Pb and without Pb Shielding and are given in Table 1.

Table 1 Exposure parameters used for various CT examinations
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The Pb shield was draped over the RANDO phantom, and did not fully surround the phantom during Pb Shielding measurements, as demonstrated in Fig. 3.

Fig. 3
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RANDO phantom with Pb Shielding for the study

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The exposed TLDs were read by the the Harshaw TLD Model 3500 Reader. The TLDs were then annealed using the Thermolyne 47,900 furnace, preparing them for the next examination. The calibration process followed in this study is explained in Appendix B. The collected data was analysed descriptively to determine the mean and standard deviation. The t-tests and two-way analysis of variance (ANOVA) tests were used to compare the means and Tukey post hoc analysis was performed.

Ethical conduct relates to avoiding error, promoting the aims of the research, prohibiting falsifying, fabricating, or misrepresenting research data, and promoting truth [20]. Ethical clearance for our study was granted.

3 Results

Measurements were undertaken three times. The mean TLD readings for the three slots in slices 4 (brain), 17 (chest), 24 (abdomen) and 30 (pelvis) with Pb and without Pb Shielding for the CT examinations of the brain, chest, abdomen and pelvis are presented in Table 2.

Table 2 Results of TLD measurements obtained with Pb and without Pb for various CT examinations
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Higher internal scatter absorbed doses were observed for measurements without Pb Shielding for all areas looked at in this study. This was attributed to scattered radiation generated in the head and gantry of the CT unit which reaches the exposed area from outside and can be reduced by Pb Shielding. Also observed was higher internal scatter for thicker areas of the phantom, this was attributed to the greater electron density of the larger volume of tissue resulting in an increase in the production of scatter, which has been well documented in the literature [4, 13, 19]. Length of the scatter area was another factor that influenced the internal scatter in our study. Internal scatter decreased further away from the exposed area, with the area adjacent to the exposed area receiving the largest internal scatter and the area furthest the least internal scatter. This was observed for internal scatter measurements both with Pb and without Pb. A similar observation was made by Brnic et al., which attributed this to the intensity of external scatter decreasing proportional to the square of the distance from its source [19]. For our study this was a statistical significant decrease of internal scatter absorbed dose for the CT examinations of the brain, chest, abdomen and pelvis.

The mean difference with Pb and without Pb by two way ANOVA in conjunction with Tukey’s test is given in Table 3.

Table 3 Differences with Pb and without Pb by ANOVA in conjunction with Tukey’s test
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The differences in the data concerning the use of Pb and without Pb followed a normal distribution in all statistical variables. There was a statistically non-significance for all CT examinations with Pb and without Pb. Our study therefore demonstrates that whether one uses Pb Shielding or not the internal scatter will be the same, and cannot be reduced with the use of Pb Shielding. This is in agreement with the AAPM positional statement on Pb Shielding [13].

4 Discussion

Exposure outside the area of the primary beam occurs from internal scatter. We demonstrated that using Pb Shielding in CT was statistically non-significant in minimising internal scatter. There was however a 2.0% reduction recorded in data with Pb Shielding, which was attributed to the absorption of scattered radiation from the head and gantry of the CT unit. Similarly, a study assessing the application of Pb Shielding for chest CT examination observed a reduction in the dose outside of the scan field, although the reduction was minimal [17]. Breast skin dose reductions were reported when Pb Shielding was employed for CT examinations of the brain, abdomen-pelvis, liver dynamic, lumbar spine, and neck. The reduction in breast skin dose with the use of Pb Shielding, due to its proximity to the exposed area and the absorption of scattered radiation from the head and gantry of the CT unit is widely recommended and reported in literature [19, 20].

This study demonstrated that internal scatter levels measured with Pb and without Pb Shielding was higher closest to the exposed area and decreased distant from it. Chung et al. [18] confirmed an inverse square law relation during CT when entrance surface doses were investigated. The entrance surface doses closest to the primary radiated area were greater than those in distant areas. The geometry of radiation is particularly important in determining the scatter exposure of an organ. It is essential to emphasise that internal scatter is the primary contributor to the radiation dose to radiosensitive tissues. The exposure to CT radiation from the primary beam is influenced by scatter radiation from the head and gantry, spreading to other secondary areas. The intensity of external scatter decreases as the distance from its source increases. Additionally, the larger the volume of tissue, the more internal scatter occurs. Most of the exposure outside the primary beam area comes from internal scatter. It is essential to note that external shielding does not reduce internal scatter. Internal scatter is a significant source of radiation exposure for sensitive tissues, particularly those deep within the body. Therefore, shielding is less effective when the scatter source is within a patient’s body.

The levels of internal scatter for the CT abdomen differ when Pb Shielding is applied, compared to when Pb shielding is absent. However, the abdomen contains a peritoneal cavity consisting of soft tissue organs, with only the spine providing a bony structure. Hsieh et al. [21] showed that the effective doses of kidney-ureter-bladder, intravenous urography and abdominal CT were 0.22, 1.51, and 8.21–9.27 mSv respectively where the CT yielded the highest effective dose. The chest, on the other hand, is filled with air due to the presence of lungs and has a thinner bony structure from the ribs. Therefore, there is minimal attenuation, and only a low radiation-absorbed dose is received. The brain has a small area of view and thinner cranial bones, requiring less dose for penetration.

A limitation of our study was the use anthropomorphic phantom to study internal scatter, as it does not fully replicate the radiation scatter that can occur in actual human bodies with organic tissues. The phantom also simulates an average human size, but skinny and obese patients may yield different radiation dose estimates. Another limitation is the use of only 0.35 mm equivalent Pb and no other thicknesses. However, the authors believe that the internal scatter reduction and no difference with Pb and without Pb Shielding will still be seen with actual human tissue and other Pb equivalent thicknesses, hence the use of the RANDO phantom and 0.35 mm equivalent Pb was acceptable.

5 Conclusion

This study wanted to investigate internal scatter in CT examinations using an anthropomorphic phantom. The study’s findings revealed that areas that were distant from the exposed area had lower internal scatter levels than those closer to it. The internal scatter levels throughout the CT examinations of the brain, chest, abdomen and pelvis exhibited no significant difference, regardless of whether Pb Shielding was used or not. This was in agreement with the AAPM positional statement. The internal scatter, was however dependent on the thickness of the RANDO phantom.