NEUROIMAGING
MR generally best tool for imaging the
brain
Non-contrast CT still used as first test for 2
common emergency indications: head
trauma and stroke
In both cases looking for acute blood
which is white relative to brain, because of
lipid content of brain
Head Trauma
3 trauma cases with blood in meningeal
compartments going from superficial to deep:
epidural, subdural, and subarachnoid bleeds
Looking for blood collections that create
mass effect on brain and need to be
evacuated surgically
Usually no mass effect with subarachnoid
bleed, but need to consider if subarachnoid
bleed was from aneurysm that then caused
patient to crash car (which came first)
Stroke
Stroke: sudden neurologic deterioration on
vascular basis (CVA)
Key reason for doing CT is to determine if
stroke is hemorrhagic or not. Causes and
treatments are different. May give
thrombolytic emergently for acute infarct, but
not for bleed
How soon does hemorrhagic stroke show up
on CT? : Immediately!
Following case is typical hypertensive basal
ganglia bleed
Next case is non-hemorrhagic left occipital
stroke (infarct)
How soon does infarct show up on CT?:
Depends on the size. Large one may have
subtle swelling in 6 hours, but smaller one
may take a couple days. Therefore, emergent
head CT is done to see if patient has bleed or
not, not to necessarily to see an infarct
Infarct becomes low density as edema
develops
This is the same patient with infarct shown on
MR (first 2 series: T1- and T2-weighted)
As opposed to CT, MR can measure more
than one variable, e.g., T1 and T2, so if
lesion not well seen on one sequence, may
be better seen on another. CT can measure
only one variable, X-ray attenuation
MR more sensitive to soft tissues differences
than CT. Can see edema of infarct sooner.
Water is dark on T1 and bright on T2
Typical MR unit is superconducting magnet
which is always on.
Need very strong magnetic field to magnetize
hydrogen nuclei. Net vector of magnetization
is then tipped by adding RF energy.
Precessing vector then creates sine wave
signal captured by coils placed around body
part of interest. The decay of the signal
depends on a couple effects, and the images
created can depend more on one factor than
the other (T1 or T2)
Same patient with additional common MR
sequence (FLAIR)
FLAIR is like T2 sequence, but pure water
signal is set to zero, getting rid of all of the
bright signal from CSF. The edema (water) in
infarct stays bright since it is in the brain with
a different surrounding molecular
environment, and therefore a different signal
than protons in pure water (CSF). It makes
the visual search for pathology easier.
MR signal depends on another variable, motion. The basilar artery
on the T2 image is a black dot anterior to the brain stem. Although
there is a lot of water in the artery, and it might be expected to be
bright on T2, it has a flow void because the protons have moved out
of the imaging volume and haven’t gone through entire process of
signal generation.
Therefore, can do MRA without contrast
Diffusion-weighted scan signal depends on microscopic
motion/diffusion. Infarct is bright on DW because acute ischemia
causes shift of water from extracellular space to intracellular where
water can’t move as much. There is less of a “microscopic flow void”
and infarct is bright. This occurs much earlier than edema, and is
sensitive and fairly specific for acute infarct.
Following is the very commonly performed diffusion-weighted
sequence on the same patient
62-year-old patient with
progressive right arm weakness
over the last week
3 images are T1, T2, and T1 gadolinium-contrast-enhanced
MR
T1 and T2 are very sensitive to pathology, but often not
specific, since most pathologies all over body are dark on T1
and bright on T2, like infarct, because they have increased
water content
T2 sequences good for screening because most pathologies
are bright
Gadolinium adds specificity. There are 2 enhancing (therefore
solid) masses. They light up because Gd doesn’t cross bloodbrain barrier into normal brain, but does enter tumor
extracellular space
Brain mets from lung CA, most common malignancy
presenting with brain mets
42-year-old obese patient with
chronic low back pain
No imaging recommended. Won’t usually
change management.
70-year-old female with sudden
severe low back pain when
lifting and turning to empty a
heavy garbage can
Sounds like osteoporotic compression fx
Well shown on plain X-ray
36 year-old-patient with severe
left leg sciatica not responding
to physical therapy, and now
with calf weakness
Imaging not usually indicated initially for sciatica,
because will usually resolve with conservative
treatment (PT, meds)
Do imaging if conservative Rx fails or if neurologic
damage develops (weakness, loss of reflex)
MR exam of choice because it has excellent soft
tissue contrast. CT has very high spatial resolution
for high-contrast tissues (like nodules in lung or
fracture in bone), but not as good for soft tissues
MR shows L4-5 disc protrusion compressing left
L5 nerve root.
67-year-old patient with bilateral
leg pain increasing with
distance walked (non-smoker,
no diabetes, normal pedal
pulses)
History suggests claudication, but also
suggests not vascular
This is spinal claudication caused by
spinal stenosis
Normal axial lumbar CT for comparison
Severe spinal stenosis on CT due to facet
joint degenerative disease and
hypertrophy
CT will show bony stenosis well, but won’t
show soft tissue component as well as MR
L4-5 spinal stenosis on MR
Lumbar normal vs spinal stenosis on MR
73-year-old male S/P
prostatectomy for aggressive
prostate cancer, now with
bilateral leg weakness
While CT will show bony anatomy well, do
MR to see soft tissue component
Cauda equina compression by prostate
CA mets
Spine Trauma
Plain X-rays still often used for lesser
degrees of trauma
CT for clarification of X-ray findings or as
initial imaging for high-suspicion cases
with greater trauma
Same as using CT to further evaluate CXR
findings. Lung and bone are both highcontrast tissues, so use the high spatial
resolution of CT. Don’t need the contrast
resolution of MR for bone
Normal open mouth view of odontoid for
comparison. Note lateral masses of C1
aligned with those of C2
C1 ring fracture. Note spreading of lateral
masses of C1
Use CT to better show anatomy
Compared to plain X-rays CT will better
show compromise of spinal canal
CT will better show injury to posterior
elements that may indicate instability and
need for orthopedic mechanical
stabilization of spine
When neurologic injury is question, such
as spinal cord injury, use MR because
that’s soft tissue issue
Facial Trauma
Compared to plain X-rays, CT provides more
definitive evaluation for possible facial bone
fractures, detecting fractures not visible on X-ray
The following axial CT image is a single slice on a
patient with a trimalar complex (tripod) fracture
The tripod fracture breaks the 3 connections of the
zygoma to the rest of the face
– Through maxilla including floor of orbit and lateral wall
of maxillary sinus
– Through lateral wall of orbit, typically at frontozygomatic suture
– Through zygomatic arch
The 2 following 3-D CT surface
reconstructions (same patient as for
preceding image) make the 3 components of
the tripod fracture easier to see
The face is constructed of multiple rigid bony
rings, and it is difficult have a displaced
fracture through one side of a ring without the
fracture completing itself in space (hence the
multiple components, analogous to fractures
of other bony rings such as the pelvis)
Following coronal CT slice is from patient
who had direct blow to left eye
Note orbital floor blow-out fracture with
prolapse of inferior rectus muscle into
fracture gap.
Orbital blow-out fractures occur due to direct
blow to eyeball resulting in increased intraorbital pressure causing bony confines of
orbit to give way at weakest point (typically
orbital floor, but also commonly medial wall of
orbit)
Following case is 3-D CT surface reconstruction
for a patient who had a blow to mid-face
This is a Le Fort I fracture extending through
superior alveolar ridge of maxilla on both sides.
LeForte fractures break different levels of the
central bony face off from the more superior face.
Le Fort II fracture is higher, extending from lateral
wall of maxillary sinus, across inferior orbital rim
and floor of orbit, to bridge of nose (on both sides)
Le Fort III fracture is higher still, extending from
lateral wall of orbit (usually at fronto-zygomatic
suture to bridge of nose (on both sides)