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.