Brain Death

Question 1

What is brain death, its causes, and pathophysiology?

Answer 1

• Brain death is the irreversible cessation of brain function
• Causes: Head trauma, cerebrovascular accidents, drug overdose, anoxia
• Pathophysiology: Increase in intracranial pressure causes cessation of rCBF, leading to brain death

Question 2

What are the indications for a brain death study?

Answer 2

-Diagnosis of irreversible cessation of all function in the cerebrum and brainstem
- For patients suspected of brain death to confirm a clinical diagnosis
- Positive EEG: Cessation of brain activity for at least 6hrs
- Poor BAER: Pupillary reaction to light, corneal reflexes, gag/swallowing reflex, oculovestibular reflex, apnea test

Question 3

What is the non-diffusible radiopharmaceutical used in brain perfusion imaging? What is the recommended dose and its properties?

Answer 3

-99m Tc DTPA
-Dose: 555-740 MBq
-Hydrophilic agent which does not cross the blood brain barrier
- rapid renal excretion allows for repeat exams

Question 4

How should 99m Tc DTPA be injected?

Answer 4

-bolus dose in an antecubital vein
-may use a tourniquet or band around scalp just above orbits to prevent flow to superficial vessels
-since it is a non-diffusible RP, high quality bolus and injection technique is important, flow studies useful
-parenchymal uptake is most important in these kinds of studies, static images best for brain death interpretation

Question 5

What are the diffusible radiopharmaceuticals used in brain perfusion imaging? What is the recommended dose and its properties?

Answer 5

• 99m Tc HMPAO or 99m Tc ECD
• Dose: 370-1100 MBq

Question 6

What are two advantages and one disadvantage of using diffusible radiopharmaceuticals?

Answer 6

Advantages: Interpretation less dependant on bolus quality
-Delayed images are usually definitive for cerebral blood flow
Disadvantages: Repeat exams on the same day are not possible due to parenchymal retention

Question 7

Patient prep for brain death?

Answer 7

None

Question 8

Collimator for brain death studies?

Answer 8

-LEHR or LEUR
-128 x 128 matrix or higher

Question 9

Positioning for brain death studies?

Answer 9

-Smallest radius of rotation
-Ensure brain is visible in all projections to avoid incomplete views

Question 10

HMPAO brain death parameters?

Answer 10

-Delayed images
-Optional flow and immediate static

Question 11

DTPA brain death parameters

Answer 11

-Flow and pool
-Optional delayed static or spect

Question 12

Why flow imaging for brain death?

Answer 12

-Evaluates dynamic blood flow to the brain
-1-3 sec per frame anterior images of the head
-Image acquisition should be started when bolus is administered and continue imaging for 2-3 minutes

Question 13

Why static imaging for brain death?

Answer 13

-Anterior and lateral blood pool images obtained 20minutes or more after completion of flow for HMPAO and ECD
- 500, 000 - 1 000, 000 counts per view
- Projections: Ant, Rlat, Llat, Post; SPECT

Question 14

Results?

Answer 14

-Flow and early images: lack of blood flow in arterial system and in superior sagittal sinus
-HMPAO delayed images: no tracer uptake in the brain

Question 15

How to interpret a brain death study?

Answer 15

-Injection of bolus into peripheral arm veins travels to the right side of the heart and becomes visible in the carotid arteries
-As it enters the brain, the tracer will become visible in the cerebral arteries and sagittal sinus
-In brain death, there will be no cerebral perfusion on the radionuclide angiogram

Question 16

What is the appearance of a positive brain death study?

Answer 16

-Internal carotid artery blood flow ceases due to increased ICP or clotting
- During flow there is visualization of both carotids, but it stops at this level
-no blush of activity in cerebral arteries or pooling in the sagittal sinus
-Blood pool image fails to demonstrate activity in the sagittal or transverse sinuses
-Internal carotid arteries are blocked, so blood is shunted to external carotid arteries.-> “Hot nose sign”

Question 17

What is the hot nose sign?

Answer 17

-Secondary sign to brain death
-It is increased blood flow to the maxillary branch of the external carotid arteries in increased focal activity in nasal area

Question 18

What are the causes of false negative brain death studies?

Answer 18

-Hyperemic scalp structures
-Drainage of blood from scalp to superior sagittal sinus

Question 19

What are the causes of false positive brain death studies?

Answer 19

-Injection of the RP
-Infiltration of RP at injection site or prolonged infusion will compromise the evaluation of flow phase
-Absence of activity in the carotid vessels on the flow suggests complete infiltration of the dose