World market or regional integration and food security in West Africa

The problem of food security in West Africa was put on the international agenda in 1974 at the international food conference in Rome following the Great Sahelian Drought of 1968-1973. In those years preoccupation with food security was limited mainly to the Sahel countries and concentrated on estimating the availability and use of resources for grain production. Nowadays, studies in the field of food security focus not only on production, but also on the functioning of the food market, and on consumption and nutrition. This paper concentrates on the debate whether sustainable food security in West Africa will be achieved by protection measures for national food production, by a liberalization of the food trade that facih'tates cheap imports of food from the world market, or by stimulating regional food trade through regional co-operation

Inefficient or just different? Effects of heterogeneity on bank efficiency scores

In this paper, we show the importance of accounting for heterogeneity among sample firms in stochastic frontier analysis. For a fairly homogenous sample of German savings and cooperative banks, we analyze how alternative theoretical assumptions regarding the nature of heterogeneity can be modeled and the extent to which the respective empirical specifications affect estimated efficiency levels and rankings. We find that the level of efficiency scores is affected in the case of both cost and profitmodels. On the cost side especially, level and rank correlations show that different specifications identify different banks as being best or worst performers. Our main conclusion is that efficiency studies in general and bank efficiency studies in particular should account for heterogeneity across sample firms. Especially when efficiency measures are employed for policy purposes, a careful choice of models and transparency regarding maximization methods are essential to be able to make inferences about managerial behavior. --Heterogeneity,X-efficiency,benchmarking,bank production

Workflow for automatic renal perfusion quantification using ASL-MRI and machine learning

Purpose: Clinical applicability of renal arterial spin labeling (ASL) MRI is hampered because of time consuming and observer dependent post-processing, including manual segmentation of the cortex to obtain cortical renal blood flow (RBF). Machine learning has proven its value in medical image segmentation, including the kidneys. This study presents a fully automatic workflow for renal cortex perfusion quantification by including machine learning-based segmentation. Methods: Fully automatic workflow was achieved by construction of a cascade of 3 U-nets to replace manual segmentation in ASL quantification. All 1.5T ASL-MRI data, including M 0, T 1, and ASL label-control images, from 10 healthy volunteers was used for training (dataset 1). Trained cascade performance was validated on 4 additional volunteers (dataset 2). Manual segmentations were generated by 2 observers, yielding reference and second observer segmentations. To validate the intended use of the automatic segmentations, manual and automatic RBF values in mL/min/100 g were compared. Results: Good agreement was found between automatic and manual segmentations on dataset 1 (dice score = 0.78 ± 0.04), which was in line with inter-observer variability (dice score = 0.77 ± 0.02). Good agreement was confirmed on dataset 2 (dice score = 0.75 ± 0.03). Moreover, similar cortical RBF was obtained with automatic or manual segmentations, on average and at subject level; with 211 ± 31 mL/min/100 g and 208 ± 31 mL/min/100 g (P <.05), respectively, with narrow limits of agreement at −11 and 4.6 mL/min/100 g. RBF accuracy with automated segmentations was confirmed on dataset 2. Conclusion: Our proposed method automates ASL quantification without compromising RBF accuracy. With quick processing and without observer dependence, renal ASL-MRI is more attractive for clinical application as well as for longitudinal and multi-center studies

Imaging of pediatric great vessel stents: Computed tomography or magnetic resonance imaging?

__Background:__ Complications might occur after great vessel stent implantation in children. Therefore follow- up using imaging is warranted. __Purpose:__ To determine the optimal imaging modality for the assessment of stents used to treat great vessel obstructions in children. __Material and methods:__ Five different large vessel stents were evaluated in an in-vitro setting. All stents were expanded to the maximal vendor recommended diameter (20mm; n = 4 or 10mm; n = 1), placed in an anthropomorphic chest phantom and imaged with a 256-slice CT-scanner. MRI images were acquired at 1.5T using a multi-slice T2-weighted turbo spin echo, an RFspoiled three-dimensional T1-weighted Fast Field Echo and a balanced turbo field echo 3D seq

Rapid 2D variable flip angle method for accurate and precise T1 measurements over a wide range of T1 values

Purpose: To perform dynamic T 1 mapping using a 2D variable flip angle (VFA) method, a correction for the slice profile effect is needed. In this work we investigated the impact of flip angle selection and excitation RF pulse profile on the performance of slice profile correction when applied to T 1 mapping over a range of T 1 values. Methods: A correction of the slice profile effect is proposed, based on Bloch simulation of steady-state signals. With this correction, Monte Carlo simulations were performed to assess the accuracy and precision of 2D VFA T 1 mapping in the presence of noise, for RF pulses with time-bandwidth products of 2, 3 and 10 and with flip angle pairs in the range [1°-90°]. To evaluate its performance over a wide range of T 1, maximum errors were calculated for six T 1 values between 50 ms and 1250 ms. The method was demonstrated using in vitro and in vivo experiments. Results: Without corrections, 2D VFA severely underestimates T 1. Slice profile errors were effectively reduced with the correction based on simulations, both in vitro and in vivo. The precision and accuracy of the method depend on the nominal T 1 values, the FA pair, and the RF pulse shape. FA pairs leading to <5% errors in T 1 can be identified for the common RF shapes, for T 1 values between 50 ms and 1250 ms. Conclusions: 2D VFA T 1 mapping with Bloch-simulation-based correction can deliver T 1 estimates that are accurate and precise to within 5% over a wide T 1 range

Arterial spin labeling using spatio-temporal encoding readout for robust perfusion imaging in inhomogenous magnetic fields

Purpose: To evaluate the feasibility of spatio-temporal encoding (SPEN) readout for pseudo-continuous ASL (pCASL) in brain, and its robustness to susceptibility artifacts as introduced by aneurysm clips. Methods: A 2D self-refocused T 2*-compensated hybrid SPEN scheme, with super-resolution reconstruction was implemented on a 1.5T Philips system. Q (=BW chirp*T chirp) was varied and, the aneurysm clip-induced artifact was evaluated in phantom (label-images) as well as in vivo (perfusion-weighted signal (PWS)-maps and temporal SNR (tSNR)). In vivo results were compared to gradient-echo EPI (GE-EPI) and spin-echo EPI (SE-EPI). The dependence of tSNR on TR was evaluated separately for SPEN and SE-EPI. SPEN with Q ˜ 75 encodes with the same off-resonance robustness as EPI. Results: The clip-induced artifact with SPEN decreased with increase in Q, and was smaller compared to SE-EPI and GE-EPI in vivo. tSNR decreased with Q and the tSNR of GE-EPI and SE-EPI corresponded to SPEN with a Q-value of approximately ˜85 and ˜108, respectively. In addition, SPEN perfusion images showed a higher tSNR (p  75) compared to SE-EPI and GE-EPI. However, the SPEN chirp-pulse saturates incoming blood, thereby reducing pCASL labeling efficiency of the next acquisition for short TRs. Future developments are needed to enable 3D scanning

Decreased native renal T1 up to one week after gadobutrol administration in healthy volunteers

Background: Gadolinium-based contrast agents (GBCAs) are widely used in MRI, despite safety concerns regarding deposition in brain and other organs. In animal studies gadolinium was detected for weeks after administration in the kidneys, but this has not yet been demonstrated in humans. Purpose: To find evidence for the prolonged presence of gadobutrol in the kidneys in healthy volunteers. Study Type: Combined retrospective and prospective analysis of a repeatability study. Population: Twenty-three healthy volunteers with normal renal function (12 women, age range 40–76 years), of whom 21 were used for analysis. Field Strength/Sequence: Inversion recovery-based T 1 map at 3T. Assessment: T 1 maps were obtained twice with a median interval of 7 (range: 4–16) days. The T 1 difference (ΔT 1) between both scans was compared between the gadolinium group (n = 16, 0.05 mmol/kg gadobutrol administered after T 1 mapping during both scan sessions) and the control group (n = 5, no gadobutrol). T 1 maps were analyzed separately for cortex and medulla. Statistical Tests: Mann–Whitney U-tests to detect differences in ΔT 1 between groups and linear regression to relate time between scans and estimated glomerular filtration rate (eGFR) to ΔT 1. Results: ΔT 1 differed significantly between the gadolinium and control group: median ΔT 1 cortex –98 vs. 7 msec (P < 0.001) and medulla –68 msec vs. 19 msec (P = 0.001), respectively. The bias corresponds to renal gadobutrol concentrations of 8 nmol/g tissue (cortex) and 4 nmol/g tissue (medulla), ie, ~2.4 μmol for both kidneys (0.05% of original dose). ΔT 1 correlated in the gadolinium group with duration between acquisitions for both cortex (regression coefficient (β) 16.5 msec/day, R 2 0.50, P < 0.001) and medulla (β 11.5 msec/day, R 2 0.32, P < 0.001). Medullary ΔT 1 correlated with eGFR (β 1.13 msec/(ml/min) R 2 0.25, P = 0.008). Data Conclusion: We found evidence of delayed renal gadobutrol excretion after a single contrast agent administration in subjects with normal renal function. Even within this healthy population, elimination delay increased with decreasing kidney function. Level of Evidence: 3. Technical Efficacy: Stage 3. J. Magn. Reson. Imaging 2020;52:622–631

Detecting low blood concentrations in joints using T1 and T2 mapping at 1.5, 3, and 7 T: an in vitro study

BACKGROUND: Intra-articular blood causes irreversible joint damage, whilst clinical differentiation between haemorrhagic joint effusion and other effusions can be challenging. An accurate non-invasive method for the detection of joint bleeds is lacking. The aims of this phantom study were to investigate whether magnetic resonance imaging (MRI) T1 and T2 mapping allows for differentiation between simple and haemorrhagic joint effusion and to determine the lowest blood concentration that can be detected. METHODS: Solutions of synovial fluid with blood concentrations ranging from 0 to 100% were scanned at 1.5, 3, and 7 T. T1 maps were generated with an inversion recovery technique and T2 maps from multi spin-echo sequences. In both cases, the scan acquisition times were below 5 min. Regions of interest were manually drawn by two observers in the obtained T1 and T2 maps for each sample. The lowest detectable blood concentration was determined for all field strengths. RESULTS: At all field strengths, T1 and T2 relaxation times decreased with higher blood concentrations. The lowest detectable blood concentrations using T1 mapping were 10% at 1.5 T, 25% at 3 T, and 50% at 7 T. For T2 mapping, the detection limits were 50%, 5%, and 25%, respectively. CONCLUSIONS: T1 and T2 mapping can detect different blood concentrations in synovial fluid in vitro at clinical field strengths. Especially, T2 measurements at 3 T showed to be highly sensitive. Short acquisition times would make these methods suitable for clinical use and therefore might be promising tools for accurate discrimination between simple and haemorrhagic joint effusion in vivo

Multiparametric Renal MRI: An Intrasubject Test-Retest Repeatability Study

BACKGROUND: Renal multiparametric magnetic resonance imaging (MRI) is a promising tool for diagnosis, prognosis, and treatment monitoring in kidney disease. PURPOSE: To determine intrasubject test-retest repeatability of renal MRI measurements. STUDY TYPE: Prospective. POPULATION: Nineteen healthy subjects aged over 40 years. FIELD STRENGTH/SEQUENCES: T1 and T2 mapping, R2 * mapping or blood oxygenation level-dependent (BOLD) MRI, diffusion tensor imaging (DTI), and intravoxel incoherent motion (IVIM) diffusion-weighted imaging (DWI), 2D phase contrast, arterial spin labelling (ASL), dynamic contrast enhanced (DCE) MRI, and quantitative Dixon for fat quantification at 3T. ASSESSMENT: Subjects were scanned twice with ~1 week between visits. Total scan time was ~1 hour. Postprocessing included motion correction, semiautomated segmentation of cortex and medulla, and fitting of the appropriate signal model. STATISTICAL TEST: To assess the repeatability, a Bland-Altman analysis was performed and coefficients of variation (CoVs), repeatability coefficients, and intraclass correlation coefficients were calculated. RESULTS: CoVs for relaxometry (T1 , T2 , R2 */BOLD) were below 6.1%, with the lowest CoVs for T2 maps and highest for R2 */BOLD. CoVs for all diffusion analyses were below 7.2%, except for perfusion fraction (FP ), with CoVs ranging from 18-24%. The CoV for renal sinus fat volume and percentage were both around 9%. Perfusion measurements were most repeatable with ASL (cortical perfusion only) and 2D phase contrast with CoVs of 10% and 13%, respectively. DCE perfusion had a CoV of 16%, while single kidney glomerular filtration rate (GFR) had a CoV of 13%. Repeatability coefficients (RCs) ranged from 7.7-87% (lowest/highest values for medullary mean diffusivity and cortical FP , respectively) and intraclass correlation coefficients (ICCs) ranged from -0.01 to 0.98 (lowest/highest values for cortical FP and renal sinus fat volume, respectively). DATA CONCLUSION: CoVs of most MRI measures of renal function and structure (with the exception of FP and perfusion as measured by DCE) were below 13%, which is comparable to standard clinical tests in nephrology. LEVEL OF EVIDENCE: 2 TECHNICAL EFFICACY: Stage 1