Components of an MRIComponents of an MRIsystemsystem••The Static MagnetThe Static MagnetThe permanent magnet. The permanent magnet. Resistive ElectromagnetResistive ElectromagnetSuperconductingSuperconductingElectromagnetElectromagnet••RF SystemRF SystemThe Transmitter is made up of an RF synthesizer, The Transmitter is made up of an RF synthesizer, power amplifier, and transmitting coil.power amplifier, and transmitting coil.The receiver consists of a pickThe receiver consists of a pick--up coil, amplifier, up coil, amplifier, and signal processing system. and signal processing system. ••The Magnet Gradients The Magnet Gradients Magnet gradients are produced by coils that are Magnet gradients are produced by coils that are orientated perpendicular to each other.orientated perpendicular to each other.Coils are resistive magnets.Coils are resistive magnets.Produce gradients from 20Produce gradients from 20mTmT/m to 100/m to 100mTmT/m./m.Magnetic Resonance ImagingMagnetic Resonance ImagingSome Nuclei Commonly Used in Some Nuclei Commonly Used in NMRNMRNucleiNucleiUnpaired Unpaired ProtonsProtonsUnpaired Unpaired NeutronsNeutronsNet Net SpinSpingg(MHz/T)(MHz/T)11HH1100½½42.5842.5822HH1111116.546.543131PP1100½½17.2517.252323NaNa11223/23/211.2711.271414NN1111113.083.081313CC0011½½10.7110.711919FF1100½½40.0840.08Spin in a Magnetic FieldSpin in a Magnetic FieldThe nuclear The nuclear magnetic momentmagnetic momentprecessesprecessesabout about the applied the applied magnetic field magnetic field Possible nuclear spin statesPossible nuclear spin statesHigh EnergyHigh EnergyLow EnergyLow EnergyPhoton AbsorptionPhoton AbsorptionTransition from low Transition from low energy state to high energy state to high energy stateenergy stateThe energy needed to The energy needed to make the transition make the transition depends on the depends on the magnetic moment of magnetic moment of the nucleus and the the nucleus and the strength of the applied strength of the applied magnetic fieldmagnetic fieldEnergy Shift=Frequency of DetectionEnergy Shift=Frequency of DetectionResonanceResonanceWhen a magnetic field is applied to a When a magnetic field is applied to a sample of material, the majority of nuclei sample of material, the majority of nuclei are in the high energy state.are in the high energy state.When the sample is immersed in radio When the sample is immersed in radio waves at the resonant frequency, the high waves at the resonant frequency, the high and low energy populations become equal.and low energy populations become equal.The radio frequency emitted as the sample The radio frequency emitted as the sample returns to its initial state can be returns to its initial state can be measured.measured.As energy of photons is raised, As energy of photons is raised, their wavelength shortens.their wavelength shortens.Therefore, the resolution of an MRI Therefore, the resolution of an MRI imageimage——A. Decreases as the magnetic field is raisedA. Decreases as the magnetic field is raisedB. Increases as the magnetic field is raised B. Increases as the magnetic field is raised C. Is not affected by the magnitude of the C. Is not affected by the magnitude of the magnetic fieldmagnetic fieldPulsed FT SpectroscopyPulsed FT SpectroscopyThe magnetic field is The magnetic field is held constant and a held constant and a pulse of radio waves pulse of radio waves containing a range of containing a range of frequencies is sent frequencies is sent through the sample.through the sample.More efficientMore efficientRichard Ernst won the Richard Ernst won the Nobel Prize in Nobel Prize in Chemistry in 1991.Chemistry in 1991.FTFTA Fourier Transform is used to A Fourier Transform is used to convert the data to the frequency convert the data to the frequency domain.domain.Intensity of received radio signalIntensity of received radio waveBig MagnetsBig MagnetsNMR spectroscopy NMR spectroscopy requires very requires very strong magnets (1 strong magnets (1 to 20 Tesla).to 20 Tesla).A stronger magnet A stronger magnet results in a larger results in a larger energy shift.energy shift.Inside theInside theDewarDewarInside the MagnetInside the MagnetBlock diagram of systemBlock diagram of systemRF coils in useRF coils in useSurface coils are commonly used for spines, shoulders, TMJ's, and other relatively small body parts. Paired saddle coils are commonly used for imaging of the knee. By running current in opposite directions in the two halves of the gradient coil, the magnetic field is made stronger near one and weaker near the other.Z-gradient used occasionally as RF coils for pelvis imaging and cervical spine imaging. This coil is commonly used as a transceiver coil for imaging of the head. What type of pickWhat type of pick--up coil is up coil is shown here?shown here?A.A.Surface coilSurface coilB.B.Birdcage coilBirdcage coilC.C.HelmholtzHelmholtzcoilcoilD.D.Saddle coilSaddle