Presentation on theme: "Magnetic Resonance Imaging"— Presentation transcript:
1Magnetic Resonance Imaging Basic PrinciplesV.G.Wimalasena PrincipalSchool of Radiography
2Introduction Modern 3 Tesla MRI unit Patient Couch RF Coil (for head) Bore of the magnetRF Coil (for head)Main magnet bodyPatient Couch
3What is MRI?Magnetic resonance imaging (MRI), or nuclear magnetic resonance imaging (NMRI), is primarily a Medical Imaging technique most commonly used in radiology to visualize the structure and function of the body.It provides detailed images of the body in any plane.
4MRI Vs CTMRI provides much greater contrast between the different soft tissues of the body than CT does, making it especially useful in neurological (brain), musculoskeletal, cardiovascular, and oncological (cancer) imaging.Unlike CT, it uses no ionizing radiation, but uses a powerful magnetic field to align the nuclear magnetization of (usually) hydrogen atoms in water in the body.
5Uses RF fieldsRadiofrequency fields are used to systematically alter the alignment of the nuclear magnetization of Hydrogen atoms, causing the hydrogen nuclei to produce a rotating magnetic field detectable by the scanner.This signal can be manipulated by additional magnetic fields to build up enough information to construct an image of the body.
6HistoryMRI is a relatively new technology, which has been in use for little more than 30 years (compared with over 110 years for X-ray radiography).The first MR Image was published in 1973 and the first study performed on a human took place on July 3, 1977.Magnetic resonance imaging was developed from knowledge gained in the study of nuclear magnetic resonance
7Brief lay explanation of MRI physics The body is mainly composed of water molecules which each contain two hydrogen nuclei or protons.When a person goes inside the powerful magnetic field of the scanner these protons align with the direction of the field.
8A second radiofrequency electromagnetic field is then briefly turned on causing the protons to absorb some of its energy.When this field is turned off the protons release this energy at a radiofrequency which can be detected by the scanner.
9The position of protons in the body can be determined by applying additional magnetic fields during the scan which allows an image of the body to be built up.These are created by turning gradients coils on and off which creates the knocking sounds heard during an MR scan.
10Diseased tissue, such as tumors, can be detected because the protons in different tissues return to their equilibrium state at different rates.By changing the parameters on the scanner this effect is used to create contrast between different types of body tissue.
11Use of contrast agentsContrast agents may be injected intravenously to enhance the appearance of blood vessels, tumours or inflammation.Contrast agents may also be directly injected into a joint, in the case of arthrograms, MR images of joints.
12Safety precautionUnlike CT scanning MRI uses no ionizing radiation and is generally a very safe procedure.But Patients with some metal implants, cochlear implants, and cardiac pacemakers are prevented from having an MRI scan due to effects of the strong magnetic field and powerful radiofrequency pulses.
13Uses of MRI MRI is used to image every part of the body, But is particularly useful inneurological conditions,disorders of the muscles and joints,for evaluating tumors andshowing abnormalities in the heart and blood vessels.
17Motion within the atom There are three types of motion within an atom Electrons spinning on their own axisElectrons orbiting the nucleusThe nucleus spinning about its own axis
18The principles of MRI rely on the spinning motion of specific nuclei present in biological tissues These are called (MR active nuclei)
19MR active nuclei ?MR active nuclei are Characterized by their tendency to align their axis of rotation to an applied magnetic fieldDue to the laws of electromagnetic induction, nuclei that have a net charge and are spinning acquire a magnetic moment and are able to align with an external magnetic field
20MR active nuclei continued.. Important ExamplesHydrogen 1Carbon 13Nitrogen 15Oxygen 17Fluorine 19Sodium 23Phosphorus 31The nuclei with odd mass numbers undergoes this interactionThe result of this interaction is angular momentum or spinAlthough neutrons have no net charge heir subatomic particles are not evenly arranged over the surface of the neutron and this imbalance enables the nucleus in which the neutron is situated to be MR active as long as the mass number is odd.
21The magnetic moment alignment The alignment of the magnetic moment is measured as the total of the nuclear magnetic moments and is expressed as a vector sumThe strength of the total magnetic moment is specific to every nucleus and determines the sensitivity to magnetic resonance
22The hydrogen nucleusThe hydrogen nucleus is the MR active nucleus used in clinical MRIVery abundant in the bodySolitary proton gives a relatively large magnetic moment
23The hydrogen nucleus as a magnet The nucleus contains one positively charged proton that spinsThe spin of the proton induces a magnetic field around it and acts as a small magnetNNSS
24The magnetic vectorThe magnetic moment of each nucleus has vector properties.i.e. it has size and direction and is denoted by an arrowsizedirection
25Alignment of the magnetic moments In the absence of an applied magnetic field the magnetic moments are randomly orientedWhen placed in a strong external magnetic field the magnetic moments of the hydrogen nuclei align with this magnetic field , parallel or anti-parallel (as shown in next slide)
26Alignment of the magnetic moments ParallelAnti-parallelRandom alignment in the absence of external magnetic fieldAlignmentExternal magnetic field
27The state of alignmentQuantum physics describes that the hydrogen nuclei only possesses two energy states or populations – low & highLow energy nuclei align their magnetic moments parallel to the external magnetic fieldHigh energy nuclei align their magnetic moments anti-parallel to the external magnetic field
28Energy levels & field strength Low energy populationEnergy difference depends on field strengthhigh energy population
29Energy levels & alignments The energy level and the number of nuclei aligned in each direction is determined by the strength of the external magnetic field and the thermal energy level of the nucleiLow thermal energy nuclei do not have enough energy to oppose the field and align parallelHigh thermal energy nuclei have sufficient energy to oppose and may align anti-parallel
30Alignment & field strength Thermal energy depends on the body temperatureThe main deciding factor to increase the number of parallel alignments is the high field strength of the external magnetic fieldAt thermal equilibrium the parallel population is higher than the anti-parallel populationTherefore there is a net magnetic moment parallel to the external magnetic field
31The net magnetization vector B0Net Magnetization Vector (NMV)
32SummaryThe magnetic moment (of hydrogen in this case) is called the Net Magnetization Vector (NMV)The static external magnetic field is called B0The interaction of the NMV with B0 is the basis of MRIThe unit of B0 is Tesla or Gauss.1 Tesla (T) = Gauss (G)
33Summary continued…When a patient is placed in the bore of the magnet the hydrogen nuclei within the patient align parallel and anti-parallel to B0.A small excess of hydrogen nuclei line up parallel to B0 and constitute the NMV of the patient.The energy difference between the two populations increases as B0 increases.The magnitude of NMV is larger at high field strengths(B0 )
34PrecessionEach hydrogen nucleus that makes up the NMV is spinning on its own axisThe influence of B0 produce an additional spin or wobbleThis path is called the precessional path and the speed at which the NMV wobbles around B0 is called the precessional frequencyPrecessional pathB0Magnetic moment of the nucleusPrecessionHydrogen nucleus
35Precession continued…. Two populations;High energy nuclei – spin downLow energy nuclei – spin upTheir magnetic moments precess on a circular path around B0 as shownSpin up nucleiB0PrecessionSpin down nuclei
36The Larmor equationThe value of the precessional frequency is governed by the Larmor equation i.eThe precessional frequency (ω0) = Magnetic field strength(B0) x Gyro-magnetic ratio(γ)ω 0 = B0 x γGyro-magnetic ratio is a constant for a specific MR active nucleus and is expressed as the precssional frequency at 1.0 tesla. The unit is MHz / T
38ResonanceResonance is a phenomenon that occurs when an object is exposed to an oscillating perturbation that has a frequency close to its own natural frequency of oscillation.At resonance the object can absorb energy from the external sourceTherefore Exchange of energy between two systems at a specific frequency is called resonance.
39Nuclear ResonanceWhen a nucleus is exposed to an external perturbation that has an oscillation similar to its own natural frequency, the nucleus gains energy from the external force.The nucleus gains energy and resonates if the energy is delivered at exactly its precessional frequency.
40RF signal & Nuclear magnetic Resonance Energy at the precessional frequency of hydrogen at all field strengths in clinical MRI corresponds to the radio frequency (RF) band of the electromagnetic spectrumFor resonance of hydrogen to occur, an RF pulse of energy at exactly the Larmor frequency of the hydrogen NMV must be appliedOther MR active nuclei that have aligned with B0 do not resonate because their precessional frequencies are different to that of hydrogen
41Excitation & RF frequency The application of an RF pulse that causes resonance to occur is termed excitation.The absorption of energy causes an increase in the number of spin down hydrogen nuclei populations as some of the spin up nuclei gain energy via resonance and become high energy nuclei (next slide)The energy difference corresponds to the energy required to produce resonance via excitationAs the field strength increases, the energy difference between the two populations also increases so that more energy (higher frequencies) are required to produce resonance.
42Energy transfer during excitation Low energy populationSome nuclei gain energy to join the high energy populationHigh energy population
43The results of resonance The first result is the NMV moves out of alignment away from B0The angle to which the NMV moves out of alignment is called the flip angleThe magnitude of the flip angle depends upon the amplitude and duration of RF pulseUsually the flip angle is 900 (see next slide). The transverse NMV rotates at the Larmor frequency
44The flip angle & Transverse plane B0 is now termed the longitudinal planeThe plane at 900 to B0 is termed the transverse planeLongitudinal planeLongitudinal planeB0NMVFlip angleFlip angle 900NMVTransverse planeTransverse plane
45In phase / out of phaseThe second result of resonance is that the magnetic moments within the transverse NMV move into phase with each otherPhase is the position of each magnetic moment on the precessional path around B0Magnetic moments that are in phase are in the same place on the precessional path around B0 at any given timeMM that are out of phase are not in the same place on the precessional path
46Phase of magnetic moments around the precessional path Out of phaseIn phase
47SummaryFor resonance of hydrogen to occur, RF at exactly the Larmor frequency of hydrogen must be appliedThe result of resonance is an NMV in the transverse plane that is in phaseThis NMV precesses in the transverse plane at the Larmor frequency
48Formation of MR signal after removal of RF pulse The MR signalFormation of MR signal after removal of RF pulse
49The MR signalAs a result of resonance the NMV is precessing in phase in the transverse plane.According to Faraday’s laws of induction,When a receiver coil (a conductive loop) is placed in the area of moving magnetic field a voltage is induced in it.This Signal is produced when coherent (in phase) magnetization cut across the coil.
50MR signal continued….Therefore the moving NMV produces magnetic field fluctuations inside the coilAs the NMV precesses at the Larmor frequency in the transverse plane a voltage is induced in the coil.This voltage constitutes the MR signalWhy would you expect the MR signal to be alternating?
51The frequency of the MR signal is the same as the Larmor frequency The magnitude of the MR signal depends on the amount of magnetization present in the transverse plane
52Generation of the MR signal in the receiver coil B0Precession of NMVReceiver coilNMV
53Relaxation & The free induction decay signal Switching off RF pulseRelaxationRecovery & decayFID
54RelaxationWhen the RF pulse is turned off the NMV is again influenced by B0 , and, it tries to realign with it.To do that it must lose the energy given to it by the RF pulse.The process by which the NMV loses energy is called relaxation
55Recovery & Decay As relaxation occurs the NMV returns to align with B0 When this happens,The amount of magnetization in the longitudinal plane gradually increases – this is called recoveryThe amount of magnetization in the transverse plane gradually decreases – this is called decay
56The free induction decay signal As the magnitude of transverse magnetization decreases so does the voltage induced in the receiver coil.The induction of this reduced signal is called the free induction decay (FID) signal
57Result of relaxation During relaxation The NMV gives up absorbed energy and returns to B0The magnetic moments of the NMV lose the transverse magnetization due to dephasingLooking down on to transverse planeIn phaseDephasingOut of phase
58T1 Recovery & T2 Decay Relaxation results in recovery of magnetization in the longitudinal planeanddecay of magnetization in the transverse plane.The recovery of longitudinal magnetization is caused by a process called T1 recoveryThe decay of transverse magnetization is caused by a process called T2 decay
59T1 RecoveryT1 recovery is caused by the nuclei giving up their energy to the surrounding environment or lattice and it is often termed spin - lattice relaxationThe rate of recovery is an exponential process with a recovery time constant called T1
60Recovery time constant -T1 T1 is the time it takes 63% of the longitudinal magnetization to recover in the tissue100%63%Signal intensityT1Time
61T2 decayThis is caused by nuclei exchanging energy with neighbouring nuclei.The energy exchange is caused by the magnetic fields of each nucleus interacting with its neighbour.It is often termed spin-spin relaxation results in a decay or loss of transverse magnetizationThe rate of decay is also an exponential process so that the T2 relaxation time is its time constant of decay
62Time constant of decay – T2 100%T2 is the time it takes 63% of the transverse magnetization to be lostSignal intensity37%T2Time
63Dephasing of the FIDA signal or voltage is only induced in the receiver coil if there is magnetization in the transverse plane that is in phaseDephasing (T2)Signal (FID)
64Pulse timing parameters The magnitude and timing of the RF pulses form the basis of MRI and are discussed in Next lesson