bk3500 User Guide
(16-01410-EN-04)
Safety Information
49
The measurement accuracies are based on the assumption of “ideal” tissue, that is, a
tissue characterized by a sound velocity of 1540 m/s. When making clinical
measurements with ultrasound, errors may arise which are not taken into account in
this section. For example,
•
The sound velocity may vary from approximately 1450 m/s in fatty tissue to
1585 m/s in muscle. This can, in simple cases, give rise to errors of up to 6% for
linear measurements. This inaccuracy may be further increased by refraction
occurring at tissue boundaries.
•
The user can introduce errors when using approximate formulas, when
positioning the system’s calipers with respect to the ultrasound image and when
outlining structures in the image.
•
The ellipsoid volume approximation, described in this user guide, is only
applicable when the cross section of the structure being studied approximates an
ellipse (the circle being a special ellipse), and when the structure is roughly
symmetrical about the selected axis of rotation.
NOTE:
The choice of the axis of rotation is important for the calculation of the
volume. A vertical axis gives a different volume than a horizontal axis.
The minimum requirement is that the cross section outlined by the user should be
convex. If the user draws a non-convex outline, an inaccuracy is introduced which is
not taken into account in this section. In this case, the system calculates and displays
the convex hull of the figure, that is, the smallest convex figure containing the non-
convex figure outlined by the user (see Fig 3-5). The system bases the calculation of
the ellipsoid volume on the convex hull.
Figure 3-5. The dotted line indicates the convex hull of the non-convex figure.
Measuring volume using a stepping method produces an approximation caused by
the finite number of steps in the measurement. The user must always try to assess
how large an inaccuracy is introduced by the selected step size, that is, the distance
between organ cross sections.
