Hello -
CT scans are generated by the reconstruction of many, many X-rays
taken at various angles through the tissue of interest into three
dimensional images. Since they are based upon X-rays, the density of
an area on a CT scan corresponds to how well a particular tissue
absorbs X-rays - as X-rays are developed on film, the more energy that
is absorbed results in a darker colour being produced - thus, on an
X-ray (and a CT scan) bone is white (it absorbs more X-rays than
surrounding tissue), air is black (it absorbs very little), and brain
matter is a midlin' shade of grey (it absorbs some). When describing
the appearance of a region of a CT scan, comparisons are usually made
to the normal appearing brain tissue - things that are the same colour
are called isodense (same density), things lighter are hyperintense
(higher density - bad grammer, but it's what's used), things darker
are hypodense (lower density). Now, CT scans can be done in at least
two ways - with and without a contrast agent. Contrast agents are
substances that have high signals on a scan, thus allowing you to
'colour' part of the scan based upon what is there - typical contrast
agents for brain CT scans are injected into the blood and cause the
blood to appear white on a scan. The type of scan which this woman
had has some impact upon the answer - I'll get to that below.
So, for a first approximation, things that are of "decreased density"
are where normal brain is not. Possibilities here include that there
may just not be any brain in that area (what would fill in the space
would be cerebrospinal fluid which has a lower absorbtionn than
brain). This can result from prior damage due to infarct (stroke),
injury, infection, birth trauma, toxin exposure, hypoxia, or atrophy.
If the inciting event occurs young enough in life (<5 years old
usually), the brain may successfully remodel remaining tissue such
that any deficit is slight to non-existant. When this occurs later in
life, there is almost always a deficit. These possibilities would
show up this way whether the CT scan was done with contrast or not.
The next possibility is that the area is filled with a serous fluid
(also less intense on a scan) - cysts, cystic tumors and chronic
bleeds are the most likely candidates here. A chronic bleed, if still
actively bleeding, could enhance (become hyperintense near the source
of bleeding) with a contrast CT.
A very dramatic example of a resolving bleed (not all bleeds produce
this large of an effect):
http://instruct.tri-c.edu/rduiker/Adjuncts/Images/Neuro/SDHhypodense.html
Air in the brain will also produce a pronounced hypointensity, but is
unheard of without history of trauma or surgery.
Infiltration of fluid into normal brain tissue can also produce a
hypointensity by shifting the average composition of an area - this is
called "edema" or swelling. It would be unusual to see swelling in
one section of the brain without evidence of something that is causing
the brain to swell (ie a tumor, bleed, abscess, etc) or without
history of a bonk on the head. Contrast versus non-contrast here
would only provide information if the causal agent of the swelling
picked up the contrast agent. A neural deficit is common with causes
of edema, but not universal if the area of edema is small.
A concussion is one cause of edema:
http://instruct.tri-c.edu/rduiker/Adjuncts/Images/Neuro/Contusion.html
A stroke can produce hypodense areas in region that is cut off from
blood flow - the vascular territory in the brain is well-defined, and
these tend to be "wedge-shaped" - this would almost exclusively come
with deficits. A contrast CT scan might better demarcate the area of
the stroke.
http://www.strokecenter.org/education/ct-mri_criteria/#ctimaging
Tumors and abscesses could also produce hypodense regions - not all
tumors are hypodense, however. Contrast CT could help distinguish
these causes from other causes as often times hypodense tumors and
abscesses will show "ring enhancement" as the contrast agent is taken
up by the tissue immediately adjacent to the tumor/abscess.
http://www.braintumorfoundation.org/tumors/astro.htm
The posterior temporal lobe is an area with many parts, more specific
localization can be useful in gauging likely causes. Information as
to whether or not the area is medial, mesial, or periventricular could
be useful in distinguishing possibilities. Additionally, any evidence
of "mass effect", "midline shift", or "hydrocephalus" could further
direct the search for a cause of the hypodensity.
A brief overview of brain CT scanning:
http://199.243.225.113/ComputerAssistedLearning/reading%20CT%27s/Analyze.htm
The same site also has an anatomy tutorial of CT scans:
http://199.243.225.113/ComputerAssistedLearning/reading%20CT%27s/Anatomy_guide/alpha.htm
Please let me know if you have further questions, or if you have more
information with which you would like for me to incorporate into the
answer.
synarchy
Answer based upon personal education (MD and PhD in neuroscience
degrees)
Search strategy for references:
neuroradiology CT
neuroradiology tutorial
neuroradiology primer |
Clarification of Answer by
synarchy-ga
on
24 Jul 2003 04:41 PDT
Hello -
The CT scan could have had technical difficulties - if the patient has
any metal near the area of interest, there will be 'scattering', but
this usually leads to star-shaped hyperintense artifacts, and would be
unusual without you providing a history of some reason for metal in/on
the skull.
CT (and MRI) scans can also be affected by patient movement/position
during the scan. The way that the scans are displayed are in linear
'cuts' showing a 2D plane through the brain. If the patient is
slightly cock-eyed to the plane of the scanner, the 'cut' won't be
perfectly perpendicular to the plane of brain - this can lead to one
side (or both) looking abnormal compared to the other side. The
suface of the brain is composed of many rolling hills of 'sulci' and
'gyri' which are normal folds in the surface. If looked at at an off
angle, a normal valley (or 'sulcus') might appear as a hypodensity in
an unusual place (it would simply represent a normal area of non-brain
seemingly appearing in the wrong place due to angle). You could think
of this as comparing a topographic map to a photograph taken
not-quite-straight down - if compared to the topo map, it could look
abnormal. Off-plane images are usually detectable by comparing
bilateral (present on both sides) structures, such as the eyes, ear
canals, and other bony points on the skull - if they don't look the
same on both sides, chances are that the patient is tipped one way or
the other.
Motion affects the scan in a slightly different way - the CT is
reconstructed from many, many individual X-Rays taken at different
angles through the skull - if the patient moved between one X-ray and
the next, the superposition used to align all of the images and create
the reconstruction would have an inherent flaw - the images taken
while the patient was in a different position would not line up
correctly - this can create all manner of artifacts, although it
usually apparent by 'ghosting' - sort of like when your TV has poor
reception, and you see multiple images staggered across the screen
(that's due to ground-loops however, a totally different discussion...
:) ).
If you still have concerns, the best recourse that I would suggest
would be to follow up with a neurologist a few times - they can
examine your daughter for subtle neurologic signs that could indicate
a problem and would be expected to change if the hypodensity
corresponds to anything that is changing. Non-changing regions
corresponding to hypodense areas on a CT that are negative on an MRI
have a very low probability of being anything serious, however.
I'm happy to keep explaining as need be - let me know.
synarchy
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