EXAM #15 — PHYSICS UNIT 08 Flashcards
a. signal.
a. signal. RF received in a coil that comes from the relaxation of excited protons
b. noise.
b. noise. RF received in a coil that comes from background electrical interference and random thermal motion in a patient
c. signal-to-noise ratio.
c. signal-to-noise ratio. a measure of image quality in MRI; the ratio of the average signal in an imaging volume and the average noise in the volume; signal intensity divided by standard deviation of noise
d. contrast-to-noise ratio.
d. contrast-to-noise ratio. ratio between the SNR values of adjacent tissues or structures
e. spatial resolution.
e. spatial resolution. ability to distinguish points as separate and distinct
f. scan time.
f. scan time. time needed to complete all data acquisition for a sequence
g. voxel.
g. voxel. 3 dimensional area of tissue that is imaged
h. rectangular FOV.
h. rectangular FOV. a method whereby outer phase encoding steps are omitted and the FOVin the phase encoding direction is smaller than the frequency encoding direction
define the following pulse sequence parameters and describe how a change in their values affects SNR in general:
a. TR. repetition time; increased TR = _____ SNR
define the following pulse sequence parameters and describe how a change in their values affects SNR in general:
a. TR. repetition time; increased TR = increased SNR
define the following pulse sequence parameters and describe how a change in their values affects SNR in general:
b. TE. echo time; increased TE = _____ SNR
define the following pulse sequence parameters and describe how a change in their values affects SNR in general:
b. TE. echo time; increased TE = decreased SNR
define the following pulse sequence parameters and describe how a change in their values affects SNR in general:
c. slice thickness. increased slice thickness= _____ SNR
define the following pulse sequence parameters and describe how a change in their values affects SNR in general:
c. slice thickness. increased slice thickness= increased SNR
define the following pulse sequence parameters and describe how a change in their values affects SNR in general:
d. FOV. the total area that is imaged; increased FOV = _____ SNR
define the following pulse sequence parameters and describe how a change in their values affects SNR in general:
d. FOV. the total area that is imaged; increased FOV = increased SNR
define the following pulse sequence parameters and describe how a change in their values affects SNR in general:
e. image matrix. number of phase and frequency encoding steps in an image, or relative number of pixels in an image;
finer matrix (more encoding steps, smaller pixels) = _____ SNR
coarser matrix (fewer encoding steps, larger pixels) = _____ SNR
define the following pulse sequence parameters and describe how a change in their values affects SNR in general:
e. image matrix. number of phase and frequency encoding steps in an image, or relative number of pixels in an image; finer matrix (more encoding steps, smaller pixels) = decreased SNR, coarser matrix (fewer encoding steps, larger pixels) = increased SNR
define the following pulse sequence parameters and describe how a change in their values affects SNR in general:
f. NSA. number of signal averages; increased NSA = _____ SNR
define the following pulse sequence parameters and describe how a change in their values affects SNR in general:
f. NSA. number of signal averages; increased NSA = increased SNR
- explain the effect that voxel volume has on SNR and why. the larger the voxel volume, the _____ signal can be obtained from it because a larger voxel has a higher spin density
- explain the effect that voxel volume has on SNR and why. the larger the voxel volume, the more signal can be obtained from it because a larger voxel has a higher spin density
- identify 3 sequence parameters that affect voxel volume.
- identify 3 sequence parameters that affect voxel volume.
FOV, slice thickness, matrix size
- identify 3 sequence parameters that affect image contrast and SNR.
- identify 3 sequence parameters that affect image contrast and SNR.
TR, TE, flip angle
- explain the relationship between voxel size and spatial resolution.
the larger th-e voxel size, the _____ the spatial resolution
- explain the relationship between voxel size and spatial resolution. the larger th-e voxel size, the lower the spatial resolution
- identify 3 sequence parameters that affect scan time and SNR.
- identify 3 sequence parameters that affect scan time and SNR.
TR, NSA, matrix size
- identify 2 sequence parameters that affect the number of slices available during each TR period.
- identify 2 sequence parameters that affect the number of slices available during each TR period.
TE, receive bandwidth
- identify 3 sequence parameters that affect spatial resolution and SNR.
- identify 3 sequence parameters that affect spatial resolution and SNR.
slice thickness, matrix size, FOV
- identify the amount by which the SNR is increased if the NSA is doubled.
- identify the amount by which the SNR is increased if the NSA is doubled.
doubling NSA results in an increase in SNR by the square root of 2 = 1.44 or 44%
- explain how a narrower RF receive bandwidth increases the SNR.
- explain how a narrower RF receive bandwidth increases the SNR.
a narrower (decreased) bandwidth allows less noise to be received relative to signal
- write and explain the formula for scan time for a spin echo pulse sequence.
scan time =
- write and explain the formula for scan time for a spin echo pulse sequence.
scan time = (TR) x (number of phase encoding steps) x ( NSA)
