MRI T1 and T2 Weighted Images Quiz

T1 Recovery/Weighted Images

• Fatty structures appear brighter in images
• White matter (containing myelin, which is fatty) appears paler than gray matter
• Fluid appears dark
• TR (repetition time) of 400-650ms allows for structure recovery, excluding fluid
• 300ms for pure fat; 400ms for fatty structures
• CSF, gray, and white matter require a longer TR for contrast differentiation.
• Images can be taken with an echo time (TE) of 15ms before fat decays.
• Short TE and short TR are used for T1 weighted images
• Fat dephases quickly; resulting in faster T1 recovery. Fat has a greater longitudinal magnetization.

T2 Decay/Weighted Images

• Fluid appears bright in images
• Golden standard for assessing cancers, edema, and inflammation
• Cancers often contain fluid.
• Edema and inflammation frequently show associated fluid.
• Fluid needs to be in transverse plane and in phase to appear as bright.
• Waiting for fluid to recover and decay allows all structures to be in the same axis.
• This isn't ideal; TR doesn't control T2 weighting.
• TE controls T2 weighted images
• Wait for fatty structures to decay to choose the right TE time before fluid dephases.

Additional Notes

• Fluid dephases rapidly at 90° TE. Higher TE with a long TR will result in a faster scan time. Longer TE is used to only see the fluid.

• TR control is used for assessing structures, and TE is used for assessing fluids.

• Best sequence is chosen for a particular pathology and then repeated if possible.

• Radiologist should be consulted for patient agitation.

• TR controls image weighting.

• Longer TR means structures are brighter

• TR determines the amount of T1 relaxation.

• Time between RF pulses; signal listens for time between pulses. Signal time will never be more than TR.

• Myographic effect for spine (CSF surrounds chord): Very bright T2 weighted image to visualize fluid, increase the TE value to almost 800ms.

• Water has higher energy than fat and doesn't absorb the RF pulse energy as much.

• Water diphases less readily than fat, because the protons do not collide as frequently. Molecular tumbling is faster than the frequency, leading to slower recovery in water.

• Matrix of 256 x 192; the second number determines the number of RF pulses (or TR's) that happen. 192 TR, gives an indication of scan time. A T1 weighted scan will have a shorter scan time. (T1 = 300ms x 192 = shorter scan time; T2 = 3000ms x 192 longer scan time).

• Decreasing matrix will decrease scan time but also decreases spatial resolution.

• High resolution = high matrix = smaller pixels

• Pulse sequences are operator controlled.

• Series of RF pulses and recovery times.

• Proton density images are based on hydrogen concentration. Some structures have more or less hydrogen. Bones minus marrow typically have little hydrogen. Bone marrow generally gives a signal on T1 due to fat. Bones show up as bright on a T1 due to fat within. CSF is rich in hydrogen causing a good signal to differentiate gray and white matter. Good for multiple sclerosis and to visualize structures in the brain.

• Always include sagittal proton density for MS and corpus collosum. The axial plane is generally best for imaging the brain, along with the corpus callosum.

• T1: short TR and short TE; T2: long TR and long TE. Never short TR and long TE.

• Premature dephasing (T2*) is from inhomogeneities within the magnet, leading to faster dephasing.

• Spin echo and gradients include 90° and 180° pulses. First RF pulse is the FID. Acts like a wall that bounces back and speeds up the slow to read the signal. The 180 degree pulse from one side to the other helps to get the nuclei phase to be the same.

• Gradient echo and pulse sequences do not have an 180° pulse but use bipolar gradients to reverse and rephase the process. This technique allows for shorter scan times in T2*; but is susceptible to metal artifacts.

• Scan time is estimated as TR x phase encoding (second number of matrix) x number of excitations.

• Compensates for premature dephasing; gradients cause loud noises, not the RF pulses.

• Short flip angle for T1 and PD, allows quick recovery with no 180 degree pulse.

• T1 weighted: larger flip angle, short TR. T2 weighted: long TR, small flip angle.