EXAM #11 — PHYSICS UNIT 04

Question 1

define the following terms as they apply to nuclear magnetism in the MRI environment:

quantum mechanical description.

Answer 1

quantum mechanical description.

study of individual units of matter or energy (i.e. protons)

Question 2

define the following terms as they apply to nuclear magnetism in the MRI environment:

classical mechanical description.

Answer 2

classical mechanical description.

study of bundles of matter or energy (i.e. net magnetization)

Question 3

define the following terms as they apply to nuclear magnetism in the MRI environment:

nuclear magnetic moment.

Answer 3

nuclear magnetic moment.

sum total of the magnetic moments of the protons in a nucleus

Question 4

define the following terms as they apply to nuclear magnetism in the MRI environment:

net magnetization.

Answer 4

net magnetization.

the sum total of the magnetic moments of all the protons in a sample of tissue

Question 5

define the following terms as they apply to nuclear magnetism in the MRI environment:

precession.

Answer 5

precession.

the motion of a proton or net magnetic vector rotating about an external magnetic field

Question 6

define the following terms as they apply to nuclear magnetism in the MRI environment:

Larmor frequency or resonant frequency, to include its unit of measurement.

Answer 6

Larmor frequency or resonant frequency, to include its unit of measurement.

the frequency at which the net magnetic vector precesses about the Z axis; or the frequency at which the net magnetic vector can absorb transmitted RF; measured in Hz

Question 7

define the following terms as they apply to nuclear magnetism in the MRI environment:

resonance.

Answer 7

resonance.

the ability of the net magnetic vector to gain energy from an external source (RF)

Question 8

define the following terms as they apply to nuclear magnetism in the MRI environment:

gyromagnetic ratio.

Answer 8

gyromagnetic ratio.

the precessional frequency of a specific nucleus at 1.0T

Question 9

define the following terms as they apply to nuclear magnetism in the MRI environment:

rotating frame of reference.

Answer 9

rotating frame of reference.

the frame of reference that assumes that the observer is rotating with the motion of the net magnetization vector; simplifies the motion into 2 dimensions

Question 10

define the following terms as they apply to nuclear magnetism in the MRI environment:

laboratory frame of reference.

Answer 10

laboratory frame of reference.

the frame of reference that assumes that the observer is stationary and all motion is being observed

Question 11

define the following terms as they apply to nuclear magnetism in the MRI environment:

spin (or proton) density.

Answer 11

spin (or proton) density.

the relative number of spins or protons in a sample

Question 12

identify the 3 subatomic particles that make up an atom, and their associated electrical charges.

Answer 12

proton (+), neutron (no charge), electron (-)

Question 13

identify the 2 subatomic particles that make up an atomic nucleus.

Answer 13

proton, neutron

Question 14

identify the subatomic particle that plays the biggest role in MRI signal formation.

Answer 14

proton

Question 15

explain the relationship between the number of protons in an atomic nucleus and the magnetic moment of the nucleus.

Answer 15

only nuclei with an odd number of protons have a nuclear magnetic moment, the more protons in a nucleus the lower the magnetic moment (hydrogen has a high magnetic moment due to the single proton in the nucleus)

Question 16

identify the element that is imaged in clinical MRI.

Answer 16

hydrogen

Question 17

identify 2 reasons that hydrogen is imaged in clinical MRI.

Answer 17

large magnetic moment, abundant in the body

Question 18

identify the energy states possible for a hydrogen atom in a static magnetic field.

Answer 18

high energy and low energy (anti- parallel and parallel)

Question 19

explain the relationship between high and low energy states and parallel and antiparallel orientation to an external magnetic field.

Answer 19

high energy protons align anti-parallel, low energy protons align parallel

Question 20

explain the orientations of atomic nuclei in the presence and absence of an external static magnetic field.

Answer 20

in the absence of an external field, nuclei are randomly oriented, in the presence of one they are aligned parallel or anti-parallel

Question 21

explain the orientation and strength of net magnetization in the presence and absence of an external static magnetic field.

Answer 21

in the absence of an external field there is no net magnetization, in the presence of one there is net magnetization aligned with the external field

Question 22

identify the gyromagnetic ratio of the hydrogen atom.

Answer 22

42.6MHz

Question 23

identify the conventional direction of an external magnetic field on a vector diagram.

Answer 23

vertical, or aligned with the Z axis

Question 24

identify the axis about which the net magnetization of a sample precesses at equilibrium.

Answer 24

Z axis

Question 25

identify the letter used to denote net magnetization.

Answer 25

M

Question 26

identify the notation for the net magnetization vector at equilibrium.

Answer 26

Mo

Question 27

identify the notation for net magnetization as it precesses about the Z axis of a vector diagram.

Answer 27

Mz

Question 28

identify the notation for net magnetization as it precesses in the XY plane of a vector diagram.

Answer 28

MXY

Question 29

identify the relationship between the number of protons in a sample and the strength of the net magnetization of the sample.

Answer 29

as the number of protons increases, net magnetization increases

Question 30

explain the relationship between the external magnetic field strength and net magnetization.

Answer 30

the higher the field strength the stronger the net magnetization

Question 31

state the 2 properties that an RF pulse must have in order to create resonance.

Answer 31

RF pulse must be at the Larmor frequency and perpendicular to the external magnetic field (Bo)