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Aorta


Blood Vessel Anatomy:
  1. Tunica Intima-inner layer endothelial cells, basement membrane is the innermost layer of intimal cells
  2. Tunica Media-middle layer, muscle, thicker and more organized layer in arteries than veins
  3. 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 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 organ it is 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 that could be 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
  • 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


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 phasicity 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
  • 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 arteries and popliteal arteries can have associated aneurysm formation
  • 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 - blood collects in the perirenal space first
  • Decreased flow to lower 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 decreased flow toward the heart and pedal edema

Sonographic Appearance:
  • Identifying the celiac axis guarantees a complete evaluation of the proximal segment
  • Identifying the aortic bifurcation guarantees a complete evaluation of the infrarenal segment
  • Goal of the exam is to find the location of the maximum diameter of the aorta
  • 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
  • Turbulence seen with Color and PW Doppler evaluation

Endovascular Aortic Repair:

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 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 leading to 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
  • Surgical intervention required immediately due to significant risk of rupture
  • Marfan syndrome:
    • Genetic disorder that affects connective tissue of the heart, vessels and bones
    • Aortic dissection and aneurysm are commonly associated with this disorder
    • The aortic root and arch are the most commonly affected blood vessel
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
Pseudoaneurysm:
  • Caused by trauma or invasive procedures
  • Blood escapes 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
  • Treatment by compression of the stalk or thrombin injection
  • Compression performed in 10 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 pseudo
  • Doppler evaluation will demonstrate high resistance to-and-fro flow in the stalk and low resistance to-and-fro flow in the body of the pseudo

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 and possibly tardus parvus waveforms
  • 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
Retroperitoneal Fibrosis:
  • AKA Ormond's 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


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