Abdominal Aorta: Sonographic Evaluation and Disease

Blood Vessel Anatomy:

Tunica Intima-inner layer endothelial cells, basement membrane is the innermost layer of intimal cells
Tunica Media-middle layer, muscle, thicker and more organized layer in arteries than veins
Tunica Adventitia/Externa-Outer Layer, epithelial cells

***Veins have thinner media layer to allow flexibility of lumen size with varied flow volume***

AORTA

Anatomy:

Trunk artery
Arises from left ventricle
Courses inferior through chest and enters the abdomen through the diaphragm
Located anterior and to the left of the spine and to the left of the IVC
Most posterior abdominal vessel
More posterior than the IVC until the umbilicus level where it lies more anterior than the IVC
Distributes oxygenated blood to organs and limbs
In most cases branches of the aorta are named after the organs they are feeding

Paired Branches:

Suprarenal arteries - supply adrenal glands
Renal arteries - supply kidneys
Gonadal arteries - supply ovaries/testicles
Lumbar arteries
Common iliac arteries

Unpaired Branches:

Celiac Axis - 1st abdominal branch of aorta, supplies blood to common hepatic artery, splenic artery and left gastric artery
Superior Mesenteric Artery(SMA)
Inferior Mesenteric Artery (IMA)
Median Sacral Artery

Anterior Branches: from superior to inferior

Celiac Axis
Superior Mesenteric Artery(SMA)
Inferior Mesenteric Artery (IMA)
Gonadal Arteries
Median Sacral Artery

Lateral Branches: from superior to inferior

Suprarenal Arteries
Renal Arteries
Common Iliac Arteries

COMMON ILIAC ARTERIES:

Bifurcation at L3-4 vertebra, umbilicus level
Supplies legs and pelvis
High resistance flow
Above the umbilicus, abdominal arteries are posterior to veins
Below the umbilicus, the abdominal/pelvic arteries are anterior to the veins
Evaluated for arterial pathology in longitudinal and transverse planes

Indications to Scan the Aorta:

Abdominal pain
Pulsatile mass
Aneurysm seen on x-ray
F/U aneurysm
Trauma

Lab Testing:

Hematocrit - decreased levels can indicate an active bleed in the body
Low hematocrit could be due to an aneurysm that is leaking into the abdomen

Exam Technique:

2.5MHz to 6.5MHz adult probe
4MHz to 8MHz pediatric probe
Patient must be NPO 8-12hrs prior to the exam to best visualize the abdominal vasculature
Simethicone may be administered to reduce gas in the digestive system
Multiple patient positions can help view the aorta behind gas containing bowel loops
Supine, oblique and decubitus positions may all be necessary for optimal evaluation
A transverse view can demonstrate a transverse image of both great vessels on the same image
To obtain a longitudinal view of the aorta and IVC on the same image:
- Position the patient in a decubitus position
- Use a coronal approach to angle through the abdomen to visualize the aorta and IVC simultaneously
- A right coronal approach will demonstrate the aorta as the posterior vessel on the image
- A left coronal approach will demonstrate the IVC as the posterior vessel on the image
Right posterior oblique position can be especially helpful for evaluating the distal aorta bifurcation into the common iliac arteries
Identifying the celiac axis guarantees a complete evaluation of the proximal segment
Identifying the aortic bifurcation guarantees a complete evaluation of the infrarenal segment
2D, Color and Doppler evaluation of proximal, mid and distal segments with iliac arteries

Sonographic Appearance:

Best anatomic landmark in abdomen
Longitudinal - hollow tube anterior to spine
Transverse - circular structure to the left of the comma shaped IVC
Gradual tapering as it courses distally
Becomes more anterior in the abdomen as it courses distally

Average Normal Measurements:

Proximal 2.0-2.6 cm

Mid 1.6-2.4 cm

Distal 1.1-2.0 cm

Iliacs 0.6-1.4 cm

AP Dimensions are obtained in longitudinal plane, from outer wall to outer wall of the vessel
Width Dimensions are obtained in the transverse plane, from outer wall to outer wall of the vessel

Doppler of the Aorta:

High resistance
Clean spectral window
Biphasic above renal arteries due to low resistance branches to organs
Triphasic below renal arteries; supplies lower extremities which cause an increase in resistance
Occlusive disease may change resistance and pulsatility of waveform
Aneurysmal disease may show increased turbulence, especially with color Doppler

PATHOLOGY

Aortic Ectasia:

Lack of tapering of the aorta as it travels distally, size remains constant from proximal to distal portions
Can be a precursor to aneurysm formation

Abdominal Aortic Aneurysm:

Focal dilatation of the aorta >3cm or increase in diameter >50% between two adjacent segments
Most commonly caused by atherosclerotic disease
All three layers of the arterial wall are stretched
Risk factors include male gender, family history, smoking, chronic HTN
Anemia and low hematocrit levels can be an indicator for a slow bleed from an aneurysm
Types:
- Saccular - localized round outpouching, may have small stalk
- Fusiform - vessel wall stretches in a circumferential manner; most common type
- Berry - tiny out pouching, usually found in the cerebrum and splanchnic arteries
- Mycotic - infected aneurysm; seen with syphilis
Above the Renal Arteries - immediate surgical intervention
Below the Renal Arteries - Most common location of fusiform aneurysms
Surgical intervention at a diameter >5.5cm
Iliac, common femoral and popliteal arteries can have associated aneurysm formation
Common iliac artery diameter >1.5 cm indicates aneurysm
If AAA identified, measure the diameters of the common femoral and popliteal arteries
CFA and popliteal artery aneurysms are defined as a >50% increase in diameter compared to adjacent segment
Requires follow up exams after AAA is first identified
- Annual follow up on AAA 4-4.4cm diameter
- Semiannual follow up on AAA >4.5cm

Complications:

Rupture
Decreased flow to lower the extremities
Blue Toe Syndrome with thrombus accumulation and embolization
Affects renal circulation and systemic blood pressure, if suprarenal or juxtarenal in location
Large AAA can compress IVC causing reduced flow toward the heart and pedal edema

Sonographic Appearance:

Identifying the celiac axis guarantees a complete evaluation of the proximal abdominal segment
Identifying the aortic bifurcation guarantees a complete evaluation of the infrarenal abdominal segment
Goal of the exam is to find the location of the maximum diameter of the aorta
The greatest diameter of the abdominal aortic segment should be at the level of the celiac axis
Longitudinal images provide the best view for accurate measurements perpendicular to the axis of the vessel
Measure true vessel size from outer edge of the wall to outer edge of the wall
Enlarged vessel >3cm or >50% increase in diameter compared to adjacent segment
Possible thrombus accumulation causes echogenic debris levels within the lumen
Measure true lumen size, if thrombus accumulation present
Measure the length of the affected segment
Document AAA location related to renal arteries
Turbulence seen with Color and PW Doppler evaluation
Yin Yang Sign - describes the swirling blood in the body of the aneurysm

Aneurysms of the Splanchnic Arteries:

Splenic Artery
- Most common splanchnic artery aneurysm
- Associated with pancreatic inflammation, peptic ulcers, patients with more than 2 pregnancies (multigravida) and trauma
- Usually multiple
- More common in women
- May cause bruit in LUQ
- Critical finding due to rupture risk
Hepatic Artery
- 2nd most common type
- Most are in the extrahepatic segment
- Associated with systemic infection and trauma
SMA
- Least common type
- Associated with cystic medial necrosis

Endovascular Aortic Repair:

Percutaneous stent or graft placement through the groin is the standard treatment for infrarenal aortic aneurysms
The grafts must be monitored by serial ultrasound exams to assess for endoleak
3-6 month serial exams in the first year, 6-12 months after the first year
Normally the native aortic sac will decrease in size over time
If a leak is present, the native sac will increase in size between exams
The native aortic sac should be measured in the transverse plane
If the aortic sac expands more than 0.6cm between exams, a leak should be suspected
Color Doppler can be helpful in identifying blood flowing out of the graft into the native sac
Four Types of Leaks
- Type I: leak at the graft attachment site
- Type II: retrograde flow into the native sac from the lumbar artery or IMA
- Type III: defect in the graft
- Type IV: graft material too porous and allowing blood through the fabric mesh

Blue Toe Syndrome:

Embolic material lodges in digital arteries in toes leading to cyanosis of the distal tissues
Causes: Thrombus in an aortic aneurysm, arteritis, ulcerated atherosclerotic lesions, some angiography procedures

Dissection:

Intimal layer tears and allows flow between the intima and media layers into a blind pocket (false lumen)
Remaining lumen (true lumen) is decreased in size due to the flap with blood flow beneath it
Causes weakened vessel wall, risk of vessel rupture
Thrombosis can occur in the false lumen which can cause a significant stenosis/occlusion in the vessel and ischemia distal to the dissection
Most commonly occurs in the aortic arch due to the shearing forces of the blood as it rounds the curve of the arch
Associated with connective tissue disorders - Marfan and Ehlers-Danlos Syndrome
Surgical intervention required immediately due to significant risk of rupture
Debakey Classification:
- Type I - involves ascending and descending aorta
- Type II - involves ascending aorta; associated with Marfan syndrome; least common
- Type III - involves the descending aorta (below the origin of the left subclavian artery); lowest mortality rate
Marfan syndrome:
- Genetic disorder that affects connective tissue of the heart, vessels and bones
- Patients are usually very tall with a thin frame, long extremities and fingers
- Abraham Lincoln is believed to have had Marfan syndrome
- The aortic root and arch are the most commonly affected blood vessel (DeBakey Type II)
- Aortic dissection, aneurysm and valve insufficiency are commonly associated with this disorder
- Mitral valve prolapse and valve insufficiency are common with this disorder

Sonographic Appearance:

Linear echogenicity seen in the lumen of the vessel, separating it into two channels; one channel is a blind ended pocket
Color flow demonstrates two lumens, both with turbulence
Bidirectional flow seen in false lumen

Aortic Rupture:

Significant risk of rupture in aneurysms >7cm in diameter
Back pain and hypotension
Hypovolemic shock

Occurs due to sudden significant blood loss
Organs do not receive the oxygen and nutrients they need to function
This can lead to organ failure and can be fatal

Critical finding
Varied sonographic appearance
Irregular hypoechoic areas near an aortic aneurysm
Hematomas can displace surrounding structures

Pseudoaneurysm:

Caused by trauma or invasive procedures
Blood escapes through all 3 layers of the artery into surrounding tissues and is encapsulated within the tissues
Forms round sac of blood
The connection with the vessel is made through a neck or stalk
Critical finding
Treatment by compression of the stalk or thrombin injection
Compression performed in 10 one-minute intervals with a re-evaluation of flow with color Doppler after each compression interval
Thrombin - clotting agent injected into a pseudoaneurysm to close the opening and clot the blood that has escaped; usually reserved for larger pseudoaneurysms with larger stalks

Sonographic Appearance:

Rounded, anechoic structure adjacent to main artery
Color demonstrates a connection between the artery and the structure
Color also demonstrates turbulent flow, Yin/Yang sign within the body
Doppler evaluation will demonstrate high resistance to-and-fro flow in the stalk and lower resistance to-and-fro flow in the body
Measuring the depth to the pseudoaneurysm is helpful for needle selection with Thrombin injection

Aortic Stenosis:

Most commonly caused by atherosclerotic changes
Can also be related to AAA thrombosis or arteritis
Resistance will increase proximal to the stenosis
Causes increased velocity at the site of the stenosis
Distal to the stenosis the flow will be dampened with tardus parvus waveforms possible
Effects will be similar to coarctation with increased brachial pressures and decreased bilateral ankle pressures
If the stenosis is superior to the renal artery origins, renal ischemia will activate the renin-angiotensin system causing systemic HTN

Leriche Syndrome:

AKA aortoiliac occlusive disease
Due to occlusion of the abdominal aorta just above the site of its bifurcation
Causes bilateral symptoms and flow changes in the lower extremities
Symptoms include fatigue of both lower limbs, intermittent bilateral claudication with ischemic pain, absent or diminished femoral pulses and pallor or coldness of both lower extremities
Doppler waveforms will demonstrate low resistance, post-stenotic flow changes throughout both legs
Can be related to erectile dysfunction as the internal iliac arteries supply the penis with blood

Retroperitoneal Fibrosis:

AKA Ormond disease
Most commonly occurs at the level of the aortic bifurcation and inferiorly in pelvis
Idiopathic(usually) overgrowth of fibrous tissue around an atherosclerotic aorta
Can be related to drugs, infection, malignancy or cancer therapy
May lead to ureteral obstruction causing hydronephrosis
May compress the IVC causing bilateral pedal edema
May compress the gonadal veins causing scrotal swelling

Sonographic Appearance:

Soft tissue mass surrounding great vessels
Hypoechoic
Smooth borders

The questions below are reading comprehension questions intended to help evaluate your retention of the material presented on the page. To access our registry exam practice questions or the CME post test, please refer to your login instructions for information on accessing the dedicated testing site (if your subscription included exam access).

Correct answer is "At the level of the umbilicus".

Correct answer is "Use a coronal view with decubitus patient positioning".

Correct answer is "Are most accurately measured with a longitudinal plane".

Correct answer is "The resistance to flow increases as you move toward the extremities".

Correct answer is "3cm".

Correct answer is "Rupture".

Correct answer is "Aneurysm".

Correct answer is "Slow leak of an AAA".

Correct answer is "Celiac axis and iliac arteries".

Correct answer is "Increase in aortic sac size".

Correct answer is "Marfan and Ehlers-Danlos syndromes".

Correct answer is "The intima and media layers".

Correct answer is "Pseudoaneurysm".

Correct answer is "To determine the length of the needle for Thrombin injection".

Correct answer is "Leriche syndrome".

Correct answer is "Ormond disease".