MRI INSTRUMENTATION

CURRENT + A LONG STRAIGHT WIRE EQUALS A

MAGNETIC FIELD

this is higher at higher field strengths

SNR

Z gradient

Superior to Inferior (head to toe)

Magnetic field strength is expressed in units of

Tesla (T) or Gauss (G) 10,000 G = 1 T

slew rate is expressed in units of

Tesla/meter/second (T/m/sec)

the spin excess is the

relevant proton density

The earth magnetic field strength is approx

0.5 G

The majority of Superconducting magnets range between

0.5 to 1.5 Tesla

typical slew rates for a gradient systems range between

10 T/m/sec to over 100 T/m/sec

1 TESLA IS = TO

10,000 GAUSS

most MR systems have gradient subsystems capable of achieving maximum amplitudes of

20 mT/m or greater

Superconducting magnets are limited by the FDA to

4 tesla in the U.S.

if B0 is 1.0 T, then the precession is equal to 1.0T x 42.6 MHz/T

42.6 MHz

THIS LINE HAS TO STAY WITHIN THE MRI ROOM

5 GAUSS LINE

typical rise time ranges between

500 usec to around 120 usec

Permanent magnets are very sensitive to temperature, and function optimally at

70 F +/- 2 F

CURRENTLY WHAT IS THE FDA LIMITS FOR ADULTS AND INFANTS IN CLINICAL IMAGING

8 TESLA

ADVANTAGES AND DISADVANTAGES OF SUPERCONDUCTING.

A - LOW POWER CONSUMPTION HIGH MAGNETIC FIELD STRENGTH D - LIMITED SPACE(INSIDE) LARGE FRINGE FIELD CRYOGENS MAINTENANCE REQUIRED

ADVANTAGES AND DISADVANTAGES OF PERMANENT MAGNETS

A = LOW POWER CONSUMPTION LOW FRINGE FIELDS D = CANNOT BE TURNED OFF LOW MAGNETIC FIELD STRENGTH

ADVANTAGES AND DISADVANTAGES OF RESISTIVE MAGNETS

A = TURN OFF MAGNETIC FIELD QUICKLY LOW FRINGE FIELD D = HIGH POWER CONSUMPTION LOW MAGNETIC FIELD STRENGTH

WHERE ARE THE MAGNETIC POLE PIECES LOCATED

ABOVE AND BELOW THE PATIENT

USES A SUPERCONDUCTING COIL LOCATED AT EACH END OT THE MAIN MAGNET INSIDE THE CRYOSTAT, THIS IS KNOWN AS WHAT TYPE OF SHIELDING

ACTIVE SHIELDING

STRENGTH OF THE MAGNETIC FIELD EQUALS THE

AMOUNT OF CURRENT PASSED THROUGH THE WIRE

Y gradient

Anterior to Posterior

this field is used to provide the necessary energy needed to cause the net magnetization of the tissues to tip and rotate through the transverse plane

B1

THESE SUBSTANCES/MATERIALS ARE SLIGHTLY REPELLED BY A MAGNETIC FIELD WHEN PLACED IN AN EXTERNAL APPLIED MAGNETIC FIELD. D = DO NOT FLY INTO THE MAGNET

DIAMAGNETIC MATERIALS

A magnet is known as a

Dipole

THESE SUBSTANCES/MATERIALS HAVE A POSITIVE INCREASE IN THEIR MAGNETIC FIELD WHEN PLACE IN AN EXTERNAL APPLIED MAGNETIC FIELD. THEY HAVE POTENTIAL TO BECOME PROJECTILES. F = FLY INTO THE MAGNET

FERROMAGNETIC MATERIALS

during relaxation a signal voltage known as what is induced in the receiver coil

FID

A MAGNETIC FIELD THAT EXERTS A FORCE OF MAGNETISM AND MAGNETIZABLE PARTICLES. THE EFFECT OF THIS FORCE LINES IS SYMBOLIZED BY MAGNETIC FIELD LINES.

FRINGE FIELD

The basic principle on which MR signals are detected

Faradays Law of Induction

exposing magnetized protons to an RF pulse causes

First, spins precess in phase, then low energy spins enter the high energy state and as a result the NMV moves into the transverse plane. The hydrogen spins to precess in phase which causes the NMV to pecess as well. Next, as the RF pulse continues, some of the spins in the low energy state absorb energy from the RF field and move to the high energy state. as more and more spins absorb energy, changing spins states, the NMV begins to tip outward away from teh longitudinal axix, or z axis. As a result, the NMV will now be precessing through the xy or transvers plane, 90 degrees away from its original orientation in the z axis. In this case, the RF pulse transferred the NMV from a position along the z axis into the transverse plane, or 90 degrees. Hence the 90 degree RF pulse

Where are the RF and gradient coils placed on a vertical magnet

Flat coils that are located on the face of the magnet

UNITS TO MEASURE FIELD STRENGTH

GAUSS (G) TESLA (T)

USED TO SPATIALLY ENCODE MR SIGNAL

GRADIENTS

This design is utilize to scan the cervical spine and neck

Helmholtz pair

WHAT TYPE OF COOLANT IS USED TO COLL A SUPERCONDUCTIVE MAGNET

LIQUID HELIUM/NITROGEN

the frequency at which hydrogen proton precess is known as the

Larmour Frequency

Superconducting magnets are immersed in what to cool the wires and remove the resistance

Liquid helium (cryogen)

THIS TYPE OF SHIELDING IS REQUIRED IN THE ROOM TO KEEP THE 5 GAUSS LINE INSIDE THE ROOM

MAGNETIC ROOM SHIELDING

THIS SHIELDING IS USUALLY MADE OUT OF STEEL, AND SURROUNDS THE ENTIRE ROOM. EVEN THE DOOR AND WINDOWS OF THE MAGNET ROOM IS SHIELDED.

MAGNETIC SHIELDING

A FUNDAMENTAL PROPERTY OF MATTER, THERFORE, ALL SUBSTANCES/MATERIALS HAVE SOME FORM OF THIS TO A VARYING DEGREE

MAGNETISM

A TYPE OF ROCK THAT EXHIBIT MAGNETIC PROPERTIES

MAGNETITE

IN ORDER TO OBTAIN AN MR SIGNAL FROM TISSUES, A LARGE STATIC MAGNETIC FIELD AND RADIOFREQUENCY FIELD ARE REQUIRED. THE MAIN PURPOSE OF THE STATIC MAGNETIC FIELD KNOWN AS B0 IS TO.

MAGNETIZE TISSUE

KNOWN AS THE PRIMARY MAGNETIC FIELD

MAIN MAGNETIC FIELD/STATIC FIELD (Bo)

the gyromagnetic ratio is measured in

MHz/T

nuclei with an uneven mass number (uneven number of protons and neutrons) are

MR active

The direction and strength of the current determines the direction and strength of the

Magnetic field

How are vertical field magnets placed and they have a reduced

Magnets are placed above and below the patient and they have a reduced fringe field. Open design is less confining

What is Faradays law of induction

Magnets or magnetic fields can be used to induce a current in electrical conductors

What is Passive magnetic shielding

Metal in the scan room walls to confine the fringe field

Faradays Law of induction.

Moving a magnet or changing its magnetic field (^B) over time (^t) in the presence of a condutor will induce a voltage (^V). ^B / ^t = ^V

WHAT MATERIALS ARE SUPER CONDUCTORS THAT HAVE NO ELECTRIC RESISTIVITY AT VERY LOW TEMPERATURES ( 0 KELVIN - 273 DEG C)

NIOBIUM/TITANIUM

Most MR magnets are made of

Niobium-titanium alloy that loses resistance at 4 deg kelvin

What is Active magnetic shielding

Other magnets and their associated magnetic field to confine the fringe field

THESE SUBSTANCES/MATERIALS HAVE A SLIGHT INCREASE IN THEIR MAGNETIC FIELD, WHEN PLACED IN AN EXTERNAL APPLIED MAGNETIC FILED. P = PARTIAL ATTRACTION TO THE MAGNET

PARAMAGNETIC MATERIALS

THE MAGNET ROOM HAS A SURROUNDING LINING (STEEL) IN THE MAGNET OR ROOM, THIS IS KNOWN AS WHAT TYPE OF SHIELDING

PASSIVE SHIELDING

CONSISTS OF LARGE BLOCKS MADE FROM FERROMAGNETIC ALLOYS SUBSTANCES THAT RETAIN THEIR MAGNETIC PROPERTIES AFTER BEING EXPOSED TO A MAGNETIC FIELD

PERMANENT MAGNET

MAIN MAGNETIC FIELDS ARE

PERPENDICULAR TO THE PATIENT

Vertical field systems most often use what type of magnet

Permanent, which have field strength of 0.06 to 0.35 T

these coils allow for increased area of coverage without the reduction in SNR

Phase array coils

refers to rather the gradient magnetic field is creating a field greater than or less than the frequency of B0

Polarity

the NMV represents the spin excess and is also known as

Proton density

THIS IS KNOWN AS THE THE SECONDARY MAGNETIC FIELD OR B1

RADIO FREQUENCY FIELD (RF FIELD)

THESE ARE CONSIDERED SOLENOID ELECTROMAGNETS. IN ORDER TO MAINTAIN THE MAGNETIC FIELD THEIR MUST BE CONSTANT CURRENT.

RESISTIVE MAGNET

these coils must be tuned to the specific larmor frequency of the MR system for which they are designed

RF coils

the slice location is determined by the transmitter frequency of the

RF pulse

Field strength and homogeneity can be increased in vertical field magnets by

Reducing the gap between the two magnets (Causes a reduction in pt area coverage)

Can be turned off when not in use

Resistive magnets

Can be used in vertical or horizontal systems and have strengths up to 0.3 Tesla

Resistive magnets

Requires constant current to stay on

Resistive magnets

Solenoid Electromagnets are known as

Resistive magnets

while the helmholtz pair can improve signal homogeneity across a region, this is not increased

SNR

as the receiver bandwidth is narrowed

SNR is increased but so is sampling time

WORKS LIKE A RESISTIVE, EXCEPT THE WIRE/COIL ARE COOLED WITH CRYOGENS TO ELIMINATE RESISTANCE. ONCE THIS IS DONE, NO POWER IS NEEDED TO MAINTAIN THE MAGNETIC FIELD.

SUPERCONDUCTING

THIS STRONG MAGNETIC FIELD IS GENERATED BY ELECTRICAL CURRENT FLOWING IN A LARGE COIL. THE CONDUCTING WIRES ARE MADE FROM NIOBIUM/TITANIUM ALLOY EMBEDDED IN THE COPPER

SUPERCONDUCTIVE MAGNETS

The Receiver or Surface coils used with vertical field magnets are

Solenoid in design

The majority of superconducting magnets are what type

Solenoid in design and thus exhibit a horizontal magnetic field

The magnetic field of a dipole runs from

South to North

Similar to resistive magnets in that they utilize direct current

Superconducting magnets

These substances demonstrate positive magnetic susceptibilities that are stronger than paramagnetic substances and weaker than ferromagnetic substances. Also used as T2 contrast agents

Superparamagnetic materials

IN RESISTIVE MAGNETS, THE STRENGTH OF THE MAGNET DEPENDS ON THE CURRENT THAT PASSES THROUGH ITS COIL OF WIRE, THIS IS CALLED

THE LAW OF ELECTROMAGNETISM

ADVANTAGE OF ACTIVE SHIELDING

THEY REDUCE THE 5 GAUSS LINE

In resistive magnets, when the current or number of turns of wire is increased

The field strength and heat is also increased

Disadvantage of permanent magnets

They are heavy

OPEN MRI MAGNETIC FIELDS ARE

VERTICAL

an arrow the posses two properties, magnitude and direction

Vector

require the use of solenoid coils due to the orientation of B0

Vertical field magnets

increasing the amount of RF energy will cause an increase in the flip angle. in order to double the flip angle

a four fold increase in RF power is required

the severity of the slope of the gradient magnetic field.

amplitude

tipping the NMV 90 degrees away from the z axis is referred to as

applying a 90 degree flip angle

For RF coils to work properly, B1 fields should be oriented how

at right angles (perpendicular to) the B0 field

used to acquire information over a large field of view

body coil

increased slew rates offer the benefit of reduced

echo times (TE), increased number of slices per TR, shorter TR for 3D sequences and improved image quality with echo planar and fast spin echo sequences

relative proton density increases as

field strength increases. the signal and thus the SNR increases as a result

Superimposed over the main magnetic field

gradient magnetic field

tissue magnetization (NMV) and the proton density, are greater at

higher field strengths

increasing B0 causes the resonant or precessional frequency of hydrogen to

increase

primary purpose of local coils

increase SNR

The B0 field must be stable and homogenous in the central area or

isocenter (where imaging takes place)

main disadvantage of body coils

low SNR

the greater B0, the greater the number of spins in the

low energy state

at any given time there will be a slight majority of spins in the

low energy state. this condition is known as thermal equilibrium

The ability of material to become magnetized

magnetic susceptibility

gradient magnetic fields are described in units of

milliTesla per meter mT/m or gauss per centimeter g/cm

the higher the maximum amplitude the smaller the

minimum FOV and the thinner the minimum slice thickness

the excess of spins in the low energy state add to form a magnetic field referred to as

net magnetization vector NMV

the number of protons in the sample of tissue is technically known as

proton density

as a general rule, which type of coil increase SNR 40% over a linear coil

quadrature coils

energy used for excitation in MRI, is low energy radiation

radio frequency energy (RF)

an oscillating electromagnetic field

radiowave

the range of frequencies sampled during the frequency encoding or readout gradient

receiver bandwidth

as the bandwidth or range of frequencies is narrowed, the thickness is

reduced

X gradient

right to left

the speed at which a gradient magnetic field attains its maximum amplitude

rise time

the main purpose of the gradient subsystem is to

select the slice and imaging plane and to spatially encode the MR signal

hydrogen atom consists of a

single proton

another way to express gradient performance is

slew rate

the acceleration of the gradient magnetic field to its maximum amplitude

slew rate

as the BW of the RF pulse is varied, so is the

slice thickness

the term gradient means

slope or incline

responsible for the MR signal

spin excess

this is the proton density that relates to image quality in MRI

spin excess or relevant proton density

a high amplitude gradient would have a

steep slope and would vary the intensity of the magnetic field in a given direction

to increase SNR, which type of coils are used

surface or local coils

The larger the magnetic field moving through the conductor

the greater the current induced in the conductor

the shorter the time, in other words, the more rapid the change in the magnetic field

the greater the induced current

the ratio of the spin angular momentum of the proton o its magnetic moment is known as

the gyromagnetic ratio

spins aligned anti-parallel or against the direction of B0 are in

the high energy state

spins aligned parallel or with the direction of B0 are in

the low energy state

moving charged particles like positively charged protons, make magnetic fields known as

the magnetic moment (u)

in order to achieve excitation or resonance, the transmitted RF pulse must match

the prcessional frequency

the magnetic moment of spins in the high energy state cancel the effect of those in the low energy state during

thermal equilibrium

the spin excess occurs in a condition known as

thermal equilibrium

the main purpose of the radio frequency subsystem

to transmit the RF pulses or field known as B1 and to receive the MR signal from the tissue being examined

the RF subsystem consist of coils which

transmit and receive

the body coil operates as what type of coil

transmit and receive

local coils can be

transmit and receive or receieve only

the range of frequencies that is transmitted

transmitter bandwidth (BW)

once the desired RF signal is achieved, the RF pulse is

turned off to allow for detection of the signal

rise time is expressed in units of

usec (micro seconds)

in the presence of a magnetic field, protons align either

with or opposed the magnetic field

the gyromagnetic ratio for hydrogen is

42.6 MHz/T

ADVANTAGES AND DISADVANTAGES OF PASSIVE SHIELDING

A - CHEAP D - NEED LARGE SCANNING ROOM LARGE WEIGHT (FLOOR MAGNET)

when two coils are combined in such a manner that they utilized the same receiver electronics, it is called

A Helmholtz pair

the NMV is aligned parallel to

B0 (external magnetic field)

HOW DOES ACTIVE SHIELDING WORK

CURRENT IS PASSED, CREATES A MAGNETIC FIELD, WHICH OPPOSES THE MAIN MAGNETIC FIELD

DIRECTION OF THE CURRENT EQUALS THE

DIRECTION OF THE MAGNETIC FIELD