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Regarding relaxation times and the signal of different tissues: a. Cortical bone has long T1 and T2 relaxation times b. Water is hyperintense on both T1- and T2-weighted sequences c. Grey matter has a lower signal than white matter on T1 sequences d. Melanoma metastases are usually of low T2 signal e. Hyperacute intracerebral haemorrhage is bright on both T1 and T2 images |
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Answer» a. False. Cortical bone is essentially a solid, despite a water content of 10–15%. It has an extremely short T2 of <1 ms; therefore, it appears black on conventional T1- and T2-weighted sequences. b. False. Water has a long T1 and T2; therefore, it appears hypointense on T1 and hyperintense on T2 sequences. This makes T2 sequences sensitive for pathology, as most pathological processes (e.g. oedema, tumour, infarction) are associated with increased tissue water content. c. True. Grey matter has slightly longer T1 and T2 relaxation times than white matter, and so is darker on T1 and brighter on T2 sequences. The way to remember this is that grey matter is ‘grey’ on the sequence that better shows anatomy (T1-weighted). d. True. There are only a few causes of a low T2 signal; these include melanin, calcification, fibrous tissue, high protein content and flow void. e. False. The appearance of haemorrhage on T1- and T2-weighted imaging changes with time due to the temporal degradation of blood products according to the table on p. 135. Hyperacute blood is isointense or dark on T1 and bright on T2 sequence. |
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