BIOE 220/RAD 220
REVIEW SESSION 8
March 19, 2012
What We’ll Cover Today
• Comments on homework 6
• Anatomy leftover review
• Physics leftover review
• Questions
• Reminder: check the fourth review session slides for
some exam preparation material
Common Issues on HW 6
• Problem 22 asked if the CBF was higher or lower at the
occluded vessel
• The difference values were more negative, but the magnitude of
the difference indicates CBF, so it was higher
• ASL (arterial spin labeling) tags spins in the neck arteries,
and then images them in the head
• If vessels are flowing slowly, we won’t see them in the head since
they won’t have arrived yet when the rest of the arterial blood does
• VSASL does not have this problem, so it sees slow flow
• Problem 24 stated “In fMRI, you get about a 1% signal
intensity change for about a 50% change in CBF”
• As we discussed in a previous homework, blood only accounts for
about 4% of the brain tissue volume, so even large relative
changes in the blood will be small overall
New physics material since the midterm
• MRI:
• Scan time to acquire Ny phase encodes: Ny*TR
• Number of slices we can interleave during acquisition: floor(TR /
(time from 90º pulse to the end of sampling) )
• How is MR image reconstructed? 2D Fourier Transform
• Ultrasound:
• Doppler shift: fshift = 2 * (v/c) * f0 * cos(θ)
• Pulsed doppler gives velocity spectrum at particular point
• Color doppler overlays doppler information as color (be aware of
doppler aliasing)
• Hope to measure whether >70% stenosis exists in carotid
• Adult transcranial doppler ultrasound can be acquired transorbital,
transtemperal, transforaminal
New physics material since the midterm
• MRI Sequences:
• Understand how gradients relate to k-space trajectory
• Be able to relate pulse sequence diagram to acquisition type (GRE,
SE, Spatial Saturation, Fat Sat (Chem Sat or STIR), FLAIR, Flow
Encoding, Diffusion, Fast Spin Echo, Spiral, EPI)
• Perfusion:
• CBF (Cerebral Blood Flow): Rate at which blood flows through
tissue
• CBV (Cerebral Blood Volume): Volume of blood in brain
• MTT (Mean Transit Time): Average amount of time it takes particles
to move through a voxel OR Average amount of time it takes
injected contrast to reach ROI
• Diffusible contrast agents cross the BBB (i.e. breathing Xenon)
New physics material since the midterm
• Perfusion:
• Arterial spin labeling (ASL) takes two images of the head, one
normal, one with the neck “tagged” prior to imaging so that arterial
blood will have lower signal. Difference image represents CBV
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Longer delay time gives better quantitation, but lower SNR
Isn’t sensitive to slow flow
VSASL is a similar measure that won’t miss slow vessels
Both approaches are non-contrast
• CT Perfusion measures the signal response to contrast bolus
injection
• Can infer MTT, CBV, CBF from the shape and timing of the response
• Stroke area takes a longer time to reach max contrast, Tmax > 6
seconds
• If CBF/CBV has also reduced noticeably, tissue is already dead
• Review the “Pros/Cons” of the different methods
New physics material since the midterm
• fMRI:
• Brain is only 2% of body weight, but uses 15% of cardiac output
and 20% of body energy
• When neurons are active, they need energy to restore equilibrium
(must be resupplied with additional blood)
• CMRO2 = OEF * [O2]a * CBF
• CMRO2 = Cerebral metabolic rate of oxygen
• OEF = oxygen extraction factor
• [O2]a = Arterial oxygen constant
• All parameters change with neural activation except [O2]a
• Contrast agent methods exploit changes in CBV
• BOLD measures changes in T2* induced by changes in
oxygenation state of hemoglobin
• Block design: Alternate periods of task/no task, and measure the
changes in brain signal between the two stages
Spinal Cord
Anterior
Right
Left
Posterior
Spinal Column
Basic plans of (A) posterior column-medial lemniscal
pathway; (B) spinothalamic pathway.
Spinal Column
• Poster columns medial lemniscus (PCML)
• Touch
• Spinothalamic
• Pain
• Temperature
• 3 neuron system
• Both cross the midline
• Pcml near sensory decussation in medulla
• ST at or near the level of entry
• All involve the thalamus
• Project to postcentral gyrus
Spinal Column
• Corticospinal tract
• Voluntary motion
• Two neuron system
• Precentral gyrus
through motor
decussation (medulla)
to contralateral anterior
horn
• Anterior horn gray
matter to muscle
Spinal Column
Sternocleidomastoid
Cavernous Sinus
Myelogram
• Myelography is an imaging test that
uses a special dye to view the
spinal cord. Along with an x-ray , the
dye can help your doctor clearly
outline the space containing the
spinal cord, nerves, to show
any disk herniation .
• Myelography is used to detect
problems in and around the spinal
cord. The problems that may be
causing ongoing pain that has not
been relieved by medication,
physical therapy, and activity
modification. Myelography can
detect issues such as:
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Spinal tumors
Spinal cord swelling
Herniated disks
Narrowing of the spinal canal due
to spinal stenosis
Dermatomes
Thank You
• Please Remember to fill out TA evaluations
• The rest of the time will be available for questions