J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.38.10.935 on 1 October 1975. Downloaded from Journal ofNeurology, Neurosurgery, and Psychiatry, 1975, 38, 935-947 Computerized axial tomography: the normal EMI scan J. GAWLER, J. W. D. BULL, G. H. DU BOULAY, AND J. MARSHALL From the National Hospitalfor Nervous Diseases, Queen Square, London SYNOPSIS Computerized axial tomography using the EMI Scanner s a new method of using x-rays in diagnosis. The technique displays intracranial and orbital structures in the transverse plane. The appearances of normal EMI Scans are described and correlated with cerebral and orbital anatomy seen in transverse section. Computerized transverse axial tomography with the EMI Scanner-a revolutionary method of + 500 utilizing x-rays in diagnosis developed by G. N. Hounsfield-provides images of transverse 'slices' of the brain without the use of contrast guest. Protected by copyright. media. Normal structures, such as the internal capsule, which could not be shown by conven- tional techniques, and structures, such as the ventricular system, which could be seen only with the aid of contrast media, may now be / displayed without prior disturbance. The present paper describes the normal intra- -J 0 100 0 cranial anatomy as revealed by the EMI Scan. cU 100- z As normal volunteers have not been examined, CO, 3: the composite picture described here has been 50- -J built from experience with 2 500 patients investi- gated for intracranial disease, in some of whom ADJUST WINDOW the EMI Scan and other investigations proved c- WIDTH -J negative. Full descriptions of the scanning C,) c1 z C' technique have been published elsewhere (Houns- LL field, 1973) and only relevant features will be a a considered here. Each transverse 'slice' of brain is considered http://jnnp.bmj.com/ as a matrix of cells. Initially, an 80 by 80 matrix \/ containing 6 400 cells each 3 mm by 3 mm was used, but currently a 160 by 160 matrix of 25 600 cells measuring 1.5 mm by 1.5 mm is employed. The depth of each cell is 8 or 13 mm depending upon the adjustable width of the x-ray beam. The image ofthe brain is constructed on September 27, 2021 by from measurements of the amount of x-ray -500 - absorbed by each of the cells as the x-ray beam FIG. 1 Oscilloscope display represented graphically. scans the head. The absorption values are The ten divisions (peak black, peak white and eight expressed on an arbitrary scale where air has a shades of grey) may be applied to the entire density range used by the EMI system or restricted to any (Accepted 23 May 1975.) segment of the range. 9: 35 J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.38.10.935 on 1 October 1975. Downloaded from 936 J. Gawler, J. W. D. Bull, G. H. du Boulay, and J. Marshall black (low density) to peak white (high density). The scale can be adjusted to cover the entire density range (-500 to + 500) or a segment from any part of the range (Fig. 1). Absorption values are calculated to an accuracy better than half per cent so that on the arbitrary scale a difference of4 or 5 points is significant. When the oscilloscope scale covers 0 to 40 of the density range, each change in the grey scale represents 4 points of density difference permitting cerebro- spinal fluid, grey and white matter to be distinguished. For examination of supratentorial structures a series of four scans yielding eight 13 mm tomo- grams is employed. The tomogram pairs are FIG. 2 Paired transverse axial tomograms, each 13 centred 3, 5.5, 8, and 10 cm above and parallel mm deep, are centred at regular intervals above the to the orbitomeatal line (Fig. 2). For posterior orbitomeatal line to cover the supratentorial compart- fossa examination the beam is centred 3 or ment. 3.5 cm above the orbitomeatal line, with the patient's head flexed 15 or 20 degrees, and for the orbits a pair of 8 mm 'slices' centred 1 orguest. Protected by copyright. value of -500, water has zero value, and dense 1.5 cm above the orbitomeatal line with the bone a mean value of + 500. Cerebrospinal fluid head extended 10 to 15 degrees are employed gives values of 0 or 1, white matter 10 to 18, and (Fig. 3). grey matter 18 to 30. Calcified structures such as the pineal body or choroid plexus give higher ANATOMY DEFINED BY THE EMI SCAN values. Intracranial compartment (Fig. 4) The absorption values may be expressed the skull varies from numerically by a line printer or displayed as an Because the size and shape of person to person a 'slice' at a given level above the image by a cathode ray oscilloscope whose orbitomeatal line will contain different structures in adjustable grey scale has 10 divisions from peak different patients. For this reason, a series of idealized 'slices' will not be described and, instead, the appearance and relationship of normal structures in the transverse plane as revealed by computerized tomography will be considered. VENTRICULAR SYSTEM The lateral ventricles are seen in three or four contiguous 'slices' (Fig. 5). Con- http://jnnp.bmj.com/ sidered from above downwards the highest 'slice' may incorporate only 1 or 2 mm of each lateral ventricle, which then appear as bilateral vague areas of slightly lower density (Fig. 6a). With slight obliquity of the head, one ventricle may just present within the 'slice', while the other lies below it. This can lead to the mistaken interpretation that a patho- logical diminution of white matter density exists in on September 27, 2021 by the centrum semiovale on this side (Fig. 6b). It must be remembered that as a 'slice' is 13 mm deep it may incorporate both cerebrospinal fluid and brain tissue with absorption values varying between FIG. 3 Tomograms are centred specifically for 1 and 10 (Fig. 7). The precise limits of the ventricles posterior fossa and orbital examination. cannot therefore be defined. In practice, this is not a J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.38.10.935 on 1 October 1975. Downloaded from Computerized axial tomography: the normal EMI scan 937 *S kt..-:X<s t. k... ..t { ,K, 18- s .. I semiovOle a.9.s#4._ ^swNRS. o X, ii _- .4, / ...,*e< .._ *. : Ps . ^.l/\' s\ X ' .-rj .a:,. *"B;S ffi (1(1. 4 1 guest. Protected by copyright. F-niC. 4 2 FIG. 4 Normal EMI Scan. Three scan pairs centred 3 (4/5 and 6), 5.5 (4/3 and 4), and 8 (4/1 and 2) cm above the orbitomeatal line provide a sequence of six contiguous tomograms. A normal brain (left) has been http://jnnp.bmj.com/ cut at appropriate levels to match the scan and the diagram (right) shows those structures which may be identified on the scan. When comparing the cut surface of the brain and EMI Scan, it is important to remember the latter represents all the tissues in a 'slice' 13 mm thick. (See over.) problem because, when the absorption values are its density, the whole cell is registered as ventricle, on September 27, 2021 by contrasted on the oscilloscope, fairly sharp definition even though only a segment of the ventricle is of the ventricles is obtained. When the ventricular projected into the cell, and this would make the margin lies within cells that contain principally ventricle appear larger than its real size. Overall, the cerebral tissues, a significant reduction in density technique makes the ventricles appear slightly may not be apparent and the ventricles would thus smaller than in a fixed specimen of a normal brain, appear smaller than their true dimensions. Con- and this is allowed for in the interpretation. versely, when a cell contains sufficient fluid to reduce The highest tomogram to pass clearly through the J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.38.10.935 on 1 October 1975. Downloaded from 938 J. Gawler, J. W. D. Bull, G. H. du Boulay, andJ. Marshall 0iy;h _W _ ,S / \\N Sephurn pe'!LIcdL Front' h. .ffi.*:11! ^-- L_ _- : r'-~~~~~~~~~~~it e r ,t ti C riPsa p s u l,; AInt.rior a.imt qJjA 5Z>---6Xi3t=: #XX .^ ffi *< . 4 t . Q. ) I_ -s ¢ s @ /~~~~~~~~~~~~~~T +~~~~~~~p h c r,: - Q. uC orda T1111-XiilEX11 il e w t~~~~~~~~~~~~~~~~~~~~~~~~~~~~~I'- r 4.C. Posta--IC.r -~ .. r .} /, I / 'd _ ____ pOsts-SclF'm-'~~~~Ari~rd ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ine rn a __,..[Icap u 'c' guest. Protected by copyright. .~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~i-,J 'C. l__ As-.-._,Sff~~~~~~~~~~~~~~~~b c-dU n L-..........Czz ., . .... ! , w H;(. 4+ http://jnnp.bmj.com/ lateral ventricles contains the superior segments of appear as a vague area of low density extending the cellae mediae (Fig. 4/2). While these are fairly backward towards the occipital pole and should not symmetrically displayed with respect to the midline, be mistaken for a lesion. The next contiguous they commonly show some difference in size and tomogram, descending through the series, incor- contour. The body of the corpus callosum lies porates the frontal horns anteriorly and the trigones between the cellae mediae on this plane, with the and occipital horns posteriorly (Fig. 4/3). A degree of forceps minor diverging anteriorly and the forceps asymmetry in lateral ventricular contour is common, major posteriorly along the medial aspects of the partly because of true anatomical difference and on September 27, 2021 by ventricles. Laterally lies the white matter of the partly because any obliquity of the head will result corona radiata.