Error introduced into velocity measurements by inappropriate Doppler angle assignment

Abstract

Doppler flow measurements require precise estimates of the flow velocity as well as of the vessel's area in a transverse cross-section. The angle assignment feature has been implemented in all ultrasound equipment for velocity measurements, which are used increasingly in the management of many medical conditions1. Even though the technique has been improved, faulty velocity measurements still occur, and this may lead to unnecessary intervention2. In clinical practice, visual error could change the velocity obtained, and the same faulty misalignment introduced at, for example, 0°, 20° or 70° would generate increasing velocity values, because Cos θ is a non-linear function. Here, we show mathematically how the same error in angle assignment may generate increasingly faulty velocity measurements when the vessel is inclined increasingly on the screen. The Vm/V formula was used to calculate error in velocity measurements introduced by faulty alignment, as a percentage of the true velocity. Alignment errors of + 5°, + 3°, + 1°, − 1°, − 3° and − 5° were introduced to imaginary vessels at different inclinations (0–80°) on the screen. The vessel's inclination on the screen determines the angle correction used. For example, for a vessel at 50° from the vertical on the screen, an assigned angle correction of 47° (i.e. a − 3° error) will produce 6% difference in velocity by applying the Vm/V formula. Figure 1 shows the effect of faulty angle assignment (by + 5° to − 5°) applied to different vessel inclinations (0–80°). An error of + 5° with the vessel at an angle of 80° (i.e. applying a correction of 85° when the vessel was in reality at 80°) generated 100% error in the measured velocity. What seems to be an insignificant visual error may involve a clinically relevant decision, for example, as to whether to perform cordocentesis. Percentage error introduced into velocity measurements at different vessel inclinations with + 5° to − 5° faulty angle assignment. The angle assignment feature is useful for measuring the velocity of any vessel. Nevertheless, the question of the position of the vessel on the screen, despite the use of this feature, has not been clarified. Our proposal for velocity measurements is to set the image of the vessel in the most vertical position possible, and then use the angle assignment feature. This process is preferable because misalignments introduce relatively less variation into the velocimetry result. One application of Doppler velocimetry in clinical practice is the assessment of fetal anemia by measuring middle cerebral artery (MCA) peak systolic velocity. According to our calculations, the best results should be obtained when angle correction is used minimally. This is in agreement with a previous study2 that showed that the lowest intra- and interobserver errors in the calculation of velocity for the assessment of fetal anemia is made when the MCA is visualized at an angle close to 0°, without using any angle correction. We further suggest that whenever velocity is calculated, the angle alignment used should be reported in order to allow quantification of the degree of error in velocimetry that may have been introduced. False-positive cases might be attributable to errors generated by angle misalignment in non-vertical vessels. The effect of visual error, introduced inevitably by the operator when angle assignment is used, is minimized when the vessel is closer to the vertical on the screen. Our results do not eliminate the necessity of the angle assignment feature; this is necessary to obtain the absolute velocity and should not be abandoned. We propose that when absolute velocity is measured, sonographers should be aware that the use of elevated angle correction might increase the effect of the operator's misalignment significantly and therefore affect the clinical value of the measurement. Our main conclusion is, therefore, that for velocity measurements, the probe should be positioned to visualize the vessel in the most vertical position possible, in order to use the smallest angle assignment that the fetal position permits. M. Yamamoto*, J. Carrillo*, A. Insunza*, G. Mari , Y. Ville , * Unidad de Medicina Perinatal, Clínica Alemana de Santiago y Hospital Padre Hurtado, Universidad del Desarrollo, Santiago, Chile, Department of Obstetrics and Gynecology, Wayne State University, 3990 John R, 7 Brush, Detroit, MI, USA, Service de Gynecologie et Obstetrique, CHI Poissy Saint Germain en Laye, Université Paris Ouest St Quentin en Yvelines, Poissy, France