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 ﺍﻟﺘﺎﺳﻌﺔ 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

Images From k-space • K-space is turned into an image using a Fourier Transformation 2 D-IFT

Center of k-space 2 D-IFT

Everything Else 2 D-IFT

Full Frequency – Half Phase 2 D-IFT

Selecting a Slice • Again use gradient to modify frequency of the proton’s spin • Slice select gradient is positive on one side of the slice and negative on the other side • At the desired slice location the slice select gradient is zero • Thus, protons in this slice and only this slice will be spinning at the center frequency of the scanner! • If this gradient is on when we apply RF pulse only protons in the slice will be tipped into x-y plane and thus measurable

Slice Select Gradient

Slice Thickness vs Gradient Strength

Slice Orientation

Putting it All Together • Basic Pulse Sequence Diagram

EPI pulse sequence and kspace trajectory

Signal loss due to susceptibility artifacts in GRE EPI images

Magnetic Susceptibility Greater on T 2* than T 2 Images Spin Gradient Echo (T 2)Echo (T 2*) Oxygenated Hemoglobin Deoxygenated Hemoglobin

Effects of field variation upon EPI images

Effects of field variation upon EPI images

Spiral imaging

Susceptibility artifacts in spiral images

Effects of field variation on spiral images

Effects of field variation on spiral images

Acquisition Matrix Size 64 x 64 Matrix 64 x 128 Matrix Isotropic (square) Anisotropic (oblong) Isotropic (square) Relative SNR = 1 Relative SNR = 0. 5 Relative SNR = 0. 25