Kanae Nishizawa

National Institute of Radiological Sciences, Chiba-shi, Chiba-ken, Japan

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Publications (14)19.04 Total impact

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    ABSTRACT: The recent broad adoption of 4-D computed tomography (4DCT) scanning in radiotherapy has allowed the accurate determination of the target volume of tumors by minimizing image degradation caused by respiratory motion. Although the radiation exposure of the treatment beam is significantly greater than that of CT scans used for treatment planning, it is important to recognize and optimize the radiation exposure in 4DCT from the radiological protection point of view. Here, radiation exposure in 4DCT was measured with a 16 multidetector CT. Organ doses were measured using thermoluminescence radiation dosimeter chips inserted at respective anatomical sites of an anthropomorphic phantom. Results were compared with those with the helical CT scan mode. The effective dose measured for 4DCT was 24.7 mSv, approximately four times higher than that for helical CT. However, the increase in treatment accuracy afforded by 4DCT means its use in radiotherapy is inevitable. The patient exposure in the 4DCT could be of value by clarifying the advantage of the treatment planning using 4DCT.
    Medical dosimetry: official journal of the American Association of Medical Dosimetrists 02/2009; 34(1):87-90. · 1.26 Impact Factor
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    ABSTRACT: The 256-multislice CT (256MSCT) obtains volumetric data with 128-mm coverage in a single rotation. This coverage allows satisfactory visualization of the whole heart, allowing the 256MSCT to visualize the cardiac chambers and coronary arteries by cine scan without ECG gating. These characteristics provide a solution to the problems of MSCT. Although a wider beam width provides more efficient imaging over a wider coverage area, patient doses with the 256MSCT are of considerable concern. We assessed potential radiation exposure with the 256MSCT in a cardiac CT protocol and compared the results to those with 16- and 64MSCT (collimated 64x0.5mm using 256MSCT). Organ or tissue doses were measured in an anthropomorphic phantom under a coronary artery imaging protocol with the 256MSCT in cine scan mode without ECG gating, and with the 16- and 64MSCT in helical scan mode with ECG gating. Average effective doses were 22.8mSv for the 16MSCT, 27.8mSv for the 64MSCT and 14.1mSv for the 256MSCT. The 16- and 64MSCT doses were thus approximately 1.6- and 2.0-fold higher than those of the 256MSCT. Use of the 256MSCT in cardiac volumetric cine imaging offers lower radiation exposure than 16- and 64MSCT, and suggests the potential of this equipment in single-beat cardiac imaging without ECG gating. This effective dose is acceptable for routine cardiac imaging.
    European Journal of Radiology 04/2008; 65(3):442-8. · 2.51 Impact Factor
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    ABSTRACT: The characteristics of a glass dosimeter were investigated for its potential use as a tool for postal dose audits. Reproducibility, energy dependence, field size and depth dependence were compared to those of a thermoluminescence dosimeter (TLD), which has been the major tool for postal dose audits worldwide. A glass dosimeter, GD-302M (Asahi Techno Glass Co.) and a TLD, TLD-100 chip (Harshaw Co.) were irradiated with gamma-rays from a (60)Co unit and X-rays from a medical linear accelerator (4, 6, 10 and 20 MV). The dosimetric characteristics of the glass dosimeter were almost equivalent to those of the TLD, in terms of utility for dosimetry under the reference condition, which is a 10 x 10 cm(2) field and 10 cm depth. Because of its reduced fading, compared to the TLD, and easy quality control with the ID number, the glass dosimeter proved to be a suitable tool for postal dose audits. Then, we conducted postal dose surveys of over 100 facilities and got good agreement, with a standard deviation of about 1.3%. Based on this study, postal dose audits throughout Japan will be carried out using a glass dosimeter.
    Radiotherapy and Oncology 03/2008; 86(2):258-63. · 4.52 Impact Factor
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    ABSTRACT: The spread of Multi-detector-row computed tomography (MDCT) has been remarkable. Here, various organ and tissue doses were evaluated with six types of MDCT scanners in common use in Japan; using thermoluminescence dosimeters and anthropomorphic phantoms under condition of routine clinical examinations of the chest in adult and child, of the head in child and of the abdomen-pelvis in adult. Estimated lung doses and averaged effective dose in chest examinations were 19.2 +/- 2.03 mGy and 9.54 +/- 0.90 mSv for the adult and 15.7 +/- 1.88 mGy and 7.42 +/- 0.82 mSv for the child phantom, respectively. The numerical difference between effective dose and organ or tissue doses was about 2-2.5 times. For the adult abdomen-pelvis examinations, averaged effective dose was 13.0 +/- 3.72 mSv. Averaged effective dose for the child head examinations was 2.6 +/- 1.32 mSv. In one case, the dose approached 80 mGy for the brain in the head examination, giving a difference from the effective dose of 10 times or more.
    Radiation Protection Dosimetry 02/2008; 128(1):98-105. · 0.91 Impact Factor
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    ABSTRACT: The internal exposures of the patients in nuclear medicine can be calculated based on the equations and data in ICRP Publications 53 and 80. Physical and biological parameters are used for the calculation, and both include uncertainties. Physical parameters can be considered as more precise than biological parameters, so that uncertainties originated from biological parameters are more important. Absorbed fractions (AFs) have been calculated by Monte-Carlo method using medical internal radiation dose (MIRD)-type mathematical phantoms. They depend on the shapes and sizes of the phantoms used in simulations. For estimating shape- and size-related uncertainties, AFs of pairs of source regions and target tissues of the patient-injected 99mTc-MDP were calculated by using EGS4 codes and a voxel phantom of Japanese male. By simply resizing the voxels of the phantom, the dependencies of size for AFs were calculated, and the uncertainties caused by the cumulated activities in source regions were also estimated by assuming these parameters distributions as Gaussian.
    Radiation Protection Dosimetry 02/2007; 127(1-4):558-62. · 0.91 Impact Factor
  • S Mori, K Nishizawa, M Ohno, M Endo
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    ABSTRACT: Recent rapid progress in CT technology has yielded equipment with large numbers of detector rows and standard computed tomography dose index (CTDI) is therefore no longer an adequate integration range. An integration range of 300 mm is necessary to accurately measure dose under a nominal beam width of 128 mm due to scattered radiation. However, such a long phantom is inconvenient to use routinely in cone-beam CT patient dose checking. To assess patient dose accurately with standard dosimetry methods, we determined a conversion factor (CF) which was calculated from the weighted dose profile integral (DPI(w)) for the 300 mm integration range with a 300 mm long CTDI phantom using a 300 mm long ionization chamber divided by that for the 100 mm integration range with a standard CTDI phantom (140 mm long) with a 100 mm long chamber. CF values increase with increasing nominal beam width and effective energy in the range from 1.5 to 2.0. CF values can also be adapted for use with other CT systems as their dose profiles are thought to be analogous to those for the 300 mm phantom and are useful in any hospital situation to assess accurate patient doses using standard dosimetry methods.
    The British journal of radiology 12/2006; 79(947):888-92. · 2.11 Impact Factor
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    ABSTRACT: The 256-slice CT-scanner has been developed at the National Institute of Radiological Sciences. Nominal beam width was 128 mm in the longitudinal direction. When scanning continuously at the same position to obtain four-dimensional (4D) images, the effective dose is increased in proportion to the scan time. Our purpose in this work was to measure the dose for the 256-slice CT, to compare it with that of the 16-slice CT-scanner, and to make a preliminary assessment of dose for dynamic 3D imaging (volumetric cine imaging). Our group reported previously that the phantom length and integration range for dosimetry needed to be at least 300 mm to represent more than 90% of the line integral dose with the beam width between 20 mm and 138 mm. In order to obtain good estimates of the dose, we measured the line-integral dose over a 300 mm range in PMMA (polymethylmethacrylate) phantoms of 160 mm or 320 mm diameter and 300 mm length. Doses for both CT systems were compared for a clinical protocol. The results showed that the 256-slice CT generates a smaller dose than the 16-slice CT in all examinations. For volumetric cine imaging, we found an acceptable scan time would be 6 s to 11 s, depending on examinations, if dose must be limited to the same values as routine examinations with a conventional multidetector CT. Finally, we discussed the studies necessary to make full use of volumetric cine imaging.
    British Journal of Radiology 02/2006; 79(937):56-61. · 1.22 Impact Factor
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    ABSTRACT: For the estimation of collective effective dose from radiopharmaceuticals used in nuclear medicine diagnosis, a national survey was carried out in Japan. The survey contents covered radiopharmaceutical use, sex, age, activity, and so on of each patient in October 1997 and the monthly number of examinations in 1997. The annual number of diagnostic examinations using radiopharmaceuticals was 0.82 million for males and 0.74 million for females. The frequency of examination was about 3% for patients less than 17 years old and about 60% for those more than 60 years old. Effective dose was calculated on the basis of such literature as ICRP publications. The dose used most frequently was 5-6mSv per examination. The collective effective doses from diagnostic nuclear medicine examinations were estimated to be 13100 man .Sv for males and 20200 man .Sv for females.
    Igaku butsuri: Nihon Igaku Butsuri Gakkai kikanshi = Japanese journal of medical physics: an official journal of Japan Society of Medical Physics 02/2006; 26(2):75-82.
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    ABSTRACT: The prototype cone-beam CT (CBCT) has a larger beam width than the conventional multi-detector row CT (MDCT). This causes a non-uniform angular distribution of the x-ray beam intensity known as the heel effect. Scan conditions for CBCT tube current are adjusted on the anode side to obtain an acceptable clinical image quality. However, as the dose is greater on the cathode side than on the anode side, the signal-to-noise ratio on the cathode side is excessively high, resulting in an unnecessary dose amount. To compensate for the heel effect, we developed a heel effect compensation (HEC) filter. The HEC filter rendered the dose distribution uniform and reduced the dose by an average of 25% for free air and by 20% for CTDI phantoms compared to doses with the conventional filter. In addition, its effect in rendering the effective energy uniform resulted in an improvement in image quality. This new HEC filter may be useful in cone-beam CT studies.
    Physics in Medicine and Biology 12/2005; 50(22):N359-70. · 2.70 Impact Factor
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    ABSTRACT: In order to examine phantom length necessary to assess radiation dose delivered to patients in cone-beam CT with an enlarged beamwidth, we measured dose profiles in cylindrical phantoms of sufficient length using a prototype 256-slice CT-scanner developed at our institute. Dose profiles parallel to the rotation axis were measured at the central and peripheral positions in PMMA (polymethylmethacrylate) phantoms of 160 or 320 mm diameter and 900 mm length. For practical application, we joined unit cylinders (150 mm long) together to provide phantoms of 900 mm length. Dose profiles were measured with a pin photodiode sensor having a sensitive region of approximately 2.8 x 2.8 mm2 and 2.7 mm thickness. Beamwidths of the scanner were varied from 20 to 138 mm. Dose profile integrals (DPI) were calculated using the measured dose profiles for various beamwidths and integration ranges. For the body phantom (320-mm-diam phantom), 76% of the DPI was represented for a 20 mm beamwidth and 60% was represented for a 138 mm beamwidth if dose profiles were integrated over a 100 mm range, while more than 90% of the DPI was represented for beamwidths between 20 and 138 mm if integration was carried out over a 300 mm range. The phantom length and integration range for dosimetry of cone-beam CT needed to be more than 300 mm to represent more than 90% of the DPI for the body phantom with the beamwidth of more than 20 mm. Although we reached this conclusion using the prototype 256-slice CT-scanner, it may be applied to other multislice CT-scanners as well.
    Medical Physics 05/2005; 32(4):1061-9. · 2.91 Impact Factor
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    ABSTRACT: We have developed a four-dimensional CT (4D CT) using continuous rotation of cone-beam x-ray. The maximum nominal beam width of the 4D CT is 128 mm at the center of rotation in the longitudinal direction. In order to obtain appropriate estimations of exposure dose, detailed single-slice dose profi les perpendicular to the rotation axis including scattered radiation were measured in PMMA cylindrical phantoms, which were cylindrical lucite phantoms of 160 mm and 320 mm diameter and 900 mm length. Dose profi les were measured with a pin photodiode detector at the center and a peripheral point of 10 mm depth. A pin silicon photodiode sensor with 3 × 3 × 3 mm sensitive region was used as an x-ray detector, which was scanned along longitudinal direction in the phantom for beam widths of 20, 42, 74, 106 and 138 mm. The dose profi les had long tails caused by scattered radiation more than 200 mm out of the beam width edge. The exposure dose covered 95 % was distributed along about 360 mm length at the center and about 310 mm at the periphery, which was independent of the beam width. Before the advent of multi-detector CT, CTDI100 was used to approximate integral dose for clinical scan conditions. However, for 4D CT employing a variable beam width, the standard CTDI was not a good estimation. This work was carried out to establish a method of the dose measurements including scattered radiation for cone-beam CT such as 4D CT. In order to perform the dose assessment including scattered radiation, dose measured length should be recommended to measure integral dose over beam widths plus at least 230 mm, which covered 95 % total exposure dose.
    Proc SPIE 05/2004;
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    ABSTRACT: Computed tomography(CT) has been established as an important diagnostic tool in clinical medicine and has become a major source of medical exposure. A nationwide survey regarding CT examinations was carried out in Japan in 2000. CT units per million people in Japan numbered 87.8. The annual number of examinations was 0.1 million in those 0-14 years old, 3.54 million for those 15 years old and above, and 3.65 million in total. Eighty percent of examinations for those 0-14 years old were examinations of the head, as were 40% for those 15 years old and above. The number of examinations per 1000 population was 290. The collective effective dose was 295 x 10(3) person.Sv, and the effective dose per caput was evaluated as 2.3 mSv.
    Nihon Igaku Hoshasen Gakkai zasshi. Nippon acta radiologica 04/2004; 64(3):151-8.
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    ABSTRACT: Computed tomography (CT) has evolved remarkably through device improvement and advancement of peripherals, including computers. In 1999, multi detector-row CT (MDCT) appeared and rapid high-speed scanning became possible. However, usefulness of MDCT in actual clinical application cannot be assessed until the exposure doses are assessed appropriately. Since CT examinations need a comparatively high dose, it is necessary to evaluate patient exposure for introduction of MDCT. Patient doses by three types of MDCTs were evaluated for cases of scanning of the chest and abdomen-pelvis. The examination conditions were the same as those in actual clinical examinations. The obtained effective doses were 9.4-28 mSv for the chest examination and 13-28 mSv for the abdomen-pelvis. The average surface doses varied between 16-43 mGy for the chest examination and 20-37 mGy for the abdomen-pelvis. The highest surface dose was 57 mGy for the abdomen-pelvis examination. The exposed doses differed according to scanning method and imaging conditions such as tube current, slice thickness and so on. It seemed that there is room for dose reduction by proper adjustment of scan conditions in MDCT examinations.
    Igaku butsuri: Nihon Igaku Butsuri Gakkai kikanshi = Japanese journal of medical physics: an official journal of Japan Society of Medical Physics 02/2002; 22(3):152-8.
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    ABSTRACT: The development of computerized tomography (CT) has made CT fluoroscopy possible with real-time CT images. However examination are expected to have high medical and occupational exposures. Then, exposures to patients and operating and assisting physicians during the CT fluoroscopy-guided lung biopsy were estimated. And changes in the examination conditions to lower the dose were made. Patient exposure was measured using an anthropomorphic phantom by simulation of clinical examination conditions. The surface dose to the physician was measured during actual clinical examinations. The average effective dose for the patient was 34+/-22mSv. The highest surface dose amounted to 1.9 Gy, although this was in a very narrow field. Patient doses could be reduced by a factor of 2.5-3 by changing examination methods while still retaining diagnostic quality. The highest dose to the operating physician was 10mGy which was recorded on the back of the hand and the average effective dose was estimated as 5.99&mgr;Sv per 1-minute examination. Doses were reduced by about a factor of 50 by lowering the tube voltage from 120kV to 80 kV and using a supplementary tool. The doses for assisting physicians were not significant. The exposure for physicians and patients was much affected by lowering the tube voltage used for fluoroscopy. Using a supplementary tool was effective for reducing the dose for physicians.
    Igaku butsuri: Nihon Igaku Butsuri Gakkai kikanshi = Japanese journal of medical physics: an official journal of Japan Society of Medical Physics 01/2001; 21(4):233-244.