Assessment of Radiation Dose and Image Quality in Non-Contrast Head Computed Tomography: A Comparative Analysis Between 16-Slice and 32-Slice Multidetector Computed Tomography Scanners

INTRODUCTION

Computed tomography (CT) techniques have undergone significant expansion and technological advancements in recent decades, leading to their establishment as the preferred method for patient examination. In recent years, notable progress has been made in CT technology, transitioning from the initial single-slice “stop-and-shoot” method to the current volumetric CT method that employs multiple slices. The evolution of multi-detector computed tomography (MDCT) scanners has progressed rapidly, from single-slice to, most recently, 320-slice CT scanners.^[1,2] Compared to conventional single-slice CT scanners, multiple-slice CT scanners provide a variety of characteristics that methodically increase or decrease the patient’s radiation dose.^[3] The deleterious risk from exposure to ionising radiation needs to be carefully considered; this is particularly true in the head and neck region, which has comparatively poor soft tissue shielding with numerous radiosensitive organs.^[4]

Notably, CT is considered a high-dose imaging modality.^[5] Therefore, conducting a comparative analysis and quantifying the radiation exposures associated with various detectors is imperative. This will enable the identification of the most suitable detector configuration that minimises radiation exposure while simultaneously producing diagnostically valuable images. The relationship between the quality of images produced by CT scans and the amount of radiation dose administered is critical to consider.

Technical variables such as tube voltage (kVp), tube current (mA), pitch, slice thickness, field of view (FOV), tube rotation time, reconstruction algorithm, tube current modulation, scan range, and beam collimation have an impact on both image quality and radiation dose. The hardware of a system is also capable of influencing the parameters of image quality. This category encompasses gantries, X-ray tubes, detectors, and data acquisition systems.^[6,7] Moreover, the image exhibits a certain degree of noise because of the listed factors. The level of CT noise is primarily influenced by the quantity of X-rays that contribute to image formation. The radiation exposure from CT scanning is correlated with image noise. Noise typically decreases with an increase in radiation dose and vice versa. The CT number standard deviation can be used to quantify image noise. A higher standard deviation is suggestive of a higher level of noise.^[8,9]

All the above technical parameters affect image quality and radiation dose directly and indirectly. The assessment of parameters that influence image quality often includes the analysis of their effect on the signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), spatial resolution, artifacts, and noise of the image. Raising the SNR results in a corresponding improvement in the overall quality. However, a rise in the SNR has the potential to result in significant increases in the radiation dose delivered to the patient.^[10] CNR is a crucial metric utilised in assessing the quality of images. Greater CNR values are indicative of superior image quality, as they signify a more prominent differentiation between the object of interest and the noise in the background. Typically, increased radiation exposure may lead to a rise in the CNR.^[11]

Due to cost effectiveness, 16- and 32-slice MDCT scanners are still widely used in hospitals. Comparing the radiation dose and image quality (CNR, SNR) of 16-and 32-slice MDCT is crucial for choosing the safest and most efficient imaging option for various clinical situations. The aim of this study was to estimate and compare the radiation dose in terms of effective dose (ED) and image quality between a 16-slice and a 32-slice MDCT scanner from different imaging centers using image quality parameters, i.e., image noise, CNR, and SNR.

MATERIAL AND METHODS

The study was reviewed and approved by the CHARUSAT-Institutional Ethics Committee (IEC/CHARUSAT/EX/22/42). The present study utilised retrospective data obtained from two MDCT scanners, namely, the 16-slice MDCT Toshiba Activion manufactured by Toshiba Medical Systems in Tokyo, Japan, and the 32-slice MDCT Somatom Scope manufactured by Siemens Healthcare in Forchheim, Germany, which are in two multispecialty hospitals in Gujarat. The study was carried out over 6 months, spanning from November 2022 to May 2023.

Image quality assessment

The current investigation assesses the quality of images through quantitative assessments of parameters such as image noise, SNR, and CNR. Four ROIs, i.e., 15-20 mm² in size, were manually placed within the distinct regions of the bilateral anterior frontal lobe (white matter-WM) and bilateral thalamus (grey matter- GM) at axial slice number 45 [Figure 1]. To minimise interpersonal variability, all ROIs were drawn by a single trained individual. The mean Hounsfield unit (HU) and image noise (standard deviation of HU) were measured for each ROI. We calculated the CNR and SNR using the following formula.^[15-17]

$\text{CNR}=\left(\text{mean}\text{grey}\text{matter}\text{ROI}-\text{mean}\text{WM}\text{ROI}\right)/{[{\left(\text{SD}\text{of}\text{GM}\right)}^{\text{2}}+{\left(\text{SD}\text{of}\text{WM}\right)}^{\text{2}}]}^{\text{1}/\text{2}}$

$\text{SNR}=\text{mean}\text{HU}/\text{SD}\text{of}\text{ROI}$

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Figure 1:

Manually positioned regions of interest (ROIs) are denoted by colour circles, positioning in the white matter and grey matter; located at the bilateral anterior frontal lobe and bilateral thalamus for (a) 32-slice MDCT somatom scope, the ROIs are positioned on slice number 45, (b) 16-slice MDCT Toshiba Activion, the ROIs are placed on slice number 50, with a slice thickness of 5.0 mm. Red dots indicate ROIs in thalamus (grey matter).

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RESULTS

From November 2022 to May 2023, a retrospective analysis was conducted on the non-enhanced head CT of 172 patients (86 patients in each MDCT). There was significant variability at P value <0.001 in the levels of radiation exposure observed across 16-slice MDCT scanners and 32-slice MDCT scanners. The ED for 16-slice MDCT was 1.46 mSv, while the ED for 32-slice MDCT was 2.37 mSv [Table 3].

The results of the qualitative comparison of image quality suggested that the SNRs for both GM and WM were greater for 16-slice MDCT, with a significant level <0.001. The obtained results indicate that the GM had an SNR of 10.24±1.41 and 8.31±2.21 for 16- and 32-slice MDCT scanners, respectively. The mean SNR of WM was 9.77±1.30 for the 16-slice MDCT scanner and 7.92±2.18 for the 32-slice MDCT scanner. The analysis for CNR comparison between both scanners indicated that the 16-slice MDCT scanner had higher CNR values, i.e., 44.76±2.34, than did the 32-slice MDCT scanner, i.e., 32.26±2.81 [Table 4].

The correlation of quantitative data ED and average SD for 16-slice MDCT was not statistically significant (rho=0.095, p=0.386) [Table 5]. Likewise, for the 32-slice MDCT images, there was no statistically significant relationship between the ED and the mean SD (rho=0.146, p=0.179) [Table 6].

DISCUSSION

The results of this study significantly contribute to the understanding of dosimetric data comparisons between 16- and 32-MDCT scanners for non-enhanced CT of the brain. The results of the study indicate that the application of a 16-MDCT scanner leads to a notable decrease in radiation dose, with an ED of 1.46 mSv in contrast to the use of a 32-MDCT scanner (ED=2.37 mSv). A decrease in radiation exposure is a crucial factor in reducing the possible hazards linked with ionising radiation, particularly in the case of children and young adults who may exhibit greater susceptibility to radiation-induced side effects.

The present study supports prior research, which suggests that the use of MDCT scanners with lower slice counts is associated with decreased levels of radiation exposure. In a study, Al Ewaidat et al. (2018) reported that the 16-slice MDCT scanner exhibited a lower radiation dose compared to the 32- and 64-slice MDCT scanners across different anatomical regions.^[2] Mori et al. (2006)^[18], Arthurs et al. (2009)^[19], and Khan et al. (2011)^[12] suggested that using higher-slice scanners can lead to lower levels of radiation exposure during CT imaging.

Multiple factors that play an essential role in reducing the dose were observed in the 16-slice MDCT scanners in our study. With the progress made in MDCT technology, it has become possible to obtain larger scan ranges and increased tube currents. However, it is important to understand that this may lead to a potential increase in radiation dose. Recently, advancements in scanner technology have introduced several features that aid in reducing the potential risks associated with radiation exposure during medical imaging procedures. These features include automatic tube current modulation, which adjusts the amount of radiation emitted based on the patient’s size and shape, dose-reducing protocols that limit the amount of radiation used, and the ability to select lower tube potentials, which reduces the energy of the X-rays used. These features work together to effectively balance the potential risks and help reduce patient radiation exposure during medical imaging procedures.^[3,11]

Although the dose for 16-MDCT is lower than that for 32-MDCT, it should be noted that 16-MDCT offers superior image quality. The investigation assessed the improvement in image quality, which was proven by the increased values of the SNR and CNR in 16-MDCT images compared to those in 32-MDCT images. 16-MDCT had greater GM and WM SNR values than did 32-MDCT. The GM SNR was 10.24 for the 16-slice MDCT scanner and 8.31 for the 32-slice MDCT scanner.

The current study compares the image quality of two distinct MDCT scanners. Our study focused on comparing the image quality of 16-slice and 32-slice scanners with that of routine brain protocols, whereas previous research has mainly concentrated on investigating the impacts of iterative reconstruction (IR) algorithms, various scanning modes, and different mAs.^[20-27] On the other hand, some studies have conducted a wider range of comparisons and assessments.^[15,28,29]

The use of an IR technique in the 16-slice MDCT scanner used in our study may have played a crucial role in the observed reduction in radiation dose and potential enhancement in image quality. The studies conducted by Guziński et al., Korn et al., Harris et al., Love et al., Nakaura et al., Hsieh et al., and Ren et al. have consistently shown that the implementation of IR algorithms, such as adaptive statistical iterative reconstruction (ASiR), sinogram affirmed iterative reconstruction (SAFIRE), iterative model reconstruction (IMR), provides multiple advantages.^[20-26] These algorithms have been shown to effectively decrease radiation doses while maintaining image quality. Additionally, they have been demonstrated to enhance image quality metrics, including image noise, the SNR, the CNR, and subjective image quality. The present study had a few limitations of the study one being that it did not consider the potential impact of technical parameters. Research on the impact of different technical factors on quantitative image quality and radiation dose for unenhanced head CT might be pursued in the future. Moreover, the current investigation utilised a limited set of ROIs; a larger number of ROIs could enhance the understanding of the quantitative evaluation of brain morphology across diverse contexts. Additionally, the manual allocation of ROIs can induce bias and variations, which can have an unfavourable effect on the accuracy of the outcomes. The absence of matched image sets may pose a possible challenge in the analysis, as the reported variations in radiation dose and image quality could be influenced by cerebral morphology. The acquisition of paired sets of images from both 16-slice MDCT and 32-slice MDCT scanners would have facilitated a more accurate comparison between the two imaging modalities.

CONCLUSION

The present study showed that the 16-slice MDCT scanner provides a safer choice for non-enhanced head CT with lower radiation exposure and image quality when examined in terms of the SNR and CNR. The utilisation of reconstruction algorithms plays a crucial role in reducing the radiation dose and potentially enhancing image quality. These findings emphasise the importance of considering both patient safety and diagnostic accuracy when selecting a CT scanner configuration for brain imaging.