DOI:10.2214/AJR.05.1462
AJR 2006; 187:965-971
© American Roentgen Ray Society
Inversion Mode Display of 3D Sonography: Applications in Obstetric and Gynecologic Imaging
Beryl R. Benacerraf1,2,3
1 Department of Radiology and Department of Obstetrics and Gynecology, Brigham
and Women's Hospital, Boston, MA.
2 Department of Obstetrics and Gynecology and Department of Radiology,
Massachusetts General Hospital, Harvard Medical School, Boston, MA.
3 Diagnostic Ultrasound Associates, PC, 333 Longwood Ave., Ste. 400, Boston, MA
02115.
Received August 19, 2005;
accepted after revision December 15, 2005.
Address correspondence to B. R. Benacerraf.
Abstract
OBJECTIVE. Three-dimensional sonography involves volume acquisitions
of sonographic data that can be displayed in a variety of ways, including
surface rendering and multiplanar reconstruction. A new method of displaying
sonographic volumes is called the inversion mode, which displays the cystic
portions within the entire volume as echogenic areas. The grayscale portion of
the image becomes transparent, and the cystic areas become brightly visible in
three dimensions.
CONCLUSION. This article shows the applications for this method of
volume sonography display and shows the importance of being able to visualize
all of the cystic areas concurrently within a volume.
Keywords: 3D sonography • pelvic imaging • sonography • volume imaging • women's imaging
Introduction
Three-dimensional sonography, or volume imaging, represents a major advance
in the field of sonography in the past several years
[1,
2]. It permits acquisition of a
volume of sonographic data to be displayed in many different ways, including
surface rendering of an interface and multislice reconstruction of any scanned
plane. This advance has permitted sonography to participate in cross-sectional
imaging, much the way contemporary MRI and CT displays reconstruct and
automate cross-sectional image planes. This feature can reduce the operator
dependency usually needed with 2D sonography
[2].
A 3D sonographic volume contains all of the sonographic information within
it and offers different options for display. Surface rendering of the entire
volume is possible only when an interface surrounded by fluid exists,
permitting the display of a surface of the volume. Alternatively, the inside
of the volume can be viewed as multiple cross-sectional images, much like CT
and MRI. Using these multislice displays, only a small portion of the volume
can be viewed at one time. However, a new way of rendering the entire volume
is now available that allows visualization of all cystic areas within the
volume in a way that neither surface rendering nor individual reconstructed
slices can achieve. These cystic areas are normally hidden within the volume
when traditional surface rendering is used; only parts of the cystic areas
that are in one plane are displayed if single-slice techniques are used.
This article describes the use of this new method of volume imaging, called
inversion mode, in which the cystic portions within the volume are displayed
in their entirety as an echogenic area, while the grayscale portions of the
image are rendered as transparent. Other studies have shown that inverting the
sonography display such that the cystic areas are echogenic rather than
hypoechoic improves our ability to see the details of the fluid collections
being imaged
[3-5].
However, the inversion mode provides a far greater benefit than previously
described: It actually allows all solid areas to become transparent and all
cystic areas within the entire volume to be seen concurrently together.
The image is created by a postprocessing function in which all the cystic
voxels within the volume are tagged as opaque or echogenic, and all the solid
or echogenic voxels are tagged as radiolucent. This technique results in a
cast of all the cystic portions within a volume, making the solid areas
transparent. This display is novel and opens many diagnostic options
heretofore unavailable or difficult to attain sonographically. Here are some
examples of when this new type of volume display can be helpful in imaging the
female pelvis and the fetus.
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Fig. 3B —Hydrosalpinx and ovarian follicle. Inversion mode shows
hydrosalpinx as echogenic. In addition, rounded cyst (C) is seen just above
hydrosalpinx, consistent with follicle in ovary. Also note fimbriated end of
tube distally (arrows). These additional features are not all in one
plane and can be visualized together only if all cystic areas in entire volume
are displayed concurrently. Inversion mode of sonographic volume containing
adnexa shows that septate cyst actually represents hydrosalpinx when viewed in
its entirety.
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Fig. 4C —Polycystic ovaries. Magnified image of multicystic ovary seen
with inversion mode. Note that all follicles are visible and can be easily
counted. Sonography machine can also calculate volume of each highlighted
follicle.
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Hydrosalpinx
With standard 2D imaging, a hydrosalpinx often appears as several cysts
separated by septa, which can be misinterpreted as a multiseptate ovarian cyst
or cystic mass (Figs. 1A and
1B). Using standard, 3D
multislice reconstruction, a more tubular appearance of the hydrosalpinx can
be displayed, although, again, the hydrosalpinx would need to be oriented in a
single plane for it to be shown in its entirety on one image
[3].
The inversion mode shows all of the cystic areas within the entire volume
by converting them from anechoic into hyperechoic areas. All of the solid
areas become transparent and are substrated from the image, leaving only the
cystic or fluid-containing areas clearly visible. This enables us to view the
entire hydrosalpinx, even if the tube is convoluted in multiple planes
[3] (Figs.
2A and
2B).
As shown in Figures 3A and
3B, the inversion mode can also
show a follicular cyst within the ovary, adjacent to the hydrosalpinx, as it
too is cystic and is contained within the volume. The image containing the
follicular cyst and the entire hydrosalpinx could not otherwise be displayed
without the use of the inverse mode, which enables us to view all of the
cystic areas contained within the entire volume at the same time.
Ovarian Cyst
Any ovarian cyst rendered with the inversion mode will be echogenic with a
black background rather than echolucent with an echogenic rim as is usually
seen in standard sonographic displays. The advantages to using this mode
include instant measurement of the cyst volume and easy determination of the
number of cysts residing in the entire volume. This information can be helpful
when measuring and counting follicles for infertile patients by documenting
all of the findings in an entire ovary using only one volume. In addition, for
patients with polycystic ovaries (Figs.
4A,
4B, and
4C), all the tiny follicles can
be shown and counted quickly.
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Fig. 6B —Third trimester fetus with hydronephrosis and hydroureter.
Inversion mode of fetal hydronephrosis and hydroureter shows far more than
just one plane. Entire distended urinary tract is displayed at once, down to
bladder, and includes blunted calyces.
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The Uterine Cavity
Saline hysterosonography is used to evaluate the endometrium in patients
with abnormal bleeding. By instilling saline into the uterine cavity, the
endometrium is outlined by fluid, and any polyp or submucous fibroid is easily
seen. The inversion mode permits the saline in the uterine cavity to be
echogenic, hence simulating positive contrast. In addition, the entire cavity
and any other fluid collections are visualized simultaneously, providing a
comprehensive picture of these cystic areas
[3] (Figs.
5A,
5B, and
5C).
Fetal Anatomy
Imaging fluid-containing structures within the fetus can be facilitated
using the inversion mode [4].
For example, when imaging a hydronephrotic kidney, the inversion mode provides
not just the image displayed in one slice but the entire urinary collecting
system showing the pyelectasis, caliectasis, and hydroureter down to the
bladder [4]. This type of
display would not be possible using any single slice, as the kidney extends
through multiple scan planes. This example shows how using the entire volume
in the display can facilitate observation of all the abnormalities
concurrently (Figs. 6A and
6B).
The inversion mode can be useful for evaluating the head of a fetus with
hydrocephalus by displaying the entire ventricular system without the
surrounding brain (Figs. 7A,
7B,
7C,
7D, and
7E). This display is possible
only if the entire volume is included and if the gray-scale portion of the
image is subtracted [4].
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Fig. 7A —Third trimester fetus with ventriculomegaly. Transverse view
(standard 2D) of fetal head at 38 weeks shows ventriculomegaly with bilateral
anterior horn cystic abnormalities. Hydrocephalus and anterior horn cysts were
unchanged at birth.
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Fig. 7D —Third trimester fetus with ventriculomegaly. Three views of
inversion mode from same patient, show echogenic display of anterior horn
cysts anteriorly and dilated ventricles posteriorly.
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Fig. 7E —Third trimester fetus with ventriculomegaly. Three views of
inversion mode from same patient, show echogenic display of anterior horn
cysts anteriorly and dilated ventricles posteriorly.
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The fetal heart and circulatory system can also be imaged using the
inversion mode, providing the opportunity to view the entire cast of the fetal
heart and circulatory system in one image
[5]. This display can be
rotated, as it includes the entire 3D information rather than only one slice
of 2D information. This cast of the circulatory system (Figs.
8A and
8B) is akin to an angiogram or
a power Doppler image without the use of Doppler sonography, thus avoiding
potential Doppler artifacts. Four-dimensional (4D) evaluation of the heart can
also include the inversion mode, showing a real-time display similar to an
angiogram.
Discussion
The applications discussed here show that the new inversion mode is
potentially a powerful way to display all the cystic or echolucent areas
within an entire volume without being hampered by the gray-scale portion of
the image. By transforming the echolucent, cystic areas into echogenic areas
and deleting the gray-scale areas, the inversion mode permits a display of
only the cystic areas within a volume, heretofore unachievable in sonography.
This novel display of volume sonography has many potential benefits and uses
in imaging, only some of which are depicted in this essay. The clinical
benefit is substantial for many different applications. For example, a
hydrosalpinx is a convoluted tube that passes through several different planes
and may not be displayed in any one slice or plane. Only if all the cystic
areas in all potential planes are displayed can a true cast of the entire
hydrosalpinx be seen. This type of display has great clinical potential to
increase diagnostic accuracy and certainty.
The inversion mode can also display complex fluid collections if the
threshold is set low enough for the hypoechoic (not necessarily only anechoic)
structures to be included in the opaque cast. The threshold is set by the
operator according to clinical and initial imaging information. The potential
artifacts created by this technique are related to the threshold at which the
inversion of the voxel displays takes place. If the threshold is set high,
only the purely cystic areas are made opaque; if it is set low, more
structures will be included, perhaps with low-level echoes in the inversion.
The image display achieved is determined by these postprocessing settings,
which make it more or less sensitive to the cystic areas within the volume. An
understanding of how to optimize the threshold settings is crucial for
producing useful images.
The inversion mode is simply the production of a cast of all the cystic
areas within a volume. The other main potential artifacts include the ones
related to 3D reconstruction of planes within a volume, such as motion
artifacts, and acoustic shadows cast in unpredictable directions unless one is
aware of how the volume was acquired. The main artifact to recognize when
specifically applying the inversion mode is the use of the threshold at which
the inversion takes place. Through-transmission is eliminated when the
inversion mode is used, because any solid or echogenic areas become
transparent by the process of producing the cast of the cystic areas. This
does not seem to be a problem, because the inversion mode images are not used
to determine whether an area is cystic or solid (that can be done with
standard displays). Rather, the inversion mode is used to view the
distribution of the cystic areas within the space of the entire volume (rather
than one plane).
Many applications are likely to benefit from the inversion mode display of
sonographic volumes other than obstetrics and gynecology. Some examples are
evaluation of the biliary tree, mapping of vascular abnormalities, and
evaluation of the adult urinary tract, in addition to many more.
Three-dimensional sonography, or volume imaging, is in its infancy and has
enormous potential to propel sonography into the next phase of
development.
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Abstract
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