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Physics of 3 Tesla MRI & Silent MRI

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3 tesla mri, its physics, advantages and disadvantages, its comparison with 1.5 tesla mri scanner, clinical applications of 3 tesla mri, various artifacts related to 3 tesla, physics of silent mri, advantages of silent mri

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Physics of 3 Tesla MRI & Silent MRI

  1. 1. 3 TESLA MRI & SILENT MRI DR VARUN BANSAL DEPTT OF RADIO-DIAGNOSIS
  2. 2. • Tissue relaxation rates • Pulse sequence changes: • timings, • radiofrequency • specific absorption rate • Pulse sequence optimization • Radiofrequency pulse limitations and specific absorption rate (SAR) • Signal to noise • Susceptibility • Contrast agents • Specific artifacts • Chemical shift artifacts of the first and second kinds • B1 inhomogeneity and standing • Wave artifacts • Steady-state pulse sequence banding artifacts
  3. 3. TISSUE RELAXATION RATES T1 RELAXATION • transfer of energy from excited protons to the surrounding structure (or spin – lattice), good ‘‘contact’’ between the spins and the lattice). • main B0 field strength increases  the resonance frequency of the excited spins also increases • 64 MHz at 1.5T to 128 MHz at 3T • higher frequency of the spins reduces the efficiency of energy transfer, resulting in longer T1 relaxation times at 3T
  4. 4. • In general, an increase in tissue T1 will cause a decrease in image SNR • lipids at 3T has been shown to increase by only approximately 20% • 40% increase in skeletal muscle, up to 62% increase for gray matter and 42% for white matter in brain, 41% increase in liver, • and up to 73% increase for kidney • Lipid signal in images remains strong at 3T resulting in increased artifacts
  5. 5. • also relative changes where the T1 relaxation time for one tissue increases at a different rate than the T1 relaxation time of another tissue.
  6. 6. T2 RELAXATION • mostly independent , • small, statistically insignificant, decrease of the transverse relaxation time T2 in certain tissues by up to 10% or more at higher magnetic field strengths • reduce the gain in SNR at high-field MR imaging for long echo time (TE) protocols.
  7. 7. T2 * EFFECT • transverse signal decay due to spin-spin interactions • field inhomogeneity and shimming difficulty both increase because of tissue susceptibility effects at high field • T2* to shorten significantly, changing image contrast owing to decay of transverse magnetization • mainly in gradient-recalled sequences, but can also be seen in fast imaging sequences that yield a mixture of T1/T2 image contrast such as true-FISP
  8. 8. PULSE SEQUENCE OPTIMISATION • increased T1 relaxation times at 3T. If sequence timings are not changed  T1-weighted contrast at 3T can actually be worse than. • extension in repetition time (TR) to match the T1 increase is one way to overcome signal and contrast changes; however, • Solution - use inversion recovery methods to maintain T1 image weighting while maintaining overall scan time. • changes in transverse relaxation time (T2) and T2* at 3T, TE values in various sequences may also need to be reoptimized.
  9. 9. • Increased TR alone can lead to • longer scan times, • increased motion artifacts, • decreased patient compliance, • fewer scan options for complex patient pathologies • other sequence parameters • Solution : • Number of signal averages, phase encodes, or echo train length for fast-imaging sequences, can be altered to decrease the overall scan time • Tradeoffs  result in decreased gain of SNR
  10. 10. • Two promising technologies • Fast three-dimensional (3D) pulse sequences and • parallel imaging techniques • exciting a slab of spins and encoding along the slice dimension, rather than acquiring images slice by slice • longer echo pulse trains or faster repetition times, allowing 3D volumes to be acquired with acquisition times similar to those for multislice 2D data.
  11. 11. PARALLEL IMAGING TECHNIQUES • take advantage of the higher SNR achieved at 3T and increased availability of phased array coils to acquire fewer phase encodes,  shortening total acquisition times, while maintaining equivalent image resolution to standard scan techniques • allows the increased SNR at 3T to be traded for spatial or temporal resolution for applications that are not SNR limited.
  12. 12. SPECIFIC ABSORPTION RATE • Measure for energy deposition within the human body • SAR required at 3T increases by a factor of four • absorbed by the subject and can cause increased tissue temperature • Depends on: • field strength, • the pulse sequence, • the RF transmit coil used • patient position inside the coil
  13. 13. • increased power RF wavelength localized ‘‘hot spots” • spin-echo and turbo spin-echo (TSE) sequences. • make use of closely spaced refocusing pulses or pulse trains that can quickly exceed SAR thresholds • Solution: • use of ‘‘variable flip angle’’ or ‘‘hyper-echo’’ RF pulse train techniques • VERSE (Variable Rate Selective Excitation) pulses, without decreasing the flip angle or increasing the excitation time
  14. 14. PARALLEL IMAGING • reducing the number of repetitions, and thus shortening the total data acquisition time, the total amount of energy absorbed for a given data acquisition is reduced. • magnetization preparation pulses. Inversion recovery pulses, magnetization transfer pulses, and fat and other saturation pulses
  15. 15. • MR body imaging is particularly impacted • almost always runs at the upper limits of the allowed SAR deposition • patients are more likely to experience an uncomfortable sensation of warmth or heating • Minimize SAR effect: • increase of the TR, • decrease in the number of slices, • decrease of the flip angle • increase scan time, reduce anatomical coverage, alter contrast, and/or further reduce the gain in SNR
  16. 16. SIGNAL TO NOISE RATIO • proportional to • main magnetic field strength • voxel volume • square root of the total sampling time, and • sequence-specific contrast-related terms. • Some of these factors, such as the longitudinal relaxation time T1 and receiver bandwidth, as well as specific absorption rate limitations can affect the SNR
  17. 17. • transverse relaxation time T2 is independent of the main magnetic field strength and assuming only an increase of the longitudinal relaxation time T1 • T2-weighted sequences • HASTE = 1.8 increase in SNR • gradient-echo-based T1-weighted sequences • in- and opposed-phase 2D dual echo and • 3D VIBE (volume interpolated breath hold examination) • a factor of about 1.6 to 1.7 increase in SNR can be obtained.
  18. 18. • Other factors, leading to a degradation: • practical limitations on sequence optimization due to SAR limits, • conservation of contrast, • various competing sequence parameter interactions • lack of certain specialized RF coils at 3T • All these reasons contribute to a gain in SNR that is less than the factor of 2.0 originally expected.
  19. 19. MAGNETIC SUSCEPTIBILITY • extent to which a material becomes magnetized when placed within a magnetic field. • result of microscopic gradients or variations in the magnetic field strength that occur near the interfaces of materials of different magnetic susceptibility, • Caused by: • bone-soft tissue or air-tissue interfaces. • metallic objects from previous surgical/interventional procedures • iron deposition in tissues, • since the susceptibility of metal is much higher than that of soft tissue.
  20. 20. • result in image nonuniformities, including • in-plane image distortion, • localized regions of high or low signal intensity, and • localized signal drop-outs • caused by T2* shortening.
  21. 21. • also occur next to gas-filled structures, • gas-filled bowel or sinuses in the head • susceptibility of gas is much smaller than that of soft tissue  for bowel wall imaging • obscure important findings at 3T MR imaging that may have been visualized at standard 1.5T MR • metal-containing devices that are considered MR safe at a field strength of 1.5T are not necessarily safe
  22. 22. • Advantages: • enlarged susceptibility artifacts  because of a gas/soft tissue interface may also be helpful, • detecting gas as in intrahepatic pneumobilia or free intraperitoneal gas • improved visualization in • T2*-weighted • perfusion studies • metal-related susceptibility • artifacts from surgical clips or surgical debris (eg, prior cholecystectomy or prior hepatic resection)
  23. 23. • techniques to minimize the influence of susceptibility artifacts: • readout direction can be changed to alter the location of the artifact, • voxel size can be reduced, • shimming of the main magnetic field can be optimized to even out field variations. • Avoid Gradient echo  do not have 180 refocusing pulses • Avoid echo-planar sequences  do have long echo trains,
  24. 24. CONTRAST AGENTS • behavior and effectiveness of contrast agents at 3T versus 1.5T depend on • relaxivity of the paramagnetic ion complex and tissue relaxation times, • both of which vary with field strength. • Relaxivity of chelated gadolinium contrast agents decrease only on the • order of 5% to 10% from 1.5 T to 3T T1 values for tissues, • can lengthen by 40% or more at 3T.
  25. 25. • relationship between contrast and its effect on tissue T1 can be given by: • C is the in vivo contrast agent concentration, • R is the relaxivity of the contrast agent, • T1(0) is baseline tissue T1 relaxation time without contrast, • T1(C) is the T1 relaxation of the tissue after contrast • T1 times at 1.5T are shorter than at 3T, an equivalent dose of contrast at 3T  more of a contrast difference. • effectiveness – • reduce the amount of contrast given in routine studies • improve CNR. • increased effectiveness of contrast-enhanced MR angiography techniques at 3T • further increases in T1 times for blood • better suppression of the background signal from fat
  26. 26. TRANSIENT EFFECTS FROM STATIC FIELD • Phenomena reported in association with patients moving in/out of high field magnets – Nausea (slight) – Vertigo – Headache – Tingling/numbness – Visual disturbances (phosphenes) – Pain associated with tooth fillings • Effects transient and cease after leaving magnet – actively shielded & short bore high-field magnets • larger spatial gradient – reduced or avoided by moving slowly in the main field
  27. 27. SPECIFIC ARTIFACTS • in general every artifact that is present at 1.5T is also present at 3T. the increase • in field strength actually causes artifacts to be more of a problem, either in absolute terms or because effective workarounds have not yet been developed
  28. 28. CHEMICAL SHIFT ARTIFACTS OF THE FIRST AND SECOND KINDS • Result of a difference in the resonant frequency between water and fat and is seen only along the frequency encoding axis and the slice selection dimension • difference in resonant frequency is directly proportional to the main magnetic field strength, and has been measured to be approximately 3.5 ppm, resulting in a difference of about 225 Hz at 1.5T, or a difference of about 450 Hz at 3T.
  29. 29. • Hypointense band, one to several pixels in width, toward the lower part of the readout gradient field, and as a hyperintense band toward the higher part of the readout gradient field • artifact of the first kind will be twice as wide at 3T compared with standard 1.5T imaging. • Problem in – small subcapsular renal hematoma or an intramural aortic hematoma. • Solve - receiver bandwidth can be increased  expense of SNR which decrease by 30%
  30. 30. CHEMICAL SHIFT ARTIFACT OF THE SECOND KIND • not limited to the frequency encoding axis, but may be seen in all pixels along a fat/water interface as it is based on an intravoxel phase-cancellation effect where fat and water exist in the same voxel • size of this artifact does not increase with the main magnetic field strength and is defined by the spatial resolution of the MR sequence • However, the echo time needs to be adjusted as the frequency difference is twice as large compared with standard 1.5T MR systems
  31. 31. • 3T MR system, both fat and water protons are • in-phase at 2.2 ms, 4.4 ms, 6.6 ms, and so on, and • out-of-phase at 1.1 ms, 3.3 ms, 5.5 ms, and so on. • at 1.5T, the fat and water are phase • opposed at 2.2 ms and • In phase at 4.4 seconds. • In short, • by doubling the field strength we have halved the echo times needed for in-phase and opposed phase imaging.
  32. 32. • Fortunately, the increased difference in resonant frequency between water and fat at 3T may also be advantageous • allows for a better separation of the fat and water peak during MR spectroscopy, • better or faster fat suppression using other chemical shift techniques as well, eg, fat saturation and water excitation.
  33. 33. B1 INHOMOGENEITY AND STANDING WAVE ARTIFACTS • exacerbation of artifacts more pronounced at 3T as they are related to the higher frequency B1 transmit fields that are used at 3T. • particular difficulty for designing RF coils at the higher frequency is achieving a homogeneous B1 RF field. • T1-weighted gradient-echo imaging is usually not compromised • oftentimes problematic in T2-weighted TSE imaging
  34. 34. • Wavelength of the RF field at 128 MHz is 234 cm in free space, (much larger than FOV) • Water (and most body tissue) has a rather high dielectric constant,  reduces both the speed and wavelength of electromagnetic radiation • reduces the RF field wavelength from 234 cm  30 cm • result in a so-called ‘‘standing wave’’ effect (often incorrectly called a ‘‘dielectric resonance’’ effect
  35. 35. • strong signal variations across an image can be seen, especially brightening or dark ‘‘holes’’ in regions away from the receive coil caused by constructive or destructive interference from the standing waves • more pronounced the • larger the region of interest • more in obese patients with a distended abdomen than in thin patients.
  36. 36. • To overcome B1 inhomogeneity challenge: • Special RF pulse designs or coil designs such as multichannel RF transmission techniques where the phase and amplitude of the various elements can be adjusted to obtain a uniform B1 field • passive coupling of coils to improve B1-homogeneity • use of dielectric pads or RF cushions • RF cushions -- consist of a gel encapsulated in synthetic material • has a higher dielectric constant alters these interference patterns and potentially reduces or eliminates the destructive interference
  37. 37. SHIELDING EFFECTS • rapidly changing magnetic field  induce a circulating electric field • electric current is established  acts like an electromagnet that opposes the changing magnetic field, reducing the amplitude and dissipating the energy of the RF field. • more conductive the medium, eg, ascites, pregnant; the stronger the opposing electromagnet and greater the attenuation of the RF field
  38. 38. STEADY-STATE PULSE SEQUENCE BANDING ARTIFACTS • gained in popularity recently because they provide both higher CNR and SNR and motion compensation • Cardiac imaging • SSFP, (bSSFP), (FIESTA), (true FISP). • vulnerable to banding artifacts because of off-resonance effects, which cause variations in signal intensities across images • local off-resonance frequency is equal to a multiple of 1/TR, dark stripes appear in images
  39. 39. • minimized at 1.5T by keeping TR as short as possible to shift these stripes outside of the FOV or by summing frequency modulated acquisitions to average images over multiple spectral offsets • At higher field, difficulties with shimming and increased susceptibility effects • Increases in tissue T1 values make it less desirable to shorten TR to remove the bands from the FOV, • Solution - increased gradient switching speed and use frequency – averaging methods.
  40. 40. SPECTROSCOPY • Increased spectral resolution and SNR – Brain, prostate, breast (+ other body applications) – Multi-nuclear: 31P, 19F, 23Na, 13C
  41. 41. PROSTATE AND BREAST SPECTROSCOPY
  42. 42. BLOOD OXYGEN LEVEL DEPENDENT IMAGING • Functional MRI relies on the BOLD effect • BOLD facilitates neuronal activation measurements without using exogenous contrast agents • Activation: Oxy-blood increases while Deoxy-blood (paramagnetic) decreases – T2* is lengthened => signal increase – BOLD contrast increased due to smaller T2* values – SNR increase also leads to higher sensitivity • CNR increases by factor of 1.8-2.2 from 1.5T to 3.0T – These net effects, and reasons for them, are complicated
  43. 43. DIFFUSION WEIGHTED IMAGING • SNR is crucial – Thinner slices • Reduce partial volume artifacts – Higher b-values • Diffusion Tensor Imaging (DTI) – Same benefits as DWI – Faster acq.=> minimize motion • Shortened T2* – limits benefits – Use parallel imaging techniques
  44. 44. PERFUSION IMAGING • Arterial Spin Labeling (ASL) – Uses and inversion pulse to “tag” blood – Images acquired as tagged blood perfuses into tissue – Long T1 results in better tagging • Dynamic Susceptibility Contrast (DSC) – Bolus of paramagnetic agent • T2* contrast – T2* effect increased by field
  45. 45. CONTRAST ENHANCED IMAGING • Higher SNR • Longer tissue T1 vs. little change in contrast agent T1 – Better contrast – Use less contrast • Perform higher resolution dynamic imaging • Applications: brain, breast and body imaging
  46. 46. MR ANGIOGRAPHY AT HIGHER FIELDS • In general SNR => better spatiotemporal resolution • Time of flight (TOF) – Relies on saturated normal tissue and bright inflow – Longer T1 time => better background tissue saturation • Magnetization Transfer Contrast can further suppress – Must be careful of SAR limits – Higher-field => increased inflow signal • Contrast bolus – Better T1-contrast • Phase contrast – More sensitive to slow flow
  47. 47. CARDIAC IMAGING AT HIGHER FIELDS • Speed is king in cardiac imaging – Use parallel imaging techniques to their fullest • CINE imaging – SAR => reduce flip angles for SSFP (trueFISP/FIESTA) • T2 weighting and SNR loss • Black blood imaging (double inversion recovery) – Increased T1 of blood (+ 30% ) => longer inversion time needed – More SNR and slow T1 relaxation • Chance to increase the limited slice efficiency of method • Cardiac Tagging methods – Persistance of tagging • Emerging techniques: Perfusion, Delayed Enhancement
  48. 48. SILENT MRI • sources are the pulses of current generated in the gradient • gradient coil is placed inside a strong magnetic field, a pulsed Lorentz force is induced, causing vibrations of the coil structure, which in turn generate a compression wave in the air perceived as the “scanner noise” coil for the spatial encoding of the NMR signal. • 70–110 dB
  49. 49. • increasing strength of the magnetic field, quadratic increase of the sound pressure. • Lorentz force is proportional to the product of magnetic field and gradient amplitude, • and the latter must increase proportionally to the field to keep chemical shift and susceptibility artifacts constant.
  50. 50. METHODS TO REDUCE NOISE 1. active noise cancellation (ANC) using an anti-phase sound 2. Construction of ‘‘quiet gradient coils’’ in which the net Lorentz force is compensated between current pathways 3. application of a reduced gradient slew rate in fMRI and a radical attempt to replace gradient pulsing with a mechanically rotated coil 4. design of the gradient waveforms 5. can also be combined with the other approaches
  51. 51. REDESIGN OF GRADIENT WAVEFORM • A. Sinusoidal gradient slopes • B. Maximum slope durations • C. Minimum number of slopes • Can bring the noise level upto 25 – 40 dB.
  52. 52. REFERENCES • A Review of MR Physics: 3T versus 1.5T. Magn Reson Imaging Clin N Am 15 (2007) 277–290 • Brain and spine MR artifacts at 3T. Neurorad 2008. • Diagnostic Radiology. Recent Advances and Applied physics in imaging. AIIMS, MAMC, PGI. 2nd edition • Grainger & Allison’s . Diagnostic Radiology. 6th edition • MRI made easy. Govind B Chavhan 2nd edition • Haaga. CT and MRI of the Whole Body. 5th edition
  53. 53. THANK YOU

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