Iterative Reconstructions in Reduced-Dose CT

Rationale and Objectives

To compare adaptive statistical iterative reconstruction (ASIR) and model-based iterative reconstruction (MBIR) algorithms for reduced-dose computed tomography (CT).

Materials and Methods

Forty-four young oncology patients (mean age 30 ± 9 years) were included. After routine thoraco-abdominal CT (dose 100%, average CTDI vol 9.1 ± 2.4 mGy, range 4.4–16.9 mGy), follow-up CT was acquired at 50% (average CTDI vol 4.5 ± 1.2 mGy, range 2.2–8.4 mGy) in 29 patients additionally at 20% dose (average CTDI vol 1.9 ± 0.5 mGy, range 0.9–3.4 mGy). Each reduced-dose CT was reconstructed using both ASIR and MBIR. Four radiologists (two juniors and two seniors) blinded to dose and technique read each set of CT images regarding objective and subjective image qualities (high- or low-contrast structures), subjective noise or pixilated appearance, diagnostic confidence, and lesion detection.

Results

At all dose levels, objective image noise was significantly lower with MBIR than with ASIR ( P < 0.001). The subjective image quality for low-contrast structures was significantly higher with MBIR than with ASIR ( P < 0.001).

Reduced-dose abdominal CT images of patients with higher body mass index (BMI) were read with significantly higher diagnostic confidence than images of slimmer patients ( P < 0.001) and had higher subjective image quality, regardless of technique.

Although MBIR images appeared significantly more pixilated than ASIR images, they were read with higher diagnostic confidence, especially by juniors ( P < 0.001).

Conclusions

Reduced-dose CT during the follow-up of young oncology patients should be reconstructed with MBIR to ensure diagnostic quality. Elevated body mass index does not hamper the quality of reduced-dose CT.

Introduction

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Materials and Methods

Patients

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CT Examinations and Image Reconstruction

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Table 1

Patients’ Exposition Dose

Type of Examination Reference CT (100% Dose) Follow-up CT (50% Dose) Follow-up CT (20% Dose) Patients’ CTDI vol Abdominal CT ( n = 10) 8.7 mGy (range 6.6–12.5 mGy) 4.4 mGy (range 3.1–6.2 mGy) 1.9 mGy (range 1.6–2.4 mGy) Thoracoabdominal CT ( n = 34) 9.2 mGy (range 4.4–17 mGy) 4.58 mGy (range 2.2–8.4 mGy) 1.9 mGy (range 0.9–3.4 mGy) Total CTDI vol 9.1 ± 2.4 mGy (range 4.4–16.9 mGy) 4.5 ± 1.2 mGy (range 2.2–8.4 mGy) 1.9 ± 0.5 mGy (range 0.9–3.4 mGy) Patients’ dose-length products (DLP) Abdominal CT ( n = 10) 675.6 mGy × cm (range 367–998) 311.8 mGy × cm (range 194–448) 115.2 mGy × cm (range 77–155) Thoraco-abdominal CT ( n = 34) 730.5 mGy × cm (range 310–1302) 355.3 mGy × cm (range 149–580) 134.4 mGy × cm (range 53–232) Total DLP 718.0 ± 225 mGy × cm (range 310–1302) 345.4 ± 102.5 mGy × cm (range 149–580) 131.1 ± 35.9 mGy × cm (range 53–232)

CT, computed tomography; CTDI, computed tomography dose index.

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Image Analysis

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Objective Image Quality

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Subjective Image Quality

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Focal Lesion Detection

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Statistical Analysis

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Results

Patients and Examination Types

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Evaluation of Reference CT Images and Comparison to ASIR and MBIR Images at 50% Dose

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Table 2

Evaluation Scores According to the Dose Reduction and the Reconstruction Algorithm (Four Readers)

Dose Reduction

Type of IR Reference 100% 50% ASIR 50% MBIR_P_ Value ( t Test) 20% ASIR 20% MBIR_P_ Value ( t Test)Objective image quality (in Hounsfield units) Average density 67.66 67.95 67.27 0.40 69.52 67.30 0.06 Standard deviation 18.68 25.80 13.34 <0.001 \* 40.82 16.94 <0.001 \* Subjective image quality__High-contrast structures (5-point scale, 1 = best, 5 = worst) Pericardium 1.22 1.61 1.65 0.58 2.76 2.77 0.95 Chest wall 1.51 2.18 2.21 0.72 3.72 3.57 0.10 Adrenal glands 1.51 2.35 2.27 0.37 3.66 2.88 <0.001 \* Superficial epigastric vessels 1.21 1.97 1.89 0.30 3.45 2.68 <0.001 \* Low- contrast structures (5-point scale, 1 = best, 5 = worst) Portal veins 1.55 2.45 2.15 <0.001 \* 3.98 3.08 <0.001 \* Hepatic veins 1.72 2.47 2.08 <0.001 \* 3.78 2.98 <0.001 \* Subjective image noise/pixilated appearance (5-point scale, 1 = best, 5 = worst) Chest noise 1.28 2.41 1.21 <0.001 \* 3.48 2.06 <0.001 \* Chest-pixilated appearance 1 1.01 2.27 <0.001 \* 1.48 2.62 <0.001 \* Abdomen noise 1.64 3.14 1.32 <0.001 \* 3.07 1.65 <0.001 \* Abdomen-pixilated appearance 1.01 1 2.64 <0.001 \* 1.72 3.08 <0.001 \* Overall diagnostic confidence (4-point scale, 1 = best, 4 = worst) Chest 1 1.53 1.5 0.73 2.47 2.45 0.85 Abdomen 1.12 2.08 1.76 <0.001 \* 3.06 2.24 <0.001 \*

ASIR, adaptive statistical iterative reconstruction; CT, computed tomography; IR, iterative reconstruction; MBIR, model-based iterative reconstruction.

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Comparison Between Reduced-Dose ASIR and MBIR Images

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Influence of BMI on Quality of ASIR and MBIR Images

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Table 3

Influence of the BMI on the Image Evaluation (Four Readers)

Group of BMI BMI ≤ 24.7 BMI >24.7 BMI ≤ 24.7 BMI >24.7P Value † Dose Reduction

Type of IR 50%

ASIR 50%

MBIR 50%

ASIR 50%

MBIR 20%

ASIR 20%

MBIR 20%

ASIR 20%

MBIR_Objective image quality (in Hounsfield units)_ Average density 70.98 70.67 64.62 63.55 74.70 72.29 64.68 62.64 <0.001 \* Standard deviation 24.16 11.76 27.60 15.08 38.07 15.34 43.39 18.43 <0.001 \* Subjective image quality__High-contrast structures (5-point scale, 1 = best, 5 = worst) Pericardium 1.70 1.74 1.52 1.56 2.82 2.80 2.70 2.73 0.051 Chest wall 2.35 2.39 1.99 2.01 3.86 3.75 3.58 3.4 <0.001 \* Adrenal glands 2.62 2.48 2.05 2.05 3.91 3.05 3.43 2.72 <0.001 \* Superficial epigastric vessels 2.10 1.93 1.82 1.83 3.57 2.82 3.33 2.55 <0.001 \* Low-contrast structures (5-point scale, 1 = best, 5 = worst) Portal veins 2.48 2.14 2.42 2.17 3.88 3.00 4.08 3.15 0.441 Hepatic veins 2.38 1.99 2.56 2.18 3.66 2.93 3.88 3.03 <0.001 \* Subjective image noise/pixilated appearance (5-pointscale, 1 = best, 5 = worst) Chest noise 2.49 1.21 2.33 1.20 3.5 2.06 3.5 2.06 0.394 Chest-pixilated appearance 1.01 2.34 1 2.19 1.52 2.67 1.44 2.58 0.193 Abdomen noise 3.18 1.27 3.08 1.37 3.04 1.63 3.1 1.67 0.817 Abdomen-pixilated appearance 1 2.65 1 2.63 1.75 3.09 1.7 3.07 0.772Overall diagnostic confidence (4-point scale, 1 = best, 4 = worst) Chest 1.59 1.55 1.45 1.44 2.52 2.49 2.42 2.42 0.067 Abdomen 2.16 1.84 1.99 1.68 3.05 2.36 3.07 2.13 <0.001 \*

ANOVA, analysis of variance; ASIR, adaptive statistical iterative reconstruction; BMI, body mass index; IR, iterative reconstruction; MBIR, model-based iterative reconstruction.

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Influence of Reader Experience on Evaluation of ASIR and MBIR Images

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Table 4

Influence of the Readers’ Experience on the Image Evaluation

Group of Reader Juniors Seniors Juniors Seniors_P_ Value † Dose Reduction

Type of IR 50%

ASIR 50%

VEO 50%

ASIR 50%

VEO 20%

ASIR 20%

VEO 20%

ASIR 20%

VEO_Objective image quality (in Hounsfield units)_ Average density 68.48 67.28 67.41 67.26 69.71 66.71 69.33 67.89 0.08 Standard deviation 25.49 14 26.11 12.68 41.16 17.71 40.48 16.17 0.18Subjective image quality__High-contrast structures (5-point scale, 1 = best, 5 = worst) Pericardium 1.81 1.97 1.42 1.34 3.17 3.26 2.34 2.28 <0.001 \* Chest wall 2.56 2.6 1.80 1.82 3.88 3.79 3.55 3.34 <0.001 \* Adrenal glands 2.38 2.32 2.32 2.23 3.76 2.57 3.58 3.19 0.55 Superficial epigastric vessels 2.13 2.09 1.81 1.68 3.72 2.45 3.17 2.91 <0.001 \* Low-contrast structures (5-point scale, 1 = best, 5 = worst) Portal veins 2.33 1.86 2.57 2.44 4.09 2.59 3.88 3.57 <0.001 \* Hepatic veins 2.42 1.84 2.51 2.32 3.78 2.57 3.78 3.49 <0.001 \* Subjective image noise/pixilated appearance (5-point scale, 1 = best, 5 = worst) Chest noise 2.64 1.41 2.19 1 3.54 3.12 3.42 1 <0.001 \* Chest-pixilated appearance 1 2.31 1.01 2.23 1.96 2.18 1 3.06 0.52 Abdomen noise 3.57 1.60 2.7 1.03 2.31 2.19 3.83 1.10 <0.001 \* Abdomen-pixilated appearance 1 2.69 1 2.59 2.43 2.55 1.02 3.60 0.11Overall diagnostic confidence (4-point scale, 1 = best, 4 = worst) Chest 1.99 1.84 1.07 1.16 2.78 2.76 2.16 2.14 <0.001 \* Abdomen 2.70 2.15 1.45 1.38 3.26 2.05 2.86 2.43 <0.001 \*

ANOVA, analysis of variance; ASIR, adaptive statistical iterative reconstruction; IR, iterative reconstruction.

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Focal Lesion Detection According to Dose and Type of IR

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Table 5

Interobserver Agreement Between the Four Readers for Detecting Focal Lesions on Reduced Dose CT Examinations

Kappa Values Focal Parenchymal Lesions Lymphadenopathies Mean Median Mean Median ASIR 50% 0.59 0.61 0.51 0.51 VEO 50% 0.62 0.64 0.53 0.55 ASIR 20% 0.31 0.33 0.09 0 VEO 20% 0.33 0.37 0.031 0.01

ASIR, adaptive statistical iterative reconstruction; CT, computed tomography.

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Discussion

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References

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Iterative Reconstructions in Reduced-Dose CT

Rationale and Objectives

Materials and Methods

Results

Conclusions

Introduction

Materials and Methods

Patients

CT Examinations and Image Reconstruction

Image Analysis

Objective Image Quality

Subjective Image Quality

Focal Lesion Detection

Statistical Analysis

Results

Patients and Examination Types

Evaluation of Reference CT Images and Comparison to ASIR and MBIR Images at 50% Dose

Comparison Between Reduced-Dose ASIR and MBIR Images

Influence of BMI on Quality of ASIR and MBIR Images

Influence of Reader Experience on Evaluation of ASIR and MBIR Images

Focal Lesion Detection According to Dose and Type of IR

Discussion

References

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