Exam 2 review questions

How to determine weighting through a sequence diagram

T1 would have short TR and TE. T2 would have long TR and TE. PD would have long TR and short TE.

What is the name of the signal produced by the 90 degree pulse in a sequence diagram?

FID

What is the formula needed to calculate scan time of fast spin echo?

TR x #PE x NEX / ETL

How do you convert scan time into minutes/seconds?

Divide by 1,000 to get to seconds. Then divide by 60 seconds for minytes.

How many RF pulses is there in a echo trail length?

Multiple 180 pulses but equals ETL + 1

How many TR periods are needed for FSSE?

TR periods = # phase encoding / ETL

What determines the depth dimension of a voxel?

Slice thickness

What is the equation to determine the dimension of a pixel?

FOV / Matrix

What is the name of voxels that are equal dimension in all planes?

Isotropic voxels

What is the name of a voxel that is NOT equal in all planes?

Anisotropic voxels

Field of view (FOV)

Total area of the body to be scanned

Matrix

Grid of pixels. ( # frequency x # phase encoding steps)

Slice thickness

The “depth” dimension of a voxel

NEX

Number of signal averages (# times data are collected at same PE step)

SNR

Signal to noise ratio. The higher the SNR, the worse the resolution.

What is the term for the difference in SNR between two areas?

Contrast to noise ratio (CNR)

A narrow bandit would increase or decrease SNR?

Increase

A narrow bandit would increase or decrease chemical shift?

Increase

4 factors affected by SNR in coils are….

The size of the coil, proximity of coil to target area, # of coil elements, position and alignment of coil

What are the 3 benefits of 3D imaging?

No gap, better SNR, better resolution, isotropic voxels

What is the main drawback of 3D imaging?

Scan time

3 functions of a gradient coils are….

Slice select, phase encoding, frequency encoding

What is the formula needed to calculate scan time of gradient echo? (2D)

TR x PE x NEX

What is the formula needed to calculate scan time of 3D gradient echo?(3D)

TR x PE x NEX x Slices

NEX x 2 =

√2

NEX x 4 =

double SNR

NEX x 9 =

triple SNR

If TR increases…

SNR increases and scan time increases

If TR decreases….

SNR decreases and scan time decreases

If TE increases….

SNR decreases

If TE decreases….

SNR increases

If NEX increases

SNR increases and scan time increases

If NEX decreases

SNR decreases and scan time decreases

If slice thickness increases

SNR increases and resolution decreases

If slice thickness decreases

SNR decreases and resolution increases

If Pixel/voxel size increase

SNR increases and resolution decreases

If pixel/voxel size decrease

SNR decrease and resolution increases

If FOV increases

SNR increases and resolution decreases

If FOV decreases

SNR decreases and resolution increases

If Matrix increases

SNR decreases and resolution increases

If Matrix decreases

SNR increases and resolution decreases