Rad T 265MRI Lecture

No Magnetic Field

= No NetMagnetization

Protons align with a magnetic field…

In a magnetic field, protons can take either high- or low-energy

states

The difference between the numbers of protons in the high-energy and low-energy states results in a net magnetization (M) and gives rise to the

Larmor Equation.

002 Bv

MRI Equipment

• Main Magnet

• Shim Magnet

• Gradient Coil

• RF system

Main Magnetic Field

▪Typically oriented to the long axis of the patient

▪Bo

Increase Bo

Homogeneity Precessional frequency Chemical shift

Type of Magnets - Resistive

▪Require constant electrical current

▪Max field is less than 0.3T

Type of Magnets - Permanent

▪Can be built in a variety of shapes and configurations

▪Tend to be heavy

▪Lower field strengths

▪Made of aluminum, nickel, and cobalt - alnico

Type of Magnets - Superconductor

▪Most common

▪Lowest electrical costs

▪Highest field strength

▪Use cryogens

Helium-450 F, -269 C, 4.2 K

Nitrogen-320 F, -160 C, 77.3 K

Shim CoilsIncrease homogeneity

▪Passive▸Steel plates attached to the magnet

▪Active▸Electromagnets with an opposite polarity

Gradient Fields

▪Measured in mT/m

▪Rise time▸1 ms for 0 to 10mT/m is good

Gradient Coils ▪Produce noise

▪They rattle in their mountings

▪Greatest stress is caused

by obliques

Faraday’s LawUsed for most MRI activities

▪Ramping the magnet

▪Shimming

▪RF field

▪Gradients

▪MR signal

RF Receiver Coil

▪Designed to detect transverse magnetization

▪Based on Faraday’s Law▸Variable magnetic fields produce an electric

current in a loop of wire

RF Shielding

▪Copper is preferred▸Expensive

▪Aluminum can be used

▪Problems with the RF shield produce zipper artifacts

Gauss

▪10,000 gauss = 1 tesla

▪Earth’s magnetic field is 0.5 g

Larmor Equation

▪Precession frequency is based on Bo

▪For a 1 T magnetic the precessional frequency is 42.6

Mhz

RF

▪Needs to be perpendicular to Bo

▪Needs to be at the precession frequencySpins are only in phase during RF pulsesWhen the pulse ends dephasing begins immediately

Inherent Tissue Characteristics

▪T1, T2, PD, flow, motion

▪We can only demonstrate these not change them

▪Pulse sequences are used to maximize differences in tissue characteristics

Weighting

• T 1

T 1 weighted images have a short TE and TR

Provide more anatomical info – better spatial resolution

• T 2T 2 weighted images have a long TE and TR

More pathologic info

MRI Contrast Agents

▪Types; Paramagnetic, Ferromagnetic

▪Administration

▪Reactions

▪Contraindications

Paramagnetics

▪Gadolinium▸Positive contrast▸Shortens T1 relaxation▸Appears brighter on the image▸Elimination half life 1 - 2 hrs

Ferromagnetics

▪Ferumoxides▸ Negative contrast▸ Shorten T2 relaxation▸ Appears darker on the image

SAR Dependent on

▪Duration of RF▸Flip angle and strength

▪Frequency▸Pulse sequence and strength

▪Patient Mass▸Weight

SAR LimitsIncrease core temp 1 C

▪Whole body▸ 0.4 W/kg

▪Head▸3.2 W/kg

▪Small Volume▸8.0 W/kg

Static Field Exposure

▪Whole body▸3T

▸ Extremities▸5T

High Field ExposurePossible effects

▪Magnetophosphenes

▪Nausea

▪Vertigo

▪Metallic taste

Fringe Field

▪Public is limited to 0.5 mT▸0.5 mT = 5 gauss

▪No pacemakers beyond this line

Noise Limitations

▪Earplugs are necessary above 100 db

▪Remember noise is related to gradient activity

▪Gradients are rattling in their supports

Quench

▪Uncontrolled release of cryogens

▪Helium and nitrogen replace oxygen

▪Asphyxiation

Non Compatible DevicesAbsolute contraindications

▪Cardiac pacemakers

▪Internal defibrillators

▪Biostimulators

▪Implanted infusion pumps

▪Cochlear implants

▪Metallic orbital FB

Non Compatible Devices Continued

Safe to image

▪Surgical hemostasis clips

▪Orthopedic prostheses

▪Dental work▸Except magnetic dentures

▪IUDs

▪Intra vascular coils

Wires

▪Important to remember that coiled wires will generate a current and that currents produce heat.

▪Faraday’s Law

MR Data AcquisitionFourier Transformation

▪Process that takes a complex signal and breaks it down into its component parts

Types of Pulse Sequences

▪SE, IR, STIR, GE

▪RARE, FLARE, FLAIR, FSE

▪EPI,

Spin Echo

▪Uses a 90 RF followed by a 180 RF

▪Traditionally the most popular sequence

▪Can provide T1 or T2 information

IR, STIR

▪Uses a 180 RF followed by a 90 RF and then a 180 RF

▪Provides heavy T1 weighting▪Can be used to minimize signal by varying the TI

time

Gradient Echo

▪Uses an initial RF pulse, usually less than 90

▪Rephases the spins by using a gradient instead of other RF pulses

FSE

▪Uses ETL▪ETL - obtain more than 1 echo per TR▪Different from regular ME because second echo and beyond

is used to fill the same k- space, not a new one

EPI

▪Similar to FSE

▪Difference is all the phase encoding steps are acquired during one TR

Length of sequence

▪SE, IR, traditional sequences

▪TR x NSA x #PE

TR Controls

▪T1 relaxation

▪Spin lattice

▪Longitudinal

TE Controls

▪T2

▪Spin spin

▪Transverse relaxation - dephasing

Affecting SNR

▪Slice gap

Increase slice gap, increase SNR, less

cross-talk

▪Slice thickness

Increase slice thickness, increase SNR,

more anatomy per slice = more signal

Also, increase partial volume and decrease

resolution

Affecting SNR

▪FoVIncrease FoV, increase SNR (more anatomy)

Decrease resolution

This is the same effect we discussed in CT

▪Increase matrixIncrease resolutionDecrease SNR, smaller pixels

Affecting SNR

▪Increasing TR increases SNR

Provides more relaxation

▪Decreasing TE increases SNRLess dephasing occurs

Types of Suppression

▪STIR; short tau inversion, suppresses fat

▪FLAIR; suppresses fluids, long T1 values

▪Heavy T2; long TE and TR, maximizes T2 values

▪Spectral fat suppression; based on freq difference between fat and water

GatingUsed to eliminate or minimize physiologic motion

▪Peripheral pulse

▪Respiratory

▪Cardiac

▪NOTE ALL INCREASE TR▸Or decrease slices

MR AngiographyTwo major techniques

▪Time of flight

▪Phase contrast

Time of Flight (ToF)Needs MIP

▪Uses slice entry phenomena

▪Can also be called▸Paradoxical enhancement▸Flow related enhancement

▪Evaluates vessel morphology

A. Safety issues surrounding static field.

• RF pulse determines “flip angle” Rotation determines amount

of magnetization measured

• Field strength determines resolution Increased magnetization

leads to increased signal

Pulse and Field Effects

cosMM Z

Images adapted from: http://www.mri.tju.edu/phys-web/1-T1_05_files/frame.htm

sinMM XY

Muscle

Tissue

Difference

B0= 0.2 T

B0 = 1.5 T

Short TR

Short TE

Long TE

Long TR

T1

T2

PD