![]() ![]() At the concentrations used, these agents have the effect of causing T1 signal to be increased (this is sometimes confusingly referred to as T1 shortening). The most commonly used contrast agents in MRI are gadolinium based. white matter: hyperintense compared to grey matter (white-ish).grey matter: intermediate signal intensity (grey).muscle: intermediate signal intensity (grey).urine, CSF): low signal intensity (black) The dominant signal intensities of different tissues are: T1 weighted (T1W) sequences are part of almost all MRI protocols and are best thought of as the most 'anatomical' of images (historically the T1W sequence was known as the anatomical sequence), resulting in images that most closely approximate the appearances of tissues macroscopically, although even this is a gross simplification. Again many use these words as if they are absolute terms and this leads to confusion (more on this issue here). When describing diffusion weighted sequences, we also use the term intensity but additionally we use the terms "restricted diffusion" and "facilitated diffusion" to denote whether water can move around less easily (restricted) or more easily (facilitated) than expected for that tissue. NB: the word density is for CT, and there are few better ways to show yourself as an MRI noob than by making this mistake. "the lesion is hyperintense to the adjacent spleen". Does this denote a lesion darker than CSF or than the adjacent brain?Īs such it is preferable to either use absolute terminology or, if using relative terms, to acknowledge the comparison tissue e.g. In many other situations however use of relative terms leads to potential confusion. Imagine a lesion within the ventricles of the brain described as "hypointense". In some instances this does not lead to any problems for example, a hyperintense lesion in the middle of the liver is clearly hyperintense compared to the surrounding liver parenchyma. hypointense = darker than the thing we are comparing it toĪnnoyingly these relative terms are used without reference to the tissue being used as the comparison.isointense = same brightness as the thing we are comparing it to.hyperintense = brighter than the thing we are comparing it to.Often we refer to the appearance by relative terms: When describing most MRI sequences we refer to the shade of grey of tissues or fluid with the word intensity, leading to the following absolute terms: This leads to a broad categorization as follows: A number of 'optional add-ons' can also be considered, such as fat or fluid attenuation, or contrast enhancement. This leads to a division of all sequences into proton density (PD) weighted, T1 weighted, T2 weighted, diffusion weighted, flow sensitive and 'miscellaneous'. The simplest way to think about the multitude of sequences available on modern scanners is to divide them according to the dominant influence on the appearance of tissues. ![]()
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