Advanced Apps Advanced MR Imaging/Artifacts Flashcards
The HEIGHT of a peak on an MR spectrum correlates to the _________ of a substance that was detected.
Amount
The HEIGHT of a peak on an MR spectrum correlates to the amount of a substance that was detected
Which of the following is NOT a metabolite found in a 1H spectrum of normal brain tissue?
Succinylcholine
Succinylcholine is a respiratory suppressant, not a metabolite found in normal brain tissue
The final result in a spectroscopy study is:
A spectrum of specific detected metabolites
Spectroscopy (MRS) allows doctors to obtain biochemical information about the chemical components in tissues of the human body in a non-invasive way.
Contrast in Diffusion Weighted Imaging (DWI) depends on:
All of the above
Contrast in Diffusion Weighted Imaging (DWI) depends on the degree to which molecular mobility is impaired (water restriction), the sensitivity of the sequence to Brownian motion, and the direction of water diffusion.
Common problems in DWI include:
All of the above
Common problems in DWI include sensitivity to magnetic susceptibility when using echo planar readout, bulk patient motion, and low SNR.
In DWI sequences, ADC stands for:
Apparent Diffusion Co-efficient
The first successful clinical application of DWI was in imaging the brain following stroke in adults. Areas which were injured during a stroke showed up “darker” on an ADC (apparent diffusion co-efficient) map compared to healthy tissue.
The final result in a perfusion study is:
A set of calculated images which indicate various flow characteristics
The final result in a perfusion study is a set of calculated images which indicate various flow characteristics
____________ can be described as water molecules moving the same way in every direction.
Isotropic diffusion
Isotropic diffusion: Water molecules move the same way in every direction.
____________ can be described as water molecules moving in a preferred direction.
Functional MRI
The imaging technique that employs a very rapid pulse sequence while the patient is at rest or performing a task is known as Functional MRI.
The echoes collected from a 256/256 Sagittal Cervical spine have a frequency range 14.1 kHz above and below the central frequency, 1.5Tesla system with 220 Hz chemical shift between fat and water, the vertebral body signal will shift _____ pixels relative to the signal in intervertebral disk.
2.0
28.2 x 1000 = 28200 Hz; 28200 ÷ 256 = 110; 220 ÷ 110 = 2 pixels shift between fat and water
What would the pixel shift be in a sequence acquired at 1.5Tesla (220 Hz) with a 224 matrix and a BW of 26.8 kHz?
1.8
26.8(bandwidth in kHz) x 1000 to convert into Hz = 26800
26800 ÷ 224(matrix) = 120 Hz frequency difference per pixel
220(chemical shift) ÷ 120 = 1.8, the fat image will be shifted 1.8 pixels along the frequency/readout axis relative to water
DWI susceptibility artifacts in or around strong air tissue interfaces can be minimized by:
Increasing parallel imaging acceleration factor
Increasing parallel imaging factors can decrease susceptibility artifacts around air tissue interfaces, such as temporal bone/auditory canal regions.
All of the following artifacts except __________ occur along the phase encoding axis.
Chemical Shift
Chemical shift primarily occurs along the frequency axis.
Motion is seen as a smearing across the image in the
phase encoding direction
Magnetic susceptibility artifacts are more prominent with:
Gradient echo
Gradient echo is more susceptible to metallic artifacts due to the lack of 180° refocussing pulse that would reduce field inhomogeneities.
A leak in the RF shielding can appear as:
A zipper artifact in the frequency direction
A remedy for a Gibbs truncation artifact would be:
Increase the number of phase encodings
To correct for Gibbs truncation artifact, increase the number of phase encodings or reduce the FOV (while maintaining matrix)
A FID crushing artifact can be eliminated with the use of:
A and/or C
Increase in NEX and/or enabling Flow Compensation can correct for FID artifacts
Chemical shift artifact occurs because:
Fat and water precess at different frequencies
The ___________ to reduce chemical shift artifacts.
Receiver bandwidth should be increased
Aliasing occurs because tissue outside the selected FOV is:
Undersampled
Aliasing (wrap-around artifact) occurs when tissue outside the FOV (in the phase direction) is undersampled.
The ________ should be _______ to reduce chemical shift artifacts.
Receiver bandwidth; increased
A higher bandwidth is used for the reduction of chemical shift artifacts (lower bandwidth - more chemical shift - but better signal to noise ratio).
Which artifact mimics a syrinx in the spine?
Truncation
________ occurs when tissue outside the Field of View is undersampled, causing a misregistration of anatomical location, in the phase direction, but on the opposite side of the anatomical location, also known as wrap-around artifact
Aliasing artifact
In what anatomical region would you expect to see chemical misregistration the least?
Cerebellum
Which artifact occurs because of chemical misregistration?
Chemical shift
Chemical shift artifacts occur because of chemical misregistration.
Which of the following influences can lead to artifacts?
All of the above
Magnetic field inhomogeneity, machine hardware, and the patient can all be influences causing artifacts.
In a sagittal cervical spine MRI, what could be done to compensate for CSF flow artifacts?
Enable gradient moment nulling
Gradient moment nulling (or flow compensation) can be used in a cervical spine MRI to minimize CSF flow artifacts.
Which would yield more chemical shift artifacts?
3T superconducting magnet
Which of the following could lead to poor fat suppression?
Inhomogeneity
A decrease in the voxel volume yields
A decrease in partial volume averaging
To reduce the appearance of a Gibbs truncation artifact, the:
Number of phase encodings must be increased
To minimize cross talk artifact,
Increase the interslice gap
To correct the artifact Gibbs truncation
Increase phase matrix
When the TE is lowered:
Susceptibility artifact decreases
When the TE is shortened, susceptibility artifacts decrease because shortened TE’s allow less time for dephasing thus reducing signal loss.
Crosstalk is cause by:
Differing frequencies of fat and water
To reduce an RF zipper artifact:
Close scan room door and ensure adequate door suction
Ensuring adequate door suction in the MR scan room will aid in preventing which artifacts from appearing in an MR image?
RF zipper
Moire artifact is caused by:
Interference of aliased signals at different phases
The Food and Drug Administration (FDA) set static magnetic field clinical use limits of _________ for the entire population and __________ for those over 1 month of age.
4 Tesla; 8 Tesla
Fine line or FID crushing artifacts are corrected through
Parallel imaging
Due to the gradient echo-based coil calibration scan required to perform parallel imaging acceleration techniques, this option should be avoided in metal artifact reduction sequences.
Magnetic resonance imaging (MRI) uses ________ separate magnetic fields to perform the task of routine image acquisition.
Gauss
Gauss is the term used to measure the magnetic field strength in the periphery of the MRI scan room, whereas Tesla measures magnetic field strength in isocenter
The __________ magnetic field poses risks to patients and staff, due to the torpedo effect when a ferromagnetic object becomes magnetized and attracts to the large magnet
main static
The main static magnetic field poses risks to patients and staff, due to the torpedo effect when a ferromagnetic object becomes magnetized and attracts to the large magnet
What is the term used to describe gradient field interactions as visual light flashes, or “seeing stars”?
magnetophosphenes
Which gradient is primarily responsible for performing the function of determining slice thickness, scan plane and orientation of a pulse sequence?
slice select gradient
The slice select gradient is primarily responsible for performing the function of determining slice thickness, scan plane and orientation of a pulse sequence
All of the following are potential risk factors for SAR thermal absorption during an MR pulse sequence EXCEPT:
Dielectric effects
Dielectric effects are caused by local eddy currents due to the increased conductivity of body tissue, and are identified by areas of shading or signal loss.
Primarily stemming from active RF coil channels or elements outside the scanning field of view, ______________ artifacts can be corrected by simply de-selecting the element(s) outside the scanning range.
Annefact
Primarily stemming from active RF coil channels or elements outside the scanning field of view, annefact ribbon like artifacts can be corrected by simply de-selecting the element(s) outside the scanning range
A _____________ artifact occurs when a tendon or ligament is positioned approximately 55° to the direction of the main magnetic field, and a short TE value pulse sequence is utilized
Magic angle
