Mod 2 notes - Brent DMS Abdo 1120 Anatomy PDF

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Brent DMS Abdo 1120 Anatomy...

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Anatomy and Sonography of the Abdominal Vasculature

Preamble These notes are meant to be supplementary information to the material in your textbooks. For complete information, please read Chapters 10-12 in Curry and Tempkin and Chapter 21 in Tortora. Within this module you will learn the anatomy of the vessels normally visualized in the abdomen, and will view their sonographic appearance. The portal venous system will be introduced in this module, but we'll get into more detail in the next module when we discuss the normal anatomy of the liver. The abdominal blood vessels are like a "map" or a "field of landmarks" of the abdomen. The vessels will form the landmarks for the various abdominal organs visualized by ultrasound, and will help you to figure out where you are while scanning. It would be lovely if our sonographic images looked like those in an anatomy textbook, but all we see are shades of grey. This can be quite daunting to the beginner. The abdominal vessels become familiar structures from which to build the rest of the anatomy on. The vessels can also have pathology within them, so it is important to not only know how the vessels lie within the body, but to understand their anatomy and the significance of patient symptoms and lab work as well, to assess for pathology.

The Circulatory System The Heart An in-depth study of the heart and systemic circulatory system will be done in your Echocardiography and vascular courses next semester, but in order to understand the vessels inside the abdomen, a basic understanding of the circulatory system is essential. Think of your heart as a pump that has 4 chambers. The right side of the heart normally contains deoxygenated blood or blood that is low in oxygen and rich in carbon dioxide. This blood generally has a bluish tinge to it and so in anatomy books it is usually colored blue. The left side of the heart contains oxygenated blood which is rich in oxygen and low in carbon dioxide. Blood that is rich in oxygen generally takes on a reddish tinge and is generally colored red in anatomy books. Arteries and veins Arteries carry blood away from the heart and veins carry blood toward the heart. Individuals that are unfamiliar with anatomy make the assumption that all arteries carry oxygenated blood and all veins carry deoxygenated blood. This is not the case actually. In our bodies we actually have two separate systems that work together transporting blood to and from the heart. Pulmonary Circulation Pulmonary arteries take blood from the right side of the heart to the lungs where the carbon dioxide will be exchanged with oxygen. 4 pulmonary veins bring blood rich in oxygen from the lungs to the left side of the heart where it will then be pumped out to the rest of the body. This is where the systemic circulation starts off. Systemic Circulation The arteries in this system carry oxygenated blood from the heart to all of the tissues and organs in the body. After the tissues and organs use up all of the oxygen and exchange it with carbon dioxide, the veins in the systemic circulation bring the deoxygenated blood back to the right side of the heart where it returns back to the pulmonary circulation.

Anatomy of Arteries and Veins Before we start to look at the vessels that we image in the abdomen, we need to take a brief look at the microanatomy of arteries and veins. Again, you will study these vessels in greater depth in your vascular course. The general pathway that blood takes after it leaves the left side of the heart is as follows: Left ventricle - Aorta - Arteries - Arterioles - Capillaries - Veinules - Veins - Superior/Inferior Vena Cava

Both arteries and veins are composed of the same basic layers or tunics.

Tunica Intima - This is the innermost layer of blood vessels. This layer is very smooth and slippery because as the blood circulates through the circulatory system, the red blood cells rub against the walls of the blood vessels. If the tunica intima is rough, the red blood cells will rupture and start to clot. Clotting is a normal process that takes place when you cut yourself, but when it happens within the lumen of a blood vessel this will cause an obstruction. One important feature that differentiates and artery from a vein is that the walls of arteries are perfectly smooth compared to the intima of veins. The tunica intima of a vein contains valves which are like a door that allows blood to travel in only one direction. The reason for this difference is that blood is driven through arteries from the pressure of the left side of the heart. Every time the left ventricle contracts the pressure from the contraction pushes the blood down the artery. By the time the blood leaves the capillaries and gets to the veins it has very low blood pressure and the valves in the veins help it return back to the right side of the heart. Tunica Media - This is the middle layer of arteries and veins and is composed of smooth muscle and elastic fibers which allow the vessel to expand and contract. Arteries have a thick tunica intima compared to veins because the blood in arteries is under a higher pressure from the heart. In veins, the pressure is quite low and so the walls are thinner. This feature makes it possible to differentiate and artery from a vein on ultrasound as veins tend to compress where arteries do not. Tunica Adventitia - This is the outermost layer of a blood vessel and is composed of collagen and elastic fibers. This provides a tough outer coating to the blood vessel and also allows it to be anchored to the tissues it is supplying. Vasa Vasorum - The walls of arteries and veins are living tissue and so they also need a blood supply. The vasa vasorum is located in the tunica adventitia and supplies blood to and drains blood from the walls of the arteries and veins. Arterioles and venules are situated on either side of the capillaries. These vessels generally have the same three tunics as the arteries and veins, but as they get very small and just before they become capillaries, the tunica media and adventitia becomes replaced with connective tissue from the surrounding organ.

Anatomy of Capillaries Capillaries are the smallest blood vessels in the body and only contain a single layer of endothelial cells. This single layer of cells is located within the organs and tissues and is the only place where oxygen and nutrients leave the circulatory system to nourish the tissues and the carbon dioxide and other end products of cellular metabolism can leave the tissues to enter the circulatory system and return back to the lungs. The capillaries are like a very fine sieve which only allows small particles through. Metarterioles A metarteriole is a special connection between arterioles and venules which bypasses the capillaries. The main difference is that they have small bands of connective tissue within their walls.

Some things to think about while reading about the different blood vessels in our body:

Think about why the aorta, the largest artery in the body, has a more developed media than the IVC, the largest vein in the body. Why do you think the aorta doesn't compress like the IVC does? Why do you think that some veins in the body have valves attached to their walls and the arteries do not?

The Abdominal Aorta and its Branches

The Abdominal Aorta We will begin our learning adventure with the abdominal aorta and its branches. The aorta is the main artery coming directly from the left ventricle of the heart. It ascends just to the right of midline to form the aortic arch. The aortic arch extends to the left of midline giving off 3 branches along the way; the brachiocephalic or innominate artery, the left common carotid artery and finally, the left subclavian artery. From here, the thoracic aorta descends down the chest slightly to the left of midline and then pierces through an opening in the diaphragm to become the abdominal aorta. You will learn more about the thoracic aorta in your Adult Echocardiography course, so we will focus on the abdominal aorta here. There are many branches off the abdominal aorta that serve the abdominal organs. We will only focus on the branches which we are capable of routinely visualizing with ultrasound.

THE ABDOMINAL AORTA AND ITS BRANCHES.

The abdominal aorta begins at the level of the diaphragm, and extends to approximately the level of the umbilicus (which is at approximately the L4 spinal level), where it bifurcates into the Common Iliac Arteries. 1. Celiac Artery (also known as Celiac Axis, or Celiac Trunk) o This is a short little artery arising from the anterior wall of the aorta o It is only about 1.25 cm long, and quickly branches into 3 other arteries which supply the liver, gallbladder, stomach, spleen and pancreas with fresh oxygenated blood o The celiac artery branches into 3 arteries: the Left gastric artery, the splenic artery and the hepatic artery Left Gastric Artery • • •

This is the most superior branch of the celiac trunk It travels superior and to the left, supplying the stomach and the esophagus with blood We don't see this artery routinely with ultrasound

Splenic Artery • •

This is usually the largest branch and it is very tortuous as it travels along the superior border of the pancreas. It sends off little branches to supply the spleen and pancreas with blood.

Common Hepatic Artery •

• •

The common hepatic artery comes off the celiac axis and heads to the right, just cephalic to the head of the pancreas. As the common hepatic artery approaches midline, it makes a turn and gives off a small branch called the gastroduodenal artery which sits in the anterolateral portion of the pancreatic head. After giving off the gastroduodenal artery, the hepatic artery heads up to enter the liver's porta hepatis within the hepatoduodenal ligament of the lesser omentum along with the main portal vein and the common bile duct. After making the turn toward the liver, the hepatic artery changes its name to the hepatic artery proper. The usual relationship of the hepatic artery proper is medial and anterior to the main portal vein. You will find the CHA anterior to the right superior aspect of the pancreatic head. This artery further divides into: Right hepatic artery (which gives off another branch - cystic artery) Left hepatic artery (which serves the left lobe) Right gastric artery (which serves the stomach) Gastroduodenal artery (which is found in the neck of the pancreas)

2. Superior Mesenteric Artery (SMA) • • • •

Supplies most of the small intestine, cecum, ascending colon, and most of transverse colon Leaves anterior aorta about 1 cm below celiac axis Travels posterior to pancreatic body and courses inferiorly The left renal vein courses between the SMA and Aorta

3. Renal Arteries • • •

Branch laterally from the aorta immediately below the SMA Right is longer than left and passes posterior to IVC and right renal vein Left lies posterior to the left renal vein

4. Inferior Mesenteric Artery (IMA) • • •

Arises from anterior aorta Supplies the transverse colon, descending colon, sigmoid and rectum Not routinely visible by ultrasound

5. Middle Suprarenals • •

Arise from each side of the aorta at approximately the level of the SMA These are not routinely visible by ultrasound

6. Testicular / Ovarian Arteries • •

Arise from the posterior aspect of the aorta These are not routinely visible by ultrasound

7. Lumbar Arteries • •

Usually 4 on each side arising from posterior aspect of aorta These are not routinely visible by ultrasound

8. Medial Sacral Artery •

Small branch just before the aortic bifurcation, not routinely visible by ultrasound

9. Common Iliac Arteries • • •

there are 2 of these, of course, because the aorta bifurcates into the common iliac arteries they lie anterior and lateral to the common iliac veins branches are the internal and external iliac arteries

Now that we have mentioned all the arteries of the aorta, you should know that the only ones we'll pay attention to sonographically in abdomen are: 1. 2. 3. 4.

celiac artery, splenic artery, hepatic artery SMA renal arteries common iliac arteries

I mentioned them all for the sake of completeness!

The Inferior Vena Cava and its tributaries

Smaller venules unite to form increasingly larger veins. Veins from all over the body, except those, which serve the lungs and myocardium, empty either into the superior vena cava (SVC) or the inferior vena cava (IVC). Both SVC and IVC enter the right atrium of the heart.

The union of the common iliac veins forms the IVC. The IVC travels lateral to the aorta and posterior to the liver to pierce the diaphragm and enter the right atrium. Branches include (in order from origin to heart):

•

Right gonadal Renal veins (with left gonadal and adrenal veins emptying into the left renal vein) Right adrenal vein

•

Hepatic veins.

• •

We'll only see the renal and hepatic veins (with ultrasound). One relationship to remember is:

The left renal vein courses anterior to the aorta and posterior to the SMA to enter the lateral aspect of the IVC.

The Portal Venous System As you are probably realizing, all the organs in the abdomen, and pretty much the entire body, are supplied with oxygenated blood which is carried by an artery and the deoxygenated blood from the organs is returned to the heart via the veins. Arteries and veins are kind of like "Buddies", in that they usually travel in pairs. If you look at the branches of the aorta and the IVC you have probably noticed that for the most part, there is an artery branching off the aorta and a corresponding vein returning to the IVC. If you take a close look at the aorta and IVC, however, you are probably noticing a few exceptions: • • •

The Splenic Artery, Superior Mesenteric Artery and Inferior Mesenteric Artery are all branches of the abdominal aorta. If you look at the IVC, you will notice that there are NO corresponding veins to go with these arteries. The Splenic Vein, Superior Mesenteric Vein and Inferior Mesenteric Vein are all part of a third system of veins called the Portal System

The Portal System The portal system of veins are the veins that drain blood from the spleen and intestines and return this blood to the liver. The blood from these organs is rich in nutrition and some waste products, so the liver gets to process all of this blood; processing all of the nutrition so that we have energy to live and destroying all of the bacteria and harmful products so that we don't get sick. All the blood comes into the liver from the portal vein. The blood travels through microscopic blood vessels in the liver lobules called liver sinusoids. It is in the liver lobules that the blood is processed; nutrients are absorbed and waste products removed. After the liver is finished with the blood, it makes its way back to the heart through the hepatic veins that connect with the IVC. Don't worry if this sounds a bit confusing, we are going to spend a lot of time talking about this in our next module which is the liver. Here is a way to remember how the veins come together •

The Splenic Vein and the Superior Mesenteric Vein get married and become the Main Portal Vein. The Main Portal Vein goes on a honeymoon to the liver. The inferior Mesenteric vein stays behind and stays attached to the splenic vein.

The veins of the portal system which you will observe sonographically are the Superior Mesenteric Vein (SMV), Splenic vein (SV) and main portal vein (MPV).

Sonography of the Abdominal Vasculature • • •

Patient Preparation: fasting Transducer: 3 – 3.5 MHz Curvilinear probe (5 MHz for thinner patients) Menu Choice: usually abdominal set-up - Use harmonics if available.

There are several questions that should be addressed if one is interested in examining the vascular system for pathology: 1. Is the size of the vessel within normal limits? 2. Is this vessel located in its normal position? If not, then how is it being displaced? What is displacing it? 3. Is the vessel patent, or are there flow abnormalities? 4. Is the motion of the vessel normal? So what does a normal blood vessel look like? In general, all blood vessels in the body are more or less hollow tubes. The layers of the vessels that make up the walls generally show up as thin, echogenic margins, and the lumens of the vessels show up more or less anechoic. this is because the lumens of the blood vessels are where the blood flows. Blood is mainly water, which, if you recall from module 1, shows up anechoic on ultrasound. A good statement to remember is that a normal vessel should have thin, echogenic walls and hypoechoic lumens.

Sonography of the Aorta and its Branches: The abdominal aorta (AA) is an anechoic tube that dips posteriorly as it courses under the liver. It is directly anterior and slightly left of the spine. If possible, the entire AA should be demonstrated from diaphragm to bifurcation. Your job is to scan the aorta in sagittal and transverse planes, looking for pathology and taking the appropriate images.

Measuring the Aorta As you will soon discover, there is a normal range of measurements that we learn for certain structures in the body. The normal measurements for the aorta are as follows: The proximal abdominal aorta • •

Men - 2.5-2.7 cm Women - 2.1-2.3 cm

The distal abdominal aorta • •

Men - 2.0-2.4 cm Women - 1.7-2.2 cm

A standard measurement that is done on pretty much any abdominal ultrasound or dedicated abdominal aorta scan, is to measure the distal end of the abdominal aorta. The reason why we spend extra time and attention to this area is because this is where the majority of abdominal aortic aneurysms occur. There is a proper way to measure the aorta to ensure an accurate representation of the diameter of the vessel and for repeat ability if the patient comes in for follow-up scans: When measuring the distal abdominal aorta, make sure that you are long-axis or sagittal on the aorta. This ensures that you are truly perpendicular to the vessel and will document the most accurate diameter. You want to make sure that you see the walls clearly, as well as the bifurcation of the aorta. Again, this ensures accurate measurement and repeat ability. When evaluating the size of a vessel, you should use the technique called “outer to outer”. This means you position your first caliper on the outermost layer of the anterior portion of the vessel (adventitia), and the 2nd caliper is placed on the outermost portion of the posterior part of the vessel (adventitia). Make sure that your calipers are perpendicular to the lumen of the vessel so that you don't make the diameter bigger than it really is. This measurement technique correlates best with CT measurements of the aorta which is often considered the gold standard for assessment of aortic pathology. Scanning Details: o o o

You should scan sagittally down the aorta from its proximal portion all the way to the distal aorta The aorta demonstrates strong pulsations, compared to the IVC which shows a soft doublebeat pattern of pulsation When you are scanning the aorta, look at its wall. The intima of the aorta is echogenic, and should be smooth. You may see the hypoechoic media of the aorta, as well as the echogenic adventitia.

Branches of the Abdominal Aorta •

The first branches seen in a sagittal plane are the celiac artery (axis) and the SMA. They both arise from the anterior wall of the aorta. The celiac artery is short and appears truncated due to its splitting into the hepatic, splenic and left gastric arteries. The SMA is long and travels inferiorly, anterior to the aorta as it supplies blood to most of the small intestine and p...