User:JS 95/sandbox

Artefacts in imaging



Artefacts within magnetic resonance imaging (MRI) are items that appear within a image that are not part of the object in which you are aiming to scan. Artefacts can be caused by magnetic susceptibility issues within the magnetic field as well as a number of other reasons. There are a wide range of artefacts that can occur within an image, all with their own cause. These causes have to be taken into account, when scanning participants/patients. However, some of these artefact issues do still arise, and some of these issues can be resolved after the images have been created, for example using motion correction on phase-encoded motion artefacts. However, not all artefacts can be corrected for after scanning, and some can greatly affect the images beyond the point of correction.

Background and General Causes
To fully understand how these artefacts occur it is best to understand how images are taken. To understand how MRI works it is helpful to first understand how the protons are effected by the magnetic field of the MRI machine, which is explained by Nuclear magnetic resonance; as well as understanding how different parts of the MRI machine work and how they are built. Images are created after protons within the body are excited out of alignment from the main magnetic field, using Radio frequency (RF) energy. Once out of alignment, the protons will spin freely and will release the RF energy they were excited with, in order to return to the equilibrium state in alignment with the main magnetic field. This release of energy is picked up by an RF receiver and transformed into a scan. Artefacts can be caused by the protons spin being altered, and this change in spin will affect how the image comes out. There are other ways in which the scanner can experience artefacts that are not just caused by the manipulation of the protons within the magnetic field. For example, stray RF energy can be picked up by the RF receiver; these stray RF signals that originate from outside the scanner can cause artefacts.

Artefact Categories
There are several different types of artefact that can occur within a scanner. Some may occur due to problems with the shielding of the room around the scanner and the hardware within the room, or due to foreign bodies within the scanner or tissue heterogeneity. One of the most prevalent artefacts is caused by motion, which is caused by the participant moving or by parts of the body moving naturally (i.e. chest moving due to breathing). Artefacts can also be caused by software issues as well as problems when capturing the data and transforming it into an image. These are very broad categories and have many different causes. The categories for these different types can be seen below:



Nyquist Sampling Theorem and Fourier Transform Artefacts:

 * Gibbs
 * Wrap around
 * Zero Fill

MR Software Artefacts

 * Cross excitation
 * Slice-overlap

Room shielding and Hardware Artefacts

 * Shading
 * Zipper
 * Zebra stripes (Umbrella term, Moire and Zero Fill artefacts fall under this)
 * Central point
 * Moiré fringes
 * RF overflow

Physiologic and Patient/Participant Motion Artefacts

 * Entry slice phenomenon
 * Phase-encoded motion

Foreign bodies and Tissue heterogeneity Artefacts

 * Magic angle effect
 * Black boundary
 * Magnetic susceptibility
 * Chemical shift

All of the above artefact types have different effects on the images. Some of which are easier to spot than others, some are harder to resolve, and some are more likely to occur compared to others.

Zipper Artefacts


An example of a zipper artefact can be seen in Figure 4. Zipper artefacts are caused by stray RF energy finding its way into the scanner room and interfering with the RF receiver. They can occur all over the image, and the location of the strip and its overall width will depend on the frequency (location) and the cause (width) of the stray frequency. They are caused by this stray RF energy getting into the RF receiver which shows the black and white strips instead of an image; this is because it replaces the RF coming from the protons that show structural data and puts random data in its place, creating this black and white strip. This is mainly due to improper shielding of the MRI room or objects being brought into the room that can emit RF energy (such as phones or light bulbs) ; therefore, it is important to ensure the room is properly shielded, the door to the room is shut correctly, and no RF emitting objects are brought into the room.

Cross-Excitation Artefacts
The presence of a cross-excitation artefact is an indicator that the protons within an acquisition slice have already been excited by the RF energy from the previous slice. This causes a suppression of the tissue and thus causes a decrease in the image quality within the slice. This affect is due to the T2 relaxation times of the protons within the tissue; if the tissue was already excited by the RF energy from the previous slice then when hit again with the RF energy, the tissue will be suppressed within the acquisition slice. This can prominently be seen in Figure 4. The best way to overcome this artefact issue is to ensure there is a gap between the slices in which you are acquiring. This gap will reduce the chances of protons being affected by previous slices.



Phase-Encoded Motion Artefacts
Phase-encoded motion effect refers primarily to artefacts caused by motion within the scanner when images are being taken (Figure 5 shows an example of this). Due to the protons being encoded so that location of the protons can be ascertained and images collected; when part of the participant is moved from where they were encoded, it causes a change in the image, one that can be seen by a shadowing across the image or duplication of a region. By ensuring the participant does not move, this will reduce the chances of the artefact occurring. There are motion effects that occur that cannot be stopped; for example, the chest cavity moves when a person within the scanner is breathing. This movement of the chest can cause movement all over the body. Unfortunately, this cannot be overcome due to participants/patients needing to breath, therefore this is overcome by setting in place algorithms that will mathematically shift the image in order to remove the shift caused by movement of the chest cavity or moving body part.

Magic Angle Effect Artefact
This artefact is caused by an increased proton density due to high levels of water bound to calogen within the joints that contain ligaments and tendons. Meaning that when on an angle of 57.4 degrees, the T2 relaxation effects of the protons within this region are slower, giving a brighter contrast to the tendons. This means that the tissue within that region when scanned, looks as though there is an issue with the tendons, displaying similar properties to tendinitis. See Figure 3 for an example. In order to resolve this issue, the angle of acquisition must be altered.



Magnetic Susceptibility Artefact
These artefacts are caused by metals being introduced to the magnetic field. They can affect an image very heavily depending on the type of metal, and only need to be introduced to the field in small quantities to affect the image. The metals will affect the spin of the protons closest to the object. This is due to the metal entering the main magnetic field and having a current induced within the object; this current then causes a magnetic field to be created around the object itself. This ‘second’ magnetic field around the object can affect the spin of the protons around it (make them move faster). These types of artefacts are shown in Figure 6. Research has shown that it is best to either keep metal based prosthesis parallel to the main magnetic field or to completely remove it from the scanner. The best way of overcoming magnetic susceptibility issues, is to remove any form of ferromagnetic or para-magnetic objects from within the room of the MRI machine.



Gibbs Artefact
The Gibbs artefact is a ringing artefact, first discussed by J.W.Gibbs. The Gibbs artefact is caused by an under sampling of imaging data, which leads to a lack of data in K-space. This causes a ringing affect within the image, especially near tissue boundaries, as seen in Figure 7. If the images are taken with a greater sampling of data, it will mean the image is clear and the ringing affect will be decreased ; the effect can also be fixed post scanning, using several mathematical techniques.

These are just a few of the artefacts from the first list shown, but all give an example of the main causes for artefacts seen in imaging. It is important to note that while a lot of the above research has been mainly brain related, artefacts will affect images from anywhere within the human body. These artefacts are not specific to the brain or brain based research.

Artefacts and Research
It is important to know as much about these artefacts as possible so that research is not affected by these issues; and so that they can be rectified, methods for resolving them can be improved upon, or they can be anticipated before scanning has begun. As stated above the majority of artefacts can be resolved by following the above procedures on how to stop the specific artefacts from occurring. It is extremely important to understand how these artefacts occur and what they look like because, when it comes to research, participant data can become unusable and set studies back; if the artefact is not recognised then the final data may be affected by this and could lead to a misrepresentation of the true workings of the human brain.

Artefacts and Medical Diagnosis
It is important that these artefacts can be identified within imaging so that there is no misdiagnosis of disorders. The magic angle artefact is a prime example of an instance in which a misdiagnosis could occur if the artefact is not identifiable, as the artefact has similar visual qualities to that of tendinitis. Other artefacts can have more severe consequences, for example, the magnetic susceptibility artefact can be mistaken for a tumour if it affects the image in a way that mimics a tumour on a T1 weighted image. This could have very serious emotional and psychological consequences if a patient is then informed they have a tumour that they do not actually have. Therefore, it is extremely important that those operating an MRI machine, as well as those assessing images, are aware of these artefacts that can be easily mistaken for serious medical issues.