Fast Imaging & Artifacts PDF
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MRI Basics - Fast Imaging & Artifacts Daniel B. Ennis, Ph.D. Magnetic Resonance Research Labs [email protected] 310.206.0713 (Office) http://mrrl.ucla.edu UCLA Radiology Spatial Localization Spatial Encoding • Three key steps: – Slice selection • You have to pick slice! – Phase Encoding • You have to encode 1 of 2 dimensions within the slice. Steps required to – Frequency Encoding (aka readout) acquire k-space data. } • You have to encode the other dimension within the slice. UCLA Radiology Where am I in k-space? RF Phase Cycling RF Spoiler Gradient Slice Select k (t) pe Phase Encode k (t) ro Freq. Encode UCLA Gradients move the acquisition through k-space. Radiology How do we calculate scan time? T = T R N N N Scan y,pe z,pe avg · · · • TR – Repetition time – Time between excitation pulses – Typically one echo per TR (but not always!) • Multi-echo imaging is faster… • N – Number of in-plane (y) phase encodes y,pe – Required for 2D and 3D imaging • N – Number of slice (z) phase encodes z,pe – Only used for 3D volume imaging • N – Number of averages avg – Averages are repeated acquisitions that boost SNR • T =1000ms•256•1=4:16 [mm:ss] Scan – MRI scanning can be slow. UCLA Radiology k-space What is k-space? • k-space is the raw data collected by the scanner. – A point in k-space tells us about the presence/absence of a spatial frequency (pattern) in the acquired image. – Each echo measures many of the spatial frequencies that comprise the object. – k-space has units of cm-1 or mm-1 – Audio signals have units of Hertz (s-1) • Gradients – Help extract spatial frequency information – Move us around in k-space • A line of k-space is filled by an echo • 2D FT of k-space produces the image UCLA Radiology What is k-space? k-space image space ➨ Fourier Transform k-space is the raw data collected by the scanner. UCLA Radiology What is k-space? k-space image space ➨ Fourier Transform One echo fills a line of k-space. UCLA Radiology What is k-space? Contrast Information Points in k-space represent different patterns in an image. UCLA Radiology
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