MAGNETIC RESONANCE IMAGING; physics

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DETAILED DISCUSSION ON MRI ... MACHINE PHYSICS

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  • have a positive electrical charge. protons possess a spin. PRODUCING A CURRENT WHICH CAUSES A magnetic fieldNormally protons are aligned in a random fashion. This, however, changes whenthey are exposed to a strong external magnetic field. Then they are aligned inonly two ways, either parallel or antiparallel to the external magnetic field
  • The equation states that the precession frequency becomes higher when the magnetic field strength increases.The exact relationship is determined by the gyro-magnetic ratioThis gyro-magnetic ratio is different for different materials(e.g. the value for protons is 42.5 MHz/T)
  • IN the picture here smaller magnets which when placed laong the magnetic field of a large bar magnet will align along the line of magnetic field of it.
  • BEFOREa vector represents a certain force (by its size) that acts in a certain direction (direction of the arrow). The force that is represented by vectors in our illustrations,is the magnetic force. Draw in whTe board
  • for MRI the mobile protons are important (which are a subset of all protonsthat are in the body). .
  • Only to specific RF not any other frequency.Which indeed is the magnetic part of the NMRWhy Odd It is impossible to arrange these nuclei so that a zero net angular momentum is achieved. Thus, these nuclei will display a magnetic moment and angular momentum necessary for NMR.
  • Larger B0 produces larger net magnetization M, lined up with B0Thermal motionstry to randomize alignment of proton magnets At room temperature, the population ratio is roughly 100,000 to 100,006 per Tesla of B0Some protons which have different energy levels align anti parallel
  • So basically two groups of protons on which are aligned( having lesser energy)And ones which are anti- parallel. (higher energy) to the magnetic fieldAnd magnetization along or,better, longitudinal to the externalmagnetic field cannot bemeasured directly. For this weneed a magnetization which isnot longitudinal, but transversalto the external magnetic field.
  • The radiowave has two effects on the protons: it lifts some protons to a higherlevel of energy (they point down), and it also causes the protons to precess instep, in phase. The former results in decreasing the magnetization along thez-axis, the so-called longitudinal magnetization. The latter establishes anew magnetization in the x-y-planeIf after the RF pulse, the number of protons on the higher energy levelequals the number of protons on the lower energy level, longitudinalmagnetization has disappeared, and there is only transversal magnetizationdue 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(—•), a new transversal magnetization, which moves around with the precessingProtonsRF pulse if cause shift to 180 then its called 180RF pulse. Also
  • The Rf pulse makes the protons in phase.When B0 (feidl) was in longitudinal direction and net magnetization so .Now the Transverse magnetization. Has occured
  • In a strong external magnetic field a newmagnetic vector along the external fieldis established in the patient (a). Sendingin an RF pulse causes a new transversalmagnetization while longitudinal magnetiationdecreases (b). Depending on theRF pulse, longitudinal magnetization mayeven totally disappear (c
  • As soon as the RF pulse is switched off, the whole system, which was disturbedby the RF pulse, goes back to its original quiet, peacefulstate, it relaxes.
  • The newly established transverse magnetizationstarts to disappear (a process called transversalrelaxation), and the longitudinalmagnetization grows back toits original size (a processcalled longitudinal relaxation).
  • This curve is going downhill, as transversal magnetization disappearswith time. And as you probably expect: there is alsoa time constant, describing how fast transversal magnetizationvanishes. This time constant is the transversalrelaxation time
  • DRAW THE MIXED GRAPHS OF T1 RELAXATION AND T2 RELAXATION
  • the field of the MR magnet,in which the patient is placed,is not totally uniform, nottotally homogenous, but variesa little, thus causing differentprecession frequencies• and each proton is influencedby the small magnetic fieldsfrom neighbouring nuclei, thatare also not distributed evenly,thus causing different precessionfrequencies too. Theseinternal magnetic field variationsare somehow characteristicfor a tissue.So after the RF pulse is switchedoff, the protons are nolonger forced to stay in step;and as they have differentprecession frequencies, theywill be soon out of phase.
  • the protons (which are on the higher energy level) cannot hand their energy over to the lattice quickly, they will only slowly go back to their lower energy levelT1 IS longer in stronger magnetic fields : PHASING rfeNERGY IS MORE .
  • Gauss was a German mathematician,who was the first tomeasure the geomagnetic fieldof the earth.Tesla is considered to be the"father" of the alternatingcurrent. He was a peculiarfellow, having refused to sharethe Nobel prize with theinventor Thomas Edison in theearly 1900s.
  • Resistive magnetsare therefore also calledelectromagnets. They are onlymagnetic as long as there isan electrical current flowingthrough them. Thus, they useelectrical energy. As there isa resistance to the flow of theelectricity through the wire,these magnets get warm whenin operation, and have to becooled.
  • This change is essentially based on difference of resultant frequency.So for each slice there will be a difference in Precessing frequency and so an RF pulse of frequency corresponding to the selected slice is applied only those protons will get excited
  • 1. Apply a Y gradient or “phase encode gradient”2. Nuclei in different rows experience different magnetic fields. Nuclei in the highest magnetic field (top row), precess fastest and advance the farthest (most cycles) in a given time.
  • A 2DFT can be accomplished around any plane, by choosing the appropriate gradients for slice selection, phase encoding and frequency encoding.
  • Physics at highschoolExperimntThe lines of arrangement of the iron pellets corresponds to the magnet field.Functional magnetic resonance imaging or functional MRI (fMRI) is a functional neuroimaging procedure using MRI technology that measures brain activity by detecting associated changes in blood flow.[1] This technique relies on the fact that cerebral blood flow and neuronal activation are coupled. When an area of the brain is in use, blood flow to that region also increases.The primary form of fMRI uses the Blood-oxygen-level dependent (BOLD) contrast
  • MAGNETIC RESONANCE IMAGING; physics

    1. 1. MRI PHYSICS (PART I) BY DR. ARIFKHAN S
    2. 2. MRI – MAGNETIC RESONANCE IMAGING • • • • ATOM : Nucleus + electrons Nucleus : Neutrons+ PROTONS PROTONS spin and creates magnetic field. The protons - being little magnets - align themselves in the external magnetic field like a compass needle in the magnetic field of the earth. • May align parallel or anti-parallel
    3. 3. LARMOR EQUATION •
    4. 4. MAIN MAGNET FIELD BO • Purpose is to align H protons in H2O (little magnets) [Main magnet and some of its lines of force] [Little magnets lining up with external lines of force]
    5. 5. A SINGLE PROTON There is electric charge on the surface of the proton, thus creating a small current loop and generating magnetic moment . J + + + The proton also has mass which generates an angular momentum J when it is spinning. Thus proton “magnet” differs from a magnetic bar in that it also possesses angular momentum caused by spinning.
    6. 6. VECTOR & COORDINATE SYSTEM
    7. 7. Protons in a Magnetic Field Bo Parallel (low energy) Anti-Parallel (high energy) Spinning protons in a magnetic field will assume two states. If the temperature is 0o K, all spins will occupy the lower energy state.
    8. 8. • Naturally the preferred state of alignment is the one that needs less energy • So more protons are on the lower energy level, parallel to the external magnetic field • Proton’s are not stationary. Precession movement is seen precession frequency : precess per second Depends upon the strength of the magnetic field The stronger the magnetic field, the faster the precession rate and the higher the precession frequency. Magnetic field : B0
    9. 9. NMR: NUCLEAR MAGNETIC RESONANCE • Resonance here refer to the change in energy states of the NUCLEI in response to A RF wave of specific Radio Frequency. • Resonance can also occur in an external magnetic field. • Criteria: • Must have ODD number of protons or ODD number of neutrons. • Examples: • 1H, 13C, 19F, 23N, • 40.08, 11.27 and 17.25 MHz/T and 31P with gyromagnetic ratio of 42.58, 10.71, • The main Resonating nuclei in Human body is H.
    10. 10. Net Magnetization Bo M M c Bo T
    11. 11. Energy Difference Between States /2 = 42.57 MHz / Tesla for proton in hydrogen atom Knowing the energy difference allows us to use electromagnetic waves with appropriate energy level to irradiate the spin system so that some spins at lower energy level can absorb right amount of energy to “flip” to higher energy level.
    12. 12. Basic Quantum Mechanics Theory of MR Spin System Before Irradiation Bo Lower Energy Higher Energy
    13. 13. RESONANCE AND RF PULSING 2 things happen to Resonating protons when RF pulse is applied 1- Energy Absorption Increase number of High energy Spin Up nuclei and these may align anti parallel 2- Phase Coherence NMV precesses in transverse plane at Larmor Frequency
    14. 14. • High Energy state but Not in phase High energy state but in phase : COHERENCE
    15. 15. Basic Quantum Mechanics Theory of MR The Effect of Irradiation to the Spin System Lower Higher
    16. 16. Basic Quantum Mechanics Theory of MR Spin System After Irradiation
    17. 17. 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. THIS RELAXATION PRODUCES
    18. 18. • T1 relaxation Curve : after switching off RF change in longitudinal MAGNETIZATION
    19. 19. • This time constant is the transversal relaxation time T2 , OR • spin-spin-relaxation TIME
    20. 20. IMAGING TRICKS • B0 MAGNETIC FIELD IS NOT UNIFORM. • INTERNAL MAGNETIC FIELD OF PROTONS: TISSUE DIFFERENCE • RF PULSING : DURATION AND INTERVAL
    21. 21. TISSUE FEATURES • LIQUIDS HAVE LONG T1 AND LONG T2 When the lattice consists of pure liquid/water, it is difficult for the protons to get rid of their energy, • FATS HAVE SHORT T1 AND T2 The carbon bonds at the ends of the fatty acids have frequencies near the Larmor frequency, thus resulting in effective energy transfer.
    22. 22. MRI : THE MACHINE
    23. 23. EQUIPMENT Gradient Coil 4T magnet RF Coil gradient coil (inside) Magnet RF Coil
    24. 24. MAIN COMPONENTS OF A SCANNER • Static Magnetic Field Coils • Gradient Magnetic Field Coils • Magnetic shim coils • Radiofrequency Coil • Subsystem control computer • Data transfer and storage computers • Physiological monitoring, stimulus display, and behavioral recording hardware
    25. 25. MRI SCANNER COMPONENTS
    26. 26. STATIC MAGNETIC FIELD COILS • Earths magnetic field is 0.3-0.7G (G: Gauss) • 1 Tesla is 10000 G • Magnets used for imaging mostly between .5 to 1.5 Tesla • PERMANENT AND ELECTROMAGNET (SUPERCONDUCTING ) • The field should be homogenous. Inhomogeniety reduces the signal to noise ratio ; i.e affects the image quality.
    27. 27. PERMANENT MAGNETS & ELECTROMAGNETS • Permanent magnet : is always magnetic and does not use any energy for work. but is thermally instable, field strength is limited, weight (a .3T magnet will be around 100 Tons!!!!) • Electromagnet: 1. Resistive Magnet: An electrical current is passed through a loop of wire and generates a magnetic field 2. Superconducting magnets: widely used in MR machines. Here the current carrying conductor is kept at very low temperatures called SUPERCONDUCTING TEMPERATURE (4 Kelvin or -269 Celsius). At this temperature the conducting material looses its resistance for electricity. And so produces constan magnetic field s. Cryogens are Helium and Nitrogen. • Advantages of superconducting magnets are high magnetic field strength and excellent magnetic field homogeneity
    28. 28. VOLUME COILS • These completely surround the part of the body that is to be imaged. • it is the transmitter for all types of examinations. It also receives the signal when larger parts of the body are imaged • E.g The helmet-type head coil acts as receiver coil, the body coil transmitting the RF pulses while imaging the head
    29. 29. SHIM COILS • Shimming: the process by which an in-homogenous magnetic filed is converted to a homogenous magnetic filed by electrical as well as mechanical adjustments. • Shim Coils are such devices used for shimming of the magnetic field (i.e the STATIC MAGNETIC FIELDS)
    30. 30. IMAGING GRADIENT MAGNETS • Used to vary magnetic field in known manner • Each point has slightly different rate of precession & Larmor Frequency. • Variety of signal released by Protons returning to z-plane can used to determine the composition of exact location of each point. Gradient Function: • Slice selection • Frequency encoding • Phase encoding
    31. 31. GRADIENT FUNCTIONS • Slice selection : We can selectively excite nuclei in one slice of tissue by incorporating a third magnetic field: the “gradient” magnetic field. • The gradient magnetic field produces a linear change in the total magnetic field. • Here, “gradient” means “change in field strength as a function of location in the MRI bore”. • Since the gradient field changes in strength as a function of position, we use the term “gradient amplitude” to describe the field
    32. 32. PHASE AND FREQUENCY ENCODING Consider an MRI image composed of 9 voxels (3 x 3 matrix) All voxels have the same precessional frequency and are all “in phase” after the slice select gradient and RF pulse
    33. 33. • When the Y “phase encode gradient” is on, spins on the top row have relatively higher precessional frequency and advanced phase. Spins on the bottom row have reduced precessional frequency and retarded phase
    34. 34. 3. Turn off the Y “phase encode gradient” 4. All nuclei resume precessing at the same frequency 5. All nuclei retain their characteristic Y coordinate dependent phase angles 6. A “read out” gradient is applied along the X axis, creating a distribution of precessional frequencies along the X axis. 7. The signal in the RF coil is now sampled in the presence of the X gradient.
    35. 35. • While the frequency encoding gradient is on, each voxel contributes a unique combination of phase and frequency. The signal induced in the RF coil is measured while the frequency encoding gradient is on.
    36. 36. • 8. The cycle is repeated with a different setting of the Y phase encoding gradient. For a 256 x 256 matrix, at least 256 samples of the induced signal are measured in the presence of an X frequency encoding gradient. The cycle is repeated with 256 values of the Y phase encoding gradient. • 9. After the samples for all rows are taken for every phase-encode cycle, 2D Fourier Transformation is then carried out along the phaseencoded columns and the frequency-encoded rows to produce intensity values for all voxels.
    37. 37. K- SPACE • The Fourier transformation acts on the observed “raw data” to form an image. A conventional MRI image consists of a matrix of 256 rows and 256 columns of voxels (an “image matrix”). • The “raw data” before the transformation ALSO consists of values in a 256 x 256 matrix • The raw data matrix is also called K-SPACE. • The 2DFT of k-space produces an image. • Each value in the resulting image matrix corresponds to a grey scale intensity indicative of the MR characteristics of the nuclei in the voxels. Rows and columns in the image are said to be “frequency encoded” or “phase encoded”.
    38. 38. k-space Image space y ky FT-1 kx x FT Acquired Data MRI task is to acquire k-space image then transform to a spatial-domain image. kx is sampled (read out) in real time to give N samples. ky is adjusted before each readout. Final Image MR image is the magnitude of the Fourier transform of the k-space image
    39. 39. • The top row of k-space would be measured in the presence of a strong positive phase encode gradient. • A middle row of k-space would be measured with the phase encode gradient turned off. • The bottom row of k-space would conventionally be measured in the presence of a strong negative phase encode gradient. • While the frequency encoding gradient is on, the voltage in the RF coil is measured at least 256 times. The 256 values measured during the first RF pulse are assigned to the first row of the 256 x 256 “raw data” matrix. The 256 values measured for each subsequent RF pulse are assigned to the corresponding row of the matrix.
    40. 40. RF BANDWIDTH • There are actually two RF bandwidths are associated with MRI: Transmit and Receive • RF Transmit bandwidth ~ +1 kHz affects slice thickness • RF Receive bandwidth ~ +16 kHz is sometimes adjusted to optimize signal-to-noise in images
    41. 41. SURFACE COILS • Surface coils are placed directly on the area of interest, and have different shapes corresponding to the part to be examined. • They are receiver coils only, most of the received signal coming from tissues near by; deeper structures cannot be examined with these coils
    42. 42. BASIC MRI TECHNIQUE • 1. PLACE A PATIENT IN A UNIFORM MAGNETIC FIELD • 2. DISPLACE THE EQUILLIBRIUM MAGNETIZATION VECTOR WITH RF – PULSE. • 3. OBSERVE THE SIGNAL AS THE MAGNETIZATION VECTOR RETURNS TO EQULLIBRIUM
    43. 43. MR Signal When RF pulse is given NMV rotates around transverse plane It passes across Receiver Coil Inducing voltage in it thus producing a signal • RF Removed  Signal decreased  Amplitude of MR Signal decreased • Free Induction Decay "FID": • Free (No RF Pulse) • ID (because of Decay of Induced signal in Receiver Coil)
    44. 44. IMAGING NMV can be separated in to Individual Vectors of tissue present in the patient • Such as Fat, CSF & Muscle
    45. 45. IMAGING High Signal Low Signal Intermediate White Black Large transverse Small transverse component of component of magnetization magnetization Grey
    46. 46. CONTRAST MECHANISMS High Signal Low Signal Intermediate White Black Large transverse Small transverse component of component of magnetization magnetization Grey
    47. 47. CONTRAST MECHANISMS (PARAMETERS) Image contrast controlled by: 1- Extrinsic Contrast parameters: TR, TE & Flip Angle 2- Intrinsic Contrast parameters: T1 Recovery, T2 Decay, Proton Density, Flow & Apparent Diffusion Coefficient
    48. 48. CONTRAST MECHANISMS (RELAXATION PROCESS) after removal of RF pulse Signal induced in Receiver Coil decreased T1-relaxation time NMV recovers and realign to B0 this process called "T1 Recovery" T1 time is an intrinsic contrast parameter that inherent to tissue being imaged T2- relaxation time 2-Nuclei loss Precessional coherence or diphase and NMV decay in the transverse plane this process called "T2 Decay“ T2 Decay is an intrinsic contrast parameter and is inherent to the tissue being imaged
    49. 49. CONTRAST MECHANISMS (T1 RECOVERY) Short TR  DIFFERNCE IN SIGNAL INTENSITY  T1 contrast (T1 Weighted) • TR 300-600 ms
    50. 50. CONTRAST MECHANISMS (DEFINITIONS) Repetition Time "TR“: Time from application of one RF pulse To the application of the next (it affects the length of relaxation period after application of one RF excitation pulse to the beginning of the next) SHORT TR : <500ms LONG TR : > 1500ms
    51. 51. CONTRAST MECHANISMS (DEFINITIONS) Echo Time "TE" Time between RF excitation pulse and collection of signal (it affects the length of relaxation period after removal of RF excitation pulse and the peak of signal received in receiver coil)
    52. 52. • TR determines T1 contrast • TE determines T2 contrast.
    53. 53. CONTRAST MECHANISMS (DEFINITIONS) Flip Angle Angle throw which the NMV moved as result of a RF excitation pulse
    54. 54. CONTRAST MECHANISMS (T1 RECOVERY) T1 Recovery Caused by exchange of energy from nuclei to their surrounding environment or lattice "Spin Lattice Energy Transfer" and realign in B0 this occur in exponential process at different rates in different tissue NB: Molecules are constantly in motion; Rotational and Transitional
    55. 55. CONTRAST MECHANISMS (T1 RECOVERY) T1 in Fat T1 in Water absorb energy quickly T1 is very short i.e. nuclei dispose their energy to surrounding fat tissue and return to B0 in very short time inefficient at receiving energy T1 is longer i.e. nuclei take allot longer to dispose energy to surrounding water tissue FAT WATER
    56. 56. PROPERTIES OF BODY TISSUES Tissue T1 (ms) T2 (ms) Grey Matter (GM) 950 100 White Matter (WM) 600 80 Muscle 900 50 4500 2200 250 60 Cerebrospinal Fluid (CSF) Fat Blood 1200 T1 values for B0 ~ 1Tesla. T2 ~ 1/10th T1 for soft tissues 100-200
    57. 57. CONTRAST MECHANISMS (T2 DECAY) T2 Decay Caused by exchange of energy from one nucleus to another "Spin-spin Energy Transfer“ as result of intrinsic magnetic fields of nuclei interlacing with each other this energy exchange  loss of coherence or dephasing and as result NMV decay in transverse plane T2 is exponential process occur at different rates in different tissues

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