Digital Chest Radiography System Research Articles

Scatter rejection and low‐contrast performance of a slot‐scan digital chest radiography system with electronic aft‐collimation: A chest phantom study

Anti-scatter grids have been widely used to reject scatter and increase the perceptibility of low-contrast object in chest radiography; however they also attenuate the primary x-rays, resulting in a substantial degradation of primary information. Compensation for this degradation requires the use of higher exposure technique hence higher dose to the patient. A more efficient approach to reject scatter is the slot-scan imaging technique which employs a narrow scanning x-ray fan beam in conjunction with a slit or slot shaped solid state detector or an area detector used with an aft-collimator. With this approach, scatter can be rejected effectively without the need to attenuate primary x-rays. This paper demonstrates an electronic aft-collimation method, referred to as the alternate line erasure and readout (ALER) technique, for implementing the slot-scan digital radiography with a modern flat-panel detector. With this technique, instead of first exposing the detector and then reading the image line by line, the image line on the leading edge of the scanning fan beam is reset to erase the scatter accumulated prior to the arrival of the fan beam x-rays, while the image line on the trailing edge of the scanning fan beam is read out to acquire the image signals following the fan-beam exposure. These reset and readout processes are alternated and repeated as the x-ray fan beam scans across the detector. An anthropomorphic chest phantom was imaged to evaluate the scatter rejection ability and the low-contrast performance for the ALER technique and compare them with those for the anti-scatter grid method in full-field chest imaging. With a projected beam width of 16 mm, the slot-scan/ALER technique resulted in an average reduction of the scatter-to-primary ratios by 81%, 84%, 82%, and 86% versus 65%, 73%, 74%, and 73% with the anti-scatter grid method in the lungs, mediastinum, retrocardium, and subdiaphragm, respectively. The average CNR for the slot-scan/ALER technique was found to improve by 135%, 133%, 176%, and 87% versus 15%, 15%, 38%, and -11% with the anti-scatter grid method in the mediastinum, retrocardium, subdiaphragm, and lungs, respectively. These results demonstrated that the slot-scan/ALER technique can be used to achieve equally effective scatter rejection but substantially higher low-contrast performance than the anti-scatter grid method.

MO‐E‐L100J‐04: Scatter Rejection and Low‐Contrast Performance of a Slot‐Scan Digital Chest Radiography System with Electronic Aft‐Collimation: A Phantom Study

Purpose: To investigate scatter rejection and low‐contrast performance of an electronic aft‐collimation based slot‐scan imaging technique for chest imaging. Method and Materials: The slot‐scan imaging technique was implemented with a 1.6 cm wide fan‐beam and a modified flat‐panel (FP) detector for electronic aft‐collimation. During the scan, the leading edge line of the scanning fan‐beam is reset to erase the scatter accumulated while the trailing edge line is read out to acquire the image signals following the fan‐beam exposure. Two images acquired with the same techniques were subtracted from each other for measuring the noise levels. A 2‐mm thick lead plate with a 2‐D array of holes was used to measure the primary signals which were then subtracted from those obtained without the lead plate to determine the scatter components. A 2‐D array of aluminum beads (3mm in diameter) is used as the low‐contrast objects to measure the contrast ratios (CRs) and contrast‐to‐noise ratios (CNRs) for evaluation of the low‐contrast performance in chest images. Results: The slot‐scan imaging method resulted in lower average scatter‐to‐primary ratios (SPRs) and improved CRs, primary signal‐to‐noise ratios (PSNRs) and CNRs than the anti‐scatter grid method. Slot‐scan imaging used in conjunction with grid resulted in further reduction of the SPRs and improvement of CRs. However, in most cases, the PSNRs were degraded and CNRs were at the same level or even degraded. Conclusion: Slot‐scan imaging with electronic aft‐collimation can effectively reject scatter without having to attenuate the primary x‐rays. It resulted in almost doubled CNR improvement as compare to the anti‐scatter grid method. Further improvement by using slot‐scan imaging with an anti‐scatter grid was found to be limited. Conflict of Interest: This work was supported in part by a research grant EB00117 from the NIH‐NIBIB and the research grants CA51248 and CA104759 from the NIH‐NCI.

A comparison of low contrast performance for amorphous Silicon/caesium iodide direct radiography with a computed radiography: A contrast detail phantom study

A range of digital image acquisition devices exists in diagnostic radiology. This study compares contrast performance of two such systems: an amorphous Silicon/caesium iodide (a-Si:CsI) based flat panel (DR) digital chest radiography system and a computed radiography (CR) system. Images of a contrast detail resolution phantom were acquired at a range of radiation doses. Three observers assessed all hardcopy images using a four-alternative forced choice observer perception technique. Contrast detail performance was calculated and low contrast performance quantified. The DR system demonstrated significantly better low contrast performance and potential dose savings of up to 75% compared to the CR system. Threshold levels of contrast detail resolution were defined and levels of under- and over-exposure, compared to the threshold level, were highlighted. Both systems were noise limited at lower exposures and latitude limited at higher exposures. The results demonstrate that the DR system should perform better than the CR system under typical clinical conditions relevant to chest radiography particularly for the detection of low contrast details such as lung metastases or pneumothoraces.

Comparative Scatter and Dose Performance of Slot-Scan and Full-Field Digital Chest Radiography Systems

To evaluate the scatter, dose, and effective detective quantum efficiency (DQE) performance of a slot-scan digital chest radiography system compared with that of a full-field digital radiography system. Scatter fraction of a slot-scan system was measured for an anthropomorphic and a geometric phantom by using a posterior beam-stop technique at 117 and 140 kVp. Measurements were repeated with a full-field digital radiography system with and without a 13:1 antiscatter grid at 120 and 140 kVp. For both systems, the effective dose was measured on posteroanterior and lateral views for standard clinical techniques by using dosimeters embedded in a female phantom. The effective DQEs of the two systems were assessed by taking into account the scatter performance and the DQE of each system. The statistical significance of all the comparative differences was ascertained by means of t test analysis. The slot-scan system and the full-field system with grid yielded scatter fractions of 0.13-0.14 and 0.42-0.48 in the lungs and 0.30-0.43 and 0.69-0.78 in the mediastinum, respectively. The sum of the effective doses for posteroanterior and lateral views for the slot-scan system (0.057 mSv +/- 0.003 [+/- standard deviation]) was 34% lower than that for the full-field system (0.086 mSv +/- 0.001, P < .05) at their respective clinical peak voltages (140 and 120 kVp, respectively). The effective DQE of the slot-scan system was equivalent to that of the full-field system in the lung region but was 37% higher in the dense regions (P < .05). The slot-scan design leads to marked scatter reduction compared with the more conventional full-field geometries with a grid. The improved scatter performance of a slot-scan geometry can effectively compensate for low DQE and lead to improved image quality.

Irreversible JPEG Compression of Digital Chest Radiographs for Primary Interpretation: Assessment of Visually Lossless Threshold

To determine if digital chest images could be compressed in a primary interpretation context without perceived loss of fidelity (below the visually lossless threshold) at transilluminated film or cathode ray tube (CRT) display. One hundred forty-four posteroanterior radiographs were obtained with a digital chest radiography system. At both film and CRT display, an identified original image was presented side by side with a replicate, which was either an unaltered image or an image that had been Joint Photographic Experts Group (JPEG) compressed to 10:1, 20:1, or 50:1 and reconstructed. Each of the 10 readers indicated whether the replicate was "indistinguishable from the original" or "degraded" at clinical reading distance and at close inspection. The readers' ability to detect compressed images was examined for patterns; 95% CIs were used for statistical testing. With transilluminated film at clinical reading distance, readers were as likely to rate originals (48 [20%] of 240 readings) as degraded as they were to rate 20:1 replicates (106 [22%] of 480 readings) as degraded, but they frequently identified 50:1 replicates (283 [59%] of 480 readings) as degraded. At close inspection, 20:1 replicates (163 [34%] of 480 readings) were often identified as degraded, but 10:1 replicates (19 [8%] of 240 readings) were not identified as degraded more often than originals (17 [7%] of 240 readings). With CRT display, the results were nearly identical. At reading distance for primary interpretation, full-size digital chest radiographs that have been JPEG compressed to 10:1 or 20:1 and reconstructed are visually lossless at film or CRT display. Images compressed to 10:1 remain visually lossless at close inspection.

Comparison of an amorphous silicon/cesium iodide flat-panel digital chest radiography system with screen/film and computed radiography systems--a contrast-detail phantom study.

Flat-panel (FP) based digital radiography systems have recently been introduced as a new and improved digital radiography technology; it is important to evaluate and compare this new technology with currently widely used conventional screen/film (SF) and computed radiography (CR) techniques. In this study, the low-contrast performance of an amorphous silicon/cesium iodide (aSi/Csl)-based flat-panel digital chest radiography system is compared to those of a screen/film and a computed radiography system by measuring their contrast-detail curves. Also studied were the effects of image enhancement in printing the digital images and dependence on kVp and incident exposure. It was found that the FP system demonstrated significantly better low-contrast performance than the SF or CR systems. It was estimated that a dose savings of 70%-90% could be achieved to match the low-contrast performance of the FP images to that of the SF images. This dose saving was also found to increase with the object size. No significant difference was observed in low-contrast performances between the SF and CR systems. The use of clinical enhancement protocols for printing digital images was found to be essential and result in better low-contrast performance. No significant effects were observed for different kVps. From the results of this contrast-detail phantom study, the aSi/CsI-based flat-panel digital chest system should perform better under clinical situations for detection of low-contrast objects such as lung nodules. However, proper processing prior to printing would be essential to realizing this better performance.

Towards image quality, beam energy and effective dose optimisation in digital thoracic radiography.

This paper outlines how objective measurements of both image quality, in terms of signal-to-noise ratio, and effective dose may be used as tools to find the optimum kVp range for a digital chest radiography system. Measurements were made with Thoravision, an amorphous selenium-based digital chest X-ray system. The entrance surface dose and the effective dose to an anthropomorphic chest phantom were determined demonstrating how effective dose is related to beam quality. The image quality was measured using detective quantum efficiency, threshold contrast and a radiologist preference trial involving 100 patients. The results show that, despite the fact that the entrance surface dose decreases as the kVp increases, the effective dose, a better measure of the risk, reaches a minimum value between 90 and 110 kVp; however, the image quality decreases as the kVp increases. In this study the optimum kVp for chest radiography, using a selenium-based radiography system, is in the range 90-110 kVp. This is contrary to the 120- to 150-kVp range that is commonly used. Also, this study shows how objective measurements can be used to optimise radiographic technique without prolonged patient trials.

A Case Study into the Effect of Radiographic Factors on Image Quality and Dose for a Selenium Based Digital Chest Radiography System

Thoravision, the first commercially available digital radiography system utilising amorphous selenium, has been evaluated from both clinical and physical perspectives. The advent of digital radiography systems has made the optimisation of both image quality and dose an important problem. Initial studies, using the manufacturer's recommended factors, demonstrated that the image quality, measured both objectively and subjectively, was significantly superior to conventional screen-film images. However, some questions still remained regarding the system's performance. A second study using alternative factors showed that the Thoravision always produced acceptable diagnostic images, regardless of the factors, and there are advantages to using lower kVp, objective measurements of image quality confirm this. It is clear that no single set of radiographic factors and image processing algorithms will produce optimised images for every patient and further elucidation of the problem is required. The difference between acceptable and a good clinical image is also highlighted and the implications discussed.

Technical evaluation of a digital chest radiography system that uses a selenium detector.

To evaluate a digital chest radiography system that uses a selenium detector. The relative amounts of scattered radiation in the images (scatter fractions), the effect of x-ray exposure levels on image appearance, the potential "throughput" in a clinical environment, and the effects of image processing options were evaluated. Scatter fractions in digital images acquired with an antiscatter grid were lower in the lung region and higher in the retrocardiac and central mediastinal regions than in conventional images. Digital images acquired without an antiscatter grid had higher scatter fractions in all areas. Increases in exposure intensity reduced the appearance of noise. A new image could be acquired every 37 seconds, and a "preview image" appeared on the monitor after approximately 23 seconds. Laser-printed images were available after at least 5 minutes; the time required increased when many images were acquired in a short time. The selenium-based chest radiography system allows for rapid chest examination and excellent image quality when used with an antiscatter grid.

Image quality of a digital chest radiography system based on a selenium detector

A digital chest radiography system has been developed, with a detector based on the photoelectric properties of amorphous selenium. The selenium layer is deposited on a cylindrical aluminium drum, large enough to cover the full field of view for chest imaging. The electrostatic charge image which is formed on the selenium surface after x-ray exposure is read out by electrometer probes using fast drum rotation. For a physical evaluation of the attainable image quality, the characteristic curve, the modulation transfer function, and the noise spectra were measured. From these measurements, the signal-to-noise properties of the detector in terms of detective quantum efficiency (DQE) and noise equivalent quanta (NEQ) were derived. The results show that the selenium-based detector has a wide dynamic range and a significantly better DQE than screen-film and storage phosphor systems for spatial frequencies below the Nyquist limit (2.7 lp/mm). As a consequence, the detectability of small, low-contrast details is considerably improved.

Radiographic reporting directly from the TV monitor of a digital chest radiography system

The practical usefulness of a digital large image intensifier system was tested on 400 consecutive, routine chest examinations. Reading and reporting was carried out directly from the digital images on the TV monitors. For each patient images were also read independently from 100 mm photofluorograms. When the reports for each examination were compared, there was total agreement in reporting in 40% of the cases and clinically insignificant differences in interpretation in 56%. In the remaining 16 patients (4%), the opinion of the observers differed as to whether significant disease was present or not. However, these disagreements could not be related to imaging technique. We believe that a majority of chest examinations can be performed with digital technique and can be read directly from the monitor screen.