Image quality evaluation of breast tomosynthesis with synchrotron radiation.

PURPOSE: This study investigates the image quality of tomosynthesis slices obtained from several acquisition sets with synchrotron radiation using a breast phantom incorporating details that mimic various breast lesions, in a heterogeneous background. METHODS: A complex Breast phantom (MAMMAX) with a heterogeneous background and thickness that corresponds to 4.5 cm compressed breast with an average composition of 50% adipose and 50% glandular tissue was assembled using two commercial phantoms. Projection images using acquisition arcs of 24°, 32°, 40°, 48°, and 56° at incident energy of 17 keV were obtained from the phantom with the synchrotron radiation for medical physics beamline at ELETTRA Synchrotron Light Laboratory. The total mean glandular dose was set equal to 2.5 mGy. Tomograms were reconstructed with simple multiple projection algorithm (MPA) and filtered MPA. In the latter case, a median filter, a sinc filter, and a combination of those two filters were applied on the experimental data prior to MPA reconstruction. Visual inspection, contrast to noise ratio, contrast, and artifact spread function were the figures of merit used in the evaluation of the visualisation and detection of low- and high-contrast breast features, as a function of the reconstruction algorithm and acquisition arc. To study the benefits of using monochromatic beams, single projection images at incident energies ranging from 14 to 27 keV were acquired with the same phantom and weighted to synthesize polychromatic images at a typical incident x-ray spectrum with W target. RESULTS: Filters were optimised to reconstruct features with different attenuation characteristics and dimensions. In the case of 6 mm low-contrast details, improved visual appearance as well as higher contrast to noise ratio and contrast values were observed for the two filtered MPA algorithms that exploit the sinc filter. These features are better visualized at extended arc length, as the acquisition arc of 56° with 15 projection images demonstrates the highest image reconstruction quality. For microcalcifications, filtered MPA implemented with a combination of median and sinc filters indicates better feature appearance due to efficient suppression of background tissue. The image quality of these features is less sensitive to the acquisition arc. Calcifications with size ranging from 170 to 500 µm, like the ones presently studied, are well identified and visualized for all arcs used. The comparison of single projection images obtained under different beam conditions showed that the use of monochromatic beam can produce an image with higher contrast and contrast to noise ratio compared to an image corresponding to a polychromatic beam even when the latter is acquired with double incident exposure. CONCLUSIONS: Filter optimization in respect to the type of feature characteristics is important before the reconstruction. The MPA combined with median and sinc filters results in improved reconstruction of microcalcifications and low-contrast features. The latter are better visualized at extended arc length, while microcalcifications are less sensitive to this acquisition parameter. Use of monochromatic beams may result in tomographic images with higher contrast acquired at lower incident exposures.

Evaluation of digital breast tomosynthesis reconstruction algorithms using synchrotron radiation in standard geometry.

PURPOSE: In this article, the image quality of reconstructed volumes by four algorithms for digital tomosynthesis, applied in the case of breast, is investigated using synchrotron radiation. METHODS: An angular data set of 21 images of a complex phantom with heterogeneous tissue-mimicking background was obtained using the SYRMEP beamline at ELETTRA Synchrotron Light Laboratory, Trieste, Italy. The irradiated part was reconstructed using the multiple projection algorithm (MPA) and the filtered backprojection with ramp followed by hamming windows (FBR-RH) and filtered backprojection with ramp (FBP-R). Additionally, an algorithm for reducing the noise in reconstructed planes based on noise mask subtraction from the planes of the originally reconstructed volume using MPA (MPA-NM) has been further developed. The reconstruction techniques were evaluated in terms of calculations and comparison of the contrast-to-noise ratio (CNR) and artifact spread function. RESULTS: It was found that the MPA-NM resulted in higher CNR, comparable with the CNR of FBP-RH for high contrast details. Low contrast objects are well visualized and characterized by high CNR using the simple MPA and the MPA-NM. In addition, the image quality of the reconstructed features in terms of CNR and visual appearance as a function of the initial number of projection images and the reconstruction arc was carried out. Slices reconstructed with more input projection images result in less reconstruction artifacts and higher detail CNR, while those reconstructed from projection images acquired in reduced angular range causes pronounced streak artifacts. CONCLUSIONS: Of the reconstruction algorithms implemented, the MPA-NM and MPA are a good choice for detecting low contrast objects, while the FBP-RH, FBP-R, and MPA-NM provide high CNR and well outlined edges in case of microcalcifications.

Signal and noise transfer properties of CMOS based active pixel flat panel imager coupled to structured CsI:Tl.

Complementary metal-oxide-semiconductors (CMOS) active pixel sensors can be optically coupled to CsI:Tl phosphors forming a indirect active pixel flat panel imager (APFPI) for high performance medical imaging. The aim of this work is to determine the x-ray imaging capabilities of CMOS-based APFPI and study the signal and noise transfer properties of CsI:Tl phosphors. Three different CsI:Tl phosphors from two different vendors have been used to produce three system configurations. The performance of each system configuration has been studied in terms of the modulation transfer function (MTF), noise power spectra, and detective quantum efficiency (DQE) in the mammographic energy range. A simple method to determine quantum limited systems in this energy range is also presented. In addition, with aid of monochromatic synchrotron radiation, the effect of iodine characteristic x-rays of the CsI:Tl on the MTF has been determined. A Monte Carlo simulation of the signal transfer properties of the imager is also presented in order to study the stages that degrade the spatial resolution of our current system. The effect of using substrate patterning during the growth of CsI:Tl columnar structure was also studied, along with the effect of CsI:Tl fixed pattern noise due to local variations in the scintillation light. CsI:Tl fixed pattern noise appears to limit the performance of our current system configurations. All the system configurations are quantum limited at 0.23 microC/kg with two of them having DQE (0) equal to 0.57. Active pixel flat panel imagers are shown to be digital x-ray imagers with almost constant DQE throughout a significant part of their dynamic range and in particular at very low exposures.

Deconvolution of x-ray phase contrast images as a way to retrieve phase information lost due to insufficient resolution.

When free-space propagation x-ray phase contrast imaging is implemented outside synchrotron radiation facilities, the combined effect of detector resolution and source size swamps the fine phase contrast fringes, often making them almost undetectable. In an attempt to mitigate this effect, a simple deconvolution procedure based on division in the Fourier space plus multiplication by an appropriate filter was applied to experimental x-ray phase contrast images of a simple geometric phantom. The filter parameter was varied in order to assess its impact on the level of retrieved phase signal. The deconvolved images were compared to simulated ones obtained under different resolution conditions, showing that this simple procedure provided signals equivalent to those that would be obtained with a detector with three times better resolution. By accepting an increase in the overall image noise, the method also appears to bring up secondary phase contrast fringes, which are not visible in the unprocessed signal.

Preliminary images from an adaptive imaging system.

I-ImaS (Intelligent Imaging Sensors) is a European project aiming to produce real-time adaptive X-ray imaging systems using Monolithic Active Pixel Sensors (MAPS) to create images with maximum diagnostic information within given dose constraints. Initial systems concentrate on mammography and cephalography. In our system, the exposure in each image region is optimised and the beam intensity is a function of tissue thickness and attenuation, and also of local physical and statistical parameters in the image. Using a linear array of detectors, the system will perform on-line analysis of the image during the scan, followed by optimisation of the X-ray intensity to obtain the maximum diagnostic information from the region of interest while minimising exposure of diagnostically less important regions. This paper presents preliminary images obtained with a small area CMOS detector developed for this application. Wedge systems were used to modulate the beam intensity during breast and dental imaging using suitable X-ray spectra. The sensitive imaging area of the sensor is 512 x 32 pixels 32 x 32 microm(2) in size. The sensors' X-ray sensitivity was increased by coupling to a structured CsI(Tl) scintillator. In order to develop the I-ImaS prototype, the on-line data analysis and data acquisition control are based on custom-developed electronics using multiple FPGAs. Images of both breast tissues and jaw samples were acquired and different exposure optimisation algorithms applied. Results are very promising since the average dose has been reduced to around 60% of the dose delivered by conventional imaging systems without decrease in the visibility of details.

Correlation of energy dispersive diffraction signatures and microCT of small breast tissue samples with pathological analysis.

Identification of specific tissue types in conventional mammographic examinations is extremely limited. However, the use of x-ray diffraction effects during imaging has the potential to characterize the tissue types present due to the fact that each tissue type produces its own unique diffraction signature. Nevertheless, the analysis and categorization of these diffraction signatures by tissue type can be hampered by the inhomogeneous nature of breast tissue, leading to categorization errors where several types are present. This work aims to reduce sample categorization errors by combining spectral diffraction signature collection with sample imaging, giving more detailed data on the composition of each sample. Diffraction microCT was carried out on 19 unfixed breast tissue samples using an energy resolving translate-rotate CT system. High-resolution transmission microCT images were also recorded for comparison and sample composition analysis. Following imaging, the samples were subjected to histopathological analysis. Reconstructing on various momentum transfer regions allows different tissue types to be identified in the diffraction images. Results show a correlation between measured x-ray diffraction images and stained histopathological tissue sections. X-ray diffraction signatures generated from the measured data were categorized and analysed, with a t-test indicating that they have the potential for use in tissue type identification.

Three-dimensional cascaded system analysis of a 50 µm pixel pitch wafer-scale CMOS active pixel sensor x-ray detector for digital breast tomosynthesis.

High-resolution, low-noise x-ray detectors based on the complementary metal-oxide-semiconductor (CMOS) active pixel sensor (APS) technology have been developed and proposed for digital breast tomosynthesis (DBT). In this study, we evaluated the three-dimensional (3D) imaging performance of a 50 µm pixel pitch CMOS APS x-ray detector named DynAMITe (Dynamic Range Adjustable for Medical Imaging Technology). The two-dimensional (2D) angle-dependent modulation transfer function (MTF), normalized noise power spectrum (NNPS), and detective quantum efficiency (DQE) were experimentally characterized and modeled using the cascaded system analysis at oblique incident angles up to 30°. The cascaded system model was extended to the 3D spatial frequency space in combination with the filtered back-projection (FBP) reconstruction method to calculate the 3D and in-plane MTF, NNPS and DQE parameters. The results demonstrate that the beam obliquity blurs the 2D MTF and DQE in the high spatial frequency range. However, this effect can be eliminated after FBP image reconstruction. In addition, impacts of the image acquisition geometry and detector parameters were evaluated using the 3D cascaded system analysis for DBT. The result shows that a wider projection angle range (e.g. ±30°) improves the low spatial frequency (below 5 mm-1) performance of the CMOS APS detector. In addition, to maintain a high spatial resolution for DBT, a focal spot size of smaller than 0.3 mm should be used. Theoretical analysis suggests that a pixelated scintillator in combination with the 50 µm pixel pitch CMOS APS detector could further improve the 3D image resolution. Finally, the 3D imaging performance of the CMOS APS and an indirect amorphous silicon (a-Si:H) thin-film transistor (TFT) passive pixel sensor (PPS) detector was simulated and compared.

Evaluation of the 3D spatial distribution of the Calcium/Phosphorus ratio in bone using computed-tomography dual-energy analysis.

PURPOSE: The Calcium/Phosphorus (Ca/P) ratio was shown to vary between healthy bones and bones with osteoporotic symptoms. The relation of the Ca/P ratio to bone quality remains under investigation. To study this relation and determine if the ratio can be used to predict bone fractures, a non-invasive 3D imaging technique is required. The first aim of this study was to test the effectiveness of a computed-tomography dual-energy analysis (CT-DEA) technique developed to assess the Ca/P ratio in bone apatite (collagen-free bone) in identifying differences between healthy and inflammation-mediated osteoporotic (IMO) bones. The second aim was to extend the above technique for its application to a more complex structure, intact bone, that could potentially lead to clinical use. METHODS: For the first aim, healthy and IMO rabbit cortical bone apatite samples were assessed. For the second aim, some changes were made to the technique, which was applied to healthy and IMO intact bone samples. RESULTS: Statistically significant differences between healthy and IMO bone apatite were found for the bulk Ca/P ratio, low Ca/P ratio proportion and interconnected low Ca/P ratio proportion. For the intact bone samples, the bulk Ca/P ratio was found to be significantly different between healthy and IMO. CONCLUSIONS: Results show that the CT-DEA technique can be used to identify differences in the Ca/P ratio between healthy and osteoporotic, in both bone apatite and intact bone. With quantitative imaging becoming an increasingly important advancement in medical imaging, CT-DEA for bone decomposition could potentially have several applications.

Size measurements on isolated rat heart cells using Coulter analysis and light scatter flow cytometry.

Isolated ventricular muscle cells from the adult rat heart have been examined by both Coulter analysis and light scatter flow cytometry. The dispersed cell preparations contain two main cell types: viable, rod-shaped cells and damaged, round cells. Coulter analytical techniques provided statistical data on cell volume for both cell types. The contribution of each population to the Coulter pulse height distributions were separated by a subtraction method using data obtained from digitonin-treated preparations that contain only round cells. A shape factor for cells aligned with the flow direction was computed from light microscope measurements and the effects of cell orientation within the Coulter aperture were approximately assessed. The estimated volumes for intact myocytes compare favourably with those reported in the literature. No significant size difference was observed between fresh and fixed cells. Narrow angle, forward light scatter measurements were made on individual cells flowing across a focused laser beam. Both scatter pulse height and pulse width (pulse duration) distributions were collected. Values for myocyte length calculated from pulse width information agree well with published data and confirm that the hydrodynamic forces in the flow system produced alignment of the cells with the flow direction. Scatter pulse width distributions reveal two distinct peaks assignable to either rod or round cells. Preliminary electronic gating experiments, using pulse height signals, suggest that signals derived from round cells could be eliminated entirely using a gating regime based on pulse width. This would enable flow cytometric measurements to be made on only the intact myocytes present in heterogeneous preparations.

50 µm pixel pitch wafer-scale CMOS active pixel sensor x-ray detector for digital breast tomosynthesis.

Wafer-scale CMOS active pixel sensors (APSs) have been developed recently for x-ray imaging applications. The small pixel pitch and low noise are very promising properties for medical imaging applications such as digital breast tomosynthesis (DBT). In this work, we evaluated experimentally and through modeling the imaging properties of a 50 µm pixel pitch CMOS APS x-ray detector named DynAMITe (Dynamic Range Adjustable for Medical Imaging Technology). A modified cascaded system model was developed for CMOS APS x-ray detectors by taking into account the device nonlinear signal and noise properties. The imaging properties such as modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) were extracted from both measurements and the nonlinear cascaded system analysis. The results show that the DynAMITe x-ray detector achieves a high spatial resolution of 10 mm(-1) and a DQE of around 0.5 at spatial frequencies <1 mm(-1). In addition, the modeling results were used to calculate the image signal-to-noise ratio (SNRi) of microcalcifications at various mean glandular dose (MGD). For an average breast (5 cm thickness, 50% glandular fraction), 165 µm microcalcifications can be distinguished at a MGD of 27% lower than the clinical value (~1.3 mGy). To detect 100 µm microcalcifications, further optimizations of the CMOS APS x-ray detector, image aquisition geometry and image reconstruction techniques should be considered.

A Monte Carlo study of multiple scatter effects in Compton scatter densitometry.

The contribution of multiple scatter to the measured signal in x- and gamma-ray Compton scatter densitometry has been investigated theoretically by the use of Monte Carlo techniques to follow individual photon life histories. A three component phantom was employed in the computer model to simulate the patient at three examination sites; the radius/ulna, the femoral neck, and the lumbar spine. Monoenergetic radiation beams of 60- and 100-keV photons and polyenergetic x-ray spectra of 100 and 140 kVp were used. Scattered events were detected over 360 degrees and classified according to their origin and frequency of scatter. The single scatter in bone to multiple scatter ratio was studied as an indication of the signal-to-noise ratio and this was found to vary with phantom size but was independent of photon energy. Correction factors to be used in a clinical densitometer to account for the inclusion of multiple scatter events were computed. These were found to be 0.65-0.58 at the optimum scattering angles for the phantoms considered.

Photon absorption measurements of bone density in the presence of scattered radiation.

Bone density measurements are frequently carried out using photon absorptiometry techniques. The analysis of data collected in this way depends upon scatter-free detector signals. In practice the detector and source collimation lead to high levels of scattered radiation being detected. An analysis of the problem using photon transport computer models has shown that the current systems are reasonably insensitive to the scatter contribution except where patient changes occur over a long series of measurements or where new results taken with DEXA are compared with those taken on older systems such as DPA. This is frequently the case since the major body of data related to serial bone density measurements has been achieved with DPA. Inclusion of scattered radiation in the bone density calculations can lead to a 0.5-3.5% reduction between system types. Patient size changes could lead to a reduction in bone density of 0.5% (femoral neck measurement) to 1% (lumbar spine measurement).

The effect of scattered radiation in dual-energy analysis.

Dual-energy techniques can be used to provide additional information during x-ray examinations. A recent development has been in the application of dual-energy analysis to fluoroscopic procedures. In this paper the effects of scattered radiation upon the results obtained with a small area, dual-energy probe used during fluoroscopy has been evaluated. The method employed has been the development and application of a Monte Carlo photon transport computer program. Experimental confirmation of the model has been performed and the model extended to study the effects of scattered radiation upon the chemical sensitivity of the dual-energy probe. These effects have been studied using a simplified phantom geometry to represent a homogeneous patient. The results indicate that for tissue analysis where the effective atomic number (Z) is less than 13 the effects of scattered radiation lead to errors in estimates of Z of +/- 0.25 when the acceptance angle of the detected radiation is below +/- 20 degrees.

Quantitative x-ray diffraction analysis of bone and marrow volumes in excised femoral head samples.

The aim of this paper is to apply the technique of quantitative x-ray diffraction analysis (QXDA) to trabecular bone tissue to demonstrate that quantitative data of the ratio of bone and marrow volumes within the trabecular region can be obtained. Apparatus has been constructed for measuring energy dispersive x-ray diffraction spectra of human femoral head samples in the diagnostic x-ray energy range. Individual diffraction peaks due to bone and marrow tissue were identified in the measured spectra. The relative intensities of the two peaks within the spectra quantify the relative proportions of the two components, and so the bone to marrow peak ratio is proposed as a parameter which is capable of providing information on the osteoporotic state of trabecular tissue. Preliminary results indicate a significant correlation between this method and the bone density measurement techniques of quantitative computed tomography and Compton scatter densitometry. Results have shown that the use of a synthetically prepared calibration curve can enable absolute measurement of bone or marrow volumes.

A real time dual-energy probe for tissue characterization during fluoroscopy.

In the past dual-energy techniques have been applied with a variety of detector systems. However, making clinical decisions during a diagnostic x-ray procedure by using dual-energy information has not been possible. This paper looks at the development of a dual-energy probe that permits real time analysis. The technique is based on the local analysis procedure introduced by Speller and co-workers and uses spectral filtering for energy separation. The split-detector probe is optimized using computer models, and the effects of beam hardening and scattered radiation are considered. It is shown that a 0.25 mm CsI/25 mm NaI combination of detector elements with a 0.3 mm Cu filter offers the best performance. Preliminary results using the probe for in vivo analysis of gall stone composition compare well with the more accepted methods of x-ray diffraction and atomic absorption spectroscopy. The two groups of stones evaluated were found to have mean effective atomic numbers of 5.6 +/- 1.7 and 9.6 +/- 0.5. It is suggested that such a procedure could prove useful in patient management in the use of lithotripsy.

A CCD based focal spot camera.

Rapid, quantative assessment of x-radiation source intensity distributions would be useful in several areas of radiation physics. A simple x-ray sensitive CCD based camera for the measurement of focal spot and radiation beam intensity profiles has been constructed. Focal spot images recorded with the CCD camera and radiographic emulsion are compared and the advantage of the CCD camera in rapidly recording true radiation intensity distributions is demonstrated.

A novel method for producing x-ray test objects and phantoms.

A novel method for producing customized x-ray test objects and clinically realistic phantoms has been developed. Test objects can be created with a drawing software package and the digital images can be printed on a standard inkjet printer but using potassium iodide solution in place of the cartridge's ink. The reproducibility and the consistency, the limiting spatial resolution, the uniformity as well as the potassium iodide thickness per print have been evaluated. The relationship between the number of prints, grey levels and the radiation contrast was investigated and quantified. A copy of the Leeds TO10 contrast detail test object was printed and the x-ray images of the Leeds TO10 and of the printed Leeds TO10 were compared. In addition, the potential use of this method was demonstrated by reproducing a percutaneous transluminal coronary angioplasty clinical digital image. The reproducibility and consistency of this method was found to be better than 0.1%. The limiting spatial resolution of the printer using ink was found to be 3.55 1p mm(-1) but it deteriorated when the ink was replaced with potassium iodide and as the print density increases. The uniformity across the printed area was found to be satisfactory although an artefact due to the printer was present in the x-ray images. The comparison between the Leeds TO10 and the printed Leeds TO10 gave differences less than 10%. A good agreement between the clinical image and the printed clinical image was found. In conclusion, the method is a reliable, cost-effective, flexible and alternative way for producing x-ray test objects and clinically related phantoms.

Monitoring and correction of multiple scatter during clinical Compton scatter densitometry measurements.

A system for monitoring multiple scatter during a clinical Compton scatter densitometry measurement of bone density is described. Multiple scatter from the measurement site was measured using a supplementary collimated detector positioned so that only multiply scattered photons could enter the detector. The data from the detector were used to form a multiple-scatter correction factor (mcf) to correct for the bias attributed to multiple scatter. The results of experimental and computer simulations are presented which demonstrate the relationship between the multiple-scatter reading and calculated mcf values. The influence of bone size on the values of mcf in large measurement sites, such as the femoral neck, was shown to be negligible. A simulation was used to produce a visualization of the multiple scatter in order to ascertain the optimum position of the supplementary detector. This technique was shown to be a rapid and accurate method of measuring the multiple-scatter bias and suitable for use during clinical CSD measurements.

X-ray scatter signatures for normal and neoplastic breast tissues.

Measurements of breast tissue scattering properties have been made in an energy dispersive x-ray diffraction system over the momentum transfer range of 0.70 to 3.50 nm(-1). One hundred samples of excised tissue have been used. Results from the diffraction system have been compared with the histological analysis for each individual sample. It has been found that tissue types can be characterized on the basis of the shape of the scatter spectrum and on its relative intensity. The shapes are significantly different between tissue types in the range 1.0 to 1.8 nm(-1) and suggest that if particular values of momentum transfer are monitored, a discriminating signal could be obtained. Analysis of the maximum intensity in the signature also reveals a change of up to a factor of 2 between adipose and fat-free tissues.

Short communication: helical computed tomography: where is the cut?

The helical scanning mode in computed tomography (CT) entails continuous table movement during image acquisition. The projections acquired in this mode will not be consistent with those in the axial scanning mode. A comparison of the helical and axial modes of scanning on the IGE HiSpeed Advantage CT system (software version ZA) has been made using a contrast-detail phantom. Differences seen between the two modes include shadow and streaking artefacts and an increase in the partial volume effect in the helical scan mode.