Model based iterative reconstruction has a distinct
correlation characteristic between image noise and radiation
dose compared to conventinal CT reconstruction methods: a
phantom study
Poster No.:
C-1218
Congress:
ECR 2012
Type:
Scientific Exhibit
Authors:
I. Matsuda, M. Katsura, M. Akahane, J. Sato, K. Yasaka, A.
Kunimatsu, K. Ohtomo; Tokyo/JP
Keywords:
Artifacts, Technology assessment, Experimental investigations,
Computer Applications-Teleradiology, Image manipulation /
Reconstruction, CT, Computer applications
DOI:
10.1594/ecr2012/C-1218
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Page 1 of 11
Purpose
1. Filtered back projection (FBP), which is widely used reconstruction method, has
intrinsic problem on trade-off between image quality and radiation exposure. This basic
relationship is expressed in a simple formula;
•
image noise is inversely proportional to square root of radiation dose.
2. Iterative reconstruction methods, such as adaptive statistical iterative reconstruction
(ASIR), have been recently increasingly known as means for reducing radiation dose. But
their noise-to-dose relationships are not well known. Model based iterative reconstruction
(MBIR) is a newly introduced reconstruction method for CT that incorporates optical
models in its iteration process to enable more accurate image reconstruction than
other conventional methods. We tried to demonstrate fundamental relationships between
radiation dose and image noise in the three reconstruction methods; MBIR, ASIR and
FBP.
3. The purpose of our study is to investigate relationships between image noise and
radiation dose with a phantom in MBIR, ASIR and FBP.
Methods and Materials
A phantom (PH-9, Kyoto-Kagaku, Kyoto, Japan) was scanned with a 64-detector CT
scanner (Discovery CT750HD, GE healthcare, Tokyo, Japan). Scanning were performed
at 400, 200, 100, 50, 10, 5 mAs in tube current-time product. The other scanning
parameters were, tube peak voltage, 120 kVp; detector coverage, 40 mm; beam pitch,
0.984; rotation speed, 0.5 sec.
The scanned images were reconstructed with MBIR, ASIR and FBP. Then blended
images of ASIR and FBP at blending ratio 50% were made, which were named ASIR50.
Images reconstructed with ASIR were named ASIR100.
Regions of interest were placed on homogenous part of the images to measure
standard deviation (SD) of CT value as image noise. An example of ROI placement is
demonstrated in Fig. 1 on page 2. We measured SD 1 time for each reconstructed
image.
Correlation between image noise and radiation dose was investigated on a
double logarithmic plot with regression coefficient because the coefficient represents
mathhmatical order of the proportional relationship between image noise and radiation
dose.
Images for this section:
Page 2 of 11
Fig. 1: An image of the phantom. 400 mAs, reconstructed with FBP. ROI was placed in
homogenous part of the phantom.
Page 3 of 11
Results
SD of the images were 11, 15.13, 21.42, 32.33, 71.01 and 101.18 with FBP; 8.18, 10.95,
15.15, 22.24, 50.40 and 76.47 with ASIR50; 5.36, 7.39, 10.41, 14.55, 33.78 and 50.23
with ASIR100; and 5.77, 6.76, 7.71. 9.65, 13.31, 17.75 with MBIR for 400, 200, 100, 50,
10, 5 mAs, respectively. The double logarithmic plot of the SDs was shown in Fig. 2 on
page 4. The regression coefficients in the double logarithmic plot were -0.51, -0.51,
-0.51, -0.25 for FBP, ASIR50, ASIR100 and MBIR, respectively.
For illustration purposes, images at 50 mAs reconstructed with FBP, ASIR and MBIR
are demonstrated in Fig. 3 on page 4, Fig. 4 on page 5 and Fig. 5 on page 6.
Images for this section:
Fig. 2: The bilogarithmic plot of the results. FBP, ASIR50 and ASIR100 seem to be
parallel on the graph and thier regression coefficients are approximately -1/2. MBIR is not
parallel to the others and its regression coefficients is approximately -1/4. MBIR reduces
more image noise than the others, especially in lower radiation dose range.
Page 4 of 11
Fig. 3: An image reconstructed with FBP at 50 mAs. Image SD was 32.33. Low contrast
structure of the phantom is hardly recognizable. Compare with Fig 4 and Fig 5 for
reference.
Page 5 of 11
Fig. 4: An image reconstructed with ASIR at 50 mAs. Image SD was 14.55. Low contrast
structure of the phantom is better perceived than in the image reconstructed with FBP
(Fig 3). Compare with Fig 3 and Fig 5 for reference.
Page 6 of 11
Fig. 5: An image reconstructed with MBIR at 50 mAs. Image SD was 9.65. Low contrast
structure of the phantom is depicted well. Compare with Fig 3 and Fig 4 for reference.
Page 7 of 11
Conclusion
MBIR and ASIR reduced image noise substantially than FBP. ASIR100 reduced image
noise to one-half of FBP. ASIR50 showed intermediate reduction of image noise between
FBP and ASIR100 as expected because it is an average image of FBP and ASIR100.
MBIR showed best noise reduction among them (Fig. 2 on page 8).
Regression coefficient of FBP approximated -1/2, which is expected by the rule
mentioned in the purpose section "image noise is inversely proportional to square root of
radiation dose". In this figure '-1/2', minus means inversely and 1/2 means mathematical
order of square root.
ASIR50 and ASIR100 seems parallel to FBP on the graph and their regression
coefficients are near -1/2. This result implies that the basic rule of image noise and
radiation dose is applicable to ASIR as well as FBP.
Unlike the other reconstruction methods, MBIR has a different characteristic of noiseto-dose relation. The regression coefficient is near -1/4, which implies the basic rule is
not applicable to MBIR. As a result, noise reduction by MBIR is relatively larger with lower
radiation dose.
The basic rule that states image noise is inversely proportional to square root of radiation
dose comes from Poisson distribution of detected X-ray photon. MBIR is considered to
escape from this statistical effect by employing more correct and intricate physical model
in its iteration process.
In conclusion, with FBP and ASIR, our results were nearly consistent with classical
theory that states image noise is inversely proportional to square root of radiation dose.
With MBIR, however, the results were essentially different in that image noise was
inversely proportional to biquadratic root of radiation dose. As a result, MBIR achieved
much more noise reduction especially in lower radiation dose range.
Images for this section:
Page 8 of 11
Fig. 2: The bilogarithmic plot of the results. FBP, ASIR50 and ASIR100 seem to be
parallel on the graph and thier regression coefficients are approximately -1/2. MBIR is not
parallel to the others and its regression coefficients is approximately -1/4. MBIR reduces
more image noise than the others, especially in lower radiation dose range.
Page 9 of 11
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Personal Information
Izuru Matsuda, MD.
Department of radiology, Graduate school of medicine, the university of Tokyo, Tokyo,
Japan.
imatsuda-tky@umin.ac.jp
Page 11 of 11