Kichiro Koshida

Misoperation of CT Automatic Tube Current Modulation Systems with Inappropriate Patient Centering: Phantom Studies

OBJECTIVE Inappropriate patient centering on the gantry changes the size of the localizer radiographs used for CT examinations, influencing the operation of CT automatic tube current modulation because tube current is controlled with information from localizer radiographs. The purpose of this study was to examine the influence of inappropriate patient centering on the gantry isocenter on automatic tube current modulation. MATERIALS AND METHODS An elliptical phantom was scanned with four automatic tube current modulation techniques after acquisition of localizer radiographs in the horizontal and vertical directions with the phantom center shifted from the gantry isocenter in the vertical direction. After scanning, the magnification rate of the frontal localizer radiographs, tube current-time product, and image noise were examined. RESULTS On phantom studies, the magnification rate of localizer radiographs showed a linear relation to the vertical deviation of the phantom from the gantry isocenter. From 50 mm above to 50 mm below the gantry isocenter, tube current-time products ranged from 75% to 141% compared with those at the gantry isocenter. In addition, increases and decreases in the amount of image noise related to changes in tube current-time product were confirmed. CONCLUSION Inappropriate patient centering causes misoperation of automatic tube current modulation systems, in which tube current is controlled with information from localizer radiographs, and thus causes increases in tube current or image noise.

Frequency analyses of CSF flow on cine MRI in normal pressure hydrocephalus

Abstract. Our objective was to clarify intracranial cerebrospinal fluid (CSF) flow dynamics in normal-pressure hydrocephalus (NPH). Frequency analyses of CSF flow measured with phase-contrast cine MRI were performed. The CSF flow spectra in the aqueduct were determined in patients (n=51) with NPH, brain atrophy or asymptomatic ventricular dilation (VD), and in healthy volunteers (control group; n=25). The changes in CSF flow spectra were also analyzed after intravenous injection of acetazolamide. Moreover, a phase transfer function (PTF) calculated from the spectra of the driving vascular pulsation and CSF flow in the aqueduct were assessed. These values were compared with the pressure volume response (PVR). The amplitude in the NPH group was significantly larger than that in the VD or control group because of a decrease in compliance. The phase in the NPH group was significantly different from that in either the VD or the control group, but no difference was found between the VD and control groups. The amplitude increased in all groups after acetazolamide injection. The PTF in the NPH group was significantly larger than in the control group, and a positive correlation was noted between PTF and PVR. Frequency analyses of CSF flow measured by cine MRI make it possible to noninvasively obtain a more detailed picture of the pathophysiology of NPH.

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

Effective dose evaluation of multidetector CT examinations: influence of the ICRP recommendation in 2007

We compared effective doses for recent computed tomography (CT) examinations calculated based on International Commission on Radiological Protection publication number 103 (ICRP 103) with those calculated based on ICRP publication number 60 (ICRP 60), and considered the usefulness of the effective dose in CT dose evaluation. After placing radiophotoluminescence glass dosimeters (RPLDs) inside or outside an anthropomorphic phantom, we examined from the chest to the pelvis, cardiac, and cranial regions of the phantom. The absorbed dose was calculated by multiplying calibrated dose values of RPLD by the mass energy coefficient ratio. The effective dose was calculated as the sum total of the value for each tissue, which was multiplied by the equivalent dose according to the tissue weighting factor recommended in ICRP 103 and ICRP 60. Calculated effective doses based on ICRP 103 were different by –11% to +82% compared with those based on ICRP 60. The values of absorbed doses for selective tissues were relatively higher than the values for the effective dose. The effective dose represents only a mean dose value for an average human. Therefore, assessing the absolute dose of particular individuals in CT examinations based exclusively on the effective dose is not recommended.

Chest CT Performed with 3D and z-Axis Automatic Tube Current Modulation Technique: Breast and Effective Doses

RATIONALE AND OBJECTIVES Chest computed tomographic (CT) scans are the most effective examinations for detecting lung cancer at an early stage. In chest CT examinations, it is important to consider the reduction of radiation dose, particularly to the mammary gland. The objective of this study was to assess breast doses and effective doses on chest CT examinations between three-dimensional and z-axis automatic tube current modulation (ATCM) techniques. MATERIALS AND METHODS Absorbed dose to the breast, lung, mediastinum, and skin was evaluated with an anthropomorphic phantom and radiophotoluminescence glass dosimeters using two different CT scanners. The dosimeters were placed inside and outside the phantom. The phantom was scanned using three-dimensional and z-axis ATCM techniques after scanning localizer radiographs from the horizontal and vertical directions. After scanning, each organ dose was calculated. Moreover, the dose-length product recorded in the dose reports was examined, and each effective dose was calculated. RESULTS Compared with z-axis ATCM, three-dimensional ATCM reduced breast dose by 0.7% to 18.6% and effective dose by 4.9% to 10.2%. In particular, three-dimensional ATCM reduced frontal breast dose. For other organs, three-dimensional ATCM reduced absorbed doses by 3.4% to 13.6% compared to z-axis ATCM. CONCLUSION Three-dimensional ATCM can reduce absorbed doses to the breast and other organs, in addition to reducing effective dose, compared to z-axis ATCM.

Polarity effect in commercial ionization chambers used in photon beams with small fields

Ionization chambers are the instruments of choice for use in photon dosimetry. Ionization chambers together with radiographic films represent the best detectors for measurement of dose distribution for a quality assurance (QA) program in intensity-modulated radiotheraphy (IMRT). The polarity effect was investigated for seven different commercially available ionization chambers. This study concentrated on measuring the dependence of the polarity effect at various energies, and for various field size, ionization chamber, and electrometer combinations. Of the seven chambers, CC01, PTW23323, and PTW31006 had the largest polarity effect for small field sizes. The materials of the central electrode of these three chambers were steel or air-equivalent plastic C-552. The magnitude of the polarity effect was shown to be strongly dependent on the material of the collecting electrode. This polarity effect dependence was observed for the ionization chambers and small field sizes studied.

Assessment of Olfactory Nerve by SPECT-MRI Image with Nasal Thallium-201 Administration in Patients with Olfactory Impairments in Comparison to Healthy Volunteers

Purpose The aim of this study was to assess whether migration of thallium-201 (201Tl) to the olfactory bulb were reduced in patients with olfactory impairments in comparison to healthy volunteers after nasal administration of 201Tl. Procedures 10 healthy volunteers and 21 patients enrolled in the study (19 males and 12 females; 26–71 years old). The causes of olfactory dysfunction in the patients were head trauma (n = 7), upper respiratory tract infection (n = 7), and chronic rhinosinusitis (n = 7). 201TlCl was administered unilaterally to the olfactory cleft, and SPECT-CT was conducted 24 h later. Separate MRI images were merged with the SPECT images. 201Tl olfactory migration was also correlated with the volume of the olfactory bulb determined from MRI images, as well as with odor recognition thresholds measured by using T&T olfactometry. Results Nasal 201Tl migration to the olfactory bulb was significantly lower in the olfactory-impaired patients than in healthy volunteers. The migration of 201Tl to the olfactory bulb was significantly correlated with odor recognition thresholds obtained with T&T olfactometry and correlated with the volume of the olfactory bulb determined from MRI images when all subjects were included. Conclusions Assessment of the 201Tl migration to the olfactory bulb was the new method for the evaluation of the olfactory nerve connectivity in patients with impaired olfaction.

Evaluation of organ doses and effective dose according to the ICRP Publication 110 reference male/female phantom and the modified ImPACT CT patient dosimetry.

We modified the Imaging Performance Assessment of CT scanners (ImPACT) to evaluate the organ doses and the effective dose based on the International Commission on Radiological Protection (ICRP) Publication 110 reference male/female phantom with the Aquilion ONE ViSION Edition scanner. To select the new CT scanner, the measurement results of the CTDI100,c and CTDI100,p for the 160 (head) and the 320 (body) mm polymethylmethacrylate phantoms, respectively, were entered on the Excel worksheet. To compute the organ doses and effective dose of the ICRP reference male/female phantom, the conversion factors obtained by comparison between the organ doses of different types of phantom were applied. The organ doses and the effective dose were almost identical for the ICRP reference male/female and modified ImPACT. The results of this study showed that, with the dose assessment of the ImPACT, the difference in sex influences only testes and ovaries. Because the MIRD‐5 phantom represents a partially hermaphrodite adult, the phantom has the dimensions of the male reference man including testes, ovaries, and uterus but no female breasts, whereas the ICRP male/female phantom includes whole‐body male and female anatomies based on high‐resolution anatomical datasets. The conversion factors can be used to estimate the doses of a male and a female accurately, and efficient dose assessment can be performed with the modified ImPACT. PACS number: 87.53.LY, 87.57.Q‐, 87.57.‐s

Accuracy of measuring half‐ and quarter‐value layers and appropriate aperture width of a convenient method using a lead‐covered case in X‐ray computed tomography

Determination of the half‐value layer (HVL) and quarter‐value layer (QVL) values is not an easy task in X‐ray computed tomography (CT), because a nonrotating X‐ray tube must be used, which requires the assistance of service engineers. Therefore, in this study, we determined the accuracy of the lead‐covered case method, which uses X‐rays from a rotating X‐ray tube, for measuring the HVL and QVL in CT. The lead‐covered case was manufactured from polystyrene foam and a 4 mm thick lead plate. The ionizing chamber was placed in the center of the case and aluminum filters were placed 15 cm above the aperture surface. Aperture widths of 1.0, 2.0, and 3.0 cm for a tube voltage of 110 kV and an aperture width of 2.0 cm for the tube voltages of 80 and 130 kV were used to measure exposure doses. The results of the HVL and QVL were compared with those of the conventional nonrotating method. A 2.0 cm aperture was believed to be adequate, because of its small differences in the HVL and QVL in the nonrotating method and its reasonable exposure dose level. When the 2.0 cm aperture was used, the lead‐covered case method demonstrated slightly larger HVLs and QVLs (0.03‐0.06 mm for the HVL and 0.2‐0.4 mm for the QVL) at all the tube voltage settings. However, the differences in the effective energy were 0.1‐0.3 keV; therefore, it could be negligible in an organ‐absorbed dose evaluation and a quality assurance test for CT. PACS numbers: 87.57.‐s; 87.57.Q‐; 87.57.uq

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