Question 19

College Answer

Many candidates restricted their answers to limitations of the scan itself, and did not consider other clinical issues related to putting a critically ill patient into a CT scanner. Candidates should consider asking themselves “why don’t we perform more of this investigation?”.

Physical limitations include:

•    patient size,

•    usually distant from resuscitation area, thus patient needs to be moved,

•    some patients may require GA for the investigation when they otherwise wouldn’t have had a GA,

•    difficulty monitoring and attending to patient during scanning (especially if intubated and ventilated and “sick”).

Clinical limitations include:

•    may be other priorities (eg urgent laparotomy which precludes early CT)

•    interpretation is difficult/impossible if other previous IV contrast Xray investigation

•    normal CT head doesn’t exclude underlying injury (eg diffuse axonal injury, vascular injury, ischaemia, hypoxic injury) – so may need to repeat it within 24 hrs

•    CT head findings do not correlate with ICP value (unless CT findings of herniation – and then ICP bound to be too high and too late a detection)

•    CT findings are not generally good predictors of patient outcome (though may be useful in some situations eg the Marshall score [used to prognosticate outcome], and the presence of traumatic SAH on CT which portends a poor prognosis).

•    poor visualisation of posterior fossa and brainstem.

Discussion

The CT scan of the brain has the following limitations when it comes to traumatic brain injury:

Logistical limitations:

• The investigation requires transport to and from the CT scanner
• It involves a degree of cooperation, and may require sedation
• Not all patients will fit into the 84cm aperture or on the table which has a 220kg fully extended loading maximum

Machine limitations:

• It involves radiation exposure
• It may involve contrast exposure
• The images of posterior fossa structures are usually poor
• Diffuse axonal injury will not be visualised
• On a non-contrast study, you will not see arterial dissection or vascular insufficiency
• Infarcted regions cannot be visualised early in the infarct
• Hypoxic injury cannot be seen early in the injury
• Artifact from metallic implants (eg. crowns, aneurysm clips, scalp staples) will obscure the view
• CT may miss small amounts of blood which occupy the space between slices, because of image averaging
• Where it comes to subtle neuronal and axonal injury, or to petechii, CT misses 10-20% of pathology seen on MRI

Interpreter limitations:

• The CT needs to be interpreted by a competent radiologist for the findings to be valid
• Subtle features and posterior fossa injuries may be missed

Relevance to clinical setting

• The CT is an assessment of structure, rather than function
• Early CT may underestimate the extent of an evoving injury
• It may not be possible to get the CT done because some sort of damage control surgery takes precedence.
• CT findings may not correlate with ICP, unless you are coning.
  • This is debated. Some people swear by their grey-white junction findings and their basal cistern size.
  • Certainly, a non-raised-ICP-looking CT should not prevent you from inserting an EVD if there are clinical indications for it.

There is an excellent article which discusses the advantages and limitations of various neuroimaging techniques:

 

Lee, Bruce, and Andrew Newberg. "Neuroimaging in traumatic brain imaging." NeuroRx 2.2 (2005): 372-383.

 

Miller, M. Todd, et al. "Initial head computed tomographic scan characteristics have a linear relationship with initial intracranial pressure after trauma." Journal of Trauma-Injury, Infection, and Critical Care 56.5 (2004): 967-973.