175
Falx and Interhemispheric Fissure on Axial CT: I. Normal Anatomy
Robert D. Zimmerman 1 To determine the normal appearance of the falx and interhemispheric fissure, 200 Emily Yurberg 1 consecutive normal CT scans were evaluated prospectively. On unenhanced scans, the Eric J. Russel1 2 normal falx is visualized in 90% of patients and therefore interhemispheric hyperdensity alone should not be considered a sign of subarachnoid hemorrhage. The falx is most Norman E. Leeds 1 often (88%) visualized in the posterior part of the interhemispheric fissure, as a hyperdense, pencil-thin line extending from the calvarium to the splenium of the corpus callosum. In the anterior part of the fissure, the falx is visualized in only 38% of patients, when its appearance differs significantly from that of the fissure. It is seen as a thin, hyperdense line extending posteriorly from the calvarium for a variable distance, but it never reaches the genu of the corpus callosum. The interhemispheric fissure is a hypodense structure broader than the falx with a zigzag configuration due to medial frontal sulci. The difference in configuration between the anterior part of the fissure and the anterior falx is very helpful in differentiating subarachnoid hemorrhage from normal falx visualization.
The falx cerebri and interhemispheric fissure, although recognized early on axial CT [1], received little attention in the literature. We have studied the normal anatomic configuration of these structures in detail and offer anatomic information for differentiation of a variety of pathologic processes that may affect the fal x and interhemispheric fissure.
Materials and Methods
Two hundred scans performed without contrast opacification on an EMI 1005 were evaluated prospectively. The consecutive cases were interpreted as norm al or showing This article appears in the March/ April 1982 only involutional changes consistent with age. An y patient with a hi story, physical findings, issue of the AJNR and the May 1982 issue of or laboratory data suggestive of intracranial hemorrh age, trauma, or surgery was eliminated the AJR. from the study even when the scans were interpreted as normal. The patients were aged 8 Received July 17, 1981; accepted after revi days to 89 years; there were 96 males and 104 females. sion September 10, 1981 . On axial projection, at 10°- 20 ° to the anthropologic baselin e, the falx and interhemi Presented at the annual meeting of the Ameri spheric fi ssure are seen as a unit divided into three segments by the interposition of the can Society of Neuroradiology, Los Angeles, curved corpus callosum (fig . 1). Anterior (precallosal), posterior (retrocallosal), and superior March 1980. (supracall osal) segments are identified. Presented at the annual meeting of the Ameri The frequency of falx visuali zation on unenhanced scans was determined. Each segment can Roentgen Ray Society, Las Vegas, April 1980. of the falx and interhemispheri c fi ssure was separately analyzed for frequency of visualiza , Department of Radiology, Montefiore Hospital tion and appearance. Particular attention was paid to the effect of age on the appearance and Medical Center, 111 E. 21 Oth SI. , Bronx, NY of the falx/ fi ssure combination. The mean and peak density of the fal x on unenhanced 10467. Address reprint requests to R. D. Zimmer man. scans was determined for each case. Areas of obvious calcification were excluded from the quantitative analysis. 2 Department of Radiology, Ru sh Presby terian -Sl. Luke·s Medical Center, Chicago, IL Contrast opacificati on was studied in 20 of the 200 cases. These were chosen because 60612. pre- and postcontrast scans were obtained at identical levels, all owing determination of the effect of enhancement on falx density and configuration. AJNR 3 :175-180, March/ April 1982 0 195-61 08/ 82/ 0302-0175 $00.00 Anatomic studies were performed on 10 brains bisected at the corpus callosum with the © American Roen tg en Ray Society falx intact. The falx and its relation to the interhemispheric fissure and corpus call osum 176 ZIMMERMAN ET AL. AJNR:3, Marchi April 1982
POSTERIOR FALX VISU ALIZATI ON
IoIALE 823"'- FEMALE 93 2 % TOTAL 68.0% ( 9!1) 100 (93.8 1 ( 912 1 ( 9 01
z (81 1, ~ ..>- 80 (751 20° N ::; ..=> " 40 0-10 10-20 20·30 30 ·40 4 0 ·50 50·60 60·70 > 70 AGE l yeorsl A Fig. 1.-Drawing of falx and corpus callosum. On axial scans, corpus ANTERIOR FAL X VI SUALIZATION callosum divides fal x and surrounding interhemispheric fissure into anterior (precallosal), posterior (retrocallosal). and superi or (supracall osal) seg ments. Division 01 falx into these segments depends on scanning angle, which does WA LE 39. 6"" 100 not cause significant problems, since changes in lalx and fissure are gradual. FEW"LE 36 ''''' TOTA L 38.0% 80 .... VI SUALIZAT IO N "" VIS UAL IZATION CO RRECTED { 66.7 1 z o FOR 4TROPHY o ( 62.9 ) were examined. Four patients with typical falx visualization on 1601 ~ 60 unenhanced scans died of intercurrent disease, and the fa lx was N ..J evaluated pathologically for presence or absence of calcification . .. ~ :;: 40 ' 33.31 A " " ( 23.5 ) ( 20 ) ,," " (1 7.'1 __ 0 20 Results " ' 0"'" ( 1• •8 ) The falx was visualized in 90% of unenhanced scans. The frequency of visualization of each fal x segment varied sig 0·10 10·20 20-30 30-40 40·50 50- 60 60 · 70 > 70 nificantly. The retrocallosal falx was seen in 88% of patients AGE ( years) regardless of age or gender (fig. 2A). It had the character B istic and virtually invariable appearance of a pencil-thin, hyperdense line extending from the calvarium to the sple SUPFRIOR F"_ ' VISU;"lIZATI,JN nium of the corpus callosum (fig . 3). Anatomic studies .1001 JJ 931 demonstrate that this falx segment is of uniform thickness ,SO) and occupies the entire interhemispheric fissure (fig. 4). In 80 this location, the fissure is extremely narrow and is almost (671 invisible on CT, even in elderly patients with relatively severe Q ::; 0 atrophy affecting other segments of the fissure, especially N ..J its anterior part (figs. 3C and 3D). Fi g. 3. -Normal precallosal and retrocallosal falx and interhemispheric visualized bu t deep falx in B. In all cases, there is gap between free inferior fissure at several ages. Scans in patients aged 3 (A), 22 (B), 46 (C), and 76 margin of falx and genu of corpus callosum (small white arrow). Retrocallosal (D). Typical appearan ce of falx and surrounding fissure in th ese ages. In all segment of interhemispheric fissure so narrow as to be virtually invisible examples, retrocallosal fal x is thin hyperdense line extending from calvarium (A-D), even in face of severe atrophy elsewhere, in particular, anterior part to splenium of corpus callosum (large white arrow). Precallosal part of falx is of interhemi sph eric fissure (C and D) . In these figures, fissure (small black more variable. Anterior falx may be nonvisualized (A) or minimally seen (B) arrows) has configuration of hypodense zigzag line extending from calvarium between frontal lobes (large black arrow). As fissure dilates, falx becomes to genu of corpus callosum . In younger patients, fissure is either not seen (A) more easily seen (large black arrow, C and D) . Depth of an terior part of falx or only intermittently seen as small focal areas of cerebrospinal flu id density varies, as demonstrated by comparing short or shallow falx in C with poorly (small black arrows). Fig. 4. - Anatomic specimens. Bisected cerebral hemispheres with falx call osu m (small arrow). In other specimens (B), precallosal falx is deeper and intact. Uniformly thick and dense posterior falx fills virtuall y entire retrocal thicker, but gap remain s between iree margin of fa lx (large arrow) and genu losal interhemispheri c fi ssure. In precallosal seg ment, falx is more variabl e in of corpus call osum (small arrow). Fenestrations near inferior margin of falx in depth. It may be shallow anteriorly (A), filling only anterior fi ssure with larg e many cases (curved arrows). gap between anterior inferior margin of falx (large arrow) and genu of corpus adults (figs. 3A and 38). It progressively dilated with age rounded by the relatively hypodense cerebrospinal fluid and became clearly visible in most patients over 30 (fig . 3C) (figs. 3C and 3D). and in all patients over 40 (fig. 3D). It produced the typical The supracallosal fal x was visualized in 68% of patients pattern of a zigzag hypodense line which extended intact (fig. 2C) and was most often seen under age 10 or over 50. from the inner table of the skull to the genu of the corpus Just above the corpus callosum, the appearance of the falx callosum. As the fissure dilated with age, its anatomic con and fissure is a combination of pre- and retrocallosal config figuration could be more clearly seen and the fal x was more urations (figs. 5A and 58). Thus, anteriorly, the falx was thin easily and more often visualized (fig. 28) within it, sur- or not visualized and the interhemi spheric fi ssure visible 178 ZIMMERMAN ET AL. AJNR:3, Marchi April 1982 Fig. 5.-Superi or fa lx. Just above corpus call osum, superior falx produces segment remain s narrow. An terior fa lx (large black arrow) visualized because configuration that is combination of that seen in pre- and retrocallosal it is surrounded by cerebrospinal fluid. C, AI the vertex, fissure is narrow and segmenls. A, Young patients. Fissure uniformly narrow (small black arrows) falx is rarely visualized in young patients. D, Older patients. Fissure shows and only posterior part of fa lx is seen (white arrow). B, Older patient. uniform dilatation (small arrows) and falx is visualized throughout its course, Anterosuperior part of fissure (small black arrows) dilates and posterior except where interrupted by small fenestrati on (open arrow). with its width dependent on age (figs. 5A and 56). Poste FAL X DENSITY riorly the fal x was usually observed as a dense thin line (fig . 100 ( 97} 5A), and the fissure was either invisible or extremely narrow. 0--0 HIGH DE NS ITY At the vertex on ly, the fissure was seen to have a uniform ~ MEAN DENSITY width from the anterior to the posterior aspect of the calvar 80 ium (fig. 50). ( 67 ) ( 69 ) '"~ {63 1 '68 ' The mean density of the falx was 46 H and did not vary z =0 60 with age, although the maxi mum density did increase, peak 0 """ ...I ing at 97 H in patients over age 70 (fig . 6). i;:"' ( ~O) (>'» ( 47 ) (41) '"0 4 0 After contrast infusion, the fa lx was visualized in every z (42. 5 J =0 ( 42 ) 0 case (fig . 7). In the retrocallosal and supracall osal regions, :I: it appeared to be slightly broader and denser, but was 20 otherwise unchanged from its precontrast appearance. In the precallosal segment, the fal x was thicker and more o L---~----~----r---~----~----~--~--__ ~ frequently seen. It appeared to extend further posteriorly 0 · '0 10- 20 20-30 30-40 40-50 50-60 60-70 >70 within th e fissure, often merging with a pericallosal vascular AGE {years } blush and thus producing a continuous dense line from the inner table of the skull to the corpus callosum (figs. 76 and Fig . 5.-Density of falx. Mean density of falx (closed circles) does not change significantly with age, but peak density (open circles) does in crease, 70). especially in patients over age 50. This rise may be related to presence of Four patients with typical falx visualization on en enhanced foci of calcifications too small to visualize on CT. scans died of intercurrent disease. Pathologic evaluation demonstrated no calcification or ossification within the fal x in these four cases. spheric fissure in patients without subarachnoid hemor rhage. Those investigators came to a similar conclusion, Discussion that this was due to an intrinsic density difference between Limitations in spatial and density resolution of early CT the fal x and surrounding tissue. The frequency of falx visu scanners prevented routine visualization of the falx on un ali zation varied between the two series, and in fact was enhanced studies; it was seen only after contrast infusion different from that obtained in our study. These differences [1]. In patients with subarachnoid hemorrhage, increased can be explained on the basis of methods of patient selec density in th e interhemispheric fissure was identified on tion and differences in scanner resol ution. In our series, unenhanced scans and, because it mimicked the appear pathologic studies confirmed that falx visuali zation is not ance of the enhanced fal x, came to be called the " falx sign " dependent on calcification (fig. 8A) and, therefore, must be of subarachnoid hemorrhage [2-5]. With improvements in accounted for by intrinsic density differences between the density and spatial resolution, Osborn et al. [6] and Akimoto falx and surrounding brain . The observation that the falx has et al. [7] demonstrated increased density in the interhemi- a greater radiographic density than surrounding brain was AJNR:3, Marchi April 1982 CT OF NORMAL FALX AND INTERHEMISPHERIC FISSURE 179 first made in 1946 by Robertson [8] wh o demonstrated visualization of the uncalcified normal falx on frontal radio graphs taken so that the long axis of the falx was parall el to the axis of the x-ray beam (fig. 8B). Robertson [8] speculated that the " dense" fibrous nature of the falx was responsible. Insights into the factors that contribute to radiographic den sity gained since the advent of CT suggest th e high vascu larity of the falx [9, 10] makes it denser than surrounding brain tissue. The key to an accurate understanding of the ax ial anat omy of the fal x and interhemispheric fi ssure li es in vi ewing these structures as a single functional unit divided into three segments by the corpus call osum . Each segment has unique characteristi cs that are useful in the evaluation of pathologic processes that affect the fal x and interhemispheric fi ssure [11]. The retrocallosal segment is characterized by its con stant appearance. The fal x is anatomically invariabl e (figs. 3 and 4) in this area, and the interhemispheri c fi ssure is extremely narrow (fig . 3) and does not dilate with age, even with relatively severe atrophic dilatation of the subarachnoid spaces elsewhere in the brain, and in parti cul ar in th e anterior part of the interhemispheric fissure (figs. 3C and 3D). Thus, the fal x is virtually always seen as a pencil-thin hyperdense line without surrounding hypodensity (fig . 3). Th e precallosal segment is as variable as the retrocallosal segment is constant (fig . 3). Anatomicall y the anterior part of the fa lx is thinner than the posterior segment and is of variable depth (fig . 4). This is reflected on CT by both the lesser frequency of visualization (fig . 2B) and th e vari ability c o of appearance of the fal x (fig . 3). The fissure is al so vari abl e in this area. It is narrow in children and young adults, but Fig . 7.- Contrast-enhanced scans. On contrast-enhanced scans (B and progressively dilates with age (fig. 3). This age-related di 0), falx is thicker and denser than on comparabl e unenhanced scans (A and latation accounts for the increase in rate of fal x vi sualization C). Configuralion of posterior fal x is unchanged (large white arrows) aft er contrast unfusion. Precallosal falx (black arrows) increases in densily and in patients over age 40 (fig. 2B). The anterior fal x is thin and depth due to beller vi sualization of thin in feri or margin of falx and visualization straight and never extends completely from the calvarium to of peri callosal artery, whose shadow merges wilh anlerior falx. Wh en arlery is slraight, (B), il merges imperceptively with falx. When artery has slighlly the corpus call osum. Th e anterior fissure is wider and zig curved course (black arrowhead), it is more easily differentiated from falx. zags (where medial gyri impinge on the fissure). It always extends from the calvarium to the corpus callosum. An understanding of these two configurations is the key to differentiati on of norm al falx visualization from subarachnoid hemorrhage in the interhemispheric fissure [11]. The supracallosal segment has two configurati ons. Above the corpus callosum, it is a combination of the pre- and retrocallosal segments. Thus, anteriorly the fi ssure is mild to moderately prominent, and the fal x is minimally vi sualized. Posteriorly the fal x is dense and the fissure so narrow, th at it is virtually invisible (figs. 5A and 5B). Only at th e vertex does the fissure have a symmetrical size (figs. 5C and 50). REFERENCES 1. Naidich TP, Pudlowski RM , Leeds NE , Naidich JB, Chi solm JJ, A B Rifkin MD. Th e normal contrast enhanced computerized axial tomogram of th e brain . J Cornput Assist Torn ogr 1977;1 : 16-29 Fig . 8.-Normal uncalcified falx. A, Retrocallosal (large white arrow) and 2. Liliequist B, Lindquist M, Valoimarsson E. Computerized to precallosal (black arrow) segments of falx are visualized in patient who mography and subarachnoid hemorrhage. Neuroradiology subsequenlly died and in whom postmortem examinalion demonstrated no 1977;14:21-26 evidence of falx calcification. Thus, falx visuali zalion is due to inlrinsic densi ty difference between this struclure and the surrounding brain, rather than 3 . Lim ST, Sage OJ . Detection of subarachnoid blood clot and cal cification. B, Frontal skull radiograph. Vi suali zation of uncalcified fal x as other thin , fl at structures by computed tomog raph y. Radiology linear midline hyperdensity. 1977;123: 79 - 8 4 18 0 ZIMMERMAN ET AL. AJNR:3, March i April 1982 4. Scotti G, Ethier R, Melancon 0, Terbrugge K, Tchang S. cerebri. Radiology 1946;56 : 320-323 Computed tomography in the evaluation of intracranial aneu 9. Newton TH , Gooding CA, Price DG . Opacificati on of th e falx rysms and subarachnoid hemorrhage. Radiology 1977; and tentorium during cerebral angiography. Invest Radiol 123:85-90 1970;5: 34 8- 354 5. Dolinskas CA, Zimmerm an RA , Bil aniuk LJ . A sign of subarach 10. Kerber CN , Newton TH . 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