Chapter 7 hydrogen in a magnetic field Flashcards
MRI takes advantage of the _________of hydrogen in tissues
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.
nuclei with an odd mass # will have a half-integer value, hydrogens value is 1/2. This means there are:
two spin states which are sometimes referred to as ‘spin up’ and ‘spin down’
When placed in an externally applied magnetic field, nuclei with spin = 1/2 will:
orient their magnetic moment one of two orientations with respect to the external field
Spins aligned parallel (with the field) to the B0 are___ energy state.
LOW
spins aligned antiparallel (against the field) are ____energy state
HIGH
The energy difference between the 2 spin states is ______to the applied magnetic field (B0)
PROPORTIONAL
the greater B0, the greater the energy difference between these 2 spin states
What is thermal equilibrium?
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.
what is net magnetization vector?
-NMV
-excess number of
hydrogen protons aligned with the static magnetic field (low energy)
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.
increasing;increasing
-this increases the net magnetization of the tissue and thus increases the available signal from the tissue
What is precession?
a rotational motion about an axis of a vector whose origin is fixed at the
origin of the coordinate system
what is gyromagnetic ratio?
the ratio of the spins angular momentum to its magnetic moment
expressed in units of megahertz/tesla (MHz/T)
What is the gyromagnetic ratio of H?
42.6 MHz/Tesla
some texts will say 42.58 MHz/Tesla
What is the Larmor Frequency?
the rate at which the magnetic moment of a given magnetically active nuclei will precess about the axis of the externally applied magnetic field.
What is the Larmor Frequency equation?
W0 = B0 x Y
(greek letter omega) = B0 x (greek letter gamma)
the precessional frequency = the magnetic field strength (in Tesla) x gyromagnetic ratio
how can the precessional frequency for hydrogen be calculated?
by the Larmor Equation
______B0 causes the precessional or resonant freq of hyrdogen to increase.
Increase.
Decreasing B0 will cause the resonant frequency to decrease.
the frequency at which the hydrogen protons precess is known as the:
larmor frequency or resonant frequency
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.
true
what is chemical shift
difference in resonant frequencies between fat and water
the amount of chemical shift between the frequencies of water and fat protons can be expressed in 2 different ways:
-units in parts per million (ppm) (independent of field strength)
-units of Hertz (Hz)
what is shimming?
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)
