MRI ARTIFACTS(FOCUS ON WATER FAT SHIFT)

1. By Emmanuel W. Fiagbedzi
Master of Medical physics
ICTP-UNITS
Trieste
06/04/15

2.  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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3.  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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4.  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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5. Fourier
Transform
Received Signal
Image
www.imaios.com/en/e-Courses/e-MRI/Image-quality-and-artifacts/chemical-shift
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6.  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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7. www.imaios.com/en/e-Courses/e-MRI/Image-quality-and-artifacts/chemical-shift
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8. Shoulder image with
clear water-fat shift.
Blue is position of
water image, yellow is
fat image
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9.  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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10.  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
06/04/15

11.  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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12.  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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13.  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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14. 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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15.  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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16.  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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17.  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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