1. T2* RELAXATION
1.1. PHASE COHERENCE
1.1.1. CAUSED BY VARIATIONS IN MAGNETIC FIELD
1.1.2. AS MORE SPINS LOSE ENERGY THEY BUMP INTO EACH OTHER
1.1.3. IN PHASE OR MAINTAINED PHASE COHERENCE= MAX SIGNAL BECAUSE ALL SPINS ARE PRECESSING IN SAME DIRECTION, AT SAME FREQUENCY.
1.1.4. OUT OF PHASE OR LOSS OF PHASE COHRENCE= EACH SPIN WILL PRECESS AT DIFFERENT FREQUENCY, IN A SLIGHTLY DIFFERENT DIRECTION
1.2. TOTAL NET MAGNETIZATION VECTOR IS SHORTEST
1.3. = T2 TISSUE + T2 MAGNETIC FIELD
1.3.1. T2 MAG FIELD OVERWHELMS T2 TISSUE CONCENTRATION
1.3.2. ONLY GETTING INFO ABOUT VARIATIONS IN MAG FIELD, NO INFO FROM PATEIENT
1.3.3. What is a free induction decay (FID)? - Destruction of the net magnetisation vector without loss of energy to the environment ("free"). - The oscillating decaying MRI signal in the transverse plane. - The process by which spins are excited by an RF pulse. - NMR Signal in the absence of any magnetic gradients. - Decays exponentially; at t = T2, 63.2% of the signal has been lost. - The decay curve is the signal envelope. The actual signal is oscillating at the resonance frequency in the MHz range.
1.4. THE ENVELOPE OF THE FREE INDUCTION DECAY
2. SIGNAL STRENGTH PROPORTIONAL TO
2.1. PROTON DENSITY
2.2. GYROMAGNETIC RATIO SQUARED
2.3. MAGNETIC FIELD STRENGTH
3. PROTON (SPIN) DENSITY
3.1. CONCENTRATION OF MOBILE H-NUCLEI IN TISSUE
3.1.1. CORTICAL BONE vs MEDULLARY BONE
3.1.2. FOUND IN FLUIDS
3.2. SPIN DENSITY OF SOME TISSUES ( APPROXIMATE VALUES OF MOBILE H-NUCLEI)
3.2.1. CSF = 100
3.2.2. FAT = 95
3.2.3. MUSCLE = 90
3.2.4. BLOOD = 85
3.2.5. GRAY MATTER = 70
3.2.6. WHITE MATTER = 60
3.2.7. CORTICAL BONE = 1 - 10
3.2.8. LUNG = 1 - 5
3.2.9. AIR < 1
3.3. SATURATION RECOVERY PULSE SEQUENCE MAKES PDW IMAGE
4. T1 RELAXATION
4.1. MR IMAGES
4.1.1. LONG T1 = DARK
4.1.2. SHORT T1 = BRIGHT
4.2. TIME NEEDED FOR Mz TO REGROW TO Mo
4.3. Mz= Mo(1-e^(-t/t1))
4.3.1. 5 T1= TOTAL RELAXATION
4.3.2. T1= 0.67 Mo
4.4. LONGITUDINAL RELAXATION
4.4.1. EVENTS OCCUR ALONG THE AXIS OF THE NET MAGNETIZATION VECTOR, PARALLEL TO THE MAGNETIC FIELD
4.5. SPIN-LATTICE RELAXATION
4.5.1. ENERGY FROM RF PULSE IS LOST FROM H-NUCLEI (SPIN) TO ENVIRONMENT/MOLECULE (LATTICE)
4.6. TI RELAXATION TIMES OF TISSUES (ms)
4.6.1. CSF= 2000
4.6.2. BLOOD = 800
4.6.3. MUSCLE = 600
4.6.4. GRAY MATTER = 520
4.6.5. WHITE MATTER = 390
4.6.6. FAT = 180
4.6.7. DISEASED TISSUE USUALLY HAS LONGER T1 TIMES THAN DOES CORRESPONDING HEALTHY TISSUE
5. T2 RELAXATION
5.1. MR IMAGES
5.1.1. SHORT T2 = DARK
5.1.2. LONG T2 = BRIGHT
5.2. TIME NEEDED FOR Mxy TO DECAY TO 0
5.3. Mxy= Mo ( 1- e^ (-T/T2) )
5.4. TRANSVERSE RELAXATION
5.4.1. EVENTS OCCUR PERPENDICULAR TO THE AXIS OF THE NET MAGNETIZATION VECTOR, PERPENDICULAR TO THE MAGNETIC FIELD
5.5. SPIN-SPIN RELAXATION
5.5.1. ENERGY FROM RF PULSE IS LOST FROM H-NUCLEI (SPIN) TO OTHER H-NUCLEI (SPIN) BY COLLISION
5.6. T2 RELAXATION TIMES OF TISSUES
5.6.1. CSF = 200
5.6.2. BLOOD = 180
5.6.3. GRAY MATTER = 90
5.6.4. WHITE MATTER = 75
5.6.5. FAT = 90
5.6.6. MUSCLE = 40
6. T1 > T2 > T2*
7. MEASURING RELAXATION TIMES
7.1. TRACK RUNNER'S ANALOGY
7.1.1. EACH SPIN IS A RUNNER, SOME RUN FASTER AROUND THE TRACK THAN OTHERS
7.1.2. THE STARTING GUN FIRES = 90 PULSE KNOCKS SPINS INTO Mxy, FID SIGNAL OCCURS
7.1.3. RUNNERS PROCESS AT DIFFERENT SPEEDS = LOSS OF PHASE COHERENCE
7.1.4. STARTING GUN = 180 PULSE
7.1.4.1. THE RUNNERS REVERSE DIRECTION AND WILL ALL END AT STARTING LINE AT THE SAME TIME
7.1.4.2. 180 PULSE BRINGS SPINS INTO PHASE COHERENCE
7.1.4.3. = MAXIMUM MR SIGNAL OCCURS AT 2t (ms) ... t = TIME OF 180 PULSE (STARTING GUN)
7.1.4.4. 2nd SPIN ECHO IS SMALLER, BUT NEEDED TO CALCULATE THE T2 FROM THE T2* IN THE SIGNAL
7.1.5. DE-PHASING WILL OCCUR AGAIN, SO ANOTHER SHOT OF GUN = 2nd 180 PULSE
7.1.5.1. NEED BOTH SIGNALS TO CALCUATE OUT THE T2 FROM THE T2* IN THE SPIN ECHO SIGNA;
7.1.6. THE RUNNERS GET TIRED = THE REDUCTION OF SIGNAL FROM REPEATED 180 PULSES
7.2. T2 RELAXATION TIME
7.2.1. SPIN ECHO PULSE SEQUENCE
7.2.1.1. Mz, Mx, My COMPONENTS
7.2.1.2. SE PULSE SEQUENCE
7.2.1.2.1. 90
7.2.1.2.2. 180
7.2.1.2.3. 180
7.2.2. T2 RELAXATION EVELOPE
7.3. T1 RELAXATION
7.3.1. USE IR PULSE SEQUENCE ... AND
7.3.1.1. 180
7.3.1.1.1. KNOCKS SPINS DOWN INTO NEGATIVE Z-AXIS (- Mz)
7.3.1.1.2. THIS STEP, THE INVERSION, ALLOWS T1 RELAXATION TO BE MEASURED BECAUSE THE SIGNAL IS FROM THE Mz VECTOR ONLY (REGROWTH OF Mz TO Mo= T1 RELAXATION)
7.3.1.1.3. Ti = TIME TO INVERSION , DELAY TIME BETWEEN 180 AND 90 PULSES
7.3.1.2. 90
7.3.1.2.1. KNOCKS SPINS INTO Mxy
7.3.1.2.2. FID FOLLOWS
7.3.1.2.3. TE = TIME TO ECHO
7.3.1.3. 180
7.3.1.3.1. RE-PHASES SPINS IN Mxy
7.3.1.3.2. CREATES SE OF T1 RELAXATION
7.3.2. CHANGE THE LENGTH (t) OF T i
7.3.2.1. TIME TO INVERSION, DELAY TIME (BTWN 180 AND 90 PULSE)
7.3.2.2. CHANGING LENGTH OF Ti CREATES T1 RELAXATION CURVE
7.4. IMAGE WEIGHTING AND PULSE SEQUENCE PARAMETERS
7.4.1. T1W IMAGES
7.4.1.1. SHORT Tr < 600 ms
7.4.1.2. SHORT Te < 20 ms
7.4.2. T2W IMAGE
7.4.2.1. LONG Tr > 200 ms
7.4.2.2. LONG Te > 90 ms
7.4.3. PDW IMAGE
7.4.3.1. LONG Tr > 2000 ms
7.4.3.2. SHORT Te < 20 ms
7.5. SPECIAL IMAGING USING INVERSION RECOVERY PULSE SEQUNCE
7.5.1. INVERSION RECOVERY PULSE SEQUENCE
7.5.2. ALLOWS YOU TO OMIT (ZERO OUT) A TISSUE'S SIGNAL BY MATCHING T1 RELAXATION TIMES OF THE TISSUE WITH THE DELAY TIME (Ti)
7.5.2.1. FAT
7.5.2.1.1. STIR
7.5.2.1.2. ANYTIME IMAGING PATHOLOGY THAT MAY BE SURROUNDED BY FAT, MUST USE FAT SAT/STIR TO REMOVE T1 SIGNAL FROM FAT
7.5.2.2. FLUID
7.5.2.2.1. FLAIR
7.6. PULSE SEQUENCES AND RESULTING IMAGE WEIGHTING
7.6.1. SATURATION RECOVERY
7.6.1.1. PDW (SDW) ONLY
7.6.1.2. USES FID SIGNAL
7.6.2. PARTIAL SATURATION RECOVERY
7.6.2.1. PDW AND TI W
7.6.2.1.1. DEPENDING ON Tr
7.6.2.1.2. INTERMEDIATE Tr TIMES CAUSE LOSS OF CONTRAST
7.6.2.2. LONG Tr =PDW
7.6.2.3. SHORT Tr ( < 500 ms) = TIW
7.6.2.4. USES FID SIGNAL
7.6.3. SPIN ECHO PULSE SEQUENCE
7.6.3.1. T1W
7.6.3.1.1. LONG T1 t = DARK
7.6.3.1.2. SHORT TI t = BRIGHT
7.6.3.2. T2W
7.6.3.2.1. SHORT T2 t = DARK
7.6.3.2.2. LONG T2 T = BRIGHT
7.6.3.3. USES SPIN ECHO SIGNAL, NOT FID
7.6.4. INVERSION RECOVERY PULSE SEQUENCE
7.6.4.1. ONLY T1W IMAGES
7.6.4.1.1. LONG T1 = DARK
7.6.4.1.2. SHORT TI = BRIGHT
7.6.4.2. USES FID SIGNAL