The document discusses magnetic resonance imaging (MRI) hardware components and artifacts. It describes the basic principles of MRI including proton imaging, spin and precession. It explains the main hardware components including the magnet, gradients, radiofrequency coils and system components. It then discusses various types of artifacts including patient-related, hardware-related and processing artifacts. Corrective measures for common artifacts like motion, chemical shift and aliasing are provided. The document emphasizes the importance of understanding artifacts to avoid confusing them with pathology.

1. PRESENTER : Dr. NAVNEET

2.  Introduction
 Part I – MRI Hardware
 Part II – MR Artifacts
 Summary

3.  MR imaging is based on proton imaging.
 Most of the signals on MR images comes from water
molecules are mostly composed of hydrogen(H+).
 Spin : rotation of a proton around its axis.
 Precession : rotation of the axis itself under the influence
of external magnetic field.
 Larmours equation

5. Four basic steps are involved in MR
imaging :
1. Placing the patient in the magnet.
2. Sending the radiofrequency pulse by coil.
3. Receiving signals from patient by coil.
4. Transformation of signals into images by Fourier
transformation.

6.  Main magnetic fields are oriented in space along x ,y and z-
axis.
- Longitudinal magnetisation.
- Transverse magnetisation

7.  3 Gradient fields :
- These have different strength in varying locations .
- produced by gradient coils.
1. Slice selection gradient
2. Frequency encoding gradient
3. Phase encoding gradient

8. 1. Cylindrical
magnet
2. Gradient coil;
3. RF body coil
4. Patient
5. Patient table;
6. Head coil
7. Transmit/receive
chain;
8. Imaging display.

9. All MRI scanners include several essential components
1. Polarizing magnetic field (B0)/Magnet
2. Secondary magnetic fields
 Gradients
 Radio frequency (RF) irradiation (B1)

13. Magnet types used in MRI may be classified into
three categories:
 Permanent.
 Resistive.
 Superconducting.

14.  Composed of one or more pieces of iron or magnetizable
alloy
 Can be open or closed cylindrical geometry type
 Good spatial homogeneity, but they are susceptible to
temporal changes in field strength.
 Maximum field strength approximately 0.3 T.
 Weight - 5 - 25 tons
 They consume no electric power, dissipate no heat, and are
very stable
 Inexpensive to maintain

15.  Generate their field by the conduction of electricity
through loops of wire.
 Resistive or superconducting depending upon whether
the wire loops have finite or zero electrical resistance.
 Lighter in weight.
 Produce an strong magnetic field.
 Used in open MRI.

16.  Magnets are manufactured using wire composed of
Nb/Ti or Nb/Sn alloys.
 Have the property of zero resistance when cooled down
to 10 K(–263°C) .
 Cooled by a bath of liquid helium.
 Current continues in the closed loop of the coil for years.
 Magnetic field is always present.

17. Quench pipe

18.  Permanent Iron Core
 Low Field “Open”
 Resistive Electromagnet
 Up to 0.2T
 Superconducting Magnet
 Cools wire coil with cryogens
 0.5T to 35T

19. Passive shims

20.  Dedicated electromagnetic coils (shim coils) are provided
to optimize the B0 field homogeneity within the design
of the main.
 In a superconducting electromagnet, superconducting
shims are additional coils of superconducting wire
wound coaxially with the main coil in such a way as to
generate specific field gradients.

21.  Types
 Passive Shimming
 Resistive Shimming
 Superconductive Shimming
 Gradient Offset Shimming
magnet

23.  Used to produce deliberate variations in the main
magnetic field.
 There are usually three sets of gradient coils, one for each
direction.
 The variation in the magnetic field permits localization of
image slices as well as Frequency and Phase Encoding.

24.  The set of gradient coils for the z axis are helmholtz pairs,
and for the x and y axis paired saddle coils.
 Gradient fields are produced by passing current through
a set of wire coils located inside the magnet bore.
 Provide linear gradations of the magnetic field means
one end of the bore of the magnet has a lesser strength
and the other a greater strength.

28.  “Antenna" of the MRI system.
 Transmit the RF signal and receives the return signal.
 Act as both transmitter and receiver
 They are simply a loop of wire either circular or
rectangular.

29. RF TRANSMITTER
 Generates RF energy in the form of RF pulses.
 Applied to coil and transmitted to patient’s body.
 Absorbed by the tissues.
RF RECEIVER
 Short time after RF pulse transmission resonating
tissue will respond by returning Signal.
 Provide data from which image is reconstructed.
 Resulting image is display of RF signal.

30.  Solenoid RF Coils
 Surface Coils and Phased Arrays
 RF Volume Resonators

35. Data acquisition control
 Acquisition of RF signal from patient body . Sequence of
RF pulse is transmitted to the body.
Image reconstruction
 computer use collected data during acquisition process to
create or construct image by Fourier transform.
Image storage
 Image are stored in the computer for future viewing.

38.  A structure not normally present.
 But visible as a result of a limitation or malfunction.
 Affect the quality of the MRI exam.
 May be confused with pathology.
 The knowledge of MRI artifacts and noise producing
factors is important.

39. Depending on their origin, artifacts are typically
classified as
 Patient-related.
 Signal processing dependent.
 Hardware (machine)-related.

40. Patient related artefacts :
 Motion artefacts.
 Flow.
 Metal artefacts.
Signal processing dependent artifacts:
 Chemical shift artefact
 Wrap around
 Gibbs phenomenon (ringing artefact)

41. Machine/hardware-related artefacts:
 Magnetic susceptibility artifacts.
 Magnetic field in homogeneities.
 Shading artefacts.
 Cross excitation and cross talk artefacts.
 parallel imaging artefacts.

42. Others :
 Gradient field nonlinearity.
 Partial volume artefacts.
 Magic angle artefacts.
 RF overflow artifacts (clipping).
 Entry slice phenomenon.

43.  Gradients applied at a very high duty cycle (eg, those in
echo-planar imaging)
 Caused by bad data points, or a spike of noise, in k-space.
 Seen as regularly spaced straight lines through MR image.

46.  Due to loose electrical connections and breakdown of
interconnections in RF coil.
 The location of spike and its distance from center of k
space determines the angulation b/w lines and distance
between them.

47. CORRECTIVE MEASURES :
 Repeat the scan.
 Avoid high duty cycle sequences.

48.  It is a line with alternating bright and dark pixels
propagating along the frequency encoding direction.
 Mainly due to RF leakage, as in defective farady cage,
FM radio station, few electronic eqipments.
- may occur in either frequency/phase encoding direction.

50. Corrective measures :
 Make sure the MR scanner room-door is shut during
imaging.
 Remove all electronic devices from the patient prior to
imaging.

51.  If the artifact persists despite all nearby electronic
equipment being turned off, it is possible that the RF
shielding is compromised.
 this usually occurs at the contacts between the door and
the jam and may need to be cleaned or repaired.
 the penetration panel where the cables enter the room
is another site to be checked.

52.  Ghosting and smearing, common artifacts produced by
voluntary or involuntary motion of the patient.
 Random motion (patient motion) produces smearing.
 Periodic motion (respiratory/cardiac/vascular) produces
discrete , well defined ghosts.
 Motion related artefacts are produced in phase encoding
direction.

55.  Esophageal contraction and vascular pulsation
during head and neck imaging.
 Respiration and cardiac activity during thoracic
and abdominal imaging.
 Bowel peristalsis during abdominal and pelvic imaging.

56.  Phase encoding axis swap i.e, changing the phase encoding
direction.
 Spatial presaturation bands placed over moving tissues.
 Spatial presaturation bands placed outside the FOV.
 Scanning prone to reduce abdominal excursion
 Cardiac/Respiratory gating
 Shorten the scan time when motion is from patient moving.

58.  Flow can manifest as altered intravascular signal (flow
enhancement or flow-related signal loss).
 When unsaturated spins in blood first enter in to a slice or
slices.
 Characterized by bright signal in a blood vessel at the first
slice.
 High velocity flow - protons do not contribute to the
echo and are registered as a signal void or flow-related
signal loss.

59.  GRE sequences are much more susceptible to flow
artifacts than are SE sequences.
 SE sequences – flow appears dark
 GRE imaging -- in-flow effect produces the bright blood
phenomenon.
 Confused with thrombosis

61.  Use spatial saturation bands before the first and after
the last

62.  Susceptibility (χ) is a measure of the extent a substance
becomes magnetized when placed in an external magnetic
field.
 MSA results from local magnetic field in homogeneities
introduced by the metallic object.
 Materials that disperse the main field are called diamagnetic.

63.  Materials that concentrate the field are called
paramagnetic, super paramagnetic or ferromagnetic.
 Ferromagnetic materials have the largest susceptibilities.
 So field distortions and MR artifacts are more prominent
around metal objects and implants.
 Axis : frequency and phase encoding

68.  Use of spin echo sequence. (Avoid echo planar imaging)
 Remove all metals.
 Increasing band width .
 Using thin slice and higher matrix.
 Use of parallel imaging.
 Using radial k-space sampling.

69. Good effects :
 Used to diagnose hemorrhage , hemosiderin deposition
and calcification.
 Forms the basis of post contrast T2* weighted MR
perfusion studies.
 Used to quantify myocardial
and liver iron overload.

70.  Chemical shift is due to the differences between resonance
(Larmour) frequencies of fat and water.
 Which leads to shift in the detected anatomy.
 At 1.5 T, protons from fat resonate at a point approximately
220 Hz downfield from the water.
 If this occurs, there will be a slight misregistration of the fat
content on images, because of the slight shift in the frequency
of the fat protons.

71.  There are two types of chemical shift artefacts:
1. Chemical shift misregistration artefacts.
2. Interference from chemical shift (in-phase/out-
phase).

73.  Based on the difference in frequency between fat and water
(about 220 Hz).
 So the fat and water signals are in phase every 1/220 Hz.
 In other words, they are in phase at 4.45 msec, 8.9 msec, and
so on.
 At the in-phase echo times, the water and fat signals are
summed up
 whereas at opposed-phase echo times, the signal in these
pixels is canceled out, leading to the appearance of a dark
band at the fat-water interface
IN-PHASE/OUT- PHASE

75. Chemical shift misregistration Interference from chemical
shift(IPOP)
Along frequency encoding axis Along phase encoding axis
Dark edge at the interface between
fat and water
Dark edge around certain organs
Corrected by :
1.Fat suppression
2.Increasing band width
1.Using spin echo sequences
2. Selecting TE that is multiple of 4.5
at 1.5T.
Good effects:
Forms the basis of MR spectroscopy In phase and out phase imaging is used
to detect fat in lesion. (Eg – Fatty
infiltration)

76. Chemical shift is inversely proportional to bandwidth.
 Increasing the bandwidth reduces the chemical shift.
 Fat suppressed imaging can be used to eliminate the
chemical shift misregistration.
 Use of a spin echo sequence instead of a gradient echo can
eliminate the black boundary artifact.

77.  When the imaging field of view is smaller than the
anatomy being imaged.
 Anatomy that appears outside the FOV appears within
image and on opposite side.
 Aliasing can occur along any axis – frequency encoding
axis/ phase encoding axis.
 Basis of aliasing lies in analog to digital conversion.

80.  Enlarging the field of view (FOV).
 Using pre-saturation bands on areas outside the FOV.
 Anti-aliasing software.
 Switching the phase and frequency directions.
 Use a surface coil to reduce the signal outside of the area of
interest.

81.  Occurs at high contrast boundaries.
 Due to truncation(omission) of sampled signals.
 Commonly seen at the low signal intensity spinal cord with
high signal intensity CSF on T2WI of the spine.
 Occurs due to the omission of some data in k-space.
 Which cause the signal intensity of a given pixel to vary
from its ideal signal intensity.
 Appearance
 Bright and dark lines.

83.  Increasing the matrix size.
 Use of smoothing filters.
 If fat is one of the boundaries, use of fat suppression.

85.  Patients who are very large promote this artifact.
 The image will have uneven contrast with loss of signal
intensity in one part of the image.
 Frequency and phase encoding direction.
- Causes :
1. Uneven excitation of nuclei within patient .
2. Abnormal loading of coil/coupling of coil
3. Inhomogeneity of magnetic field.
4. Over flow of analog to digital converter.

87.  Load the coil correctly.
 Shimming to reduce inhomogenecity of field.
 Acquire the images with less amplification to avoid ADC
overflow.

89.  During slice selection there is some degree of excitation
of adjacent slices as well.
 Partial saturation of the signal in that section leads to a
lower signal intensity.
 If that slice is imaged soon after without living a gap
there will be a loss of signal.

92.  Same as cross excitation.
 Produced when RF pulse is switched off.
 Due to dissipation of energy to nuclei from neighboring
nuclei.
 Seen in slice selection gradient.

93.  Increase inter slice gap.
 Scanning with interleaved slices with odd and even
numbered slice respectively.
 Optimized RF pulse that have a more rectangular slice
profile can be implemented.

94.  Parallel imaging allows for faster image acquisition and
potentially improved image quality by under sampling k-
space.
 The reduced acquisition time can be used to improve spatial
or temporal resolution.
 High acceleration factor and small FOV is used.
 Graininess in the image.
 Can be reduced by using lower acceleration factor and
increasing the FOV.

96. Significant image degradation due to noise
amplification with parallel imaging factors
greater than 3.

97.  Seen when doing gradient echo images with the body
coil.
 Seen in heavy pt.
 Seen in coronal imaging when large FOV is used
which is still smaller than the object in view.
 Because of lack of perfect homogeneity of the main
magnetic field from one side of the body to the other
(at edges).

98.  Aliasing of one side of the body to the other results in
superimposition of signals of different phases that
alternatively add and cancel.
 This causes the banding appearance and is similar to the
effect of looking though two screen windows.

99. • Use Surface coil
• Shimming
• Arms by the side of
pt. so that they are
within the FOV.

100.  An artifactual T2 bright signal seen in tendons and
ligaments that are oriented at about a 55 degree angle to
the main magnetic field.
 A bright signal from this artifact is commonly seen in the
rotator cuff and distal patellar tendon.
- Not to be mistaken for tear.
- Tends to occur only on short TE sequences (e.g. T1, GRE, PD).

103.  Lengthen TE
 Use T1 weighted sequences since T1 relaxation is
unaffected by this.

104.  A focal dot of increased or decreased signal in the
center of an image.
 Caused by a constant offset of the DC voltage in the
amplifiers.
 Remedy
 Requires recalibration by engineer
 Maintain a constant temperature in equipment room
for amplifiers.

106.  MR has achieved widespread clinical acceptance as A
noninvasive, radiation–free alternative to
radiography and computed tomography.
 MR artefacts hinders the image quality.
 MR artefacts are confused with pathology.
 Identifying them and correcting them by radiologist.