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so in this lecture we'll be reviewing
some normal abdominal and pelvic CT
anatomy and we'll be going over a
standard approach to when we're looking
at a CT scan so what I do when I look
through a CT scan is I use multiple
window levels and I scroll through
multiple times it's important to
evaluate only one organ at a time the
abdominal CT scan has a lot of different
organs in there and so if you try to
take a look at everything at once it's
very likely that you'll miss something
so it's important to take a look at one
organ at a time and scroll through
multiple times through that organ before
moving on to the next one by focusing on
only one organ you'll be less likely to
miss findings so let's come up with a
standard approach what I start off with
is taking a look at the lung windows so
I start off by looking at the lung basis
and then I scroll all the way through
the abdomen and pelvis and I look for
any kind of free or abnormal collections
of air I then look through the bone
windows and I take a look for any kind
of a cult fracture it's important in all
of Radiology that when someone comes in
with a clinical finding you don't focus
on only that one finding it's important
to take a look at the entire scan and
make sure that you're seeing even the
incidental findings that the patient may
have I then take a look at the liver and
I scroll multiple times through the
liver I then go on to the spleen the
gallbladder and the pancreas and again
through each of these I scroll through
multiple times to make sure that I'm
evaluating the entire organ before I
move on to the next one I then take a
look at the renal collecting system and
the kidneys the adrenal glands and then
I go further down and take a look at all
of the pelvic organs I then evaluate the
entire mesentery the blood vessels and
then you want to take a look at the
entire bowel always check the appendix
in a patient that's coming in with
abdominal pain or even in a patient
that's not coming in with abdominal pain
because oftentimes you may find an
occult appendicitis that just hasn't
become symptomatic I then take a look at
all of the soft tissues so long windows
are very useful in identifying small air
bubbles so every abdominal CT should be
evaluated in lung windows to look for
free internal air or abnormal
collections of air
so as you can see on the single slice
this is all air that's located within
the bowel and let's take a look at these
CT images so can you see where the free
air is
so here's a collection of abnormal air
if you compare it with the air that's
located just posterior to it you can
tell the difference the air that's
located posterior to it appears to form
within the bowel wall while the air
that's circled is actually outside of
any kind of structure and so this is how
you can identify free air when you look
through long windows next I take a look
at the CT scan and bone window and I
evaluate all of the different bony
structures this is just an example of a
couple of bony structures that you can
see so we have multiple ribs and then
you have the vertebral body as well as
the spinous process of the vertical body
I then go on and take a look at the
liver so the liver is divided into right
left and caudate lobes and those are
then subdivided into multiple segments
by the vessels you may often hear about
the quadrate lobe as well and that's
actually now known as the medial segment
of the left lobe so the middle hepatic
vein divides the liver into the right
and the left lobes the right hepatic
vein divides the right lobe into its
anterior and posterior segments and the
falciform ligament divides the left lobe
into its medial and lateral segments the
portal vein then divides the liver into
upper and lower segments so this is an
example of the medial segment of the
left lobe of the liver here we have the
right lobe of the liver
we don't definitely see the hepatic
veins here but it's probably located
somewhere around here and you would see
it as you scroll through the liver this
is the lateral segment of the left lobe
of the liver and as you remember the
falciform ligament right here is what
divides the medial and lateral segments
of the left lobe down here we see the
caudate lobe so about 80% of the
vascular supply to the liver is portal
vein but it also has a partial supply by
the hepatic artery about 20% or so
normally the liver should have a very
smooth contour and if it doesn't do you
want to suspect an abnormality the
density of the liver should equal that
of the spleen so as you can see here
this is the normal appearing liver
which appears about the same density as
a normal appearing spleen the spleen is
about 12 centimeters in longitudinal
dimension it's actually best measured on
ultrasound and it does tend to appear
heterogeneous on arterial phase imaging
but it should appear homogeneous on
portal venous phase imaging and again
about the same density as the liver so
here we have the pancreas just to take a
look at the surrounding structures we
can see here the liver which is adjacent
to the pancreatic head just anterior to
the pancreas we have the stomach which
is partially filled with contrast here
and then just posterior to the pancreas
we have the kidney here and this is the
left kidney that I'm pointing at right
now so the pancreas is not normally seen
on a single slice you actually have to
scroll through multiple slices to see
the pancreas the normal pancreatic duct
is about three to four millimeters and
in general it shouldn't really be
visualized on a CT scan as you can see
here we actually don't see a normal duct
within this pancreas and if you do see
it you want to suspect that it may be
slightly dilated or abnormal the
pancreas consists of the head which is
located approximately here the uncinate
process which comes down a little
inferior ly the body and the tail so
each of these are general definitions
there's really no anatomical division
here and again it's located in multiple
planes and really can't be seen on a
single axial slice so now let's move on
to the gallbladder the gallbladder is
kind of embedded underneath the liver
but it's pretty well seen on a CT scan
here we have an axial CT scan which
shows you this fluid-filled sac and here
we have a coronal CT scan which shows
you the gallbladder as well it's
actually located between the right and
left lobe of the liver and it's really
best evaluated by ultrasound so if there
is an abnormality that's suspected
within the gallbladder an abdominal
ultrasound is really the first line of
imaging the gallbladder wall should not
measure more than about three
millimeters in thickness and again
that's really best to see it on
ultrasound but may be suspected on CT
the adrenal glands are located above the
kidneys they're shaped like an
upside-down Y and you can see them here
they're small and they can be sometimes
hard to identify especially when there's
adjacent pathology so now let's take a
look at a renal anatomy so here we have
a diagram of the kidney which
demonstrates a cortex right here we have
multiple renal pyramids which comprise
the renal medulla and then we have the
renal hilum where we have the ureter and
blood vessels going in and out of the
kidney here we have a portion of the
proximal ureter coming out of the kidney
so kidneys are retroperitoneal organs
their hilum again consists of the renal
pelvis the artery and the vein the
pelvis is what drains out into the
ureter renal lesions are often localized
into the upper Pole the lower pole or
the inter polar region the inter polar
region is located around the level of
the hilum and again there's no real
anatomical division but just an
approximate division the urinary bladder
is seen within the pelvis as a
fluid-filled structure the wall is
usually about five millimeters or less
and you can see it here in both the
axial and the coronal CT scans so what
is intraperitoneal versus
retroperitoneal the parietal peritoneum
is a thin membrane that lines the
abdominal and pelvic walls and the
visceral peritoneum is an invagination
of the parietal peritoneum that lines
most of the abdominal organs so if a
structure is located intraperitoneal it
means that the organ is lined all around
by the visceral peritoneum if it's
retroperitoneal that means it's lined on
only one side by the visceral peritoneum
so there's a long list of organs that
are retroperitoneal i remember it by the
mnemonic sad pucker so s stands for
super renal or adrenal glands AAA is for
a Horta and IVC the D is the duodenum
but that excludes the first two to three
centimeters which
actually intraperitoneal we have the
pancreas except for the pancreatic tail
the ureters the colon predominantly the
ascending and the descending colon the
transverse is actually intraperitoneal
the kidneys are retroperitoneal organs
the esophagus or the portion of the
esophagus that's within the abdomen and
then the rectum so let's take a look at
the vessels within the abdomen when the
scan is performed in arterial phase
imaging we can see the arteries very
well and this is a coronal and a
sagittal reconstruction of a CT scan
performed in the arterial phase here you
can see the aorta coming down and then
you can see both her renal arteries on
the sagittal view or the view where
you're looking at the side from the side
of the patient you can again see the
aorta coming down and then you have the
celiac artery superiorly here and the
superior mesenteric artery as you scroll
through these CT images you'll be able
to follow each of these vessels out this
is actually a 3d rendering that that was
created off of the CT scan that was
performed in the venous phase you can
see the venous system very well so again
we have an axial CT image and then we
have a coronal CT image in the portal
venous phase and here you can see the
renal veins you can see a portion of the
inferior vena cava and you still have a
little bit of contrast within the aorta
here you can actually see the inferior
vena cava a little bit better on this
coronal image and you can see one of the
renal veins coming out this is again a
portion of the abdominal aorta that has
calcification and than the walls because
of atherosclerosis 'yes this is another
slice within the portal venous phase and
now you can see the portal vein really
well adjacent to the liver hilum and
then here we have it branching into the
superior mesenteric vein and the splenic
vein so when you take a look at the
bowel you really need proper distension
for accurate assessment the normal bowel
wall thickness is about less than 3
millimeters and as you can see here this
is a normal appearing colon it actually
looks like there are areas of wall
thickening
and some of this could be because of
incomplete distension from the oral
contrast and it could also be because of
peristalsis of the bowel here we have an
axial CT image and you can see the
difference so when the small bowel here
is fully distended you actually don't
see any of the wall around it here we
have a section of small bowel that's not
fully distended and it appears to have
wall thickening but this is probably
because it's in completely distended so
when you are evaluating for bowel wall
thickening you want to look for other
secondary changes may be surrounding
inflammatory change would help you and
you also want to see whether that
portion of the barrel is fully distended
or not so let's take a look at this case
what are the different phases that are
depicted here and do you see the
pathology
so this is an example of renal calculi
if you look at the first scan this is
performed without intravenous contrast
and you can see the arrow points to an a
high density within the kidney which
represents a stone the second image is
performed in the portal venous phase and
here you can also see the stone and you
can also see a little bit of contrast
within the surrounding structures
however let's take a look at the delayed
phase so this is the phase that's
performed at about ten minutes or so and
you can see a lot of contrast within the
renal pelvis
this actually obscures the valuation of
the stone and this is the reason why you
don't want to give contrast in a patient
where you're suspecting a renal calculus
because you may actually miss the stone
because the contrast will hide it so
we've gone over some normal abdominal
anatomy and we've gone over some normal
techniques and a good approach to the CT
of the abdomen and pelvis hopefully this
will provide a good groundwork as we
move on to some pathology
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