University:
University of Michigan
Course:
PHYSICS 250 | Fundamental Physics for the Life Sciences II
Academic year:

2021
Views:

127

Pages:

14
Author:

J Clay

MRI Physics: Magnetic Field Gradients Pulse Sequences • Contrast (Spin Preparation) What kind of contrast does the image have? What is the TR, TE, Flip Angle, etc.? Gradient echo/spin echo/etc. • Localization (Image Acquisition) How is the image acquired? How is “k-space” sampled? Spatial Resolution? Field-of-View? Goals of Image Acquisition • • • • • • Acquire 2D (or 3D) Fourier data Acquire samples finely enough to prevent aliasing (FOV) Acquire enough samples for the desired spatial resolution (Dx) Acquire images with the right contrast Do it fast as possible Do it without distortions and other artifacts Magnetic Fields in MRI B0 – The main magnetic field Always on (0.5-7 T) Magnetizes the object to be imaged After excitation, the magnetization precesses around B0 at w0 = gB0 B1 – The rotating RF magnetic field Tips magnetization into transverse plane Used for “excitation” On for brief periods, then off Magnetic Fields in MRI The last magnetic field to be used in MRI are the gradient fields 3 of them: Gx, Gy, Gz Used primarily for spatial localization Make the magnetic field different in different parts of the body, e.g. for the x-gradient: B(x) = B0 + Gx.x Magnetic Field Gradients z x y z x x-gradient (Gx) z x y z-gradient (Gz) y y-gradient (Gy) Precession 𝜔 = 𝛾 $ 𝐵! Larmor Frequency Angular Frequency Main Magnetic Field Gyromagnetic Ratio B0=1.5T à w = 64 MHz à 64 million rounds/sec B0=3.0T à w = 128 MHz à 128 million rounds/sec Stronger Magnetic Field = Faster precession Precession Frequency can be altered with magnetic field gradients 𝐵(𝑥) = 𝐵! + Gx x 𝜔 =𝛾$𝐵 𝜔(𝑥) = 𝛾𝐵(𝑥) = 𝛾(𝐵! + Gx x) Precession Frequency can be altered with magnetic field gradients 𝐵(𝑥) = 𝐵! + 𝐺" $ 𝑥 𝜔(𝑥) = 𝛾𝐵(𝑥) = 𝛾(𝐵! + 𝐺" $ 𝑥) x=0 x=X Gx 1D “Image” B=B0 BX=B0+GxX w= w0 wX= w0+gGxX x 0 w0 freq X wX= w0+gGxX The Challenge Pulse Sequence (with Gradients) Data Collection x 0 w0 X wX= w0+gGxX freq ???? time Fourier Transform time w0 wX= w0+gGxX The Challenge Pulse Sequence (with Gradients) Data Collection x 0 w0 X wX= w0+gGxX freq Fourier Transform!! time Signal Length à Freq Resolution Few Freq (w) Components can be separated Short Signal (t) Fourier Transform Signal Length à Freq Resolution Many Freq (w) Components can be separated Long Signal (t) Fourier Transform

MRI Physics - Magnetic Field Gradients

Related Documents

Recommended Documents

Description

Related Documents

Free up your schedule!

Take 5 seconds to unlock