Basic MRIBasic MRI

Chapter 1 LectureChapter 1 Lecture

IntroductionIntroduction

MRI uses radio waves and a MRI uses radio waves and a magnetic field to make imagesmagnetic field to make images

Other methods make images in other Other methods make images in other waysways Radiography – x-ray attenuationRadiography – x-ray attenuation CT – x-ray with computerCT – x-ray with computer NM – gamma rays from radionuclidesNM – gamma rays from radionuclides US – sound wavesUS – sound waves

MR ImagesMR Images

The image is a display of the radiofrequency The image is a display of the radiofrequency (RF) signal intensity(RF) signal intensity

The source of the RF signal from the patient is The source of the RF signal from the patient is the “condition of magnetization produced the “condition of magnetization produced when the patient is placed in the magnetic when the patient is placed in the magnetic field.”field.”

Magnetization occurs when magnetic nuclei Magnetization occurs when magnetic nuclei (like H nuclei or protons) are present(like H nuclei or protons) are present

Magnetization is changed during imaging, Magnetization is changed during imaging, and the rate of change depends on tissue and the rate of change depends on tissue characteristicscharacteristics

Tissue CharacteristicsTissue Characteristics

PDPD T1T1 T2T2 FlowFlow DiffusionDiffusion Spectroscopy/chemical shiftSpectroscopy/chemical shift

Major Tissue Major Tissue CharacteristicsCharacteristics

PD– proton densityPD– proton density T1– longitudinal relaxation time; T1– longitudinal relaxation time;

spin-lattice relaxation timespin-lattice relaxation time T2– transverse relaxation time; spin-T2– transverse relaxation time; spin-

spin relaxation timespin relaxation time

Minor Tissue Minor Tissue CharacteristicsCharacteristics

FlowFlow DiffusionDiffusion Spectroscopy/chemical shiftSpectroscopy/chemical shift

What do you see on an MR What do you see on an MR image?image?

RF signal intensity, influenced byRF signal intensity, influenced by Tissue magnetization, including Tissue magnetization, including

saturation pulsessaturation pulses Proton (hydrogen nuclei) densityProton (hydrogen nuclei) density Relaxation effects from T1 and T2Relaxation effects from T1 and T2

Spatial CharacteristicsSpatial Characteristics

SlicesSlices VoxelsVoxels PixelsPixels

Image QualityImage Quality

Detail/ResolutionDetail/Resolution Noise/Signal-to-Noise ratioNoise/Signal-to-Noise ratio ArtifactsArtifacts

In-Plane ResolutionIn-Plane Resolution

This is a photo that This is a photo that has been taken at has been taken at 165x256 resolution165x256 resolution

In-Plane ResolutionIn-Plane Resolution

This is a photo that This is a photo that has been taken has been taken with 329x512 with 329x512 resolutionresolution

In-Plane ResolutionIn-Plane Resolution

Original Resolution Original Resolution 720x1150720x1150

Signal-to-NoiseSignal-to-Noise

Signal-to-NoiseSignal-to-Noise

Signal-to-NoiseSignal-to-Noise

Signal-to-NoiseSignal-to-Noise

Aliasing: ExampleAliasing: Example

Wraparound Artfacts in 3DWraparound Artfacts in 3D

Chemical Shift EffectChemical Shift Effect

Chemical Shift Chemical Shift

Chemical Shift - ExampleChemical Shift - Example

Chemical Shift ArtifactsChemical Shift Artifacts

Signal TruncationSignal Truncation

Truncation ArtifactsTruncation Artifacts

Partial Volume EffectPartial Volume Effect

Motion Artifacts - PeriodicMotion Artifacts - Periodic

Motion Artifacts - PeriodicMotion Artifacts - Periodic

Motion Artifacts - RandomMotion Artifacts - Random

Flow Motion Artifacts - CSFFlow Motion Artifacts - CSF

Magic Angle ArtifactsMagic Angle Artifacts

FT of Realistic RF SignalFT of Realistic RF Signal

RF Zipper ArtefactRF Zipper Artefact

RF Feed through Zipper RF Feed through Zipper ArtifactArtifact

RF NoiseRF Noise

Magnetic Inhomogeneity Magnetic Inhomogeneity ArtifactsArtifacts

Diamagnetic Susceptibility Diamagnetic Susceptibility ArtifactArtifact

Paramagnetic Effect of Paramagnetic Effect of DeoxyhemoglobinDeoxyhemoglobin

Ferromagnetic Ferromagnetic Susceptibility ArtifactsSusceptibility Artifacts

Susceptibility ArtifactsSusceptibility Artifacts

Gradient Non-linearityGradient Non-linearity

Geometric Distortion: Geometric Distortion: Gradient Non-LinearityGradient Non-Linearity