Lecture 10 - Brain Imaging Flashcards
CT is essentially a
Rotating x-ray beam where the brain is imaged from several directions
For CT what happens to the beam as it passes through brain tissue?
It attenuates (ie weakens)
In CT, what is the significance of beam attenuation as it passes through tissue?
Detectors on opposite side of source of beam pick up rate of attenuation (ie., rate at which x-ray beam weakens as it passes through tissue)
Rate of attenuation in CT
varies by tissue composition
Radiodensity information in CT
(ie., attenuation rate) is detected
How might CT allow for reconstruction of a 3D image?
If patient is moved through the scanner slowly, allows us to acquire information at multiple levels of the brain, and therefore reconstruct 3-D image
CT enhances visualization of what?
1) bony anatomy 2) acute hemorrhagte or stroke 3) elements with high atomic numbers show up better (calcium, iron, iodine, barium, lead)
For CT what are the advantages?
It is faster and less expensive than MRI, can be used as an initial screening and assessment tool (bone fragments, etc)
What are CT disadvantages compared to MRI?
1) Use of X-ray; 2) less contrast differences between soft tissues 3) lower spatial resolution, several millimeters in CT scan VS one millimeterin MRI scan
hypodense (dark) structures in CT can indicate
edema and infarction
hyperdense (bright) structures in CT
indicate calcifications and hemorrhage and bone trauma can be seen as disjunction in bone windows.
CT can detect tumors by
Tumors can be detected by the swelling and anatomical distortion they cause, or by surrounding edema
Infarct in CT VS MRI
1) CT: loss of gray / white differentiation in CT scan 2) Infarct in MRI: greater resolution to see the difference
Anatomic MRI is based on
principles of nuclear magnetic resonance
Anatomic MRI produces what?
High resolution images of the brain and spine
Advantage of MRI
no radiation; radio frequency waves instead of x-rays
Clinical applications of MRI
1) high resolution and detailed visualization of soft tissue 2) visualizes anatomy (gray and white matter; CSF) 3) identifies a wide range of pathological processes
What the body is comprised of that the MRI takes advantage of?
63% hydrogen atoms
What property of H atoms in the body does the MRI use?
protons in H atoms have a spin, like a top that produces a small magnetic field which aligns with or against the large externally applied magnetic field resulting in a precess or wobble at a frequency propotional to the magnetic field
What is the effect of H atoms lining up with the external magnetic field?
Net magnetization of the tissue
In MRI, how do you detect the net magnetization of the tissue?
By applying a radiofrequency pulse which tips the protons away from the direction of the magnetization
In MRI, what happens after you turn the RF pulse off?
The protons realign with the external magnetic field resulting in the decay of the energy that each spinning proton absorbed from the RF pulse
In MRI, what happens as the energy decays?
RF signal is emitted which is picked up with an antennae and decoded into images with Fourier transform algorithms
In MRI, explain the digital image construction
1) spin of proton decays 2) emits RF signals at different rates 3) Pixel = RF signal strength on a grey scale
What does the rate of RF emission depend on in MRI?
The composition of the tissue in which they are located
In MRI, what is the significance of pulse sequences?
1) vary timing of the RF pulse which attenuates the tissue of interest
Give an example of pulse sequences in MRI.
Rapid repetitions of the RF pulse enhances grey – white contrast
What is the best image strategy for the MRI of lesions?
Infrequent repetitions of RF pulse enhances signal from water which is usually increased in pathologic conditions
What are the diagnostic applications of anatomic MRI?
1) inflammatory disease (multiple sclerosis) 2) neoplastic disease (tumors) 3) epilepsy 4) cerebrovascular disease (stroke)
What is MRS?
1) Permits us to study chemical structure of the brain 2) separates out chemical mixtures in the brain 3) can derive concentrations and ratios of chemicals (metabolites)
How does MRS work?
1) an RF pulse is applied, after which each chemical component (metabolite) emits a specific frequency
What happens to the signals of MRS?
They are analyzed with Fourier transforms to generate NMR spectra which consists of multiple peaks
For MRS, what does the size of the peak correspond to?
The concentration of each chemical component
What are the MRS diagnostic markers?
1) NAA 2) Choline 3) Creatine 4) Lactate
MRS diagnostic marker NAA
located in cell bodies and dendrites; considered a neuronal marker
MRS diagnostic marker choline
cell membrane synthesis and degradation: marker for demyelination
MRS diagnostic marker creatine
glial marker
MRS diagnostic marker lactate
found following ischemic events
MRS shows what for MS?
1) acute stage shows presence of Lactate, slight decrease in N-acetylaspartate and an increase in choline 2) longitudinal study = disappearance of lactate at 3 months, persistent low levels of NAA, progressive choline increase during the first weeks follo
Diffusion weighted imaging and diffusion tensor imaging allows for what?
Allows us to visualize and measure the integrity of white matter tracts in the brain
In diffusion weighted images, what is a voxel?
The intensity of each image element
In diffusion weighted images, what does a voxel reflect?
the best estimate of the rate of water diffusion at that location
DWI
3 gradient gradient-directions are applied, sufficient to estimate the trace of the diffusion tensor or ‘average diffusivity’, a putative measure of edema
Anisotropy
measurement of water diffusion along different orientations within axons
DTI measures
water diffusion along different orientations within axons.
What is the assumption for DTI in an axon?
it is that features of the axon, such as cell membranes and myelin, restrict the flow of water through the axon, so that the water flows in a relatively organized manner along the longitudinal and perpendicular orientations of the axon
In DTI, if water is not diffusing along these axes, but instead seems to be diffusing in many directions, what is it thought to represent?
Pathology
Measurement is of diffusion along length of axis: the larger the coefficient
the greater the diffusion of water along the longitudinal axis
Measurement is of diffusion along length of axis: the smaller the coefficient
the less diffusion along longitudinal axis, and presumably the more the water is diffusing in many different directions
Greater diffusion along the longitudinal axis is thought to represent what?
intactness of axonal development
In DTI, if the axon is long what do you expect?
If relatively long, represented as an ellipsoid and indicates that water is diffusing along long axis and is thought to represent the integrity of myelin
In DTI, Radial diffusivity represents what?
diffusion along perpendicular axes of voxel, in both directions. It is thought to represent integrity of myelin; myelin would restrict the flow of water in the perpendicular direction
Fractional anisotriopy, or FA, is calculated based on
both axial and radial diffusivity and represents the overall direction of water diffusion, and is thought to reflect the overall organization and integrity of the axon
High degree of anisotropy indicates that
more water is diffusing along the longitudinal axis of the voxel relative to the perpendicular axis, and would be represented by an ellipsoid
low degree of anisotropy indicates
that water is diffusing equally in all directions, and would be represented as a sphere
In DTI, red is
lateral to medial
In DTI, green is
anterior to posterior
In DTI, blue is
superior to inferior
fMRI
allows us to acquire images of the brain while patients are performing cognitive tasks in the MRI scanner
what are the advantages of fMRI?
1) allows evaluation of brain functioning in vivo 2) Understand the neural systems that make performance of cognitive tasks possible 3) Understand changes in brain function associated with disorders and with aging 4) Understand sites of neural reorganizat
What is the basis for BOLD in an fMRI?
Oxyhemoglobin is paramagnetic and can be measured by small changes in intensity values on the MR images
How does fMRI work?
1) Measure blood-oxygen-level dependant signal with MRI 2) compare baseline and experimental conditions
In the fMRI, why do we measure the changes in intensity values?
to determine how much a task is stimulating neural activity and in what regions of the brain neural activity is stimulated
What are the limitations of fMRI?
1) limitations of temporal resolution 2) limitations of spatial resolution 3) relation b/w neuronal activity, blood flow and fMRI signals has not been definitively established
What is PET?
1) Use of tracers that are incorporated into a biologically active molecule 2) scanner images the positron-emitting tracer upon its decay
What are the applications of PET?
1) blood flow and perfusion 2) Metabolism (resting and task dependant)
PET 18-FDG
traces glucose uptake, thought to reflect synaptic activity
PET 15-O
traces oxygen uptake
Ligands and neuroreceptor imaging?
1) radiotracers bind to pre and post synaptic neuroreceptors 2) EG dopamine synthesis and reuptake 3) advanced pharmacological treatments
What are the limitations for PET?
1) need cyclotron 2) injection of radioactive tracer 3) poor spatial resolution
How does PET apply to tumor management?
Although [18F]-FDG–PET seems to be useful in grading brain tumors and determining their prognosis, PET also has another advantage over anatomic imaging. Unlike CT or MR imaging, PET can distinguish radiation necrosis from tumor recurrence
How can PET distinguish radiation necrosis from tumor recurrence?
areas of radiation necrosis are hypometabolic, whereas tumor recurrence appears hypermetabolic on FDG-PET
radiation necrosis was associated with what?
hypometabolism in the white matter only
necrosis caused by chemotherapy was associated with what?
gray matter changes in addition to white matter abnormalities
Can you you distinguish an area of tumor recurrence among necrotic changes?
Yes with PET
fMRI showed activation of what area in mapping emotion?
Amygdala activation during presentation of all emotions
What does fMRI mapping of declarative memory show?
Indicates that prefrontal cortex regions are important for memory formation and they have prolonged maturational trajectory
PET and Alzheimers.
It has been used in the management of AD, since hypometabolism in various brain regions have been associated with severity of clinical symptoms. Has the ability to aid in the diagnosis and the determination of the course and severity of the disease.
fMRI and Alzheimers
1) APOE-4 allele carriers show increased brain activation during memory tasks 2) After 2 years degree of baseline brain activity correlated with memory decline 3) shows clinical predictive significance
Intraoperative MRI
awake real time neurosurgery, don’t forget to ask the patient to count in their native tongue
Measurements provided for CT
brain structure
Measurements provided for MRI
1) brain structure 2) vasculature 3) chemical structure 4) fiber tracts
Measurements provided for PET
1) perfusion 2) metabolism 3) neurotransmitter integrity
What are the disorders for CT?
1) hemorrhages 2) trauma 3) generalized atrophy
What are the disorders for MRI?
1) neoplasm (tumor) 2) demyelination (MR spectroscopy) 3) degenerative disorders (cortical atrophy)
What are the disorders for PET?
1) psychiatric 2) addictive 3) degenerative disorders 4) epilepsy
What are the advantages of CT?
1) bone imaging 2) hemorrhage detection 3) patient flexibility
What are the advantages of MRI?
1) high spatial resolution 2) no radiation 3) excellent contrast 4) functional / chemical imaging
What are the advantages of PET?
Functional imaging 2) physiological variables
What are the disadvantages for CT?
1) ionizing radiation 2) not as good for soft tissue
What are the disadvantages for MRI?
1) long study duration 2) no ferromagnetic or electronic devices 3) claustrophobic
What are the disadvantages for PET?
1) ionizing radiation 2) costly, limited access 3) tracer production
Neuroimaging and Knowledge
1) increased understanding of neural mechanisms in learning, aging and disease 2) development of effective, well-targeted pharmacological agents 3) identification of functionally important areas prior to neurosurgery 4) understanding of neural reorganizat
