To prospectively evaluate dose reduction and image quality characteristics of chest CT reconstructed with model-based iterative reconstruction (MBIR) compared with adaptive statistical iterative reconstruction (ASIR).
One hundred patients underwent reference-dose and low-dose unenhanced chest CT with 64-row multidetector CT. Images were reconstructed with 50 % ASIR-filtered back projection blending (ASIR50) for reference-dose CT, and with ASIR50 and MBIR for low-dose CT. Two radiologists assessed the images in a blinded manner for subjective image noise, artefacts and diagnostic acceptability. Objective image noise was measured in the lung parenchyma. Data were analysed using the sign test and pair-wise Student’s t-test.
Compared with reference-dose CT, there was a 79.0 % decrease in dose–length product with low-dose CT. Low-dose MBIR images had significantly lower objective image noise (16.93 ± 3.00) than low-dose ASIR (49.24 ± 9.11, P < 0.01) and reference-dose ASIR images (24.93 ± 4.65, P < 0.01). Low-dose MBIR images were all diagnostically acceptable. Unique features of low-dose MBIR images included motion artefacts and pixellated blotchy appearances, which did not adversely affect diagnostic acceptability.
Diagnostically acceptable chest CT images acquired with nearly 80 % less radiation can be obtained using MBIR. MBIR shows greater potential than ASIR for providing diagnostically acceptable low-dose CT images without severely compromising image quality.
• Model-based iterative reconstruction (MBIR) creates high-quality low-dose CT images.
• MBIR significantly improves image noise and artefacts over adaptive statistical iterative techniques.
• MBIR shows greater potential than ASIR for diagnostically acceptable low-dose CT.
• The prolonged processing time of MBIR may currently limit its routine use in clinical practice.
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Model-based iterative reconstruction
Adaptive statistical iterative reconstruction
Filtered back projection
Modulation transfer function
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We gratefully acknowledge Kosuke Sasaki, M.S., and Koji Segawa, R.T., for their technical support and assistance in data acquisition.
ASIR and MBIR are new iterative reconstruction (IR) algorithms. Unlike the conventional FBP, which is based on simpler mathematical assumptions of the tomographic imaging system, IR generates a set of synthesised projections by accurately modelling the data collection process in CT. The model incorporates statistical system information (including photon statistics and electronic noise in the data acquisition system) and details of the system optics (including the size of each detector cell, dimensions of the focal spot, and the shape and size of each image voxel). The synthesised image is mathematically compared and corrected with the actual measurement in order to adjust estimation of the object’s image. The technique then iterates this comparison and correction step in order to achieve close proximity between actual and measured projections. Inconsistencies in the projection measurement due to limited photon statistics and electronic noise are corrected with multiple iterations. These data-processing steps help to improve image quality from the noise and resolution perspectives, but prolong the reconstruction duration compared with FBP because of the intensive computations particularly required for incorporating system optics information.
The ASIR technique models just the photons and electronic noise statistics that primarily affect image noise, which are not as computationally intensive or time-consuming. This enables near real-time display of images at the time of imaging. ASIR also differs from other IR techniques in that the vendor provides a blending tool to blend the FBP with the ASIR images. This is accomplished by reconstruction of CT raw data with both FBP and ASIR techniques and then performing a weighted summation of each data set for the final reconstructed images. Prior phantom and clinical studies have already shown that ASIR provides diagnostically acceptable images with a reduction in image noise for low-radiation dose CT compared with the FBP algorithm [9–18].
The MBIR technique, on the other hand, is a pure IR technique that does not involve blending with FBP images, and is mathematically more complex and accurate than ASIR. MBIR not only incorporates modelling of photon and noise statistics like ASIR, it also involves modelling of system optics. This is unlike ASIR, which uses an idealised set of system optics (as does FBP), resulting in similar data utilisation per image. MBIR analyses the x-ray beam at the focal spot, then as it passes through the patient’s body and again as it strikes the detector. The algorithm weights each data point so that noisy projections have less influence on the final results, and this allows more accurate image reconstruction. Phantom experiments have shown that MBIR has the potential to further reduce image noise, improve spatial resolution and thereby allow further dose reduction without compromising image quality . With incorporation of system optics information and therefore a more accurate account of voxel and focal spot size and geometry, one can expect improvements in spatial resolution [19, 20]. Because MBIR is a complicated algorithm, using multiple iterations and multiple models, the reconstruction time is significantly longer than FBP as well as the other IR techniques, even with dedicated state-of-the-art parallel processors. The reconstruction time in the present study was about 1 h per case, although the exact time was not recorded as it was not a feature of the application software.
The rationale for the NI setting in the present study is as follows. As the specified slice thickness for a given NI setting decreases by a factor of x, maintenance of the same radiation dose requires an increase in NI by a factor of 1 divided by the square root of x. According to the previous radiology literature [13, 23], the NI of 15.75 at a slice thickness of 2.5 mm has been used for reference-dose chest CT. Multiplying 15.75 by the square root of 4 (= 2.5/0.625) results in 31.5. Theoretically, the fixed NI of 31.5 at 0.625 mm should achieve the same radiation dose for reference-dose chest CT from the previous radiology literature. As for the NI setting for low-dose CT, we referred to the average ED for low-dose CT described in the National Lung Screening Trial (NLST, 1.5 mSv) , which is about one fifth of the radiation dose for reference-dose chest CT described in the previous radiology literature [13, 23]. As the radiation dose decreases by 1/y, the image noise increases by the square root of y. Multiplying the NI for reference-dose CT in the present study (= 31.5) by the square root of 5 results in an NI of 70.44.
Subjective image noise was defined as overall graininess or mottle in the lung parenchyma, and was assessed in the lung window setting on a five-point scale (1 = no or only minimal image noise, 2 = less than average image noise, 3 = average image noise, 4 = more than average or substantial image noise that may interfere with diagnostic decision-making in less than half of the lung parenchyma, and 5 = more than average or substantial image noise that may interfere with diagnostic decision-making in more than half of the lung parenchyma). Artefacts were graded on a three-point scale (1 = artefacts unapparent or only minimally recognisable, 2 = artefacts recognised but not interfering with diagnostic decision-making, and 3 = substantial artefacts recognised affecting diagnostic decision-making). The following artefacts were assessed: streak artefacts, motion artefacts due to heart wall motion and blotchy pixellated appearance at the tissue interface (Fig. 2b). Critical reproduction of visually sharp anatomical structures was assessed. The following anatomical structures were evaluated: pulmonary fissures; secondary pulmonary lobular structures such as interlobular arteries; large- and medium-sized pulmonary vessels; small pulmonary vessels; large- and medium-sized bronchi; small bronchi; the pleuromediastinal border; the border between the pleura and the thoracic wall; the thoracic aorta; anterior mediastinal structures including thymic residue; the trachea and main bronchi; paratracheal tissue; the carina and lymph node area; and the oesophagus. Diagnostic acceptability was assessed on a four-point scale (1 = fully acceptable, 2 = probably acceptable, 3 = deemed acceptable only for limited clinical conditions, and 4 = unacceptable).
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Katsura, M., Matsuda, I., Akahane, M. et al. Model-based iterative reconstruction technique for radiation dose reduction in chest CT: comparison with the adaptive statistical iterative reconstruction technique. Eur Radiol 22, 1613–1623 (2012). https://doi.org/10.1007/s00330-012-2452-z
- Model-based iterative reconstruction
- Adaptive statistical iterative reconstruction
- Radiation dose reduction
- Image noise
- Spatial resolution