A Review of Dual Energy CT: Principles, Applications, and Future Outlook (Chinese Journal of) CT Theory and Applications
Wang, A., S., Hsieh, S., S., Pelc, N., J.
2012; 3 (21): 367-386
The contrast properties of computed tomography (CT) depend significantly on the X-ray energy spectrum used to measure the object. Conventional CT uses only a single energy spectrum and at times suffers from ambiguity so that two different materials can appear identical. Dual energy CT (DECT) uses two different energy spectra that can be used to remove this ambiguity. Although the basic concept of DECT is not new, its popularity has recently skyrocketed because of the availability of commercial implementations. We review the basic principles of DECT physics and estimation, as well as the technologies that have enabled DECT to enter modern clinical imaging. Applications of DECT abound, and we familiarize the reader with a selection of the most common clinical applications. Finally, we conclude by touching on two areas of current technical development: photon-counting detectors and synthetic CT.
Synthetic CT: Simulating low dose single and dual energy protocols from a dual energy scan MEDICAL PHYSICS
Wang, A. S., Pelc, N. J.
2011; 38 (10): 5551-5562
The choice of CT protocol can greatly impact patient dose and image quality. Since acquiring multiple scans at different techniques on a given patient is undesirable, the ability to predict image quality changes starting from a high quality exam can be quite useful. While existing methods allow one to generate simulated images of lower exposure (mAs) from an acquired CT exam, the authors present and validate a new method called synthetic CT that can generate realistic images of a patient at arbitrary low dose protocols (kVp, mAs, and filtration) for both single and dual energy scans.The synthetic CT algorithm is derived by carefully ensuring that the expected signal and noise are accurate for the simulated protocol. The method relies on the observation that the material decomposition from a dual energy CT scan allows the transmission of an arbitrary spectrum to be predicted. It requires an initial dual energy scan of the patient to either synthesize raw projections of a single energy scan or synthesize the material decompositions of a dual energy scan. The initial dual energy scan contributes inherent noise to the synthesized projections that must be accounted for before adding more noise to simulate low dose protocols. Therefore, synthetic CT is subject to the constraint that the synthesized data have noise greater than the inherent noise. The authors experimentally validated the synthetic CT algorithm across a range of protocols using a dual energy scan of an acrylic phantom with solutions of different iodine concentrations. An initial 80/140 kVp dual energy scan of the phantom provided the material decomposition necessary to synthesize images at 100 kVp and at 120 kVp, across a range of mAs values. They compared these synthesized single energy scans of the phantom to actual scans at the same protocols. Furthermore, material decompositions of a 100/120 kVp dual energy scan are synthesized by adding correlated noise to the initial material decompositions. The aforementioned noise constraint also allows us to compute feasible mAs values that can be synthesized for each kVp.The single energy synthesized and actual reconstructed images exhibit identical signal and noise properties at 100 kVp and at 120 kVp, and across a range of mAs values. For example, the noise in both the synthesized and actual images at 100 kVp increases by 2 when the mAs is halved. The synthesized and actual material decompositions of a dual energy protocol show excellent agreement when the decomposition images are linearly weighted to form monoenergetic images at energies from 40 to 100 keV. For simulated single energy protocols with kVp between 80 and 140, the highest feasible mAs exceeds that of either initial scan.This work describes and validates the synthetic CT theory and algorithm by comparing its results to actual scans. Synthetic CT is a powerful new tool that allows users to realistically see how protocol selection affects CT images and enables radiologists to retrospectively identify the lowest dose protocol achievable that provides diagnostic quality images on real patients.
Effect of detector lag on CT noise power spectra MEDICAL PHYSICS
Baek, J., Pelc, N. J.
2011; 38 (6): 2995-3005
The authors examined the effect of detector lag on the noise power spectrum (NPS) of CT images reconstructed with filtered backprojection (FBP).The authors derived an analytical expression of the NPS with detector lag, and then verified it using computer simulations with parallel beam and fan beam geometries. The dependence of the NPS on the amount of lag, location within the scanned field of view (FOV), and the number of views used in the reconstruction (samples per rotation) was investigated using constant and view dependent noise in the raw data.Detector lag introduces noise correlation in the azimuthal direction. The effect on the NPS is a frequency dependent reduction in amplitude. In small regions of the image, the effect is primarily in the frequencies corresponding to the azimuthal direction. The noise blurring and NPS filtering increases with increasing radial distance, and therefore regions at larger radial distances have lower noise power. With the same detector lag response function, the amount of noise correlation and NPS filtering decreases with increasing number of views.The shape of the NPS depends on the detector lag coefficients, location of the region, and the number of views used in the reconstruction. In general, the noise correlation caused by detector lag decreased the amplitude of the NPS.
Local and global 3D noise power spectrum in cone-beam CT system with FDK reconstruction MEDICAL PHYSICS
Baek, J., Pelc, N. J.
2011; 38 (4): 2122-2131
The authors examine the nonstationary noise behavior of a cone-beam CT system with FDK reconstruction.To investigate the nonstationary noise behavior, an analytical expression for the NPS of local volumes and an entire volume was derived and quantitatively compared to the NPS estimated from experimental air and water images.The NPS of local volumes at different locations along the z-axis showed radial symmetry in the f(x)-f(y) plane and different missing cone regions in the f(z) direction depending on the tilt angle of rays through the local volumes. For local volumes away from the z-axis, the NPS of air and water images showed sharp transitions in the f(x)-f(y) and f(y)-f(z) planes and lack of radial symmetry in the f(x)-f(y) plane. These effects are mainly caused by varying magnification and different noise levels from view to view. In the NPS of the entire volume, the f(x)-f(y) plane showed radial symmetry because the nonstationary noise behaviors of local volumes were averaged out. The nonstationary sharp transitions were manifested as a high-frequency roll-off.The results from noise power analysis for local volumes and an entire volume demonstrate the spatially varying noise behavior in the reconstructed cone-beam CT images.
Sufficient Statistics as a Generalization of Binning in Spectral X-ray Imaging IEEE TRANSACTIONS ON MEDICAL IMAGING
Wang, A. S., Pelc, N. J.
2011; 30 (1): 84-93
It is well known that the energy dependence of X-ray attenuation can be used to characterize materials. Yet, even with energy discriminating photon counting X-ray detectors, it is still unclear how to best form energy dependent measurements for spectral imaging. Common ideas include binning photon counts based on their energies and detectors with both photon counting and energy integrating electronics. These approaches can be generalized to energy weighted measurements, which we prove can form a sufficient statistic for spectral X-ray imaging if the weights used, which we term ?-weights, are basis attenuation functions that can also be used for material decomposition. To study the performance of these different methods, we evaluate the Cramér-Rao lower bound (CRLB) of material estimates in the presence of quantum noise. We found that the choice of binning and weighting schemes can greatly affect the performance of material decomposition. Even with optimized thresholds, binning condenses information but incurs penalties to decomposition precision and is not robust to changes in the source spectrum or object size, although this can be mitigated by adding more bins or removing photons of certain energies from the spectrum. On the other hand, because ?-weighted measurements form a sufficient statistic for spectral imaging, the CRLB of the material decomposition estimates is identical to the quantum noise limited performance of a system with complete energy information of all photons. Finally, we show that ?-weights lead to increased conspicuity over other methods in a simulated calcium contrast experiment.
An inverse geometry CT system with stationary source arrays MEDICAL IMAGING 2011: PHYSICS OF MEDICAL IMAGING
Hsieh, S. S., Heanue, J. A., Funk, T., Hinshaw, W. S., Pelc, N. J.
Traditional CT systems face a tradeoff between temporal resolution, volumetric coverage and cone beam artifacts and also have limited ability to customize the distribution of incident x-rays to the imaging task. Inverse geometry CT (IGCT) can overcome some of these limitations by placing a small detector opposite a large, rotating scanned source array. It is difficult to quickly rotate this source array to achieve rapid imaging, so we propose using stationary source arrays instead and investigate the feasibility of such a system. We anticipate that distinct source arrays will need to be physically separated, creating gaps in the sinogram. Symmetry can be used to fill the missing rays except those connecting gaps. With three source arrays, a large triangular field of view emerges. As the small detector orbits the patient, each source spot must be energized at multiple specifically designed times to ensure adequate sampling. A timing scheme is proposed that avoids timing clashes, efficiently uses the detector, and allows for simple collimation. The two-dimensional MTF, noise characteristics, and artifact levels are all found to be comparable to parallel-beam systems. A complete, 100 millisecond volumetric scan may be feasible.
Synthetic CT: simulating arbitrary low dose single and dual energy protocols MEDICAL IMAGING 2011: PHYSICS OF MEDICAL IMAGING
Wang, A. S., Pelc, N. J
CT protocol selection (kVp, mAs, filtration) can greatly affect the dose delivered to a patient and the quality of the resulting images. While it is imperative to get diagnostic quality images from a study, the dose to the patient should be minimized. With synthetic CT, protocol optimization is made simple by simulating realistic scans of arbitrary low dose protocols from a previously acquired dual energy scan. For single energy protocols, the simulated projections have the same statistical properties as projections from an actual scan. The reconstruction of these synthesized projections then provides realistic images at a different protocol. For dual energy protocols, the material decomposition of the simulated protocol is directly synthesized. Moreover, the dose distribution from an arbitrary protocol (single or dual energy) can be found and used in conjunction with the predicted image quality for protocol design. We demonstrate single energy synthetic CT on a clinical study by synthesizing a 120 kVp image from a dual energy dataset. The synthesized image is compared to an actual 120 kVp image on the same patient, showing excellent agreement. We also describe a framework for implementing synthetic CT in software that is intuitive to use and allows radiologists to see the impact of protocol selection on image quality and dose distribution. A simple GUI demonstrates the vision for synthetic CT by allowing for the comparison of several dose reduction techniques: filtration, mA modulation, partial scan, or shielding. In particular, objects such as a breast shield can be simulated and virtually inserted as part of the original scan. In each case, the kVp and mAs can be adjusted while the synthesized image and dose profile are updated in real-time. With such software, synthetic CT can be applied as an educational and scientific tool for radiologists concerned with dose and image quality.
A comparison of four algorithms for metal artifact reduction in CT imaging MEDICAL IMAGING 2011: PHYSICS OF MEDICAL IMAGING
Golden, C., Mazin, S. R., Boas, F. E., Tye, G., Ghanouni, P., Gold, G., Sofilos, M., Pelc, N. J.
Streak artifacts caused by the presence of metal have been a significant problem in CT imaging since its inception in 1972. With the fast evolving medical device industry, the number of metal objects implanted in patients is increasing annually. This correlates directly with an increased likelihood of encountering metal in a patient CT scan, thus necessitating the need for an effective and reproducible metal artifact reduction (MAR) algorithm. Previous comparisons between MAR algorithms have typically only evaluated a small number of patients and a limited range of metal implants. Although the results of many methods are promising, the reproducibility of these results is key to providing more tangible evidence of their effectiveness. This study presents a direct comparison between the performances, assessed by board certified radiologists, of four MAR algorithms: 3 non-iterative and one iterative method, all applied and compared to the original clinical images. The results of the evaluation indicated a negative mean score in almost all uses for two of the non-iterative methods, signifying an overall decrease in the diagnostic quality of the images, generally due to perceived loss of detail. One non-iterative algorithm showed a slight improvement. The iterative algorithm was superior in all studies by producing a considerable improvement in all uses.
Use of sphere phantoms to measure the 3D MTF of FDK reconstructions MEDICAL IMAGING 2011: PHYSICS OF MEDICAL IMAGING
Baek, J., Pelc, N. J.
To assess the resolution performance of modern CT scanners, a method to measure the 3D MTF is needed. Computationally, a point object is an ideal test phantom but is difficult to apply experimentally. Recently, Thornton et al. described a method to measure the directional MTF using a sphere phantom. We tested this method for FDK reconstructions by simulating a sphere and a point object centered at (0.01 cm , 0.01 cm, 0.01 cm) and (0.01 cm, 0.01 cm, 10.01 cm) and compared the directional MTF estimated from the reconstructed sphere with that measured from an ideal point object. While the estimated MTF from the sphere centered at (0.01 cm , 0.01 cm, 0.01 cm) showed excellent agreement with that from the point object, the estimated MTF from a sphere centered at (0.01 cm , 0.01 cm, 10.01 cm) had significant errors, especially along the fz axis. We found that this is caused by the long tails of the impulse response of the FDK reconstruction far off the central plane. We developed and tested a new method to estimate the directional MTF using the sphere data. The new method showed excellent agreement with the MTF from an ideal point object. Caution should be used when applying the original method in cases where the impulse response may be wide.
Frequency-Combined Extended 3D Reconstruction for Multiple Circular Cone-Beam CT Scans 2011 IEEE NUCLEAR SCIENCE SYMPOSIUM AND MEDICAL IMAGING CONFERENCE (NSS/MIC)
Grimmer, R., Baek, J., Pelc, N., Kachelriess, M.
In circular cone-beam CT a single circle scan often does not cover the complete z-range of interest. If this is the case, two or more circle scans are acquired. The standard combination of the separate reconstructions has two disadvantages: 1) The reconstructable volume is smaller than possible, thus dose remains unused and the noise level is higher than necessary. 2) The cone-beam artifacts are increased at the edges of the partial volumes which have a large distance to the midplanes. To overcome these disadvantages we developed a method that simultaneously reconstructs all circle scans and thereby is able to reconstruct larger segments from each circle scan on the one hand and that reduces cone-beam artifacts by combining the segments in frequency domain on the other hand. The proposed method was evaluated using a simulation study as well as measured data from different flat-panel cone-beam CT scanners, as for example the Varian OBI scanner. In the example geometry we used the maximal reconstructable z-range can be increased by 25% and our approach additionally leads to a noise reduction of about 40% in the overlap region. Regarding the cone-beam artifacts, we were able to reduce the artifact level to a value as low as achievable by more complex algorithms that perform a voxel-wise weighted backprojection to favor voxels seen under small cone-angles. This paper demonstrates that the proposed algorithm is able to significantly improve the reconstruction of sequence scans while the reconstruction time is kept equivalent to the standard approach. We further demonstrate that the method can be used in clinical practice.
For publications before 2011, view Dr. Pelc's publication page.