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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 cortical 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.