MRI sequence

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  1. Extrinsic parameters affecting contrast and signal to noise ratio include the echo time (TE) and repetition time (TR)
    1. T1 weighted – short TE (15-25) and short TR (500-700)
      1. in T1 weighted images, tissues that have a short T1 relaxation time, particularly fat, appear bright while those with long T1 relaxation times such as cysts, CSF and edema appear dark
    2. T2 weighted – long TE (80-100) and long TR (2500-3500)
      1. tissues with a long T2 relaxation time such as any fluid, appear bright
    3. Proton weighted – short TE (15-25) and long TR (2500-3500)
      1. in general, tissue signal intensities on proton density weighted images mirror those obtained on T2 weighted images however some white matter lesions such as MS plaques and small cortical stokes appear hyperintense relative to CSF making proton density images preferable in such cases; CSF is isointense on the proton density but bright on T2 weighted
  2. Bright on T1 weighted images:
    1. Fat and yellow marrow, proteinaceous material, intracellular methemoglobin, extracellular methemoglobin, melanin, calcium, paramagnetic contrast agent
  3. Bright on T2:
    1. Cysts, edema, extracellular methemoglobin, vitreous humor, CSF, MS plaques, tumors, abscess, infarct (NOTE: iron is dark on both T1 and T2)
  4. A pulse sequence defines the manner in which the radiofrequency pulses, which generate the detectable signals, and magnetic gradient fields, which provide the spatial encoding of the signals, are applied
    1. Gradient recalled echo pulse sequences
      1. faster than the fast spin echo technique but has enhanced signal to noise ratio
  5. paramagnetic IV contrast agents such as gadolinium that are able to cross the blood brain barrier only in areas where the blood brain barrier has been broken down and cause a local shortening of T1 and T2 relaxation times
  6. Fluid attenuated inversion recover pulse sequences (FLAIR)
    1. Allows for acquisition of images with T2 contrast but with CSF and other low viscocity fluids suppressed
    2. Particularly helpful in periventriuclar lesions such as MS plaques
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