[Purpose] Statistically-based image reconstruction (SIR) methods that have been incorporated into commercial CT scanners have succeeded in promoting low-dose CT with high image quality in comparison with scanners using the filtered back-projection (FBP) method. Not only researchers but also medical doctors and technologists engaged in CT studies have an interest in the algorithms of the SIR methods, however, the algorithms have not been made available to users by the CT manufacturers. Kudo reviewed the fundamentals of SIR methods on the basis of the articles published by the joint research group of each manufacturer released before product development (Med Imag Tech 32: 239–248, 2014). He classified the SIR methods into true iterative reconstruction (true IR), hybrid IR, and image space denoising (ISD) methods. His review article has made a significant contribution to the CT community of users. However, the reconstructed images obtained by those methods have not been presented. Our purpose in this study is to implement the mathematical equations of three IR methods, one each of the true IR, hybrid IR and ISD methods, and evaluate their image quality.

[Methods] The system matrix of IR methods used in commercial CT scanners uses a physical photon detection process based on the finite size of an X-ray focal spot, the beam width, and the X-ray detector. However, we assumed the X-ray beam was a pencil beam and the system matrix was then given by the line integral of linear attenuation coefficients because we focus on the image quality in the ideal photon detection system equations given by Kudo. Total variation (TV) was used for regularization of the true IR, hybrid IR and ISD methods. Four kinds of numerical phantoms with 256×256 pixels were used as test images. Gaussian noise of 15, 20, 25, and 30 dB was added to the projection data with 256 linear samplings and 256 views over 180°.

[Results] Root mean square errors (RMSEs) of the true IR, hybrid IR, and ISD methods were 4.28–5.70, 15.87–16.47, and 16.94–17.17, respectively. RMSE of the FBP method ranged from 27.64–33.02 and that of the FBP method processed with a Gaussian filter of FWHM (full width at half maximum) of 3 pixels ranged from 8.14–17.28. The image quality of the true IR method was superior to that of the hybrid IR and ISD methods and the FBP method.

[Discussion] The noise was slightly suppressed by including the variance of projection data; however, the regularization was inevitable even if the noise levels were in the range of 25–30 dB. The noise was not suppressed sufficiently by the hybrid IR and ISD methods because the noise due to the FBP image used as the initial image for these IR methods has a dominating effect in successive reconstruction or denoise processing. Mathematical equations of each IR method were easily realized by observing the intermediate images such as the regularization term of the iteration process. In addition to these equations, the reconstructed images by the SIR methods and their RMSEs presented in this study are useful in CT research.

[Conclusions] The fundamental point of SIR methods is the regularization term used in minimizing the object function.

Advanced radiotherapy such as intensity-modulated radiotherapy offers many advantages of high accuracy and efficiency of radiotherapy. To date, many technical guidelines with description of quality assurance and quality control have been reported. However, some reports indicated that human factor and environment is major cause of radiotherapy incidents. If radiotherapy systems depend on automation and computer system, individual risk management is degraded and ability of preventing radiotherapy incidents weaken. Recently, the American Association of Physicists in Medicine (AAPM) task group-100 was reported and it has a new concept guideline, which proposed the comprehensive risk management and education of non-technical skills for overall radiotherapy processes. The TG-100 recommends implementation of process map, reporting system, risk assessment such as failure mode and effects analysis (FMEA) and fault tree analysis (FTA) especially for advanced radiotherapy. In this paper, we described effective and efficient procedures to improve the treatment processes and education of non-technical skills using the such management tools proposed by the TG-100 guide-lines.

In past decade, several reports for patient safety in radiotherapy were published. Process of radiotheray has been recognized complex because its sub-processes are performed with interaction by multidisciplinaly team. Thus, there are many opportunities to occur human failure such as communication error and equipment operation error in the sub-process. This tutorial paper was focused non-technical issues towards patient safety in external radiotherapy.

Quantification of PET/CT and SPECT/CT using standardized uptake value (SUV) is affected by many factors related to technical factors, such as scanner calibration, imaging physics related factors and patient related factors. Here, I briefly describe some world trend in accuracy of SUVs in PET/CT and SPECT/CT, followed by present and future strategies for clinical practice using SUVs. Finally, I also provided the results of multi-center phantom studies in Japan using SUVs of PET/CT and SPECT/CT.

18F-FDG PET/CT has an important role in radiation therapy planning. FDG PET/CT parameters such as standard uptake value and metabolic tumor volume provide important prognostic and predictive information. Importantly, FDG PET/CT for radiation planning has added biological information in defining the gross tumor volume (GTV) as well as involved nodal disease. Several studies have shown that PET has an impact on radiation therapy planning in an important proportion of patients.

On the other hands, FDG PET/CT for radiation therapy planning has several limitations. First of all, the method to determine the optimal threshold of FDG PET/CT images that generates the best volumetric match to GTV is not established. The size of the GTV derived from FDG accumulation changes significantly depending on the threshold value, the threshold value can affect the clinical target delineation. Secondly, FDG is not a cancer-specific agent, and false positive findings in benign diseases have been reported. PET/CT simulation for radiation therapy planning requires cooperation of other professions and sufficient physical assessment.

Medical Physics education and clinical training should be available at the university with the attachment in clinical practice at department of radiology. The curriculum should cover physics in all three specialties of radiation oncology, diagnostic radiology and nuclear medicine. Anatomy and physiology for medical physicist, Radiation physics, Radiation dosimetry, Radiation Biology and Radiation Protection should be core subjects. Selected subjects, Medical Physics Seminar, Research Methodology and thesis are included in the curriculum.