MRI Artifacts Flashcards
(22 cards)
What is the purpose of gradients in MRI?
To locally alter the magnetic field for selective excitation and localization of protons
Gradients allow for the creation of slices, phase encoding, and frequency encoding in MRI.
What is a slice encoding gradient?
It creates slices of the patient by allowing RF pulses to interact with protons processing at the same speed
The slice encoding gradient is essential for determining the spatial location of signals in MRI.
What is K space in MRI?
The area where MRI systems collect raw data over time
K space is fundamental in the reconstruction of MRI images.
Define T1 relaxation time.
Short TR and short TE; fat appears bright and fluid appears dark
T1 relaxation time is critical for differentiating between various tissues in MRI.
What is the significance of T2 relaxation time?
Long TR and long TE; fluid appears bright while fat appears variable
T2 relaxation time helps in identifying fluid-filled structures and pathologies.
What does PD stand for in MRI?
Proton Density
PD imaging minimizes T1 and T2 effects, with signal intensity determined by mobile hydrogen content.
What is inversion recovery?
An initial 180-degree pulse (TI time inversion) used in modified SE sequences
Inversion recovery techniques are useful in suppressing certain signals, such as fluid.
What does FLAIR stand for?
Fluid Attenuated Inversion Recovery. Long inversion time
FLAIR sequences are used to suppress fluid signals in MRI.
What is STIR used for in MRI?
Short TI to suppress fat signals
STIR is particularly useful in certain imaging protocols where fat suppression is needed.
What is the effect of fast or turbo sequences in MRI?
Fills multiple k space voxels at once, reducing scan times and improving image quality
These sequences are beneficial for faster imaging while maintaining resolution.
What are single shot techniques in MRI?
All lines of k space are filled in one TR. HASTE sequences
Single shot techniques can lead to a poor signal-to-noise ratio.
What is the HASTE sequence known for?
Myelogram effect with very thin slices
HASTE is beneficial for specific applications in neurology and musculoskeletal imaging.
True or False: Gradient echo sequences have a 180-degree pulse.
False
Gradient echo sequences do not have a 180-degree pulse, making them different from traditional T2-weighted images.
What is T2*?
Long TR, long TE, and a small flip angle
T2* is sensitive to microbleeds and other susceptibility artifacts.
What are common motion-related artifacts in MRI?
Pulsatile motion, blurred bands, and regularly spaced ghosting
These artifacts can significantly affect image quality and diagnostic accuracy.
What is the entry slice phenomenon?
Artificially high signal at the site where slow-moving fluids enter the imaging slice
This phenomenon can mimic pathology and must be carefully evaluated.
What causes the zipper artifact?
Stray RF signals entering the scan room
The zipper artifact appears as narrow bands extending from all images.
What is a chemical shift artifact?
Occurs due to the difference in precession frequencies of protons in fat and water
This artifact typically appears as dark and bright crescents at fat-water interfaces.
Define susceptibility artifact.
Distortion of the field due to variability in magnetic susceptibility within the field of view
Susceptibility artifacts can reveal important clinical information but may also distort images.
What is partial volume averaging?
Averaging of intensities of different tissues/materials within the same voxel
This can result in inaccurate intensity representation, leading to potential pseudolesions.
What are pseudolesions in MRI?
Structures due to normal anatomic variation that may mimic pathologic lesions
Examples include trigeminal nerve enhancement and choroid plexus enhancement after contrast.
What can cause hyperintensity of the neurohypophysis on T1?
Vasopressin in secretory vessels
Normal pituitary gland enhancement may occur post-contrast.
