ppMRIARTIFACTS.pptx
- 8. Gradient coils generate secondary magnetic fields over the primary field. The arrangement of these coils gives mri the capability to image directionally along the 3 axis. Z gradient- long axis-axial Y gradient – vertical axis –coronal X gradient – horizontal axis - saggital
- 9. RF coils are used for transmitting the RF pulse and receiving signals.
- 15. The digital signal representing the imaged body part is stored in the temporary image space called K- space. The digital signal is then sent to an image processor where a mathematical formula called fourier transformation is applied and the image of the mri scan is dispayed on a monitor
- 16. Introduction • An Artifact is something observed in a scientific investigation that is not naturally present but occurs as a result of the investigative procedure. • It is a structure not normally present, but visible as a result of malfunction in the hardware or software of the device, or a consequence of environmental influences
- 17. Types of MRI image artifacts MR hardware • Zipper artifact • Herringbone artifact • Zebrastripes • Moire fringes
- 18. MR software Slice-overlap artifact (cross talkartifact) Cross excitation
- 19. Patient and physiologic motion -Phase-encoded motion artifact -Entry slice phenomenon
- 20. Tissue heterogeneity and foreign bodies -Magic angleeffect -Magnetic susceptibility artifact -Chemical shift artifact
- 21. Fourier transform -Gibbs artifact /truncation artifact --Aliasing/wrap around artifact
- 22. • Cause: This type of artifact results from the contamination of the MR scanner environment (shielded MR suite) by spurious Radio Frequency signals. Zipper artifact
- 23. A Faraday cage is a conductive enclosure used to shield a space from electromagnetic interference. In MR imaging, this type of structure provides radiofrequency shielding to the scanning room to minimize occurrence of imaging artifacts.
- 24. • The contamination can result dueto 1)BreechintheRadiofrequencycagei.e. RF entering the shielded room from outside 2) Illumination devices & other medical equipment (e.g pulse oximeter )
- 25. Appearance- A dense broken line of alternatively dark and bright pixels running across the image perpendicular to the frequency encoding direction.
- 27. Solution: 1) Scan door should be closed 2) Breech in RF cage should be located and repaired 3) Equipments in the scan room should be serviced
- 28. Herringbone artifact • Herringbone artifact (also known as ascrisscross artifact or corduroyartifact) • Due to interferences which occur while filing‐in of the k‐space • These interferences are usually RF spikes which lead to points in k‐space with high intensity • Causes ; • Fluctuating power supply • RF pulse discripencies
- 29. Appearance- It appears as obliquely oriented stripes seen throughout the image. Artifact is scattered all over the image in a single slice or multiple slices .
- 30. Moire fringes • Moire fringes are an interference pattern most commonly seen when doing gradient echo images with the body coil. • Because of lack of perfect homogeneity of the main magnetic field from one side of the body to the other , aliasing of one side of the body to the other occurs resultsing 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 screenwindows.
- 31. Wavy appearance at the edge of the images
- 32. Solutions: 1) Use of spin echo sequences 2) Use of anti Aliasing remedies 3) Plug on the receiver coil should be firmly put in the socket
- 33. Slice-overlap artifact • Loss of signal in an image due to a multi‐slice, multi‐angle acquisition or imperfect sliceprofile • The mechanism behind this artifact is spinsaturation
- 34. Image depicts Two sets of spine images, at different angles, covering L4‐5 and L5‐S1 respectively. The set acquired later(red) will have image spins that have already been excited and that are saturated,leading to signal loss If slices at different angles cross, then spins that have been previously excited (saturated) can not be excited again, thus leading to a signal drop in the crossing region
- 36. Solution
- 37. Cross excitation artifact • There is loss of signal within a slice due to pre-excitation from RF pulse meant for an adjacent slice • Appearance- ThisArtefact causes a reduction in SNR in adjacent slices in a Slice stack. .
- 38. Cause : Ideally the profile of a slice should be square or rectangular when viewed from the edge, but in practice a radio frequency excitation pulse is not able to achieve this. The adjacent slices receive energy from the RF excitation pulse of their neighbors . When the tissues in next slice receive their own RF pulse they are unable to produce a signal as they are already saturated
- 39. Solution:- • Cross excitation can be reduced by insuring that there is at least at 30% gap between the slices (Gap= 30% of the Slice thickness ) This reduces the likelihood of RF pulse exciting adjacent slice. • Interleaving between slices i.e. FIrst acquiring the odd number slices(1,3,5,7..) Then acquiring the even number slices( 2,4,6,8..)
- 40. Effect of cross-talk on image contrast. On left is a SE image with 50% gap showing expected spin- density contrast. On right the same sequence with 10% gap demonstrating impairedcontrast.
- 41. On the coronal scout, the lumen of the gallbladder is sharply divided into low signal medially and high signal laterally with a "level" appearance suggesting layering sludge. However, the axial T2W shows no sludge in the gallbladder. This finding is attributed to cross-excitation artifact.
- 42. In this case, the scouts were acquired in the following order: axial, sagittal and finally coronal. The sagittal FOV involved the medial gallbladder. The protons in the medial half of GB are already saturated. Therefore, the immediate subsequent RF pulse of the coronal acquisition will result in a low signal in the medial gallbladder and can mimic sludge.
- 43. Phase-encoded motion artifact • Phase-encoded motion artifact occurs as a result of tissue/fluid moving during the scan. It manifests as ghosting in the direction of phase-encoding. • These artifacts may be seen from arterial pulsations, swallowing, breathing, peristalsis, and physical movement of a patient. When projected over anatomy it can mimic pathology, and needs to be recognized. • Motion that is random such as the patient moving produces a smear in the phase direction. Periodic motion, such as respiratory or cardiac/vascular pulsation, produces discrete, well-defined ghosts.
- 44. This is a typical example of phase-encoded artifact secondary to aortic pulsation. The ghost images of the aorta are: discrete, well defined, lining with the aorta along the short axis of the abdomen (AP axis) at regular intervals and extending beyond the field of view. These features help recognize them and not to confuse them with any pathology.
- 45. Blood flow in the sagittal sinus and carotids causes Periodic ghosting which gives the impression of an enhancing lesion in the occipital lobe as well as anterior to the right temporal horn. Wide windowing demonstrates the ghosting extending beyond the confines of the patient's anatomy. Apparent enhancement in the occipital lobe and right temporal lobe in a patient with malignancy. windowed slice demonstrates the phase mismapping artefact from the superior sagittal sinus.
- 46. Swallowing artifact has produced horizontal band of noise along phase-encoded direction. Where it passes over vertebral body and spinal cord, there are local intensity and contrast changes, rendering these portions of scan suspect.
- 47. Solutions cardiac/respiratory gating spatial presaturation bands placed over moving tissues (e.g. over the anterior neck in sagittal cervical spines) spatial presaturation bands placed outside the FOV, especially before the entry or after the exit slice for reducing ghosting from vascular flow scanning prone to reduce abdominal excursion increasing the number of signal averages shorten the scan time when motion is from patient movement
- 48. Cardiac Gating This involves the acquistion of a series of MR images of a single anatomic section during a specific point in the cardiac cycle ,which is being monitored with ECG. Data within the k-space is linked with specific time point in cardiac cycle. The acquired datasets are then sorted according to time stamp to produce sequential images allowing a dynamic evaluation of myocardial function.
- 49. Respiratory gating respiration related motion is monitored, using bellows or breathing belt, and image acquisition is timed to take place at end expiration, when there is little or no motion. however this is infrequently used in clinical imaging. -Breath holding method most often used -Sequences with short acquisition time may be used, or if breath holding possible for limited time, exam may be divided into several brief acquisitions.
- 50. Saturation pulses Saturation pulses involve the application of RF energy to suppress the MR signal from moving tissues outside the imaged volume to reduce or eliminate motion artifacts.
- 51. Saturation pulse
- 52. -Most commonly used in abdominal imaging at the interface of lung and diaphragm -Application of small, one dimensional spatial encoding gradient in a plane perpendicular to diaphragm. - Echo measured at this location allows correction of imaging dataset and ensures that only the imaging data acquired when diaphragm is at its peak (end expiration), is used in image reconstruction. Navigator Echo
- 53. Entry slice phenomenon Entry slice phenomenon occurs when unsaturated spins in blood first enter into a slice . Unsaturated spins = protons without transverse magnetisation When a large number of such protons encounter a RF pulse for the first time(i.e. at the entry slice),they undergo transverse magnetisation and emit a strong signal. - It is characterized by the bright signal in a blood vessel (artery or vein) at the first slice that the vessel enters. -
- 56. This artefact can be confused with thrombosis Solutions- -The characteristic location and if necessary, the use of gradient echo flow techniques can be used to differentiate entry slice artefacts from occlusions. -Spatial saturation bands place before the first slice and after the last can also be used to eliminate this artefact.
- 57. Magic angle artefact • The magic angle effect is important in the clinical MR imaging of certain tissues that are highly structured and are oriented obliquely to the main magnetic field (especially tendons, cartilage, and peripheral nerves). • In tendons, the water molecules are aligned along the tendon sheath and have strong dipolar interactions. Due to these dipolar interactions, protons in the collagen rapidly dissipate their energy after encountering a RF pulse i.e. tendons have a short intrinsic T2 value (1-10 ms). The signal of tendons is uniformly low on all conventional MR sequences due to this rapid T2 decay.
- 60. however, when the tendons are aligned at an angle of 55degree to the main magnetic field , these dipolar interactions go to zero, leading to an increase in T2 relaxation time and consequent sudden increase in signal.
- 66. Common sites: proximal part of the Posterior cruciate ligament infrapatellar tendon at the tibial insertion peroneal tendons as they hook around the lateral malleolus Supraspinatus tendon
- 67. High signal within a tendon can mimic tendinopathy so it is important that radiologists are aware of the magic angle phenomena. Look for it in the outer portions of the rotator cuff, supraspinatus tendon, the distil patellar tendon, tendons of the ankle, extensor and flexor policis tendons of the wrist or any part made of collagen that lies in a curve
- 68. Susceptibility artifact • Due to the presence of substances ( e.g.ferromagnetic materials) that causes variation in the local magneticfield • Results in increased magnetic field inhomogeneity ,leading to spin dephasing and consequent signal drop • Becomes worse for long echo times and GRE sequences( no RF 180 pulse) • Less visible in fast spin echosequence • Worse for higher fieldstrength
- 69. Susceptibility artefact due to hip prosthesis
- 70. Susceptibility artefact due to dental fillings
- 71. Susceptibility artefact due to dental fillings
- 72. Remedies • Use spin-echo sequences A spin echo sequence uses a 180-degree RF pulse . The main purpose of this 180-degree pulse is to correct local field inhomogeneities. A gradient echo pulse sequence does not utilize such a “refocusing” pulse. As such, local field inhomogeneities are much more readily apparent when gradient echo sequences are utilized
- 73. Chemical Shift -Chemical Shift is the difference between precessional frequencies of water and fat.(3.5ppm) -Artefact is the displacement of signal intensity due to this difference
- 74. AT 1.5 T W>F by 220Hz At 3T W>F by 440Hz WATER FAT Water molecules have low electron shielding and their nucleus is more susceptible to external magnetic field. As a result water molecules precess at a higher frequency at the same magnetic field. Fat molecules have higher electron shielding and their nucleus is comparitively less susceptible to external magnetic field. Fat molecules precess at a lower frequency at the same mag field
- 75. water and lipid protons coexist at the interface, the signal emitted by the lipid protons will have a lower frequency than that of the water protons,leading to mismapping of fat.
- 76. CHEMICAL SHIFT TYPE 1: MISMAPPING ARTIFACT -Most commonly seen around water containg structures sorrounded bt Fat(Liver,Kidneys,Orbits) -Artifact appears as a dark band on one side of the water/fat interface and a bright band on the other side
- 77. a black band is seen on the left side of each kidney and a white band on the right.
- 78. T2 weighted image demonstrates chemical shift artefact around the lipoma. Note the signal loss anteriorly (black arrows) and hyperintensity posteriorly (white arrows). This is the result of inaccurate spatial encoding, resulting in the lipoma being incorrectly placed posterior to its actual location. LIPOMA
- 79. axial T1 image also demonstrates a chemical shift artifact at the edges between the mass and the orbital fat. ORBITAL HEMANGIOMA
- 80. Chemical Shift: 2nd Kind The chemical shift artifact of the second kind is sometimes known as the India Ink or black line artifact because it looks like organs and muscle bundles have been outlined with a black pen. The line does not correspond to a true anatomic structure, but results from destructive interference of signals at the boundary pixels that contain both water and fat. For this reason it is sometimes often called the phase cancellation artifact. Examples in the spine, thigh, and abdomen.
- 82. Gibbs artifact / truncationartifact • It refers to a series of bright or dark lines, parallel & next to borders of abrupt intensity change This artefact occurs at areas of abrupt intensity change such as -the CSF-Spinal Cord Interface. -the Skull-Brain Interface.
- 83. Bright and dark bands are seen parallel and adjacent to the outer convexity of calvaria
- 85. . Figure illustrates a syrinx-like artifact (red arrow) within the center of the cord, a classic truncation artifact that appears as a result of alternating dark and light signal bands overlying the spinal cord. This artifact can be easily mistaken for hydromyelia (dilatation of the central canal, more commonly referred to as a “syrinx”),
- 86. Truncation artifacts are reduced by the use of a smaller field of view (FOV) or a larger matrix size.
- 87. Aliasing / wrap aroundartifact • It occurs when the field of view (FOV) is smaller than the body part being imaged. The part of the body that lies beyond the edge of the FOV is projected onto the other side of the image • Caused by under‐sampling in the phase encoding direction
- 88. -phase shifts of between 0° and 360° encompass the field-of-view. The subject's left flank in the drawing extends outside the field-of-view and encompasses phase shifts from 361° to 450°. Since all meaningful frequencies have been defined over the range of 0° to 360°, a phase shift of 361° will be assigned to the spatial position of 1°, and a shift of 450° will be assigned to 450°-360° = 90°. Hence ,the left side of the patient's body will therefore be "wrapped around" and spatially mismapped to the opposite (right) side of the image.
- 90. Solution -Enlarging the field of view (FOV) -Using pre-saturation bands on areas outside the FOV -Anti-aliasing software -Use a surface coil to reduce the signal outside of the area of interest
- 91. PARTIAL VOLUME ARTIFACT -Partial volume artefact occurs if slice thickness > thickness of the tissue of interest. -Occurs when multiple tissue types are encompassed within a single voxel -If a small structure is entirely contained within the slice thickness along with other tissue of differing signal intensities, the resulting signal displayed on the image is a combination of these two intensities. This reduces contrast of the small structure.
- 92. Slice thickness 10mm Unable to depict finer structures due to partial volume artefact Slice thickness 3mm Depicts cranial nerves
- 93. THANKS