Andreas Korn

Sinogram affirmed iterative reconstruction in head CT: Improvement of objective and subjective image quality with concomitant radiation dose reduction

PURPOSE Iterative reconstruction has recently been revisited as a promising concept for substantial CT dose reduction. The purpose of this study was to assess the potential benefit of sinogram affirmed iterative reconstruction (SAFIRE) in head CT by comparing objective and subjective image quality at reduced tube current with standard dose filtered back projection (FBP). MATERIALS AND METHODS Non-contrast reduced dose head CT (255 mAs, CTDIvol 47.8 mGy) was performed in thirty consecutive patients and reconstructed with SAFIRE and FBP. Images were assessed in terms of quantitative and qualitative image quality and compared with FBP of standard dose acquisitions (320 mAs, CTDI vol 59.7 mGy). RESULTS In reduced dose CT examinations, use of SAFIRE versus FBP resulted in 47% increase in contrast-to-noise ratio (CNR) (2.49 vs. 1.69; p<0.0001). While reduction of tube current was associated with 13% decrease in CNR, quantitative degradation of image quality at lower dose was more than compensated through SAFIRE (2.49 vs. 1.96; p=0.0004). Objective measurements of image sharpness were comparable between FBP and SAFIRE reconstructions (575.9 ± 74.1 vs. 583.4 ± 74.7 change in HU/Pixel; p=0.28). Compared to standard dose FBP, subjective grading of noise as well as overall image quality scores were significantly improved when SAFIRE was used in reduced dose exams (1.3 vs. 1.6, p=0.006; 1.3 vs. 1.7, p=0.026). CONCLUSION At 20% dose reduction, reconstruction of head CT by SAFIRE provides above standard objective and subjective image quality, suggesting potential for more vigorous dose savings in neuroradiology CT applications.

Clinical Evaluation and Potential Radiation Dose Reduction of the Novel Sinogram-affirmed Iterative Reconstruction Technique (SAFIRE) in Abdominal Computed Tomography Angiography

RATIONALE AND OBJECTIVES Computed tomographic angiography is the standard in routine follow-up after endovascular aneurysm repair, causing radiation exposure; thus, dose-saving strategies should be applied. The aim of this study was to evaluate the novel sinogram-affirmed iterative reconstruction (SAFIRE) algorithm in terms of clinical usability and potential reduction of radiation exposure. MATERIALS AND METHODS Forty-six patients underwent computed tomographic angiographic follow-up after endovascular aneurysm repair. Data were acquired using a dual-source computed tomographic scanner running both x-ray tubes at the same voltage (120 kV). Raw data were reconstructed using projections of both tubes with filtered back projection (FBP) and of only one tube with FBP and SAFIRE, corresponding to synthetic acquisition with half the radiation dose of the clinical routine radiation dose. Image sets were objectively compared regarding signal-to-noise ratio and edge sharpness. Two radiologists independently assessed a set of subjective criteria, including diagnostic usability, depiction of contrasted vessels, and image noise. RESULTS Half-dose (HD) SAFIRE images showed significantly higher signal-to-noise ratios compared to full-dose FBP images (P < .001), while having equal edge sharpness (P = .56). Most of the subjectively assessed parameters, such as diagnostic usability and depiction of contrasted vessels, were rated similar for HD SAFIRE and full-dose FBP images. Full-dose FBP images depicted fine anatomic structures more clearly (P < .05), while HD SAFIRE data sets showed less noise (P < .01). HD FBP images performed worse on all criteria (P < .001). Interrater agreement was good (κ = 0.74-0.80). CONCLUSIONS Using the SAFIRE algorithm, the radiation dose of high-contrast abdominal computed tomographic angiography is reducible from routine clinical levels by up to 50% while maintaining good image quality and diagnostic accuracy.

Assessment of the hepatic veins in poor contrast conditions using dual energy CT: evaluation of a novel monoenergetic extrapolation software algorithm.

PURPOSE To evaluate a novel monoenergetic post-processing algorithm (MEI+) in patients with poor intrahepatic contrast enhancement. MATERIALS AND METHODS 25 patients were retrospectively included in this study. Late-phase imaging of the upper abdomen, which was acquired in dual-energy mode (100/140 kV), was used as a model for poor intrahepatic contrast enhancement. Traditional monoenergetic images (MEI), linearly weighted mixed images with different mixing ratios (MI), sole 100 and 140 kV and MEI+ images were calculated. MEI+ is a novel technique which applies frequency-based mixing of the low keV images and an image of optimal keV from a noise perspective to combine the benefits of both image stacks. The signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) of the intrahepatic vasculature (IHV) and liver parenchyma (LP) were objectively measured and depiction of IHV was subjectively rated and correlated with portal venous imaging by two readers in consensus. RESULTS MEI+ was able to increase the SNR of the IHV (5.7 ± 0.4 at 40keV) and LP (4.9 ± 1.0 at 90keV) and CNR (2.1 ± 0.6 at 40keV) greatly compared to MEI (5.1 ± 1.1 at 80keV, 4.7 ± 1.0 at 80keV, 1.0 ± 0.4 at 70keV), MI (5.2 ± 1.1 M5:5, 4.8 ± 1.0 M5:5, 1.0 ± 3.5 M9:1), sole 100 kV images (4.4 ± 1.0, 3.7 ± 0.8, 1.0 ± 0.3) and 140 kV images (2.8 ± 0.5, 3.1 ± 0.6, 0.1 ± 0.2). Subjective assessment rated MEI+ of virtual 40 keV superior to all other images. CONCLUSION MEI+ is a very promising algorithm for monoenergetic extrapolation which is able to overcome noise limitations associated with traditional monoenergetic techniques at low virtual keV levels and consequently does not suffer from a decline of SNR and CNR at low keV values. This algorithm allows an improvement of IHV depiction in the presence of poor contrast. KEY POINTS • The evaluated new image-based algorithm for virtual monoenergetic imaging allows calculating low virtual keV images from dual energy datasets with significantly improved contrast-to-noise ratios. • The image based novel monoenergetic extrapolation algorithm applies frequency-based mixing of the low keV images and an image of optimal keV from a noise perspective to combine the benefits of both image stacks.• When compared to traditional monoenergetic images, the novel monoenergetic algorithm has improved contrast-to-noise ratios for both low and high virtual keV images.• Contrast-enhanced dual energy images with poor contrast conditions can be significantly improved, e.g. late phase imaging of the liver.

Automatic Lumen Segmentation in Calcified Plaques: Dual-Energy CT Versus Standard Reconstructions in Comparison With Digital Subtraction Angiography

OBJECTIVE Dual-energy CT has the potential to automatically remove calcified plaques from angiographic data sets. The objective of this study is to compare the accuracy of visual grading of stenoses after plaque removal with visual grading in standard reconstructions. Digital subtraction angiography (DSA) was used as a reference standard. SUBJECTS AND METHODS Twenty-five patients underwent dual-energy CT (140 kV and 80 mAs; 80 kV and 234 mAs) angiography and DSA. Plaque and bone removal was performed. Twenty-nine calcified stenoses were quantified using standard reconstructions, plaque and bone removal maximum intensity projections after plaque and bone removal, and DSA images, according to the North American Symptomatic Carotid Endarterectomy Trial criteria. The accuracy of the detection of relevant stenoses (> 70%) and occlusions was assessed. Correlation coefficients of the grades of stenoses with DSA were calculated. The influence of vessel enhancement on the accuracy of plaque removal was analyzed. RESULTS The average postprocessing time was 45 seconds. After plaque removal, all 25 relevant and four nonrelevant stenoses were correctly detected. Six relevant stenoses were overestimated as complete occlusions. With the standard reconstructions, two nonrelevant stenoses were overestimated as relevant. Correlation coefficients (r(2)) for the grading of stenoses after plaque removal and with standard reconstructions versus DSA were 0.7694 and 0.4329, respectively. Vessel contrast enhancement correlated weakly (r(2) = 0.2072) with the accuracy of plaque removal. CONCLUSION Dual-energy CT with plaque removal automatically delivers CT luminograms with a high sensitivity for the detection of relevant stenoses and a higher correlation to DSA than standard reconstructions but frequently leads to an overestimation of high-grade stenoses as occlusions. Thus, dual-energy CT plaque and bone removal should be used complementary to standard reconstructions, and not exclusively.

Dual energy CTA of the carotid bifurcation: Advantage of plaque subtraction for assessment of grade of the stenosis and morphology

OBJECTIVES Calcifications adjacent to the vessel lumen often limit the assessment of stenoses at the carotid bifurcation in 3D multi intensity projection images (3D-MIP) using conventional single energy CT. Aim of the study was to evaluate the diagnostic value of 3D-MIP images after subtraction of bone and calcified plaques (PBS) using dual energy CT for the assessment of carotid bifurcation stenoses. MATERIALS AND METHODS 36 patients with a total of 46 stenoses at the carotid bifurcation were examined with a dual energy CT system. Grade of the stenoses and plaque morphology were assessed in axial multi planar projections (axMPR) and freely rotatable 3D-MIP images before and after PBS and compared with results from DSA. RESULTS Grade of the stenosis could be evaluated in all 46 cases in DSA, axMPR and 3D-MIP after PBS. However, in 25 cases grade of the stenosis was not assessable prior to PBS. The average grade of the stenosis increased from DSA (81.4%) to axMPR (83.5%) to 3D-MIP before and after PBS (86.5% and 85.6%). The amount of pseudo-occlusions increased in concordance with the grade of the stenosis (0<9<16). Using 3D-MIP reconstructions, plaque morphology could be evaluated in 32/46 stenoses before PBS and in 44/46 cases after PBS. CONCLUSIONS PBS facilitated the evaluation of grade of the stenosis in all cases. Nevertheless, after PBS stenoses were overrated in 3D-MIP in comparison to DSA and axMPR. Moreover, plaque morphology, as an independent risk factor for stroke, can be evaluated even in calcified plaques after PBS. Therefore dual energy CTA with plaque subtraction has the potential to identify patients with vulnerable plaques better than conventional CTA.

Diffusion restriction of the optic nerve in patients with acute visual deficit

Diffusion magnetic resonance imaging (MRI) is commonly used in acute stroke, but not considered diagnostic in ischemic optic neuropathy. This study evaluates the presence of diffusion restriction in patients with acute visual loss by analyzing diffusion‐weighted images (DWI).

Automatic bone and plaque removal using dual energy CT for head and neck angiography: Feasibility and initial performance evaluation

PURPOSE We sought to evaluate the feasibility and efficiency of dual energy (DE) bone and plaque removal in head and neck CT angiography. MATERIALS AND METHODS 20 patients with suspected carotid stenoses received head and neck DE-CTA as part of their pre-interventional workup. Visual grading using multiplanar reformations (MPR), thick slab maximum intensity projections (MIP) and quantitative vessel analysis (QVA) of stenoses was performed prior and after DE bone removal. Results were evaluated for the detection of relevant stenoses (vessel area reduction >70%). Vessel segmentation errors were analyzed. RESULTS Segmentation errors occurred in 19% of all vessel segments. Nevertheless, most post-bone removal artifacts could be recognized using the MPR technique for reading. Compared to MPR reading prior to bone removal, sensitivity, specificity, positive and negative predictive values after bone removal were 100%, 98%, 88% and 100% for MPR reading and 100%, 91%, 63% and 100% for exclusive MIP reading, respectively. There was a good agreement between the QVA results prior and post-DE plaque removal (r(2)=0.8858). CONCLUSION DE bone and plaque removal for head and neck angiography is feasible and offers a rapid and highly sensitive overview over vascular head and neck studies. Due to a slightly limited specificity of the MIP technique due to segmentation errors, possible stenoses should be verified and graded using MPR techniques.

Grading of Carotid Artery Stenosis in the Presence of Extensive Calcifications: Dual-Energy CT Angiography in Comparison with Contrast-Enhanced MR Angiography

PurposeWe investigated the agreement of dual-energy computed tomography angiography (DE-CTA) and contrast-enhanced magnetic resonance angiography (CE-MRA)in the quantitative measurement of stenoses of the internal carotid artery in comparison with digital subtraction angiography (DSA).MethodsA total of 21 patients with stenoses of the external carotid artery were investigated with a DE-CTA and CE-MRAbefore undergoing carotid angioplasty. The grade of the stenoses was assessed in axial multiplanar reformations (MPR) before and multi-intensity projections (MIP) after plaque subtraction (PS) and compared with results from CE-MRA and DSA according to the North American Symptomatic Carotid Endarterectomy Trial.ResultsAverage grades of stenoses were 80.7 ± 16.1 % (DSA), 81.4 ± 15.3 % (MRA), 80.0 ± 16.7 % (DE-CTA-MPR), and 85.2 ± 14.7 % (DE-CTA-PS-MIP). Of 21 stenoses, 6 were filiform (stenosis grade, 99 %) in the DSA examination. Five of these cases were identified as pseudo-occlusions in MRA, while four were considered as occlusions in DE-CTA-PS-MIP. Another four cases were identified as pseudo-occlusion in DE-CTA-PS-MIP, which were identified as 90 % stenosis in the DSA examination.ConclusionsIn comparison with the gold standard DSA, DE-CTA-MPR had a slightly better agreement in measuring the degree of stenosis of the internal carotid arteries than CE-MRA. In DE-CTA-PS-MIP images, a systematic overestimation has to be taken into account due to partial extinction of the lumen by the PS algorithm. Nevertheless, DE-CTA should be preferred in imaging patients with carotid artery stenosis in the presence of extensive calcifications.

High-Pitch Computed Tomography Coronary Angiography—A New Dose-Saving Algorithm: Estimation of Radiation Exposure

Purpose. To estimate effective dose and organ equivalent doses of prospective ECG-triggered high-pitch CTCA. Materials and Methods. For dose measurements, an Alderson-Rando phantom equipped with thermoluminescent dosimeters was used. The effective dose was calculated according to ICRP 103. Exposure was performed on a second-generation dual-source scanner (SOMATOM Definition Flash, Siemens Medical Solutions, Germany). The following scan parameters were used: 320 mAs per rotation, 100 and 120 kV, pitch 3.4 for prospectively ECG-triggered high-pitch CTCA, scan range of 13.5 cm, collimation 64 × 2 × 0.6 mm with z-flying focal spot, gantry rotation time 280 ms, and simulated heart rate of 60 beats per minute. Results. Depending on the applied tube potential, the effective whole-body dose of the cardiac scan ranged from 1.1 mSv to 1.6 mSv and from 1.2 to 1.8 mSv for males and females, respectively. The radiosensitive breast tissue in the range of the primary beam caused an increased female-specific effective dose of 8.6%±0.3% compared to males. Decreasing the tube potential, a significant reduction of the effective dose of 35.8% and 36.0% can be achieved for males and females, respectively (P < 0.001). Conclusion. The radiologist and the CT technician should be aware of this new dose-saving strategy to keep the radiation exposure as low as reasonablly achievable.

Graft versus self (GvS) against T-cell autoantigens is a mechanism of graft–host interaction

Significance As the mechanism of graft-versus-host disease (GVHD) after allogeneic hematopoietic cell transplantation (HCT), recognition of the recipient’s body by donor immune cells was previously believed to be based on genetic and immunological differences between donor and recipient. However, evidence in murine models and in autologous HCT has shown that also autoimmunity contributes to GVHD. In this study, we show the development of auto-reactivity after human allogeneic HCT and characterize the specific self-epitopes of T cells that may contribute to mediation of GVHD. Such autoantigens have never been characterized before. These observations contribute to a better understanding of the immune responses that are activated following hematopoietic cell transplantation. Graft-versus-host disease (GVHD) represents the major nonrelapse complication of allogeneic hematopoietic cell transplantation. Although rare, the CNS and the eye can be affected. In this study, manifestation in the retina as part of the CNS and T-cell epitopes recognized by the allogeneic T cells were evaluated. In 2 of 6 patients with posttransplantation retina diseases and 6 of 22 patients without ocular symptoms, antigen-specific T-cell responses against retina-specific epitopes were observed. No genetic differences between donor and recipient could be identified indicating T-cell activation against self-antigens (graft versus self). Transplantation of a preexisting immunity and cross-reactivity with ubiquitous epitopes was excluded in family donors and healthy individuals. In summary, an immunological reaction against retina cells represents a mechanism of graft-versus-host interaction following hematopoietic cell transplantation.