Introduction to Neuroimaging
G. Hathout, M.D.
Professor, UCLA Dept of Radiology
Division of Neuroradiology, West LA VA
Acknowledgements:
While most of the slides are mine, I have liberally borrowed slides from
the Web to enhance this presentation. I would like to gratefully
acknowledge those sources, and to apologize if any have been
inadvertently overlooked.
Acknowledgements, cont’d
http://www.cs.sunysb.edu/~mueller/teaching/cse377/mriPhysics.pdf
http://hsc.uwe.ac.uk/radscience/MRI/MRI_Physics.pdf
http://www.mri.tju.edu/phys-web/1-T1_05_files/frame.htm
http://www.biac.duke.edu/education/courses/fall03/fmri/handouts/W5_Physics_
2003.ppt#256,1,Principles of MRI Physics and Engineering
UC San Diego Neuroradiology Learning Files, Dr. Hesselink.
56 y.o. patient, nearly comatose, not moving left side. Diagnosis ?
Neuroradiology:
The Old Days!
Introduction to CT (Computer
Assisted Tomography).
Neuroimaging’s first big step:
-image in slices
-avoid superposition of structures
CT generates images by taking numerous 2D projections and mathematically
reconstructing them to make a planar image.
Rotating X-ray tube and detector array allows multiple projections at
different angles.
Impact CT scan
The projections can be reconstructed in a number of ways, such as
filtered back projection, or using Fourier methods, to form an image.
Each image is made of pixels (picture elements), typically 256 x 256.
Each pixel is given a shade of gray, corresponding to its linear attenuation
coefficient u. CT (Hounsfield) numbers can be generated from this.
The added information of CT comes at a high price: big radiation doses!
Some actual Hounsfield unit measurements from the scanner – by assigning a CT
number to each pixel, CT can make a planar image with good contrast and no overlap!
Hounsfield Units
White matter: 28
Gray matter: 34
CSF:
4
Fat:
-87
Bone:
733
Remember: The higher the electron density, the higher the linear x-ray attenuation
coefficient u. The higher u, the higher the CT number (HU or Hounsfiled unit).
The higher the HU, the brighter something looks on CT.
Gray matter is about 6 HU higher than white matter (therefore slightly brighter on CT).
-
Gray matter contains more water and less fat than white matter
This means about 8% more oxygen and 8% less carbon
This leads to a slightly higher electron density (O : 16 vs. C :12)
This, in turn, leads to higher linear x-ray attenuation
Now, back to our patient. What’s wrong?
Can you see better now?
We’ve come a long way!
What CT can show:
Images can be windowed
differently. On “bone” windows,
we can distinguish metal from bone.
For example, this man decided to stop
a bullet with his head.
Notice that on bone windows,
the brain detail is lost.
Scans show a lentiform hyperdense extra-axial collection pressing on the
right cerebral hemisphere (your left, the patient’s right), diagnostic of
an epidural hematoma.
There is significant mass effect on the lateral ventricles and midline shift.
On bone windows, the hematoma seems to disappear (brain detail is lost), but
we can now appreciate the fracture (arrow) which has lacerated the…
(fill in the blank).
Which patient is normal?
The patient on your right has blood in the subarachnoid spaces. What is the leading
cause of atraumatic subarachnoid bleed?
Baby with big head. Diagnosis?
The ventricles are too big = Hydrocephalus.
Notice – the aqueduct of Sylvius is missing (arrow).
Diagnosis: Congenital aqueductal stenosis.
Contrast (iodinated compound) can be added to CT. In areas of pathology, it leaks
out of a damaged blood-brain barrier. Notice that normal brain does not enhance.
Patient has a deep ring-enhancing lesion with surrounding edema, seen only after
contrast. Diagnosis: abscess versus tumor (metastatic or primary).
Now, on to the star of neuroimaging: Magnetic Resonance
Imaging, a.k.a. MRI
MR is a method of mapping hydrogen protons. It not only reflects proton density (like
CT reflects electron density), but also reflects magnetic properties of hydrogen protons.
This means …. Better tissue contrast!
MRI Imaging in 3 Easy Steps:
1. Establish equilibrium
2. Disturb Equilibrium
3. Allow protons to return to equilibrium
At equilibrium, the multiple hydrogen protons oriented “parallel,”
and “anti-parallel,” to the magnetic field produce a state of net vertical
magnetization (Mz), but no horizontal magnetization (Mxy = 0).
****Key Point:
At equilibrium, there is
a net vertical magnetization,
but no horizontal
magnetization.
The equilibrium is disturbed by radiofrequency pulses which destroy the vertical
magnetization Mz and create a horizontal magnetization Mxy. It is this horizontal
magnetization Mxy that produces the signal that makes the MR image.
After the RF pulse stops, the system returns to equilibrium.
Important: definition of T1 and T2.
T1 and T2 are time constants that measure the speed at which
various proton populations return to equilibrium.
The T1 time of a tissue reflects how quickly vertical
magnetization recovers in that tissue. T1 weighted images
reflect the relative T1 times of different tissues.
The T2 time reflects how quickly horizontal magnetization
disappears in that tissue. T2 weighted images reflect the relative
T2 times of different tissues.
Solid tissue has a SHORTER T1 time than free water, so it recovers its longitudinal
magnetization faster.
Therefore, it has a higher signal than water on T1 weighted images.
For example, pure fat has a shorter T1 time (recovers longitudinal magnetization
faster) than white matter, so it is brighter on a T1 weighted image.
Water has a longer T2 time than solid tissue (loses transverse magnetization
more slowly), so it is brighter on a T2 weighted image.
Pure water has a longer T2 time than gray matter, so it is brighter (has a higher
MR signal) on a T2 weighted image.
Take home message:
*On T1 weighted images, tissues with SHORTER T1
times
are brighter.
*On T2 weighted images, tissues with LONGER T2
times
are brighter.
Some representative T1 and T2 times of various tissues
(measured in milliseconds)
-Fat has a short T1 time, and water has a long T1 time, so scalp fat is bright and CSF
is dark on a T1 weighted image.
-Water has a long T2 time, so CSF is bright on a T2 weighted image.
-White matter (myelinated) has more fat and less water than gray matter
-Therefore, on a T1 weighted image, white matter is brighter than gray matter,
while on a T2 weighted image, white matter is darker than gray matter.
The ability to reflect T1 and T2 times of protons gives much more tissue contrast
than just being able to reflect the relative proton density (as in the bland proton density
or PD image on the far right, which has very little tissue contrast).
The benefits of contrast resolution!
Diagnosis?
Hypoplastic cerebellar vermis leads to
an in utero diagnosis of Dandy-Walker
malformation!
Getting better with age!
Another benefit of MR: no ionizing radiation!
MR’s multiplanar capability
The benefits of multiplanar imaging
Anything missing? (Patient on your left, control on right)
Patient is missing his corpus callosum.
Diagnosis: Agenesis of the corpus callosum
Guess the weighting
T1 weighted MR images of a child – little obvious pathology visible.
After contrast administration, numerous ring enhancing
abscesses are visible at the base of the brain.
Diagnosis: CNS tuberculosis.
Teaching point: Utility of gadolinium contrast in MRI.
42 year old with headache
(Slide 1)
Ring enhancing lesions in right
parietal lobe with edema. Ddx:
abscess vs. tumor
Lesion is bright on DWI diffusion-weighted
image (bottom left), and dark on ADC
(bottom right).
Diagnosis: Brain abscess.
Teaching point: MR can reach into yet more
tissue parameters, such as water diffusion, to
be even more specific. Abscess is bright on
DWI, while tumor is dark, although both are
ring-enhancing lesions on conventional MRI.
Ring enhancing lesion, just like the last
case, but this time dark on DWI (bottom
right), and bright on ADC (bottom left).
Opposite to the previous case, there is no
restricted diffusion here.
Diagnosis: Necrotic tumor.
Teaching point: Diffusion weighted MRI
allows even greater tissue characterization.
PET -- aka Positron Emission Tomography:
The world of metabolic imaging
List of elemental isotopes that are positron emitters and
can be used for PET imaging.
Whole body FDG
PET image.
Notice how
hypermetabolic the
brain is compared to
the rest of the body.
PET shows a very hypermetabolic nodule
in the right upper lobe of this patient’s lung,
nonspecific on CT (above), but diagnosed as
a malignancy based on PET hypermetabolism
(right).
Teaching point: PET is very useful in
cancer imaging.
FDG PET Scan: Patient with movement disorder
and normal MRI
A
B
1
2
3
Patient
Normal Control
The PET scans of our patient show absent flourordeoxyglucose uptake in the
striatum. The normal control shows uptake in the caudate (arrow 1) and putamen
(arrow 2), not present in our patient. Both patients show uptake in the thalami
(arrow 3).
Diagnosis: Huntington’s disease.
Teaching point: Don’t do drugs!
Just how amazing is MRI ?
CT scan:
Abnormalities hard to detect.
T2 weighted MRI, on right, shows obvious bilateral subdural hygromas pushing on the
brain. Hard to see on CT because they have similar electron density to brain, easy to see
on MR because they have a different T2 time.
CT with only very subtle
findings.
MR of the same patient, obtained just after the CT, shows an obvious large
left MCA stroke with tissue edema
too subtle to be easily detectable on CT.
MR can also obtain completely non-invasive angiographic pictures, which
show occlusion of the left ICA.
ICA
Vert
CCA
Right
Left
33 yo in status epilepticus
CT without obvious
abnormality.
MRI shows abnormal tissue
edema in the left mesial
temporal lobe, too subtle for
CT, essentially pathognomonic
for Herpes encephalitis.
FLAIR
T2