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109
8
Radiographic appearance of the
mesenteryandperitoneum
J. CALVIN COFFEY AND MARTIN SHELLY
Learn to see things as they really are, not as
weimagine they are.
Vernon Howard
AIMS
e rst aim is to summarize the historical development
of the fi ld of mesenteric and peritoneal reection radiol-
ogy. e second aim is to present an atlas of the radiologic
appearance of the mesentery and associated peritoneal
reection, based on recent developments in our under-
standing of their anatomy.
INTRODUCTION
Radiologic investigation is a noninvasive means of visu-
alizing structures. It is a key adjunct in the diagnosis and
management of diseases. Given recent developments in our
understanding of mesenteric and peritoneal reection anat-
omy, it is important to reappraise the radiologic appearance
of these in normality and disease states.
e radiologic appraisal of intraperitoneal or intra-
abdominal structures is invaluable in the assessment of
intra-abdominal diseases. e development of computer
tomography (CT) and magnetic resonance (MR)–based
imaging modalities transformed abdominal imaging
generating real-time snapshots of structures inside and out-
side the intestine. For decades, the emphasis in abdominal
imaging was delineation of the intestinal tract itself using
contrast-based techniques. With CT and MR and the col-
lation of axial, sagittal, and coronal views, extra-intestinal
structures could be identifi d and reconstructed to provide
an improved appraisal of diseases [1]. It is interesting that
despite this renaissance in noninvasive imaging, the resolu-
tion of fatty structures has remained relatively underdevel-
oped even to the present.
HISTORY OF MESENTERIC IMAGING
With the development of CT- and MR-based imaging
modalities, radiologists realized that the appearances of
mesenteric regions were dicult to reconcile with prevail-
ing anatomic teaching. As classic interpretations of mesen-
teric anatomy emphasized a discontinuous and fragmented
structure, it is not surprising that this was described as a
complex fi ld [2,3]. e current and simpler principle of
mesenteric anatomy, which emphasizes continuity through-
out, is a recent development and has yet to penetrate the
general body of radiologic literature [3–9].
In 1982, Oliphant proposed that the CT appearances of
the mesentery could be explained by a continuity between
mesentery and retroperitoneum (Figure 8.1) [10]. e retro-
peritoneum was called “the posterior central core” and the
Aims 109
Introduction 109
History of mesenteric imaging 109
Reappraisal of the radiologic appearance of the
mesentery andperitoneal reection 110
Difculties encountered in establishing aradiologic atlas 110
Recent advances permit generation of avalidated
atlas of mesenteric and peritoneal radiology 110
The Visual Human Project: Collating fullcolor and
radiologic images 110
OsiriX: Enhancing visualization and model generation 112
Point cloud 112
Reference atlas 116
Implications for the radiologic appraisal of abdominal
disease 117
Future studies: Assembling a comprehensive model
of abdominal anatomy 117
Summary 117
References 117
110 Radiographic appearance of the mesenteryandperitoneum
continuous space called the “subperitoneal space of Oliphant.”
Several publications emerged, which adapted this concept
tothe CT appearance of intra-abdominal pathology [11–17].
In 1986, Willie Dodd commented on how the CT appraisal
of intra-abdominal pathology was dicult to reconcile with
anatomic concepts [18]. He presciently suggested that CT
appearances may be better understood by considering the
mesenteric organ as fully outside the retroperitoneum (i.e.,
extra-retroperitoneal) (Figure 8.2). Although this suggestion
resonated with the earlier anatomic descriptions of Carl Toldt
(see Chapter 1), there was little uptake of this concept [19].
In 1993, Charnsangavej etal. reappraised the radiologic
appearance of the mesocolon using vascular markings to
identify separate mesocolic regions [20,21]. is greatly
simplifi d identication of mesocolic segments in normality
and disease states. e practicality of this approach led to its
widespread adoption [20–23]. However, they did not recon-
cile radiologic appearances with then prevailing concepts of
mesocolic anatomy. In the most recent review of the topic,
Ramachandran etal. commence their review by stating that
the radiologic appearance of the mesentery and peritoneal
reections remains a dicult fi ld [24]. eir interpretation
of the mesentery is based on classic interpretations of mes-
enteric anatomy (i.e., the mesentery is discontinuous).
Recent anatomic fi dings (see Chapter 2) resonate with
the suggestions of Dodd and Toldt, that is, the mesocolon
persists throughout its length into adulthood as an entirely
extra-retroperitoneal organ [18]. As is seen in the preceding
chapters, Culligan etal. demonstrated, using a combination
of dierent investigatory modalities, that the adult mesen-
tery is continuous from the duodenojejunal flexu e to the
anorectal junction (Figure 8.3a and b) [4,5]. Crucially, it fans
out from a root region at the SMA origin, to span the entire
intestine from duodenum to anorectal junction. Although it
attaches to the retroperitoneum in several regions, it remains
separated from this by Toldt’s fascia. While a vascular conti-
nuity occurs at the superior and inferior mesenteric vascular
pedicles, there is no adipose contiguity [3,6,25–28].
REAPPRAISAL OF THE RADIOLOGIC
APPEARANCE OF THE MESENTERY
ANDPERITONEAL REFLECTION
Difculties encountered in establishing
aradiologic atlas
Ideally, for radiologists to interpret the radiologic appear-
ance of the mesentery, a reference atlas should be developed,
which collates anatomic and radiologic images. Several fac-
tors hampered the development of one such atlas for the
mesentery. First, the shape of the mobilized mesentery bares
little if any resemblance to that occupied in the undisturbed
state. As a result, it is impossible to infer mesenteric shape
from resected specimens. In turn, this has implications for
related structures such as the peritoneal reection.
Second, the fascia separating the mesentery from under-
lying retroperitoneum is exquisitely thin (see Chapter 5) and
thus subject to obliteration during xation processes. In addi-
tion, the resolution of current imaging is such that the fascia
is extremely dicult to identify in nonpathologic settings.
Finally, as the boundary of the mesentery is mesothelial,
it is not possible to identify it on current CT or MRI images.
is in turn means that it is dicult to diffe entiate meso-
colic from small intestinal mesenteric regions of the mes-
enteric organ. If one is overlying the other, then one cannot
determine where the interface between both occurs.
Recent advances permit generation
of avalidated atlas of mesenteric and
peritoneal radiology
Fortunately, a number of recent advancements converged to
permit development of a reference atlas for mesenteric radi-
ology. ese include digitalization of radiologic imagery,
the Visual Human Project (VHP), and the development of
sowares that permit 3D reconstruction (e.g., OsiriX) [28].
e combination of these facilities, with recent advances in
mesenteric anatomy and histology, has enabled the devel-
opment of the rst, validated reference atlas of mesenteric
and peritoneal radiology. Before describing this atlas, the
instruments involved in its generation will be described.
The Visual Human Project: Collating
fullcolor and radiologic images
Key in the development of a reference atlas was identifying
an archive in which cadaveric anatomy (in the undisturbed
pcl
tm
srl
sim
im
dcl
hdl
lt
gsl
ghl
Figure 8.1 Schematic illustration of the subperitoneal space
and posterior abdominal core, as described by Oliphant.
Reappraisal of the radiologic appearance of the mesentery andperitoneal reection 111
Mesenteric root region
Retroperitoneum
Aorta
Mesenteric
root region
Superior
mesenteric
artery
(a)
Retroperitoneum
Mesenteric
root region
(b)
Vasculature on CT at mesenteric root region Higher-magnification view of mesenteric root region(c) (d)
Figure 8.2 (a) Axial section through the full color male dataset of The Visual Human Project at the level of the origin of
the superior mesenteric artery (SMA). The mesentery converges on (or diverges from) the SMA root. Otherwise, a direct
mesenteric continuity does not occur with the retroperitoneum. Importantly, this (the “root region”) is the origin of the
mesenteric organ distal to the fourth part of the duodenum. (b) Wider eld view of the same region as in (a) demonstrat-
ing the relationship of the mesenteric root region to adjacent structures. Coronal computerized axial tomographic illustra-
tion of the root of the mesentery under low (c) and high (d) magnication, respectively.
112 Radiographic appearance of the mesenteryandperitoneum
state) could be collated with CT-based imaging. e VHP
is a remarkable archive that permits this form of collation
[29–31]. e VHP is housed at the U.S. National Library of
Medicine and is a digital archive of human anatomy derived
from cryosectioning of a male and female cadaver [32]. Full
color photographs are available of each cryosection and
stored at a pixel resolution that permits identication of
subtle fascial planes and peritoneal reections. As these are
full color photographs, it means that mesenteric, mesocolic,
peritoneal, and fascial structures can be observed as one
might in a cadaveric dissection. Crucially, however, they
occur in the undisturbed and insitu state.
e scale of the opportunity represented by the VHP
increases further given the provision of corresponding
axial CT and MRI images (Figure 8.4a through c). As a
result, if one identifi s a particular structure in full color
images, then one can identify the same structure in the cor-
responding CT and MRI images. e VHP thus provides a
unique and invaluable opportunity to develop a CT-based
(or MRI-based) reference atlas of mesenteric anatomy and
radiolog y [7,31].
OsiriX: Enhancing visualization and model
generation
e data contained in the VHP digital imagery are such
that once imported into soware such as OsiriX (Pixmeo,
Switzerland), then the corresponding axial, coronal, and
sagittal images can be generated from full color, CT, and
MRI datasets (Figure 8.4) [28,33]. e versatility of OsiriX
is considerable and by utilizing a custom lookup table
(i.e.,CLUT), one can articially subtract mesenteric adipose
tissue leaving all remaining fascia, vessels, and peritoneal
reections (Figure 8.5). Serial axial sections can then be
stacked conceptually to generate a complete 3D model cor-
responding to the invivo mesentery, associated fascia, and
peritoneal reections. is is invaluable in demonstrating
insitu and undisturbed mesenteric, fascial, and peritoneal
anatomy in general.
By way of example, this process has been applied to
demonstrate the region where the small intestinal mes-
entery continues laterally as right mesocolon (Figure 8.5).
is region was rst identifi d in the VHP and then on the
corresponding CT image. CT images were next imported
into OsiriX. In the later, all mesenteric fat was conceptually
subtracted to retain only the associated vessels, peritoneal
reection, and underling fascia [7].
Point cloud
Nonexural mesenteric regions (i.e., the left and right
mesocolon) are readily identiable in the VHP. Flexural
regions (i.e., ileocolic, hepatic, splenic, sigmoid, and rec-
tosigmoid) are more dicult to interpret. is diculty
can be overcome using a “point cloud”–based approach.
First, in the VHP, boundaries of the mesentery are demar-
cated by a series of points. is is repeated for serial images
in the axial, coronal, and sagittal views, and the overall
point cloud is imported into a soware to “join the dots”
and create a 3D shape (Figure 8.6). e resultant model
is in a digital format, which means it can be imported
into a range of soware such as Cinema 4D, Z-Brush, and
OsiriX. e model can also be used to generate 3D printed
models of the mesentery [7,28,34].
(a) (b)
Mesentery and intestine
The mesenteric organ
Mesentery without intestine
Figure 8.3 2.5D snapshots of 3D digital sculpture demonstrating the mesenteric organ and associated intestine distal to
the SMA root region. (a) (See also QR 1/1.) Mesentery and intestine. (b) Mesentery without intestine.
Reappraisal of the radiologic appearance of the mesentery andperitoneal reection 113
Toldt’s fascia and peritoneal reflection
Retroperitoneum
Retroperitoneum
Toldt’s fascia
Toldt’s fascia
Left mesocolon
Left mesocolon
Peritoneal reflection
Peritoneal reflection(b)
(a)
Retroperitoneum
Toldt’s fascia
(c)
Left mesocolon
Left
peritoneal
reflection
Figure 8.4 (a) Axial view of left mesocolon as seen in full color version of the visual human project. (b) Corresponding
appearance on the computerized tomographic section at the same level and (c) image in OsiriX. In the OsiriX image, the
fat has been removed, leaving only the peritoneal reection for visualization. When axial images are stacked in OsiriX,
then a 3D model of the reection is apparent. This is the left peritoneal reection.
114 Radiographic appearance of the mesenteryandperitoneum
Small intestinal mesentery(c)
Right mesocolon
Left mesocolon
Mesenteric regions
Right mesocolon
Right mesocolon
Small intestinal mesentery Small intestinal mesentery(b)(a)
Left mesocolon
Figure 8.5 Panel demonstrating (a) region of continuity between small intestinal mesentery and right mesocolon in the
visual human project (VHP), (b) corresponding appearance on computerized tomographic imaging in the VHP and
(c)corresponding appearance once (b) has been imported in OsiriX.
Reappraisal of the radiologic appearance of the mesentery andperitoneal reection 115
Extraction of all
points as point cloud
Point cloud
3D digital sculpture
derived from point cloud
3D digital print
of mesentery and
intestine
CT appearance of mesorectum
(a)
(b)
(c)
Corresponding appearance of mesorectum
Development of 3D printed model of mesorectum
Points around
mesorectal margin
Points correspondin
g
to above
Figure 8.6 Flow chart demonstrating the process whereby (a) the mesentery is outlined in the visual human project,
(b)theoutline (in point format) is exported in the form of a point cloud, and (c) a 3D model of the mesorectum is
developed from the point cloud.