Basic Principles MRI related to Neuroimaging.ppt

1、Basic Principles MRI related to Neuroimaging,Xiaoping Hu Department of Biomedical Engineering Emory University/Georgia Tech ,Outline,Basic NMR/MRI Physics Imaging sequences Contrast Mechanisms Pitfalls and Limitations,In the absence of magnetic field,In the presence of magnetic field

2、,Bulk Nuclear Magnetization in the Presence of a Static Magnetic Field,nuclear spin inside a magnetic field,Precession,Larmor Frequency, is frequency of precession and resonance usually in the radiofrequency (RF) range,Resonance,Resonance occurs when the external influence exerted to a system matche

3、s the systems 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).,Generation of NMR signal,Excitation an RF pulse is applied to tip the

4、 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,Spatial discriminationachieved

5、with magnetic field gradients,B0,x,B0,w,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,Lauterbur, 242, 190, Nature, 1973.,B0,w,frequency,phase,FT,RF,Gss,Gpe,Gro,Signal,timing diagram of a spin-

6、echo sequence,k-space traversal of a spin-echo sequence,frequency encoding,phase encoding,slice #1 acquisition,slice #2 acquisition,slice #n acquisition, ,TR,Temporally interleaved multislice imaging,nominal thickness,1,2,7,8,9,10,11,12,3,4,5,6,1,7,4,10,5,11,6,12,2,8,3,9,with gap or skip,no interlea

7、ve,interleave,Effects of Slice Spacing and Order,RF,Gss,Gpe,Gro,Signal,timing diagram of a blipped EPI sequence,k-space traversal of an EPI sequence,frequency encoding,phase encoding,Spiral Pulse Sequence,Spiral k-space trajectory,k = k(t) e k(t) = C t (t) = C k(t) (Archimedian),i(t),1,2,CONTRAST ME

8、CHANISMS in MRI T1 (Spin-lattice Relaxation time) relaxation along Bo T2 (Spin-spin relaxation time) relaxation perpendicular to Bo T2* (Signal decay perpendicular to Bo ) due to dephasing plus T2,x,z,y,Relaxation and Contrast,T1-relaxation,T2-relaxation,T1 relaxation,TR,90 pulse,90 pulse,TR,M0,M,Si

9、gnal decay due to transverse relaxation Irreversible processes (T2) Dephasing due to different frequency of precession in the presence of magnetic field inhomogeneities (reversible) (T2).,1/T2*=1/ T2 + 1/T2 Characterizes decay due to both processes.,180 pulse,90 pulse,TE,time,S(TE) = So e,-TE/T2*,90

10、 pulse,Relaxation and Contrast,T1-relaxation: Growth of magnetization for next nutation,T2-relaxation: decay of magnetization being detected,T1w Imaging at 3 Tesla,Brain Tumor Imaging,T1W Pre-contrast,T1W Post-contrast,T2W Pre-contrast,MRI for brain tumor,Spatial resolution,Signal-to-noise ratio Ima

11、ging time Gradient performance parameters Physics Diffusion Signal decay,State of the Art,Structural imaging of human subjects 1mm 1mm 1mm Anatomic imaging of rodents 50m 50 m 50 m NMR microscopy (of samples) 10m 10 m 10 m Functional studies Humans: 3mm 3mm 5mm Animals: 100m 100 m 500 m In vivo prot

12、on spectroscopy Human: 7mm 7mm 7mm Animal: 1mm 1mm 1mm,Temporal resolution,Signal-to-noise ratio Image resolution Gradient performance parameters Physics Relaxation,State of the Art,High resolution 3-D structural imaging 10-20 min Multislice imaging minutes Anatomic imaging of animals hours NMR micr

13、oscopy (of samples) hours to days Functional studies Sec/image, minutes/study In vivo proton spectroscopy Human: 10s of minutes Animal: hours,High-resolution imaging with reduced FOV Zoomed imaging by outer volume saturation,Limitations of ultrafast sequences,EPI Nyquist ghost Spatial distortion Spi

14、ral Blurring EPI and Spiral Signal dropout Resolution degradation due to T2* decay,k-space data,image,Nyquist ghost,k-space data,image,B0 inhomogeneity induced distortion,Several possible causes Static field inhomogeneity Subject-dependent susceptibility Field inhomogeneity disturbs the conditions o

15、f Fourier imaging Image distortion and artifacts are encountered with severe inhomogeneity,EPI image distortion due to field inhomogeneity,Single-Shot EPI,Segmented EPI,flash,corrected,original,Phase map,Spiral (before correction),Spiral (after correction),Problems in both EPI and Spiral,signal loss

16、 due to T2* decay resolution degraded and limited by T2*,7 Tesla T2*-weighted images (TE: 15 msec),5-mm, 1-mm,z-shim,Pulse Sequence for a Single-Shot EPI with Susceptibility Compensation,Song, MRM 46, 407, 2001.,Combined images from the single-shot acquisitioncompared with conventional single-shot acquisition at 4T