This article gives dominant research contributions of the late Dr. Eiichi Tanaka in the field of very weak radioactivity measurements with a new type of an extremely low background β-ray scintillation spectrometer, which consists of a large plastic scintillator and a gas-flow GM counter located in the small hollow in the bottom part of the plastic scintillator. The pulse signals from plastic scintillator are coincidence-gated by the pulses from the GM counter. Dominant background signals due to the energetic muons still appear in the higher region than 3.5 MeV even with coincidence gating mode, but extreme reduction of background can be attained in the 0～3 MeV region which is essentially important in the β-ray spectrometry for most of radionuclides.

The purpose of this article is to introduce fundamental studies on improvement of count rate performance of scintillation detectors which Dr. Eiichi Tanaka dedicated himself to carrying out. He proposed a new technique based on the combination of pulse shortening and selective integration in which the integration period is not fixed but shortened by the arrival of the following pulse. Theoretical analysis of the degradation of the statistical component of resolution is made for the proposed system with delay line pulse shortening, and the factor of resolution loss is formulated as a function of the input pulse rate. A new method is also presented for determining the statistical component of resolution separately from the non-statistical system resolution. Preliminary experiments with a NaI (Tl) detector had been carried out, the results of which are consistent with the theoretical prediction. The related works are also introduced.

At present scintillation gamma camera plays key role in nuclear medicine as planner and tomographic single photon imaging modality. The basic technology of gamma camera made remarkable progress carried out by many researchers and engineers from around 1970 to 1980 and reached some matured stage as analog signal processing. Among them Dr. E. Tanaka made key contribution through the proposal of “Ideal arithmetics for position computer” (1969) and its implementation by the delay line time conversion (1970). These works triggered other unique position computing method applied to gamma camera products. This article reviews these technological progresses and outlines the next advances of gamma camera in the digital computer era.

This article gives a commentary on the development of positron emission tomography scanners based on Positology which was proposed by Dr. Eiichi Tanaka. Circular detector rings of an equally spaced detector arrangement have a problem of linear sampling density distributions in the projections. The problem can be solved by Positology in which a continuously rotating detector ring of an unequally spaced detector arrangement obtained by computer iterative searching provides a fine and uniform sampling density distribution. The imaging performance of multilayer positron emission tomography scanners for extended sources is evaluated analytically. The analysis is simplified by “rotation transform,” by which three-dimensional photon detection problems are solved by two-dimensional treatment. Event rates of singles, unscattered true coincidence, and single- and double-scattered coincidence are formulated for a uniform cylindrical phantom. Positron emission tomography scanners based on Positology, named Positologica, were developed for brain, whole-body and animal study. A high resolution stationary PET scanner, SHR-1200 was also developed for brain study.

The purpose of this article is to introduce achievement and philosophy on studies of SPECT reconstruction algorithms which Dr. Eiichi Tanaka dedicated himself to carrying out. The method that Dr. Tanaka suggested first (in 1974) was to reconstruct analytically a section image from the projection data convoluted the correction functions. The correction functions were optimized to yield a maximum ‘signal-to-noise ratio’. In 1983 and 1984, two image reconstruction algorithms, WBP and RPC were suggested assuming a uniform attenuation in the object. Dr. Tanaka understood intuitively that these methods had few noise and distortions in reconstructed images. WBP was an approximate solution, but three type analytically methods were reported by Tretiak O, et al., Bellini S, et al., and Inoyue T, et al. In 1995, Metz CE, et al. and Kudo H, et al. succeeded independently each other in finishing a general theory of the analytic absorption Radon Conversion by using parameter “n”. The relations among the above three analytically methods and WBP were explained, beautifully. The important thing in the study is a flexible idea such as catching a problem as the issue of black box or reverse and the result to be concise and elegant.

In memoriam of Dr. Eiichi Tanaka who passed away on August 21, 2021, I review his achievement in the research of tomographic image reconstruction. Tomographic image reconstruction is an important research area which has wide applications including X-ray CT, nuclear medicine imaging such as PET and SPECT, and electron microscopy. Since 1970’s, Dr. Tanaka has worked on numerous important topics in tomographic reconstruction fields aiming at using them in image reconstruction for PET and SPECT. Among them, in this paper, I will introduce his research on Filtered BackProjection (FBP) method, analytical attenuation correction in SPECT, image reconstruction in Time-of-Flight PET, image reconstruction for 3-D PET imaging, and iterative image reconstruction method called Dynamic Row-Action Maximum Likelihood Algorithm (DRAMA).

This is a review on history of medical physics in Radiological Physics and Technology published by JSRT and JSMP (https://www.jsmp.org/en/).