36 y/o M with Marfan syndrome and chest pain.
Type B aortic dissection.
* LOW YIELD FOR OVERNIGHT CALL ALERT*
Per the HMC ED Radiology website, "Acute aortic dissection (thoracic or abdominal) and unable to go to CT within 30 minutes" is an indication for emergent MRI.
Dating back 5 years in zVision, we did not find a single case of an MRI (either unenhanced or enhanced MRA) that was obtained at HMC "after-hours" to assess for acute aortic pathology. It's possible that something slipped through our diligent zVision review, but per our discussions with one of the UW/HMC attendings, this will likely never happen overnight.
However, if for some unfortunate reason you do have to read one of these independently, and you are unfamiliar with the labyrinth of MRI acronyms / uncomfortable assessing for acute aortic pathology, we recommend identifying the contrast enhanced arterial phase T1 MRA images (like the series we included here). This will feel essentially identical to reading a CTA to rule out dissection. It never hurts to refresh yourself on acute aortic syndrome with the help of our friends at The Radiology Assistant. Here is a helpful pictorial essay on the contrast enhanced MRA appearance of aortic dissection from Radiographics. Look for a hypointense dissection flap or a contrast filled outpouching of the lumen into the aortic wall to indicate a penetrating atherosclerotic ulcer (PAU).
Similar to a CT, an intramural hematoma (IMH) may be difficult to detect on the contrast-enhanced series. The appearance of an IMH will be variable, dependent on the stage of hemoglobin evolution. An acute IMH may be T1 iso-intense with the aortic wall. In that case, look for an area of unusual (possibly crescentic) wall thickening and corresponding T2 signal abnormality. This article from the Annals of Cardiothoracic Surgery provides a nice example of a T2 hyperintense, subacute IMH (figure 1B, middle image). This article provides a nice example of a T1 hyperintense, subacute (7 days old) IMH (figure 4). Here is a low signal IMH on a contrast-enhanced MRI from our PACS. Ultimately, the dating of extra-cranial hemorrhage with MRI is unreliable. If you see an area of crescentic wall thickening with some corresponding signal abnormality, raise the concern for IMH. Don't get too bogged down trying to date the hematoma, it's too frustrating and not worth your while.
Unenhanced MRA has also been described in the assessment of acute aortic pathology, using techniques such as fast spin-echo and gradient echo sequences. It is even less likely you will have to interpret an unenhanced MRA overnight. These studies take longer to perform than their gadolinium enhanced counterparts. If an ED provider is demanding that an MRI be performed emergently due to lack of CT scanner availability AND gadolinium is contra-indicated, you'd likely be better advised to simply wait for the scanner to open up than to pursue an unenhanced MRA. If you're still interested, however:
The contrast in an unenhanced MRA is provided by inherent differences in the T2/T1 ratios of different tissues, and this can be utilized to yield either bright or dark blood images. For black blood imaging, a very rudimentary understanding of the concept is as such: The magnetic signal of moving blood is identified and nulled, causing blood to appear black and allowing the aortic wall to be clearly depicted. In our case, there is sluggish flow in the false lumen which results in incomplete nulling of the blood signal and a heterogenous appearance of signal in the false lumen.
Surgical consult. Type A dissections usually mandate procedural intervention. Type B dissections may be managed medically as long as there are no indications for surgery (e.g. dissection flap occludes visceral arterial branches).
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