For a patient with suspected aortic dissection who is hemodynamically stable enough for contrast, what is the first‑line diagnostic imaging (computed tomography angiography) and which alternative modalities (trans‑esophageal echocardiogram or magnetic resonance angiography) should be used if CTA is contraindicated due to severe renal insufficiency, iodinated contrast allergy, or instability?

Diagnostic Imaging for Suspected Aortic Dissection

Computed tomography angiography (CTA) is the first-line diagnostic imaging modality for hemodynamically stable patients with suspected aortic dissection, given its wide availability, speed, accuracy (sensitivity 93-100%, specificity 98-99%), and comprehensive anatomic detail. 1, 2

Primary Diagnostic Approach: CTA

CTA should extend from the thoracic inlet to the pelvis and include both non-contrast and contrast-enhanced phases. 2 This modality not only confirms the diagnosis but also:

• Classifies the dissection extent (Stanford Type A vs. B) 1
• Differentiates true from false lumen 1
• Localizes intimal tear sites 1
• Assesses branch vessel involvement and malperfusion 1
• Detects aortic regurgitation, pericardial effusion, hemopericardium, and periaortic hematoma 1
• Identifies alternative diagnoses in 13% of cases without aortic pathology 1

The critical advantage of CTA is that it can be performed rapidly in the emergency department at all hours, which is essential given the 1-2% per hour mortality rate in untreated dissection. 2, 3

Alternative Modalities When CTA is Contraindicated

Transesophageal Echocardiography (TEE)

TEE is the preferred alternative for hemodynamically unstable patients or those with severe renal insufficiency, offering bedside diagnosis with sensitivity 98-100% and specificity 95-100%. 1, 2

TEE should be performed:

• At the bedside in the intensive care unit for unstable patients 2
• In the operating room immediately before emergency surgery 2, 4
• When the patient cannot be safely transported for CT imaging 4

The key limitation is that TEE provides a more limited field of view compared to CTA and may miss distal aortic involvement. 1

Magnetic Resonance Angiography (MRA)

MRI demonstrates the highest sensitivity (100%) for aortic dissection but should be reserved for stable patients when CT is contraindicated (iodinated contrast allergy, severe renal insufficiency) or for follow-up imaging, not acute diagnosis. 1, 2, 5

MRI is rarely used acutely because:

• Longer acquisition times (potentially 9+ hours delay) 6
• Limited availability, especially after hours 1
• Patient instability precludes safe scanning 2
• Monitoring difficulties in the MRI environment 5

Critical Algorithm for Imaging Selection

For stable patients: CTA immediately → If negative but high clinical suspicion persists, obtain second imaging study (TEE or MRI) 1

For unstable patients: TEE at bedside or in operating room as sole diagnostic procedure before emergency surgery 2, 4

For contrast allergy/severe renal insufficiency (stable): MRI first-line, or TEE if MRI unavailable 2

For contrast allergy/severe renal insufficiency (unstable): TEE exclusively 2

Important Caveats and Pitfalls

Never use sequential multiple imaging techniques when one high-quality study is diagnostic, as this causes unnecessary time loss and increases mortality risk. 2, 7 Each hour of delay increases mortality by 1-2%. 3

Transthoracic echocardiography (TTE) has insufficient sensitivity (60-80% for Type A, 70% for descending aorta) to exclude dissection and should not be used as the sole diagnostic test. 1, 2 However, if TTE shows clear evidence of dissection (intimal flap, pericardial effusion), proceed directly to surgical consultation without delay. 1

Chest radiography should never delay definitive imaging in high-risk patients, even though abnormalities appear in 60-90% of cases. 1, 7 Use chest x-ray only in low-to-intermediate risk patients to identify alternative diagnoses. 1

Aortography is now obsolete for primary diagnosis and should only be considered when noninvasive imaging cannot establish the diagnosis or when coronary anatomy must be delineated before surgery in patients with known coronary disease. 1, 2

Time-Sensitive Considerations

When clinical probability of dissection is high (>50%), excessive delays in obtaining the "ideal" test may be fatal. 6 In this scenario:

• CTA obtained within 2 hours yields higher survival than MRI obtained within 9 hours 6
• TEE obtained within 6 hours yields higher survival than delayed MRI 6
• The threshold for ordering a second confirmatory test (if first negative) is only justified if delay does not exceed 10 hours 6

The practical reality: order the most accurate test that can be performed within 2 hours, not the theoretically "best" test that requires prolonged waiting. 6