Faculty Profiles - SAITO Masatoshi

Papers - SAITO Masatoshi

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A simple algorithm to derive virtual non‐contrast electron density from dual‐energy computed tomography data for radiotherapy treatment planning Reviewed

Masatoshi Saito, Tatsuki Saito

Medical Physics 52 ( 5 ) 3107 - 3116 2025.1

Authorship：Lead author,　Corresponding author Publishing type：Research paper (scientific journal) Publisher：Wiley

Abstract

Background

The use of iodinated contrast‐enhancing agents in computed tomography (CT) improves the visualization of relevant structures for radiotherapy treatment planning (RTP). However, it can lead to dose calculation errors by incorrectly converting a CT number to electron density.

Purpose

This study aimed to propose an algorithm for deriving virtual non‐contrast (VNC) electron density from dual‐energy CT (DECT) data. This algorithm was developed by extending the formula previously developed by Saito, which enables the calculation of the electron density of human tissue through weighted subtraction of CT numbers acquired from DECT scans.

Methods

To investigate the feasibility of the proposed VNC algorithm, we performed analytical DECT image simulations at 90 and 150 kV/Sn on virtual phantoms consisting of various tissue/iodine surrogates with known mass densities and elemental compositions. Two different shapes of phantoms made of water‐mimicking surrogates were generated as inputs: a circular phantom (33 cm diameter) for calibration and an elliptical phantom (33 cm width and 28 cm height) for validation. The circular phantom was equipped with inserts of human‐tissue‐mimicking substitutes, pure water, and iodine‐enhanced soft‐tissue substitutes (2, 5, 10, and 15 mg/mL iodine). The elliptical phantom contained inserts of reference human tissues, iodine‐enhanced soft‐tissue substitutes (2, 2.5, 5, 7.5, 10, 15, and 20 mg/mL iodine), and a 10‐mm‐diameter core of 4 mg/mL iodine surrounded by a blood‐mimicking base material. The performance of the proposed algorithm was evaluated by comparing the accuracy of VNC electron densities with those of non‐contrast (NC) base materials (water‐ or blood‐mimicking surrogates).

Results

The derived algorithm enabled the calculation of VNC electron density in a manner similar to that of unenhanced human tissues by adapting a VNC‐specific weighting factor, thereby eliminating the intermediate step of converting CT numbers to electron density. The simulated results showed that the VNC algorithm could almost completely remove the contrast in the electron density image between iodine‐enhanced and base materials. The relative deviations of simulated VNC electron density values from the corresponding pre‐contrast value were within ± 0.4% for all tested materials, with a root‐mean‐square error (RMSE) of 0.2%.

Conclusions

Within the limits of the analytical DECT image simulation used in this study, the simple VNC algorithm could effectively provide accurate VNC electron densities for iodine‐enhanced materials. This may allow the contrast agent to be used for CT scans during RTP without compromising dose calculation accuracy.

DOI： 10.1002/mp.17648

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Technical note: Implementing MRI/CT‐based elemental concentration data to Monte Carlo simulation for yielding positron emitters in proton therapy Reviewed

Masatoshi Saito, Ryuga Matsumoto

Medical Physics 51 ( 4 ) 2861 - 2870 2023.12

Authorship：Lead author,　Corresponding author Publishing type：Research paper (scientific journal) Publisher：Wiley

Abstract

Background

The elemental concentration (especially oxygen and carbon) and mass density must be accurately assigned to perform Monte Carlo (MC) simulations for predicting proton‐induced nuclear reactions in the human body. We recently proposed an approach to quantify elemental concentrations and mass densities of human soft tissues from water content (WC) data obtained by quantitative magnetic resonance (MR) imaging (which we called “MRWC”).

Purpose

This study presents the first implementation of MRWC‐derived elemental concentrations and mass densities as complementary inputs into MC simulations on a virtual head phantom, and demonstrates the simulation of positron emitter production yields in proton therapy.

Methods

An MC code, PHITS, was used to simulate proton therapy with a monoenergetic 140 MeV beam for a digital head phantom provided by BrainWeb. Three different head images were synthesized as inputs: a conventional CT image, an ideal CT image as a reference, and a WC image coupled with the bone‐only CT image for a hybrid approach (MRWC/CT). Thereafter, the performance of the MRWC/CT method was evaluated by comparing its accuracy in predicting the production yields of positron emitters (11C and 15O) with the gold‐standard CT‐only method.

Results

The MRWC/CT method could predict 11C and 15O production yield maps that closely resembled the corresponding reference maps, while the CT‐only method failed. The structural similarity index measures between the reference and CT‐ or MRWC/CT‐derived maps were improved from 0.67 (CT‐only) to 0.87 (MRWC/CT) for 11C and 0.76 (CT‐only) to 0.93 (MRWC/CT) for 15O. Furthermore, applying post‐processing normalizations to account for elemental density variations in the production yields of positron emitters facilitated the determination of distal fall‐off positions in depth activity profiles.

Conclusion

At least in the head area, the MRWC/CT method demonstrated potential for more precise predictions of proton‐induced positron emitter distributions via MC simulations than that of the CT‐only method.

DOI： 10.1002/mp.16896

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MRI‐based quantification of carbon and oxygen concentrations in human soft tissues for range verification in proton therapy Reviewed

Masatoshi Saito

Medical Physics 50 ( 9 ) 5671 - 5681 2023.3

Authorship：Lead author,　Corresponding author Publishing type：Research paper (scientific journal) Publisher：Wiley

ABSTRACT

Background

In‐situ range verification of particle therapy based on the detection of secondary emitted radiation requires highly accurate assignment of elemental concentrations (particularly carbon and oxygen) in the human body.

Purpose

A method for quantitatively predicting carbon and oxygen concentrations in human soft tissues is proposed. This method relies on an empirical one‐to‐one correspondence between the mass fraction and water content (WC), which is a measurable tissue quantity based on magnetic resonance (MR) imaging (referred to as “MRWC‐based method”).

Methods

A numerical analysis of the MRWC‐based method was performed for 47 standard human soft tissues tabulated in the literature as objects of interest with unknown mass fractions of the four main elements—C, O, H, and N. Thereafter, the method was evaluated in terms of the mass‐fraction quantification accuracy by comparing it with the gold‐standard CT‐based method developed by Schneider et al. The MRWC‐based method was also applied to the MR imaging data of a virtual head phantom obtained from a three‐dimensional MRI‐simulated brain database.

Results

The predicted mass fractions in a range of human soft tissues were in better agreement with the reference values than those predicted by the CT‐based method. The mean absolute errors of the predicted mass% values for the overall standard soft tissues could be reduced from 4.8 percentage points (pp) (CT‐based) to 0.5 pp (MRWC‐based) for carbon and from 5.2 pp (CT‐based) to 0.4 pp (MRWC‐based) for oxygen. The application to the simulated MRI data confirmed the capability of the sufficient recognition of the boundaries between the white matter and gray matter in the brain that could not be realized by the CT‐based method. Thus, the MRWC‐based method exhibits superior performance in the prediction of carbon and oxygen concentrations in soft tissues.

Conclusions

This study is limited to a proof‐of‐concept scope but demonstrates the feasibility of the MRWC‐based method for the generation of elemental images of human soft tissues from MRI‐derived water‐content images.

DOI： 10.1002/mp.16353

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1002/mp.16353

Quadratic relation for mass density calibration in human body using dual‐energy CT data Reviewed

Masatoshi Saito

Medical Physics 48 ( 6 ) 3065 - 3073 2021.6

Proton dose calculation based on converting dual-energy CT data to stopping power ratio (DEEDZ-SPR): a beam-hardening assessment Reviewed International journal

Sodai Tanaka, Yoshiyuki Noto, Satoru Utsunomiya, Takaaki Yoshimura, Taeko Matsuura, Masatoshi Saito

Physics in Medicine & Biology 65 ( 23 ) 235046 - 235046 2020.12

Authorship：Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：IOP Publishing

To achieve an accurate stopping power ratio (SPR) prediction in particle therapy treatment planning, we previously proposed a simple conversion to the SPR from dual-energy (DE) computed tomography (CT) data via electron density and effective atomic number (Z eff) calibration (DEEDZ-SPR). This study was conducted to carry out an initial implementation of the DEEDZ-SPR conversion method with a clinical treatment planning system (TPS; VQA, Hitachi Ltd., Tokyo) for proton beam therapy. Consequently, this paper presents a proton therapy plan for an anthropomorphic phantom to evaluate the stability of the dose calculations obtained by the DEEDZ-SPR conversion against the variation of the calibration phantom size. Dual-energy x-ray CT images were acquired using a dual-source CT (DSCT) scanner. A single-energy CT (SECT) scan using the same DSCT scanner was also performed to compare the DEEDZ-SPR conversion with the SECT-based SPR (SECT-SPR) conversion. The scanner-specific parameters necessary for the SPR calibration were obtained from the CT images of tissue substitutes in a calibration phantom. Two calibration phantoms with different sizes (a 33 cm diameter phantom and an 18 cm diameter phantom) were used for the SPR calibrations to investigate the beam-hardening effect on dosimetric uncertainties. Each set of calibrated SPR data was applied to the proton therapy plan designed using the VQA TPS with a pencil beam algorithm for the anthropomorphic phantom. The treatment plans with the SECT-SPR conversion exhibited discrepancies between the dose distributions and the dose-volume histograms (DVHs) of the 33 cm and 18 cm phantom calibrations. In contrast, the corresponding dose distributions and the DVHs obtained using the DEEDZ-SPR conversion method coincided almost perfectly with each other. The DEEDZ-SPR conversion appears to be a promising method for providing proton dose plans that are stable against the size variations of the calibration phantom and the patient.

DOI： 10.1088/1361-6560/abae09

PubMed

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Other Link： https://iopscience.iop.org/article/10.1088/1361-6560/abae09/pdf

Simulation of photon-counting detectors for conversion of dual-energy-subtracted computed tomography number to electron density Reviewed

Masatoshi Saito

Radiological Physics and Technology 12 ( 1 ) 105 - 117 2019.3

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：Springer Science and Business Media LLC

DOI： 10.1007/s12194-018-00497-0

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Other Link： http://link.springer.com/article/10.1007/s12194-018-00497-0/fulltext.html

Comment on: Methodological accuracy of image‐based electron density assessment using dual‐energy computed tomography [Med. Phys. 44(6), 2429–2437 (2017)] Reviewed

Masatoshi Saito

Medical Physics 46 ( 2 ) 1075 - 1076 2019.2

Comment on: Dual-energy CT quantitative imaging: A comparison study between twin-beam and dual-source CT scanners [Med. Phys. 44(1), 171-179] Reviewed

Masatoshi Saito

Medical Physics 45 ( 8 ) 3996 - 3996 2018.8

Simplified derivation of stopping power ratio in the human body from dual-energy CT data Reviewed

Masatoshi Saito, Shota Sagara

MEDICAL PHYSICS 44 ( 8 ) 4179 - 4187 2017.8

A simple formulation for deriving effective atomic numbers via electron density calibration from dual-energy CT data in the human body Reviewed

Masatoshi Saito, Shota Sagara

MEDICAL PHYSICS 44 ( 6 ) 2293 - 2303 2017.6

Technical Note: Relation between dual-energy subtraction of CT images for electron density calibration and virtual monochromatic imaging Reviewed

Masatoshi Saito

MEDICAL PHYSICS 42 ( 7 ) 4088 - 4093 2015.7

Initial implementation of the conversion from the energy-subtracted CT number to electron density in tissue inhomogeneity corrections: An anthropomorphic phantom study of radiotherapy treatment planning Reviewed

Masayoshi Tsukihara, Yoshiyuki Noto, Ryuta Sasamoto, Takahide Hayakawa, Masatoshi Saito

MEDICAL PHYSICS 42 ( 3 ) 1378 - 1388 2015.3

Technical Note: Exploring the limit for the conversion of energy-subtracted CT number to electron density for high-atomic-number materials Reviewed

Masatoshi Saito, Masayoshi Tsukihara

MEDICAL PHYSICS 41 ( 7 ) 071701 2014.7

Conversion of the energy-subtracted CT number to electron density based on a single linear relationship: an experimental verification using a clinical dual-source CT scanner Reviewed

Masayoshi Tsukihara, Yoshiyuki Noto, Takahide Hayakawa, Masatoshi Saito

PHYSICS IN MEDICINE AND BIOLOGY 58 ( 9 ) N135 - N144 2013.5

Potential of dual-energy subtraction for converting CT numbers to electron density based on a single linear relationship Reviewed

Masatoshi Saito

MEDICAL PHYSICS 39 ( 4 ) 2021 - 2030 2012.4

Optimized low-kV spectrum of dual-energy CT equipped with high-kV tin filtration for electron density measurements Reviewed

Masatoshi Saito

Medical Physics 38 ( 6 ) 2850 - 2858 2011

Structural Study of Zr_<50>Cu_<50> Amorphous Alloy by Anomalous X-ray Scattering Coupled with Reverse Monte-Carlo Simulation

KAWAMATA T., YOKOYAMA Y., SAITO M., SUGIYAMA K., WASEDA Y.

Materials Transactions, JIM 51 ( 10 ) 1796 - 1801 2010.10

Language：English Publisher：The Japan Institute of Metals and Materials

Anomalous X-ray scattering (AXS) analysis was applied for the structural analysis of a rapidly quenched Zr50Cu50 amorphous alloy. The present AXS analysis provided the environmental structural information round Cu and Zr, and the subsequent reverse Monte Carlo (RMC) simulation allowed us to obtain three partial pair distribution functions together with a three-dimensional structural model. The Voronoi polyhedral analysis of the nearest-neighbor region confirms that the structural features are similar to those of the dense random packing of hard spheres (DRPHS) model. The chemical oscillatory structure composed of alternating Cu-rich and Zr-rich coordination shells was realized in the middle range, up to 0.8 nm. The shell structure is suggested to be one of the fundamental structural features of medium-range ordering in the Zr50Cu50 amorphous alloy.

DOI： 10.2320/matertrans.MAW201016

CiNii Article

CiNii Books

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Other Link： https://jlc.jst.go.jp/DN/JALC/00356579564?from=CiNii

Technical note: Optimization for improved tube-loading efficiency in the dual-energy computed tomography coupled with balanced filter method Reviewed

Masatoshi Saito

Medical Physics 37 ( 8 ) 4182 - 4185 2010

Electron density distribution in liquid Ga estimated from the anomalous X-ray scattering measurements Reviewed

Masatoshi Saito, Kazumasa Sugiyama, Yoshio Waseda

High Temperature Materials and Processes 29 ( 5-6 ) 393 - 403 2010

Structural study of liquid LiNbO3 by the high-temperature energy dispersive X-ray diffraction coupled with reverse Monte Carlo simulation Reviewed

K. Sugiyama, M. Saito, Y. Waseda

Journal of Crystal Growth 311 ( 3 ) 966 - 969 2009.1

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