PULSE SEQ Flashcards
Conventional Spin Echo
The SE is the most basic imaging pulse sequence in MRI. It is used to produce T1, T2 and PD weighted sequences.
Spin Echo
Spins loses their precessional coherence after the 900 excitation pulse because of an increase or decrease in their precessional frequencies caused by magnetic field inhomogeneities.
Fast or Turbo Spin Echo Sequences (FSE or TSE)
These sequences are much faster than the conventional SE sequences.
They use a train of 1800 rephasing pulses and each one produce a spin echo.
This is called an echo train and the number of 1800pulses and resultant echoes is called echo train length or turbo factor.
Because these sequences are faster than conventional SE sequences we can increase the matrix to improve the spacial resolution.
Inversion Recovery (IR)
The IR sequence is a spin echo sequence that begins with an 1800 inverting pulse that inverts the NMV through 1800.
The TR is the time between successive inverting 1800 inverting pulses for a specific slice.
STIR (Short TI Inversion Recovery)
This sequence uses very short TI’s.
A 900 excitation pulse is applied at a time when the NMV of fat is passing exactly through the transverse plane.
It is called the null point because there is no longitudinal component in fat and therefore also no signal.
FLAIR (Fluid Attenuated Inversion Recovery)
This sequence uses long TI’s to null the signal from CSF in exactly the same way as a STIR sequence.
Gradient Echo
These sequences use a gradient to rephrase the spins as opposed to an 1800 RF pulse as in the SE sequence.
The frequency encoding gradient performs
the rephrasing in a
gradient echo scan.
Diffusion Weighted Imaging (DWI)
The term diffusion describes moving molecules.
They move due to their random thermal motion.
This motion can be restricted by ligaments, membranes, macro-molecules and pathology.
ADC (Apparent Diffusion Coefficient) is the term used to describe the rate of diffusion.
K - Space
Spacial encoding allows spins to be shifted along the phase and frequency axis of an image.
Data Matrix
Pixel – picture element/ Voxel – volume element
Each pixel possesses a specific gray value - When viewed together the pixel matrix creates an image.
Signal intensity of pixel depends on number of excited spins in corresponding voxel.
Contrast to Noise Ratio (CNR)
CNR is the difference in the SNR between two adjacent areas.
It is controlled by the same factors that affect the SNR.
Images should always have a good CNR to demonstrate the difference between anatomy and pathology.
Spacial Resolution
It is the ability to distinguish between two points as separate and distinct and is controlled by the voxel size.
Scan Time (ST)
Scan time is the time required to complete the acquisition of data.
Magnetic Field Strength
Increasing field strength will increase the number of excited protons; therefore there is a stronger net magnetization vector.
