0:01

It is a common fallacy to think that the density of a

0:05

hematoma on CT is due to the iron content

0:09

within the blood products. In point of fact,

0:12

the density on CT is dependent on the hemoglobin protein

0:17

concentration rather than iron, and because of that,

0:22

the density of a hematoma on CT will vary depending upon

0:26

hemoglobin concentration or the patient's hematocrit.

0:29

With a hematocrit of 45,

0:32

a typical hematoma will have a Hounsfield unit of

0:35

approximately 56.

0:37

This is to be contrasted, for example,

0:41

with the normal Hounsfield units of gray matter of between

0:44

35 and 40 Hounsfield units, and white matter,

0:48

which is less dense than gray matter

0:50

at 30 to 35 Hounsfield units.

0:52

However,

0:53

you'll note that if one is anemic at a

0:56

hematocrit of between 24 and 30,

0:59

the Hounsfield units associated with that hematoma of an

1:03

anemic patient will simulate that of gray matter

1:08

or white matter, and therefore become isodense.

1:11

This is most frequently seen with isodense subdural

1:16

hematomas, which have low hematocrit concentration.

1:21

Over the course of time,

1:23

the density of a hematoma decreases as the body

1:28

removes the blood products from the hematoma,

1:32

whether it's in the intracranial intraparenchymal

1:35

compartment or in the extra-axial compartment.

1:39

And that density decrease in the parenchyma

1:42

is usually by 1.5 Hounsfield units per day.

1:46

So, as you can see,

1:47

it would take probably around ten days

1:51

for the hematoma of a hematocrit of 45 to get to the

1:57

similar density as that of, for example, white matter.