Variation in digital breast tomosynthesis image quality at differing heights above the detector.

INTRODUCTION: The aim of this preliminary work was to determine if image quality in digital breast tomosynthesis (DBT) changes when tomosynthesis image slices were obtained at differing heights above the detector and in differing breast thicknesses. METHODS: A CIRS Model 020 BR3D breast imaging phantom was used to obtain the DBT images. The images were also acquired at different tube voltages, and each exposure was determined by the automatic exposure control system. Contrast-to-noise ratio (CNR) and figure-of-merit (FOM) values were obtained and compared. RESULTS: At a phantom thickness of 5 cm or greater, there was a significant reduction (P ≤ 0.05) of image CNR values obtained from the images near the top of the phantom to those obtained near the bottom of the phantom. When the phantom thickness was 4 cm, there was no significant difference in CNR values between DBT images acquired at any height in the phantom. FOM values generally showed no difference when images were obtained at differing heights above the detector. CONCLUSION: Image quality, as measured by the CNR, was reduced when tomosynthesis slice image heights were at the top of the phantom and when the thickness of the phantom was more than 4 cm. From this preliminary work, clinicians need to be aware that DBT images obtained near the top of the breast, when breast thickness is greater than 4 cm, may have reduced image quality. Further work is needed to fully assess any DBT image quality changes when images are obtained near the top of the breast.

Using aluminum for scatter control in mammography: preliminary work using measurements of CNR and FOM.

Full-field digital mammography (FFDM) systems provide the current gold standard in mammographic examinations. Although FFDM provides the lowest mammographic doses, the radiation dose to the breast during mammographic examinations is still a concern. Thus, image quality optimization at the lowest dose is a major goal. In planar X-ray imaging, thin sheets of aluminum (Al) are used as filtration to reduce the number of low-energy X-ray photons reaching the patient. The goal of this work was to evaluate whether Al can be used in FFDM to remove scatter radiation from reaching the image detector, hence improving image quality. Doses were compared with the use of a grid. A Hologic Selenia mammographic unit was used to acquire images of two phantoms, namely, the ACR phantom and a Perspex phantom of 5 cm. Images were acquired using two tube voltages (kVp) and filter combinations under two exposure/dose conditions. Al sheets of various thicknesses were placed between the phantom and the image detector. Contrast-to-noise ratio (CNR) and figure of merit (FOM) values were measured and compared with images acquired using a grid. When a constant dose was delivered to the image detector, the highest CNR was achieved using a grid; however, the highest FOM values were achieved when using 0.05 mm thick Al sheets. This study successfully demonstrates that thin sheets of Al can be used in mammography examinations to reduce scattered radiation and improve image quality, as indicated by the measured CNR values. Given the limitations of this work, further kVp and target/filter combinations and various methods of image quality measurement need to be studied.

Validity of Using Accreditation Phantom in Quality Control of Digital Tomosynthesis.

AIM: This study was undertaken to compare the two image-quality phantoms commonly used in full-field digital mammography (FFDM) and digital breast tomosynthesis (DBT) imaging. METHODS: Mammography units with two targets and three filters resulting in three possible target/filter combinations and two kVp values which are widely used (28 and 32) were used for the comparison. The automatic exposure control system was used in combination with the selected kVp. The CIRS 15 mammographic accreditation phantom (MAP) and CIRS 20 (BR3D) breast imaging phantom were used with the three target/filter combinations and two kVp values. A total of 24 images were acquired and evaluated. Image score was determined as the smallest sized object detectable. The data were analyzed by using Mann-Whitney test. RESULTS: There were significant (p<0.001) differences between the detectability of fibers present in the two phantoms, but there were no differences in the detectability of specks. CONCLUSION: The finding in FFDM and DBT showed there were significant differences between the two phantoms (p<0.02) in fibers and specks visibility. The CIRS 20 phantom provided greater visibility of smaller structures, while the MAP was more suitable for assessing image quality of both FFDM and DBT imaging systems.

Effect of mA Reduction on Image Quality Parameters and Patient Dose in Computed Tomography Imaging.

PURPOSE: To establish the extent of dose reduction that can be achieved using a computed tomography (CT) quality assurance phantom with various user-defined protocols at different milliamperage (mA) settings and kilovoltage peak (kVp) without affecting image quality. METHODS: Images of a CT quality assurance phantom containing line-pair patterns and 3 tissue-equivalent inserts in water were acquired with different mA settings on 4 different CT scanners. The kVp was kept constant at 120 while the mA was changed in increments of 20, from 30 mA to 190 mA. Three images from each series acquired at different mA settings were selected: one each for the quantitative analysis of spatial resolution, contrast-to-noise ratio (CNR), and image noise using image analysis freeware. RESULTS: Statistical analysis was carried out with a statistical probability of P < .05 indicating significant difference. Results of the statistical analysis revealed significant differences in the CNR of all 3 types of tissue-equivalent inserts (P < .001) and in image noise (P < .001) as the mA was changed. However, changes in mA did not produce significant differences in the spatial resolution of the images (P > .06). CONCLUSION: A radiation dose reduction of 25% was achieved in 4 different CT scanners without significantly affecting the 3 image quality parameters.

Quality of images acquired with and without grid in digital mammography.

In this study, we assessed the quality of digital mammography images acquired with a grid and without a grid for different kVp values. A digital mammography system was used for acquisition of images of the CIRS Model 015 Mammography Accreditation Phantom. The images were obtained in the presence of the grid and then with the grid removed from the system. The energy of the X-rays was varied between 26 and 32 kVp. The images were evaluated by five senior radiologic technologists with extensive experience in mammography. Statistical analysis was carried out with the Mann-Whitney non-parametric test with the level of significance set at p = 0.05. The comparison between images obtained with a grid and without a grid indicated that, for the visibility of fibers, the non-grid images at 28 kVp were significantly (p = 0.032) better than the images acquired with a grid. At all other kVp values, the images were not statistically different regarding the visibility of fibers. For the visibility of specks and masses, the images did not show any significant differences at any of the kVp values of the study. Imaging with kVp higher than 30 requires a grid to improve the visibility of fibrous calcifications and specks. For the visibility of masses at 32 kVp, no statistically significant differences between the grid and non-grid images were found.

Effect of Heat on Image Plates in Computed Radiography Systems.

PURPOSE: The sensitivity of computed radiographic (CR) image plates to ionizing and/or nonionizing radiation such as light, heat, and scatter radiation have been known to cause effects such as electronic noise and to degrade image quality. The purpose of this study was to investigate the effects of image plate storage temperature on dark noise. METHOD: Image plates from Fuji and AGFA CR systems were stored at various room temperatures for 5-hour durations and then processed using the respective systems. The average pixel value in several regions of interest in the resultant images were used an indication of dark noise. The variation in the average pixel values was used to calculate image noise levels. RESULTS: Image plates of the Fuji system only showed significant increase in dark noise and image noise levels when the temperature exceeded 40°C. The image plates of the AGFA system showed increases in both quantities at all temperatures. CONCLUSION: It is recommended that storage temperatures of image plates be kept below 35°C to minimize dark noise effects.

Effect of delay between erasure and image acquisition in CR.

OBJECTIVE: The effect of time delay between last erasure and image acquisition using computed radiography (CR) image plates (IPs) was investigated using an Agfa ADC Compact CR system. METHOD: IPs of various sizes were erased by the IP processing cycle of the CR system. These IPs then were stored in their normal storage place for various durations, ranging from 0 hours to 168 hours. The IPs then were processed, and dark noise images were obtained. The dark noise in each plate was estimated by measuring the average pixel value in several places on the image. Statistical analysis was carried out using 1-way analysis of variance and Kruskal-Wallis testing for any significant differences in dark noise between time delays at P = .05. Regression analysis was carried out to test any relationship between dark noise and time delay. RESULTS: Statistically significant (P < .05) increases in dark current were observed when the time delay increased, with increase greater than 40% for time delays more than 24 hours. In the short term (< 1 hour), a quadratic relationship was observed, and in the long term (> 24 hours) a linear relationship was observed between dark current and time delay. CONCLUSION: Dark noise increases with time delay, and it is useful to erase the IP before image acquisition if time delays of more than 24 hours are experienced, such as over a weekend.

Optical density changes in dry-processed films.

CONTEXT: Dry-processed radiographic films should be stored in a temperature-controlled environment away from direct light to prevent image deterioration. However, films occasionally are subjected to other conditions, such as when they are left inadvertently on viewing boxes or removed from the radiology department. OBJECTIVE: To assess the effects of heat and light on optical density in radiographic films produced with dry laser imagers. METHODS: Step-wedge images generated by a dry laser imaging system were placed on an illuminated viewing box, inside the trunk of a car and in direct sunlight on a hot day for varying lengths of time. Changes in optical density (OD) then were measured with a densitometer. Changes of more than 2 standard deviations from the original OD were attributed to the effects of light and heat. RESULTS: The films left on a viewing box and inside a car's trunk did not show significant changes in OD. However, films subjected to direct sunlight for 30 minutes or more at an average temperature of 40 degrees C (104 degrees F) showed significant changes in OD in the straight-line portion of the Hurter and Driffield curve. CONCLUSION: Dry-processed films can degrade when exposed to temperatures higher than 35 degrees C (95 degrees F) and should be stored properly to avoid affecting their diagnostic quality.

Radiation dose in pelvic imaging.

OBJECTIVE: To compare the radiation dose during pelvic x-ray examinations using computed radiography (CR) and film-screen (FS) radiography at various x-ray tube voltages (kV) and tube-current time product (mAs) values. METHODS: A pelvic phantom was imaged using FS and CR systems. The entrance surface dose was measured using an ionization chamber, and the gonadal dose and effective dose were calculated using the XDOSE program. The diagnostic quality of the images was assessed using a 5-point subjective scoring system. RESULTS: At standard kV values, the image quality did not vary significantly between the CR and the FS system, but at higher kV values, the CR images werefound to be of better quality than FS images. In addition, the lower limit of entrance skin dose consistent with diagnostically acceptable CR images was 50% lower than that for FS images. CONCLUSION: The gonadal dose and effective dose for pelvic x-ray examinations can be reduced by 50% when CR systems are used and appropriate exposure factors are established.