Introduction to CT Abdomen and Pelvis: Anatomy and Approach

The purpose of this video is to give you a solid introduction to abdominal CT. After watching this two-part video, you should be ready to start looking at CT scans of the abdomen and pelvis. Before watching this, I strongly recommend that you take the time to watch the video, a practical introduction to CT on this channel. The video covers the basic principles of CT hounds units including the density of commonly imaged tissues And the application of these basic principles in a few abdominal CT cases. Uh the video also covers the basic principles of windowing which is

obviously very important uh as well as an introduction to IV contrast and the phases uh of contrast in abdominal imaging. We're not going to cover these topics again in this talk. So again, I suggest that you watch that before watching this video. In this first video, we're going To focus on abdominal CT anatomy, mainly by scrolling through a normal CT of the abdomen and pelvis, as well as some cases. We're going to focus specifically on the most clinically essential anatomy, namely the things that you need to know before you're looking at an abdominal CT scan

efficiently. The goal is to really get you familiar with what normal looks like, including normal measurements when important. We're going to briefly touch on some Anatomic variance and some clinical pearls where appropriate, and we'll introduce several abnormal cases to illustrate the clinical importance of some of the anatomy. We'll finish with a basic approach to abdominal CT. In the second video, we'll talk about the things that you'll commonly see, the things that you should never miss, and some clinical pearls that are going to help you deal with common clinical situations. Before we jump into a normal

scan and go through basic anatomy, I think it's really important to understand a few basic things about the peritineal cavity. I start with this because when people are first starting out, they often have a difficult time conceptualizing parinal anatomy and the basics of parinal anatomy are frequently clinically important. So I do think it's a good place to start before jumping into scrolling through a CT. The abdominal cavity can be separated into the paritinal cavity here in red and the retroparitinium or behind the parinium here in blue. Detailed parinal anatomy is well beyond the scope of

this talk. Uh we could probably spend a few hours talking about parinal anatomy but it is important to understand a few basic things about parinal anatomy. So we need to introduce some basic terminology. The parinium is a thin membrane or sheet Of a single layer of epithelial cells. The parnium is a large and complex sheet. Simply put, the paritinium lines the abdominal cavity and envelops the intraaritinal or some of the abdominal cavity organs. The parinium that is lining the abdominal wall here in red is called the parietal paritinium. And the part that envelops the organs

is called the visceral paritinium here in blue. These names Aren't important. What's important to know is that all of this parnium is one continuous sheet. So the pryoparinium and the visceral parnium is one continuous sheet. And this creates a number of complex folds. What is important to know is that the space between the parinium that lines the abdominal wall and the parinium that lines the organs is called the paritinal cavity. When you hear people saying that there is free intraparitinal fluid or Free intraparitinal gas, they're talking about gas and fluid in this paritinal cavity. not

extraparal like the retroparinium here but in the parinal cavity itself as I mentioned the paritinium has a bunch of complex folds which includes uh a bunch of named paritinal ligaments peritineal ligaments are nothing more than a double layer of paritinium that supports a structure within the peritineal cavity Mentum and mezentary that you've probably heard before are nothing more than specially named paritinal ligaments or double layers of paritinium. When you hear the word mentum, it's just referring to paritinal ligaments aka a double layer of paritonyium that extends from the stomach to another structure. Mezzentary is a

paritinal ligament or a double layer of paritinium that connects an organ to the retroparitinium or posterior abdominal wall. Inside the Mezzent or between the two sheets of parinium, there are blood vessels, lymph nodes, and fat that we can actually see on CT. We don't see the paritinium itself. If that's a little bit confusing, it'll clear up very shortly. Let's look at this same diagram that we were looking at on the last slide. We can see that there are multiple paritinal ligaments. All of these are double layers of paritonyium. Here we've highlighted one of them in

red. And now We've highlighted the others on this diagram in red. These are all paral ligaments or double layers of paritonyium. Here labeled is the stomach. The ligaments that extend from the stomach here are specially named. They're just ligaments, but they're called momentum. We have the lesser momentum above here that extends from the stomach towards the liver. And we have the greater um down here, the bigger one that extends From the bottom of the stomach down the anterior abdominal cavity and then back up to the transverse colon here. These two ligaments back here that connect

the transverse colon to the retroparinium or posterior abdominal wall and the small bowel to the posterior abdominal wall are so-called mezzentary. This one here that supports the small bowel is called the small bowel mezent and it contains arteries that feed the Small bowel, veins that drain the small bowel, lymph nodes and a bunch of fat. And we can see those things on CT and we're going to see those shortly when we go through a normal CT. As I mentioned before, there are many named ligaments that are beyond the scope of this talk, but it's worth

going through a few that have some clinical relevance. When you surgically open the abdomen, the first thing that you see is the Greater momentum. And this is a picture of what that greater um looks like surgically. The greater um is a ligament that hangs like an apron and covers the abdominal contents anteriorly. Remember ligaments are just a double layer of parneium and um by definition originates from the stomach. So if we look at this diagram here again we pointed out on the last slide but the greater momentum is a double layer of Parneium coming from

the stomach and joining the transverse colon and hangs down uh along the anterior abdominal cavity. The diagrams show them as individual layers but these all fuse and this becomes a four layered structure and on CT you don't actually see these layers of parnium but again you see the fat and vessels and things that are in the greater um so if we look at this case here this is a 70-year-old female who comes to the emergency department With abdominal distension we'll orient you to the anatomy when we start scrolling through CTS but here we have the

right side of the patient here left side of the patient here back and front. There's all this low density free fluid in the parinal cavity. This is ascites or free intraarinal fluid. This is the kidney back here. This is a retroparinal structure. You can see the fluid's not surrounding that because it's intraarinal fluid. I'll briefly point Out these loops of small bowel that are filled with oral contrast here. And this structure here that has a bunch of fat in it. Remember fat is very dark. and all of these vessels that are coursing through it. This

is the small bowel mezentary here along the anterior abdominal cavity where we'd expect the greater momentum. We have all of this soft tissue filling that space which is abnormal. If we compare that to a normal CT here on the Right, we see that the normal greater momentum should be fatty density with small vessels coursing through it. This one is clearly abnormal. This soft tissue in the greater omum is a case of omenal caking. There are parinal metastases from a primary malignancy that are spreading through the parnium. When you have omenal caking and parinal metastases in

a female. The most common cause is ovarian cancer. Other things that can cause disappearance include Other GI cancers, things like colon cancer, pancreatic cancer, gallbladder cancer, etc. The main point here is to illustrate why parinal anatomy is important when looking at a CT scan of the abdomen and pelvis. The parinal ligaments are a highway for disease spread. When tumor invades the parnium, it often spreads along the parnium in a sheetlike fashion. And although this case here is very obvious and the greater momentum is A very big structure, if you're further along, it's worth learning the

more intricate parinal anatomy. The more of the anatomy you know to a point, the more subtle pathology you're going to pick up on CT scans. The lesser um is actually composed of two parts. The gastrohypatic ligament. So from the stomach to the liver, gastro hippatic as well as the hpatouiadena ligament from the liver to the douadinum. But for all intents and Purposes, most people just call it the gastroatic ligament when they're describing it on imaging. The main thing to know here is that this space contains things in the periportal region, including the portal triad. So

you'll see a lot of things here, mainly big lymph nodes and direct tumor spread in abnormal situations. And we're going to have a look at that on a CT scan shortly. So let's open up a CT scan. Okay, so this is a CT scan of the Abdomen and pelvis in a female. And I'll briefly point out these small bowel loops here. Centrally, all this fatty attenuation. When I scroll up and down, you see it's mainly vessels, small lymph nodes that are popping in and out and fat attenuation. These vessels are feeding and draining the small

bowel. This is the small bowel mezentary that we pointed out earlier. And anteriorly we have the greater momentum where we see fat attenuation And small vessels. In abnormal cases like the case we showed earlier, you'll see soft tissue density here or other abnormalities in these parinal ligaments. The other piece of basic level parinal anatomy that you need to know is some of the parinal spaces. And the reason why you need to know this is to be able to describe any abnormalities that you see as well as know where to look for things when you're looking

for things that are abnormal. So if we start In the right upper quadrant here, we have the liver and around the liver and under the diaphragm here, although you can't see the space now, if there was free fluid or something abnormal, you can have abnormalities in that space. The space here is called the right subfrenic space. Frenic means diaphragm. So subfrenic is under the right or under the diaphragm on this case the right diaphragm. So you have right subrenic space. This is the liver and this is the kidney. And in between the two you have

what's called Morrison's pouch also known as the hippatrenal recess. This is an important space because when the patient is lying flat, anything that causes there to be free fluid or blood often collects in this space around the liver. So for example, if you had an abdominal trauma and a laceration to the liver, blood may collect in this space in the hpatarrenal recess, also known as Morrison's pouch. So you need to look here to make sure that there's no free fluid. In the left upper quadrant we have the spleen here and similar to the right we

have the left subfrenic space again under the diaphragm and the left perisenic space around the spleen a little bit lower down in the flanks near the descending and ascending colons we have the right Paracolic gutter and the left paricolic gutter. This is another location where you may see parinal metastasizes or free fluid. And then if we get down to the pelvis very briefly, we have the bladder here anteriorly. We have the rectum here posteriorly. This is the vagina. And if I scroll up here, we have the uterus here. The spaces in the pelvis are important

because these are gravity dependent Spaces. And if you're looking for free fluid, this is a place that you're going to see it often early on or when you have small volumes of free fluid. So often times you'll see free fluid here in the recto uterine space or pouch of Douglas. This is where small amounts of free fluid are often seen in females. If you're trying to remember what to call this space, is it uterine? It's always posterior to anterior in the pelvis here. So it's recto uterine space Or pouch of Douglas here and then anterior

to the uterus and behind the bladder you have this space here which is called the uterero vesicle space again posterior to anterior it's another space where you can get free fluid or uh parinal metastases as well. Jumping over to a male pelvis. Here we have the rectum. Here we have the bladder anteriorly here. So the most dependent space here is called the rectto vesicle space. Often times When we're describing free fluid here we just say free fluid in the pelvis. Here we have the paired seminal vesicles and the prostate inferiorly here. But we'll talk about

the organ anatomy in a bit. Okay, now that we've talked a bit about the parinium, uh it's time to talk a little bit about the retroparitinium. In the center of the retroparinium here, we have the kidneys. And the kidneys sit in this space here in orange called the Perrenal space. The perrenal space is an ice cream cone shaped space that's bound by the perrenal fascia. Okay. In the perrenal fascia, there's the anterior perrenal fascia and posterior perrenal fascia. The perrenal fascia is just dense connective tissue that surrounds the perrenal space. The anterior leaf is also

known as Gerro's fascia and the posterior leaf is also known as Zuker Candle's fascia. I should mention that the anterior and posterior fascia are not just one layer, it's multiple layers, and that creates some potential spaces where things can spread, but that's not that important for now. If we look at the perrenal space on a chronal CT, here we have the kidneys and the adrenals up here. So, adrenal up here, kidney down here. And you can see this patient has a bunch of fluid Outside of the perrenal space. And the fluid happens to nicely outline

the maintained perrenal space. And this really illustrates the shape of this perinal space and the main structures that are within it. And we've highlighted it here for you in red. Anterior to the perrenal space, we have the anterior parrenal space. And posteriorly here where it says PPS we have the posterior parah renal space. Things that are important to know are the structures in the anterior para renal space. Again these are all retroparinal structures. We have the pancreas here. We have the ascending and descending colon that are also retroparinal structures sitting in the anterior parrenal space.

Also portions of the douadinum, the second and third portions of the dudinum that surround the pancreatic head uh are also in the anterior paratren renal space. And this Anatomy is clinically important. In a patient who has pancreatitis, for example, and inflammation surrounding the pancreas, you may only have subtle inflammation that tracks along this anterior pernal space and along the anterior aspect of Gerot's fascia. And this might be your only sign of inflammation of some structure in this anterior parrenal space. So let's go back to our CT. Okay. So let's review these retroparinal spaces and Structures.

So this is the left kidney and this is the right kidney. This is the adrenal here on the left and this is the right adrenal here. And the adrenal and kidney are in this perrenal space. Okay. And you can see the anterior perrenal fascia and posterior perrenal fascia here as well. Remember the anterior perrenal fascia is called Gerro's fascia and the posterior pernal fa fascia is called Zechroandals fascia. We have the posterior parrenal space Back here mainly containing fat in a normal patient. And then we have the anterior parrenal space which is anterior to gerro's

fascia containing the pancreas which is here. So we have the pancreas here. It also contains the douadinum. So this is the second portion of the due here and the third portion of the dudum across the midline. The ascending colon over here and the descending colon over here. Those are all retroparal Structures. see how they're anterior to the anterior pernal fascia or gerotus fascia. Again, this can be clinically relevant. For example, a case of pancreatitis may have inflammation tracking down the anterior gerotus fascia. There are also other extraparal spaces low in the pelvis that are worth

mentioning. This includes the prevesicle space here anterior to the bladder that extends superiorly along the margin of The bladder here. The portion of the prestical space that is behind the pubic symphysis here is called the space of rezius. These are all extraparal spaces. We also have the perry vesicle spaces here around the bladder or beside the bladder. Posteriorly, another named space that's extraparal is the pre-sacral or retrectyl space here. The reason why this anatomy is Important, there are many clinical reasons. Uh one clinical example, we've put up a picture uh over here. This is a

patient who had pelvic fractures and hematia or blood in the patient's urine. And this is what is called a CT systogram. Patients who have abdominal trauma or or pelvic trauma with pelvic fractures and hematia, uh the first thing you need to think about or rule out is a rupture of the bladder. Um when you're diagnosing a Rupture of the bladder, uh the type of bladder rupture is very clinically important. The two main types of bladder rupture are extraparal bladder rupture where the bladder is ruptured into the extra pararineal space only and intraarinal bladder rupture where

the rupture where the bladder is ruptured into the intraarinal space. The intraarinal bladder ruptures or any bladder rupture that has an intraarinal component is usually treated surgically Whereas extraparal bladder ruptures are treated conservatively. So in this picture here, we've injected contrast through a foley catheter into the bladder and distended it. And you can see that contrast has extravisated outside of the bladder and into the perves vesicle space and space around the bladder. And this is all extraparal. There's no intraarinal contrast. This is an extra parinal bladder rupture. And this does not need to go to

surgery Immediately. Okay, great. Now that we've gotten that out of the way, it's time to get down to business and start going through the intraabdominal organs. Let's start with the liver. Okay, so the liver is split up into eight segments. I've included a diagram here briefly and we'll come back to this. Radiographically, to split up the liver, which I've pointed at here, uh into the eight Segments, we use the vascule as landmarks to define the segment. So let's look at the vessels of the liver first. This here is the IVC or inferior vennea. Draining into

the inferior vennea are the hypatic veins. So we have the right hypatic vein here, middle hpatic vein here and left hypatic vein here. And you can see you can see that they all join the IVC. In most patients, the left and middle apatic veins join just before going into the IVC. Common variance of the hippatic veins. Often times or many times you'll see veins directing directly in the IVC a little bit lower, usually draining from somewhere down here in segment six. And we'll talk about that the segments in a bit. But you'll have accessory veins

that are draining directly in the IVC. Um, this can be important if the Patient's going for surgery. Uh, for example, this here is the portal vein. This is a portal venus scan. So, the portal vein is quite bright. And the portal vein goes into the liver here and supplies the majority of the blood to the liver. The portal vein is composed or made up of two vessels. the SMV here and the splenic vein here. So splenic vein is draining the spleen And hugs the back of the pancreas and the SMV is coming up here and

when they join they are now by definition the portal vein. The portal vein splits into the right and left portal vein. So this here is the left portal vein here and this here is the right portal vein over here. The right portal vein Splits into the anterior division which is going this way and the posterior division which is moving this way. So again, main portal vein, left portal vein here. If I scroll down, we have our right portal vein that goes into the anterior division here and the posterior division here. Common variants include a triurcation

where the right anterior, right posterior and left all come to the same point. Uh or the right Posterior vein coming off early or coming off first. That's also known as the Ztype configuration of the portal vein. If you're starting off and this is early on, don't worry. Just remember this is the portal vein and it splits into right and left. So, back to the segmental anatomy. The hippatic veins that we've now defined right, middle, and left split the liver into four slices. So, one, two, three, four, split by the right, middle, and Left hippatic veins.

And you can also see this in the diagram. The portal vein where it the left portal vein is and where the right portal vein is splits the liver in half. Superior to the portal vein is up here. And the inferior half that's inferior to the portal vein. So if we split the liver into four pieces based on the hpatic veins and then into two halves the superior and inferior half based on the portal vein we have a total of eight Segments. They're numbered as follows and you can follow along in the diagram that's on the

screen. So first of all we have segment one that is called the codate lobe that I've outlined here. We have segment two that sits to the left of the left hypatic vein and superior to the left portal vein. So this is all segment two up here inferior to the left portal vein but left of the left hpatic vein. Here we have the segment three. We have segment four that sits between the left and middle hypatic vein here. So segment 4 A is above the portal vein and segment 4B is below the portal vein. Below the

portal vein and here is segment five. That is between the middle and right hippatic vein. But way down here under the portal vein is segments five. Segment six is posteriorly here. Segment seven is superiorly, so above the right portal vein, but to the right of the right hpatic vein here. And Segment eight is up here. Okay, if you didn't follow that, have a look at the diagram and scroll through through some anatomy to uh better understand the segments. The segments are important to describe abnormalities uh when you're reporting as well as have the same nomenclature

as the people who you're communicating with. Other anatomy to know this here is the falsifform ligament separating the left anatomic lobe from The right anatomic lobe. Functionally we split segments one, two, three and four as the functional left lobe and the functional right lobe are segments five to eight over here. This here is called the ligamentum venosum and that separates segment two from the codate lobe. Very briefly, common locations to have focal fat in a normal or relatively normal liver include adjacent to the Falsifform ligament in posterior segment four here as well as adjacent to

the gallbladder fossa. Okay, so we're done with the liver. This here is the gallbladder. gallbladder is split up into the fundus, the body, and the neck. Oftentimes you have stones at the neck that can obstruct it and cause acute colaccyitis. The neck goes into the cystic duct which can be difficult to see on normal CTS And the cystic duct inserts into the common hpatic duct that turns into the common bile duct. And the common bile duct is small but can be seen here. This is a very small common bile duct. So it's difficult to see

uh but essentially the normal common bile duct should be less than 6 millime at the age of 60 or less and you can add a millimeter for every decade after that. So if they're 70 they can be up to 7 millime 80 up to 8 millime 90 up to 9 Millimeters in diameter. That's the common duct. Uh if the patient's had a colcysteectctomy you can uh have a little bit more prominent of a duct and that's normal. So this is a patient who came in with right upper quadrant pain and the reason why we're looking

at this abnormal case is because the bilary tree is dilated here. So it's a lot easier to illustrate the anatomy. So first of all how do we Know that the bile ducts are dilated? Well you have this hypoattenuating structure or all of these hypotenuating structures that are running along the portal veins. And you see that this hypoattenuating tubular structure here is on one side of the portal venous structure. Sometimes you'll see hypoattenuation on both sides and that usually is seen with periportal edema. When you see hypoattenuation on one side that is tubular and connects we're

Dealing with intropatic bilary ductilitation and we can see these ducts very well. So here on the right we have the right anterior bile duct. The right anterior bile duct drains from superior to inferior in an oblique fashion. Over here more posteriorly and more horizontally oriented is the right posterior bile duct. So the right posterior and right anterior bile ducts drain to make up the right main bile Duct. And the left intropatic bile duct here or left main bile duct also drains to join the right main bile duct to make the common hippatic duct which is

here in the portaepus. The common hpatic duct is joined by the cystic duct here. So this is the gallbladder down here with stones in it. And if I scroll up, you can see the cystic duct here because it's dilated and it joins the common hippatic duct to distally become the common bile duct or The CBD. This common bile duct is dilated as well. And if you look closely distally, you can actually see small dense structures in the distal common bile duct. These are biliary duct stones or coladocalithsis. And this is the cause of the patient's

right upper quadrant pain. Okay, here is the coronal images of that same patient. And you can see that in this plane we can see the common bile duct quite nicely and we can see the Stones distally obstructing. This duct is too dilated. If I measured it or gave a rough measurement, it's about 1.7 cm, which is way too big. I can also quite nicely follow the common duct and see the right main interropatic duct that is drained by the right anterior duct which is more oblique. And then if I scroll more posteriorly you can see

the posterior duct which is more horizontal. When it comes to bilary Variance we more commonly discuss it when we're looking at MRI. Um but the most common variants have to do with the insertion of the right posterior duct. The right posterior duct can join at the same uh location as the left main and the right anterior duct. That's called a triurcation. Uh or the right posterior duct can drain directly into the common duct. There are other uh variants as well but those are some of the most common. This over here is the spleen in the

left upper quadrant. You'll commonly see normal splenules around the spleen just like this one here. This is just an accessory spleen. The spleen size varies by sex and height of the patient and you can look up the normal reference range. I'll put a link in the description. Uh but for practical purposes, usually people use a cut off of about 13 cm in maximum axial or coronal dimension. This here is the pancreas. Starting over here, we have the pancreatic head. Inferiorly and posterior to the SMB, we have the unsenet process. So, unsenet process, pancreatic head here.

Pancreatic neck refers to the portion that's anterior to the SMV and SMA. So, it's the pancreatic neck. pancreatic body here again hugs the splenic vein. So if you can't find the pancreas, look to the splenic vein. And then the tail Here, this is called the pancreatic tail. Often comes close to the spleen. It's also worth knowing what the what people mean when they refer to the dorsal and vententral pancreas. Remember dorsal usually means posterior or along the back. If you think about the dorsal fin for example in uh certain mammals uh they're referring to the

back portion. Uh however in the pancreas it's reversed. So the dorsal pancreas actually ends up twisting around and Refers to this larger portion of the pancreas that ends up anterior and the vententral pancreas refers to this posterior component that actually ends up dorsal. So remember in the pancreas dorsal and vententral pancreas are reversed. This is important to know because you can have a genesis of the dorsal pancreas and this is all fat here and you have some normal pancreas down here. Um you can also normally see some asymmetric fatty infiltration. Often Times you see that

on ultrasound as a pseudo mass here in the vententral pancreas and that can mimic a mass. The pancreatic duct you can see faintly here in the middle of the pancreas. This is a normal or very minuscule normal pancreatic duct, but it should run along here. Normally, you shouldn't see it or it should be very thin. A normal pancreatic duct should be less than 3 mm. And that's absolutely key because when you have pancreatic duct Dilotation, you got to be worried uh about an obstructing pancreatic mass. And we'll talk about pathology in the next video. Again,

uh the main pancreatic duct that runs along the uh pancreas here uh predominantly usually drains at the major papila and joins the CBD. Occasionally the most common variant is that the main pancreatic duct drains into the minor papula above and does not join the common bile duct. That's called Pancreatic divism. The classic form involves complete separation of the main pancreatic duct and the CBD. Uh people often mix up the Santorini and Worung. Uh but just remember that uh Santorini comes up here at the minor is the one that inserts up here at the minor papula

and wor is the one that inserts uh down here where the CBD is. Uh but if you remember anything just remember main pancreatic duct and CBD and then remember divvis which is the Most common uh most common variant. These are the adrenal glands here. So the right adrenal gland here and the left adrenal gland over here. Uh I won't talk about these much but essentially they're adrenal glands. So they're above the renal or above the kidney. They're adrenal glands. They should generally be less than 1 cm in thickness. And they should have concave margins. So

see how these margins are concave. There's no convexities. If you see Convexities or anything that's thicker than a centimeter, that's abnormal. Essentially, most commonly, you're looking for nodules here. Okay, let's look at the kidneys now. So, this is the right kidney and the left kidney on these chronal images. And when it comes to renal anatomy, there are a few things you need to know. Uh so the kidney itself can be separated into the cortex which is more peripherally the medula which are more Relatively hypoattenuating here and then the collecting system itself. So we can see

here the renal pelvis that is non-dilated that drains into the right urer here. How a normal kidney looks is going to depend on the phase of contrast. So when you initially give contrast and look at kidneys closer to an arterial phase image, a lot of the contrast is still in the kidneys peripherally, the contrast is going to fill the renal cortex first And then it's going to move towards the center or towards the hilum. So early on in the arterial phase, you're going to see only more peripheral enhancement and then over time you're going to

have more homogeneous enhancement of the entire cortex uh and then it's going to enter the uh collecting system and you can do delayed imaging if you want contrast in the collecting system to better assess the collecting system. So when you're doing a renal protocol for example there Are different phases that you look at including the cortical medularary phase where the majority of contracts is in the cortex the nephrographic phase where the majority of the kidney is enhanced and we're better going to pick up on abnormalities in the kidney uh parankma itself and then we have

uh delayed phased imaging as well that we can do as I mentioned to opacify uh the collecting systems. So in this patient here uh there is contrast filling the uh Collecting systems. Uh these again are non-dilated collecting systems. Uh but you can clearly and very nicely see contrast in these minor callouses here that drain into major callouses here and multiple major callouses drain to become the renal pelvis here in the center. and the renal pelvis then drains into the urer here. Some of the reasons to perform delayed phased imaging or imaging where you fill the

collecting system with contrast uh Include looking for cancer of the collecting system. For example, if the patient has a history of TCC and you're looking for a recurrence or if you're looking for injury to the collecting system like in a trauma or say a gynecologist may be worried that they may have nicked a urer. Uh if we fill the collecting system with contrast and the contrast ends up outside of the urer, then that's obviously abnormal. And that's just a general principle of Imaging. If you fill something with contrast and it goes somewhere else where it's

not supposed to be, that's abnormal. Of course, uh this case here uh demonstrates one of the abnormal enhancement patterns of the kidney. So in this phase, you'd expect the kidneys to enhance relatively homogeneously. And and in the right kidney specifically, also a little bit on the left, but mainly on the right, uh you'll notice multiple areas of patchy hypotenuation In the cortex that are somewhat linear and wedge-shaped in many locations. This is called a striated nephrogram appearance and there are a number of causes of that. One of the more common being pylonritis and you'll also

notice some stranding and inflammation around the right kidney. Uh this was a patient who had a pylonritis or infection of their right kidney. There are a few other abnormal enhancement patterns of the kidney that You're going to need to learn at some point. Uh but the general approach to all of these abnormal uh all of these abnormal enhancement patterns is this. Um when you have abnormal enhancement patterns, it can be due to an obstruction of the collecting system, an abnormal arterial supply or abnormal artery, an abnormal venus drainage like thrombosis for example, or an issue

with the parankma itself. So if you break it down that way and look at each of those Structures, you can often figure out what's going on. Okay. So, moving on to the pelvis. This is a female. Anteriorly here we have the bladder. Posteriorly, we have the rectum. And in between, we have the vagina. And you can see that there are lossles of gas uh within the vagina here, which you can see normally. If I scroll up, uh above the vagina, here is the cervix. And then here is the uterus. In this patient On the left,

we can see the left ovary over here. I'll scroll through it. And you can see the right ovary over here with small follicles within it. These ovaries are easy to find because they're in the expected location. Sometimes it can be difficult to find the ovaries. The best way to find the ovaries when you can't find them uh specifically if you're looking to see if a mass is coming from uh the adexa or the ovary specifically uh rather than a mass That's not associated with the ovary um is by following the vascular pedacles. If you follow

the vessels down you can often figure it out. So for example uh here is the IVC on the right here and draining into the IVC directly on the right is the gonatal vein. So you see this little tiny structure here uh that is coming that is draining into the IBC. If I follow that down, okay, it's right here in front here. I follow that down, it Goes right into this structure here, which is the right ovary. So if you can't find the ovary, follow the vascular drainage/supply, and you'll uh be much more successful. Okay. Next,

we're going to go through the bowel anatomy. Starting at the esophagus here in the chest. As I scroll down here, we have the GE junction, the gastroosophageal junction where the esophagus joins the stomach below the Diaphragm. So this is the stomach here. It's filled with oral contrast. This patient had oral contrast. The stomach can be separated into the fundus which is superior here. the body centrally and distally the antrum. You should also know that the medial aspect here is called the lesser curvature and this side over here is called the greater curvature. The stomach connects

directly to the Start of the douadinum. This here is the douadal cap and the first portion of the douadinum. There are four parts to the to the douadinum. The second portion portion of the douadinum here runs along the right aspect of the pancreatic head. If I follow the second portion of the duodinum down, I can connect it across the midline to the third portion of the douodenum. And if I follow this out here to the Fourth portion of the douadinum. So the second and third portions of the douadinum are retroparitinal and the fourth enters back

into the pareneal cavity that we described earlier. We then connect to these ginal loops or junal loops that are often occupied in the left upper quadrant but it's variable. And we don't usually follow the bowel in every case unless it's important like if there's a bowel Obstruction. Um but if we were to follow all these we would see that they would connect to these loops here that are more filled with contrast and have a tendency to be in closer to the right lower quadrant. And these are alol loops or ilial loops and the ilial loops

uh if we continue to follow them. So this is one of the more distal ilia loops. If I follow it inferiorly here will eventually come to the terminal illium here and then drain into the Seeum which is part of the colon or large bowel. The point where it drain the terminal aliam drains into the seeum is called the ilioal valve. You can often identify the ilioal valve by the small amount of fat attenuation in it. And I'm pointing that out here. But you can also know its location by following the bowel itself. If I scroll

inferiorly here, we have the secl pole or the secl base. And we also in this patient can see the appendix quite nicely here. And if we follow that up, you can see that it's blind ending. it disappears and doesn't lead to anything aka its blind ending and it is the appendix. We'll talk about how to find the appendix uh every single time when it's present at least uh in a bit. Let's follow the colon first. So the seeum joins to become the ascending colon here. Notice the appearance difference uh of the large and small Bowel

owing to the hostra or saculations. The ascending colon comes up here to near the liver and this is called the hippatic flexure. It then comes across the midline as the transverse colon here to the left upper quadrant near the spleen as the splenic flexure. The descending colon here a retroparitinal structure just like the Ascending colon was. the sigmoid colon here which can be very redundant and long following it down here to the rectum and the rectum if I scroll down here at this angle becomes the anus and this is the anal canal here. Now that

we've gone through the basic bowel anatomy let's talk about how we find the appendix. So when people are first learning how to find the appendix, they're often taught that you should Find the terminal alium or terminal ilium and then scroll down a little bit and you'll find the appendix. And that happens to work here. The way that I like to put it is you should find the terminal illium and the IC valve specifically and then scroll more approximately in the seeum and that could be either upwards or downwards. It depends on the orientation of the

seeum is that's variable. In this patient happens to be more inferior. The other thing that is very important to know is that when you know when you see the terminal illium it's not only more proximal in the seeum but also it always arises from the same aspect or same side as the terminal illium. So if I were to scroll down here I'm not looking for the appendix out on this side or out over here. I'm specifically looking along this margin here at that location in the SECL base. If I scroll down, sure enough, you can

see That in this patient, this is the appendix here. It arises from that same aspect. And I can follow it up now and see that it's normal and filled with contrast with no inflammation around it. This patient does not have appendicitis. very briefly uh to know if bowel is dilated or not. Uh a lot of people use the so-called 36-9 rule. So normal small bowel should not be larger than 3 cm in diameter. Large bowel should not be greater than 6 cm in diameter and the Seeum here in the right lower quadrant should not be

larger than 9 cm in diameter. But what matters more than the actual measurements is the appearance of the bowel itself. So you know I've seen lots of bowel obstructions and a lot of resources actually use 2.5 cm is a cut off for small bowel. But what what again matters more is the appearance. If it's tensely distended proximally there's a point of transition and then collapsed distally that is way more important than Any measurement of bowel. Okay, let's briefly talk about the vascule. So, we talked about the hippatic veins and the portal venus system already with

the SMV and the uh splenic vein. As a quick review, let's talk about arterial anatomy. So, this is the aorta here. As I scroll down, the first major branch in the midline is the celiac axis coming off anteriorly here. The second major branch off the aorta just inferiorly inferior to the celiac Axis is the SMA here. The SMA runs inferiorly here adjacent to the SMV. More inferiorly here, just before the aorta bifurcates, we see the IMA here where my arrow is. And you can follow that tiny inferior mesenteric artery down here as well. The aorta

then bifurcates into the right and left common iliac arteries. Common iliac arteries then bifurcate Here into the external iliac anteriorly and internal iliac posteriorly. We'll follow the external iliacs for now which become after it gives off the inferior epigastric the common femoral artery superficial femoral artery here and the deep femoral artery more posteriorly but again don't worry about these things for now just know the basic anatomy on the way up we can follow the veins and they're named just like the arteries So we have the external iliac vein here which joins the internal iliac vein

here to make the common iliac vein which drains uh joins the left common iliac to become the IVC or inferior vennea. You can see the renal veins here on both sides um the apatic veins that join and then where the IVC joins to the right atrium or drains into the right atrium. Next we'll cover the lymph node stations. So these are things that you're going to commonly see and things That you should be looking for on every CT scan. So where do you look? Well, let's start with the retroparitinal nodes. So if I scroll down

here, I often start by looking here in the paraortic region. So these nodes here, we can actually see small ones. These are not abnormal, but these small nodes that are popping in and then popping out of view when I'm scrolling through this axial image are all small paraortic lymph nodes. In the space Between the aorta and the IVC, we have the aortocal nodes. The nodes behind the IVC are called the retrocaveal nodes. And if they're in front of the IVC, they're called precaval nodes. Inferiorly we call the nodes as the arteries that they follow along.

So we have common iliac nodes here. We have external iliac nodes and internal iliac nodes And then out here we have uh several small normal inguinal nodes. Well, we're down here in the pelvis, although there's not much fat in this patient. Around the rectum, this space here is called the misoctyl space, and there's misrectal fat here around the rectum. In patients with colon cancer and prostate cancer, uh often times you'll see abnormal misoctyl nodes. These are called misoctal nodes if they're present. And blood from the colon drains into these superior rectal vessels. And you'll often

see nodes. If you see these linear things here, those are little vessels. You'll often see abnormal rounded nodes when rectal cancer metastasizes. So, uh any nodes that are seen along these vessels are called superior rectal nodes. Back to the upper abdomen, uh we have the adjacent to the celiac artery. If you see abnormal nodes there, we call them celiac nodes. A Common place to see nodes is here along the lesser curvature of the stomach. over here. This is all gastropatic ligament. So you can call these gastropatic ligament nodes uh or nodes along the lesser curvature.

Often times people refer to nodes in the portepus uh as either gastropatic nodes or uh specifically you can say periportal nodes is a colloquial way to refer to them. The node refer the node between the portal vein and the uh IVC Here is a so-called porttoal node. Uh but again it's all in the gastropedic ligament. So you could just refer it to it as that. Another nodal station is the mezentic node. So you can see several normal messentic nodes here in the small bowel mezent. Again notice that the vessels are tubular and you can always

connect them to something. Whereas these small normal nodes pop in and out of sight when you're scrolling through them. If They were large or had an abnormal appearance, that would be abnormal and we would draw attention to them. These are all normal lymph nodes. How can you tell if a lymph node is abnormal? Well, we generally measure lymph nodes in the abdomen and pelvis in short axis, which means the shorter dimensions. So, for example, for this tiny node here, we wouldn't measure it from this side to this side. We would measure it from posterior to

anterior. And this is a few millimeters. it's a normal lymph node. If we thought the lymph node was abnormal and measured it, you have to remember that size is not everything. There are different size cut offs for various uh stationed lymph nodes. But in general, if you remember around 1 cm is a cutff. Uh you'll probably be pretty safe. But again, size is just one part of the picture. things like rounded lymph nodes, heterogeneous or cystic Change, those things can strongly suggest that the lymph nodes are abnormal um even if they're much smaller than a

centimeter. And the clinical context is also very important as well. If the patient again has a rectal cancer for example and has a 7 millm rounded node along the superior rectal chain that is very suspicious for being involved with cancer. Okay, so let's go through our approach on this patient who comes into the Emergency department with right lower quadrant pain. The first thing that I do when I look through a scan is just take a general overview of what's going on. So, I'm scrolling down and looking for any fat stranding or anything that's abnormal. And

it's important to do this because not only do you want to know uh kind of what you're getting yourself into, but you also want to have a general overview Of what's going on with this patient first of all, and second of all, if you see something that's absolutely emergent, you want to look at it more closely uh and call the referring clinician. The other thing is that when you're so focused on looking at, you know, very small areas of the scan and you look at the liver, then the spleen, the kidneys, etc., you really get

uh tunnel vision at Looking at those specific organs and you you miss the big picture and often can miss findings even uh when you're so zoned in. So I always start with a scroll down and a scroll up uh looking for obvious abnormalities. In this case we obviously see there's inflammatory stranding in the right lower quadrant. I see a stone here at the base of the appendix and a dilated inflamed appendix. This is acute appendicitis. Um so even just with our first two scrolls We can see that uh there are abnormalities and make the diagnosis.

Uh but the purpose of this is to go through the whole approach. So after looking at the entire scan quickly I then suggest that beginners look for free inparial gas. So you do that by windowing. Usually people use lung window but the important thing to do is just to widen the window. So if I just widen the window like that now you can see that the gas stands out Very well. And I can see that all of this gas is inside the bowel or where it's supposed to be. I don't see any free interparental gas

in the abdomen. I then look for free inparental fluid or ascites. And as I'm scrolling through, there's special attention to areas like the hippatarino fossa or Morrison's pouch, uh the paropatic spaces. Um and mainly you're most commonly going to see a little bit in the pelvis. So here there is just a trace amount of free Fluid here in the pelvis. Uh that's intraarinal. So there's a little bit of free fluid in this patient. I then go through and assess each organ. So I start by looking at the liver for any focal lesions. Don't see anything.

Gallbladder. Look for any intrahypatic or extraatic bilaryation. And we showed you a case earlier of that. This is a normal common duct. Since I'm already here, I look at the pancreas starting from the unsenit proess in head to the neck to the body all the way to the tail. The duct is not dilated. Then look at the spleen here, the adrenals bilaterally, left adrenal here, right adrenal here. Kidneys looking for hydronosis, focal lesions, other abnormalities. Then follow the urgers down on both sides. You don't have to follow Them in every patient if they don't have

hydronosis, but I generally look in the general region and follow them down to the bladder. Now that I'm here in the bladder or in the pelvis, I look at the pelvic organs. So the bladder, seminal vesicles, prostate here, and the rectum. And then I take a second more close look at the parnium. So I look at the uh rectto vescal space here and then as I scroll up I'm looking at the I usually Start with the left parakolic gutter and follow it all the way up and look at the left upper quadrant in this region.

I'm also looking in the greater momentum for any abnormal soft tissue nodularity. as I scroll down and then back up the right paracolic gutter and looking specifically at Morrison's pouch and subfrenic space here. And the reason you do this is not only to see little bits of fluid, but you can see Metastases in patients who have cancer. I then look at the bowel. And so when I'm looking at the bowel, I start at the esophagus here. Look at the stomach itself briefly and just follow it down to the dueum first, second, third across the midline

and fourth portion of the dudium here. Unless it's a case that requires following the bowel like a bowel obstruction for example, the bowel is normal caliber. I generally then look at the small bowel in sweeps. So I split It up into halves. So I look at this half of all the bowel first and then on the scroll up look at this half of the small bowel there. At the same time when I look at the small bowel I treat the mezzent as its own organ and as an organ as part of the small bowel. So

I look at the small bowel mezent here and here on the second run. So I do two runs of the small bow mezentary looking for nodes and other abnormalities And then I follow the colon. So you always want to run the colon and I start from the anus to the rectum to the sigmoid here to the descending colon to the splenic flexure and transverse colon hpatic flexure and I'm following it backwards down the ascending colon all the way to the secl base here. Once I've got to the seagull base, I look at the terminal illium,

which is here. And you can see that there's some thickening of the terminal illium, likely reactive to this patient's appendicitis. And then in every patient, even if they don't have right lower quadrant pain, you want to look for the appendix. Even in patients who you're not worried about appendicitis, you'll often pick up uh abnormalities of the appendix such as mucousils, a term used to refer to a mucus filled appendix. Um, and the cause Of that can be neoplastic or cancer. Um and again we talked about how to find the appendix. In this patient it's very

clear that there is an appendicolith or a calcified stone here at the base of the appendix obstructing the lumen causing a dilated inflamed appendix with a lot of inflammation uh around it. This is acute appendicitis. Satisfaction of search don't stop there. You got to keep looking through the case. So once I've looked at the bowel And the mezzentary I look at the vascule. Um so there are three sets of vasculature to look at. So the arteries first I generally follow the aorta down and looking at the major branches. So just the celiac axis the SMA

I follow down so you don't miss an SMA thrombosis. Then I follow the uh aorta to its branches including the external iliac arteries uh briefly and on the way back up I'm looking at the veins. So Specifically, you're going to pick up things like DVTs or clots in these veins. And I follow the veins back up to the IVC briefly and quickly follow the IVC back up to the heart. I then uh look at the apatic vascule. So look at the apatic veins very briefly. And then every time you want to look at the portal

venous system. Lastly, I look at the bones and soft tissues. So the soft tissues I do runs of the soft Tissues looking at the musculature and subcutaneous tissues and abdominal wall and then look at the bones. So we showed you that earlier with anatomy but again bone window and then looking at it an axial first the pelvic bones the femurss the spine and then the lower ribs and then I look at the lungs here on lung window and the heart on soft tissue window. Then I look at my reformat specifically Looking at the sagittal images

which we have up here in bone window with focus on the spine but looking at all the bones and also the chronal images specifically to look at things that are often missed on axial images. So I look at the kidneys on every single patient on the coronal images and the pancreas on every single patient in the chronal images. So here is the pancreas here. Pancreatic head, neck, Body, and tail. And and then I quickly scroll through the rest of it looking for any obvious abnormalities usually with special attention on the mezzent. You can see how

nicely you can see the small bowel mezentary here. Okay, so that's it for this uh particular talk looking at uh mainly anatomy uh and what normal should look like as well as a uh a detailed approach to the abdomen and pelvic CT. In the Future videos, we're going to look at abnormal cases and specifically talk about things that you should never miss on an abdominal pelvic CT um when you're looking at each of the organs. So things like pancreatic cancer and how to make sure you never miss a pancreatic cancer or when you see liver

hypodensities, how you know that they're benign or if they're worrisome or what you need to do when you see various things in each of the organs. And this will likely take Place over a few uh videos in the future. Um so please stay tuned. Thanks.