Korean J Radiol.  2018 Feb;19(1):119-129. 10.3348/kjr.2018.19.1.119.

Quantitative Image Quality and Histogram-Based Evaluations of an Iterative Reconstruction Algorithm at Low-to-Ultralow Radiation Dose Levels: A Phantom Study in Chest CT

• Lee KB
• Goo HW

• KMID: 2425117
• DOI: http://doi.org/10.3348/kjr.2018.19.1.119

Abstract

OBJECTIVE
To describe the quantitative image quality and histogram-based evaluation of an iterative reconstruction (IR) algorithm in chest computed tomography (CT) scans at low-to-ultralow CT radiation dose levels.
MATERIALS AND METHODS
In an adult anthropomorphic phantom, chest CT scans were performed with 128-section dual-source CT at 70, 80, 100, 120, and 140 kVp, and the reference (3.4 mGy in volume CT Dose Index [CTDIvol]), 30%-, 60%-, and 90%-reduced radiation dose levels (2.4, 1.4, and 0.3 mGy). The CT images were reconstructed by using filtered back projection (FBP) algorithms and IR algorithm with strengths 1, 3, and 5. Image noise, signal-to-noise ratio (SNR), and contrast-to-noise ratio (CNR) were statistically compared between different dose levels, tube voltages, and reconstruction algorithms. Moreover, histograms of subtraction images before and after standardization in x- and y-axes were visually compared.
RESULTS
Compared with FBP images, IR images with strengths 1, 3, and 5 demonstrated image noise reduction up to 49.1%, SNR increase up to 100.7%, and CNR increase up to 67.3%. Noteworthy image quality degradations on IR images including a 184.9% increase in image noise, 63.0% decrease in SNR, and 51.3% decrease in CNR, and were shown between 60% and 90% reduced levels of radiation dose (p < 0.0001). Subtraction histograms between FBP and IR images showed progressively increased dispersion with increased IR strength and increased dose reduction. After standardization, the histograms appeared deviated and ragged between FBP images and IR images with strength 3 or 5, but almost normally-distributed between FBP images and IR images with strength 1.
CONCLUSION
The IR algorithm may be used to save radiation doses without substantial image quality degradation in chest CT scanning of the adult anthropomorphic phantom, down to approximately 1.4 mGy in CTDIvol (60% reduced dose).

Keyword

Iterative reconstruction; Low-dose chest CT; Image quality evaluation; Histogram-based analysis

MeSH Terms

Adult
Cone-Beam Computed Tomography
Humans
Noise
Signal-To-Noise Ratio
Thorax*
Tomography, X-Ray Computed*

Figure

• 

  Fig. 1 Axial CT images of chest phantom obtained at reference dose level and 70 kVp.A. Axial CT image reconstructed with FBP (B30f) demonstrates locations of three regions of interest (N, lung nodule; B1, air outside anterior chest wall; B2, right posterior lung). B. Axial CT image reconstructed with sinogram-affirmed IR with strength of 5 (I30f_5) shows decrease in image noise and increase in image blurring, compared with corresponding FBP image (A). In contrast, beam-hardening artifacts caused by simulated coronary arteries in phantom remain largely unchanged between two (A, B). C. Subtraction image between two CT images (B30f-I30f_5) clearly reveals subtle differences caused by application of IR algorithm that can be difficult to recognize by visual comparison. In addition to noise pattern, distinct outlines of chest phantom are seen on subtraction image, which can explain image blurring caused by IR technique. FBP = filtered back projection, IR = iterative reconstruction

• 

  Fig. 2 Histograms of subtraction images between FBP and sinogram-affirmed IR images.Histograms of three subtraction images (B30f-I30f_1, B30f-I30f_3, and B30f-I30f_5) acquired at 70 kVp and reference radiation dose (A) and acquired at 70 kVp and 90%-reduced radiation dose (B) show gradually increased horizontal stretching with increased strength of IR algorithm. Of note, degree of their horizontal stretching is more pronounced at 90%-reduced radiation dose than at reference radiation dose for corresponding subtraction pairs. HU = Hounsfield units

• 

  Fig. 3 Graphs demonstrating effects of radiation dose, tube voltage, and image reconstruction algorithm on image noise.A. Graph shows markedly increased image noise of CT image reconstructed with sinogram-affirmed IR with strength of 5 (I30f_5) between 60%- and 90%-reduced radiation dose levels. B, C. Graphs demonstrate greater image noise reduction with higher strength of IR algorithm at 70 kVp (B) and 120 kVp (C). Greatest image noise change is also noted between 60%- and 90%-reduced radiation dose levels at both 70 kVp (B) and 120 kVp (C).

• 

  Fig. 4 Graphs demonstrating effects of radiation dose, tube voltage, and image reconstruction algorithm on SNR and CNR.A, B. Graphs show greater SNR increase with higher strength of IR algorithm at 70 kVp (A) and 120 kVp (B). Greatest SNR change is noted between 60%- and 90%-reduced radiation dose levels at both 70 kVp (A) and 120 kVp (B). C, D. Graphs show greater CNR increase with higher strength of IR algorithm at 70 kVp (C) and 120 kVp (D). In contrast to SNR, greatest SNR change is noted between 60%- and 90%-reduced radiation dose levels at 70 kVp (C) but not at 120 kVp (D). CNR = contrast-to-noise ratio, SNR = signal-to-noise ratio

• 

  Fig. 5 Magnified standardized histograms of subtraction images between FBP and IR algorithm at 70 kVp.A. At reference radiation dose, all three histograms are almost normally distributed and slightly skewed to right. B. At 90%-reduced radiation dose, histogram shows almost normal distribution for strength 1, but histograms show deviated and ragged appearance for strengths 3 and 5.

• 

  Fig. 6 Magnified standardized histograms of subtraction images between IR algorithms at 70 kVp.A. At reference radiation dose, all three histograms are almost normally distributed. B. At 90%-reduced radiation dose, all three histograms appear minimally deviated and ragged as well as slightly skewed to right for all strengths.

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