MR Terminology
How do you describe abnormalities on MR?
When describing findings on MR, we use words that refer to signal intensity. "Attenuation" used in CT does not apply in MR. However, like CT, abnormalities are described by their relationship to the intensity of a reference structure (brain tissue, CSF etc.). The descriptors used are hypointense, isointense and hyperintense.
Hypointense
If an abnormality is dark on MR, we describe it as hypointense.
On the T1 sequence, the right parieto-occipital lobe mass is less intense than the adjacent parenchyma and is therefore hypointense
Isointense
If an abnormality is similar to a reference structure, we describe it as isointense.
On this T2 sequence, the right parieto-occipital lobe mass is similar in intensity as the CSF and could be desrcribed as isointense to CSF
Hyperintense
If an abnormality is brighter than a reference structure it is hyperintense.
On this FLAIR sequence, both the mass (blue) and surrounding edema (green) are brighter, or hyperintense to the adjacent brain parenchyma.
MR Sequences
There are several types of MR sequences/images, each of which have unique characteristics and are good for different purposes or in combination can help discern tissue composition. The two most basic image types are T1 and T2 images. T1 or T2 images are obtained by manipulating two basic parameters, TR and TE. TR, repetition time, is the time between one RF transmission, or excitation, and the next. TE, echo time, is the time between the excitation and when the coil is programmed to receive the resultant signal. Other image types include T2 FLAIR, T2*, PD (proton density), and DWI (diffusion weighted imaging). Fat saturation can also be applied to make fat look dark instead of bright. Contrast, in the form of gadolinium, can be administered to highlight different structures or pathology. The following images and table highlight the qualities and differences between sequences that you will encounter in an MR exam.
Sagittal T1
Useful for: Evaluating midline structures
CSF: Dark
White Matter: White
Gray Matter: Gray
Vessels: Dark
Axial T1
Useful for: Evaluating anatomic detail
CSF: Dark
White Matter: White
Gray Matter: Gray
Vessels: Dark
Axial T1 with contrast
Useful for: Evaluating for BBB breakdown in the setting of tumor, infection, MS etc.
CSF: Dark
White Matter: White
Gray Matter: Gray
Vessels: Bright
Axial T2
Useful for: Looking at areas of edema & pathology
CSF: Bright
White Matter: Gray
Gray Matter: Lighter than white matter
Vessels: Dark
Axial FLAIR
Useful for: Evaluating areas of edema or disease within the subarachnoid space with CSF subtraction. Edema stands out because is CSF dark
CSF: Dark
White Matter: Gray
Gray Matter: Lighter than white matter
Vessels: Dark
Axial DWI
Useful for: stroke imaging, abscess, cellular tumors
CSF: Dark
White Matter: Gray
Gray Matter: Lighter than white matter
looks like a FLAIR, but appears a little fuzzier
Diffusion Weighted Imaging
Diffusion weighted imaging (DWI) has many applications, but it most often thought of in work-ups for ischemia/stroke. It is important to have a rough understanding how DWI works to properly interpret the images. DWI detects when water (hydrogen protons) are unable to diffuse freely. Normally cells are packed loosely enough that the water surrounding them can move or diffuse rather easily. With cytotoxic edema in ischemia the cells swell secondary to failure of the Na/K pump and the space between the cells becomes very narrow. Subsequently, the water that normally surrounds the cells cannot diffuse as easily and becomes "restricted." DWI is very sensitive at detecting this change, becoming bright/positive within minutes of acute ischemia.
When there is a tumor, inflammation, or other process that breaks down the capillary endothelium water leaks out of the vessels and there is an increasing amount of water surrounding the brain cells. This is called vasogenic edema. In this case the water can actually diffuse more easily than usual because of the larger gaps between cells. DWI will not be positive in this circumstance.
Courtesy of Aaron Field, MD PhD