Stein Samstad

Dynamic three-dimensional freehand echocardiography using raw digital ultrasound data.

In this paper, we present a new method for simple acquisition of dynamic three-dimensional (3-D) ultrasound data. We used a magnetic position sensor device attached to the ultrasound probe for spatial location of the probe, which was slowly tilted in the transthoracic scanning position. The 3-D data were recorded in 10-20 s, and the analysis was performed on an external PC within 2 min after transferring the raw digital ultrasound data directly from the scanner. The spatial and temporal resolutions of the reconstruction were evaluated, and were superior to video-based 3-D systems. Examples of volume reconstructions with better than 7 ms temporal resolution are given. The raw data with Doppler measurements were used to reconstruct both blood and tissue velocity volumes. The velocity estimates were available for optimal visualization and for quantitative analysis. The freehand data reconstruction accuracy was tested by volume estimation of balloon phantoms, giving high correlation with true volumes. Results show in vivo 3-D reconstruction and visualization of mitral and aortic valve morphology and blood flow, and myocardial tissue velocity. We conclude that it was possible to construct multimodality 3-D data in a limited region of the human heart within one respiration cycle, with reconstruction errors smaller than the resolution of the original ultrasound beam, and with a temporal resolution of up to 150 frames per second.

The Velocity Distribution in the Aortic Anulus in Normal Subjects: A Quantitative Analysis of Two-dimensional Doppler Flow Maps

The velocity distribution in the aortic anulus is commonly assumed to be uniform. A skewed velocity profile may have consequences for the accuracy of volume flow estimates by the Doppler echocardiographic technique. To assess this issue, the velocity distribution in the aortic anulus in 12 normal subjects was studied by computer analysis of digital velocity data from two-dimensional Doppler ultrasound flow maps. The velocity profiles in the aortic anulus were found to be flat but slightly skewed, with the highest velocities toward the septum. There was little interindividual variation. Our findings imply that the centerline velocity is the best estimate for the spatial mean velocity at the aortic anulus in normal subjects. The importance of this finding in patients is unknown. In normal subjects, the results suggest that stroke volume might be overestimated by approximately 15% by Doppler echocardiography if the cross-sectional velocity profile is not accounted for.

Intraoperative color Doppler ultrasound assessment of LIMA-to-LAD anastomoses in off-pump coronary artery bypass grafting

BACKGROUND Although techniques for off-pump coronary artery bypass grafting (CABG) are continually being refined, angiographic follow-up studies have indicated a higher rate of anastomoses-related stenoses than expected after traditional on-pump CABG. This study was performed to evaluate the use of intraoperative epicardial color Doppler ultrasound to quality-assess left internal mammary artery (LIMA) to left anterior descending coronary artery (LAD) anastomoses performed on the beating heart. METHODS Twenty-four LIMA-to-LAD anastomoses were evaluated with real-time epicardial ultrasound imaging using an ultrasound transducer positioned between the paddles of the stabilizer during off-pump procedures. The length of the anastomosis (D(A)), diameters of LIMA (D(M)), LAD at the toe of the anastomosis (D1), and 5 mm distally to the anastomosis (D2) were measured, and the ratios between these variables were calculated. The flow velocity through the anastomoses was visualized by color Doppler coding, and flow was assessed with transit-time flowmetry. RESULTS The epicardial color Doppler ultrasound allowed accurate assessment of the anastomoses. Twenty-three (96%) of the primary anastomoses were confirmed as patent. Mean ratios of D1/D2, D(A)/D2, and D(M)/D2 were 0.89 +/- 0.13, 3.01 +/- 1.04 and 1.32 +/- 0.32, respectively. One anastomosis had a stenosis more than 50% detected by color Doppler ultrasound. After surgical revision, transit-time flow increased from 22 to 40 ml/min. CONCLUSIONS Intraoperative color Doppler ultrasound allowed adequate imaging for quality assessment of LIMA-to-LAD anastomoses performed on the beating heart. One anastomosis was revised due to a technical error detected by epicardial color Doppler imaging. Epicardial ultrasound scanning is a valuable tool for intraoperative assessment of LIMA-to-LAD anastomoses during off-pump coronary surgery.

Volumetric Blood Flow Measurement with the Use of Dynamic 3-Dimensional Ultrasound Color Flow Imaging

We describe a new method for measuring blood volume flow with the use of freehand dynamic 3-dimensional echocardiography. During 10 to 20 cardiac cycles, the ultrasonographic probe was slowly tilted while its spatial position was continuously recorded with a magnetic position sensor system. The ultrasonographic data were acquired in color flow imaging mode, and the separate raw digital tissue and Doppler data were transferred to an external personal computer for postprocessing. From each time step in the reconstructed 3-dimensional data, one cross-sectional slice was extracted with the measured and recorded velocity vector components perpendicular to the slice. The volume flow rate through these slices was found by integrating the velocity vector components, and was independent of the angle between the actual flow direction and the measured velocity vector. Allowing the extracted surface to move according to the movement of anatomic structures, an estimate of the flow through the cardiac valves was achieved. The temporal resolution was preserved in the 3-dimensional reconstruction, and with a frame rate of up to 104 frames/s, the reconstruction jitter artifacts were reduced. Examples of in vivo blood volume flow measurement are given, showing the possibilities of measuring the cardiac output and analyzing blood flow velocity profiles.

Cross-sectional Left Ventricular Outflow Tract Velocities Before and After Aortic Valve Replacement: A Comparative Study With Two-dimensional Doppler Ultrasound

To assess whether aortic valve replacement (AVR) results in changes in the flow velocity distribution in the left ventricular outflow tract (LVOT), 10 patients undergoing AVR for aortic stenosis were studied. By extracting velocity information from color flow maps as digital data, instantaneous cross-sectional velocity profiles were constructed. Velocity profiles obtained 1 to 3 days before AVR were compared with recordings made 3 months later. The LVOT velocity profiles were variably skewed both before and after surgery, and no systematic or uniform changes could be detected after AVR. The highest velocities were most often localized in the region from the center of the outflow tract diameter toward the septum both before and after surgery. At the time of peak flow the ratio of the maximum to the cross-sectional mean velocity was 1.38 +/- 0.13 before and 1.39 +/- 0.08 after AVR (NS), and the ratio of the maximum to the mean velocity time integral was 1.47 +/- 0.10 before and 1.56 +/- 0.10 after (NS). We conclude that AVR in patients with aortic stenosis does not result in a change in LVOT velocity profiles that will influence stroke volume estimates with the Doppler technique.

Estimation of arterial compliance in aortic regurgitation: three methods evaluated in pigs.

Three methods for measuring arterial compliance when aortic regurgitation is present are examined. The first two methods are based on a Windkessel model composed of two elements, compliance C and resistance R. Arterial compliance was estimated from diastolic pressure waveforms and diastolic regurgitant flow for one method, and from systolic aortic pressure waveforms and systolic flow for the other method. The third method was based on a three-element Windkessel model, composed of characteristic resistance r, compliance C and resistance R. In this method arterial compliance was calculated by adjusting the model to the modulus and phase of the first harmonic term of the aortic input impedance. The three methods were compared and validated in six anaesthetised pigs over a broad range of aortic pressures. The three methods were found to give quantitatively similar estimates of arterial compliance at mean aortic pressures above 60 mm Hg. Below 60 mm Hg, estimates of arterial compliance varied widely, probably because of poor validity of the Windkessel models in the low pressure range.

Estimation of regurgitant volume and orifice in aortic regurgitation combining CW Doppler and parameter estimation in a Windkessel-like model

A method for noninvasive estimation of regurgitant orifice and volume in aortic regurgitation is proposed. The method was tested in anesthetized open-chested pigs. It can be used with noninvasive measurement of regurgitant jet velocity using continuous-wave ultrasound Doppler measurements together with cuff measurements of systolic and diastolic systemic pressure in the arm. These measurements are then used for parameter estimation in a Windkessel-like model which includes the regurgitant orifice as a parameter. The aortic volume compliance and the peripheral resistance are also included as parameters and estimated in the same process. Electromagnetic flow measurements in the ascending aorta and pulmonary artery are used for control. The correlation obtained between regurgitant volume derived from parameter estimation and from electromagnetic flow measurements is 0.95 over a range from 2.1 to 17.8 mL.<<ETX>>

Quantification of aortic regurgitation by Doppler echocardiography: a new method evaluated in pigs

We have developed a method to quantify aortic regurgitant orifice and volume, based on measurements of the velocity of the regurgitant jet, aortic systolic flow, the systolic and diastolic arterial pressures, a Windkessel arterial model, and a parameter estimation technique. In six pigs we produced aortic regurgitant flows between 2·1 and 17·8 ml per beat, i.e. regurgitant fractions from 0·06 to 0·58. Pulmonary and aortic flows were measured with electromagnetic flow probes, aortic pressure was measured invasively, and the regurgitant jet velocity was obtained with continuous-wave Doppler. The parameter estimation procedure was based on the Kalman filter principle, resulting primarily in an estimate of the regurgitant orifice area. The area was multiplied by the velocity integral of the regurgitant jet to estimate regurgitant volume. A strong correlation was found between the regurgitant volumes obtained by parameter estimation and the electromagnetic flow measurement. These results from our study in pigs suggest that it may be possible to quantify regurgitant orifice and volume in patients completely noninvasively from Doppler and blood pressure measurements.

Impact of changes in heart rate and stroke volume on the cross sectional flow velocity distribution of diastolic mitral blood flow. A study on 6 patients with pacemakers programmed at different heart rates.

The effect of changes in stroke volume on the cross sectional velocity distribution in the mitral orifice during passive mitral inflow was studied in six patients with total atrioventricular block, atrial fibrillation and VVI pacemakers during periods with different heart rates. The time velocity integrals recorded both in the left ventricular outflow tract and at the mitral orifice decreased significantly as the heart rate was increased from 60 to 80 and from 80 to 100 beats per minute.Instantaneous cross sectional flow velocity profiles were constructed by time interpolation of the velocity data from each point in sequentially delayed two dimensional digital ultrasound maps. Each patient had a characteristic cross sectional flow velocity profile in the mitral orifice recorded at the level of the leaflet tips in a four chamber view. The velocity profiles varied between the patients. With increase in heart rate only minimal changes in the flow profiles from individual patients were seen.The maximum velocity through the mitral orifice overestimated the cross sectional mean velocity at the same time by a factor of 1.4–1.9. The maximum time velocity integral overestimated the cross sectional mean by a factor of 1.4–1.8. The observed cross sectional skew varied between patients but did not change significantly with increasing heart rate and decrease in stroke volume.

Realtime Automatic Assessment of Cardiac Function in Echocardiography

Assessment of cardiac function by echocardiography is challenging for nonexperts. In a patient with dyspnea, quantification of the mitral annular excursion (MAE) and velocities is important for the diagnosis of heart failure. The displacement of the atrioventricular (AV) plane is a good indicator of systolic left ventricular function, while the peak velocities give supplementary information about the systolic and diastolic function. By measuring these parameters automatically, a preliminary diagnosis can be given by the nonexpert. We propose an automatic algorithm to localize the mitral annular points in an apical four-chamber view and estimate the MAE, as well as the systolic, early diastolic, and late diastolic tissue peak velocities, by using a deformable ventricle model for orientation and tissue Doppler data for tracking. Automatic parameter estimates from 367 tissue Doppler recordings were compared to reference measurements by experienced cardiologists to assess the accuracy of the estimation, as well as the ability to correctly detect reduced MAE, which we defined as less than 10 mm. The dataset consisted of 200 recordings from a patient population and 167 healthy from a population study. When considering the average of the septal and lateral values, the estimation error for the MAE had a standard deviation of 2.1 mm, which was reduced to 1.9 mm when excluding recordings for which the automatic segmentation failed to locate the AV plane (41 recordings). The corresponding standard deviations for the peak velocities were around 1 cm/s. The classification of MAE was correct in 90% of the cases and had a sensitivity of 83% and a specificity of 92%. We conclude that the algorithm has good accuracy and note that the estimation error for the MAE was comparable to interobserver and methodology agreements reported in the literature.