Physical Principles Artifacts Lecture 11

Artifacts in MRI

  • Artifacts are distortions or errors in MRI images that do not accurately represent the patient's anatomy or pathology. They can manifest as missing signals, added signals, or geometric distortions.

  • Artifacts can significantly reduce the diagnostic quality of MRI images. They may mimic pathology, obscure true pathology, or lead to misdiagnosis.

  • Identifying and understanding the causes of artifacts is a critical skill for MRI technologists to ensure accurate diagnoses.

  • Radiologists' confidence in their diagnoses is reduced when artifacts are present in MRI scans.

  • Excessive artifacts may render images non-diagnostic, necessitating repeat scans, which increase costs and patient exposure to magnetic fields and radiofrequency energy

  • Given the multitude of potential artifacts, this lecture will focus on common ones, addressing approximately 90% of the artifacts encountered in clinical practice.

Sources of Artifacts

The generation of an MRI image involves a complex chain of events, each step of which can introduce artifacts:

  • Patient: Physiological processes (e.g., motion, breathing).

  • Magnetic field: Field inhomogeneity, magnetic susceptibility.

  • RF waves: Imperfect RF pulses, RF interference.

  • Spatial encoding: Gradient non-linearity, pulse sequence limitations.

  • Signal recording: Analog-to-digital conversion errors.

  • Scanning the signal: Data acquisition errors.

  • Signal processing: Reconstruction algorithm errors.

Artifacts can originate from any of these stages.

  • Patient-related: Motion artifacts due to patient movement during the scan.

  • Magnetic field/RF waves: Metal implants causing susceptibility artifacts due to distortions in the magnetic field.

  • Spatial encoding/signal processing: Aliasing artifacts due to undersampling of data, truncation artifacts from limited data points.

It's not always possible to eliminate artifacts entirely. Strategies often involve minimizing their impact or moving them away from the region of interest.

Chemical Shift Artifact

  • Can happen anywhere in the body but primarily in the abdomen due to the high amounts of water and fat, especially in organs like the kidneys.

  • Kidneys are very watery because they produce urine. Surrounding the kidney there is a lot of fat because we have fat between our organs in order to protect our organs no matter how thin we are!

  • Chemical Shift Artifact is the machine misplaces where fat and water are inside the scan

  • the shift that happens is water - this only happens in the frequency direction

  • quiz we need to know in what dirrection certain aftifacts go- not every artifact is a problem with spatial encoding

  • spatial encoding is the gradient ! the gradient that turns on in order to make the resolution

  • Water and fat precess at different frequencies. Water precesses faster than fat. This difference affects the artifact's manifestation. this confuses the machine and begins to think that alll of the water is a little bit farther over becaus eof the differnence of frequency.

  • when it comes to recovery or decay FAT is fast but water precesses fatser

  • example on lecture 23:44 dark line where the kidney acutually is and bight line where the kidney is over lapping the FAT

  • chemical shift artifact is the displacemnt of signal between fat and water along the frequency axis of the image due to there different precession frequencies

  • water and fat have different presessional frequencys most of the time that doesnt matter but it can if we get chemical shift artifact

  • chemical shift artifact causes a signal void between areas of fat and water and only happens in frequency encoding direction

Chemical Shift- Remedy

  • This artifact becomes worse the higher the strength of the magnet strengths gets- the higher the strength of the magnet the more differnce in processional frequency between fat and water

  • stronger magnet = bigger difference between fat and water

  • we can remove STIR or fat saturation to get rid of fat signal leading to not seeing the artifact

  • Broaden means we are increasing the number - Broaden the recive bandwith is a way to get rid of chemical shift !

  • always happens in the frequency direction and its because water and fat shift over

Out-of-Phase Artifact

  • There are times we use Out-of-phase artifacts to create in- and out-of-phase images, which can highlight the presence of fat within tissues.

  • Out-of-phase artifact is when fat and water are out of phase

  • For example, if an echo is acquired at a TE of 2.3 milliseconds on a 1.5 Tesla scanner, water and fat will be out of phase, leading to signal cancellation. If the TE is doubled to 4.6 milliseconds, water and fat will be back in phase. 34mins

  • Out-of-phase artifact helps us see patholgy in abdomin and pelvis exams

  • Dixon technique uses in and out of phase images to create images where fat is dark

Remedy

  • We can Switch to a spin echo sequence or a fast spin echo, which because a 180-degree RF pulse is going to force water and fat back into phase, eliminating the artifact.

  • or you can use a TE that matches the periodicity of fat and water - meaning we chose a time fat and water are both in phase and they will both be bright - 36?

Phase Mismapping Artifact (Motion Artifact)

  • when a patient moves they cause Phase mismapping artifact, also known as motion artifact

  • occures when anatomy is moving between the phase encoding gradient and the frequency encoding gradient

  • Also referred to as ghosting because the displaced signal often appears as a faint "ghost" of the moving anatomy

  • Periodic motion (such lungs moving during breathing or heartbeat) is more predictable and easier to compensate for

  • Random motion like aptient moving during an exam is harder to compensate for

  • This artifact only happens in the phase encoding direction, making it easier to identify and address

  • final question which direction is the phase encoding direction

  • chemical shift only happens in the frequency

  • phase mismappinmg only happens in the phase

phase mismapping - repitory compensation

  • repitory compensationuse air filled bellows or navigatior to track breathing of a patient

  • Respiratory Gating: This technique involves not scanning when breathing when the patient has exhaled fully (lungs not moving) we then start scanning to collect echos

  • Bellows: A physical device consisting of an air-filled cord placed around the patient's abdomen to monitor respiratory motion

  • Navigators: uses a small RF pulse to track the position of the liver, allowing for motion correction without direct physical contact

  • Pre-saturation:known as a saturation band- when we saturate the signal we talk about turning something dark

  • saturation band- we place a band and make everything under it dark because anything dark not generating signal

  • Applying a saturation pulse to elimiates signal from moving tissues, effectively making everything under the band dark. Commonly used to suppress blood flow artifacts

  • Increasing NSA (number of signal averages): reduces noise and phase mapping artifact by collecting each echo multiple times

  • NOTE: While increasing NSA can reduce motion artifacts, it also increases scan time, which may be limited by the patient's ability to hold still

Aliasing Artifacts

  • Aliasing artifacts- known as wrap or foldover artifacts, this occur when signal from outside (FOV) is ends up showing up on the opposite side of the FOV.

  • This artifact arises because anatomy outside the FOV, but still within the the coil - everything inside of the coil is the echo - phase direction only !!!

  • Increase the field of view (FOV) to encompass all anatomy the coil, ensuring that no signal originates from outside the FOV.

  • Saturation bands can be positioned outside the FOV to suppress signal from regions prone to wrap-around.

  • Oversampling (no phase wrap): The MRI system can collect additional phase-encoding steps beyond what is necessary for the prescribed FOV and discard the extra data. This eliminates wrap-around but increases scan time.

    Magnetic Susceptibility:

  • occures because all tissues magnetize differently depending on magnetic characteristics 1:34

  • These artifacts result in signal voids (regions of signal loss) and areas of high signal intensity surrounding the metal, distorting the local magnetic field.

  • Different tissues magnetize to varying degrees based on their magnetic properties, leading to local field inhomogeneities

  • 1:47 end

Remedy

  • Use spin echo or turbo spin echo sequences, which employ RF rephasing pulses to mitigate the effects of local field inhomogeneities.

  • Remove all external metal objects from the patient, if possible. For implanted devices, assess the potential for artifacts before scanning.

  • Increase the receiver bandwidth to reduce the sensitivity to local field variations.

  • Employ STIR sequences when metal interferes with fat saturation techniques, as STIR is less sensitive to magnetic susceptibility artifacts.

  • Maverick/Warp is a metal artifact reduction sequence (MARS) protocol designed to reduce artifacts caused by metallic implants.

  • Consider the composition of surgical steel implants, as variations in alloy composition can affect the severity of artifacts

Crosstalk Artifacts- 1:47 start

  • Crosstalk artifacts occur when RF energy from one slice leaks over to another slice

  • Crosstalk cause by RF pulses and there fourier transforms are not exactly squared

  • These artifacts can be caused by misaligned slices or imperfections in the shape of RF pulses used for slice selection. The RF pulses may not have perfectly square profiles.

  • Crosstalk results in altered image contrast and reduced signal-to-noise ratio (SNR) due to the mixing of signals from different slices

Remedy

  • Introduce a gap between slices, typically around 10% of the slice thickness, to minimize RF energy leakage between adjacent slices.

  • Use a 3D imaging sequence, where the entire volume is excited simultaneously, eliminating the need for slice gaps.

  • Interleaving involves collecting odd-numbered slices first, followed by even-numbered slices. This allows for partial recovery of magnetization in each slice before adjacent slices are excited.

  • In lumbar spine imaging, ensure that there is no overlap between slices to prevent crosstalk artifacts.

  • Multiple acquisitions can help improve image quality but do not directly address crosstalk artifacts. Acquisitions refer to the number of times the data is acquired, influencing SNR and scan time

Truncation Artifacts -2:09

  • Truncation artifacts known as Gibbs artifacts, can mimic the appearance of motion caused by undersampling because too few K space lines Results from undersampling due to a low phase matrix.

  • Increasing phase matrix to take it back up (simple remedy)

Zipper Artifacts

  • Zipper artifacts are caused by extraneous RF energy interfering with the MRI scan, typically appearing as a bright line or series of lines in the frequency-encoding direction

Remedy

  • Identify and remove any interfering electronic devices from the vicinity of the MRI scanner.

  • Repair any RF leaks in the shielding of the MRI room. Addressing issues with copper plating or other shielding components can reduce RF interference.