Rationale and Objectives
To compare image quality on computed tomographic colonography (CTC) acquired at standard dose (STD) and low dose (LD) using filtered-back projection, adaptive statistical iterative reconstruction, and model-based iterative reconstruction (MBIR) techniques.
Materials and Methods
A total of 65 symptomatic patients were prospectively enrolled for the study and underwent STD and LD CTC with filtered-back projection, adaptive statistical iterative reconstruction, and MBIR to allow direct per-patient comparison. Objective image noise, subjective image analyses, and polyp detection were assessed.
Results
Objective image noise analysis demonstrates significant noise reduction using MBIR technique ( P < .05) despite being acquired at lower doses. Subjective image analyses were superior for LD MBIR in all parameters except visibility of extracolonic lesions (two-dimensional) and visibility of colonic wall (three-dimensional) where there were no significant differences. There was no significant difference in polyp detection rates ( P > .05). Doses: LD (dose-length product, 257.7), STD (dose-length product, 483.6).
Conclusions
LD MBIR CTC objectively shows improved image noise using parameters in our study. Subjectively, image quality is maintained. Polyp detection shows no significant difference but because of small numbers needs further validation. Average dose reduction of 47% can be achieved. This study confirms feasibility of using MBIR in this context of CTC in symptomatic population.
Computed tomographic colonography (CTC) is a widely accepted procedure for the investigation of colorectal cancer, both in the context of symptomatic and screening population . Diagnostic performance has been well validated previously , but the procedure is still associated with high radiation dose especially for symptomatic population with dose estimates of around 7–10 mSv with some studies showing that this trend is not decreasing despite increasing tools available for dose reduction . Using new iterative reconstruction techniques, investigators have performed work on phantom models to assess the accuracy of polyp detection and also there are ongoing clinical trials . Some work has also been performed using other versions of hybrid iterative reconstruction . The aim of our study was to perform low-dose (LD) feasibility study in clinical setting in the symptomatic population. This cohort allows iterative reconstruction to be evaluated on many different levels and for assessment of colonic and extracolonic findings with the methodology allowing direct per-patient comparison of standard-dose (STD) and LD scans based on the findings from previous phantom studies . Our primary objectives were (1) to compare objective image noise between traditional reconstruction method of filtered-back projection (FBP), adaptive statistical iterative reconstruction (ASIR), and model-based iterative reconstruction (MBIR) using STD and LD CT scans; and (2) to compare subjective image noise and quality analysis of STD ASIR scans versus LD MBIR scans to assess feasibility of LD scanning in clinical practice. Our secondary objective was to compare diagnostic accuracy of detected polyps between STD and LD scans across all three reconstruction algorithms.
Materials and methods
Patient Selection
Institutional and regional ethical review board approved this prospectively enrolled study. Informed consent was obtained from all patients. Inclusion criteria were age >50 years, scheduled for a standard-of-care CTC for symptomatic investigation, and fit to undergo the procedure. All patients had standard departmental protocol bowel preparation. Two days before the examination, patient was advised to have low-fiber diet only. The day before the examination, laxatives (sodium picosulfate; Ferring Pharmaceuticals, Saint-Prex, Switzerland) were given as two sachets of 15.08 g dissolved each in one glass of water for morning and afternoon. On the day of examination at 3 hours before examination, Gastrograffin (diatrizoate dimeglumine; Bayer Pharma, Leverkusen, Germany) was given to drink for fecal residue tagging (50 mL in 500 mL of water). Our departmental protocol is in line with the recent European Society of Gastrointestinal and Abdominal Radiology consensus statement . Exclusion criteria included age <50 years, inability to give informed consent, hemodynamic instability, and prior contrast agent reaction. Sixty-five patients were prospectively recruited (26 men and 39 women). Sample size was based on calculation of number that would be needed to reduce mean image noise by a value of 1 (standard deviation (SD) of 1.75) using significance criterion of 0.05 (95% confidence interval) and statistical power of 90%. Referring indications were as follows (and some patients had more than one): weight loss, n = 43; changing bowel habits, n = 34; unexplained anemia, n = 17; abdominal pain, n = 22; palpable mass, n = 13; per rectal bleeding, n = 10; staging scan, n = 8; failed colonoscopy, n = 8; and suspected recurrent disease, n = 5. In terms of prior investigation, other than the eight patients who had failed colonoscopy before this examination, no patients had prior optical colonoscopy or CTC in the preceding 12 months. Mean weight was 75.4 ± 12.1 kg (range, 53–109 kg). Mean age was 75 ± 9.5 years (range, 55–92 years). Scans were performed between October 28, 2012 and March 31, 2013.
Scanning Techniques
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Image Reconstruction
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Objective Image Noise Analysis
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Subjective Image Noise and Quality Analysis
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Polyp Detection
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Radiation Dose
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Statistical Analysis
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Results
Objective Image Noise Analysis
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Subjective Image Noise and Quality Analysis
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Table 1
Subjective Analysis (Two-Dimensional [2D] Datasets)—Average Scores Between Two Observers
2D Image Noise Visibility of Colonic Wall Overall Diagnostic Confidence Visibility of Extracolonic Lesions Supine Prone Supine Prone Supine Prone Supine Prone ASIR STD MBIR LD ASIR STD MBIR LD ASIR STD MBIR LD ASIR STD MBIR LD ASIR STD MBIR LD ASIR STD MBIR LD ASIR STD MBIR LD ASIR STD MBIR LD Mean 2.2 2 3.1 2.9 2.2 2.1 3.3 3.2 2.1 1.9 2.8 2.7 2.3 2.3 3.3 3.4 SD 0.37 0.26 0.31 0.26 0.38 0.26 0.44 0.44 0.55 0.47 0.81 0.71 0.43 0.42 0.41 0.6 SE 0.046 0.032 0.038 0.033 0.048 0.032 0.055 0.055 0.069 0.059 0.1 0.088 0.054 0.053 0.051 0.074 Lower 95% CI 2.1 1.9 3.1 2.8 2.1 2 3.2 3.1 1.9 1.8 2.6 2.5 2.2 2.2 3.2 3.2 Upper 95% CI 2.3 2.1 3.2 3 2.3 2.1 3.4 3.3 2.2 2 3 2.9 2.4 2.4 3.4 3.5 Kappa 0.891 0.883 0.581 0.633 0.808 0.702 0.725 0.693 0.874 0.881 0.81 0.799 0.814 0.711 0.548 0.73P value 0.0039 0.0003 0.0083 0.0034 0.0027 0.0034 0.3458 0.1983 ∗ ∗ ∗ ∗ ∗ ∗
ASIR, adaptive statistical iterative reconstruction; CI, confidence interval; LD, low dose; MBIR, model-based iterative reconstruction; SE, standard error; SD, standard deviation; STD, standard dose.
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Table 2
Subjective Analysis (Three-Dimensional [3D] Datasets)—Average Scores Between Two Observers
3D Image Noise Visibility of Colonic Wall Overall Diagnostic Confidence Supine Prone Supine Prone Supine Prone ASIR STD MBIR LD ASIR STD MBIR LD ASIR STD MBIR LD ASIR STD MBIR LD ASIR STD MBIR LD ASIR STD MBIR LD Mean 2.1 2 3.2 2.9 2.1 2.1 3.3 3.3 2.1 2 2.8 2.7 SD 0.34 0.22 0.34 0.32 0.41 0.2 0.43 0.48 0.59 0.54 0.8 0.78 SE 0.042 0.027 0.043 0.04 0.051 0.025 0.053 0.059 0.073 0.067 0.099 0.096 Lower 95% CI 2.1 1.9 3.1 2.8 2 2 3.2 3.1 1.9 1.9 2.6 2.5 Upper 95% CI 2.2 2 3.2 2.9 2.2 2.1 3.4 3.4 2.2 2.1 3 2.9 Kappa 0.871 0.851 0.641 0.572 0.814 0.548 0.814 0.711 0.875 0.901 0.812 0.805P value 0.0054 <0.0001 0.0961 0.1489 0.031 0.0107 ∗ ∗ ∗ ∗
ASIR, adaptive statistical iterative reconstruction; CI, confidence interval; LD, low dose; MBIR, model-based iterative reconstruction; SE, standard error; SD, standard deviation; STD, standard dose.
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Polyp Detection
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Table 3
Per-Polyp Sensitivity, Specificity, PPV, and NPV Between Two Observers
Reconstruction Sensitivity Specificity PPV NPV Reader 1 Reader 2 Reader 1 Reader 2 Reader 1 Reader 2 Reader 1 Reader 2 FBP STD 0.94 0.94 0.88 0.88 0.89 0.89 0.94 0.94 FBP LD 0.82 0.88 0.76 0.82 0.78 0.83 0.81 0.88 ASIR STD 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 ASIR LD 0.94 0.94 1.00 1.00 1.00 1.00 0.94 0.94 MBIR STD 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 MBIR LD 0.94 0.94 1.00 1.00 1.00 1.00 0.94 0.94
ASIR, adaptive statistical iterative reconstruction; FBP, filtered-back projection; LD, low dose; MBIR, model-based iterative reconstruction; NPV, negative predictive value; PPV, positive predictive value; STD, standard dose.
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Radiation Dose
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Table 4
Dose-Length Products and Effective Dose for STD and LD Scans
Reconstruction STD LD DLPs (mGy cm) ED ∗ (mSv) DLPs (mGy cm) ED ∗ (mSv) Supine Prone Total Total Supine Prone Total Total Average 346.0 137.6 483.67.3 165.3 92.4 257.73.9 SD 222.3 105.8 129.8 71.9 Range (112.0–1130.0) (23.0–547.3) (27.3–675.0) (15.9–36.48) CTDIs (mGy) CTDIs (mGy) Average 7.05 2.8 3.36 1.88 SD 4.51 2.16 2.63 1.47 Range (2.24–21.4) (0.51–11.3) (0.56–13.97) (0.35–7.63)
CTDIs, computed tomography dose index; DLP, dose-length product; ED, effective dose; LD, low dose; SD, standard deviation; STD, standard dose.
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Discussion
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Conclusions
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