nuclear spin
- 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)
Precession frequency
- Nuclei in a magnetic field rotate, or precess, like gyroscopes around the direction of the field
- Larmor Frequency
Longitudinal magnetization M0
- The sum of all spins is a vector that lies along B0 and is called longitudinal magnetization M0, also “net magnetization” or “total magnetization”
Magnetic Resonance
- Net magnetization M0 lies along the z-axis, along B0
- Magnetization is tipped in the xy-plane by the RF-pulse
Relaxation
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!
Relaxation times
- 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
TR
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
TE
Zeitspanne, die man nach der Anregung bis zur Messung des MR-Signals verstreichen lässt; bestimmt Einfluss von T2 auf den Bildkontrast
Hemoglobin (HB)
- 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
BOLD
- 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
Spatial Encoding: Frequency encoding
- 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
