By Emmanuel W. FiagbedziMaster of Medical physicsICTP-UNITSTrieste

04/15/23

Image Artifact is something observed in a scientific investigation that is not naturally present but occurs as a result of the investigative procedure. (oxford dictionary).

All MRI images have artifacts to some degree. Some are irreversible and may only be reduced while others can be totally eliminated. Knowledge of artifacts is a must in order to maintain optimum image quality.

Artifacts are classified as to their basic principles, Physiologic (motion, flow), Hardware (electromagnetic spikes, ringing),Inherent physics (chemical shift, susceptibility) etc

Examples of artifacts include chemical shift artifacts, motion, zipper,eddy current etc.

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Water-fat shift (WFS) is defined as the displacement of the water signal with respect to fat signal in an IMR image.

Water fat shift artifacts are common in vertebral bodies, orbits, solid organs surrounded by fat

Fat protons resonate at slightly lower frequencies than water. The frequency difference is called chemical shift. The amount of WFS is proportional to the main magnetic field. Amount of chemical shift is expressed in arbitrary units known as parts per million (ppm) .

Two types of chemical shift artifacts exist :

Type 1 is seen in the frequency-encoding direction and only concerns field strengths higher than 1 T

Type 2 can be found at any field strength but requires GE sequences with particular TEs.

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In type 1,during frequency encoding, fat protons preccess slower than water protons in the same slice because of their magnetic shielding.

Through the difference in resonance frequency between water and fat, protons at the same location are misregistrated (dislocated) by the Fourier transformation, when converting MRI signals from frequency to spatial domain.

This chemical shift misregistration cause accentuation of any fat-water interfaces along the frequency axis and may be mistaken for pathology. Where fat and water are in the same location, this artifact can be seen as a bright or dark band at the edge of the anatomy

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FourierTransform

Received SignalImage

www.imaios.com/en/e-Courses/e-MRI/Image-quality-and-artifacts/chemical-shift04/15/23

This frequency difference results from the different electron environments

of the protons of water and of fat.

The difference in chemical shift is approximately 3.5 parts-per-million

(ppm) which at 1.5 Tesla corresponds to a frequency difference between that of fat and water of approximately 220 Hz.

As a result, fat containing structures are shifted in the frequency direction

from their true positions.

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www.imaios.com/en/e-Courses/e-MRI/Image-quality-and-artifacts/chemical-shift

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Shoulder image withclear water-fat shift.

Blue is position ofwater image, yellow isfat image

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The chemical shift artifact of the second kind only occurs with gradient echo sequences. With spin echo sequences, the 180° pulse refocuses spins to create the echo.

The absence of a 180° RF pulse in gradient echo sequences causes a phase shift between protons of fat and water when the (gradient) echo is formed. This phase shift depends on their resonance frequency shift due to the chemical shift.

With a 1.5 T field strength, the frequency shift is 225 Hz, corresponding to a period of 4.4 ms. Therefore, at 1.5 T, protons of fat and water will be in phase every 4.4 ms : their signals are additive. For TEs between this interval of 4.4 ms, their phases are shifted and for a TE at the middle of this interval (2.2 ms), they are out of phase.

The signal intensity of a voxel containing fat and water oscillates with an increasing echo time, with a minimum when fat and water are out of phase, and a maximum when they are in phase

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For TE corresponding to fat and water out of phase (2.2 msec), the signal of voxels containing the same proportion of fat and water is canceled, producing a black line at all fat/tissue borders.

This contour artifact is known as the chemical shift artifact of the second kind.

It is never seen with spin echo sequences as the phase shifts due to chemical shift are canceled by the 180° refocusing pulse

• http://www.imaios.com/en/e-Courses/e-MRI/Image-quality-and-artifacts/artifacts

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Fat suppression is the process of utilizing specific MRI parameters to remove the deleterious effects of fat from the resulting images, e.g. with Spectral fat saturation, STIR(short inversion time inversion recovery),water selective excitation techniques, or pulse sequences based on the Dixon method.

We shall discuss about three of these methods: Spectral fat saturation, STIR and the Dixon method.

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With this form of fat suppression the fat resonance is excited selectively and then the signal is “spoiled”using gradient pulses. The fat spins are initially tipped into the transverse plane using a special 90° pulse that affects only the fat spins.

After the rf pulse, the fat spins are aligned perpendicular to the main magnetic field, B0, while the water spins are still parallel to B0

If a signal were to be measured at this point it would have contributions from fat spins only. However, spoiler gradient pulses are used to dephase the fat spins causing the fat signal to decay to zero without affecting the water spins, which are still in equilibrium. At this point, the fat signal is said to be “saturated

The fat signal has beensuppressed and a standard MR sequence can now be initiated. The resulting image should, in principle,have no contribution from fat spins.

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Short inversion-Time Inversion Recovery (STIR) employs a 180° inversion pulse to invert all magnetization.

This 180° RF pulse causes an initial inversion of the longitudinal magnetization (so that it is aligned in the z direction), as shown in the next slide

The magnetization then begins to grow back in the direction of the main magnetic field(z).

The magnetization of different tissues will grow back at different rates. When the signal from the tissue to be suppressed crosses the zero axis, application of a 90° RF pulse will rotate all other signals into the transverse plane.

Since the signal from the tissue at the zero point is zero, there is nothing to rotate into the transverse plane. Thus, this tissue will not contribute any brightness to the resulting image

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Inversion of the signal in the inversion-recovery sequence. After initial inversion of the longitudinal magnetization, T1 relaxation occurs and the signals from different tissues cross the zero axis at different times. When the signal to be suppressed crosses the zero axis, a 90° RF pulse will rotate all other signals into the transverse plane for image formation. TI inversiontime.

NB: STIR is based on the difference in T1 relaxation times between water and Fat.

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The principle upon which the Dixon technique is based is that, since fat and water have different resonance frequencies, they will also precess in the transverse plane at different rates (i.e. they have different Larmor precession frequencies).

By adjusting the sequence timing, the phase of the fat spins relative to the water spins in the transverse plane can be adjusted to whatever phase angle is desired when the signal is acquired.

In this method two images are acquired; one with the fat and water spins in‐phase and the other with them out‐of‐phase. These images can be obtained from separate acquisitions or as different echoes of the same acquisition.

If these two images are added together pixel by pixel the result will be a fat suppressed image

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Chemical shifts artifacts(water fat shift) are evident in MRI and shouldnot be confused with pathology.

Every effort should be made to correct them in order to improve image quality and aid effective diagnosis.

THANK YOU

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oxford dictionary Prof longo lecture notes on MR spectroscopy www.mr-tip.com .www.revisemri.com Morelli JN, Runge VM, Ai F, et.al. An Image-based Approach to

Understanding the Physics of MR Artifacts. RadioGraphics2011; 31:849–8662

http://www.imaios.com/en/e-Courses/e-MRI/Image-quality-and-artifacts/artifacts

W.T. Dixon, Simple Proton Spectroscopic Imaging, Radiology 153 : 189‐194.

M.A. Bernstein, K.F. King, X.J. Zhou, Handbook of MRI Pulse Sequences,New York, Elsevier Academic Press, 2004. p. 857‐887.

G.H. Glover, E. Schneider, Multipoint Dixon Technique for Water and Fat Proton and Susceptibility Imaging, J. Magn. Reson. Imaging 1:521‐530

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