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Postgrad Med J 1996; 72: 386-390 (© The Fellowship of Postgraduate Medicine, 1996

New techniques

Intracranial imaging

M Gibson, G Cook, A Al-Kutoubi

SummaryThis article concentrates on theimaging of intracranial structuresand outlines some basic imagingstrategies for common clinicalpresentations.

Keywords: intracranial structures, imaging

Imaging modalities inneuroradiology

* plain X-rays* computed tomography* magnetic resonance imaging* angiography* nuclear medicine* ultrasound

Figure 1 MRA of the carotid arteriesqpi

There is an area of signal fall out (shortstraight arrow) in the left carotid bulbindicating a stenosis. The normal rightinteral carotid artery (ong arrow) and leftvertebral artery (short curved arrow) areshown

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* : .. LWW E s sseSsesssSeSsb s,,, ,: . ...~~~~~~~~~~~~~~~~............. ...~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.....: ??W, ...................:Figure 2 Intra-arterial DSA of left carotidartery, lateral view. There is a coflection ofabnormal vessels (arrow) typical of an arter-iovenous malformnation

Department of Radiology, St Mary'sHospital, London W2, UKM GibsonA Al-Kutoubi

Department of Nuclear Medicine,Guys Hospital, London, UK

G~~~ ~Cook _ _

Acepe 16 October_1995

Imaging modalities

The various imaging modalities used in neuroradiology (shown in the box) arediscussed below.

PLAIN RADIOGRAPHYThe plain skull X-ray has a much diminished role in neuroradiology. Even intrauma its value is limited and should be restricted to specific indications, suchas those outlined in the Harrogate criteria; loss of consciousness, amnesia,neurological signs, cerebrospinal fluid (CSF) from nose to ears, penetratinginjury, alcohol intoxication and difficulty in assessing the patient.' Even then itshould be used in conjunction with computed tomography (CT) scanning.

COMPUTED TOMOGRAPHY 4

The introduction ofCT in the early 1980s revolutionised neuroradiology. Sincethat time it has become increasingly available and is now almost essential for adistrict general hospital. The head is usually scanned in contiguous 10-mmslices from vertex to posterior arch of the atlas using a small X-ray source and arotating detector. This protocol is often modified, depending on the area ofinterest, with thinner slices or different planes. Intravenous contrast media canbe administered to improve visualisation of lesions and blood vessels.

MAGNETIC RESONANCE IMAGINGIn many areas of neuroradiology magnetic resonance imaging (MRI) hasreplaced CT as the cross-sectional imaging modality of choice. At present it isless widely available. The physics ofMRI are complex but usually Ti-weightedimages (useful for delineating anatomy) and T2-weighted images (useful fordetecting pathology) are obtained. These can be displayed in any planealthough usually orthogonal planes (sagittal, coronal and axial) are used. Thesignal from different tissues depends on the weighting, for instance, water is lowsignal (black) on Ti and high signal (white) on T2. Contraindications to MRIinclude ferromagnetic intracranial aneurysm clips, cochlear implants, ocularforeign bodies and cardiac pacemakers. Contrast media are available for MRIand have the same properties as conventional X-ray contrast media (ie, they aretaken up by pathologies that disrupt the blood-brain barrier). The indicationsfor contrast media are not well defined. In general, they do not increase theyield of detected lesions (as the tissue contrast in unenhanced MRI is alreadygood) but do help clarify abnormal or equivocal lesions and outline the vessels.MRI has the advantage over CT that it does not utilise ionising radiation.

Magnetic resonance angiography (MRA) can be performed on the samescanner as MRI, although special software is required. The physical propertyof the flowing blood, rather than contrast media, is used to generate images. Ithas been used to evaluate the carotid vessels (figure 1). It is also used toimage the intracranial vasculature although, as yet, the resolution is not goodenough to replace angiography. Future advances in technology may wellachieve this.At present magnetic resonance spectroscopy (MRS) and functional MRI

(fMRI) are little used in clinical practice. MRS offers an insight intoneurometabolism and will complement morphological information. fMRI is atechnique that correlates function and anatomy and both modalities have greatpotential.

ANGIOGRAPHYThe use of angiography in neuroradiology has diminished since the advent ofCT and MRI. However, it does still have a role. It can be performed by anintravenous injection (poor contrast resolution but very safe) or intra-arterially(with a 1-2% risk of permanent neurological damage but excellent resolu-tion). Digital subtraction angiography (DSA) is useful for evaluating thevessels of the neck and intracerebral arteriovenous malformations (figure 2) or

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Intracranial imaging 387

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Figure 3 Entraarterial DSA of right caro-tid, lateral view. There is a dense area ofcontrast that is typical of an aneurysm(arrow)

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Figure 4 Doppler ultrasound of the carotidbifurcation. A plaque of atheroma (arrow)projects into the lumen of the intenal carotidartery at its origin causing a stenosis

aneurysms (figure 3). It is the only approach for therapy, for instance, toembolise arteriovenous malformations. More recently angioplasty andendovascular stenting of carotid stenoses has been developed.

NUCLEAR MEDICINEConventional, planar, nuclear medicine brain scans have been replaced by MRIand CT. However, scintigraphic cisternography remains a valuable functionaltechnique in differentiating normal pressure hydrocephalus from other typesand is a sensitive method for locating CSF leaks.Advances in single photon emission computed tomography (SPECT) and

positron emission tomography (PET) have lead to these becoming importantmodalities in neuro-imaging. SPECT uses compounds labelled with conven-tional radioisotopes such as Technetium-99m (99mTc) or Idoine-123 (123I).The gamma camera rotates around the patient. Axial, sagittal and coronalimages can be produced from the reconstructed data. The commonest agentused is 99mTc-hexamethyl propyleneamine oxime (HMPAO). This lipophilicsubstance is taken up and trapped in the brain in proportion to blood flow andso images produced with this radiopharmaceutical represent regional cerebralblood flow, ie, a functional map rather than an anatomical image.PET scanning makes use of short-lived, cyclotron-produced radionuclides

(eg, "8F, 'IC, 150). The PET scanner detects paired annihilation photonsemitted at 180 degrees to each other, to form tomographic images in any plane.As with SPECT, functional images can be produced, such as glucosemetabolism (18F deoxyglucose), amino acid metabolism ("IC methionine) andblood flow (H2 'S0). The advantages ofPET over SPECT include better spatialresolution, its use of organic substances rather than analogues and its ability toquantitate function accurately. There is limited availability, however, due to thehigh capital costs in setting up a cyclotron unit.

ULTRASOUNDUltrasound can be used to image the brains of neonates with patent fontanellesto detect hydrocephalus, haemorrhage or other structural abnormalities. In theolder child or adult the skull prevents the ultrasound beam from reaching thebrain. However, it can be used to image the arteries of the neck, particularly todetect stenoses of the carotid arteries (figure 4). This technique is operatordependent but when quality control is sufficiently high, some surgeons willoperate on ultrasound findings alone. Alternatively it is combined withangiography or used as a screening tool.

Figure 5 CT scan of the brain. A large lowattenuation area in the territory of the middlecerebral artery indicates a infarct (arrow)

tEf~~~~~~Figure 6 Intra-arterial DSA of the carotidbifurcation. There is a very tight stenosis atthe origin of the internal carotid artery(arrow)

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Figure7Intravenous DSA of the origin of~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.......

thegreat vessels. The normal great.......eseenintheupper chest and neck~~~~~~2=


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388 Gibson, Cook, Al-Kutoubi

Figure 8 MRI of the brain, T -weightedimages post contrast. There are large areas ofabnormal enhancement (arrows) in the leftcerebral hemisphere due to herpes encepha-litis Figure 9 CT scan of the brain The scan

shows fresh subarachnoid blood in theSylvian fissures (arrows) mn a patient present-ing with sudden onset of severe occipitalheadache

Figure 10 CT scan of the brain. A patientpresents with increasing confusion severalweeks after a minor head trauma. The scanshows an acute on chromic subdural haema-toma. Fresh blood is shown as hyperdense(curved arrow) and the chronic collection isshown as isodense (straight arrow)

...

........ ................

Figure 11 99mTc-HMPAO-SPECT trans-axial images. An elderly woman with demen-tia. The cortex is generally thinned in keepingwith generalised atrophy and there are addi-tional focal areas ofreduced uptake bilaterallyand nonsymmetrically, consistent with amulti-infarct aetiology

Imaging strategies

The basic strategies set out below are designed to encompass some commonclinical presentations. They represent our opinion and are not intended to bedogmatic. Obviously their use depends on local availability and local clinicalopinion. Clinical presentations can be divided into chronic and acute.

ACUTE PRESENTATIONS

Established neurological deficit (ie, stroke persisting longer than 24 h)CT or MRI are the modalities of choice. In early infarction MRI is moresensitive than CT. However CT does exclude most other important,particularly surgical, causes. Three days after the infarction CT reveals thediagnosis in 80% of cases (figure 5). Both CT and MRI can accurately detectintracerebral haemorrhage. This is important as the management of haemor-rhage is different to that ofnonhaemorrhagic infarction where aspirin or heparinmay be used. For suspected cerebellar or brainstem lesions MRI is preferred toCT as the latter suffers from artefact due to the adjacent petrous bone whichobscures much of this region.

Transient neurological deficitTransient neurological deficits include transient ischaemic attacks (TIAs)lasting less than 24 h or reversible ischaemic neurological disability. Usually CTor MRI is performed to identify any infarction, although in TIAs they are oftennormal. Assessment of the arteries of the neck is undertaken to identify carotidstenoses which, if treated surgically, will improve the prognosis.2 This can bedone with duplex ultrasound scanning (dependent on operator skill and often asa complimentary or screening procedure rather than a definitive investigation).Intra-arterial angiography provides high resolution images but carries a 1 -2%risk of permanent neurological deficit (figure 6). Intravenous angiography issafer and in most patients adequate diagnostic images are obtained (figure 7).Some patients however will need intra-arterial studies.3 MRA can provide clearimages of the neck vessels. It has drawbacks such as overestimation of stenoses,artefacts and long imaging times. However, technical advances make this apromising noninvasive investigation for the near future.

Acute loss of consciousnessAgain neither CT or MRI are usually used. CT can be (and usually is) negativein encephalitis or early infarction. MRI is more sensitive than CT but less


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Intracranial imaging 389

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Figure 13 Study 1: "1C-methionine. Study2: 18FDG-PET brain scans. A patient with ahistory of astrocytoma ofthe right frontal loberesected surgically with possible recurrence.CT was unable to differentiate postsurgicaland radiotherapy changes from recurrence.Both "C-methionine and 18FDG scans showincreased uptake into an area of concurrenttumour (bright areas). High 18FDG uptakeprobably means that this is a high-gradetumour. Some low-grade tumours showreduced 18FDG metabolism in comparisonto normal cortex. A right frontal defect isseen, consistent with previous surgery

accessible (figure 8). It also takes longer, makes monitoring the patient difficultand is more dependent on the patient lying still.

Severe headacheSubarachnoid haemorrhage needs to be confirmed or refuted. CT will usuallyshow subarachnoid blood (figure 9), although if the clinical suspicion is highand the CT scan negative then a lumbar puncture should be performed. Thetiming of the CT depends on whether the local neurosurgeon requires urgentinvestigations to precede urgent operation. Angiography is essential beforesurgery for subarachnoid haemorrhage to identify aneurysms and arteriovenousmalformations. It is important to localise the site of the bleeding and to detectany synchronous intracranial aneurysms. Spiral CT and MRA are also beingused to detect intracranial aneurysms.

Head injuryThe indications for skull X-rays are given above. A cervical spine X-ray can bedifficult to obtain but can be vital in trauma to detect associated spine fractures.Further imaging is required if the level of consciousness or neurological deficitsdoes not improve after resuscitation. CT is often used as it is available and easy,when the patient is uncooperative, or in need of close observation. Although it isless sensitive than MRI it detects the majority of lesions that require urgentsurgical intervention (ie, subdural and extradural haematomas) (figure 10).

CHRONIC PRESENTATIONS

Chronic headacheThis is a common clinical problem and most causes do not require imaging (ie,migraine). However, the presence of atypical or other suspicious features canwarrant imaging. The choice between MRI and CT depends on localavailability.

EpilepsyImaging in epilepsy is often negative. However, recently interest has focused onthe use of MRI in intractable temporal lobe epilepsy. The coronal imagesthrough the temporal lobes allow inspection of the hippocampus. Lesions suchas small gliomas can be detected. Alteration in signal or size ofthe hippocampusmay indicate mesial temporal sclerosis. Both conditions are amenable tosurgery. This can lead to improvement or even cure of the epilepsy.4 PETscanning with 18fluorodeoxyglucose (FDG) is the most sensitive method ofdetecting epileptogenic foci. These cause hypometabolic defects on theinterictal scans or hypermetabolic foci during an ictus. Concurrent electro-encephalographic monitoring is therefore useful. SPECT is also able to locatethese foci due to their reduced blood flow interictally.

DementiaImaging seldom contributes to the investigation of dementia. If clinical featuresare suggestive of a chronic subdural haematoma then this can be detected byeither CT or MRI. Both 99mTc-HMPAO-SPECT and '8FDG-PET candifferentiate between the various types of dementia (figure 11).

Neurological impairment from a cerebral causeBoth CT and MRI can reveal the conditions that may require surgical


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390 Gibson, Cook, Al-Kutoubi

Figure 14 MRI of the brain, sagittal Ti-weighted scan. The scans shows a largetumour occupying and expanding the pons(arrow). The size and position of this tumourmade it unresectable. Treatment was withpalliative radiotherapy

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intervention (ie, primary or secondary tumours, abscesses or chronic subduralhaematomas). Other nonsurgical lesions such as old infarctions or demyelina-tion can also be detected. MRI is more sensitive that CT in detectingdemyelination in both the brain and spinal cord (figure 12). The functionalnature of PET allows differentiation of recurrent or residual tumour fromreactive gliosis secondary to radiotherapy (figure 13). It is the most sensitivemodality in this situation.

Visual disturbanceUsually the site of the abnormality can be determined clinically. MRI is usefulfor imaging the visual pathway from the orbit to the visual cortex.

Cranial nerve palsiesMRI is used to image the brianstem (figure 14) and exit regions of most cranialnerves. The more peripheral lesions are difficult to detect. Investigation oftinnitus, vertigo or sensorineural hearing loss is usually directed towards thedetection of acoustic neuromas. This can be done with contrast-enhanced CTbut many centres are switching to MRI due to its higher sensitivity.

Endocrine abnormalitiesThese are usually investigated with blood tests. However, detection of apituitary adenoma is important, especially if surgery is a possibility. This can beachieved with either MRI or CT (figure 15). The former is preferred, especiallyfor follow-up to avoid exposure to ionising radiation.

1 Harrogate seminar report. The management ofacute head injury London: Department of Healthand Social Security, 1983; no 8.

2 North American Symptomatic EndarterectomyTrial collaborators. Beneficial effect of carotidendarterectomy in symptomatic patients withhigh grade carotid stenosis. N EnglJ Med 1991;325: 445-53.

3 Stevens JM, Barter S, Kerslake R, Schneidau A,Barber C, Thomas DJ. Relative safety of IVDSA over other methods of carotid angiographyand impact on clinical management of cerebro-vascular disease. Br J Radiol 1989; 62: 813- 6.

4 Jack CR. Epilepsy - surgery and imaging.Radiology 1993; 189: 635-46.


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