Author Correction: Spatial and temporal heterogeneity of mouse and human microglia at single-cell resolution.

In this Letter, Dominic Grün and Sagar have been added to the author list (affiliated with Max-Planck-Institute of Immunology and Epigenetics (MPI-IE), Freiburg, Germany). The author list, 'Author contribution' and 'Acknowledgements' sections have been corrected online. See accompanying Amendment.

Spatial and temporal heterogeneity of mouse and human microglia at single-cell resolution.

Microglia have critical roles not only in neural development and homeostasis, but also in neurodegenerative and neuroinflammatory diseases of the central nervous system1-4. These highly diverse and specialized functions may be executed by subsets of microglia that already exist in situ, or by specific subsets of microglia that develop from a homogeneous pool of cells on demand. However, little is known about the presence of spatially and temporally restricted subclasses of microglia in the central nervous system during development or disease. Here we combine massively parallel single-cell analysis, single-molecule fluorescence in situ hybridization, advanced immunohistochemistry and computational modelling to comprehensively characterize subclasses of microglia in multiple regions of the central nervous system during development and disease. Single-cell analysis of tissues of the central nervous system during homeostasis in mice revealed specific time- and region-dependent subtypes of microglia. Demyelinating and neurodegenerative diseases evoked context-dependent subtypes of microglia with distinct molecular hallmarks and diverse cellular kinetics. Corresponding clusters of microglia were also identified in healthy human brains, and the brains of patients with multiple sclerosis. Our data provide insights into the endogenous immune system of the central nervous system during development, homeostasis and disease, and may also provide new targets for the treatment of neurodegenerative and neuroinflammatory pathologies.

5 kHz Transcranial Alternating Current Stimulation: Lack of Cortical Excitability Changes When Grouped in a Theta Burst Pattern.

Background: Suprathreshold transcranial single pulse electrical stimulation (tES) is painful and not applicable in a repetitive mode to induce plastic after-effects. Objective: In order to circumvent this pain problem, we applied here a 5 kHz transcranial alternating current stimulation (tACS) theta burst protocol with a field intensity of up to 10 mA to the primary motor cortex (M1). Furthermore, we were interested in finding out whether electrical theta burst stimulation (eTBS) is able to induce lasting after-effects on cortical plasticity. Methods: Three different eTBS protocols were applied at 5 mA in a sham controlled, double blinded cross-over design on the M1 region of seventeen healthy subjects during the first part of the study. The second study part consists of three different eTBS protocols ranging from 5 mA to 10 mA and 1 ms to 5 ms sinusoidal bursts, applied to the M1 region of 14 healthy subjects. Results: We were able to apply all eTBS protocols in a safe manner, with only six subjects reporting mild side effects related to the stimulation. However, no eTBS protocol induced lasting effects on muscle- evoked potential (MEP) amplitudes when compared to sham stimulation. Significant inhibition of MEP amplitude was only seen in the lower intensity protocols as compared to baseline. Conclusion: eTBS is a safe method to apply high frequency tACS with up to 10 mA intensity. Future studies need to explore the parameter space to a larger extent in order to assure efficacy.

Self-adapting cyclic registration for motion-compensated cone-beam CT in image-guided radiation therapy.

PURPOSE: In image-guided radiation therapy an additional kV imaging system next to the linear particle accelerator provides information for an accurate patient positioning. However, the acquisition time of the system is much longer than the patient's breathing cycle due to the low gantry rotation speed. Our purpose is a cyclic registration in the context of motion-compensated image reconstruction that provides high quality respiratory-correlated 4D volumes for on-board flat panel detector cone-beam CT scans. METHODS: Based on the small motion assumption, widely used within registration algorithms, a strategy is developed for motion estimation. In this strategy temporal restrictions are incorporated, for example, the cyclic motion patterns of respiration. The resultant cyclic registration method is to show less sensitivity on image artifacts, in particular on artifacts due to projection data sparsification. Using a new cyclic registration method a motion estimation is performed on respiratory-correlated reconstructions, and the obtained motion vector fields are used for motion compensation. RESULTS: The proposed cyclic registration is evaluated in the context of motion-compensated image reconstruction using simulated data and patient data. Motion artifacts of 3D standard reconstructions can be significantly reduced by the resulting cyclic motion compensation. The method outperforms the respiratory-correlated reconstructions regarding sparse-view artifacts and maintains the high temporal resolution at the same time. Image artifacts show only minor to almost no effect on the motion estimation using the cyclic registration. CONCLUSIONS: The cyclic motion compensation approach provides respiratory-correlated volumes with high image quality. The cyclic motion estimation is of such low sensitivity to sparse-view artifacts, that it is capable to determine high quality motion vector fields based on reconstructions of low sampled data.

An investigation of 4D cone-beam CT algorithms for slowly rotating scanners.

PURPOSE: To evaluate several algorithms for 4D cone-beam computed tomography (4D CBCT) with slow rotating devices. 4D CBCT is used to perform phase-correlated (PC) reconstructions of moving objects, such as breathing patients, for example. Such motion phase-dependent reconstructions are especially useful for updating treatment plans in radiation therapy. The treatment plan can be registered more precisely to the motion of the tumor and, in consequence, the irradiation margins for the treatment, the so-called planning target volume, can be reduced significantly METHODS: In the study, several algorithms were evaluated for kilovoltage cone-beam CT units attached to linear particle accelerators. The reconstruction algorithms were the conventional PC reconstruction, the McKinnon-Bates (MKB) algorithm, the prior image constrained compressed sensing (PICCS) approach, a total variation minimization (ASD-POCS) algorithm, and the auto-adaptive phase correlation (AAPC) algorithm. For each algorithm, the same motion-affected raw data were used, i.e., one simulated and one measured data set. The reconstruction results from the authors' implementation of these algorithms were evaluated regarding their noise and artifact levels, their residual motion blur, and their computational complexity and convergence. RESULTS: In general, it turned out that the residual motion blur was lowest in those algorithms which exclusively use data from a single motion phase. Algorithms using the image from the full data set as initialization or as a reference for the reconstruction were not capable of fully removing the motion blurring. The iterative algorithms, especially approaches based on total variation minimization, showed lower noise and artifact levels but were computationally complex. The conventional methods based on a single filtered backprojection were computationally inexpensive but suffered from higher noise and streak artifacts which limit the usability. In contrast, these methods showed to be less demanding and more predictable in their outcome than the total variation minimization based approaches. CONCLUSIONS: The reconstruction algorithms including at least one iterative step can reduce the 4 CBCT specific artifacts. Nevertheless, the algorithms that use the full data set, at least for initialization, such as MKB and PICCS in the authors' implementation, are only a trade-off and may not fully achieve the optimal temporal resolution. A predictable image quality as seen in conventional reconstruction methods, i.e., without total variation minimization, is a desirable property for reconstruction algorithms. Fast, iterative approaches such as the MKB can therefore be seen as a suitable tradeoff.

Autoadaptive phase-correlated (AAPC) reconstruction for 4D CBCT.

PURPOSE: Kilovoltage cone-beam computed tomography (CBCT) is widely used in image-guided radiation therapy for exact patient positioning prior to the treatment. However, producing time series of volumetric images (4D CBCT) of moving anatomical structures remains challenging. The presented work introduces a novel method, combining high temporal resolution inside anatomical regions with strong motion and image quality improvement in regions with little motion. METHODS: In the proposed method, the projections are divided into regions that are subject to motion and regions at rest. The latter ones will be shared among phase bins, leading thus to an overall reduction in artifacts and noise. An algorithm based on the concept of optical flow was developed to analyze motion-induced changes between projections. The technique was optimized to distinguish patient motion and motion deriving from gantry rotation. The effectiveness of the method is shown in numerical simulations and patient data. RESULTS: The images reconstructed from the presented method yield an almost the same temporal resolution in the moving volume segments as a conventional phase-correlated reconstruction, while reducing the noise in the motionless regions down to the level of a standard reconstruction without phase correlation. The proposed simple motion segmentation scheme is yet limited to rotation speeds of less than 3 degrees/s. CONCLUSIONS: The method reduces the noise in the reconstruction and increases the image quality. More data are introduced for each phase-correlated reconstruction, and therefore the applied dose is used more efficiently.

A qualitative and a quantitative analysis of an auto-segmentation module for prostate cancer.

PURPOSE: This work describes the clinical validation of an automatic segmentation algorithm in CT-based radiotherapy planning for prostate cancer patients. MATERIAL AND METHODS: The validated auto-segmentation algorithm (Smart Segmentation, version 1.0.05) is a rule-based algorithm using anatomical reference points and organ-specific segmentation methods, developed by Varian Medical Systems (Varian Medical Systems iLab, Baden, Switzerland). For the qualitative analysis, 39 prostate patients are analysed by six clinicians. Clinicians are asked to rate the auto-segmented organs (prostate, bladder, rectum and femoral heads) and to indicate the number of slices to correct. For the quantitative analysis, seven radiation oncologists are asked to contour seven prostate patients. The individual clinician contour variations are compared to the automatic contours by means of surface and volume statistics, calculating the relative volume errors and both the volume and slice-by-slice degree of support, a statistical metric developed for the purposes of this validation. RESULTS: The mean time needed for the automatic module to contour the four structures is about one minute on a standard computer. The qualitative evaluation using a score with four levels ("not acceptable", "acceptable", "good" and "excellent") shows that the mean score for the automatically contoured prostate is "good"; the bladder scores between "excellent" and "good"; the rectum scores between "acceptable" and "not acceptable". Using the concept of surface and volume degree of support, the degree of support given to the automatic module is comparable to the relative agreement among the clinicians for prostate and bladder. The slice-by-slice analysis of the surface degree of support pinpointed the areas of disagreement among the clinicians as well as between the clinicians and the automatic module. CONCLUSION: The efficiency and the limits of the automatic module are investigated with both a qualitative and a quantitative analysis. In general, with efficient correction tools at hand, the use of this auto-segmentation module will lead to a time gain for the prostate and the bladder; with the present version of the algorithm, modelling of the rectum still needs improvement. For the quantitative validation, the concept of relative volume error and degree of support proved very useful.

Effect of ingestion order of the fat component of a solid meal on intragastric fat distribution and gastric emptying assessed by MRI.

PURPOSE: To develop an MRI technique to investigate how varying the ingestion order of nonfat and fat components of a solid meal influences three-dimensional intragastric distribution and gastric emptying (GE). MATERIALS AND METHODS: Eight healthy subjects were studied twice in randomized order. On one occasion (condition F-NF), the fat component (40 g mayonnaise on toast) was served before the nonfat component (270 g pasta, 200 g tomato sauce, 100 mL water); on the other (condition NF-F), the ingestion order was reversed. GE and intragastric distribution of both components were assessed by MRI for 180 minutes. RESULTS: During condition F-NF, GE of fat was significantly faster than during condition NF-F (T(25) [min]: F-NF: 20 +/- 9; NF-F: 40 +/- 7; P < 0.05), a larger amount of fat was observed in the antrum during condition F-NF, and more fat layering occurred. No differences were observed in total GE between the two conditions. CONCLUSION: Meal ingestion order influences GE and intragastric distribution of fat, which can be assessed by MRI techniques, providing new insights into the physiology of gastric processing and intragastric distribution of different meal phases.

Improved MR coronary angiography with use of a new rapid clearance blood pool contrast agent in pigs.

PURPOSE: To evaluate in an animal model the potential for clinical use of a new rapid clearance blood pool contrast agent to improve free-breathing and breath-hold magnetic resonance (MR) coronary angiography. MATERIALS AND METHODS: Free-breathing and breath-hold MR coronary angiography were performed in a pig model (n = 9) (a) without use of a contrast agent; (b) with P792 (Guerbet Research, Aulnay Sous Bois, France), a monodisperse monogadolinated macromolecular compound that acts as a blood pool contrast agent with rapid clearance properties; and (c) with an extravascular gadolinium-based contrast agent. This resulted in six imaging options, which were compared in terms of contrast-to-noise ratio (CNR), signal-to-noise ratio, and vessel length measurements by using the Student t test. RESULTS: Use of P792 improved CNR and visible vessel length significantly with both MR respiratory motion correction approaches, as compared with nonenhanced MR imaging (P <.05). CNR was improved by 76% (from 5.0 to 8.6) with the free-breathing approach and by 34% (from 6.2 to 8.2) with the breath-hold approach. Visible vessel length was increased by 27% (from 79.7 to 99.2 mm) with the free-breathing approach and by 90% (from 48.2 to 86.5 mm) with the breath-hold approach. The P792-enhanced free-breathing approach allowed more distal visualization of the coronary arteries than did the P792-enhanced breath-hold approach (P <.05). Use of the extravascular contrast agent did not improve image quality significantly when compared with that of nonenhanced MR images. CONCLUSION: Use of P792 improves coronary artery MR imaging in conjunction with free-breathing and breath-hold approaches.

Value of magnetic resonance imaging for the noninvasive detection of stenosis in coronary artery bypass grafts and recipient coronary arteries.

BACKGROUND: Magnetic resonance imaging (MRI) is a potential noninvasive diagnostic tool to detect coronary artery bypass graft stenosis, but its value in clinical practice remains to be established. We investigated the value of MRI in detecting stenotic grafts, including recipient vessels. METHODS AND RESULTS: We screened for inclusion 173 consecutive patients who were scheduled for coronary angiography because of recurrent chest pain after coronary artery bypass grafting (CABG). We studied 69 eligible patients with 166 grafts (81 single vein, 44 sequential vein, and 41 arterial grafts). MRI with baseline and stress flow mapping was performed. Both scans were successful in 80% of grafts. Grafts were divided into groups with stenosis > or =50% (n=72) and > or =70% (n=48) in the graft or recipient vessels. Marginal logistic regression was used to predict the probability for the presence of stenosis per graft type using multiple MRI variables. Receiver operator characteristics (ROC) analysis was performed to assess the diagnostic value of MRI. Sensitivity (95% confidence interval)/specificity (95% confidence interval) in detecting single vein grafts with stenosis > or =50% and > or =70% were 94% (86 to 100)/63% (48 to 79) and 96% (87 to 100)/92% (84 to 100), respectively. CONCLUSIONS: MRI with flow mapping is useful for identifying grafts and recipient vessels with flow-limiting stenosis. Flow scans could be obtained in 80% of the grafts. This proof-of-concept study suggests that noninvasive MRI detection of stenotic grafts in patients who present with recurrent chest pain after CABG may be useful in selecting those in need of an invasive procedure.

Tetralogy of Fallot: postoperative delayed recovery of left ventricular stroke volume after physical exercise assessment with fast MR imaging.

In six asymptomatic patients with corrected tetralogy of Fallot and nine healthy control subjects, the authors assessed left ventricular (LV) function during recovery from supine bicycle exercise by performing fast magnetic resonance (MR) flow mapping in the ascending aorta. Abnormal recovery of LV function after exercise was observed in the patients. MR flow mapping allows assessment of cardiac recovery after exercise.

MR-velocity mapping in vascular stents to assess peak systolic velocity. In vitro comparison of various stent designs made of Stainless Steel and Nitinol.

INTRODUCTION: Peak systolic velocity (PSV) measurements of blood flow inside vascular stents allow reliable detection of in-stent re-stenosis. The purpose of this in vitro study was to evaluate the feasibility of obtaining PSV measurements inside vascular stents made of Stainless Steel and Nitinol, using a velocity encoded MR technique. MATERIALS/METHODS: In a flow phantom, stents of Stainless Steel and Nitinol were studied. The phantom was integrated into a closed-tubing circuit driven by a MR dedicated pulsatile flow pump. MR imaging was performed on a 1.5 T system. The PSV in the tube without stent was used as the gold standard to determine the accuracy and the variability (paired t-test and Pittman's test) of the PSV measurements inside the stents. RESULTS: PSV values inside the stents showed percentual difference in mean of -15 to 21% (P < 0.05) at a pump setting of 10 and 20 ml/s. CONCLUSION: PSV measurements can be accurately obtained inside stents made of Stainless Steel and Nitinol. MR-velocity measurements may be used in patients to non-invasively evaluate stent patency and in-stent re-stenosis.

Reproducibility of total cerebral blood flow measurements using phase contrast magnetic resonance imaging.

PURPOSE: To evaluate reproducibility of total cerebral blood flow (CBF) measurements with phase contrast magnetic resonance imaging (pcMRI). MATERIALS AND METHODS: We repeated total CBF measurements in 15 healthy volunteers with and without cardiac triggering, and with and without repositioning. In eight volunteers measurements were performed at two different occasions. In addition, measurement of flow in a phantom was performed to validate MR measurements. RESULTS: A difference of 40.4 ml/minute was found between CBF measurements performed with and without triggering (P < 0.05). For repeated triggered measurements, the coefficient of variation (CV) was 7.1%, and for nontriggered measurements 10.3%. For repeated measurements with repositioning, the CV was 7.1% with and 11.2% without triggering. Repeated measurements at different occasions showed a CV of 8.8%. Comparing measured with real flow in the phantom, the triggered differed 4.9% and the nontriggered 8.3%. CONCLUSION: The findings of this study demonstrate that pcMRI is a reliable method to measure total CBF in terms of both accuracy and reproducibility.

Gastric motility: comparison of assessment with real-time MR imaging or barostat measurement initial experience.

Measurements of gastric volume and motility with magnetic resonance (MR) imaging were compared with simultaneously performed measurements with a barostat in six healthy volunteers. Three-dimensional volume and two-dimensional dynamic MR images and barostat measurements were obtained at rest. Alterations in gastric volume and motility were induced by means of infusion of glucagon and erythromycin, respectively. There is strong evidence to have the opinion that MR imaging is as accurate as barostat measurement in determining changes in gastric volume, and it yields additional information about gastric contractions.

Exercise MR imaging in the assessment of pulmonary regurgitation and biventricular function in patients after tetralogy of fallot repair.

PURPOSE: To assess the responses of pulmonary regurgitation (PR) and biventricular function to submaximal exercise by using a magnetic resonance (MR) imaging exercise protocol with young adult patients who underwent tetralogy of Fallot repair at a young age. MATERIALS AND METHODS: Fifteen patients with corrected tetralogy of Fallot (mean age, 17.5 years +/- 2.5 [SD]) underwent MR imaging at rest and during exercise for the evaluation of PR and biventricular function. Results were compared with findings from 16 control subjects (mean age, 17.5 years +/- 2.3). Mean age at tetralogy of Fallot repair was 2.1 years +/- 1.6, and mean follow-up time after repair was 15.4 years +/- 2.6. Exercise level at MR imaging was calculated individually and corresponded to 60% of peak oxygen uptake. The parameters of cardiac function obtained at rest and during exercise were compared by using a paired t test. An unpaired t test was used to compare parameters of cardiac function between patients and control subjects. RESULTS: PR decreased during exercise (from 27 mL/m(2) +/- 17 to 23 mL/m(2) +/- 15; P =.012). At rest, right ventricular (RV) ejection fraction was normal (>47%) in 80% of patients. RV response to exercise in the patient group was abnormal compared with response in the control group, as demonstrated by an increase in RV end-diastolic volume index (132 mL/m(2) +/- 36 to 137 mL/m(2) +/- 38; P =.041) and no significant change in end-systolic volume index or ejection fraction. In only one patient, RV ejection fraction increased by more than 5%. Left ventricular response was not different between patients and control subjects. CONCLUSION: MR imaging is well suited to assess cardiac response to exercise, and findings revealed a decrease in PR and an abnormal RV response to exercise in patients with corrected tetralogy of Fallot.

Diffusion-weighted MRI in the characterization of soft-tissue tumors.

PURPOSE: To explore the potential of perfusion-corrected diffusion-weighted magnetic resonance imaging (MRI) in characterizing soft-tissue tumors. METHODS AND MATERIALS: Diffusion-weighted MRI was performed in 23 histologically proven soft-tissue masses using a diffusion-weighted spin-echo sequence with diffusion gradient strengths yielding five b-values (0-701 seconds/mm(2)). True diffusion coefficients and perfusion fractions were estimated and compared with apparent diffusion coefficients (ADCs). RESULTS: ADC values of all tumors, subcutaneous fat, and muscle were significantly higher than true diffusion coefficients, indicating a contribution of perfusion to the ADC. True diffusion coefficients of malignant tumors (1.08 x 10(-3) mm(2)/second) were significantly lower than those of benign masses (1.71 x 10(-3) mm(2)/second), whereas ADC values between these groups were not significantly different. CONCLUSION: Perfusion-corrected diffusion-weighted MRI has potential in differentiating benign from malignant soft-tissue masses.

MR flow mapping in coronary artery bypass grafts: a validation study with Doppler flow measurements.

PURPOSE: To validate fast magnetic resonance (MR) flow mapping with intravascular Doppler flow measurements in vitro and in patients with nonstenotic and stenotic coronary artery bypass grafts. MATERIALS AND METHODS: MR and Doppler flow measurements were performed in a small-diameter flow phantom with physiologic flow conditions and at baseline and during adenosine stress in 27 grafts in 23 patients, who were scheduled for cardiac catheterization. At invasive analysis, the grafts were divided into those with stenosis of less than 50% (nonstenotic) and those with stenosis greater than or equal to 50% (stenotic). In vitro velocity values and velocity values in nonstenotic and stenotic grafts were compared with linear regression analysis, and the in vitro interstudy variability was determined. RESULTS: Excellent correlations in average peak velocity (r = 0.99, P <.001) and diastolic peak velocity (r = 0.99, P <.001) were demonstrated in vitro between MR and Doppler flow measurements, with less than 5% interstudy variability. MR and Doppler flow measurements revealed good correlations in peak velocity and velocity reserve both in nonstenotic (n = 20) (average peak velocity: r = 0.81, P <.001; diastolic peak velocity: r = 0.83, P <.001; velocity reserve: r = 0.56, P =.010) and stenotic (n = 7) (average peak velocity: r = 0.83, P <.001; diastolic peak velocity: r = 0.78, P =.001; velocity reserve: r = 0.70, P =.078) grafts. CONCLUSION: Fast MR flow mapping provides noninvasive measures of peak velocity and velocity reserve, which closely correlate with Doppler values both in vitro and in nonstenotic and stenotic grafts.

Detection of vein graft disease using high-resolution magnetic resonance angiography.

BACKGROUND: The application of previous magnetic resonance (MR) angiography techniques has enabled noninvasive differentiation between patent and occluded coronary artery bypass grafts. However, the detection of graft stenosis remains difficult. The purpose of our study was to determine the accuracy of high-resolution navigator-gated 3-dimensional (3-D) MR angiography in detecting vein graft disease. Methods and Results- MR angiography was performed in addition to coronary angiography with quantitative coronary analysis in 56 vein grafts from 38 patients (mean age 66.6+/-9.3 years), who presented with recurrent chest pain after bypass surgery. Eighteen grafts showed a luminal stenosis >/=50%, 11 grafts a stenosis >/=70%, and 6 grafts were occluded. All MR angiograms were evaluated independently by 2 blinded observers, who scored the presence of graft occlusion and graft stenosis >/=50% and >/=70% with a confidence level of 1 to 10. MR image quality was judged as insufficient in 6 grafts and these were excluded. Receiver-operator characteristic analysis revealed an area under the curve of 0.89 and 0.89 for identifying graft occlusion, 0.81 and 0.87 for stenosis >/=50%, and 0.82 and 0.79 for stenosis >/=70% for the 2 observers, respectively. Interobserver agreement in assessing graft occlusion and stenosis >/=50% and >/=70% was 94% (kappa=0.74, r=0.81), 72% (kappa=0.40, r=0.66), and 82% (kappa=0.53, r=0.72), respectively. CONCLUSIONS: High-resolution navigator-gated 3-D MR angiography allows not only good differentiation between patent and occluded vein grafts but also the assessment of vein graft disease with a fair diagnostic accuracy. This approach offers perspective as a noninvasive diagnostic tool for patients who present with recurrent chest pain after vein graft surgery.