Acute aortic dissection: utilizing imaging modalities to improve detection

Acute aortic dissection is a serious condition in eld-erly patients and may become fatal rapidly if left undiag-nosed and untreated. As pointed out by Welch et al1 chest radiography is a screening test for this condition by detecting these signs such as widening mediastinum, blurring of the aortic knob, left apical cap, etc. Unfortu-nately these signs on chest radiography may not be sensi-tive enough to detect acute aortic dissection.1'2 There-fore, if patients have high clinical likelihoods of ihe aortic dissection including old age, crushing chest pain, hyper-tension, pulse deficit, known history of thoracic aneu-rysm, or Marfan' s syndrome, etc, further diagnostic test is required to exclude this deadly disease even normal chest radiography.2'3 Other imaging modalities include aortography, transesophageal echocariogrpahy ( TEE ) , magnetic resonance imaging ( MRI) , or computerized to-mography (CT). "'? Currently, aortography is not routine-ly performed to diagnose this condition because it may worse the situation if the catheter is placed into the false lumen. Noninvasive modalities have increasing role be-cause they can detect dissection very accurate and provide the information of the wall and near by structures. " This is so important because it has been recognized that acute aortic syndrome is a spectrum of the diseases including acute aortic dissection, penetrating aortic ulcer, intramu-ral hematoma, and aortic rupture.4' 5 Each imaging mo-dality has strength and weakness for detecting this syn-drome. Transesophageal echocardiography provides ade-quate diagnostic accuracy, evaluates the involvement of aortic valve, determines cardiac function, and importantly can be performed at the bedside in this sick individual, however, the distal part of ascending thoracic aorta and proximal aortic arch area is a blind spot in a number of patients due to the air-filled trachea and left mainstem bronchus interposed between the esophagus and this part of the aorta." The false positive results may be due to reverberation artifacts, calcified plaque, therefore, other imaging modality may help to improve the detection if TEE is nondiagnostic. Magnetic resonance imaging is known to be an excellent modality to determine these con-ditions with high accuracy.3'4'6 The extent of the disease can be completely assessed with MR angiography and car-diac and valve function can be assessed as well. Due to large field of view, the involved surrounding structure and aortic wall can be assessed. Unfortunately, the examina-tion is in a closed space and if patients are in critical con-dition, it may not be an ideal situation due to safety con-cern. The MRI examination is contraindicated in patients with cardiac pacemaker, automatic implantable cardio-verter-defibrillators, intracranial aneurysm clips, or oto-logic implants, etc." Recently, the development of multi-detectors CT scanner allows very fast image acquisition and provides information in aortic pathology, as well as cardiac function and coronary artery disease. Coronary ar-tery involvement from dissection or existing significant coronary arterial stenoses can be determined as well with CT examination. " This will improve and shorten diag-nostic process and patient may be omitted from diagnostic coronary angiography to exclude significant coronary arte-rial stenoses prior to surgery if needed.8'1 '" Cardiac CT has been evolving to be utilized for the evaluation of chest pain patients presented in emergency unit because it can detect various causes of chest pain including acute aortic syndrome, coronary artery disease, pulmonary embo-lolistn, or pneumothorax, etc.9 One important limitation is that CT examination requires iodinated contrast which may not be used in some patients who have concomitant renal failure.