 Hello everyone, my name is Dr. Sanjeev Mani from Indian Radiologist and along with Dr. Srijita Ghosh and Dr. Mithusha Vadma, we will be talking today on MRI artifacts. Now this is normally a very boring topic but we are going to make it a little bit interesting. It is a very simple way to remember MRI artifacts by thinking of the world's favorite food, pizza, okay? And not just any other pizza, pizza, which is a classic one, Chef Maggie's Pizza. Hi, this is Dr. Srijita Ghosh from Kolkata. Welcome to Indian Radiologist. First of all, thank you all of you for continuing watching Indian Radiologist. Please continue following our channel and keep subscribing and click the bell icon to get instant notifications. Now let me share a small secret. I think Dr. Sanjeev just now showed an empty pizza packet. You know why? Because he's gobbled up everything inside but I will not do the same. I will show you this entire juicy pizza but stressing on something. This is not an ordinary pizza. It is Chef Maggie's Pizza which will help you to remember all the MRI artifacts and make it much, much more interesting. So before I gobble up, here is Chef Maggie's Pizza for everyone. Thank you. So we start with chemical shift artifact and let's have a close look at this image. This is of course the amount of the abdomen and you can see a normal looking kidney here. Kidneys surrounded by perinephric fat but if we look at the lower pole of the kidney, what you can see is a black band coming out here whereas if we look at the superior pole of the kidney you see a white band here. Same over here in the anterior aspect you can see a white band however at the posterior aspect you see a black band. Now this is no abnormality. This is no pathology. This is what is known as the chemical shift artifact. Now the chemical shift artifact or the chemical shift phenomenon is also known as misregistration or mismapping that is occurring between the fat and the water molecules. Now we know that there is a difference in the resonance frequencies between fat and water with fat resonating at a lower frequency than water. However in this artifact what happens is that the fat and water pixels get incorrectly registered in the image and fat hence shows up as a bright band on one side and a dark band on the other side of a fat soft tissue interface. Now this artifact is seen in water containing structures which are surrounded by fat. So namely the liver, the kidney as we saw here, the optic nerve as well as the thecal sac. This artifact is also more intensive when the magnetic field is stronger and also more frequently seen on T2 and less on T1 sequences. To reduce this what can be done is a selection of a larger matrix or a spin eco sequence can be employed. Special pulse sequences like fat saturation or inversion recovery can also help and also by decreasing the voxel size. Now obviously an MR device with a weaker magnet would reduce this artifact or eliminate it completely but that of course is not recommended. Now coming to the H of Chef Maggie's Pizza and this H stands for herring bone artifact. Before we go into the details what is herring? Herring is a fish and the herring bone is like a pattern consisting of columns of short parallel lines as we see here in the MR axial plane brain showing these parallel lines running through. Is this normal? No, it's an artifact. This is called herring bone artifact also known as spike artifact, crisscross artifact or corduroy artifact. In corduroy pants and all that we wear we have these parallel lines. So this is known as corduroy artifact. This MRI artifact is related to one or few aberrant data points in case space. In image space the regularly spaced stripes resemble the appearance of a fabric with a herring bone pattern. This artifact covers the entire image in a single slice as we see here or multiple slices. This occurs due to fluctuating power supply, RF pulse discrepancy, electromagnetic spikes by gradient coils. How do we improve on this? By eliminating or removing any other equipment or turn off other equipment and then repeat the sequence in most of the cases once we do so this artifact is gone. Equipment related artifacts is a big subject and many of the artifacts here could fall into this category. However we are going to mention a few such as radio frequency detection circuit failure basically which can occur because of improper detector channel operation or sometimes just failure of the RF coil. But one important artifact that we need to discuss here is the one that we call crosstalk artifact. So what is crosstalk artifact? Actually there's a loss of signal which occurs in a slice from a multi-angle multi-slice acquisition. If you have a look at this image what you can see is that during the stage of planning what has happened is that the slices have crisscrossed each other at the level of the posterior elements. Now when that happens sometimes you will get these linear shadows that we are experiencing here on this image. So when the slices are obtained at different spaces and they are not parallel then these slices may overlap and give rise to this sort of an artifact known as crosstalk artifact. Now coming to F of Chef Maggie's Pizza and it stands for flow artifact. Now flow can manifest as either an altered intravascular signal, flow enhancement or flow related signal loss or as flow related artifacts, ghost images or spatial misregistration. The fully magnetized protons yield a high signal in comparison with the rest of the surroundings. High velocity flow causes the protons entering the image to be removed from it by the time the 180 degree pulses administered. The effect is that these protons do not contribute to the echo and are registered as a signal void or flow related signal loss like we get in a vessel like the intracerebral artery, bicellar in the brain. Flow artifacts are caused actually so by flowing blood or fluid in the body. A liquid flowing through a slice can experience an RF pulse and then flow out of the slice by the time the signal is recorded. Flow artifacts are also common especially in gradient eco sequences due to the nature of GRE sequences blood flow will produce a bright signal. Usually saturation bands can minimize this artifact saturating the slice upstream so that the blood will not produce a signal. Gradient moment nulling can also be applied to try to correct flow artifacts. So students we not only have to know the artifacts, see how they look like but also we have to know what are the methods to reduce it. So in this slide on the left we see the flow artifact due to flow of the pulsatile CSF but see on the right hand side this artifact is not there and how has it been done? The gradient moment nulling is applied in the second image to eliminate the artifact. So this is about F of Chef Maggie's pizza or flow artifact. The next artifact which we are going to discuss is moist fringe artifact. It's an interference pattern most commonly seen when acquiring gradient eco images using the body coil. So I'm sure you must have seen it at least once or twice during your experience and MR postings because of lack of the 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 results in superimposition of signals of different phases that alternatively adds and cancel and that results to this kind of a pattern. This causes the banding appearance which we call as the moist fringe artifact. So it actually looks like black and white banding at the edge of the field of view as you can see in this image and this happens when the field of view is really large like when you are scanning the abdomen or when you're scanning the pelvis with both joints and it is seen with the GRE images. So it's two things in combination one is aliasing that is wrap around the kind of effect and magnet in homogeneity and how do we fix it because whenever you discuss artifacts or whenever you are reading about it or learning about it you should know also how to mitigate these artifacts. So you can shift from GRE to spin echo sequences and also try to bring all the anatomy within the FOV so that you can just mitigate this kind of wrapping around. The next artifact we are going to discuss is motion artifact. We know that motion artifact is pretty common in fact in almost 5 to 10 percent of all examinations we inevitably have to repeat some sequence because of patient motion. Now this could be because the patient is uncomfortable and in fact even claustrophobic within the MR gantry so care should be taken to actually talk to the patient and train the patient before starting the MR scan and that may help us in fact reduce this motion artifact. So besides talking to the patient other solutions include motion correction by using radial case space sequences or faster sequences like the blade sequence. We can also use immobilization aids besides that cardiac and respiratory gating scanning the patient in a prone position to reduce the abdominal excursion caused by respiration and also shortening the scan time can all help prevent motion artifacts. Besides respiratory motion artifacts we also have to worry about peristaltic artifacts especially when scanning the abdomen and pelvis and this can be tricky especially because of peristaltic motion of the bowel on which we have no control. So the solutions for this also include fast imaging technique and also sometimes patient preparation in the form of a low residue diet which sometimes can help in improving the image of the study. If nothing else works then IV administration of buscopan is quite effective in improving the scan quality as we can see on this image we see here bowel motion and post IV buscopan we can see a much clearer image because of the reduction in the peristalsis. Sometimes if all options fail then we may have to refer to a mild sedation or a short GA to complete the study. Some patients may also not want the GA and then you may have to move towards another imaging technique if nothing else is working. Aliasing artifact is also known as the wraparound artifact and it's a pretty frequently encountered artifact. To understand this artifact we have to understand what exactly is the field of view. Now field of view refers to the dimensions of the anatomic region to be included in the imaging and usually it is a tradeoff between the signal-to-noise ratio and the spatial resolution. So when we keep the imaging matrix constant a smaller field of view results in a higher spatial resolution but lower signal-to-noise ratio. Now if we happen to choose a field of view that is smaller than the area to be imaged that is what can lead to the aliasing or wraparound artifact. Hence basically this occurs when the field of view is smaller than the body part being imaged and hence the part of the body that now lies beyond the edge of the field of view is projected onto the other side of the image resulting in this artifact. Now this artifact can be corrected by increasing the number of phase encoding steps or very simply enlarging the field of view. Sometimes some machines do have anti-aliasing software which help reduce this artifact. You can also use surface coils to diminish the amount of signal received from outside the region of interest to reduce this artifact. We should always try to exclude the aliasing artifact as it creates quite an irritant while reporting. However in some areas for example like the pituitary gland a degree of aliasing artifact may be acceptable provided of course it does not interfere with the imaged anatomy. Now coming to the G of Chef Maggie's Pizza and it stands for Gibbs effect or Gibbs artifact. Now Gibbs artifact or Gibbs ringing artifacts are also known as truncation artifacts and are caused by under sampling of high spatial frequencies at sharp boundaries in the image. They are seen as dark lines paralleling and adjacent to borders of abrupt and intense change in object like in the CSF the spinal cord in the brain the skull interface. In spinal cord this artifact can actually simulate a small syringe so we have to be aware of this. It is seen in both frequency and phase encoding direction. It is related to the finite number of encoding steps used by the Fourier transform to reconstruct an image. How can we get rid of it? By increasing the matrix size use of smoothening filters if fact is one of the boundaries use of a fat suppression can help also. So this is what we need to know about the G of Chef Maggie's Pizza that is Gibbs effect or Gibbs truncation effect. Now the reason we have put ghosting artifacts here is that because sometimes they are discussed as a separate entity altogether but they are nothing else but artifacts caused by movements especially motion artifacts like respiratory motion, bowel motion as well as the heartbeat. Now these artifacts have already been discussed earlier in the motion artifacts section so we move on to the next one. Next very useful artifact which is in phase and out of phase based imaging artifacts also known as chemical shift imaging that is based on the difference in precision frequencies of water and fat molecules which causes them to be in different relative phases during the acquisition sequence and allow in phase and out opposite phase images to be obtained. Intra cytoplasmic fat is depicted as signal drop on opposed phase images when compared with the in phase images. So here you can see example the first image on the left is in phase while the other one which has a dark rim to the organs that is the out of phase. So here you can see that there is a small lesion in the left adrenal which loses or shows signal drop on the opposite phase or opposed phase and that is because of the intracytoplasmic fat content in the adrenal adenoma. So the same example with a zoomed image how we can see that clearly there is intracytoplasmic fat content and with this we can confidently come to a diagnosis. The way you can identify out of phase images is by the dark rim which you see around the organs on out of phase images. So it has lot of application and the main application is based upon the fact that we can identify pathological fat contents of the tissue in the abdomen by showing signal intensity drop on opposed or out of phase images. So where all can we use it? We can use it to quantify to identify fatty liver. We can identify areas of focal fat sparing. We can identify adrenal adenomas, adrenal myolipomas, lipid poor angiomyolipoma in RCCs to identify presence or absence of fat, thymic hyperplasia, osteoporotic versus neoplastic vertebral edging very important situation because here we have to differentiate between the two and sometime it can be really confusing. Whenever there is fat we can understand that it is osteoporotic fracture whereas neoplastic lesions they will display persistence of the high signal on out of phase images, vertebral metastasis, hemochromatosis of liver, intrahepathic pneumonia and metallic objects by the effect of susceptibility. So you can utilize this artifact to make a confident diagnosis. Next coming to is susceptibility artifact very commonly seen artifact and that is why whenever you are taking in the patient we are also demetalizing the patient while preparing for MRI. One of the factors whenever you have a free projectile or a ferromagnetic object getting into the magnetic field it can actually create haverks by being acting as a high speed projectile but the second fact is like if somebody has a hair clip or a hairpin that can create a lot of distortion the image quality but sometimes there are implants which cannot be taken out of the body during the imaging and they create these kind of susceptibility artifacts. Why do we have this? Because of the result of variations in the magnetic field strength that occurs near the interface of the subject and substance of different magnetic susceptibility. Large artifacts are commonly seen surrounding ferromagnetic objects inside of a diamagnetic material. Human body is a diamagnetic material so whenever you have something which is ferromagnetic inside the body we see a lot of susceptibility artifact. These gradient cause defacing of spin and frequency shift of the surrounding tissue. So we see bright and dark areas with spatial distortion of the surrounding anatomy. This is how these artifacts are seen. They help us to identify presence of some metallic object or implant within the body. This is how we see very commonly with the pedical fixation screws in the spine. Again this is a spotter maybe asked it's mascara which is causing the susceptibility artifacts or sometimes the mascara can have some ferromagnetic particles and you can get this kind of an artifact because of the susceptibility. How do we fix it? One and the most easiest way is remove all metal objects if whatever possible then again shift to spin echo sequences instead of GRE. Increase the ETL which is the cotrain length decrease the TE and increase the matrix. Again as we see these are useful sometimes to make a confident diagnosis. Similar is the case with susceptibility artifact. So susceptibility artifact can help us pick up hemorrhage and we understand that there is a whole lot of imaging sequence like GRE and SWI based on this particular artifact. We can identify hemorrhage, we can identify calcification sometime. Fungal infection can produce paramagnetic substances and that can also create this kind of appearance or artifact. Coming next to pulsation artifact yet another commonly seen artifact and the importance of understanding this is that sometime we may just create a wrong diagnosis after seeing this artifact as it mimics lesions. So here in this image you can see there is a vessel and just by the side of the vessel at equivalent or equal distances we see some kind of grossing or shadowing. So what is this? Yet another name to this is face encoded motion motion artifact and it is one of the various MR artifact which is the result of tissue slash fluid moving during the scan. So as we understand blood vessels they carry blood so that is anything fluid tissue which is mobile during the scan. It manifests as ghosting in the direction of face encoding so we can remember it as P for pulsation P for face encoding it is in the direction of face encoding usually in the direction of the short axis of the image. So for abdomen the axial and terraposterior direction will be the short axis. So that can be seen due to arterial pulsations, hollowing, breathing, peristalsis or a physical movement by the patient. How to identify that this is artifact and not an actual pathology? So identifying known moving flowing structures and noting that the artifact is in line with them. Second matching shape of ghost to that of the fluid vessel. Very commonly to mention is that they can mimic liver lesions whenever scanning abdomen because aorta is going to create this kind of artifact and some lesions similar to the shape and size of aorta will be overlapping the image of your liver parent schema. So that is where you can actually confuse white windowing to see repetitive ghost beyond confines of anatomy. They can be distinguished from gibbs or truncation artifact because they extend across the entire field of view unlike truncation artifact that diminishes quickly away from the boundary causing them. So this is another example of pulsation artifact. So this is what we discussed blood vessels pulsation artifact and that is how these are created. What is the solution? How can we remove them? You can use cardiac respiratory gating. You can use special pre-saturation band placed over moving tissues. Then your fat sat or saturation bands are being placed outside the FOV especially before the entry or after the exit slides for reducing ghosting from vascular flow. Scanning prone in abdomen to reduce abdominal excursion, switching face and frequency direction, increasing the number of signal averages or shorten the scan time. Now coming to the Z of Chef Maggie's pizza. Z stands for zipper artifact. Zipper artifacts appear as dashed lines. Now there are various causes of this MRI artifact. They may be due to radio frequency entering the scanning room. We see these artifacts as lines oriented perpendicular to the frequency axis of the image. Frequently there is more than one artifact line on an image and they may be present in all the image of an entire series. Now software and equipment problems can also cause zipper lines in both directions. There are various causes of zipper artifacts and various images. Most of them are related we have to remember. They are related to hardware or software problems which are beyond the radiologists immediate control. The zipper artifacts that can be controlled easily are those that occur when the door is open during acquisition of images due to the RF entering the scanning room from electronic equipment. For example, say a mobile device or aircraft and are being picked up by the receiver chain of imaging subsystems. So if we can actually get hold of any of these devices or anything which is on we can get rid of this artifact. Next is asymmetric fat suppression artifact. Another important artifact to understand because you can again misdiagnose few pathologies if you do not identify that it is an artifact and not an actual area of non-fat suppression or altered signal intensity. This happens due to asymmetry of body parts only a small band of tissues present where the fat protons are processing at the frequency expected resulting in frequency selective fat saturation working only in that area. So whenever you have a large field of view like in abdomen or whenever you have a curved surface like in ankle joint you can have asymmetric fat suppression and you may feel that this is an area of soft tissue edema or abnormality. Frequency specific fat set pulses become less effective when the FOV is increased so you have to use smaller FOV. The magnetic field homogeneity decreases as more tissue is imaged as a result. The precision frequency change across the imaging volume thus fat is processing at the expected frequency only in the center of the imaging volume. So whatever is your field of view or the specific area you want to see you have to keep the FOV small and the area of interest should be placed at the center. It is best to use small field of use when applying fat saturation pulses. Hope you like this video so to remember MR artifacts remember Chef Magnus Pisa do subscribe to our channel Indian ideologist thank you