The ED Guide to Neuroimaging: Part 2

Written by: Justin Seltzer, MD (NUEM PGY-3) Edited by: Priyanka Sista, MD, (NUEM PGY-4) Expert commentary by:  Peter Pruitt, MD, MS

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Make sure to check out The ED Guide to Neuroimaging: Part 1

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Part two of this series examines the literature regarding the appropriate use of the head CT in blunt head trauma, a common clinical grey zone in emergency medicine.

The Canadian Head CT Rule (Canadian), New Orleans Criteria (New Orleans), NEXUS II Head CT Rule (NEXUS), and PECARN Pediatric Head Injury Algorithm (PECARN) are four major decision rules designed to assist clinicians with this often difficult decision. This article is dedicated to comparing these rules and providing a reasonable guide for maximizing their individual utility. The provided infographics detail the specifics of each rule for quick reference. 

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To start, there are many shared characteristics between the rules. All apply to blunt head trauma only and, except for NEXUS, specifically to those presenting within 24 hours of injury. They utilize criteria to characterize high risk populations for which emergent head CT is appropriate as well as those low risk enough to forego it. Each boasts near perfect sensitivity and negative predictive values for clinically significant acute intracranial processes. Finally, all were prospective cohort studies and, aside from New Orleans, multi-center. 

However, there are differences between each rule that can impact their applicability to certain situations and populations. 

• Study population: The single center New Orleans Criteria had the smallest study population, with 1429 total patients, while the largest, PECARN, had over 42,000 patients. All aside from NEXUS had some age restrictions. Canadian included adults and pediatric patients older than 16 years and New Orleans included adults and pediatric patients older than 3 years. PECARN was exclusively pediatric and excluded anyone over 18 years old. 

• Inclusion and exclusion criteria: There was significant heterogeneity between the studies on what qualified for inclusion. New Orleans only included patients with known loss of consciousness or post-injury amnesia with a normal neurologic exam. Similarly, Canadian involved patients with GCS ≥13 and witnessed alteration or loss of consciousness. PECARN, on the other hand, was most concerned with mechanism and excluded patients with trivial mechanisms or injuries, such as ground level falls, walking into objects, and isolated scalp involvement. These are further contrasted with NEXUS, which included “all patients with blunt trauma with minor head injury (Glasgow Coma Scale [GCS] score of 15) who present to participating study center.”[1] 

• Decision rule criteria: Certain criteria, such as evidence of skull fracture, persistent vomiting, older age (>60-65 years), were nearly universally present. However, beyond these there is little consensus. NEXUS, likely because it applies to all ages, includes criteria such as alertness, behavior changes, and scalp hematoma similar to PECARN. Only New Orleans included clinical intoxication, while NEXUS was the only rule to include coagulopathy. Mechanism-based criteria were only considered by Canadian and PECARN. 

• Primary outcome: There is significant similarity in terms of primary outcome. NEXUS criteria sought “clinically important intracranial injury,” New Orleans any acute abnormality on CT, and PECARN “clinically important traumatic brain injury.” The definitions varied somewhat but were generally similar. Only Canadian stratified differently, with a set of criteria geared towards identifying the need for neurosurgical intervention specifically and another set for the more familiar “clinically important brain injury.” 

• Methods of Application: NEXUS, Canadian, and New Orleans are all or nothing; meeting even one element results in a head CT and not meeting any means a head CT is likely unnecessary. PECARN is unique in that if the major criteria are not met, minor criteria defer to observation or head CT based in part on non-standardized elements such as physician experience and parental preference. Only in the absence of major and minor criteria can a child be cleared immediately. 

Finally, it is important to understand the level of external validation and comparison to which each of these studies has been subjected. Boudia and colleagues performed an external validation study of both Canadian and New Orleans involving 1582 patients 10 years and older over a 3-year period. They noted some key differences between reported performance and performance between the two metrics. Canadian had 100% sensitivity for need for neurosurgical intervention, while New Orleans was 82% sensitive. Canadian was 95% sensitive for clinically significant head CT findings, compared with 86% sensitivity for New Orleans. Negative predictive values were 100% and 99% for Canadian and New Orleans, respectively.[5] Mower, Gupta, Rodriguez, and Hendey recently published a nearly 10 year validation of NEXUS involving 11,770 patients from four centers, which showed improved sensitivity (99% versus 98.3%), specificity (25.6% versus 13.7%), and negative predictive value (99.7% versus 99.1%) compared with the original study for clinically significant intracranial injury. This study also compared NEXUS and Canadian performance within the same study population for those who met Canadian criteria. NEXUS was found to have superior sensitivity (100% versus 97.3%) but worse specificity (32.6% versus 58.8%) for neurosurgical intervention while having worse sensitivity and specificity (97.7%/12.3% versus 98.4%/33.3%) compared with Canadian medium risk criteria for identification of significant brain injury.[6] Schachar and colleagues compared New Orleans, Canadian, and NEXUS in 2,101 pediatric patients over nearly seven years at a non-trauma center; all showed negative predictive values over 97% however Canadian and NEXUS both showed dramatically lower (65.2% and 78.3%, respectively) sensitivities in this population.[7] Smits and colleagues concluded from a Dutch cohort of 3,181 adult patients that Canadian had a lower sensitivity than New Orleans for traumatic intracranial findings but still identified all neurosurgical cases and had a much higher specificity, resulting in a greater number of avoided unnecessary scans.[8] In contrast, PECARN has been externally validated multiple times, all with near perfect sensitivity and negative predictive value;[9-13] of note, in one study two physically abused children with clinically important traumatic brain injury were misclassified as low risk, highlighting a gap in its criteria.[11]

In summary, the four major head CT decision rules all boast impressive sensitivity and negative predictive value for significant traumatic intracranial injury, though external validation and comparison studies have shown that some rules perform better than others under less controlled conditions. When properly applied to the intended patient populations, we can conclude that these are all useful clinical decision making tools, in particular to identify low risk patients and avoid unnecessary radiation exposure, costs, and resource utilization.

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Expert Commentary

I applaud Dr. Seltzer for his interesting and informative summary of decision instruments for patients with blunt head trauma. It is important to have a clear strategy for managing patients with this complaint, since traumatic brain injury is one of the most common ED complaints, accounting for an estimated 2.8 million annual visits in 2013, and the number of visits are steadily increasing.[1] Using a well validated decision instrument, such as the Canadian CT Head Rule in adults or the PECARN rule in children, reduces the frequency of unnecessary imaging and decreases length of stay while increasing the diagnostic yield (frequency of positive tests) amongst those patients that are imaged.[2,3] With this in mind, integration of these rules into clinical practice is a key component of appropriate resource utilization, and is recommended by multiple clinical practice guidelines.[4–6] However, the use of decision instruments cannot completely replace clinical gestalt, defined as the impression of the patient derived from the clinical evaluation. Unfortunately, studies comparing decision instruments to gestalt are extremely limited.[7] One study compared the PECARN decision instrument to clinician gestalt and found gestalt to be much more specific with similar sensitivity, although clinicians were asked about the criteria used in the decision instruments prior to making their “gestalt” decision.[8] There are no studies comparing the decision instruments used in adults to gestalt, so their relative performance is still open to assessment. Clinical instinct is still a valuable tool, and decision instruments only function to support this core skill. It is also important to consider what constitutes a positive outcome in these studies. Most notably, the Canadian CT Head Rule in its simplest form does not attempt to identify individuals who will have no hemorrhage at all.[9] Instead, the authors pre-defined defines a “clinically important injury”, which allowed patients to have small subdural hematomas or trace subarachnoid hemorrhage while still being considered low risk by the rule. Because these lesions rarely require intervention, the clinical significance of identifying them is minimal.

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Peter Pruitt, MD, MS

Assistant Professor

Department of Emergency Medicine

Northwestern University

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 How to cite this post

[Peer-Reviewed, Web Publication]  Seltzer J,   Sista P, (2019, December 15 ). The ED Guide to Neuroimaging: Part 2.  [NUEM Blog. Expert Commentary by Pruitt P ]. Retrieved from http://www.nuemblog.com/blog/emergency-neuroimaging-pt2.

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References

1. Mower WR, Hoffman JR, Herbert M, Wolfson AB, Pollack CV Jr, Zucker MI; NEXUS II Investigators. Developing a decision instrument to guide computed tomographic imaging of blunt head injury patients. J Trauma. 2005 Oct;59(4):954-9.

2. Kuppermann N, Holmes JF, Dayan PS, Hoyle JD Jr, Atabaki SM, Holubkov R, Nadel FM, Monroe D, Stanley RM, Borgialli DA, Badawy MK, Schunk JE, Quayle KS, Mahajan P, Lichenstein R, Lillis KA, Tunik MG, Jacobs ES, Callahan JM, Gorelick MH, Glass TF, Lee LK, Bachman MC, Cooper A, Powell EC, Gerardi MJ, Melville KA, Muizelaar JP, Wisner DH, Zuspan SJ, Dean JM, Wootton-Gorges SL; Pediatric Emergency Care Applied Research Network (PECARN). Identification of children at very low risk of clinically-important brain injuries after head trauma: a prospective cohort study. Lancet. 2009 Oct 3;374(9696):1160-70.

3. Haydel MJ, Preston CA, Mills TJ, Luber S, Blaudeau E, DeBlieux PM. Indications for computed tomography in patients with minor head injury. N Engl J Med. 2000 Jul 13;343(2):100-5.

4. Stiell IG, Wells GA, Vandemheen K, Clement C, Lesiuk H, Laupacis A, McKnight RD, Verbeek R, Brison R, Cass D, Eisenhauer ME, Greenberg G, Worthington J. The Canadian CT Head Rule for patients with minor head injury. Lancet. 2001 May 5;357(9266):1391-6.

5. Bouida W, Marghli S, Souissi S, Ksibi H, Methammem M, Haguiga H, Khedher S, Boubaker H, Beltaief K, Grissa MH, Trimech MN, Kerkeni W, Chebili N, Halila I, Rejeb I, Boukef R, Rekik N, Bouhaja B, Letaief M, Nouira S. Prediction value of the Canadian CT head rule and New Orleans for positive head CT scan and acute neurosurgical procedures in minor head trauma: a multicenter external validation study. Ann Emerg Med. 2013 May;61(5):521-7.

6. Mower WR, Gupta M, Rodriguez R, Hendey GW. Validation of the sensitivity of the National Emergency X-Radiography Utilization Study (NEXUS) Head computed tomographic (CT) decision instrument for selective imaging of blunt head injury patients: An observational study. PLoS Med. 2017 Jul 11;14(7):e1002313.

7. Schachar JL, Zampolin RL, Miller TS, Farinhas JM, Freeman K, Taragin BH. External validation of New Orleans (NOC), the Canadian CT Head Rule (CCHR) and the National Emergency X-Radiography Utilization Study II (NEXUS II) for CT scanning in pediatric patients with minor head injury in a non-trauma center. Pediatr Radiol. 2011 Aug;41(8):971-9.

8. Smits M, Dippel DW, de Haan GG, Dekker HM, Vos PE, Kool DR, Nederkoorn PJ, Hofman PA, Twijnstra A, Tanghe HL, Hunink MG. External validation of the Canadian CT Head Rule and New Orleans for CT scanning in patients with minor head injury. JAMA. 2005 Sep 28;294(12):1519-25.

9. Schonfeld D, Bressan S, Da Dalt L, Henien MN, Winnett JA, Nigrovic LE. Pediatric Emergency Care Applied Research Network head injury clinical prediction rules are reliable in practice. Arch Dis Child. 2014 May;99(5):427-31.

10. Lorton F, Poullaouec C, Legallais E, Simon-Pimmel J, Chêne MA, Leroy H, Roy M, Launay E, Gras-Le Guen C. Validation of the PECARN clinical decision rule for children with minor head trauma: a French multicenter prospective study. Scand J Trauma Resusc Emerg Med. 2016 Aug 4;24:98.

11. Ide K, Uematsu S, Tetsuhara K, Yoshimura S, Kato T, Kobayashi T. External Validation of the PECARN Head Trauma Prediction Rules in Japan. Acad Emerg Med. 2017 Mar;24(3):308-314.

12. Babl FE, Borland ML, Phillips N, Kochar A, Dalton S, McCaskill M, Cheek JA, Gilhotra Y, Furyk J, Neutze J, Lyttle MD, Bressan S, Donath S, Molesworth C, Jachno K, Ward B, Williams A, Baylis A, Crowe L, Oakley E, Dalziel SR; Paediatric Research in Emergency Departments International Collaborative (PREDICT). Accuracy of PECARN, CATCH, and CHALICE head injury decision rules in children: a prospective cohort study. Lancet. 2017 Jun 17;389(10087):2393-2402.

13. Nakhjavan-Shahraki B, Yousefifard M, Hajighanbari MJ, Oraii A, Safari S, Hosseini M. Pediatric Emergency Care Applied Research Network (PECARN) prediction rules in identifying high risk children with mild traumatic brain injury. Eur J Trauma Emerg Surg. 2017 Dec;43(6):755-762.

References (Expert Commentary)

1. Taylor CA, Bell JM, Breiding MJ, Xu L. Traumatic Brain Injury–Related Emergency Department Visits, Hospitalizations, and Deaths — United States, 2007 and 2013. MMWR Surveill Summ. 2017;66(9):1-16.

2. Sharp AL, Huang BZ, Tang T, et al. Implementation of the Canadian CT Head Rule and Its Association With Use of Computed Tomography Among Patients With Head Injury. Ann Emerg Med. 2017;33(0):1505-1514.

3. Stiell IG, Clement CM, Rowe BH, et al. Comparison of the Canadian CT Head Rule and the New Orleans Criteria in patients with minor head injury. JAMA. 2005;294(12):1511-1518.

4. Rosenberg A, Agiro A, Gottlieb M, et al. Early Trends Among Seven Recommendations From the Choosing Wisely Campaign. JAMA Intern Med. October 2015:1.

5. Schuur JD, Carney DP, Lyn ET, et al. A top-five list for emergency medicine a pilot project to improve the value of emergency care. JAMA Intern Med. 2014;174(4):509-515.

6. Mills AM, Raja AS, Marin JR. Optimizing Diagnostic Imaging in the Emergency Department. Acad Emerg Med. 2015:n/a-n/a.

7. Schriger DL, Elder JW, Cooper RJ. Structured Clinical Decision Aids Are Seldom Compared With Subjective Physician Judgment, and are Seldom Superior. Ann Emerg Med. 2016.

8. Babl FE, Oakley E, Dalziel SR, et al. Accuracy of Clinician Practice Compared With Three Head Injury Decision Rules in Children: A Prospective Cohort Study. Ann Emerg Med. 2018;71(6):703-710.

9. Stiell IG, Lesiuk H, Wells GA, et al. The Canadian CT head rule study for patients with minor head injury: Rationale, objectives, and methodology for phase I (derivation). Ann Emerg Med. 2001;38(2):160-169.

The ED Guide to Neuroimaging: Part 1

Written by: Justin Seltzer, MD (NUEM PGY-1) Edited by: Andrew Cunningham, MD, (NUEM PGY-3) Expert commentary by:  David Rusinak, MD

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Neuroimaging, mainly using CT, has become an indispensable part of our emergency diagnostic process, but, all too often we rely on radiologists to interpret what we ordered. The goal of this multi-part blog is as follows:

• To cover the basics of how to look at a CT brain and quickly identify life threat

• Review the literature supporting the major ED indications

• Discuss special considerations, such as when to use contrast, angiography, or MRI instead.

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Systematic Reading of a CT Brain

The first portion of this blog will focus on how to read a CT brain quickly with a focus on life threats.

The classic mnemonic, “Blood Can Be Very Bad,” is a pathology oriented, step-wise method to look for blood, cistern changes, and alterations to the brain parenchyma, ventricle appearance, and bony anatomy.  Applying this approach to each image cut individually can help reveal subtle findings that would otherwise be easily missed by quick scrolling.

Blood:

Blood can collect both intra-axially (parenchymal) and extra-axially (outside the parenchyma).

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Figure 1

• Classically, spontaneous intra-axial bleeding originates in deep structures such as the basal ganglia and thalamus (Figure 1).

  • In the setting of trauma, intra-axial bleeding is often ipsilateral or directly contralateral to the injury site (coup-contrecoup) but can be anywhere

  • Inferior frontal and anterior temporal lobes are high risk for traumatic contusions due to close proximity to bone (Figure 2)

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Figure 2

• Extra-axial bleeding is defined by location: mainly subdural, epidural, subarachnoid, and intraventricular hemorrhages. The patterns for these are well known and readily identified, however below are some key points on extra-axial bleeding.

  • Finding chronic subdural hematomas can be difficult as older blood and grey matter are similar appearing

  • Mass effect, abnormal appearing brain folds on that side, and use of coronal reconstructions can help identify

  • Subarachnoid hemorrhage becomes difficult to see within hours to days but acutely is often observed well in the cisterns (see below)

  • Be careful not to mistake choroid or pineal calcifications for hemorrhage

  • Don’t forget about scalp hematomas

Cisterns:

The cisterns are not ventricles but rather outpouchings of the subarachnoid space. When evaluating a CT brain the following, certain cisterns have clinical relevance for potential herniation syndromes, layering of subarachnoid blood, and/or the significant structures that run through them. Figures 3-5 show the locations of the major cisterns described below.

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Figure 3

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Figure 4

• Suprasellar: Located in the area of the sella turcica, forms a pentagon/star shape

  • Classic location of subarachnoid hemorrhage due to proximity to circle of Willis

  • Obliteration associated with downward transtentorial (i.e. uncal) herniation or due to severe elevated ICP

• Perimesencephalic cistern: A group of interconnected basal cisterns surrounding the midbrain (mesencephalon), important location of subarachnoid hemorrhage, may see effacement (reduction or loss) with tonsillar herniation

  • Interpeduncular: Located in the area of the cerebral peduncles

  • Quadrigeminal: Classically forms a W shape, obliteration associated with upward herniation

  • Ambient and crural: Connections between quadrigeminal and interpeduncular cisterns

• Cerebellopontine: Located between anterior cerebellum and lateral pons, synonymous with area of cerebellopontine angle

• Cisterna magna: Located between the cerebellum and medulla, receives fourth ventricular CSF outflow (Figure 4)

• Prepontine: Located at the anterior aspect of the pons

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Figure 5

Brain parenchyma:

CT allows for gross evaluation of the major structures as well as a differentiation of grey and white matter by Hounsfield units. The focus here is major parenchymal disruptions.

• Mass lesions, mass effect, midline shift: Because of the fixed nature of the skull, mass lesions of any type easily exert pressure on the surrounding tissue (mass effect) that can result in increased ICP, midline shift, and herniation.

  • Midline shift is measured in millimeters of displacement of the septum pellucidum at the level of the foramen of Monro from the midline of the skull

• Ischemic changes: depending on the size of the involved territory and duration, may be subtle or obvious density or architectural changes.

  • Early signs of infarction: reduced grey-white matter distinction and loss of insular hyperdensity

Ventricles:

The ventricular system is where CSF is produced and the route by which it travels into the subarachnoid space. The lateral ventricles drain via the foramina of Monro to the third ventricle, which then drains via the cerebral aqueduct (aqueduct of Sylvius) to the fourth ventricle and then to the cisterna magna and the rest of the subarachnoid space via the median and lateral apertures (foramina of Magendie and Lushka, respectively). Figures 3-5 also show the locations of the major ventricles.

• Interruption of ventricular CSF flow will cause proximal ventricular dilation that helps localize the level of obstruction

• If unsure between hydrocephalus and atrophy, dilation of temporal horns of the lateral ventricles can be helpful as it occurs in hydrocephalus involving the lateral ventricles but not with hydrocephalus ex vacuo

Bones:

Intimate knowledge of bony anatomy is not essential fracture evaluation. However, it is crucial that the bony anatomy be viewed with a dedicated bone window. Skull and facial fractures can be subtle and the presence of blood, especially an epidural hematoma, may help localize them. As noted above, soft tissue findings such as scalp hematomas are important to rule out as well.

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Key Learning Points and Conclusions

• A systematic approach is essential to avoid missing significant findings, especially with complex neuroimaging—remember “Blood Can Be Very Bad”

• Immediately look for: blood anywhere (don’t forget the scalp!), effacement of major cisterns, mass effect/midline shift, enlarged ventricles (temporal horns), skull fractures

• Older blood, such as a chronic subdural hematoma, can be hard to find and may require different cuts or inference from mass effect or effaced sulci

• Signs of infarction may be subtle (more on this later)

In the next installation, we will discuss the major indications for CT brain and the utility of CT for these indications.

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Expert Commentary

Overall, this is a very nice approach to head CT interpretation.  The classic mnemonic, “Blood Can Be Very Bad,” is not something I’ve heard of before, but it works.  Let’s take each search item in turn.

Blood

A helpful way to think about intracranial hemorrhage is to consider the causes of hemorrhage and the most common location for each pathology.  Common causes include trauma, stroke (hypertensive or hemorrhagic conversion of a venous or arterial infarct), neoplasm (primary or secondary), vascular (aneurysm, AVM, dural AV fistula), and spontaneous (anticoagulation, amyloid angiopathy, vasculitis).  If you consider the location of each of these pathologies, the hemorrhage will typically be primarily in this location.  A tumor, for example, will cause a parenchymal bleed, a ruptured aneurysm will cause subarachnoid hemorrhage, an AVM will result in a parenchymal bleed, etc.  Often with parenchymal bleeds additional imaging, vascular and MRI, as well as follow up imaging will be necessary to determine the underlying cause.

A correction is that subacute hemorrhage, not chronic, has a density similar to gray matter.  Chronic subdural hemorrhages are usually very hypodense and easy to detect on CT. So, from a practical perspective, a patient experiencing headaches from subarachnoid hemorrhage that is greater than 3 or 4 days old may be occult by CT.  This underscores the role of lumbar puncture and vascular imaging in working up patients with headaches.

Another important concept to keep in mind is window and level when interpreting CTs. Different substances (air, metal, bone, blood, fat, etc) have different and defined densities.  The pathologies associated with each of these substances (fractures, edema in the setting of stroke, etc) can be better seen by adjusting the window and level settings.  This can be done manually or, typically, PACS viewers have preset brain, bone, lung and soft tissue windows that can be displayed by pressing different numbers on the keypad.  Subtle subdural hemorrhages are often only seen with the appropriate window and level that allows distinction of the hemorrhage from the overlying calvarium.

Cisterns

The blood vessels course through the cisterns, so these must be scrutinized for the presence of hemorrhage secondary to a ruptured aneurysm in a patient presenting with an atraumatic headache.  The cisterns are also effaced in the setting of mass effect. Mass effect may be from a space occupying lesion; such as a tumor, abscess, or hemorrhage; or from diffuse cerebral edema with generalized brain swelling.  Often the absence of something (i.e. patent basal cisterns) can be harder to detect than the presence of something, like hemorrhage.  It is, therefore, important to examine the basal cisterns on each case to get comfortable with their normal variation of appearance so that their absence, such as in diffuse cerebral edema, is not missed.

Brain parenchyma

Subtle changes in parenchymal density can be difficult to detect.  It is important to get acquainted with ideal window and level settings to uncover subtle parenchymal changes.  Also comparison with prior imaging, if available, is necessary to determine the chronicity of parenchymal findings.  Understanding where a physical exam finding localizes intracranially can also be very useful- aphasia or left upper extremity weakness localize to very different locations, for example.  Lastly, always look at the vessels in the subarachnoid space to identify hyperdense thrombus in the setting of a suspected stroke.

Ventricles

Distinguishing volume loss from ventricular dilatation takes experience to understand the variation of normal across the entire age spectrum.  If hydrocephalus is suspected, determining if it is obstructive or communicating can help to understand the underlying cause. The temporal horns are the most elastic portion of the ventricles and dilate first in the setting of hydrocephalus. 

Bones

Depressed skull fractures and easy to see on routine bone windows.  Things get complicated when subtle non-displaced fractures mimic normal sutures or if the fracture involves the skull base/temporal bones.  It is probably not within the normal ED physician’s scope of practice to have a detailed knowledge of skull base anatomy, but if a skull base fracture is suspected (loss of hearing, hemorrhage in the external auditory canal, facial nerve paralysis, etc) it is important or order the proper test for further evaluation, like a temporal bone CT.  A helpful tip is to look for subtle foci of intracranial air and soft tissue swelling which may direct you to a subtle fracture.    

David Rusinak, MD

Assistant Professor of Radiology, Northwestern Medicine

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 How to cite this post

[Peer-Reviewed, Web Publication]  Whipple T,   Gappmeier V (2018, April 23 ). Demystifying the Hand Exam.  [NUEM Blog. Expert Commentary by Rusinak D ]. Retrieved from http://www.nuemblog.com/blog/neuroimaging

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References

1. Shaw AS, Prokop M. Computed Tomography. In Grainger & Allison's Diagnostic Radiology, 6th Edition (2016).

2. McKetty MH. The AAPM/RSNA physics tutorial for residents. X-ray attenuation. RadioGraphics, 1998; 18(1):151-163

3. Cadogan M. CT Head Scan. Life in the Fast Lane. Retrieved from https://lifeinthefastlane.com/investigations/ct-head-scan/.

4. Nadgir R, Yousem DM. Approach and Pitfalls in Neuroimaging. In Neuroradiology: The Requisites, 4th Edition (2017).

5. Mehta A, Jones BP. Neurovascular Diseases. In Grainger & Allison's Diagnostic Radiology, 6th Edition (2016). Chapter 62, 1456-1496.

6. Jones J. Subarachnoid Cisterns. Radiopaedia. Retrieved from https://radiopaedia.org/articles/subarachnoid-cisterns.

7. Nadgir R, Yousem DM. Cranial Anatomy. In Neuroradiology: The Requisites, 4th Edition (2017).

8. Skalski M, Dawes L. Cerebral herniation. Radiopaedia. Retrieved from  https://radiopaedia.org/articles/cerebral-herniation.

9. Baron EM, Jallo JI. TBI: Pathology, Pathophysiology, Acute Care and Surgical Management, Critical Care Principles, and Outcomes. In Brain Injury Medicine: Principles and Practice, 2nd Edition (2012). Chapter 18: 265-282.

10. Nadgir R, Yousem DM. Head Trauma. In Neuroradiology: The Requisites, 4th Edition (2017).

11. Waxman SG. Ventricles and Coverings of the Brain. In Clinical Neuroanatomy, 28th Edition (2013).

12. Nadgir R, Yousem DM. Neurodegenerative Diseases and Hydrocephalus. In Neuroradiology: The Requisites, 4th Edition (2017).

13. Galliard F, Jones J. Intraventricular haemorrhage. Radiopaedia. Retrieved from https://radiopaedia.org/articles/intraventricular-haemorrhage