# Spatial Distribution of Noise Reduction in Four Iterative Reconstruction Algorithms in CT—A Technical Evaluation

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## Abstract

**:**

## 1. Introduction

## 2. Materials and Methods

## 3. Results

#### 3.1. Noise Maps

#### 3.2. Noise Profiles across Edges

## 4. Discussion

#### 4.1. Noise Reduction Properties of the IR Algorithms

#### 4.2. Scan and Reconstruction Parameters

#### 4.3. Method for Measuring Noise Reduction

## 5. Conclusions

## Author Contributions

## Funding

## Conflicts of Interest

## Abbreviations

AAPM | American Association of Physicists in Medicine |

ADMIRE | Advanced modeled iterative reconstruction |

AIDR 3D | Adaptive iterative dose reduction 3D |

ASIR-V | Adaptive statistical iterative reconstruction V |

CT | Computed tomography |

CTDI${}_{\mathrm{vol}}$ | Volume computed tomography dose index |

FBP | Filtered back projection |

HU | Hounsfield unit |

IR | Iterative reconstruction |

NM | Noise map |

NRM | Noise reduction map |

NPS | Noise power spectrum |

org | original |

pp | percentage points |

ROI | Region of interest |

SD | Standard deviation |

std | standard |

str | strong |

## Appendix A. Noise Reduction Maps

**Figure A1.**Noise reduction maps created by calculating the pixelwise relative noise reduction between the noise maps of each IR level compared to FBP for all vendors. A lighter color indicates a larger amount of noise reduced in the given pixel compared to FBP. The darkest blue and black colors, representing values smaller than 0%, indicates a noise increase compared to FBP.

## References

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**Figure 1.**(

**a**) Anthropomorphic abdomen phantom; (

**b**) computed tomography (CT) image of the phantom illustrating the positions of the three edges used to measure noise profiles in the noise maps. The difference in tissue density over the three edges were 1000 HU, 70 HU and 30 HU.

**Figure 2.**Noise maps showing the inter-image pixel standard deviation for 30 images, each reconstructed with filtered back projection (FBP), a medium level of iterative reconstruction (IR) and a high level of IR for all vendors. A lighter color indicates a higher standard deviation, an thus a higher level of noise, in the given pixel.

**Figure 3.**Edge profiles showing the standard deviation (SD) measured over the 1000 HU edge in the noise maps reconstructed with FBP, a medium level of IR and a high level of IR for each vendor. The dashed line shows the average CT number in the FBP images.

**Figure 4.**Edge profiles showing the standard deviation (SD) measured over the 70 HU edge in the noise maps reconstructed with FBP, a medium level of IR and a high level of IR for each vendor. The dashed line shows the average CT number in the FBP images.

**Figure 5.**Edge profiles showing the standard deviation (SD) measured over the 30 HU edge in the noise maps reconstructed with FBP, a medium level of IR and a high level of IR for each vendor. The dashed line shows the average CT number in the FBP images.

Parameter | Canon | GE | Philips | Siemens |
---|---|---|---|---|

Scanner type | Aquilion Prime | Revolution Evo | Ingenuity | Somatom Definition Flash |

CTDI${}_{\mathrm{vol}}$ [mGy] | 15 | 15 | 12 | 15 |

Tube potential [kV] | 120 | 120 | 120 | 120 |

Tube current product [mAs] | 160 | 250 | 150 | 222 |

Reconstruction kernel | FC18 | Standard | B | B30f / I30f * |

IR algorithm | AIDR 3D | ASIR-V | iDose | ADMIRE |

IR levels used | org/std/str | 0%/50%/100% | 0/3/6 | 0/3/5 |

**Table 2.**Average amount and standard deviation of noise reduced outside and at three anatomical edges for two levels of IR relative to FBP, measured in the calculated noise maps (see Appendix A). The difference in noise reduced outside and at each anatomical edge is listed in percentage points (pp).

Edge | IR Level | Position | Noise Reduction Relative to FBP | |||
---|---|---|---|---|---|---|

Canon | GE | Philips | Siemens | |||

1000 HU | Medium | Outside edge | 36% ± 3% | 31% ± 8% | 22% ± 1% | 29% ± 2% |

At edge | 8% ± 2% | 6% ± 0.2% | 0% ± 1% | 8% ± 3% | ||

Difference | (28 ± 3) pp | (25 ± 8) pp | (22 ± 1) pp | (21 ± 4) pp | ||

High | Outside edge | 41% ± 4% | 51% ± 15% | 43% ± 2% | 52% ± 2% | |

At edge | 10% ± 1% | 6% ± 0.4% | 0% ± 2% | 15% ± 6% | ||

Difference | (31 ± 4) pp | (45 ± 15) pp | (44 ± 3) pp | (37 ± 6) pp | ||

70 HU | Medium | Outside edge | 38% ± 2% | 39% ± 1% | 23% ± 0.4% | 30% ± 1% |

At edge | 34% ± 1% | 29% ± 2% | 7% ± 3% | 20% ± 5% | ||

Difference | (4 ± 2) pp | (10 ± 2) pp | (16 ± 3) pp | (10 ± 5) pp | ||

High | Outside edge | 44% ± 2% | 65% ± 1% | 46% ± 1% | 53% ± 1% | |

At edge | 40% ± 1% | 44% ± 4% | 12% ± 6% | 34% ± 9% | ||

Difference | (4 ± 2) pp | (21 ± 4) pp | (33 ± 6) pp | (18 ± 9) pp | ||

30 HU | Medium | Outside edge | 42% ± 2% | 40% ± 1% | 23% ± 1% | 30% ± 1% |

At edge | 40% ± 1% | 33% ± 2% | 16% ± 2% | 18% ± 3% | ||

Difference | (2 ± 2) pp | (7 ± 2) pp | (7 ± 2) pp | (11 ± 3) pp | ||

High | Outside edge | 49% ± 2% | 67% ± 1% | 46% ± 1% | 52% ± 1% | |

At edge | 46% ± 1% | 52% ± 4% | 32% ± 3% | 33% ± 5% | ||

Difference | (3 ± 2) pp | (15 ± 4) pp | (14 ± 3) pp | (19 ± 5) pp |

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**MDPI and ACS Style**

Guleng, A.; Bolstad, K.; Dalehaug, I.; Flatabø, S.; Aadnevik, D.; Pettersen, H.E.S. Spatial Distribution of Noise Reduction in Four Iterative Reconstruction Algorithms in CT—A Technical Evaluation. *Diagnostics* **2020**, *10*, 647.
https://doi.org/10.3390/diagnostics10090647

**AMA Style**

Guleng A, Bolstad K, Dalehaug I, Flatabø S, Aadnevik D, Pettersen HES. Spatial Distribution of Noise Reduction in Four Iterative Reconstruction Algorithms in CT—A Technical Evaluation. *Diagnostics*. 2020; 10(9):647.
https://doi.org/10.3390/diagnostics10090647

**Chicago/Turabian Style**

Guleng, Anette, Kirsten Bolstad, Ingvild Dalehaug, Silje Flatabø, Daniel Aadnevik, and Helge E. S. Pettersen. 2020. "Spatial Distribution of Noise Reduction in Four Iterative Reconstruction Algorithms in CT—A Technical Evaluation" *Diagnostics* 10, no. 9: 647.
https://doi.org/10.3390/diagnostics10090647