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Symmetry in computed tomography of the brain: the pitfalls
Clinical Radiology (2009) 64, 298e306
PICTORIAL REVIEW
Symmetry in computed tomography of the brain: the pitfalls J.J. Downera,*, P.M. Pretoriusb Departments of aClinical Radiology and bNeuroradiology, John Radcliffe Hospital, Oxford, UK Received 3 July 2008; received in revised form 4 August 2008; accepted 25 August 2008
Computed tomography (CT) studies of the brain are one of the most frequent examinations interpreted by radiologists out of hours. Apparently normal appearances in patients with significant neurological morbidity can be perplexing. As the contents of the cranium are normally remarkably symmetrical on axial CT, disease entities that result in symmetrical appearances are the most difficult to detect. In this review we highlight a spectrum of important acute neurological conditions that result in abnormal but symmetrical appearances on CT. ª 2008 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.
Introduction Computed tomography (CT) images of the brain are one of the most frequent examinations interpreted on-call by radiology trainees. Apparently normal appearances in patients with significant neurological disability can be perplexing. As the contents of the cranium are normally remarkably symmetrical about the mid-sagittal plane, disease entities that result in symmetrical appearances are, therefore, the most difficult to detect. Symmetrical but abnormal appearances can occur in four circumstances: bilateral lesions, midline lesions, the absence of a normal midline feature, and the bilateral absence of normal features (Table 1). In this review we highlight a spectrum of important acute neurological conditions that result in abnormal but symmetrical appearances on CT structured around a series of specific review areas (Table 2). In this way we emphasize the importance of avoiding the temptation to treat the interpretation of axial images of the brain as an exercise in the detection of asymmetry and offer * Guarantor and correspondent: J. J. Downer, Department of Clinical Radiology, John Radcliffe Hospital, Oxford OX3 9DU, UK. Tel.: þ44 1865 220800. E-mail address: [email protected] (J.J. Downer).
an approach to the review of the symmetrical brain study to aid the detection of disease. We present practical advice on image interpretation rather than an exhaustive review of the conditions described.
Midline Basilar artery thrombosis The basilar artery runs in the midline over the ventral surface of the pons in the prepontine cistern. Acute thromboembolic basilar artery occlusion accounts for 6e10% of large vessel ischaemic stroke and is associated with a poor prognosis.1 Mortalities as high as 95% are reported.2 Early aggressive treatment may increase survival and limit disability. As a result, early diagnosis on neuroimaging is of importance. Intra-arterial thrombolysis and mechanical thrombectomy are treatment options where interventional neuroradiology is available. In other centres, intravenous thrombolysis may be considered.3 Patients generally present with sudden onset severe motor and bulbar symptoms and reduced conscious level. A stuttering onset or prodrome is seen in a number of patients with vertebrobasilar
0009-9260/$ - see front matter ª 2008 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.crad.2008.08.012
Symmetry in computed tomography of the brain: the pitfalls
Table 1 brain CT
299
Explanation for symmetrical appearances on
Normal Bilateral lesions Midline lesion Absence of a normal midline feature Bilateral absence of normal features
territory transient ischaemic attacks preceding the occlusive event.4 Acute intra-arterial thrombus appears hyperdense on unenhanced CT. The density of the proximal part of the basilar artery can be accentuated by superimposition of streak artefact from the petrous bones. The distal part of the artery is more easily assessed (Fig. 1) and its density can be compared with that of the internal carotid arteries on the same sections to control for individual variations in haematocrit. It is prudent to widen the window settings when a hyperdense artery is seen to ensure that the density is not due to focal arterial calcification.
Sagittal sinus thrombosis Cerebral venous thrombosis is a relatively rare disorder when compared with arterial disease.5 However, it is an important and under-diagnosed condition. Clinically, diagnosis can be difficult as the presentation is highly variable and sometimes non-specific. A favourable prognosis is dependent on prompt diagnosis and appropriate therapy.6 Most patients are treated with systemic anticoagulation. Less commonly transcatheter thrombolysis is performed. Unenhanced CT is the initial imaging investigation in the majority of patients presenting with acute neurological illness. Demonstration of a hyperattenuating acutely thrombosed vein or sinus is a direct sign of the diagnosis. When this involves the sagittal sinus and parenchymal changes are absent, imaging is symmetrical. When the thrombosis involves only part of the sinus, differences in
Table 2 Suggested review areas for the symmetrical CT brain study Midline Extra-axial spaces Cortex Deep grey matter White matter Ventricles
Figure 1 Basilar artery thrombosis. The basilar artery (arrow) is significantly denser than the internal carotid artery (arrowhead) on this unenhanced CT study indicating acute thromboembolic occlusion.
the density between the anterior and posterior part of the sinus makes detection easier (Figs. 2 and 3). Venous sinus thrombosis can be missed if only contrast-enhanced images are obtained as acute thrombus may be of very similar density to enhancing venous blood within a patent venous structure (Fig. 3b).
Deep cerebral venous thrombosis Deep cerebral vein thrombosis is less common than dural sinus thrombosis but is associated with a poorer prognosis.7 The deep cerebral veins include the ICV (internal cerebral veins), basal veins, and the great cerebral vein (Fig. 3). The internal cerebral veins drain the thalami, basal ganglia, and adjacent white matter.8 Classically internal cerebral vein thrombosis presents with severe dysfunction of the diencephalon, resulting in coma and disturbances of eye movements and pupillary reflexes. However, partial syndromes occur, without disturbance of consciousness or brain stem signs.9 Imaging in internal cerebral vein thrombosis is often symmetrical with hyperattenuation of the paired internal cerebral veins on unenhanced studies that fail to enhance following the injection of contrast material (Fig. 4). Symmetrical hypoattenuation within the drainage territory of the veins indicates insipient venous infarction and carries a poor prognosis.
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Figure 2 Superior sagittal sinus thrombosis. Thrombosis of the superior sagittal sinus is easily appreciated on CT (a) due to the marked difference in the density of the thrombosed anterior part of the sinus compared with the patent posterior part. Thrombus of different ages may also explain this differential density, but magnetic resonance venography (b) confirmed the CT findings in this case.
Extra-axial spaces Subdural haematoma A significant number of on-call CT studies are performed in patients following head injury. Acute subdural haematoma is important to identify as emergent neurosurgical intervention may be indicated. Subdural haematomas commonly occur along the convexities, falx cerebri, or tentorium cerebelli. Detection of convexity subdural haematoma on unenhanced CT is more challenging when it is isodense with brain. Displacement of the greyewhite matter interface inward from the calvarium and apparently unexplained mass effect are indirect signs of the diagnosis. When bilateral, appearances may be symmetrical, making detection particularly difficult.10 Hence, when a study is symmetrical, consider whether the appearances of the extra-axial cerebrospinal fluid (CSF) spaces and ventricles are appropriate for a patient’s age, examine the greyewhite matter interface for displacement. When looking for displacement of the greyewhite matter interface, keep in mind that the normal cortical thickness is less than 4 mm, and in cases where the cortex appears to be against the inner surface of the skull, white matter should be seen extending to within 4 mm of the skull. Narrow window settings should also be used to aid detection of isodense subdural collections (Fig. 5).
Symmetrical appearances may also be observed in the presence of parafalcine or bilateral tentorial subdural haematomas. In a published series, one of the most common locations for subdural haematoma missed by radiology residents on call was parafalcine11 (Fig. 6). This study also highlighted the interpeduncular cistern as the most common location for missed subarachnoid haemorrhage, again a midline abnormality resulting in symmetrical appearances.
Cortex Hypoxiceischaemic brain injury Due to its high metabolic demands, the brain is particularly sensitive to hypoxiceischaemic insults secondary to global disturbances in perfusion or, less commonly, oxygenation. Common causes include prolonged hypotension, cardiac arrest, birth asphyxia, and near drowning. Hypoxiceischaemic brain damage can also occur in non-accidental injury in infants. The pattern of brain damage is determined by factors such as the severity and duration of the insult, and the maturity of the brain.13 Different patterns of brain damage for instance are encountered in birth asphyxia depending on the gestational age of the foetus at the time of the insult. As a general rule, grey matter structures, in
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Figure 3 Superior sagittal sinus thrombosis. On this unenhanced CT image the posterior portion of the sagittal sinus is hyperdense (a). A corresponding contrast-enhanced image demonstrates no further enhancement of the thrombosed sinus but normal enhancement of other venous and arterial structures (b). Midline vascular anatomy is reviewed with reference to a sagittal reconstruction from a normal CT venogram on a different patient (c). Arrows indicate: anterior cerebral arteries (i); thalamostriate veins (ii), which drain into the internal cerebral veins (iii), which unite to form the great cerebral vein, which in turn drains into the straight sinus (iv) and the superior sagittal sinus (v).
particular the peri-rolandic cortex and basal ganglia, are more sensitive to hypoxiceischaemic insults than white matter structures. The vertebro-basilar (posterior) circulation territory is preferentially perfused at the expense of the anterior circulation territory during acute severe hypoxiceischaemic episodes. This autoregulatory mechanism is presumably intended to protect vital structures in the brainstem. Symmetrical imaging appearances characterized on CT by low density and loss of or decreased greyewhite matter differentiation are often encountered. The ‘‘reversal sign’’ on CT was first described by Han et al. in 1990,12 and is the CT manifestation of the relative sparing of the posterior circulation perfusion territory. The cerebellar cortex is spared from the ischaemic insult,
retaining its normal attenuation and, therefore, looks hyperdense compared with the decreased density in the affected adjacent posterior temporal cortex on the other side of the tentorium. This abnormality can be very subtle in the early stages on standard brain window settings and is best appreciated on narrower window settings (Fig. 7).
Deep grey matter structures As a rule, deep grey matter structures (basal ganglia and thalami) are of identical attenuation to cortical grey matter on CT. Basal ganglia calcification is an easily recognized d and usually clinically insignificant d exception to this rule. Hypoattenuation of any or all of the deep grey
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Figure 4 Internal cerebral vein thrombosis. These images demonstrate hyperdense Internal cerebral vein (internal cerebral veins) on unenhanced CT (a) that fail to enhance (arrow) following intravenous contrast medium administration (b). There are parenchymal changes with low density within the drainage territory of the ICV involving the left thalamus (open arrow) and lentiform nucleus (arrowhead) with haemorrhage in the anterior limb of the right internal capsule.
matter structures compared with cortex should be considered pathological. Symmetrical hypoattenuation of these structures is a grave sign but can easily be overlooked. A range of disease entities may cause acute bilateral symmetrical abnormality of deep grey
matter structures on CT. Vascular causes include deep cerebral venous thrombosis (as previously described, Fig. 4) and bilateral thalamic infarction. Hypoxia can also result in bilateral abnormality of deep grey matter structures (Fig. 8). Other causes include toxic insult, for example, following
Figure 5 Bilateral convexity subdural haematoma. Bilateral isodense extra-axial collections result in displacement of the greyewhite matter interface inward from the calvarium and mass effect with effacement of cortical sulci on these unenhanced axial CT images.
Symmetry in computed tomography of the brain: the pitfalls
Figure 6 Acute parafalcine subdural haematoma. Widened window settings (not shown) prevent confusion with falcine calcification.
methanol ingestion, and metabolic disorders, for example hypoglycaemia.14
White matter Posterior reversible encephalopathy syndrome (PRES) PRES is an uncommon disorder with characteristic clinical and radiological features typically associated with acute hypertension. Patients present
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Figure 8 Bilateral hypoattenuation of deep grey matter structures. Attenuation of the basal ganglia and thalami is significantly reduced in comparison with cortical grey matter in this case due to hypoxic insult in near drowning. A chronic left convexity subdural haematoma is also demonstrated overlying the frontal lobe.
with headache, confusion, seizures and visual disturbance. CT images are characterized by hypoattenuation affecting cortex and subcortical white matter within posterior circulation territories bilaterally (Fig. 9). The absence of diffusion restriction on diffusion-weighted magnetic resonance imaging (MRI) and reversibility over time classically differentiate this condition from infarction.15
Figure 7 Hypoxiceischaemic brain injury. Unenhanced CT images demonstrating the ‘‘reversal sign’’ emphasizing the importance of using narrow window settings when this diagnosis is expected. (a) Window width (WW)110, Window level (WL)35; (b) WW30, WL35.
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of greyewhite differentiation may be the only sign in early disease, be it due to ischaemia or PRES.
Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL)
Figure 9 Posterior reversible encephalopathy syndrome. Unenhanced CT demonstrating bilateral hypoattenuation involving cortex and subcortical white matter in posterior circulation territories with no enhancement following intravenous contrast medium administration.
Unfortunately, PRES is somewhat of a misnomer as neuroimaging abnormalities are not always posterior or reversible with blood pressure control.16 The commonest symmetrical white matter abnormality noted on CT is periventricular white matter low attenuation due to small vessel disease. Recognizing other, less common, symmetrical white matter abnormalities is important. The pattern of this abnormality, in an appropriate clinical setting, may suggest a specific diagnosis. Loss
CADASIL is an uncommon inherited small vessel disease that causes stroke in young adults. Mutations in the Notch3 gene on chromosome 19 are responsible. Patients typically have a history of migraine with aura and may present with recurrent transient ischaemic attacks or subcortical strokes. Early cognitive decline and depression are also features. An autosomal dominant family history of migraine, young stroke, or early onset dementia may be elicited.17 Neuroimaging is characterized by subcortical infarcts and diffuse white matter ischaemic change. Subcortical infarcts characteristically involve the anterior temporal pole (Fig. 10) and external capsule, but may also involve centrum semiovale, deep grey matter structures, and pons.18 Recognizing this pattern of abnormality , given an appropriate clinical context, should suggest the diagnosis.
Ventricles Hydrocephalus There is a wide variation in the appearance of the ventricles between individuals. Diagnosing
Figure 10 CADASIL. Bilateral subcortical white matter hypoattenuation involving the anterior temporal lobes (arrows) consistent with CADASIL in a young adult patient with a history of migraine who presented with stroke.
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Figure 11 Obstructive hydrocephalus. Unenhanced CT in a child demonstrating supratentorial hydrocephalus. There is ballooning of the normally slit-like third ventricle and ventricular size out of proportion to the appearance of the cortical sulci in the supratentorial compartment. The forth ventricle is effaced, consistent with an obstruction at the level of the posterior fossa. There is hypoattenuation in the cerebellar white matter. In this case, obstructive hydrocephalus resulted from cerebellar swelling due to cerebellitis.
hydroce-phalus involves recognizing ventricular enlargement, and distinguishing where this appearance is due to parenchymal atrophy. In hydrocephalus, sulci are relatively effaced in comparison with ventricular size. Periventricular white matter low attenuation may be present in acute hydrocephalus due to transependymal cerebrospinal fluid spread as a result of raised intraventricular pressure. In obstructive hydrocephalus, the level of
obstruction can be determined by the level above which ventricular enlargement is observed (Fig. 11). Obstruction may be intra- or extra-ventricular.
Raised intracranial pressure Imaging appearances correlate poorly with intracranial pressure.19 However, concrete signs of raised intracranial pressure may be evident with
Figure 12 Effacement of the third ventricle consistent with raised intracranial pressure in a middle-aged woman presenting with headache and papilloedema secondary to unexpected malignant obstruction of the superior vena cava. The effacement of the extra-axial CSF spaces and relatively small size of the lateral ventricles would be normal in a teenager or young adult but should raise the suspicion of brain swelling in this age group.
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symmetrical appearances. Fig. 12 is an illustrative case demonstrating complete effacement of the third ventricle. This sign was originally described in diffuse post-traumatic brain swelling,20,21 but can be seen in severe diffuse brain swelling of any cause. The third ventricle should be visible in all age groups and complete effacement should prompt a review for other signs of raised intracranial pressure, such as effacement of the extra-axial CSF spaces and tonsillar herniation.
Conclusion Symmetry is a poor marker of normality in brain imaging. Apparently normal brain CT appearances in a patient with suspected intracranial pathology should prompt a careful review to detect abnormalities leading to symmetrical appearances. Although this review is not intended to be exhaustive, we hope that by highlighting important conditions leading to symmetrical but abnormal brain appearances on CT, we can encourage the inexperienced radiologist to take interpretation of cranial CT beyond the detection of asymmetry.
References 1. Smith WS. Intra-arterial thrombolytic therapy for acute basilar occlusion: pro. Stroke 2007;38(Suppl.):701e3. 2. Lindsberg PJ, Soinne L, Roine RO, et al. Options for recanalization therapy in basilar artery occlusion. Stroke 2005; 36:203e4. 3. Lindsberg PJ, Mattle HP. Therapy of basilar artery occlusion. a systematic analysis comparing intra-arterial and intravenous thrombolysis. Stroke 2006;37:922e8. 4. Idicula TT, Joseph LN. Neurological complications and aspects of basilar artery occlusive disease. Neurologist 2007;13:363e8. 5. Ferro JM, Canhao P, Stam J, et al. Prognosis of cerebral vein and dural sinus thrombosis: results of the International Study on Cerebral Vein and Dural Sinus Thrombosis (ISCVT). Stroke 2004;35:664e70.
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6. Rodallec MH, Krainik A, Feydy A, et al. Cerebral venous thrombosis and multidetector CT angiography: tips and tricks. RadioGraphics 2006;26(Suppl. 1):S5eS18. 7. Crawford SC, Digre KB, Palmer CA, et al. Thrombosis of the deep venous drainage of the brain in adults. Analysis of seven cases with review of the literature. Arch Neurol 1995;52:1101e8. 8. Leach JL, Fortuna RB, Jones BV, et al. Imaging of cerebral venous thrombosis: current techniques, spectrum of findings, and diagnostic pitfalls. RadioGraphics 2006;26(Suppl. 1):S19e41. 9. Herrmann KA, Sporer B, Yousry TA. Thrombosis of the internal cerebral vein associated with transient unilateral thalamic edema: a case report and review of the literature. AJNR Am J Neuroradiol 2004;25:1351e5. 10. Provenzale J. CT and MR imaging of acute cranial trauma. Emerg Radiol 2007;14:1e12. 11. Strub WM, Leach JL, Tomsick T, et al. Overnight preliminary head CT interpretations provided by residents: locations of misidentified intracranial hemorrhage. AJNR Am J Neuroradiol 2007;28:1679e82. 12. Han BK, Towbin RB, De Courten-Myers G, et al. Reversal sign on CT: effect of anoxic/ischemic cerebral injury in children. AJR Am J Roentgenol 1990;154:361e8. 13. Chao CP, Zaleski CG, Patton AC. Neonatal hypoxiceischemic encephalopathy: multimodality imaging findings. RadioGraphics 2006;26:S159e72. 14. Finelli PF, DiMario Jr FJ. Diagnostic approach in patients with symmetric imaging lesions of the deep gray nuclei. Neurologist 2003;9:250e61. 15. Bartynski WS. Posterior reversible encephalopathy syndrome, part 1: fundamental imaging and clinical features. AJNR Am J Neuroradiol 2008 Jun;29(6):1036e42. 16. Stott VL, Hurrell MA, Anderson TJ. Reversible posterior leukoencephalopathy syndrome: a misnomer reviewed. Intern Med J 2005;35:83e90. 17. Gladstone JP, Dodick DW. Migraine and cerebral white matter lesions: when to suspect cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL). Neurologist 2005;11:19e29. 18. Singhal S, Rich P, Markus HS. The spatial distribution of MR imaging abnormalities in cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy and their relationship to age and clinical features. AJNR Am J Neuroradiol 2005;26:2481e7. 19. Harden SP, Dey C, Gawne-Cain ML. Cranial CT of the unconscious adult patient. Clin Radiol 2007;62:404e15. 20. Teasdale E, Cardoso E, Galbraith S, et al. CT scan in severe diffuse head injury: physiological and clinical considerations. J Neurol Neurosurg Psychiatry 1984;47:600e3. 21. Colquhoun IR, Burrows EH. The prognostic significance of the third ventricle and basal cisterns in severe closed head injury. Clin Radiol 1989;40:13e6.
PICTORIAL REVIEW
Symmetry in computed tomography of the brain: the pitfalls J.J. Downera,*, P.M. Pretoriusb Departments of aClinical Radiology and bNeuroradiology, John Radcliffe Hospital, Oxford, UK Received 3 July 2008; received in revised form 4 August 2008; accepted 25 August 2008
Computed tomography (CT) studies of the brain are one of the most frequent examinations interpreted by radiologists out of hours. Apparently normal appearances in patients with significant neurological morbidity can be perplexing. As the contents of the cranium are normally remarkably symmetrical on axial CT, disease entities that result in symmetrical appearances are the most difficult to detect. In this review we highlight a spectrum of important acute neurological conditions that result in abnormal but symmetrical appearances on CT. ª 2008 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.
Introduction Computed tomography (CT) images of the brain are one of the most frequent examinations interpreted on-call by radiology trainees. Apparently normal appearances in patients with significant neurological disability can be perplexing. As the contents of the cranium are normally remarkably symmetrical about the mid-sagittal plane, disease entities that result in symmetrical appearances are, therefore, the most difficult to detect. Symmetrical but abnormal appearances can occur in four circumstances: bilateral lesions, midline lesions, the absence of a normal midline feature, and the bilateral absence of normal features (Table 1). In this review we highlight a spectrum of important acute neurological conditions that result in abnormal but symmetrical appearances on CT structured around a series of specific review areas (Table 2). In this way we emphasize the importance of avoiding the temptation to treat the interpretation of axial images of the brain as an exercise in the detection of asymmetry and offer * Guarantor and correspondent: J. J. Downer, Department of Clinical Radiology, John Radcliffe Hospital, Oxford OX3 9DU, UK. Tel.: þ44 1865 220800. E-mail address: [email protected] (J.J. Downer).
an approach to the review of the symmetrical brain study to aid the detection of disease. We present practical advice on image interpretation rather than an exhaustive review of the conditions described.
Midline Basilar artery thrombosis The basilar artery runs in the midline over the ventral surface of the pons in the prepontine cistern. Acute thromboembolic basilar artery occlusion accounts for 6e10% of large vessel ischaemic stroke and is associated with a poor prognosis.1 Mortalities as high as 95% are reported.2 Early aggressive treatment may increase survival and limit disability. As a result, early diagnosis on neuroimaging is of importance. Intra-arterial thrombolysis and mechanical thrombectomy are treatment options where interventional neuroradiology is available. In other centres, intravenous thrombolysis may be considered.3 Patients generally present with sudden onset severe motor and bulbar symptoms and reduced conscious level. A stuttering onset or prodrome is seen in a number of patients with vertebrobasilar
0009-9260/$ - see front matter ª 2008 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.crad.2008.08.012
Symmetry in computed tomography of the brain: the pitfalls
Table 1 brain CT
299
Explanation for symmetrical appearances on
Normal Bilateral lesions Midline lesion Absence of a normal midline feature Bilateral absence of normal features
territory transient ischaemic attacks preceding the occlusive event.4 Acute intra-arterial thrombus appears hyperdense on unenhanced CT. The density of the proximal part of the basilar artery can be accentuated by superimposition of streak artefact from the petrous bones. The distal part of the artery is more easily assessed (Fig. 1) and its density can be compared with that of the internal carotid arteries on the same sections to control for individual variations in haematocrit. It is prudent to widen the window settings when a hyperdense artery is seen to ensure that the density is not due to focal arterial calcification.
Sagittal sinus thrombosis Cerebral venous thrombosis is a relatively rare disorder when compared with arterial disease.5 However, it is an important and under-diagnosed condition. Clinically, diagnosis can be difficult as the presentation is highly variable and sometimes non-specific. A favourable prognosis is dependent on prompt diagnosis and appropriate therapy.6 Most patients are treated with systemic anticoagulation. Less commonly transcatheter thrombolysis is performed. Unenhanced CT is the initial imaging investigation in the majority of patients presenting with acute neurological illness. Demonstration of a hyperattenuating acutely thrombosed vein or sinus is a direct sign of the diagnosis. When this involves the sagittal sinus and parenchymal changes are absent, imaging is symmetrical. When the thrombosis involves only part of the sinus, differences in
Table 2 Suggested review areas for the symmetrical CT brain study Midline Extra-axial spaces Cortex Deep grey matter White matter Ventricles
Figure 1 Basilar artery thrombosis. The basilar artery (arrow) is significantly denser than the internal carotid artery (arrowhead) on this unenhanced CT study indicating acute thromboembolic occlusion.
the density between the anterior and posterior part of the sinus makes detection easier (Figs. 2 and 3). Venous sinus thrombosis can be missed if only contrast-enhanced images are obtained as acute thrombus may be of very similar density to enhancing venous blood within a patent venous structure (Fig. 3b).
Deep cerebral venous thrombosis Deep cerebral vein thrombosis is less common than dural sinus thrombosis but is associated with a poorer prognosis.7 The deep cerebral veins include the ICV (internal cerebral veins), basal veins, and the great cerebral vein (Fig. 3). The internal cerebral veins drain the thalami, basal ganglia, and adjacent white matter.8 Classically internal cerebral vein thrombosis presents with severe dysfunction of the diencephalon, resulting in coma and disturbances of eye movements and pupillary reflexes. However, partial syndromes occur, without disturbance of consciousness or brain stem signs.9 Imaging in internal cerebral vein thrombosis is often symmetrical with hyperattenuation of the paired internal cerebral veins on unenhanced studies that fail to enhance following the injection of contrast material (Fig. 4). Symmetrical hypoattenuation within the drainage territory of the veins indicates insipient venous infarction and carries a poor prognosis.
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Figure 2 Superior sagittal sinus thrombosis. Thrombosis of the superior sagittal sinus is easily appreciated on CT (a) due to the marked difference in the density of the thrombosed anterior part of the sinus compared with the patent posterior part. Thrombus of different ages may also explain this differential density, but magnetic resonance venography (b) confirmed the CT findings in this case.
Extra-axial spaces Subdural haematoma A significant number of on-call CT studies are performed in patients following head injury. Acute subdural haematoma is important to identify as emergent neurosurgical intervention may be indicated. Subdural haematomas commonly occur along the convexities, falx cerebri, or tentorium cerebelli. Detection of convexity subdural haematoma on unenhanced CT is more challenging when it is isodense with brain. Displacement of the greyewhite matter interface inward from the calvarium and apparently unexplained mass effect are indirect signs of the diagnosis. When bilateral, appearances may be symmetrical, making detection particularly difficult.10 Hence, when a study is symmetrical, consider whether the appearances of the extra-axial cerebrospinal fluid (CSF) spaces and ventricles are appropriate for a patient’s age, examine the greyewhite matter interface for displacement. When looking for displacement of the greyewhite matter interface, keep in mind that the normal cortical thickness is less than 4 mm, and in cases where the cortex appears to be against the inner surface of the skull, white matter should be seen extending to within 4 mm of the skull. Narrow window settings should also be used to aid detection of isodense subdural collections (Fig. 5).
Symmetrical appearances may also be observed in the presence of parafalcine or bilateral tentorial subdural haematomas. In a published series, one of the most common locations for subdural haematoma missed by radiology residents on call was parafalcine11 (Fig. 6). This study also highlighted the interpeduncular cistern as the most common location for missed subarachnoid haemorrhage, again a midline abnormality resulting in symmetrical appearances.
Cortex Hypoxiceischaemic brain injury Due to its high metabolic demands, the brain is particularly sensitive to hypoxiceischaemic insults secondary to global disturbances in perfusion or, less commonly, oxygenation. Common causes include prolonged hypotension, cardiac arrest, birth asphyxia, and near drowning. Hypoxiceischaemic brain damage can also occur in non-accidental injury in infants. The pattern of brain damage is determined by factors such as the severity and duration of the insult, and the maturity of the brain.13 Different patterns of brain damage for instance are encountered in birth asphyxia depending on the gestational age of the foetus at the time of the insult. As a general rule, grey matter structures, in
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Figure 3 Superior sagittal sinus thrombosis. On this unenhanced CT image the posterior portion of the sagittal sinus is hyperdense (a). A corresponding contrast-enhanced image demonstrates no further enhancement of the thrombosed sinus but normal enhancement of other venous and arterial structures (b). Midline vascular anatomy is reviewed with reference to a sagittal reconstruction from a normal CT venogram on a different patient (c). Arrows indicate: anterior cerebral arteries (i); thalamostriate veins (ii), which drain into the internal cerebral veins (iii), which unite to form the great cerebral vein, which in turn drains into the straight sinus (iv) and the superior sagittal sinus (v).
particular the peri-rolandic cortex and basal ganglia, are more sensitive to hypoxiceischaemic insults than white matter structures. The vertebro-basilar (posterior) circulation territory is preferentially perfused at the expense of the anterior circulation territory during acute severe hypoxiceischaemic episodes. This autoregulatory mechanism is presumably intended to protect vital structures in the brainstem. Symmetrical imaging appearances characterized on CT by low density and loss of or decreased greyewhite matter differentiation are often encountered. The ‘‘reversal sign’’ on CT was first described by Han et al. in 1990,12 and is the CT manifestation of the relative sparing of the posterior circulation perfusion territory. The cerebellar cortex is spared from the ischaemic insult,
retaining its normal attenuation and, therefore, looks hyperdense compared with the decreased density in the affected adjacent posterior temporal cortex on the other side of the tentorium. This abnormality can be very subtle in the early stages on standard brain window settings and is best appreciated on narrower window settings (Fig. 7).
Deep grey matter structures As a rule, deep grey matter structures (basal ganglia and thalami) are of identical attenuation to cortical grey matter on CT. Basal ganglia calcification is an easily recognized d and usually clinically insignificant d exception to this rule. Hypoattenuation of any or all of the deep grey
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Figure 4 Internal cerebral vein thrombosis. These images demonstrate hyperdense Internal cerebral vein (internal cerebral veins) on unenhanced CT (a) that fail to enhance (arrow) following intravenous contrast medium administration (b). There are parenchymal changes with low density within the drainage territory of the ICV involving the left thalamus (open arrow) and lentiform nucleus (arrowhead) with haemorrhage in the anterior limb of the right internal capsule.
matter structures compared with cortex should be considered pathological. Symmetrical hypoattenuation of these structures is a grave sign but can easily be overlooked. A range of disease entities may cause acute bilateral symmetrical abnormality of deep grey
matter structures on CT. Vascular causes include deep cerebral venous thrombosis (as previously described, Fig. 4) and bilateral thalamic infarction. Hypoxia can also result in bilateral abnormality of deep grey matter structures (Fig. 8). Other causes include toxic insult, for example, following
Figure 5 Bilateral convexity subdural haematoma. Bilateral isodense extra-axial collections result in displacement of the greyewhite matter interface inward from the calvarium and mass effect with effacement of cortical sulci on these unenhanced axial CT images.
Symmetry in computed tomography of the brain: the pitfalls
Figure 6 Acute parafalcine subdural haematoma. Widened window settings (not shown) prevent confusion with falcine calcification.
methanol ingestion, and metabolic disorders, for example hypoglycaemia.14
White matter Posterior reversible encephalopathy syndrome (PRES) PRES is an uncommon disorder with characteristic clinical and radiological features typically associated with acute hypertension. Patients present
303
Figure 8 Bilateral hypoattenuation of deep grey matter structures. Attenuation of the basal ganglia and thalami is significantly reduced in comparison with cortical grey matter in this case due to hypoxic insult in near drowning. A chronic left convexity subdural haematoma is also demonstrated overlying the frontal lobe.
with headache, confusion, seizures and visual disturbance. CT images are characterized by hypoattenuation affecting cortex and subcortical white matter within posterior circulation territories bilaterally (Fig. 9). The absence of diffusion restriction on diffusion-weighted magnetic resonance imaging (MRI) and reversibility over time classically differentiate this condition from infarction.15
Figure 7 Hypoxiceischaemic brain injury. Unenhanced CT images demonstrating the ‘‘reversal sign’’ emphasizing the importance of using narrow window settings when this diagnosis is expected. (a) Window width (WW)110, Window level (WL)35; (b) WW30, WL35.
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of greyewhite differentiation may be the only sign in early disease, be it due to ischaemia or PRES.
Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL)
Figure 9 Posterior reversible encephalopathy syndrome. Unenhanced CT demonstrating bilateral hypoattenuation involving cortex and subcortical white matter in posterior circulation territories with no enhancement following intravenous contrast medium administration.
Unfortunately, PRES is somewhat of a misnomer as neuroimaging abnormalities are not always posterior or reversible with blood pressure control.16 The commonest symmetrical white matter abnormality noted on CT is periventricular white matter low attenuation due to small vessel disease. Recognizing other, less common, symmetrical white matter abnormalities is important. The pattern of this abnormality, in an appropriate clinical setting, may suggest a specific diagnosis. Loss
CADASIL is an uncommon inherited small vessel disease that causes stroke in young adults. Mutations in the Notch3 gene on chromosome 19 are responsible. Patients typically have a history of migraine with aura and may present with recurrent transient ischaemic attacks or subcortical strokes. Early cognitive decline and depression are also features. An autosomal dominant family history of migraine, young stroke, or early onset dementia may be elicited.17 Neuroimaging is characterized by subcortical infarcts and diffuse white matter ischaemic change. Subcortical infarcts characteristically involve the anterior temporal pole (Fig. 10) and external capsule, but may also involve centrum semiovale, deep grey matter structures, and pons.18 Recognizing this pattern of abnormality , given an appropriate clinical context, should suggest the diagnosis.
Ventricles Hydrocephalus There is a wide variation in the appearance of the ventricles between individuals. Diagnosing
Figure 10 CADASIL. Bilateral subcortical white matter hypoattenuation involving the anterior temporal lobes (arrows) consistent with CADASIL in a young adult patient with a history of migraine who presented with stroke.
Symmetry in computed tomography of the brain: the pitfalls
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Figure 11 Obstructive hydrocephalus. Unenhanced CT in a child demonstrating supratentorial hydrocephalus. There is ballooning of the normally slit-like third ventricle and ventricular size out of proportion to the appearance of the cortical sulci in the supratentorial compartment. The forth ventricle is effaced, consistent with an obstruction at the level of the posterior fossa. There is hypoattenuation in the cerebellar white matter. In this case, obstructive hydrocephalus resulted from cerebellar swelling due to cerebellitis.
hydroce-phalus involves recognizing ventricular enlargement, and distinguishing where this appearance is due to parenchymal atrophy. In hydrocephalus, sulci are relatively effaced in comparison with ventricular size. Periventricular white matter low attenuation may be present in acute hydrocephalus due to transependymal cerebrospinal fluid spread as a result of raised intraventricular pressure. In obstructive hydrocephalus, the level of
obstruction can be determined by the level above which ventricular enlargement is observed (Fig. 11). Obstruction may be intra- or extra-ventricular.
Raised intracranial pressure Imaging appearances correlate poorly with intracranial pressure.19 However, concrete signs of raised intracranial pressure may be evident with
Figure 12 Effacement of the third ventricle consistent with raised intracranial pressure in a middle-aged woman presenting with headache and papilloedema secondary to unexpected malignant obstruction of the superior vena cava. The effacement of the extra-axial CSF spaces and relatively small size of the lateral ventricles would be normal in a teenager or young adult but should raise the suspicion of brain swelling in this age group.
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symmetrical appearances. Fig. 12 is an illustrative case demonstrating complete effacement of the third ventricle. This sign was originally described in diffuse post-traumatic brain swelling,20,21 but can be seen in severe diffuse brain swelling of any cause. The third ventricle should be visible in all age groups and complete effacement should prompt a review for other signs of raised intracranial pressure, such as effacement of the extra-axial CSF spaces and tonsillar herniation.
Conclusion Symmetry is a poor marker of normality in brain imaging. Apparently normal brain CT appearances in a patient with suspected intracranial pathology should prompt a careful review to detect abnormalities leading to symmetrical appearances. Although this review is not intended to be exhaustive, we hope that by highlighting important conditions leading to symmetrical but abnormal brain appearances on CT, we can encourage the inexperienced radiologist to take interpretation of cranial CT beyond the detection of asymmetry.
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