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