What is T1-weighted MRI good for? What is it based on?
- shows the anatomy at high resolution with good gray/white matter differentiation
- Some pathology is not well differentiated
- T1 is based on hydrogen ions
SPGR = SPoiled, Gradient Recalled at steady state
- Provides more detail than the T1-weighted MRI
- The deck allows you to individually adjust each type of image and to re-arrange their stacking order
- analogous to a deck of cards
T1-Weighted MRI
SPGR MRI
Fluid Attenuated Inversion Recovery = FLAIR
- highlights the tumor better
- reveals edema within the tumor and surrounding tissue
- gray & white matter anatomy are less defined
Threshold FLAIR + SPGR
- “threshold” the FLAIR image to allow the detailed SPGR anatomy
- good image of the tumor bed
- Thresholding is a mathematical manipulation of the image so that only voxels with intensities above the threshold limit are displayed
What the heck is a “voxel”?
Voxel = a small 3-dimensional imaging volume
- 3-dimensional equivalent of a “pixel”
- more voxels ⇒ higher spatial resolution
- more detail in the MR image
What does a BOLD fMRI show? What is it used for?
Brain Function
- Functional MRI provides images of highly localized changes in blood flow and oxygenation
- driven by changes in net neural activity due to a sensory, motor or cognitive event
- BOLD = Blood Oxygenation Level Dependent
What is the BOLD mechanism for a fMRI?
- Neural activity triggers local vasodilation and resultant increase in highly oxygenated hemoglobin
- Water protons within the blood and tissue are induced to emit radio frequency signals:
- high (strong) when the local magnetic field created by the scanner is undisturbed
- low (weak) when the local field is disrupted by the presence of poorly oxygenated hemoglobin
- Neural activity causes increased blood flow which removes poorly oxygenated hemoglobin which allows the protons to emit a strong signal
fMRI Mapping of the Visual Field:
based on hemodynamic changes
- relatively short scanning session of 10-15 min
- a visual stimulus consists of a flickering, checkered annulus slowly expanding over a 32 second period and then repeats 5 times
- Color codes visual field eccentricity
How is imaging used as presurgical mapping?
- “virtual surgery”
- add distance contours showing an estimated “no-fly zone” around the tumor
- Different colors within the no-fly zone mark “shells” at different distances
- Gray and white matter structures within 5-10 mm of a resection site are likely to be at risk for damage caused by the surgery
- "”shells” can be turned into an outline
- assess peripheral and central visual field risk of damage
Diffusion Tensor Imaging (DTI):
- patient was instructed to move different body parts to identify the arrangement of the “motor homunculus” representation near the AVM
- here activation was generated by movement of the right hand
- close proximity of the superior longitudinal fasciculus
- connects Broca’s and Wiernicke’s areas
- elect use of a “Gamma knife” (focused gamma irradiation) to lesion the AVM
How does advanced imaging impact surgery?
allows surgeons to be MORE aggressive with the resection because they don’t have to guess where the crucial structures are located
Besides visual field representation and “motor homunculus” representation, what are some other applications that advanced imaging is used for?
- Placement of the electrodes for therapeutic brain stimulation
- Intra-Op guidance during surgery
- track the location of a surgical instrument
- Planning “shaped” radiation treatment
- custom “shaping” of the radiation field so as to maximally impact the tumor while minimizing dosage to eloquent cortex nearby
