Brain MRI Sequences

Brain MRI Sequences

08/21/2015

1. Sequences a. Dominant weighting b. Modifiers 2. T1 and T2-weighted a. Two fundamental anatomic sequences b. T1 – TR short and TE short (repetition time, echo time) c. T2 – TR long and TE long d. Best way to differentiate is the relationship between gray matter and white matter i. T1 – gray matter is gray (hypointense) and white matter is white (hyperintense) 1. Hyperintense on T1: blood product, fat, proteinaceous material and contrast enhancement ii. T2 – gray matter is white and white matter is gray

3. Proton-density a. Intermediate sequence characterized by long TR but short TE b. Makes white matter lesions easier to identify than on T2 and T1 c. Not used as much anymore because it has been superseded by FLAIR

i. However, this is used still in other parts of the body (particularly MSK)

4. C+ a. Gadolinium-enhanced b. Administer contrast and then perform T1-weighted sequences c. E.g. in a thalamic primary brain tumor i. Hemorrhagic change – intrinsically high signal in T1 ii. Another component of it demonstrates contrast-enhancement

5. FLAIR (fluid attenuation inversion recovery) a. Most common modifier for T2-weighted b. Attenuates CSF c. Relationship of white matter and gray matter is the same as T2 i. Making the relationship of gray and white matter a better determinant whether it is T2 or T1

6. Fat suppression + T1 a. A modifier of T1 b. STIR – short tau inversion recovery c. Can be done pre-contrast or post-contrast d. Not subcutaneous fat becoming hypointense e. Lipoma in the brain

T1-weighted (lipoma)

It is a bit difficult to tell whether there is contrast-enhancement at the margins or whether this represents merely fat. Fat is hyperintense on T1. So you can perform fat saturation or fat suppression where only fat loses signal and contrast remains high signal (hyperintense)

You can see at the margins that there’s only very minimal contrast enhancement with the bulk of the tumor completely attenuating out. Note that it is the subcutaneous fat that becomes dark and the scalp remaining bright post contrast Note: fat-saturated post-contrast T1 sequences are routine in most parts of the body because of the presence of significant amounts of fat. This is not the case in the brain where fat is an abnormal substance. And as such for purely parenchymal lesions fat saturation is not usually performed. The exception to this is intracranial masses that are involving the skull or skull base where an extracranial extension is being sought. So the most common situation for this to be performed is in base of skull meningiomas or CPA masses where potential for extracranial spread is present 7. Fat suppression + T2 a. Performed as part of a gradient echo sequence where the intention is not particularly to suppress fat, but to make it more susceptible to paramagnetic effects

b. In the orbits, or base of the skulls, to examine structures that are otherwise closely related to fat i. Common in the case of orbits where the extraocular muscles and optic nerve are surrounded by fat and that examining for abnormal signal within either of those structures is much more easily performed with fat attenuation c. For intracranial imaging, also important in looking for CSF leak

8. Susceptibility weighted (SWI) a. Represent a number of different sequences that share the propensity to have signal loss due to paramagnetic or diamagnetic effects b. Calcium or blood product will result in dark or black signal c. Exquisitely sensitive to very small amounts of such materials d. Photo below: familial autosomal dominant multiple cavernoma syndrome i. If you look at the T1 or T2 sequences, these abnormalities would be much smaller ii. The phenomenon is called blooming: signal loss extends beyond the anatomical confines of the lesion – due to the fact that paramagnetic or diamagnetic materials distort the magnetic field locally beyond their margins e. SWI at higher field strengths are particularly sensitive

f. Susceptibility sensitive sequences gradient echo imaging

g. May deserve their own column but placed under T2-weighted because many of these are T2* weighted 9. Diffusion weighted imaging

a. Encompass DWI (or isotropic imaging) and ADC; can include diffusion tensor imaging b. DWI i. Dirty sequence – made up of both diffusion information and T2 information ii. Examining DWI alone can lead you to erroneously interpret high signal as representing true abnormal restricted diffusion when in fact what you are seeing is so called T2 shine-through

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Apparent Diffusion Coefficient a.

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Diffusion tensor a. Allows for tractography b. Research based or operative planning

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Flow sensitive sequences a. MR angiography i. Usually without intravenous contrast and relies on blood bringing with it signal ii. Usually shown as MIPs 1. Because there is little background for the vessels to be localized against, it can be difficult to know exactly where one is on a single image iii. Photo: young patient with vein of Galen malformation

b. MR i. ii. iii.

venography Can use phase contrast or similar to MR angiography For dural venous sinuses or cerebral veins Photo: dural venous thrombosis (posterior part of the superior sagittal sinus)

c. CSF flow studies

i. Same principles as phase contrast venography can be used to look at the pulsatile flow of CSF in cisterns and aqueduct as shown in photo ii. Can be useful to exclude aqueduct stenosis or hyperdynamic flow of normal pressure hydrocephalus needs to be evaluated

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Miscellaneous a. MR spectroscopy i. Performed routinely as part of brain tumor or mass work up

b. Functional MRI i. Image during memory or verbal tasks

c. MR Perfusion i. Routine in assessment of tumors and neurodegenerative conditions

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Summary table