Basic Image Formation

Part IMagnets

Amanda Golsch BSc RT(R)(MR)

Electromagnetism/Magnetism

• 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.

Paramagnetic• 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• 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.

Superparamagnetic• 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.

Magnets

• 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 South pole.

• 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 Field?

• 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 wire.

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 MagnetsAre 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 imaging occurs.

Permanent Magnets• 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 resonant frequency.

Solenoid Electromagnets• 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.

Resistive Magnets• 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.

Superconducting Magnets• 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.

Superconducting Magnets• Most superconducting magnets are solenoid by design and exhibit a

horizontal magnetic field.• 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 conductor.

PART II

The RF SystemGradient Coils

Transmit and Receive Bandwidth

RF Coils• 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 field.• 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.

Transmit/Receive Coils• 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 Coils• 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.

Gradient Coils• The coil that is used to vary the

intensity of the magnetic field in the left to right direction is the X gradient coil.

Gradient Coils• 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.

Gradient Coils• The gradient coil that is used to

vary the magnetic field in the head to foot direction is the Z gradient coil.

Amplitude• 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 thickness changes. • 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 those samples.• 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

Question 1• 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

Question 2• An example of a paramagnetic substance is:

– A. Gadolinium– B. Wood– C. Iron– D. Plastic

Question 3• T or F: Current that flows through a long straight wire creates a magnetic

field about the wire.

Question 4• Transmit bandwidth controls

– A. Slice Thickness– B. Scan Time– C. The number of frequency samples collected– D. The slew rate

Question 5• The Z gradient varies the intensity of the magnetic field

– A. Anterior to Posterior– B. Right to Left– C. Head to Foot

Question 6• T or F: Amplitude refers to the severity of the slope of the gradient

magnetic field.

Question 7• 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

Question 8• 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.