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# Mri basic principle and sequences

MRI,BASIC PRINCIPLE,HOW TO IDENTIFY IMAGES,ARTIFACTS

MRI,BASIC PRINCIPLE,HOW TO IDENTIFY IMAGES,ARTIFACTS

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### Mri basic principle and sequences

1. 1. PRESENTER: DR PAWAN KUMAR MRI-BASIC PRINCIPLE/TECHNIQUE/READIN G
2. 2. Nuclear magnetic resonance ►Dr Isidor Rabi (Nobel in 1944), discovered NMR (Nuclear Magnetic Resonance) in the late 1930s, but considered it to be an artefact of his apparatus ►CJ Gorter, coined the term ‘Nuclear Magnetic Resonance’ in 1942. ►Bloch and Purcell (Nobel Prize in 1952 )if Certain nuclei are placed in magnetic field ,absorb energy in electromagnetic spectrum and re emit energy when regain their original position.
3. 3. MRI HARDWARE  Permanent magnets  Resistive magnets: In a resistive magnet, an electrical current is passed through a loop of wire and generates a magnetic field.  Superconducting magnets: Superconducting magnets are the ones most widely used in MR machines at the present time. They also make use of electricity, but they have a special current carrying conductor.  This is cooled down to superconducting temperature (about 4° K or -269° C). At this temperature, the current conducting material loses its resistance for electricity.
4. 4.  In MRI radio frequency coils are necessary to send in the RF pulse to excite the protons, and to receive the resulting signal. The same or different coils can be used for transmission of the RF pulse and receiving the signal.
5. 5. Necessary Equipment Magnet Gradient Coil RF Coil Source: Joe Gati, photos RF Coil 3T magnet gradient coil (inside)
6. 6. MRI principle  MRI is based on the principle of nuclear magnetic resonance (NMR)  Two basic principles of NMR: 1.Atoms with an odd number of protons or neutrons have spin 2.A moving electric charge, be it positive or negative, produces a magnetic field
7. 7. Protons  Protons possess a positive charge.  Like the earth they are constantly turning around an axis and have their own magnetic field.
8. 8. Protons  Normally protons are aligned in a random fashion. This, however, changes when they are exposed to a strong external magnetic field.  Then they are aligned in only two ways, either parallel or antiparallel to the external magnetic field
9. 9. Precession  Protons perform a wobbling type of motion in a strong magnetic field ,called precession.
10. 10. Precession Frequency  To calculate the precession frequency. Larmor equation:  B0 = is the strength of the external magnetic field, which is given in Tesla (T)  is the angular frequency, and ‘y’is the so- called gyromagnetic ratio.
11. 11. EFFECT OF MAGNETIC FIELD  The z-axis runs in the direction of the magnetic field lines,  The five protons, which "point" down cancel out the magnetic effects of the same number of protons, which "point"up
12. 12. Radio Frequency (RF)  A short burst of some electromagnetic wave, which is called a radio frequency (RF) pulse.  The purpose of this RF pulse is to disturb the protons, which are peacefully precessing in alignment with the external magnetic field.  Not every RF pulse disturbs the alignment of the protons. For this, we need a special RF pulse, one that can exchange energy with the protons.  Only when the RF pulse and the protons have the same frequency, can protons can pick up some energy from the radio wave, a phenomenon called Resonance.
13. 13. Longitudinal magnetization  the vectors along the z-axis point in the same direction, and thus add up to a new magnetic sum vector pointing up. As this magnetization is in direction along/longitudinal to the external magnetic field, it is also called longitudinal magnetization
14. 14. EFFECT OF RF PULSE ON MAGNETIZATION  The radiofrequency pulse exchanges energy with the protons (a), and some of them are lifted to a higher level of energy, pointing downward in the illustration (b).  In effect the magnetization along the z-axis decreases, as the protons which point down "neutralize" the same number of protons pointing up.
15. 15. Transversal Magnetization  When the protons randomly point left/right, back/forth and so on, they also cancel their magnetic forces in these directions  The RF pulse synchronize them - "in phase". They now point in the same direction at the same time, and thus their magnetic vectors add up in this direction. This results in a magnetic vector pointing to the side to which the precessing protons point, and this is in a transverse direction.  This is why it is called transversal magnetization.
16. 16.  As the transversal magnetic vector moves around with the precessing protons, it comes towards the antenna, goes away from it, comes towards it again and so on, also with the precession frequency.  The resulting MR signal therefore also has the precession.
17. 17. Longitudinal and transversal relaxation  The newly established transverse magnetization starts to disappear (a process called transversal relaxation), and the longitudinal magnetization grows back to its original size (a process called longitudinal relaxation).
18. 18. Longitudinal/Spin lattice relaxation  After the RF pulse is switched off, protons go back from their higher to the lower state of energy, i.e. point up again.  This energy is just handed over to their surroundings, the so called lattice. And this is why this process is not only called longitudinal relaxation, but also spin-lattice-relaxation
19. 19. T1-Curve  If one plots the longitudinal magnetization vs. time after the RF pulse was switched off, one gets a so-called T1-curve.
20. 20. SPIN SPIN RELAXATION  After the RF pulse is switched off, protons lose phase coherence, they get out of step thus transversal magnetization decreases.
21. 21.  Plot showing transversal magnetization vs. time after the RF pulse is switched off, one gets a curve as illustrated,which is called a T2-curve.
22. 22. Characteristics of T1 and T2  T1 is about 300 to 2000 msec, and T2 is about 30 to 150 msec.  It is difficult to pinpoint the end of the longitudinal and transversal relaxation exactly.  T1 =63% of the original longitudinal magnetization is reached.  T2 =37% of the original value,T1 is longer than T2  T1 varies with the magnetic field strength; it is longer in stronger magnetic fields.
23. 23. 90°pulse  If after the RF pulse, the number of protons on the higher energy level equals the number of protons on the lower energy level, longitudinal magnetization disappears, and there is only transversal magnetization due to phase coherence.  The magnetic vector seems to have been "tilted“ 90° to the side. The corresponding RF pulse is thus also called a 90° pulse
24. 24. TE vs TR  • TE: the time between the 90° pulse and the echo.  • TR: the time between two pulse sequences, i.e. from one 90° pulse to the next.
25. 25. HOW TR AFFECTS THE SIGNAL INTENSITY OF TISSUE
26. 26.  A and B are two tissues with different relaxation times.  Frame 0 shows the situation before, frame 1 immediately after a 90° pulse.  When we wait for a long time (TRlong) the longitudinal magnetization of both tissues will have totally recovered (frame 5).  A second 90° pulse after this time results in the same amount of transversal magnetization (frame 6) for both tissues, as was observed
27. 27.  When we do not wait as long , but send in the second RF pulse after a shorter time (TRShort), longitudinal magnetization of tissue B, which has the longer T1, has not recovered as much as that of tissue A with the shorter T1.  The transversal magnetization of the two tissues after the second RF pulse will then be different (frame 5). Thus, by changing the time between successive RF pulses, we can influence and modify magnetization and the signal intensity of
28. 28. TR AND SIGNAL INTENSITY  Brain has a shorter longitudinal relaxation time than CSF.  With a short TR the signal intensities of brain and CSF differ more than after a long TR.
29. 29. T2*/T2effect/spin echo sequence  After the RF pulse is switched off, the protons dephase (a-c).  The 180°pulse causes them to precess in the opposite direction and so they rephase again (d-f).
30. 30. T2* curve  The 180° pulse refocusses the dephasing protons which results in a stronger signal, the spin echo after the time TE.  The protons then dephase again and can be refocussed another time by a 180° pulse and so on. Thus it is possible to obtain more than one signal, more than one spin echo.  The spin echoes, however, differ in intensity due to so-called T2- effects.  A curve connecting the spin echo intensities is the T2 curve. If we did not use the 180° pulse, the signal intensity would decay much faster. A curve describing the signal intensity in that case is the T*2 (T2 star) curve, The type of pulse sequence, that we used in our experiment, is called a spin echo sequence, consisting of a 90° pulse and a 180° pulse
31. 31.  T2-curves for two tissues with different transversal relaxation times; tissue A has a shorter T2 than tissue B, thus loses transversal magnetization faster.  With a short TE (TEshort) the difference in signal intensity is less pronounced than after a longer TE (TElong).
32. 32. Short vs Long TE/TR  A TR of less than 500 msec is considered to be short, a TR of more than 1500 msec to be long  A short TE is one that is as short as possible, a long TE also is about 3 times as long.  A TE of less than 30 msec is considered to be short, a TE greater than 80 msec to be long.
33. 33. spin echo pulse sequence  1. (90° - TE/2 -180° - TE/2 -> record signal at TE)  after TR (time from the beginning of one 90° pulse to the next 90° pulse) follows another pulse cycle and signal b measurement:  2. (90° - TE/2 -180° - TE/2 -> record signal at TE)
34. 34. EFFECT OF TR/TE ON IMAGE  By combining T1- and T2-curves signal intensity of certain tissues can be determined for a pulse sequence using TR and TE as illustrated.  With a long TR, differences in T1, in longitudinal magnetization time are not very important any more, as all tissues have regained their full longitudinal magnetization.  When we only wait a very short TE then differences in signal intensity due to differences in T2 have not yet had time to become pronounced.  The resulting picture is thus neither T1- nor T2-weighted, but mostly determined by the proton density of the tissues (for this, ideally TE should be zero).
35. 35. T2-weighted picture  When we wait a long TR and a long TE, differences in T2 have had time enough to become pronounced, the resulting picture is T2-weighted
36. 36. T1-weighted picture  When we wait a shorter time TR, differences in T1 influence tissue contrast to a larger extent, the picture is T1-weighted, especially when we also wait a short TE (when signal differences due to differing T2s have not had time to become pronounced)
37. 37.  T2-curves of different tissues can intersect. The signal intensity of the tissues is reversed choosing a TE beyond the crossing point (TEC): before this crossing point (e.g. at TE1) tissue A has a higher signal intensity than tissue B.  This means that image contrast is still determined by differences in T1: the tissue A with the shorter T1 has the stronger signal intensity.  At TEC both tissues have the same signal intensity, and thus cannot be differentiated.  After the crossing point (e.g. at TE2) the relative signal intensities are reversed, and tissue B has the stronger signal
38. 38. T1W/T2W IMAGES
39. 39. Useful for: Evaluating anatomic detail CSF: Dark White Matter: White Gray Matter: Gray Vessels: Dark Useful for: Looking at areas of edema & pathology CSF: Bright White Matter: Gray Gray Matter: Lighter than white matter Vessels: Dark
40. 40. T1 Recovery Short TR T1 contrast (T1 Weighted)  TR 300-600 ms  TE 10-30 ms
41. 41. Bright on T1  Fat, subacute hemorrhage, melanin, protein rich fluid.  Slowly flowing blood  Paramagnetic substances(gadolinium,copper,manganese)  9
42. 42. Dark on T1  Edema, tumor, infection, inflammation, hemorrhage(hyperacute, chronic)  Low proton density, calcification  Flow void
43. 43. T2 Decay Long TE T2 contrast (T2 Weighted)  TR 2000 ms  TE 70 ms
44. 44. Bright on T2  Edema, tumor, Infection, inflammation, subdural collection  Met hemoglobin in late sub acute hemorrhage
45. 45. Dark on T2  Low proton density,calcification,fibrous tissue  Paramagnetic substances(deoxy hemoglobin,methemoglobin(intracellular),ferritin,h emosiderin,melanin.  Protein rich fluid  Flow void
46. 46. Which scan best defines the abnormality  T1 W Images:  Subacute Hemorrhage  Fat-containing structures  Anatomical Details  T2 W Images:  Edema  Demyelination  Infarction  Chronic Hemorrhage  FLAIR Images:  Edema,  Demyelination  Infarction esp. in Periventricular location
47. 47. Infarct  Acute : T1W –Isointense hypo intense T2W-Hyper intense  Sub acute: T1W-Low signal,increasedsignal in peripheral region..hemorrhage(metHb) T2W- High signal  Chronic:T1W-low signal T2W-High siignal
48. 48. Hemorrhage
49. 49. FLOW VOID-HOW  Flow effects are responsible for the black appearance of flowing blood, the signal void in blood vessels
50. 50.  Paramagnetic substances like Gadolinium shorten the T1 and the T2 of their surroundings.  The respective T1- and T2-curves are shifted towards the left.  In effect, that means that for a certain TR there is more, for a certain TE there is less signal EFFECT OF CONTRAST MATERIAL
51. 51. When tissue A and B has less contrast:  T1-curves for tissue A and B are very close to each other, resulting in only a small difference in signal intensity between the tissues at TR.  NOW- the T1-curve of tissue A is shifted to the left, as contrast agent entered tissue A but not tissue B. At the same time TR there now is a much greater difference in signal intensity, i.e. tissue contrast
52. 52. The inversion recovery sequence  The inversion recovery sequence uses a 180° pulse which inverts the longitudinal magnetization, followed by a 90° pulse after the time TI.  The 90° pulse "tilts“ the magnetization into the transversal (x-y-) plane, so it can be measured/received.  The tissue in the bottom row goes back to its original longitudinal magnetization faster, thus has the shorter T1.  For the time TI, which is illustrated, this results in less transversal magnetization after the 90° pulse
53. 53. Short TI inversion-recovery (STIR) sequence Longer TE used..both long T1 and T2 …bright
54. 54.  Substances having both LONG T1 and T2 will be bright.  T1 and T2 of most pathologic lesion are prolonged.  And substances with short T1 will be suppressed eg. hemorrhage, gd- enhancement.
55. 55. Fluid-attenuated inversion recovery (FLAIR)  First described in 1992 and has become one of the corner stones of brain MR imaging protocols  An IR sequence with a long TR and TE and an inversion time (TI) that is tailored to null the signal from CSF
56. 56.  Water bound to complex molecule with in plaque has relatively shorter T1 than free water with in ventricle.  Long inversion time effectively suppress free water eg./csf.  Lesion that contain complex, partially bound water (less mobile)…shorter T1 than free water appears bright.  Effective in high lightening lesion eg demyelination, stroke, Ischemic gliosis and tumor.
57. 57.  Useful in diffrentiating acute infarct from cystic encephalomalacia  Useful in SAH …removes CSF signal
58. 58. GRE
59. 59. GRE  In a GRE sequence, an RF pulse is applied that partly flips the NMV into the transverse plane (variable flip angle).  Gradients, as opposed to RF pulses, are used to dephase (negative gradient) and rephase (positive gradients) transverse magnetization.
60. 60.  GRE Sequences contd:  This feature of GRE sequences is exploited- in detection of hemorrhage, as the iron in Hb becomes magnetized locally (produces its own local magnetic field) and thus dephases the spinning nuclei.  The technique is particularly helpful for diagnosing hemorrhagic contusions such as those in the brain .
61. 61. GREFLAIR Hemorrhage in right parietal lobe
63. 63. Diffusion-weighted MRI  DWI images is obtained by applying pairs of opposing and balanced magnetic field gradients (but of differing durations and amplitudes) around a spin- echo refocusing pulse of a T2 weighted sequence.  Stationary water molecules are unaffected by the paired gradients, and thus retain their signal.  Non stationary water molecules acquire phase information from the first gradient, but are not re phased by the second gradient, leading to an overall loss of the MR signal
64. 64.  The normal motion of water molecules within living tissues is random (brownian motion).  In acute stroke, there is an alteration of homeostasis  Acute stroke causes excess intracellular water accumulation, or cytotoxic edema, with an overall decreased rate of water molecular diffusion within the affected tissue.  Tissues with a higher rate of diffusion undergo a greater loss of signal in a given period of time than do tissues with a lower diffusion rate.  Therefore, areas of cytotoxic edema, in which the motion of water molecules is restricted, appear brighter on diffusion-weighted images because of lesser signal losses
65. 65. Apparent Diffusion Coefficient  Calculated by acquiring two or more images with a different gradient duration and amplitude .  To differentiate T2 shine through effects or artifacts from real ischemic lesions.
66. 66. ADC  useful for estimating the lesion age and distinguishing acute from subacute DWI lesions.  Acute ischemic lesions can be divided into  1- hyperacute lesions (low ADC and DWI- positive)  2-subacute lesions (normalized ADC).  3-Chronic lesions can be differentiated from acute lesions by normalization of ADC and DWI.
67. 67. Nonischemic causes for decreased ADC  Abscess  Lymphoma and other tumors  Multiple sclerosis  Seizures  Metabolic (Canavans )
68. 68. Evaluation of acute stroke on DWI  The DWI and ADC maps show changes in ischemic brain within minutes to few hours  The signal intensity of acute stroke on DW images increase during the first week after symptom onset and decrease thereafter, but signal remains hyper intense for a long period (up to 72 days in the study by Lausberg et al)  The ADC values decline rapidly after the onset of ischemia and subsequently increase from dark to bright 7-10 days later .
69. 69.  This property may be used to differentiate the lesion older than 10 days from more acute ones .  Chronic infarcts are characterized by elevated diffusion and appear hypo, iso or hyper intense on DW images and hyperintense on ADC maps
70. 70. DW MR imaging characteristics of Various Disease Entities MR Signal Intensity Disease DW Image ADC Image ADC Cause Acute Stroke High Low Restricted Cytotoxic edema Chronic Strokes Variable High Elevated Gliosis Hypertensive encephalopathy Variable High Elevated Vasogenic edema Arachnoid cyst Low High Elevated Free water Epidermoid mass High Low Restricted Cellular tumor Herpes encephalitis High Low Restricted Cytotoxic edema CJD High Low Restricted Cytotoxic edema MS acute lesions Variable High Elevated Vasogenic edema Chronic lesions Variable High Elevated Gliosis
71. 71. Slice selection and thickness  There are two ways to determine slice thickness. (a). By using certain bandwidth of RF pulse eg. If frequencies between 64 and 65 mHz, protons in slice 1 will be influenced by the RF pulse.  When the RF pulse only contains frequencies between 64 mHz and 64.5 mHz, thus has a smaller bandwidth, slice 2, which is half as thick as slice 1 will be imaged.  When there is more difference in magnetic field strength between the level of the feet and the head, i.e. The magnetic gradient is steeper, the resulting slice will be thinner, even though the RF pulse bandwidth is the same.