Photostimulable Phosphor Screen Research Articles

Evaluation of the modulation transfer function of megavoltage X-rays

The modulation transfer function (MTF) must be measured against the requirements of the specific digital imaging application under consideration in order to evaluate the performance of detector equipment and to further develop the detector by performing quality-assurance (QA) procedures. The purpose of this study was to measure the MTF in digital megavoltage images (DMVs) for radiation therapy (RT) by using an edge block. We used 6 megavolts (MVs) of energy for the pre-sampling MTF, along with a photo-stimulating phosphor-based computed radiography (CR) system and a hexalon lead screen that contained a terbium-doped gadolinium oxysulfide granular phosphor (Gd2O2S:Tb). The DMV MTF was measured at both low and high resolutions according to changes in the dose in monitor units (MUs) by using four different detector combinations: CR-IP (image plate: photo-stimulable phosphor screen), CR-IP-Lead (image plate + lead screen), CR-IP-Regular (fast front screen + image plate + fast front screen) and CR-IP-Fast (fast back screen + image plate + fast front screen). At a low resolution, the MTF 50% and the MTF 10% when using the CR-IP detector increased by about 30% and 46%, in proportion to the increase in the dose from 1 to 20 MU, respectively. At a high resolution, the CR-IP and the CR-IP-Lead detectors showed increases in the MTF of about 8% or 10% when the dose increased from 1 to 20 MU. The present study, therefore, evaluates how edge methods can be helpful in taking MTF measurements during QA tests of a megavoltage imaging (MVI) system.

Performance of an edge block used in a configuration detector: Image quality measurements

In megavoltage X-ray imaging (MVI), computed radiography (CR) has required a very simple and general quality assurance (QA) method based on a radiotherapy imaging technique. The purpose of this study was to develop a general QA method for evaluating the modulation transfer function (MTF), the noise power spectrum (NPS) and the detective quantum efficiency (DQE) for MVI. Thus, a simple and compact edge block made of tungsten, with dimensions of 19 × 10 × 1 cm3, was efficiently designed and developed. The pre-sampling MTF was evaluated with the following settings: a 6 megavoltage (MV) energy, four different types of detectors, CR-IP (image plate: photo-stimulable phosphor screen), CR-IP-Lead (image plate + lead screen), CR-IP-Regular (fast front screen + image plate + fast front screen) and CR-IP-Fast (fast back screen + image plate + fast front screen). The MTF values at 1 mm−1 for CR-IP-Regular showed the highest resolution of 0.18. The best noise distribution was observed in the measurement of the NPS with CR-IP-Regular. With increasing spatial frequency, our results showed that value of the DQE for the four types of detectors was approximately 1.0 mm−1, which is comparable to the values reported in the literature. In this report, measurements for megavoltage imaging based on line-pair modulations by using an edge block are presented. When the wide side of the tungsten block was reduced, the accuracy of the edge measurement was improved to match those offered by edge techniques. The present study demonstrates that the QA method with our home-made edge block can be used to evaluate the MTF, NPS and DQE for MVI.

Comparison of noise power spectrum methodologies in measurements by using megavoltage X-ray energies

The noise power spectrum (NPS) is one of the most general methods for measuring the noise amplitude and the quality of an image acquired from a uniform radiation field. The purpose of this study was to compare different NPS methodologies by using megavoltage X-ray energies. The NPS evaluation methods in diagnostic radiation were applied to therapy using the International Electrotechnical Commission standard (IEC 62220-1). In order to measure the region of interest (ROI) of the NPS, we used the following five factors: the overlapping impact, the non-overlapping impact, the penumbra, the flatness and different ROI sizes. We used NPS from four different types of detectors, the CR-IP (computed radiography image plate: photo-stimulable phosphor screen), the CR-IP-lead (hexalon lead screen), the CR-IP-back [lanex TM fast back screen: {terbium-doped gadolinium oxysulfide granular phosphor screen (Gd2O2S:Tb,133 mg/cm2)} + 1-mm-thick copper plate and the CR-IP-front (lanex TM fast front screen). A Kodak 2000 RT photo-stimulable phosphor-based computed radiographic (CR) system showed that the normalized noise power spectrum (NNPS) curve gradually decreased, in compliance with increasing spatial resolution. In addition, each detector showed a different reactivity of the NPS to megavoltage. The results of multivariate analysis of variance (MANOVA) test (methods × detectors) revealed significant main effects of the methods [F(1, 4) = 53.543, P = 0.001 and of the detectors [F(1, 4) = 17.556, P = 0.001]. The present study revealed that various factors could be employed to produce megavoltage imaging (MVI) of the NPS and as a baseline standard for NPS control in MVI.

診療用Ｘ線撮影システムおよびＣＲシステムにおける簡便な始業点検プログラムの評価

We investigated the feasibility of a simple, easy method that uses a digital radiography (DR) phantom for daily operational checks of the medical X-ray system and computed radiography (CR) system. First, we measured exposure indicator calibration (S value) from 12 seconds to 11 minutes after irradiation to examine the effect of fading of the photostimulable phosphor screen. We then examined the management width of the exposure indicator calibration ratio and limiting resolution of the CR system. Finally, we studied the exposure indicator calibration ratio, limiting the resolution and time necessary for the initial check-up by running the program 20 times for five weeks. Under the influence of the fading, the S value increased gradually, although the level of variation was small. From the mean and coefficient of variation of the exposure indicator calibration ratio, the management width was determined as +/-10%. The exposure indicator calibration ratio and limiting resolution were within +/-10% and +/-20%, respectively. It took three minutes to accomplish the initial check-up. The initial check-up using a DR phantom is simple and can perform the operational check of the medical X-ray system and the CR system with sufficient accuracy in only 3 minutes.

Radiation monitoring using imaging plate technology: A case study of leaves affected by the Chernobyl nuclear power plant and JCO criticality accidents

This paper describes the use of a photostimulable phosphor screen imaging technique to detect radioactive contamination in the leaves of wormwood (Artemisia vulgaris L) and fern (Dryopteris filix-max CL. Schoff) plants affected by the Chernobyl nuclear power plant accident. The imaging plate technology is well known for many striking performances in two-dimensional radiation detection. Since imaging plate comprises an integrated detection system, it has been extensively applied to surface contamination distribution studies. In this study, plant samples were collected from high- and low-contaminated areas of Ukraine and Belarus, which were affected due to the Chernobyl accident and exposed to imaging technique. Samples from the highly contaminated areas revealed the highest photo-stimulated luminescence on the imaging plate. Moreover, the radio nuclides detected in the leaves by gamma and beta ray spectroscopy were 137Cs and 90Sr, respectively. Additionally, in order to assess contamination, a comparison was also made with leaves of plants affected during the JCO criticality accident in Japan. Based on the results obtained, the importance of imaging plate technology in environmental radiation monitoring has been suggested.

Field ion microscope with photostimulable phosphor screen — new approach for quantitative analysis of FIM images-

Adaptability of a photostimulable phosphor screen, called an imaging plate (IP), for field emission and field ion microscopies was examined. The photostimulated luminescence (PSL) of the IP for He + is strong enough for field ion microscopy, but that for Ne + is less than one tenth of He +, like an ordinary fluorescent material. The digitized computer-processed images of W, W-Re and Pt-Ir are unexpectedly clear. The image brightness seems to increase with the number of incoming ions and the PSL intensity ratio of a dark atom to a bright atom is found to be as large as 1 to 160, suggesting the possibility of a quantitative analysis of electron tunneling and a field ionization probability over individual surface atoms. However, this ratio is significantly smaller than the expected dynamic range of the IP, 10 4, possibly due to the charge up of the IP's surface, low ion energies and an IP's high outgassing rate, which deteriorates vacuum and induces high background counting. Accordingly, darkly imaged “invisible atoms” of the alloys are hardly recognizable at present.

Accuracy of Intensity Measurement by Use of an Area Detector with a Photostimulable Phosphor Screen, as Confirmed by Measuring Scattering Intensity from a Liquid

An area detector using a photostimulable phosphor screen, namely an imaging plate (IP), has been applied to the intensity measurement of X-ray scattering from liquid CCl4. The scattering intensity in the present experiment has been compared with that determined by means of an energy-dispersive method. The two intensities have agreed well with each other. It has been proved that the use of the IP detector makes it possible to measure the scattering intensities of halo patterns precisely with much shorter accumulating time with a simple procedure of analysis and that there is no effect of the oblique incidence of scattering X-rays to the IP at high scattering angles.

Design and performance of an imaging plate system for X-ray diffraction study

A new readout system for a BaFBr: Eu 2+ photostimulable phosphor screen (imaging plate) was constructed by modifying a drum scanner, with a design optimized for X-ray diffraction and scattering applications. An effort was made to achieve a high detective quantum efficiency below 20 keV, a small pixel size (25 μm × 25 μm), a low quantization noise (0.22%) using 12-bit A/D converters, and the capability to cover an inherent dynamic range (1:10 5) of the photostimulated luminescence by using two photomultiplier tubes. This system is being used in several synchrotron radiation experiments: Laue diffraction of protein crystals, small angle diffraction from a single muscle fiber, powder diffraction from crystals in a diamond anvil cell, and time-resolved small-angle X-ray scattering from a synthetic polymer during stretching.