Rationale and Objectives
The purpose of this study was to assess the effect of a low-tube-voltage technique and iterative reconstruction (IR) on the radiation dose and image quality of computed tomography colonography (CTC).
Materials and Methods
We studied 30 patients (14 women and 16 men; mean age, 64.5 ± 13.1 years; range, 39–90 years) with colorectal cancer referred for surgical treatment. All underwent CTC with fecal tagging under a standard 120-kVp protocol in the supine position and a 100-kVp protocol in the prone position. The 120-kVp images were reconstructed with filtered back projection (FBP). The 100-kVp images were postprocessed using FBP and a hybrid type of IR (adaptive iterative dose reduction 3D). The effective radiation dose (ED), image noise, and contrast-to-noise ratio (CNR) were compared among the three protocols. The visual image quality was scored on a four-point scale.
Results
The mean ED was significantly lower under the 100-kVp protocol than the 120-kVp protocol, resulting in a 27% radiation dose decrease (3.5 ± 2.0 vs 2.5 ± 1.5 mSv; P < .01). Image noise decreased by 48%, and the mean attenuation of tagged fluid increased from 452 to 558 HU on images acquired at 100 kVp with IR compared to that in the 120-kVp protocol; these differences were significant. The mean CNR was significantly higher under the 100 kVp with IR than the other two protocols. We found no significant differences in the visual scores for diagnostic utility between the 100 kVp with IR and the 120 kVp with FBP protocol ( P = .10).
Conclusions
Low-tube-voltage CTC reduced the radiation dose by approximately 27% while maintaining the image quality.
Computed tomography (CT) colonography (CTC) is a widely accepted screening tool for colorectal disease because of its excellent diagnostic performance and good patient tolerance . As it is useful for the preoperative evaluation of patients with colorectal carcinoma, it helps with surgical planning . It enables evaluation of the entire colon, even in patients with obstructive lesions and allows segmental localization of the tumor. CTC also facilitates the evaluation of locoregional and distant extracolonic tumor spread. However, there are concerns with respect to potential risks related to the ionizing radiation of CTC . Consequently, the radiation dose of CTC should be kept at the minimum needed for diagnostically adequate image quality. As a reduction in the radiation dose results in increased image noise and decreased image quality, it is critically important to balance the delivery of low radiation doses with the likelihood of obtaining diagnostically useful images.
Low-tube-voltage techniques yield higher contrast enhancement than the standard 120-kVp tube voltage method at lower radiation doses . However, increased image noise, a byproduct of low-tube-voltage settings, is a serious problem . Chang et al. assessed the effect of low-tube-voltage techniques on the image quality of CTC in 63 patients. They found that a decrease in the tube voltage from 120 to 100 kVp with filtered back projection (FBP) resulted in a significant decrease in the radiation dose and a decrease in the three-dimensional (3D) image quality. They suggested that the iterative reconstruction (IR) technique helped to reduce the image noise while preserving or improving the image quality at a low-tube-voltage settings for CTC.
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Materials and methods
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Patients and Bowel Preparation
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Table 1
Patient Characteristics
Demographics Number Number of patients 30 Age (years) 64.5 ± 13.1 Female/male 14/16 Body weight (kg) 59.0 ± 14.6 Body mass index (kg/m 2 ) 23.1 ± 3.9 Tumor size (mm) 28.7 ± 16.0 T stage and tumor size (n/mm) T1 6/13.8 ± 7.4 T2 7/27.9 ± 9.5 T3 11/40.9 ± 17.1 T4 6/26.3 ± 4.8 N stage ( n ) N0 21 N1 5 N2 4 Tumor location (n) Ascending 8 Transverse 4 Descending 1 Sigmoid 10 Rectum 7
Data are mean ± standard deviation.
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CTC Acquisition and Image Reconstruction
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Table 2
Scan Parameters of Each Protocol
CT acquisition characteristics 100 kVp with FBP 100 kVp with IR 120 kVp with FBP Beam collimation (mm) 80 × 0.5 80 × 0.5 80 × 0.5 Tube voltage (kVp) 100 100 120 Tube current (mA) ∗ 132.5 ± 65.0 132.5 ± 65.0 112.8 ± 55.6 Rotation time (s) 0.5 0.5 0.5 IR (AIDR 3D) NA Standard NA Slice thickness (mm) † 5 and 0.5 5 and 0.5 5 and 0.5 Slice intervals (mm) † 5 and 0.5 5 and 0.5 5 and 0.5 Scanning position Prone Prone Supine Bowel preparation quality score ‡ 3.8 ± 0.4 3.8 ± 0.4 3.9 ± 0.3
AIDR, adaptive iterative dose reduction; FBP, filtered back projection; HU, Hounsfield unit; IR, iterative reconstruction; NA, not applicable; 3D, three dimensional.
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Radiation Dose Measurements
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Quantitative Image Analysis
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Qualitative Image Analysis
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Table 3
Definition of the Four-Point Subjective Image Quality Scale
Image quality score Description Mural surface nodularity 1, poor Too much mural surface nodularity 2, fair Substantial mural surface nodularity 3, good Some mural surface nodularity 4, excellent Minimal mural surface nodularity Image noise 1, poor Too much image noise 2, fair Substantial image noise 3, good Some image noise 4, excellent Minimal image noise Tumor depiction 1, poor Poor visualization of the tumor 2, fair Insufficient visualization of the tumor 3, good Good visualization of the tumor 4, excellent Clear visualization of the tumor Depiction of the extracolonic organs 1, poor Poor visualization of the extracolonic organs 2, fair Insufficient visualization of the extracolonic organs 3, good Good visualization of the extracolonic organs 4, excellent Clear visualization of the extracolonic organs Diagnostic utility 1, poor Insufficient diagnostic information 2, fair Partially limited diagnostic information 3, good Sufficient diagnostic information 4, excellent Useful diagnostic information
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T and N Staging
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Table 4
Tumor and Node Staging by the Tumor–Nodes–Metastases Classification System ( Seventh Edition )
Stage Description Tumor (T) staging T0 No evidence of primary tumor T1 Tumor invades submucosa T2 Tumor invades muscularis propria T3 Tumor invades through the muscularis propria into the subserosa or into nonperitonealized pericolic or perirectal tissues T4 Tumor directly invades other organs or structures and/or perforates visceral peritoneum Node (N) staging N0 No regional lymph node metastasis N1 Metastasis in one to three regional lymph nodes N2 Metastasis in four or more regional lymph nodes
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Statistical Analysis
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Results
Radiation Exposure
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Quantitative Image Analysis
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Table 5
Summary of Our Quantitative Image Quality Analysis
Image quality parameters 100 kVp with FBP 100 kVp with IR 120 kVp with FBP_P_ Value Tagged fluid (HU) 566.9 ± 157.6 558.8 ± 154.7 452.2 ± 136.3 <.01 ∗ † Psoas muscle (HU) 54.0 ± 6.6 53.8 ± 6.8 50.3 ± 8.2 .09 Contrast enhancement (HU) 512.8 ± 157.4 504.9 ± 154.5 401.9 ± 136.8 <.01 ∗ † Contrast-to-noise ratio 27.3 ± 8.8 55.5 ± 17.2 22.5 ± 7.5 <.01 ∗ ‡ Image noise (HU) 19.0 ± 3.4 9.3 ± 2.2 17.9 ± 2.5 <.01 ∗ ‡
FBP, filtered back projection; HU, Hounsfield unit; IR, iterative reconstruction.
Data are the mean ± standard deviation.
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Qualitative Image Analysis
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Table 6
Summary of Our Qualitative Image Quality Analysis
Image quality parameters 100 kVp with FBP 100 kVp with IR 120 kVp with FBP_P_ Value Mural surface nodularity Observer 1 3.1 ± 0.3 3.9 ± 0.3 3.6 ± 0.5 <.01 ∗ † ‡ Observer 2 3.1 ± 0.4 3.9 ± 0.2 3.7 ± 0.5 <.01 † ‡ Mean 3.1 ± 0.3 3.9 ± 0.3 3.7 ± 0.5 <.01 ∗ † ‡ Image noise Observer 1 3.0 ± 0.2 3.9 ± 0.3 3.2 ± 0.4 <.01 ∗ ‡ Observer 2 3.0 ± 0.3 3.9 ± 0.2 3.3 ± 0.5 <.01 ∗ † ‡ Mean 3.0 ± 0.3 3.9 ± 0.2 3.2 ± 0.4 <.01 ∗ † ‡ Tumor depiction Observer 1 3.0 ± 0.3 3.2 ± 0.4 3.1 ± 0.4 .16 Observer 2 3.0 ± 0.2 3.3 ± 0.5 3.1 ± 0.3 .04 ‡ Mean 3.0 ± 0.3 3.2 ± 0.4 3.2 ± 0.4 .01 ‡ Extracolonic organs Observer 1 3.1 ± 0.3 3.6 ± 0.5 3.4 ± 0.5 <.01 † ‡ Observer 2 3.0 ± 0.2 3.6 ± 0.5 3.5 ± 0.5 <.01 † ‡ Mean 3.1 ± 0.2 3.6 ± 0.5 3.4 ± 0.5 <.01 † ‡ Diagnostic utility Observer 1 3.1 ± 0.3 3.9 ± 0.2 3.7 ± 0.5 <.01 ∗ † ‡ Observer 2 3.1 ± 0.3 3.9 ± 0.3 3.8 ± 0.4 <.01 † ‡ Mean 3.1 ± 0.3 3.9 ± 0.3 3.8 ± 0.4 <.01 ∗ † ‡
FBP, filtered back projection; IR, iterative reconstruction.
Data are the mean ± standard deviation.
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Table 7
Percent of Cases with Appropriate Image Quality (Image Quality Score ≥3)
Image quality parameters 100 kVp with FBP (%) 100 kVp with IR (%) 120 kVp with FBP (%) Mural surface nodularity Observer 1 96.7 100 100 Observer 2 96.7 100 100 Tumor size ≥20 mm 91.7 100 100 Tumor size <20 mm 100 100 100 Image noise Observer 1 93.3 100 100 Observer 2 93.3 100 100 Tumor size ≥20 mm 91.7 100 100 Tumor size <20 mm 94.4 100 100 Tumor depiction Observer 1 93.3 100 100 Observer 2 96.7 100 100 Tumor size ≥20 mm 87.5 100 100 Tumor size <20 mm 100 100 100 Extracolonic organs Observer 1 93.3 100 100 Observer 2 96.7 100 100 Tumor size ≥20 mm 95.8 100 100 Tumor size <20 mm 94.4 100 100 Diagnostic utility Observer 1 93.3 100 100 Observer 2 96.7 100 100 Tumor size ≥20 mm 91.7 100 100 Tumor size <20 mm 97.2 100 100
FBP, filtered back projection; IR, iterative reconstruction.
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T and N Staging
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Discussion
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Conclusions
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