Question 13

Outline the advantages and disadvantages of a CT scan, Transoesophageal echocardiography, MRI and an aortogram for the evaluation of suspected aortic dissection.

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College Answer

•    CT Advantages:
•    easy availability on an emergency basis
•     high sensitivity and specificity
•    can pick up complications involving the branches ( e.g. ischaemic gut) and extent of dissection into abdominal aorta
•    easier to monitor the patient than MRI
•    detects pericardial effusion.

Disadvantages:
•    have to move the patient
•    iodinated contrast
•    cannot assess for AR, LV function or coronaries

•    TOE Advantages:
•    bedside test
•    can detect intimal flap, true and false lumen AR, tamponade
•    assess LV function
•    no contrast needed

Disadvantages:
•     semi-invasive
•    may need anaesthesia/intubation
•    may cause undesirable hypertension
•    not widely available
•    special expertise required

•     MRI Advantages:
•    High sensitivity and specificity
•    MR contrast (Gadolinium) has more favourable safety profile
•    can detect AR

Disadvantages:
•    Not readily available
•    inconvenient (patient motionless for 30 minutes)
•    access and monitoring difficult
•    limited applicability (claustrophobia, pacemakers)

•     Aortography
Advantages:
•    will detect intimal flap, AR
•    assess LV, tamponade, blocked coronaries (important for surgery in type A
dissection)

Disadvantages:
•    not readily available
•     invasive
•    large contrast load

Discussion

The college answer can be more easily reduced into a table:

Imaging Modalities for the Evaluation of Aortic Dissection
Imaging modality	Advantages	Disadvantages
Aortogram	High sensitivity (86-88%) and specificity (75-94%) Can detect blocked coronaries in Type A dissection Can assess valves May allow endoluminal repair during the same procedure, expertise permitting	Not easily available Large contrast load Time consuming Ineffective in detecting intramural haematoma (the contrast cannot get in there!) Potential for false negative results when a thrombosed false lumen prevents contrast entry Slightly lower sensitivity and specificity than TOE, CT or MRI; has been largely replaced by them.
CT	Easily available High sensitivity (83-94%) and specificity (87-100%) Information about end-organ ischaemia Imaging of the vascular tree allows planning of surgical or endovascular approach ECG-gated CT = cardiac motion artifact is abolished Able to exclude conditions which mimic aortic dissection	Contrast exposure No information about the valves Risk of transfer to CT Motion artifact could be an issue in ungated studies
MRI	High sensitivity and specificity (95-100% for both) Contrast is less nephrotoxic Information about end-organ ischaemia Imaging of the vascular tree allows planning of surgical or endovascular approach Occasionally allows assessment of aortic valve pathology, coronary arteries and the LV	Significant risk of transfer Not easily available Certain patient groups excluded (eg. recent trauma with surgical staples) Often fails to characterize the relationship of an intimal flap and aortic root structures, specifically the coronary arteries
TOE	Can assess valves Decent sensitivity (35-80%) and specificity(39-96%) Performed at the bedside- no risk of transfer Contrast not required Allows detection of tamponade Allows assesment of proximal coronary arteries Able to detect intramural haematoma	Invasive Accuracy is operator dependent Requires sedation May cause hypertension Limited by a blind spot caused by interposition of the trachea and left main bronchus between the oesophagus and aorta Unable to visualise the abdominal aorta
CXR	Rapidly available Immediate evidence of widened mediastinum A completely normal CXR in low risk patients may be meaningful as a means of excluding dissection	Inadequately sensitive (~71%) Rarely able to exclude dissection in most patients

A good article on this topic is available. It illuminates some of the finer points which the college answer has omitted:

In high risk patients, all the modalities are more or less equal in accuracy
In moderate risk patients, positive predicitive values are >90% for CT, MRI and TOE but only 65% for aortography
MRI is the most sensitive of the lot - in low risk patients, it picks up close to 100% of the dissections
All four modalities have a 85% negative predictive value.

References

Khan, Ijaz A., and Chandra K. Nair. "Clinical, diagnostic, and management perspectives of aortic dissection." Chest Journal 122.1 (2002): 311-328.

The canonical source for this information would have to be the most recent iteration of theACCF/AHA Guidelines for Diagnosis and Management of Patients With Thoracic Aortic Disease.

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