3D transvaginal ultrasound: interactive teaching models

3D principle

3D transvaginal ultrasound: how it works

How many 2D images become one 3D view of the uterus.

2D ultrasound

Optimising a 2D ultrasound image

Start from a poor image and adjust the angle of insonation, frequency and harmonics, focal point, contrast, zoom and gain. Each control changes the image the way it does on the machine, and the numbers beside it show what changed physically.

2D

How this simulation works

Simulated image, not patient data, built from a simplified model: speed of sound 1540 m/s, soft-tissue attenuation 0.5 dB/cm/MHz (round trip 2 × for the fundamental; harmonics transmit at half the image frequency and receive at the image frequency, 1.5 ×), smooth interfaces reflect most strongly at 90°, a Gaussian transmit beam, and log compression to the dynamic range. Read zoom stretches the existing pixels about the focal zone; write zoom keeps the fan angle, enlarges everything about the fan tip and scans the shorter depth again.

Angle of insonation
This is the angle of the beam fan, up to 180°. At a narrow fan the uterus is only partly visible. Open the fan until the whole uterus is visible; wider than that only lowers the frame rate.
Frequency and harmonics
Higher frequency gives finer detail but less penetration; lower frequency penetrates deeper with coarser detail. Harmonics (transmit at half the image frequency, receive at the image frequency) give a cleaner, sharper image.
Focal point
Put the focus at the depth of the target. The beam is narrowest there, so detail is sharpest and the echo brightest.
Contrast
Turn contrast up to make white and black more distinct, so the endometrium stands out from the myometrium. Turn it down and the picture becomes a flatter grey scale. Too high loses the mid greys.
Zoom
Read zoom (like digital zoom) enlarges the stored picture about the focal zone and adds no detail. Write zoom (like optical zoom) keeps the fan angle and the fan tip fixed and makes everything bigger: the depth shortens and is scanned again, so the frame rate rises and real detail appears.
Gain
Set gain last so the myometrium is mid-grey and fluid stays black. Too much gain washes the image out and fills cysts with noise.

Image quality

1 · Angle of insonation (fan angle)

2 · Frequency

3 · Focal point

4 · Contrast

5 · Zoom

6 · Gain

Multiplanar

Z technique for the mid-coronal plane of the uterus

Three orthogonal planes cut through one simulated volume. Move the reference dot, or rotate any plane about its X, Y or Z axis, and the other two follow.

OmniView

Start from the mid-sagittal plane on the left. Draw freehand along the endometrium and the coronal view builds up on the right as you draw. Rotate X, Y or Z in either image to refine it.

Drag does

Fine adjustment

Xabout the image's horizontal axis Yabout the image's vertical axis Zin the plane of the image ↕parallel shift

VCI slice thickness

Uterus

Simulated volume, not patient data. The line is drawn freehand. X, Y and Z rotate the volume about the axes of the image you work in, pivoting on the middle of the line. Volume contrast imaging (VCI) averages a slab of the chosen thickness around the OmniView plane.

Volume contrast imaging

VCI of the uterus

VCI shows a slab of the chosen thickness instead of a single plane: speckle is suppressed and contrast improves, but a thick slab blurs edges. Drag the divider in the VCI image to compare VCI off with VCI on.

TUI

TUI: tomographic ultrasound imaging

TUI shows parallel slices through the volume at a set interslice distance, like CT or MRI. The reference image marks where each slice lies.

SonoAVC

SonoAVC: automated follicle measurement

Fit the fan-shaped ROI to the ovarian borders in planes A, B and C, run SonoAVC, then check the result and correct it with Add and Remove.

Set the ROI
Choose ROI, then drag the edges of the fan in planes A, B and C to the ovarian borders.
Run SonoAVC
Automatic measures every fluid-filled structure inside the ROI. Manual starts empty so you tap each follicle.
Check the result
Move through the ovary. Add a follicle that was missed; remove anything that is not a follicle.

Follicles by d(V)

#	d(V)	dx	dy	dz	mn(d)	V (mL)

Simulated ovary, not patient data. Values are calculated from the model geometry, in mm, and only the part of a structure inside the ROI is measured. The ROI is a fan whose apex lies above the image, like the transvaginal sector. d(V) is the diameter of a sphere with the same volume (relaxed sphere diameter); dx, dy and dz are the maximum diameter and the two diameters perpendicular to it; mn(d) is their mean. In this model the automatic run demarcates rounded fluid-filled structures of 0.1 mL or more; the elongated pelvic vessel beside the ovary is left out, but can still be measured by hand with Add. Raine-Fenning N et al. Ultrasound Obstet Gynecol 2008;31:691–696.

Fibroid mapping

Fibroid mapping in 3D

Plane A (mid-sagittal) is on the left and the right image starts blank. Trace the endometrial cavity freehand and the machine renders the uterus in 3D on the right. Then tap a fibroid, in plane A or in the 3D image: it is detected, its mean diameter is measured and its FIGO type is assigned.

Trace the cavity
Choose Trace cavity, then draw freehand along the endometrium in plane A. The uterus is rendered in 3D on the right.
Tap a fibroid
Choose Tap fibroid and tap a fibroid in plane A or in the 3D image. Sweep plane A to the left or right to reach the others.

Drag does

Sweep plane A:

FIGO leiomyoma types

0 Pedunculated intracavitary
1 Submucosal, <50% intramural
2 Submucosal, ≥50% intramural
3 Contacts endometrium, 100% intramural
4 Intramural
5 Subserosal, ≥50% intramural
6 Subserosal, <50% intramural
7 Subserosal, pedunculated
8 Other (e.g. cervical, parasitic)
Hybrid (e.g. 2–5): submucosal and subserosal, each with less than half the diameter in the endometrial and peritoneal cavities

Simulated uterus, not patient data. Mean diameter is the mean of the three orthogonal diameters. Diameters, volume, volume shares and distances are calculated from the model geometry; contact with the endometrium is taken as ≤1 mm. In the 3D image the colours show where the surface of the fibroid lies: in the cavity, within the myometrium, or beyond the serosa. Volume uses the ellipsoid formula (π/6 × d1 × d2 × d3). FIGO system: Munro MG et al. Int J Gynaecol Obstet 2018;143:393–408.

Doppler

SlowflowHD and Radiantflow

A simulated sagittal plane of the uterus with arcuate vessels in the outer myometrium, radial arteries crossing it and spiral arteries near the endometrium. Compare colour Doppler with SlowflowHD, then add Radiantflow.

Colour Doppler
At a standard scale the wall filter removes slow flow: arcuate and radial flow show, spiral arteries do not.
Lower the scale
Slower flow starts to pass the wall filter, but faster flow aliases and tissue motion causes colour flash.
SlowflowHD
Low-PRF continuous acquisition with wall filters that adapt to tissue movement: slow flow in small vessels in fine detail, without flash.
Radiantflow
Uses the colour Doppler amplitude to give a 3D-like appearance, with clearer vessel edges.

Flow mode

Radiantflow

Scale (PRF)

Colour gain

How this simulation works

Simulated image, not patient data; vessel sizes and velocities are illustrative, not reference values. Nyquist velocity = PRF × 1540 m/s ÷ (4 × 5 MHz) at 0° to the beam. Colour Doppler: wall filter at 10% of the Nyquist velocity. SlowflowHD: low-PRF continuous acquisition and wall filters that subtract tissue motion (Drukker et al., Ultrasound Med Biol 2022). Radiantflow is shown by shading the colour with the Doppler amplitude.