Relaxation rates of neoplastic tissues

A seminal finding that has facilitated the identification of malignancy by magnetic resonance is that neoplastic tissue generally relaxes at rates different from the surrounding tissue [1].

An early NMR study [2] in rats revealed that Ti relaxation times were nearly three times longer in 3 Novikoff hepatomas compared to 5 normal liver specimens. The T2 relaxation times were about twice as long in the tumors. Subsequently, these authors reported that a combined Ti and T2 normalized NMR malignancy index distinguished cancerous and normal tissues in 36 colon samples, 22 of 23 breast samples and 26 of 29 lung samples [3]. However, in a more recent study [4] human surgical samples of malignant and normal large bowel, while showing statistically significant differences in Ti and T2 relaxation times, these differences were considered "too small to use the NMR method alone for diagnosis" (p. 51).

Many cancers appear hyper-lucent on T2 weighted images. However, if the surrounding tissue, e.g. endometrium is hyper-intense on T2 weighted images, neoplastic tissue, i.e. endometrial carcinoma, can be of intermediate intensity [1]. Prostate cancer, on the other hand, is generally of low signal intensity on T2 weighted images. There are also exceptions based upon histological type. Sometimes, tumors may be iso-intense to their surroundings. On the other hand, a difference in signal intensity compared to surrounding tissue is by no means pathognomonic for neoplasia.

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