Physics

Question 1

nuclear spin

Answer 1

• When placed in a magnetic field, nuclei experience an external magnetic torque that aligns them either in a parallel or an antiparallel direction with respect to the applied magnetic field (B0)

Question 2

Precession frequency

Answer 2

• Nuclei in a magnetic field rotate, or precess, like gyroscopes around the direction of the field
• Larmor Frequency

Question 3

Longitudinal magnetization M0

Answer 3

• The sum of all spins is a vector that lies along B0 and is called longitudinal magnetization M0, also “net magnetization” or “total magnetization”

Question 4

Magnetic Resonance

Answer 4

• Net magnetization M0 lies along the z-axis, along B0

- Magnetization is tipped in the xy-plane by the RF-pulse

Question 5

Relaxation

Answer 5

T1 : restoring the equilibrium (restoring the original spin states)
- T1 measures how quickly the protons realign with the main magnetic field
T2: tissue-dependent loss of coherence in the xy-plane
- T2 measures how quickly the protons give off energy
T2: field-dependent loss of coherence
- T2 relaxation refers to other dephasing effects that cause a signal decay in addition to T2, such as magnetic field inhomogeneity
- T2* contrast good for imaging veins; dark contrast, very sensitive
T2* contrast is the basis for fMRI!

Question 6

Relaxation times

Answer 6

• Echo-time (TE) and repetition-time (TR) indicate the time-points where the data is acquired
• By selecting TR and TE, we can choose T1 vs. T2 weighting

Question 7

TR

Answer 7

Zeit, die zwischen zwei aufeinander folgenden Anregungen derselben Schicht verstreicht – beeinflusst T1 Kontrast, da sie bestimmt, wie lange die Spins Zeit haben, sich von der letzten Anstrengung zu erholen; wenn kurz gewählt, beeinflusst T1 wesentlich den Bildkontrast
o Gewebe mit kurzem T1 relaxieren rasch und erzeugen nach erneuter Anregung ein starkes Signal (im Bild hell)
o Mit langer T1 – alle geben ähnlich intensives Signal ab

Question 8

TE

Answer 8

Zeitspanne, die man nach der Anregung bis zur Messung des MR-Signals verstreichen lässt; bestimmt Einfluss von T2 auf den Bildkontrast

Question 9

Hemoglobin (HB)

Answer 9

• Four goblin chains, each contains a heme group
• At the center of each heme group is an iron atom (Fe)
• Each heme group can attach an oxygen molecule
• Oxy-Hb is diagmagnetic
• Deoxy-Hb I paramagnetic -> it changes the local magnetic field

Question 10

BOLD

Answer 10

• The increase in fresh blood dilutes the deoxyhemoglobin
• More fresh blood than is needed and less deoxyhemoglobin in a voxel causes the image intensity to increase in activated areas
   Blood Oxygenation Level Dependent signal  BOLD signal
• If more deoxyhemoglobin is produced, wouldn’t the signal in the image decrease?
• Yes, but there is also an increase in fresh blood – more fresh blood than is needed and this dilutes the dHb. Less deoxyhemoglobin in a voxel causes the image intensity to increase in activated areas

Question 11

Spatial Encoding: Frequency encoding

Answer 11

• The gradient makes the field at each location slightly different, so that each location has a different frequency; if we know the frequency, we know the location
• We measure frequency  field  location