Hybrid spectral CT reconstruction

Darin P. Clark, Cristian T. Badea, Yuanquan Wang
2017 PLoS ONE  
Current photon counting x-ray detector (PCD) technology faces limitations associated with spectral fidelity and photon starvation. One strategy for addressing these limitations is to supplement PCD data with high-resolution, low-noise data acquired with an energy-integrating detector (EID). In this work, we propose an iterative, hybrid reconstruction technique which combines the spectral properties of PCD data with the resolution and signal-to-noise characteristics of EID data. Our hybrid
more » ... truction technique is based on an algebraic model of data fidelity which substitutes the EID data into the data fidelity term associated with the PCD reconstruction, resulting in a joint reconstruction problem. Within the split Bregman framework, these data fidelity constraints are minimized subject to additional constraints on spectral rank and on joint intensity-gradient sparsity measured between the reconstructions of the EID and PCD data. Following a derivation of the proposed technique, we apply it to the reconstruction of a digital phantom which contains realistic concentrations of iodine, barium, and calcium encountered in small-animal micro-CT. The results of this experiment suggest reliable separation and detection of iodine at concentrations ! 5 mg/ml and barium at concentrations ! 10 mg/ml in 2-mm features for EID and PCD data reconstructed with inherent spatial resolutions of 176 μm and 254 μm, respectively (point spread function, FWHM). Furthermore, hybrid reconstruction is demonstrated to enhance spatial resolution within material decomposition results and to improve low-contrast detectability by as much as 2.6 times relative to reconstruction with PCD data only. The parameters of the simulation experiment are based on an in vivo micro-CT experiment conducted in a mouse model of soft-tissue sarcoma. Material decomposition results produced from this in vivo data demonstrate the feasibility of distinguishing two K-edge contrast agents with a spectral separation on the order of the energy resolution of the PCD hardware. OPEN ACCESS Citation: Clark DP, Badea CT (2017) Hybrid spectral CT reconstruction. PLoS ONE 12 (7) : e0180324. https://doi.org/10.many diagnostic imaging applications, including the characterization of vascular diseases, lung perfusion and ventilation, and kidney stones [1] . Preclinically, many translational applications of EID-based spectral CT have been developed, including the characterization of myocardial infarction [2], atherosclerotic plaque composition [3] , and tumor aggressiveness and therapy response in primary sarcoma tumors [4] and in lung cancer [5] . These preclinical applications typically use one (or more) contrast agent(s) based on a heavy metal (e.g. iodine, barium, gold) which can be semi-quantitatively separated from soft tissues based on its spectral signature [6, 7] . Despite these promising applications, future advancements in spectral CT imaging and quantitative material differentiation are largely limited by the spectral sensitivity of EIDs. Nominally, photon counting x-ray detector (PCD) technology delivers superior spectral sensitivity to EIDs by binning incoming x-ray photons as a function of their energy, achieving detector-centric spectral differentiation with a single, polychromatic source spectrum. Current PCD hardware uses a semiconductor sensor (e.g. silicon, CdTe, CdZnTe) which absorbs energy from incident x-rays through scattering and absorption interactions. Free electrons within the semiconductor material, which are produced by these interactions, are read out by a pixelated anode. The resulting electrical signal is thresholded based on its amplitude as an indirect measure of the energy of the incident x-ray photon. Unfortunately, the recorded energy of x-ray photons is often distorted due to technical challenges associated with the readout electronics and to physical phenomena such as charge sharing, pulse pileup, and K-escape [8] . Addressing these challenges and meeting the design goals of specific clinical and preclinical imaging applications has led to a number detector designs. For instance, a silicon-based PCD with 50 μm 2 pixels and slits to reduce scatter has been integrated into a commercial mammography system (MicroDose mammography; Koninklijke Philips N.V., Amsterdam, Netherlands). More recently, a dual-source clinical CT scanner produced by Siemens (Munich, Germany) and installed at the Mayo Clinic (Rochester, MN) was modified, replacing one of its EIDs with a PCD. The CdTe-based photon counting detector has 0.5-mm detector pixels (effective size, z dimension) and covers a 275-mm field of view. Preliminary cadaver studies performed with this hybrid clinical scanner demonstrated improved signal-to-noise characteristics and reduced beam-hardening and calcium-blooming artifacts relative to clinical EID data at diagnostically relevant tube currents and kVp settings [9] . PCD-based preclinical CT, which we focus on in this work, requires much higher spatial resolution than clinical CT imaging (micro-CT; typical voxel sizes:~5-100 μm 3 ). When highresolution imaging is performed with a low-power, micro-focus x-ray source and a small detector pixel pitch, common PCD issues associated with count-rate performance and charge collection efficiency can be mitigated. However, PCD-based micro-CT suffers from high noise levels due to photon starvation encountered with small pixels and energy binning. Spectral distortions due to charge sharing between small detector pixels (<<0.5 mm 2 ) are also a significant [8] . It follows that PCD-based micro-CT system design requires a balance between design parameters related to these issues to achieve an optimum level of performance within the resolution constraints. Currently, there is one commercially available PCD-based micro-CT system, the Medipix All Resolution System (MARS Bioimaging Ltd.; Christchurch, New Zealand) [10] . The MARS is based on the Medipix3 detector chip developed at CERN (Geneva, Switzerland). In addition to its 55 μm 2 (110 μm 2 ) pixel pitch, a unique feature of the Medipix3 detector is its charge-summing circuitry which can be used to compensate for charge sharing between neighboring detector pixels [11] . Several other custom-built, PCD-based micro-CT scanners have been demonstrated [12] [13] [14] [15] [16] . In both the clinical and preclinical arenas, hardware development will continue to improve semiconductor readouts, count-rate performance, field-of-view coverage, etc.; however, fundamental issues such as photon starvation, quantitative material decomposition accuracy, and
doi:10.1371/journal.pone.0180324 pmid:28683124 pmcid:PMC5500339 fatcat:qkm25ykjfrbezg4qztbclpqtcq