optic spinecho · Meningiomasinvolvingthe optic nerve Meningiomas may arise directly from the orbital dural sheath orthe optic nerve maybe compressed by tumour arising intracranially

56ournal ofNeurology, Neurosurgery, and Psychiatry 1995;58:562-569

High resolution magnetic resonance imaging ofthe anterior visual pathway in patients with optic

neuropathies using fast spin echo and phasedarray local coils

A Gass, G J Barker, D MacManus, M Sanders, P Riordan-Eva, P S Tofts, J Thorpe,W I McDonald, I F Moseley, D H Miller

NMR Research Group,Institute ofNeurology,National Hospital forNeurology andNeurosurgery, QueenSquare, London, UKA GassG J BarkerD MacManusP S ToftsJ ThorpeW I McDonaldD H MillerLyshohm Departmentof Radiology, NationalHospital for Neurologyand Neurosurgery,Queen Square,London, UKI F MoseleyDepartnent ofNeuro-Ophthahmology,National Hospital forNeurology andNeurosurgery, QueenSquare, London, UKM SandersP Riordan-EvaMoorfields EyeHospital, City Road,London, UKW I McDonaldI F MoseleyCorrespondence to:Dr DH Miller, NMRResearch Group, Institute ofNeurology, NationalHospital, Queen Square,London WC IN 3BG, UK.

Received 14 September 1994and in revised form5 January 1995.Accepted 19 January 1995

AbstractHigh resolution MRI of the anteriorvisual pathways was evaluated using fre-quency selective fat suppressed fast spinecho (FSE) sequences in conjunctionwith phased array local coils in patientswith optic neuropathies. Fifteen normalcontrols and 57 patients were examined.Coronal T2 weighted fat suppressed FSEimages were obtained in 11 minutes withan in plane resolution of O039 x 0O39 mm.The optic nerve and its sheath containingCSF were clearly differentiated. Centralretinal vessels were often visible. Indemyelinating optic neuritis and in ante-rior ischaemic optic neuropathy high sig-nal within the nerve was readilydelineated. Meningiomas and gliomasinvolving the optic nerve were preciselyvisualised both in the orbit and intra-cranially. Extrinsic compression of theoptic nerves was readily visualised incarotid artery ectasia and dysthyroid eyedisease. Enlarged subarachnoid spacesaround the optic nerves were demon-strated in benign intracranial hyper-tension. High resolution MRI of theanterior visual pathway represents an

advance in the diagnosis and manage-ment of patients presenting with opticneuropathy.

(3NeurolNeurosurg Psychiatry 1995;58:562-569)

Key words: magnetic resonance imaging; fast spinecho; fat suppression; phased array coil; optic neuro-

pathy

Each optic nerve consists of about 1 millionmyelinated nerve fibres; the optic nerve-

sheath complex measures 4 to 6 mm in diam-eter and its length from the globe to the opticchiasm varies between 45 and 55 mm.'

Successful imaging of the optic nerve is diffi-cult as it is a small, often tortuous, and mobilestructure surrounded by fat within the orbit,by bone within the optic canal, and by CSF.Intracranially it lies very close to the carotidarteries. There are many causes of optic neu-

ropathy, and optimal management is depen-dent on making a correct diagnosis.Neuroradiological procedures are especiallyimportant in the diagnosis of extrinsic com-

pressive lesions and primary tumours of theoptic nerve; CT has been the preferred investi-gation for orbital pathology as conventionalMBI sequences are impeded by high signalfrom orbital fat. Magnetic resonance imagingis more effective in the region of the opticcanal and the intracranial portion of the nervewhere bone hardening artefacts limit the valueof CT. Even the combination of high resolu-tion CT and conventional MRI, however, isnot always satisfactory, and subtle lesionsexerting little or no mass effect are not infre-quently missed.2 Short tau inversion recovery(STIR) and frequency selective fat suppressedspin echo sequences have proved useful inreducing orbital fat signal, but only relativelylow resolution images are possible within clin-ically acceptable time constraints.3-7 Using aconventional surface coil placed in front of theeyes, signal to noise ratio (SNR) is improvedwithin the orbit, but decreased in the opticchiasm and adjacent regions.8 9 Two recenttechnical developments, however-phasedarray local coils and the fast spin echo (FSE)sequence-now make it possible to obtainhigh resolution images of the anterior visualpathways in an acceptable time.'0-'3We reportour findings of the use of a combination of fatsuppressed FSE and phased array coils in an"optimised" imaging protocol for the anteriorvisual pathways. The advantages and poten-tial of such an imaging approach are outlined.

Materials and methodsMAGNETIC RESONANCE IMAGINGImaging was performed on a 1-5T Signa(General Electric, Milwaukee, USA) MRIunit. All subjects were studied in the coronalplane using an "optimised" (see appendix)mildly T2 weighted FSE sequence with fre-quency selective fat suppression with a pair of3 inch circular coils.5 6 Imaging parameterswere: TR3250/Tef68, ETL 16, NEX = 6, fieldof view 20 cm, 512 x 512 matrix, rectangularfield of view, 3 mm interleaved contiguousslices, 11 minutes. In 28 patients fat sup-pressed Ti weighted spin echo or gradientecho sequences were used for imaging beforeand after injection of gadolinium-DTPA(0 1 mmol/kg), using a field of view of16-20 cm and a matrix of 256 x 256. In sixpatients with optic nerve tumours additional

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High resolution MRI of the anterior visual pathway in patients with optic neuropathies usingfast spin echo and phased array local coils

axial images were obtained using the samesequences.The phased array local coils were posi-

tioned symmetrically over the temples, suchthat optimum proximity to the optic nerveswas obtained. Patients were asked to avoidexcessive eye movement and to keep theireyes closed to avoid globe motion artefact.After axial localising scans at the level of theoptic nerve entering the globe, coronal sliceswere adjusted to the angle needed for sym-metric imaging of the two nerves.

All films were reviewed by an experiencedneuroradiologist (IFM). In normal controls,patients with benign intracranial hyper-tension, optic neuritis, optic nerve sheathmeningiomas, and optic nerve gliomas, struc-tured reporting schemes were used. The scansof patients with benign intracranial hyper-tension, optic neuritis, anterior ischaemicoptic neuropathy, and of normal controlswere mixed and reported by IFM withoutknowledge of the clinical details.

PATIENTS AND CONTROLSWe studied 57 patients with optic neuro-pathies and 15 normal controls. Patients wererecruited from the National Hospitals ofNeurology and Neurosurgery and the physi-cians' clinic and orbital clinic of MoorfieldsEye Hospital. Informed consent was obtainedin writing from all subjects. The research pro-tocol had prior approval of the NationalHospital's joint medical ethics committee.

Normal controlsFifteen normal controls (age range 20-41)were studied.

Optic neuritisFourteen patients (age range 19-41) with aclinical diagnosis of demyelinating optic neu-ritis, supported by abnormal visual evokedpotentials, were studied after three days to sixmonths (mean 2-4 months) of visual loss.'4

Anterior ischaemic optic neuropathyFour patients with the clinical diagnosis ofanterior ischaemic optic neuropathy werestudied within four weeks of the onset ofsymptoms and again after six months.'5

Benign intracranial hypertensionBenign intracranial hypertension is a condi-tion of obscure aetiology; patients wereincluded in this category if they had increasedintracranial pressure, papilloedema, and nor-mal CSF constituents in the absence of anintracranial mass or hydrocephalus; 13patients with benign intracranial hypertensionwere examined.'6

Meningiomas involving the optic nerveMeningiomas may arise directly from theorbital dural sheath or the optic nerve may becompressed by tumour arising intracraniallyfrom the clinoid or tuberculum sellae."7 18 Westudied 12 patients with meningiomas involv-ing the optic nerve. Histological confirmationwas available in six patients.

Optic nerve gliomaOptic nerve gliomas often have a benign histo-logical appearance and although they mayoccur at any age, most patients present in thefirst decade.'8 '9We studied five patients withoptic nerve gliomas. Histological confirmationwas available in two patients.

Central retinal artery occlusionTwo patients with sudden onset visual fielddefects due to central retinal artery occlusionwere studied within four weeks of visual lossand again after six months.20

Leber's hereditary optic neuropathyTwo patients with visual loss due to Leber'shereditary optic neuropathy, confirmed bygenetic testing, were studied 11 months andthree years after onset of visual loss.21-23

Dysthyroid eye diseaseWe studied two patients with symptomaticdysthyroid eye disease.2425

Carotid ectasiaOne patient with optic nerve compression dueto ectatic carotid arteries was studied.26

Tobacco-alcohol neuropathyTwo patients with a clinical diagnosis oftobacco-alcohol amblyopia were studied.2728

ResultsLocal coil positioning was well tolerated by allsubjects. Motion artifact from globe move-ment was minimal and image quality was con-sistently good. Fat suppression was achievedin all patients without major artefact or varia-tion of contrast.

Normal controlsVisualisation of the optic nerve and its inter-nal structure was provided. The differentia-tion of the nerve from surrounding CSFwithin its dural sheath was possible in theanterior orbit in all studies. The width of theoptic nerve sheath was symmetric in normalcontrols but varied between subjects withCSF visible on two to five slices in the orbit. Itnarrowed progressively towards the orbitalapex with no CSF signal seen in the opticcanal. None of the controls showed changesin morphology or signal suggesting a lesion inthe optic nerve. High signal from the centralretinal vessels was visible in 10 of 15 controls.The image quality of the chiasmal area wassimilar to that of the intraorbital portion of theoptic nerve.

Optic neuritisHyperintense lesions were seen within allsymptomatic nerves. Lesion length variedfrom one to 12 slices (3-36 mm). The intra-orbital portion of the optic nerve was involvedin 13, the intracanalicular portion in four, andthe intracranial portion in five studies. In longlesions, visible on more than five slices, signalchange was visible in both the intracanalicu-lar and intracranial portions. The coronal

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A B

imaging plane showed smaller degrees of sig-nal abnormality at the edge of lesions and intwo patients partial cross sectional involve-ment of the optic nerve was seen. In the acutestage, swelling of the nerve attributable tooedema, with no CSF visible surrounding thenerve, was seen in three patients (fig 1 A-C).

Benign intracranial hypertensionThe ring of CSF surrounding the optic nervewas demonstrated throughout the orbit and

Figure 2 Coronal T2weighted FSEfatsuppression image of theanterior orbit in a patientwith benign intracranialhypertension. Asymmetricwidening of thesubarachnoid space with awider, enlarged CSF spaceon the left can be seen(arrow). Signalcharacteristics and size ofthe optic nerves arenormal. Asymmetry of discswelling was seen onfundoscopy withpronounced papilloedemaon the left.

Figure 1 Coronal T2 weighted FSEfat suppressionimages showing high signal lesions in different locations inthree patients with optic neuritis (A) Lesion of theintracanalicular portion of the right optic nerve (arrow);(B) high signal lesion of the posterior orbital portion of theright optic nerve, afine ring ofCSF surrounding the opticnerve can be seen bilaterally; the olfactory tracts are seen astriangular structures (arrowed); (C) high signal lesion ofthe left optic nerve intracranially (arrow up); normalsignal intensity similar to cerebral white matter is shown inthe right optic nerve (arrow down).

into the optic canal (on eight to 11 slices). Infour patients with asymmetric disc swelling onfundoscopy, the asymmetry of papilloedemacorrelated with the width of the optic nervesheath seen on MRI (figure 2).

Meningiomas involving the optic nerveTwo cases showed mainly intraorbital tumourgrowth, 10 of 12 showed mainly intracranialtumour masses. The tumour tissue was clearlyidentified and delineated from surroundingstructures in all patients. Gadolinium-enhanced imaging and T2 weightedsequences demonstrated the presence orabsence of tumour growth in the optic canaland dural abnormalities intraorbitally. Thetopographic relation of nerve and tumour wasclearly demonstrated intracranially. Tumoursurrounding the optic nerve in the optic canalor intraorbitally was shown in eight patients.Gadolinium enhanced Ti weighted imagesgave a target like appearance in six patients; intwo patients enhancement of the optic nervewas also demonstrated (fig 3).

Optic nerve gliomasThe appearance of the gliomas was that of aswollen, fusiform, enlarged optic nerve.Intrinsic optic nerve signal was abnormal, butto different degrees in different parts of thelesion, quite unlike the rather homogeneoushyperintensity seen in demyelinating opticneuritis lesions. Three patients showed anenlarged optic canal on the side of the lesion.The nerve kinked intraorbitally and in fourpatients there was no subarachnoid CSFspace identifiable. Chiasmal involvement wasdemonstrated in one patient. The T2weighted images showed intrinsic signal

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High resolution MRI of the anterior visual pathway in patients with optic neuropathies usingfast spin echo and phased array local coils

Figure 3 Coronal T2 weighted FSEfat suppression image of a patient with anintraorbital optic nerve sheath meningioma showing a "target" like formation(arrow) with an optic nerve in the centre of white matter intensity, bright CSFaround it, and the tumour mass surrounding this concentrically.

abnormalities of the chiasm in one case inwhich its size and shape were normal (fig 4).

Anterior ischaemic optic neuropathyNo signal abnormality could be demonstratedat presentation, but after six months theaffected nerves showed long high signallesions involving at least five slices, whichwere of normal size (fig 5 A, B).

Dysthyroid eye diseaseImages clearly showed the involved extraocularmuscles and to what extent orbital fat andextraocular muscles contributed to soft tissueenlargement. No intrinsic optic nerve lesionswere seen (fig 6 A, B)

Leber's hereditary optic neuropathyBright signal was seen in the optic nerves oftwo virtually blind patients. The optic nerve-sheath complex was small and no CSF wasvisible surrounding the nerves.

Tobacco-alcohol neuropathyNo signal abnormality was visible in the opticnerves intracranially or in the optic-sheathcomplexes.

Ectatic carotid arteriesProminent enlarged carotid arteries weredemonstrated intracranially and were associ-ated with compression of the optic nerves,~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~...:....~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~..........X,y. ,~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.i.

... .. ..... .... .: :::.:}

|k: ...... .~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~. : .:..

Z. . ..... . > m~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.

A

Figure 4 Coronal T2 weighted FSEfat suppression image showing a right optic nerveglioma with a grossly enlarged optic nerve with heterogeneous intrinsic signalfrom thetumour (arrow). High signal surrounds the tumour representing CSF or mucoiddegeneration, which causes compression of the extraocular muscles; the left opticnerve-sheath complex shows normal signal characteristics.

BFigure 5 Coronal T2 weighted FSEfat suppression imagesin a patient with anterior ischaemic optic neuropathy. (A)Showing no abnormality in the right optic nerve 13 daysafter symptom onset; (B) on the follow up examinationafter six months a high signal lesion is demonstrated(arrow).

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which were displaced by the arteries, andindentation of brain parenchyma. The opticnerves and chiasm were extremely small andthin (fig 7).

DiscussionBy combining the FSE sequence with phasedarray coils we have shown that high resolu-tion, high signal to noise ratio images of theanterior visual pathway, from the globe to thechiasm, can be collected in just over 10 min-utes. The ability to provide fine anatomicaldetail and images that are sensitive to patho-logical changes offers an important advance inthe differential diagnosis of optic neu-ropathies, and the monitoring of their treat-ment, and in some instances should providenew insights into the underlying pathophysio-logical mechanisms. High anatomical resolu-tion offers a more confident interpretationof the small area of the optic nerve-sheathcomplex.A problem with small conventional surface

coils was the signal drop off in the optic canaland chiasm. 9With lateral positioning, the sig-nal from phased array coils decreased towardsthe centre of the head, but the signal to noiseratio was adequate in all parts of the anteriorvisual pathway. By magnifying the centralportion of the image slightly, the extremelyhigh signal areas of subcutaneous fat andgrey matter in the lateral temporal lobesimmediately under the coils were excluded,making it easier to window the images opti-mally. This was the only postprocessingrequirement in our study. Homogeneity cor-rection can be achieved by division of phan-tom image, but did not offer additionalinformation and did not help in the descrip-tion and interpretation of normal anatomy orpathological findings.'0

In demyelinating optic neuritis, althoughdiagnosis on clinical grounds, supported byvisual evoked potentials, is usually straightfor-ward, occasional difficulty may arise in mak-ing a distinction from acute anteriorischaemic optic neuropathy." Optic nerveMRI in such instances should be useful in theacute phase, when signal abnormality in thenerve will occur in demyelinating but not inischaemic optic neuropathies. In optic neuritiswe could demonstrate high signal lesions in allsymptomatic nerves, which underlines thehigh sensitivity of this imaging approach tothe inflammatory-demyelinating optic nervelesion.A particular value in accurately characteris-

ing the symptomatic lesion in demyelinatingoptic neuritis is the opportunity to elucidatethe pathophysiological mechanisms ofdemyelinating CNS disease. This is possiblein the optic nerve, where a unique correlationof the evolving clinical, electrophysiological(visual evoked potentials), and pathological(MRI) events can be monitored. On lowerresolution scans a correlation between acutevisual loss in optic neuritis and gadoliniumenhancement, the enhancement indicating thepresence of acute inflammation, has been

A

BFigure 6 Coronal T2 weighted FSEfat suppression imagesofa patient with dysthyroid eye disease after surgicaldecompression of the medial wall of the right orbit. (A) Onthe midorbital image displacement and enlargement ofintraorbital soft tissues is shown (solid arrow). The opticnerve-sheath complex has normal signal characteristics(open arrow). (B) In the posterior orbitflattening of theright optic nerve (solid arrow) is seen by grossly enlargedextraocular muscles.

Figure 7 Coronal T2 weighted FSEfat suppression imageshowing indentation of the gyrus rectus by an enlargedright carotid artery. Thin. flattened optic nerves (arrows)more so on the right can be seen medially adjacent to thecarotid arteries.

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found. 14 In the future, the combination ofhigh resolution imaging with quantitative MRapproaches thought to be sensitive to demy-elination and axonal loss (for example, mag-netisation transfer imaging32 33 and T2magnetisation decay curve analysis34) shouldgive further insights into the mechanisms ofrelapse, recovery, and failure to recover.The features we demonstrated in benign

intracranial hypertension were diagnosticallyuseful and cannot be reliably shown witheither lower resolution MRI or high resolutionCT." The increased size of the subarachnoidspace correlates with raised intracranial pres-sure, provided that the optic nerves them-selves are of normal size. As medicaltreatment of benign intracranial hypertensionis largely empirical and surgical interventionsuch as fenestration of the optic nerve sheathis sometimes advocated, high resolution MRImay well be a useful objective tool for show-ing that the sheaths are dilated and may safelybe incised, and for monitoring treatment.'6

Meningiomas involving the optic nerve areoften diagnosed relatively late. Radiotherapymay provide visual stabilisation.'7 If surgicalintervention is to be undertaken, completeresection of the tumour is needed to provideany chance of a permanent cure.'7 Intra-orbitally growing tumour may be inoperablebecause of the inevitable risk of damage to theoptic nerve and complete removal of intracra-nial tumour has often been impossible as thetopographic relation of the tumour to thecarotid arteries is so intimate.'7 In focalintracranially arising meningiomas the pres-ence of intracanalicular tumour extension andthe extent of intracranial tumour is importantin identifying patients at the greatest risk oflosing vision. We could demonstrate boththese features. In two patients surgery wasperformed after demonstration that intra-cranial tumour caused compression of theoptic nerve but had not yet entered the opticcanal. Whether such presurgical imaginginformation may be of prognostic valueregarding the long term outcome remains tobe seen. Exact topographic information aboutthe degree of encasement of the carotid arterieswas clearly provided in all cases with intra-cranial tumour growth. High resolution MRIshould also become an important follow upinvestigation to monitor the speed and loca-tion of tumour growth in patients treated con-servatively or with radiotherapy and to detectrecurrence in those treated with surgery.Tumour spread across the sphenoid bonetowards the opposite side into the periforami-nal area can also be clearly seen with highresolution T2 weighted and postgadoliniumenhanced scans.

Optic nerve gliomas are usually slow grow-ing tumours, and are therefore often treatedconservatively. The rate of tumour growthand involvement of the optic chiasm andretrochiasmal structures are of prognosticimportance.19 Therefore, clinical assessmentand MRI are the mainstays of monitoringtumour progression and extent. High resolu-tion MRI is perfectly suited to this purpose.

Optic nerve compression is an importantcause of visual loss in dysthyroid eye diseaseand may require surgical decompression,systemic corticosteroid treatment, or radio-therapy.'2 Magnetic resonance imaging offersan objective measure of orbital soft tissueswelling (extraocular muscles, intraorbitalfat), and optic nerve compression. Theresponse to surgical decompression or med-ical treatment can be monitored accuratelywith a T2 weighted FSE study.

High signal lesions in the optic nerve havebeen previously demonstrated in Leber'shereditary optic neuropathy, but the small sizeof the optic nerve-sheath complex along withthe lack of CSF around the nerve is a newfinding.'9 The specificity of this pattern needsto be assessed by further study of other condi-tions that are associated with chronic opticnerve atrophy.The compression of the optic nerves pro-

duced by enlarged intracranial carotid arterieshas not been previously demonstrated byMRI. Even though this is a rarely diagnoseddisorder, coronal high resolution MRI mayhelp to identify vascular compression causingvisual loss, especially in the elderly popula-tion.

Finally, the value of gadolinium enhance-ment has been already noted in optic neuritisand in optic nerve tumours.'4 40-42 Post-gadolinium images of either conventional(256 x 128) or high (512 x 512) resolutionare a very useful complement to the dataobtained with high resolution T2 weightedFSE images in the differential diagnosis ofoptic neuropathies.

We are grateful for the provision of fast spin echo and phasedarray local coils by General Electric, Milwaukee, USA. Wethank Mr John Wright for kindly agreeing to the study of hispatients.The NMR Research Group is funded by a generous grant

from the Multiple Sclerosis Society of Great Britain andNorthern Ireland, and supported by the Brain Research Trustand the Medical Research Council.

AppendixTECHNICAL NOTE: OPTIMISATION OF IMAGINGPROTOCOLThe anterior visual pathways are difficult to image witha clinically acceptable resolution and signal to noiseratio (SNR), in a clinically acceptable time, by conven-tional spin echo based techniques. We therefore per-formed a stepwise optimisation of an imaging protocolin normal controls, to investigate the utility of phasedarray coils (to increase SNR) and FSE (to increasespeed) in this anatomical area.

Phased array coilsPhased array coils combine the high SNR of a localcoil with the field of view (FOV) of a much largercoil.9 '° They are particularly useful for imaging largestructures close to the surface, or for simultaneouslyimaging several small structures. In this study a pair of3 inch circular coils (provided by General ElectricMedical Systems, Milwaukee, USA), originallyintended for temporomandibular joint imaging, wereplaced over the temples to determine whether this pro-vided higher SNR than the standard head coil forimages of the anterior visual pathways from the globeto the optic chiasm.

SNR comparison quadrature head coil: phased array coilsWe performed an SNR comparison of phased arraylocal coils and the standard head coil using identical

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fat suppressed FSE imaging parameters. For any localcoil the received signal, and thus the SNR, varies frompoint to point; for phased array coils the situation isfurther complicated by the processing used to combinethe data from each coil into a single image, which alsovaries from point to point. We therefore defined SNR asthe ratio of mean signal over a small area to the meanbackground intensity in an adjacent "zero" signal area.A mean signal from the optic nerves themselves wasmeasured in both optic nerves on all slices. The meanbackground signal (representing the noise) was mea-sured in the sinuses. In all cases the same regions wereused on both the head coil and phased array coil scans.With the phased array coil, a 26-39% SNR gain wasseen relative to the head coil along the length of theoptic nerve.

Fat suppressed SEAssuming that the relaxation properties of the nerveare similar to those of cerebral white matter a fatsuppressed spin echo sequence (SE2500/70) allows 10slices (5 mm thickness, 1 mm gap) to be collected in asingle acquisition. A matrix of 256 x 128 and twoaverages (NEX) takes just under 11 minutes and with a16 cm FOV yields an in plane resolution of 1-25 x0-625 mm. Fat suppression can be achieved by usingfrequency selective presaturation pulses.45 This pro-vides good fat suppression T2 weighted spin echoimages and allows the nerve to be clearly distinguishedfrom its surroundings, but the sequence does notprovide enough resolution to see details of the nervestructure.

Fast spin echo (FSE)Fast spin echo is based on the rapid acquisition withrelaxation enhancement (RARE) sequence firstdescribed by Hennig et al.'3 In essence, the techniqueacquires a multiecho train (typically four, eight, 16 ormore echoes) following each excitation. A differentphase encoding pulse is applied in conjunction witheach echo, which allows multiple lines of image data tobe collected alter each excitation. Thus the timerequired is radically reduced compared with a spinecho sequence at the expense of slightly less slices peracquisition. An FSE 2500/95 (echo train length (ETL)16) with fat suppression and the same in plane resolu-tion as the above SE sequence takes only 45 seconds,and covers seven slices in a single acquisition.Increasing the matrix size to 256 x 256, and setting thenumber of excitations (NEX) = 4 increases the scantime to two minutes 45 seconds, and a 512 x 512matrix, again with NEX = 4, takes 11 minutes for thetwo acquisitions necessary to collect sufficient slices tostudy the whole optic nerve. Resolution in the 512 x512 image is very good, but the 5 mm slice thicknessleads to blurring of the edges and large partial volumeeffects if the nerve is not aligned perpendicular to theslice. The SNR of the images is also too low to be clin-ically acceptable.

RectangularFOVRectangular FOV, which collects only the central 50%of an image, reduces imaging time by a factor of two. Arequirement for the technique to be successful, how-ever, is that very little signal should be present outsidethis reduced field of view, as it will wrap back into theimage producing unacceptable artefacts. The naturaldrop off of the 3 inch coils, combined with superiorand inferior spatial presaturation pulses, fulfils thiscondition when imaging the optic nerves. The extratime gained by using a rectangular FOV can be used toincrease the number of aquisitions, which in turnallows the slice thickness to be reduced to 3 mm. Thisgives more isotropic resolution, reducing partial vol-ume effects, but reduces the SNR of the resultingimages still further.

An "optimal" combinationThe SNR lost by moving to 512 x 512 images and3 mm slices (with a 16 cm FOV) can be partiallycompensated for by the use of the phased array coils,

but the average 30% gain in SNR is not in itselfenough to produce clinically acceptable images.Slightly increasing the FOV and increasing the numberof averages recovers more SNR, and increasing the TR(to reduce TI weighting) and reducing the TE slightly(to decrease T2 weighting) gives a further increase inSNR. This leads to an "optimal" sequence: FSE3250/68 (ETL = 16, NEX = 6, FATSAT, rectangularFOV, 20 cm FOV, 512 x 512 matrix, 3 mm slices)which gives 14 slices in 11 minutes with the addedbonus that the two acquisitions required allow con-tiguous interleaved slices to be collected.

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