After reading this chapter, you should be able:
Which also modify the available sequence parameters (TE) and the artifacts.
|The signal to noise ratio goes up||Penalty|
|With the voxel size||Limits the spatial resolution|
|When a local or surface coil is used instead of a large or body coil||Reduced field of exploration|
|With the number of averagings (square root factor)||Longer scan time|
|When the receiver bandwidth decreases (square root factor)||Higher chemical shift artifacts|
Motion is responsible for a corruption in spatial localization in the phase-encoding direction, resulting in a blur and ghost images propagated in the phase-encode direction.
|Reduce body movements (physical restraint, sedation)||Patient's comfort|
|Breath-hold sequences||Requires apnea and fast sequences|
|Gating and movement compensation||
Increase the scan time, restrict the available TRs,
a blur persists with some methods
|Signal suppression of moving tissues (fat of abdominal wall...)||Increased TR due to magnetization preparation or saturation bands|
|Swapping phase-encode and frequency-encode directions||Only shifts the artifacts, consequences on sequence parameters and other artifacts|
At the interface between two tissues with different magnetic susceptibilities, there are local distortions in the magnetic field responsible for a signal loss (and sometimes an image distortion). These artifacts are much stronger in presence of metal.
|SE rather than GE sequences|
|Short TE||Restricts available contrasts|
|Increased receiver bandwidth to reduce the minimal TE||Decreased SNR|
The image is reconstructed from k-space by a 2D inverse Fourier transform. As k-space data are finite and defined by the matrix size, the reconstruction of high-contrast interfaces is imperfect and causes visible artifacts. These artifacts appear as multiple parallel lines adjacent to the interface or as false widening of the edges at this interface. Truncation artifacts can occur in both the frequency and phase-encode directions.
|Incread matrix size||
Increased scan time
Aliasing or wrap-around results from a spatial mismapping caused by an undersampling in the phase-encode direction. Consequently, objects outside of the FOV overlap on the opposite side of the image.
|Swapping the frequency-encode and phase-encode directions|
|Increasing the FOV||Decreased spatial resolution|
|Phase oversampling||Increased scan time|
|No-phase wrap or Anti-aliasing||Decreased SNR|
The chemical environment of protons can cause a shift in their precessional frequency. The shift in resonance frequency between protons in fat and water is roughly equal to 3.5 ppm, corresponding to a difference of about 225 Hz at 1.5 T.
This frequency shift is responsible for a spatial misregistration in the frequency-encode direction, resulting in a contour artifact with dark and white bands at the interfaces between fat and tissues in the frequency-encode direction : the chemical shift artifact of the first kind.
As the resonance frequency shift causes a phase shift which is not canceled in gradient echo sequences, there is a black line at all fat/tissue borders when fat and water are out of phase (TE = 2.2 ms at 1.5 T) : the chemical shift artifact of the second kind. This artifact occurs in both the frequency-encode and phase-encode directions.
|Fat suppression||Increased scan time|
|Swapping of the frequency-encode and the phase-encode directions||Rotates the artifact without eliminating it|
|Increased receiver bandwidth||Decreased SNR|
Tendons and ligaments imaging (T1-weighted, +/- gado / MTC)