Nuclear: Central Nervous System Flashcards by John Mahon

What is the mechanism of localization for HMPAO?

HMPAO gets trapped in the brain where it accumulates in the cortex proportional to blood flow

Note: It gets trapped in the brain because it is initially lipophilic (can cross the BBB), but once in the brain it becomes hydrophilic (can’t cross the BBB to get out).

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What is the mechanism of localization for ECD?

ECD gets trapped in the brain where it accumulates in the cortex proportional to blood flow

Note: It gets trapped in the brain because it is initially lipophilic (can cross the BBB), but once in the brain it becomes hydrophilic (can’t cross the BBB to get out).

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What is HMPAO used for?

Dementia imaging
Seizure focus localization

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What is ECD used for?

Dementia imaging
Seizure focus localization
Stroke imaging

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What is the major difference between HMPAO and ECD?

ECD washes out more slowly and has better background clearance

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What is intravenous Tc-99m DTPA used for?

Shunt studies
Normal pressure hydrocephalus
Brain death

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What is the mechanism of localization for Tc-99m DTPA?

It is hydrophilic and stays suspended in the blood (best thought of as an angiographic tracer)

Note: It does not enter brain parenchyma because it is lipophobic and cannot pass the BBB.

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Which intravenous radiotracer is incompatible with SPECT imaging: HMPAO, ECD, or DTPA?

You can’t do SPECT imaging with Tc-99m DTPA (an “angiographic tracer”) because it stays in the blood (there isnt enough time for SPECT imaging)

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How can you identify a seizure focus using nuclear imaging?

HMPAO or ECD will localize to the seizure focus during the ictal period (tracer should be injected within 30 seconds of the seizure to get a good study)

Note: The seizure focus will then be cold after the seizure (during the interictal period).

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How is thallium-201 produced?

Cyclotron

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What is the half life of thallium-201?

3 days

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What are the major energy peaks for thallium-201?

69 keV
81 keV

Note: These are actually the emission energies of thalliums daughter product Mercury-201.

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What is the mechanism of localization for thallium?

Thallium acts as a potassium analog, entering metabolically active cells via the Na/K ATPase pump

Note: Acts as a viability tracer because only living cells will take it up.

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If performing a dual gallium/thallium study, which radiotracer needs to be injected first?

Thallium must be done first (the gallium photopeaks will drown out the thallium ones)

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What radiotracer was used?

Thallium-201

Note: Living tissue lights up (e.g. subcutaneous soft tissue) except the brain (due to BBB).

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What type of infection is clasically thallium-negative?

Toxoplasa

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How do tumor and necrosis appear on Thallium-201 scans?

Tumor is thallium-positive

Necrosis is thallium-negative

Note: Thallium is taken up by pretty much all living cells.

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What radiotracers are used for CNS tumor imaging?

Thallium-201 (most common)
Tc-99m sestamibi

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Thallium-201

Think brain cancer

Note: Infections are less bright with thallium than tumors. Abscess/necrosis will not show up on thallium (requires living cells).

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Brain tissue that is thallium hot and HMPAO cold…

Think tumor

Note: Necrosis would be both thallium and HMPAO cold.

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How can you distinguish CNS lymphoma and CNS infections (e.g. toxoplasma, abscess, etc.) using nuclear imaging?

Thallium-201 scan

Note: Toxoplasmosis will be cold on thallium and lymphoma will be hot.

Why is a scalp tourniquet used for brain death studies?

To suppress scalp perfusion (which could be interpreted as brain perfusion)

How can you tell a brain death study is adequate?

You should be able to identify radiotracer in the common carotid artery

What radiotracer is used for brain death studies?

Tc-99m DTPA

Brain death Note: "Hot nose sign," perfusion stops at the skull base.

What radiotracers can be used to identify cerebral infarctions?

- Tc-99m ECD - Tc-99m HMPAO

How does a stroke appear on nuclear imaging?

Acute stroke: cold (decreased perfusion) Subacute stroke (B): hot (increased "luxury" perfusion) Chronic (C): cold (no perfusion)

Luxury perfusion

The paradoxical finding of increased blood flow in the region of a subacute stroke (due to vascular dilatation and excessive blood flow to the region of dead tissue) Note: This is seen 48-72 hours after stroke onset (subacute phase).

What is the point of a Diamox study?

To evaluate cerebrovascular reserve (areas that have already maxed out their auto regulatory vasodilation will appear relatively photopenic on the post-diamox study because these regions will not be able to increase perfusion as much as the rest of the brain can)

What medication is used for Diamox studies?

Acetazolamide (a vasodilator)

Would this pt benefit from revascularization (post Diamox image on the right)?

Yes, there is an area in the left MCA territory that is relatively photopenic on the post Diamox image, indicating ischemia (would benefit from revascularization because it has already maxed out autoregulatory vasodilation and it isn't enough)

How will a seizure focus appear on an interictal FDG-PET of the brain?

Decreased FDG uptake at the seizure focus (during the interictal period)

Can a pt sleep while getting an FDG-PET of the brain?

No, they should be woken up if they fall asleep (a sleeping brain has more variable uptake, skewing results)

What features indicate a normal FDG-PET of the brain?

- Symmetry - Basal ganglia is at least equal to cortex (up to 15% greater) - Cerebellum is 15% lower than cortex - Thalamus is equal to cortex

What type of dementia fits this FGD-PET pattern?

Alzheimers Note: Low activity in the posterior temporoparietal cortex.

What is the first area of the brain to have decreased perfusion/metabolism in Alzheimers dementia?

Posterior cingulate gyrus (see image of posterior cingulate gyrus infarction) Note: The precuneus is also affected early on.

What type of dementia fits this FGD-PET pattern?

Multi-infarct (vascular) dementia

What type of dementia fits this FGD-PET pattern?

Dementia with Lewy bodies Note: Low perfusion/metabolism in the lateral occipital cortex with preservation of the mid-posterior cingulate gyrus ("cingulate island sign").

FDG-PET

Dementia with Lewy bodies Note: Low perfusion/metabolism in the lateral occipital cortex with preservation of the mid-posterior cingulate gyrus ("cingulate island sign").

What type of dementia fits this FGD-PET pattern?

Frontotemporal dementia (Picks) Note: Low perfusion/metabolism in the frontal and temporal lobes.

What radiotracer is used most often for CSF studies?

In-111 DTPA (intrathecal administration via lumbar puncture)

Normal cisternography study Note: Radiotracer reaches basal cisterns by 2 hours and flows around the cerebral convexities with clearance from the basal cisterns by 24 hours.

What is abnormal about this cisternography study?

There should not be radiotracer in the lateral ventricles (normally CSF flows out of the ventricles and is absorbed at the arachnoid granulations in the skull)

At what point should radiotracer clear from the basal cisterns during a cisternography study?

By 24 hours Note: If it takes longer than this, consider normal pressure hydrocephalus.

Cisternography findings of normal pressure hydrocephalus

- Early entry (4-6 hours) of tracer into the lateral ventricles - Persistence of tracer in the lateral ventricle for > 24 hours - Delay is ascent of tracer to the parasagittal region (longer than 24 hours)

How can you differentiate NPH from communicating hydrocephalus from a cisternography study?

The opening pressure when you do the lumbar puncture will be normal in normal pressure hydrocephalus (and increased in communicating hydrocephalus)

What are the most common locations for a CSF leak?

- Cribriform plate -> ethmoid sinuses - Sella turcica -> sphenoid sinus - Ridge of sphenoid -> ear

How can you tell whether there is a CSF leak on cisternography

- Look for abnormal radiotracer location on images - Place pledgets in pts nose prior to cisternography and see if they have radiotracer after the study (1.5x serum radiotracer is considered positive for leak)

How can you differentiate communicating vs noncommunicating hydrocephalus using cisternography?

You can't, the radiotracer shouldn't enter the ventricles on either

How can you use cisternography to evaluate VP shunt patency?

Usually, Tc-99m DTPA is injected into the ventriculoperitoneal shunt tubing (if radiotracer ends up in the peritoneal cavity the distal end is patent, you can then manually occlude the distal tubing to force radiotracer into the cerebral ventricles to show that the proximal end is also patent) Note: Tracer should show up in the peritoneal cavity within 10 minutes (otherwise partial/complete obstruction) and should clear from the cerebral ventricles after refluxing through the proximal end.