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Sidky E.Y.,University of Chicago | Duchin Y.,University of Chicago | Pan X.,University of Chicago | Ullberg C.,XCounter AB
Medical Physics | Year: 2011

Purpose: The authors developed an iterative image-reconstruction algorithm for application to low-intensity computed tomography projection data, which is based on constrained, total-variation (TV) minimization. The algorithm design focuses on recovering structure on length scales comparable to a detector bin width. Methods: Recovering the resolution on the scale of a detector bin requires that pixel size be much smaller than the bin width. The resulting image array contains many more pixels than data, and this undersampling is overcome with a combination of Fourier upsampling of each projection and the use of constrained, TV minimization, as suggested by compressive sensing. The presented pseudocode for solving constrained, TV minimization is designed to yield an accurate solution to this optimization problem within 100 iterations. Results: The proposed image-reconstruction algorithm is applied to a low-intensity scan of a rabbit with a thin wire to test the resolution. The proposed algorithm is compared to filtered backprojection (FBP). Conclusions: The algorithm may have some advantage over FBP in that the resulting noise level is lowered at equivalent contrast levels of the wire. © 2011 American Association of Physicists in Medicine.


Carton A.-K.,University of Pennsylvania | Bakic P.,University of Pennsylvania | Ullberg C.,XCounter AB | Maidment A.D.A.,University of Pennsylvania
Progress in Biomedical Optics and Imaging - Proceedings of SPIE | Year: 2010

Analysis of complex imaging tasks requires a phantom that simulates the patient anatomy. We have developed a technique to fabricate 3D physical anthropomorphic breast phantoms for image quality assessment of 2D and 3D breast x-ray imaging systems. The phantom design is based on an existing computer model that can generate breast voxel phantoms of varying size, shape, glandularity, and internal composition. The physical phantom is produced in two steps. First, the computer model of the glandular tissue, skin and Coopers' ligaments is separated into sections. These sections are fabricated by high-resolution rapid prototype printing using a single tissue equivalent material. The adipose tissue regions in the sections are filled using an epoxy-based resin combined with phenolic microspheres. The phantom sections arc then stacked. The phantom is provided with an extra section modified to include iodine-enhanced masses. We fabricated a prototype phantom corresponding to a 450 ml breast with 45% dense tissue deformed to represent a 5 cm compressed thickness. The rapid prototype and epoxy based resin phantom materials attenuate x rays similar to 50% glandular tissue and 100% adipose tissue, respectively. The iodinated masses arc between 4.0 and 9.6 mm thick and contain 2.5 mg/ml and 5 mg/ml iodine. Digital mammography and digital breast tomosynthesis images of the phantom are qualitatively similar in appearance to clinical images. In summary, a method to fabricate a 3D physical anthropomorphic breast phantom has been developed with known ground truth in the form of a companion voxel phantom. This combined system of physical and computational phantoms allows for both qualitative and quantitative image quality assessment. © 2010 SPIE.


Svane G.,Karolinska University Hospital | Azavedo E.,Karolinska University Hospital | Lindman K.,XCounter AB | Urech M.,XCounter AB | And 4 more authors.
Acta Radiologica | Year: 2011

Background: In two-dimensional mammography, a well-known problem is over- and underlying tissue which can either obstruct a lesion or create a false-positive result. Tomosynthesis, with an ability to layer the tissue in the image, has the potential to resolve these issues. Purpose: To compare the diagnostic quality, sensitivity and specificity of a single tomosynthesis mammography image and a traditional two-view set of two-dimensional mammograms and to assess the comfort of the two techniques. Material and Methods: One hundred and forty-four women, mainly chosen because of suspicious features on standard mammograms (76 malignant), had a single tomosynthesis image taken of one breast using a novel photon counting system. On average, the dose of the tomosynthesis images was 0.63 times that of the two-view images and the compression force during the procedure was halved. The resulting images were viewed by two radiologists and assessed both individually and comparing the two techniques. Results: In 56% of the cases the radiologists rated the diagnostic quality of the lesion details higher in the tomosynthesis images than in the conventional images (and in 91% equal or higher), which means there is a statistically significant preference for the tomosynthesis technique. This included the calcifications which were rated as having better quality in 41% of the cases. While sensitivity was slightly higher for traditional mammography the specificity was higher for tomosynthesis. However, neither of these two differences was large enough to be statistically significant. Conclusion: The overall accuracy of the two techniques was virtually equal despite the radiologist's very limited experience with tomosynthesis images and vast experience with two-dimensional mammography. As the diagnostic quality of the lesion details in the tomosynthesis images was valued considerably higher this factor should improve with experience. The patients also favored the tomosynthesis examination, rating the comfort of the procedure as much higher than regular mammography which might affect screening attendance.


Carton A.-K.,University of Pennsylvania | Ullberg C.,XCounter AB | Maidment A.D.A.,University of Pennsylvania
Medical Physics | Year: 2010

Purpose: Previously, the authors developed a dual-energy (DE) acquisition technique for a photon-counting digital breast tomosynthesis (DBT) imaging system. Low-energy (LE) and high-energy (HE) images are acquired in a single scan by covering alternate slits of a multislit prepatient collimator with Sn and Cu, respectively. A theoretical model was used to optimize the technique. In this article, an experimental validation of this technique is presented. Methods: Experiments were performed on a prototype DBT system. LE and HE projection images were acquired sequentially; either a Sn or a Cu filter was positioned in the filter holder at the exit window of the x-ray tube. Sn filters from 0.113 to 0.242 mm thick and Cu filters from 0.103 to 0.267 mm were used. The images were acquired with a W target at 49 kV. Tomographic images, hereafter referred to as DBT images, were reconstructed using a shift-and-add algorithm. DE-DBT images were obtained by weighted logarithmic subtraction of the LE and HE images. Weighting factors w t that optimally cancel breast tissues with two different glandularities were assessed for 20-80 mm thick phantoms with 0%, 50%, and 100% glandularity. The mean and standard deviation in the per-pixel signal intensity (SI) were calculated in the DBT images. These data were used to calculate signal-difference-to-noise ratios (SDNRs) between iodine enhanced and nonenhanced polymethyl methacrylate backgrounds. To illustrate the feasibility of the technique, DE-DBT images of a structured phantom containing iodine disks were assessed. The experimental results were compared against the values obtained from a theoretical model of the imaging system. Results: The average difference between theoretical and experimental w t was found to range from 8% to 21%. Experimental w t values increase with phantom thickness and Cu thickness, depend somewhat on Sn thickness, and vary more as a function of breast composition in thick breasts than in thin breasts. Theoretical and experimental mean and standard deviation in the per-pixel SI differ by -7% to 10% and by -3% to 4%. Theoretical and experimental SDNR values differ, on average, by 1.5%. Iodine concentrations can be predicted from SDNR; the relationship can be accurately fit to a quadratic. In the images of the structured phantom, iodine concentrations of 1 mg/cm 2 and larger are discernable. Conclusions: The strong agreement between experimental and theoretical results in this article indicates that the authors' computer model is accurate. © 2010 American Association of Physicists in Medicine.


Carton A.-K.,University of Pennsylvania | Ullberg C.,XCounter AB | Lindman K.,XCounter AB | Acciavatti R.,University of Pennsylvania | And 2 more authors.
Medical Physics | Year: 2010

Purpose: Dual-energy (DE) iodine contrast-enhanced x-ray imaging of the breast has been shown to identify cancers that would otherwise be mammographically occult. In this article, theoretical modeling was performed to obtain optimally enhanced iodine images for a photon-counting digital breast tomosynthesis (DBT) system using a DE acquisition technique. Methods: In the system examined, the breast is scanned with a multislit prepatient collimator aligned with a multidetector camera. Each detector collects a projection image at a unique angle during the scan. Low-energy (LE) and high-energy (HE) projection images are acquired simultaneously in a single scan by covering alternate collimator slits with Sn and Cu filters, respectively. Sn filters ranging from 0.08 to 0.22 mm thickness and Cu filters from 0.11 to 0.27 mm thickness were investigated. A tube voltage of 49 kV was selected. Tomographic images, hereafter referred to as DBT images, were reconstructed using a shift-and-add algorithm. Iodine-enhanced DBT images were acquired by performing a weighted logarithmic subtraction of the HE and LE DBT images. The DE technique was evaluated for 20-80 mm thick breasts. Weighting factors, w t, that optimally cancel breast tissue were computed. Signal-difference-to-noise ratios (SDNRs) between iodine-enhanced and nonenhanced breast tissue normalized to the square root of the mean glandular dose (MGD) were computed as a function of the fraction of the MGD allocated to the HE images. Peak SDNR/MGD and optimal dose allocations were identified. SDNR/MGD and dose allocations were computed for several practical feasible system configurations (i.e., determined by the number of collimator slits covered by Sn and Cu). A practical system configuration and Sn-Cu filter pair that accounts for the trade-off between SDNR, tube-output, and MGD were selected. Results: w t depends on the Sn-Cu filter combination used, as well as on the breast thickness; to optimally cancel 0% with 50% glandular breast tissue, w t values were found to range from 0.46 to 0.72 for all breast thicknesses and Sn-Cu filter pairs studied. The optimal w t values needed to cancel all possible breast tissue glandularites vary by less than 1% for 20 mm thick breasts and 18% for 80 mm breasts. The system configuration where one collimator slit covered by Sn is alternated with two collimator slits covered by Cu delivers SDNR/MGD nearest to the peak value. A reasonable compromise is a 0.16 mm Sn-0.23 mm Cu filter pair, resulting in SDNR values between 1.64 and 0.61 and MGD between 0.70 and 0.53 mGy for 20-80 mm thick breasts at the maximum tube current. Conclusions: A DE acquisition technique for a photon-counting DBT imaging system has been developed and optimized. © 2010 American Association of Physicists in Medicine.


Carton A.-K.,University of Pennsylvania | Bakic P.,University of Pennsylvania | Ullberg C.,XCounter AB | Derand H.,XCounter AB | Maidment A.D.A.,University of Pennsylvania
Medical Physics | Year: 2011

Purpose: Develop a technique to fabricate a 3D anthropomorphic breast phantom with known ground truth for image quality assessment of 2D and 3D breast x-ray imaging systems. Methods: The phantom design is based on an existing computer model that can generate breast voxel phantoms of varying composition, size, and shape. The physical phantom is produced in two steps. First, the portion of the voxel phantom consisting of the glandular tissue, skin, and Cooper's ligaments is separated into sections. These sections are then fabricated by high-resolution rapid prototyping using a single material with 50% glandular equivalence. The remaining adipose compartments are then filled using an epoxy-based resin (EBR) with 100% adipose equivalence. The phantom sections are stacked to form the physical anthropomorphic phantom. Results: The authors fabricated a prototype phantom corresponding to a 450 ml breast with 45% dense tissue, deformed to a 5 cm compressed thickness. Both the rapid prototype (RP) and EBR phantom materials are radiographically uniform. The coefficient of variation (CoV) of the relative attenuation between RP and EBR phantom samples was <1% and the CoV of the signal intensity within RP and EBR phantom samples was <1.5% on average. Digital mammography and reconstructed digital breast tomosynthesis images of the authors' phantom were reviewed by two radiologists; they reported that the images are similar in appearance to clinical images, noting there are still artifacts from air bubbles in the EBR. Conclusions: The authors have developed a technique to produce 3D anthropomorphic breast phantoms with known ground truth, yielding highly realistic x-ray images. Such phantoms may serve both qualitative and quantitative performance assessments for 2D and 3D breast x-ray imaging systems. © 2011 American Association of Physicists in Medicine.


Ullberg C.,XCounter AB | Urech M.,XCounter AB | Weber N.,XCounter AB | Engman A.,XCounter AB | And 2 more authors.
Progress in Biomedical Optics and Imaging - Proceedings of SPIE | Year: 2013

Photon counting detectors offer some unique features for x-ray imaging. If designed correctly, photon counting detectors have no readout noise and no dark counts. This is an important feature in for example low dose CT imaging where the total dose is distributed over a large number of projections from different angles. In addition to this it is also possible to incorporate pulse height discrimination of each photon event, thus enabling the recording of images from multiple energy intervals in a single exposure. We demonstrate the performance of a newly developed dual-energy fast photon counting detector with 100μm pixel size that can be read out up to 1000fps. It consists of a three side buttable ASIC that is bump bonded to a CdTe converter. The very high conversion efficiency of CdTe makes the detector suitable for a wide range of applications requiring high spatial resolution at low doses. The efficiency of the detector is maintained all the way out to the edge of the chip which opens up the possibility to build larger detectors still fulfilling medical requirements. The novel detector incorporates a charge sharing correction feature and the effect of this function is demonstrated using the DQE measurements for different spectra as well as with spectrum reconstruction from Cd109and Am 241 radioactive sources. We show that this charge sharing correction feature affects the properties of NPS and MTF, and the energy resolution is greatly enhanced. Measurements are also compared to a simulation model for the detector system. © 2013 SPIE.


Patent
XCounter AB | Date: 2012-01-18

A system for recording computed tomography image data of an object in an object area (4) comprises an X-ray source (5) and an X-ray detector (6) arranged at either side of the object area, the X-ray source having a flying focal spot (5a) from which X-rays is emitted and the X-ray detector comprising pixels (6a) arranged in at least one row for recording images of the object. A device (10a) is provided for rotating the X-ray source and the X-ray detector with respect to the object around an axis of rotation (12), while the at least one row of pixels record images of the object. The X-ray source comprises means (5b) for moving the flying focal spot of the X-ray source from an original position and in a direction essentially opposite to the direction the X-ray source moves during the rotation, and the X-ray detector is provided with means (6b) for time delay summation such that pixel signal values of the at least one row of pixels are shifted one pixel and summed with pixel signal values obtained in a following recording of an image, wherein the shifting and summing of pixel signal values are performed repeatedly and the pixel signal values are shifted in a direction essentially opposite to the direction the X-ray detector moves during the rotation.


A semiconductor based photon counting detector comprising a substrate (11) of semiconductor material; a detector bias voltage supply (12) for applying a detector bias voltage over the substrate, each time during a data acquisition period (t_(1)); a readout arrangement (13) for repetitively reading out data indicative of charges freed in, and transported through, the substrate (11) in response to photons being absorbed, each time during a readout period (t_(2)) following a data acquisition period, wherein the data contain number of charge pulses of photons being absorbed; an external light source (15) for exposing the substrate for light to enable trapped charge carriers to escape from defect levels in the substrate; and a control device (14) operatively connected to the detector bias voltage supply, the readout arrangement, and the external light source. The control device (14) is configured to control the detector bias voltage supply to switch off the detector bias voltage over the substrate and the external light source (15) to switch on the light, thus exposing the substrate (11) for light to enable trapped charge carriers to escape from defect levels in the substrate, concurrently during at least some of said readout periods.


PubMed | XCounter AB and University of Chicago
Type: Journal Article | Journal: Journal of medical imaging (Bellingham, Wash.) | Year: 2015

One of the challenges for iterative image reconstruction (IIR) is that such algorithms solve an imaging model implicitly, requiring a complete representation of the scanned subject within the viewing domain of the scanner. This requirement can place a prohibitively high computational burden for IIR applied to x-ray computed tomography (CT), especially when high-resolution tomographic volumes are required. In this work, we aim to develop an IIR algorithm for direct region-of-interest (ROI) image reconstruction. The proposed class of IIR algorithms is based on an optimization problem that incorporates a data fidelity term, which compares a derivative of the estimated data with the available projection data. In order to characterize this optimization problem, we apply it to computer-simulated two-dimensional fan-beam CT data, using both ideal noiseless data and realistic data containing a level of noise comparable to that of the breast CT application. The proposed method is demonstrated for both complete field-of-view and ROI imaging. To demonstrate the potential utility of the proposed ROI imaging method, it is applied to actual CT scanner data.

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