Visual grading analysis of digital neonatal chest phantom X-ray images: Impact of detector type, dose and image processing on image quality.

OBJECTIVES: To evaluate the impact of digital detector, dose level and post-processing on neonatal chest phantom X-ray image quality (IQ). METHODS: A neonatal phantom was imaged using four different detectors: a CR powder phosphor (PIP), a CR needle phosphor (NIP) and two wireless CsI DR detectors (DXD and DRX). Five different dose levels were studied for each detector and two post-processing algorithms evaluated for each vendor. Three paediatric radiologists scored the images using European quality criteria plus additional questions on vascular lines, noise and disease simulation. Visual grading characteristics and ordinal regression statistics were used to evaluate the effect of detector type, post-processing and dose on VGA score (VGAS). RESULTS: No significant differences were found between the NIP, DXD and CRX detectors (p>0.05) whereas the PIP detector had significantly lower VGAS (p< 0.0001). Processing did not influence VGAS (p=0.819). Increasing dose resulted in significantly higher VGAS (p<0.0001). Visual grading analysis (VGA) identified a detector air kerma/image (DAK/image) of ~2.4 µGy as an ideal working point for NIP, DXD and DRX detectors. CONCLUSIONS: VGAS tracked IQ differences between detectors and dose levels but not image post-processing changes. VGA showed a DAK/image value above which perceived IQ did not improve, potentially useful for commissioning. KEY POINTS: • A VGA study detects IQ differences between detectors and dose levels. • The NIP detector matched the VGAS of the CsI DR detectors. • VGA data are useful in setting initial detector air kerma level. • Differences in NNPS were consistent with changes in VGAS.

Task-based detectability in anatomical background in digital mammography, digital breast tomosynthesis and synthetic mammography.

Objective.Determining the detectability of targets for the different imaging modalities in mammography in the presence of anatomical background noise is challenging. This work proposes a method to compare the image quality and detectability of targets in digital mammography (DM), digital breast tomosynthesis (DBT) and synthetic mammography.Approach. The low-frequency structured noise produced by a water phantom with acrylic spheres was used to simulate anatomical background noise for the different types of images. A method was developed to apply the non-prewhitening observer model with eye filter (NPWE) in these conditions. A homogeneous poly(methyl) methacrylate phantom with a 0.2 mm thick aluminium disc was used to calculate 2D in-plane modulation transfer function (MTF), noise power spectrum (NPS), noise equivalent quanta, and system detective quantum efficiency for 30, 50 and 70 mm thicknesses. The in-depth MTFs of DBT volumes were determined using a thin tungsten wire. The MTF, system NPS and anatomical NPS were used in the NPWE model to calculate the threshold gold thickness of the gold discs contained in the CDMAM phantom, which was taken as reference. Main results.The correspondence between the NPWE model and the CDMAM phantom (linear Pearson correlation 0.980) yielded a threshold detectability index that was used to determine the threshold diameter of spherical microcalcifications and masses. DBT imaging improved the detection of masses, which depended mostly on the reduction of anatomical background noise. Conversely, DM images yielded the best detection of microcalcifications.Significance.The method presented in this study was able to quantify image quality and object detectability for the different imaging modalities and levels of anatomical background noise.

Investigation of test methods for QC in dual-energy based contrast-enhanced digital mammography systems: II. Artefacts/uniformity, exposure time and phantom-based dosimetry.

Part II of this study describes constancy tests for artefacts and image uniformity, exposure time, and phantom-based dosimetry; these are applied to four mammography systems equipped with contrast enhanced mammography (CEM) capability. Artefacts were tested using a breast phantom that simulated breast shape and thickness change at the breast edge. Image uniformity was assessed using rectangular poly(methyl)methacrylate PMMA plates at phantom thicknesses of 20, 40 and 60 mm, for the low energy (LE), high energy (HE) images and the recombined CEM image. Uniformity of signal and of the signal to noise ratio was quantified. To estimate CEM exposure times, breast simulating blocks were imaged in automatic exposure mode. The resulting x-ray technique factors were then set manually and exposure time for LE and HE images and total CEM acquisition time was measured with a multimeter. Mean glandular dose (MGD) was assessed as a function of simulated breast thickness using three different phantom compositions: (i) glandular and adipose breast tissue simulating blocks combined to give glandularity values that were typical of those in a screening population, as thickness was changed (ii) PMMA sheets combined with polyethylene blocks (iii) PMMA sheets with spacers. Image uniformity was superior for LE compared to HE images. Two systems did not generate recombined images for the uniformity test when the detector was fully covered. Acquisition time for a CEM image pair for a 60 mm thick breast equivalent phantom ranged from 3.4 to 10.3 s. Phantom composition did not have a strong influence on MGD, with differences generally smaller than 10%. MGD for the HE images was lower than for the LE images, by a factor of between 1.3 and 4.0, depending on system and simulated breast thickness. When combined with the iodine signal assessment in part I, these tests provide a comprehensive assessment of CEM system imaging performance.

Investigation of test methods for QC in dual-energy based contrast-enhanced digital mammography systems: I. Iodine signal testing.

The technique of dual-energy contrast enhanced mammography (CEM) visualizes iodine uptake in cancerous breast lesions following an intravenous injection of a contrast medium. The CEM image is generated by recombining two images acquired in rapid succession: a low energy image, with a mean energy below the iodine K-edge, and a higher energy image. The first part of this study examines the use of both commercially available and custom made phantoms to investigate iodine imaging under different imaging conditions, with the focus on quality control (QC) testing. Four CEM equipped systems were included in the study, with units from Fujifilm, GE Healthcare, Hologic and Siemens-Healthineers. The CEM parameters assessed in part I were: (1) image signal as a function of iodine concentration, measured in breast tissue simulating backgrounds of varying thickness and adipose/glandular compositions; (2) normal breast texture cancellation in homogeneous and structured backgrounds; (3) visibility of iodinated structures. For all four systems, a linear response to iodine concentration was found but the degree to which this was independent of background composition differed between the systems. Good cancellation of the glandular tissue inserts was found on all the units. Visibility scores of iodinated targets were similar between the four systems. Specialized phantoms are needed to fully evaluate important CEM performance markers, such as system response to iodine concentration and the ability of the system to cancel background texture. An extensive evaluation of the iodine signal imaging performance is recommended at the Commissioning stage for a new CEM device.

Seven general radiography x-ray detectors with pixel sizes ranging from 175 to 76µm: technical evaluation with the focus on orthopaedic imaging.

Aim. Flat panel detectors with small pixel sizes general can potentially improve imaging performance in radiography applications requiring fine detail resolution. This study evaluated the imaging performance of seven detectors, covering a wide range of pixel sizes, in the frame of orthopaedic applications.Material and methods. Pixel sizes ranged from 175 (detector A175) to 76µm (detector G76). Modulation transfer function (MTF) and detective quantum efficiency (DQE) were measured using International Electrotechnical Commission (IEC) RQA3 beam quality. Threshold contrast (CT) and a detectability index (d') were measured at three air kerma/image levels. Rabbit shoulder images acquired at 60 kV, over five air kerma levels, were evaluated in a visual grading study for anatomical sharpness, image noise and overall diagnostic image quality by four radiologists. The detectors were compared to detector E124.Results. The 10% point of the MTF ranged from 3.21 to 4.80 mm-1, in going from detector A175to detector G76. DQE(0.5 mm-1) measured at 2.38µGy/image was 0.50 ± 0.05 for six detectors, but was higher for F100at 0.62. High frequency DQE was superior for the smaller pixel detectors, howeverCTfor 0.25 mm discs correlated best with DQE(0.5 mm-1). Correlation betweenCTand the detectability model was good (R2= 0.964).CTfor 0.25 mm diameter discs was significantly higher for D150and F100compared to E124. The visual grading data revealed higher image quality ratings for detectors D125and F100compared to E124. An increase in air kerma was associated with improved perceived sharpness and overall quality score, independent of detector. Detectors B150, D125, F100and G76, performed well in specific tests, however only F100consistently outperformed the reference detector.Conclusion. Pixel size alone was not a reliable predictor of small detail detectability or even perceived sharpness in a visual grading analysis study.

Performance evaluation of digital breast tomosynthesis systems: comparison of current virtual clinical trial methods.

Virtual clinical trials (VCT) have been developed by a number of groups to study breast imaging applications, with the focus on digital breast tomosynthesis imaging. In this review, the main components of these simulation platforms are compared, along with the validation steps, a number of practical applications and some of the limitations associated with this method. VCT platforms simulate, up to a certain level of detail, the main components of the imaging chain: the x-ray beam, system geometry including the antiscatter grid and the x-ray detector. In building VCT platforms, groups use a number of techniques, including x-ray spectrum modelling, Monte Carlo simulation for x-ray imaging and scatter estimation, ray tracing, breast phantom models and modelling of the detector. The incorporation of different anthropomorphic breast models is described, together with the lesions needed to simulate clinical studies and to study detection performance. A step by step comparison highlights the need for transparency when describing the simulation frameworks. Current simulation bottlenecks include resolution and memory constraints when generating high resolution breast phantoms, difficulties in accessing/applying relevant, vendor specific image processing and reconstruction methods, while the imaging tasks considered are generally detection tasks without search, evaluated by computational observers. A number of applications are described along with some future avenues for research.

Performance evaluation of digital breast tomosynthesis systems: physical methods and experimental data.

Digital breast tomosynthesis (DBT) has become a well-established breast imaging technique, whose performance has been investigated in many clinical studies, including a number of prospective clinical trials. Results from these studies generally point to non-inferiority in terms of microcalcification detection and superior mass-lesion detection for DBT imaging compared to digital mammography (DM). This modality has become an essential tool in the clinic for assessment and ad-hoc screening but is not yet implemented in most breast screening programmes at a state or national level. While evidence on the clinical utility of DBT has been accumulating, there has also been progress in the development of methods for technical performance assessment and quality control of these imaging systems. DBT is a relatively complicated 'pseudo-3D' modality whose technical assessment poses a number of difficulties. This paper reviews methods for the technical performance assessment of DBT devices, starting at the component level in part one and leading up to discussion of system evaluation with physical test objects in part two. We provide some historical and basic theoretical perspective, often starting from methods developed for DM imaging. Data from a multi-vendor comparison are also included, acquired under the medical physics quality control protocol developed by EUREF and currently being consolidated by a European Federation of Organisations for Medical Physics working group. These data and associated methods can serve as a reference for the development of reference data and provide some context for clinical studies.

A methodology to estimate the patient diameter and thickness from thoracic and abdominal projection radiographs of adult patients.

Patient dose management systems can be part of a department's quality management tools to estimate items such as the radiation burden in specific groups or list dose outliers for further follow up. Patient size information could improve both aspects, but is not generally available. A new metric is proposed to estimate patient size for thorax and abdominal projection radiography from parameters available in thedicomheader and therefore accessible by dose management software. The tested hypothesis was that an attenuation metric, related to the ratio of detector air-kerma to incident air-kerma, inversely correlates with patient size. Such a metric was defined and then worked out for thorax and abdomen projection radiography. Input material consisted of the thorax or abdominal radiographs of 137 cases, completed with a recent CT scan as ground truth size. From the CT, the water equivalent diameter (WED) and water equivalent thickness (WET) were calculated. The correlation between the attenuation metric and the patient size was established separately for thorax and abdomen. Validation of the attenuation metric predicting the patient size was performed using extra sets of examinations on three more radiographic x-ray devices, with available CT scan. The attenuation metric had a good correlation (R2) of 0.91 and 0.84 with the WED for thorax and abdomen respectively. The corresponding values for the WET were 0.89 and 0.78. Validation of the methodology on the devices with standardized exposure index in thedicomheaders showed that the WED could be estimated within ±15% and the WET within ±30% for thorax and abdomen examinations. The ground truth and estimated size were found statistically equivalent. An attenuation metric based ondicomtags allows to estimate the patient size in projection radiography. This could now be implemented in patient dose management systems.

A study of the impact of x-ray tube performance on angiography system imaging efficiency.

This work compared the impact of x-ray tube performance and automatic dose rate control (ADRC) parameter selection on system imaging efficiency of two Siemens angiography systems: a Siemens Megalix x-ray tube installed on an Artis Zee system (denoted 'MEGALIX') and a newer generation Gigalix x-ray tube installed on an Artis Q (denoted 'GIGALIX'). A method was used that accounted for two potential sources of bias in this comparison: differences in radiation output between the x-ray tubes and differences between the x-ray detectors on the two systems. First, ADRC x-ray factors (tube voltage, tube current, pulse length, focus size, spectral prefilter) and radiation output were recorded as a function of poly(methyl) methacrylate (PMMA) thickness on the MEGALIX unit. These factors were then applied manually on the GIGALIX system and incident air kerma rate (IAKR) and signal difference to noise ratio (SDNR) were measured. Second, the ADRC on the GIGALIX system was used to give the x-ray factors and both IAKR and SDNR relevant to the GIGALIX based system directly. This method enabled the SDNR to be measured from images acquired on the same x-ray detector. SDNR and IAKR were measured on both systems using a PMMA phantom covering thicknesses from 6 cm to 40 cm. A small 0.3 mm iron insert was used to measure SDNR, which was then multiplied by modulation transfer function based weighting factors for focal spot blurring and motion blurring. These factors were evaluated for an object motion of 25 mm s-1 and at a spatial frequency of 1.4 mm-1 in the object plane, relevant to interventional cardiology, giving a spatial frequency dependent SDNR(u). In the second phase of the study, a technical figure of merit (FOM) was used to express imaging performance of both systems, calculated as SDNR2(u)/IAKR. Averaged over all phantom thicknesses, the FOM of the GIGALIX-based system was 42% and 73% higher compared to that of the MEGALIX based system, for fluoroscopy and acquisition mode respectively. The results indicate that increased x-ray tube power and smaller foci can improve overall system efficiency and reduce doses.

X-ray image quality and system exposure parameters for a hybrid Angio-MR system.

This study investigates the exposure parameters and required x-ray tube output when performing neurological procedures on a hybrid Angio-MR concept system proposed by Siemens Healthineers. The x-ray part of this system uses a longer source to detector distance than conventional (C-arm) systems and will have a fixed amount of filtration. Additionally, as the x-ray source is situated inside a magnetic field, the focal spot size and shape may be slightly distorted. In order to compare the Angio-MR system to a typical C-arm system, the exposure parameters of 60 thrombectomy procedures, performed in our hospital over the course of one year, were investigated in detail and a set of median values was determined. An analytical simulation platform was then developed to calculate the required tube voltage, tube current and pulse length to reach similar spatial frequency dependent signal difference to noise ratio (SDNR(u)) values as a conventional C-arm angiography system. These simulations were performed for a variety of focal spot sizes for the Angio-MR system. Results show that a standard current x-ray tube has sufficient power to reach similar SDNR(u) values as obtained in a conventional system if the focal spot size between both systems is comparable.

Radiation protection of operators and patients in a hybrid Angio-MR suite.

This work investigates the patient eye lens dose and x-ray scatter to the operator expected for a proposed hybrid Angio-MR concept. Two geometries were simulated for comparative assessment: a standard C-arm device for neuro-angiography applications and an innovative hybrid Angio-MR system concept, proposed by Siemens Healthineers. The latter concept is based on an over-couch x-ray tube and a detector inside an MRI system, with the aim of allowing combined, simultaneous MRI and x-ray imaging for procedures such as neurovascular interventions (including x-ray fluoroscopy and angiography imaging, 3D imaging, diffusion, and perfusion). To calculate the scattered radiation dose to the physician, Monte Carlo simulations were performed. Dose estimates of simplified models of the brain and eyes of both the patient and the physician and of the physician's torso and legs have been calculated. A number of parameters were varied in the simulation including x-ray spectrum, field of view (FOV), x-ray tube angulation, presence of shielding material and position of the physician. Additionally, 3D dose distributions were calculated in the vertical and horizontal planes in both setups. The patient eye lens dose was also calculated using a detailed voxel phantom and measured by means of thermoluminescent dosimeters (TLDs) to obtain a more accurate estimate. Assuming the same number of x-rays and the same size of the irradiated area on the patient's head, the results show a significant decrease in the scattered radiation to the physician for the Angio-MR system, while large increases, depending on setup, are expected to patient eye lens dose.

In-plane image quality and NPWE detectability index in digital breast tomosynthesis.

A rigorous 2D analysis of signal and noise transfer applied to reconstructed planes in digital breast tomosynthesis (DBT) is necessary for system characterization and optimization. This work proposes a method for assessing technical image quality and system detective quantum efficiency (DQEsys) for reconstructed planes in DBT. Measurements of 2D in-plane modulation transfer function (MTF) and noise power spectrum (NPS) were made on five DBT systems using different acquisition parameters, reconstruction algorithms and plane spacing. This work develops the noise equivalent quanta (NEQ), DQEsys and detectability index (d') calculated using a non-prewhitening model observer with eye filter (NPWE) for reconstructed DBT planes. The images required for this implementation were acquired using a homogeneous test object of thickness 40 mm poly(methyl) methacrylate plus 0.5 mm Al; 2D MTF was calculated from an Al disc of thickness 0.2 mm and diameter 50 mm positioned within the phantom. The radiant contrast of the MTF disc and the air kerma at the system input were used as normalization factors. The NPWE detectability index was then compared to the in-plane contrast-detail (c-d) threshold measured using the CDMAM phantom. The MTF and NPS measured on the different systems showed a strong anisotropy, consistent with the cascaded models developed in the literature for DBT. Detectability indices calculated from the measured MTF and NPS successfully predicted changes in c-d detectability for details between 0.1 mm and 2.0 mm, for DBT plane spacings between 0.5 mm and 10 mm, and for air kerma values at the system input between 157 µGy and 1170 µGy. The linear Pearson correlation between the detectability index and threshold gold thickness of the CDMAM phantom was -0.996. The method implements a parametric means of assessing the technical image quality of reconstructed DBT planes, providing valuable information for optimization of DBT systems.

Detective quantum efficiency measured as a function of energy for two full-field digital mammography systems.

This paper presents detective quantum efficiency (DQE) data measured for a range of x-ray beam qualities for two full-field digital mammography (FFDM) systems: a caesium iodide (CsI) detector-based unit and a system designed around an amorphous selenium (a-Se) x-ray detector. Four beam qualities were studied for each system, covering mean energies from 17.8 keV to 23.4 keV for the CsI system and 17.8 keV to 24.7 keV for the a-Se unit. These were set using 2, 4, 6 and 7 cm polymethylmethacralate (PMMA) and typical tube voltage and target/filter combinations selected by the automatic exposure control (AEC) program used clinically on these systems. Normalized noise power spectra (NNPS) were calculated from flood images acquired at these beam qualities for a target detector air kerma of 100 microGy. Modulation transfer function (MTF) data were acquired at 28 kV and Mo/Mo target/filter setting. The DQE was then calculated from the MTF and NNPS results. For comparison, the quantum detective efficiency (QDE) and energy absorption efficiency (EAE) were calculated from tabulated narrow beam spectral data. With regard to detector response, some energy dependence was noted for pixel value plotted against air kerma at the detector. This amounted to a change in the gradient of the detector response of approximately 15% and 30% per keV for the CsI- and a-Se-based systems, respectively. For the DQE results, a reduction in DQE(0) of 22% was found for the CsI-based unit as beam quality changed from 25 kV Mo/Mo and 2 cm PMMA to 32 kV Rh/Rh and 7 cm PMMA. For the a-Se system, a change in beam quality from 25 kV Mo/Mo and 2 cm PMMA to 34 kV Mo/Rh and 7 cm PMMA led to a reduction in DQE(0) of 8%. Comparing measured data with simple calculations, a reduction in x-ray quantum detection efficiency of 27% was expected for the CsI-based system, while a reduction of 11% was predicted for the a-Se system.

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.

Implementation of a spatio-temporal figure of merit for new automatic dose rate control regimes in dynamic x-ray imaging.

This work describes a new approach to automatic dose rate control (ADRC) for dynamic x-ray imaging, utilizing a spatial frequency weighted signal difference to noise ratio (SDNR(u)). Three aspects of ADRC programming using SDNR(u), which contains information on the target material, velocity and size, were examined. First, whether SDNR(u) can be held constant at the requested level over some patient thickness range for five materials relevant to interventional imaging (iron, gadolinium, platinum, bismuth, and tantalum). Second, the efficiency of the new ADRC was compared to the current settings using a figure of merit (FOM), defined as SDNR(u)2/reference air kerma rate for iron and platinum, over a range of simulated patient thicknesses. Third, the ability of the new ADRC to optimize exposure parameters for iron, iodine, gadolinium, tantalum, platinum and bismuth was examined. A phantom of 20 mm PMMA and 2 mm Al sheets was used to simulate patient equivalent thicknesses between 25 mm to 375 mm. The relevant metal foil targets were placed at the phantom centre and imaged on a Siemens Artis Q cardio-angiography system. SDNR(u) and reference air kerma were measured, along with the FOM for the relevant conditions. The optimal exposure factor study was made for patient equivalent thicknesses of 100 mm, 200 mm and 300 mm. The new ADRC regulation held SDNR(u) constant versus phantom thickness within 5%, for the five materials studied. FOM increase compared to the current regulation used on the Artis Q ranged between 18% and 296% (averaged over all thicknesses), and depended on acquisition mode and material. Material optimization via the new ADRC increased FOM by 68%, 165%, 164% and 32% for gadolinium, tantalum, platinum and bismuth respectively, corresponding to potential dose savings of 40%, 62%, 62% and 24% for the same target SDNR(u). An SDNR(u) driven approach to the ADRC logic of dynamic imaging systems is a viable alternative to current programming, with a resulting improvement in imaging efficiency and corresponding dose reduction.

Generalized SDNR analysis based on signal and noise power.

The standard approach to signal difference-to-noise ratio (SDNR) analysis requires a region of interest (ROI) positioned within the object to measure signal-difference, restricting this metric to flat-topped objects with large, sharply delineated areas. This work develops a generalized expression for SDNR (SDNRg) calculated from a ROI encompassing the object. Signal power, defined as the deviation of pixel values from the mean background due to the object, is used instead of signal-difference. Comparison was first made by simulating ideal flat-topped discs with sharp edges and diameters between 1 and 80 pixels, into a uniformly noisy background using a known signal-difference. For discs covering more than 20 pixels, SDNRg and standard SDNR (SDNRst) were within 3%, while for discs of less than 20 pixels, SDNRg was within 26% of the truth compared to 58% for SDNRst. Generalized and standard SDNR were compared for radiography images of three different phantoms with microcalcification-like objects (MTM-100 phantom), hemispheric objects of different thicknesses with a Gaussian intensity distribution and mammography quality control (QC) images. Applied to Gaussian details, SDNRg was between 20% and 45% higher than SDNRst, depending on object thickness, while for the QC images, SDNRg was with 1.7% of the standard SDNR. Compared to the standard SDNR, SDNRg is applicable to non-uniform signals, where an explicit contrast measurement is not suitable, and has improved accuracy when assessing SDNR of small objects.

A hybrid simulation framework for computer simulation and modelling studies of grating-based x-ray phase-contrast images.

Clinical studies performed using computer simulation are inexpensive, flexible methods that can be used to study aspects of a proposed imaging technique prior to a full clinical study. Typically, lesions are simulated into (experimental) data to assess the clinical potential of new methods or algorithms. In grating-based phase-contrast imaging (GB-PCI), full wave simulations are, however, computationally expensive due to the high periodicity of the gratings and therefore not practically applicable when large data sets are required. This work describes the development of a hybrid modelling platform that combines analytical and empirical input data for a more rapid simulation of GB-PCI images with little loss of accuracy. Instead of an explicit implementation of grating details, measured summary metrics (i.e. visibility, flux, noise power spectra, presampling modulation transfer function) are applied in order to generate transmission and differential phase images with large fields of view. Realistic transmission and differential phase images were obtained with good quantitative accuracy. The different steps of the simulation framework, as well as the methods to measure the summary metrics, are discussed in detail such that the technique can be easily customized for a given system. The platform offers a fast, accurate alternative to full wave simulations when the focus switches from grating/system design and set up to the generation of GB-PCI images for an established system.

Early experience in the use of quantitative image quality measurements for the quality assurance of full field digital mammography x-ray systems.

Quantitative image quality results in the form of the modulation transfer function (MTF), normalized noise power spectrum (NNPS) and detective quantum efficiency (DQE) are presented for nine full field digital mammography (FFDM) systems. These parameters are routinely measured as part of the quality assurance (QA) programme for the seven FFDM units covered by our centre. Just one additional image is required compared to the standard FFDM protocol; this is the image of an edge, from which the MTF is calculated. A variance image is formed from one of the flood images used to measure the detector response and this provides useful information on the condition of the detector with respect to artefacts. Finally, the NNPS is calculated from the flood image acquired at a target detector air kerma (DAK) of 100 microGy. DQE is then estimated from these data; however, no correction is currently made for effects of detector cover transmission on DQE. The coefficient of variation (cov) of the 50% point of the MTF for five successive MTF results was 1%, while the cov for the 50% MTF point for an a-Se system over a period of 17 months was approximately 3%. For four a-Se based systems, the cov for the NNPS at 1 mm(-1) for a target DAK of 100 microGy was approximately 4%; the same result was found for four CsI based FFDM units. With regard to the stability of NNPS over time, the cov for four NNPS results acquired over a period of 12 months was also approximately 4%. The effect of acquisition geometry on NNPS was also assessed for a CsI based system. NNPS data acquired with the antiscatter grid in place showed increased noise at low spatial frequency; this effect was more severe as DAK increased. DQE results for the three detector types (a-Se, CsI and CR) are presented as a function of DAK. Some reduction in DQE was found for both the a-Se and CsI based systems at a target DAK of 12.5 microGy when compared to DQE data acquired at 100 microGy. For the CsI based systems, DQE at 1 mm(-1) fell from 0.49 at 100 microGy to 0.38 at 12.5 microGy. For the a-Se units, there was a slightly greater reduction in average DQE at 1 mm(-1), from 0.53 at 100 microGy to 0.31 at 12.5 microGy. Somewhat different behaviour was seen for the CR unit; DQE (at 1 mm(-1)) increased from 0.40 at 100 microGy to 0.49 at 12.5 microGy; however, DQE fell to 0.30 at 420 microGy. DQE stability over time was assessed using the cov of DQE at 1 mm(-1) and a target DAK of 100 microGy; the cov for data acquired over a period of 17 months for an a-Se system was approximately 7%. For comparison with conventional testing methods, the cov was calculated for contrast-detail (cd) data acquired over the same period of time for this unit. The cov for the threshold contrast results (averaged for disc diameters between 0.1 mm and 2 mm) was 6%, indicating similar stability.

Evaluation of automatic dose rate control for flat panel imaging using a spatial frequency domain figure of merit.

Current automatic dose rate controls (ADRCs) of dynamic x-ray imaging systems adjust their acquisition parameters in response to changes in patient thickness in order to achieve a constant signal level in the image receptor. This work compares a 3 parameter (3P) ADRC control to a more flexible 5-parameter (5P) method to meet this goal. A phantom composed of 15 composite poly(methyl) methacrylate (PMMA)/aluminium (Al) plates was imaged on a Siemens Artis Q dynamic system using standard 3P and 5P ADRC techniques. Phantom thickness covered a water equivalent thickness (WET) range of 2.5 cm to 37.5 cm. Acquisition parameter settings (tube potential, tube current, pulse length, copper filtration and focus size) and phantom entrance air kerma rate (EAKR) were recorded as the thickness changed. Signal difference to noise ratio (SDNR) was measured using a 0.3 mm iron insert centred in the PMMA stack, positioned at the system isocentre. SDNR was then multiplied by modulation transfer function (MTF) based correction factors for focal spot penumbral blurring and motion blurring, to give a spatial frequency dependent parameter, SDNR(u). These MTF correction factors were evaluated for an object motion of 25 mm s-1 and at a spatial frequency of 1.4 mm-1 in the object plane, typical for cardiac imaging. The figure of merit (FOM) was calculated as SDNR(u)²/EAKR for the two ADRC regimes. Using 5P versus 3P technique showed clear improvements over all thicknesses. Averaged over clinically relevant adult WET values (20 cm-37.5 cm), EAKR was reduced by 13% and 27% for fluoroscopy and acquisition modes, respectively, while the SDNR(u) based FOM increased by 16% and 34% for fluoroscopy and acquisition. In conclusion, the generalized FOM, taking into account the influence of focus size and object motion, showed benefit in terms of image quality and patient dose for the 5-parameter control over 3-parameter method for the ADRC programming of dynamic x-ray imaging systems.

A comprehensive model for x-ray projection imaging system efficiency and image quality characterization in the presence of scattered radiation.

This work proposes a method for assessing the detective quantum efficiency (DQE) of radiographic imaging systems that include both the x-ray detector and the antiscatter device. Cascaded linear analysis of the antiscatter device efficiency (DQEASD) with the x-ray detector DQE is used to develop a metric of system efficiency (DQEsys); the new metric is then related to the existing system efficiency parameters of effective DQE (eDQE) and generalized DQE (gDQE). The effect of scatter on signal transfer was modelled through its point spread function (PSF), leading to an x-ray beam transfer function (BTF) that multiplies with the classical presampling modulation transfer function (MTF) to give the system MTF. Expressions are then derived for the influence of scattered radiation on signal-difference to noise ratio (SDNR) and contrast-detail (c-d) detectability. The DQEsys metric was tested using two digital mammography systems, for eight x-ray beams (four with and four without scatter), matched in terms of effective energy. The model was validated through measurements of contrast, SDNR and MTF for poly(methyl)methacrylate thicknesses covering the range of scatter fractions expected in mammography. The metric also successfully predicted changes in c-d detectability for different scatter conditions. Scatter fractions for the four beams with scatter were established with the beam stop method using an extrapolation function derived from the scatter PSF, and validated through Monte Carlo (MC) simulations. Low-frequency drop of the MTF from scatter was compared to both theory and MC calculations. DQEsys successfully quantified the influence of the grid on SDNR and accurately gave the break-even object thickness at which system efficiency was improved by the grid. The DQEsys metric is proposed as an extension of current detector characterization methods to include a performance evaluation in the presence of scattered radiation, with an antiscatter device in place.