Chapter 5
1. What are the differences between static and motion contrast?
A) Static Contrastcontrastsmechanisms sensitive to type, number, and relaxation properties of spin. This also includes a contrast in density, the content of a particular molecular type, and general chemical content
B) Motion Contrast contrasts mechanism sensitive to movement of nuclei
2. What are the differences between endogenous and exogenous contrast?
A) Endogenous Contrast depends on intrinsic properties of biological tissue (BOLD- depends on the amount of deoxygenated hemoglobin within a brain region)
B) Exogenous Contrast depends on foreign substance injected into body (injection of gadolinium-DTPA). Exogenous is used in clinical MRI, but not often in functional studies due to its invasiveness.
3. What is TR?
"TR" means repetition time. This is the time interval between successive excitatory pulses.
4. What is TE?
"TE" means echo time. This is the time interval between the excitatory pulse and receiving the data.
5. What property of tissue does a proton density image show? What TR and TE are needed to achieve proton density imaging?
It shows the proton density in a given tissue. It provides contrast through differences in number of protons in a voxel. To have a proton-density weighted (and sufficient) MR signal, pulse sequences are used that have a long TR and a short TE. Researchers use pulse sequences that minimize T1 and T2 contrasts and maximize proton-density contrast. The practical range is a TR greater than T1 effects and a TE less than T2 effects. Longer TR can be detrimental to the experiment due to participant unease.
6. What do T1-weighted images show? What TR and TE are needed to achieve T1-weighting?
T1 images give information about the T1 values of tissues. T1 needs pulse sequences with an intermediate TR and a short TE value.
7. What do T2-weighted images show? What TR and TE are needed to achieve T2-weighting?
T2 is used to provide information on fluid filled regions. This is important for diagnosis of tumors and T2 images are used frequently when looking for anatomical references. T2 requires a long TR and intermediate TE.
8. How are T1 and T2 images different in what they show? That is, why would someone use a T2 image instead of a T1 image, or vice versa?
T2 images will show the cerebrospinal fluid; where as T1 images cannot show fluid. Fluid in a T1 image appears to be the same as air.
9. What do T2* images show? What TR and TE are needed to achieve T2*-weighting?
T2*Â are sensitive to deoxygenated hemoglobin and requires a long TR and intermediate TE.
10. Do T2* images require the use of magnetic gradients or electromagnetic pulses to form the MR signal echo? Why?
T2 * images use magnetic gradients because the gradients are used to reform the pulses which will eliminate field imhomgeneuity effects.
11. What is MR angiography (MRA)? Why might exogenous contrast agents be used in MRA?
MRA( aka time of flight) is a type of MR angiography that generates contrast by suppressing signal from spins within an imaging plane so that voxels from inflowing spins have a high signal. Exogenous contrast agents might be used to help see blood vessels more clearly. This replaces classic angiography which used a contrast agent injection through a catheter.
12. What are the differences between time-of-flight and velocity-encoded MRA?
The velocity-encoded MRA uses velocity of the vessels to show contrast where as TOF uses spin suppression to arise to a contrast image by suppressing signals from the spin within an imaging plane so that voxels with inflowing spins have high signals.
13. What does diffusion imaging measure? What is the ADC, and what does it represent?
Diffusion imaging uses the movement or amplitude of diffusion to create a MR signal. ADC is apparent diffusion coefficient, which is the quantification of diffusivity assuming isotropic (equal) diffusion.
14. Why might diffusion be anisotropic? What does FA represent?
Diffusion may be anisotropic (unequal) because certain tissue may have a higher preference for water molecules. FA is the preference for molecules to diffuse in an anisotropic manner.
15. What does perfusion MRI measure?
Perfusion MRI measures how the blood is delivered to the body (i.e. through capillaries). Perfusion MRI can use both exogenous and endogenous contrast. Exogenous contrast uses intravascular contrast agents that are freely perfused into the vascular system. Endogenous contrast, which is used more commonly in fMRI, uses arterial spin labeling approaches.
16. What is arterial spin labeling? What are the differences between continuous and pulsed ASL?
ASL is a perfusion imaging technique where blood is excited (spun) and then data is not collected until the excited blood enters the imaging area. Continuous ASL uses two transmitters, one to label and one to collect the data. Pulsed ASL uses only one coil as both a labeler and a recorder. There is a brief delay period in pulsed ASL in order to have a imaging plane.
17. How does echo-planar imaging sample k-space?
Echo-planar imaging fills all of k-space during a single excitation pulse. In order to accomplish this with a reasonable spatial resolution, k-space must be filled very rapidly; this requires a very strong gradient system. In order to fill k-space, EPI scans alternate lines in opposite directions.
18. What are magnetic susceptibility artifacts?
MSAs are loses in the MR image due to field imhomogeneity resulting from the boundary between brain tissue and air-filled cavities. For example, signal loss in the ventral frontal region is due to the sinus cavities.
19. How does spiral imaging sample k-space?
Spiral imaging uses sinusoidal changes in gradients to trace a corkscrew path through k-space. Like EPI, spiral imaging is prone to signal losses due to magnetic field inhomogeneities.
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