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Neuroradiology (1987) 29:333-338
© Springer-Verlag 1987
MR imaging of epidermoid cysts J.Vion-Dury 1, F. Vincentell?, M.Jiddane 1, Y.Van Bunnen 1, C. Rumeau 1, F. Grisol?, and G. Salamon 1 Departments of 1 Neuroradiology and 2 Neurosurgery, CHU Tirnon~, Marseille, France
Summary. After a short review of literature, MRI assessments of four cases of epidermoid cysts (EC) are reported. EC, (characterized in computed tomography by hypo or iso-density areas nonenhanced by contrast), are characterized in MRI by: 1) an important variability of signal intensity between the different cases, and sometimes between the different parts of the same cyst, 2) the absence of edema in surrouding parenchyma, in spite of important mass effect, 3) well defined limits, permitting certainty of the extra-cerebral nature of this tumor, 4) the presence of calcifications easily perceptible in MRI. It is proposed that the variability of signal intensity of EC is caused by different relaxation time values corresponding to different concentrations of keratin, cholesterol and water content. K e y w o r d s : Epidermoid cysts - MR imaging tumors - T2 weighted images
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Brain
Epidermoid cysts (EC) or pearly tumors, were described by Cruveilhier [1]. It is now generally accepted that epidermoid cysts result from migration abnormalities of epiblastic cells during closure of the neural tube, in the 3rd to 5th weeks of embryonic life [2]. This theory of "epiblastic inclusions" was suggested by Bostroem [3] and confirmed by Bayley [4]. Dandy [5] described these tumors as the "most beautiful tumors of the body". These totally benign tumors are rare and constitute about 1% of brain tumors [6, 7]. Although they can appear anywhere in the central nervous system, they show a predilection for cerebello-pontine angle and sub-arachnoid cisterns in the midline of the base of the skull [8]. Less frequently, there are located within the 4th ventricle.
They present a very slow evolution and are clinically revealed late in life in spite of their congenital origin. This fact explains their considerable size. Before computed tomography (C.T.), pneumoencephalography was the specific radiologic investigation [9]. C.T. shows the precise topography, size and density of these tumors and provides the diagnosis. Pearly tumors are characterized by an homogenous low density lesion, fairly well delimited and classically non-enhanced by contrast [2, 10-15]. EC do not produce edema, and the mass effect is slight, compared to tumor size. Calcifications can be seen in these tumors, M.R. imaging (MRI) of EC is particularly useful in the definition of anatomical limits of tumoral tissue as suggested Mironov [16]. But it is also necessary to define MRI characteristics of EC. When, in different works, EC are mentioned with other brain tumors, as shown in Table 1, the different authors diverge about their MRI characteristics. We report here four cases of these benign tumors and discuss their MRI characteristics.
Material
and methods
Four patients (varying in age from 45 to 68 years) who presented a neurological syndrome suggesting an intra-cranial process were studied by computed tomography and then with MR imaging. After surgery, pathological examination showed in all cases, characteristic epidermoid tumors. C.T. scan were performed on an EMI 1010 cranial scanner or an ND 8000 CGR scanner, permitting axial brain slices of respectively 13 and 10 mm thickness before and after contrast injection. MRI studies were carried out on two Technicare imagers (Technicon, 0.15 T, resistive magnet) be-
334 Table 1. T1 and T2 relaxation times o f some EC described in the literature. T1 and T2 are estimated from visual assesment o f comparison with normal brain tissue References
Number o f cases o f EC
Magnetic Field
Spin exitation sequence
TE (ms)
TR (ms)
TI (ms)
T1 of EC
T2 of EC
Rink [17]
1
0,14 T
SE
28 346
860 1260
-
-
enhanced enhanced
MacKay [18]
3
0,15 T
SE
86 160
1580 1160 1500
-
0 no change
400
reduced
1600
-
enhanced
enhanced
-
reduced
-
no change
l reduced 2 enhanced 2 enhanced
1 reduced and 2 enhanced 2 enhanced
-
enhanced enhanced
IR Mironov [16]
2
0,5 T
SE
43 + 8 6
Lee [19]
1
0,5 T
SE
30
500 2000
Brandt-Zawadski [20]
2
0,35 T
SE
56
Latack [21]
5
0,35 T
SE
28+56
500 1000 2000
Davidson [22]
3
0,15 T
SE
40 80
1080 1160 1400 1200 1800
IR
Table 2. Parameters of SE pulse sequences (CPMG) Spin TR exitation (ms) Sequence
N u m b e r o f Echo delay N u m b e r o f Average echoes time slices TE (ms)
A B C
2 2 8
2060 2060 2240
30+60 60+120 30 x n 8>n>l
13 13 1
2 2 2
longing to both Clairval Clinic and Residence du Parc Clinic (Marseille, France). Section thickness was 10 m m and slices were adjacent. All images were produced using spin echo sequences (SE). Table 2 gives the parameters of SE pulse sequences.
Results (summarized in Table 3) Case i
This patient (woman, 46 years old) was investigated for a slow and insidious cerebellar syndrome with ataxia and nausea. C.T. scan showed an homogeneous median hypodensity area in the posterior fossa, non-enhanced by contrast (Fig. 1 a). MRI sagittal slices precisely showed the topography of the tumor characterized by considerable decrease of signal intensity and the displacement of the vermis above and forward, suggesting an extracranial process. The difference in the morphology of the tumor between echo 60 ms and echo 30 ms suggests that the periphery of the tumor (which is not seen in the sec-
-
400 200 600
enhanced enhanced
ond echo and the signal intensity of which is near to that of normal brain), is probably the cyst membrane (Fig. 1 b, c). The signal of the cyst is always less than the signal of normal parenchyma, suggesting a reduction of spin-spin relaxation time. Case 2
This male patient (58 years old) presented with frontal headaches and was admitted after a convulsive seizure. C.T. scan showed an inter-hemispheric and fronto-basal hypodensity in the midline, predominantly on the right side, non-enhanced by contrast, and containing some calcifications. MRI in the sagittal plane (sequence C) shows a tumor characterized by a great increase of signal intensity, non homogenous and well demarcated, suggesting increase of T2 relaxation times. The intra cystic heterogenity of signal would correspond to calcification (Fig.2). Case 3
This patient was a 68-year-old man, presenting a progressive exophthalmos of the right eye with oculomotor paralysis for ten years, corneal anaesthesia and trigeminal hypo-esthesia. C.T. scan showed (Fig. 3 a, b) a voluminous mass in the fight temporal lobe, varying in density (hypo and isodense) and presenting some calcifications. MRI clearly shows the extracerebral origin of this tumor and the severity of ihe temporal lobe atrophy. Double echo sequences in frontal and sagittal planes show that this
335 Table 3. Summary of clinical, C.T., and M. R. I. features of cases reported Case
Clinical data
C.T.
M.R.I.
Relaxation time T2
• hypodensity in posterior fossa
• extra-cranial process displacing vermis forward • density reduced • cyst membrane seen
reduced
• frontobasal hypodensity • calcifications
• heterogenous enhanced density area • calcifications
enhanced
Case 1
• ataxia
woman 46 years
• cerebellar syndrome • nausea
Case 2
• frontal headache
man 58 years
• convulsive seizures
Case 3
man 68 years
• exophtalmos • oculomotor paralysis • corneal anaesthesia • trigeminal hypoesthesia
• hypo and isodense • area intemporal lobe • calcifications
• heterogenous signal intensity • absence of edema • monstrous tumor involving temporal fossa, pterygo maxillary fossa and orbit • temporal lobe atrophy
enhanced in temporal part of the tumor, but not modified in the maxillary part
Case 4
• blindness of the right eye
• sellar and supra sellar
• reduction of visual acuity of left eye
• sellar and supra-sellar tumor, with signal intensity enhanced
enhanced
man 45 years
Fig.1 a-c. Case 1: CT (a) and MRI (b, c) of an epidermoid cyst of the posterior fossa. Note that in SE (sequence A) imaging, EC is dark compared to normal parenchyma suggesting short T2 values. (b = SE 30 ms, c SE 60 ms)
Fig.2 a-c. Case 2: CT (a) and MRI (b, e) of epidermoid cyst of frontal region. Note in this case, the increase of T2 relaxation time attested by an increase of signal intensity in echoes 60 and 150 ms of a multi S.E. sequence (type C)
336 tumor, which involves the pterygo-maxillary fossa and orbit, presents an heterogenous increase of signal intensity. The lower part (maxillary) shows a weak and heterogenous decrease of intensity suggesting different modifications in T2 relaxation time compared to the upper part. It is noteworthy that this enormous tumor does not induce any edema in the temporal lobe, confirming the very slow ~evolution. The heterogenicity of the lower part of the tumor was confirmed by surgery. Also note the calcifications in the medial side of the temporal fossa.
Case 4 This man (45 years old) presented an epidermoid cyst previously partially excised in 1969, and 1976. He was investigated for headaches, failing eyesight on the fight side, and reduction of visual acuity of the other eye. C.T. shows a sellar and suprasellar hypodensity non-enhanced by contrast. MRI shows (Fig. 4) in coronal plane an inhomogenous increase of signal intensity (in a SE type C sequence) suggesting an increase of T2 relaxation time. In the frontal lobe, the increase of intensity in the white matter cannot be due to edema in relation with the mass effect: because this patient was previously treated this hypersignal might rather correspond to scarring.
Relation between X-ray density and M R I Except for calcifications, no relationship between density observed in C.T. scan and the response of these tumors to spin exitation was found. Unfortunally, no density evaluation and no T2 calculation were made because such cases were studied without this prior development of experimental procedure. All epidermoid tumors show hypo or isodensity, independant of the strength of the MRI signal intensity.
Relation between pathology and M R I In these four patients histology confirmed the presence of epidermoid cysts. Unfortunately, it was not possible to quantify the proportions of keratin and cholesterol and to relate histologic aspects to MRI characteristics.
Discussion Fig.3 a-g. Case3: CT (a, b) and MRI (e-g) of a monstrous epidermoid cyst of the temporal fossa. Note 1) the presence of calcifications on the internal part of tumor (3c), 2) the increase of signal intensity on the echo at 120 ms. e-f (sequence B) suggesting a weak increase in T2 value, g shows that this increase of signal intensities is not homogenous, when the inferior part of the tumor fails to present high T2 values
Although in CT epidermoid tumors are constantly defined by an hypo (or sometimes iso) density area, non-enhanced by contrast, in MRI, both published
337
Fig.4 a-d. Case 4: CT (a) scan and MRI (b-d) of a supra sellar epidermoid cyst. The MRI SE sequence shows an increase of T2 relaxation time. (b = SE 60 ms, and e SE 240 ms of a SE type C sequence). Not in white matter of frontal lobe, an hypersignal the significance of which is probably related to scarfing from previously surgery (d)
data and our personal experience suggest that these tumors present a considerable variability concerning the T2 relaxation time characteristics and consequently their visual characteristics (signal intensity, contrast). Authors who described EC in MRI used different strengths of magnetic field, different imagers and different spin-excitation modalities. Yet, it appears from table 1, that the variability in MRI characteristics of EC does not seem dependant on TE or TR; contrast between EC and normal tissue is probably little or none affected by variations of TR used [23]. This fact is confirmed by our own results. As demonstrated for X-ray attenuation in CT [2], MRI characteristics of these tumors are dependant on their relative composition of cholestero and keratin. High concentration in lipid underly a low Xray attenuation. In MRI, fat [17, 22, 23], fatty tumors [18, 27] and especially EC [28] exhibit short T1 or T2 values. It must be suggested that a proportion of cholesterol and keratin are in a crystalline form, and that, perhaps, in these tumors, there are many microcalcifications: all these elements might participate in a reduction of relaxation time values. For these reasons, variability of signal intensities, and occurence of enhancement of T1 and T2 values in some EC might rather be related to a differential hydratation of these tumors [29], as suggested previously for other expansive processes [28-31]. Finally, it appears from the literature and the four cases presented, that about 57% of EC are characterized by an increase, and about 15% by a decrease of T2 relaxation time; in 26% of EC, the T2 relaxation time is not different from that of the surrounding parenchyma. This variability in N M R characteristics was also found in pineal germinomas [32]. As suggested by Lebas et al. [28], it clearly appears for EC that no correlation between X-ray attenuation and MRI signal intensity or relaxation
time is found. These results are not in agreement with Borello et al. [33], because it must be assumed that the parameters implicated in MRI signal are more complex and numerous than for X-rays attenuation in CT, and not reducible to the variations of density. MRI in EC is of great interest because no modifications in T2 relaxation time are seen in surrounding tissues, suggesting absence of edema in the nontumoral parenchyma as explained by the slow evolution of these tumors. If it is accepted that MRI is a very sensitive method of demonstrating edema, this absence of edema in surrounding tissue, may be an important pointer to the nature of the tumor. MRI assessment of EC also provides useful evidence of the extra-cranial nature of these tumors. The margins of an expansive process are often very well revealed by MRI, and probably more precisely than with CT. More, in MRI, calcifications are well seen in these tumors as focusses of signal diminution or signal void [34]. In conclusion, MRI is a necessary exploration in the diagnosis of EC If the characteristics of the tumor are not specific, the absence of edema in surrounding tissue is strong evidence of the diagnosis. Anatomical evaluation of these tumors is very well provided by MRI, permitting a better choice of surgical access. Acknowledgements. We would like to thank Dr. Bouscarle, Dr. Debaene and Dr. Giudicelli for permitting to realize MRI imaging of E.C. in Residence du Parc Clinic and Clairval Clinic.
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