Presentation on theme: "Proton Spin In absence of a magnetic field, protons spin at random"— Presentation transcript:
1 Proton Spin In absence of a magnetic field, protons spin at random A magnetic field is used to align them (B0)Current MRI fields run at 0.5 and 3.0 Teslas1 Tesla is equal to 10,000 GaussEarth's magnetic field varies from between Gauss.Refrigerator magnet ~ Gauss
2 MR visible nucleiProtons must be unbound to allow them to spin and change spinsBound protons are not MR visible as they don’t emit RF from energy changesCortical bone contains bound H and thus always appears black on MRFat and Water contain lots of unbound H and make soft tissue imaging so sensitive to MR
3 PrecessionIn a magnetic field the proton precesses at a given frequency based on the Larmor equationThis frequency is based on field strength and the specific nucleus – each has a different frequencyIn MRI the precession frequencies occur in the RF range and are detected by an antenna
4 Pulse sequence Tipping the spin into a different plane from B0 B1 is the field used to tip the spin measured in degrees from B090° RF pulse tips it into the transverse plane or x’180° RF pulse inverts it to a –z direction
5 Pulse SequenceTipping the spin into another plane puts them into a high energy state once the new field is removedDecay happens rapidly back to the original state and the energy given off by the decay is in the radiofrequency rangeThis energy is called the FID (Free Induction Decay)This is received by an antenna
6 Signal to Noise Ratio Signal to Noise Ratio The amount of useful signal compared to useless noise from RFCoherent signal is additive while incoherent noise tends to cancel outSNR is proportional to the square root of the number of averages.Doubling the # of slices doubles time but improves SNR by 1.4
7 T2Having been tipped into the transverse plane, the net magnetisation begins to dephase (T2*)Rapid decay of unified spin based on tissue and magnetic field
8 T1 Once fully dephased the spins return to equilibrium (T1) Takes longer than T2 to reach equilibrium
9 TE and TR TE = Echo Time TR = Repetition time Time from 90° pulse to the time after the 180° that the echo from the tissue is detectedTR = Repetition timeTime between pulses (ie ms spacing between 90°-180°-180° pulses)
10 TE governs T2Long TE allows for differences in T2 times to emerge. Water, with a long T2 time takes longer to decay and thus gives a stronger signal. T2 weighted = H20 brightUsing a long TE interval, tissues with long TE becomes brighter. A short TE would make long TE tissues dark
11 TR governs T1Tissue with a long TR (like water) takes a long time to recover to equilibriumUsing a short TR interval, tissues with short TR (like fat) have given off more signal (brighter) than a slower decaying tissue
12 PD weighting Proton Density weighting Density of spins or number of protons in a tissueShort TE – decreases water signalLong TR – decreases fat signalSuppression of fat and waterBetter anatomic detail
13 MRI Contrast T2-weighting requires long TE, long TR T1-weighting requires short TE, short TRPD-weighting requires short TE, long TR
15 Figure 1. Representative axial brain images of the eight patients with ACA infarctions. Patient numbers appear in the upper left corner of each panel. Patient 1, T1-weighted axial MRI; patient 2, CT scan; patient 3, T2-weighted MRI; patient 4, T1-weighted MRI; patient 5, proton density MRI; patient 6, CT scan; patient 7, T2-weighted MRI; and patient 8, T2-weighted MRI.
16 MRASpecific gradients are applied to phase and dephase the free H rapidlyH that is flowing into an area that was just dephased has not yet been dephased, thus producing high signalThis is repeated rapidly and flow can be monitored
17 Spin-Echo MRI This “family” includes T1, T2 & Proton Density RF pulses excite the protons to flip 90 degrees from the main magnetic field, then 180 degrees (to refocus the phase of precession). The first “echo” (RF from the tissue) is obtained. This first echo provides the data for either T1 or proton density contrast. To obtain the data for T2 contrast, a second 180 degree pulse is then applied to obtain a second “echo”This is termed the pulse train.The times between the RF excitations (TR & TE) vary depending on the desired contrast. This produces various tissue signal intensity relationships which are referred to as “weighting” characteristics
18 Spin Echo (Gp) phase encoding gradient (y plane) (Gf) frequency encoding during signal detection(Gs) slice-selection gradient during RF pulse to spin up and refocus only the slice of interest
19 Inversion Recovery/ IR In this family the pulse train isthe time of inversion / TI (“tee eye”) = the time between the first 180 and the 90 pulseTI times can be tailored by suppressing signal from specific types of tissue. Greater contrast
20 STIR/ Short TI Inversion Recovery A short TI value suppresses the signal from fat and causes abnormal water destiny to “light up” brightly.one of the best ways to image bone marrow and spinal cord pathology. (T2 with fat saturation also works well.)
21 T2W fast-spin echo with fat-sat (saturation>suppression) shows normal meniscus, but increased signal at the posterior of the distal femoral metaphysis in teenager with a cortical desmoid
22 FLAIR= Fluid Attenuated Inversion Recovery Long TI inversion recovery produces bright signal from brain pathology but CSF signal is suppressedThis allows better contrast of CNS lesions when they are located at CSF interfaces (common in MS)
23 True Inversion Recovery aka T1 FLAIR New pulse sequence on 3T systemsProduces true anatomic contrast between grey vs. white brain matter
24 Gradient Echo or Field Echo In this technique, the protons are excited to flip angles of less than 90 degrees (which is faster than SE)instead of using a 180-degree refocusing RF pulse the precessions are “refocused” by reversing the polarity of the magnetic field gradient (The higher magnetic field side becomes the lower magnetic field side and vice versa.).This can be used to rapidly produce extremely bright water signal in the CSF (the Rapid Myelographic technique) or in synovial fluid.
25 T2* (tee 2 star) gradient echo/GE with fat saturation a variation of GE and FSE that suppresses the signal from fat while imaging fluids brightly but is extremely fast.another fine technique for imaging bone marrow pathology and pathology of articular cartilage.
26 Gradient Echo or Field Echo Rapid imaging produces myelographic effectSensitive to magnetic susceptibility (Hgb)
27 3D ImagingThis modification of Gradient/ Field Echo, and Fast Spin Echo/FSE, acquires data for imaging as a block or cube rather than as slices. The data can later be “cut up” into slices of any thickness in any desired plane.Allows for ultra thin slice images (1mm or less) when necessary.
28 69 yr old patient presented with loss of consciousness. A) CT demonstrates hypodensity consistent with infarction of basilar artery distribution in the left cerebellar lobe,vermis and medial portion of the right cerebellar lobe.B,C) MRI FLAIR and MRI T2WI demonstrate vasogenic edema in the same distribution.D) ADC map (b=1000) demonstrates diffusion restriction due to cytotoxic edema in the same distribution.E,F) DWI (b=500, b=1000) also demonstrates diffusion restriction due to cytotoxic edema.This case demonstrates acute infarction in the left and right cerebellar lobes and the vermis in a stage of intermixed vasogenic and cytotoxic edema.