BIOE 220/rad 220 Review session 6

1. BIOE 220/rad 220 Review session 6 March 5, 2012

2. 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?

3. Nasal and Oral cavities Nasal Cavity Nasopharynx Uvula/Soft Palate Oropharynx Tongue Epiglottis

4. Parotid glands Stensen’s Duct Parotid Gland

5. Salivary glands Sublingual gland Submandibular duct Submandibular gland

6. Thyroid

7. Thyroid

8. 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

9. Cervical Spine Posterior Anterior

10. Cervical spine

11. Cervical spine

12. Thoracic Spine

13. Lumbar Spine

14. Lumbar Spine

15. Lumbar Spine

19. How does fat look in MRI? • Without fat suppression, fat always appears bright • Why? Short T1, normalT2but 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

20. 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

22. 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

23. 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)

24. 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