Signal-to-noise ratio improvements using anti-scatter grids with different object thicknesses and tube voltages

A deep-learning-based scatter correction with water equivalent path length map for digital radiography

We proposed a new deep learning (DL) model for accurate scatter correction in digital radiography. The proposed network featured a pixel-wise water equivalent path length (WEPL) map of subjects with diverse sizes and 3D inner structures. The proposed U-Net model comprises two concatenated modules: one for generating a WEPL map and the other for predicting scatter using the WEPL map as auxiliary information. First, 3D CT images were used as numerical phantoms for training and validation, generating observed and scattered images by Monte Carlo simulation, and WEPL maps using Siddon's algorithm. Then, we optimised the model without overfitting. Next, we validated the proposed model's performance by comparing it with other DL models. The proposed model obtained scatter-corrected images with a peak signal-to-noise ratio of 44.24 ± 2.89 dB and a structural similarity index measure of 0.9987 ± 0.0004, which were higher than other DL models. Finally, scatter fractions (SFs) were compared with other DL models using an actual phantom to confirm practicality. Among DL models, the proposed model showed the smallest deviation from measured SF values. Furthermore, using an actual radiograph containing an acrylic object, the contrast-to-noise ratio (CNR) of the proposed model and the anti-scatter grid were compared. The CNR of the images corrected using the proposed model are 16% and 82% higher than those of the raw and grid-applied images, respectively. The advantage of the proposed method is that no actual radiography system is required for collecting training dataset, as the dataset is created from CT images using Monte Carlo simulation.

Radiation dose considerations in digital radiography with an anti-scatter grid: A study using adult and pediatric phantoms

BACKGROUND

When using an anti-scatter grid, a decrease in receptor dose caused by its X-ray absorption seems to lead to the misperception that radiation dose needs to be increased even in digital radiography (DR).

OBJECTIVE

To demonstrate that there is no need to increase radiation dose in DR with a grid, based on a visual evaluation using an adult and a pediatric abdomen phantom (PAD and PPD , respectively).

MATERIALS AND METHODS

Phantom images with and without a grid were obtained with exposure parameters determined based on a preliminarily measured signal-to-noise ratio improvement factor (SIF), an index for potential dose reduction when using a grid. In visual evaluation, four radiologists compared phantom images with a grid applied at different dose reduction rates (0% [no reduction], 18%, 36%, and 59% for PAD and 0% and 11% for PPD ) against an image without a grid at the baseline dose (as the reference). They graded the overall image quality of the former relative to that of the latter (reference) on a 3-point scale (3 = better, 2 = almost equal, 1 = worse).

RESULTS

The mean scores for dose reduction rates of 0%, 18%, 36%, and 59% were 3.00, 3.00, 2.75, and 1.00, respectively, for PAD ; those for 0% and 11% were 2.13 and 1.63, respectively, for PPD . These results support the validity of our view that no dose increase is necessary when using an anti-scatter grid. Actually, there is even a potential for improvement in image quality with dose reduction rates of ≤36% for PAD .

CONCLUSION

It is worth reconsidering the necessity of increasing radiation dose in the DR imaging of the adult and pediatric abdomens with an anti-scatter grid.

Evaluating the use of anti-scatter grids in adult knee radiography

INTRODUCTION

Anti-scatter grids efficiently reduce scatter radiation from reaching the imaging receptor, enhancing image quality; however, the patient radiation dose increases in the process. There is disagreement regarding the thickness thresholds for which anti-scatter grids are beneficial. This study aims to establish a thickness threshold for the use of anti-scatter grids to optimise adult knee radiography.

METHODS

The study consisted of two phases. In Phase 1 phantom knee radiographs were acquired at varying thicknesses (10-16 cm) and tube voltages (60-80 kV). For each thickness and tube voltage, images with and without an anti-scatter grid were obtained. In Phase 2, two radiologists and three radiographers, evaluated the image quality of these images. Visual Grading Analysis (VGA) scores were analysed using Visual Grading Characteristics (VGC) based on the visualisation of five anatomic criteria.

RESULTS

The average DAP decreased by 72.1% and mAs by 73.1% when removing the anti-scatter grid. The VGC revealed that overall images taken with an anti-scatter grid have better image quality (AUC ≥0.5 for all comparisons). However, the anti-scatter grids could be removed for thicknesses 10, 12 and 14 cm in conjunction with using 80 kVp,.

CONCLUSION

Anti-scatter grids can be removed when imaging adult knees between 10 and 12 cm using any kVp setting since the radiation dose is reduced without significantly affecting image quality. For thicknesses >12 cm, the use of anti-scatter grids significantly improves image quality; however, the radiation dose to the patient is increased. The exception is at 14 cm used with 80 kVp, where changes in image quality were insignificant.

IMPLICATIONS FOR PRACTICE

Optimisation by removing anti-scatter grids in adult knee radiography seems beneficial below 12 cm thickness with any kVp value. Since the average knee thickness ranges between 10 and 13 cm, anti-scatter grid can be removed for most patients. Nevertheless, further studies are recommended to test if this phantom-based threshold applies to human subjects.

A new stationary grid, with grid lines aligned to pixel lines with submicron-order precision, to suppress grid artifacts

PURPOSE

We have developed a new stationary grid named a pixel-aligned grid (PA grid), in which the grid lines are aligned to the pixel lines with submicron-order precision. Further, we have evaluated its performance relative to that of a conventional grid combined with grid-line removal (GLR) processing.

METHODS

A flat-panel detector system of an indirect type, with a pixel pitch of 150 μm, was employed. Four PA grids having a grid ratio of 6:1 associated with abdominal bedside radiography, with the grid-line pitch (GP) varied around the target value of 150 μm, were produced. Blank images were obtained with four PA grids for measuring the period and amplitude of the grid artifact. In performance evaluation, acrylic and anthropomorphic abdominal phantom images were used with the PA grid, a conventional grid (40 lines/cm, grid ratio 6:1), and no grids. The grid artifacts were evaluated by power spectrum (PS) analysis. Also, the signal-to-noise ratio (SNR) improvement factor (K_SNR ) was measured.

RESULTS

Grid artifacts were hardly recognizable with PA grids with GP errors of 0.3 μm and 0.6 μm because of the prolonged grid artifact periods. The measured artifact amplitudes of these PA grids were less than 0.6%. Furthermore, the PA grids did not produce notable frequency peaks in PS. In contrast, the conventional grid without GLR processing produced two conspicuous peaks. With GLR processing, notable reductions in PS were observed around the two peak frequencies, which caused blurring in bone structures. For the acrylic thickness of 20 cm, the K_SNR s for the PA grid were around 1.4, suggesting some SNR improvement in abdominal bedside radiography.

CONCLUSION

The present study has demonstrated that PA grids with their grid-line pitches close to the pixel-line pitch within errors of 0.6 μm produce grid artifact-free images without any signal losses. Thus, the proposed PA grid will prove to be effective and useful in various clinical applications.

Technical evaluation of a prototype ratio 29:1 grid for adult patient cardiovascular angiography imaging conditions

The purpose of this work was to assess technical performance of a prototype high-ratio (r29), 80 line cm^-1grid for imaging conditions which mimic those for adult cardiovascular angiography. The standard equipment r15, 80 line cm^-1grid was used as a reference. Plastic Water^®LR phantoms with thickness in the range 20-44 cm were used to simulate adult patient attenuation and scatter. Grids were tested using x-ray field of view 20 and 25 cm and x-ray source to detector distance (SID) 107 and 120 cm. The primary transmission fraction (T_P) was measured using both narrow beam geometry and a lead beam stop (BS) technique. Scatter transmission (T_S) was measured with the lead BS technique. The quantum signal to noise ratio improvement factor (K_SNR) was used to describe relative grid performance. The experimental conditions required revised theory to assess grid performance. Theory to account for the detector glare and underestimation of scatter intensity by the lead BS method was developed. Also, novelK_SNRtheory was developed to allow direct comparison of two grids operated at different SID. MeanT_Pwas modestly lower for the r29 versus r15 grid (0.69 versus 0.75). When tested under equivalent scatter condition, T_Sof the r29 grid was approximately ½ that of the r15 grid (0.18 versus 0.34).K_SNRof the r29 grid at SID 120 cm compared to the r15 grid at SID 107 cm increased linearly with phantom thickness (range 1.0 to ∼1.16). Findings of this work indicate that the r29 grid used at SID 120 cm is expected to provide improved image quality (or reduced patient radiation dose) when compared to the r15 grid used at SID 107 cm for adult cardiovascular patients and that the potential benefit of the r29 grid increases with patient thickness >20 cm.

Characterization of anti-scatter grids via a modulation transfer function improvement factor using an edge device

In optimizing the imaging conditions, changes in image quality due to scattered radiation are important evaluation targets. This study focuses on the evaluation of the image quality improvement characteristics obtained using anti-scatter grids in digital x-ray imaging, and proposes a frequency-dependent modulation transfer function (MTF) improvement factor,MIFG(u),as a new evaluation index. Accordingly, the purpose of this study is to clarify the validity and the usefulness of this proposed index in the performance evaluation of grids. The proposedMIFG(u)method is applied to evaluate several types of grids with different grid densities and ratios, and the characteristics of grids exhibiting different performances are examined. The proposed index is calculated based on the MTF measurement by using an edge test device. The results show thatMIFG(u)changed according to grid type and scatter conditions. In particular, a remarkable difference was observed in the high scatter condition compared with the low condition.MIFG(u)in the vertical direction with regards to the absorbing strips shows a peak at 0.2-0.5 cycles/mm and be a constant value from approximately 1 cycle/mm; whileMIFG(u)in the parallel direction is a constant value with respect to changes in spatial frequency. It is shown thatMIFG(u)could be used to accurately describe the characteristics of a grid under different imaging conditions. We believe that the use of the proposed index could expand the options for optimizing imaging conditions when using grids.

[Relationship between Radiation Quality and Image Quality in Digital Chest Radiography: Validation Study Using Human Soft Tissue-equivalent Phantom]

PURPOSE

To evaluate image quality for chest radiography at different radiation qualities, using phantoms with scatter fractions similar to those of lungs.

METHODS

Two base phantoms with 10 and 4 cm thicknesses, respectively, made of a soft tissue-equivalent material, were used to mimic the X-ray attenuation of the human lung. Two plates with soft tissue- and bone-equivalent materials, respectively, were placed on the base phantom as contrast objects. The image data were obtained with the same entrance surface dose in each radiation quality. Six radiation qualities generated using 120 and 90 kV, and additional copper filters with thicknesses 0, 0.1, and 0.2 mm were selected. The signal-difference-to-noise ratio (SdNR) and a contrast ratio of the soft tissue to the bone were measured for the six radiation qualities.

RESULTS

The thicker the additional filter, the better the SdNR at both tube voltages. The SdNR values were not significantly different between 120 and 90 kV for the same filter thickness. The contrast ratio was higher at 120 than at 90 kV by approximately 8%.

CONCLUSIONS

Because of the advantage of the contrast ratio and the highest SdNR, the radiation quality with 120 kV and 0.2-mm copper filtration was the best. It was indicated that the conventional tube voltage of 120 kV remains to be better than the lower tube voltage of 90 kV.