Imaging Methods Flashcards
Give 3 imaging methods.
CT. PET, MRI/fMRI
Computerized tomography uses:
X-ray
Positron emission tomography uses
radiation
Magnetic Resonant Tomography uses:
Magnetic fields
What do PET and fMRI both have in common?
They are Multiphoton microscopy
How does X-ray and radiography linked to CT? (3)
Laminography based upon rotation of source and detector,
Laminography at multiple planes produces polytomography
Radio beam and absorption (Different tissue has different absorption rate)
How does CT scan work?
Scanning with focused beam on thin layer of tissue. Show image according to absorption, Reconstruction by computer software
CT scan provides high _____________________ and is best for _____________________ and shows ______________
CT scan provides high spatial resolution and is best for bony structures and shows soft tissue better.
In CT scan, the contrast across different soft tissue is _____ as compared with MRI
low
What is the difference between regular X-ray and CT scan?
Regular x-ray shows the total absorption rate along the pathway of beam.
CT shows the absorption rate of any points along the pathway of the x-ray beam.
What is a collimator?
Acollimatoris a device that narrows a beam of particles or waves.
How can we increase the contrast in X-ray and CT?
Inject substance into body space and blood,
Increase absorption (opacification: filling space with material with larger X-ray attenuation
Iodinated compounds
Air injection—pneumoencephalography
What does this image show?
Acoustic neuroma is shown in CT scan by enhancer dye injected
What do these images show?
Arterial phase (left) : abnormal course of arterial vessels to the inner ear
Venous phase (right): displacement of petrosal veins on the left
How does MRI work?
Atoms with odd protons spin -> micro atom magnetic fields
They can be aligned by external magnetic field
And excited by radio impulse at Larmor frequency to high energy status
Jumping from high to low energy status accomplished by radiation of weak signal
In MRI, different __________, different _________ -> _________ and offers better _____________________________
In MRI, Different tissue, different signals -> images and offer better resolution for soft tissues
What is Lamor Frequency?
Different tissue has a different nucleus so different Larmor Frequency (Natural Frequency)
What is occurring in this image?
High energy status returning to a low energy one accomplished with an emission of RF energy (at Larmor frequency)
Recovery to alignment and the status of losing phase (related to T1 and T2): T1: return to aliment, T2: return to random status without external MF.
What are the three status of atoms in magnetic fields?
- Randomized (low energy) which is natural
- Alignment (forced in phase)
- Irritated (high-energy status
In MRI, the emission is ________________________________ and provides _______________________________
The emission is picked up by the detector and provides information about the structure
How are images formed from MRI?
Release of RF is tissue (nucleus) specific, RF detected by receiving coils, Amplitude mapped by Fourier transformation, Image created by using the signal
What is the difference between T1 and T2 -weighted images?
T1>T2 (T2 relaxation is quicker than T1) for most the tissues
In T1 weighted time water appears _________
dark
What are two functional imaging technologies related to metabolism and blood flow?
FMRI and PET
How does fMRI work? (5)
- Blood oxygenation level-dependent (BOLD) contrast fMRI
*Increase brain activity—increase deoxyhemoglobin—increase blood flow (2-6 seconds later), surplus
*Deoxyhemoglobin has a greater magnetic susceptibility
*MRI signal represents the ratio that indicates brain activity
*T2-weighted image for quicker imaging (better temporal resolution)
How does PET work?
Radioactive material is injected into blood, and stronger signals from areas with high blood flow
What are the limitations of fMRI in imagess related to hearing tasks? (3)
- Limitation by the huge scanner noise (>100 dB SPL).
- Limited by temporal resolution
- Methods to reduce the impact of background noise
What is the purpose of the Resting-status fMRI (rs-fMRI)?
To overcome the difficulties from the image related to the hearing task we developed the Resting-status fMRI (rs-fMRI)
What are the 3 steps in Resting-status fMRI (rs-fMRI)?
Look for the low-frequency vibration of BOLD
Calculate correlation across different regions
Strong correlation represents strong connectivity (functional connectivity)
What are the advantages of MAGNETOENCEPHALOGRAPHY? (2)
High-time resolution (comparable to EEG,»_space;MRI/PET/CT
Better spatial resolution than EEG/EP
What is MAGNETOENCEPHALOGRAPHY?
Measures the magnetic field produced by electrophysiological responses
Provide safety comparison between methods.
- Effective ionizing radiation for a PET test is 14 micro-Sievert (mSv)—the allowed dose for one year
- 0.02 mSv for a chest x-ray
- 6.5-8 mSv for a chest CT scan.
- 23-36 mSv for a PET-CT scan
MRI is generally safe
How does fluorescence microscopy work?
Fluorescent dyes made highly selective to a molecule of interest (DNR/RNA, protein, lipid), diff. colors for different molecules
Illuminated by light of a specific wavelength, in visible wavelengths (390-700 nm)
Absorption of the light by fluorophores, jumping to a high-energy state, emitting light when returning back
The emitted light is different from that of the illumination
The emitted light is separated from the illumination by an emission filter and dichroic beamsplitter
What are the visible wavelengths in Fluorescence microscopy?
390-700 nm
What is laser confocal scanning microscopy (LCSM)/Confocal microscopy?
Wide-field fluorescence microscopes have poor resolution due to the emitted light from unfocused background
In LCSM, only one point is illuminated at a time, and a pinhole is used to block further from the unwanted area, to increase the resolution
What are the limitations of LCSM?
- Visible light is largely SCATTERED AND ABSORBED before it reaches a deep target
- Limited the depth upon which the observation can be done.
Limited the use in vivo.
Advantage of 2 photon
Allows visualization of living tissue at depths
How does 2-photon imaging works?
Photon absorptions excite a molecule to higher energy state (similar to any fluorescent microscopy.
Instead of using laser in visible range, 2-photon microscopes use infrared light to reduce absorption and scattering
The 2-photons are applied within 1 femtosecond of each other (10-15 seconds) so to add together.
In this way, the two photons, each with wavelength of 400-500 nm, make the wavelength of excitation to be 800-1000 nm (infrared).
However, only one photon will release energy at a time, so to present visible light
Better focus (no out-of-focus light)
What are the methods of psychological evaluation? (6)
Classical methods of limit
Method of constant stimuli
Method of adjustment
Adaptive procedure and derived
Two-interval forced choice
Scales of measurement
How does the method of limit work? (6)
What determines the threshold in the method of limit?
At each run, the turning point is identified. The average of reversals is the threshold.
How do we identify the differential Limen using the method of limit?
There are two reversal points in each series, the range between which (equals) suggests uncertainty.
Reversal points: Louder Equal and then Equal soft
What are the limitations or biases due to guessing? (3)
Anticipation
Habituation
time-consuming
How does the method of Constant Stimuli work? (5)
What is an advantage of the method of Constant stimuli?
Advantage: avoid bias from guessing (no anticipation)
What is the criterion for the threshold in the psychometric function?
50% corresponds to the middle point in the psychometric function.
The threshold is determined statistically and is defined in the linear, dynamic range of the psychometric function.
What is important about this graph?
At the 10 and 11 dB stimulus levels, responses were at 100 %, and at 4 and 5 dB responses were 0%. This is shown in the graph.
Table 7.1 states that 50% response level occurs between 7 dB and 8 dB (because the performance is 70% at 8 dB and 34% at 7 dB).
In the psychometric function, the_____________________________, the more __________________________
Step size and accurate estimation (the smaller the steps, the more accurate the estimation)
Method of Constant Stimuli can be used for _____________________
Differential limen
____ correct as the threshold for differential limen (DL) in the Method of constant stimuli.
75%
How does the method of adjustment work?
In the method of adjustment, there is a
perseveration of response or persistence of the stimulus:_______________________________________________________________________________________
a lower threshold is obtained on a descending run and a higher threshold is reached in an ascending run.
What is an adaptative procedure?
The stimulus presented depends on the previous response
Make changes around the threshold (target), not far away
More efficient method because (1) close to the target (2) no need for estimation of the target (3) the adjustable step size leads to more precision
Many alternative procedures in practice: e.g. modified method of limit by an adaptive procedure
What method is more used in our clinic?
Up-Down or staircase method
What does this technique show?
Parameter estimation by sequential testing (PEST) adaptive changes in both direction and step size
How does the method of tracking work (example of the adaptative method)?
The subject controls the stimuli but only the direction (ascending or descending) (i.e if detected turn it down, if not detected turn it up)
Fixed-rate of change
Bekesy Audiometry
How does Bekesy tracking work?
Both frequency and intensity are changed
Tester controls only the direction of intensity change
When the sound is audible, tune it down, not audible, tune it up
Frequency is changed at a fixed rate
Two different trials with (1) continuous tone, (2) pulsed tone
Diagnosis mainly based upon the difference between the two trials
In Bekesy trackin, Type l have more chances to be seen in______________________
Normal or conductive
In Bekesy tracking, type ll had more chances to be seen in ____________________
Cochlear HL
In Bekesy tracking, type lll and lV have more chances to be seen in _________________________.
retro-cochlear HL
What is the Two Alternative/Interval Forced (TAFC) Choice?
In any test in which the subject can make a “correct” response by guessing, the chance of guessing can’t be used as a threshold.
Compare standard tone and probe tone or other alternative comparisons (e.g., one interval has noise, the other has noise + signal)
The threshold must be 75% or higher
Response modification
Calculate mean
Variations in procedure regarding this method
What would the % threshold be if 3IFC?
If it is 3 intervals, forced choice, the % correctness from guessing is 33% and the criterion for threshold can be 50%
What are 4 types of scale measurements?
Nominal scales
Ordinal scales
Interval scales
Ratio scales
Nominal scales: (3)
- The least restrictive
- Assign observations to groups
Ex: male-female, normal-hearing impaired - No ordering
Ordinal scales (2):
*Rank order of observations (1st, 2nd, etc)
* Does not define the size of the difference between the categories
Interval scales (2):
specify order and distance
Ordered metric scale
Ratio scales: (2)
- define distance in ratio instead of absolute unit
- understand “true zero point”—not actually zero, otherwise the ratio will be indefinitely large (e.g. sound level)
Explain Direct Scaling:
- Ratio Estimation (how many times louder) and Production (make the sound 2 times louder)
- Magnitude Estimation (assign a number to a given sound) and Production (match a sound to a given number)
- Cross-Modality Matching
What does this graph show?
Estimation with reference to other sti
Modulus can be removed:
Absolute magnitude estimation (AME
Explain magnitude:
The reverse of magnitude estimation
Subject makes sound level based upon the number that is given
Can be done with and without modulus (absolute magnitude production (AMP)
Subject bias: tend not to assign extreme values in magnitude estimation, but to make extreme level adjustments in magnitude production
What does this graph demonstrate?
Estimation/production without reference to modulus and other stimuli
Note: Bias and balancing procedure (PMB—psychological magnitude balance)
Sensitivity refers to:
real threshold
Proclivity refers to:
response bias or uncertainty
What are biases in the method of limit related proclivity? (2)
Anticipation
Habituation
What are biases in the method of adjustment proclivity? (2)
Persistence of the stimulus, or perseveration of the response
What are 4 Yes-No possible response outcomes?
Hit
Correct Rejection
Miss
False Alarm
Hit =
Miss =
False alarm =
Correct rejection =
Hit = true failed (TF)
Miss = false pass (FP)
False alarm = false failed (FF)
Correct rejection = true pass (TP)
High sensitivity is critical for ______________________________
High specificity is critical for _______________________
High sensitivity is critical for a screening tool
High specificity is critical for terminal diagnosis tool
Assumptions of Theory of signal detection (TSD):]
Noise exists:
Function:
Decision:
Noise exists: external and internal
Function: Function changes
Decision: noise alone (N) or signal-plus-noise (SN)
Decision axis
Or magnitude of sensory activation
Probability
What is important about this graph?
Larger the separation between N and SN, and the smaller the σ, the bigger the d’—the measure of sensitivity
(Noise = N SN= Signal + Noise)
Why are clear instructions important during the testing of anomalies?
Instruction can be critical! Internal criteria setup for optimum b
Why are clear instructions important during the testing of anomalies?
Instruction can be critical! Internal criteria setup for optimum b
How to show the quality of signal detection?
How probabilities change with different d’.
Receiver-operating characteristic—ROC curve
Show sensitivity and criterion at the same time
Sensitivity: rep by distance of ROC from the diagonal
Criterion: particular points along the curve
Larger the d’, smaller the false alarm when sensitivity is high
What are the implications of Theory of signal detection (TSD)? (4)
Important for psychoacoustic/neurological experiment
Design and assumption
Clear-cut of threshold does not exist
Outcome depends on methods
