Basic MRI Chapter 1 Lecture

Introduction MRI uses radio waves and a magnetic field to make images MRI uses radio waves and a magnetic field to make images Other methods make images in other ways Other methods make images in other ways Radiography – x-ray attenuation Radiography – x-ray attenuation CT – x-ray with computer CT – x-ray with computer NM – gamma rays from radionuclides NM – gamma rays from radionuclides US – sound waves US – sound waves

MR Images The image is a display of the radiofrequency (RF) signal intensity The image is a display of the radiofrequency (RF) signal intensity The source of the RF signal from the patient is the “condition of magnetization produced when the patient is placed in the magnetic field.” The source of the RF signal from the patient is the “condition of magnetization produced when the patient is placed in the magnetic field.” Magnetization occurs when magnetic nuclei (like H nuclei or protons) are present Magnetization occurs when magnetic nuclei (like H nuclei or protons) are present Magnetization is changed during imaging, and the rate of change depends on tissue characteristics Magnetization is changed during imaging, and the rate of change depends on tissue characteristics

Tissue Characteristics PD PD T1 T1 T2 T2 Flow Flow Diffusion Diffusion Spectroscopy/chemical shift Spectroscopy/chemical shift

Major Tissue Characteristics PD– proton density PD– proton density T1– longitudinal relaxation time; spin-lattice relaxation time T1– longitudinal relaxation time; spin-lattice relaxation time T2– transverse relaxation time; spin-spin relaxation time T2– transverse relaxation time; spin-spin relaxation time

Minor Tissue Characteristics Flow Flow Diffusion Diffusion Spectroscopy/chemical shift Spectroscopy/chemical shift

What do you see on an MR image? RF signal intensity, influenced by RF signal intensity, influenced by Tissue magnetization, including saturation pulses Tissue magnetization, including saturation pulses Proton (hydrogen nuclei) density Proton (hydrogen nuclei) density Relaxation effects from T1 and T2 Relaxation effects from T1 and T2

Spatial Characteristics Slices Slices Voxels Voxels Pixels Pixels

Image Quality Detail/Resolution Detail/Resolution Noise/Signal-to-Noise ratio Noise/Signal-to-Noise ratio Artifacts Artifacts

In-Plane Resolution This is a photo that has been taken at 165x256 resolution

In-Plane Resolution This is a photo that has been taken with 329x512 resolution

In-Plane Resolution Original Resolution 720x1150 Original Resolution 720x1150

Signal-to-Noise

Signal-to-Noise

Signal-to-Noise

Signal-to-Noise

Aliasing: Example

Wraparound Artfacts in 3D

Chemical Shift Effect

Chemical Shift

Chemical Shift - Example

Chemical Shift Artifacts

Signal Truncation

Truncation Artifacts

Partial Volume Effect

Motion Artifacts - Periodic

Motion Artifacts - Random

Flow Motion Artifacts - CSF

Magic Angle Artifacts

FT of Realistic RF Signal

RF Zipper Artefact

RF Feed through Zipper Artifact

RF Noise

Magnetic Inhomogeneity Artifacts

Diamagnetic Susceptibility Artifact

Paramagnetic Effect of Deoxyhemoglobin

Ferromagnetic Susceptibility Artifacts

Susceptibility Artifacts

Gradient Non-linearity

Geometric Distortion: Gradient Non-Linearity