Shared Flashcard Set

Details

09 - Instrumentation and equipment
Several processes must be completed to produce magnetic resonance images, including image acquisitions and image formation.
57
Medical
Professional
05/08/2015

Additional Medical Flashcards

 


 

Cards

Term

In increasing order of magnetic strength these categories are:

 

 

309

Definition
  • diamagnetism
  • paramagnetism
  • superparamagnetism
  • ferromagnetism
Term

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

 

 

309

Definition
diamagnetic
Term

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

 

 

309

Definition
Diamagnetic
Term

____________ substances have a low, positive magnetic susceptibility.

 

 

310

Definition
Paramagnetic
Term

_____________ materials have a very high magnetic susceptibility.

 

 

311

Definition
Ferromagnetic
Term

_______________ materials have 'medium' positive magnetic susceptibility.

 

 

311

Definition
Superparamagnetic
Term

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

 

 

311

Definition
ferromagnetic
Term

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

 

 

312

Definition
  • gauss (G)
  • tesla (T)
Term

The unit _______ is used to measure low field strengths.

 

 

312

Definition
gauss
Term

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

 

 

312

Definition
5 G
Term

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

 

 

312

 

Definition
tesla
Term

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

 

 

313

Definition
aluminum, nickel, cobalt, alnico
Term

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

 

 

313

Definition
permanent
Term

Two disadvantages of permanent magnet systems are:

 

 

313 - 314

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

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

Definition
open MRI
Term

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

 

 

314

Definition

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.

Term

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

 

 

315

Definition
solenoid
Term

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

 

 

315

Definition
Ohms law
Term

Ohms law states that:

 

 

315-316

Definition

V = IR

 

where

 

V is equal to the applied voltage

I is the current

R is the resistance within the wire.

Term

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

 

 

316

Definition
resistive
Term

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

 

 

317

Definition
magnet coils
Term

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

 

 

317

Definition
niobium, titanium
Term

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

 

 

317

Definition
superconductivity
Term

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

 

 

318

Definition
ramping up
Term

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

 

 

318

Definition
4 K, approximately -269ºC or -450ºF
Term

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

 

 

319

Definition
quench
Term

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

 

 

321

Definition
fringe
Term

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

 

 

322

Definition
Passive
Term

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

 

 

322

Definition
Active
Term

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

 

 

322

Definition
Shimming
Term

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

 

 

322

Definition
Passive
Term

___________ 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

Definition
Active
Term

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

 

 

325

Definition
polarity
Term

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

 

 

326

Definition
  • gradient strength
  • gradient speed
  • the combination of strength and speed
  • the duty cycle
Term
_________ or __________ defines how steep or strong a particular gradient is. It is measured in milliTesla per meter (mT/m) or gauss per centimeter (G/cm), where 1 G/cm = 10 mT/m.
Definition
Gradient strength, gradient amplitude
Term

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

 

 

326

Definition
Gradient speed, gradient rise time
Term

__________ 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

Definition
Slew rate
Term

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

 

 

326

Definition
Duty cycle
Term

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

 

 

326

Definition
rise time
Term

Stonger gradients generally equal shorter/longer rise times.

 

 

327

Definition
longer
Term

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

 

 

327

Definition
bipolar, balanced
Term

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

Definition
ramp sampling
Term

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

 

 

329

Definition
High-speed
Term

Faraday's law of induction:

 

 

331

Definition

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)

Term

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

 

 

332

Definition
Volume
Term

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

 

 

332

Definition
linear
Term

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

 

 

332

Definition
greater
Term

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

Definition
0.75
Term

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

 

 

333

Definition
size
Term

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

Definition
phased array coil
Term

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

Definition
transmit, receive
Term

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

 

 

336

Definition
cables
Term

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

 

 

336

Definition
tuned
Term

MRI computer systems vary by manufacturer. Most consist of:

 

 

337

Definition
  • 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
Term

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

Definition
pulse programmers
Term

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

 

 

338

Definition
pulse control unit
Term

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

Definition
pulse control unit
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