Dynamic Measurement of Arterial Liver Perfusion With an Interventional C-Arm System.

PURPOSE: Objective intraprocedural measurement of hepatic blood flow could provide a quantitative treatment end point for locoregional liver procedures. This study aims to validate the accuracy and reproducibility of cone-beam computed tomography perfusion (CBCTp) measurements of arterial liver perfusion (ALP) against clinically available computed tomography perfusion (CTp) measurements in a swine embolization model. METHODS: Triplicate CBCTp measurements using a selective arterial contrast injection were performed before and after complete embolization of the left lobe of the liver in 5 swine. Two CBCTp protocols were evaluated that differed in sweep duration (3.3 vs 4.5 seconds) and the number of acquired projection images (166 vs 248). The mean ALP was measured within identical volumes of interest selected in the embolized and nonembolized regions of the perfusion map generated from each scan. Postembolization CBCTp values were also compared with CTp measurements. RESULTS: The 2 CBCTp protocols demonstrated high concordance correlation (0.90, P < 0.001). Both CBCTp protocols showed higher reproducibility than CTp in the nontarget lobe, with an intraclass correlation of 0.90 or greater for CBCTp and 0.83 for CTp (P < 0.001 for all correlations). The ALP in the embolized lobe was nearly zero and hence excluded for reproducibility. High concordance correlation was observed between the CTp and each CBCTp protocol, with the shorter CBCTp protocol reaching a concordance correlation of 0.75 and the longer achieving 0.87 (P < 0.001 for both correlations). CONCLUSIONS: Dynamic blood flow measurement using an angiographic C-arm system is feasible and produces quantitative results comparable to CTp.

Reproducibility of Parenchymal Blood Volume Measurements Using an Angiographic C-arm CT System.

RATIONALE AND OBJECTIVES: Intra-procedural measurement of hepatic perfusion following liver embolization continues to be a challenge. Blood volume imaging before and after interventional procedures would allow identifying the treatment end point or even allow predicting treatment outcome. Recent liver oncology studies showed the feasibility of parenchymal blood volume (PBV) imaging using an angiographic C-arm system. This study was done to evaluate the reproducibility of PBV measurements using cone beam computed tomography (CBCT) before and after embolization of the liver in a swine model. MATERIALS AND METHODS: CBCT imaging was performed before and after partial bland embolization of the left lobe of the liver in five adult pigs. Intra-arterial injection of iodinated contrast with a 6-second x-ray delay was used with a two-sweep 8-second rotation imaging protocol. Three acquisitions, each separated by 10 minutes to allow for contrast clearance, were obtained before and after embolization in each animal. Post-processing was carried out using dedicated software to generate three-dimensional (3D) PBV maps. Two region-of-interest measurements were placed on two views within the right and left lobe on each CBCT 3D PBV map. Variation in PBV for scans acquired within each animal was determined by the coefficient of variation and intraclass correlation. A Wilcoxon signed-rank test was used to test post-procedure reduction in PBV. RESULTS: The CBCT PBV maps showed mean coefficients of variation of 7% (range: 2%-16%) and 25% (range: 13%-34%) for baseline and embolized PBV maps, respectively. The intraclass correlation for PBV measurements was 0.89, demonstrating high reproducibility, with measurable reduction in PBV displayed after embolization (P = 0.007). CONCLUSIONS: Intra-procedural acquisition of 3D PBV maps before and after liver embolization using CBCT is highly reproducible and shows promising application for obtaining intra-procedural PBV maps during locoregional therapy.

Time-resolved perfusion imaging at the angiography suite: preclinical comparison of a new flat-detector application to computed tomography perfusion.

OBJECTIVES: The objective of this study was to compare the parameter maps of a new flat-panel detector application for time-resolved perfusion imaging in the angiography room (FD-CTP) with computed tomography perfusion (CTP) in an experimental tumor model. MATERIALS AND METHODS: Twenty-four VX2 tumors were implanted into the hind legs of 12 rabbits. Three weeks later, FD-CTP (Artis zeego; Siemens) and CTP (SOMATOM Definition AS +; Siemens) were performed. The parameter maps for the FD-CTP were calculated using a prototype software, and those for the CTP were calculated with VPCT-body software on a dedicated syngo MultiModality Workplace. The parameters were compared using Pearson product-moment correlation coefficient and linear regression analysis. RESULTS: The Pearson product-moment correlation coefficient showed good correlation values for both the intratumoral blood volume of 0.848 (P < 0.01) and the blood flow of 0.698 (P < 0.01). The linear regression analysis of the perfusion between FD-CTP and CTP showed for the blood volume a regression equation y = 4.44x + 36.72 (P < 0.01) and for the blood flow y = 0.75x + 14.61 (P < 0.01). CONCLUSIONS: This preclinical study provides evidence that FD-CTP allows a time-resolved (dynamic) perfusion imaging of tumors similar to CTP, which provides the basis for clinical applications such as the assessment of tumor response to locoregional therapies directly in the angiography suite.

Quantitative liver tumor blood volume measurements by a C-arm CT post-processing software before and after hepatic arterial embolization therapy: comparison with MDCT perfusion.

PURPOSE: We aimed to determine whether the C-arm computed tomography (CT) blood volume (BV) imaging of hepatic tumors performed with a new prototype software is capable of measuring the BV changes in response to hepatic arterial treatments and to validate these quantitative measurements with commercially available multidetector computed tomography (MDCT) perfusion software. METHODS: A total of 34 patients with hepatic tumors who underwent either radioembolization (RE, n=21) or transarterial chemoembolization (TACE, n=13) were included in the study. Using a prototype software by Siemens Healthcare, 74 C-arm CT BV measurements were obtained in both pre- and postembolization settings (three patients had additional BV measurements before and after work-up angiography for RE). Ten of 34 patients underwent MDCT perfusion study before embolization, enabling comparison of BV measurements using C-arm CT versus MDCT methods. RESULTS: The mean BV of 14 tumor lesions in 10 patients on MDCT perfusion was highly correlated with the BV values on C-arm CT (r=0.97, P < 0.01). The BV values obtained by C-arm CT decreased from 140.6±28.3 mL/1000 mL to 45.9±23.5 mL/1000 mL after TACE (66.37% reduction) and from 175.6±29.4 mL/1000 mL to 84.1±22.5 mL/1000 mL after RE (53.75% reduction). DISCUSSION: Quantitative BV measurement with C-arm CT is well-correlated with MDCT BV measurements, and it is a promising tool to monitor perfusion changes during hepatic arterial embolization.

Denoising and artefact reduction in dynamic flat detector CT perfusion imaging using high speed acquisition: first experimental and clinical results.

Flat detector CT perfusion (FD-CTP) is a novel technique using C-arm angiography systems for interventional dynamic tissue perfusion measurement with high potential benefits for catheter-guided treatment of stroke. However, FD-CTP is challenging since C-arms rotate slower than conventional CT systems. Furthermore, noise and artefacts affect the measurement of contrast agent flow in tissue. Recent robotic C-arms are able to use high speed protocols (HSP), which allow sampling of the contrast agent flow with improved temporal resolution. However, low angular sampling of projection images leads to streak artefacts, which are translated to the perfusion maps. We recently introduced the FDK-JBF denoising technique based on Feldkamp (FDK) reconstruction followed by joint bilateral filtering (JBF). As this edge-preserving noise reduction preserves streak artefacts, an empirical streak reduction (SR) technique is presented in this work. The SR method exploits spatial and temporal information in the form of total variation and time-curve analysis to detect and remove streaks. The novel approach is evaluated in a numerical brain phantom and a patient study. An improved noise and artefact reduction compared to existing post-processing methods and faster computation speed compared to an algebraic reconstruction method are achieved.

Hepatic blood volume imaging with the use of flat-detector CT perfusion in the angiography suite: comparison with results of conventional multislice CT perfusion.

PURPOSE: To prospectively determine the feasibility of flat-detector (FD) computed tomography (CT) perfusion to measure hepatic blood volume (BV) in the angiography suite in patients with hepatocellular carcinoma (HCC). MATERIALS AND METHODS: Twenty patients with HCC were investigated with conventional multislice and FD CT perfusion. CT perfusion was carried out on a multislice CT scanner, and FD CT perfusion was performed on a C-arm angiographic system, before transarterial chemoembolization procedures. BV values of conventional and FD CT perfusion were measured within tumors and liver parenchyma. The arterial perfusion portion of CT perfusion BV was extracted from CT perfusion BV by multiplying it by a hepatic perfusion index. Relative values (RVs) for CT perfusion arterial BV and FD CT perfusion BV (FD BV) were defined by dividing BV of tumor by BV of parenchyma. Relationships between BV and RV values of these two techniques were analyzed. RESULTS: In all patients, both perfusion procedures were technically successful, and all 33 HCCs larger than 10 mm were identified with both imaging methods. There were strong correlations between the absolute values of FD BV and CT perfusion arterial BV (tumor, r = 0.903; parenchyma, r = 0.920; both P < .001). Bland-Altman analysis showed a mean difference of -0.15 ± 0.24 between RVs for CT perfusion arterial BV and FD BV. CONCLUSIONS: The feasibility of FD CT perfusion to assess BV values of liver tumor and surrounding parenchyma in the angiographic suite was demonstrated.

A prospective, multicenter pilot study investigating the utility of flat detector derived parenchymal blood volume maps to estimate cerebral blood volume in stroke patients.

PURPOSE: Newer flat panel angiographic detector (FD) systems have the capability to generate parenchymal blood volume (PBV) maps. The ability to generate these maps in the angiographic suite has the potential to markedly expedite the triage and treatment of patients with acute ischemic stroke. The present study compares FP-PBV maps with cerebral blood volume (CBV) maps derived using standard dynamic CT perfusion (CTP) in a population of patients with stroke. METHODS: 56 patients with cerebrovascular ischemic disease at two participating institutions prospectively underwent both standard dynamic CTP imaging followed by FD-PBV imaging (syngo Neuro PBV IR; Siemens, Erlangen, Germany) under a protocol approved by both institutional review boards. The feasibility of the FD system to generate PBV maps was assessed. The radiation doses for both studies were compared. The sensitivity and specificity of the PBV technique to detect (1) any blood volume deficit and (2) a blood volume deficit greater than one-third of a vascular territory, were defined using standard dynamic CTP CBV maps as the gold standard. RESULTS: Of the 56 patients imaged, PBV maps were technically adequate in 42 (75%). The 14 inadequate studies were not interpretable secondary to patient motion/positioning (n=4), an injection issue (n=2), or another reason (n=8). The average dose for FD-PBV was 219 mGy (median 208) versus 204 mGy (median 201) for CT-CBV. On CT-CBV maps 26 of 42 had a CBV deficit (61.9%) and 15 (35.7%) had a deficit that accounted for greater than one-third of a vascular territory. FD-PBV maps were 100% sensitive and 81.3% specific to detect any CBV deficit and 100% sensitive and 62.9% specific to detect any CBV deficit of greater than one-third of a territory. CONCLUSIONS: PBV maps can be generated using FP systems. The average radiation dose is similar to a standard CTP examination. PBV maps have a high sensitivity for detecting CBV deficits defined by conventional CTP. PBV maps often overestimate the size of CBV deficits. We hypothesize that the FP protocol initiates PBV imaging prior to complete saturation of the blood volume in areas perfused via indirect pathways (ie, leptomeningeal collaterals), resulting in an overestimation of CBV deficits, particularly in the setting of large vessel occlusion.

Dynamic iterative reconstruction for interventional 4-D C-arm CT perfusion imaging.

Tissue perfusion measurement using C-arm angiography systems capable of CT-like imaging (C-arm CT) is a novel technique with potentially high benefit for catheter guided treatment of stroke in the interventional suite. However, perfusion C-arm CT (PCCT) is challenging: the slow C-arm rotation speed only allows measuring samples of contrast time attenuation curves (TACs) every 5-6 s if reconstruction algorithms for static data are used. Furthermore, the peak values of the TACs in brain tissue typically lie in a range of 5-30 HU, thus perfusion imaging is very sensitive to noise. We present a dynamic, iterative reconstruction (DIR) approach to reconstruct TACs described by a weighted sum of basis functions. To reduce noise, a regularization technique based on joint bilateral filtering (JBF) is introduced. We evaluated the algorithm with a digital dynamic brain phantom and with data from six canine stroke models. With our dynamic approach, we achieve an average Pearson correlation (PC) of the PCCT canine blood flow maps to co-registered perfusion CT maps of 0.73. This PC is just as high as the PC achieved in a recent PCCT study, which required repeated injections and acquisitions.

A model based algorithm for perfusion estimation in interventional C-arm CT systems.

PURPOSE: Interventional C-arm CT imaging, today, plays an important role in the diagnosis and treatment of patients. The main part of the 3D imaging techniques, currently used in interventions, are morphological imaging techniques. So far, the ability for functional or perfusion imaging is limited, e.g., only static cerebral blood volume measurement [A. S. Ahmed, Y. Deuerling-Zheng, C. M. Strother, K. A. Pulfer, M. Zellerhoff, T. Redel, K. Royalty, D. Consigny, M. J. Lindstrom, and D. B. Niemann, "Impact of intra-arterial injection parameters on arterial, capillary, and venous time-concentration curves in a ca480 nine model," AJNR Am. J. Neuroradiol. 30, 1337-1341 (2009)] is available. The sample rate of current C-arm CT systems is not fast enough yet to measure dynamic parameters like cerebral blood flow using standard Feldkamp reconstruction. METHODS: The authors propose a reconstruction algorithm that models the time-dependent attenuation values of each voxel using a gamma-variate function. The method can be divided into a segmentation-based initialization and an iterative optimization step. For the initialization, a threshold-based segmentation of vessel, tissue, and nondynamic structures (e.g., bone and air) is performed on the filtered backprojection (FBP) reconstructions. For each of these regions, homogeneous time-attenuation curves are estimated to initialize all the voxels within the region. The scaling-factor is then adjusted for each voxel using the attenuation values of the static reconstructions. The second part of the algorithm is an iterative optimization of the gamma-variate parameters of each voxel, based on a simultaneous algebraic reconstruction technique. Within each iteration, a Levenberg optimization is applied to minimize the backprojected errors. RESULTS: The algorithm is quantitatively evaluated with simulated forward projections as well as real C-arm CT projection data. In the phantom experiments, penumbra and infarct core could be segmented with an adjusted Rand index of up to 0.95 for a noise level of 10(5) photons. Perfusion CT data sets from three patients were used to compare the iterative reconstruction approach to the interpolated FBP reconstruction using different sweep times. In their experiments, a sweep time of 4 s using iterative reconstruction would be equivalent to that using interpolated FBP with a sweep time of around 1 s. The reconstruction results of the animal study are compared to a perfusion CT acquisition, sampled with 1 frame per second. A correlation coefficient of 0.75 between the original and the reconstructed CBF-maps could be reached with the iterative approach compared to 0.56 using the interpolated FBP reconstruction. CONCLUSIONS: In their experiments, the quality of dynamic perfusion measurements was improved using the proposed reconstruction algorithm compared to static reconstruction followed by interpolation. It could be used to increase the temporal resolution of current C-arm CT system without hardware modification to make them feasible for dynamic perfusion measurement. Furthermore, radiation dose could be reduced using their method to increase temporal resolution than using static reconstruction with a higher sampling frequency.

Performance evaluation of a C-Arm CT perfusion phantom.

PURPOSE: Brain perfusion measurement in stroke patients provides important information on the infarct area and state of involved tissue. Interventional C-Arm angiography systems can provide perfusion measurements. A CT perfusion phantom was developed for C-Arm perfusion imaging to test and evaluate this method and to aid in the design and validation of scan protocols. METHODS: A phantom test device was designed based on the anatomy of the human head. Four feeding arteries divided into sixteen sub-branches that lead into a sintered board simulating brain parenchyma. Perfusion measurements were performed using two conventional clinical CT scanners as the gold standard and with a C-Arm CT system to test and compare the implementations. The phantom's input parameters, contrast medium and flow properties were varied. A cerebral perfusion deficit was simulated by occlusion of a feeding artery tube. RESULTS: CT perfusion maps of the sintered board brain tissue surrogate were computed and qualitatively compared for both conventional CT and C-Arm CT systems. A characteristic flow pattern of the tissue board was identifiable in both modalities. The characteristic flow pattern of the resulting perfusion maps is reproducible. The calculated flow and volume were directly related. CONCLUSIONS: A new CT perfusion phantom was developed and tested. This phantom is an appropriate model for CT-based tissue perfusion measurements in both conventional CT scanners and C-Arm CT scanners. The influence of input parameter changes can be visualized. Perfusion deficits after occlusion of a feeding artery are readily simulated and identified with CT.

Interventional 4-D C-arm CT perfusion imaging using interleaved scanning and partial reconstruction interpolation.

Tissue perfusion measurement during catheter-guided stroke treatment in the interventional suite is currently not possible. In this work, we present a novel approach that uses a C-arm angiography system capable of computed tomography (CT)-like imaging (C-arm CT) for this purpose. With C-arm CT one reconstructed volume can be obtained every 4-6 s which makes it challenging to measure the flow of an injected contrast bolus. We have developed an interleaved scanning (IS) protocol that uses several scan sequences to increase temporal sampling. Using a dedicated 4-D reconstruction approach based on partial reconstruction interpolation (PRI) we can optimally process our data. We evaluated our combined approach (IS-PRI) with simulations and a study in five healthy pigs. In our simulations, the cerebral blood flow values (unit: ml/100 g/min) were 60 (healthy tissue) and 20 (pathological tissue). For one scan sequence the values were estimated with standard deviations of 14.3 and 2.9, respectively. For two interleaved sequences the standard deviations decreased to 3.6 and 1.5, respectively. We used perfusion CT to validate the in vivo results. With two interleaved sequences we achieved promising correlations ranging from r=0.63 to r=0.94. The results suggest that C-arm CT tissue perfusion imaging is feasible with two interleaved scan sequences.

A model for filtered backprojection reconstruction artifacts due to time-varying attenuation values in perfusion C-arm CT.

Filtered backprojection is the basis for many CT reconstruction tasks. It assumes constant attenuation values of the object during the acquisition of the projection data. Reconstruction artifacts can arise if this assumption is violated. For example, contrast flow in perfusion imaging with C-arm CT systems, which have acquisition times of several seconds per C-arm rotation, can cause this violation. In this paper, we derived and validated a novel spatio-temporal model to describe these kinds of artifacts. The model separates the temporal dynamics due to contrast flow from the scan and reconstruction parameters. We introduced derivative-weighted point spread functions to describe the spatial spread of the artifacts. The model allows prediction of reconstruction artifacts for given temporal dynamics of the attenuation values. Furthermore, it can be used to systematically investigate the influence of different reconstruction parameters on the artifacts. We have shown that with optimized redundancy weighting function parameters the spatial spread of the artifacts around a typical arterial vessel can be reduced by about 70%. Finally, an inversion of our model could be used as the basis for novel dynamic reconstruction algorithms that further minimize these artifacts.

Flat-detector computed tomography in the assessment of intracranial stents: comparison with multi detector CT and conventional angiography in a new animal model.

OBJECTIVE: Careful follow up is necessary after intracranial stenting because in-stent restenosis (ISR) or residual stenosis (RS) is not rare. A minimally invasive follow-up imaging technique is desirable. The objective was to compare the visualisation of stents in Flat Detector-CT Angiography (FD-CTA) after intravenous contrast medium injection (i.v.) with Multi Detector Computed Tomography Angiography (MD-CTA) and Digital Subtracted Angiography (DSA) in an animal model. METHODS: Stents were implanted in the carotid artery of 12 rabbits. In 6 a residual stenosis (RS) was surgically created. Imaging was performed using FD-CTA, MD-CTA and DSA. Measurements of the inner and outer diameter and cross-section area of the stents were performed. Stenosis grade was calculated. RESULTS: In subjective evaluation FD-CTA was superior to MD-CTA. FD-CTA was more accurate compared with DSA than MD-CTA. Cross-sectional area of the stent lumen was significantly larger (p < 0.05) in FD-CTA in comparison to MD-CTA. Accurate evaluation of stenosis was impossible in MD-CTA. There was no statistically significant difference in the stenosis grade of DSA and FD-CTA. CONCLUSION: Our results show that visualisation of stent and stenosis using intravenous FD-CTA compares favourably with DSA and may replace DSA in the follow-up of patients treated with intracranial stents.

Cerebral blood volume imaging by flat detector computed tomography in comparison to conventional multislice perfusion CT.

OBJECTIVE: We tested the hypothesis that Flat Detector computed tomography (FD-CT) with intravenous contrast medium would allow the calculation of whole brain cerebral blood volume (CBV) mapping (FD-CBV) and would correlate with multislice Perfusion CT (PCT). METHODS: Twenty five patients were investigated with FD-CBV and PCT. Correlation of the CBV maps of both techniques was carried out with measurements from six anatomical regions from both sides of the brain. Mean values of each region and the correlation coefficient were calculated. Bland-Altman analysis was performed to compare the two different imaging techniques. RESULTS: The image and data quality of both PCT and FD-CBV were suitable for evaluation in all patients. The mean CBV values of FD-CBV and PCT showed only minimal differences with overlapping standard deviation. The correlation coefficient was 0.79 (p < 0.01). Bland-Altman analysis showed a mean difference of -0.077 ± 0.48 ml/100 g between FD-CBV and PCT CBV measurements, indicating that FD-CBV values were only slightly lower than those of PCT. CONCLUSION: CBV mapping with intravenous contrast medium using Flat Detector CT compared favourably with multislice PCT. The ability to assess cerebral perfusion within the angiographic suite may improve the management of ischaemic stroke and evaluation of the efficacy of dedicated therapies.

Optimized intravenous Flat Detector CT for non-invasive visualization of intracranial stents: first results.

OBJECTIVE: As stents for treating intracranial atherosclerotic stenosis may develop in-stent re-stenosis (ISR) in up to 30%, follow-up imaging is mandatory. Residual stenosis (RS) is not rare. We evaluated an optimised Flat Detector CT protocol with intravenous contrast material application (i.v. FD-CTA) for non-invasive follow-up. METHODS: In 12 patients with intracranial stents, follow-up imaging was performed using i.v. FD-CTA. MPR, subtracted MIP and VRT reconstructions were used to correlate to intra-arterial angiography (DSA). Two neuroradiologists evaluated the images in anonymous consensus reading and calculated the ISR or RS. Correlation coefficients and a Wilcoxon test were used for statistical analysis. RESULTS: In 4 patients, no stenosis was detected. In 6 patients RS and in two cases ISR by intima hyperplasia perfectly visible on MPR reconstructions of i.v. FD-CTA were detected. Wilcoxon's test showed no significant differences between the methods (p > 0.05). We found a high correlation with coefficients of the pairs DSA/ FD-CT MIP r = 0.91, DSA/ FD-CT MPR r = 0.82 and FD-CT MIP/ FD-CT MPR r = 0.8. CONCLUSION: Intravenous FD-CTA could clearly visualise the stent and the lumen, allowing ISR or RS to be recognised. FD-CTA provides a non-invasive depiction of intracranial stents and might replace DSA for non-invasive follow-up imaging.

Motion compensation in digital subtraction angiography using graphics hardware.

An inherent disadvantage of digital subtraction angiography (DSA) is its sensitivity to patient motion which causes artifacts in the subtraction images. These artifacts could often reduce the diagnostic value of this technique. Automated, fast and accurate motion compensation is therefore required. To cope with this requirement, we first examine a method explicitly designed to detect local motions in DSA. Then, we implement a motion compensation algorithm by means of block matching on modern graphics hardware. Both methods search for maximal local similarity by evaluating a histogram-based measure. In this context, we are the first who have mapped an optimizing search strategy on graphics hardware while paralleling block matching. Moreover, we provide an innovative method for creating histograms on graphics hardware with vertex texturing and frame buffer blending. It turns out that both methods can effectively correct the artifacts in most case, as the hardware implementation of block matching performs much faster: the displacements of two 1024 x 1024 images can be calculated at 3 frames/s with integer precision or 2 frames/s with sub-pixel precision. Preliminary clinical evaluation indicates that the computation with integer precision could already be sufficient.