Diffusion/Perfusion
Case1. An 80 year old male presents to the ER with aphasia and right arm
and leg weakness. The turboflair
images
demonstrate diffuse white
matter disease. The diffusion
images show acute infarction in the left
perisylvian region. The perfusion
images show a slightly larger zone of ischemia
indicating only minimal additional tissue at risk for infarction. The patient
subsequently made an excellent recovery.
Take Home Message: Diffusion/Perfusion MRI can demonstrate
areas of infarction and at risk for infarction as well as the
mechanism of infarction in acute stroke patients. It thus may
allow better therapeutic decision making including selecting
appropriate patients for thrombolysis.
MRA Circle of Willis
High
resolution 3DTOF MRA
gives
non-invasive evaluation of intracranial vessels without injections and can
reliably assess atherosclerosis and aneurysms.
MRA Carotid and Vetebral Arteries
High resolution 3DTOF
MRA
gives excellent detail of
atherosclerosis and degree of stenosis for endarterectomy candidates. The
bolus gadolinium MRA performed in only 11 seconds is useful in problem solving
(< or > 70% stenosis) and in uncooperative patients. Compare with the
opposite carotid artery in this patient, which remains widely
patent.
Real-time 3D Carotid MRA
Quicktime movie 
- Bolus Gadolinium MRA acquisition
obtained in 20 seconds demonstrating ulceration and stenosis of left carotid
bifurcation. This technique allows assessment of uncooperative patients.
High Resolution Imaging
This 2
mm thick 512 resolution T2W image
through
the midbrain demonstrates a small aneurysm of the left carotid siphon and
exquisite detail of the brain parenchyma. Incidently noted is an arachnoid
cyst at the left temporal pole.
Functional MRI
Due to minute changes in blood flow and oxygenation with brain activation,
MRI can map out specific functional areas of the brain. This
echo planar sequence
demonstrates the cortical motor strip in
the left hemisphere during tapping of the right thumb and second digit. This
technique can now be used in patients such as in assessing the normal tissue
surrounding brain tumors who are operative candidates.
Case Presentation: This patient presented with right foot weakness. The gadolinium
enhanced MR images 
demonstrate a necrotic mass at the left
frontoparietal junction with surrounding edema. A stereotactic biopsy was
planned but the neurosurgeon needed to know where the motor cortex was so
as to avoid paralyzing the patient. The functional
MRI images 
demonstrate the motor cortex
associated with right hand and leg movement to be located above the tumor.
Brain surface cortival
vein mapping 
was also performed
as part of the biopsy evaluation. The CT images 
performed before and after
stereotactic biopsy demonstrate an anterolateral approach to avoid the motor
strip and major veins. The biopsy was performed without complication and demonstrated
metastatic disease.
The sensory cortex for the right thumb and second digit is
shown to be immediately posterior to the motor cortex in the
volunteer.
Stroke Diagnosis and Therapy:
There are over 700,000 new cases of stroke per year in the
U.S. Until recently, the diagnosis and treatment of acute stroke
was limited to excluding other conditions and preventing recurrence.
Diffusion and perfusion MRI allow the detection of acute stroke
and assessment of the brain territory at risk for stroke within
minutes of onset of clinical deficit. Magnetic resonance angiography
can noninvasively show the arterial blood supply to the brain.
These techniques allow physicians to predict patient outcome
and select patients who may benefit from emergency intervention
such as stroke thrombolysis. This involves dissolving blood clots
within the artery(ies) supplying the dying brain. Currently,
only tissue plasminogen activating factor (tPA) is FDA approved
for intravenous use (IV) in acute stroke. However, intra-arterial
(IA) administration (infusing medication directly into clot via
microcatheter passed into the blocked artery in brain) can be
more effective and allow patients to be treated even six hours
after onset of clinical deficit.
Illustrative cases:
CASE A.
1. This patient presented with fluctuating speech difficulties and left arm
and leg weakness of uncertain duration. The initial diffusion MRI demonstrates
abnormal signal in the right frontoparietal brain
, indicating
early infarction (death). The turbo FLAIR MRI images
also
demonstrate the relatively acute infarction. A perfusion MRI 
demonstrates
a much larger zone at risk for infarction. The MRA 
demonstrates occlusion of the right internal
carotid artery with poor collateral flow to the right MCA territory.
2. This patient presented with left hand weakness with MRI performed two
hours after onset of clinical symptoms. The diffusion and conventional MRI
demonstrate early infarction in the right perisylvian region
. The
perfusion MRI demonstrates only a small additional territory at risk
.
The MRA 
shows
patency of the right MCA and its primary branches. The patient received IV-tPA
and was clinically normal the next day. Follow-up CT 
performed the next day delineates the infarction extension as predicted.
3. The next patient presented with dense paralysis involving the entire left
side of the body with obtundation (NIH stroke scale = 19) following cardiac
catheterization. MRI was performed only two hours later demonstrating thrombotic
occlusion 
of the right middle cerebral artery. The
diffusion images show a very subtle abnormality only in the right basal ganglia
with an extensive perfusion
deficit 
of the right cerebral hemisphere. Emergency
angiography confirms thrombus (clot) in the right middle cerebral artery.
A microcatheter was placed 
through a guiding catheter in the right
carotid artery and tPA administered directly into the thrombus. A total of
five microdoses was administered and the thrombus was completely dissolved
after two hours.
The follow-up MRI the next day demonstrates a right basal ganglia infarction
and
normal perfusion
. The patient made a complete
clinical recovery over the next several days as predicted (NIH stroke scale
= 1).
Case B.
75yo male 3 days post right subdural evacuation who developed
paralysis of the left arm but could not undergo MRI due to pacemaker.
CT
Angiogram 
shows right MCA occlusion with large parietal
collaterals. Perfusion maps 
show delayed arrival of blood flow (TTP),
but near normal arterial transit time (MAT) and symetrical relative cerebral
blood volume (rCBV) and cerebral blood flow (rCBF). The patient recovered
over 48 hours (RIND) as predicted by the CT perfusion study.
SPECTROSCOPY
Although water and fat contribute virtually all of the signal
in proton magnetic resonance imaging, it is possible to suppress
these signals and assess the signal from other metabolites in
the brain including choline, creatine, and NAA. These metabolites
are altered in concentration in various disease processes, particularly
tumors. Higher field instruments with very homogenous magnetic
fields and the necessary software are necessary to perform spectroscopy.
Illustrative cases follow demonstrating spectroscopy's use
in brain tumor evaluation and surgical planning.
Case #1 - This middle-aged patient presented with a brain tumor detected
on an outside MRI. Ultrathin T1
and T2 weighted sections
were
obtained for stereotactic brain biopsy localization. Multivoxel spectroscopy
demonstrates normal spectra from normal white matter.
The
most malignant portion of the tumor 
demonstrates markedly elevated choline.
There
are also abnormal spectra from the brain lateral to the visible tumor on the
standard MRI images
. Metabolic mapping of the spectra allows
a rapid assessment of the spectral peaks and the choline map 
also
demonstrates the most malignant site to biopsy. Elevated choline probably
represents the cell membrane breakdown secondary to the tumor, while NAA is
a metabolite of normal neuronal tissue.
Case #2 - This patient also presented with an extensive tumor 
which
was further evaluated with multivoxel spectroscopy which also demonstrates
the marked choline elevation in portions of the tumor
. Spectral
mapping demonstrates lactic acid in another portion of the tumor indicating
necrosis
. Stereotactic brain biopsy was performed
using the spectroscopic results for targeting and confirmed the diagnosis
of glioblastoma multiforme as suspected.
CSF Flow
Phase Contrast MRI can demonstrate qualitatively and quantitatively,
alterations in CSF flow during the cardiac cycle. This case demonstrates
obstruction to downward CSF flow at the C6 level secondary to
a post-traumatic syrinx which has expanded the cervical cord
and associated adhesions. Lysing the adhesions should collapse
the syrinx and obviate surgery of the syrinx itself.
