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![T2* and T2 ContrastT2* and T2 Contrast
• Technique: use long TR and intermediate to
long TE in SE & FSE
• Use an itermediate TE & TR for GRE with a
narrow flip angle ( 15-30)
• In F-GRE, use a very short TE & TR & very
narrow flip angle.
• Useful for functional (T2* contrast) and
anatomical (T2 contrast to enhance fluid contrast)
studies
• 1mm resolution for anatomical scans or 4 mm
resolution [better is possible with better gradient
system, and a little longer time per volume]](https://image.slidesharecdn.com/mriimagequality-gamalmahdaly-151005145245-lva1-app6892/85/Mri-image-quality-gamal-mahdaly-27-320.jpg)
![Echo Planar Imaging (EPI)Echo Planar Imaging (EPI)
Methods shown earlier take multiple RF shots to readoutMethods shown earlier take multiple RF shots to readout
enough data to reconstruct a single imageenough data to reconstruct a single image
– Each RF shot gets data with one value of phase encodingEach RF shot gets data with one value of phase encoding
If gradient system (power supplies and gradient coil) areIf gradient system (power supplies and gradient coil) are
good enough, can read out all data required for one imagegood enough, can read out all data required for one image
after one RF shotafter one RF shot
– Total time signal is available is aboutTotal time signal is available is about 22⋅⋅T2*T2* [80 ms][80 ms]
Must make gradients sweep back and forth, doing allMust make gradients sweep back and forth, doing all
frequency and phase encoding steps in quick successionfrequency and phase encoding steps in quick succession
Can acquire 10-20 low resolution 2D images per secondCan acquire 10-20 low resolution 2D images per second](https://image.slidesharecdn.com/mriimagequality-gamalmahdaly-151005145245-lva1-app6892/85/Mri-image-quality-gamal-mahdaly-62-320.jpg)
Uploaded byGamal Mahdaly
Mri image quality gamal mahdaly
MRI image quality is affected by several factors including signal-to-noise ratio, receive bandwidth, noise sources, and spatial resolution. A higher signal-to-noise ratio provides better image quality. Receive bandwidth determines the range of frequencies received and affects chemical shift artifact and readout time. Noise in MRI comes from electronic noise and fluctuations in body tissues. Spatial resolution is influenced by receive bandwidth, matrix size, slice thickness, and field of view. Contrast resolution depends on tissue T1 and T2 values and the timing of pulse sequences. Motion contrast methods like diffusion weighting and flow weighting provide additional tissue information.
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Mri image quality gamal mahdaly
- 1. MRI Image Quality GGamalamalFFathallaathalla MM.. MMahdalyahdaly gamal_mahdaly@hotmail.comgamal_mahdaly@hotmail.com Experta Medica
- 2. Factors affecting digitalimage QualityFactors affecting digital image Quality Signal to Noise Ratio:Signal to Noise Ratio: The amount of true data building the imageThe amount of true data building the image relative to the amount of false datarelative to the amount of false data interfering with the image data duringinterfering with the image data during image reconstruction.image reconstruction. In MRI, being unavoidable, noise should notIn MRI, being unavoidable, noise should not exceed signal & the ratio SNR should beexceed signal & the ratio SNR should be around 1.around 1. In CT, the noise sources are much less, soIn CT, the noise sources are much less, so a different noise index was proposed.a different noise index was proposed. Experta Medica
- 3. Factors affecting Signalto NoiseFactors affecting Signal to Noise Ratio (SNR)Ratio (SNR) Receive Bandwidth (RBW) describes theReceive Bandwidth (RBW) describes the window of frequencies, the receive coilwindow of frequencies, the receive coil tuned to receive.tuned to receive. The received frequencies are of bothThe received frequencies are of both Signal & Noise.Signal & Noise. Experta Medica
- 4. Sources of theNoise in MRISources of the Noise in MRI • Random fluctuations in electrical current (electronic noise): exists in all electrical conductors including the MR coils with which we measure the signal, but it also includes the electrically conducting tissues of the patient. •Human tissue contains many ions such as sodium, potassium and chloride which are electrically charged atomic particles carrying electrical currents within the body, e.g. in nerve conduction. •These currents generate fluctuating magnetic fields which induce a noise voltage in the coil. •Pulsatile flow is also implicated in noise generation as the pulsed spins generate random electric fluctuations as well. • Remedy Tips: use a small or dedicated anatomy coil. Where large fields of view are essential, phased array coils are usually best. Experta Medica
- 5. Receive Bandwidth (RBW)ReceiveBandwidth (RBW) The frequency range of the receiver canThe frequency range of the receiver can be related to the frequency used tobe related to the frequency used to encode each pixel and thereforeencode each pixel and therefore determine the extent of the chemical-shiftdetermine the extent of the chemical-shift artifact.artifact. For example, for a 256 frequency matrixFor example, for a 256 frequency matrix and bandwidth of 32 kHz, there are 125and bandwidth of 32 kHz, there are 125 Hz per pixel so the fat–water shift isHz per pixel so the fat–water shift is approximately 2 pixels. Typicalapproximately 2 pixels. Typical bandwidths vary from 6.5 kHz to 1 MHz.bandwidths vary from 6.5 kHz to 1 MHz. Experta Medica
- 6. Bandwidth ( backto Larmor Equation) ν = γ/2π Βο Where ν is the larmor frequency Gyromagnetic ratioGyromagnetic ratio γ/2π = 42.57 MHz / Tesla for proton= 42.57 MHz / Tesla for proton RBWRBW being the whole repertoire of received frequencies, we can say:being the whole repertoire of received frequencies, we can say: RBW = 42.57RBW = 42.57 Βο For 1T systems, RBW should be in the range of 42.57 KHzFor 1T systems, RBW should be in the range of 42.57 KHz For 1.5T systems, RBW is in the range of 64 KHzFor 1.5T systems, RBW is in the range of 64 KHz For 3T systems, RBW is in the range of 128 KHzFor 3T systems, RBW is in the range of 128 KHz The question is how can we select the suitable RBW from these ranges & is itThe question is how can we select the suitable RBW from these ranges & is it dependable on the pulse sequence?dependable on the pulse sequence? Experta Medica
- 7. Effect of RBWon SNR We should consider three factors: •The Fat/water phase shift •The SNR •The Spatial Resolution Good SNR with Narrow RBW Poor SNR with Wide RBW Noise remains always Constant & Fluctuating Experta Medica
- 8. 88 Effect of RBWon SNR Sampling rate controls the receive bandwidth •To narrow the receive bandwidth the system samples slower & consequently the read out time increases & vice versa •Slower sampling requires a longer readout time •Faster sampling allows a shorter readout time Select BW to balance minimum TE, minimum ES versus SNR Consider the impact on chemical shift Experta Medica
- 9. Factors affecting Signalto Noise Ratio (SNR) RBW : As the RBW widens, SNR decreases. Number of Excitations ( NEX): As NEX increases, SNR also increases because each excitation generates part of the signal with constant fluctuating noise. Field of View (FOV): As FOV increases, SNR also increases due to increase of signal producing spins. Slice Thickness: As it increases, SNR also increases. Experta Medica
- 10. Spatial Resolution It isthe ability to discriminate between 2 neighboring points in any image i.e. to see image details. To be read on a 2D screen, the 3D voxel content is interpreted into 2D pixel Experta Medica
- 11. Voxel and pixelVoxeland pixel Voxel =Voxel = (Volume element)(Volume element) Pixel =Pixel = (Picture element)(Picture element) Experta Medica
- 12. To be readon a 2D screen, the 3D voxel content is interpreted into 2D pixel The system averages the intensities In the pixel, thereby decreasing spatial resolution Partial volume averaging (PVA) Most approaches to enhance spatial resolution are in deed to overcome PVA
- 13. Field of view(FOV):Field of view (FOV): The diameter of reconstructed imageThe diameter of reconstructed image MatrixMatrix:: Orderly array of cells fashioned in rowsOrderly array of cells fashioned in rows and columns.and columns. In MRI, we have a frequency matrix &In MRI, we have a frequency matrix & a phase matrixa phase matrix
- 14. Spatial Resolution Factors affectingspatial resolution: • RBW: as RBW widens, spatial resolution increases as the data building the image ( signal frequencies) increases. • Matrix (Phase & Frequency dimensions): as matrix increases, PVA decreases increasing spatial resolution. • Thickness (Slice dimension): as thickness decreases, PVA decreases increasing the spatial resolution. • FOV: as FOV decreases, PVA decreases, increasing the spatial resolution
- 15.
- 16. 2020 512 512 256 256 Matrix ZIP isa zero-fill interpolation to reconstruct images in a 512x512 or 1024x1024 matrix even though acquired in a different gradient encoding configuration ( i.e. to spread image data on a larger matrix). Spatial Resolution Matrix ZIP
- 17. 2222 Matrix ZIP increasesthe visibility of resolution inherent in the Image: Note the increased sharpness of the vessel edges 256 encoded/512 recon 256 encoded/256 recon Spatial Resolution Matrix ZIP
- 18. Contrast Resolution Contrast orGrey Scale Resolution is the ability to resolve the grey color into more color grades. Contrast Resolution demonstrates the ability to differentially diagnose different lesions according to their color index on the grey scale. In MRI, we have several contrasts in each pulse sequence. Each pulse sequence has its weighting (contrast – creating) factor(s).
- 19. Grey Scaling The MRIreconstruction process results in a 2D matrix of floating point numbers in the computer which range from near 0.0 up to value equal to 1.0 These numbers correspond to the average signal intensities of the tissue contained in each voxel The MR images are normalized and truncated to integer values that encompass 4096 values, between -1000 and 3095 (typically) Rescaling these values to definite shades of grey produces the Grey Scale.
- 20. Window width /window levelWindow width / window level The human tissue MR signal intenseties extend over a narrow range (window) of t Window width represents the MR signal intensities of all the tissues of interest wh Tissues with intensities outside this range are displayed as either black or white. Window Level represents the central number of all the intensities covered by the w Both WW & WL can be set independently & their respective settings affect the con Unlike CT, in MRI, we have multiple contrasts in each pulse sequence with differen
- 21. The Concept ofContrast (or Weighting)The Concept of Contrast (or Weighting) ContrastContrast = difference in RF signals — emitted= difference in RF signals — emitted by water protons — between different tissuesby water protons — between different tissues T1 weighted example: gray-white contrast isT1 weighted example: gray-white contrast is possible becausepossible because T1T1 is different between theseis different between these two types of tissuetwo types of tissue
- 22. T2 Decay MR Sign al T1 Recovery MR Sign al 50 ms50 ms1 s1 s Static Contrast Imaging Methods
- 23. Optimal TR andTE for Proton Density Contrast 0 10 20 30 40 50 60 70 80 90 100 0 0.5 1 1.5 2 2.5 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 0 0.5 1 1.5 2 2.5 T2 Decay MRSignal t (ms)t (s) MRSignal TRTR TETE T1 Recovery Optimum TR corresponds to the end of T1 recovery Optimum TE corresponds to the beginning of T2 decay
- 24. Proton Density ContrastProtonDensity Contrast • Technique: use very long time between RF shots (large TR) and very short delay between excitation and readout window (short TE) • Useful for anatomical reference scans • Several minutes to acquire 256×256×128 volume • ~1 mm resolution
- 25. Proton Density WeightedImageProton Density Weighted Image
- 26. Optimal TR andTE for T2* and T2 Contrast 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 10 20 30 40 50 60 70 80 90 100 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 T2 Decay MRSignal MRSignal T1 Recovery TRTR TETE T1 ContrastT1 Contrast T2 ContrastT2 Contrast
- 27. T2* and T2ContrastT2* and T2 Contrast • Technique: use long TR and intermediate to long TE in SE & FSE • Use an itermediate TE & TR for GRE with a narrow flip angle ( 15-30) • In F-GRE, use a very short TE & TR & very narrow flip angle. • Useful for functional (T2* contrast) and anatomical (T2 contrast to enhance fluid contrast) studies • 1mm resolution for anatomical scans or 4 mm resolution [better is possible with better gradient system, and a little longer time per volume]
- 28.
- 29. Optimal TR andTE for T1 Contrast 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 10 20 30 40 50 60 70 80 90 100 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 T1 contrast T2 contrast T2 Decay MRSignal MRSignal T1 Recovery TRTR TETE
- 30. T1 Contrast • Technique:use intermediate timing between RF shots (intermediate TR) and very short TE, also use large flip angles for GRE & MEMP • Useful for creating gray/white matter contrast for anatomical reference • ~1 mm resolution
- 31.
- 32. -S-So SSo S = SS= Soo * (1 – 2 e* (1 – 2 e –t/T1–t/T1 )) S = SS = Soo * (1 – 2 e* (1 – 2 e –t/T1’–t/T1’ )) Inversion Recovery for Extra T1Inversion Recovery for Extra T1 ContrastContrast
- 33. T2T2 Inversion RecoveryInversion Recovery (CSFAttenuated)(CSF Attenuated)
- 34. In summary, TRcontrols T1 weighting andIn summary, TR controls T1 weighting and TE controls T2 weighting. Short T2 tissuesTE controls T2 weighting. Short T2 tissues are dark on T2 images, but short T1 tissuesare dark on T2 images, but short T1 tissues are bright on T1 images.are bright on T1 images.
- 35. Motion Contrast ImagingMethodsMotion Contrast Imaging Methods Can “prepare” magnetization to makeCan “prepare” magnetization to make readout signal sensitive to different motionreadout signal sensitive to different motion propertiesproperties – Flow weighting (bulk movement of blood)Flow weighting (bulk movement of blood) – Diffusion weighting (scalar or tensor)Diffusion weighting (scalar or tensor) – Perfusion weighting (blood flow into capillaries)Perfusion weighting (blood flow into capillaries)
- 36. MR AngiogramMR Angiogram •Time-of-FlightTime-of-Flight ContrastContrast •PhaseContrastPhase Contrast
- 37. Time-of-Flight ContrastTime-of-Flight Contrast NoFlow Medium Flow High Flow No Signal Mediu m Signal High Signal Vessel Acquisitio n Saturation Excitatio n Vessel Vessel
- 38. 90o Excitation Image Acquisition RF Gx Gy Gz 90o Saturation Time to allowfresh flow enter the slice Pulse Sequence: Time-of-Flight Contrast
- 39. Phase Contrast (VelocityEncoding) Externally Applied Spatial Gradient G Externally Applied Spatial Gradient -G Blood Flow v 2 0 2 )()( GvT dtvtxGdtvtxG T T T γ γγφ = +−+= ∫ ∫ Time T 2T0
- 40.
- 41.
- 42. Diffusion Weighted ImagingDiffusionWeighted Imaging SequencesSequences Dtl 2= Externally AppliedExternally Applied Spatial GradientSpatial Gradient GG Externally AppliedExternally Applied Spatial Gradient -Spatial Gradient -GG TimeTime TT 2T2T00 322 3 2 TGD oeSS γ⋅− =
- 43. Pulse Sequence: Gradient-EchoDiffusionPulse Sequence: Gradient-Echo Diffusion WeightingWeighting 90o Excitation Image Acquisition RF Gx Gy Gz G -G
- 44.
- 45. Advantages of DWIAdvantagesof DWI 1.1. The absolute magnitude of the diffusionThe absolute magnitude of the diffusion coefficient can help determine proton poolscoefficient can help determine proton pools with different mobilitywith different mobility 2.2. The diffusion direction can indicate fiber tracksThe diffusion direction can indicate fiber tracks 3.3. DWI defines the diffusion characteristics of tumorsDWI defines the diffusion characteristics of tumors
- 46.
- 47. Determination of fMRIUsing the Directionality of Diffusion Tensor
- 48. Display of DiffusionTensor Using Ellipsoids
- 49.
- 50. PerfusionPerfusionDiffusionDiffusion Diffusion and PerfusionContrastDiffusion and Perfusion Contrast
- 51. Part III.3Part III.3 Somefundamental acquisition methoSome fundamental acquisition metho And their k-space viewAnd their k-space view
- 52. k-Space Recap Kx =Kx= γγ/2/2π ∫π ∫00 tt Gx(t) dtGx(t) dt Ky =Ky = γγ/2/2π ∫π ∫00 tt Gx(t) dtGx(t) dt Equations that govern k-space trajectoryEquations that govern k-space trajectory These equations mean that the k-space coordThese equations mean that the k-space coord are determined by the area under the gradienare determined by the area under the gradien dxdyeyxIkkS ykxki yx yx )(2 ),(),( +− ∫∫= π
- 53. Gradient Echo ImagingGradientEcho Imaging Signal is generated by magnetic fieldSignal is generated by magnetic field refocusing mechanism only (the use ofrefocusing mechanism only (the use of negative and positive gradient)negative and positive gradient) It reflects the uniformity of the magnetic fieldIt reflects the uniformity of the magnetic field Signal intensity is governed bySignal intensity is governed by S = So eS = So e-TE/T2*-TE/T2* wherewhere TETE is the echo time (time fromis the echo time (time from excitation toexcitation to the center of k-space)the center of k-space) Can be used to measure T2* value of theCan be used to measure T2* value of the tissuetissue
- 54. MRI Pulse Sequencefor GradientMRI Pulse Sequence for Gradient Echo ImagingEcho Imaging digitizer ondigitizer on ExcitationExcitation SliceSlice SelectionSelection FrequencyFrequency EncodingEncoding PhasePhase EncodingEncoding ReadoutReadout
- 55. K-space view ofthe gradient echoK-space view of the gradient echo imagingimaging Kx Ky 1 2 3 . . . . . . . n
- 56. Multi-slice acquisitionMulti-slice acquisition Totalacquisition time =Total acquisition time = Number of views * Number of excitations * TRNumber of views * Number of excitations * TR Is this the best we can do?Is this the best we can do? Interleaved excitation methodInterleaved excitation method
- 57.
- 58. Spin Echo ImagingSpinEcho Imaging ♦Signal is generated by radiofrequency pulse refocusing mechanism (the use of 180o pulse ) ♦It doesn’t reflect the uniformity of the magnetic field ♦Signal intensity is governed by S = So e-TE/T2 where TE is the echo time (time from excitation to the center of k-space) ♦Can be used to measure T2 value of the tissue
- 59. MRI Pulse Sequencefor Spin Echo Imaging digitizer ondigitizer on ExcitationExcitation SliceSlice SelectionSelection FrequencyFrequency EncodingEncoding PhasePhase EncodingEncoding ReadoutReadout 9090 180180
- 60. K-space view ofthe spin echo imaging Kx Ky 1 2 3 . . . . . . . n
- 61. Fast ImagingFast Imaging Howfast is “fast imaging”?How fast is “fast imaging”? In principle, any technique that can generate anIn principle, any technique that can generate an entire imageentire image with sub-second temporal resolution can be calledwith sub-second temporal resolution can be called fast imaging.fast imaging. For fMRI, we need to have temporal resolution on theFor fMRI, we need to have temporal resolution on the order oforder of a few tens ofa few tens of msms to be considered “fast”. Echo-planarto be considered “fast”. Echo-planar imaging,imaging, spiral imaging can be both achieve such speed.spiral imaging can be both achieve such speed.
- 62. Echo Planar Imaging(EPI)Echo Planar Imaging (EPI) Methods shown earlier take multiple RF shots to readoutMethods shown earlier take multiple RF shots to readout enough data to reconstruct a single imageenough data to reconstruct a single image – Each RF shot gets data with one value of phase encodingEach RF shot gets data with one value of phase encoding If gradient system (power supplies and gradient coil) areIf gradient system (power supplies and gradient coil) are good enough, can read out all data required for one imagegood enough, can read out all data required for one image after one RF shotafter one RF shot – Total time signal is available is aboutTotal time signal is available is about 22⋅⋅T2*T2* [80 ms][80 ms] Must make gradients sweep back and forth, doing allMust make gradients sweep back and forth, doing all frequency and phase encoding steps in quick successionfrequency and phase encoding steps in quick succession Can acquire 10-20 low resolution 2D images per secondCan acquire 10-20 low resolution 2D images per second
- 63.
- 64. Why EPI?Why EPI? Allows highest speed for dynamic contrastAllows highest speed for dynamic contrast Highly sensitive to the susceptibility-Highly sensitive to the susceptibility- induced fieldinduced field changes --- important for fMRIchanges --- important for fMRI Efficient and regular k-space coverageEfficient and regular k-space coverage and goodand good signal-to-noise ratiosignal-to-noise ratio Applicable to most gradient hardwareApplicable to most gradient hardware
- 65. Spiral ImagingSpiral Imaging t= TE RFRF GxGx GyGy GzGz t = 0
- 66. KK-Space-Space Representation ofRepresentation of SpiralImageSpiral Image AcquisitionAcquisition
- 67. Why Spiral?Why Spiral? •More efficientMore efficient kk-space trajectory to improve-space trajectory to improve throughput.throughput. • Better immunity to flow artifacts (no gradient atBetter immunity to flow artifacts (no gradient at the center of k-space)the center of k-space) • Allows more room for magnetization preparation,Allows more room for magnetization preparation, such as diffusion weighting.such as diffusion weighting.
- 68. Under very homogeneousmagnetic field, images look good …
- 69. Gradient-Recalled EPI ImagesUnder Homogeneous Field
- 70. Gradient Recalled SpiralImages Under Homogeneous Field
- 71. However, if wedon’t have a homogeneous field … (That is why shimming is VERY important in fast imaging)
- 72. Distorted EPI Imageswith Imperfect x-Shim A B C D
- 73. Distorted Spiral Imageswith Imperfect x-Shim A B C D
- 74. Any Question???. Again AnyQuestion???. Thank you!
- 75. Have a nicedayHave a nice day