bioe 220/rad 220 review session 6

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