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Learning Modules ❯ Vascular

CT of Abdominal Aorta: Aneurysms, Dissections and Repair

Elliot K. Fishman M.D.
Johns Hopkins Hospital

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Abdominal Aortic Aneurysm

Aneurysm defined as:

Diameter >3.0 cm
Dilated region with diameter >50% of normal region (1.5x its normal size)

Causes:

Hypertension/smoking
Connective tissue disorders/familial
Infection (mycotic aneurysm)

Source: 2014 ESC Guidelines on diagnosis and treatment of thoracic and aortic disease. Eur Hrt Jnl.

Abdominal Aortic Aneurysm: Facts

The infrarenal aorta is the most common site of aortic aneurysm formation
Aortic aneurysms are three to four times more common in men than in women
The incidence in men older than 60 years of age is 4% to 8%, while that in women is 0.5% to 2%
Risk factors for AAA include age, hypertension, chronic obstructive pulmonary disease (COPD), history of cigarette smoking, male sex, and family history of an aortic aneurysm.
The prevalence of AAA among those who had a first-degree relative with an AAA is as high as 30%

Abdominal Aortic Aneurysms: Genetic Disease

Type IV Ehlers-Danlos syndrome
Marfan syndrome
Loeys Dietz syndrome

Thresholds for AAA repair

Abdominal Aorta:

Fusiform > 5.5 cm warrants repair (1-year incidence of rupture was 9.4% for AAAs measuring 5.5 to 5.9 cm, 10.2% for AAAs of 6.0 to 6.9 cm, and 32.5% for AAAs of ≥7.0 cm)
Rapid growth > 1cm/yr
Symptomatic AAA should be repaired
Consider elective repair of any saccular aneurysms

Society for Vascular Surgery Practice Guidelines. J Vasc Surg 2009; 50: S2-S49

“Endovascular aneurysm repair (EVAR) has become the standard of care (given appropriate anatomy) based on its lower morbidity and mortality compared with open surgery. The use of EVAR continues to be limited by anatomic factors, including neck angulation, a short or wide neck, severe calcification, access difficulties, and the presence of thrombus, but modern grafts implanted in patients with proper anatomy seem to be extremely durable.”
Overview of aortic aneurysm management in the endovascular era.
Calero A, Illig KA.
Semin Vasc Surg. 2016 Mar;29(1-2):3-17

Vascular CTA Protocols

Injection rate
Contrast volume
Scan delay
Single vs dual phase acquisition
Site of injection

Vascular CTA Protocols

Injection rate 4-5 cc/sec
Contrast volume 100-120 cc
Contrast type Omnipaque-350
Scan delay variable 15-60 sec
Single vs dual phase acquisition
Site of injection usually antecubital

MDCT Protocols 64/ DSCT

Scan Protocol Optimization: Timing

Preset delay vs Bolus tracking
If bolus tracking where do you track from?

3 Post Processing Tools

Vessel tracking
Automated Bone removal
Negative angiogram display

Runoff with Bypass Graft (Fem-Pop)

CT of Abdominal Aorta: Aneurysms, Dissections and Repair

Dual Energy: Facts

The energy dependence of the photoelectric effect and the variability of K edges forms the basis for dual energy techniques
K edge refers to the spike in attenuation that occurs at energy levels just greater than that of the K-shell binding because of the increased photoelectric absortion at these energy levels
K edge values vary for each element and they increase as atomic number increases

K edges and Atomic Numbers of Key Materials

Dual Energy Technology

Dual source CT: 2 x-ray tubes run at different kVps so data is perfectly aligned
Single source CT: rapid kVp switching is needed to get the two series aligned

Imaging at Lower kVp (80 or 100 kvp vs 120 or 140 kVp): The Good News

Iodinated contrast shows an approximately 80% increase in CT attenuation at 80 kVp compared with 140 kVp
Lower radiation dose
Less IV contrast needed for select examinations
Reduction in artifacts like pseudoenhancement

“ Dual-energy bone subtraction has been shown to be faster and technically superior to threshold-based bone subtraction techniques even when the latter are manually corrected. However, even dual-energy bone subtraction is not infrequently incomplete.”
Dual-Energy CT: Vascular Applications
Vlahos I et al.
AJR 2012; 199:S87-S97

“ The clinical interpretation advantage of 3D-rendered volumetric data are limited by the need for software assisted Hounsfield unit-threshold-based bone subtraction. This is not only time consuming, but the quality of bone subtraction is subject to patient and user dependent variation.”
Dual-Energy CT: Vascular Applications
Vlahos I et al.
AJR 2012; 199:S87-S97

CT Scan Protocols for Stent Repair Follow-Up Studies

Non-contrast CT
Arterial phase imaging
- Scan at 30 sec post injection
- 5cc/sec injection of 100-120 cc iohexol or Iodixanol
Delayed Phase Imaging
- Scan at 70-80 seconds post injection

Post Processing of CT Datasets

Axial
MPR Coronal
MPR Sagittal
Curved Planar Reconstruction (CPR)
MIP 3D Imaging
VRT 3D Imaging

Endografts and Complications: What We Need to Know

Post-stent CT findings

Normal

Aneurysm sac decreases in size progressively

Complications

Aneurysm size increasing (endoleak)
Change in stent positioning (migration, kinking)
Branch vessel compromise
Infection
Limb thrombosis
Aneurysm formation elsewhere in aorta

Successful Stent Placement

Complications and Outcome Timing to the Initial Procedure

Immediate 1–30 days after the interventional procedure
Short term 30 days–12 months after the procedure
Long term >12 months after the procedure

Types of Endoleaks

Type I Flow from inadequate graft apposition at proximal or distal seal zones
Type II Retrograde flow from aortic or iliac artery branches (i.e., patent lumbar, inferior mesenteric, or internal iliac arteries)

Types of Endoleaks

Type III Flow from inadequate graft to graft apposition at component junctions or from tears in the endograft fabric
Type IV Flow through the pores of the fabric
Type V Refractory occult endoleak

Device failures include

Migration of graft
Graft infolding
Fabric tears
Limb disconnections
Stent fractures

“ Operator experience and familiarity with EVAR have improved during the past 20 years. These advances have resulted in decreased procedure times and blood loss and improved intraprocedural and perioperative complication rates. The incidence of type I and type III endoleaks has decreased, and the need for reintervention has declined. Aneurysm-related mortality rates after EVAR are lower now than in the past, and these results continue to improve. The most obvious innovation accounting for these improvements is the transition from use of physician-made devices to the use of industry-made devices. “

“Over an 8-year period, 988 patients underwent EVAR, of whom 42 (4.3%) required secondary interventions involving placement of additional endovascular devices.”
Endovascular stent-graft repair of failed endovascular abdominal aortic aneurysm repair.
Baril DT et al.
Ann Vasc Surg. 2008 Jan;22(1):30-6.

“The mean time from initial operation until second operation was 34.1 months. Failures included type I endoleaks in 38 patients (90.5%), type III endoleaks in two patients (4.8%), and enlarging aneurysms without definite endoleaks in two patients (4.8%).”
Endovascular stent-graft repair of failed endovascular abdominal aortic aneurysm repair.
Baril DT et al.
Ann Vasc Surg. 2008 Jan;22(1):30-6.

“Endovascular repair is the treatment of choice for high-risk patients. A small but significant number of clinical failures were observed during the long-term follow-up.”
Experience and outcomes after a decade of endovascular abdominal aortic aneurysm repair: a retrospective study from a community-based single center.
Kalteis M et al.
Ann Vasc Surg. 2012 Apr;26(3):330-7

“A total of 106 patients were treated in a period of 9 years. A Zenith stent-graft was used in 95% of cases. No deaths occurred during the first 30 days post surgery. The complication rate was 4.7% (n = 5). The overall clinical and technical success rate at 30 days was 93.4%.”
Experience and outcomes after a decade of endovascular abdominal aortic aneurysm repair: a retrospective study from a community-based single center.
Kalteis M et al.
Ann Vasc Surg. 2012 Apr;26(3):330-7

“After a mean follow-up period of 52 months (range, 13-112 months), the overall mortality rate was 25.4%. Aneurysm-related mortality was 2.1%. Rupture of the aneurysm occurred in four cases (4.3%). The final clinical failure rate was 13.8%.”
Experience and outcomes after a decade of endovascular abdominal aortic aneurysm repair: a retrospective study from a community-based single center.
Kalteis M et al.
Ann Vasc Surg. 2012 Apr;26(3):330-7