Principles of the MRI Signal Contrast Mechanisms MR

ﻣﺤﺎﺿﺮﺍﺕ ﺍﻟﻔﻴﺰﻳﺎﺀ ﺍﻟﻄﺒﻴﺔ Principles of the MRI Signal Contrast Mechanisms MR Image Formation

Contrast Principles of the MRI Signal MR Image Formation Mechanisms ﺍﻟﻤﺤﺎﺿﺮﺓ ﺍﻟﺴﺎﺑﻌﺔ ﺳﻌﻴﺪ ﺳﻠﻤﺎﻥ ﻛﻤﻮﻥ. ﺍﻋﺪﺍﺩ ﺩ ﻛﻠﻴﺔ ﻣﺪﻳﻨﺔ ﺍﻟﻌﻠﻢ ﺍﻟﺠﺎﻣﻌﺔ ﻗﺴﻢ ﺍﻟﻔﻴﺰﻳﺎﺀ ﺍﻟﻄﺒﻴﺔ– ﺍﻟﻤﺮﺣﻠﺔ ﺍﻟﺮﺍﺑﻌﺔ John Van. Meter, Ph. D. Center for Functional and Molecular Imaging Georgetown University Medical Center

Gradients • A gradient is simply a deliberate change in the magnetic field • Gradients are used in MRI to linearly modify the magnetic field from one point in space to another • Gradients are applied along an axis (i. e. Gx along the x-axis, Gy along the y-axis, Gz along the z-axis) • What happens to the frequency at which the precess when we turn on a gradient?

Effect of Gradient on Rate of Precession = g. Bw B= B 0+ B 1 -r 0 1 2 3 4 5 6 7 8 9 +r

Effect of a Gradient

From Proton Signal to Pixel Intensities • Amplitude of the sinusoidal wave at a pixel used to determine the brightness of the pixel (i. e. color)

Signal from Multiple Pixels Pixel 1. . . Pixel n Net Signal at Coil +

Decomposing Received Signal • Left unchanged the signal received cannot be broken down by location of individual pixels • Need method for efficiently pulling out the signal from many pixels at once • Gradients used to relate where a particular signal is coming from

Frequency Encoding • Use a gradient to modify the rate at which the protons spin based on location of the proton • Requires the gradient to remain on

Uniform Field Col 1 Col 2 Col 3 Uniform Field Prior to Gradient

Lower Field Col 1 Col 2 Col 3 Higher Field Gradient Applied

Frequency Encoding • Apply gradient in one direction and leave it on • Result: ØProtons that experience a decrease in the net magnetic field precess slower ØProtons that experience an increase in the net magnetic field precess faster

Side-Effect of Gradient • Gradient also causes phase of the protons to change • Application of a second gradient of opposite polarity will undo this

Frequency Encode Gradient The area under the second gradient must be equal to that of the first gradient

Phase Encoding • Turn gradient on briefly then turn it off • Turning on the gradient will cause some protons to spin faster others to spin slower depending on where they are located • Turning off the gradient will make them all spin at the same rate again • BUT they will be out of ‘phase’ with one another based on where they are located

Phase Encoding

Uniform Field Row 1 Row 2 Row 3 Uniform Field Prior to Gradient

Lower Field Row 1 Row 2 Row 3 Higher Field Gradient Applied

Uniform Field Row 1 Row 2 Row 3 Uniform Field Gradient Turned Off