Flat-panel Detector System Research Articles

Impact of flat panel‐imager veiling glare on scatter‐estimation accuracy and image quality of a commercial on‐board cone‐beam CT imaging system

The purposes of this study is to measure the low frequency drop (LFD) of the modulation transfer function (MTF), associated with the long tails of the detector point spread function (PSF) of an on-board flat panel imager and study its impact on cone-beam CT (CBCT) image quality and scatter measurement accuracy. Two different experimental methods were used to characterize LFD and its associated PSF of a Varian OBI flat-panel detector system: the edge response function (ERF) method and the disk transfer function (DTF) method. PSF was estimated by fitting parametric models to these measurements for four values of the applied voltage (kVp). The resultant PSF was used to demonstrate the effect of LFD on image contrast and CT number accuracy in CBCT images reconstructed from synthetic datasets, as well as, accuracy of scatter measurements with the beam-stop method. The MTFs derived from the measured ERF data revealed LFDs varying from 8% (at 60 kVp) to 10.5% (at 120 kVp), while the intensity of the long PSF tails was found to increase with increasing kVp. The veiling glare line spread functions derived from the ERF and DTF methods were in excellent agreement. Uncorrected veiling glare reduced contrast and the image intensity in CBCT reconstruction, near the phantom periphery (by 67 Hounsfield units in a 20 cm-in-diameter water phantom) and (to a smaller degree) near inhomogeneities. Use of the bow-tie filter mitigated these effects. Veiling glare also resulted in about 10%-15% overestimation of the scatter-to-primary ratio when measured with the beam-stop or beam-stop array method. The long tails of the detector PSF were found to have a modest dependence of beam spectrum, which is reflected on the MTF curve LFD. Our findings show that uncorrected veiling glare can affect quantitative accuracy and contrast in CBCT imaging, based on flat panel imager. In addition, it results in overestimation of the scatter-to-primary ratio, measured with the beam-stop methods.

胸部 Flat Panel Detector System と Computed Radiography Systemの被曝線量と画像評価

胸部 Flat Panel Detector System と Computed Radiography Systemの被曝線量と画像評価

Occupational Radiation Dose:Percutaneous Interventional Procedures on Hemodialysis Arteriovenous Fistulas and Grafts

To evaluate operator radiation exposure during percutaneous interventions on hemodialysis arteriovenous fistulas (AVFs) and arteriovenous grafts (AVGs). Seventy-seven procedures were performed in 57 patients while the interventional radiologist wore two ring dosimeters, two thermoluminescent dosimeters at the lower legs, and a thermoluminescent dosimeter on the forehead. Dose-area product, fluoroscopy time, total procedure time, and radiation exposures to eye lens, hands, and legs were recorded. Variables were procedure type, access site side, and angiography equipment. Statistical analysis was performed with the signed-rank and Mann-Whitney U tests. Mean operator radiation doses for the left hand, right hand, eye lens, left leg, and right leg were 0.28, 0.28, 0.03, 0.11, and 0.12 mSv, respectively. Radiation exposure to the hands was significantly higher compared with that to the legs (P<.0001). In recanalization procedures, fluoroscopy time, total procedure time, and mean number of angiographic runs were higher (all P<.001) than those for percutaneous transluminal angioplasty (PTA), as were radiation exposures to the hands and left leg (all P<.05). Left-sided access interventions resulted in higher doses to the right hand and leg (both P<.05). For right-sided access interventions, doses to the left hand and leg were higher (P<.0001). Eye lens radiation dose was significantly higher for procedures with the flat-panel detector system (P=.002). Operator radiation exposure to the hands, legs, and eyes during percutaneous interventional procedures performed on hemodialysis AVFs and AVGs is relatively low. Radiation exposure to the hands was higher than that to the legs, and the hand and leg closest to the AVF or AVG received a higher dose. Recanalization procedures resulted in higher doses to the hands and left leg than did PTA. Eye lens radiation dose may be higher with a flat-panel detector system.

Review of a simple noise simulation technique in digital radiography

Reduction of exposure dose and improvement in image quality can be expected to result from advances in the performance of imaging detectors. A number of researchers have reported on methods for simulating reduced dose images. The simplest method provides reduced dose images by adding white Gaussian noise with a certain standard deviation to the original image. Our aim in this study was to develop and validate a system with a graphic user interface for simulating reduced dose images by a simple method. Here, we describe a technical approach with the use of a flat-panel detector system, and we validated the simulation performance in reducing the dose objectively and subjectively. In addition, the technical limitations and possible solutions to the simple method are suggested based on the validation results presented in this paper.

Comparison of Digital Flat-Panel Detector and Conventional Angiography Machines: Evaluation of Stent Detection Rates, Visibility Scores, and Dose-Area Products

The objective of this study was to compare the performance and radiation doses of a flat-panel detector (FPD) angiography machine with an image intensifier (II) angiography machine. Images of four nitinol stents (Sinus-SuperFlex, SMART, Luminexx, and Zilver stents) in a phantom of a human pelvis were acquired on an FPD system (Axiom Artis) and an II system (Fluorospot TOP) using the following modes: spot-film, continuous fluoroscopy (4, 7.5, 15, and 30 pulses/s), and three amplification modes. Objective stent detection rates and subjective radiopacity scores (scale: 0 [not visible] to 4 [excellent visibility]) were calculated. The radiation doses evaluated by the respective machines were compared. Over all modes and stents, the mean objective correct stent detection rates and mean subjective radiopacity scores were 89.49% and 1.81, respectively, for the Axiom Artis and 91.00% and 2.26 for the Fluorospot TOP. The stent detection rates over all modes for the SMART and Luminexx stents were better using the Axiom Artis machine (97.61% vs 93.55% and 98.28% vs 90.41%, respectively) and those for the Sinus-SuperFlex and Zilver stents were better using the Fluorospot TOP machine (90.83% vs 83.56% and 89.29% vs 80.50%). The subjective radiopacity scores of stent visibility were worse for the Axiom Artis than the Fluorospot TOP for all stents except the Luminexx stent (mean score, 2.34 vs 2.21, respectively). The objective stent detection rates and subjective radiopacity scores improved using the spot-film mode and with raising amplification, whereas increases in the fluoroscopy pulsing frequency did not improve stent detection rates or radiopacity scores for either machine. The radiation doses at continuous fluoroscopy were approximately 90% higher for the Axiom Artis than for the Fluorospot TOP (2.60 vs 1.41 μGy/m(2) at 30 pulses/s, respectively). The objective correct stent detection rates were similar for both machines with differences in detection for the respective stents. The subjective radiopacity scores were almost always better for the Fluorospot TOP machine. Also, the Axiom Artis machine generated approximately 90% higher radiation doses in fluoroscopy. For both machines, using a higher fluoroscopy pulsing frequency had no positive effect on objective correct stent detection rates or subjective radiopacity scores.

A semiempirical linear model of indirect, flat‐panel x‐ray detectors

It is important to understand signal and noise transfer in the indirect, flat-panel x-ray detector when developing and optimizing imaging systems. For optimization where simulating images is necessary, this study introduces a semiempirical model to simulate projection images with user-defined x-ray fluence interaction. The signal and noise transfer in the indirect, flat-panel x-ray detectors is characterized by statistics consistent with energy-integration of x-ray photons. For an incident x-ray spectrum, x-ray photons are attenuated and absorbed in the x-ray scintillator to produce light photons, which are coupled to photodiodes for signal readout. The signal mean and variance are linearly related to the energy-integrated x-ray spectrum by empirically determined factors. With the known first- and second-order statistics, images can be simulated by incorporating multipixel signal statistics and the modulation transfer function of the imaging system. To estimate the semiempirical input to this model, 500 projection images (using an indirect, flat-panel x-ray detector in the breast CT system) were acquired with 50-100 kilovolt (kV) x-ray spectra filtered with 0.1-mm tin (Sn), 0.2-mm copper (Cu), 1.5-mm aluminum (Al), or 0.05-mm silver (Ag). The signal mean and variance of each detector element and the noise power spectra (NPS) were calculated and incorporated into this model for accuracy. Additionally, the modulation transfer function of the detector system was physically measured and incorporated in the image simulation steps. For validation purposes, simulated and measured projection images of air scans were compared using 40 kV∕0.1-mm Sn, 65 kV∕0.2-mm Cu, 85 kV∕1.5-mm Al, and 95 kV∕0.05-mm Ag. The linear relationship between the measured signal statistics and the energy-integrated x-ray spectrum was confirmed and incorporated into the model. The signal mean and variance factors were linearly related to kV for each filter material (r(2) of signal mean to kV: 0.91, 0.93, 0.86, and 0.99 for 0.1-mm Sn, 0.2-mm Cu, 1.5-mm Al, and 0.05-mm Ag, respectively; r(2) of signal variance to kV: 0.99 for all four filters). The comparison of the signal and noise (mean, variance, and NPS) between the simulated and measured air scan images suggested that this model was reasonable in predicting accurate signal statistics of air scan images using absolute percent error. Overall, the model was found to be accurate in estimating signal statistics and spatial correlation between the detector elements of the images acquired with indirect, flat-panel x-ray detectors. The semiempirical linear model of the indirect, flat-panel x-ray detectors was described and validated with images of air scans. The model was found to be a useful tool in understanding the signal and noise transfer within indirect, flat-panel x-ray detector systems.

低線量領域におけるDigital Radiographyの物理的画質特性の検討

We measured physical image properties of a flat panel detector (FPD) system and a computed radiography (CR) system, targeting to a low dose range (reference dose: 2.58×10(-7) C/kg: to 1/20 dose). Input-output properties, pre-sampled modulation transfer functions (pre-sampled MTFs), and normalized noise power spectra for an FPD system equipped with a CsI scintillator (FPDcsi) and a CR system with an imaging plate coated with storage phosphor (CR) were measured at the low dose range for radiation quality of RQA3 (≍50 skV) and RQA5 (≍70 kV), and detective quantum efficiencies (DQEs) were calculated. In addition, in order to validate the DQE results, component fractions of Poisson and multiplicative and additive noise were analyzed using relative standard deviation analysis. The DQE values of FPDcsi were decreased with dose decrease, and contrarily to these, those of CR were increased. At the 1/10 and 1/20 doses for RQA3, the DQEs of FPDcsi and CR became almost the same. From the results of RSD analysis, it was proved that the main cause of DQE deterioration on FPDcsi are non-negligible additive (electronic) noise, and the DQE improvement on CR was caused by both of significant multiplicative (structure) noise and very low electronic noise. The DQE results were validated by comparing burger phantom images of each dose and radiation quality.

Comparison of anti-scatter grids for digital imaging with use of a direct-conversion flat-panel detector

Our purpose in this study was to establish a selection standard for anti-scatter grids for a direct conversion flat-panel detector (FPD) system. As indices for grid evaluation, we calculated the selectivity, Bucky factor, and the signal-to-noise ratio improvement factor (SIF) by measuring rates of scatter transmission, primary transmission, and total transmission (based on the digitally displayed measurement values of the FPD system), using 4 acrylic phantoms of different thicknesses. The results showed that the SIF was less than 1.0 when the phantom thickness was 5 cm. When the phantom thickness was 25 cm and the grid ratio was 16:1, the SIF was 1.505 and 1.518 (maximum value) at 90 and 120 kV, respectively. Compared with the grid ratio of 12:1, the SIF at the grid ratio 16:1 was improved by 6.1% at 90 kV, and by 7.0% at 120 kV. In a direct-conversion FPD system, the grid ratio of 16:1 is considered adequate for eliminating the scattered-radiation effect when much scattered radiation is present, such as with a thick imaged object or a high X-ray tube voltage.

Geometric Calibration of Novel Cone-Beam CT Developed For Dedicated Stereotactic Radiotherapy Unit

A novel Cone-Beam CT system has been under development for a dedicated stereotactic radiosurgery/radiotherapy unit (Gamma Knife PerfeXion) to be equipped with image guidance. The stability and the reproducibility of the imaging system were investigated using the full geometric calibration. The imaging system consists of a rotational arm (Parker system) holding X-ray tube (Dunlee PX 1483), flat panel detector (Varian 4030 CB) and in-house system control and reconstruction software (Pilot 1.0). The arm travels in the vertical direction through the linear guide system (Parker 406xr) from its home position to the imaging position by 1250 mm. The accuracy and precision of the vertical motion is measured using dial gauge each time before geometric calibration. Geometric calibrations were repeated over several days with 1) the fixed calibration phantom position and 2) repositioning the imaging system in order to measure its stability and reproducibility. The calibration phantom consists of 16 ball bearings precisely located in two circular trajectories in a cylindrical plastic phantom. All the geometric parameters including source position, detector position and detector tilt angles with respect to gantry angle were determined. The calibration algorithm previously developed and verified at the cone-beam CT systems on optical bench and clinical linear accelerator was applied. Positioning accuracy of the vertical linear guide measured with dial gauge is better than 0.02mm (average absolute error). More than 200 projection images of calibration phantom were acquired each time over 210 degrees of gantry rotation for geometric calibration. Piercing points was reliably reproduced and its position was always in radius of 0.2mm or half pixel radius. In the current design, the reproducibility of vertical position, V, is limited to the precision of mechanical pin structure which allows three different X-ray tube positions. Gantry angles are noticed to have the maximum error of 0.2 degrees. Detector tilt angles are all within 0.5 degrees. Considering the uncertainty of the calibration algorithm of 0.4o (roll and pitch) and 0.01o (yaw) in detector tilt angles, the yaw is found to be the only angle that is significantly tilted. Adjusting this angle can impact the image quality. Source and detector trajectories show interesting behavior of gantry tilt and wobbling motion although the deviation from the ideal trajectory is less than 0.2 mm, which has negligible impact on image quality. Geometric calibration showed excellent stability and reproducibility of the prototype imaging system. More extensive work on quality assurance including full geometric calibration will be done after the imaging system is installed on the treatment unit.

Digital tomosynthesis of the chest: Utility for detection of lung metastasis in patients with colorectal cancer

To evaluate the performance of digital tomosynthesis (DT) of the chest for detection of lung nodules in patients with colorectal cancer (CRC). The institutional review board approved this study, and all patients provided informed consent. A commercial caesium iodide/amorphous silicon (CsI/a-Si) flat-panel detector system was used to verify the performance of the DT and chest radiography (XR) methods. DT was performed in 142 patients with CRC. All 142 patients underwent chest computed tomography (CT) within a week of DT. As a reference standard, two radiologists reviewed the chest CT in consensus and recorded the presence of pulmonary nodules. Another two radiologists independently observed the DT images and recorded the presence of pulmonary nodules. The status of all lung nodules was assessed either histologically or by follow-up over a period of 1 year. The nodules were classified into metastasis, benign, and uncertain. Statistical analysis of the results was performed. Two hundred and thirty-seven nodules from 142 patients were found at CT. These included 71 proven metastases and 126 benign nodules; 40 nodules were uncertain. Observers detected 83% of all lung nodules and 93% of proven metastases using DT. Among 237 nodules, 147 nodules were larger than 4mm in diameter on the CT images. Observers detected 87% of lung nodules that were larger than 4mm. Despite a reasonably low radiation dose, DT is a sensitive method, and is comparable to chest CT for the detection of lung nodules, particularly metastatic lung nodules in patients with CRC.

Structural noise from automatic exposure control device and its relationship to X-ray tube voltage used for calibration of a flat-panel detector system

In flat-panel detector (FPD) systems, the ion-chamber dosimeters used for automatic exposure control (AEC), which are placed between the detector and the source, should not affect clinical images because of FPD gain correction, but can sometimes still introduce fixed-pattern noise. In this study, we investigated whether such artifacts were caused by structural noise from the AEC detector on the basis of the noise power spectrum (NPS) and the mean square error (MSE) of FPD images taken at various tube voltages either with or without the AEC detector. When the NPS was measured without the AEC detector, the NPS did not increase in the low special-frequency band at all radiation qualities tested, irrespective of X-ray calibration tube voltages. However, when the NPS was measured while the AEC detector was used, the NPS increased in the low special-frequency band at all radiation qualities when the X-ray calibration tube voltages were at low levels. Similarly, the MSE increased when the X-ray calibration tube voltages were at low levels. From these results, artifacts in the AEC detector appear to be suppressed when a radiation quality of approximately 90kV is used at four different standardized radiations quality (RQA3, RQA5, RQA7, and RQA9).

フラットパネルディテクタ (FIPD) の画質特性の現状について

フラットパネルディテクタ (FIPD) の画質特性の現状について

フラットパネルディテクタ (FPD) の画質特性の評価

フラットパネルディテクタ (FPD) の画質特性の評価

SU-E-I-188: Potential for Dose Reduction in An Indirect Flat Panel Detector System Employing Irradiation Side Sampling

Purpose: To investigate possibility of dose reduction for a newly developed indirect flat panel detector(FPD)system employing irradiation side sampling (ISS), physical imaging properties were measured and compared with those of a conventional computed radiography(CR)system. Method and Materials:The FPDsystem employing ISS (CALNEO MT, Fuji Film) and the CRsystem (FCR PROFECT CS, Fuji Film) including the imaging plate (ST‐VI, Fuji Film) were used in this study. The Wiener spectrum (WS), the presampled modulation transfer function(MTF), and the detective quantum efficiency (DQE) for both systems were measured. To evaluate image quality of the detector for chest radiography, the RQA 9 in the International Electrotechnical Commission (IEC) 62220‐1 was used. Results: The WS of the FPDsystem showed considerably lower than that of the CRsystem at every spatial frequency. The WS of the FPDsystem with a 50 % dose reduction of the CRsystem was almost identical to that of the CRsystem. The presampled MTF of the FPDsystem was comparable to that of the CRsystem. The DQE of the FPDsystem obtained at zero spatial frequency was almost twice than that of the CRsystem. Conclusion: The radiographic noise property and the detection efficiency of the new ISS FPD were significantly superior to those of the conventional CRsystem. It is expected that the use of the new ISS FPDsystem for chest radiography can reduce the patient skin dose by 50 % compared with the conventional CRsystem.

Radiation Dose Reduction in Scoliosis Patients: Low-Dose Full-Spine Radiography with Digital Flat Panel Detector and Image Stitching System

To evaluate the exposure dose reduction with a digital flat panel detector (FPD) and an image stitching system (ISS) in full-spine radiography for scoliosis patients. During a 6-month period, all consecutive scoliosis patients with a clinical indication for full-spine radiography (n=50) were examined with an FPD and ISS. Automatic exposure control adjusted to speed class 1600 was used together with age-adjusted tube voltage and filtration. Dose area products were recorded for all images (antero-posterior n=50, lateral n=18). Images were evaluated by two radiologists for the possibility (possible, impossible) of typical scoliosis measurements (Cobb angle, Stagnara angle, lateral deviation, Risser stage). All measurements assessed as impossible underwent a second evaluation categorizing the reason why a measurement was impossible (underlying pathology, projection, image quality). Patient characteristics influencing exposure were recorded (sex, age, weight, height). Mean dose area products were compared to the literature with consideration of patient group and image quality. The mean dose area product was 16.8µGym (2) for antero-posterior images and 26.6µGym (2) for lateral images. A comparison to published values showed an exposure dose reduction of 47% to 93%. Measurement of the Cobb and Stagnara angle, lateral deviation and Risser stage was possible in 96% (n=50), 83% (n=18), 100% (n=50) and 100% (n=50) of cases. The reasons for impossible measurements were independent of image quality (underlying pathologies, projection). When imaging scoliosis patients, an FPD combined with an ISS can substantially reduce the exposure dose.

AAPM/RSNA Physics Tutorial for Residents: Physics of Flat-Panel Fluoroscopy Systems

This article reviews the design and operation of both flat-panel detector (FPD) and image intensifier fluoroscopy systems. The different components of each imaging chain and their functions are explained and compared. FPD systems have multiple advantages such as a smaller size, extended dynamic range, no spatial distortion, and greater stability. However, FPD systems typically have the same spatial resolution for all fields of view (FOVs) and are prone to ghosting. Image intensifier systems have better spatial resolution with the use of smaller FOVs (magnification modes) and tend to be less expensive. However, the spatial resolution of image intensifier systems is limited by the television system to which they are coupled. Moreover, image intensifier systems are degraded by glare, vignetting, spatial distortions, and defocusing effects. FPD systems do not have these problems. Some recent innovations to fluoroscopy systems include automated filtration, pulsed fluoroscopy, automatic positioning, dose-area product meters, and improved automatic dose rate control programs. Operator-selectable features may affect both the patient radiation dose and image quality; these selectable features include dose level setting, the FOV employed, fluoroscopic pulse rates, geometric factors, display software settings, and methods to reduce the imaging time.

Abstract No. 138: Radiation dose during tunneled central venous catheter placement: Effect of trainee participation and flat panel detector use

Radiation caused by medical imaging and its potential effects on patients has caused considerable interest in the health care community as well as in the popular media. While most of the attention has focused on CT, fluoroscopy-guided procedures can deliver a significant dose to patients. The purpose of this retrospective review is to investigate factors that influence fluoroscopy time and thus patient radiation during fluoroscopy-guided procedures. After IRB approval, the HI-IQ database was queried for tunneled central venous catheter placement (tCVC) returning 1,743 complete cases from January 2007 to June 2010. The data in the report included fluoroscopy time (f), dose (mGy), operator(s) and whether it was performed in an angriography suite with a flat panel detector (FPD) sytstem or an image intensifier (II) system. The cases were performed by one of 13 staff (range 7–232 tCVCs placed). Variables from the data were analyzed using a T-test after log transformation and testing for variance with an F-test. Results of the statistical analysis are summarized in the Table.Tabled 1FPD-systemII-systemp-valueMean fluoroscopy time⁎Numbers presented are after log transformation. (minutes ± standard deviation)0.11 ± 0.150.19 ± 0.13<0.0001Mean patient dose⁎Numbers presented are after log transformation. (mGy ± standard deviation)1.11 ± 0.281.18 ± 0.290.005Trainee involvementNo traineep-valueMean fluoroscopy time⁎Numbers presented are after log transformation. (minutes ± standard deviation)0.23 ± 0.130.11 ± 0.13<0.0001Mean patient dose⁎Numbers presented are after log transformation. (mGy ± standard deviation)1.25 ± 0.321.09 ± 0.26<0.0001 Numbers presented are after log transformation. Open table in a new tab Our data show that the use of flat panel detector-systems for the placement of tunneled central venous catheters resulted in both decreased fluoro-time and patient dose (mGy). We also show that the involvement of a trainee (resident or fellow) in the procedure increases the fluoro-time of the case. While the differences are small for tCVC placement, similar results are likely for other IR procedures and should be taken into consideration when considering room utilization and trainee involvement.

Super-resolution variable-dose imaging in digital radiography: quality and dose reduction with a fluoroscopic flat-panel detector

Super-resolution (SR) image processing produces a high-resolution image from a series of low-resolution images. The aim of this study was to evaluate SR-images based on fluoroscopic flat-detector (FD) acquisition at different frame rates. Fluoroscopic FD-sequences with 20 frames were obtained with varying pulse frequencies of (1) a line pair resolution phantom; (2) a low-contrast resolution phantom, and (3) a human knee specimen. Super-resolution digital radiographs (SR-Radiographs) were generated from each sequence. Variable-dose images were simulated by constructing SR-Radiographs using 6 and 12 frames from the corresponding fluoroscopic sequence. "Single Shot" and Computed Radiography (CR) images were obtained for comparison based on dynamic range and sharpness of bone detail structures. Patient-derived SR-Radiographic images were constructed to demonstrate clinical examples. The spatial resolution of SR-radiographs obtained at 12.5 frames per second (fps) and 6 fps were comparable with CR and "Single Shot" images, providing ~3.5 line pairs per mm (l p/mm). Similarly, low-contrast resolution of SR-radiographs obtained at 12.5, 6, and 30 fps were equivalent to CR and "Single Shot" images. The human knee specimen SR-radiograph obtained using 12 FD images at 12.5 fps was superior to a CR image in overall image quality, with a dose reduction of 75%. Variable-dose SR-radiographic simulations suggest a dose saving potential of 90-95% when using 6 FD images at 12.5 fps or 6 fps, respectively. The phantom testing images and simulation results demonstrate that diagnostic quality SR-radiographic images of skeletal extremities can be synthesized using a flat-panel detector system designed primarily for angiography. SR-images obtained with substantially reduced radiation dose are feasible, and this technology may improve digital radiography for pediatric, neonatal radiography, or mammography applications. Further testing is needed to validate super-resolution techniques in other body regions and for different flat-detector systems.

直接型および間接型Ｆｌａｔ Ｐａｎｅｌ Ｄｅｔｅｃｔｏｒｓと Ｃｏｍｐｕｔｅｄ Ｒａｄｉｏｇｒａｐｈｙにおける物理的画質評価を利用した画質同一化の試み

The purpose of this study was to investigate image quality identification methods among direct- and indirect-type flat panel detectors (FPDs) and a computed radiography (CR) system using two radiation qualities RQA3 and RQA5 defined in the IEC 61267 standard. For each system, the digital characteristic curve, the presampled modulation transfer function (MTF), and the normalized noise power spectrum (NNPS) were measured. Images for a burger phantom and a foot-bone phantom were processed by resolution identification utilizing two-dimensional Fourier transform, and then contrast-to-noise ratio (CNR) for each image was measured. For the RQA3, the direct FPD system indicated the highest DQE value, and for the RQA5 DQE value of indirect FPD system, it was a little higher than that of direct FPD system. The CNR results with the resolution identification displayed good accordance with the DQE results in both phantoms. From the DQE results, dose ratios for image quality identification were determined, and the CNRs of the dose-adjusted images were measured. The results for, the CNRs of all systems showed good coincidence. From these findings, they indicated that the DQE measurement is effective to determine the exposure parameters for equalizing the image quality of different types of radiographic systems.

Overlap of radiation field and radiation field shape in cardiac catheterization

The radiation field shape for cardiac catheterization has changed from being circular to being rectangular with the spread of flat-panel detector systems. A rectangular radiation field provides a wide fluoroscopy field to the four corners of a subject area; however, in cardiac catheterization, there is not much usable information around the four corners at several angles, and this tends to be regarded as a meaningless radiation exposure. Hence, overlap of radiation fields has been of concern recently. The authors changed field sizes/fluoroscopy angles and examined entrance dose rates to study radiation field shapes and configurations of radiation exposure to patients, and they discussed a radiation exposure reduction method. In measurements of phantom entrance dose rates, we considered right anterior oblique (RAO) directions, cranial (CRA) directions and RAO-CRA directions and found that entrance dose rates rose considerably, particularly at the RAO-CRA. In the study of radiation field overlap, we discuss radiation field shapes (rectangular/circular) as well as angles. In the RAO-CRA directions, differences occurred in angles of non-overlapping radiation field by differences in radiation field shapes. For RAO-CRA, compared with the RAO and CRA directions, entrance dose rates increased with an increase in angle. When convenience in clinics is considered, the utilization frequency of the four corners of a rectangular field is low. When one considers the increases in radiation exposure caused by radiation field overlap, it is more effective to use circular radiation fields.