So, in this lecture, we?ll be discussing abnormalities of the thoracic aorta
and the pulmonary arteries.
These great vessels are usually best evaluated on a contrast enhanced CT
and abnormalities associated with them may be life threatening.
So, it?s important that when these are suspected,
we perform imaging very quickly.
Some of the common abnormalities include thoracic aortic dissection,
thoracic aortic aneurysm, pulmonary embolism,
and pulmonary artery hypertension.
So, aortic dissection is a result of an intimal defect that causes blood
to enter the aortic wall and it creates a true and a false lumen.
It?s categorized into two major types, so there?s the Stanford type A
which involves the ascending aorta and may or may not involve
the descending aorta and usually, these are treated surgically.
Then, there?s the Stanford type B which involves primarily the descending aorta
and may or may not involve the arch and usually, these are treated medically.
So, let?s take a look at this case here.
What type of Stanford classification would this be?
So, we have a flap that involves both the ascending aorta
and the descending aorta, so this is actually a Stanford type A classification
and this patient would need surgical management.
Thoracic dissections could be missed on a radiograph
and they can actually be missed on a non-contrast enhanced CT,
if a patient is unable to have contrast such as a patient that has renal failure
or a contrast allergy, an MRI can be performed without contrast
and that will demonstrate it.
So, let?s take a look at the different types of imaging
and see the differences in the detection of aortic dissection.
So the image on the left is a non-contrast enhanced CT.
If you take a look at the descending aorta here,
you see no abnormalities, however this patient had a contrast enhanced CT
which shows an intimal flap
and this is an example of a descending aortic dissection,
so this would have been missed if the patient only had this non-contrast CT.
On this image here which is the fiesta image from an MRI,
you can actually see that the flap is seen
and so this is another way of performing this if a patient had a contrast allergy
and couldn?t have the contrast enhanced CT.
So, how can you differentiate between the true lumen
and the false lumen of an aortic dissection?
So, the true lumen is continuous with the aortic valve,
it has a smaller cross sectional area,
and it may be compressed by the false lumen.
The false lumen however has delayed flow within it
and it has a much larger cross sectional area.
Complete thrombosis of the false lumen indicates that there?s less of a chance
of aortic dilatation and so that is less urgent of a finding
than if the false lumen is not completely thrombosed.
So, now let?s move on to thoracic aortic aneurysm.
An aneurysm of the aorta is when the diameter enlarges
to more than 50% of the normal diameter.
Thoracic aneurysms are usually defined as a diameter of the aorta
of about greater than four centimeters in size.
So, this is a radiographic image of a patient
that has a proximal descending aortic aneurysm.
On the radiograph, the aneurysm actually appears to be a large mediastinal mass
as you can see here but radiography is not very sensitive,
particularly for ascending and arch aneurysms.
So, thoracic aneurysms can be divided into two major categories,
there?s the fusiform type and then there?s the saccular,
and this is really defined by the appearance of the aneurysm
and the shape of it. So the fusiform is a long aneurysm
while the saccular represents more of a globular shape.
Fusiform aneurysms are usually caused by atherosclerotic disease
while saccular aneurysms are usually caused by an infectious cause
and usually, the fusiform aneurysms are seen predominantly
within the aortic arch and then least likely in the ascending aorta,
while saccular aneurysms really can be located anywhere within the aorta.
So when would you do a surgical repair of an aortic aneurysm?
So, you wanna monitor the growth,
if it grows for more than a centimeter per year,
that?s one of the reasons why you would want to do a surgical repair,
if the ascending aneurysm is greater than about five and half centimeters in size,
that?s another indication, and for descending aneurysms,
if they?re greater than about six and a half centimeters in size,
that?s when you would go on to surgery.