B0B0 M Bulk Nuclear Magnetization in the Presence of a Static Magnetic Field
nuclear spin inside a magnetic field gyroscope influenced by gravity Precession
Larmor Frequency is frequency of precession and resonance usually in the radiofrequency (RF) range
Resonance Resonance occurs when the external influence exerted to a system matches the system’s natural frequency. E.g., pushing a swing In MRI, the natural frequency, called the Larmor frequency, is proportional to the applied magnetic field. At 1.5 T, it is ~64 Mhz (1Mhz=1000,000 hz; FM radio uses 88-106 Mhz).
Generation of NMR signal Excitation –an RF pulse is applied to tip the magnetization such that it has a transverse component Reception –precessing transverse component of M induces an emf in a receiving RF coil Relaxation –The processes with which the magnetization returns to equilibrium. They determine the intensity/contrast of the image
Spatial discrimination achieved with magnetic field gradients B0B0 x
B0B0 RF power Selective Excitation Application of a band-limited RF pulse in the presence of a gradient along the direction perpendicular to the desired slice
Spiral k-space trajectory k = k(t) e k(t) = C t (t) = C k(t) (Archimedian) i (t) 1 2
CONTRAST MECHANISMS in MRI T 1 (Spin-lattice Relaxation time) relaxation along B o T 2 (Spin-spin relaxation time) relaxation perpendicular to B o T 2 * (Signal decay perpendicular to B o ) due to dephasing plus T 2
x z y Relaxation and Contrast T1-relaxation T2-relaxation
Signal decay due to transverse relaxation Irreversible processes (T 2 ) Dephasing due to different frequency of precession in the presence of magnetic field inhomogeneities (reversible) (T 2 ’). 1/T 2 *=1/ T 2 + 1/T 2 ’ Characterizes decay due to both processes.
Spatial resolution Signal-to-noise ratio Imaging time Gradient performance parameters Physics –Diffusion –Signal decay
State of the Art Structural imaging of human subjects –1mm× 1mm× 1mm Anatomic imaging of rodents –50 m× 50 m × 50 m NMR microscopy (of samples) –10 m× 10 m × 10 m Functional studies –Humans: 3mm× 3mm × 5mm –Animals: 100 m× 100 m × 500 m In vivo proton spectroscopy –Human: 7mm × 7mm × 7mm –Animal: 1mm × 1mm × 1mm
State of the Art High resolution 3-D structural imaging –10-20 min Multislice imaging –minutes Anatomic imaging of animals –hours NMR microscopy (of samples) –hours to days Functional studies –Sec/image, minutes/study In vivo proton spectroscopy –Human: 10s of minutes –Animal: hours
High-resolution imaging with reduced FOV Zoomed imaging by outer volume saturation
Limitations of ultrafast sequences EPI –Nyquist ghost –Spatial distortion Spiral –Blurring EPI and Spiral –Signal dropout –Resolution degradation due to T2* decay
B0 inhomogeneity induced distortion Several possible causes –Static field inhomogeneity –Subject-dependent susceptibility Field inhomogeneity disturbs the conditions of Fourier imaging –Image distortion and artifacts are encountered with severe inhomogeneity
EPI image distortion due to field inhomogeneity
Single-Shot EPISegmented EPI flash corrected original Phase map
Pulse Sequence for a Single-Shot EPI with Susceptibility Compensation TE 1 RF Gx Gy Gz Compensatory Gradient TE 2 Song, MRM 46, 407, 2001.
Combined images from the single-shot acquisition compared with conventional single-shot acquisition at 4T New Single-shot Two partial-k TE1: 36 ms TE2: 44 ms Conventional Single-shot One full-k TE: 40 ms Song, MRM 46, 407, 2001.