British Journal of Radiology 75 (2002),563-564 © 2002 The British Institute of Radiology
Intracranial lithography?
M I Tawil, MRCP,
J P Wilson, MB BCh BAO and
N B Wright, DMRD, FRCR
Department of Radiology, Royal Liverpool Children's Hospital NHS Trust, Alder Hey, Eaton Road, Liverpool L12 2AP, UK
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Introduction
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A 14-year-old female presented with increasing seizure activity. She had been born prematurely at 26 weeks gestation, developed hydrocephalus secondary to intraventricular haemorrhage during the neonatal period and subsequently required ventriculoperitoneal shunt insertion. She was severely developmentally delayed and suffered from seizures, which were normally controlled with anticonvulsants.
Cranial MRI was performed with a 0.5 T superconducting magnet. Both T1 weighted spin echo (SE) (TR, 475 ms; TE, 25 ms) and T2 weighted turbo spin echo (TR, 3000 ms; TE, 125 ms) images were obtained (Figure 1
). What are the findings?
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Figure 1. (a) Sagittal and (b) axial T1 weighted spin echo images. (c) Axial T2 weighted turbo spin echo image.
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Answer
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On T1 weighted MRI, a midline lesion of high signal intensity with a low signal rim was present in the interhemispheric fissure. On T2 weighted MRI the lesion appeared as an intermediate signal area, again with a low signal rim. The images also showed considerable dilatation of lateral ventricles with periventricular gliosis and thinning of the cerebral cortex. There was also marked cerebellar atrophy and thinning of the brain stem, while the skull vault was markedly thickened. The midline abnormality demonstrated on T1 weighted images could be confused with an area of intracerebral clot. The child had undergone previous cranial CT, shown in Figure 2, which confirmed that the midline area of high attenuation was calcification in the falx. MRI demonstrated an unusual appearance in that hyperintense signal was seen on T1 weighted SE images in an area that was obviously seen as a calcification on CT. This paradoxical appearance of calcification is particularly important to recognize as it can cause diagnostic problems. The cause becomes clear when the lesion is compared with the thickened skull vault and the previous CT images.
The causes of high signal intensity on T1 weighted SE MRI are fat, methaemaglobin, melanin and, rarely, proteinaceous fluid. Slow flowing blood can also cause an increase in signal intensity. Causes for low signal include air, flowing blood, haemosiderin and calcium. The low signal appearances in calcified tissues are a product of the decreased proton density and the changes in T1 and T2 relaxation time.
One of the disadvantages of MRI is that it cannot reliably demonstrate calcification. The most common appearance of calcified tissue on MRI is either null or low signal intensity on T1 weighted images.
Calcified tissue causing high signal intensity has not been widely reported and the reasons for this phenomenon are not fully explained. Experimental models have shown that the increase in signal intensity is owing to shortening of the T1 relaxation time as a result of the paramagnetic effect of the calcium on surrounding water. The degree of T1 shortening may also be related to the surface area of the calcium particulate; the greater the surface area the greater the T1 relaxivity [1]. However, the changes in T1 relaxation cannot be considered in isolation as there are also changes in the T2 relaxation and proton density, which cause opposite signal effects. The T1 shortening is counteracted by a decreasing proton density and shortening of the T2 relaxation time, the latter two factors acting to reduce the MR signal and so offset the high signal T1 effect. This process becomes dominant as the density of the calcification increases [1–3].
Other explanations include the presence in the calcified tissue of paramagnetic metals such as manganese and iron. However, iron must be in a free form to cause T1 shortening and evidence is lacking that it is present in this state [2]. Iron is present in the blood, which can obviously cause confusion.
The MR appearance of calcification may therefore be variable, although it is commonly seen as low intensity signal on T1 weighted images. However, under certain circumstances it can be demonstrated as an area of hyperintensity. Although this appearance is unusual, not every intracranial hyperintense lesion on T1 weighted MR images with corresponding high attenuation on CT imaging necessarily indicates haemorrhage.
Received for publication December 21, 2000.
Accepted for publication January 31, 2001.
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References
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- Dell LA, Brown MS, Orrison WW, Eckel CG, Matwiyoff NA. Physiologic intracranial calcification with hyperintensity on MR imaging: case report and experimental model. Am J Neuroradiol 1988;9:1145–8.[Abstract]
- Henkelman RM, Watts JF, Kucharczyk W. High signal intensity in MR images of calcified brain tissue. Radiology 1991;179:199–206.[Abstract/Free Full Text]
- Hendrick RE, Russ PD, Simon JH. MRI: principles and artefacts. The Raven MRI Teaching File. New York: Raven Press, 1993:189–91.
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