Comparison of selected photon shield and organ-based tube current modulation for radiation dose reduction in head computed tomography: A phantom study

Abstract

OBJECTIVE:

The aim of this study is to investigate the radiation dose and image quality of head CT using SPS and OBTCM techniques.

METHODS:

Three anthropomorphic head phantoms (1-yr-old, 5-yr-old, and adult) were used. Images were acquired using four modes (Default protocol, OBTCM, SPS, and SPS+OBTCM). Absorbed dose to the lens, anterior brain (brain_A), and posterior brain (brain_P) was measured and compared. Image noise and CNR were assessed in the selected regions of interest (ROIs).

RESULTS:

Compared with that in the Default protocol, the absorbed dose to the lens reduced by up to 28.33%,71.38%, and 71.12% in OBTCM, SPS, and SPS+OBTCM, respectively. The noise level in OBTCM slightly (≤1.45HU) increased than that in Default protocol, and the SPS or SPS+OBTCM mode resulted in a quantitatively small increase (≤2.58HU) in three phantoms. There was no significant difference in CNR of different phantoms under varies scanning modes (p > 0.05).

CONCLUSIONS:

During head CT examinations, the SPS mode can reduce the radiation dose while maintaining image quality. SPS+OBTCM couldn’t further effectively reduce the absorbed dose to the lens for 1-yr and 5-yr-old phantoms. Thus, SPS mode in pediatric and SPS+OBTCM mode in adult are better than other modes, and should be used in clinical practice.

Keywords

Selected photon shield organ-based tube current modulation computed tomography (ct)lens

1 Introduction

Increased computed tomography (CT) examination in diagnostic radiology is a large contributor of the increased radiation dose to the public from medical exposures [1]. Repeated head CT scans can deliver the lens with a total dose exceeding the limit of public exposure(i.e., 15 mSv annually) [2]. However, the lens is a radiation-sensitive organ that is susceptible to radiation expose during head CT scans, which could cause diseases such as lens opacity and cataracts [3]. Based on over 3.2 million CT surveys, it found that nearly 500,000 cases underwent brain CT scans, of which 5.7% were for pediatrics and young adults (0–24 years old) [4]. Therefore, any method that can reduce the radiation dose of head CT scans is currently very valuable.

Traditional strategies to reduce CT radiation exposure in the clinical practice include iterative reconstruction [5], automatic tube current modulation based on CT attenuation (ACTM) [6], and automatic tube voltage modulation based on CT attenuation [7]. However, those methods were not intended for tissues and organs highly sensitive to radiation, and some technologies may produce higher radiation doses for radiosensitive organs [8]. The bismuth shields are an effective protection method for radiosensitive organs such as lens [9, 10], thyroid [11, 12], and breast [13, 14], during clinical CT examination; however, it relatively causes some image artifacts and may be difficult to integrate into a clinical routine due to the required patient cooperation, especially for children [15].

Organ-based Tube Current Modulation (OBTCM) is a technique used in CT imaging to optimize the amount of radiation delivered to different organs in the body. This technique uses data from the CT scanner to determine the thickness and density of the patient’s tissues and adjusts the tube current to optimize the amount of radiation delivered to each organ. It maily adjusts the X-ray tube current in front of the patient based on the angle of the X-ray tube to achieve the purpose of protecting radiosensitive organs from radiation exposure experienced during CT procedure [9 , 15–21]. Many studies [9 , 19] proved the effectiveness of OBTCM in reducing the radiation dose to the lens. Yamauchi-Kawaura et al. [19] reported a decreased radiation dose to the lens in head CT by 20% –30% using OBTCM, without compromising the image quality in the pediatric head phantoms. Wang et al. [9] described that the dose to the lens in the head CT examination reduced by 30.4% with OBTCM using an adult anthropomorphic phantom.

Moreover, newly developed selected photon shield (SPS) technique could filter low-energy rays to decrease radiation dose by a tin filter. The tin filter works by selectively filtering out low-energy X-rays that are not useful for image formation. These low-energy X-rays contribute to patient radiation dose but do not contribute much to the image quality. By filtering out these low-energy X-rays, the tin filter reduces patient radiation dose while improving the signal-to-noise ratio of the CT image [22]. The SPS has been demonstrated to have huge potential to reduce the radiation dose to chest [23, 24], abdominopelvic [25, 26], sinus/temporal bone [22 , 27–30], and dental [31] CT scan. To date, OBTCM can be used in conjunction with other techniques, such as iterative reconstruction and SPS technology, to further reduce patient radiation dose while maintaining high image quality. To the best of our knowledge, the application of SPS or the combination of the SPS and the OBTCM techniques to reduce the radiation dose to the lens in head CT examination has not yet been reported. This study aimed to compare the dose and noise level of the SPS and the OBTCM technique, and to explore the potential value in reducing the radiation dose to the lens in head CT examination.

2 Materials and methods

2.1 Anthropomorphic phantoms

This study was conducted using three anthropomorphic head phantoms (CIRS, Norfolk, VA, USA, models 704, 705 and 701), which simulated 1-yr-old, 5-yr-old, and adult patient, respectively. Each phantom age accounts for key variations in tissue composition due to age or sex and accounts for variations in bone mineral density. This results in a more precise physical model with which to investigate the interaction of radiation with different tissues. It allows for more accurate dose calculations in different age groups. These sectioned and drilled anthropomorphic phantoms can readily accept thermoluminescence dosimetry (TLD), which is a versatile tool for assessing absorbed doses from different organs. It should note that the selection of the socket position is based on the detailed information of the average position of the radiosensitive internal organs, which helps users to use the minimum number of detection devices for accurate calculation. Thus, in this study, 2 TLDs were placed in the eye for the three phantoms. 9, 10 and 11 pieces were placed in the brain for 1-yr-old, 5-yr-old, and adult phantom, respectively, as presented in Fig. 1. Moreover, only 1 TLD was used in one location. The whole dosimetry evaluation was divided into three areas, including the lens, the anterior brain (brain_A), and the posterior brain (brain_P), which are represented by red, green, and blue marks, respectively.

Fig. 1

Anteroposterior and lateral volume rendering images of different phantoms with pre-set TLDs. The red, green, and blue marks represent the locations of TLDs in the lens, the anterior brain (brain_A), and the posterior brain (brain_P), respectively.

2.2 Scanning protocols

All phantoms were scanned using a clinical 64-MDCT scanner (go. Top, Siemens Healthcare, Erlangen, Germany). A commercial OBTCM technique (XCARE, Siemens Healthcare, Erlangen, Germany) was implemented on the CT scanner. The OBTCM works by automatically adjusting the tube current during the scan based on the patient’s anatomy, such as the size and shape of the head. This technique allows tube current decrease at ∼75% during the scan over an anterior 120° arc of the head (for protecting the lens) and relatively increase by 25% over the remaining 240° during one rotation, to preserve image quality [32]. In comparison, SPS works by using a filter made of a tin alloy to selectively remove low-energy photons from the X-ray beam before it reaches the patient’s head. This reduces the radiation dose to sensitive organs, while still maintaining image quality for diagnostic purposes. The filter is placed in front of the X-ray tube and can be adjusted based on the patient’s age and size to ensure the appropriate level of radiation dose reduction [22]. The advantage of SPS is that it can provide targeted radiation dose reduction to sensitive organs, while OBTCM can provide more general dose reduction based on overall anatomy. However, both techniques have been shown to effectively reduce radiation dose in CT imaging while maintaining image quality.

Four scanning modes were performed using the Default protocol, the Default protocol with OBTCM (OBTCM), the SPS mode corresponding to Default protocol (e.g., 120 kV vs. Sn 120 kV), and the SPS mode with OBTCM (SPS+OBTCM) (Table 1). In the Siemens go. Top CT scanner, the default protocol for a non-contrast head CT for adults typically uses a tube voltage (kVp) of 120 and a tube current (mAs) of 230. For pediatric head CT scans, the kVp and mA settings may be adjusted based on the patient’s age and size to achieve the appropriate image quality while minimizing radiation dose. The scanner may also use pediatric-specific protocols, such as reducing the radiation dose in low-dose modes or using iterative reconstruction techniques to further reduce dose. In this study, the default protocol for pediatric uses 100 kVp tube voltage with 220 mAs tube current, and 110 kVp tube voltage with 220 mAs tube current for 1-yr-old and 5-yr-old phantom, respectively. Other scanning parameters for different protocols were consistent, and performed with spiral acquisition with CARE Dose 4D, filtered back-projection (FBP) reconstruction with SAFIRE (value = 3), rotation time = 1s, and pitch = 0.55, positioning line: listen to the angle line (OM).

Table 1
The imaging parameters of different scanning modes in the three anthropomorphic head phantoms

Phantoms Scanning Mode Tube Voltage (kV) Care Dose 4D (Ref.mAs) OBTCM mode CTDI_vol(mGy)

1-yr-old Default protocol 100 220 off 27.47

OBTCM 100 on 27.04

SPS Sn100 off 9.7

SPS+OBTCM Sn100 on 9.6

5-yr-old Default protocol 110 220 off 30.5

OBTCM 110 on 30.5

SPS Sn110 off 16.21

SPS+OBTCM Sn110 on 16.07

Adult Default protocol 120 230 off 42.29

OBTCM 120 on 41.61

SPS Sn120 off 21.8

SPS+OBTCM Sn120 on 22.16

Phantoms	Scanning Mode	Tube Voltage (kV)	Care Dose 4D (Ref.mAs)	OBTCM mode	CTDI_vol(mGy)
1-yr-old	Default protocol	100	220	off	27.47
	OBTCM	100		on	27.04
	SPS	Sn100		off	9.7
	SPS+OBTCM	Sn100		on	9.6
5-yr-old	Default protocol	110	220	off	30.5
	OBTCM	110		on	30.5
	SPS	Sn110		off	16.21
	SPS+OBTCM	Sn110		on	16.07
Adult	Default protocol	120	230	off	42.29
	OBTCM	120		on	41.61
	SPS	Sn120		off	21.8
	SPS+OBTCM	Sn120		on	22.16

2.3 Dose measurement

In this study, TLDs (LiF: Mg, Cu, P) were preset according to the position demonstrated in Fig. 1. To evaluate the variability and reproducibility, the TLDs used in this study were in the same batch (dispersion is 3%, coefficient of variation is 2%), and all of those were annealed for 10 min at 240° before radiation exposure. For each phantom, we first put the annealed TLDs into the position of the corresponding organ and tissue in the phantom and then assemble it; after scanning the phantom, TLDs were removed, numbered, and replaced with a new TLD for the subsequent scanning protocol. Since each phantom needs to be performed with four different scanning protocols, a total of 144 TLDs were numbered and measured within 2 days after radiation exposure, and all the TLDs were read in a Thermo Scientific Harshaw 3500 TLD^® reader (Thermo Fisher Scientific Inc., Reading, UK). The readout TLD values in nanocoulombs were multiplied by an individual calibration factor, which was carried out in the standard radiation field with reference energies of 65, 83 and 662 keV, respectively. The tested TLD was placed in the radiation field to obtain three calibration factors of radiation energy. The mean value of the three calibration factors was taken as the calibration factor of 0.00121 mSv/nC. The mean value of the measured dose in the bilateral lens area was defined as the absorbed dose to the lens, and the same definitions are applicable for brain_A and brain_P.

2.4 Image quality evaluation

In this study, image noise and contrast noise ration (CNR) calculated in both the orbital and intracranial regions was used to evaluate the impact of different dose reduction strategies on image quality. As shown in Fig. 2, the peri-orbital layer was first selected, nine ROIs were then included in the study, two of which were in the left and right orbitals, one was in the air space outside the anterior forehead, and the remaining six were in the intracranial. It should note that relatively homogenous regions around the reference TLD based on its location were selected, so as to better represent the noise level in that area. The CT numbers in ROIs of ∼30 mm² were recorded among different scanning modes. The SD value of CT value in each ROI was recorded as noise level. All ROIs were measured repeatedly on five consecutive images along z-axis direction to avoid selection bias. CNR was calculated as follows: CNR = (CT_ROI-CT_air)/SD_air. The average value of multiple ROI noise and CNR values in the orbital region and brain parenchyma were taken as the noise level and CNR of the corresponding region. Statistical analysis was performed using GraphPad Prism software (v8.0, GraphPad Software, LaJoIIA, USA). The average noise level and CNR within each group were compared among the four scanning modes. Statistical analysis of noise level and CNR were performed between various scan modes by analysis of variance with Tukey’s Multiple Comparison Test. A P-value of < 0.05 was considered statistically significant.

Fig. 2

ROI placement for image noise measurement in different scanning modes for head phantom.

3 Results

3.1 Dose evaluation

The head CT examinations based on Default protocol revealed that the average absorbed dose to the lens ranges from 46.54 mGy for the 1-yr-old phantom to 58.39 mGy for the adult phantom (Fig. 3A). The corresponding absorbed doses in OBTCM mode range from 34.60 to 42.01 mGy, resulting in 21.89% and 28.33% reduction, respectively (Fig. 3B). Whereas, the SPS mode depicted a lower dose level that ranges from 13.32 to 27.26 mGy, exhibiting 71.38% and 53.30% reduction, respectively. The SPS+OBTCM mode exhibited nearly same dose levels (ranged from 13.43 to 21.35 mGy), with 71.12% and 55.71% dose reduction, respectively. Specifically, for 5-yr-old and adult phantoms, SPS+OBTCM mode led to a further dose reduction of lens compared with SPS mode, with the decreasing from 22.30 to 21.35 mGy and 27.26 to 20.68 mGy, respectively. However, in 1-yr-old phantom, the radiation dose increased slightly from 13.32 to 13.43 mGy.

Fig. 3

The absorbed dose to the lens and brain (brain_A and brain_P) and percentage dose changes associated with different modes. ^*Dose increase rate than that of Default protocol.

The dose evaluation of the brain_A shows a similar dose change trending with a lens between different scanning modes. In the Default protocol, the average absorbed dose to the brain_A ranges from 38.71 mGy for 1-yr-old to 46.61 mGy for adult phantom (Fig. 3C). The corresponding absorbed doses in the OBTCM mode range from 36.06 to 45.44 mGy, resulting in a slight dose reduction ranging from 6.69% to 19.69%, respectively (Fig. 3D). Again, the SPS mode depicts a lower dose level ranging from 12.72 to 25.95 mGy, exhibiting 67.09% and 43.54% reduction, respectively. The absorbed dose in SPS+OBTCM mode also indicated at same levels (ranged from 12.23 to 23.79 mGy), with 68.53% and 48.20% dose reduction, respectively.

The dose evaluation of the brain_P revealed that the average dose to Default protocol ranges from 34.28 mGy for 1-yr-old to 41.95 mGy for adult phantom (Fig. 3E). Whereas, the OBTCM mode shows a slightly higher dose ranging from 36.55 to 44.50 mGy, resulting in a slight dose increase (from 0.75% to 6.73%) (Fig. 3F). Contrarily, the dose in the SPS mode ranges from 12.43 to 20.9 mGy, resulting in a higher dose reduction from 41.56% to 63.60%. Similar results with 13.07–23.61 mGy absorbed dose and 41.74% –61.8% dose reduction can also be observed in the SPS+OBTCM mode.

3.2 Image quality evaluation

Both dose reduction strategies for different phantoms will increase image noise in the head CT scan relative to that on the Default protocol (Table 2); however, the absolute value of the difference with Default protocol was not larger than 2.58 Hounsfield units (HU). As for the CNR comparison, there was no significant difference in CNR of different phantoms under varies scanning modes (p > 0.05) (Table 3).

Table 2
Image noise for the two ROI groups was determined using four scanning modes

ROI Mean Image Noise of Default protocol (HU) OBTCM SPS SPS+OBTCM

Mean Image Noise (HU) Change From Default protocol (HU) Mean Image Noise (HU) Change From Default protocol (HU) Mean Image Noise (HU) Change From Default protocol (HU)

Orbit

1-yr-old 3.28±0.11 3.82±0.20 0.54±0.08 5.07±0.07^ab 1.79±0.04 5.05±0.07^ab 1.77±0.44

5-yr-old 3.12±0.54 4.57±0.01^a 1.45±0.55 5.7±0.51^ab 2.58±0.03 5.58±0.48^ab 2.46±0.06

Adult 4.66±0.20 5.05±0.44 0.39±0.24 5.62±0.14^a 0.96±0.06 5.63±0.10^a 0.97±0.30

Brain

1-yr-old 3.65±0.10 4.59±0.37^a 0.94±0.37 5.57±0.22^ab 1.92±0.19 6.00±0.35^ab 2.35±0.29

5-yr-old 3.75±0.14 4.65±0.21^a 0.89±0.27 5.08±0.25^ab 1.33±0.21 5.10±0.23^ab 1.35±0.32

Adult 5.01±0.32 5.44±0.17 0.43±0.45 5.51±0.30 0.51±0.48 5.88±0.46^a 0.87±0.61

ROI	Mean Image Noise of Default protocol (HU)	OBTCM	SPS	SPS+OBTCM
Orbit
1-yr-old	3.28±0.11	3.82±0.20	0.54±0.08	5.07±0.07^ab	1.79±0.04	5.05±0.07^ab	1.77±0.44
5-yr-old	3.12±0.54	4.57±0.01^a	1.45±0.55	5.7±0.51^ab	2.58±0.03	5.58±0.48^ab	2.46±0.06
Adult	4.66±0.20	5.05±0.44	0.39±0.24	5.62±0.14^a	0.96±0.06	5.63±0.10^a	0.97±0.30
Brain
1-yr-old	3.65±0.10	4.59±0.37^a	0.94±0.37	5.57±0.22^ab	1.92±0.19	6.00±0.35^ab	2.35±0.29
5-yr-old	3.75±0.14	4.65±0.21^a	0.89±0.27	5.08±0.25^ab	1.33±0.21	5.10±0.23^ab	1.35±0.32
Adult	5.01±0.32	5.44±0.17	0.43±0.45	5.51±0.30	0.51±0.48	5.88±0.46^a	0.87±0.61

^aStatistically significant difference with the value from the Default protocol. ^bStatistically significant difference with the value from the OBTCM mode. Note. Data is shown as means±standard deviation; SPS = Selected photon shield; OBTCM = Organ-based tube current modulation; HU = Hounsfield unit.

Table 3

The CNR for the two ROI groups was determined using four scanning modes

ROI	Default protocol	OBTCM	SPS	SPS+OBTCM	F	p
Orbit
1-yr-old	13.17±0.45	13.11±0.52	12.93±0.21	12.89±0.28	1.30	0.29
5-yr-old	13.24±0.22	13.23±0.22	13.09±0.59	13.07±0.58	0.38	0.76
Adult	13.18±0.33	13.05±0.57	12.97±0.58	12.83±0.37	0.94	0.43
Brain
1-yr-old	13.49±0.52	13.37±0.51	13.27±0.22	13.25±0.21	2.26	0.09
5-yr-old	13.58±0.21	13.53±0.23	13.52±0.53	13.41±0.54	0.91	0.44
Adult	13.38±0.46	13.36±0.39	13.28±0.77	13.13±0.72	1.08	0.36

Note. Data is shown as means±standard deviation; SPS = Selected photon shield; OBTCM = Organ-based tube current modulation; HU = Hounsfield units.

The noise evaluation of orbit revealed that the noise level of the Default protocol varies between 3.12 HU and 4.66 HU. Using OBTCM based on Default protocol, it varies between 3.82 HU and 5.44 HU; however, it only shows a statistically significant increase (1.45 HU, p < 0.0001) in the 5-yr-old phantom compared with the Default protocol. Similarly, the noise level of the SPS mode (range: 5.07 ∼ 5.70 HU) or the SPS+OBTCM mode (range: 4.91 ∼ 5.58 HU) shows a significant image noise increase (0.96 ∼ 2.58 HU, p < 0.0001) compared with the Default protocol for all phantoms. Statistically significant noise increase (range: 0.97 ∼ 2.46 HU, p = 0.0004 ∼ 0.0041, p < 0.05) can also be observed for 1-yr-old and 5-yr-old phantoms in the SPS+OBTCM mode.

The scope of the noise level in the brain was 3.65∼5.01 HU in the Default protocol. Those of the OBTCM mode for 1-yr-old and 5-yr-old phantoms (4.59 and 4.65 HU, respectively) show a slight increase (0.94 and 0.89 HU, respectively, p < 0.0001) compared with the Default protocol. Similarly, the noise level of the SPS mode (range:5.08 ∼ 5.57 HU) or the SPS+OBTCM mode (range: 5.10 ∼ 6.00 HU) also exhibits a significant image noise increase (0.87 ∼ 2.35 HU, p < 0.0001) compared with the Default protocol for all phantoms, except for the adult phantom in the SPS mode. Meanwhile, a similar statistical noise increase was found for 1-yr-old and 5-yr-old phantoms compared to the SPS or SPS+OBTCM and OBTCM modes (p < 0.0001).

4 Discussion

To the best of our knowledge, this is the first study that aimed to compare the effect of the SPS and the OBTCM techniques, and the combination of these two techniques on the dose and noise level in head CT scan. Results suggested that SPS reduces the radiation dose to the whole image plane with relatively small increase in noise (≤2.58 HU) and up to 71.38% dose reduction to the lens. Based on the SPS technique, OBTCM couldn’t effectively reduce the absorbed dose to the lens for 1-yr and 5-yr-old phantoms.

OBTCM has been clinically used in head CT to reduce the lens dose. Specifically, 28.07% dose reduction to the lens for the adult phantom agrees with the previous studies [9, 19]; however, few studies have reported the effect of the OBTCM technique in reducing the radiation dose to the lens in the pediatric population. Markart et al. [33] showed that the combination of OBTCM and automatic tube voltage modulation could reduce 32% absorbed dose to lens for pediatric. Yamauchi-Kawaura et al. [19] demonstrated a 20% –28% dose reduction to the lens of pediatric using OBTCM, which agreed with our study results. Whereas, Papadakis et al. [18] found only up to 14% dose reduction (Monte Carlo for dose estimation) for the lens in pediatric (neonate, 1-yr-old, 5-yr-old, and 10-yr-old anthropomorphic phantom) using another commercial OBTCM technique (ODM, GE Medical Systems). The lower dose reduction noted for the lens may be the result of the smaller reduction of the tube current in the head. Previous studies showed that only 33% of the tube current reduction can be observed using ODM, whereas ∼75% of the tube current can be reduced when using OBTCM (XCARE) [34].

SPS reduces radiation dose by filtering low-energy X rays, and has traditionally been used in high-contrast or hard tissue imaging, with a large number of studies focusing on head and neck CT imaging, including temporal bone or paranasal sinuses [22 , 35] and dental [31]. For example, prospective CT studies of the paranasal sinus and temporal bone using SPS demonstrated 67% ∼85% reduction than that of conventional protocol (Sn100 kV with 120 kV; Sn100 kV with 100 kV; Sn150 kV with 120 kV) in delivered radiation doses to patients while preserving the image quality [22 , 30]. Sn100 kV with iterative reconstruction in a third-generation dual-source CT can also be achieved at a low effective dose of 0.03 mSv for paranasal sinuses, which is comparable with that of conventional radiography [28]. Petritsch et al. [27] showed that Sn100 kV of the paranasal sinus, with an iterative reconstruction algorithm, can provide reliable exclusion of suspected acute inflammatory sinus disease in 100% of cases at an ultra-low effective dose of 0.012 mSv per scan. Ha et al. [35] found that Sn100 kV showed a 50% dose reduction than 120 kV scanning in patients with craniofacial trauma, with sufficient diagnostic image quality. Hackenbroch et al. [31] demonstrated that Sn100 kV can result in a 96% dose reduction in dental CT examinations without sacrificing the image quality compared with standard low-dose examinations(100/120 kV);however, these studies did not evaluate the effect on the radiation dose to radiosensitive organs, such as lens in the head and neck CT examination.

Recently, some studies focused on the possibility of SPS in low-contrast or soft-tissue imaging. Such as, Silkwood et al. [36] demonstrated that adaptive filter could improves image quality in photon counting spectral (PCS) breast CT by decreasing beam hardening artifacts and by eliminating spatial non uniformities of CT numbers, noise, and CNR. Kimura et al. [37] showed that Sn100 kV could substantially reduce the radiation dose by 89% (1.93 vs 17.9 mSv) compared to standard protocol (120 kV) while maintaining good diagnostic performance in colorectal cancer patients. Leyendecker et al. [38] performed SPS technique in abdominopelvic CT enhanced examination, results showed that Sn100 kV can achieve similar diagnostic performance and CNR compared to an automated kV-selection protocol (90∼150 kV), while reducing the radiation dose by 81% (1.14±0.34 vs. 5.99±2.66 mSv). Meanwhile, studies also showed that Sn 140 kV could be used in breast cancer [39, 40], pancreatic cancer [41] and orbital tumor [42] CT imaging. All the above studies have proved the clinical value of SPS in low contrast or soft tissue imaging. To our best knowledge, such an approach has not been yet explored in head CT examination. While current study also demonstrated that SPS can reduce radiation dose by up to 71.38% for lens by a series of phantoms, without image quality loss. In the meanwhile, we found that the dose reduction is nearly same for SPS mode and SPS+OBTCM mode as a whole, one probable reason is that SPS is good enough in reducing radiation dose, thus makes the effect of OBTCM negligible. Specifically, on the basis of SPS, we didn’t observe noticeable dose reduction caused by OBTCM in 1-yr and 5-yr-old phantoms for len dose, the small size and lower density of these phantoms may account for this, which result in the difference of dose reduction between 1-yr and 5-yr-old phantoms with adult phantom.

Image quality must be considered when evaluating dose reduction strategies. Our study in noise evaluation showed a quantitatively small increase in the SPS and SPS+OBTCM modes without CNR lose. The absolute value of the difference in noise varies a lot, especially in the orbit of a 5-yr-old child (2.58 HU) in the SPS mode. However, the number of photons passing through the head decreases with the SPS mode, and the region of the orbital and skull base will produce more artifacts caused by lesser photons, which may lead to increased local image noise. The increased noise can be compensated by other reconstructed techniques, such as increasing the level of iterative reconstruction.

Our study had some limitations. Firstly, this study only involves a few limited age head phantoms; different imaging modes may have various effects on the head of different age groups to affect the radiation dose to the lens. Therefore, conducting further clinical research with numerous people from different age groups is necessary. Secondly, the phantoms were formed, which made the position of the slightly deeper inside the eyes than the real-world lens dose measurement. The TLDs we used in this study were placed on the material of equivalent material, which gave us the reasonable CT values to find out the most suitable scanning protocols. However, we need further study for real-world CT examination, especially the measurement of dose reduction in lens. In addition, only the absorbed dose to the lens and brain was evaluated, and the radiation sensitive organs in other non-scanning areas, such as thyroid, were not evaluated. Finally, noise and contrast-to-noise ratio were used to evaluate image quality in this study, but might not enough to indicate frequency characteristics of the clinical image [43]. We notice that evaluation indicators such as NPS measurement could better reflect the noise characteristics between images. However, the changes of TLDs’ positions and FOVs between all scan sequences in three anthropomorphic phantoms made it difficult to calculate and compare NPS characteristics between groups in this study. And there are no specific commercial phantoms to simulate the visual evaluation of the head of infant, child and adult and the evaluation of the noise contrast characteristics in those phantoms. Further study is needed to determine whether it can be significantly improved the noise characteristics with dose-saving protocols by home-made phantoms and the performance in clinical settings.

In conclusion, our results show that the SPS mode can vastly reduce the radiation dose in the head CT scan with quantitative and small increase in noise level. Additionally, the SPS+OBTCM mode couldn’t effectively reduce the absorbed dose to the lens for 1-yr and 5-yr-old phantoms, compared with SPS mode. Thus, SPS mode in pediatric and SPS+OBTCM mode in adult are better than other modes, and should be used in clinical practice.

Footnotes

Acknowledgments

None.

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