Size-Specific Dose Estimates for Thoracic and Abdominal Computed Tomography Examinations at Two Moroccan Hospitals

Abstract

Size-specific dose estimates (SSDE) are the latest topic of interest in patient radiation–dose studies in computed tomography (CT). The aim of this study is to calculate and evaluate the doses (SSDE) by measuring the effective diameter (ED) of cross-sectional images collected during CT examinations of the chest and abdomen in Moroccan hospitals. Doses (SSDE) were calculated based on cross-sectional images by measuring the effective diameters of 75 patients in both examinations (45 for the thorax and 30 for the abdomen). Specific conversion factors for (ED) were used to convert the registered CTDI_vol to SSDE, according to the instruction in the American Association of Physicists (AAPM) Report 204. In thoracic CT, the CTDI_vol and SSDE values ranged from 5.8 to 10.7 mGy (mean: 8.08) and 9.55 to 15.37 mGy (mean: 12.13), respectively. For abdominal CT, CTDI_vol and SSDE values ranged from 4.8 to 12.2 mGy (mean: 7.95) and 8.01 to 14.15 mGy (mean: 11.31), respectively. The results show that the SSDE is a useful tool and could potentially educate CT operators on its effective use as a way to optimize radiation dose instead of CTDI_vol, in particular to establish diagnostic reference levels.

Introduction

In the last few decades, computed tomography (CT) has rapidly progressed and become an important part of the diagnostic arsenal in hospitals around the world [1]. According to the International Atomic Energy Agency (IAEA), computed tomography (CT) was used for about 25% of all radiological examinations and contributed about 60–70% of the total dose from radiological examinations [2]. Anam et al. have reported that the radiation doses associated with a CT scan are in the range of 1–15 mSv and that the effective doses from abdominal and thoracic CT scans were about 5–7 mSv [3].

In order to reduce the potential risks associated with these high doses, it is recommended to apply the principles of radiation protection listed by the International Commission on Radiological Protection (ICRP), including justification and optimization [4]. Justification is the fundamental principle of radiation protection, which consists in irradiating only if the information is really useful for the treatment of the patient [5–7]. If irradiation is required, the optimization consists in trying to irradiate as little as possible, by limiting the dose received while maintaining good image quality [8–11].

Nowadays, all new scanners are equipped with information on the volumetric computed tomography dose index (CTDIvol) and the dose-length product (DLP), displayed in the CT dose report image [12]. The CTDI_vol is measured using a polymethylmethacrylate (PMMA) phantom with a diameter of 32 cm to represent the patient’s body and 16 cm to represent the patient’s head and also a 100-mm pencil ionization chamber. While the DLP is obtained by multiplying the CTDI_vol and the scan length [13], the CTDI_vol and DLP are dependent on scan parameters such as tube voltage (kVp), tube current–time product (mAs), and pitch [14]. But they are independent of patient size and do not provide accurate dose estimates for each patient. Thus, the CTDI_vol measures the dose at the scanner output and not the dose received by the patient; this was demonstrated by several studies [15–17].

To rectify this discrepancy, the American Association of Physicists in Medicine (AAPM) has developed an appropriate tool for estimating the patient dose during a CT scan. In its reports 204 and 220, they have recommended a size-specific dose estimate (SSDE) as a tool for estimating patient dose [18, 19]. The AAPM Task Group has suggested the use of two conversion factors based on effective diameter (ED) by measuring lateral diameter (LAT) and anteroposterior diameter (AP) on the one hand and water equivalent diameter (Dw) on the other hand. Given that the SSDE takes into account body size, it has proven to be a useful indicator for estimating exposure dose by establishing diagnostic reference levels [20, 21]. The SSDE is calculated by multiplying the CTDI_vol by the size-dependent conversion factors introduced by the AAPM 204 report. These conversion factors were developed in the AAPM reports for body scans and for head scans [19, 22]. Four different methods were used to determine the conversion factors cited in Report 204; namely, anteroposterior dimension (AP), lateral dimension (LAT), sum of both dimensions (AP + LAT), and effective diameter (ED).

This work highlights the relationship between patient size, CTDI_vol and SSDEs of CT images of the abdomen and thorax undergone on two brands of CT scanners in Morocco. Thus, the objective is to calculate and evaluate the SSDE values based on the determination of the effective diameter and to update the radiation exposure for adult patients undergoing abdominal and thoracic CT examinations.

Materials and Methods

Patient data collection was performed while abdominal and thoracic CT examinations were being performed, because information on age, weight, and other parameters was not registered by the users while they were performing an examination. Furthermore, the measurements of SSDE values were performed on cross-sectional images of 75 adult patients who underwent both examinations on 2 scanners of different brands (Table 1). In addition, all scan acquisition parameters and patient information were extracted from the Picture Archiving and Communication System (PACS) (Table 2).

Table 1.

CT scanner characteristics per hospital

Hospital	Scanner model	Slice
H1	HITACHI Supria	16
H2	Philips Brilliance	16

Table 2.

Acquisition parameters applied in abdomen and chest exam

Exposure parameters	Chest exam	Abdomen exam
No of patients	45	30
Sex	28 M, 17 F	16 M, 14 F
Age (year)	32–82	25–67
Weight (kg)	50–98	41–85
Height (cm)	155–182	154–177
Tube voltage (KVp)	120	120
Product mAs (mAs)	120–191	87–275
Slice thickness (mm)	3.75	5
Rotation time (s)	0.70–0.75	0.70–0.75
Pitch	1–1.06	1–1.06
FOV	275–393	350–394
Collimation	1.25 × 16	1.25 × 16
	0.6 × 16	0.6 × 16

The effective diameter (ED) is based on the measurement of the anteroposterior (AP) and lateral (LAT) dimensions of the patient’s cross-section in the middle of the scanned region (Fig. 1). It represents the patient’s size as the diameter of a circle that has an area similar to that of the patient’s cross section with this relationship:

$$effectivediameter=\sqrt{\left(A,P,\times ,L,A,T\right)}(\mathrm{cm})$$ 1

Fig. 1.

Measurement of the patient’s diameter (AP + LAT)

Both LAT and AP diameters were measured at each examination (45 for the thorax and 30 for the abdomen) in a single transverse CT image.

After measuring the effective diameters, factors (f_size) were taken from the tables in AAPM Report 204 which, when multiplied by CTDI_vol, give SSDE, according to this equation [18]:

$$SSDE=f_{\mathrm{size}}\times {\mathit{CTDI}}_{\mathrm{vol}}(\mathrm{mGy})$$ 2

CTDI_vol values were obtained at each examination from the scanner console.

The results obtained from doses (SSDE and CTDI_vol) of the two examinations were evaluated as quantitative variables which are shown with arithmetic values, mean, and median. In addition, these results were compared with those of other countries in India and Saudi Arabia.

Results

Figures 2 and 3 show the relationship between SSDE values and patient size (LAT + AP) for the thoracic and abdominal examinations, respectively. At the thoracic CT examination, slightly significant correlations appeared between SSDE and patient size (R² = 0.54). In contrast, for the abdominal CT examination, strong correlations were observed between SSDE and patient size (R² = 0.69).

Fig. 2.

Relationship between body size (LAT + AP) and SSDE for thoracic examinations

Fig. 3.

Relationship between body size (LAT + AP) and SSDE for abdominal examinations

Several methods were used to estimate the SSDE, such as Monte Carlo measurements on simple cylindrical phantoms, Monte Carlo measurements on voxelized phantoms, physical measurements using anthropomorphic, and PMMA phantoms. The size-specific dose estimates (SSDE) is defined as a patient-dose estimate which take into consideration corrections based on the size of the patient, using dimension measured on patients’ images.

In this study, all 75 patients underwent CT scans without the use of contrast medium and under a tube voltage of (120 KVp). Among them, 45 adult patients underwent thoracic CT examinations (28 males and 17 females); mean age was 60 years; mean height was 169 cm; mean weight was 74 kg; mean AP + LAT was 49.33 cm. These 45 patients had a mean effective diameter (ED) of 24.35 cm with mean CTDI_vol doses of 8.08 mGy and mean SSDE of 12.13 mGy. Concerning the abdominal CT examinations, 30 patients (16 males and 14 females) had a mean age of 51 years, mean height of 164 cm, mean weight of 62.71 kg, mean effective diameters (ED) of 25.27 cm with mean CTDI_vol doses of 7.95 mGy and mean SSDE of 11.31 mGy. In addition, the other minimal and maximal values of CTDI_vol, as well as the (ED) and SSDE are presented in Table 3.

Table 3.

Minimal and maximal values of CTDIvol, SSDE as well as the (ED), and (LAT + AP) for chest and abdomen examinations

Examination	(LAT + AP)	Effective diameter	CTDIvol	SSDE
	Min–Max	Min–Max	Min–Max	Min–Max
Chest	43.35–55.84	21.57–27.73	5.8–10.7	9.55–15.37
Abdomen	43.06–64.25	21.43–31.42	4.8–12.2	8.01–14.15

Discussion

CTDI_vol is a useful tool for comparing output doses between different scanners because CTDI_vol depends directly on the acquisition parameters, but does not take into account the physical characteristics of the patient. This is significant because the dose received depends on both the patient size and the radiation output of the scanner. The SSDE is determined from the CTDI_vol using conversion factors that depend on the effective diameter of the patient.

In thoracic and abdominal examinations, the CTDI_vol increases with the increase of the patient’s diameter values. The increase in CTDI_vol can be explained by the activation of tube current modulation (TCM) in these two protocols. This is consistent with the results found by Gabusi et al. [23]. In effect, the scanners determine the patient’s attenuation and prescribe dosimetric modifications appropriate to the patient and the body region to meet the quality of the image required. In this technique (TCM), if the diameter decreases, the tube current decreases proportionally, which reduces the CTDI_vol. This is consistent with the findings of Anam et al. [24]. In contrast, with a non-(TCM) technique, CTDI_vol is constant with decreasing or increasing diameters because the tube current is constant and independent of patient diameter. Furthermore, increasing CTDI_vol results in an increase in SSDE in both CT examinations, which is consistent with the nature of the conversion factors provided by AAPM Report No. 204 [18].

In order to evaluate the results obtained from the measurements, a comparison of the median CTDI_vol and SSDE values was done with other results in India [25] and Kingdom of Saudi Arabia [26]. The evaluation of the median CTDI_vol and SSDE values in both examinations showed that they are lower than those of the other studies (Table 4). This discrepancy may be due to the CT parameter settings of different scanner brands. In addition, there are large variations in the mAs applied in scanning between these selected studies, and thus variations in the CTDI_vol and SSDE values also have a difference in the use of pitch from one scanner to another.

Table 4.

Comparison of median CTDI_vol and SSDE values obtained from this study with other studies

CT exam	CTDIvol (mGy)			SSDE (mGy)
	This study	India	KSA	This study	India	KSA
Chest	7.90	16.27	14.48	12.16	23.1	21.14
Abdomen	8.95	14.74	16.15	12.15	20.1	25.84

Therefore, the (ED) is not sufficient to determine the patient characteristics and is unable to distinguish regions of low or high attenuation [27]. Hence, to overcome this problem, a more appropriate metric is the water equivalent diameter (Dw) which was suggested to assess the patient size, taking into account the attenuation in the scanned region. According to Report 220, it was found that the difference between (Dw) and (ED) is less than 5% in the abdomen-pelvis region, so both can be used in this region. In contrast to the abdomen-pelvis region, using (ED) in the thoracic region instead of (Dw) will lead to an overestimation of tissue attenuation and an underestimation of SSDE of approximately 4.3 to 21.5% [19].

This study had several limitations. First, these measurements were performed at only two Moroccan hospitals. Future studies could consider including cranial examination, pediatric patients, and increasing the number of hospitals involved, and the number of patient samples. Second, the protocols used were from only two scanner models, whereas there are several brands and models of CT scanners installed in Morocco. Third, all the scanners involved in this study were only 16-slice. Finally, difficulty in traveling between hospitals for data collection, because it is necessary to collect on site important information about the patients who underwent CT scans, such as age and weight.

Conclusion

The size-specific dose estimate (SSDE) is a tool for estimating the average absorbed dose to the scan volume that takes into account the body size of the region being scanned (using the effective diameter ED) and the radiation output of the scanner (using CTDI_vol). In this study, the measurement of SSDE was based on the effective diameters of patients from cross-sectional images collected during thoracic and abdominal CT examinations in Moroccan hospitals. In these two examinations, the SSDE increases with patient size due to automatic modulation of the tube current. Thus, the SSDE is a useful way and could potentially give awareness to CT users on its effective use as a tool for optimizing radiation dose in computed tomography instead of CTDI_vol, especially for establishing diagnostic reference levels. It is thus essential that the Moroccan agency for nuclear and radiological safety and security (AMSSNuR) should establish in the future national DRLs using (SSDE).

Declarations

Ethical Approval

This study did not carry out activities that would require approval by a research ethics committee.

Informed Consent

This article does not contain any studies involving human subjects.

Conflict of Interest

The authors declare no competing interests.