Nuclear Magnetic Resonance-Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course Resonance and Excitation Changing Reference Frames M-Longhudinal Magetization 67 E .Why is MRI so hard to learn? .Changing reference frames M Classical versus Quantum 胎 Mechanical explanation .Lab and rotating frames 成 ◆Changing scales 品 。Macroscopic Intermediate(spin isochromats) mm 马家 ha vo 180 then split up later into smaller and smaller pieces 4a Resonance and Excitation Resonance and Excitation .B,field component rotating at Larmor f (off-freg.-litle effect) Time required 10-100 usec .Rotating reference frame:B,stationary in x-y plane B,applied torque to Mrotation:B ◆90°pulse一largest My (signal)generated describes the on th ◆For flip angle(g<90 。smaller M gneri Common angles:90(/2 radians:/2 pulse)and 180(radians) ◆less time necessary to displace M 9,19and26May2005 6 Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course 9, 19 and 26 May 2005 6 © UW and Brent K. Stewart, PhD, DABMP 21 Resonance and Excitation c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 382. RF Pulse Angle Tip: 0° 90° 180° Higher energy state © UW and Brent K. Stewart, PhD, DABMP 22 Changing Reference Frames Changing Reference Frames ™ Why is MRI so hard to learn? ™ Changing reference frames ™ Classical versus Quantum Classical versus Quantum Mechanical explanation ™ Lab and rotating frames ™ Changing scales ™ Macroscopic ™ Intermediate (spin Intermediate (spin isochromats) ™ Microscopic/QM ™ Start with a voxel of 1 mm x 1 mm Start with a voxel of 1 mm x 1 mm x 10 mm as a starting point and x 10 mm as a starting point and then split up later into smaller and then split up later into smaller and smaller pieces © UW and Brent K. Stewart, PhD, DABMP 23 Resonance and Excitation ™ B1 field component rotating at Larmor f field component rotating at Larmor f0 (off-freq. → little effect) little effect) ™ Rotating reference frame: B1 stationary in x-y plane ™ B1 applied torque to applied torque to Mz → rotation: rotation: θ = γ · B1· t ™ Flip angle ( Flip angle (θ) describes the rotation through which the longitudinal ) describes the rotation through which the longitudinal magnetization (Mz) is displaced to generate transverse ) is displaced to generate transverse magnetization (Mxy) ™ Common angles: 90° (π/2 radians: /2 radians: π/2 pulse) and 180° (π radians) radians) c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 384. Rotating Frame Lab Frame © UW and Brent K. Stewart, PhD, DABMP 24 Resonance and Excitation ™ Time required 10-100 µsec ™ 90° pulse → largest Mxy (signal) generated ™ For flip For flip angle ( angle (θ) < 90° ™ smaller Mxy component component generated and less signal generated and less signal ™ less time necessary to less time necessary to displace Mz ™ greater amount of greater amount of Mxy (signal) (signal) per excitation time ™ Low flip Low flip angle (θ) very important in rapid MRI important in rapid MRI scanning c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 384