image contrast

Question 1

Why is contrast needed?

Answer 1

To demonstrate the difference between anatomy and pathology also between normal anatomical structures.

MRI has excellent soft tissue discrimination.

MRI contrasts depend on many variables and the mechanisms of these variables will be discussed.

Question 2

Image Contrast 2 parts

Answer 2

Intrinsic imaging parameters (those you cannot change and they are inherent to the body’s tissues).
Extrinsic contrast parameters (those you can manipulate).

Question 3

Intrinsic parameters

Answer 3

T1 recovery
T2 decay
Proton density
Flow
ADC (apparent diffusion coefficient) measures the magnitude of diffusion (of water molecules) within cerebral tissue.

Question 4

Extrinsic parameters:

Answer 4

TR (repetition time)
TE (echo time)
FA (flip angle) 
TI ( Inversion time)
Echo train length/turbo factor
b-value (diffusion sensitivity is controlled by modification of the duration and amplitude of the diffusion gradient)

Question 5

MRI Images Obtain Contrast Mainly From

Answer 5

From T1 Recovery
T2 Decay and
Proton Density (PD)

Question 6

On T1 images

Answer 6

Fat is bright and water dark

Question 7

On T2 images:

Answer 7

Water is bright and fat darker

Question 8

Saturation

Answer 8

The NMV is only partially saturated when it is pushed beyond 900.

It is fully saturated when it is pushed to a full 1800.

T1 weighting occurs when there is partial saturation of the fat and water vectors.

Proton density weighting of occurs when there is no saturation of the fat and water vectors.

Let’s go back and look at T1 recovery.

Question 9

Summary

Answer 9

TR controls T1 weighting.
A short TR maximizes T1 weighting.

TE controls T2 weighting.
A long TE maximizes T2 weighting.

Long TR maximizes PD weighting.

Spin echo sequences produce either T1, T2 of PD weighted images.