| 1. |
|
|
| 2. |
- Danielsson, Mats, Professor, et al.
(författare)
-
Photon-counting x-ray detectors for CT
- 2021
-
Ingår i: Physics in Medicine and Biology. - : IOP Publishing. - 0031-9155 .- 1361-6560. ; 66:3, s. 03TR01-
-
Forskningsöversikt (refereegranskat)abstract
- The introduction of photon-counting detectors is expected to be the next major breakthrough in clinical x-ray CT. During the last decade, there has been considerable research activity in the field of photon-counting CT, in terms of both hardware development and theoretical understanding of the factors affecting image quality. In this article, we review the recent progress in this field with the intent of highlighting the relationship between detector design considerations and the resulting image quality. We discuss detector design choices such as converter material, pixel size, and readout electronics design, and then elucidate their impact on detector performance in terms of dose efficiency, spatial resolution, and energy resolution. Furthermore, we give an overview of data processing, reconstruction methods and metrics of imaging performance; outline clinical applications; and discuss potential future developments.
|
|
| 3. |
|
|
| 4. |
- Sundberg, Christel, et al.
(författare)
-
Silicon photon-counting detector for full-field CT usingan ASIC with adjustable shaping time
- 2020
-
Ingår i: Journal of Medical Imaging. - : SPIE - International Society for Optical Engineering. - 2329-4302 .- 2329-4310. ; 7:5
-
Tidskriftsartikel (refereegranskat)abstract
- Purpose: Photon-counting silicon strip detectors are attracting interest for use in next-generation CT scanners. For CT detectors in a clinical environment, it is desirable to have a low power consumption. However, decreasing the power consumption leads to higher noise. This is particularly detrimental for silicon detectors, which require a low noise floor to obtain a good dose efficiency. The increase in noise can be mitigated using a longer shaping time in the readout electronics. This also results in longer pulses, which requires an increased deadtime, thereby degrading the count-rate performance. However, as the photon flux varies greatly during a typical CT scan, not all projection lines require a high count-rate capability. We propose adjusting the shaping time to counteract the increased noise that results from decreasing the power consumption.Approach: To show the potential of increasing the shaping time to decrease the noise level, synchrotron measurements were performed using a detector prototype with two shaping time settings. From the measurements, a simulation model was developed and used to predict the performance of a future channel design.Results: Based on the synchrotron measurements, we show that increasing the shaping time from 28.1 to 39.4 ns decreases the noise and increases the signal-to-noise ratio with 6.5% at low count rates. With the developed simulation model, we predict that a 50% decrease in power can be attained in a proposed future detector design by increasing the shaping time with a factor of 1.875.Conclusion: Our results show that the shaping time can be an important tool to adapt the pulse length and noise level to the photon flux and thereby optimize the dose efficiency of photon-counting silicon detectors.
|
|
| 5. |
|
|
| 6. |
|
|
| 7. |
|
|
| 8. |
|
|
| 9. |
|
|
| 10. |
- Grönberg, Fredrik, et al.
(författare)
-
Count statistics and pileup correction for nonparalyzable photon counting detectors with finite pulse length
- 2018
-
Ingår i: Medical Imaging 2018. - : SPIE - International Society for Optical Engineering. - 9781510616363
-
Konferensbidrag (refereegranskat)abstract
- Photon counting detectors are expected to be the next big step in the development of medical computed tomography. Accurate modeling of the behavior of photon counting detectors in the high count rate regime is therefore important for detector performance evaluations and the development of accurate image reconstruction methods. The commonly used ideal nonparalyzable detector model is based on the assumption that photon interactions are converted to pulses with zero extent in time, which is too simplistic to accurately predict the behavior of photon counting detectors in both low and high count rate regimes. In this work we develop a statistical count model for a nonparalyzable detector with finite pulse length and use it to derive the asymptotic mean and variance of the output count distribution using tools from renewal theory. We use the statistical moments of the distribution to construct an estimator of the true number of counts for pileup correction. We con firm the accuracy of the model and evaluate the pileup correction using Monte Carlo simulations. The results show that image quality is preserved for surprisingly high count rates.
|
|
| 11. |
- Grönberg, Fredrik, et al.
(författare)
-
Count statistics of nonparalyzable photon-counting detectors with nonzero pulse length
- 2018
-
Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405. ; 45:8, s. 3800-3811
-
Tidskriftsartikel (refereegranskat)abstract
- Purpose: Photon-counting detectors are expected to be the next big step in the development of medical computed tomography (CT). Accurate modeling of the behavior of photon-counting detectors in both low and high count rate regimes is important for accurate image reconstruction and detector performance evaluations. The commonly used ideal nonparalyzable (delta pulse) model is built on crude assumptions that make it unsuitable for predicting the behavior of photon-counting detectors at high count rates. The aim of this work is to present an analytical count statistics model that better describes the behavior of photon-counting detectors with nonzero pulse length. Methods: An analytical statistical count distribution model for nonparalyzable detectors with nonzero pulse length is derived using tools from statistical analysis. To validate the model, a nonparalyzable photon-counting detector is simulated using Monte Carlo methods and compared against. Image performance metrics are computed using the Fisher information metric and a comparison between the proposed model, approximations of the proposed model, and those made by the ideal nonparalyzable model is presented and analyzed. Results: It is shown that the presented model agrees well with the results from the Monte Carlo simulation and is stable for varying x-ray beam qualities. It is also shown that a simple Gaussian approximation of the distribution can be used to accurately model the behavior and performance of nonparalyzable detectors with nonzero pulse length. Furthermore, the comparison of performance metrics show that the proposed model predicts a very different behavior than the ideal nonparalyzable detector model, suggesting that the proposed model can fill an important gap in the understanding of pileup effects. Conclusions: An analytical model for the count statistics of a nonparalyzable photon-counting detector with nonzero pulse length is presented. The model agrees well with results obtained from Monte Carlo simulations and can be used to improve, speed up and simplify modeling of photon-counting detectors.
|
|
| 12. |
- Hsieh, Scott S., et al.
(författare)
-
Digital count summing vs analog charge summing for photon counting detectors : A performance simulation study
- 2018
-
Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405. ; 45:9, s. 4085-4093
-
Tidskriftsartikel (refereegranskat)abstract
- PurposeCharge sharing is a significant problem for CdTe-based photon counting detectors (PCDs) and can cause high-energy photons to be misclassified as one or more low-energy events. Charge sharing is especially problematic in PCDs for CT because the high flux necessitates small pixels, which increase the magnitude of charge sharing. Analog charge summing (ACS) is a powerful solution to reduce spectral distortion arising from charge sharing but may be difficult to implement. We investigate correction of the signal after digitization by the comparator (digital count summing), which is only able to correct a subset of charge sharing events but may have implementation advantages. We compare and quantify the relative performance of digital and ACS in simulations. MethodsTransport of photons in CdTe was modeled using Monte Carlo simulations. Energy deposited in the CdTe substrate was converted to electrical charges of a predetermined shape, and all charges within the detector pixel are assumed to be perfectly collected. In ACS, the maximum charge received over any 2x2 block of pixels was grouped together prior to digitization. In digital count summing (DCS), the charge was digitized in each pixel, and subsequently, adjacent pixels that detected events grouped their charge to record a single, higher energy event. All simulations were performed at the limit of low flux (no pileup). The default tube voltage was 120kVp, object thickness was 20cm of water, pixel pitch was 250m, and charge cloud modeled as a Gaussian with sigma=40m. Variation of these parameters was examined in a sensitivity analysis. ResultsDetectors that used no correction, DCS, and ACS misclassified 51%, 39%, and 15% of incident photons, respectively. For iodine basis material imaging, DCS exhibited 100% greater dose efficiency compared to uncorrected, and ACS exhibited an additional 111% greater dose efficiency compared to digital charge summing. For a nonspectral task, the dose efficiency improvement as estimated by improvement of zero-frequency detective quantum efficiency, DQE(0) was 10% for DCS compared to uncorrected and 10% for ACS compared to DCS. A sensitivity analysis showed that DCS generally achieved half the benefit of ACS over a range of conditions, although the benefit was markedly less if the charge cloud was instead modeled as a small sphere. ConclusionsSumming of counts after digitization may be a simpler alternative to summing of charge prior to digitization due to the relative complexity of analog circuit design. Over most conditions studied, it provides roughly half the benefit of ACS and may offer certain implementation advantages.
|
|
| 13. |
- Liu, Xuejin, et al.
(författare)
-
Count rate performance of a silicon-strip detector for photon-counting spectral CT
- 2016
-
Ingår i: Nuclear Instruments and Methods in Physics Research Section A. - : Elsevier. - 0168-9002 .- 1872-9576. ; 827, s. 102-106
-
Tidskriftsartikel (refereegranskat)abstract
- A silicon-strip detector is developed for spectral computed tomography. The detector operates in photon counting mode and allows pulse-height discrimination with 8 adjustable energy bins. In this work, we evaluate the count-rate performance of the detector in a clinical CT environment. The output counts of the detector are measured for x-ray tube currents up to 500 mA at 120 kV tube voltage, which produces a maximum photon flux of 485 Mphotons/s/mm(2) for the unattenuated beam. The corresponding maximum count-rate loss of the detector is around 30% and there are no saturation effects. A near linear relationship between the input and output count rates can be observed up to 90 Mcps/mm(2), at which point only 3% of the input counts are lost. This means that the loss in the diagnostically relevant count rate region is negligible. A semi-nonparalyzable dead-time model is used to describe the count-rate performance of the detector, which shows a good agreement with the measured data. The non-paralyzable dead time tau(n) for 150 evaluated detector elements is estimated to be 20.2 +/- 5.2 ns. (C) 2016 Elsevier B.V. All rights reserved.
|
|
| 14. |
- Sjölin, Martin, 1987-, et al.
(författare)
-
A method for geometric calibration of edge-on detectors in a CT-gantry
- 2016
-
Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405. ; 43:11, s. 6165-6174
-
Tidskriftsartikel (refereegranskat)abstract
- Purpose: Photon-counting edge-on detectors are currently being considered for usein clinical computed tomography (CT) systems. A method for geometric calibrationof edge-on detectors mounted in a CT gantry has been developed and evaluated.The method is complementary to the geometrical calibration methods developed forCT systems using flat-panel detectors and takes the extra dimension of the edge-ondetectors (along the direction of the x-rays) into account.Methods: The method uses projection images of a simple phantom together withgeometrical arguments to accurately estimate the orientation and relative position ofthe edge-on detectors. Both computer simulations and experimental measurementswere used to verify the method.Results: It is experimentally demonstrated that the method can determine the orientationof the detector with an accuracy of 0.08 degrees. The method is also shownto be insensitive to errors in the modeled parameters used in the algorithm.Conclusions: The presented method can accurately determine the orientation andrelative position of edge-on detectors mounted in a CT-gantry and can be used toevaluate the detector mounting and to produce an accurate forward model of theimaging system. Also, the method has potential to reduce the dimensionality of thegeometric calibration of the full CT system since the direction of the x-rays withrespect to the detector is measured.
|
|
| 15. |
- Sjölin, Martin, 1987-, et al.
(författare)
-
Compression of CT sinogram data by decimation in the view direction
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Purpose: In clinical computed tomography (CT), the image data is acquired during continuousrotation. If the time during which the signal is integrated (the frame time) is too long, the datais blurred in the view direction. This can be overcome by having a high angular sampling rate,but for systems with limited bandwidth, the increased amount of data can be a problem. In thispaper, we evaluate the benefit of maintaining a high angular sampling rate on the CT gantryand performing a decimation (digital low-pass filtration followed by a downsampling) in the viewdirection before the bottleneck of the data transfer chain.Methods: A theoretical evaluation of the effects of the decimation is presented and theimplementation of the digital filter is discussed. The compression scheme is evaluated on imagedata of a CATPHANR 504 phantom.Results: From a resolution point-of-view, it is beneficial to sample at more than twice theNyquist frequency (the minimum rate required for not introducing aliasing). At this samplingrate, a decimation step can reduce the amount of data by a factor of two without loosing any ofthe signal. It is demonstrated that a 2:1 compression can be achieved without compromising thespatial resolution or increasing the noise.Conclusions: In continuous rotation CT, the angular sampling rate must be high in ordernot to suffer from blur in the view direction. From a sampling point-of-view, the sampling rateis higher than necessary and the signal can be decimated (low-pass filtered and downsampled)without losing any signal. The proposed compression scheme can be implemented on the CTgantry and thus reduce the bandwidth requirements on the data transfer.
|
|
| 16. |
- Sjölin, Martin, 1987-, et al.
(författare)
-
Compression of CT sinogram data by decimation in the view direction
- 2017
-
Ingår i: Medical physics (Lancaster). - : WILEY. - 0094-2405. ; 44:9, s. E138-E146
-
Tidskriftsartikel (refereegranskat)abstract
- Pupose: In clinical computed tomography (CT), the image data is acquired during continuous rotation. If the time during which the signal is integrated (the frame time) is too long, the data is blurred in the view direction (i.e., azimuthal blur). This can be overcome by having a high angular sampling rate, but for systems with limited bandwidth, the increased amount of data can be a problem. In this paper, we evaluate the benefit of maintaining a high angular sampling rate on the CT gantry and performing a decimation (digital low-pass filtration followed by a downsampling) in the view direction before the bottleneck of the data transfer chain. Methods: A theoretical evaluation of the effects of the decimation is presented and the implementation of the digital filter is discussed. The compression scheme is evaluated on image data of a CATPHAN (R) 504 phantom and a human skull phantom. Results: It is shown that digital decimation can be used to compress data before read-out with more remaining data fidelity compared to having longer frame times. Specifically, the method is shown to preserve the detail in the reconstruction of the CATPHAN resolution patterns and the human skull phantom. It is also demonstrated that the method can be used to prevent aliasing artifacts. Conclusions: Decimation in the view direction is presented as an alternative to increasing the frame time for CT systems with limited bandwidth of the data read-out. The method can be used to either remove aliasing artifacts or preserve spatial resolution. The proposed compression scheme can be implemented on the CT gantry and thus reduce the bandwidth requirements on the data transfer.
|
|
| 17. |
|
|
| 18. |
- Sjölin, Martin, 1987-, et al.
(författare)
-
Energy calibration by simultaneous model-fitting to threshold scans for several kVp
- 2016
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Accurate calibration of the energy thresholds on spectral x-ray detectors used for computed tomography is essential for avoiding ring artifacts and bias when using basis material decomposition. Previously, it has been proposed to calibrate the energy thresholds by fitting simulated data to a threshold scan of a polychromatic spectrum from a medical x-ray tube. Here, we suggest that the energy calibration is performed by fitting simulated data to threshold scans acquired at several different kVp settings simultaneously. The method is evaluated experimentally by its capacity to correctly position the noise floor and the k-edge of tungsten. It is shown that using several kVp settings largely improves the accuracy of the energy calibration, particularly in the low energy range. Since the method only relies on the spectra from the x-ray tube (without any specific filtration), it is ideal for the use in a clinical computed tomography system. It is also shown that the method can operate with a fairly low number of detected photons, whichis promising in terms of calibration time.
|
|
| 19. |
|
|
| 20. |
|
|
| 21. |
- Sjölin, Martin, 1987-
(författare)
-
Measurement circuit for an x-ray detector, and a corresponding method and x-ray imaging system
- 2019
-
Patent (populärvet., debatt m.m.)abstract
- Disclosed is a measurement method performed by a Computed Tomography, CT, system. The CT system includes an x-ray source (60) and an x-ray detector (50) array of photon counting edge-on detectors (5), wherein each photon counting edge-on detector has a number of depth-segments, also referred to as detector elements, arranged at different spatial locations in the direction of incoming x-rays (45). The method includes to apply a time offset measurement scheme that provides a time offset between measurement periods for at least two different detector elements located at different depths, wherein the time offset is chosen so that at least two measurement periods at least partially overlaps in time. Disclosed is also a corresponding CT system (10), a control unit for a CT system and a measurement circuit for a CT system. A computer program (225) controlling a CT system is also disclosed. The disclosed technology provides for a higher sampling frequency in the angular direction.
|
|
| 22. |
|
|
| 23. |
- Sjölin, Martin, 1987-
(författare)
-
Methods of image acquisition and calibration for x-ray computed tomography
- 2016
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- X-ray computed tomography (CT) is a common medical imaging device for acquiring high-resolution 3D images of the interior of the human body. The images are formed by mathematical reconstruction from hundreds of planar x-ray images that have been acquired during less then a second.Photon-counting spectral detectors are seen by many as the next big step in the development of medical CT. The potential benefits include: quantitative CT, ultra-low dose imaging and optimal contrast-to-noise performance. The current aim for the research pursued by the Physics of Medical Imaging Group at KTH is to develop, and commercialize, a photon-counting spectral detector using silicon wafers in edge-on geometry. With the introduction of a new detector comes many challenges, some of which this Thesis aims to address.Efficient calibration schemes will be an essential part of the realization of photon-counting spectral detectors in clinical CT. In the first part of the Thesis, three calibration methods are presented: two methods for calibration of the energy thresholds on multi-bin spectral detectors and one method for geometric calibration of edge-on detectors that are mounted in a CT gantry.The CT image acquisition produces large amounts of data that have to be transported out of the system, preferably in real-time. Already today, fewer samples are acquired when operating at very high rotation speeds due to bandwidth limitations. For photon-counting spectral detectors, the amount of data will be even larger due to the additional energy information and the generally smaller pixels, and it is therefore desirable to minimize the number of angular samples acquired per revolution. In the second part of the Thesis, two methods for relaxing the angular sampling requirement are presented. The first method uses the built-in redundancy of multi-layer detectors to increase the angular sampling rate via a temporal offset between the detector layers. The second method uses decimation in the view (angular) direction as a means for compression of CT sinogram data. The compression can be performed on the CT gantry and thus lower the required bandwidth of the data transfer.Although the overall aim of this work has been to develop methods that facilitate the introduction of photon-counting spectral detectors for medical CT, the presented methods are also applicable in the broader context of calibration of x-ray detectors and CT image acquisition.
|
|
| 24. |
|
|
| 25. |
- Sjölin, Martin, 1987-, et al.
(författare)
-
Relative calibration of energy thresholds on multi-bin spectral x-ray detectors
- 2016
-
Ingår i: Nuclear Instruments and Methods in Physics Research Section A. - : Elsevier. - 0168-9002 .- 1872-9576. ; 840, s. 1-4
-
Tidskriftsartikel (refereegranskat)abstract
- Accurate and reliable energy calibration of spectral x-ray detectors used in medical imaging is essential for avoiding ring artifacts in the reconstructed images (computed tomography) and for performing accurate material basis decomposition. A simple and accurate method for relative calibration of the energy thresholds on a multi-bin spectral x-ray detector is presented. The method obtains the linear relations between all energy thresholds in a channel by scanning the thresholds with respect to each other during x-ray illumination. The method does not rely on a model of the detector's response function and does not require any identifiable features in the x-ray spectrum. Applying the same method, the offset between the thresholds can be determined also without external stimuli by utilizing the electronic noise as a source. The simplicity and accuracy of the method makes it suitable for implementation in clinical multi-bin spectral x-ray imaging systems.
|
|
| 26. |
|
|
| 27. |
|
|
| 28. |
|
|
| 29. |
- Sundberg, Christel, et al.
(författare)
-
Increased count-rate performance and dose efficiency for silicon photon-counting detectors for full-field CT using an ASIC with adjustable shaping time
- 2019
-
Ingår i: MEDICAL IMAGING 2019. - : SPIE-INT SOC OPTICAL ENGINEERING. - 9781510625440
-
Konferensbidrag (refereegranskat)abstract
- Photon-counting silicon strip detectors are attracting interest for use in next generation CT scanners. For silicon detectors, a low noise floor is necessary to obtain a good dose efficiency. A low noise floor can be achieved by having a filter with a long shaping time in the readout electronics. This also increases the pulse length, resulting in a long deadtime and thereby a degraded count-rate performance. However, as the flux typically varies greatly during a CT scan, a high count-rate capability is not required for all projection lines. It would therefore be desirable to use more than one shaping time within a single scan. To evaluate the potential benefit of using more than one shaping time, it is of interest to characterize the relation between the shaping time, the noise, and the resulting pulse shape. In this work we present noise and pulse shape measurements on a photon-counting detector with two different shaping times along with a complementary simulation model of the readout electronics. We show that increasing the shaping time from 28.1 ns to 39.4 ns decreases the noise and increases the signal-to-noise ratio (SNR) with 6.5% at low count rates and we also present pulse shapes for each shaping time as measured at a synchrotron source. Our results demonstrate that the shaping time plays an important role in optimizing the dose efficiency in a photon-counting x-ray detector.
|
|
| 30. |
- Sundberg, Christel, et al.
(författare)
-
Increasing the dose efficiency in silicon photon-counting detectors utilizing dual shapers
- 2018
-
Ingår i: Medical Imaging 2018. - : SPIE - International Society for Optical Engineering. - 9781510616363
-
Konferensbidrag (refereegranskat)abstract
- Silicon photon-counting spectral detectors are candidates for the next generation of medical CT. For silicon detectors, a low noise floor is necessary to obtain good detection efficiency. A low noise floor can be obtained by having a slow shaping filter in the ASIC, but this leads to a long dead-time, thus decreasing the count-rate performance. In this work, we evaluate the benefit of utilizing two sub-channels with different shaping times. It is shown by simulation that utilizing a dual shaper can increase the dose efficiency for equal count-rate capability by up to 17%.
|
|
