Biomedical subjects

Steffen Ringgaard

Publications and source records attributed to Steffen Ringgaard.

10 recordsLinked to original sources

PubMed2006

The effects of time varying curvature on species transport in coronary arteries.

Alterations in mass transport patterns of low-density lipoproteins (LDL) and oxygen are known to cause atherosclerosis in larger arteries. We hypothesise that the species transport processes in coronary arteries may be affected by their physiological motion, a factor which has not been considered widely in mass transfer studies. Hence, we numerically simulated the mass transport of LDL and oxygen in an idealized moving coronary artery model under both steady and pulsatile flow conditions. A physiological inlet velocity and a sinusoidal curvature waveform were specified as velocity and wall motion boundary conditions. The results predicted elevation of LDL flux, impaired oxygen flux and low wall shear stress (WSS) along the inner wall of curvature, a predilection site for atherosclerosis. The wall motion induced changes in the velocity and WSS patterns were only secondary to the pulsatile flow effects. The temporal variations in flow and WSS due to the flow pulsation and wall motion did not affect temporal changes in the species wall flux. However, the wall motion did alter the time-averaged oxygen and LDL flux in the order of 26% and 12% respectively. Taken together, these results suggest that the wall motion may play an important role in coronary arterial transport processes and emphasise the need for further investigation.

Animals↗

PubMed2005

Spiral coronary angiography using a blood pool agent.

PURPOSE: To experimentally investigate the optimum dose of an iron-oxide-based blood pool agent for spiral coronary MR angiography (MRA), and the difference between single and multiple spiral excitations in each cardiac cycle. MATERIALS AND METHODS: Images using single and triple spiral excitations in each cardiac cycle were obtained in late diastole of the left main coronary artery in eight pigs following an inversion prepulse. Measurements were obtained before and after injection of increasing doses of an iron oxide blood pool agent (Clariscan) corresponding to concentrations of 0.8, 2.2, and 3.9 mg Fe/kg BW. The signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were measured. RESULTS: For 0.8 mg Fe/kg BW relative to precontrast values, a significant increase was observed for both one (SNR: 2.3, CNR: 3.8) and three (SNR: 1.4, CNR: 2.2) excitations (P < 0.01). When the dose was increased from 0.8 mg Fe/kg BW to 2.2 mg Fe/kg BW, only the SNR (P < 0.01) increased further. Significantly higher CNR (1.6-1.8) and SNR (1.4-1.6) values were seen for one excitation relative to three excitations at all concentrations (P < 0.05). CONCLUSION: At low concentrations, an iron oxide blood pool agent can increase SNR and CNR significantly with both single excitation and triple excitations using an inversion-prepared spiral acquisition scheme. At higher concentrations, T2* effects reduce image quality.

Animals↗

PubMed2005

Strut chordal-sparing mitral valve replacement preserves long-term left ventricular shape and function in pigs.

OBJECTIVE: Mitral valve replacement with preservation of the entire subvalvular apparatus entails superior postoperative left ventricular function compared with other techniques. However, this option is often not possible because of valve pathology. We hypothesized that preservation of only 4 mitral valve secondary ("strut") chordae would be functionally and geometrically equivalent to total valve preservation in the long-term setting. In a porcine mitral valve replacement model we investigated the long-term effects of 3 surgical techniques on left ventricular function and geometry: (1) total preservation of the native valve, (2) strut chordal preservation, and (3) total excision of the subvalvular apparatus. METHODS: Forty 60-kg pigs were randomized to 1 of the 3 techniques. Global and regional left ventricular function and dimensions were assessed with cardiovascular magnetic resonance and conductance catheter 90 days after mitral valve replacement. Groups were compared by multivariate analysis of variance. RESULTS: There was no overall difference between groups 1 and 2. Group 3 animals had (1) greater base-apex diastolic and systolic lengths, and smaller short-axis diameters, and (2) lower sphericity indices, and greater base-apex and short-axis fractional shortening than groups 1 and 2. Regional analysis showed slimming and elongation to occur primarily in the basal left ventricular segments. Left ventricular contractility and hemodynamic parameters did not differ between groups. CONCLUSIONS: Strut chordal preservation was equivalent to total valve preservation during mitral valve replacement, whereas total chordal resection caused significant left ventricular slimming with compensatory increases in fractional shortening. Therefore, to preserve left ventricular geometry, special attention must be paid to maintain the valvular-ventricular continuity through the strut chordae during mitral valve replacement. This concept may have important therapeutic implications for chordal-sparing mitral valve replacement.

Animals↗

PubMed2004

Wall shear rates differ between the normal carotid, femoral, and brachial arteries: an in vivo MRI study.

PURPOSE: To investigate wall shear rates in vivo in the common carotid, brachial, and superficial femoral arteries using very high resolution magnetic resonance imaging (MRI) phase contrast measurements. MATERIALS AND METHODS: Mean, maximum, and minimum wall shear rate and an oscillatory shear index were measured for 20 volunteers, aged 23.3 +/- 1.9 years, in the three arteries, using phase contrast imaging with 0.0625 mm2 resolution and three-dimensional paraboloid fitting. RESULTS: The superficial femoral artery had the lowest mean (130.3 +/- 13.1 second(-1)), maximum (735.8 +/- 32.4 second(-1)), and minimum (-224.5 +/- 17.0 second(-1)) wall shear rate, as well as the highest oscillatory shear index (0.21 +/- 0.02). All values were significantly different (P < 0.05) from both the brachial artery and the common carotid artery values. The highest mean (333.3 +/- 13.6 second(-1)) and minimum (117.9 +/- 24.5 second(-1)) wall shear rates and the lowest oscillatory shear index (0 +/- 0) were found in the common carotid artery. CONCLUSION: It is possible to measure wall shear rate in vivo in different arteries using MRI with very high resolution. The findings exhibit the in vivo environment of wall shear rates and suggest a nonuniform distribution of wall shear rates throughout the arterial system.

Adult↗

PubMed2004

Three-dimensional phase contrast velocity mapping acquisition improves wall shear stress estimation in vivo.

The purpose of this study was to evaluate and apply high-resolution three-dimensional phase contrast mapping for estimation of wall shear stress in vivo. A silicon pipe of 8 mm diameter with a 8.3 ml/s steady flow and the entrance of the carotid bifurcation in 10 young healthy volunteers aged 23.6 +/- 3.1 years was studied. Very high resolution three-dimensional and two-dimensional phase contrast mapping sequences with spatial resolutions of 0.31 x 0.31 x 1.5 mm(3) and 0.31 x 0.31 x 3 mm(3), respectively, were compared in vivo and in vitro. Wall shear stress was calculated using multi-sectored, three-dimensional paraboloid fitting. In comparison to the two-dimensional measurements, the three-dimensional method with only half the slice thickness gave higher signal-to-noise ratio and velocity-to-noise ratios both in vivo and in vitro. Wall shear stress derived from the three-dimensional velocity measurements did not differ from the two-dimensional velocity measurements either in vitro or in vivo. Mean wall shear stress was lowest and oscillatory shear index was highest at the outer wall, towards the carotid bifurcation. Three-dimensional velocity mapping increases resolution and image quality and allows estimation of wall shear stress patterns circumferentially and longitudinally in human arteries.

Adult↗

PubMed2004

MRI assessment of the influence of body position on the shape and position of the urinary bladder.

OBJECTIVE: Continuous surveillance of patients' bladder volume may be of benefit during the treatment of various urologic disorders. In order to design optimal bladder volume monitoring equipment, information about the shape and position of the bladder, not at least in relation to posture, is crucial. The purpose of this study was consequently to evaluate the influence of body position on the change in shape and position of the bladder. MATERIAL AND METHODS: MRI was performed in three young volunteers during posture changes. MRI scans in the transverse, frontal and sagittal planes were recorded in bladders of various sizes. The position of the bladder midpoint in the three planes was determined. Moreover, to account for its shape, the compactness of the cross-section of the bladder was calculated on the basis of its perimeter and area. RESULTS: For the medial-lateral and cranial-caudal positions, the maximal displacements were 4 and 5 mm, respectively. In the ventral-dorsal direction the displacement varied, but was <18 mm. The compactness changed maximally by 11%, indicating a minimal change in shape. CONCLUSION: In young subjects, the position and shape of the bladder only change modestly for different body positions, thus eliminating the need for special correction measures when designing bladder volume monitoring equipment.

Adult↗

PubMed2004

Effect of bladder volume, gender and body position on the shape and position of the urinary bladder.

OBJECTIVE: The optimal design of bladder volume-monitoring equipment, e.g. regarding transducer design and software algorithms, depends greatly on the shape and position of the bladder or on potential changes due to body position and bladder volume. The purpose of this study was to study the influence of body position, bladder volume and gender on the shape and position of the bladder. MATERIAL AND METHODS: MRI scans of 11 young volunteers were performed during changes in posture. Images were recorded in the transverse and frontal planes and repeated four to six times through the bladder-filling cycle from empty to full. The position of the bladder midpoint was measured in the three planes. Moreover, to account for shape, the compactness of the cross-section of the bladder was calculated based on its perimeter and area. The effect of body position and gender was analyzed using repeated-measures ANOVA and the independent samples t-test, respectively. RESULTS: No significant effect of body position on bladder position or shape was found. Gender was shown to have a significant impact on both the position and shape of the bladder: the female bladder was positioned more caudally and was less compact in the transverse plane than the male bladder. Furthermore, there was significant cranial-dorsal movement of the bladder during filling. CONCLUSION: No significant changes in the position or shape of the bladder were observed during changes in body position in young subjects, regardless of bladder volume or gender. However, changes in both parameters were found due to bladder filling.

Adult↗

PubMed2004

Arterial MR imaging phase-contrast flow measurement: improvements with varying velocity sensitivity during cardiac cycle.

To reduce noise in velocity images of magnetic resonance (MR) phase-contrast measurements, the authors implemented and evaluated a pulse sequence that enables automatic optimization of the velocity-encoding parameter V(enc) for individual heart phases in pulsatile flow on the basis of a rapid prescan. This sequence was prospectively evaluated by comparing velocity-to-noise ratios with those from a standard MR flow scan obtained in the carotid artery in eight volunteers. This sequence was shown to improve velocity-to-noise ratios by a factor of 2.0-6.0 in all but the systolic heart phase and was determined to be an effective technique for reducing noise in phase-contrast velocity measurements.

Adult↗

PubMed2004

Quantitative haemodynamic evaluation of aortic cannulas.

We have designed a new type of aortic cannula for extracorporeal circulation which we hypothesize has superior haemodynamic characteristics compared with existing types. The aim of the study was to perform a quantitative evaluation of the haemodynamic performance of commercially available aortic cannulas against our new design. Six existing cannulas and two self-designed, modified DLP cannulas were investigated. Pressure drop, peak velocity and wall thickness were obtained from in vitro experiments. Pressure drop and turbulent stresses were obtained by in vivo experiments. At 5 L/min the pressure drops of the new design were 18-25 mmHg in vitro and 20 mmHg in vivo compared with 25-92 mmHg in vitro and 28-90 mmHg in vivo for the six commercially available cannulas. Turbulent stresses of the new design varied between 25 and 50 N/m2 in vivo compared with 125-500 N/m2. Both in vivo and in vitro results showed a clear haemodynamic superiority of the new cannula-tip design compared with all existing types.

Animals↗

PubMed2002

Long-term effects of partial unilateral ureteral obstruction on renal hemodynamics and morphology in newborn rats: a magnetic resonance imaging study.

We assessed the longitudinal changes in renal vein blood flow (RVBF) and kidney volume in response to neonatally induced partial unilateral ureteral obstruction (PUUO) in rats with a magnetic resonance imaging (MRI) technique. During anesthesia, either the upper third or two thirds of the left ureter was embedded into the psoas muscle in newborn rats, creating either a mild (n=20) or a severe (n=9) partial obstruction. Control groups consisted of sham-operated (n=12) and non-operated (n=15) rats. During the following 24 weeks, RVBF and kidney volume were measured sequentially every 2-6 weeks with MRI, beginning 9 days after the operation. Both mild and severe obstruction caused a time-dependent decrease in RVBF. At week 24, the mean RVBF had decreased to 79% of the controls in the mildly obstructed kidneys (mean+/-SE: 1.45+/-0.14 vs 1.84+/-0.08 ml/min/100 g body weight, P<0.05) and to 57% of controls in the severely obstructed kidneys (1.05+/-0.10 vs 1.84+/-0.08 ml/min/100 g body weight, P<0.05). The renal pelvis volume increased and the renal parenchymal volume decreased significantly in the severely obstructed kidneys compared to the mildly obstructed kidneys. A good correlation was found between kidney volume measured in vivo using MRI and that measured in vitro (r>0.8, P<0.05), and between RVBF and renal parenchymal volume (r=0.758, P<0.01). In conclusion, the degree of reduction in RVBF depends on the severity and the duration of the PUUO. MRI can safely and reliably be used to monitor the longitudinal changes in RVBF and kidney volume in rats from early life.

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