Takashi Yokoi

Osaka University, Ōsaka-shi, Osaka-fu, Japan

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Publications (7)10.41 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: In nuclear medicine, cerebral vascular reserve(CVR) is evaluated using technetium-99m ethyl cysteinate dimer [99mTc-ECD] and acetazolamide(ACZ). We developed a protocol involving the intravenous injection of 99mTc-ECD in three divided doses(TIE method), and have found that the cerebrovascular response to ACZ depended on time after ACZ administration. However, it was difficult to obtain high-precision quantitative SPECT images by the conventional method because of complicated image processing and image degradation accompanying image subtraction. We recently developed software known as the Automatic Quantitative CVR Estimation Tool(hereinafter referred to as Triple AQCEL), which, after the input of simple parameters, enables us to carry out automatic reconstruction of quantitative SPECT images without image degradation due to subtraction. Triple AQCEL was determined to reduce image degradation caused by subtraction and to provide valid quantitative data. Because Triple AQCEL does not require manual determination of ROI or image selection for the reconstruction of quantitative SPECT images, reproducibility of regional cerebral blood flow by 3DSRT is ensured. Since all analyses in evaluation by the TIE method are automated and the operator plays no part in them, with the resulting increase of throughput, this software will contribute to improved reproducibility of regional cerebral blood flow data, and will be useful in clinical pathophysiological assessment both preoperatively and during postoperative follow-up.
    Nippon Hoshasen Gijutsu Gakkai zasshi 06/2007; 63(5):563-9. DOI:10.6009/jjrt.63.563
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    ABSTRACT: The following process conventionally has been followed to develop quantitative images of cerebral blood flow: (1) mean cerebral blood flow (mCBF) is calculated by the Patlak plot method; (2) a SPECT slice that includes the basal ganglia is selected; and (3) based on the value of mCBF calculated by the Patlak plot method, the SPECT slice is corrected by the Lassen method and developed into a SPECT image of quantitative regional cerebral blood flow. However, this process is complicated, and the values of rCBF have been reported to fluctuate because selection of the SPECT slice and the ROI setting are in the hands of the operator. We have developed new software that automates this analysis. This software enables automatic processing simply by inputting the value of mCBF in the normal hemisphere. Since there is no need for manual operations such as setting the ROI, reproducibility is improved as well. Regional cerebral blood flow as determined by this software is quite similar to that calculated by the conventional method, so the existing clinical evaluation does not need to be changed. This software is considered to be useful.
    Nippon Hoshasen Gijutsu Gakkai zasshi 06/2006; 62(5):729-33. DOI:10.6009/jjrt.62.729

  • Annals of Nuclear Medicine 03/2005; 19(2):165-166. DOI:10.1007/BF03027398 · 1.68 Impact Factor
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    ABSTRACT: Mean cerebral blood flow (mCBF) in the slice including the basal ganglia (reference slice) is necessary for the quantification of regional CBF using Patlak plot and BUR methods on 99mTc-ECD cerebral perfusion SPECT. The mCBF was calculated from the mean counts of this slice. A region of interest (ROI) has been manually set on the reference slice to obtain the mean counts (manual ROI method). However, there was large variability observed in the value of rCBF in this method. We developed a 3DSRT method for improving the accuracy of the mean counts in the reference slice and evaluated the difference between the value of rCBF on manual ROI method and that on 3DSRT method in consecutive 11 patients with cerebral vascular disease. Difference in the value of mean counts of the reference slice was distributed within the 2 standard deviations (SD) with Blant-Altman analysis in 9 of 11 patients. Significant difference in the value of mean counts between two methods was observed in 2 of 11 patients. 3DSRT method is superior accuracy to the manual ROI method in the evaluation of the counts in the ROI. Lower accuracy in manual ROI method, therefore, results in the difference of the value of mean counts. 3DSRT method provides high accuracy with the various quantitative methods for the evaluation of rCBF using 99mTc-ECD.
    Kaku igaku. The Japanese journal of nuclear medicine 03/2005; 42(1):11-6.
  • Source
    Takashi Yokoi · Tsutomu Soma · Hiroyuki Shinohara · Hiroshi Matsuda ·
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    ABSTRACT: We implemented a 3D co-registration technique based on mutual information (MI) including 2D image matching as a coarse pre-registration. The 2D coarse pre-registration was performed in the transverse, sagittal and coronal planes sequentially, and all six parameters were then optimized as fine registration. Normalized mutual information (NMI) was also examined as another entropy-based measure that was invariant to the overlapped area of two images. In order to compare accuracy and precision of the present method with a conventional two-level multiresolution approach, simulation was performed by 100 trials with the random initial mismatch of +/-10 degrees and +/-17.92 mm (Type-I) and +/-20 degrees and +/-40.32 mm (Type-II). For Type-I, no significant differences were found between registration errors of the multiresolution approach and the present method with the MI criterion. No biases were observed (< or =0.13 degrees and < or =0.57 mm for the multiresolution approach; < or =0.12 degrees and < or =0.57 mm for the present method) and the SDs were very small (< or =0.18 degrees and < or =0.12 mm for the multiresolution approach; < or =0.11 degrees and < or =0.11 mm for the present method). For Type-II, SDs for the multiresolution approach (< or =1.8 degrees and < or =0.88 mm) were markedly larger than those for the present method (< or =0.64 degrees and < or =0.20 mm) with MI. Success rate for the present method was 99.9%, which was higher than 97.6% for the multiresolution approach. Simulation also revealed that MI and NMI performance were almost equivalent. The choice of optimization strategy more affected accuracy and reproducibility than the choice of the registration criterion (MI or NMI) in our simulation condition. The present method is sufficiently accurate and reproducible for MRI-SPECT registration in clinical use.
    Annals of Nuclear Medicine 01/2005; 18(8):659-67. DOI:10.1007/BF02985959 · 1.68 Impact Factor
  • Takashi Yokoi · Hiroyuki Shinohara · Akihiro Takaki ·
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    ABSTRACT: Split-dose injection using technetium-99m ethyl cysteinate dimer ((99m)Tc-ECD) and consecutive SPET measurements performed before and after acetazolamide (ACZ) loading was used to estimate the cerebral perfusion reserve. The disadvantage of the split-dose method is that the signal-to-noise ratio (S/N) of ACZ-loaded images is decreased by subtraction of the 1st SPET data (rest) from the 2nd SPET data (ACZ loaded). To improve the S/N of reconstructed images, we implemented an iterative reconstruction algorithm including the term of remaining radioactivity in the brain from the 1st injection. It was expected that this method (the "addition method") would improve the S/N of rest and ACZ images compared with the conventional subtraction method owing to exclusion of the subtraction process. To evaluate the effect of statistical noise, we estimated the percentage coefficient of variation (%COV) as a function of total photon counts (from 1.35 to 86.5 Mcounts/slice) by Monte Carlo simulation with equal-volume split-dose injection. The %COV of the 2nd SPET study was higher than that of the 1st (e.g. 50.3% for the 1st and 80.5% for the 2nd at a total count of 2.70 Mcounts/slice) when using the conventional subtraction method. By contrast, the %COV of the 1st and 2nd SPET studies was almost equivalent (e.g. 43.1% for the 1st and 41.4% for the 2nd at a total count of 2.70 Mcounts/slice) when using the addition method. We also determined the optimal injection dose ratio of the 2nd to the 1st SPET study, which provides the equivalent %COV value between the 1st and 2nd images. With the subtraction method, the optimal injection dose ratio of the 2nd to the 1st SPET study was approximately 2.0, while with the addition method it was approximately 1.0. The absolute value of %COV at the optimal injection dose was about 54% and 43% with the subtraction method and the addition method, respectively. The addition method gave a lower %COV value than the subtraction method even at the optimal injection dose ratio. In a clinical study, the addition method provided better quality images than the subtraction method. The ROI values of rest images estimated by the subtraction method were close to the results obtained with the addition method (ROI(sub)=1.01 ROI(add)-0.312, r=0.999). The ROI values of the ACZ images estimated by the subtraction method also agreed with the results obtained using the addition method, but the correlation was slightly worse (ROI(sub)=1.03 ROI(add)-2.23, r=0.995). Quantitative ROI values were quite similar between the methods. Our results demonstrated that the quality of reconstructed rest and ACZ-loaded images were significantly better with the addition method than with the conventional subtraction method. We conclude that the proposed method will be useful as a practical reconstruction algorithm to improve the S/N in an equal-volume split-dose injection protocol using (99m)Tc-ECD.
    European journal of nuclear medicine and molecular imaging 09/2003; 30(8):1125-33. DOI:10.1007/s00259-003-1183-4 · 5.38 Impact Factor
  • Source
    Takashi Yokoi · Hiroyuki Shinohara · Hideo Onishi ·
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    ABSTRACT: Iterative reconstruction techniques such as an ordered subsets-expectation maximization (OSEM) algorithm can easily incorporated various physical models of attenuation or scatter. We implemented OSEM reconstruction algorithm incorporating compensation for distance-dependent blurring due to the collimator in SPECT. The algorithm was examined by computer simulation to estimate the accuracy for brain perfusion study. The detector response was assumed to be a two-dimensional Gauss function and the width of the function varied linearly with the source-to-detector distance. The attenuation compensation (AC) was also included. To investigate the properties of the algorithm, we performed computer simulations with the point source and digital brain phantoms. In the point source phantom, the uniformity of FWHM for the radial, tangential and longitudinal directions was evaluated on the reconstruction image. As for the brain phantom, quantitative accuracy was estimated by comparing the reconstructed images with the true image by the mean square error (MSE) and the ratio of gray and white matter counts (G/W). Both noise free and noisy simulations were examined. In the point source simulation, FWHM in radial, tangential and longitudinal directions were 14.7, 14.7 and 15.0 mm at the image center and were 15.9, 9.83 and 10.6 mm at a distance of 15 cm from the center by using FBP, respectively. On the other hand, they were 8.12, 8.12 and 7.83 mm at the image center, and were 7.45, 7.44 and 7.01 mm at 15 cm from the center by OSEM with distance-dependent resolution compensation (DRC). An isotropic and stationary resolution was obtained at any location by OSEM with DRC. The spatial resolution was also improved about 6.5 mm by OSEM with DRC at the image center. In the brain phantom simulation, the blurring at the edge of the brain structure was eliminated by using OSEM with both DRC and AC. The G/W was 2.95 and 2.68 for noise free and noisy cases, respectively, when no compensation was performed. But the values for G/W without and with noise became 3.45 and 3.21 with AC only and were improved to 3.75 and 3.71 with both AC and DRC. The G/W approached the true value (4.00) by using OSEM with both AC and DRC even when there was statistical noise. In conclusion, OSEM reconstruction including the distance-dependent resolution compensation algorithm was reasonably successful in achieving isotropic and stationary resolution and improving the quantitative accuracy for brain perfusion SPECT.
    Annals of Nuclear Medicine 03/2002; 16(1):11-8. DOI:10.1007/BF02995286 · 1.68 Impact Factor

Publication Stats

63 Citations
10.41 Total Impact Points


  • 2006-2007
    • Osaka University
      • Department of Medical Physics and Engineering
      Ōsaka-shi, Osaka-fu, Japan