Computed Radiography Detector Research Articles

Optimization of image quality and dose in adult and pediatric chest radiography via Monte Carlo simulation and experimental methods

Optimization in radiography aims to achieve adequate image quality to enable diagnosis by adjusting the dose to the patient. The figure of merit (FOM) is a well-known optimization method that balances image quality (IQ) and Dose. However, different authors use distinct definitions. This work aims to compare several FOM definitions to determine the most adequate for chest radiography optimization via Monte Carlo (MC) simulation. Moreover, this study aims to perform a systematic study, evaluating the influence of tube potential, additional filtration, detector technologies, antiscatter grids, and phantom thickness on FOM. The dose was evaluated by the mean absorbed dose (MAD), the entrance skin dose (ESD), and the effective dose (E). The signal-to-noise ratio (SNR) and the contrast-to-noise ratio (CNR) were the IQ indexes. FOM was calculated as IQ²/Dose. Two types of antiscatter grids were simulated. One grid had cotton fiber as an interspace material, and the other grid had aluminum. The simulated detectors were composed of CsI and a-Se, used in digital radiography (DR), and BaFBr, used in computed radiography (CR). The results showed that tube potentials close to 120 kV optimized the SNR for all cases of phantom thickness, dosimetric quantities, and additional filtrations. On the other hand, the optimum tube potentials for CNR are dependent on these parameters. It was shown that optimizing the CNR provides better overall image quality. Additionally, MAD and E are suitable for optimization studies based on FOM. Additional filtration showed a better dose reduction, with 1 mm Cu having the best performance. Considering both optimization and clinical applicability, additional filters of 0.2 mm Cu + 2 mm Al are indicated. The optimum tube potentials are similar for all antiscatter girds evaluated. When different detectors are applied, the one with a higher effective atomic number provides larger FOM values and optimum tube potential. Experimental validation was performed only with the CR detector due to limitations in the detection technologies available. The dependency of FOM on the tube potential was slightly different between the experimental and MC simulation cases; however, there was an intersection between the optimum tube potential range for both cases. The results in this study yielded a general overview of optimization, analyzing several parameters, providing an improvement in other optimization studies that were limited by either one FOM definition or experimental factors such as one detector and/or one antiscatter grid configuration.

Technical characterization of five x-ray detectors for paediatric radiography applications

Physical image quality of five x-ray detectors used in the paediatric imaging department is characterized with the aim of establishing the range/scope of imaging performance provided by these detectors for neonatal imaging. Two computed radiography (CR) detectors (MD4.0 powder imaging plate (PIP) and HD5.0 needle imaging plate (NIP), Agfa HealthCare NV, B-2640 Mortsel, Belgium) and three flat panel detectors (FPD) (the Agfa DX-D35C and DX-D45C and the DRX-2530C (Carestream Health Inc., Rochester, NY 14608, USA)) were assessed. Physical image quality was characterized using the detector metrics given by the International Electrotechnical Commission (IEC 62220-1) to measure modulation transfer function (MTF), the noise power spectrum (NPS) and the detective quantum efficiency (DQE) using the IEC-specified beam qualities of RQA3 and RQA5. The DQE was evaluated at the normal operating detector air kerma (DAK) level, defined at 2.5 µGy for all detectors, and at factors of 1/3.2 and 3.2 times the normal level. MTF curves for the different detectors were similar at both RQA3 and RQA5 energies; the average spatial frequency for the 50% point (MTF0.5) at RQA3 was 1.26 mm−1, with a range from 1.20 mm−1 to 1.37 mm−1. The DQE of the NIP CR compared to the PIP CR was notably greater and similar to that for the FPD devices. At RQA3, average DQE for the FPD and NIP (at 0.5 mm−1; 2.5 µGy) was 0.57 compared to 0.26 for the PIP CR. At the RQA5 energy, the DRX-2530C and the DX-D45C had the highest DQE (~0.6 at 0.5 mm−1; 2.5 µGy). Noise separation analysis using the polynomial model showed higher electronic noise for the DX-D35C and DRX-2530C detectors; this explains the reduced DQE seen at 0.7 µGy/image. The NIP CR detector offers notably improved DQE performance compared to the PIP CR system and a value similar to the DQE for FPD devices at the RQA3 energy.

Evaluation of Doses and Image Quality in Mammography with Screen-Film, CR, and DR Detectors - Application of the ACR Phantom.

SummaryBackgroundDifferent methods of image quality evaluation are routinely used for analogue and digital mammography systems in Poland. In the present study, image quality for several screen-film (SF), computed radiography (CR), and fully digital (DR) mammography systems was compared directly with the use of the ACR mammography accreditation phantom.Material/MethodsImage quality and mean glandular doses were measured and compared for 47 mammography systems in the Mazovia Voivodeship in Poland, including 26 SF systems, 12 CR systems, and 9 DR systems. The mean glandular dose for the breast simulated by 4.5 cm of PMMA was calculated with methods described in the “European guidelines for quality assurance in breast cancer screening and diagnosis”. Visibility of the structures in the image (fibers, microcalcifications, and masses) was evaluated with the mammographic accreditation ACR phantom.ResultsImage quality for DR systems was significantly higher than for SF and CR systems. Several SF systems failed to pass the image quality tests because of artifacts. The doses were within acceptable limits for all of the systems, but the doses for the CR systems were significantly higher than for the SF and DR systems.ConclusionsThe best image quality, at a reasonably low dose, was observed for the DR systems. The CR systems are capable of obtaining the same image quality as the SF systems, but only at a significantly higher dose. The ACR phantom can be routinely used to evaluate image quality for all types of mammographic systems.

Poster — Thur Eve — 02: Measurement of CT radiation profile width using Fuji CR imaging plate raw data

Measuring the CT collimation width and assessing the shape of the overall profile is a relatively straightforward quality control (QC) measure that impacts both image quality and patient dose, and is often required at acceptance and routine testing. Most CT facilities have access to computed radiography (CR) systems, so performing CT collimation profile assessments using CR plates requires no additional equipment. Previous studies have shown how to effectively use CR plates to measure the radiation profile width. However, a major limitation of the previous work is that the full dynamic range of CR detector plates are not used, since the CR processing technology reduces the dynamic range of the DICOM output to 210, requiring the sensitivity and latitude settings of CR reader to be adjusted to prevent clipping of the CT profile data. Such adjustments to CR readers unnecessarily complicate the QC procedure. These clipping artefacts hinder the ability to accurately assess CT collimation width because the full‐width at half maximum value of the penumbras are not properly determined if the maximum dose of the profile is not available. Furthermore, any inconsistencies in the radiation profile shape are lost if the profile plateau is clipped off. In this work we developed an opensource Matlab script for straightforward CT profile width measurements using raw CR data that also allows assessment of the profile shape without clipping, and applied this approach during CT QC.

The use of computed radiography plates to determine light and radiation field coincidence

Photo-stimulable phosphor computed radiography (CR) has characteristics that allow the output to be manipulated by both radiation and optical light. The authors have developed a method that uses these characteristics to carry out radiation field and light field coincidence quality assurance on linear accelerators. CR detectors from Kodak were used outside their cassettes to measure both radiation and light field edges from a Varian linear accelerator. The CR detector was first exposed to a radiation field and then to a slightly smaller light field. The light impinged on the detector's latent image, removing to an extent the portion exposed to the light field. The detector was then digitally scanned. A MATLAB-based algorithm was developed to automatically analyze the images and determine the edges of the light and radiation fields, the vector between the field centers, and the crosshair center. Radiographic film was also used as a control to confirm the radiation field size. Analysis showed a high degree of repeatability with the proposed method. Results between the proposed method and radiographic film showed excellent agreement of the radiation field. The effect of varying monitor units and light exposure time was tested and found to be very small. Radiation and light field sizes were determined with an uncertainty of less than 1 mm, and light and crosshair centers were determined within 0.1 mm. A new method was developed to digitally determine the radiation and light field size using CR photo-stimulable phosphor plates. The method is quick and reproducible, allowing for the streamlined and robust assessment of light and radiation field coincidence, with no observer interpretation needed.

A phantom for verification of dwell position and time of a high dose rate brachytherapy source

Accuracy of dwell position and reproducibility of dwell time are critical in high dose rate (HDR) brachytherapy. A phantom was designed to verify dwell position and dwell time reproducibility for an Ir-192 HDR stepping source using Computed Radiography (CR). The central part of the phantom, incorporating thin alternating strips of lead and acrylic, was used to measure dwell positions. The outer part of the phantom features recesses containing different absorber materials (lead, aluminium, acrylic and polystyrene foam), and was used for determining reproducibility of dwell times. Dwell position errors of < 1 mm were easily detectable using the phantom. The effect of bending a transfer tube was studied with this phantom and no change of clinical significance was observed when varying the curvature of the transfer tube in typical clinical scenarios. Changes of dwell time as low as 0.1 s, the minimum dwell time of the treatment unit, could be detected by choosing dwell times over the four materials that produce identical exposure at the CR detector.

TU‐A‐BRB‐08: A Technique to Determine the Gantry Laser and the Proton Pencil Beam Coincidence Using Computer Radiography Detector for Routine Quality Checks

Purpose: A computed radiography (CR) detector with photostimulable phosphors can detect both direct and indirect ionization radiation, and is even sensitive to photons in the visible range. The purpose of this work is to develop and validate a technique using CR to check both proton radiation versus localization laser coincidence and spot positioning accuracy for magnetically scanned proton pencil beam spots (PPBS). Methods: CR plate is removed from its cassette and set up perpendicular to the incident beam and exposed to an array of PPBS irradiated at predefined coordinates. Upon exposure of the particle radiation, the CR detector is then exposed to the gantry's laser systems. The plate is then put through a digital scanner which then reads the photo stimulated luminescence and converts the signal into a digital image pixel map. MATLAB®‐based analysis software and ImageJ® image processing tools were used to compute the dose gradient from the laser bleaching of the proton exposure of the detector and for quantification of the results of this QA check procedure. The technique further allows quantitative determination of both spot positions, and the relationship between X ray system used for image guided patient setup and the PPBS delivery. Results: Both the coincidence between laser representing the gantry isocenter and center of PPBS along the central axis and spot positions can be checked within 1mm accuracy with the CR detector. Conclusions: We have demonstrated that our technique allows determining the coincidence between the laser and radiation beam. The technique described here is found to be suitable for periodic QA checks of relative positions of the spot center relative to the gantry isocenter and beam central axis. Improvements of this technique by considering the effect of detector's quantum yield and modulation transfer function on the data remain subject of further study.

Physical evaluation of a needle photostimulable phosphor based CR mammography system

Needle phosphor based computed radiography (CR) systems promise improved image quality compared to powder phosphor based CR units for x-ray screening mammography. This paper compares the imaging performance of needle CR cassettes, powder based CR cassettes and a well established amorphous selenium (a-Se) based flat panel based mammography system, using consistent beam qualities. Detector performance was assessed using modulation transfer function (MTF), normalized noise power spectrum (NNPS), and detective quantum efficiency (DQE). Mammography system performance was assessed against levels from the European Guidelines, including threshold gold thickness (c-d), relative signal difference to noise (SdNR) and mean glandular dose, for automatic exposure control settings suggested by the manufacturers. The needle based Agfa HM5.0 CR detector was compared against the single sided readout Agfa MM3.0R and dual sided readout Fuji Profect CS powder CR plates using a 28 kV Mo/Rh spectrum, while a 28 kV W/Rh spectrum was used to compare the Agfa HM5.0 against the Siemens MAMMOMAT Inspiration a-Se based system. MTF at 5 mm(-1) was 0.16 and 0.24 for the needle CR detector in the fast and slow scan directions, respectively, indicating a slight improvement (∼20%) over the two powder CR systems but remained 50% lower than the result at 5 mm(-1) for the a-Se detector (∼0.55). Structured screen noise was lower for the needle phosphor compared to the powder plates. CR system gain, estimated from the measured absorption fraction and NNPS results, was 6.3 for the (single sided) needle phosphor and 5.1 and 7.2 for the single sided and dual sided powder phosphor systems. Peak DQE at ∼100 μGy was 0.47 for the needle system compared to peak DQE figures of 0.33 and 0.46 for the single sided readout powder plates and dual sided readout plates. The high frequency DQE (at 5 mm(-1)) was 0.19 for the needle CR plates, a factor of approximately 3 greater than for the powder CR plates. At 28 kV W/Rh, 2 mm Al, peak DQE for the needle CR system was 0.45 against a value of 0.50 for the a-Se detector. The needle CR detector reached the Acceptable limit for 0.1 mm details in the European Guidelines at a mean glandular dose (MGD) of approximately 1.31 mGy imaged at 28 kV Mo/Rh, compared to figures of 2.19 and 1.43 mGy for the single sided and dual sided readout powder CR systems. The a-Se detector could reach the limit at 0.65 mGy using a 28 kV W/Rh spectrum, while the needle CR system required 1.09 mGy for the same spectrum. Imaging performance for the needle CR phosphor technology, characterized using MTF and DQE and threshold gold thickness demonstrated a clear improvement compared to both single and dual sided reading powder phosphor based CR systems.

Image quality assessment in digital mammography: part I. Technical characterization of the systems

In many European countries, image quality for digital x-ray systems used in screening mammography is currently specified using a threshold-detail detectability method. This is a two-part study that proposes an alternative method based on calculated detectability for a model observer: the first part of the work presents a characterization of the systems. Eleven digital mammography systems were included in the study; four computed radiography (CR) systems, and a group of seven digital radiography (DR) detectors, composed of three amorphous selenium-based detectors, three caesium iodide scintillator systems and a silicon wafer-based photon counting system. The technical parameters assessed included the system response curve, detector uniformity error, pre-sampling modulation transfer function (MTF), normalized noise power spectrum (NNPS) and detective quantum efficiency (DQE). Approximate quantum noise limited exposure range was examined using a separation of noise sources based upon standard deviation. Noise separation showed that electronic noise was the dominant noise at low detector air kerma for three systems; the remaining systems showed quantum noise limited behaviour between 12.5 and 380 µGy. Greater variation in detector MTF was found for the DR group compared to the CR systems; MTF at 5 mm−1 varied from 0.08 to 0.23 for the CR detectors against a range of 0.16–0.64 for the DR units. The needle CR detector had a higher MTF, lower NNPS and higher DQE at 5 mm−1 than the powder CR phosphors. DQE at 5 mm−1 ranged from 0.02 to 0.20 for the CR systems, while DQE at 5 mm−1 for the DR group ranged from 0.04 to 0.41, indicating higher DQE for the DR detectors and needle CR system than for the powder CR phosphor systems. The technical evaluation section of the study showed that the digital mammography systems were well set up and exhibiting typical performance for the detector technology employed in the respective systems.

An examination of automatic exposure control regimes for two digital radiography systems

The influence of two methods of an automatic exposure control (AEC) setup using a simple measure of detectability is examined as a function of x-ray beam quality for a computed radiography (CR) system and for an indirect conversion digital radiography (DR) system. The regimes assessed were constant air kerma at the detector and the constant contrast noise ratio (CNR). A low scatter geometry was employed with x-ray spectra varying from 60 kV and 1 mm Cu to 125 kV and 2 mm Cu. The CNR was measured using a 2 mm thick Al square of dimension 1 cm by 1 cm. Detectability was quantified via a nominal contrast for a fixed beam quality of 70 kV and 1 mm added Cu filtration, taken from c-d curves measured using the Leeds TO20 test object, for the four x-ray spectra. In addition, objective image quality parameters including modulation transfer function (MTF), noise power spectrum (NPS) and detective quantum efficiency (DQE) were also measured for both systems at the four different x-ray spectra. It was found that the constant air kerma strategy did not maintain threshold nominal contrast (simple detectability) constant as the x-ray beam mean energy increased, for either the CR system or the DR system. For the CR detector, the threshold nominal contrast for a 1 mm disc increased by a factor of 4.4 from 3.50% to 15.4% as the tube potential was raised from 60 kV to 125 kV, while for the DR detector, the threshold nominal contrast increased by a factor of 3.4, from 2.27% to 7.67% as the tube potential increased from 60 kV to 120 kV. The constant CNR method was more successful at maintaining constant detectability for the c-d discs. The threshold nominal contrast for the 1 mm disc changed by a factor of 1.2, from 4.80% to 5.70% for the CR system, as the spectrum changed from 60 kV to 125 kV. For the indirect conversion detector, the threshold nominal contrast increased from 2.27% to 5.66% (a factor of 1.4 increase). The constant CNR strategy required an increase in air kerma by factors of 14.5 and 4.5 for the CR and DR detectors, respectively. The Rose SNR was used to show that the air kerma at the detector should be modulated by the inverse of the measured contrast loss squared and by the inverse of the reduction in the low frequency DQE of the detector, in order to maintain the Rose SNR constant. Calculations of air kerma required for a constant Rose SNR, made using the measured contrast and the extrapolated DQE(0), were within 25% of the air kerma for the measured CNR method for both detectors. These results suggest that the constant CNR strategy keeps simple object detectability constant by increasing air kerma to overcome (a) the contrast loss and (b) the photon loss, due to a reduction in DQE as the mean energy is increased. AEC systems set up to hold air kerma constant or nearly constant at the x-ray detector will not maintain a constant level of contrast-detail detectability as x-ray energy changes. The constant CNR method is considerably more successful if fixed contrast-detail detectability is required for different x-ray beam qualities.

Calibrating automatic exposure control devices for digital radiography

The energy responses of digital radiography detectors differ from those of screen-film systems. To provide a consistent level of image quality at different tube potentials automatic exposure control (AEC) devices must be calibrated to suit the energy response of the image receptor with which they are intended for use. AEC calibration for digital radiography systems requires an alternative parameter to optical density, ideally one related to the quality of a digital image. Energy responses of computed radiography (CR) and indirect digital radiography (IDR) image receptors have been calculated, and compared with those for screen-film systems. Practical assessments of the relative sensitivities of a CR detector made using the detector dose indicator (DDI), pixel value and signal-to-noise ratio showed similar variations with tube potential. The DDI has been used to determine the correct kV compensation curve required to calibrate the AECs for the loss in detector sensitivity with tube potential. AECs are set up relative to a predetermined air kerma incident on the detector at 80 kV for CR and IDR systems using this curve and the method used is described. Factors influencing the calibration of AECs for digital radiography including techniques, types of phantom and contributions from scatter are reviewed, and practical methods recommended for use.

AEC set-up optimisation with computed radiography imaging

Phototimer set-up is a critical procedure for dose and image quality optimisation in computed radiography (CR) systems. While a conventional radiography automatic exposure control device (AEC) can be calibrated in order to gain a constant optical density on the film independent of beam quality and patient size, CR detectors present a high dynamic range which allows a much larger dose interval, but with different image quality levels. CR leads to a less frequent exam repetition, but may produce quite noisy images if the exposure level on the plate is not correct. The aim of this work is to evaluate the performance of a CR plate (Agfa MD40) in order to optimally calibrate an AEC device. The plate response has been characterised in terms of digital signal, exposure on the plate and signal-to-noise ratio for different beam qualities, in a patient of standard size.