Brain Perfusion Imaging

Question 1

What is the role of nuclear medicine in CNS imaging?

Answer 1

-Dementia characterization
-Parkinsonian syndromes
-Epilepsy seizure foci localization (pre-surgery)
-Brain tumour recurrence
-Brain death
-Vascular assessment
-CSF evaluation

Question 2

What are the ideal properties of a RP used in CNS imaging?

Answer 2

-Readily passes the blood brain barrier (high first-pass extraction)
-Localization proportional to rCBF (allows for delayed imaging which reflects perfusion at the time)
-Desirable 140 keV photons
- 99m Tc-ECD & 99m Tc-HMPAO

Question 3

What are the properties of 99m Tc-ECD?

Answer 3

-Stable for 6 hours post reconstitution
-Neutral & lipophilic, crosses the cell membrane of neurons and undergoes enzymatic de-esterification (polar metabolites get trapped)
-Enzymatic

Question 4

When to image after injecting
ECD?

Answer 4

-Image up to three hours later
- 30-60 min wait between injection and images creates ideal image
- 20 min wait images are still interpretible

Question 5

What are the properties of 99m Tc- HMPAO?

Answer 5

-Stable 30 min to 4 hrs post reconstitution
-Neutral, lipophilic: once tracer crosses cell membrane, is transformed to hydrophilic and trapped in membrane
-Gluthione- mediated
- Appx 5% of the activity injected goes to brain with no significant late distribution
-40% GU and 15% GI excretion

Question 6

When to image HMPAO?

Answer 6

• Image 20 mins to 2 hours post-injection
  -90 min delay is best
  -40 min delay is interpretable

Question 7

Advantages of ECD?

Answer 7

• More stable
  + less restrictive production requirements
  + Longer shelf life
• Less background activity
  + Rapid blood clearance
  + Better images
  + Earlier imaging

Question 8

Advantages of HMPAO?

Answer 8

• Better uptake in medial temporal lobes, ideal for diagnosis of Alzheimer’s disease
• Greater uptake at high perfusion rates
  + demonstrates luxury perfusion
• Higher first pass extraction

Question 9

What are 3 precautions and considerations of brain perfusion imaging?

Answer 9

• Dementia patients must be monitored closely
• Patients with neurologic deficits may require more care and procedure modification and monitoring
• Sedation should be administered after RP injection if possible

Question 10

Patient Prep?

Answer 10

• No prep
  + Ideal to avoid agents increasing brain bloodflow
• Before injection
  + Quiet dimly lit room
  + Eyes and ears open, no sleeping
  + No stimulus
  + Laying down or sitting
  + Iv should be placed 10 minutes pre injection
• During/after injection
  +Do not interact with patients before, during, or up to 5 minutes after injection, to be clarified during interview

Question 11

What is relevant history to note for brain perfusion imaging?

Answer 11

• Past drug use or trauma
• Neurologic examination
• Psychiatric examination
• Mental status examination
• Recent imaging
• Current medications and last taken

Question 12

What is the time reconstitution to injection for 99mTc- HMPAO?

Answer 12

• Stabilized: 10 minutes post reconstitution; within 4 hours
• Unstabilized: 10 minutes post reconstitution; within 30 minutes
  +Exception: patients with seizure disorders to be injected as soon as possible after reconstitution

Question 13

What is the time reconstitution to injection for 99mTc- ECD?

Answer 13

• 10 minutes post reconstitution; within 6 hours

Question 14

How long till you can image an injected HMPAO?

Answer 14

• 40 minutes; optimal 90 min

Question 15

How long till you can image an injected ECD?

Answer 15

• 20 minutes; optimal 45 min

Question 16

Dose given for brain imaging?

Answer 16

• Adult: 555- 1110MBq
• Pediatric: 7.4- 11.1MBq/kg

Question 17

What are the acquisition parameters for brain imaging?

Answer 17

• Smallest radius of rotation
• 128 x 128 matrix or greater
• 128 projections, 40 stops/head
• 20- 40 seconds/stop
• zoom 2
• FBP

Question 18

Processing Recommendations

Answer 18

• Low pass filters are recommended
• Reconstruction includes entire brain
• Reformat transaxial slices in all three orthogonal planes

Question 19

Processing and display parameters

Answer 19

• Reconstruct entire brain
• Attenuation correction such as Chang
• Generate transverse sections relative to a repeatable anatomic orientation
• Align axes, including sagittal along ac-pc line

Question 20

Oxygen demands of the brain

Answer 20

• Areas with increased synaptic activity require greater blood flow
• Change in blood flow shows abnormalities in RP uptake and distribution
  + Decrease in perfusion

Question 21

Normal results

Answer 21

• 2:1 - 4:1 differential in rCBF comparing grey to white
• Relative equal distribution among lobes, normal distribution cahnges with age

Question 22

Normal distribution differences among the RP

Answer 22

HMPAO- higher in frontal lobes, thalamus, cerebellum
ECD- higher in parietal and occipital lobes