After completing this laboratory exercise you should feel comfortable identifying abdominal and pelvic organs and blood vessels on axial and coronal CT scans.
Click here for the lecture.
The computer screen is divided into three parts. On the top of the screen are axial and coronal CT scans. To scroll through the scans move the mouse over the image and while depressing the mouse button move the mouse up and down. To stop scrolling un-click the mouse.
Beneath the scans are a list of body part categories: organs, bowel, arteries, veins, and muscles. When you click on a category a list of body parts will appear in the bottom left corner of the screen.
Click on a body part and an image will appear in the bottom right corner of the screen with the body part annotated. You can view the annotated body part in the axial and coronal plane and toggle the annotations on and off.
Remember, radiographic images are oriented so that the patient’s left is on the right side of the image. The patient’s right is located on the left side of the image. If you are in doubt, check which side the heart is on.
Read through the following text which is a virtual tour of the abdomen and pelvis. Find each body part listed in bold on the CT scans. Use the annotated images to verify that you are correct. Find each body part in both the axial and coronal planes. Practice scrolling through all the parts like I did in the lecture to get a sense of how the parts are interconnected with each other.
Click here to open the laboratory exercise.
The liver is the large organ in the right upper quadrant. It is located immediately inferior to the right hemi-diaphragm. Find the gall bladder along the undersurface of the liver. Be sure you see its relationship to the liver in the axial and coronal plane. Gall-stones are sometimes seen as bright round or faceted structures within the gall bladder. However, most gall stones are radio-lucent, meaning that they cannot be seen on x-rays or CT.
The spleen is the organ in the left upper abdomen located immediately beneath the left-hemidiaphragm.
The pancreas is a deceptively large organ in the mid abdomen. It has a head, body, tail, and uncinate process. The head of the pancreas abuts the duodenum (discussed later). The uncinate process is located posteriorly. Notice how the uncinate process is located behind the superior mesenteric artery and superior mesenteric vein (also discussed later) and notice how the left lateral margin of the uncinate process comes to a sharp point. Uncinate means hook, and this is the part of the pancreas that hooks around the vessels in the root of the mesentery. Be sure you can follow the pancreas in both the axial and coronal plane around the duodenum and behind the SMA and SMV.
The kidneys are paired organs on either side of the spine. Find the left kidney and right kidney. Notice how the renal cortex is brighter than the renal medulla. This is because the intra-venous contrast administered for the exam is filtered in the glomeruli which are located in the cortex. If the images were obtained about 3 minutes later, the medulla would also be bright as the contrast is filtered through the renal tubules. The central portions of the kidneys consist of the collecting systems. Urine is concentrated into the collecting systems and then drain via the ureters to the bladder. Follow each ureter to the bladder in both the axial and coronal planes. Notice how they dive posteriorly within the pelvis.
The adrenal glands are located just anterior and superior to the kidneys. Each adrenal gland has a medial and a lateral limb. They look like little Star Trek insignias or Acura automobile logos. Find them in both the axial and coronal planes. The right adrenal gland is whispy and thin.
The uterus is located in the deep pelvis between the bladder and the rectum (discussed later) and often wraping along the bladder dome (superior surface of the bladder). The darker portion in the center of the uterus is the endometrium. The brighter portion is the enhancing myometrium. The left and right ovaries are located between the uterus and pelvic side walls, almost always along the course of the broad ligament which is the thin line connecting each ovary to the uterus, The ovaries are mobile. Notice how the left ovary is located slightly higher than the right ovary.
The esophagus terminates at the stomach at the level of the diaphragm. The stomach begins posteriorly at the gastro-esophageal junction. The gastric fundus/body courses anteriorly and rightward and then courses posteriorly towards the gastric antrum. Follow the stomach. The stomach terminates at the pylorus (not distinctly identified) and the duodenum begins. The duodenum wraps around the head of the pancreas and then crosses the midline posterior to the superior mesenteric vein and artery and anterior to the spine. Be sure you can follow the duodenum and see its relationship to the pancreas. Notice how the pancreatic head and uncinate process are in close proximity to the duodenum and superior mesenteric arteries and veins. Cancers in the head of the pancreas like to surround and infiltrate these structures, making surgical removal difficult. (The surgeon can’t remove the SMA or the bowel will die). After crossing the midline, the duodenum comes superiorly and is suspended at the ligament of Trietz. Distal to the ligament of Trietz, the bowel becomes the jejunum.
The jejunum is predominantly located in the left upper quadrant. The ileum is the distal portion of the small bowel and is located primarily in the right lower abdomen. The mucosa of the ileum and jejunum have different appearances on CT. This is not discussed in this laboratory exercise. Don’t try to follow the jejunum and ileum. This is exceedingly difficult.
The ileum terminates at the ileocecal valve. Distal to this valve is the colon, or large intestine. The portion of the ileum just proximal to the ileocecal valve is called the terminal ileum. This is a common place for inflammation from Crohns disease, tuberculosis, and bacterial bowel infections.
The cecum is the first portion of the colon and is located just distal to the ileocecal valve. Look again at the ileocecal valve and be sure you can follow the terminal ileum into the valve and recognize the cecum distal to the valve. The appendix originates from the base of the cecum, often several cm inferior to the ileocecal valve. Find the appendix. Be sure you can connect the appendix to the cecum. Use the coronal images to see its relationship to the ileocecal valve.
The colon continues superiorly as the ascending colon. Distal to the hepatic flexure, in the right upper quadrant near the liver, the colon becomes the transverse colon. At the spenic flexure, near the spleen, the colon becomes the descending colon. When the colon begins to move medially (actually, when it changes from retroperitoneal to intraperitoneal) it is called the sigmoid colon. The sigmoid colon becomes the rectum, and then the anus. Notice how the sigmoid colon wraps around the bladder. Notice how the rectum passes posterior to the uterus. Trace the entire colon on the axial and coronal images.
This concludes the tour of the bowel. Now on to the arteries.
The aorta is the major blood vessel in the abdomen and pelvis. It is located just to the left of midline anterior to the spine.
The first major branch of the aorta is the celiac artery. The celiac has three major branches: the left gastric artery, the splenic artery, and the common hepatic artery. These vessels are small and not well seen on these CT images but you should try to find them anyway. The left gastric artery courses upwards and courses along the lesser curvature of the stomach. The splenic artery is often very loopy and goes to the spleen. The common hepatic artery gives off a gastroduodenal artery that courses just anterior to the head of the pancreas. Distal to this branch, the common hepatic artery becomes the proper hepatic artery, divides and enters the liver.
The second major branch of the aorta is the superior mesenteric artery (SMA). The SMA courses in the root of the mesentery and roughly parallels the aorta. It runs to the left of the superior mesenteric vein (SMV). The SMA gives off branches to the jejunum, ileum, and colon. It supplies the bowel from the ligament of Trietz to the splenic flexure. See if you can follow the branches of the SMA on the coronal images and appreciate how they fan out into the mesentery. The celiac supplies the bowel from the GE junction to the ligament of Trietz.
The paired renal arteries are the next major branches off the aorta. There is great variation in the number of renal arteries from person to person. Notice how the right renal artery travels behind the inferior vena cava (IVC).
The inferior mesenteric artery (IMA) is a small vessel that originates from the anterior portion of the lower aorta. The IMA travels into the deep pelvis but gives off branches that track into the mesentery. The IMA supplies the bowel from the splenic flexure to the mid rectum. Find the SMA in both planes.
The aorta bifurcates into the common iliac arteries. The common iliac arteries bifurcate into internal and external iliac arteries. The internal iliac artery supplies the pelvic viscera (rectum, anus, uterus, bladder, uterus, penis) and also gives off branches to the gluteal muscles. These vessels are too small to see well on these images. The external iliac artery has no branches. The origin of the inferior epigastric artery, which courses upward along the anterior abdominal wall, marks the transition of the external iliac artery and the common femoral artery. The common femoral artery gives off a profunda artery which supplies the large muscles of the thigh. The common femoral artery then continues as the superficial femoral artery en route to the lower leg.
The largest vein in the abdomen is the inferior vena cava (IVC). It runs in the cranio-caudad direction parallel to and to the right of the aorta. Blood from the lower extremities, genitals, distal rectum, and kidneys returns to the heart via the IVC.
The renal veins drain into the IVC in the mid abdomen. The left renal vein travels in a predominantly transverse (left-right) direction and drapes anterior to the aorta on its way to the IVC (though variants do exist). The left renal vein actually travels between the SMA and the IVC. Confirm this on the images. The right renal vein is a shorter vessel as the distance between the IVC and right kidney is small. It can be very difficult to see on the axial images.
The venous return from the bowel travels through the liver on its way back to the heart via the portal vein. In fact, 85% of the blood flow to the liver is via the portal vein, not via the hepatic arteries (though the bile ducts are supplied almost exclusively by the hepatic arteries). You should notice that the IVC also travels through the liver on its way back to the heart. However, blood from the IVC does not actually flow into the liver, just through the liver. Follow the IVC through the liver. In many places it is hard to see because it is isodense (i.e. the same color) as the liver.
The portal vein is formed by the junction of the splenic vein and the superior mesenteric vein (SMV). The SMV carries the venous return from bowel segments between the ligament of Trietz and the splenic flexure. Find it on the CT images just to the right of the SMA. The splenic vein carries the venous return from the spleen. It runs along the posterior surface of the pancreas. Be sure you can see this on the CT images. These two veins join at the porto-splenic confluence to become the portal vein. Be sure to trace the splenic vein and SMV to the porto-confluence and see the origin of the portal vein.
The portal vein flows into the liver and branches into a left portal vein and a right portal vein. The right portal vein again branches into a right anterior portal vein and a right posterior portal vein. These veins branch into smaller and smaller vessels and supply small sinusoids within the liver. It is here the liver absorbs carbohydrates from the bowel to produce and store glycogen.
Blood from the portal vein filters through the liver, eventually returning to the systemic venous circulation via the hepatic veins. There is a left hepatic vein, right hepatic vein, and middle hepatic vein. These three veins course in a dome shaped configuration towards the IVC at the liver dome. They enter the IVC just inferior to the diaphragm and right atrium. The hepatic and portal veins are important anatomical landmarks because they divide the liver into segments, each with relatively independent arterial perfusion, portal vein perfusion, and biliary drainage. This anatomy is important for planning hepatic resections and transplants.
The inferior mesenteric vein (IMV) provides venous drainage from the bowel between the splenic flexure and mid rectum. It runs roughly parallel to the left ureter and merges with the splenic vein adjacent to the porto-splenic confluence. It is a small vessel but can be seen on these CT scans. Follow this vessel throughout its length.
There are few important muscles in the abdomen and pelvis.
The levator-aniis the muscle that supports the pelvic floor. In the coronal plane it has a “V” shaped appearance superiorly. Since portions of it run in the axial plane it is difficult to see on the axial images. Confirm its course on the coronal images. You can see the muscle course towards the rectum inferiorly, where the levator-ani merges with the pubococcygeus. The pubococcygeus attaches to the pubis anteriorly and then wraps around the ureter, vagina, and rectum in a sling like fashion. This provides support. Damage to this muscle as a consequence of vaginal delivery can result in incontinence. The pubococcygeus muscle contributes to the external anal sphincter. It is a skeletal muscle under voluntary control. The internal anal sphincter is comprised of smooth muscle intrinsic to the wall of the anus. When you perform a “Kegel” exercise, or squeeze to hold in urine or a bowel movement, you are contracting your levator-ani and pubococcygeus muscles.
The psoas muscles are large paired muscles that run in the retroperitoneum along either side of the spine. These are the “Filet Mignon” cut. If you are ever forced to resort to cannibalism then this is useful information. Since the psoas lies adjacent to the spine it is often involved in spinal pathology. For example, infection of the disks and vertebral bodies secondary to tuberculosis often results in psoas abscesses. Conversely, infection in the psoas (from IV drug use or an adjacent diverticular abscess) can spread through the psoas into the spinal canal with devastating effects.
The ilacus muscles are located along the anterior portion of the iliac wings. The psoas and ilacus muscles join together to become the ileopsoas muscle, which can be traced inferiorly to its insertion onto the lesser trochanter of the hip. The ileopsoas is the dominant hip flexor (i.e. it brings your knees to your belly). When you do sit ups incorrectly you are exercising your ileopsoas muscle and this is the muscle in your groin that hurts after long hikes in heavy boots.
Several additional pelvic and hip muscles are better introduced as part of a lower extremity lab, but since they are so well seen here we will look at them.
Three knee extensors originate from the pelvis. The tensor fascia lata and sartorious muscles originate from the anterior superior iliac spine. The tensor fascia lata runs laterally along the thigh. The sartorious muscle crosses medially and runs along the medial thigh and eventually inserts onto the proximal tibia. The sartorious is the longest muscle in the body. The rectus femoris originates from the anterior inferior iliac spine. Proximally, it runs between the tensor fascia lata and sartorious. The rectus femoris is the fourth muscle in the quadriceps group and inserts onto the patella.
The obturator internus muscle lies within the pelvis along the inner surface of the obturator foramen. It wraps around the posterior aspect of the pelvis and inserts onto the lesser trochanter. The obturator externus muscle lies along the outer surface of the obturator foramen. It also inserts onto the lesser trochanter. These muscles are external rotators of the hip.
The pectineus muscle is the anterior most adductor of the hip. It is worth knowing since this muscle makes a nice trivia question.