Metastases

Metastatic tumours are actually more common than primary brain tumours and their incidence is increasing [56, 57]. They occur in middle-aged and older adults and can originate from almost any systemic can cers, although carcinomas oflung, breast, skin and kidneys, and melanomas are the most common causes of brain metastases [57]. On conventional MRI, most brain metastases are seen as multiple, nodular, well-circumscribed enhancing lesions, often at the grey-white matter junction, with central necrosis or haemorrhage and surrounding vasogenic oedema [58]. The diagno

Fig. 18.9. Contrast-enhanced T1-weighted (a), FLAIR (b) and T2-weighted (e) images, ADC (c), rCBV (d), Cho (f),NAA (g), Cr (h) andLL (i) maps, and proton MR spectra (1-9) from selected ROIs in a 60-year-old man with a left frontal solitary metastasis from a thyroid carcinoma. ROIs are a necrotic core surrounded by a thin ring-like enhancement (1,2); vasogenic oedema (3,4); homolateral (5, 6) and contralateral (7-9) normal ROIs. Note: the absence of Cho, Cr and NAA, and the presence of lipid resonance at 2.02 ppm in the spectra of the necrotic core; the lower levels of metabolites, and the higher ADC and lower rCBV in oedema ROIs as compared to normal ROIs

Fig. 18.9. Contrast-enhanced T1-weighted (a), FLAIR (b) and T2-weighted (e) images, ADC (c), rCBV (d), Cho (f),NAA (g), Cr (h) andLL (i) maps, and proton MR spectra (1-9) from selected ROIs in a 60-year-old man with a left frontal solitary metastasis from a thyroid carcinoma. ROIs are a necrotic core surrounded by a thin ring-like enhancement (1,2); vasogenic oedema (3,4); homolateral (5, 6) and contralateral (7-9) normal ROIs. Note: the absence of Cho, Cr and NAA, and the presence of lipid resonance at 2.02 ppm in the spectra of the necrotic core; the lower levels of metabolites, and the higher ADC and lower rCBV in oedema ROIs as compared to normal ROIs sis of intracranial metastases on conventional imaging is usually uncomplicated and straightforward. However, differentiation between solitary metastases, which occur in 30-50% of cases, and high-grade gliomas is difficult because both may exhibit oedema, mass effect and central necrosis, especially if there is no history of systemic cancer [54]. It is also difficult on conventional imaging to distinguish among metastases from different origins. *H-MRSI usually shows an absence of metabolites or high LL peak in the necrotic core, and a variable amount of Cho and LL along the enhanced margins; NAA and Cr are absent or markedly reduced [3]. Perienhancing regions of metastasis show Cho and rCBV lower [34,36] and ADC higher [36,59] than those of high-grade gliomas. Perienhancing regions of metastasis, in fact, represent pure vasogenic oedema since it does not contain infiltrating tumour cells [39], while those of high-grade gliomas contain tumour cells infiltrating along the perivascular spaces [37]. *H-MRSI, DWI and PWI may be helpful then to distinguish pre-operatively solitary metastases from high-grade glio-mas (Fig. 18.9). PWI may also allow the distinction of hypervascular metastases, such as those from renal carcinoma or melanoma, from other types of metastases [60].

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