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Basic Principles MRI related to Neuroimaging Xiaoping Hu Department of Biomedical Engineering Emory University/Georgia Tech xhu@bme.emory.edu
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Outline Basic NMR/MRI Physics Imaging sequences Contrast Mechanisms Pitfalls and Limitations
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In the absence of magnetic field
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In the presence of magnetic field
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B0B0 M Bulk Nuclear Magnetization in the Presence of a Static Magnetic Field
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nuclear spin inside a magnetic field gyroscope influenced by gravity Precession
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Larmor Frequency is frequency of precession and resonance usually in the radiofrequency (RF) range
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Resonance Resonance occurs when the external influence exerted to a system matches the system’s natural frequency. E.g., pushing a swing In MRI, the natural frequency, called the Larmor frequency, is proportional to the applied magnetic field. At 1.5 T, it is ~64 Mhz (1Mhz=1000,000 hz; FM radio uses 88-106 Mhz).
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Generation of NMR signal Excitation –an RF pulse is applied to tip the magnetization such that it has a transverse component Reception –precessing transverse component of M induces an emf in a receiving RF coil Relaxation –The processes with which the magnetization returns to equilibrium. They determine the intensity/contrast of the image
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Spatial discrimination achieved with magnetic field gradients B0B0 x
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B0B0 RF power Selective Excitation Application of a band-limited RF pulse in the presence of a gradient along the direction perpendicular to the desired slice
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Lauterbur, 242, 190, Nature, 1973.
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B0B0
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frequency phase
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FT
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RF G ss G pe G ro Signal timing diagram of a spin-echo sequence
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k-space traversal of a spin-echo sequence frequency encoding phase encoding
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slice #1 acquisition slice #2 acquisition slice #n acquisition TR Temporally interleaved multislice imaging
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nominal thickness 127891011123456 174105116122839 with gap or skip no interleave interleave Effects of Slice Spacing and Order
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RF G ss G pe G ro Signal timing diagram of a blipped EPI sequence
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k-space traversal of an EPI sequence frequency encoding phase encoding
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Spiral Pulse Sequence
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Spiral k-space trajectory k = k(t) e k(t) = C t (t) = C k(t) (Archimedian) i (t) 1 2
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CONTRAST MECHANISMS in MRI T 1 (Spin-lattice Relaxation time) relaxation along B o T 2 (Spin-spin relaxation time) relaxation perpendicular to B o T 2 * (Signal decay perpendicular to B o ) due to dephasing plus T 2
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x z y Relaxation and Contrast T1-relaxation T2-relaxation
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T1 relaxation TR 90° pulse TR M0 M
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Signal decay due to transverse relaxation Irreversible processes (T 2 ) Dephasing due to different frequency of precession in the presence of magnetic field inhomogeneities (reversible) (T 2 ’). 1/T 2 *=1/ T 2 + 1/T 2 ’ Characterizes decay due to both processes.
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180° pulse 90° pulse TE
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time TE S(TE) = S o e -TE/T 2 * 90° pulse
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Relaxation and Contrast T1-relaxation: Growth of magnetization for next nutation T2-relaxation: decay of magnetization being detected
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T 1 w Imaging at 3 Tesla
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Brain Tumor Imaging T1W Pre-contrast T1W Post-contrast T2W Pre-contrast MRI for brain tumor
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Spatial resolution Signal-to-noise ratio Imaging time Gradient performance parameters Physics –Diffusion –Signal decay
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State of the Art Structural imaging of human subjects –1mm× 1mm× 1mm Anatomic imaging of rodents –50 m× 50 m × 50 m NMR microscopy (of samples) –10 m× 10 m × 10 m Functional studies –Humans: 3mm× 3mm × 5mm –Animals: 100 m× 100 m × 500 m In vivo proton spectroscopy –Human: 7mm × 7mm × 7mm –Animal: 1mm × 1mm × 1mm
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Temporal resolution Signal-to-noise ratio Image resolution Gradient performance parameters Physics –Relaxation
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State of the Art High resolution 3-D structural imaging –10-20 min Multislice imaging –minutes Anatomic imaging of animals –hours NMR microscopy (of samples) –hours to days Functional studies –Sec/image, minutes/study In vivo proton spectroscopy –Human: 10s of minutes –Animal: hours
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High-resolution imaging with reduced FOV Zoomed imaging by outer volume saturation
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Limitations of ultrafast sequences EPI –Nyquist ghost –Spatial distortion Spiral –Blurring EPI and Spiral –Signal dropout –Resolution degradation due to T2* decay
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k-space dataimage Nyquist ghost
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k-space data image
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B0 inhomogeneity induced distortion Several possible causes –Static field inhomogeneity –Subject-dependent susceptibility Field inhomogeneity disturbs the conditions of Fourier imaging –Image distortion and artifacts are encountered with severe inhomogeneity
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EPI image distortion due to field inhomogeneity
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Single-Shot EPISegmented EPI flash corrected original Phase map
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Spiral (before correction)
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Spiral (after correction)
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Problems in both EPI and Spiral signal loss due to T2* decay resolution degraded and limited by T2*
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7 Tesla T2*-weighted images (TE: 15 msec) 5-mm 1-mm z-shim
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Pulse Sequence for a Single-Shot EPI with Susceptibility Compensation TE 1 RF Gx Gy Gz Compensatory Gradient TE 2 Song, MRM 46, 407, 2001.
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Combined images from the single-shot acquisition compared with conventional single-shot acquisition at 4T New Single-shot Two partial-k TE1: 36 ms TE2: 44 ms Conventional Single-shot One full-k TE: 40 ms Song, MRM 46, 407, 2001.
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Acquisitions - spiral-out (A) - spiral-in (B) - combined spiral-in/out (C) Spiral-In/Out Experiments TE Glover & Law, MRM, 45, 515, 2001
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Spiral-In/Out Combination spiral-out spiral-in wtd ave Glover & Law, MRM, 45, 515, 2001
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