Nuclear Magnetic Resonance-Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course Pulse Sequences Spin Echo(SE)-Echo Time (TE) .Tailoring pulse sequence ◆Initial 90°oulse (t=O)一maximal M,and phase coherence emphasizes the image contrast FID exponentially decays via T2 relaxation dependent on p.T1 and T2- ◆Att=TEf2a180°oulse is applied一induces spin rephasing contrast weighted images .Timing,order,polarity,pulse ·22s调oePe2 undoing all the shaping.and repetition An FID waveform echo ("spin echo)produced att=TE frequency of RF pulses and gradient(later)application Three major pulse sequences 。Spin echo Inversion recovery Gradient recalled echo ☒mg含m分 Spin Echo(SE)-Echo Time(TE) SE-Repetition Time(TR)&Partial Saturation Maximum echo amplitude depends on T2 and not T2 Standard SE pulse sequences use a series of 90pulses separated FID envelope decay still dependent on T2 by At TR (repetition time,msec):[300,3000] SE formation separates RF excitation and signal acquisition events +This At allows recovery of M,through T1 relaxation processes .FID echo envelope centered at TE sampled and digitized with ADC After the 2nd 90 pulse,a steady-state M,produces the same FID .Multiple echos generated by successive 180pulses allow amplitude from subsequent 90 pulses:partial saturation determination of sample T2-exponential curve fiting:M (t) Degree of partial saturation dependent on T1 relaxation and TR Ta deeay -osanWmm 出24 40 9,19and26May2005 10 Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course 9, 19 and 26 May 2005 10 © UW and Brent K. Stewart, PhD, DABMP 37 Pulse Sequences Pulse Sequences ™ Tailoring pulse sequence Tailoring pulse sequence emphasizes the image contrast emphasizes the image contrast dependent on dependent on ρ, T1 and T2 , T1 and T2 → contrast weighted images ™ Timing, order, polarity, pulse Timing, order, polarity, pulse shaping, and repetition shaping, and repetition frequency of RF pulses and frequency of RF pulses and gradient (later) application ™ Three major pulse sequences ™ Spin echo ™ Inversion recovery ™ Gradient recalled echo c.f. http://www.indianembassy.org/dydemo/page3.htm © UW and Brent K. Stewart, PhD, DABMP 38 Spin Echo (SE) Spin Echo (SE) - Echo Time (TE) Echo Time (TE) ™ Initial Initial 90° pulse (t = 0) pulse (t = 0) → maximal Mxy and phase coherence ™ FID exponentially decays via T2* relaxation ™ At t = TE/2 a 180° pulse is applied pulse is applied → induces spin rephasing ™ Spin inversion: spins rotate in the opposite direction, undoing Spin inversion: spins rotate in the opposite direction, undoing all the all the T2* dephasing through through ∆t = TE/2 at t = TE ( = TE/2 at t = TE (∆t = 2·TE/2) ™ An FID waveform echo ( An FID waveform echo (“spin echo”) produced at t = TE c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 392. © UW and Brent K. Stewart, PhD, DABMP 39 Spin Echo (SE) Spin Echo (SE) - Echo Time (TE) Echo Time (TE) ™ Maximum echo amplitude depends on T2 and not T2* ™ FID envelope decay still dependent on T2* ™ SE formation separates RF excitation and signal acquisition events ™ FID echo envelope centered at TE sampled and digitized with ADC ™ Multiple echos generated by successive Multiple echos generated by successive 180° pulses allow pulses allow determination of sample T2 determination of sample T2 - exponential curve fitting: Mxy(t) ∝ e-t/T2 c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 393. © UW and Brent K. Stewart, PhD, DABMP 40 SE - Repetition Time (TR) Repetition Time (TR) & Partial Saturation ™ Standard SE pulse sequences use a series of Standard SE pulse sequences use a series of 90° pulses separated pulses separated by ∆t = TR (repetition time, msec): [300,3000] t = TR (repetition time, msec): [300,3000] ™ This ∆t allows recovery of Mz through T1 relaxation processes ™ After the 2nd 90° pulse, a steady-state Mz produces the same FID produces the same FID amplitude from subsequent 90° pulses: pulses: partial saturation ™ Degree of partial saturation dependent on T1 relaxation and TR c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 394