In this study we used a large previously built database of 2,892 mammograms and 31,650 single mammogram radiologists’ assessments to simulate the impact of replacing one radiologist by an AI system in a double reading setting. The double human reading scenario and the double hybrid reading scenario (second reader replaced by an AI system) were simulated via bootstrapping using different combinations of mammograms and radiologists from the database. The main outcomes of each scenario were sensitivity, specificity and workload (number of necessary readings). The results showed that when using AI as a second reader, workload can be reduced by 44%, sensitivity remains similar (difference -0.1%; 95% CI = - 4.1%, 3.9%), and specificity increases by 5.3% (P<0.001). Our results suggest that using AI as a second reader in a double reading setting as in screening programs could be a strategy to reduce workload and false positive recalls without affecting sensitivity.
Contrast-Enhanced Digital Breast Tomosynthesis (CEDBT) provides quasi three-dimensional contrast enhancement of breast lesions and has been investigated for breast cancer detection and lesion assessment. The acquisition geometry of CEDBT may affect its ability to detect and assess contrast-enhanced lesions. In this study, we investigate the effects of angular range of CEDBT on lesion margin assessment. The CIRS BR3D phantom with iodine inserts was imaged for four angular ranges between 15 and 45 degrees with same total glandular dose using a prototype CEDBT system. The artifact spread functions of iodine objects with various sizes were measured. The detectability of iodine objects overlaid in the depth direction with different separation distances was evaluated using signal-difference-to-noise ratio. Clinical images of malignant lesions were acquired with 25 projections over approximately 50 degrees, and CEDBT for various angular ranges were generated using a subset or all of the projection images and were assessed for lesion margins. Our results show that increasing angular range of CEDBT improves the separation of overlapping iodine signals in phantom images, and the margins of malignant mass lesions are better identified. In conclusion, CEDBT with wide angular range may improve lesion characterizations, e.g. lesion size, morphology and location, and provide better performance than contrast enhanced digital mammography (CEDM) for applications such as guidance of biopsy and evaluation of treatment response.
Contrast-enhanced digital mammography (CEDM) reveals neovasculature of breast lesions in a two-dimensional contrast enhancement map. Contrast-enhanced digital breast tomosynthesis (CEDBT) provides contrast enhancement in three dimensions, which may improve lesion characterization and localization. We aim to compare CEDM and CEDBT for lesion assessment. Women with breast imaging-reporting and data system 4 or 5 suspicious breast lesion(s) were recruited in our study and were imaged with CEDM and CEDBT in succession under one breast compression. Two radiologists assessed CEDM and CEDBT with both images displayed side-by-side and compared (1) contrast enhancement of lesions and (2) lesion margin using a five-point scale ranging from −2 (CEDM much better) to +2 (CEDBT much better). Biopsy identified 19 malignant lesions with contrast enhancement. Our results show that CEDBT provides better lesion margins than CEDM with limited reduction in contrast enhancement. CEDBT delivers less radiation dose compared to CEDM + DBT. Synthetic CEDM can be generated from CEDBT data and provides lesion contrast enhancement comparable to CEDM. CEDBT has potential for clinical applications, such as treatment response monitoring and guidance for biopsy.
KEYWORDS: Breast, Digital breast tomosynthesis, Tissues, Visualization, Mammography, Breast cancer, Medicine, Magnetic resonance imaging, X-ray imaging, X-rays
Assessment of breast density at the point of mammographic examination could lead to optimized breast cancer screening pathways. The onsite breast density information may offer guidance of when to recommend supplemental imaging for women in a screening program. A software application (Insight BD, Siemens Healthcare GmbH) for fast onsite quantification of volumetric breast density is evaluated. The accuracy of the method is assessed using breast tissue equivalent phantom experiments resulting in a mean absolute error of 3.84%. Reproducibility of measurement results is analyzed using 8427 exams in total, comparing for each exam (if available) the densities determined from left and right views, from cranio-caudal and medio-lateral oblique views, from full-field digital mammograms (FFDM) and digital breast tomosynthesis (DBT) data and from two subsequent exams of the same breast. Pearson correlation coefficients of 0.937, 0.926, 0.950, and 0.995 are obtained. Consistency of the results is demonstrated by evaluating the dependency of the breast density on women’s age. Furthermore, the agreement between breast density categories computed by the software with those determined visually by 32 radiologists is shown by an overall percentage agreement of 69.5% for FFDM and by 64.6% for DBT data. These results demonstrate that the software delivers accurate, reproducible, and consistent measurements that agree well with the visual assessment of breast density by radiologists.
Contrast-Enhanced Digital Breast Tomosynthesis (CEDBT) provides a three-dimensional (3D) contrast-enhancement map with co-registered anatomical information from low-energy DBT. It combines the benefits from Contrast-Enhanced Digital Mammography (CEDM) and Digital Breast Tomosynthesis (DBT), and may improve breast cancer detection and assessment of lesion morphology. We investigate the efficacy of CEDBT in the assessment of lesion contrast enhancement and margin identification, and evaluate the dose efficiency. We generate synthetic CEDM images from CEDBT data, similar to synthesis of 2D mammograms from DBT data, which may facilitate overall lesion assessment without additional radiation dose. Preliminary results from a patient study show that CEDBT depicts lesion margins better compared to CEDM, while the contrast-enhancement level for in-plane slice is not as high as in CEDM. CEDBT delivers less radiation dose compared to CEDM + DBT. Synthetic CEDM is able to provide lesion contrast-enhancement level comparable to CEDM.
Digital breast tomosynthesis (DBT) is a three-dimensional (3-D) X-ray imaging modality that allows the breast to be viewed in a 3-D format, minimizing the effect of overlapping breast tissue. DBT is commonly known for its high in-plane spatial resolution allowing to detect very small structures inside the breast which makes it a powerful tool in the clinical environment. However, since DBT is a limited angle tomography, artifacts are inevitable. In this paper, we investigate the influence of the angular scanning range as well as the slice thickness, i. e. the distance between two adjacent slices, on the in-plane spatial resolution of calcifications and present an analytic model to describe the imaging process. For the validation of the analytic model, 54 datasets with varying calcification diameter, slice thickness, and angular scanning range, were used and compared to a ray-casting simulation. It could be shown that the relative mean error between the analytic model and the generated ground truth over all datatsets is ε- = 0.0137. The results indicate that both investigated parameters affect the in-plane spatial resolutio
Assessment of breast density at the point of mammographic examination could lead to optimized breast cancer screening pathways. The onsite breast density information may offer guidance when to recommend supplemental imaging for women in a screening program. In this work, performance evaluation of a new software (Insight BD, Siemens Healthcare GmbH) for fast onsite quantification of volumetric breast density is presented. Accuracy of volumetric measurement is evaluated using breast tissue equivalent phantom experiments. Reproducibility of measurement results is analyzed using 8150 4-view mammography exams. Furthermore, agreement between breast density categories computed by the software with those determined visually by radiologists is examined. The results of the performance evaluation demonstrate that the software delivers accurate and reproducible measurements that agree well with the visual assessment of breast density by radiologists.
The detection of cancerous mass lesions using digital breast tomosynthesis (DBT) has been shown to be limited in patients with dense breasts. Detection may potentially be improved by increasing the DBT angular range (AR), which reduces breast structural noise and increases object contrast in the reconstructed slice. We investigate the impact of DBT AR on the detection of masses in a simulation study using a cascaded linear system model (CLSM) for DBT. We compare the mass conspicuity between wide- and narrow-AR DBT system in a clinical pilot study. The simulation results show reduced in-plane breast structural noise and increased in-plane detectability of masses with increasing AR. The clinical results show that masses are more conspicuous in wide-AR DBT than narrow-AR DBT. Our study indicates that the detection of mass lesions in dense breasts can be improved by increasing DBT AR.
KEYWORDS: Digital breast tomosynthesis, Signal attenuation, Signal detection, Breast, Tissues, Image quality, Clinical trials, Digital mammography, 3D image reconstruction, Psychophysics
Detection of lesions is an essential part of making a diagnosis in mammography and therefore is a main focus in the development of algorithms built for image quality assessment. We propose a hybrid approach with an accurate lesion projection model and embedding of lesions into clinical images that already contain relevant structures of anatomical noise. Using an algebraic lesion model, lesions with different sizes and contrasts are generated. The projection algorithm incorporates the modeling of blur effects due to system movement and the physical extent of the anode. Signal and background patches are extracted and used to evaluate channelized Hotelling observers with Laguerre-Gauss channels and with Gabor channels. A four-alternative forced-choice study with five medical imaging experts is performed and the inter-reader agreement with and without the model observers is determined by using Fleiss' kappa. Analyzing three different sizes for tiny, dense lesions and four density levels for larger mass-like lesions we find a good detection rate of the tiny lesions for both human as well as model observers. The inter-reader agreement using the common interpretation of Fleiss' kappa is substantial or better. Comparing full-field digital mammography and digital breast tomosynthesis w.r.t. the different mass densities we find that human readers and model observers perform well on the DBT data and the detection rate drops with lesion contrast as expected. The inter-reader agreement here is fair for the lowest contrast and substantial for the denser cases. Both human readers and model observers show difficulty in detecting the low contrast lesions in FFDM images. The inter-reader agreement is rather poor among all readers. Overall, the results indicate a good agreement between human observers and model observers and a distinctive benefit of 3-D reconstruction over FFDMs for low contrast lesions.
X-ray cone-beam (CB) imaging is moving towards playing a large role in diagnostic radiology. Recently, an innovative, versatile X-ray system (Multitom Rax, Siemens Healthcare, GmbH, Forchheim, Germany) was introduced for diagnostic radiology. This system enables taking X-ray radiographs with high flexibility in patient positioning, as well as acquiring semi-circular short CB scans in a variety of orientations. We show here that this system can be further programmed to accurately scan the entire spine in the weight-bearing position. Such a diagnostic imaging capability has never been demonstrated so far. However, we may expect it to play an important clinical role as clinicians agree that spine diseases would be more accurately interpretable in the weight-bearing position. We implemented a geometry that provides complete data so that CB artifacts may be avoided. This geometry consists of two circular arcs connected by a line segment. We assessed immediate and short-term motion reproducibility, as well as ability to image the entire spine within a Rando phantom. Strongly encouraging results were obtained. Reproducibility with sub-mm accuracy was observed and the entire spine was accurately reconstructed.
The upsurge in interest of digital tomosynthesis is mainly caused by breast imaging; however, it finds more and more attention in orthopedic imaging as well. Offering a superior in-plane resolution compared to CT imaging and the additional depth information compared to conventional 2-D X-ray images, tomosynthesis may be an interesting complement to the other two imaging modalities. Additionally, a tomosynthesis scan is likely to be faster and the radiation dose is considerably below that of a CT. Usually, a tomosynthetic acquisition focuses only on one body part as the common acquisition techniques restrict the field-of-view. We propose a method which is able to perform full-body acquisitions with a standard X-ray system by shifting source and detector simultaneously in parallel planes without the need to calibrate the system beforehand. Furthermore, a novel aliasing filter is introduced which addresses the impact of the non-isotropic resolution during the reconstruction. We provide images obtained by filtered as well as unfiltered backprojection and discuss the influence of the scanning angle as well as the reconstruction filter on the reconstructed images. We found from the experiments that our method shows promising results especially for the imaging of anatomical structures which are usually obscured by each other since the depth resolution allows to distinguish between these structures. Additionally, as of the high isotropic in-plane spatial resolution of the tomographic volume, it is easily possible to perform precise measurements which are a crucial task, e. g. during the planning of orthopedic surgeries or the assessment of pathologies like scoliosis or subtle fractures.
Talbot-Lau X-ray imaging (TLXI) provides information about scattering and refractive features of objects – in addition to the well-known conventional X-ray attenuation image. We investigated the potential of TLXI for the detection of hairline fractures in bones, which are often initially occult in conventional 2D X-ray images. For this purpose, hairline fractures were extrinsically provoked in a porcine trotter (post-mortem) and scanned with a TLXI system. In the examined case, hairline fractures caused dark-field and differential-phase signals, whereas they were not evident in the conventional X-ray image. These findings motivate a comprehensive and systematic investigation of the applicability of TLXI for diagnosing hairline fractures.
Tomosynthesis images of the breast suffer from artifacts caused by the presence of highly absorbing materials. These can be either induced by metal objects like needles or clips inserted during biopsy devices, or larger calcifications inside the examined breast. Mainly two different kinds of artifacts appear after the filtered backprojection procedure. The first type is undershooting artifacts near edges of high-contrast objects caused by the filtering step. The second type is out-of-plane (ripple) artifacts that appear even in slices where the metal object or macrocalcifications does not exist. Due to the limited angular range of tomosynthesis systems, overlapping structures have high influence on neighboring regions. To overcome these problems, a segmentation of artifact introducing objects is performed on the projection images. Both projection versions, with and without high-contrast objects are filtered independently to avoid undershootings. During backprojection a decision is made for each reconstructed voxel, if it is artifact or high-contrast object. This is based on a mask image, gained from the segmentation of high-contrast objects. This procedure avoids undershooting artifacts and additionally reduces out-of-plane ripple. Results are demonstrated for different kinds of artifact inducing objects and calcifications.
In this work, we provide an initial characterization of a novel twin robotic X-ray system. This system is equipped
with two motor-driven telescopic arms carrying X-ray tube and flat-panel detector, respectively. 2D radiographs
and fluoroscopic image sequences can be obtained from different viewing angles. Projection data for 3D cone-beam
CT reconstruction can be acquired during simultaneous movement of the arms along dedicated scanning
trajectories. We provide an initial evaluation of the 3D image quality based on phantom scans and clinical
images. Furthermore, initial evaluation of patient dose is conducted. The results show that the system delivers
high image quality for a range of medical applications. In particular, high spatial resolution enables adequate
visualization of bone structures. This system allows 3D X-ray scanning of patients in standing and weight-bearing
position. It could enable new 2D/3D imaging workflows in musculoskeletal imaging and improve diagnosis of
musculoskeletal disorders.
Anti-scatter grids used in full-field digital mammography not only attenuate scattered radiation but also attenuate
primary radiation. Dose saving could be achieved if the effect of scattered radiation is compensated with a software-based scatter correction not attenuating the primary radiation. In this work, we have carried out phantom studies in order to investigate dose saving and image quality of grid-less acquisition in combination with software-based scatter correction. The results show that similar image quality (contrast-to-noise ratio and contrast-detail visibility) can be obtained with this alternative acquisition and post-processing scheme at reduced dose. The relative dose reduction is breast-thickness-dependent and is >20% for typical breast thicknesses. We have carried out a clinical study with 75 patients that showed non-inferior image quality at reduced dose with our novel approach compared to the standard method.
A new algorithm is suggested to compute one or several virtual projection images directly from cone-beam data
acquired in a tomosynthesis geometry. One main feature of this algorithm is that it does not involve the explicit
reconstruction of a 3D volume, and a subsequent forward-projection operation, but rather operates using solely
2D image processing steps. The required 2D processing is furthermore based on the use of pre-computed entities,
so that a significant speed-up in the computations can be obtained. The presented algorithm can be applied
to a variety of CT geometries, and is here investigated for a mammography application, to simulate virtual
mammograms from a set of low-dose tomosynthesis projection images. A first evaluation from real measured
data is given.
Digital breast tomosynthesis (DBT) is a three-dimensional X-ray imaging modality that has the potential to decrease the
superimposition effect of breast structural noise, thereby increasing lesion conspicuity. To further improve breast cancer
detection, our work has been devoted to develop a prototype for contrast-enhanced dual-energy tomosynthesis
(CEDET). CEDET involves the injection of an iodinated contrast agent and measures the relative increase in uptake of
contrast in the suspected breast cancer lesion. Either temporal or dual-energy subtraction techniques may be used to
implement CEDET. Both 2D contrast-enhanced dual-energy mammography and 3D tomosynthesis can be applied.
Here we present the design of a prototype CEDET system based on the Siemens MAMMOMAT Inspiration and
employing two additional high-energy filters in addition to the standard Rh filter, the latter being used for the low-energy
acquisitions. A quality factor of squared
signal-difference-to-noise-ratio of iodine per pixel area and average glandular
dose as a function of breast thickness is used to optimize the filter material, the filter thickness, and the tube voltage. The
average glandular dose can be calculated from the entrance surface air kerma using computed conversion coefficients
DgN for the used X-ray spectra. We also present the results of DQE measurements of the amorphous selenium detector
involved. Finally, results of phantom tests for tomosynthesis acquisition and first clinical data in the 2D mode will be
shown.
In breast tomosynthesis (BT) a number of 2D projection images are acquired from different angles along a limited arc.
The imaged breast volume is reconstructed from the projection images, providing 3D information. The purpose of the
study was to investigate and optimize different reconstruction methods for BT in terms of image quality using human
observers viewing clinical cases. Sixty-six cases with suspected masses and calcifications were collected from 55
patients.
KEYWORDS: Reconstruction algorithms, Breast, Digital breast tomosynthesis, Expectation maximization algorithms, Image restoration, Mammography, Tissues, Spatial resolution, Stochastic processes, Signal to noise ratio
Digital Breast Tomosynthesis (DBT) suffers from incomplete data and poor quantum statistics limited by the total dose absorbed in the breast. Hence, statistical reconstruction assuming the photon statistics to follow a Poisson distribution may have some advantages. This study investigates state-of-art iterative maximum likelihood (ML) statistical reconstruction algorithms for DBT and compares the results with simple backprojection (BP), filtered backprojection (FBP), and iFBP (FBP with filter derived from iterative reconstruction).
The gradient-ascent and convex optimization variants of the transmission ML algorithm are evaluated with phantom
and clinical data. Convergence speed is very similar for both iterative statistical algorithms and after approximately 5
iterations all significant details are well displayed, although we notice increasing noise. We found empirically that a
relaxation factor between 0.25 and 0.5 provides the optimal trade-off between noise and contrast. The ML-convex
algorithm gives smoother results than the ML-gradient algorithm. The low-contrast CNR of the ML algorithms is between CNR for simple backprojection (highest) and FBP (lowest). Spatial resolution of iterative statistical and iFBP algorithms is similar to that of FBP but the quantitative density representation better resembles conventional mammograms. The iFBP algorithm provides the benefits of statistical iterative reconstruction techniques and requires much shorter computation time.
Breast tomosynthesis is currently an investigational imaging technique requiring optimization of its many combinations
of data acquisition and image reconstruction parameters for optimum clinical use. In this study, the effects of several
acquisition parameters on the visual conspicuity of diagnostic features were evaluated for three breast specimens using a
visual discrimination model (VDM). Acquisition parameters included total exposure, number of views, full resolution
and binning modes, and lag correction. The diagnostic features considered in these specimens were mass margins,
microcalcifications, and mass spicules. Metrics of feature contrast were computed for each image by defining two
regions containing the selected feature (Signal) and surrounding background (Noise), and then computing the difference
in VDM channel metrics between Signal and Noise regions in units of just-noticeable differences (JNDs). Scans with
25 views and exposure levels comparable to a standard two-view mammography exam produced higher levels of feature
contrast. The effects of binning and lag correction on feature contrast were found to be generally small and isolated,
consistent with our visual assessments of the images. Binning produced a slight loss of spatial resolution which could
be compensated in the reconstruction filter. These results suggest that good image quality can be achieved with the
faster and therefore more clinically practical 25-view scans with binning, which can be performed in as little as 12.5
seconds. Further work will investigate other specimens as well as alternate figures of merit in order to help determine
optimal acquisition and reconstruction parameters for clinical trials.
KEYWORDS: Sensors, Modulation transfer functions, X-rays, 3D modeling, Breast, Spatial frequencies, Optical filters, Mammography, Systems modeling, 3D acquisition
In breast tomosynthesis there are tradeoffs between resolution, noise and acquisition speed for a given glandular dose. The purpose of the present work is to investigate the dependence of tomosynthesis imaging performance on system configuration, which includes detector operational modes and image acquisition geometry. A prototype Siemens breast tomosynthesis system with maximum angular range of +/- 25 degrees was used in our investigation. The system was equipped with an amorphous selenium (a-Se) full field digital mammography detector with pixel size of 85µm. The detector can be read out with full resolution or 2x1 binning (binning in the tube travel direction), which increases the image readout rate and decreases the degradation effect of electronic noise. The total number of views can be varied from 11 to 49, and filtered back projection (FBP) method was used to reconstruct the tomosynthesis images. We investigated the effects of detector operational modes (binning) and imaging geometry (view angle and number) on temporal performance and spatial resolution of the projection images. The focal spot blur due to continuous tube travel was measured for different acquisition geometry, and its effect on in-plane presampling modulation transfer function (MTF) was compared to that due to pixel binning. A three-dimensional cascaded linear system model was developed for tomosynthesis to predict the 3D MTF, NPS and DQE. The results were compared with experimental measurements, and reasonable agreement was achieved. The understanding of the relationship between the 3D and projection image quality will lead to optimization of the x-ray spectrum, imaging geometry and reconstruction filters for digital breast tomosynthesis.
Digital breast tomosynthesis is a new technique intended to overcome the limitations of conventional projection mammography by reconstructing slices through the breast from projection views acquired from different angles with respect to the breast. We formulate a general theory of filtered backprojection reconstruction for linear tomosynthesis. The filtering step consists of an MTF inversion filter, a spectral filter, and a slice thickness filter. In this paper the method is applied first to simulated data to understand the basic effects of the various filtering steps. We then demonstrate the impact of the filter functions with simulated projections and with clinical data acquired with a research breast tomosynthesis system.** With this reconstruction method the image quality can be controlled regarding noise and spatial resolution. In a wide range of spatial frequencies the slice thickness can be kept constant and artifacts caused by the incompleteness of the data can be suppressed.
A prototype breast tomosynthesis system has been developed, allowing a total angular view of ±25°. The detector used in this system is an amorphous selenium direct-conversion digital flat-panel detector suitable for digital tomosynthesis. The system is equipped with various readout sequences to allow the investigation of different tomosynthetic data acquisition modes. In this paper, we will present basic physical properties -- such as MTF, NPS, and DQE -- measured for the full resolution mode and a binned readout mode of the detector. From the measured projections, slices are reconstructed employing a special version of filtered backprojection algorithm. In a phantom study, we compare binned and full resolution acquisition modes with respect to image quality. Under the condition of same dose, we investigate the impact of the number of views on artifacts. Finally, we show tomosynthesis images reconstructed from first clinical data.
KEYWORDS: Signal to noise ratio, Monte Carlo methods, Sensors, Mammography, Breast, Systems modeling, Digital mammography, Collimators, Scattering, Image processing
We are investigating the advantage of scatter removal by the slot-scanning method compared to antiscatter grids. We carry out model calculations for the signal-to-noise ratio simulating different geometrical settings for slot-scan systems. The results are compared with those for standard nonscanning mammography systems with and without anti-scatter grid. Monte Carlo simulations are performed in order to get a realistic amount of scatter radiation as input for the model estimates. We present the results as function of the compressed breast thickness equivalent to the scatter fraction. It is demonstrated that a perfect slot-scan system with 100% transmission of primary radiation and 100% suppression of scattered radiation improves SNR2, and correspondingly reduces dose, by a factor of less than 1.8, compared with conventional anti-scatter grids and otherwise the same detector DQE. For realistic geometry the advantage is considerably smaller. The advantage of scatter removal by employing a slot-scanning method is moderate because the scatter fraction is relatively low in mammography. For breast thickness up to 5 cm it turns out that it is advantageous to work without a grid due to the low scatter fraction, which questions a scatter reduction method in that region at all. The model can be used as a simple design tool.
C-arm volume reconstruction has become increasingly popular over the last years. These imaging systems generate 3D data sets for various interventional procedures such as endovascular treatment of aneurysms or orthopedic applications. Due to their open design and mechanical instability, C-arm imaging systems acquire projections along non-ideal scan trajectories. Volume reconstruction from filtered 2D X-ray projections requires a very precise knowledge of the imaging geometry. We show that the 3D image quality of C-arm cone beam imaging devices can be improved by proper design of the calibration phantom.
We report on the German standardization activities for acceptance testing of medical soft copy displays. The goal is to assure image quality of imaging systems in radiology considering that the display device is part of the imaging system consisting furthermore of the image acquisition system, of the human visual systems (HVS), and of the ambient light conditions. We analyze the properties of the HVS with respect to soft copy reading. The contrast sensitivity gives a measure for the display's spatial resolution that should be aimed at. The contrast ratio of maximum to minimum luminance is limited by the adaptation process in realistic complex radiological images. Furthermore, the nonlinear behavior of the HVS requires to establish a certain display function to provide a tone scale that is approximately perceptually linear. These HVS properties have to be compared with typical electronic display parameters and with conventional film/screen images under realistic conditions. As a result we end up with recommendations for the display size, for spatial resolution, for luminance, and for contrast ratio to be fulfilled by display systems. These parameters among others are subject to acceptance testing. We describe the classification of displays into application categories, the test equipment, and the test procedures that can and shall be applied in clinical practice. In particular, we analyze the luminance measurement proposed by the acceptance testing standard.
A multitude of approximate cone-beam algorithms have been proposed suited for reconstruction of small cone angle CT data. The goal of this study is to identify a practical and efficient approximate cone-beam method, and to investigate its performance at medium cone angles associated to area detectors. Three different approximate algorithms for spiral cone-beam CT will be compared: the (pi) -method, the Multirow- Fourier-Reconstruction and the Advanced Single-Slice Rebinning method. These algorithms are different in the way how the two- dimensional detector images are filtered. In each view x-ray samples are identified which describe an approximation to a virtual reconstruction plane. The image quality of the respective reconstruction will be assessed with respect to image artifacts, the slice sensitivity profile, and the in- plane modulation transfer function. It turns out that the performance of approximate reconstruction improves as the virtual reconstruction plane better fits the spiral focus path. The Advanced Single-Slice Rebinning method using tilted reconstruction planes is a practical algorithm, providing image quality comparable to that of a single-row scanning system even with a 46 row detector at a table feed of 64 mm per rotation of the gantry.
Functional cardiac imaging with 3rd generation CT scanners is challenging, because the temporal resolution seems to be limited to approximately 2/3 of the rotation time of the gantry. We propose a new method for high temporal resolution volume heart imaging with multirow detectors based on a retrospective electrocardiogram-gated rebinning procedure. The limited time resolution is overcome using time consistent projection data retrieved from more than one cardiac cycle. In principle the method provides volume heart imaging with adjustable time resolution at arbitrary cardiac phases. It can be applied both for spiral and axial scan imaging. The presented study is based on computer simulations incorporating a model of the human heart taking into account anatomy, motion and heart rate variability. For multirow detectors we were able to show that good image quality can be obtained even during systole with temporal resolution which even exceeds that provided by an Electron Beam Scanner in standard mode of operation. Using an area detector with detector height > 3 cm (center of rotation) the total measurement time is within one breathhold for complete volume imaging of the heart. Furthermore, freezing motion of the coronary arteries during enddiastole allows high quality 3D display of coronary anatomy.
We studied the impact of CRT spot size, phosphor luminance noise and image noise on the specification of high- resolution CRT displays that address the critical needs of general chest radiography. Using Argus CRT simulation software, the design of high-resolution CRTs for the display of adult chest radiographs was studied. The simulated images were printed on a laser printer and evaluated by a board- certified radiologist, RMS. The validity of the Argus simulation was assessed by modeling a 1k X 1k pixels CRT, whose technical parameters were sufficiently well known. Comments from the observer are presented comparing the simulated 2k display and a size-matched replicate of the original screen/film image. Critical parameters like phosphor luminance efficiency and its impact on electron beam size and phosphor luminance noise and its impact on radiographic image noise are discussed. We conclude that Argus CRT simulation software can successfully model the performance of CRTs intended to display medical images permitting consideration of critical parameters without costly manufacturing trials. Based on the 2k CRT simulation results, we suggest that a low luminance noise phosphor such as type p45 be used to ensure that specifying a small spot size would yield the anticipated sharpness improvements.
We investigate a simulation tool for the optimization of soft copy displays in radiology. The digital image is traced through the individual components of a black and white cathode ray tube (CRT) monitor and the luminance image observed at the glass faceplate is simulated. The simulated images can be evaluated numerically or rendered on film by a high-resolution printer for viewing. We validated the program simulating a real existing monitor by comparing the results with measured values, as well as by visually comparing the actual image with the simulated one. The gross properties like luminance, dynamic range, and spatial resolution are sufficiently well described. The visual impression of the simulated image is very similar to that of the real soft copy. We investigate the influence of individual parameters on image quality. We find that the bandwidth of the video amplifier has to be larger than half the pixel rate. We demonstrate the influence of the electron beam spot size on spatial resolution. It is shown how the spatial resolution depends on phosphor luminous efficiency and on glass transmission. Furthermore, for a given target display curve, it is found that the current-to- voltage relationship of the electron gun influences the number of perceived gray values. Finally we discuss phosphor noise in context with dynamic range.
In order to develop image pick-up tubes with high and uniform resolution over a large image size, the electron optical design consisting of magnetic focusing and electrostatic deflection (MS-type) was investigated and optimized by 3-dimensional numerical calculations. With optimal electromagnetic configuration, the calculated intensity distribution of the beam spot in the focal plane showed a circular shape of nearly constant width even for elongations of the beam far from the image axis, which promises negligible image aberrations. Test tubes of 1 in. format were built up using Saticon photoconductive layer and low output capacitance (LOC). The overall length of the pick-up tube is less than 125 mm. Experimental investigations resulted in a modulation depth (square wave response) of 60% and more at 1600 TVL using an image size of 19 mm diameter (interlaced system, 1249 lines). The uniformity of resolution at the whole image area was confirmed in agreement with theory. An analysis of image aberrations showed very small curvature of field, landing error, astigmatism and geometric distortion. In comparison to conventional all magnetic tubes (MM-type), the MS- type tube yields superior image quality especially at the corners, more flexibility for using different scanning modes and a reduction in power consumption of the tube and in total costs for the camera system.
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