MRI PHYSICS chapter 9

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What are the 3 different types of MRI scanners?

Closed bore systems, Open systems and Extremity systems.

Describe the characteristics of the closed bore system.

-Most abundant in the field

-Higher field strength

-Solenoid electromagnets

-Horizontal Bo

Describe the characteristics of the open system.

-Unobstructed view on all sides

-Helps with claustrophobia

-Usually lower field strengths

-Vertical Bo

Describe the characteristics of the extremity system.

-Designed to scan limbs

-Smaller in design

-Can be portable

An MRI imaging system uses a combination of how many electromagnetic fields to acquire images of the body? What are they?

There are 3 electromagnetic fields which are:

-primary magnetic field (Bo)

-gradient field

-RF field

What are the five pieces of hardware that are needed to produce a magnetic resonance image?

-a magnet

-a radio frequency source

-a magnetic field gradient system

-a computer system

-an imaging processor

What hardware is needed for a nuclear alignment?

a magnet (Bo)

What hardware is used for RF excitation?

a radiofrequency source

What type of hardware is needed for spatial and encoding?

a magnetic field gradient system

What type of hardware is needed for an image formation?

a computer system

What type of hardware is needed to convert signals into images?

an Imaging processor

Which type of spin gives a higher signal and better image quality?

low energy spin or high energy spin

low energy spin

The stronger the magnet the more spins are in what energy state?

low or high

low energy spins ie parallel

What is needed to maintain magnetic evenness or homogeneities?

a shim system

What does the word susceptibility refer to?

how easily something can be influenced

Materials that have the ability to be influenced by an external magnetic field is determined by a property known as _________?

magnetic susceptibility

The primary magnetic field (aka main magnetic field) is used to prepare tissues to be imaged by aligning them with the main B0.

True or False?

True

The law of electromagnetic induction indicates that whenever a charged particle has a spinning motion, a corresponding magnet field is induced.

True or False?

true

What are the four main categories of magnetic elements?

diamagnetic

paramagnetic

super paramagnetic

ferromagnetic

What magnetic elements has paired electrons?

diamagnetic

Which magnetic element repels the main magnetic field?

diamagnetic - weakens the field strength

What are some examples of diamagnetic substances?

lead, helium, gold and silver

Which magnetic element materials have unpaired electrons?

paramagnetic

Which magnetic element has a small positive magnetic susceptibility and slightly increases the field strength?

paramagnetic

What are some examples of paramagnetic materials?

calcuim/oxygen/ aluminum/ titanium/ and gadolinium chelates

What is the magnetic field symbol?

Bo

What magnetic elements have large positive magnetic susceptibility?

ferromagnetic

What magnetic material subsequently can become permanent magnets?

ferromagnetic

What are three examples of ferromagnetism material?

iron/ steel/ and gadolinium

What magnetic element has an intermediate positive magnetic susceptibility that is greater than the exhibited paramagnetic materials and less than the ferromagnetic material?

superparamagnetic

What is 1 example of superparamagnetic?

iron oxide particles

What happens to a ferromagnetic material that is exposed to an external magnetic field even when the external magnetic field is no longer present?

it becomes a permanent magnet

The magnetic field strength is measured in what two units?

Gauss or Tesla

What is the Earth's magnetic field strength

0.5 G

In MRI what unit is used to measure the fringe magnetic field that extends beyond the bore of the main magnet

Gauss

What is the FDA limit in the strength of the stray field?

5G

What is the FDA limit in field strength for infants until 1 month of age?

4T

What is the FDA limit in field strength for any age above one month?

8T

One Tesla is equal to ___________?

10,000 G

What are the two different types of primary magnets?

1. permanent magnet

2. electromagnet (resistive & superconducting)

What type of magnet does passing an electrical current through a conductor produce?

electromagnet

The type of magnet that is produced by permanently magnetizing a ferromagnetic substance is know as a ______________?

Permanent magnet

In clinical MRI, permanent magnets can be produced to operate up to what field strength?

8T

What are some substances that are used in the production of permanent magnets?

Neodymium, boron and ion OR aluminum, cobalt and nickel.

What is the main advantages of permanent magnets

-Lower power consumption and operational costs

-Lower initial capital cost

-Small fringe field due to Bo being weak (open MRI)

What type of magnet has almost no discernible fringe field?

permanent magnet system

What is a disadvantage of a permanent magnet?

-Low field strength. Usually .5T or less

-Heavy and difficult to site

-Bo cannot be turned off

What can be expressed by the right hand thumb rule?

It tells us if we curl our fingers, it will show the direction of the current flowing through a coil of wire and our thumb will tell us where the North Pole is.

There are coils that both transmit and receive signals known as transceivers.

True or False?

true

What type of coil is the main magnet?

body coil

What type of coil is the main RF transmitter and transmits RF for most examinations that are acquired without a transmitted/ receive coil?

body coil

What type of coil can be a bird cage type configuration?

head coil, body coil and knee coil

Extremity coils are generally used to image what type of extremities?

knee, ankle, and foot but can be used for upper extremities such as elbow and wrist

To accomplish signal reception in a receiver coil the secondary B1 field (RF) must be situated at what type of angle to the magnetic field?

perpendicular

Can the configuration of RF transmitter and receiver coils directly affect the quality of signal and hence the quality of the image?

yes

What type of coil is tube-shaped for vertical field systems?

solenoid coil

What type of coil has a saddle configuration?

body coil

The surface coil sensitivity is related to ___________?

its size

Surface coils can only transmit RF.

True or False

False - they can only receive RF

In an attempt to get both good SNR and large coverage manufactures have combined multiple small coils within multiple receivers this is known as ______________?

phased array coil technology

RF coils are ALWAYS oriented parallel to Bo. Signal is transmitted and received perpendicular to Bo.

True or False?

True

What are five types of phased array coils available?

-spine

-pelvic

-breast

-cardiac

-temporomandibular joint

The degree of resistance along a wire is determined by what law?

Ohm's Law

The magnetic field in a resistance magnet depends on the current that passes through its coils of wire.

True or False?

true

An electromagnet at room temperature is subject to Ohm's law and is said to be a ___________ magnet?

a resistive magnet

Can the direction of a magnetic field be either horizontal or vertical?

Yes, it could be either depending upon the configuration of the magnet

What is the maximum field strength in a system of a resistive magnet?

less than 0.2 t or 0.3 t

Which type of magnet can be turned off instantly at a flick of a switch?

a resistive magnet

When coils that are constructed of alloy of niobium and titanium and then are cooled to below a certain critical temperature this property is known as ______________?

superconductivity

What type of magnet has virtually zero resistance and will continue to carry a powerful electrical current indefinitely and without heating up?

a superconducting electromagnet

What type of coils in superconducting electromagnets is used to help oppose the fringe field?

bucking coils

What temperature is needed to become superconducting?

4 Kelvin (-452F)

What typically is the field strength for superconducting magnets?

1.5T up to 3T

Once a superconducting magnet is ramped up to the desired field strength, the power source is disconnected and current continues to flow due to no resistance.

True or False?

True

What substance is used to maintain superconductivity and eliminate resistance?

cryogens

Cryogenics used in MRI include what type of substances?

liquid helium and liquid nitrogen

What is the primary safety concern related to helium gas?

that it displaces oxygen

In the event of a spontaneous helium boil-off how many liters of gas are liberated and what is this known as?

1 million liters/ quenching

What type of MRI system (magnet) uses superconducting solenoid magnets above and below the patient creating a vertical magnetic field?

Open systems

The magnetic field outside the bore of the magnet is known as __________?

stray field or fringe field

What can significantly reduce the area affected by the fringe field?

magnetic shielding

What are two types of magnetic shielding?

passive and active

What type of shielding is accomplished by surrounding the magnet with steel plates? (Hint: in the walls, ceiling and floors)

passive shielding

What type of shielding uses additional solenoid electromagnetics located around the outside of the magnetic coils at each end of the magnetic bore?

active shielding

What is used to maintain even homogeneity of the magnet?

shimming

What are two types of shimming?

active and passive

What type of shimming is used that makes the field even and is achieved by the use of metal discs?

passive shimming

What is a secondary electro magnetic field over Bo to make it more homogenous?

active shim

What type of coil is found in the bore of the magnet that is a cylindrical structure containing three individual electromagnetics?

gradient coil

What are the four factors that change the strength of an electromagnet?

1. the amount of current passing through the windings

2. the number of windings in the coil

3. diameter of the wire used in the windings

4. the distance or spacing between the windings

Gradient means ___________?

Gradual change - they change linerarly over distance.

Gradient coils are powered by what?

gradient amplifiers

What type of gradient characteristic defines how steep or strong a particular gradient is?

gradient strength or gradient amplitude

What gradient characteristic defines the time it takes to get to a particular amplitude measured in microseconds?

gradient rise time

What combines the amplitude and the rise time and is measured in mT/m/ms (millitesla per meter per microsecond)?

gradient slew rate

How quickly a gradient can attain a particular gradient slope is called ________________?

rise time

What causes the acoustic noise generated by the scanner?

the vibrations of the gradient set

What is the very center of the bore called?

Isocenter

What instrument is used to achieve resonance?

RF coils

Is RF considered an oscillating magnetic field due to the way the RF is sent and received?

Yes