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MRI touch


Keign Palma

on 21 September 2017

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Transcript of MRI touch

Basic Principles
Move the elements
Create a unique layout
Keign Orson J. Palma, RRT
Magnetic Theory
Atomic Magnetism
The negatively charged electrons orbiting the nucleus
spin on their axis in one of two directions: clockwise
or anticlockwise. In a non-magnetic element there are
the same number of electrons spinning in each direction.
In a magnetic element there is an imbalance,
with more electrons spinning in one direction than the
other. It is this imbalance that creates the potential for
Nuclear Magnetism
all atomic nuclei (except for hydrogen) contain
both protons and neutrons. When there is an
imbalance between the number of protons and the
number of neutrons the nucleus will spin upon its axis,
and the spinning of the positively charged protons
within the nucleus creates a magnetic field.
As shown in Fig. 1.3 the spinning protons do not
align themselves exactly with the main magnetic field
but more at an angle to it.
Larmor frequency
Precessional frequency
Net magnetization (M), the averaged sum of many individual quantum spins, can be treated as a regular vector in classical physics.
flip angle
Flip angle,
also called tip angle, is the amount of rotation the net magnetization (M) experiences during application of a radiofrequency (RF) pulse. It is often designated by the Greek letter alpha (α)

T2 is the time required for the transverse magnetization to fall to approximately 37% (1/e) of its initial value.
T1 can be viewed as the time required for the z-component of M to reach (1 − 1/e) or about 63% of its maximum value
Origin of the MR Signal
The MR signal is a small electrical current induced in the receiver coil by the precession of the net magnetization (M) during resonance
This is a manifestation of Faraday's Law of Induction, wherein a changing magnetic field induces a voltage in a nearby conductor.
Origin of MRI Signal
Free Induction Decay
Gradient Echo
A gradient echo (GRE) is simply a clever manipulation of the FID signal that begins by applying an external dephasing gradient field across the specimen or tissue. This gradient (produced by special coils hidden within the magnet housing) causes a calibrated change in local magnetic fields and hence alters the resonance frequencies slightly across the specimen. This results in accelerated dephasing and 'squelching/ scrambling' of the FID.
First step in GRE formation. A gradient is applied across the specimen, resulting in accelerated dephasing and squelching of the FID.

Second step in GRE formation. A rephasing gradient is applied (opposite in polarity to the dephasing gradient). This reverses the phase shifts induced by the dephasing gradient and resurrects the FID as GRE. Note that T2* decay continues unabated.
TR and TE
Spin Echo
A single RF pulse generates a free induction decay (FID), but two successive RF pulses produce a spin echo (SE). The time between the middle of the first RF pulse and the peak of the spin echo is called the echo time (TE).
The time between the 180° inverting pulse and the 90°-pulse is called the inversion time (TI)
selective tissue suppression
tissue sensitivity to T1 differences
additive T1 and T2 contrast
longer imaging time
high SAR (heating)
Inversion Recovery
Short Tau Inversion Recovery
FLuid Attenuation Inversion Recovery
Spectral Pre-saturation Inversion Recovery
Gradient Echo
Pulse Sequences
Spin Echo
Turbo Spin Echo
Fast Spin Echo
Multiple Spin Echo
uses less than 90 degree flip angle
faster imaging technique
uses gradient fields to flip (M)
prone to inhomogeneity artifact
Erwin Hahn (1949)
Susceptibility Weighted Imaging
Diffusion Weighted Imaging
Hi Keign!
Closed bore (cylindrical) configuration with superconducting solenoidal design. The coils are bathed in liquid helium allowing a stable, homogeneous field to be created, typically 1T and higher.
Most open bore scanners utilize permanent magnets in a C-shaped or horseshoe configuration. These operate at field strengths typically ranging from 0.2T to 0.7T

The magnet is the heart of the MRI system.
It is the largest and most costly item within the system and there are three primary requirements of it:
(1) to produce a magnetic field of a known strength,
(2) to produce a homogeneous field and
(3) to allow access for patients.

1. Low capital and running costs
2. No power requirements
3. Small fringe field
4. No liquid gases required for cooling
5. No heat generated
6. transverse field
usually made up of iron, cobalt, nickel
Resistive Magnet
i. Iron core
i. iron core
also known as the hybrid system.
Resistive Magnet
ii. Air Core
Resistive Magnets
ii. Air core
Superconducting Magnet
1. low capital cost
2. low running cost
3. increase field strength
without increasing weight
4. small fringe fields
1. high power requirements
2. potential instabilities in MF
1. limited field strength
2. very large mass
3. difficult to shim
4. only useful for hydrogen imaging
1. low capital cost
2. low running cost
3. easy accessible for
4. can be switched off
5. no liquid gas for cooling
6. no vacuum required
1. high power requirements
2. water cooling system required
3. instabilities in MF due to fluctuations of power supply
1. high field strength
2. high field homogeneity
3. extremely stable
4. low power requirements
5. adjustable field strength
1. high capital cost
2. high cost for cryogens
3. careful handling of cryogens
4. coils not easily accesible
5. problems with high fringe fields
A superconductor is a material that loses all electrical resistance below a critical temperature
made up of alloy of titanium and niobium,
embedded in a copper matrix
The critical temperature required to produce zero
resistance for the alloy is -269°C (4 K)
helium reliquefication
cryostat vs Dewar
three implications of high MF:
(1) the space required for an independent site,
(2) the high cost of building the
extra site, and
(3) the fringe or stray fields produced in
a three-dimensional volume around the magnet and
which can have an effect on the surrounding environment.
In the past these problems were overcome by:
1. Building on a green-field site -
2. Yoke shielding (self shielding),
3. Room shielding, where the magnet room is converted into an iron box -
Active Shielding
Passive Shielding
is the process of eliminating any inhomogeneities within the magnetic field
Passive Shimming
Active Shimming
In active shimming ten to twelve current-carrying
shim coils are placed within the bore of the magnet.
Passive shimming involves the correction of inhomogeneities
by the placement of iron plates inside and/or
outside the magnet bore
special array of superconducting coils wound
around the outside of the main magnet
In passive shielding iron beams or steel plates are incorporated into the walls, ceiling, and/or floor of the magnet room.
Gradient Coils
Gradients are loops of wire or thin conductive sheets on a cylindrical shell lying just inside the bore of an MR scanner. When current is passed through these coils a secondary magnetic field is created
RF coils
RF-coils are responsible for detecting the MR signal.
RF-coils generate an oscillating/rotating magnetic field (denoted B1) that is perpendicular to the static main magnetic field (Bo)
Surface coils
Zone 1
All areas freely accessible to the general public without supervision. Magnetic fringe fields in this area are less than 5 Gauss (0.5 mT).
Still a public area, but the interface between unregulated Zone I and the strictly controlled Zones III and IV. MR safety screening typically occurs here under technologist supervision.
Zone 2
An area near the magnet room where the fringe, gradient, or RF magnetic fields are sufficiently strong to present a physical hazard to unscreened patients and personnel.
Zone 3
Zone 4
Synonymous with the MR magnet room itself. Has the highest field (and greatest risk) and from which all ferromagnetic objects must be excluded.
RF shielding
1) to prevent extraneous electromagnetic radiation from contaminating/distorting the MR signal, and
2) to prevent electromagnetic radiation generated by the MR scanner from causing interference in nearby medical devices.
Faraday cage
In MRI the signal is derived from resonating hydrogen nuclei: the greater the number resonating, the greater the signal. The signal produced is detected by the receiver or body/head coil and analysed by the computer in order to locate spatially the data acquired
Image Production
coils (
slice select
) create a magnetic field gradient along the length of the magnet bore.
coils (
) create a magnetic field gradient from left to right.
coils (
frequency-encoding or readout
) create a magnetic field gradient superior to inferior.
Spatial Location of the MR Image
Image Production
Slice Selection

is the frequency of the RF pulse (Hz),
is the Larmor frequency of hydrogen (Hz/Gauss),
is the gradient amplitude (Gauss/ cm)
slice thickness
is in centimeters.
F = H X G X Slice thickness
Slice Position
Image Production
The zero point of the gradient coils is the isocentre of
the magnet. With this in mind it is possible to excite a
slice which is not at the isocentre by changing one of
two things:
(1) the zero point of the slice selection
gradient or
(2) the frequency of the RF pulse so that it
corresponds to a resonant frequency which is either higher or lower than the resonant frequency at the isocentre of the magnet.
Slice Profile
Image Production
Ideally the shape of the slice profile should be rectangular.
spatial location within the excited slice the Y and X magnetic field gradients are activated independently.
These gradients produce:
(1) a change in the phase of precession of the excited protons and (2) a change in the frequency
of precession of the excited protons.

They are therefore labelled
phase-encoding gradient (Y)
frequency-encoding gradient or readout gradient (X).
Phase-Encoding Gradient (Y-Axis)
Image Production
The phase-encoding gradient is activated in the time between slice excitation and signal collection (readout).
Frequency-Encoding Gradient (X-Axis)
Image Production
The frequency-encoding gradient is the last to be activated and is applied across the slice in the opposite direction to the phase-encoding gradient
Image Production
MR image
k-space is an array of numbers representing spatial frequencies in the MR image.
The Fourier transform is a mathematical technique that allows an MR signal to be decomposed into a sum of sine waves of different frequencies, phases, and amplitudes
Image Quality
1. Voxel size
2. Number of signal averages
3. Pulse sequence
4. Interslice gap
5. Magnetic field strength
6. Coils
7. Receive bandwidth
8. Use of contrast media
factors which determine
the intensity of the signal received
Image Quality
Signal to Noise Ratio (SNR)
The signal-to-noise ratio (SNR) is a central theme in image
quality and is the relationship between the signal intensity
and the amount of noise detected in each voxel
The contrast-to-noise ratio is the ratio rif the signal difference
between two tissues relative to the amount of noise present
Contrast to Noise Ratio (CNR)
the larger the voxel the higher the SNR.
Voxel size
the larger the voxel imaged the poorer the spatial resolution
The thicker the slice the larger the
voxel size and therefore the greater SNR.
Slice Thickness.
A small FOV will reduce pixel size and therefore give good spatial resolution but at the expense of grainy images due to the reduction in signal.
Field of View (FOV)
Matrix size
the number of views
along both the phase-encoding and frequency-encoding
axes (typically 256 X 256).
Number of Signal Averages (NSA) or
Number of Excitations (NEX)
Image Quality
Increasing the NSA results in the overall scan time being increased accordingly. For example, doubling the NSA would double the scan time. The advantage of increasing the NSA would be a direct improvement
in the SNR.
Image Quality
Magnetic Field Strength
The SNR is approximately proportional to the magnetic
field strength. The higher the field strength the greater the SNR.
SNR decreases with increasing distance from the coil.
Interslice Gaps(spacing)
The noise within the MR system is present over a
wide range of frequencies. The wider the bandwidth
(BW) the more noise goes into the formation of the image.
Use of Contrast Media
Image Artifacts
An artifact can be described as a signal void or intensity which
appears on the image and bears no resemblance to the actual
anatomy of the volume being imaged
different sources of artifacts:
1. Magnetic field
2. Hardware-related
3. motion-related
4. technique-related
Magnetic field related
spatial misregistration of image data
(internal cause of inhomogeneity)
Magnetic field related
(External cause)
Susceptibility Artifacts
Radiofrequency Artifacts
cause an increase in image noise.
RF pulses used in MRI share the same frequency
range of many other RF sources
Banding (zebra artifact) due to coupling of eddy
currents to gradient and RF body coils in variable
(multi) echo techniques.
Inhomogeneity of RF
Surface coil
Centre Line Artifacts
zipper artifact
Motion Related
Motion-related artifacts can be caused by periodic
physiological motion (such as cardiovascular motion,
cerebrospinal fluid pulsation or respiratory motion) or
by voluntary (aperiodic) motion in which the patient
moves. Anyone of these will cause some form of
image artifact - usually
blurring or ghosting

Wraparound, or aliasing, occurs when the FOV selected
is smaller than the area of anatomy being sampled.
The area outside the FOV is excited by RF and consequently
the signals are detected and present themselves
as artifacts overlying the tissues within the FOV.
The artifacts appear as mismapped signals transposed
onto the opposite side of the image and hence the
ternl "wraparound".
Technique related
Wraparound (Aliasing)
Technique related
Truncation (Gibbs)
Gibbs artifacts (also known as truncation, ringing, or spectral leakage artifacts) typically appear as multiple fine parallel lines immediately adjacent to high-contrast interfaces.
or spectral leakage artifact
Tissue related
Chemical shift
loss of signal seen in an image from a multi-angle, multi-slice acquisition, as is obtained commonly in the lumbar spine
Slice overlap (cross-talk) artifact
Partial volume artifact
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