Basic Image Formation
Amanda Golsch BSc RT(R)(MR)
• Magnetism is a fundamental property of matter, therefore, all substances have some form of
magnetism to a varying degree.
• Diamagnetic- Exhibit a slight negative effect (-1) when placed in an externally applied magnetic
field. This negative susceptibility is not very strong and can be seen in glass, wood, and plastic.
• These substances have a slight increase in their magnetic field when placed in an externally
applied magnetic field. A common example of a paramagnetic substance is gadolinium.
• If a substance has slight diamagnetic and paramagnetic properties, the paramagnetic properties
are slightly stronger and take on those characteristics.
• Ferromagnetic substances have positive magnetic susceptibilities. However, unlike paramagnetic
substances when they are exposed to an externally applied magnetic field they remain
magnetized after the magnetic field is removed.
• Iron is an example of a ferromagnetic material.
• These substances have positive magnetic susceptibilities. The positive susceptibilities
of a superparamagnetic substance are stronger than paramagnetic substances but
weaker than ferromagnetic substances.
• Often superparamagnetic substances are used as T2 contrast agents.
• Ferromagnetic materials when
exposed to an externally applied
magnetic field become magnetized.
Therefore, the material becomes a
magnet that is known as a dipole.
This means that there is a North and
• Note that the magnetic field runs from
the South pole to the North pole.
• When like poles are brought together
the resultant fields repel each other.
• When unlike poles are brought
together the resultant fields add and
pull toward each other.
How Do You Make A Magnetic
• Current + a long straight wire = a
magnetic field about the wire.
• Direction of the current = Direction of
the magnetic field.
• Strength of the magnetic field =
Amount of current passed through the
Magnetic Field Strength
• Magnetic field strength can be measured in Tesla (T) or Gauss (g).
• 10,000 g =1T
• Therefore, a 3T MRI system = 30,000g
Vertical Field Magnets
Are sometimes referred to as an “open MRI system”.
Utilizes two magnets. One magnet is positioned above the patient and one is positioned
below the patient.
Have a reduced fringe field in comparison to conventional horizontal magnets.
Gradient and RF coils are flat and located on the face of the magnet.
Receive/Surface coils are solenoid in design.
Homogeneity and field strength can be increased by reducing the space between the two
magnets, but this is at the expense of patient area.
Regardless of magnet type, the Bo field must be homogeneous at isocenter where
• Permanent Magnets are constructed of
blocks or slabs of naturally occurring
ferrous material. Increasing the
amount of ferrous material increases
the weight, size, and field strength.
• Generally, these magnets range from
0.06T to 0.35T.
• Sensitive to ambient room
temperature. Permanent magnets
function optimally at 70°F +/-2°F.
• Changes in temperature can cause
changes in field strength. The field
strength can vary several Gauss per
degree of change. The changes in
field strength can result in changes in
• A wire is placed in a solenoid
configuration while current is passed
through the wire.
• Resistance is a property of the wire
that can pose as an obstacle.
Resistance will convert the current into
heat. In order to maintain the
magnetic field, there must be a
constant current. This type of magent
is called a resistive magnet.
• Used in horizontal or vertical field systems.
• Have field strengths up to 0.3T
• Needs constant current to be applied to create a static magnetic field.
• Needs for coils to be cooled because the result of electrical resistance is heat.
• Resistive magnets can be turned off.
• Can be temperature sensitive.
• Utilize a direct current that is applied
to a coil of wire in order to produce a
static magnetic field.
• Resistance is reduced by cooling the
coils. Superconducting magnets have
their coils immersed in liquid helium to
cool the wires and remove resistance.
• Without resistance, the electrical
current can flow within a closed circuit.
There is no need for any external
power to be applied. The flowing of
electrical current without resistance is
known as superconductivity.
• As long as the wires stay cool and the
current flows, the magnet is on.
• Most superconducting magnets are solenoid by design and exhibit a horizontal
• Superconducting magnets can achieve very high field strengths, so the FDA has a 4T
limit. Generally, clinical scanners range between 0.5T and 1.5T.
Faraday’s Law of Induction
• A magnet/magnetic field, when passed through a conductor will induce an electrical
current. The larger the magnet/magnetic field, the greater the electrical current
induced in the conductor.
• Faraday’s Law can be expressed as ∆B/∆t=∆V. Moving a magnet or changing a
magnetic field over time in the presence of a conductor will induce a voltage in the
The RF System
Transmit and Receive Bandwidth
• The purpose of the RF subsystem is to transmit the RF pulses (B1 field) and to
receive MR signal from the tissue of interest.
• For an RF coil to work appropriately, the B1 field should be perpendicular to the B0
• The B1 field provides enough energy to allow the net magnetization of the tissues to
tip and rotate through the transverse plane were the receiver coil(s) are located.
• The RF subsystem has transmit and/or receive coils.
• The RF subsystem is generally digital.
• Coils are designed to receive only, transmit only, or transmit and receive.
• The body coil located within the bore of the magnet is a transmit and receive (TR)
coil. This large coil can be used to gain information over a large field of view.
However, the trade-off of using the body coil is a loss in signal to noise.
• Generally, a smaller surface or local coil will result in greater signal to noise (SNR).
• Local coils can be a transmit/receive coil or a receive only. If the local coil is a
receive only coil, the body coil acts as a transmit coil. The result is an increase in
SNR, but there is a reduction in the area that is covered.
Types of Local Coils
• Linear coils were the first type of coils to be used in MRI.
• Quadrature coils are designed with additional loops and circuitry to improve the
efficiency with which the MR signal is induced in the coil.
• Quadrature coils increase the SNR by 40% in comparison to linear coils.
• To improve signal uniformity, it is possible to pair coils. This is known as a
Helmholtz pair. This can be done when imaging the cervical spine.
• Phased Array coils allow for greater coverage of the region of interest while
maintaining SNR. There are multiple coils and multiple receivers.
• Gradient magnetic fields are superimposed over the main magnetic field.
• These fields are produced by applying a current in the gradient coils.
• There are three sets of gradient coils in MR systems.
• The coil that is used to vary the
intensity of the magnetic field in the
left to right direction is the X
• The gradient coil that is used to vary
the intensity of the magnetic field in
the anterior to posterior direction is
the Y gradient coil.
• The gradient coil that is used to vary
the magnetic field in the head to foot
direction is the Z gradient coil.
• The amplitude is the severity of the slope of the gradient magnetic field.
• A high gradient amplitude would indicate that there is a steep slope and therefore
would greatly vary the intensity of the magnetic field in a given direction.
• Polarity can be positive or negative and refers to whether the gradient field is
creating a field greater or less than the frequency of the B0 field.
• With a higher gradient amplitude you can obtain thinner slices thicknesses and
smaller fields of view.
• Gradient amplitude is measured in mT/m.
Transmit and Receive Bandwidth
• Transmit bandwidth is a range of frequencies that are transmitted.
• Transmit bandwidth is responsible for slice thickness. As the RF pulse is varied, slice
• When the transmit bandwidth or range of frequencies are narrowed, the slice
thickness is reduced. Slice thickness is increased as transmit bandwidth or the range
of frequencies are increased.
• Slice location is also determined by the transmit frequency of the RF pulse.
• Receiver bandwidth is the range of frequencies that are sampled during the
frequency encoding gradient (read-out gradient)
• It is determined by the number of frequencies sampled and the time took to obtain
• As receiver bandwidth is narrowed, SNR is increased and so is sampling time.
Rise Time/Slew Rate
• The Rise Time is the time that it takes for the gradient magnetic field to reach it’s
maximum amplitude. This time is measured in microseconds.
• Slew Rate is the acceleration of the gradient magnetic field to it’s maximum
amplitude. This is measured in T/m/sec.
• Benefits of an increased slew rate:
• Reduced Echo Time
• Increased Slices per TR
• Shorter TR for 3D sequences
• Improved Image Quality for EPI and FSE
• If you are working on a 3T magnet, what is the gauss equivalent?
– A. 5,000
– B. 10,000
– C. 30,000
– D. 37,000
• An example of a paramagnetic substance is:
– A. Gadolinium
– B. Wood
– C. Iron
– D. Plastic
• T or F: Current that flows through a long straight wire creates a magnetic field about
• Transmit bandwidth controls
– A. Slice Thickness
– B. Scan Time
– C. The number of frequency samples collected
– D. The slew rate
• The Z gradient varies the intensity of the magnetic field
– A. Anterior to Posterior
– B. Right to Left
– C. Head to Foot
• T or F: Amplitude refers to the severity of the slope of the gradient magnetic field.
• Phased Array Coils
– A. Have multiple coils and multiple receivers
– B. Increase SNR by 40%
– C. Were the first coils used in MRI
– D. Increase scan time
• T or F: Moving a magnet or changing a magnetic field over time in the presents of a
conductor will not induce a voltage in the conductor.