What is the most highly sensitive imaging modality for diagnosing pulmonary embolism (PE), acute coronary syndrome (ACS), or aortic dissection in a patient with recent ACS and on anticoagulation?

Highly Sensitive Imaging for PE, ACS, and Aortic Dissection

CT angiography (CTA) is the most highly sensitive imaging modality for diagnosing all three conditions—pulmonary embolism, acute coronary syndrome, and aortic dissection—and can be performed as a "triple rule-out" protocol in patients with undifferentiated acute chest pain, though dedicated CT pulmonary angiography (CTPA) remains the standard for suspected PE. 1

Primary Imaging Recommendation for Pulmonary Embolism

CTPA is the imaging modality of choice for diagnosing PE, with sensitivity >95% for segmental or larger emboli and specificity approaching 96-100%. 1, 2, 3

• CTPA has become the de facto clinical "gold standard" for acute PE diagnosis, replacing catheter pulmonary angiography and ventilation-perfusion scintigraphy as the first-line imaging method. 4, 5

• Modern multidetector CTPA demonstrates sensitivities of 99-100% and specificities of 100% for detecting thromboembolic disease at the segmental level. 6

• Clinical outcome studies demonstrate it is safe to withhold anticoagulation when PE is excluded on CTPA, with subsequent PE occurring in only 1.1% of patients at 3 months. 2

• CTPA enables direct visualization of intravascular thrombus, webs, bands, and vessel occlusion while simultaneously evaluating alternative diagnoses when PE is excluded. 2, 6

Imaging for Aortic Dissection

CT is recommended as the initial diagnostic imaging modality for suspected acute aortic syndrome, given its wide availability, accuracy, speed, and the extent of anatomic detail it provides. 1

• CT, TEE, and MRI all have high sensitivity and specificity for aortic dissection, but CT has become the preferred modality for evaluating most patients with suspected acute aortic syndrome. 1

• CT not only diagnoses the underlying aortic dissection, but also shows the full extent of the dissection, the entry tear site, aortic branch vessel involvement and patency, signs of malperfusion, pericardial effusion and hemopericardium, periaortic or mediastinal hematoma, and pleural effusion. 1

• TEE and MRI are reasonable alternatives for initial diagnostic imaging when CT is contraindicated (e.g., contrast allergy) or when the patient is too unstable to travel to the radiology suite. 1

Triple Rule-Out CT Protocol

Technological advancements in ECG-gated CT allow accurate evaluation of the pulmonary vasculature, thoracic aorta, and coronary arteries on a single CT study for patients with acute chest pain, though this approach has not been proven superior through large-scale clinical trials. 1

• The "triple rule out" CT protocol to evaluate for PE, acute aortic syndrome, and acute coronary syndrome has been shown to be technically feasible in some patient groups. 1

• In one study, the prevalence of acute aortic syndrome and acute coronary syndrome among patients suspected clinically of having PE was 5.5% and 0.5%, respectively, leading authors to conclude that patients suspected for PE could be evaluated with dedicated CTPA. 1

• Isolated PE and acute aortic syndrome occurring outside the field of view of dedicated coronary CTA are rare (1.9% for PE, 0% for aortic dissection), suggesting that extended coverage may not be necessary in all cases. 7

Critical Considerations for Patients on Anticoagulation

• In patients with recent ACS already on anticoagulation who develop new symptoms, do not delay imaging based on anticoagulation status—both PE and aortic dissection can occur despite therapeutic anticoagulation. 3

• CTPA should be obtained in all patients with high pretest probability of PE, and in patients with low or intermediate pretest probability who have elevated D-dimer levels. 2

• For hemodynamically unstable patients, bedside echocardiography can reliably diagnose clinically massive PE and should be performed within 1 hour, while also evaluating for complications of aortic dissection such as aortic regurgitation or pericardial effusion. 1, 2

Alternative Imaging Modalities

• V/Q scanning should be used only when CTPA is unavailable or contraindicated, such as contrast allergy or renal insufficiency, as it is diagnostic in only 30-50% of cases. 2, 3, 8

• MRA has limited utility for acute PE diagnosis, with sensitivity of only 78% in adequate studies, and is most commonly used as a follow-up imaging modality when there is diagnostic uncertainty. 1

• Pulmonary angiography is not typically useful as a first-line test for suspected PE, though catheter-based angiography has a role in confirmation of residual disease if CT is negative and high clinical suspicion exists. 1

Important Caveats

• A small proportion of CTPA examinations will be technically unsatisfactory; quality control is critical for reliable interpretation. 2

• Subsegmental clot may be less reliably detected than proximal clot, though most patients with subsegmental PE also have more proximal clot that can be identified. 2

• In patients with high clinical probability and negative CTPA, consider additional testing (lower extremity venous ultrasound, V/Q scan) before definitively ruling out PE, due to a false-negative rate ranging from 5.3% to 40%. 3

• CTPA has about a 6-fold greater whole body effective dose than V/Q scintigraphy, and a much higher dose to breast tissue in particular, which should be considered in young women. 8