MRI Technique

General MRI Terms

Signal intensity:
In MRI the terms low, intermediate and high signal intensity are used. Depending on the scan protocol, tissue is imaged as white (= high signal intensity), as a gray tone (= intermediate signal intensity) or as dark gray/black (= low signal intensity).
 
Gradient  
An additional magnetic field that may be added manually to the MRI device's magnetic field. This creates an additional subdivision in the ‘total’ magnetic field. Gradient coils are used among other things to determine the location of the protons in the X, Y and Z axis.

Sequence:  
Combination of radiofrequent pulses and gradients (= ‘added magnetic fields’), which together constitute the building blocks for an MRI series. For instance, the terms ‘T1 weighted sequence’ and ‘T2 weighted sequence’ are used.

MRI Sequences

• T1 weighted sequence
• T2 weighted sequence
• PD weighted sequence
• Gradient & spin echo sequence
• Fat suppression
• MRI contrast
• Diffusion weighted image
• In/out-of-phase

Contrast differences are required to distinguish normal anatomy from pathology. Contrast is improved when two adjacent areas have high and low signal intensities. There are many different MRI sequences (>100) and all attempt to optimize tissue contrast.
Each MRI image consists of a T1 component and a T2 component (see also Relaxation section). It is possible to switch off most of one of either components, creating a T1 weighted or T2 weighted image respectively. A special form is the proton density (PD) weighted image. This sequence enables the visualization of the number of protons per volume. In order to achieve this, both the T1 and T2 components must be switched off.
The following briefly describes some commonly used MRI sequences.