BIOE 220/RAD 220 REVIEW SESSION 6March 5, 2012
What We’ll Cover Today• General questions?• Spinal cord anatomy review• Fat in images• T2* vs T2 decay• Review of sequences• Questions on the hw?
Nasal and Oral cavities
Nasal Cavity
Nasopharynx
Uvula/Soft Palate
Oropharynx
EpiglottisTongue
Parotid glands
Parotid Gland
Stensen’s Duct
Salivary glandsSublingual gland
Submandibular gland
Submandibular duct
Thyroid
Thyroid
Spinal Vertebrae• Breakfast at 7 • Lunch at 12• Dinner at 5
• Cervical roots: C1-C8 (all above their vertebrae except 8)• Thoracic: T1-T12, Lumbar: L1-L5, Sacral: S1-S5
Cervical Spine
Anterior
Posterior
Cervical spine
Cervical spine
Thoracic Spine
Lumbar Spine
Lumbar Spine
Lumbar Spine
How does fat look in MRI?• Without fat suppression, fat always appears bright
• Why? Short T1, normalT2 but high PD• Fat precesses at a different frequency (3.5 ppm shift –
how fast?) so it appears shifted in images• This effect can be minimized by maximizing the strength of readout
gradients (why?)• How can we remove fat from the signal? Take advantage
of difference in precession frequency• Chemical saturation: Hit fat with selective 90º pulse, spoil the
transverse signal, then do a normal measurement afterwards• Short TI Inversion recover: After a 180º pulse, wait for the fat to
recover to 0 (shorter T1), then measure the other signals
T2 vs T2* decay• When do we see T2* decay, when do we see T2 decay?• T2* decay is observed after a 90º pulse if no other
preparation is done (GRE sequence)• T2 decay is observed if a 180º refocusing pulses is used
to “unroll” any off-resonance defocusing (SE sequence)• After the 180º pulse, must wait the same amount of time as we
waited before the pulse until things have refocused
What sequences have we learned about?
• Simplest: GRE 2DFT• Excite with 90º pulse, then read out• K-space trajectory will be lines in the frequency encode direction,
stepped in the phase encode direction between every TR• Spin echo 2DFT
• Same as before, except now we add a 180º refocusing pulse after the 90º so that we’ll obtain T2 weighting instead of T2*
• 180º pulse occurs at TE/2• Spatial saturation
• Add a 90º pulse to excite a slab and then spoil it, before regular sequence• This nulls the signal in the slab, so that only fresh spins flowing in will be
visible
What sequences have we learned about?
• Fat suppression: Chem Sat• Use a 90º selective pulse on the fat signal and spoil it, before
regular sequence• Ideally nulls the fat signal without effecting rest of image
• Fat suppression: STIR• Use a 180º pulse to flip everything, then wait until the fat passes
through 0 to do our readout• Diffusion weighting
• Use strong bipolar (sums to zero) gradient, so that spins will dephase based on their movement during gradient
• Flow encoding• Similar to diffusion weighting, except that we’re interested in much
higher velocities (of bulk spins)
What sequences have we learned about?
• Fast spin echo• Perform multiple 180º - readout – 180º - readout in a single TR• Allows faster acquisition of SE image, but fat appears brighter
• EPI• Instead of recording line by line, traverse 2DFT grid very quickly in
single sequence• Fast, but very susceptible to artifacts
• Spiral• Instead of collecting k-space in grid like 2DFT, traverse k-space in
a spiral, to be more efficient with gradients• Efficient/fast, but susceptible to artifacts and leads to spatially
variant resolution/blurring