In increasing order of magnetic strength these categories are:

309

• diamagnetism
• paramagnetism
• superparamagnetism
• ferromagnetism

When an external magnetic field is applied, ____________ substances show a small magnetic moment that opposes the applied field.

309

_____________ substances have low negative magnetic susceptibilities and show a slight decrease in magnetic field strength within the sample.

309

____________ substances have a low, positive magnetic susceptibility.

310

_____________ materials have a very high magnetic susceptibility.

311

_______________ materials have 'medium' positive magnetic susceptibility.

311

Once a __________ object is exposed to an external magnetic field it retains magnetization and therefore becomes magnetized.

311

The magnetic field strength is measured in one of two units:

312

• gauss (G)
• tesla (T)

The unit _______ is used to measure low field strengths.

312

The FDA limits the strength of the stray field, located in areas accessible by the general public, to _______.

312

The ________ is the unit used to measure higher magnetic field strengths.

312

The most common material used to produce a permanent magnet is an alloy of _________, __________ and _________ called __________.

313

The main advantage of __________ magnets is that they require no power supply or cryogenic cooling and are therefore relatively low in operating costs.

313

Two disadvantages of permanent magnet systems are:

313 - 314

• The weight of a permanent magnet system can be 15,000 kg.
• Permanent magnet scanners are temperature sensitive.

Permanent magnet systems are typically designed so that the magnetic plates are located above and below the patient, providing good all-around access. These are known as ____________ systems.

314

The magnetic field strength created by introducing current through a wire is calculated using the following equation:

314

B₀ = kI

where

I is the current flowing through the wire

k is the proportionality constant (quantity of charge on each body)

B₀ is the strength of the magnetic field.

Loops of wire form a coil and act as though they are parallel straight wires. This 'spring-like' electromagnet is called a ___________ electromagnet.

315

The degree of resistance along a wire is determined by _________.

315

Ohms law states that:

315-316

V = IR

where

V is equal to the applied voltage

I is the current

R is the resistance within the wire.

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

316

The direction of B₀ in resistive systems can be either horizontal or vertical, depending on the orientation of the __________.

317

To allow the high current necessary to obtain a very high magnetic field, the coils are constructed from an alloy of _________ and _________.

317

This alloy of niobium and titanium exhibits a property known as ____________ when cooled to below a certain critical temperature.

317

The process of creating an electomagnetic field initially involves passing current through the main superconducting coil of the scanner. This process is called _________.

318

The temperature at which the niobium-titanium wire becomes superconducting is ______.

318

In the event of a spontaneous helium boil-off, over 1,000,000 liters of gas are liberated. This is known as a __________.

319

The stray magnetic field outside the bore of the magnet is know as the stray field or ________ field.

321

_________ shielding is accomplished by surrounding the magnet (or lining the magnet room) with steel plates.

322

____________ shielding uses additional solenoid electromagnets located around the outside of the main magnet coils at each end of the magnet bore.

322

____________ makes the field even and is achieved by the use of metal discs/plates and an additional solenoid magnet.

322

__________ shimming is performed by scanning a phantom and adjusting the position of the shim plates until optimum field homogeneity is achieved.

322

___________ shimming is performed by an electromagnetic coil and can be used to shim the system for each patient or even sequence within a protocol.

322

The direction of current flowing through the coils determines the ________ of the gradient.

325

To evaluate speed and strength of gradients, gradient characteristics need to be understood. These gradient characteristics include:

326

• gradient strength
• gradient speed
• the combination of strength and speed
• the duty cycle

___________ or __________ defines the time it takes for a given gradient to reach maximum amplitude. It is measured in microseconds (μs).

326

__________ defines the time it takes for a given gradient to reach maximum amplitude and what that amplitude is. It is the speed and strength of the gradient and is measured in units of milliTesla per meter per second (mT/m/s)

326

___________ defines the percentage of time that the gradient is permitted to work. It is expressed in units of percentage (%).

326

How quickly a gradient can attain a particular gradient slope is called the __________.

326

Stonger gradients generally equal shorter/longer rise times.

327

In a _________ or _________ gradient system, each gradient pulse is balanced by an equal but opposite gradient pulse.

327

Time within the sequence is reduced if sampling is performed while the frequency encoding gradient is changing. This is accomplished with a technique known as ____________, in which data points are collected when the rise time is almost complete.

329

____________ gradients allow for rapid imaging sequences. Such savings result in shorter imaging times, more slices and higher resolution than conventional imaging.

329

Faraday's law of induction:

331

dB/dt = dv or ΔB/Δτ = Δv

where

dB (ΔB) is the changing magnetic field (oscillating magnetic field caused by RF signals)

dt (Δτ) is the changing time

dv (Δv) is the changing voltage (MR signal)

_________ coils can be configured in a solenoidal, saddle and/or birdcage configuration.

332

Coils configured with a simple loop of wire and other components are known as _________ coils.

332

Generally the nearer the coil is situated to the structure to be imaged, the lower/greater the SNR.

332

The size of this area extends to the diameter of the coil and at a depth into the patient equal to the diameter of the coil x _______.

332

The range of the area that can be imaged is limited by the ________ of the coil.

333

In an attempt to get good SNR and large coverage, manufacturers have combined multiple small coils with multiple receivers. This is known as _________________ technology.

333

When using local coils, a body coil is generally used to _________ RF and the local coil is used to __________ the MR signal unless the local coil is also a transmitter.

334

Always make sure the __________ are not looped and do not touch the patient or the bore of the magnet.

336

To receive optimum signal from the patient, the coils must be correctly _________.

336

MRI computer systems vary by manufacturer. Most consist of:

337

• the computer system
• pulse control unit
• an array processor for Fourier transformation
• an image processor
• hard disc drives
• a power distribution mechanism
• the operator console for user interface

For the most part, MR computers are programmed at the system manufacturer by ________________, who pre-program pulse sequences, scan parameters and image processing functions.

338

The ____________ oversees or controls the order and timing of RF and/or gradient pulses.

338

The _____________ co-ordinates the functions of the gradient amplifiers and the coils so that they can be switched on and off at the appropriate times and for the appropriate duration of time.

339