Neuroimaging - PDF

FOCUS Neuroimaging Julius Tamangani T Background he central nervous system (CNS) is primarily imaged...

FOCUS Neuroimaging Julius Tamangani T Background he central nervous system (CNS) is primarily imaged using computed tomography (CT) and magnetic Neuroimaging is the use of various techniques that directly resonance imaging (MRI). Other modalities, such as or indirectly assess the structure or function of the nervous positron emission tomography (PET), are used in specific system. Computed tomography (CT) and magnetic resonance specialist applications or research in close collaboration with imaging (MRI) are the most widely used modalities in routine neurologists, neurosurgeons and psychiatrists. The use of clinical practice. ultrasonography is mainly limited to assessment of infants prior to closure of the fontanelles. Objectives This article discusses the common indications and limitations of Computed tomography CT and MRI, and the importance of clinical correlation. Advantages and disadvantages Compared with MRI, the main advantages of CT are greater Discussion availability, lower cost and shorter acquisition time. It is generally the modality of choice for the initial assessment There is a wide range of applications for neuroimaging but with of acute intracranial conditions. Its shorter acquisition time some limitations, including potential unnecessary anxiety in allows reasonable image quality in noncompliant patients who patients and unwarranted further investigations as a result of incidental findings. The reliability of imaging studies depends may be confused and/or restless. on careful correlation with the clinical picture. The main drawbacks of CT are the use of ionising radiation and the potential risks of intravenous contrast agents when required (contrast-induced nephropathy and allergic reactions). Soft tissue resolution of CT is not as good as that of MRI1 and images of the posterior fossa undergo a greater degree of degradation because of artefact from adjacent bony structures. Requesting CT scans CT is the modality of first choice in acute stroke because of ready availability and quick acquisition time (Table 1). Its primary role in the setting of acute stroke is the exclusion of intracranial haemorrhage and stroke mimics, such as tumours. In hyperacute ischaemic stroke, CT may be negative for the first few hours. Early signs of brain infarction are detected on a non-contrast CT in up to 61% of cases within six hours. The sensitivity increases substantially after 24 hours.2 CT is also the modality of choice in trauma. It is very sensitive for acute haemorrhage, skull fractures and spinal fractures. In spinal imaging, CT is useful in trauma, atypical back and neck pain when aggressive pathology is suspected, and in back and neck pain with associated radicular symptoms. 788 AFP VOL.45, NO.11, NOVEMBER 2016 © The Royal Australian College of General Practitioners 2016  NEUROIMAGING FOCUS Table 1. Common indications for computed tomography The Royal Australian and New Zealand College of brain and spine Radiologists (RANZCR) recently changed its guidelines on the administration of intravenous contrast. It has lowered the Brain Spine estimated glomerular filtration rate (eGFR) threshold for which Stroke Trauma no special precautions are required prior to administration of Trauma Back and neck pain with radicular intravenous contrast. The guidelines also recommend that symptoms emergency radiological examinations that require intravenous Thunderclap headache Back and neck pain with sinister features contrast administration should not be delayed in order to obtain renal function tests prior to the procedure.3 The latter Acute headache Spinal infection when magnetic resonance imaging or nuclear medicine are not includes tests such as CT angiography and perfusion in the available setting of acute stroke. Despite the potentially high doses of intravenous contrast required if both tests are performed Atypical headache as part of an acute stroke workup, it is deemed that in the Delirium appropriate clinical setting the benefits outweigh the small Coma risk of acute renal injury. Ventriculo-peritoneal shunt assessment Case 1 A previously fit and well woman, 28 years of age, presented to her general practitioner (GP) with a one-day history of vertigo and nystagmus. The GP suspected vestibular Contrast or non-contrast study neuronitis, but wanted to exclude a central cause. Following Non-contrast studies of the brain and spine are sufficient for a phone discussion with the radiologist, the GP decided to most indications. Diagnosis of common conditions such as investigate the brain with CT because MRI was not readily cerebral infarction and haemorrhage can be made without available. intravenous contrast. A plain brain CT showed a 15 mm acute parenchymal A contrast-enhanced CT of the brain is of value in specific haemorrhage of the left middle cerebellar peduncle (Figure 1) situations (eg known CNS tumour, investigation for potential with no significant associated oedema or mass effect. In intracranial metastases, CNS infection). In addition, the imaging view of the patient’s age and the atypical location of the team may detect an unexpected abnormality on the plain scan haemorrhage, CT angiography was performed. This showed at the time of scanning that warrants contrast administration. an arteriovenous malformation that was thought to be CT neck and cerebral angiography is a specific contrast- supplied by the posterior inferior cerebellar artery and drained enhanced technique that is valuable in the assessment of by the inferior vermian vein. intracranial and neck vessels. The most common applications The patient remained stable and was transferred to the are in the assessment of patients with acute stroke who are neurosurgical centre for further management. potentially eligible for thrombolysis or mechanical clot retrieval, assessment of the extracranial carotid arteries in place of Doppler ultrasound or magnetic resonance angiography, and in Magnetic resonance imaging subarachnoid haemorrhage. Advantages and disadvantages Spinal CT imaging is usually performed without contrast. This is The main strengths of MRI are its superior soft tissue adequate for the assessment of fractures, degenerative disease, contrast and lack of ionising radiation. The images are not acute disc herniation, metastatic lesions, large epidural soft tissue affected by adjacent bone structures as much as they are with lesions and established infection (eg discitis, osteomyelitis). Spinal CT, allowing better visualisation of the posterior fossa. MRI cord detail is poor on CT; this is much better assessed with MRI. provides exquisite detail of the spinal cord that CT cannot approximate. Although CT is better at depicting abnormalities Contrast and renal dysfunction of cortical bone, MRI is better at assessing bone marrow. MRI In the past decade, a substantial amount of new data has also finds wider applications in functional imaging. emerged questioning the causal nature of the relationship In contrast to CT, there are numerous available MRI between intravenous iodinated contrast and clinically significant sequences, each emphasising soft tissue contrast in a unique acute renal dysfunction (contrast-induced nephropathy). New manner. However, only a limited number of sequences evidence suggests that intravenous contrast is safer than are acquired as part of one study. The sequences used will previously thought in patients with mild to moderate renal depend on the body part being imaged, the clinical questions/ dysfunction.3 expected pathology and patient compliance. © The Royal Australian College of General Practitioners 2016 AFP VOL.45, NO.11, NOVEMBER 2016 789  FOCUS NEUROIMAGING The main drawbacks of MRI include limited availability and Cervical radiculopathy long acquisition time. The images are thus more prone to MRI is of value when conservative treatment has failed and movement artefact. Patient claustrophobia is also a greater intervention is being considered (spinal steroid injection or issue with MRI. Given the use of a strong magnet, metallic surgery). Because cervical degenerative changes are common surgical implants may either be completely contraindicated or and often asymptomatic, the reliability of MRI depends on may result in artefact that degrades image quality. Great care careful correlation with the clinical findings. is required in monitoring equipment and other objects taken into the scanning room as any ferromagnetic objects may Trauma become dangerous projectiles once close to the magnetic In cervical trauma, CT is superior to MRI and plain radiographs in field. identifying cervical spine fractures and is more readily available. A further potential disadvantage of MRI is that its higher MRI is superior in identifying soft tissue injuries (particularly sensitivity to pathology may result in higher rates of detecting spinal cord and ligaments). However, the identified abnormalities incidental abnormalities of no clinical significance. This has the may not all be clinically significant. potential to generate further patient anxiety4 and unnecessary follow-up investigations. This is also an issue with CT, but to a Chronic headache lesser degree. MRI is indicated for persistent/atypical headache or migraine when there is a change of neurological status. In most headache Requesting MRI syndromes, there is a lack of evidence to suggest that imaging Medicare Benefits Schedule (MBS) requesting rights of GPs improves health outcomes. Imaging is thus not indicated were expanded in 2013 to allow the request of MRI studies in patients with primary headache disorder6,7 (eg migraine, of the head and cervical spine (as well as of the knee) for tension-type, cluster headaches). In a meta-analysis of multiple specific clinical indications in patients aged 16 years and older. studies looking at imaging in patients with migraines and a The permissible indications for head MRI are unexplained normal neurological examination, the summary prevalence of seizure(s) and chronic headache with suspected intracranial significant intracranial abnormalities (eg aneurysm, arteriovenous pathology. For cervical spine MRI, the indications are cervical malformation, tumour) was 0.2% with an upper 95% confidence radiculopathy and trauma.5 interval (CI) limit of 0.6%.8 Potential ‘beneficial’ alleviation of anxiety from a negative scan was shown to be a non-sustained effect in one study, being present at three months but not maintained after a year.9 Seizures MRI is the imaging modality of choice in non-urgent assessment of apparently unprovoked seizures. In emergency situations, CT is recommended because of its accessibility and speed. Other neurological conditions MRI is the imaging modality of choice for several other neurological conditions, including multiple sclerosis, brain and spinal cord tumours, stroke when the diagnosis is not clear on CT, and dementia. In suspected cases of dementia, MRI and CT are equally useful in excluding other intracranial pathologies that may cause cognitive impairment (eg tumours, haematomas).10 MRI is more sensitive in detecting changes that can help with dementia subtype diagnosis (eg early subcortical vascular changes, differential regional atrophy patterns in Alzheimer’s disease, Figure 1. Plain CT brain and CT angiogram demonstrating an AVM normal pressure hydrocephalus). However, this requires specialist A, B. Axial and coronal plain CT of the brain showing an acute parenchymal input for appropriate correlation of clinical and radiological haematoma of the left cerebellar peduncle findings as there is often overlap in the clinical and radiological C–E. Axial, coronal and sagittal post-contrast angiography images showing the findings of the dementia subtypes. For example, normal pressure nidus of an underlying AVM AVM, arteriovenous malformation; CT, computed tomography hydrocephalus is a relatively rare condition (incidence ranges from 2–20 per million per year),11 but is often overcalled on imaging 790 AFP VOL.45, NO.11, NOVEMBER 2016 © The Royal Australian College of General Practitioners 2016  NEUROIMAGING FOCUS studies because other conditions such as atypical Alzheimer’s disease and atypical frontotemporal dementia can mimic the radiological findings. Case 2 A man, 40 years of age, with a history of stable pulmonary sarcoidosis, presented to his GP with a two-week history of progressively worsening headache and visual disturbance. He denied fevers, seizures and nausea/vomiting, but on examination the GP suspected papilledema. Blood tests showed moderate leucocytosis and serum C-reactive protein was also elevated. In view of the new onset nature of the headache, associated visual disturbance and suspicious fundoscopy, the patient was referred for urgent CT. Diffuse low attenuation change in the left occipital lobe was seen on the plain CT scan. Post-contrast scans confirmed a solitary rim-enhancing lesion of the left occipital lobe with white matter oedema and mild mass effect leading to mild Figure 2. CT and MRI brain in cerebral abscess displacement and distortion of the occipital horn of the left A. Axial post-contrast CT brain showing a ring enhancing lesion (arrow) of the lateral ventricle (Figure 2). left occipital lobe and white matter oedema (star) At this point, the main differential diagnosis of this lesion B. Axial T2-weighted MRI sequence showing the lesion, which is was thought to include cerebral abscess, high-grade glioma hyperintense (bright) centrally (blue dot) C. Axial FLAIR MRI sequence better demonstrating the surrounding oedema and solitary cerebral metastasis (although there was no (star) known primary malignancy). To try to differentiate tumour D. Axial T1 post-contrast MRI demonstrating rim enhancement (arrows) but from abscess, an MRI was performed. The diffusion-weighted no central enhancement (blue dot) E, F. Axial diffusion-weighted ADC map showing restricted diffusion in the sequence showed striking restricted diffusion throughout the lesion (blue dots; bright on diffusion-weighted, dark on ADC map) centre of the lesion most compatible with an abscess in this ADC, apparent diffusion coefficient; CT, computed tomography; FLAIR, fluid- clinical setting. The patient was urgently transferred to the attenuated inversion recovery; MRI, magnetic resonance imaging neurosurgical centre for further management. Further illustrative cases are shown of various common pathologies as seen on MRI (Figure 3) and CT (Figure 4). Nuclear medicine PET is a nuclear medicine molecular imaging technique that consistently maps and quantifies the distribution of metabolic changes in the brain using various radioisotopes, most commonly 18-fluorodeoxyglucose (18FDG). It finds application in various neurological conditions, including identifying epileptogenic foci in intractable seizures and the differential diagnosis of neurodegenerative disease that cause dementia but have differing patterns of hypometabolism.12 Single photon emission computed tomography (SPECT) uses gamma ray–emitting radioisotopes incorporated into tracers that are rapidly taken up by the brain, but do not redistribute, and Figure 3. Various MRI images showing normal anatomy and pathology. reflect cerebral blood flow at the time of injection. It can aid in A. Normal brain axial T2 image the diagnosis and ongoing evaluation of multiple neurological B. Normal brain sagittal T1 image conditions including (but not limited to) cerebrovascular disease, C. L5/S1 disc protrusion; sagittal T1 and T2 images D. Multiple sclerosis, axial FLAIR image dementia, epilepsy and traumatic brain injury. E. Cerebral metastases axial T1 post-contrast FLAIR, fluid-attenuated inversion recovery; MRI, magnetic resonance imaging Key points CT and MRI are the workhorses of routine clinical neuroimaging. Given its wider availability and quicker © The Royal Australian College of General Practitioners 2016 AFP VOL.45, NO.11, NOVEMBER 2016 791  FOCUS NEUROIMAGING 4. Jagadeesh H, Bernstein M. Patients’ anxiety around incidental brain tumours: A qualitative study. Acta Neurochirugica 2014;156(2):375–81. 5. The Royal Australian College of General Practitioners. Clinical guidance for MRI referral. Melbourne: RACGP, 2013. Available at www.racgp.org.au/your- practice/guidelines/mri-referral [Accessed 11 October 2016]. 6. Royal Australian and New Zealand College of Radiologists. Headache imaging – Guidance for GP referrals for MRI studies. Sydney: RANZCR, 2013. 7. Sempere AP, Porta-Etessam J, Medrano V, et al. Neuroimaging in the evaluation of patients with non-acute headache. Cephalgia 2005;25(1):30–35. 8. Neff MJ. Evidence-based guideline for neuroimaging in patients with nonacute headache. Am Fam Physician 2005;71(6):1219–22. 9. Scottish Intercollegiate Guidelines Network. Diagnosis and management of headache in adults. A national clinical guide. Edinburgh: SIGN, 2008. 10. Health Quality Ontario. The appropriate use of neuroimaging in the diagnostic work-up of dementia: An evidence-based analysis. Ont Health Technol Assess Ser 2014;14(1):1–64. 11. Klassen BT, Ahlskog JE. Normal pressure hydrocephalus: How often does the diagnosis hold water? Neurology 2011;77(12):1119–25. 12. Nazrallah I, Dubroff J. An overview of PET neuroimaging. Semin Nucl Med 2013;43(6):449–61. Figure 4. Various intracranial pathologies on CT A. Established left MCA infarct B. Intracerebral haemorrhage C. subarachnoid haemorrhage D. Acute left epidural haematoma E. Acute left subdural haematoma Note brain swelling and early dilatation of temporal horns of lateral ventricles in association with subarachnoid haemorrhage and various degrees of mass effect on all other images. CT, computed tomography; MCA, middle cerebral artery acquisition time, CT is used more frequently in the initial evaluation of most neurological emergencies. MRI has the advantages of not using ionising radiation and providing greater soft tissue resolution. It is vital to include relevant clinical information on the request form to ensure the most useful images are acquired (especially important for MRI) and are interpreted appropriately. In complex cases, discussion with the radiologist prior to imaging is particularly helpful. Imaging is not indicated in straightforward primary headache syndromes. Author Julius Tamangani MBChB, FRCR, FRANZCR, Consultant Radiologist, Bendigo Healthcare, Bendigo, Vic. [email protected] Competing interests: None. Provenance and peer review: Commissioned, externally peer reviewed. References 1. Paty DW, Oger JJ, Kastrukoff LF, et al. MRI in the diagnosis of MS: A prospective study with comparison of clinical evaluation, evoked potentials, oligoclonal banding, and CT. Neurology 1988;38(2):180–85. 2. Wardlaw JM, Mielke O. Early signs of brain infarction at CT: Observer reliability and outcome after thrombolytic treatment – A systematic review. Radiology 2005;235(2):444–53. 3. The Royal Australian and New Zealand College of Radiologists. Iodinated contrast media guideline. Sydney: RANZCR, 2016. 792 AFP VOL.45, NO.11, NOVEMBER 2016 © The Royal Australian College of General Practitioners 2016