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found that the overall rate of decrease in the thick-ness of the patient’s diaphragm averaged 6% per day of MV11). Furthermore, in clinical practice, diaphragm thickness has been shown to decrease rapidly within a few days of MV in approximately 40% of patients10, 12). Recently, diaphragm thickness measured during resting tidal breathing, in a cycle of spontaneous breathing, was shown to predict extubation success13, 14). However, the rate of rein-tubation within 24-72 hours of planned extubation ranged from 2% to 25%, with medical, pediatric, and multidisciplinary ICU patients at the highest risk15, 16).To date, diaphragm volume evaluations, which are also commonly used in the context of research areas such as sarcopenia, physical development, and aging17), 18), have only been determined using muscle thickness on echography. However, using echography alone to measure muscle thickness is inadequate19); echography cannot be used to evaluate the overall morphology and volume of the diaphragm. By contrast, three-dimensional computed tomog-raphy (3D-CT) can clearly reveal the cross-sec-tional thickness and can also be used to evaluate morphology and volume. Sometimes, it is very diffi-cult to differentiate the diaphragm from the liver and that is why some researchers report only the left diaphragm volume associated with the pulmo-nary function test result20). To our knowledge, several studies used workstation software applica-tions to evaluate diaphragm volume20-22); however, until now, there has been no definitive standard for diaphragm volume measurements23). Therefore, this study aimed to standardize the method of measuring diaphragm volume using 3D-CT with new criteria and to examine the validity of the new method.Materials and MethodsAll procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Declaration of Helsinki and its later amendments or comparable ethical standards. The study design was approved by the institutional review board of Juntendo University School of Medicine (approval number: 20-342). For retrospective medical record surveys that handle only existing samples, instead of omit-ting informed consent, information on the imple-mentation of research, including the purpose of the 482research, is posted by the Department of Cardio-vascular Surgery on the Juntendo University website. We guaranteed the opportunity for the research subjects to refuse. The information disclo-sure document used was approved by the institu-tional review board of Juntendo University School of Medicine. Information that can identify the research subjects is not included. In addition, we did not use the samples of the research subjects for purposes other than research purposes.Research subjectsThis was a retrospective study of existing samples. The participants comprised patients who were hospitalized or admitted to the Department of Cardiovascular Surgery at Juntendo University School of Medicine between June 2017 and January 2019. The selection criteria were as follows: patients underwent chest CT to evaluate the graft before coronary artery bypass grafting during hospitaliza-tion, and CT was performed with a 0.5-mm slice thickness (320 row multidetector Aquilion ONE ViSION Edition, Canon Medical Systems, Japan). Five patients (all male: age, 68.2±1.5 years; weight, 70.7±8.1 kg; height, 164.7±3.1 cm; BMI, 25.96±2.16 kg/m2; BSA, 1.77±0.10 m2) met the abovemen-tioned selection criteria during the study period.As 3D-CT can evaluate, not only the difference in thickness but also the morphology and total volume, this method was used in the current study (Figure 1).3D-CT assessment of the diaphragmDiaphragm volume was measured based on the method used by previous researchers24, 25). To eval-uate the total volume of the diaphragm as much as possible, in this study, we standardized the method of measuring diaphragm volume using 3D-CT with a new standard and verified the validity of the new method. To confirm the accuracy and reproduc-ibility of diaphragm muscle volume measurements on CT, measurements were recorded three times by two observers, and intraclass correlation coeffi-cients (ICCs) as well as interobserver correlations were determined. The detailed explanation of this method is provided below.All CT data were imported into the workstation software application, Attractive Medical Image Processor (PixSpace Co. Ltd., Japan) (Figure 2a).