Relaxation II - Clinical

  1. The MR signal in adipose tissue comes primarily from hydrogen protons in
    1. Free fatty acids
    2. Long-chain triglycerides
    3. Short-chain triglycerides
    4. Cholesterol

    The major MR signal in adipose tissue comes from long-chain aliphatic triglycerides, 16-20 carbon atoms in length, that are mostly saturated (−(CH2)n−). Link to Q&A discussion

  2. Protons in which lipid elements do not contribute significantly to the bright MR signal of fat on routine T1-weighted imaging?
    1. Cell membrane phospholipids
    2. Liquid forms of cholesterol
    3. Free fatty acids
    4. Triglyceride methyl groups

    Phospholipids and sphingolipids, present in cell membranes and myelin respectively, have exceedingly short T2 values and their MR signal is not directly recorded using routine MR sequences. They can be recorded using ultrashort TE (UTE) methods. Link to Q&A discussion

  3. Examples of bright T1 signal due to exogenous lipids include all of the following except
    1. Intrathecal Pantopaque™ from myelography in the 1980s
    2. A commercial MR skin marker
    3. A vitamin E capsule skin marker
    4. Vaseline (petroleum jelly) on the skin

    Although vitamin E capsules, fish oil capsules, and bath oil beads may be used as skin markers in MRI, they cannot be seen on fat-suppressed images. Commercial MR skin markers contain a dilute paramagnetic solution visible on all pulse sequences. Link to Q&A discussion

  4. T1 shortening due to cholesterol substantially explains the MR signal seen in which of the following diseases?
    1. Cholesterol granuloma of the petrous apex
    2. Cholesterol gallstones
    3. Craniopharyngioma
    4. All of the above
    5. None of the above

    Cholesterol in all its states has a short T1 value and range of chemical shifts similar to other lipids. The T2 values of cholesterol, however, differ significantly according to state. Liquid cholesterol esters have fairly short T2 values (~5-10 msec), but these times are long enough to be detectable/imageable by routine MRI. By contrast, the solid and crystalline forms of cholesterol associated with cell membranes have extremely short T2 times (measured in microseconds); their signals decay too quickly for detection using routine MR techniques. In craniopharyngiomas and cholesterol granulomas, cholesterol is in a solid/crystalline state and bright signal is likely due to blood products. Cholesterol gallstones are also in solid state and do not typically have bright MR signals. Link to Q&A discussion

  5. Why do areas of microcalcification sometimes appear bright on T1-weighted images?
    1. The calcium nucleus also undergoes NMR and mixes with the ¹H signal
    2. You are seeing a susceptibility artifact, not a true change in relaxation time
    3. Interaction with salts on porous calcium surfaces slows rotation of water molecules
    4. Microcalcifications often have fat-containing bone marrow within them

    The high signal is not coming from the calcium itself, as Ca is an even-numbered element with zero spin and no intrinsic MR signal. The signal is coming from water protons, whose molecular rotation rates have been slowed to near the Larmor frequency by surface interactions with the calcium salts. This rotational slowing results in short T1 and hence brightness on T1-weighted images. Link to Q&A discussion

  6. The T1-bright signal of meconium is primarily due to
    1. Paramagnetic minerals
    2. Free fatty acids
    3. Carbohydrates
    4. Mucin

    Although several physical properties contribute to the relaxation times of meconium, the dominant process is likely the accumulation of paramagnetic (Fe, Mn, Mg) minerals within it. Meconium also contains free fatty acids, carbohydrates, and mucin, but their effects on T1 relaxation are thought to be minor. Link to Q&A discussion

  7. The “magic angle” effect takes place when two magnetic dipoles form an angle of about how many degrees with each other?
    1. 30º
    2. 45º
    3. 55º
    4. 60º

    The dipolar interaction due to the static field vanishes at the magic angle of approximately 54.7º. Link to Q&A discussion

  8. Interaction of two dipoles at the magic angle affects
    1. T1 only
    2. T2 only
    3. T1 and T2
    4. Neither T1 nor T2

    The magic angle effect lengthens T2 with theoretically no effect on T1. Link to Q&A discussion

  9. Which tissue in the list below does not normally demonstrate signal changes on MRI due to the magic angle effect?
    1. Liver
    2. Tendon
    3. Cartilage
    4. Peripheral nerve

    The magic angle effect is important in the clinical MR imaging of certain tissues that are highly structured and are oriented obliquely to the main magnetic field (especially tendons, cartilage, and peripheral nerves). The MR signal may spuriously increase near the magic angle mimicking pathology. Liver has no single direction anatomic structure and would not display a magic angle effect. Link to Q&A discussion

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