Nuclear Magnetic Resonance-Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course IR-Field Attenuated IR and Contrast Comparison Raphex 2001 Diagnostic Questions Long Tl increases the signal levels of CSF other long T1 tissues ◆D44.InMR FLuid Attenuated IR (FLAIR):bounce point at CSF T1(3.500 msec) A.For most soft tissues.T2 is longer than T1 .Contrast comparison:T1-,p-,and T2-weighted plus FLAIR B.T1 decreases with field strength. C.T1 of CSF is longer than T1 of soft tissue. D.T2 increases with field strength. E.T2 of soft tissue is longer than T2 of CSF 色U Gradient Recalled Echo(GRE) Gradient Recalled Echo(GRE) Magnetic field gradient used to induce the formation of an echo Echo time conrolled through gradient magnitude or time offse ◆Gradient changes local magnetic field(B。+aB:f=2m)-(B。+AB) Flip angle(B)a major variable determining contrast in GRE seq. .FID signal generated under a linear gradient dephases quickly ◆Less time to excite the spins→short TR一→smaller flipe Inverted gradient(opposite polarity)used to produce an FID echo For short TR(<200 msec)more M:generated w small flip e Not a spin-echo technique:does not cancel Teffects 至尊 散wo一一 tor esro 9.19and26May2005 15Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course 9, 19 and 26 May 2005 15 © UW and Brent K. Stewart, PhD, DABMP 57 IR - Field Attenuated IR and Contrast Comparison ™ Long TI increases the signal levels of CSF & other long T1 tissues ™ FLuid Attenuated ttenuated IR (FLAIR): bounce point at CSF T1 (3,500 msec) (FLAIR): bounce point at CSF T1 (3,500 msec) ™ Nulling CSF requires: TI = ln(2) Nulling CSF requires: TI = ln(2) · T1 = 2,400 msec ™ TR = 7,000 typically employed to allow reasonable Mz recovery ™ Contrast comparison: T1-, ρ-, and T2-weighted plus FLAIR c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 403. © UW and Brent K. Stewart, PhD, DABMP 58 Raphex 2001 Diagnostic Questions Raphex 2001 Diagnostic Questions ™ D44. In MRI: In MRI: ™ A. For most soft tissues, T2 is longer than T1. A. For most soft tissues, T2 is longer than T1. ™ B. T1 decreases with field strength. B. T1 decreases with field strength. ™ C. T1 of CSF is longer than T1 of soft tissue. C. T1 of CSF is longer than T1 of soft tissue. ™ D. T2 increases with field strength. D. T2 increases with field strength. ™ E. T2 of soft tissue is longer than T2 of CSF. E. T2 of soft tissue is longer than T2 of CSF. © UW and Brent K. Stewart, PhD, DABMP 59 Gradient Recalled Echo (GRE) Gradient Recalled Echo (GRE) ™ Magnetic field gradient used to induce the formation of an echo ™ Gradient changes local magnetic field (B0+∆B): f = ( B): f = (γ/2π)·(B0+∆B) ™ FID signal generated under a linear gradient dephases quickly ™ Inverted gradient (opposite polarity) used to produce an FID echo ™ Not a spin-echo technique; does not cancel T2* effects c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 404. © UW and Brent K. Stewart, PhD, DABMP 60 Gradient Recalled Echo (GRE) Gradient Recalled Echo (GRE) ™ Echo time controlled through gradient magnitude or time offset Echo time controlled through gradient magnitude or time offset ™ Flip angle Flip angle (θ) a major variable determining contrast in GRE seq. a major variable determining contrast in GRE seq. ™ Less time to excite the spins Less time to excite the spins → short TR short TR → smaller flip smaller flip θ ™ For short TR (< 200 msec) more Mz generated w/ small flip generated w/ small flip θ c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 404