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At least 253 records · Page 14Linked to original sources

PubMed2002

Influence of connection geometry and SVC-IVC flow rate ratio on flow structures within the total cavopulmonary connection: a numerical study.

The total cavopulmonary connection (TCPC) is a palliative cardiothoracic surgical procedure used in patients with one functioning ventricle that excludes the heart from the systemic venous to pulmonary artery pathway. Blood in the superior and inferior vena cavae (SVC, IVC) is diverted directly to the pulmonary arteries. Since only one ventricle is left in the circulation, minimizing pressure drop by optimizing connection geometry becomes crucial. Although there have been numerical and in-vitro studies documenting the effect of connection geometry on overall pressure drop, there is little published data examining the effect of SVC-IVC flow rate ratio on detailed fluid mechanical structures within the various connection geometries. We present here results from a numerical study of the TCPC connection, configured with various connections and SVC:IVC flow ratios. The role of major flow parameters: shear stress, secondary flow, recirculation regions, flow stagnation regions, and flow separation, was examined. Results show a complex interplay among connection geometry, flow rate ratio and the types and effects of the various flow parameters described above. Significant changes in flow structures affected local distribution of pressure, which in turn changed overall pressure drop. Likewise, changes in local flow structure also produced changes in maximum shear stress values; this may have consequences for platelet activation and thrombus formation in the clinical situation. This study sheds light on the local flow structures created by the various connections andflow configurations and as such, provides an additional step toward understanding the detailed fluid mechanical behavior of the more complex physiological configurations seen clinically.

Algorithms↗

PubMed2000

Coronary sinus flow measurement by means of velocity-encoded cine MR imaging: validation by using flow probes in dogs.

PURPOSE: To validate coronary sinus flow measurements for quantification of global left ventricular (LV) perfusion by means of velocity-encoded cine (VEC) magnetic resonance (MR) imaging and flow probes. MATERIALS AND METHODS: Measurements of coronary sinus flow were performed in seven dogs by using VEC MR imaging at baseline, single coronary arterial stenosis, dipyridamole stress, and reactive hyperemia. These measurements were compared with flow probe measurements of coronary blood flow (CBF) in the left anterior descending coronary (LAD) and circumflex (CFX) arteries (CBF(LAD+CFX)) and coronary sinus. LV blood perfusion was calculated in milliliters per minute per gram from coronary sinus flow, and LV mass was obtained by using VEC and cine MR imaging. LV mass was validated at autopsy. RESULTS: CBF(LAD+CFX) and coronary sinus flow at VEC MR imaging showed close correlation (r = 0.98, P: <.001). The difference between CBF(LAD+CFX) and MR coronary sinus flow was 3.1 mL/min +/- 8.5 (SD). LV mass at cine MR imaging was not significantly different from that at autopsy (73.2 g +/- 12.8 vs 69. 4 g +/- 12.8). At baseline, myocardial perfusion was 0.40 mL/min/g +/- 0.09 at VEC MR imaging, and CBF(LAD+CFX) was 0.44 mL/min/g +/- 0. 08 (not significant). Reactive hyperemia resulted in 2.7- and 2. 3-fold increases in coronary sinus flow at VEC MR imaging and flow probe CBF(LAD+CFX), respectively. CONCLUSION: VEC MR imaging has the potential to measure coronary sinus flow during different physiologic conditions and can serve as a noninvasive modality to quantify global LV perfusion in patients.

Animals↗

PubMed1996

Fetal atrioventricular flow-velocity waveforms and their relation to arterial and venous flow-velocity waveforms at 8 to 20 weeks of gestation.

BACKGROUND: Doppler ultrasonography was used to determine the nature and gestational age-related changes of human fetal AV flow-velocity waveforms and to establish their relationship with arterial impedance indexes and venous flow velocities in normal human fetuses between 8 and 20 weeks of gestation. METHODS AND RESULTS: Flow-velocity waveform recordings were attempted in 318 singleton pregnancies. After the exclusion criteria were applied, data on 214 women were available for further analysis. Differentiation between E wave and A wave became possible at 9 weeks, whereas distinction between transmittal and transtricuspid valve flow velocities was first achieved at 10 to 11 weeks. A statistically significant nonlinear gestational age-dependent increase was established for all AV waveform parameters, which became linear when related to logarithmically estimated fetal crown-to-rump length. Transtricuspid valve flow velocities were significantly higher than transmittal valve flow velocities. Transmittal valve time-averaged flow velocities were positively correlated with peak diastolic velocities and time-velocity integral of late-diastolic reverse flow in the inferior vena cava. No correlation existed between AV time-averaged velocities and arterial impedance indexes. CONCLUSIONS: Monophasic AV flow-velocity waveforms can be recorded as early as 8 weeks of gestation and become biphasic as early as 8 weeks. They demonstrate a linear increase relative to logarithmically estimated fetal crown-to-rump length, suggesting that fetal growth-related increase in volume flow plays a role in this velocity rise. Transtricuspid valve A-wave and E-wave velocities suggest right ventricular predominance as early as the late first trimester of pregnancy. AV flow velocities are not related to arterial downstream impedance.

Adolescent↗

PubMed1999

Influence of acetazolamide and CO2 on extracranial flow volume and intracranial blood flow velocity.

BACKGROUND AND PURPOSE: The vasomotor response can be tested by means of transcranial Doppler sonography. If a constant vessel diameter is assumed, the flow velocity changes will reflect blood flow volume changes. This hypothesis is difficult to verify. Simultaneous assessment of intracranial flow velocity and extracranial flow volume changes may solve this problem. METHODS: We tested vasomotor response in 32 volunteers (age, 42+/-18 years) with 5% CO2. Acetazolamide (1 g) was tested in 15 volunteers (age, 28+/-8 years). To evaluate drug-dependent flow changes in the external carotid artery territory, acetazolamide was administered in 7 patients with unilateral occlusion of the internal carotid artery without evidence of collateralization through the ophthalmic artery (age, 67+/-12 years). Simultaneous recording included measurements of flow volume in the common carotid arteries (M-mode color duplex system) and flow velocity in the middle cerebral arteries. RESULTS: With CO2 and acetazolamide, intracranial flow velocity increased by 31% and 39%, respectively, with a simultaneous increase of common carotid artery flow volume of 47% and 50%, respectively. No change in extracranial flow volume was observed in patients with an occluded internal carotid artery. CONCLUSIONS: These data show not only the expected increase of flow velocity in the middle cerebral artery but also suggest an increase in cross-sectional vessel diameter of 6% and 4% with CO2 and acetazolamide, respectively. It remains unresolved whether this observation is due to a direct effect of the drug on the vessel walls or is simply pressure dependent.

Acetazolamide↗

PubMed2006

The utility of intraoperative blood flow measurement during aneurysm surgery using an ultrasonic perivascular flow probe.

OBJECTIVE: Inadvertent vessel compromise is one major cause of unfavorable outcome from aneurysm surgery. Existing strategies for intraoperative assessment of this complication have potential limitations and disadvantages. We assessed the utility of quantitative intraoperative flow measurements using the Transonic ultrasonic flow probe (Transonic Systems, Inc., Ithaca, NY) during aneurysm surgery. METHODS: Of all aneurysms treated surgically at our institution from 1998 to 2003, 103 patients with 106 aneurysms were identified in whom intraoperative flow measurements were available for analysis. We assessed the frequency of flow compromise and clip repositioning and correlated these with postoperative angiography and stroke. RESULTS: Significant (>25%) reduction in flow rate was apparent in 33 (31.1%) cases, and resulted in clip repositioning in 27 (25.5%), with return to baseline flow except for two cases with vessel thrombosis/dissection. In the other six cases, flow reduction was owing to spasm resolving with papaverine (n = 3) or responded to retractor repositioning (n = 3). In another six (5.7%) cases, unnecessary clip repositioning was avoided (n = 3) or safe occlusion of the parent vessel for trapping of the aneurysm was allowed by confirming adequate distal flow (n = 3). Aneurysms of the basilar, middle cerebral, anterior communicating, or carotid terminus were more likely to be associated with flow compromise (odds ratio, 4.3; P = 0.03). Postoperative angiography corroborated vessel patency in all cases, and no unexpected large vessel occlusions or strokes were evident. CONCLUSION: Use of the ultrasonic flow probe provides real-time immediate feedback concerning vessel patency. Vessel compromise is easier to interpret than with Doppler, and faster/less invasive than intraoperative angiography. Intraoperative flow measurement is a valuable adjunct for enhancing the safety of aneurysm surgery.

Adult↗

PubMed2000

Evaluation of flow volume and flow patterns in the patent false lumen of chronic aortic dissections using velocity-encoded cine magnetic resonance imaging.

In 21 patients with chronic aortic dissections and proven patent false lumens, the flow volume and flow patterns in the patent false lumens was evaluated using velocity-encoded cine magnetic resonance imaging (VENC-MRI) and the relationship between the flow characteristics and aortic enlargement was retrospectively examined. Flow patterns in the false lumen were divided into 3 groups: pattern A with primarily antegrade flow (n=6), pattern R with primarily retrograde flow (n=3), and pattern B with bidirectional flow (n=12). In group A, the rate of flow volume in the false lumen compared to the total flow volume in true and false lumens (%TFV) and the average rate of enlargement of the maximum diameter of the dissected aorta per year (deltaD) were significantly greater than in groups R and B (%TFV: 74.1+/-0.07 vs 15.2+/-0.03 vs 11.8+/-0.04, p<0.01; deltaD: 3.62+/-0.82 vs 0 vs 0.58+/-0.15 mm/year, p<0.05, respectively). There was a significant correlation between %TFV and deltaD (r=0.79, p<0.0001). Evaluation of flow volume and flow patterns in the patent false lumen using VENC-MRI may be useful for predicting enlargement of the dissected aorta.

Adult↗

PubMed1996

[A laser flow meter method for evaluating the peripheral blood flow in mice].

We have developed a method using a laser flow meter for investigating the effect of compounds on peripheral blood flow. The differences of blood flow in several portions and areas of the probe of a laser flow meter in the mouse and the effect of solvents for dissolving compounds on the blood flow were observed. The results indicated that the invisible area of blood vessels at the back or hind paw of the mouse was the most suitable placement position for the probe. In these placement positions, none of the tested solvents, 0.9% physiological saline solution, dimethylsulfoxide and 50% ethanol, had any significant effect on the peripheral blood flow. Next, using this method, we determined how the peripheral blood flow is affected by several drugs active on the circulatory system. By comparison between the present results from mice and previously reported data from other animals, it was shown that the effects of these cardiovascular agents on the peripheral blood flow in mouse skin showed similar tendencies to those in the previous reports using larger animals and sites other than the skin. However, a few of the drugs used in this experiment showed characteristic action on the peripheral blood flow in mice. Consequently, it was demonstrated that this method using the laser flow meter was useful as a simple first screening method to evaluate the effect of a compound on the peripheral blood flow.

Animals↗

PubMed1998

Pulmonary venous flow in normal dogs recorded by transthoracic echocardiography: techniques, anatomic validations and flow characteristics.

To observe pulmonary venous flow in dogs, the echocardiographic imaging planes and the techniques for examination, and the validations of anatomic location were investigated. Then, the velocity pattern of pulmonary venous flow was recorded in normal conscious dogs. Six imaging planes were available for observing the right or left caudal lobe pulmonary venous flow with two-dimensional or pulsed Doppler echocardiography. Of these, the left lateral apical 4-chamber view can be applied as standard view, because the pulmonary venous flow and transmitral flow could be recorded in this view simultaneously with small sampling angle. The velocity pattern of pulmonary venous flow demonstrated two forward waves in 19 of 20 dogs examined, with one peak occurring during ventricular systole and another during ventricular diastole. A reversed flow during atrial contraction was also seen in 11 dogs. In the two forward waves, the mean peak velocity and velocity-time integral of ventricular diastolic forward flow were significantly higher than those of systolic forward flow (46.49 +/- 6.79 vs. 31.13 +/- 4.92 cm/s, p < 0.0001 and 8.18 +/- 1.84 vs. 5.14 +/- 0.82 cm, p < 0.0001, respectively). The deceleration time of diastolic forward flow shortened with the increase of heart rate (r = -0.87, p < 0.0001). Pulmonary venous flow in dogs can be observed under transthoracic two-dimensional or pulsed Doppler echocardiography.

Animals↗

PubMed2001

Left atrial appendage flow in nonrheumatic atrial fibrillation : relationship with pulmonary venous flow and ECG fibrillatory wave amplitude.

OBJECTIVE: This study was conducted (1) to examine the relationship between left atrial appendage (LAA) flow velocity and pulmonary venous flow (PVF) variables during nonrheumatic atrial fibrillation (AF), and (2) to determine whether a reduction in LAA flow is reflected by the fibrillatory wave amplitude on the surface ECG. BACKGROUND: Although LAA Doppler echocardiographic signals provide information regarding the velocity and direction of flow only for a localized narrow sample, systolic PVF represents in part the global left atrial function, mainly relaxation. Controversy exists about whether the amplitude of fibrillatory waves recorded on the surface ECG correlates with LAA flow velocity during AF. MEASUREMENTS AND RESULTS: Thirty-three patients (20 men, 13 women; mean [+/- SD] age, 61 +/- 11 years) with nonrheumatic AF undergoing transthoracic and transesophageal echocardiography were studied. A correlation between LAA flow velocity and systolic PVF variables (peak systolic velocity, R: = 0.450, p = 0.009; velocity-time integral of systolic flow, R = 0.491, p = 0.004; systolic fraction of PVF, R: = 0.627, p < 0.0001) was observed. Patients with a low LAA flow profile (< 25 cm/s) had a reduced systolic PVF. Longer AF duration and the occurrence of moderate mitral regurgitation were related to reduced LAA flow. AF was subdivided into coarse (peak-to-peak fibrillatory amplitude > or = 1 mm) or fine (< 1 mm) in standard ECG lead V1. There was no association between the coarseness of AF and the LAA flow profile. CONCLUSION: In patients with nonrheumatic AF, a reduction in LAA flow velocity correlates with a reduction in systolic PVF. These hemodynamic changes are not reflected by the ECG fibrillatory wave amplitude.

Aged↗

PubMed1992

Femoral artery blood flow monitoring has distinct advantages for examining redistribution of blood flow in fetal acidosis.

Changes of blood flow in umbilical artery, carotid artery and femoral artery were examined during the progression of acidemia in fetal sheep by means of indwelling transit-time ultrasonic blood flow meters. Moreover, catecholamines in fetal blood were measured and its interrelation to the alteration in blood flow was examined. Gradually progressing fetal acidemia was induced by repeated cord compression. Umbilical blood flow showed a initial increase thereafter maintaining a plateau through the experiment, which seemed to be dependent on fetal arterial pressure. Carotid artery flow gradually increased until the arterial pH in fetal blood declined to 7.20 and remained at this level even though the acidemia further progressed. Femoral artery flow markedly decreased around fetal arterial blood pH 7.20 and its change correlated well with the plasma level of catecholamines. This change of femoral artery flow may be evaluated by examination of the flow index as well as flow volume. Redistribution of blood flow in the progression of fetal acidemia may be initiated at around fetal arterial pH 7.20 and can be detected by studying femoral artery flow.

Acidosis↗

PubMed1999

[Evaluation of blood flow in transplanted kidney by flow profile].

BACKGROUND: The clinical usefulness of color Doppler method with flow profile for examination of blood flow in the transplanted kidney was evaluated. PROCEDURE: We measured blood flow in the transplanted kidney by flow profile in 14 renal recipients. RESULTS: Blood flow in the renal arteries in recipients with graft dysfunction was significantly lower than those in recipients with good graft function. There was significant correlation between blood flow in the renal arteries and graft function. In addition, in the segmental arteries, there was significant correlation between peak flow velocity and blood flow. But there was no significant correlation between index of resistance and blood flow in the transplanted kidney. CONCLUSIONS: Measurement of blood flow by flow profile may improve the examination capability of color Doppler ultrasonography of the transplanted kidney.

Blood Flow Velocity↗

PubMed1999

[Effect of long-term cilostazol administration on coronary flow velocity and coronary flow reserve].

Cilostazol, a novel potent inhibitor of phosphodiesterase, increases coronary flow. The effects of cilostazol on coronary flow velocity and coronary flow reserve were studied in 103 patients with coronary artery disease who underwent coronary angiography. Cilostazol 200 mg/day was administered for 3 months (31 patients) or 6 months (37 patients), and coronary flow reserve were measured before and after the cilostazol administration. Coronary flow reserve were measured twice at an interval of 6 months in the control group (35 patients). The Doppler guide wire was advanced into the coronary artery with no significant vessel stenosis. After obtaining continuous baseline coronary flow velocity, an intracoronary infusion of papaverine (10 mg) was performed to measure coronary flow reserve. There were no significant differences in coronary flow velocity just before intracoronary papaverine infusion between the initial and follow-up studies in any of the 3 groups. Coronary flow reserve increased significantly after cilostazol administration in the 3 months and 6 months groups compared with before administration (3 months group: 2.8 +/- 0.8 vs 2.4 +/- 0.9, p < 0.05; 6 months group: 2.8 +/- 1.0 vs 2.4 +/- 0.7, p < 0.01). However, there was no significant difference in coronary flow reserve in the control group between follow-up and initial studies (2.7 +/- 0.8 vs 2.5 +/- 0.8, NS). In conclusion, the long-term oral administration of cilostazol for 3 or 6 months improves coronary flow reserve.

Administration, Oral↗

PubMed2000

Particle image velocimetry investigation of intravalvular flow fields of a bileaflet mechanical heart valve in a pulsatile flow.

BACKGROUND AND AIM OF THE STUDY: Our previous studies of bileaflet mechanical heart valves (MHV) explanted from sheep revealed patterns of localized platelet aggregation on valve surfaces, which may have clinical relevance. Since flow phenomena may promote localized platelet aggregation, an evaluation of flow within a valve lumen was conducted. METHODS: Phase-locked particle image velocimetry (PIV) measurements were obtained within the lumen of a 'mitral' model bileaflet MHV with transparent acrylic leaflets and housing, in a pulsatile flow loop. Instantaneous, two-dimensional flow maps of a central plane, perpendicular to the flow and leaflet pivot axes, were obtained at discrete times during the simulated cardiac cycle. Flow conditions were cardiac output, 3.5 l/min; rate, 72 beats/min; and systolic duration, 300 ms, using blood analog fluid refractive index-matched to acrylic. Leaflet closing velocities and angles were found using double-exposure imagery, and maximum leaflet closing velocity was extrapolated from regression analysis. RESULTS: During full opening, flow within the three lumenal orifices formed a three-peak axial velocity profile. Vorticity was concentrated in shear layers adjacent to downstream leaflet surfaces and in downstream wakes. Forward flow peak velocity was 90 cm/s, with a steep velocity gradient in the central orifice. During closing, the central-gap regurgitant flow formed a jet (peak velocity, 144 cm/s). High vorticity occurred near leaflet leading and trailing edges. During full closure, first a transient (<3 ms) 'stopping vortex' developed near the leaflet trailing edge, followed by a wall jet which formed at the leaflet-housing junction. Maximum leaflet closing velocity was 1.4 m/s. CONCLUSION: Localized jets, steep velocity gradients, high vorticity and vortex recirculation have been observed in vitro near model MHV surfaces. In vivo, each of these flow phenomena, when occurring near valve surfaces, may promote localized platelet aggregation. For the acrylic leaflets, maximum velocity was comparable with results reported for pyrolytic carbon leaflets. PIV of fully transparent models is a promising method for evaluating lumenal flows.

Cardiac Output↗

PubMed2004

Analysis of Y-graft blood flow and flow reserve in conditions of increased myocardial oxygen consumption.

BACKGROUND: It is not well established whether early following surgery the blood flow of arterial composite Y-grafts may efficiently meet the flow demand of the coronary system. The aim of this study was to evaluate whether early after surgery arterial composite Y-grafts may increase the blood flow in response to an increase in myocardial oxygen consumption (MVO2). METHODS: Twenty-seven patients who received complete arterial myocardial revascularization using the left internal thoracic artery (LITA) and the radial artery (RA) as composite Y-graft gave their consent to a pre-discharge coronary angiography and intravascular flow velocity measurements using a Doppler guidewire. Flow measurements were performed in the LITA main stem, the distal LITA and the RA, both at rest and during atrial pacing at 85% of the patient age-predicted maximum heart rate. The heart rate-systolic blood pressure product was considered as an indirect index of MVO2. Hyperemic flow was determined after injection of adenosine. The flow reserve was defined as the ratio of blood flow during maximal hyperemia (Qmax) to baseline flow (Qbasal). RESULTS: Atrial pacing significantly increased MVO2 (p < 0.000). None of the patients developed ischemic ST-segment modifications or complained of chest pain. Qbasal increased significantly in the LITA main stem (p = 0.001), distal LITA (p = 0.041) and RA (p = 0.004) while Qmax did not change significantly. As a consequence the flow reserve decreased in the LITA main stem (p = 0.002), distal LITA (p = 0.000) and RA (p = 0.000) but was not completely exhausted. CONCLUSIONS: Early after surgery arterial composite Y-grafts may significantly increase blood flow in response to conditions of increased MVO2 and maintain a normal myocardial oxygen supply-to-demand ratio.

Aged↗

PubMed2005

The effect of LVAD aortic outflow-graft placement on hemodynamics and flow: Implantation technique and computer flow modeling.

Axial-flow ventricular assist devices (VADs) can be implanted either through a left thoracotomy with outflow-graft anastomosis to the descending thoracic aorta or through a midline sternotomy with anastomosis to the ascending aorta. Each method has advantages and disadvantages. Because these VADs produce nonpulsatile flow, their hemodynamic characteristics differ from those of pulsatile devices. These differences may have important clinical consequences, particularly in relation to the outflow-graft configuration. We describe a computer-generated flow model that we created to illustrate the flow dynamics and possible clinical consequences of each method. The simulations indicate that the location of the anastomosis has important qualitative effects on flow in the ascending aorta and aortic arch. At high VAD outputs (> or =75%), native cardiac output cannot supply the carotid and subclavian arteries. With a descending aortic anastomosis, net backward flow occurs in the descending aorta to supply these branches. Consequently, the aortic arch has a region with almost no net flow, where fluid particles stagnate over many cardiac cycles, possibly causing thrombogenesis. With an ascending aortic anastomosis, the arch has no stagnant region, although flow turbulence still occurs. When the aortic valve remains closed, so that the total output occurs through the VAD, the aortic root has a region of nearly stagnant flow. With an ascending aortic anastomosis, a small degree of recirculatory flow may prevent complete stagnation at the aortic root. With the descending aortic anastomosis, however, no recirculation occurs. These results help delineate the complex flow dynamics and the advantages and drawbacks of each technique.

Aortic Valve↗

PubMed1995

Mural blood flow distribution in the large colon of horses during low-flow ischemia and reperfusion.

Six horses were subjected to 3 hours of low-flow ischemia and 3 hours of reperfusion of the large colon. After induction of anesthesia, the large colon was exteriorized through a ventral midline celiotomy. Colonic blood flow was measured continuously, using Doppler ultrasonic flow probes placed on the colonic arteries supplying the dorsal and ventral colons and was allowed to stabilize for 15 to 30 minutes after instrumentation. Low-flow ischemia was induced by reducing colonic arterial blood flow to 20% of baseline (BL) flow. Colonic mucosal, seromuscular, and full-thickness blood flow were determined on a tissue-weight basis by injecting colored microspheres proximally into the colonic artery supplying the ventral colon. Reference blood samples were obtained at a known flow rate from the colonic artery and vein at a site more distal to the site of injection. Left ventral colon biopsy specimens were harvested at BL, 3 hours of ischemia, and 15 minutes of reperfusion. Blood and tissue samples were digested and filtered to collect the microspheres, and dimethylformamide was added to release the colored dyes. Dye concentration in blood and tissue samples was measured by use of spectrophotometry, and tissue-blood flow was calculated. Data were analyzed, using two-way ANOVA for repeated measures; statistical significance was set at P < 0.05. Doppler blood flow decreased to approximately 20% of BL, whereas microsphere blood flow ranged between 13.7 and 15.5% of BL at 3 hours of ischemia.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed2004

Separation of carbon nanotubes by frit inlet asymmetrical flow field-flow fractionation.

Flow field-flow fractionation (flow FFF), a separation technique for particles and macromolecules, has been used to separate carbon nanotubes (CNT). The carbon nanotube ropes that were purified from a raw carbon nanotube mixture by acidic reflux followed by cross-flow filtration using a hollow fiber module were cut into shorter lengths by sonication under a concentrated acid mixture. The cut carbon nanotubes were separated by using a modified flow FFF channel system, frit inlet asymmetrical flow FFF (FI AFIFFF) channel, which was useful in the continuous flow operation during injection and separation. Carbon nanotubes, before and after the cutting process, were clearly distinguished by their retention profiles. The narrow volume fractions of CNT collected during flow FFF runs were confirmed by field emission scanning electron microscopy and Raman spectroscopy. Experimentally, it was found that retention of carbon nanotubes in flow FFF was dependent on the use of surfactant for CNT dispersion and for the carrier solution in flow FFF. In this work, the use of flow FFF for the size differentiation of carbon nanotubes in the process of preparation or purification was demonstrated.

Journal Article↗

PubMed1992

Separation of diffusion and slow flow effects by use of flow rephasing and dephasing.

Spin-echo signal intensity alterations due to diffusion and slow flow are investigated in connection with modified Stejskal-Tanner pulse sequences for 2D Fourier MR imaging, one for flow rephasing and the other for flow dephasing gradient waveforms. The theoretical considerations and experimental results concerning the diffusion coefficient measurements of slowly flowing material by these sequences are summarized as follows: (a) By using the flow dephasing sequence with different diffusion and flow-sensitive gradients, slow flow effects can be distinguished from diffusion effects based on the quantitative difference between the diffusion- and flow-sensitive gradient amplitude dependences of these processes. (b) By using the combination of the flow rephasing and dephasing sequences with the same diffusion sensitivity, slow flow effects can be distinguished from diffusion effects directly based on their qualitative difference. By use of the latter approach, several in vivo images are also presented, which mainly represent macroscopic motions of spins, including slow flow such as tissue perfusion.

Acetone↗