Week 8 Flashcards
As discussed in week five, what is the slice select gradient?
The third event of the pulse sequence in which spatial localization occurs via linear variation of the magnetic field
Explain linear variation.
The positive and negative linear manipulation of the magnetic field, while allowing center to remain at true strength.
What is the amount of electrical energy that must be used to create linear variation within the magnetic field?
The gradient amplitude
What parameter determines whether the gradient amplitude will increase or decrease?
Slice thickness
How must the gradient amplitude change to create a steeper tilt to the magnetic field?
Increase
If the gradient amplitude is increased, the MRI scanner will be able to attain ________ slices. Therefore, a possible ______ in slice thickness.
Thinner; decrease.
Where is signal gathered from?
Echoes
If we recall the directions of the matrix, what are the numerical values given to the stored signal generated by the pulse sequence?
Frequency and phase encoding
Where are the raw signal values stored after conversion into spatial frequency?
K-space
What is K space?
The collection of numerical values which correspond to spatial frequency. Raw data storage.
If we recall the five events of the pulse sequence, when is k-space filled with raw data?
After the slice select gradient and during the phase gradient
What are the two conditions that must be met for K space to be filled?
- A slice must be selected
- Phase and frequency encoding values must be converted into spatial frequency.
What are the two areas of k space mapping?
- Center.
- Periphery (boundary)
What information is stored in the center of a line of K space?
Contrast information of an MRI image, like TR, TE, and TI
What information is stored in the periphery of a line of k space?
Spatial resolution information of an MRI image
Can a diagnostic MRI image be visually determined from a k space image?
No, raw data is separated into contrast and resolution
What is the similarity between matrix and k space?
Both are methods of storing phase and frequency encoding information
What’s the difference between K space and matrix?
- K space is the direct storage of raw data information and is represented as more “list-like”
- Matrix is the spatial representation of processed data and is represented as more “grid-like”
What is the mathematical algorithm that transforms raw data into a diagnostic image?
Fourier transformation
The Fourier transformation converts k space into what?
The matrix
If we think of k space as a numbered list filled with raw data, what do the numbers of this list correspond to?
Echo train length
If the number of echoes sampled in one pulse sequence is increased, what factors are affected and how? How is k space affected?
- Acquisition time decreases.
- SNR decreases.
- The number of lines of k space that must be filled increases.
The raw data values that are filled into each line of k space are known as what?
Spatial frequencies
How are the raw values of phase and frequency encoding converted into spatial frequency for k space filling?
With the pixel or voxel size formula, depending on 2-D or 3-D imaging
How are the spatial frequency values arranged within a line of K space?
Low values are in the center of the line, and the high values are at the beginning and end.
A single line of K space can also be referred to as a line of what?
Phase
What three points of information are true in regards to contrast information in a line of phase?
- Contrast information is found in the center of the phase line
- Contrast information are low spatial frequency values.
- Contrast information is achieved with a high gradient amplitude
What three points of information are true in regards to resolution information in a line of phase?
- Resolution information is found on the boundaries of the phase line
- Resolution information are high spatial frequency values.
- Resolution information is achieved with a low gradient amplitude
What are the three methods of k space filling?
- Cartesian
- Spiral.
- Keyhole
Which method of k space filling is traditional, filling line by line?
Cartesian
Spiral k space filling is also known as what?
Elliptical filling
Explain spiral k space filling.
Spatial frequencies fill k space from the center out, like a backwards maze
Which method of K space filling is used in dynamic studies?
Keyhole filling
Keyhole filling occurs when resolution data is filled ______ and contrast data is filled ________.
Once; repeatedly
Why are low spatial frequency values filled multiple times during keyhole filling?
To show the active infiltration of IV contrast
What are the five post processing techniques?
- MaxIP
- MinIP
- MPR
- Subtraction
- ADC
Of the five post processing techniques, which two are used in MRAs?
- Maximum intensity projection.
- Minimum intensity projection.
MaxIP technique _____ high intensity blood vessels, and ____ the dark background.
Highlights; omits
Which MRA post processing technique allows for the manipulation of blood vessel images in 3-D?
Maximum intensity projection
MinIP is a post processing technique that _____ bright structures, and is effective at imaging _________ structures.
Omits; hypodense
What are three examples of hypodense anatomical structures?
- Lungs.
- Biliary tree.
- Pancreatic duct.
What kind of image acquisition is required to utilize multi-planar reconstruction?
3-D isotropic volume
Which post processing technique allows for the reformatting of both slice and acquisition time?
Multi-planar reconstruction
What are the three reasons that MPR requires 3-D isotropic volume?
- High SNR.
- Thinner slices
- No gapping
What does isotropic volume refer to?
A volumetric voxel with equal width, height, and depth. A perfect cube.
What does anisotropic volume referred to and when is it utilized?
A volumetric voxel with uneven width, height, and depth; for 3-D acquisition of linear anatomical structures, like long bones
What is the formula for maintaining isotropic volume?
Slice thickness/number of slices = FoV/phase
*Remember, thickness and FoV must both be in millimeters!
Subtraction is the post processing technique that digitally subtracts pre-contrast T2 from the identical post contrast T2 images. True or false?
False; T1 imaging is used for contrast studies
What does ADC stand for?
Apparent diffusion coefficient
What is apparent diffusion coefficient imaging?
A post processing technique that measures how freely water molecules diffuse amongst tissue cells
How is the ADC calculated?
Using signal intensities at various B values during a DWI MRI
What are the most commonly used B values for ADC calculation?
B=0 and B=1000
What are the ADC values of normal CSF tissue?
3000-4000 mm2/s
What are the ADC values of normal gray matter tissue?
700-1000 mm2/s
What are the ADC values of normal white matter tissue?
670-800 mm2/s
What are the ADC values of pathological grade II, III, and IV glioblastoma tissue?
1200, 1000, and 800 mm2/s
What is the ADC value of an acute infarct?
400 mm2/s
THIS imaging technique is based on gradient echoes and visualizes arteries.
Magnetic resonance angiography
What are the three types of MRA’s?
- Time of flight (ToF)
- Phase contrast (PCA)
- Contrast enhanced (CEMRA)
Which type of MRA relies on the flow phenomenon?
Time of flight
How are ToF MRA’s achieved?
With a gradient echo applied perpendicular to blood flow
Time of flight MRAs can only be 3-D. True or false?
False; They can be 2-D or 3-D
What is the flow phenomenon?
A technique that visualizes any protons that flow into the imaging plane
What are 2D TOF MRAs ideal for?
Longer vascular anatomical structures, like arms and legs.
What are 3-D TOF MRA’s ideal for?
Compact detailed areas of vascularity, like the brain.
As discussed in week six, saturation bands are physical bands that are placed at specific locations to limit undesired signals. During an MRA, saturation bands are placed outside the field of view to de-phase ________ flow from being sampled.
Venous
Any MRA performed above the heart will need a saturation placed where?
Superiorly
Inferior saturation band placement are required for MRAs performed where?
Below the heart
How are phase contrast MRA’s achieved?
By using phase information (remember, motion is always found in the phase) from high velocity blood flow, with the complete suppression of stationary tissue
What are the two main differences between PCA and ToF MRAs?
- PCA applies two pulses, while TOF uses one pulse.
- TOF uses saturation bands to achieve suppression, while PCA uses that second pulse
PCA MRAs apply two phase encoding pulses to achieve suppression. How does this work?
Any stationary protons in the region of interest will receive both pulses, thus canceling each other out. Any moving protons in the region will only get one pulse, thus emitting a readable signal.
What two pieces of information are obtained from the moving protons during a PCA MRA?
- Signal
- Velocity
What is used to calculate the velocity of each moving proton during a PCA MRA?
The speed and direction of the protons
Why is calculating the moving protons’ velocity important in PCA MRA?
Allows for the PCA to distinguish how fast blood is moving towards or away from the region of interest
What does VENC stand for?
Velocity Encoding
What is velocity encoding?
A manually adjusted parameter that sets acceptable velocities for PCA MRA’s
What risk is associated with imaging below VENC parameters?
Aliasing artifacts
What risk is associated with imaging above VENC parameters?
Inaccurate flow capture and loss of information
Contrast enhanced MRA rely on IV contrast to _______ T1 relaxation times, thus _______ T1 information.
Shorten; increase
CEMRA requires both pre-and post-contrast images. True or false?
True
In which direction does oxygenated blood flow?
From the heart to peripheral body parts
From which direction does deoxygenated blood flow?
From the body periphery back to the heart
What are the four types of blood flow and how are they characterized?
- Laminar: non-turbulent, straight blood flow.
- Spiral: normal, spiraling blood flow in one direction
- Turbulent: obstructed laminar, resulting in random flow pattern.
- Vortex: obstructed laminar, resulting in regurgitated flow.
What does diffusivity refer to?
The rate of movement of a water molecule from intra- to extracellular
High diffusivity is associated with what type of tissue?
Healthy
Increased diffusivity within a particular voxel of tissue is known as what?
Facilitated diffusion
What does restricted diffusion refer to?
An abnormal decrease in diffusivity within a particular Voxel of tissue
Low diffusivity is associated with what type of tissue?
Unhealthy tissue/pathology
Describe the series of events during a stroke and how it affects diffusivity
- Stroke occurs, there is a loss of blood flow to a part of the brain
- Brain cells begin to die without blood flow
- Dead cells cause an influx of sodium into the surrounding area
- All the cells of the area absorbed the sodium and swell.
- The swollen cells limit water molecule diffusivity
What does DWI stand for?
Diffusion weighted imaging
What does the diffusion weighted imaging technique visualize?
The diffusion of water molecules in a particular region of interest
What are the two processes that DWI utilizes?
- Diffusion sensitization.
- Signal attenuation.
How does the DWI technique achieve diffusion sensitization?
By using gradient pulses to enhance the signal of water molecules as they diffuse amongst cells
*this is where B values come in to play
What are three diagnostic uses of DWI?
- Confirmation of stroke within the first 48 hours of onset
- Multiple sclerosis.
- Concussions.
After the gradient pulse that causes diffusion sensitization, what areas cause greater signal attenuation and how do they appear on images?
Areas of restricted diffusion; bright
What is another term for the B value?
Brownian motion
The B value is an automatically selected parameter. True or false?
False; the b value is a manually adjusted parameter
How is the B value calculated?
With the strength and duration of the gradient pulse used during diffusion sensitization
How does increasing the B value affect the DWI image and the SNR?
- Decreases the brightness of CSF.
- Decreases SNR.
Define perfusion.
The process of blood being delivered to tissues throughout the body
Which type of MRI has the ability to differentiate the grading levels of metastatic and vascular diseases?
Perfusion MRI
What does metastasis refer to?
The spread of a pathogen or disease from an initial site, to a secondary site within the hosts body, like cancer.
What are the three types of perfusion MRIs?
- Dynamic susceptibility contrast (DSC)
- Dynamic contrast enhanced (DCE)
- Arterial spin labeling (ASL)
Which of the three types of perfusion MRIs evaluate T2* signal loss after contrast injection?
Dynamic susceptibility contrast (DSC)
DCE perfusion MRIs evaluate _____________ parameters after bolus interaction.
T1 shortening
Which perfusion MRI technique does not use IV contrast?
Arterial spin labeling (ASL)
What is the purpose of ASL perfusion MRI?
To identify cerebral blood flow spins by magnetically labeling
MR spectroscopy is an imaging technique that evaluates _______________________________, allowing the prediction of prognosis severity.
The histology of abnormal tissue
What are the five chemicals evaluated during MR spectroscopy?
- Myo-inositol
- Choline
- Creatine
- NAA
- Lactate
What type of k space filling is employed by dynamic imaging?
Keyhole
Give two examples of dynamic imaging studies.
- MRI prostate.
- MRI pituitary gland.
What does dynamic imaging allow the evaluation of?
The “wash-in/wash-out” effect of IV contrast in a particular area of tissue
If we recall keyhole filling, dynamic imaging studies are achieved by obtaining both contrast and resolution data on the first image, with a series of contrast only images after. What is the purpose of this?
This details the contrast changes over time in a specific region of interest, all while keeping acquisition time moderate. Acquisition time decreases if you only need half the data from most of the study.
The methods in which IV contrast is injected and monitored for its appearance is known as what?
Contrast bolus detection
What are the three types of contrast bolus detection?
- Fluoroscopic triggering.
- Timing bolus method.
- Automatic triggering
What does CINE refer to in regard to contrast bolus detection?
Fluoroscopic triggering
Describe fluoroscopic triggering for contrast bolus detection.
Detection method that allows the technologist to view “fluoro-like”, live gradient images to monitor the active flow of IV contrast
Approximately how much IV contrast is used during the timing bolus method?
1-3 ml
The timing bolus method uses contrast in conjunction with what?
Rapid gradient pulses
When does the technologist begin the actual MRI scan during the timing bolus method of contrast bolus detection?
Once the technologist confirms the presence of IV contrast in the area of interest
What is a timing bolus?
The time it takes from initial injection to the presence of the contrast on the image
Which contrast bolus detection method uses a trigger marker placed over the region of interest in order to detect the presence of gadolinium and automatically begin the scan?
Automatic triggering
