echo time

The echo time (TE) refers to the time between the application of the radiofrequency excitation pulse and the peak of the signal induced in the coil. It is measured in milliseconds. The amount of T2 relaxation is controlled by the TE.

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Physics and Imaging Technology: MRI

• MRI (introduction)
• MRI physics
  • B0
  • chemical shift
  • dependence of magnetization (proton density, field strength and temperature)
  • echo time
  • eddy currents
  • electromagnetic induction
  • Ernst angle
  • flip angle
  • Larmor frequency
  • magnetic dipole
    • magnetic dipole moment
    magnetic field gradient
    • effect of gradient strength and bandwidth on slice thickness
  • magnetic susceptibility
  • magnetism​
    • diamagnetism
    • ferromagnetism
    • longitudinal and transverse magnetization
    • nuclear magnetization
    • paramagnetism
    • superparamagnetism
  • molecular tumbling rate effects on T1 and T2
  • net magnetization vector (NMV)
    • energy difference between spin up and spin down states
  • relaxation
    • T1 relaxation
      • T1 values (1.5 T)
      • factors affecting T1
    • T2 relaxation
    • T2* relaxation
  • repetition time
  • resonance and radiofrequency (RF)
  • units of magnetism
    • SI units
      • henry
      • tesla
      • weber
    • legacy units
      • gauss
      • maxwell
• MRI hardware 
  • coils
    • gradient coil
    • phased array coil
    • radiofrequency coil
    • volume coil
    • surface coil
  • electronics and data processing
  • magnets
    • quenching
    • magnetic field
  • radiofrequency receiver
  • radiofrequency transmitter
  • 1.5 T vs 3 T
  • ultrahigh field MRI
• signal processing
  • acquisition time
  • Fourier transformation
  • k-space
  • motion compensation
  • spatial resolution
  • temporal resolution
  • signal-to-noise ratio (SNR)
  • trade-offs
  • zero filling interpolation
• MRI pulse sequences (basics | abbreviations | parameters)
  • CSF flow studies
  • diffusion weighted sequences (DWI)
    • apparent diffusion coefficient (ADC)
    • b-values
    • diffusion tensor imaging and fiber tractography​
  • echo-planar pulse sequences
  • fat-suppressed imaging sequences
  • gradient echo sequences
    • in-phase and out-of-phase
    • spoiled gradient echo MRI
    • steady-state free precession
  • inversion recovery sequences
    • short tau inversion recovery (STIR)
    • fluid attenuation inversion recovery (FLAIR)
    • turbo inversion recovery magnitude (TIRM)
    • double inversion recovery (DIR)
  • metal artifact reduction sequence (MARS)
  • perfusion-weighted imaging
    • techniques
      • dynamic susceptibility contrast (DSC) MR perfusion
      • dynamic contrast enhanced (DCE) MR perfusion
      • arterial spin labeling (ASL) MR perfusion
    • derived values
      • cerebral blood flow (CBF)
      • cerebral blood volume (CBV)
      • k-trans
      • mean transit time (MTT)
      • negative enhancement integral (NEI)
      • time to peak (TTP)
  • saturation recovery sequences
  • spin echo sequences
    • T1 weighted image
    • T2 weighted image
    • fast spin echo (FSE)
    • proton density weighted image
  • spiral pulse sequences
  • susceptibility-weighted imaging (SWI)
  • T1 rho
• MR angiography (and venography)
  • contrast-enhanced MRA
  • non-contrast-enhanced MRA
    • phase contrast imaging
    • time of flight angiography
    • TRICKS
• MR spectroscopy (MRS)
  • 2-hydroxyglutarate peak: resonates at 2.25 ppm
  • alanine peak: resonates at 1.48 ppm
  • choline peak: resonates at 3.2 ppm
  • citrate peak: resonates at 2.6 ppm
  • creatine peak: resonates at 3.0 ppm
  • functional MRI (fMRI)
    • BOLD imaging
  • gamma-aminobutyric acid (GABA) peak: resonates at 2.2-2.4 ppm
  • glutamine-glutamate peak: resonates at 2.2-2.4 ppm 
  • Hunter's angle
  • lactate peak: resonates at 1.3 ppm
  • lipids peak: resonates at 1.3 ppm
  • myoinositol peak: resonates at 3.5 ppm
  • MR fingerprinting
  • N-acetylaspartate (NAA) peak: resonates at 2.0 ppm
• MRI artifacts
  • MRI hardware and room shielding
    • central point artifact
    • herringbone artifact
    • inhomogeneity artifact
    • moiré fringes
    • RF overflow artifact
    • zebra stripes
    • zipper artifact
  • MRI software
    • cross-excitation
    • fat-water swapping
    • slice-overlap artifact a.k.a. cross-talk artifact
  • patient and physiologic motion
    • phase-encoded motion artifact
    • entry slice phenomenon
    • flow void
  • tissue heterogeneity and foreign bodies
    • black boundary artifact
    • chemical shift artifact
    • dielectric effect artifact
    • magic angle effect
    • magnetic susceptibility artifact
      • ​blooming artifact
  • Fourier transform and Nyquist sampling theorem
    • aliasing (wrap around) artifact
    • Gibbs (truncation) artifact
    • zero-fill artifact
• MRI contrast agents
  • gadolinium contrast agents
  • gastrointestinal MRI contrast agents
  • reticuloendothelial MRI contrast agents
  • tumor-specific MRI contrast agents
  • intravascular (blood pool) MRI contrast agents
    • chromium-labeled red blood cells
    • Gd-DTPA-labeled dextran
    • Gd-DTPA-labeled albumin
    • gadofosveset trisodium
  • hepatobiliary MRI contrast agents
    • gadoxetate disodium
  • extracellular MRI contrast agents
    • gadobenate dimeglumine
    • gadobutrol
    • gadodiamide
    • gadopentetate dimeglumine
    • gadoterate meglumine
    • gadoteridol
    • gadoversetamide
  • contrast media safety
    • nephrogenic systemic fibrosis (NSF)
    • contrast media and breastfeeding
• MRI safety
  • magnetic shielding