mri6 - 13.0 Magnetic Resonance Imaging We have previously...

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13.0 Magnetic Resonance Imaging We have previously covered the basic principles of the formation of a sample’s magnetization when placed in a magnetic field. This is the way NMR was done since the 1940’s. In the early 1970’s, Paul Lauterbur had the idea to spatially encode the NMR signal to make images. Now we explore the instrumentation and schemes devised since then to make images and what the important parameters are that affect image quality. •Instrumentation • magnet • gradient coils • RF coils • spectrometer for phase sensitive detection •MRI Data Acquisition • encoding spatial position: gradients, slice selection • frequency encoding, k-space diagrams • phase encoding • spin-echo pulse sequence •MRI Image Reconstruction •MR Image Quality, Sampling Requirements, SNR
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Important Points from MRI Lecture 5 • Chemical shift artifact: fat and water protons resonate at frequencies 3.5ppm apart. Fat and water may be shifted if pixels span less than this difference. At 1.5T this is 225 Hz so pixels should be wider than this. •We summarized the Nyquist criteria for the parameter relations as: γG Δt =1/D •We looked at ways to speed up acquisition: double voxel size, ok but loses resolution; double k-space step: reduced FOV •Looked at SNR and saw that: SNR ~ V v (Ny*Nex*Nx/ Δ f) 1/2 =V v (T A ) 1/2 where V v is the voxel volume: FOVx*FOVy/(Nx*Ny), T A = time the ADC is ‘on’. Note: scan time is TR*Ny*Nex • Looked at pulse sequences: Spin echo, PD, T1, T2 weighting and combined weighting • multiple echo spin echo, fast spin echo (FSE) : PD, T2 weighting depending on how k-space is filled •“Prepping” the sequence with IR or some other weighting to null fat or water (STIR, FLAIR) •Gradient recalled echo (GRE) pulse sequences: importance of using flip angle to maximize SNR, contrast •Clear example of T2* and susceptibility related contrast and signal loss
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Interleaved Multislice Acquisition RF Gs Gp Gr Tr Te Slice 1 Slice 2 Slice 3 Next phase encoding value •Slice selection center frequency is adjusted so that each interleave excites just the desired slice •Each slice gets a full Tr relaxation time •Scan time for whole volume is same as for a single slice! •# slices interleaved depends on Te
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3D Imaging: phase encode in slice direction: F (z) = g G z zt on From M. Wagshul via web. „Slab‟ selected
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Reduced gradient for “slab” selection 3D imaging From M. Wagshul via web.
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2D vs. 3D imaging Slice selection replaced with large slab selection Slice selection secondary phase encoding Repetition times: 2D – slices excited serially, TR = N sl * TR ’, imaging time = N PE * TR * Nex 3D – all slices excited simultaneously, but we have 2 phase encodings so imaging time = N PE * N sl * TR * Nex so only useful for very fast sequences, short TR SNR: 2D – signal contributions from the slice, but noise from the entire volume 3D – signal and noise contributions from the entire volume
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This note was uploaded on 10/30/2010 for the course MP 230 taught by Professor Macfall during the Fall '10 term at Duke.

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mri6 - 13.0 Magnetic Resonance Imaging We have previously...

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