Object shape dependency of in-plane resolution for iterative reconstruction of computed tomography.

The present study aimed to investigate whether the in-plane resolution property of iterative reconstruction (IR) of computed tomography (CT) data is object shape-dependent by testing columnar shapes with diameters of 3, 7, and 10cm (circular edge method) and a cubic shape with 5-cm side lengths (linear edge method). For each shape, objects were constructed of acrylic (contrast in Hounsfield units [ΔHU]=120) as well as a soft tissue equivalent material (ΔHU=50). For each shape, we measured the modulation transfer functions (MTFs) of IR and filtered back projection (FBP) using two multi-slice CT scanners at scan doses of 5 and 10mGy. In addition, we evaluated a thin metal wire using the conventional method at 10mGy. For FBP images, the MTF results of the tested objects and the wire method showed substantial agreement, thus demonstrating the validity of our analysis technique. For IR images, the MTF results of different shapes were nearly identical for each object contrast and dose combination, and we did not observe shape-dependent effects of the resolution properties of either tested IR. We conclude that both the circular edge method and linear edge method are equally useful for evaluating the resolution properties of IRs.

[Investigation of radiation dose for lower tube voltage CT using automatic exposure control].

The purpose of our study was to investigate radiation dose for lower tube voltage CT using automatic exposure control (AEC). An acrylic body phantom was used, and volume CT dose indices (CTDIvol) for tube voltages of 80, 100, 120, and 135 kV were investigated with combination of AEC. Average absorbed dose in the abdomen for 100 and 120 kV were also measured using thermoluminescence dosimeters. In addition, we examined noise characteristics under the same absorbed doses. As a result, the exposure dose was not decreased even when the tube voltage was lowered, and the organ absorbed dose value became approximately 30% high. And the noise was increased under the radiographic condition to be an equal absorbed dose. Therefore, radiation dose increases when AEC is used for lower tube voltage CT under the same standard deviation (SD) setting with 120 kV, and the optimization of SD setting is crucial.

[Evaluation of dose output according to the objects using organ-based tube-current modulation in thoracic computed tomography].

The purpose of this study was to evaluate the dose output according to the object using organ-based tube-current modulation in thoracic CT. The output doses with elliptical and circular shaped phantoms were measured using an ionizing CT chamber. The image noise was quantitatively measured in images obtained from the elliptical phantom. Although total dose outputs with and without the modulation were almost the same, dose outputs at a frontal angle of 120° decreased and those at another angle of 240° increased with the modulation. When the same-shaped phantoms were used, the differences in variation of dose outputs due to the difference in phantom size were small and those due to the difference in the percentage ratio of long- to short-axis diameter of the cross-section were large. There was no significant difference in the amount of noise with and without the dose modulation except for the case of overdose to the small phantom. Therefore, organ-based tube current modulation does not change the total dose output and it maintains the amount of noise by controlling the dose output for each projection angle. Additionally, this dose control is independent of the object size.

[Evaluation of over ranging in 64-slice multidetector-row computed tomography examinations for upper abdomen].

64-slice multidetector-row computed tomography (MDCT) have become widely used in recent years, but there is a possibility that a large volume of dose is radiated at the outside of the scope of scanning due to the effect of over ranging (OR). In this research, the dose volume change at the outside of the scope of scanning in the case of X-ray beam width (BW) and pitch factor (PF) is measured, and the effect of BW and PF on OR is considered. We scanned the upper abdomen of an acrylic human phantom using 64-slice multidetector-row CT in 4 conditions while changing BW and PF. We then measured the dose volume absorbed in the breasts, ovary, uterus, and the dose profile on the surface of phantom. As a result, when BW and PF are increased, the dose volume radiated at the outside of the scope of scanning increased, and the effect of OR appeared. Therefore, when BW and PF are set, X-ray radiated at the outside of the scope of scanning should also be considered.

[Image quality evaluation of new image reconstruction methods applying the iterative reconstruction].

The purpose of this study was to evaluate the image quality of an iterative reconstruction method, the iterative reconstruction in image space (IRIS), which was implemented in a 128-slices multi-detector computed tomography system (MDCT), Siemens Somatom Definition Flash (Definition). We evaluated image noise by standard deviation (SD) as many researchers did before, and in addition, we measured modulation transfer function (MTF), noise power spectrum (NPS), and perceptual low-contrast detectability using a water phantom including a low-contrast object with a 10 Hounsfield unit (HU) contrast, to evaluate whether the noise reduction of IRIS was effective. The SD and NPS were measured from the images of a water phantom. The MTF was measured from images of a thin metal wire and a bar pattern phantom with the bar contrast of 125 HU. The NPS of IRIS was lower than that of filtered back projection (FBP) at middle and high frequency regions. The SD values were reduced by 21%. The MTF of IRIS and FBP measured by the wire phantom coincided precisely. However, for the bar pattern phantom, the MTF values of IRIS at 0.625 and 0.833 cycle/mm were lower than those of FBP. Despite the reduction of the SD and the NPS, the low-contrast detectability study indicated no significant difference between IRIS and FBP. From these results, it was demonstrated that IRIS had the noise reduction performance with exact preservation for high contrast resolution and slight degradation of middle contrast resolution, and could slightly improve the low contrast detectability but with no significance.

[New method of measuring effective energy using copper-pipe absorbers in X-ray CT].

The general method of measuring the half-value layer (HVL) for X-ray computed tomography (CT) using square aluminum-sheet filters is inconvenient in that the X-ray tube has to be set to stationary mode. To avoid this inconvenience, we investigated a new method using copper-pipe filters that cover the ionization chamber (copper-pipe method). Using this method, the HVL can be measured at the isocenter in the rotation modes of CT. We examined the accuracy and reproducibility of the copper-pipe method compared with those of the general method. The effective energy measured using the copper-pipe method correlated well with the general method (y=1.064x, r=0.987), and its error was 1.81±1.38%. The results indicate that the copper-pipe method enables accurate measurement of the effective energy of X-ray CT and is a convenient method suited to all general X-ray equipment as well as all X-ray CT.

Assessment of patient exposure to X-radiation from SPECT/CT scanners.

UNLABELLED: In the operation of any SPECT/CT system, in addition to internal radiation exposure (gamma-ray) resulting from administration of radiopharmaceuticals, external radiation exposure (x-ray) from the CT device has to be taken into consideration in the light of recommendations from the International Commission on Radiological Protection. These recommendations include justification of practices (the use of radiation produces sufficient benefit to offset any risks caused by the use of radiation), optimization (the incurred exposure by the use of radiation should be kept as low as reasonably achievable), and dose limitation. The internal radiation exposures of each organ after administration of radiopharmaceuticals are calculated by the MIRD Committee method. For example, the internal radiation exposure index for brain perfusion scintigraphy is 0.8 mGy/37 MBq for N-isopropyl-4-iodoamphetamine((123)I) hydrochloride or 0.19 mGy/37 MBq for ethyl cysteinate dimer. On the other hand, the external radiation exposure from a CT device is calculated using the CT dose index volume (CTDIvol)--a measured and calculated value unique to the CT scanner and scan parameters used--and a dose-length product, which is obtained from CT conditions and generally used as a reference value for CT radiation exposure. However, CTDIvol and dose-length product are calculated values unique to each device, not the value of external radiation exposures of each organ. Therefore, we believe that it is necessary to measure the total (internal plus external) radiation exposure dose from CT. In the present study, using an anthropomorphic phantom for deep-body total absorbed dose measurement, we evaluated the radiation exposure doses (organ-absorbed doses) of each organ under various CT conditions. METHODS: The radiation exposure doses of each organ were measured by inserting thermoluminescent dosimeter elements into the phantom under various CT conditions. RESULTS: The following were brain radiation exposure doses in the head region. For 90 kVp and 25 mAs, 1.39 mGy (CTDIvol, 1.8 mGy), for 90 kVp and 300 mAs, 17.00 mGy (CTDIvol, 21.2 mGy), for 120 kVp and 25 mAs, 3.21 mGy (CTDIvol, 3.8 mGy), for 120 kVp and 300 mAs, 37.79 mGy (CTDIvol, 47.7 mGy), for 140 kVp and 25 mAs, 5.08 mGy (CTDIvol, 5.5 mGy), and for 140 kVp and 300 mAs, 65.07 mGy (CTDIvol, 65.6 mGy). The eye radiation exposure doses were as follows. For 90 kVp and 25 mAs, 1.94 mGy (CTDIvol, 1.8 mGy), for 90 kVp and 300 mAs, 20.31 mGy (CTDIvol, 21.2 mGy), for 120 kVp and 25 mAs, 3.71 mGy (CTDIvol, 3.8 mGy), for 120 kVp and 300 mAs, 49.72 mGy (CTDIvol, 47.7 mGy), for 140 kVp and 25 mAs, 5.44 mGy (CTDIvol, 5.5 mGy), and for 140 kVp and 300 mAs, 69.76 mGy (CTDIvol, 65.6 mGy). In addition, the radiation exposure doses of the cervical, thoracic, abdominal, and pelvic regions were measured in detail. CONCLUSION: Our estimated external radiation exposure doses (x-ray) of each organ under various CT conditions, along with the internal radiation exposure doses (gamma-ray) resulting from the administration of radiopharmaceuticals, seem to be useful as reference values in understanding the radiation exposure doses for performing various nuclear medicine examinations.

[Estimation of the effective dose of patient in interventional radiology: study of coronary angiography].

The applications of interventional radiology (IVR) increasingly are being used in clinical examinations, where they tend to extend examination time. In addition, the risk of occupational exposure necessarily is increasing with this technology. In this study, the dose distributions in a sliced acrylic-acid phantom involving the bore for each irradiation condition were measured using a thermoluminescence dosimeter (TLD). Four patterns of set-up for the fluoroscopy unit were chosen as references for the conditions generally used clinically. Exposure also was measured with dose area product (DAP), and we then calculated the entrance skin dose and effective dose for the patient. The results showed that the effective dose was 7.0 mSv to 8.0 mSv at LAO45 degrees and RAO30 degrees; 100 kV, 2.3 mSv to 3.3 mSv at LAO45 degrees and RAO30 degrees; 80 kV. The effective dose is greatly influenced by the setup of fluoroscopy in IVR. The change in DAP is especially influenced. We found that the relation between DAP and effective dose was corrected with the exponential function. The effective doses were not necessarily less than those of other radiation examinations, and increase. When PCI and TAE are repeated many times in IVR, we propose that the effective dose should be taken into consideration together with the skin dose for dose control management.

[Shielding effect of clinical X-ray protector and lead glass against annihilation radiation and gamma rays of 99mTc].

Various pharmaceutical companies in Japan are making radioactive drugs available for positron emission tomography (PET) in hospitals without a cyclotron. With the distribution of these drugs to hospitals, medical check-ups and examinations using PET are expected to increase. However, the safety guidelines for radiation in the new deployment of PET have not been adequately improved. Therefore, we measured the shielding effect of a clinical X-ray protector and lead glass against annihilation radiation and gamma rays of (99m)Tc. We then calculated the shielding effect of a 0.25 mm lead protector, 1 mm lead, and lead glass using the EGS4 (Electron Gamma Shower Version 4) code. The shielding effects of 22-mm lead glass against annihilation radiation and gamma rays of (99m)Tc were approximately 31.5% and 93.3%, respectively. The clinical X-ray protector against annihilation radiation approximately doubled the skin-absorbed dose.

[Method of estimating patient skin dose from dose displayed on medical X-ray equipment with flat panel detector].

The International Electrotechnical Commission has stipulated that medical X-ray equipment for interventional procedures must display radiation doses such as air kerma in free air at the interventional reference point and dose area product to establish radiation safety for patients (IEC 60601-2-43). However, it is necessary to estimate entrance skin dose for the patient from air kerma for an accurate risk assessment of radiation skin injury. To estimate entrance skin dose from displayed air kerma in free air at the interventional reference point, it is necessary to consider effective energy, the ratio of the mass-energy absorption coefficient for skin and air, and the backscatter factor. In addition, since automatic exposure control is installed in medical X-ray equipment with flat panel detectors, it is necessary to know the characteristics of control to estimate exposure dose. In order to calculate entrance skin dose under various conditions, we investigated clinical parameters such as tube voltage, tube current, pulse width, additional filter, and focal spot size, as functions of patient body size. We also measured the effective energy of X-ray exposure for the patient as a function of clinical parameter settings. We found that the conversion factor from air kerma in free air to entrance skin dose is about 1.4 for protection.

[Influence of electromagnetic waves on portable electronic instruments in medicine].

In this work, we considered the effect of electromagnetic compatibility (EMC) on electronic instruments used in medical facilities. We simulated and measured the intensity distribution of the electromagnetic field around portable phones and examined the influence of electromagnetic field on telemeter-type electrocardiograms (ECG) and computer display cathode ray tubes (CRT) when portable phones were brought close to them. Results showed that the waves observed on ECG are affected up to a distance of 55cm (3.55E+03V/m) from the portable phone, and the grid of the computer display is affected up to a distance of 9cm (7.16E+04V/m) in the front and 14cm (2.97E+04V/m) in the back. Therefore, to reduce electromagnetic intensity (EMI) maintaining a distance far enough from the source is effective.