Physics of ct mri


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Physics of ct mri

  1. 1. Fundamentals of CT/MRI Lokendra Yadav
  2. 2. Introduction  Computed Tomography (CT) was introduced into clinical practice in 1972 and revolutionized X Ray imaging by providing high quality images which reproduced transverse cross sections of the body.  Tissues are therefore not superimposed on the image as they are in conventional projections  The technique offered in particular improved low contrast resolution for better visualization of soft tissue, but with relatively high absorbed radiation dose
  3. 3. Introduction (contd.)  Computed tomography (CT), originally known as computed axial tomography (CAT or CT scan) is a medical imaging method employing tomography where digital geometry processing is used to generate a three-dimensional image of the internal structures of an object from a large series of two-dimensional X-ray images taken around a single axis of rotation.  The word "tomography" is derived from the Greek tomos (slice) and graphia (describing). CT produces a volume of data which can be manipulated, through a process known as windowing, in order to demonstrate various structures based on their ability to block the x-ray beam.
  4. 4. Evolution of CT  X-Ray image formation – 2D with super imposition of tissues  Conventional Tomography – due to blurring of non focused tissues  Computed axial tomography-Images of exquisite clarity, no superimposition
  5. 5. Definition & Types ECAT TCAT • CTCT is a process of creating a cross -sectionalis a process of creating a cross -sectional tomo- graphic plane or slice of any part of the bodytomo- graphic plane or slice of any part of the body in which computer is used to make a mathematicalin which computer is used to make a mathematical reconstruction of a tomo-graph (CT image).reconstruction of a tomo-graph (CT image). • CT is mainly two typesCT is mainly two types
  6. 6. ECAT (EMISSION TYPE)  It needs Gamma Camera  After administration of radionuclide to patient, patient becomes temporary source of emitted radiation, so it is called ECAT
  7. 7. TCAT (Transmission)  In this type of CT x-ray emitted from x-ray tube and passes through the body of a patient to a sensitive recorder so here patient is acts as transmitter.  Is generally called CT Scan  In this x-ray examination depends upon attenuation of x-ray beam
  8. 8. BASIC PRINCIPLE:-The Internal Structures of An Object Can Be Reconstructed From Multiple Projections Of The Object  A narrow beam of X ray scans across a patient in synchrony with a radiation detector on the opposite side of the patient.  Sufficient no. of transmission measurements are taken at different orientation of X ray source & detectors, the distribution of attenuation coefficients within the layer may be determined.  By assigning different levels to different attenuation coefficients, an image can be reconstructed with aid of computer that represent various structures with diff attenuation properties.
  9. 9. EVOLUTION OF CT SCAN Various generations • First generation - One detector, translation- rotation Pencil-beam • Second generation - Multiple detectors, translation- rotation Small fan-beam • Third generation - Multiple detectors, rotation- rotation Large fan-beam • Fourth generation - Detector ring, source-rotation Large fan-beam • Spiral / Helical scanning - Cone-beam geometry
  10. 10.  The first generation of CT scanners employed a rotate / translate, pencil beam system FIRST GENERATION The x-ray tube and a single detector (per CT slice) translate across the field of view, producing a series of parallel rays. The system then rotates slightly and translates back across the field of view, producing ray measurements at a different angle. This process is repeated at 1-degree intervals over 180 degrees, resulting in the complete CT data set. ROTATE/TRANSLATE, PENCIL
  11. 11. First Generation (Brain Scanner)  Head was enclosed in water bath b/w X ray tube & a pair of detectors below .  A third reference detector intercepted a portion of the beam before it reached the patient.  Patient remain stationary & Gantry moves through two types of motion: one is linear & other rotary  Beam- narrow pencil beam, filtered with 6mm Al eq.  Tube - oil cooled stationary anode, focal spot 2.25 x12mm operated at 120kvp & 33mA  Each slice of 180 degree rotation took 5 min so total time for clinical study was approx 25-30 min
  12. 12. The first CT scanner, an EMI Mark 1, produced images with 80 x 80 pixel resolution (3-mm pixels), and each pair of slices required approximately 4.5 min-of scan time and 1.5 minutes of reconstruction time. The First CT Scanner
  13. 13. Advantages:  It employed pencil beam geometry which allowed very efficient scatter reduction. Limitations  The detector suffered from significant amount of “afterglow,”  It took 4.5 to 5.5 minutes to complete one scan resulting to limited patient throughput.  Only head scan possible. Advantages & Limitations
  14. 14.  The next incremental improvement to the CT scanner was the incorporation of a linear array of 30 detectors.  A relatively narrow fan angle of 10 degrees was used SECOND GENERATION Rotate/Translate, Narrow Fan
  15. 15. Second Generation (ROTATE-TRANSLATE)  A fan beam with 20-30 degree divergence.  Number of detectors were increased i.e. up to 30.  Rotary movement was in arc of 30º & linear movements were 6 as compare to EMI scanner  Scan time for head 10-90 sec. Body scanning was also possible  Advantage  The shortest scan time with a second- generation scanner was 18 seconds per slice, 15 times faster than with the first- generation system.  Limitations  more scattered radiation detected than the pencil beam used in first-generation CT.
  16. 16. Third Generation: Rotate/Rotate, Wide Fan Beam  The mechanically joined x-ray tube and detector array rotate together around the patient without translation.  The detector array is long enough so that the fan angle encompasses the entire width of the patient. The translational motion of first- and second- generation CT scanners was a fundamental impediment to fast scanning. Multiple detectors, rotate-rotate, Large fan- beam
  17. 17.  Advantages  The early third-generation scanners could deliver scan times shorter than 5 seconds.  Newer systems have scan times of ½ second.  Limitations  Third-generation scanners suffered from the significant problem of ring artifacts.  Detectors and the associated electronics are expensive, this led to more expensive CT scanners. Advantages & Limitations
  18. 18. Fourth-generation CT scanners were designed to overcome the problem of ring artifacts. Fourth Generation Detector ring, Source- rotation, Large fan-beam  Based on Rotate-fixed systemBased on Rotate-fixed system i.e. tube rotates through 360i.e. tube rotates through 360ºº && detectors stationarydetectors stationary  A ring of detectors (1000-2000)A ring of detectors (1000-2000) surrounds the patient.surrounds the patient.  Fan shaped beamFan shaped beam  Scan time very short i.e. 1sec.Scan time very short i.e. 1sec. DisadvantagesDisadvantages  High cost because more no ofHigh cost because more no of detector usedetector use  More scatter radiationMore scatter radiation
  19. 19. MILLISECOND SCANNER SYSTEM Multiple X ray Tubes(5th Gen.)  First used by Mayo Clinic’s  They used 28 X-ray tubes position around a semicircular gantry, aligned with 28 light amplifiers & TV cameras that are placed behind a single curved fluorescent screen  Gantry rotates about 15 revolution per sec  Data can be acquired in 16 ms. Disadvantages  High cost  Heavy structure mechanical motion difficult
  20. 20.  Developed by Imatron Inc. which was a result of of work by Dr. Douglas & colleagues during late 1980s  Commonly referred to CVCT Scanner Basic components-  An electron gun 320cm long with its focusing & deflecting coils (electron are accelerated at 130keV  4 Tungsten targets rings180cm in dia.  A ring of detectors arranged in an arc of 210 degree  The transmitted X ray photons are measured by integrated crystals photo-diode detector system and digitized by an acquisition system.  Scan time very less 50-100 msec. because there is no mechanical rotation of the X ray source and gantry Fifth Generation:Fifth Generation: E-Beam CT Stationary/StationaryE-Beam CT Stationary/Stationary
  21. 21.  The gantry had to be stopped after each slice was acquired, because the detectors (in third-generation scanners) and the x-ray tube (in third- and fourth-generation machines) had to be connected by wires to the stationary scanner electronics.  The ribbon cable used to connect the third- generation detectors with the electronics had to be carefully rolled out from a cable spool as the gantry rotated, and then as the gantry stopped and began to rotate in the opposite direction the ribbon cable had to be retracted. LIMITATIONS OF THIRD AND FOURTH GENERATION CT SCANNERS
  22. 22. HELICAL/SPIRAL CT SCANNER  Introduced in 1989 by Dr. Kalender  Spiral CT is made possible by the use of slip ring technology. Slips rings are an electromechanical devices that conduct electricity and electrical signals through ring & brushes from a rotating surface onto fixed surface & vice-versa.  Composite brushes are made up of conductive material (silver graphite)  Brushes are to be replaced every yr. or during preventive maintenance.  3 kind of slip rings are used  -1st provide high & low voltage to X ray tube  -2nd provide low voltage to control system on gantry  -3rd transfer signal from rotating detectors array to DAS
  23. 23.  In the early 1990s, the design of third- and fourth-generation scanners evolved to incorporate slip ring technology.  A slip ring is a circular contact with sliding brushes that allows the gantry to rotate continually, untethered by wires.  The use of slip-ring technology eliminated the inertial limitations at the end of each slice acquisition, and the rotating gantry was free to rotate continuously throughout the entire patient examination. EVOLUTION OF SLIP RING TECHNOLOGY
  24. 24. Helical CT  Helical CT (also called spiral CT) scanners acquire data while the table is moving;  As a result, the x-ray source moves in a helical pattern around the patient being scanned.
  25. 25. Helical CT (Contd.)  Helical CT scanners use either third- or fourth- generation slip-ring designs.  the total scan time required to image the patient is much shorter (e.g., 30 seconds for the entire abdomen).  helical scanning allows the use of less contrast agent and increased patient throughput.  Entire scan can be performed within a single breath-hold of the patient, avoiding inconsistent levels of inspiration.
  26. 26. Helical CT (Contd.)  The advent of helical scanning has introduced many different considerations for data acquisition.  In order to produce reconstructions of planar sections of the patient, the raw data from the helical data set are interpolated to approximate the acquisition of planar reconstruction data.
  27. 27. Advantages of spiral CT Advantages  No motion artifacts  Improved lesion detection.  Reduced partial volume  Multiplanar Imaging  Improved Pt throughput  Optimized IV contrast How  Removes respiratory misregistration.  Reconstruction at arbitrary intervals.  Allow reconstruction at overlapping intervals.  Scanning time is reduced.  Data obtained during peak of contrast enhancement.
  28. 28.  When multiple detector arrays are used, the collimator spacing is wider and therefore more of the x-rays that are produced by the x-ray tube are used in producing image data.  With conventional, single detector array scanners, opening up the collimator increases the slice thickness, which is good for improving the utilization of the x-ray beam but reduces spatial resolution in the slice thickness dimension.  With the introduction of multiple detector arrays. the slice thickness is determined by the detector size and not by the collimator.  This represents a major shift in CT technology. MULTI DETECTOR CT
  29. 29. DEVELOPMENTS IN MULTI DETECTOR CT  Multi-detector CTs debuted in 1992 when Elscint introduced its CT Twin, the first dual-slice scanner.  The first four-slice scanners were presented in 1998, followed by 16-slice systems in 2001; 32- and 40-slice scanners followed within a short period. A 64-slice scanner was unveiled during the 2005 annual Radiological Society of North America scientific meeting,  128- and 256-detector scanners appear to be on the horizon.
  30. 30.  x-ray tube/generator systems.x-ray tube/generator systems.  x-ray detectors,x-ray detectors,  computer hardware,computer hardware,  motor control systems,motor control systems,  sophisticated reconstruction algorithms.sophisticated reconstruction algorithms. CT Scanners represent a marriage of diverseCT Scanners represent a marriage of diverse technologies comprising:technologies comprising:
  31. 31. X-Ray Generators for CT (Contd.)  In X-ray generators of the ct scanners, low voltage low frequency alternating current from the main power supply is converted into high voltage, high frequency (500- 25000 Hz), direct current of almost constant potential supply to X-Ray tube.  the voltage ripples is less than 1% compared to 4% from a three phase 12 pulse generator.  Current CT generators have maximum power rating of about 60KW that allows KV in the range of 80-140KVps and tube current in the range of 100mA-400mA.
  32. 32. COLLIMATORS  Beam collimation at 2 points, one close to X ray tube & other at detectors  Collimators regulates the slice thickness  Each detector has its own collimators  In some volume scanner the beam is collimated through multiple slits to reduced scatter produced before striking the patient, known as Multi-slit Multi-slice CT scanner.
  33. 33. DETECTOR TECHNOLOGY CT detectors capture the radiation beam from the patient, convert it into electrical signals, which are subsequently converted into binary coded information for onward transmission to computer system for further processing
  34. 34. TYPES OF DETECTORS Three types of detection systems are available for CT machines:  Multiple scintillation detectors with photo multiplier tubes  Multiple scintillation detectors with photo-diodes  A single multi chamber inert gas (xenon) detectors.
  35. 35. IMAGE RECONSTRUCTION  In computed tomography, a cross sectional layer of the body is divided into tiny blocks  Each blocks is assigned a no. proportional to the degree that block attenuated the X ray beam.  This block individually called voxel  The linear attenuation coefficient is used to quantitative attenuation N = Nºe-µx If the block of material with different attenuation coefficient placed in the path then, N = Nºe-[(µ1+µ2+……+µn)x ] The values of µ1,µ2,….µn can not be
  36. 36. IMAGE DISPLAY  A CT imaged displayed is consist of a matrix of picture elements called ‘pixels’  Each pixel represent the linear attenuation values of X ray at the point of body .  Pixel is a 2D display of a voxel  Matrix used are  *256x256(over 65000 pixels)  *512x512(over 260000 pixels)  *1024x1024(app.1050000 pixels)
  37. 37. CT NUMBER  It is defined as a relative comparison of x-ray attenuation of each voxel of tissue with an equal volume of water. CT no=k(µρ - µω) µω To honour Hounsfield CT no. based on magnification constant of 1000 are also called HU (Hounsfield unit)
  38. 38. Windowing is a system where the CT no. range of interest is spread cover the full grey scale available on the display system WINDOW WIDTH –Means total range of CT no. values selected for gray scale interpretation. It corresponds to contrast of the image. WINDOW LEVEL– represents the CT no. selected for the centre of the range of the no. displayed on the image. It corresponds to brightness of image . WindowingWindowing
  39. 39. Hounsfield Values WaterWater AirAir FatFat FluidFluid Soft tissueSoft tissue CalcificationCalcification BoneBone 0 HU0 HU -1000 HU-1000 HU -20 to - 200 HU-20 to - 200 HU 0 to 15 HU0 to 15 HU 20-60 HU20-60 HU 150-200 HU150-200 HU 1000 HU1000 HU
  40. 40. ADVANCEMENTS  Detector miniaturization, faster gantry rotation and enhanced computerization.  Number of detectors has increased, so has rotational speed (presently 0.33 s per rotation.).  Dual-slice scanners permitted either resolution, speed, volume or power enhancements but scanners with a minimum of 16/64 slices allow unlimited improvement in all four areas.  Applications also includes Cardiac CT, 3DCT, CT Angiography, CT Fluoroscopy, Virtual endoscopy and traditional CT.
  41. 41. Advancements of CT 19721972 19801980 19901990 20002000 Minimum scan timeMinimum scan time 300 s300 s 5-10 s5-10 s 1-2 s1-2 s 0.3-1s0.3-1s Data acquired per 360°Data acquired per 360° 57.6 kB57.6 kB 1 MB1 MB 2MB2MB 42 MB42 MB Data per spiral sequenceData per spiral sequence -- -- 24-48 MB24-48 MB 200-500 MB200-500 MB Image matrixImage matrix 808022 25625622 51251222 51251222 Power (generator)Power (generator) 2 kW2 kW 10 kW10 kW 40 kW40 kW 60 kW60 kW Slice thicknessSlice thickness 13 mm13 mm 2-10 mm2-10 mm 1-10 mm1-10 mm 0.5-5 mm0.5-5 mm
  42. 42. Dual Energy CT Methods  Dual Source-Siemens  Energy discriminating Detectors –Philips  kVp Switching-GE
  43. 43. CONCLUSION  During the coming years, cone-beam CT with large-area detectors may allow coverage of entire organs in a single axial scan  In the meantime, 64-detector systems are the best available technology, and some believe a critical point has been reached:  The best study obtainable may not be necessary. Thus, protocols are designed to reasonably bridge the possible and the necessary.  Even so, more advanced systems are fast deluging physicians with incredibly high volumes of CT images.  The respective technical developments in CT detectors will have to be reassessed constantly in the future, whereas the development of detector systems which is equally suited both for radiography and CT is the need of the day.
  44. 44. 47 Introduction MRI is a computer based cross sectional imaging modality Which can provide both anatomic as well as physiological Information non invasively, without the use of ionizing Radiation . Definition : MRI is a diagnostic imaging modality in which a Magnetic resonance , MR active nuclei ,RF pulses and computer are used to generate the MR images in transverse, coronal and saggital planes for diagnostic purpose. MRI PRINCIPLE and PHYSICS
  45. 45. 48Basic of MRI  Atomic structure Atom : matter is composed of atoms, which are composed proton, neutron and electron. having the central nucleus and orbital electrons  Atomic number: Sum of the protons and neutron in the nucleus.  Mass number: Sum of the proton and neutron in the nucleus. Proton spinning on their own axis. Electron orbiting the nucleus, spin but very less in comparison to protons. Nucleus itself spins about its own axis.
  46. 46. 49MR active nuclei  MR active nuclei are characterized by their tendency to align their axis of rotation to an externally applied magnetic field.  According to law of quantum mechanics nuclei with odd number of protons have a total magnetic moment.  Some important MR active nuclei.  HYDROGEN 1  CARBON 13  NITROGEN 15  OXYGEN 17  FLUORINE 19  SODIUM 23  PHOSPORUS 31
  47. 47. 50HYDROGEN NUCLEUS  Biological tissues are predominantly made of 12 C ,16 O , 1 H, and 14 N.  Hydrogen is the major species that is MR sensitive.  Hydrogen is most abundant atom in body.  The majority of hydrogen is in water (H2O).
  48. 48. 2) MR PROTON ALIGNMENT  Hydrogen is the most abundant element in the human body. Hydrogen protons align with the magnetic field when the human body is placed in an MR magnet.  In a magnetic field, the protons line up in the direction of the magnetic field, similar to the way a compass lines up in the earth’s magnetic field. 51 Nucleus of an atom has magnetic properties. When nucleus has an odd number of protons (or neutrons), there is a magnetization. E.g. Hydrogen-1 Proton  Nucleus behaves like a dipole magnet
  49. 49. 52 A Single Proton There is electric chargeThere is electric charge on the surface of theon the surface of the proton, thus creating aproton, thus creating a small current loop andsmall current loop and generating magneticgenerating magnetic momentmoment µµ.. Thus proton “magnet” differs from the magnetic bar in that itThus proton “magnet” differs from the magnetic bar in that it also possesses angular momentum caused by spinning.also possesses angular momentum caused by spinning.
  50. 50. MR PROTON ALIGNMENT  All the protons pointing in the direction of the magnetic filed act together to produce a net magnetization, as if they were combined into one larger magnet.  When a patient’s body is placed in a magnetic field, the hydrogen protons line up in the direction of that magnetic field. 53 No external magnetic field External magnetic field B0
  51. 51. NET MAGNETIZATION VECTOR NET MAGNETIZATION VECTOR An excess of hydrogen nuclei will line up parallel to B0 and create the NMV of the patient. NMV or longitudinal magnetization along external magnetic field cannot be measured directly. for measurement it has to be transverse. 54
  52. 52. Transverse magnetization  After forming the longitudinal magnetization R .F pulse is sent.  Precessing protons pick up some energy from R F pulse.  Some protons go to higher level and starts precessing anti-parallel.  This results in the reduction of magnitude of longitudinal magnetization.  Forces of protons add up to form a new magnetic vector (x-y) plane this is called transverse magnetization. 55
  53. 53. 56 N S The magnetic vector direction size N S
  54. 54. 4) PRECESSION A spinning top, which is hit, performs a wobbling type of motion . Protons in a strong magnetic field also show this type of motion, which is called precession. The precession actually goes very fast, the precession frequency for hydrogen protons is somewhere around 42.3 MHz in a magnetic field strength of 1 Tesla. 57
  55. 55. 58 Larmor equation. Precession speed of proton can be measured as a precessional frequency. It depends upon on magnetic field strength. According Larmor frequency. W = r Χ Bo. Where. W= PF in MHz. Bo= strength of M G in Tesla. r = gyro magnetic ratio.
  56. 56. 3) RESONANCE Resonance is the absorption or emission of energy only at certain specific frequencies. Exchange of energy between two systems at a specific frequency is called resonance. Magnetic resonance corresponds to the energetic interaction between spins and electromagnetic radiofrequency (RF). The resonance frequency, called Larmor frequency (ω0) or precessional frequency, is proportional to the main magnetic field strength: ω0 = γ B0. 59
  57. 57. PRECESSIONAL FREQUENCY OF HYDROGEN AT DIFFERENT MAGNETIC FIELD STRENGTH  At 0.5 Tesla -21.28 MHz.  At 1.0 Tesla -42.57 MHz.  At 1.5 Tesla -64 MHz.  At 3 Tesla - 127.71 60
  58. 58. Prerequisite for resonance Protons resonate if energy delivered by RF waves is- Delivered at exactly its precessional frequency of proton. and 90 degree NMV and B0 61
  59. 59. How to measure longitudinal magnetization for the measurement longitudinal magnetization it has to be transverse. How it can be transverse. It can be transverse with the application of R F waves . 62
  60. 60. What are radio frequency waves  Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Naturally-occurring radio waves are produced by lightning, or by astronomical objects. 63
  61. 61. Effects of radio frequency waves on protons R F pulse is sent . When R F pulse and proton have same frequency protons pick up some energy from RF pulse and starts wobble this is called resonance. 64
  62. 62. 65 RF Pulse Radio- wave :Radio- wave : It is an electro magnetic wave and a short burst ofIt is an electro magnetic wave and a short burst of pulse which is called RF Pulse. We need a specialpulse which is called RF Pulse. We need a special (RF) pulse that can exchange energy with protons.(RF) pulse that can exchange energy with protons. Energy exchange is possible when protons and RFEnergy exchange is possible when protons and RF wave have same frequency. So energy transformationwave have same frequency. So energy transformation is possibleis possible Apply RF pulse at resonance frequency Protons absorb energy Protons ‘jump’ to a higher state
  63. 63. Steps involved during resonance  Flip angle  Phase  MR signal  FID  Relaxation  T1 recovery 66
  64. 64. Flip angle  The NMV (net magnetization vector) moves out of alignment away from BO is called flip angle . Magnitude of the flip angle is depends upon. Amplitude and duration of RF pulse. 67
  65. 65. 68 The flip Angle
  66. 66. 69 Phases of magnetic movement Phase :Phase : NMV move into phase with each other. The phase isNMV move into phase with each other. The phase is the position of each magnetic moment as the precessionalthe position of each magnetic moment as the precessional path around Bo. Two types 1) Out of phase 2 ) In phasepath around Bo. Two types 1) Out of phase 2 ) In phase Out of phase In phase
  67. 67. 70 MR Signal : If a receiver coil is placed in the transverse plane, a voltage is induced in the receiver coil. When in phase. Magnetization cuts across the coil and produces Magnetic field Fluctuation inside the coil. Therefore NMV precess at the Larmor frequency in transverse plane, a voltage is induced in the coil. It Constitutes MR Signal The MR Signal
  68. 68. 71 FID : When the RF pulse is switched off the NMV is again Influenced by Bo and, it tries to realign it. In order to do so NMV must loss the energy given to it by the RF pulse, as relaxation occurs, the NMV returns to realign with Bo Relaxation : During relaxation the NMV gives up absorbed RF energy and return to Bo. The magnetic movements of NMV loss transverse magnetization due to dephasing. Relaxation results : - The recovery of LM is caused by a process termed T1 recovery - The decay of TM is caused by a process termed T2 deacy Result of resonance Contd..Result of resonance Contd..
  69. 69. Free induction decay  As the magnitude of the transverse magnetization decreases .so does the magnitude of the voltage induced in the receiver coil. This decrease is called free induction decay. 72
  70. 70. 73 FID
  71. 71. Relaxation  Relaxation – it means recovery of protons back towards equilibrium after been disturbed by RF pulse. Type of relaxation  Longitudinal relaxation or spin lattice relaxation.  Transverse relaxation or spin-spin relaxation 74
  72. 72. 75
  73. 73. 76
  74. 74. 77 Summary MRI Signal
  75. 75. Longitudinal relaxation (T1)  When RF pulse is switched off ,protons starts losing their energy. They transfer their energy to surrounding or lattice hence it is called spin-lattice relaxation. 78
  76. 76. 5) T1 RELAXATION TIMES  The time it takes for a proton to process back into alignment with the external magnetic field is called the T1 relaxation time.  Differences in T1 relaxation times depend on binding of the proton in different tissues.  Protons in different types of tissues have different relaxation times because their elasticity and chemical bonds are different. 79
  77. 77. 80
  78. 78. T2 RELAXATION TIMES  T2 relaxation time of a tissue is the time it takes for the protons to lose their phase.  The T2 relaxation time of a tissue is always shorter than its T1 relaxation time.  Protons in a magnetic field also have a second relaxation time called T2 relaxation time depends on interactions between the protons in small volume of tissue. 81
  79. 79. 82
  80. 80. T1 AND T2 VALUES FOR VARIOUS ORGANS AT 1T MAGNETIC FIELD STRENGTH OrganOrgan T1 (ms)T1 (ms) T2 (ms)T2 (ms) FatFat LiverLiver SpleenSpleen MuscleMuscle White matterWhite matter Gray matterGray matter CSFCSF BloodBlood WaterWater 220220 440440 460460 600600 700700 820820 20002000 800800 25002500 9090 5050 8080 4040 9090 100100 300300 180180 25002500 83
  81. 81. PULSE TIMING PARAMETER  A pulse sequence is a combination of RF pulses, signals intervening periods of recovery.  A pulse sequence consists of several components two most important are. the time of repetition (TR). the time of echo (TE). 84
  82. 82. 85 Repetition time-TR  Time interval between application of two RF pulses  Measured in ms  Determine amount of relaxation allowed occur between two RF pulses  Determine T1 relaxatation
  83. 83. 86 Echo time - TE  Time between application of RF pulse to peak of signal induced.  Measured in ms  Determine amount of transverse magnetization decay allowed to occur before signal is read.  TE controls amount of T2 relaxation.
  84. 84. 87 Basic Pulse Sequences TR,TE
  85. 85. 88 TR & TE - applications T1T1 PDPD T2T2 TRTR SHORTSHORT LONGLONG LONGLONG TETE SHORTSHORT SHORTSHORT LONGLONG  Long TR 2000 ms+  Short TR 250-700 ms  Long TE 60 ms+  Short TE 10-25 ms
  86. 86. 89 T1 vs. T2 T1 weighted images  Short TR & TE  Black fluid (csf, urine etc)  White fat  Anatomical detail  High SNR  CE T1 for pathology T2 weighted images  Long TR & TE  White fluid  Relatively Black fat  Detect of pathology  ↑ H2O - ↑ signal
  87. 87. CONTRAST MECHANISM  Image obtain contrast mainly through the mechanism of:  TI recovery  T2 decay  Proton or spin density (no of proton per unit volume of tissue) 90
  88. 88. 91 T1 Recovery in FAT Occurs due to the nuclei giving up their energy to surrounding. Slow molecular tumbling in fat allows the recovery process in FAT is rapid. T1 time of Fat is short T1 Recovery in fat
  89. 89. 92 T1 recovery in water Occurs due to nuclei giving up the energy acquired from the RF excitation to the surrounding lattice. In water molecular mobility is high. Resulting in less efficient T1 recoverey.T1 recovery of water is longer so T1 time of water is long T1 recovery in water
  90. 90. 93 T1 contrast : T1 time of fat is shorter than water fat vector realigns with Bo faster than water. Longitudinal components of magnetization of fat is larger than water T2 decay in FAT : It occurs as the result of magnetic fields of the nuclei interacting each other. Energy exchange is more efficient in the hydrogen in fat. The T2 time of fat is short (80ms) T2 decay in fat
  91. 91. 94 T2 decay in water :T2 decay in water : Energy exchange in water is less efficientEnergy exchange in water is less efficient than in fat, T2 time of hydrogen in water is long. T2 time of waterthan in fat, T2 time of hydrogen in water is long. T2 time of water (200ms)(200ms) T2 Contrast :T2 Contrast : T2 time of Fat is shorter than that of water.T2 time of Fat is shorter than that of water. Transverse components of magnetization of fat decays fasterTransverse components of magnetization of fat decays faster water has high signal, appear bright. on T2 contrast image fatwater has high signal, appear bright. on T2 contrast image fat has low signal and appears darkhas low signal and appears dark on T2 contrast imageson T2 contrast images T2 decay in water
  92. 92. 95 Proton density contrast It is the difference in signal intensity between tissues of their relative number of protons per unit volume. High signal (on brain tissue) Bright on proton density contrast. Tissue on low proton density have low signal e.g. Cortical bone – dark on proton density image. Proton density is the basic of MRI contrast
  93. 93. 96
  94. 94. MRI EQUIPMENT  The component of MRI system. Magnet. Radio Frequency source. Image processor. computer. Types of magnetism Para magnetic Diamagnetic. Ferromagnetic. 97
  95. 95. 7) MR EQUIPMENT 98
  96. 96. Advantages of MRI    MRI does not use ionizing radiation, and is thus preferred over CT in children and patients requiring multiple imaging examinations  MRI has a much greater range of available soft tissue contrast, depicts anatomy in greater detail, and is more sensitive and specific for abnormalities within the brain itself  MRI scanning can be performed in any imaging plane without having to physically move the patient  MRI contrast agents have a considerably smaller risk of causing potentially lethal allergic reaction  MRI allows the evaluation of structures that may be obscured by artifacts from bone in CT images 99
  97. 97. Thank You 100