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MR Image quality and artifacts

Signal-to-noise ratio

by Denis Hoa

 

 

 

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Noise is like interferences which present as a irregular granular pattern. This random variation in signal intensity degrades image information. The main source of noise in the image is the patient's body (RF emission due to thermal motion). The whole measurement chain of the MR scanner (coils, electronics...) also contributes to the noise. This noise corrupts the signal coming from the transverse magnetization variations of the intentionally excited spins (on the selected slice plane).
The signal to noise ratio (SNR) is equal to the ratio of the average signal intensity over the standard deviation of the noise.

 

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The signal to noise ratio depends both on some factors that are beyond the operator's control (the MR scanner specifications and pulse sequence design) and on factors that the user can change:

  • Fixed factors : static field intensity, pulse sequence design, tissue characteristics
  • Factors under the operator's control
    • RF coil to be used
    • Sequence parameters : voxel size (limiting spatial resolution), number of averagings, receiver bandwidth

 

RF coil

 

The smaller the sensitive volume of a coil, the lower the noise from the adjacent structures of the selected slice plane which it can detect, and the better the signal to noise ratio will be.
A local coil, or better, a surface coil have a higher signal to noise ratio than a body coil.

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Paramètres de séquence

Voxel volume

The signal comes from the excited protons on the selected slice plane. The number of spins in parallel state in excess is proportional to the static magnetic field intensity. The larger the field intensity is, the higher the excess number of spins in parallel state (available to make the MR signal) will be. Thus, the signal intensity varies almost linearly with the main field intensity.
Assuming a uniform proton density, the number of excited spins is proportional to the voxel size and so is the signal intensity. The signal goes up linearly with the voxel size.

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To sum up, MRI is a compromise between:

  • Spatial resolution:limited by the voxel size which is determined by the matrix size, the field of view and slice thickness
  • Signal to noise ratio:depending on the voxel size, the number of averagings and the receiver bandwidth
  • Total scan time

Which also modify the available sequence parameters (TE) and the artifacts.

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Number of excitations

When the number of excitations (or averagings) for the same slice increases:

  • The signal is identical for each measure
  • The noise is random and is not the same for each measure

Therefore, the signal sum goes up linearly with the number of excitations but the noise only goes up with the square root of the number of excitations.
In other words, the average signal remains constant, but the average noise goes down with the square root of the number of excitations.
The signal to noise ratio goes up with the square root of the number of excitations.

 

En cumulant les mesures pour une même coupe, le signal augmente de façon proportionnelle au nombre de mesure, alors que le bruit ne va augmenter que proportionnellement à la racine carré du nombre de mesures. En moyennant plusieurs mesures pour une même coupe, la moyenne du signal reste donc constante alors que la variabilité du bruit diminue.
Le rapport signal / bruit augmente proportionnellement à la racine carré du nombre de mesures.


Given a voxel size and static field strength, the number of excited spins is defined and so is the amount of MR signal. The readout of the MR signal can take more or less time, depending on the receiver bandwidth. The relation between the receiver bandwidth and the strength of the readout gradient is such that:

  • a broad bandwidth corresponds to a fast sampling of the MR signal and a high-intensity readout gradient
  • a narrow bandwidth corresponds to a slow sampling of the MR signal and a low-intensity readout gradient

Background noise has a constant intensity at all frequencies (white noise). Therefore, the larger the receiver bandwidth is, the more noise is recorded (and the higher is the readout gradient intensity and the faster the MR signal is sampled).

 

To sum up, MRI is a compromise between:

 

 

  • Spatial resolution:limited by the voxel size which is determined by the matrix size, the field of view and slice thickness
  • Signal to noise ratio:depending on the voxel size, the number of averagings and the receiver bandwidth
  • Total scan time

Which also modify the available sequence parameters (TE) and the artifacts.

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Receiver bandwith

Given a voxel size and static field strength, the number of excited spins is defined and so is the amount of MR signal. The readout of the MR signal can take more or less time, depending on the receiver bandwidth. The relation between the receiver bandwidth and the strength of the readout gradient is such that:

  • a broad bandwidth corresponds to a fast sampling of the MR signal and a high-intensity readout gradient
  • a narrow bandwidth corresponds to a slow sampling of the MR signal and a low-intensity readout gradient

Background noise has a constant intensity at all frequencies (white noise). Therefore, the larger the receiver bandwidth is, the more noise is recorded (and the higher is the readout gradient intensity and the faster the MR signal is sampled).

 

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To sum up, MRI is a compromise between:

  • Spatial resolution:limited by the voxel size which is determined by the matrix size, the field of view and slice thickness
  • Signal to noise ratio:depending on the voxel size, the number of averagings and the receiver bandwidth
  • Total scan time

Which also modify the available sequence parameters (TE) and the artifacts.

Javascript is required to display Flash animations