MRI Safety, hardware, and magnetism

When moving blood enters a magnetic field, an effect on the ECG seen as a "T wave swelling" or spike occurs, also known as the

magnetohydrodynamic effect

FDA limits for whole body SAR is _________ while Head limits are________

4 W/kg over 15 minutes; 3 W/kg over 10 minutes

As the flip angle is doubled, the RF absorption increases by

A factor of 4

visual stimulation, described as "seeing stars", is known as

Magnetophosphenes

FDA recommends posting warning signs to indicate the

5 Gauss line

RF energy absorption produces a maximum safe increase in body temperature of

1 degree celcius (1.8F )

FDA regulations for magnetic field strength are ____ for infants 1 month old or younger, and ___ for anyone older than 1 month.

4 Tesla; 8 tesla

Public access area outside MRI environment

Zone 1

Interface between public area and controlled MRI area, where patients are greeted and screened

Zone 2

Restricted area with access controlled by MRI personnel, before entering magnet room

Zone 3

Magnet room itself, where the scanner is located and the magnetic field is always present

Zone 4

The FDA limit on time varying magnetic fields is

once the patient experiences peripheral nerve stimulation.

The general public is limited to a magnetic field of

5 Gauss

What body parts don’t dissipate SAR heat as quickl

Eyes and Testicles

The time it takes for the gradient to reach its full amplitude, measured in microseconds

Rise time

speed rate of ascent or descent of a gradient from zero to its maximum amplitude (Negative or positive) And what does it affect

Slew rate (Image resolution)

1 Tesla/meter = in gauss?

100 Gauss / cm (Move decimal over twice)

The time the gradients are on during a TR period, the “gradient working time.”

Duty cycle

Sagittal slice plane is controlled by the

X gradient

Coronal slice plane is controlled by the

Y Gradient

Transverse’axial slice plane is controlled by the

Z gradient

What is the order of coils from innermost to outermost?

RF coils, Gradient coils, Shim coils, and main magnet

Formula for calculating slew rate?

Gradients Amplitude / Rise time

What percentage increase of SNR occurs when using a quadrature coil vs a linear coil?

40%

SAR increases with the _____ of the magnet’s field strength

Square (Doubling field strength quadruples SAR)

SAR is measured in what?

Watts/kg

Passive (magnetic) shielding can be accomplished by

lining the MR room with steel (Or other ferromagnetic mat.)

Active shielding requires the implementation of

superconducting windings within the construction of the MR scanner

The liquid cryogens are cooled in a superconducting magnet at a temperature of

4 Kelvin (-270 C; -452 F)

Lining the walls with copper/steel/aluminum is for what?

RF Shielding

High field strength scanner typically have field strengths greater or equal to?

1.5 T

The main unit of measurement of the magnetic field SURROUNDING the periphery of the scanner is expressed as?

Gauss (Smaller unit weaker field)

Accuracy of magnetic field in determing coil tuning and consistent magnetic field strength?

Center frequency

Larmor equation for precessional frequency

Precessional frequency = Gyromagnetic ratio x magnetic field strength (ω = γ x B)

Precessional frequency for hydrogen is

42.58

Magnetic field inhomogeneity is expressed in

parts per million (ppm)

T1 relaxation time

recovery of 63% longitudinal magnetization

T2 relaxation time is defined as the time it takes for

spins to de-phase to 37% of their original value.

how well a material attracts the imaginary lines of the magnetic field.

Magnetic permeability

______ _____ is the extent to which a material or tissue becomes magnetized in an external magnetic field.

Magnetic susceptibility

The energy used to form MRI images is _____ the pt’s tissues

Emitted from

Which type of magnet can be shut off the quickest

Resistive

Contrast reactions in MRI are incredibly rare happening __

~1%

_______ is the magnetic and thermal environment which nuclei exchange energy in longitudinal (T1) relaxation

Lattice

MR technique which assess properties of tissues based on propagation of shear waves

Magnetic Resonance Elastography (MRE)

MR Technique involving generating a spectrum of metabolites in a specific VOI

Magnetic Resonance Spectroscopy (MRS) _

____ gradient magnetic field gradient applied during the moment when the echo is formed

Readout gradient (Frequency encoding gradient)

A multiprocessor that performs calculations for Fast Fourier Transform (FFT) ; converting raw K-space data in final image

Array Processor

The array processor takes the raw k-space ____ and turns it into the ____ image

data; final

K-space is measured in

Radians/cm

After the Echo; the analog signal is then converted to?

Digital Data points

The center of the K-space matrix is reserved for?

High signal and contrast information

The outside edges of K-space is reserved for?

Highest resolution information

What process converts frequency amplitudes in the TIME domain to frequency amplitudes in the FREQUENCY domain

Fast Fourier Transform (FFT)

Once protons are hit with an RF pulse; the precessing magnetization induces a current in the receiver coil called what?

Free Induction Decay