3D/ 4D Pelvic Floor Ultrasound
Three- and four- dimensional ultrasound has enhanced our diagnostic capabilities enormously, mainly because we now have access to the axial plane. In the past, the axial plane could only be seen with magnetic resonance imaging, or by using intracavitary, side- firing transducers, which are rarely used and impede a Valsalva maneuver. The quality of translabial 4D US is at least comparable to MR (see Fig. 1), and of course it is far superior to dynamic MR due to the fact that we can obtain volumes rather than slices, and at much higher temporal resolutions.
Video 1 demonstrates the two basic display modes currently in use on 3D ultrasound systems. The multiplanar or orthogonal display mode shows cross- sectional planes through the volume in question. For pelvic floor imaging, this most conveniently means the midsagittal (top left), the coronal (top right) and the axial plane (bottom left). Appearances such as in Video 1 are obtained by placing an abdominal 4D transducer on the perineum, with the B mode orientation as described in the 2D section of this website.
Imaging planes on 3D ultrasound can be varied in a completely arbitrary fashion in order to enhance the visibility of a given anatomical structure, either at the time of acquisition or offline at a later time. The levator ani for example usually requires an axial plane that is slightly tilted in a cranioventral to dorsocaudal direction. The three orthogonal images are complemented by a ‘rendered image’, i.e., a semitrans-parent representation of all voxels in an arbitrarily definable ‘box’, the ‘region of interest’. The bottom right hand image in Video 1 shows a standard rendered image of the levator hiatus, with the rendering direction set from caudally to cranially, which seems to be most convenient for pelvic floor imaging. Video 2 demonstrates the effect of varying the position of the region of interest, showing a fly- through of a normal pelvic floor.
The ability to perform a realtime 3D (or 4D) assessment of pelvic floor structures makes the technology superior to MRI imaging. Prolapse assessment by MRI requires ultra-fast acquisition which is of limited availability and will not allow optimal resolutions. The sheer physical characteristics of MRI systems make it much harder for the operator to ensure efficient maneuvers as over 50% of all women will not perform a proper pelvic floor contraction when asked, and a Valsalva maneuver is often confounded by concomitant levator activation. Without real-time imaging, these confounders are impossible to control for.
The possibilities for postprocessing are restricted only by the software used for this purpose; programmes such as GE Kretz 4D view (Kretztechnik Gmbh, Zipf, Austria) allow extensive manipulation of image characteristics and output of stills, cine loops and rotational volumes in bitmap and AVI format, as seen on Video 3.
Technical developments such as VCI (volume contrast imaging, see Video 3) and SRI (speckle reduction imaging) employ rendering algorithms as a means of improving resolutions in the coronal plane. By using VCI on slices of a thickness of 1-3 mm, resolutions of about 1 mm can now be reached on axial or oblique axial slices (see Video 4 for normal C plane imaging and VCI in the axial plane in a patient with major bilateral levator trauma after rotational Forceps delivery) that allow distance and area measurements both on the system and offline. Speckle reduction imaging has resulted in substantially improved tissue discrimination (see Video 5).
During or after acquisition of volumes it is now possible to process imaging information into slices of predetermined number and spacing, reminiscent of computer tomography or nuclear magnetic resonance imaging (see Figure 2). This technique has been termed ‘multislice imaging’ or Tomographic Ultrasound Imaging (TUI) by manufacturers. As opposed to CT or MRI, the location, number, depth and tilt of slices can be adjusted at will after volume acquisition. The combination of true 4D (volume cine loop) capability and TUI or multislice imaging allows simultaneous observation of the effect of manoeuvres at multiple different levels.
The pelvic floor easily lends itself to such techniques, and the author suggests using the plane of minimal dimensions (defined in the midsagittal plane as the shortest line between the posterior surface of the symphysis pubis and the levator muscle behind the anorectal angle- see the levator biomechanics page) as plane of reference, with 2.5 mm steps recorded from 5 mm below this plane to 12.5 mm above (see the section on levator trauma).
Tomographic ultrasound allows an assessment for pelvic floor trauma at a glance. The width and depth of defects can be measured or estimated, and both such measurements as well as the number of abnormal slices correlate with the likelihood of prolapse and symptoms of prolapse (Dietz 2007)
4D ultrasound also simplifies the measurement of hiatal dimensions (see Figure 3), as explained in more detail in Levator Biomechanics.
Video 5: Midsagittal plane imaging of a Monarc suburethral sling using Speckle Reduction Imaging.
Figure 1: A comparison of axial plane imaging in a nulliparous asymptomatic volunteer (MR on left, 3D pelvic floor US on right). From: Dietz and Lanzarone, Obstet Gynecol 2005; 106: 707-712.
Figure 2: Axial TUI in a patient with unilateral left-sided levator avulsion. The defect is about 1.5-2 cm wide on PFM contraction.
Figure 3:Measurement of hiatal area and circumference on Valsalva in a 2 cm rendered volume placed at the plane of minimal hiatal dimensions. This is impermissible in Euclidean geometry, but some systems let you perform such measurements and show a warning icon in the top left hand corner (arrow).
Video 1: Standard orthogonal views (A plane, top left, B plane, top right, C plane, bottom left) and rendered volume, axial plane, on bottom right, at rest and during Valsalva.
Video 2: Shifting the region of interest (box on left) shows the puborectalis muscle at varying depths. The pelvic floor is completely normal, and the urethral rhabdosphincter can be seen as a donut-shaped structure.
Video 3: Rotational volume obtained from single volume dataset, showing a Monarc suburethral sling.
Video 4: Midsagittal plane (left) and VCI C plane (right), in a patient with bilateral levator trauma