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 hounsfield 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 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 variants 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 peritoneal cavity i start with this because when people are first starting out they often have a difficult time conceptualizing peritoneal anatomy and the basics of peritoneal 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 peritoneal cavity here in red and the retroperitoneum or behind the peritoneum here in blue detailed peritoneal anatomy is well beyond the scope of this talk we could probably spend a few hours talking about peritoneal anatomy but it is important to understand a few basic things about peritoneal anatomy so we need to introduce some basic terminology the peritoneum is a thin membrane or sheet of a single layer of epithelial cells the peritoneum is a large and complex sheet simply put the peritoneum lines the abdominal cavity and envelops the intraperitoneal or some of the abdominal cavity organs the peritoneum that is lining the abdominal wall here in red is called the parietal peritoneum and the part that envelops the organs is called the visceral peritoneum here in blue these names aren't important what's important to know is that all of this peritoneum is one continuous sheet so the parietal peritoneum and the visceral peritoneum is one continuous sheet and this creates a number of complex folds what is important to know is that the space between the peritoneum that lines the abdominal wall and the peritoneal that lines the organs is called the peritoneal cavity when you hear people saying that there is free intraperitoneal fluid or free intraperitoneal gas they're talking about gas and fluid in this peritoneal cavity not extra peritoneal like the retroperitoneum here but in the peritoneal cavity itself as i mentioned the peritoneum has a bunch of complex folds which includes a bunch of named peritoneal ligaments peritoneal ligaments are nothing more than a double layer of peritoneum that supports a structure within the peritoneal cavity the words omentum and mesentery that you've probably heard before are nothing more than specially named peritoneal ligaments or double layers of peritoneum when you hear the word omentum it's just referring to peritoneal ligaments aka a double layer of peritoneum that extends from the stomach to another structure mesentery is a peritoneal ligament or a double layer of peritoneum that connects an organ to the retroperitoneum or posterior abdominal wall inside the mesentery or between the two sheets of peritoneum there are blood vessels lymph nodes and fat that we can actually see on ct we don't see the peritoneum 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 perineal ligaments all of these are double layers of peritoneum here we've highlighted one of them in red and now we've highlighted the others on this diagram in red these are all peritoneal ligaments or double layers of peritoneum here labeled is the stomach the ligaments that extend from the stomach here are specially named they're just ligaments but they're called omentum we have the lesser omentum above here that extends from the stomach towards the liver and we have the greater momentum 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 retroperitoneum or posterior abdominal wall and the small bowel to the posterior abdominal wall are so-called mesentery this one here that supports the small bowel is called the small bowel mesentery 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 name 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 omentum and this is a picture of what that greater momentum looks like surgically the greater momentum is a ligament that hangs like an apron and covers the abdominal contents anteriorly remember ligaments are just a double layer of peritoneum and momentum 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 peritoneum coming from the stomach and joining the transverse colon and hangs down 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 peritoneum but again you see the fat and vessels and things that are in the greater momentum 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 peritoneal cavity this is ascites or free intraperitoneal fluid this is the kidney back here this is a retroperitoneal structure you can see the fluid is not surrounding that because it's intraperitoneal 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 mesentery here along the anterior abdominal cavity where we'd expect the greater momentum we have all 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 omentum is a case of omental caking there are peritoneal metastases from a primary malignancy that are spreading through the peritoneum when you have a mental caking and peritoneal metastases in a female the most common cause is ovarian cancer other things that can cause this appearance include other gi cancers things like colon cancer pancreatic cancer gallbladder cancer etc the main point here is to illustrate why peritoneal anatomy is important when looking at a ct scan of the abdomen and pelvis the peritoneal ligaments are a highway for disease spread when tumor invades the peritoneum it often spreads along the peritoneum in a sheet-like 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 peritoneal 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 omentum is actually composed of two parts the gastro hepatic ligament so from the stomach to the liver gastro hepatic as well as the hepato duodena ligament from the liver to the duodenum but for all intents and purposes most people just call it the gastro-hepatic 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 gonna 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 that'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 mesentery 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 peritoneal ligaments the other piece of basic level peritoneal anatomy that you need to know is some of the peritoneal 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 subphrenic space phrenic means diaphragm so subphrenic is under the right or under the diaphragm on this case the right diaphragm so you have right subphrenic 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 hepatorenal 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 hepatorenal recess also known as morrison's pouch so you need to look here to make sure there's no free fluid in the left upper quadrant we have the spleen here and similar to the right we have the left subphrenic space again under the diaphragm and the left perisplenic 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 paracolic gutter this is another location where you may see peritoneal metastases 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 oftentimes 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 utero rectal erector uterule erector uterine it's always posterior to anterior and 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 utero vesicle space again posterior to anterior it's another space where you can get free fluid or peritoneal 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 rectal vesicle space oftentimes 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 peritoneum uh it's time to talk a little bit about the retroperitoneum in the center of the retroperitone here we have the kidneys and the kidneys sit in this space here in orange called the peri renal space the peri renal space is an ice cream cone shaped space that's bound by the peri renal fascia okay in the peri renal fascia there's the anterior peri-renal fascia and posterior peri-renal fascia the perirenal fascia is just dense connective tissue that surrounds the peri-renal space the anterior leaf is also known as giroda's fascia and the posterior leaf is also known as zooker candles 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 peri-renal space on a coronal 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 perirenal space and the fluid happens to nicely outline the maintained peri-renal space and this really illustrates the shape of this perional space and the main structures that are within it and we've highlighted it here for you in red anterior to the peri-renal space we have the anterior para renal space and posteriorly here where it says pps we have the posterior para renal space things that are important to know are the structures in the anterior para renal space again these are all retroperitoneal structures we have the pancreas here we have the ascending and descending colon that are also retroperitoneal structures sitting in the anterior para renal space also portions of the duodenum the second and third portions of the duodenum that surround the pancreatic head are also in the anterior perinal 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 perineal space and along the anterior aspect of gerotos fascia and this might be your only sign of inflammation of some structure in this anterior perirenal space so let's go back to our ct okay so let's review these retroperitoneal 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 perirenal space okay and you can see the anterior perirenal fascia and posterior perirenal fascia here as well remember the anterior perirenal fascia is called gerotis fascia and the posterior perirenal fat fascia is called zechar candles cat fascia we have the posterior para renal space back here here mainly containing fat in a normal patient and then we have the anterior perirenal space which is anterior to devotos fascia containing the pancreas which is here so the pancreas here it also contains the duodenums this is the second portion of the duty in here and the third portion of the duty across the midline the ascending colon over here and the descending colon over here those are all retroperitoneal structures see how they're anterior to the anterior perirenal fascia or gerotos fascia again this can be clinically relevant for example a case of pancreatitis may have inflammation tracking down the anterior derodus fascia there are also other extraperitoneal spaces low in the pelvis that are worth mentioning this includes the pre-vesicle space here anterior to the bladder that extends superiorly along the margin of the bladder here the portion of the pre-vesicle space that is behind the pubic symphysis here is called the space of rezias these are all extraperitoneal spaces we also have the peri vesicle spaces here around the bladder or beside the bladder posteriorly another named space that's extraperitoneal is the pre sacral or retrorectal space here the reason why this anatomy is important there are many clinical reasons uh one clinical example we've put up a picture over here this is a patient who had pelvic fractures and hematuria or blood in the patient's urine and this is what is called a ct systogram patients who have abdominal trauma or pelvic trauma with pelvic fractures and hemateria the first thing you need to think about or rule out is a rupture of the bladder when you're diagnosing a rupture of the bladder the type of bladder rupture is very clinically important the two main types of bladder rupture are extraperitoneal bladder rupture where the bladder is ruptured into the extra peritoneal space only and intraperitoneal bladder rupture where the rupture where the bladder is ruptured into the intraperitoneal space the intraperitoneal bladder ruptures or any bladder rupture that has an intraperitoneal component is usually treated surgically whereas extraperitoneal 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 extravasated outside of the bladder and into the perivesical space and space around the bladder and this is all extra peritoneal there's no intraperitoneal contrast this is an extraperitoneal 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 intra-abdominal 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 out here into the eight segments we use the vasculature as landmarks to define the segment so let's look at the vessels of the liver first this here is the ivc or inferior vena cava draining into the inferior vena cava are the hepatic veins so we have the right hepatic vein here middle hepatic vein here and left hepatic vein here and you can see you can see that they all join the ivc in most patients the left and middle hepatic veins join just before going into the ivc common variants of the hepatic veins oftentimes 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 this can be important if the patient's going for surgery for example this here is the portal vein this is a portal venous 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 triforcation where the right anterior right posterior and left all come to the same point or the right posterior vein coming off early or coming off first that's also known as the z-type 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 hepatic 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 hepatic veins and you can also see this in the diagram the portal vein where 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 hepatic veins and then into two halves the superior and inferior half based in the portal vein we have a total of eight segments they are 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 caudate lobe that i've outlined here we have segment two that sits to the left of the left hepatic 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 hepatic vein here we have the segment three we have segment four that sits between the left and middle hepatic vein here so segment 4a is above the portal vein and segment 4b is below the portal vein below the portal vein and here is segment 5 that is between the middle and right hepatic vein but way down here under the portal vein is segment five segment six is posteriorly here segment seven is superiorly so above the right portal vein but to the right of the right hepatic 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 better understand the segments the segments are important to describe abnormalities 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 falsiform ligament separating the left and atomic 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 5 to 8 over here this here is called the ligamentum venosum and that separates segment 2 from the caudate lobe very briefly common locations to have focal fat in a normal a relatively normal liver include adjacent to the fossil form 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 cholecystitis the neck goes into the cystic duct which can be difficult to see on normal cts and the cystic duct inserts into the common hepatic duct that turns into the common bile duct 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 but essentially the normal common bile duct should be less than six millimeters 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 millimeters 80 up to 8 millimeters 90 up to 9 millimeters in diameter that's the common duct uh if the patient's had a cholecystectomy you can 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 biliary 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 hypoattenuating 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 intrapatic biliary 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 to the right posterior and right anterior bile duct strain to make up the right main bile duct and the left intrapatic bile duct here left main bile duct also drains to join the right main bile duct to make the common hepatic duct which is here in the porta hepatis the common hepatic 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 hepatic 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 colidocolothiasis 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 centimeters which is way too big i can also quite nicely follow the common duct and see the right main intrapatic 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 billiard variants we more commonly discuss it when we're looking at mri 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 trifurcation or the right posterior duct can drain directly into the common duct there are other 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 spleniuls 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 but for practical purposes usually people use a cutoff of about 13 centimeters 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 unseenet process so unscented process pancreatic head here pancreatic neck refers to the portion that's anterior to the smv and sma says the pancreatic neck pancreatic body here again hugs the splenic veins if you can't find the pancreas look at 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 ventral pancreas remember dorsal usually means posterior uh or along the back if you think about the dorsal fin for example and certain mammals they're referring to the back portion 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 ventral pancreas refers to this posterior component that actually ends up dorsal so remember in the pancreas dorsal and ventral pancreas are reversed this is important to know because you can have agenesis of the dorsal pancreas and this is all fat here and you have some normal pancreas down here you can also normally see some asymmetric fatty infiltration oftentimes you see that on ultrasound as a pseudomass here in the ventral 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 three millimeters and that's absolutely key because when you have pancreatic duct dilatation you got to be worried uh about an obstructing pancreatic mass and we'll talk about pathology in the next video again the main pancreatic duct that runs along the pancreas here predominantly usually drains at the major papilla and joins the cbd occasionally the most common variant is that the main pancreatic duct drains into the minor papilla above and does not join the common bile duct that's called pancreatic divisum the classic form involves complete separation of the main pancreatic duct and the cbd uh people often mix up the santorini in warsang but just remember that santorini comes up here at the minor is the one that inserts up here at the minor papilla and we're sung 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 divisum 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 i won't talk about these much but essentially their adrenal glands so they're above the renal or above the kidney their adrenal glands they should generally be less than one centimeter 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 coronal images it comes to renal anatomy there are a few things you need to know so the kidney itself can be separated into the cortex which is more peripherally the medulla 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 ureter 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 and then it's going to enter the 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 corticomedulary 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 parenchyma itself and then we have delayed phased imaging as well that we can do as i mentioned to opacify the collecting systems so in this patient here 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 acids here that drain into major calyces here and multiple major kelseys drain to become the renal pelvis here in the center and the renal pelvis then drains into the ureter 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 ureter if we fill the collecting system with contrast and the contrast ends up outside of the ureter then that's obviously abnormal and that's just a general principle of imaging if you feel something with contrast and it goes somewhere else where it's not supposed to be that's abnormal of course this case here 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 then the right kidney specifically also a little bit on the left but mainly on the right you'll notice multiple areas of patchy hypoattenuation in the cortex that are somewhat linear and wedge shaped in many locations this is called a strated nephrogram appearance and there are a number of causes of that one of the more common being pilonephritis and you'll also notice some stranding and inflammation around the right kidney this was a patient who had a pilonophritis 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 but the general approach to all of these abnormal uh all of these abnormal enhancement patterns is this 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 venous drainage like thrombosis for example or an issue with the parenchyma 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 molecules of gas within the vagina here which you can see normally if i scroll up 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 specifically if you're looking to see if a mass is coming from uh the adnexa or the ovary specifically uh rather than a mass that's not associated with the ovary um is by following the vascular pedicles if you follow the vessels down you can often figure it out so for example here is the ivc on the right here and draining into the ivc directly on the right is the gonadal vein so you see this little tiny structure here uh that is coming that is draining into the ivc if i follow that down okay it's right here in front here with all 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 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 gastroesophageal junction where the esophagus joins the stomach below the diaphragm so this is the stomach here it's filled with oral contrast patient net 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 duodenum this here is the duodenal cap and the first portion of the duodenum there are four parts to the to the duodenum the second portion of the portion of the duodenum here runs along the right aspect of the pancreatic head if i follow the second portion of the duodenum down i can connect it across the midline to the third portion of the duodenum and if i follow this out here to the fourth portion of the duodenum so the second and third portions of the duodenum are retroperitoneal and the fourth enters back into the peritoneal cavity that we described earlier we then connect to these judgmental loops or jejunal 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 allele loops or ileal loops and the ileal loops if we continue to follow them so this is one of the more distal ilia loops if i follow it inferiorly here we'll eventually come to the terminal ilium here and then drain into the cecum which is part of the colon or large bowel the point where it drains the terminal allium drains into the cecum is called the ileocecal valve you can often identify the ileocecal 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 sql poll or the sql 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 it's blind ending and it is the appendix we'll talk about how to find the appendix uh every single time uh when it's present at least uh in a bit let's follow the colon first so the cecum joins to become the ascending colon here notice the appearance difference of the large and small bowel owing to the hostra or circulations the ascending colon comes up here to near the liver and this is called the hepatic 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 retroperitoneal 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 allium 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 ilium and the ic valve specifically and then scroll more proximally in the cecum and that could be either upwards or downwards it depends on the orientation the cecum is that's variable and 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 ilium it's not only more proximal in the cecum but also it always arises from the same aspect or same side as the terminal ilium 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 the sql 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 a lot of people use the so-called 369 rule so normal small bowel should not be larger than three centimeters in diameter large bowel should not be greater than six centimeters in diameter and the cecum here in the right lower quadrant should not be larger than nine centimeters 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 centimeters is a cut off for small bowel but 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 vasculature so we talked about the hepatic veins and the portal venous system already with the smv and the 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 of 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 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 be gastric the common femoral artery superficial thermal artery here and the deep femoral artery more posteriorly but again don't worry about these things for now i 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 aliac vein here to make the common iliac vein which drains uh joins the left common iliac to become the ivc or inferior vena cava you can see the renal veins here on both sides the hepatic 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 retroperitoneal nodes so if i scroll down here i often start by looking here in the para aortic 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 a view when i'm scrolling through this axial image are all small para aortic lymph nodes in the space between the aorta and the ivc we have the aorto caval nodes the nodes behind the ivc are called the retro cable nodes and if they're in front of the ivc they're called pre-cable 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 and normal inguinal nodes while 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 mesorectal space and there's mesorectal fat here around the rectum in patients with colon cancer and prostate cancer oftentimes you'll see abnormal mesorectal nodes these are called mesorectal 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 uh you'll often see abnormal rounded nodes when rectal cancer metastasizes so any nodes that are seen along these vessels are called superior rectal nodes back to the upper abdomen 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 gastropathic ligament nodes or nodes along the lesser curvature oftentimes people refer to nodes in the porta hepatis as either gastropathic nodes or 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 ivc here is a so-called portal caval node but again it's all in the gastropedic ligament so you could just refer to it as that another nodal station is the mesenteric nodes you can see several normal mesenteric nodes here in the small bowel mesentery 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 note 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 cutoffs for various uh stationed lymph nodes but in general if you remember around one centimeter is a cut off 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 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 seven millimeter 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 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 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 you look at the liver and the spleen the kidneys etc you really get a tunnel vision at looking at those specific organs and you you miss the big picture and often can miss findings even when you're so zoned in so i always start with a scroll down and you 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 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 entrepreneurial gas so you do that by windowing usually people use long 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 entrepreneurial gas in the abdomen i then look for free intraperitoneal fluid or ascites and as i'm scrolling through their special attention to areas like the patterino fossa or morrison's pouch peripatic 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 intraperitoneal so there's a little bit of free fluid in this patient and 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 intra-hepatic or extra pedophilia reductalitation 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 unseen process 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 hydronephrosis focal lesions other abnormalities then follow the ureters down on both sides you don't have to follow them in every patient if they don't have hydronephrosis 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 the bladder seminal vesicles prostate here and the rectum and then i take a second more close look at the peritoneum so i look at the rectovesico space here and then as i scroll up i'm looking at the i usually start with the left paracolic 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 subphrenic 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 and 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 duodenum first second third across the midline and fourth portion of the duty in 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 and 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 mesentery as its own organ and as an organ as part of the small bowel so i look at the small bowel mesentery here and here on the second run so i do two runs of the small bowel mesentery 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 hepatic flexure and i'm following it backwards down the ascending colon all the way to the cecal base here once i've got to this eagle base i look at the terminal ilium which is here and you can see that there's some thickening of the terminium 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 are not worried about appendicitis you'll often pick up abnormalities of the appendix such as mucoceles a term used to refer to a mucous filled appendix and the cause of that can be neoplastic or cancer and again we talked about how to find the appendix in this patient it's very clear that there is an appendiculath or a calcified stone here at the base of the appendix obstructing the lumen causing a dilated inflamed appendix with a lot of inflammation around it this is acute appendicitis satisfaction of search don't stop there you gotta keep looking through the case so once i've looked at the bowel and the mesentery i look at the vasculature so there are three sets of vascular sure to look at so the arteries first i generally follow the aorta down and looking at the major branches of just the celiac axis the sma i followed down so you don't miss an sma thrombosis then i follow the 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 hepatic vasculature so look at the hepatic veins very briefly and then every time you want to look at the portabena 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 an abdominal wall and then look at the bones so we showed you that earlier with anatomy but again bone window and then looking at an axial first the pelvic bones the femurs 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 reformats 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 coronal 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 mesenter you can see how nicely you can see the small bowel mesentery here okay so that's it for this particular talk looking at mainly anatomy and what normal should look like as well as a 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 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 what you need to do when you see various things in each of the organs and this will likely take place over a few videos in the future so please stay tuned thanks

Channel: Navigating Radiology

Views: 171,321

Rating: 4.9579968 out of 5

Keywords: CT, Abdomen, Pelvis, Radiology, Reading, Xray, Abdominal, Imaging, Anatomy

Id: Nnr4ZB8e4nc

Length: 65min 31sec (3931 seconds)

Published: Thu Jan 30 2020