applied resulting in an image commonly used for anatomic depiction. Water signal is very low and is displayed as dark gray to black pixels on the gray scale. Fat is very bright, allowing tissue planes to be delineated. Fast flowing blood is devoid of signal and is therefore very black. Muscle tissue is an intermediate gray. Bone which has few free protons is also largely devoid of signal. Bone marrow, however, will vary depending on the relative percentage of red versus yellow marrow. Red marrow will have a signal slightly lower than muscle, whereas yellow marrow (fat replaced) will be bright. In the brain, cerebrospinal fluid (CSF) is dark, and flowing blood is black. Gray matter is dark relative to white matter (contains fatty myelin) but both are higher than cerebrospinal fluid (CSF) but less than fat. Cysts (simple) are dark in signal unless they are complicated by hemorrhage or infection or have elevated protein concentration, which results in an increased signal and slightly brighter display (Figure 2.8) (Table 2.2).
Figure 2.8. Axial MRI T1 weighted image at level of the skull base and brainstem without contrast demonstrating high signal in the subcutaneous fat, intermediate signal of the brain, and low signal of the CSF and mucosa. Note dilated right parotid duct (arrow).
Spin‐echo T2
The T2 images are obtained with a long tr and te. The T2 image is sensitive to the presence of water in tissues and depicts edema as a very bright signal. Therefore, CSF or fluid containing structures such as cysts is very bright. Complicated cysts can vary in T2 images. If hemorrhagic, they can have heterogenous or even uniformly dark signal caused by a susceptibility artifact. These artifacts can be caused by metals, melanin, forms of calcium and the iron in hemoglobin. Increased tissue water from edema stands out as bright relative to the isointense soft tissue. The fast spin‐echo T2 is a common sequence, which is many times faster than the conventional spin‐echo T2 but does alter the image. Fat stays brighter on the fast spin‐echo (FSE) sequence relative to the conventional (Figure 2.9) (Table 2.2).
Proton density images (PD)
Proton density images are obtained with a long tr but short te, resulting in an image with less tissue contrast but high signal‐to‐noise ratio. These are uncommonly used in the head and neck.
Table 2.2. Tissue characteristics on T1 and T2 MRI.a
| T1 | T2 | |
| Increased signal | FatCalciumbProteinaceous fluid (high)cSlow flowing bloodMelaninHyperacute hemorrhaged (oxyhemoglobin)Subacute hemorrhage (intracellular and extracellular methemoglobin)Gadolinium contrastManganeseCholesterol | Water (CSF) or edemaProteinaceous fluidHyperacute hemorrhage (oxyhemoglobin)Subacute hemorrhage (extracellular methemoglobin)Slow flowing bloodFat (FSE T2 scans) |
| Intermediate signal | Hyperacute hemorrhage (oxyhemoglobin)Acute hemorrhage (deoxyhemoglobin)CalciumbGray matterWhite matter (brighter than gray matter)Soft tissue (muscle)Proteinaceous fluidc | Grey matter (brighter than white matter)White matterProteinaceous fluidcCalciumb |
| Decreased signal | Water (CSF) or edemaFast flowing bloodCalciumbSoft tissueAcute hemorrhage (deoxyhemoglobin)Chronic hemorrhage (hemosiderin)CalcificationAirSimple cyst (low protein) | CalciumbMelaninHemosiderinFlowing bloodhemorrhagic cystIron depositionAcute hemorrhage (deoxyhemoglobin)Early subacute hemorrhage (intracellular methemoglobin)Chronic hemorrhage (hemosiderin)AirFast flowFat (conventional or non‐FSE T2 scan) |
a MRI signal on T1 and T2 predominantly from intracranial exam at 1.5T (Tesla).
b Signal from calcium deposition is complex. Calcium concentrations of under 30% by weight have high T1 signal and intermediate T2 signal, but over 40% have decreasing signal on T1 and T2. The surface area of the calcium particle also has an effect, with large surface area resulting in increased T1 signal (Henkelman et al. 1991).
c Depends on the protein concentration (complex cysts, abscess).
d MRI signal of intracranial hemorrhage is quite complex and dependent on multiple factors with degrees of variability.
CSF = cerebrospinal fluid.
Gradient recalled echo imaging (GRE)
Gradient recalled echo imaging is the second most common type of imaging sequence after the spin‐echo. This sequence is very susceptible (more than spin‐echo T2) to magnetic field inhomogeneity and is commonly used in the brain to identify blood products, metal deposition such as iron, manganese, and nonmetals such as calcium. This sequence is very sensitive but not specific. The “flip angle” used in obtaining GRE can be altered, resulting in either T1 weighted (long flip angle) or T2 weighted (short flip angle) images (Figure 2.10).
Short tau inversion recovery (STIR)
Short tau inversion recovery (STIR) is commonly acquired because of its very high sensitivity to fluid and readily detects subtle edema in tissues. When acquired in the conventional method, it also results in nulling the fat signal, thereby further increasing the signal of tissue fluid relative to background. This is the best sequence for edema, particularly when trying to determine bone invasion by tumors. It can also be useful in assessing skull base foramina (Figures 2.11 and 2.12).