Hiroji Iida

Assessment of an organ-based tube current modulation in thoracic computed tomography

Recently, specific computed tomography (CT) scanners have been equipped with organ‐based tube current modulation (TCM) technology. It is possible that organ‐based TCM will replace the conventional dose‐reduction technique of reducing the effective milliampere‐second. The aim of this study was to determine if organ‐based TCM could reduce radiation exposure to the breasts without compromising the image uniformity and beam hardening effect in thoracic CT examinations. Breast and skin radiation doses and the absorbed radiation dose distribution within a single section were measured with an anthropomorphic phantom and radiophotoluminescent glass dosimeters using four approaches to thoracic CT (reference, organ‐based TCM, copper shielding, and the combination of the above two techniques, hereafter referred to as the combination technique). The CT value and noise level were measured using the same calibration phantom. Organ‐based TCM and copper shielding reduced radiation doses to the breast by 23.7% and 21.8%, respectively. However, the CT value increased, especially in the anterior region, using copper shielding. In contrast, the CT value and noise level barely increased using organ‐based TCM. The combination technique reduced the radiation dose to the breast by 38.2%, but greatly increased the absorbed radiation dose from the central to the posterior regions. Moreover, the CT value increased in the anterior region and the noise level increased by more than 10% in the entire region. Therefore, organ‐based TCM can reduce radiation doses to breasts with only small increases in noise levels, making it preferable for specific groups of patients, such as children and young women. PACS numbers: 87.53.Bn; 87.57.Q‐; 87.57.qp

Radiation dose and physical image quality in 128-section dual-source computed tomographic coronary angiography: a phantom study

One‐hundred‐and‐twenty‐eight–section dual X‐ray source computed tomography (CT) systems have been introduced into clinical practice and have been shown to increase temporal resolution. Higher temporal resolution allows low‐dose spiral mode at a high pitch factor during CT coronary angiography. We evaluated radiation dose and physical image qualities in CT coronary angiography by applying high‐pitch spiral, step‐and‐shoot, and low‐pitch spiral modes to determine the optimal acquisition mode for clinical situations. An anthropomorphic phantom, small dosimeters, a calibration phantom, and a microdisc phantom were used to evaluate the radiation doses absorbed by thoracic organs, noise power spectrums, in‐plane and z‐axis modulation transfer functions, slice sensitivity profiles, and number of artifacts for the three acquisition modes. The high‐pitch spiral mode had the advantage of a small absorbed radiation dose, but provided low image quality. The low‐pitch spiral mode resulted in a high absorbed radiation dose of approximately 200 mGy for the heart. Although the absorbed radiation dose was lower in the step‐and‐shoot mode than in the low‐pitch spiral mode, the noise power spectrum was inferior. The quality of the in‐plane modulation transfer function differed, depending on spatial frequency. Therefore, the step‐and‐shoot mode should be applied initially because of its low absorbed radiation dose and superior image quality. PACS numbers: 87.57.‐s; 87.57.C‐; 87.57.cf; 87.57.cm; 87.57.cp; 87.57.Q‐; 87.57.qp; 87.57.uq

Evaluation of patient doses due to fluoroscopic exposures.

The present study measures entrance surface doses of radiation administered to patients during various fluoroscopic procedures using a dose-area product meter as well as the duration of each procedure. A conversion factor for entrance skin dose to patients was calculated. The average dose to patient during the insertion of intravenous hyperalimentation was 10.2 (maximum, 74.0) mGy, during a barium meal, 58.4 (maximum, 184.0) mGy, for endoscopic retrograde cholangio-pancreatography (ERCP), 97.3 (maximum, 376.0) mGy and for a barium enema, 86.1 (maximum, 271.0) mGy. Doses tended to increase in the abdominal domain and when patients undergo not only diagnostic evaluation but also treatment requiring procedures such as drainage. The management of radiation doses determined using a dose-area product meter is very important and that of individual patients is enabled using the Radiation Information System (RIS).

Investigation of image lag and modulation transfer function in fluoroscopy images obtained with a dynamic flat-panel detector.

Digital imaging with a dynamic flat-panel detector (FPD) is commonly used in clinical practice. However, several factors reduce the accuracy of target tracking in fluoroscopic imaging, including image lag and blurring. There have been several reports focusing on the modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) in different types of FPD. However, there have been no studies comparing image lag and MTF properties in dynamic images obtained with indirect- and direct-conversion FPDs. We investigated the image lag and MTF under several imaging conditions in fluoroscopic images obtained with an indirect-conversion and a direct-conversion FPD system. The measurements of image lag and MTF were obtained under several conditions according to IEC 62220-1-3 standards. We examined whether the image lag and MTF were influenced by the dose level and target movement speed. Indirect-conversion FPD showed dependence on the dose level, which was not observed for direct-conversion FPD. Furthermore, there were large differences in MTF between images of static and moving plate with indirect-conversion FPD in comparison to the differences observed with direct-conversion FPD. These results will be useful for the determination of imaging conditions for target tracking and other types of dynamic imaging.

Algorithm for estimation of brain structural location from head surface shape in young children.

If the brain structural coordinates could be estimated using the individual head shape, magnetoencephalography and near-infrared spectroscopy would be more ideal brain functional imaging methods especially for young human children. First, we propose an algorithm to estimate brain coordinates with reference to the head surface shape in preschool children. Second, we examined its spatial error range using a leave-one-out procedure within 38 samples of child head and brain structures. The mean error of landmarks was 13.6±5.3 mm on the head surface and 18.5±12.4 mm in the intracranial structure between estimated and actual brain coordinates.

Evaluation of transmission data of diagnostic X rays through concrete using Monte Carlo simulation

The transmittance of diagnostic X-ray beams through concrete of two different densities were measured. Broad-beam X-ray transmissions were estimated using a Monte Carlo simulation at tube voltages of 50-150 kV and compared with National Council on Radiation Protection and Measurements (NCRP) data. The observed levels were higher than those predicted by the NCRP data; that is, the NCRP data underestimate the leakage radiation from widely used inverter systems. Some of the issues were resolved and new data were proposed.

SU‐D‐217BCD‐02: Online Angular Tube Current Modulation in X‐Ray Computed Tomography: Can Tube Current Be Modulated Appropriately?

PURPOSE In X-ray computed tomography (CT), X-rays are significantly less attenuated in the anteroposterior direction and more in the lateral direction. Therefore, the tube current should be adjusted within one gantry rotation using angular tube current modulation (TCM). The aim of this study was to evaluate whether online angular TCM could reduce radiation dose appropriately. METHODS A 128-detector dual-source CT (SOMATOM Definition Flash; Siemens Healthcare, Erlangen, Germany) and an online TCM system (CARE Dose 4D; Siemens Healthcare) were used. Dose profiles were acquired using the CT Dose Profiler (RTI Electronics, Molndal, Sweden) and an elliptical cylindrical phantom (MHT; Kyoto Kagaku, Kyoto, Japan) for helical CT scans with and without TCM. In addition, absorbed dose distributions within a single section were acquired using an anthropomorphic phantom (RANI 10; The Phantom Laboratory, Salem, NY) and radiophotoluminescent glass dosimeters (RPLDs) (GD- 302M; Chiyoda Technol, Tokyo, Japan) for helical CT scans with and without TCM after placing RPLDs within all holes of one section and pasting them around the section. A graph of each absorbed dose distribution was drawn using graphing software (ORIGIN 8.6; OriginLab, Northampton, MA). RESULTS The acquired dose profiles suggested that online angular TCM could adjust the tube current in near-real time according to the attenuation measured from the previous projection. The profiles gradually stabilized because the tube current was adjusted properly. The absorbed doses were low and the distributions were stable with TCM compared with those without TCM. CONCLUSIONS In X-ray CT, an online angular TCM can reduce radiation dose effectively by adjusting tube current within one gantry rotation in near-real time.