Brain Imaging (10) Flashcards
Does X-Ray Computerized Tomography have radiation?
Yes it does
What is the theory behind CT?
It images the brain from several directions via a rotating X- Ray beam. The beams are attenuated as they go though the body’s tissue and the rate of attenuation varies based on tissu composition. Therefore radiodensity information is detected.
What are the clinical applications of CT?
It enhances the visualization of bony anatomy,acute hemorrhage or stroke, and elements with high atomic number (calciu, iron, lead)
What are the advantages of CT?
It is faster and less expensive than MRI and can be used as an initial screening and assessment tool
What are the disadvantages of CT?
There is radiation, and less contrast between soft tissues and lower spacial resolution when compared to MRI
On a CT, what is a hypodense region?
It is dark and usually indicates edema or infarct
On a CT, what is a hyperdense region?
It is light and usually indicates calcification or hemorrhage or bone trauma
On a CT what are tumors indicated by?
Swelling/ edema and anatomical distortion
What is the science of Anatomic MRI based on?
Nuclear Magnetic Resonance (NMR)
Is there radiation in MRI?
No, there is not
What are the advantages of MRI?
High resolution and detailed visualization of soft tissue
Describe the physics of MRN
Hydrogen atoms have a “spin” and the “Spin” produces a small magnetic field. When the “Spinning” proton is placed in a large, external magnetic field it will wither align with or against the external field. It then wobbles at a frequency proportional to the magnetic field.
A radiofrequency (RF) pulse is applied to tip the proton away from the direction of the magnetization (helps to detect the signal more readily)
When the pulse is turned off the protons realign and the energy absorbed from the pulse decays
As the energy decays, a RF signal is emitted and recorded.
In and MRI, why is the decay rate sometimes different?
Because of the composition of tissue
In an MRI, why is the RF pulse sequence varied?
To accentuate the tissue of interest
What are the 3 things needed for an MRI?
Magnet, RF coil, Gradient coil
How are the digital images of MRI constructed?
each pixel of the image is encoded with a numerical index that represents the relative strength of the RF signal in the area of the brain to which that pixel corresponds.
Each numerical index is assigned a gray scale value.
What are the four major diagnostic applications of anatomic MRI?
1- Inflammatory disease (like MS)
2- Neoplastic Disease (tumors)
3- Epilepsy
4- Cerebrovascular Disease (stroke)
Which imaging modality permits the study of the chemical structure of the brain?
Magnetic Resonance Spectroscoy (a form of MRI)
What are the 4 chemical substances that are studied in MRS?
N-acetylaspartate(NAA), Choline (Cho), Creatine (Cr), Lactate (Lac)
In MRS, how are different tissues defined?
Different ratios of the 4 chemical substances studied by the modality
What is physics behind MRS?
When a RF pulse is applied, each chemical component emits a specific frequency and an NMR spectrum is generated
What is NAA a diagnositic marker for?
It is a marker for cell bodies/ dendrites….so it is a neuronal marker.
What is Cho a diagnostic marker for?
It is a marker for cell membrane systhesis and degradation….so it a marker for demylination
What is Cr a marker for?
It is a glial marker
What is Lac a marker for?
It is found following ischemic events
What imaging modality allows the visualization and measurement of the integrity of white matter tracts in the brain?
Diffusion Weighted Imaging (DWI)
In DWI, what the intensity of the signal reflect?
It reflects the rate of water diffusion at each particular location
How many gradients are typically used at DWI?
3 gradient directions are usually applied
What is DWI a good diagnostic measure for?
Edema
Which imaging modality allows for the measurement of water diffusion along different orientations within axons?
Diffusion Tensor Imaging (DTI)
What is the theory behind DTI?
The assumptin is that feature of the axon restrict the flow of water through the axon.
Water will flow in a relatively organized manner along the orientation of the axon
In DTI, when water diffuses randomly, not in an organized manner, what is this indicative of?
It is indicative of a pathology!
Will the water diffusion coefficient be lower or higher during a pathology in DTI?
It will be lower
What imaging modality allows for the acquisition of images of the brain while patients are performing cognitive tasks in the MRI scanner?
Functional MRI (fMRI)
What is the major benefit of fMRI?
It allows clinicians to understand changes in brain function associated with disorders and aging and understand sites of neural recognition following stroke or injury
When we process information, what happens to both neural activity and blood flow?
Neural activity increases and blood flow increases to brain regions of activity
What does the FMRI compare?
It compares the signal during activity to a control or baseline.
It measures the changes in intesity values to determine how much a task is stimulating neural activity and in what regions of the brain neural activity is stimulated
What is the signal recorded in fMRI based on?
Blood-oxygen level dependent (BOLD)
In fMRI, what is the experimental condition?
The task is comprised of the specific cognitive variable of interest
In fMRI, what is the control condition?
The task is comprised of all features of the experiment/ task, except the specific cognitive variable of interest
What is a limitation of fMRI?
Both temporal and spacial resolution.
What imaging modality uses radioative isotope tracers?
Positron Emission Tomography (PET)
In PET scans, what are the tracers incorporated into?
They are incorporated into biologically active molecules (usually glucose)
In PET scans, what is the scanner imaging?
It is imaging teh positron- emitting tracer upon its decay
What clinical scenarios are PET scans useful for?
Blood flow/ perfusion evaluation, metabolism issues, and ligand/ neuroreceptor imaging, and tumor management
What are some limitations of PET scans?
Need a cyclotron to make the radioactive isotope tracers, it requires the injection of radioactive tracers and it has poor spatial resolution
Why are imaging modalities used to understanding normal brain development?
they can map cortical development and clinicians and researchers can compare development of controls to people with particular pathologies
In fMRI mapping, where are all emotions mapped to?
The amygdala
In all ages, what area of the brain does fMRI indicate is needed for declarative memory?
Mesial temporal lobes
In adolescents to adults, what area of the brain does fMRI mapping indicate is important for prolonged memory and memory maturation?
Prefrontal cortex
In Alzheimer’s disease, is there a loss of white or gray matter?
Gray matter
Which types of scans are most useful for Alzeimer’s Disease diagnosis and tracking?
PET scans (to track demylination) to help determine the course and severity of disease
fMRI scans for comparisons of effect of allelic variation during memory
When can image guided neurosurgery come in handy?
In pre-op planning and intra operative guidance
In pre-op planning, what is generated by the imaging modalities?
a 3D volume data set is generated by integrating all the imaging modalities.
