During the slice selection process there is a slight offset between the location of the fat and water spins which have been rotated by an RF pulse.
This difference is exaggerated in this animation. During the phase encoding gradient the fat and water spins acquire phase at different rates. The effect being that fat and water spins in the same voxel are encoded as being located in different voxels. In this example all nine voxels have a red water vector. The center voxel has some fat magnetization in addition to the water.
In a uniform magnetic field the vectors precess at their own Larmor frequency. When a gradient in the magnetic field is applied, such as the phase encoding gradient, spins at different x positions precess at a frequency dependent on their Larmore frequency and field. In this example the fat vector has the same frequency as the water vector in the voxel to its right. When the phase encoding gradient is turned off each vector has acquired a unique phase dependent on its x position.
During the frequency encoding gradient, fat and water spins located in the same voxel precess at rates differing by 3. The net effect is that the fat and water located in the same voxel are encoded as being located in different voxels. In this example the fat vector in the center voxel possesses a phase and precessional frequency as if it was located in the upper right voxel. The resultant image places the fat in the voxel to the top rather than in the center.
Even though the phase is different, the fat is not encoded as being in a different phase encoding direction voxel. What matters in phase encoding is the difference in phase between the steps and this is not changing. For a constant sampling rate, the larger B o , the greater the artifact. A reason for going to higher sampling rates is to minimize the chemical shift artifact. In this axial slice image through the legs there is a chemical shift artifact between the fat and muscle in the legs.
In general, the term partial-volume artifact describes any artifact that occurs when the size of the image voxel is larger than the size of the feature to be imaged. For example, if a small voxel contains only fat or water signal, and a larger voxel might contain a combination of the two, the large voxel possess a signal intensity equal to the weighted average of the quantity of water and fat present in the voxel.
Another manifestation of this type of artifact is a loss of resolution caused by multiple features present in the image voxel. For example, a small blood vessel passing diagonally through a slice may appear sharp in a 3 mm thick slice, but distorted and blurred in a 5 mm or 10 mm slice.
Here is a comparison of two axial slices through the same location of the head. One is taken with a 3 mm slice thickness and the other with a 10 mm thickness. Notice the loss of resolution in the 10 mm Thk image.
The solution to a partial volume artifact is a smaller voxel, however this may result in poorer signal-to-noise ratios in the image. A wraparound artifact is the appearance of a part of the imaged anatomy, which is located outside of the field of view, inside of the field of view. For example, an image of the human head may have a part of the nose outside the field of view. The nose, however, appears in the image, but at the back of the head.
In this artifact, objects located outside the field of view appear at the opposite side of the image, as if one took the image and wrapped it around a cylinder. This artifact occurs when the selected field of view is smaller than the size of the imaged object, or, more specifically, when the digitization rate is less than the range of frequencies in the FID or echo. The origin of this problem was first presented in the chapter on Fourier Transforms. The solution to a wraparound artifact is to choose a larger field of view, adjust the position of the image center, or select an imaging coil that does not excite or detect spins from tissues outside the desired field of view.
The accompanying sagittal images of the head and breast contain wraparound artifacts. In the image of the head, the nose extends beyond the field of view on the left, and its imaged position is wrapped around and appears on the right of the image. In terms of frequency and digitization rate, the nose is located at a position that has a greater resonance frequency than the digitization rate.
Consequently, it is wrapped around, and it appears at the right end of the image. Many newer imagers employ a combination of oversampling, digital filtering, and decimation to eliminate the wrap around artifact in the frequency encoding direction. This point was discussed in the detector section of the Hardware chapter. Wraparound in the phase encoding direction can be minimized using a no phase wrap option which applies a saturation pulse to spins outside of the field of view in the phase encoding direction.
Hence, minimal signal is us present in tissue which are wrapped around into the phase encoding direction FOV. Gibbs ringing is a series of lines parallel to a sharp intensity edge in an image. The ringing is caused by incomplete digitization of the echo. This means the signal has not decayed to zero by the end of the acquisition window, and the echo is not fully digitized.
The reader is encouraged to prove this using the convolution theorem. This artifact is seen in images when a small acquisition matrix is used. Therefore, the artifact is more pronounced in the point dimension of a x acquisition matrix. In the following example, a rectangular object with a spatially uniform signal is imaged.
An inadequate number of points are collected in the horizontal x direction. The resultant image displays a ringing in the intensity at the edge.
The animation window displays the upper right hand corner of this image and a plot of signal intensity. The solution to Gibbs-ringing artifact is to use a larger image matrix. All of magnetic resonance imaging requires the spins to be free to rotate and tumble freely in the tissue. In solids this does not happen. As a consequence, the chemical shift and the spin-spin coupling are dependent on the orientation of the molecule.
The dipole interaction. The day of contamination. J Nucl Med Technol ; Contamination, a major problem in nuclear medicine imaging: How to investigate, handle, and avoid it. ADAC Laboratories. Matador electronics overview manual, Rev A. The gamma camera: Basic principles. In: Physics in Nuclear Medicine.
Philadelphia: Elsevier Saunders; J Nucl Cardiol ; Editorial Board Subscribe Advertise Contact. Year : Volume : 36 Issue : 1 Page : Indian J Nucl Med ; Indian J Nucl Med [serial online] [cited Nov 13]; Methods and Findings.
Figure 1: Cinematic raw images 32 projections from the left posterior oblique to the right anterior oblique a and corresponding single-photon emission computed tomography image b of the initial rest image. A bright hotspot indicated by arrows is visible in the lateral wall of the left ventricular myocardium roughly from projection 17— UML is a way of visualizing and plotting out the way a piece of software works.
It works to map out links, processes, etc. Like UML, a class diagram is a way to map out the structure of a piece of software or application. Class diagrams are used to map out links and processes that happen between clicks in a more visual way.
Any images used to help develop the piece of software are considered artifacts. These might be example images used to help in the design of the product, or preliminary design images. It could even be simple sketches and diagrams used to help map out the software. The majority of the artifacts are software documents. Any document that describes the characteristics or attributes of a piece of software is an artifact.
Much of it is technical, and simply not of interest to the typical user. The source code is the language used to program a given piece of software. This, too, is an artifact according to software developers. Yes, even meeting notes are artifacts in the world of software design. This may include full transcripts of meetings, or just jotted notes. Important design choices and aspects may have been made during these meetings, making it important to include these in your repository.
A risk assessment offers a look at the potential risks and downfalls of a piece of software. It helps tell a developer what not to do and lists problems that the developer needs to find a way around. In some ways, these are some of the most crucial artifacts for developers to consider.
Any prototype of your program is an artifact. These might be fully-functioning pieces of the software or previews of certain parts of the program. Either way, they help a developer see what has been done and tried, and give them an idea of where to go next. This is the final artifact, and one of the only ones a typical user will care about. The compiled application will let the user install it onto their machine, and use it as its meant to be used. There may be a number of these in the artifact repository.
There could be different versions, from early prototypes to experimental builds and the final compilation. Artifacts are important to hold onto throughout the development process of any piece of software, and even long after. Without each and every artifact, it can make developing a piece of software much more difficult over time. This is especially true if development switches hands. If an artifact is missing, that leaves a developer in the dark. This is why most artifacts are kept in a repository.
This lets relevant developers access the artifacts at any time, all from one place. An artifact repository is a location for all of the necessary artifacts that might be used to develop a piece of software.
It is often hosted on a local server or in the cloud for easy access by developers. Using an artifact repository is absolutely necessary for all software development. It makes a complex task easier by giving developers all the resources they need in one place. It helps cut down on searching and gives developers the ability to move, add, and delete artifacts with ease.
Remote Repository: A remote repository is hosted on a remote URL, sometimes by a third-party company. You cannot add new artifacts to a remote repository, but you can remove them. Local Repository: A local repository is stored in-house, usually on a dedicated server. Virtual: A combination of the two above. This repository is held under one URL, allowing access to local and remote artifact files. This lets you add and remove artifacts from each repository with ease.
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