Parameters Study Guide

Parameters affecting Image Contrast

TR, TE, TI, Flip Angle, ETL

Repetition time (TR) describes

regrowth of longitudinal magnetization

Longer TR values ____ overall SNR in the image due to

increase

increased restoration of the Net Magnetization Vector (NMV)

TR values affect what contrast

T1

shorter TR values affect what contrast

maximize T1 weighting, highlighting tissues with short T1 relaxation times (fat, Gadolinium enhancement)

Longer TR values _____ scan time

increase
1. TR x Phase Matrix x NSA/NEX – spin echo
2. TR x Ph. Matrix x NEX ÷ETL – fast spin echo
3. TR x Ph. Matrix x NEX x #slices – 3D scan time

TR and slices

increase for more

TR and anatomical coverage

increase for more

Adding presaturation / fat suppression / increased ETL / increased TE all require

longer TR

Use ____ TR values to reduce SAR

longer

why would you be careful in T1 sequences with SAR

longer TR values reduce T1w

Echo Time (TE)

Timing of the application of the variable (gradient echo) or 90° (spin echo) RF excitation pulse to the peak signal for echo induced in the coil
Describes the decay of transverse magnetization

longer TE values result in _____ SNR due to

lower
increased decay of transverse magnetization detected in the receiver coil

TE impacts what image weighting

Impacts T2 weighting – Longer TE values result in increased T2 contrast (brighter fluid signal)

Increases in TE result in _____ scan times, and/or ______ # available slices

longer (lengthens TR) 
less # available slices

½ TE

time TAU

time TAU in GRE

= to half the total TE value

time TAU in SE occurs when

occurs at the position of the 180° refocusing RF pulse, which is at the timepoint of half the TE

Decreases in TE will ____ susceptibility artifact, because

reduce
longer TE values permit more dephasing between tissues with susceptibility differences.

____ receiver bandwidth (rBw) to allow shorter TE ranges

wider

Inversion Time (TI)

Describes the time from the 180° inverting pulse to the 90° excitation RF pulse

Short TI times null signal from

short T1 tissues (fat) = STIR sequence

Long TI times null signal from

long TI tissues (fluid, CSF) = FLAIR sequence

Longer TI times ____ overall SNR, _____ scan time bc

decrease
increase (longer TR required)

TI Nulling relative to field strength – timed at

@ 69% of the T1 relaxation time for the specific tissue to be nulled.

Longer TI times required for _____ because

higher field strengths (longer relaxation times).

STIR sequence TI range @ 1.5 Tesla________to null signal from all fat tissues

= 140- 160ms

STIR sequence TI range @ 3.0 Tesla ______ to null signal from all fat tissues

= 200- 210 ms

can you run a STIR after contrast. why?

T1-nulling effect of STIR will negate any signal from enhancing tissues due to the T1-shortening effect of the Gadolinium

Flip angle controls

the degree of separation from longitudinal magnetization following RF excitation

flip angle and image weighting

longer flip angles = T1 weighting
shorter flip angles = T2* weighting

longer flip angles = ___ weighting

T1

shorter flip angles = ___ weighting

T2*

T2* images good for

hemorrhage / bleed visualization

Ernst angle

maximum SNR achieved with optimal flip angle

_____ flip angles create more saturation

larger

Partial saturation – FA

>90 degrees

Full saturation – FA

180 degrees

shorter flip angles create

more susceptibility and rapid T2* decay

Echo Train Length (ETL)

describes the # of additional 180° RF refocusing pulses used to fill rows of kspace per TR period

ETL and scan time

inversely proportional rate

ETL and SNR

inversely proportional rate

ETL and artifacts

potential for blurring

ETL and contrast why

more T2 weighting (moves effective TE to later echo, resulting in k-space centering @ longer TE value)

Parameters affecting Imaging Voxel / Pixel Size

FOV, Phase Matrix, Slice Thickness

Field of View (FOV) and SR

inversely proportional

Field of View (FOV) and SNR

directly proportional

doubling FOV and SNR

4x SNR bc (double phase pixel size and double frequency pixel size)

small FOV optimally acquired with

smaller receiver coil, targeting SNR to region of interest

Rectangular FOV

shortens the dimensions of the acquired field in the phase encoding dimension only, accelerating overall scan time but increasing the potential for aliasing artifacts

rectangualr FOV potential artifact

aliasing

Use a _____ rBw if a smaller FOV is required clinically

narrow

Increased Phase Matrix = _____ pixel size = ___ spatial resolution

decreased
improved

phase matrix and scan time

direct
(TR x Phase Matrix x NEX)

______ phase gradient slope = larger phase matrix = ______ SNR = ______ scan time

steeper
decreased
increased

Increases in ETL/TSE factor

lessen workload of phase encoding gradient, resulting in accelerated scan times

Decreased phase matrix = ___ scan time but

faster
increased potential for Gibbs truncation / ringing artifacts

Slice thickness and SNR why

directly proportional (more protons detected per voxel0

Slice thickness and SR

inversely proportional (Thinner slab = more detail)

what is used to  achieve thin slice thickness

steep slope of Slice Select gradient

Thicker slices lead to what artifact

increased partial volume
more tissues in slice than intended)

Thicker slice thickness = _____ anatomic coverage

increased

Anatomic coverage equation

(Slice thickness + Slice Gap) x # slices

Parameters increasing Signal-to-Noise Ratio (SNR)

]Increased NSA / NEX, Decreased TE, Decreased ETL, Decreased Parallel Imaging factor, Decreased Phase Matrix, Increased FOV, Decreased rBW

what does NSA do for noise

Averaging out inherent noise
SNR increased by the √NSA/NEX increase

examples of math when increasing NEX for SNR

1. 2 NEX 4NEX = SNR increase ~40%

a. √2 = 1.41 or approx. 40%

b. 2x increase in NEX averages out 40%

noise in the image

2. 2 NEX 6NEX = SNR increase ~70%

a. √3 = 1.70 or approx. 70%

b. 3x increase in NEX averages out 70%

noise in the image

____ TR increased SNR why

increased
Longer time for regrowth of longitudinal magnetization results in improved SNR

_____ TE increases SNR why

Decreased
Less decay of transverse magnetization results in better overall SNR

At the expense of ______, _____ ETL’s result in better SNR

longer scan time
shorter

_______ in the parallel imaging reduction factor result in________ with improved SNR

decreases
longer scan times

______ in the phase matrix result in _____ but with improved SNR

decreases
less detail

________ FOV results in _______ with more signal but _______

increased
larger pixel sizes
decreased SR

_______ in the receiver bandwidth result in _______ but improved SNR

Decreases
longer sampling time

Parameters / Imaging Options for Acceleration

Reduced NSA/NEX

Reduced matrix

Halfscan and partial echo

Rectangular FOV

Parallel Imaging

Compressed SENSE (Compressed SENSING / SMS)

Flip angle modulation / refocusing control

Wider receiver bandwidth

Single shot techniques

Rapid Gradient Echo

Parallel Imaging and what options it has

Using phased array coils to fill lines of k-space
K-space based or Image based options

2D Parallel Imaging

2D only accelerates Phase dimension acquisition time

3D Parallel Imaging

allows multi-directional (phase and slice direction) acceleration

Compressed SENSE (Compressed SENSING / SMS)

Randomization of k-space filling to accelerate scan time Filtering options to retain / eliminate image noise

Rapid Gradient Echo consists of

T2* -- Steady State Sequences

T1* -- SPGR or Spoiled Gradient Echo

T1* -- SPGR or Spoiled Gradient Echo

Spoil away residual transverse magnetization through long TR, spoiler gradients, or RF spoiling

Parameters reducing SAR

Longer TR

Less slices and/or removal of fat suppression / presaturation pulses

Refocusing control / flip angle modulation

Reduced ETL

Parallel imaging

Low SAR sequences (anti-fast)

Optimal positioning pads / bore pads

Transmit/Receive coil

Parameters/Options Compensating for Motion

MultiVane / Blade / Propeller

Respiratory trigger / navigator

Optimal breath hold

Reduced NSA / NEX

Optimal coil selection

Selection of optimal phase direction

Cardiac / PPU triggering

Single shot techniques