Chapter 7 hydrogen in a magnetic field

Question 1

MRI takes advantage of the _________of hydrogen in tissues

Answer 1

magnetic properties
-by placing the pt in an externally applied magnetic field of the scanner (B0) we can create tissue magnetization which can be sampled.

Question 2

nuclei with an odd mass # will have a half-integer value, hydrogens value is 1/2. This means there are:

Answer 2

two spin states which are sometimes referred to as ‘spin up’ and ‘spin down’

Question 3

When placed in an externally applied magnetic field, nuclei with spin = 1/2 will:

Answer 3

orient their magnetic moment one of two orientations with respect to the external field

Question 4

Spins aligned parallel (with the field) to the B0 are___ energy state.

Answer 4

LOW

Question 5

spins aligned antiparallel (against the field) are ____energy state

Answer 5

HIGH

Question 6

The energy difference between the 2 spin states is ______to the applied magnetic field (B0)

Answer 6

PROPORTIONAL
the greater B0, the greater the energy difference between these 2 spin states

Question 7

What is thermal equilibrium?

Answer 7

after a few seconds of being exposed to B0, the spins reach a condition known as thermal equilibrium where there is a slight majority of the spins(protons) in the low energy state as opposed to the high energy state.

Question 8

what is net magnetization vector?

Answer 8

-NMV
-excess number of
hydrogen protons aligned with the static magnetic field (low energy)

Question 9

Given that it is net magnetization which will produce the MR signal, _____the strength of B0 _____the number of spins in the low energy state.

Answer 9

increasing;increasing
-this increases the net magnetization of the tissue and thus increases the available signal from the tissue

Question 10

What is precession?

Answer 10

a rotational motion about an axis of a vector whose origin is fixed at the
origin of the coordinate system

Question 11

what is gyromagnetic ratio?

Answer 11

the ratio of the spins angular momentum to its magnetic moment
expressed in units of megahertz/tesla (MHz/T)

Question 12

What is the gyromagnetic ratio of H?

Answer 12

42.6 MHz/Tesla
some texts will say 42.58 MHz/Tesla

Question 13

What is the Larmor Frequency?

Answer 13

the rate at which the magnetic moment of a given magnetically active nuclei will precess about the axis of the externally applied magnetic field.

Question 14

What is the Larmor Frequency equation?

Answer 14

W0 = B0 x Y
(greek letter omega) = B0 x (greek letter gamma)
the precessional frequency = the magnetic field strength (in Tesla) x gyromagnetic ratio

Question 15

how can the precessional frequency for hydrogen be calculated?

Answer 15

by the Larmor Equation

Question 16

______B0 causes the precessional or resonant freq of hyrdogen to increase.

Answer 16

Increase.
Decreasing B0 will cause the resonant frequency to decrease.

Question 17

the frequency at which the hydrogen protons precess is known as the:

Answer 17

larmor frequency or resonant frequency

Question 18

true or false
A hydrogen proton in a water molecule will resonate at a slightly higher frequency than a hydrogen proton in a fat molecule.

Answer 18

true

Question 19

what is chemical shift

Answer 19

difference in resonant frequencies between fat and water

Question 20

the amount of chemical shift between the frequencies of water and fat protons can be expressed in 2 different ways:

Answer 20

-units in parts per million (ppm) (independent of field strength)
-units of Hertz (Hz)

Question 21

what is shimming?

Answer 21

The process of improving the homogeneity of the magnetic field,
either through active shimming (use of shim coils) or passive shimming (adding or
removing steel from the magnet)