Inflow Effect Research Articles

Non-Contrast Renal MR Angiography: Value of Subtraction of Tagging and Non-Tagging Technique

The aim of this study was to examine the usefulness of the subtraction technique of non-contrast renal magnetic resonance angiography (MRA) between tagged and non-tagged data collection. We performed renal MRA on eleven healthy volunteers using a 3T MRI unit. For renal MRA, a three dimensional balanced type steady-state free precession (SSFP) sequence (True FISP, Siemens) was used with diaphragmatic navigator gating. We tried to acquire selective arterial images by subtracting black-blood images (tagged images, on which arterial longitudinal magnetization was nearly zero by selective inversion of upper-stream aortic flow) from bright-blood images (non-tagged images, on which arterial flow is bright due to inflow effect). For analysis, two radiologists independently evaluated the visual quality of the axial and coronal targeted maximum intensity projection images (MIP) of original bright-blood MRA and subtraction MRA. Visualization of the main stem of the renal arteries and their 1st branches were satisfactory on both techniques, and there was no statistically significant difference. The score of 2nd branch appeared superior with the subtraction method, but only the right side showed a statistically significant difference (P <0.01). Visualization of small intraparenchymal arteries was significantly superior with subtraction method on both sides. We tried to improve selective demonstration of renal arterial branches using subtraction technique. Although full sequence optimization was not performed, this pilot study showed this technique to be slightly time-consuming but superior in visualization of peripheral branches and possibly more sensitive in detecting small vessel abnormalities.

Effect of concentration of contrast agent on the inflow effect for measuring absolute perfusion by use of inversion recovery T1-weighted TurboFLASH images

Previous studies have shown that the organ blood flow (OBF) calculated from the T₁-weighted MRI technique was lower than expected. The inflow correction was one of the main corrections for measuring the absolute OBF. Our aim in this study was to investigate the influence of the contrast agent concentration on inflow effect on signal intensity (SI) and to find the gradient of different flow rates at different concentrations by use of inversion recovery T₁-weighted turbo fast low angle shot (TurboFLASH) MRI images. We performed these studies at six different concentrations of gadolinium-diethylenetriamine-pantaacetic acid (Gd-DTPA) to find the inflow effect on the SI. A flow phantom made of Perspex was designed that produced four different flow rates at the same time. The non-uniformity of the clinical head and neck coil was measured and applied to the SI for measuring the corrected SI. The corrected SI was measured at different flow rates and concentrations. The results indicated that an increase in the flow rate and in the concentration of the contrast agent was associated with an increase in the gradient of the flow rate. The results also indicated that the inflow correction could be ignored when the concentration was low for measurement of the absolute OBF. At high concentrations, the inflow effect should be considered in calculations of the absolute OBF.

Effects of inflow and radiofrequency spoiling on the arterial input function in dynamic contrast‐enhanced MRI: A combined phantom and simulation study

The arterial input function is crucial in pharmacokinetic analysis of dynamic contrast-enhanced MRI data. Among other artifacts in arterial input function quantification, the blood inflow effect and nonideal radiofrequency spoiling can induce large measurement errors with subsequent reduction of accuracy in the pharmacokinetic parameters. These errors were investigated for a 3D spoiled gradient-echo sequence using a pulsatile flow phantom and a total of 144 typical imaging settings. In the presence of large inflow effects, results showed poor average accuracy and large spread between imaging settings, when the standard spoiled gradient-echo signal equation was used in the analysis. For example, one of the investigated inflow conditions resulted in a mean error of about 40% and a spread, given by the coefficient of variation, of 20% for K(trans). Minimizing inflow effects by appropriate slice placement, combined with compensation for nonideal radiofrequency spoiling, significantly improved the results, but they remained poorer than without flow (e.g., 3-4 times larger coefficient of variation for K(trans)). It was concluded that the 3D spoiled gradient-echo sequence is not optimal for accurate arterial input function quantification and that correction for nonideal radiofrequency spoiling in combination with inflow minimizing slice placement should be used to reduce the errors.

Thermo-Hydrodynamic Analysis of the Flowfield in an Underground Concentrating Solar Tower Under Unsteady Solar Radiation

The purpose of the work presented in this study is related to the optimal design of an underground concentrating solar tower with sunlight using CFD (Computational Fluid Dynamics) method to analyze the matching in chimney height and heat collector size. The present study applied the finite volume method, pressure-based algorithms, and coupled with P-1 radiation model to investigate the inflow effect under the solar radiation through glasses roof. Thereafter, design and simulation of different chimney height have been presented to study the greenhouse effect and application. Analysis of the results from CFD show that the flow can reach to higher temperature in the bottom of a solar chimney with N2O filled. The matching of the height of chimney and temperature of collector remain the optimal aspect value.

The effect of blood inflow and B1‐field inhomogeneity on measurement of the arterial input function in axial 3D spoiled gradient echo dynamic contrast‐enhanced MRI

A major potential confound in axial 3D dynamic contrast-enhanced magnetic resonance imaging studies is the blood inflow effect; therefore, the choice of slice location for arterial input function measurement within the imaging volume must be considered carefully. The objective of this study was to use computer simulations, flow phantom, and in vivo studies to describe and understand the effect of blood inflow on the measurement of the arterial input function. All experiments were done at 1.5 T using a typical 3D dynamic contrast-enhanced magnetic resonance imaging sequence, and arterial input functions were extracted for each slice in the imaging volume. We simulated a set of arterial input functions based on the same imaging parameters and accounted for blood inflow and radiofrequency field inhomogeneities. Measured arterial input functions along the vessel length from both in vivo and the flow phantom agreed with simulated arterial input functions and show large overestimations in the arterial input function in the first 30 mm of the vessel, whereas arterial input functions measured more centrally achieve accurate contrast agent concentrations. Use of inflow-affected arterial input functions in tracer kinetic modeling shows potential errors of up to 80% in tissue microvascular parameters. These errors emphasize the importance of careful placement of the arterial input function definition location to avoid the effects of blood inflow.

Numerical investigation of unsteady cavitating turbulent flow around a full scale marine propeller

This paper treats the unsteady cavitating turbulent flow around a full scale marine propeller operated in non-uniform ship wake. The RANS method combined with k-ω SST turbulence model and the mass transfer cavitation model was applied for the flow simulation. It is noted that both the propeller performance and the unsteady features of cavitating turbulent flow around the propeller predicted by the numerical calculation agreed well with the experimental data. Due to the non-uniform wake inflow and gravity effect, there occurred periodical procedure for cavity development such as cavitation inception, growth, shrinking, etc near the blade tip for the propeller. The study also indicated that there was considerably large pressure fluctuation near the propeller during the operation. The 1st order frequency of pressure fluctuation predicted by numerical simulation equaled the rotating frequency of propeller blades. Both amplitude and frequency agreed with the experimental results fairly well.

Multi‐echo acquisition of MR angiography and venography of the brain at 3 Tesla

To improve the visibility of veins in susceptibility-weighted imaging (SWI) using a multi-gradient echo acquisition. A three-dimensional multi-echo gradient-echo pulse sequence was developed for simultaneous acquisition of MR angiography and multiple volumes of MR venography (MRV) of the brain. The first echo was acquired for MR angiography using the time-of-flight in-flow effect. The subsequent echoes were acquired for SWI-based MRV at different echo times (TEs). Multiple MRV datasets acquired at different TEs were complementary in depicting the venous vasculature. MRV data acquired at a longer TE had a higher venous contrast and stronger susceptibility weighting, whereas MRV data acquired at a shorter TE had a higher signal-to-noise ratio and less severe off-resonance artifacts. Three-dimensional mapping of local field gradients was calculated and the transverse relaxivity (R(2)) at each voxel was quantified using multi-TE exponential fitting. Multi-echo acquisition of MR angiography and venography demonstrated improved visibility of venous vasculature especially in regions with severe field inhomogeneity compared with conventional acquisition of SWI and dual-echo acquisition of MR angiography and venography.

Estimating cerebral blood volume with expanded vascular space occupancy slice coverage

A model for quantifying cerebral blood volume (CBV) based on the vascular space occupancy (VASO) technique and varying the extent of blood nulling yielding task-related signal changes with various amounts of blood oxygenation level-dependent (BOLD) and VASO weightings was previously described. Challenges associated with VASO include limited slice coverage and the confounding inflow of fresh blood. In this work, an approach that extends the previous model to multiple slices and accounts for the inflow effect is described and applied to data from a multiecho sequence simultaneously acquiring VASO, cerebral blood flow (CBF), and BOLD images. This method led to CBV values (7.9 +/- 0.3 and 5.6 +/- 0.3 ml blood/100 ml brain during activation [CBV(ACT)] and rest [CBV(REST)], respectively) consistent with previous studies using similar visual stimuli. Furthermore, an increase in effective blood relaxation (0.65 +/- 0.01) compared to the published value (0.62) was detected, likely reflecting inflow of fresh blood. Finally, cerebral metabolic rate of oxygen (CMRO(2)) estimates using a multiple compartment model without assumption of CBV(REST) led to estimates (18.7 +/- 17.0%) that were within published ranges.

3D TOF MRA of Intracranial Aneurysms at 1.5 T and 3 T: Influence of Matrix, Parallel Imaging, and Acquisition Time on Image Quality–A Vascular Phantom Study

A 3-T magnetic resonance imaging system provides a better signal-to-noise ratio and inflow effect than 1.5 T in three-dimensional time-of-flight (3D TOF) magnetic resonance angiography (MRA). The purpose of this study is to analyze the influence of matrix, parallel imaging, and acquisition time on image quality of 3D TOF MRA at 1.5 and 3 T, and to illustrate whether the combination of larger matrixes with parallel imaging technique is feasible, by evaluating the visualization of simulated intracranial aneurysms and aneurysmal blebs using a vascular phantom with pulsatile flow. An anthropomorphic vascular phantom was designed to simulate the various intracranial aneurysms with aneurysmal bleb. The vascular phantom was connected to an electromagnetic flow pump with pulsatile flow, and we obtained 1.5- and 3-T MRAs altering the parameters of 3D TOF sequences, including acquisition time. Two radiologists evaluated the depiction of simulated aneurysms and aneurysmal blebs. The aneurysmal blebs were not sufficiently visualized on the high-spatial resolution 1.5-T MRA (matrix size of 384 x 256 or 512 x 256), even with longer acquisition time (9 or 18 min). At 3 T with acquisition time of 4.5 min using parallel imaging technique, however, the depiction of aneurysmal blebs was significantly better for the high-spatial resolution sequence than for the standard resolution sequence. For the high-spatial resolution sequence, the longer acquisition times did not improve the depiction of aneurysmal blebs in comparison with 4.5 min at 3 T. For 3D TOF MRA, the combination of the large matrix with parallel imaging technique is feasible at 3 T, but not at 1.5 T.

Diffusion generated T1 and T2 contrast

In MR images of porous organic samples (such as roots or wood) in water media, the sample is often surrounded by a bright ring, with a corresponding decreased T 1 value in T 1 maps. When the medium is removed, or contrast agents are added, the ring disappears, indicating that the signal does not originate in the outer layers of the sample, but from the medium itself. It can be shown that this “bright ring effect” is only observed when the medium experiences a reduction in T 1 when permeating the sample. In order to investigate this effect, a computer model was used to simulate the diffusion of magnetisation between regions that exhibit different relaxation constants. Using this model, the origin of the signal increase was found to be an inflow effect, as diffusion transports relaxed magnetisation from the boundary regions of the sample into the surrounding medium. In the case of the “bright ring” around the plants described above, a mixing of short T 1 values from within the sample and long T 1 values within the medium occurs, yielding a “transition region” between the two values. There, a signal increase can be observed at T 1 weighted images, compared to the signal from the medium beyond this transition region. The width of the transition region is on the order of magnitude of the diffusion displacement that is calculated from the T 1 value as diffusion time. In addition to causing the bright ring around the plant samples, this diffusion effect also limits the resolution of the relaxation time maps. This effect is not limited to T 1 relaxation but also applies to T 2 relaxation. However, at high B 0 field strengths such as those used in this study (11.7 T), a T 2 effect is not usually observed due to the considerably shorter T 2 times in plants (about 50 ms, compared to T 1 times of higher than 1 s). Because the diffusion length during this T 2 relaxation is short with respect to the resolution of the imaging experiments, no T 2 ring effect is seen.

３．０Ｔ ＭＲＩ装置を用いた３Ｄ Ｄｏｕｂｌｅ ＩＲ ＦＳＰＧＲ法による血液のアーチファクトの低減

The purpose of this study was to decrease vascular artifacts caused by the in-flow effect in three-dimensional inversion recovery prepared fast spoiled gradient recalled acquisition in the steady state (3D IR FSPGR) at 3.0 Tesla. We developed 3D double IR FSPGR and investigated the signal characteristics of the new sequence. The 3D double IR FSPGR sequence uses two inversion pulses, the first for obtaining tissue contrast and the second for nulling vascular signal, which is applied at the time of the first IR period at the neck region. We have optimized scan parameters based on both phantom and in-vivo study. As a result, optimized parameters (1st TI=700 ms, 2nd TI=400 ms) successfully have produced much less vessel signal at reduction than conventional 3D IR FSPGR over a wide imaging range, while preserving the signal-to-noise ratio (SNR) and gray/white matter contrast. Moreover, the decreased artifact was also confirmed by visual inspection of the images obtained in vivo using those parameters. Thus, 3D double IR FSPGR was a useful sequence for the acquisition of T1-weighted images at 3.0 Tesla.

Fund Flows, MERs and Managed Fund Ratings Initiations

Published star ratings for managed funds, issued by independent agencies, are increasing in popularity, coverage in the academic literature, and influence as measured by the market share and money inflow dominance of rated funds. Hypothesising that fund ratings serve as a proxy for a fund's reputation, this article examines the managed fund inflow effect of the awarding of an initial rating by ASSIRT, Australia's largest fund rating agency. Retail equity funds record significant unexpected size changes at the announcement of the initial rating and over a one-year post-rating analysis period. Wholesale fund sizes take over six months to show the inflow effects of a ratings initiation. Cash and fixed interest funds exhibit volatile size changes whose pattern does not bear a significant relationship to the initial rating phenomenon. Finally, fund management expense ratios remain stable after the rating.

３．０Ｔ ＭＲＩ装置を用いた血液のアーチファクト抑制を目的とした３Ｄ Ｔｒｉｐｌｅ ＩＲ ＦＳＰＧＲの開発

The purpose of this study was to decrease vascular artifacts caused by the in-flow effect in three-dimensional inversion recovery prepared fast spoiled gradient recalled acquisition in the steady state (3D IR FSPGR) at 3.0 Tesla. We developed 3D triple IR (3IR) FSPGR and examined the signal characteristics of the new sequence. We have optimized scan parameters based on simulation, phantom, and in-vivo studies. As a result, optimized parameters (1st TI=600 ms, 3rd TI=500 ms) successfully have produced the vessel signal at more than 40% reduction, while gray-white matter contrast was preserved. Moreover, the reduced artifact was also confirmed by visual inspection of the in-vivo images for which this condition was used. Thus, 3D 3IR FSPGR was a useful sequence for the acquisition of T1-weighted images at 3.0 Tesla.

Optimization of Three-dimensional Time-of-Flight Magnetic Resonance Angiography of the Intracranial Arteries

The signal-to-noise ratio obtained from arteries in three-dimensional (3D) time-of-flight (TOF) magnetic resonance (MR) angiography is often too low to allow clinical diagnosis because the radiofrequency pulse decreases the magnetization of protons in the blood and suppresses the in-flow effect in the slab. The present study adjusted the position of the head coil to boost arterial signal intensity. Ten healthy volunteers, eight men and two women aged 24-78 years, underwent 3D TOF MR angiography of the intracranial arteries with the same standard GE transmit-receive birdcage head coil using both normal and half position (lower edge of the coil level with the mouth) methods. Our subjects were divided into Group 1 consisted of five relatively young volunteers aged 24-42 years (mean 31.2 years), and Group 2 consisted of five older volunteers aged 70-78 years (mean 73 years). The following four arteries were chosen for analysis: the internal carotid artery (ICA), the proximal middle cerebral artery segment (M1), and the two distal middle cerebral artery segments (M2, M31). The half position method increased the signal-to-noise ratio in the ICA, M1, M2, and M3 by 15%, 25%, 36%, and 44%, respectively. In general, this method resulted in the generation of stronger signals in the M2 and M3 in younger subjects and in all arteries examined in older subjects. The half position method can provide better MR angiograms in certain brain regions of younger people, and in all brain regions in older patients.

３．０Ｔ ＭＲＩ装置を用いた３Ｄ Ｔｉｍｅ ｏｆ Ｆｌｉｇｈｔ ＭＲ Ａｎｇｉｏｇｒａｐｈｙにおける血管のコントラストおよび飽和効果の検討

The purpose of this study was to clarify the effects of the increased field strength of 1.5T vs. 3.0T on blood vessel contrast and saturation in 3D time of flight MR angiography. Vessel contrast and saturation effect were evaluated for various flow velocities and flip angles using a steady-state flow phantom. The results showed increased vessel contrast and decreased saturation of blood on 3.0T compared to 1.5T, due to the following factors: 1) sufficient saturation of stationary tissue by extended T1 relaxation time, 2) increased signal-to-noise ratio (SNR), and 3) high in-flow effect. However, the blood signal on 3.0T revealed a strong tendency to phase dispersion compared to that of 1.5T. Therefore, in order to exploit the characteristic of high field strength in 3.0T, it is suggested that adequate consideration of these factors is important in setting the image acquisition parameters.

Inflow effect correction in fast gradient-echo perfusion imaging.

The purposes of this study were to assess the extent of the inflow effect on signal intensity (SI) for fast gradient-recalled-echo (GRE) sequences used to observe first-pass perfusion, and to develop and validate a correction method for this effect. A phantom experiment with a flow apparatus was performed to determine SI as a function of Gd-DTPA concentration for various velocities. Subsequently a flow-sensitive calibration method was developed, and validated on bolus injections into an open-circuit flow apparatus and in vivo. It is shown that calibration methods based on static phantoms are not appropriate for accurate signal-to-concentration conversion in images affected by high flow. The flow-corrected calibration method presented here can be used to improve the accuracy and robustness of the arterial input function (AIF) determination for tissue perfusion quantification using MRI and contrast media.

Noninvasive measurement of absolute renal perfusion by contrast medium-enhanced magnetic resonance imaging.

The aim of this study was to validate the quantification of absolute renal perfusion (RP) determined by dynamic magnetic resonance imaging (MRI) and contrast media using an experimental model in the rabbit and a transit-timed ultrasound flow probe around the left renal artery as comparison. An MR-compatible ultrasonic time-of-flight flow-probe was placed around the left renal artery in 9 New Zealand white rabbits. Absolute RP in basal state, after mechanical renal artery stenosis, intravenous dopamine, angiotensin II, or colloid infusion was measured using dynamic MRI and intravenous injection of gadoteridol. The results were correlated to the renal artery flow measured inside the magnet with the transit-timed flow-probe. For the signal intensity concentration conversion, we applied different calibrations according to various velocities measured in the aorta by a phase contrast sequence to correct for inflow effect. MRI-derived RP (in mL/min) was calculated by the maximum upslope method, where RP/volume was defined as the ratio of the cortex contrast enhancement slope over the maximum of the arterial input function determined in the aorta. Reproducible arterial and renal transit curve with excellent contrast to noise ratio were obtained. The MRI derived perfusion was systematically underestimated by comparison to the ultrasonic transit-timed flow-probe but was linearly correlated with these measures (r = 0.80, P < 0.001). Using a flow-sensitive calibration, an accurate arterial input function can be measured from the blood MR signal and used in a realistic model to assess the RP. There was a good correlation between the MR-derived RP and the renal artery blood flow measured by the flow-meter. This experimental study validates absolute RP quantification by MRI and contrast media injection and justifies further clinical studies.

Inflow effect in first-pass cardiac and renal MRI.

To estimate the effect of the inflow effect on the arterial input function in vivo in cardiac and renal MR perfusion imaging using fast gradient echo (GRE) sequences and contrast media. The MR exam protocol was designed to acquire images at different phases of the cardiac cycle. The arterial input was thus influenced by various blood flow velocities. It was found that the inflow effect was negligible in the left ventricle of the heart, while it was significantly higher in the aorta for the kidney perfusion measurement. This was principally due to the higher through-the-plane component of the blood flow velocity in the aorta than in the left ventricle. The inflow effect can be neglected in the heart cavity, but should be taken into account in renal perfusion.

High-field MRI of the Central Nervous System: Current Approaches to Clinical and Microscopic Imaging

Introduction of clinical high-field MRI has raised interest in the use of clinical imaging--the efficacy of which has not yet been fully established--in daily practice. A high signal-to-noise ratio and profound susceptibility effects can improve the spatial resolution and image contrast of clinical imaging, whereas the heating effects of the radio frequency tend to prolong acquisition time. As well, inhomogeneities in the static or local magnetic fields can have a negative effect on image quality. The T(1) prolongation may affect T(1) contrast yet improve the enhancement effect of gadolinium chelate and the inflow effect of MR angiography. High-contrast imaging, such as the short inversion-time inversion recovery technique, can provide excellent intra- and extracerebral contrast comparable to microscopic or macroscopic specimens. High-field systems can also be applied to microscopic imaging. High-field MRI is expected to have an increased clinical impact in the near future. Technological advances tailored to high-field systems, as well as the accumulation of scientific evidence, will be necessary to establish its predominance over conventional MRI.

MR Angiography in the Diagnosis of Brain Aneurysms

MR angiography represents a useful non-invasive tool to explore cerebral vessels. Two techniques of acquisition are generally used, based respectively on the “inflow effect” (time-of-flight technique) and on motion-related phase changes (phase contrast technique). The main advantage of MR angiography over conventional digital subtraction angiography is the lack of invasiveness; because of this it requires less technical training and may be easily repeated for follow-up purposes. On the other hand, the main disadvantages of this technique are limited spatial resolution, incomplete fresh blood saturation and difficulties in monitoring critical patients. In our experience, MRA is routinely used in the screening of patients at risk for cerebral aneurysms, and in the follow-up of patients submitted to endovascular treatment.