Model-based Iterative Reconstruction in Low-radiation-dose Computed Tomography Colonography

Rationale and Objectives

To assess the effect of model-based iterative reconstruction (MBIR) on image quality and diagnostic performance of low-radiation-dose computed tomography colonography (CTC) in the preoperative assessment of colorectal cancer.

Materials and Methods

This study included 30 patients with colorectal cancer referred for surgical treatment. All patients underwent CTC with a standard dose (SD) protocol in the supine position and a low-dose (LD; radiation dose reduction of approximately 85%) protocol in the prone position. The SD protocol images were post-processed using filtered back projection (FBP), whereas the LD protocol images were post-processed using FBP and MBIR. Objective and subjective image quality parameters were compared among the three different methods. Preoperative evaluations, including site, length, and tumor and node staging were performed, and the findings were compared to the postsurgical findings.

Results

The mean image noise of SD-FBP, LD-FBP, and LD-MBIR images was 17.3 ± 3.2, 40.5 ± 10.9, and 11.2 ± 2.0 Hounsfield units, respectively. There were significant differences for all comparison combinations among the three methods ( P < .01). For image noise, the mean visual scores were significantly higher for SD-FBP and LD-MBIR than for LD-FBP, and the scores for SD-FBP and LD-MBIR were equivalent (3.9 ± 0.3 [SD-FBP], 2.0 ± 0.5 [LD-FBP], and 3.7 ± 0.3 [LD-MBIR]). Preoperative information was more accurate under SD-FBP and LD-MBIR than under LD-FBP, and the information was comparable between SD-FBP and LD-MBIR.

Conclusion

MBIR can yield significantly improved image quality on low-radiation-dose CTC and provide preoperative information equivalent to that of standard-radiation-dose protocol.

Introduction

Colorectal cancer (CRC) is a significant cause of cancer-related death globally, and early detection and appropriate treatment are critical . In patients with CRC, accurate preoperative staging is indispensable and can be generally performed using conventional colonoscopy, double-contrast barium enema, computed tomography (CT), magnetic resonance imaging, and positron emission tomography . More recently, it has been suggested that computed tomography colonography (CTC) is of value in the preoperative evaluation of patients known to have CRC . However, there has been some concern about ionizing radiation exposure associated with increased use of CT in medical practice . As patients with CRC frequently undergo repeated diagnostic and follow-up CT examinations, radiologists must consider radiation dose reduction techniques during CT examinations while maintaining image quality in accordance with the as low as reasonably achievable principle .

Iterative reconstruction (IR) for CT is currently widely used and helps to reduce the quantum noise associated with filtered back projection (FBP) reconstruction. Thus, with the introduction of IR, significant radiation dose reduction during CTC has become possible . Recently, the next generation of IR algorithms have been developed to focus on data restoration and noise reduction using a model-based process (model-based iterative reconstruction [MBIR]) . Previous investigations reported that the radiation dose in screening CTC using MBIR can be reduced by 47%–60% while maintaining diagnostic image quality . However, to the best of our knowledge, there have been no attempts to assess the feasibility of MBIR for radiation dose reduction at preoperative CTC in patients with CRC.

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Materials and Methods

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Study Population and Bowel Preparation

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Table 1

Patient Demographics

Sex (male/female) 18/12 Age (years) 67.7 ± 12 Body weight (kg) 60.3 ± 12.9 Body mass index (kg/m 2 ) 23.1 ± 4.1 Tumor size (mm) 40 ± 18 T stage and tumor size (n/mm) T1 3/34.3 ± 23.5 T2 6/15.7 ± 5.5 T3 11/43.2 ± 16.1 T4 10/49.8 ± 12.4 N stage (N0/N1/N2) 16/5/9 Tumor location (ascending colon/transverse colon/descending colon/sigmoid colon/rectum) 5/3/2/10/10

Data are presented as mean ± standard deviation or number.

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CTC Protocol and Image Reconstruction

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Table 2

Imaging Parameters for the Standard-dose and Low-dose Protocols

SD-FBP LD-FBP LD-FIRST Detector collimation (mm) 80 × 0.5 Tube voltage (kVp) 120 Tube current (mA) 3D automodulation \* Gantry rotation time (s) 0.5 Helical pitch 0.813 Scanning position Supine Prone Prone Image reconstruction FBP FBP FIRST Section thickness/interval (mm) 5.0/5.0 for routine axial and 0.5/0.5 for 3D image

FBP, filtered back projection; FIRST, Forward Projected Model-based Iterative Reconstruction Solution; LD, low dose; SD, standard dose.

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Radiation Dose Evaluation

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Quantitative Image Quality Analysis

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Qualitative Image Quality Analysis

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Preoperative Diagnostic Performance

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Statistical Analysis

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Results

Radiation Dose

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Quantitative Image Quality Analysis

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Table 3

Quantitative Image Noise Assessment

SD-FBP LD-FBP LD-FIRST_P_ Value Splenic flexure (HU) 15.1 ± 2.6 33.5 ± 3.0 10.9 ± 2.6 <.01 \* , †,‡ Mid-descending colon (HU) 15.7 ± 2.5 34.6 ± 3.9 11.2 ± 2.0 <.01 \* , †,‡ Sigmoid-descending colon junction (HU) 19.4 ± 2.5 56.4 ± 9.1 11.4 ± 1.6 <.01 \* , †,‡ Rectum (HU) 19.0 ± 2.7 37.6 ± 5.1 11.3 ± 1.8 <.01 \* , †,‡ Total average (HU) 17.3 ± 3.2 40.5 ± 10.9 11.2 ± 2.0 <.01 \* , †,‡

FBP, filtered back projection; FIRST, Forward Projected Model-based Iterative Reconstruction Solution; HU, Hounsfield unit; LD, low dose; SD, standard dose.

Data are presented as mean ± standard deviation.

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Qualitative Image Quality Analysis

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Table 4

Qualitative Image Quality Score Assessment

SD-FBP LD-FBP LD-FIRST_P_ Value Mural surface nodularity 4.0 ± 0.2 2.0 ± 0.5 3.6 ± 0.6 <.01 \* , †,‡ Image noise 3.9 ± 0.3 2.0 ± 0.5 3.7 ± 0.3 <.01 \* , ‡

.06 † Depiction of the tumor 3.6 ± 0.6 2.6 ± 0.8 3.6 ± 0.6 <.01 \* , ‡

.97 † Overall image quality 4.0 ± 0.2 2.0 ± 0.5 3.6 ± 0.5 <.01 \* , †,‡

FBP,  filtered back projection; FIRST, Forward Projected Model-based Iterative Reconstruction Solution; LD, low dose; SD, standard dose.

Data are presented as mean ± standard deviation. A 4-point subjective scale was used (1 for unacceptable, 2 for limited diagnostic value, 3 for good, 4 for excellent).

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Preoperative Diagnostic Performance

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Table 5

Preoperative Diagnostic Performance

SD-FBP LD-FBP LD-FIRST Tumor location \* 1.00 1.00 1.00 Morphological types \* 1.00 0.73 1.00 Tumor length (long axis of the bowel) † 0.86 0.78 0.85 T staging \* 0.63 0.37 0.63 N staging \* 0.65 0.14 0.68

FBP, filtered back projection; FIRST, Forward Projected Model-based Iterative Reconstruction Solution; LD, low dose; SD, standard dose.

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Discussion

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