Rationale and Objectives
This study aimed to determine the appropriate body mass index (BMI)-dependent noise index (NI) setting in computed tomography pulmonary angiography (CTPA) with automatic tube current modulation with adaptive statistical iterative reconstruction (ASiR).
Materials and Methods
A total of 480 patients who had a CTPA were divided into group A (18.5 kg/m 2 ≤ BMI < 25 kg/m 2 ), group B (25 kg/m 2 ≤ BMI < 30 kg/m 2 ), and group C (BMI ≥ 30 kg/m 2 ), according to their BMI values; each group had 160 patients. The three groups were further randomly divided into four subgroups: A1, A2, A3, A4; B1, B2, B3, B4; and C1, C2, C3, C4, with corresponding NI values of 26, 36, 40, and 46, respectively. All images were restructured with the ASiR algorithm, and the images with the lowest NI (26 Hounsfield units) in each group were used as reference standard. The signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) for the pulmonary artery of each group were calculated. Subjective image quality was evaluated using a five-score method by two independent radiologists. The CT dose index of volume and dose-length product were recorded and were converted to effective dose (ED). SNR and CNR in the group A, B, and C subgroups were compared to repeated measures analysis of variance, and the subjective score, Volumetric CT dose index of volume, dose-length product, and ED were compared to one-way analysis of variance.
Results
For groups A and B, the SNR, CNR, and subjective scores of the images in their subgroups showed no statistical differences ( P > .05). The ED in subgroups A4 and B4 was significantly lower than that in subgroups A1 (by 33.24%) and B1 (by 34.47%) ( P < .01). For group C, there was no significant difference in the SNR, CNR, and the subjective image scores between subgroups C3 and C1 ( P > .05). The ED in subgroup C3 was significantly lower than the ED in subgroup C1 (by 47.75%) ( P < .01)
Conclusions
Patient BMI-dependent NI settings that are higher than the recommended value may be used in CTPA with automatic tube current modulation and ASiR to effectively reduce radiation dose while maintaining diagnostic image quality.
Introduction
Computed tomography pulmonary angiography (CTPA) is currently a widely accessible and quick to perform technique for patients suspected of having pulmonary embolism (PE) in clinic , and it has a high sensitivity (94%–100%) and specificity (89%–100%) for the diagnosis of acute PE . However, the amount of radiation exposure to the patient population in CTPA and its risks have also increased concerns regarding potential radiation damage . Thus, reducing the radiation dose on CTPAstudies has become a priority for radiologists. For clinical populations in particular, it is important to reduce radiation dose by reducing the tube current . The automatic tube current modulation (ATCM) technique selects the optimal tube current in an automated manner by using the attenuation values on anteroposterior and lateral scanogram, and it is an important method to reduce radiation dose. In one implementation of ATCM technique, the noise index (NI) value can be used to control the output of the tube current, so the NI is used as an indirect representation of image quality. However, reducing the tube current can increase image noise and adversely affect image quality. Higher image noise can be reduced by different advanced reconstruction algorithms in maintaining low radiation doses; adaptive statistical iterative reconstruction (ASiR) is a widely used and effective way to reduce noise and maintain image quality .
To our knowledge, no one has compared image quality and radiation dose in CTPA for the combination of different NI settings and different body mass index (BMI) values. The purpose of our study was to determine the appropriate BMI-dependent noise NI setting in CTPA with ATCM and ASiR algorithm by comparing radiation dose and image quality to the combination of different NI values and BMI values to maximize dose reduction while maintaining image quality.
Materials and Methods
Patient Population
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Table 1
Patient Characteristics
Parameter Group A (n = 160) Group B (n = 160) Group C (n = 160)P Value Patient characteristic Age (y) \* 56.5 ± 17.8 57.7 ± 18.2 55.9 ± 17.6 .6567 Weight (kg) \* 62.5 ± 8.1 67.7 ± 7.8 79.5 ± 7.4 .0971 Height (cm) \* 169 ± 12.7 168 ± 13.2 160 ± 14.5 .4157 F/M 81/79 77/83 74/86 .7344 Axial length (mm) \* 313 316 310 .551
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CT Scan Protocols and Radiation Dose Estimation
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Objective Evaluation of Image Quality
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Subjective Evaluation of Image Quality
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Statistical Analysis
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Results
Patient Demographics
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Quantitative and Qualitative Image Analysis
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Table 2
Objective Image Quality and Subjective Image Quality with ATCM Combination of Different NI Setting
Mean Signal Intensity (HU) Mean Image Noise (HU) SNR CNR Subjective Image Scores Group A (18.5 kg/m 2 ≤ BMI < 25 kg/m 2 ) A1 385.7 ± 132.3 15.8 ± 3.2 24.6 ± 2.4 26.6 ± 2.6 4.54 ± 0.61 A2 382.7 ± 135.5 15.6 ± 3.3 24.5 ± 2.6 26.2 ± 2.4 4.55 ± 0.65 A3 378.6 ± 123.7 15.4 ± 3.1 24.2 ± 1.8 25.9 ± 3.5 4.53 ± 0.62 A4 379.6 ± 115.3 15.5 ± 3.7 24.3 ± 2.8 26.3 ± 2.3 4.52 ± 0.59P value all: n/s all: n/s all: n/s all: n/s all: n/s Group B (25 kg/m 2 ≤ BMI < 30 kg/m 2 ) B1 373.6 ± 123.7 16.2 ± 3.5 23.2 ± 2.3 25.3 ± 2.5 4.53 ± 0.58 B2 370.3 ± 122.3 16.1 ± 2.7 23.3 ± 2.6 24.9 ± 2.1 4.49 ± 0.60 B3 369.6 ± 127.6 16.3 ± 3.2 23.1 ± 2.1 24.8 ± 2.2 4.50 ± 0.53 B4 370.5 ± 125.8 16.5 ± 31 22.9 ± 1.6 24.7 ± 1.9 4.52 ± 0.48P value all: n/s all: n/s all: n/s all: n/s all: n/s Group C (BMI ≥ 30 kg/m 2 ) C1 364.6 ± 123.7 16.3 ± 3.5 22.1 ± 2.3 22.8 ± 2.5 4.53 ± 0.58 C2 365.3 ± 122.3 16.6 ± 2.7 21.6 ± 2.6 22.5 ± 2.1 4.49 ± 0.60 C3 364.6 ± 123.7 16.8 ± 3.3 21.7 ± 2.4 22.4 ± 2.4 4.48 ± 0.53 C4 349.5 ± 116.8 18.9 ± 3.6 18.7 ± 1.6 18.9 ± 1.9 4.12 ± 0.36P value 4vs1: .0082 4vs1: .0065 4vs1: .0038 4vs1: .0043 4vs1: .0016 4vs2: .0076 4vs2: .0057 4vs2: .0031 4vs2: .0055 4vs2: .0025 4vs3: .0071 4vs3: .0078 4vs3: .0052 4vs3: .0037 4vs3: .0021 rest: n/s rest: n/s rest: n/s rest: n/s rest: n/s
ATCM, automatic tube current modulation; BMI, body mass index; CNR, contrast-to-noise ratio; HU, Hounsfield unit; NI, noise index; SNR, signal-to-noise ratio.
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Radiation Dose
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Table 3
Radiation Dose with ATCM Combination of Different NI Setting at Different BMI
Different NI Value 18.5 kg/m 2 ≤ BMI < 25 kg/m 2 25 kg/m 2 ≤ BMI < 30 kg/m 2 BMI ≧ 30 kg/m 2 CTDI vol DLP ED CTDI vol DLP ED CTDI vol DLP ED 26 7.21 ± 1.58 245.49 ± 21.68 3.61 ± 0.42 8.89 ± 1.43 266.53 ± 20.79 3.83 ± 0.62 12.88 ± 2.28 386.69 ± 29.68 6.22 ± 1.36 36 6.25 ± 1.42 185.33 ± 20.16 2.58 ± 0.31 7.63 ± 1.51 231.47 ± 19.58 2.93 ± 0.51 9.16 ± 1.63 255.23 ± 21.62 3.96 ± 0.48 40 5.91 ± 1.31 173.21 ± 19.86 2.49 ± 0.24 6.45 ± 1.44 202.73 ± 20.76 2.68 ± 0.44 8.02 ± 1.28 223 ± 20.32 3.25 ± 0.39 46 5.56 ± 1.22 160.08 ± 19.67 2.41 ± 0.18 5.93 ± 1.36 178.21 ± 20.26 2.51 ± 0.23 6.96 ± 1.22 190.08 ± 19.67 2.54 ± 0.33P value 46vs26: .00 46vs26: .00 46vs26: .00 46vs26: .00 46vs26: .00 46vs26: .00 46vs26: .00 46vs26: .00 46vs26: .00 40vs26: .03 40vs26: .00 40vs26: .02 40vs26: .00 40vs26: .00 40vs26: .00 40vs26: .00 40vs26: .00 40vs26: .00 36vs26: .04 36vs26: .02 36vs26: .00 36vs26: .05 36vs26: .01 36vs26: .00 36vs26: .00 36vs26: .00 36vs26: .00 rest:n/s rest:n/s rest:n/s rest:n/s rest:n/s rest:n/s 46vs36: .02 46vs36: .00 46vs36: .03 rest:n/s rest:n/s rest:n/s
ATCM, automatic tube current modulation; BMI, body mass index; CTDI vol , CT dose index of volume; DLP, dose-length product; ED, effective dose; NI, noise index.
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Discussion
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