Physics CH. 4&5

What is the formula for calculating effective TE?

(2xTR) - TE

What is the formula for voxel volume?

(STHxGAP) x FOVpe/Npe x FOVro/Nro

What mechanism rephases the dephasing hydrogen nuclei in a SE sequence?

180 degree RF pulse

Place the following SE pulse sequence events in the correct order 1. Signal measurement 2. 180 degree RF pulse 3. 90 degree RT pulse

3, 2 , 1

Reducing the FOV by a factor of 2 reduces the voxel volume by what factor?

4

Reducing the FOV by a factor of 2 will reduce the voxel volume by a factor of __?

4

How many coils should you use for reducing scan time to 1/4 of that of a single coil?

4 coils

how many times is the phase encoding gradient turned on to acquire each image for a SE pulse sequence using a TR of 2500ms, a TE of 35ms, a matrix of 256 by 256, a FOV of 24cm, and a NEX of 2?

512

Of the following parameters which would give the highest SNR? a. 512x512, 6mm STH, 16cm FOV, 4NEX b. 256x256, 4mm STH, 12cm FOV, 6NEX c. 256x128, 3mm STH, 10cm FOV, 10NEX

A

By increasing the TE from 80ms to 120ms on a SE pulse sequence, which of the following is expected? a. Fewer protons experience spin-spin interactions b. Better tissue contrast based on the tissues T2 characteristics c. A stronger signal in the MR echo d. Better tissue contrast based on tissues T1 characteristics

B

Of the following parameters which would give the highest Spatial Res? a. 512x512, 6mm STH, 16cm FOV, 4NEX b. 256x256, 4mm STH, 12cm FOV, 6NEX c. 256x128, 3mm STH, 10cm FOV, 10NEX

B

What percentage of SNR remains when the matrix size is reduced from 256 to 128? a. 66 b. 71 c. 75 d. 87

B

Changing the TR will directly affect which of the following? a. Slice thickness b. Orientation of the image plane c. Acquisition time d. FOV

C

Of the following parameters which would give the shortest scan time? a. 512x512, 6mm STH, 16cm FOV, 4NEX b. 256x256, 4mm STH, 12cm FOV, 6NEX c. 256x128, 3mm STH, 10cm FOV, 10NEX

C

What are the major differences between coherent and inchoherent gradient echo pulse sequences?

Coherent - Sampled from both FID&Echo Incoherent - Sampled from FID only

In which sequence is both the stimulated echo and FID signal sampled?

Coherent Gradient Echo

If 3 180 RF pulses are applied to generate 3 echos, which of the following is/are responsible for the thrid echo being smaller than the first?? a. spin-spin interaction b. magnetic susceptibility c. magnetic field inhomogeneities d. all of the above

D

Which of the following events would be appropriate to acquire a true T2 weighted image? a. 23 degree RF pulse followed by a 180 RF pulse b. 90 degree RF pulse followed by a gradient reversal c. 25 degree RF pulse followed by a gradient reversal d. 90 degree RF pulse followed by a 180 degree RF pulse

D

Which of the parameters below will affect SNR? a. TR, TE, Flip Angle b. PD of the FOV c. Voxel Volume d. All of the above

D

List 3 paramters that can inrease spatial res?

Decrease FOV, STH Increase matrix

Increasing the number of phase encodings will produce and image with ___ voxel volume?

Decreased

What is the EPI?

Echo Planar Imaging

The SNR will increase in a 3D sequence with an increase in __?

FOV and # of slices

SNR increases in a 3D sequence when which other parameter is increased?

FOV and Slab Thickness

A short T1 recovery (STIR) sequence can suppress the signal from?

Fat and Gad

What function does the initial 180 degree RF pulse have in an IR pulse sequence?

Flips the net magnetization into the negative longitudinal plane for greater T1 relaxation range

Due to the decreased efficiency in refocusing spins, this pulse sequence is extremely sensitive to magnetic field homogeneity?

GRE

Which pulse sequence is most "flow sensitive"?

GRE

How does spatial resolution affect the minimum TE?

High res = High TE (steep slope increase the min TE)

What is the parallel imaging?

I-PAT

The effective TE in a fast spin echo pulse sequence determine the ?

Image contrast

Doubling the number of NSA will ___ SNR?

Increase

Increasing the TR will have what affect on the image?

Increase T2 weighting and SNR Derease T1 weighting

Using a conventional spin echo multislice sequence, the number of slice allowed when increasing the TR will ___?

Increase by a factor of TR/TE

Decreasing the Reciever Bandwidth ___ SNR?

Increases

The time during which the frequency encoding gradient is on ____ with reduction in reciever bandwidth?

Increases

List 3 parameters that can increase SNR ?

Increasing TR, NEX, STH

What is the term for the time from the excitation pulse to the refocusing pulse and from the refocusing pulse to the echo?

Interpulse time

What are the additional RF pulses called that are used to saturate out hydrogen nuclei prior to excitation?

Presaturation

SNR increases as which of the following occur? a. Receiver bandwidth decreases b. Reveiver bandwidth increases c. Sampling time increases

Receiver bandwidth decreases

What is a way to reduce the scan time without directly affecting image contrast?

Reduce the Npe

In a GRE sequence, if the flip angle is lowered while holding the TR constant, what would be reduced?

Saturation

If six successive 180RF pulses are applied, following a 90RF pulse, how many echos are formed?

Six

What does SSFP stand for? , which signal can be obtained by using SSFP?

Stead State Free Precession SE

The peak signal strength of a spin echo is less than the inital strength of the FID b/c of ?

T2 relaxation

An image acquired using a TR of 2500ms and a TE of 90ms produces what type of image contrast?

T2 weighted

In a conversional spin-echo multi-echo sequence, it is possible to create multiple images, each with different amount of ___?

T2 weighting

The time from the inital RF pulse to the middle of the readout window is called?

TE

Which of the following parameters control the amount of contrast seen in an image due to T2 relaxation?

TE

In an inversion recovery pulse sequence, the time between the initializing 180 pulse and the 90 pulse is known as?

TI

Which of the following parameters control the amount of contrast seen in an image due to T1 relaxations?

TR

In a 2D conversional spin echo multislice pulse sequence, scan time is given by the equation ___?

TR x NEX x # of phase encodings

What is the correct formula for determining the total scan time, in minutes, for a 3DFT image?

TR(sec) x Npe x NEX x Nslices/60(sec)

In an inversion recovery pulse sequence, image contrast is controlled by?

TR, TE, and TI

Another name for TI is?

Tau

Which pulse sequence would you choose for each of the following and why? a. T1 weighting breath hold b. Steady State T2* c. True T2 weighting volume

a. FSE b. GE c. SE

Increasing the matrix in the frequency direction from 256 to 512 will ___ scan time?

have no effect

List 3 parameters that can increase CNR?

use of contrast, T2 weighting, use of MTC