Validation of MRI assessment of foot perfusion for improving treatment of patients with peripheral artery disease.

INTRODUCTION: Information on tissue perfusion in the foot is important when treating patients with chronic limb-threatening ischemia. This study aims to test the reliability of different magnetic resonance sequences when measuring perfusion in the foot. METHODS: Sixteen healthy volunteers had their right foot scanned in a test/retest study with six different magnetic resonance sequences (BOLD, multi-echo gradient echo (mGRE), 2D and 3D pCASL, PASL FAIR, and DWI with intravoxel incoherent motion (IVIM) with quantitative measurements of perfusion. For five sequences, cuff-induced ischemia followed by a hyperactive response was measured. Images of the feet were segmented into angiosomes and perfusion data were extracted from the five angiosomes. RESULTS: BOLD, PASL FAIR, mGRE, and DWI with IVIM had low mean differences between the first and second scans, while the results of 2D and 3D pCASL had the highest differences. Based on a paired t-test, BOLD, and FAIR were able to distinguish between perfusion and no perfusion in all angiosomes with p-values below 0.01. This was not the case with 2D and 3D pCASL with p-values above 0.05 in all angiosomes. The mGRE could not distinguish between perfusion and no perfusion in the lateral side of the foot. CONCLUSION: BOLD, mGRE, pASL FAIR, and DWI with IVIM seem to give more robust results compared to 2D and 3D pCASL. Further studies on patients with peripheral artery disease should explore if the sequences can have clinical relevance when assessing tissue ischemia and results of revascularization. IMPLICATIONS FOR PRACTICE: This study provides knowledge that could be used to improve the diagnosis of patient with chronic limb-threatening ischemia to explore tissue perfusion.

Water-content calculation in growth plate and cartilage using MR T1-mapping design and validation of a new method in a porcine model.

OBJECTIVE: There is a close relation between cartilage health and its hydration state. Current magnetic resonance methods allow visualizing this tissue. However, a quantitative analysis is more useful when studying disease. The purpose of this study was to quantify water content in cartilage using magnetic resonance without contrast agents. MATERIALS AND METHODS: Water-content estimations using T1 magnetic resonance mapping were done first in eight gelatin samples where the water content was previously known. The same method was used in the physeal areas of eight skeletally immature 30-kg pigs. To calculate accuracy, T1 calculations were compared to dry-freeze, which is considered the gold standard because it can remove the total water content form a tissue. Four fresh cartilage and seven gelatin samples were dry-frozen. Water content obtained from dry-freeze was compared to the one calculated from T1 map values. A mathematical model and statistical analysis were used to calculate the predictive value of the method and its significance. RESULTS: T1-map-based magnetic resonance method can calculate water content in cartilage with an accuracy of 97.3 %. We calculated a coefficient of variance for this method against dry-frozen sample of 3.68 (SD = 1.2) in gelatin samples, and 2.73 (SD = 1.3) in in vivo samples. Between two independent observers, the coefficient of variance was 0.053, which suggests it can be easily reproduced. CONCLUSIONS: Magnetic resonance was able to calculate, with high accuracy, the cartilage water content using T1 mapping sequences.

Interval and strength training in CAD patients.

This study sought to study the effect of high intensity aerobic interval endurance training on peak stroke volume and maximal strength training on mechanical efficiency in coronary artery disease (CAD) patients. 8 CAD patients (age 61.4 ± 3.7 years) trained 30 interval training sessions with 4 × 4 min intervals at 85-95% of peak heart rate while 10 CAD patients (age 66.5 ± 5.5 years) trained 24 sessions of maximal horizontal leg press. In the interval training group peak stroke volume increased significantly by 23% from 94.1 ± 23.0 mL · beat (-1) to 115.8 ± 22.4 mL · beat (-1) (p<0.05). Peak oxygen uptake increased significantly by 17% from 27.2 ± 4.5 mL · kg (-1) · min (-1) to 31.8 ± 5.0 mL · kg (-1) min (-1) (p<0.05) in the same group. In contrast, there was no such exercise training-induced change in peak stroke volume or peak oxygen uptake in the maximal strength training group, despite a 35% improvement in sub maximal walking performance.

Infants with congenital heart defects have reduced brain volumes.

Children with congenital heart defects (CHDs) have increased risk of cognitive disabilities for reasons not fully understood. Previous studies have indicated signs of disrupted fetal brain growth from mid-gestation measured with ultrasound and magnetic resonance imaging (MRI) and infants with CHDs have decreased brain volumes at birth. We measured the total and regional brain volumes of infants with and without CHDs using MRI to investigate, if certain areas of the brain are at particular risk of disrupted growth. MRI brain volumetry analyses were performed on 20 infants; 10 with- (postmenstrual age 39-54 weeks, mean 44 weeks + 5 days) and 10 without CHDs (postmenstrual age 39-52 weeks, mean 43 weeks + 5 days). In six infants with- and eight infants without CHDs grey and white matter were also differentiated. Infants with CHDs had smaller brains (48 ml smaller; 95% CI, 6.1-90; p = 0.03), cerebrums (37.8 ml smaller; 95% CI, 0.8-74.8; p = 0.04), and cerebral grey matter (25.8 ml smaller; 95% CI, 3.5-48; p = 0.03) than infants without CHD. Brain volume differences observed within weeks after birth in children with CHDs confirm that the brain impact, which increase the risk of cognitive disabilities, may begin during pregnancy.

Diffusion tensor imaging MR Neurography detects polyneuropathy in type 2 diabetes.

AIM: To evaluate if diffusion-tensor-imaging MR-Neurography (DTI-MRN) can detect lesions of peripheral nerves due to polyneuropathy in patients with type 2 diabetes. METHODS: Ten patients with type 2 diabetes with polyneuropathy (DPN), 10 patients with type 2 diabetes without polyneuropathy (nDPN) as well as 20 healthy controls (HC) were included. DTI-MRN covered proximal (sciatic nerve) and distal regions (tibial nerve) of the lower extremity. Fractional-anisotropy (FA) and diffusivity (mean (MD), axial (AD) and radial (RD)) were calculated and compared to neuropathy severity. Conventional T2-relaxation-time and proton-spin-density data were obtained from a multi-echo SE sequence. Furthermore, we evaluated sensitivity and specificity of DTI-MRN from receiver operating characteristics (ROC). RESULTS: The proximal and distal FA was lowest in patients with DPN compared with nDPN and HC (p < 0.01). Likewise, proximal and distal RD was highest in patients with DPN (p < 0.01). MD and AD were also significantly different though less pronounced. ROC curve analyses of DTI separated nDPN and DPN with area-under-the-curve values ranging from 0.65 to 0.98. T2-relaxation-time and proton-spin-density could not differentiate between nDPN and DPN. CONCLUSION: DTI-MRN accurately detects DPN by lower nerve FA and higher RD. These alterations are likely to reflect both proximal and distal nerve fiber pathology in patients with type 2 diabetes.

Accelerated atrophy of lower leg and foot muscles--a follow-up study of long-term diabetic polyneuropathy using magnetic resonance imaging (MRI).

AIMS/HYPOTHESIS: The aim of the study was to determine the loss of muscle volume in the lower leg and foot in long-term diabetic patients in relation to the presence of neuropathy. METHODS: We re-examined 26 type 1 diabetic patients who had participated in magnetic resonance imaging (MRI) studies on muscle volume in the lower leg and foot 9 to 12 years earlier. Re-examination involved MRI, isokinetic dynamometry, clinical examination, electrophysiological studies and quantitative sensory examinations. RESULTS: Annual loss of muscle volume of ankle dorsal and plantar flexors was 4.5 (5.5-3.9)% (median [range]) and 5.0 (7.0-4.2)% in neuropathic patients, 1.9 (3.2-1.0)% and 1.8 (2.6-1.3)% in non-neuropathic patients, and 1.7 (2.8-0.8)% and 1.8 (2.4-0.8)% in controls, respectively (p < 0.01). Annual change of volume and strength correlated for ankle dorsal flexors (r (s) = 0.73, p < 0.01) and for ankle plantar flexors (r (s) = 0.63, p < 0.05) in diabetic patients. In addition, annual change of muscle volume for dorsal and plantar flexors was related to the combined score of all measures of neuropathy (r (s) = -0.68, p < 0.02 and r (s) = -0.73, p < 0.01, respectively). Foot muscle volume declined annually by 3.0 (3.4-1.0)% in neuropathic patients and by 1.1 (4.0-0.2)% in non-neuropathic patients, both values being significantly different from controls (0.2 [-2.5 to 2.4]%). Loss of foot muscle volume was related to severity of neuropathy assessed at clinical evaluation (r (s) = -0.6, p < 0.05). CONCLUSIONS/INTERPRETATION: Muscular atrophy in long-term diabetic neuropathy occurs early in the feet, progresses steadily in the lower legs, relates to severity of neuropathy and leads to weakness at the ankle.

Accurate noninvasive quantitation of blood flow, cross-sectional lumen vessel area and wall shear stress by three-dimensional paraboloid modeling of magnetic resonance imaging velocity data.

OBJECTIVES: We present a new method in which a priori knowledge of the blood velocity fields within the boundary layer at the vessel wall, combined with acquisition of high resolution magnetic resonance imaging (MRI) blood velocity data, allow exact modeling at the subpixel level. BACKGROUND: Methods are lacking for accurate, noninvasive estimation of blood flow, dynamic cross-sectional lumen vessel area and wall shear stress. METHODS: Using standard acquisition of MRI blood flow velocity data, we fitted all data points (n = 69) within the boundary layer of the velocity profile to a three-dimensional paraboloid, which enabled calculation of absolute volume blood flow, circumferential vessel wall position, lumen vessel area and wall shear stress. The method was tested in a 8.00 +/ 0.01-mm diameter glass tube model and applied in vivo to the common carotid artery of seven volunteers. RESULTS: In vitro the lumen area was assessed with a mean error of 0.6%. The 95% confidence interval included the specified tube dimensions. Common carotid mean blood flow was 7.42 ml/s, and mean (standard error) diastolic/systolic vessel area was 33.25 (0.72 [2.2%])/43.46 (0.65 [1.5%]) mm2. Mean/peak wall shear stress was 0.95 (0.04 [4.2%])/2.56 (0.08 [3.1%]) N/m2. CONCLUSIONS: We describe a new noninvasive method for highly accurate estimation of blood flow, cross-sectional lumen vessel area and wall shear stress. In vitro results and statistical analysis demonstrate the feasibility of the method, and the first in vivo results are comparable to published data.

Quantitative abdominal aortic flow measurements at controlled levels of ergometer exercise.

Measuring the exercise-induced flow changes in the arteries of the body is a major challenge. The use of quantitative MR flow measurements for this purpose is hampered by movement artifacts and ECG triggering problems. To quantify exercise-induced flow changes in the abdominal aorta, we applied a fast hybrid phase contrast sequence with K-space segmentation and echo planar imaging readouts during a 12 heart beat, single breathhold post exercise scanning window after ergometer exercise in nine volunteers. Central k-space was acquired first. The changes in heart rate throughout the scanning window were quantified. The mean decrease in heart rate after six heart beats post exercise was less than 4% and less than 14% after 11 heart beats indicating that the exercise state was very well represented during the acquisition of central k-space. Abdominal aortic flow increased from 1.4+/-0.3 l/min at rest to 7.9+/-1.1 l/min at 131 watt. Retrograde flow reached a maximum value of 1.2 l/min at rest, and lasted 140 ms on average. Only for one out of the nine volunteers was there any retrograde flow present during exercise (at 33 watt and 65 watt exercise). It was concluded that retrograde flow patterns in the abdominal aorta associated with oscillating wall shear stresses and development of atherosclerosis disappeared with increasing levels of exercise. The feasibility of using fast quantitative phase contrast measurements during a post exercise scanning window to represent controlled exercise levels was demonstrated.

High-resolution assessment of velocity fields and shear stresses distal to prosthetic heart valves using high-field magnetic resonance imaging.

BACKGROUND AND AIM OF THE STUDY: Complications after replacement of diseased heart valves with mechanical prostheses have been shown to be related to the hemodynamics distal to the valve. For this reason, the velocity patterns have been disclosed in vitro with a variety of different techniques. This study introduces a magnetic resonance imaging (MRI) -based technique, which entails easy acquisition of fluid velocity field data with a high accuracy and spatial resolution. METHODS: A high-field magnetic resonance scanner equipped with short echo time phase-contrast velocity measurement software was applied in a detailed mapping of the axial velocity profile across the entire vessel area at two positions downstream of a bileaflet valve prosthesis inserted in a pulsatile flow system in vitro. The laminar shear forces were calculated from the fluid velocity field data. RESULTS: The velocity profiles very close to the valve reflected the bileaflet design as also shown in previous studies, but the extent and velocities of the jet emanating from the slit between the leaflets were clearly better visualized. However, one diameter downstream of the valve the central jet was completely dispersed and the hemodynamic complexity was significantly reduced. Recirculation and retrograde flow regions that might be relevant for understanding typical long-term complications after implantation were observed close to the valve. CONCLUSIONS: In one scan experiment the presented method provides information on flow characteristics that previously required application of different types of experiment. Thus, the method seems promising as a new technique for detailed and extensive mapping of the velocities and laminar shear stresses downstream of prosthetic heart valves in vitro.

Magnetic resonance velocity imaging: a new method for prosthetic heart valve study.

The aim of this study was to compare different (long/short echo time, whole body/small bore scanner) magnetic resonance velocity measurement techniques and their applicability to the measurement of blood velocity downstream of prosthetic heart valves. In-vitro magnetic resonance velocity measurements were performed downstream of four normal and stenotic prosthetic heart valves (St. Jude Medical bileaflet, Monostrut tilting disc, Ionescu-Shiley Pericardial and Starr-Edwards caged-ball) under steady flow conditions in an aortic test chamber. Cross-sectional and longitudinal velocity images were obtained downstreamed of the valves. Magnetic resonance was able to measure all three components of fluid velocity downstream of the valves under normal and stenotic conditions except in regions of turbulence. The velocity was measured across the tube cross-section in 10-15 minutes producing a good visualization of the axial velocity profile. High velocity regions, shear layers and reversed/stagnant regions were identified. The flow rate calculated by integration of the magnetic resonance velocity across the cross-section of the tube was accurate to 5-6% in normal cases and slightly less accurate for stenotic valves. Although signal loss on the modulus image was adverse to the velocity images, it was found that these regions could be used to identify areas of flow disturbance. The high magnetic field, small bore scanner was able to produce images with a resolution of 0.2 x 0.2 x 1.0 mm and was less affected by turbulence producing more detailed flow images. Magnetic resonances has been shown to be a useful new tool in the measurement of the velocity downstream of prosthetic heart valves. In particular it's short data acquisition time and the possibilities to reproduce the same measurements in-vivo make it an attractive alternative to traditional methods.

Prosthetic heart valve evaluation by magnetic resonance imaging.

OBJECTIVE: To evaluate the potential of magnetic resonance imaging (MRI) for evaluation of velocity fields downstream of prosthetic aortic valves. Furthermore, to provide comparative data from bileaflet aortic valve prostheses in vitro and in patients. METHODS: A pulsatile flow loop was set up in a 7.0 Tesla MRI scanner to study fluid velocity data downstream of a 25 mm aortic bileaflet heart valve prosthesis. Three dimensional surface plots of velocity fields were displayed. In six NYHA class I patients blood velocity profiles were studied downstream of their St. Jude Medical aortic valves using a 1.5 Tesla MRI whole-body scanner. Blood velocity data were displayed as mentioned above. RESULTS: Fluid velocity profiles obtained from in vitro studies 0.25 valve diameter downstream of the valve exhibited significant details about the cross sectional distribution of fluid velocities. This distribution completely reflected the valve design. Blood velocity profiles in humans were considerably smoother and in some cases skewed with the highest velocities toward the anterior-right ascending aortic wall. CONCLUSION: Display and interpretation of fluid and blood velocity data obtained downstream of prosthetic valves is feasible both in vitro and in vivo using the MRI technique. An in vitro model with a straight tube and the test valve oriented orthogonally to the long axis of the test tube does not entail fluid velocity profiles which are compatible to those obtained from humans, probably due to the much more complex human geometry, and variable alignment of the valve with the ascending aorta. With the steadily improving quality of MRI scanners this technique has significant potential for comparative in vitro and in vivo hemodynamic evaluation of heart valves.

A mathematical model of the mechanical link between shortening of the cardiomyocytes and systolic deformation of the left ventricular myocardium.

BACKGROUND: Left ventricular myocytes are arranged in a complex three-dimensional mesh. Since all myocytes contract approximately to the same degree, mechanisms must exist to enable force transfer from each of these onto the framework as a whole, despite the transmural differences in deformation strain. This process has hitherto not been clarified in detail. OBJECTIVE: To present a geometrical model that establishes a mechanical link between the three-dimensional architecture and the function of the left ventricular myocardium. METHODS: The left ventricular equator was modeled as a cylindrical tube of deformable but incompressible material, composed of virtual cardiomyocytes with known diastolic helical and transmural angles. By imposing reference circumferential, longitudinal, and torsional strains onto the model, we created a three-dimensional deformation field to calculate passive shortening of the myocyte surrogates. We tested two diastolic architectures: 1) a simple model with longitudinal myocyte surrogates in the endo- and epicardium, and circular ones in the midwall, and 2) a more accurate architecture, with progressive helical angle distribution varying from -60° in the epicardium to 60° in the endocardium, with or without torsion and transmural cardiomyocyte angulation. RESULTS: The simple model caused great transmural unevenness in cardiomyocyte shortening; longitudinal surrogates shortened by 15% at all depths equal to the imposed longitudinal strain, whereas circular surrogates exhibited a maximum shortening of 23.0%. The accurate model exhibited a smooth transmural distribution of cardiomyocyte shortening, with a mean (range) of 17.0 (13.2-20.8)%. Torsion caused a shortening of 17.0 (15.2-18.9)% and transmural angulation caused a shortening of 15.2 (12.4-18.2)%. Combining the effects of transmural angulation and torsion caused a change of 15.2 (13.2-16.5)%. CONCLUSION: A continuous transmural distribution of the helical angle is obligatory for smooth shortening of the cardiomyocytes, but a combination of torsional and transmural angulation changes is necessary to execute systolic mural thickening whilst keeping shortening of the cardiomyocytes within its physiological range.

Localization and quantification of muscle damage by magnetic resonance imaging following step exercise in young women.

Eccentric exercise affects muscles differentially according to intensity, duration, and previous exposure to the specific exercise activity. We used T2-weighted magnetic resonance imaging sequences to localize and quantify muscle damage following step exercise and to determine correlations between transverse relaxation time (T2) and other markers of muscle damage. Eight women performed two-step exercise bouts (30 min) separated by 8 weeks. Blood samples, MR scans, measurements of muscle strength, and muscle soreness were obtained immediately before, after, and up to 9 days after each bout. Resting muscle T2 (40.3+/-0.6 ms) increased exclusively in m. Adductor magnus (AM) in the thigh performing eccentric contractions and peaked 3 days after bout 1 (73.5+/-9.7 ms, P<0.05). Plasma creatine kinase (CK) activity peaked on day 3 after bout 1 and correlated with T2 in AM (r=0.96, P<0.001). After bout 2 CK and T2 were almost unaffected. This indicates that T2-weighted MRI can be applied to identify muscles from which enzymes are being released into the circulation.

In vivo wall shear stress measured by magnetic resonance velocity mapping in the normal human abdominal aorta.

OBJECTIVE: To apply a new non-invasive method for quantification of in vivo wall shear stress (WSS) by magnetic resonance (MR) FAcE velocity mapping and measure WSS in the human abdominal aorta. DESIGN: Prospective, open study. MATERIAL: Six volunteers. METHODS: MR FAcE velocity method was developed for measurements of mean, maximum, minimum WSS and oscillating shear index (OSI) values at the anterior and posterior walls of suprarenal and infrarenal abdominal aorta. RESULTS: The mean, maximum and minimum WSS values were 0.63/0.28, 4.07/2.72 and -0.71/-1.00 N/m2, respectively, in the suprarenal/infrarenal aorta. The mean WSS was 0.35 N/m2 (p < 0.001) and the maximum WSS was 1.36 N/m2 (p < 0.0001) lower in the infrarenal aorta than in the suprarenal aorta. Mean, maximum minimum WSS and OSI values in the infrarenal position differed (p < 0.01) between the anterior and posterior walls. CONCLUSION: WSS can be determined in vivo by MR FAcE velocity technique. Since the lowest WSS values were measured in the infrarenal, posterior blood-to-wall interface, the theory of more pronounced atherosclerosis development in low and oscillating WSS domains was not contradicted by the results of the present study.

Measurement of renal vein blood flow in rats by high-field magnetic resonance.

The aim of the present study was to examine whether magnetic resonance imaging (MRI) based method for non-invasive in vivo measurement of vein blood flow in rats could be used to estimate renal blood flow (RBF). Measurements were performed using a high-field (7 Tesla) MRI scanner with a short echo time phase contrast velocity measurement pulse sequence. The method was evaluated in vitro by flow measurements in an acrylic pipe and in vivo by recording left renal vein blood flow in normal and unilaterally nephrectomized rats. In a subset of animals RBF was measured by a direct method using 14C-tetraethylammoniumbromide. In vitro a high accuracy was found between applied and MRI measured flow rates in the range from 0.5 to 33 ml/min (r = 0.997; P < 0.001). In vivo the MRI measured left renal vein blood flow was 70% higher in unilaterally nephrectomized animals compared to control animals (3.4 +/- 0.4 ml/min/ 100 g body wt vs. 2.0 +/- 0.1 ml/min/100 g body wt, P < 0.001). Direct measurements of RBF revealed comparable values (3.4 +/- 0.3 ml/min/100 g body wt vs. 2.3 +/- 0.4 ml/min/100 g body wt, P = 0.05). In addition, the left kidney volume was recorded by MRI with an increase amounting to 40% (1.18 +/- 0.05 ml vs. 0.84 +/- 0.02 ml; P < 0.001) in the nephrectomized group compared to controls. Finally, a positive correlation was seen between left renal vein blood flow and MRI measured renal volume (r = 0.91; P < 0.001). In summary, MRI is a non-invasive tool by which measurement of renal vein blood flow can be performed, and it is concluded that MRI-based renal vein flow measurements can be used to estimate RBF in small rodents.

Determination of wall shear rate in the human carotid artery by magnetic resonance techniques.

OBJECTIVES: to measure wall shear rates around the circumference of the human carotid bifurcation throughout the heart cycle. DESIGN: prospective, open study. Materials eight healthy volunteers. METHODS: wall shear rates were determined at the carotid bifurcation using magnetic resonance techniques with high resolution and individually adjusted velocity encoding for imaging and haemodynamic mapping. Wall shear stresses were calculated assuming a constant value of 4 centiPoise. RESULTS: data suitable for postprocessing were obtained in all subjects. The main findings were: unidirectional wall shear rate waveforms and high wall shear rate (775 s(-1)+/-167 s(-1)) at the flow divider; low wall shear rate (60 s(-1+/-40 s(-1)) and a high oscillation index with huge interindividual variation (85+/-65) at the lateral wall. CONCLUSION: these are the first in vivo data describing, in detail, the forces of the blood acting on the wall of the carotid bifurcation. The results do not contradict the hypotheses associating low and oscillating wall shear stress with the development of atherosclerosis.)

Effects of temperature and histopathologic preparation on the size and morphology of atherosclerotic carotid arteries as imaged by MRI.

Using magnetic resonance imaging the effects of temperature, formalin fixation, and decalcification on the size and morphology of atherosclerotic arteries were evaluated. Ten ex vivo carotid arteries were scanned fresh at body and room temperature and formalin-fixed and decalcified at room temperature. Different spin-echo pulse sequences were used and absolute T2 values calculated. During processing for histopathology, the contrast between the arterial layers increased. From body to room temperature there were significant increases in size (4%-7%), T2 of media (60--> 68 msec), and fibrous plaque component (95--> 110 msec). Formalin fixation caused significant increases in size (2%-3%) and media T2 (68--> 74 msec). Decalcification caused significant shrinkage (2%-5%) and decrease in T2 of media (74--> 53 msec) and fibrous plaque component (118--> 76 msec). Thus temperature and preparation have profound effects on contrast, size, and T2 of atherosclerotic arteries. Ex vivo experiments should be performed on fresh specimens at body temperature. J. Magn. Reson. Imaging 1999;10:876-885.

Quantitative evaluation of flow patterns in perfusion cannulae by a new magnetic resonance imaging method.

The arterial cannula is a critical part of any extracorporeal circulation system due to the high flow rates which must pass a small cross-sectional area, resulting in high blood velocities. The aim of this study was to examine whether high-field magnetic resonance scanning is applicable for detailed mapping of velocity fields around the tip of such arterial cannulae in vitro. The investigated cannula was an angled, open-tip traditional design aortic cannula with an internal tip diameter of 5.5 mm. The velocity fields were measured at two different flow rates (2 and 4 l/min) at various positions in the lumen and outside the cannula using magnetic resonance imaging (MRI). All three components of the velocity vectors were measured. The study showed that MRI can provide a clear quantitative visualization of the velocity field around the tip of arterial cannulae at lower flow rates. At higher flow rates it can provide information about localization of regions with turbulent or disturbed flow.

Automatic accurate non-invasive quantitation of blood flow, cross-sectional vessel area, and wall shear stress by modelling of magnetic resonance velocity data.

OBJECTIVES: To apply a new, automatic and non-invasive method for quantification of blood flow, dynamic cross-sectional vessel area, and wall shear stress (WSS) by in vivo magnetic resonance velocity mapping of normal subjects. DESIGN: Prospective, open study. MATERIALS: Six young volunteers. METHODS: A three-dimensional paraboloid model enabling automatic determination of blood flow, vessel distensibility and WSS was applied to blood velocity determinations in the common carotid artery. Blood flow was also determined by a manual edge detection method. RESULTS: Using the new method, the common carotid mean blood flow was 7.28 (5.61-9.63) (mean (range)) ml/s. By the manual-method blood flow was 7.21 (5.55-9.60) ml/s. Mean luminal vessel area was 26% larger in peak systole than in diastole. Mean/peak WSS was 0.82/2.28 N/m2. Manually and automatically determined flows correlated (r2 = 0.998, p < 0.0001). WSS and peak centre velocity were associated (r2 = 0.805, p < 0.0001). CONCLUSIONS: Blood flow, luminal vessel area dilatation, and WSS can be determined by the automatic three-dimensional paraboloid method. The hypothesis of association between peak centre velocity and WSS was not contradicted by the results of the present study.

Quantitation of circumferential subpixel vessel wall position and wall shear stress by multiple sectored three-dimensional paraboloid modeling of velocity encoded cine MR.

Methods are lacking for accurate, noninvasive circumferential edge detection and wall shear stress calculation. Using standard MR phase contrast sequences, parts of the velocity profiles were fitted to a multiple sectored three-dimensional paraboloid model enabling exact calculation of vessel wall position and wall shear stress in 24 locations evenly distributed around the luminal vessel wall. The model was evaluated by in vitro scans and computer simulations and applied to the common carotid artery of humans. In vitro, the luminal area of a glass tube was assessed with an error of 0.9%. Computer simulations of peak systolic data revealed errors of +/-0.9% (vessel area) and +/-3.25% (wall shear stress). The in vivo results showed substantial difference between anterior and posterior wall shear stress values due to skewed velocity profiles. A new noninvasive method for highly accurate measurement of circumferential subpixel vessel wall position and wall shear stress has been developed.