Magnetic resonance imaging (MRI) is considered a powerful diagnostic method which enables the visualization of soft tissue contrast without the use of ionizing radiation (Costa et al.,2009). MRI is a nonivasive procedure for the purpose of taking the pictures of the body. As compared to conventional radiography and computed tomography (CT) scan, MRI uses powerful magnets and radiowave instead of radiation. The magnetic field produced by an MRI is about 10 thousand times greater than the earth's. In many cases, MRI will gives different information about structure in the body as shown in conventional X-ray, ultrasound or CT scan. Problem that cannot be seen with other imaging modalities also may be showed by MRI because the images produce can be highly sensitive and specific.
The magnetic field forces hydrogen atoms in the body to line up in a certain way. When radio waves are sent toward the lined-up hydrogen atoms, they bounce back, and a computer records the signal. Different types of tissues send back different signals. Vlaardingerbroek and den Boer (as cited in Shafiei et al., 2003) state that MRI is based on the signal of nuclear magnetic resonance (NMR) emitted by the interaction of atomic nuclei that possess spin with incident radiofrequency within a static magnetic field. The NMR signal, which is primarily related to the proton density of the sample or tissue, also varies according to T1 or T2 relaxation time and velocity of fluid in the sample that is influenced by other internal and external factors.
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Magnetic resonance imaging (MRI) is done for many reasons. It is used to find problems such as tumors, bleeding, injury, blood vessel diseases, or infection. MRI also may be done to provide more information about a problem seen on an X-ray, ultrasound scan, or CT scan. Contrast material may be used during MRI to show abnormal tissue more clearly.
MRI thus gives excellent images of anatomical structures differing in proton density and other tissue characteristics.
ARTIFACTS IN MRI
Bushong (2003) state that image artifact in MRI is a pattern or structure in the image caused by a signal distortion related to the technique of producing image. Therefore it is unwanted pattern or structure that does not represent the actual anatomy. It is important to know and recognise all these artifacts and have an understanding of their causes especially for those which mimicking pathology. Recognising the artifacts and understanding the causes are important when interpreting and choosing the correct approach.
For optimal image produce in any type of modalities used, there must be a high degree of spatial and contrast resoloution with a strong signal, as well as minimal artifacts. However, artifacts can occur as in all imaging modalities, resulting in degraded image quality which can compromise imaging evaluation. Bui (as cited in Shafiei et al., 2003) emphasize that MRI has the shortcoming of being prone to magnetic susceptibility difference artifacts caused by the presence of metallic materials such as dental or orthopedic implants, dental cast restorations and aneurysm clips.
A variety of artifacts can occur in MRI images due to several reasons. It can be devided into four categories which is hardware related artifacts, sequence-related artifacts, patient-related artifacts and artifacts of special MR techniques. MRI is more susceptible to artifacts than other imaging techniques. This is because due to the facts that MRI signal depends on variety of parameters such as tissue parameters, scanner parameters, sequence type and sequence parameters. This multiparametric dependency is the reason for the high and variable soft-tissue contrast. The drawback, however, is that unwanted effects may occur, if the MR sequence or hardware is not chosen properly.
However, numerous correction method have been developed to reduce the corruptive effects of artifacts and improves the quality of images. These include special pulse sequence design, improve scanning procedures and equipment and advanced postprocessing algorithm.
Motion is the most prevalent source of imaging artifacts. A standard image reconstruction assume that the patient, object or sample will remain stationary during the entire data collection process. Owing to voluntary or involuntary patient movement caused by physiological processes is rarely occur. There are many potential sources of image artifacts associated with MRI that can potentially degrade images to the extent that they are insufficient for accurate diagnosis. Basically there are two types of patient motion which is irregular bulk motion (random) of body parts such as peristalsis, swallowing and eye movement and the other one is regular motion (periodic) with continuous velocity such as pulsatility of vascular flow and cerebrospinal fluid, and cardiac and respiratory cycle.
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The appearance of motion artifact is dependent to the type of motion whether it is random or periodic. Periodic movement such as cardiac movement and blood vessel or CSF pulsation will cause ghost image whereas non-periodic movement such as peristalsis and swallowing will cause diffusing image noise. Ghost image intensity increases with amplitude of movement and the signal intensity from the moving tissue (Erasmus et al., 2004). Degradation of image quality in motion depend on the time of scan. If the motion occur during data acquisition interval, additional phase term related to the motion path will created in the data. Otherwise, if the motion occur between acquisition interval causes inconsistencies because data acquired before and after movement do not superimpose each other.
As motion artifacts arise from different origins, there are several ways to avoid or compensate for them. One way to eliminate motion artifacts is to reduce the cause of the artifacts to a minimum, at least for the duration of scanning process. For example, involuntary bulk motion can be reduced by using antispasmodic drugs and voluntary motion such as respiratory motion can be reduced by using short sequence during breathhold. Clear breathing instruction also is very important in reducing respiratory motion. For imaging of the heart or the great vessels, triggering the acquisition of phase encoding steps at fixed time in cardiac cycle can be done which means that in every phase encoding step the structure is in the same position.
In the study by Hirokawa (2008), image artifacts were shown to be reduced by the BLADE technique. MRI with the BLADE technique did not generate ghosting artifact and also showed reduction of other artifacts including respiratory motion and cardiovascular pulsation. In all cases, image artifacts were fewer in the images with the BLADE technique (BLADE is proprietary name for periodically rotated overlapping parallel lines with enhanced reconstruction [PROPELLER] in MR systems from Siemens Medical Solutions). Other than that, respiratory gating can eliminate ghosting and blurring from diaphragm motion. However it is inefficient because only the data collected in specific parts of the cardiac or respiratory cycle can be used for image formation. This will result in cosiderable increase of scan time and long TR times. The more subtle techniques are based on re-ordering the k-space lines such that adjacent samples in the final data set have minimal differences in respiratory phase. These so-called respiratory compensation methods have only little time penalty, but are less effective in non-periodic breathing.
Fig. 15 Motion artifact. More random respiratory motion (left) shows more general blurring of the T2 weighted TSE image (TR2200 ms, TE 103ms, ETL 21), periodic breathing leads to ghost artifacts (right). (adapted from Stadler,2008)
CHEMICAL SHIFT ARTIFACTS
Chemical shift artifact is a very common artifact that display on the image especially at the region of abdomen and spine. It is manifest as bright or dark outlines at fat-water interphase. It occur in some molecule such as lipids because the proton in the nuclei are magnetically shielded by their surrounding electrons and having a difference in resonance of protons as a result of their micromagnetic environment. CS artifact are due to the phenomenon on whereby different proton precess in different frequencies. Proton in different molecules have their own specific characteristic of Larmor frequency which is called 'chemical shift' and is exploited in MR spectroscopy in order to differentiate the molecules in sample. Therefore, Larmor frequency between fat and water are different in the relative frequency shift between main fat peak and water peak is 3.5 ppm, resulting in a difference of only 73 Hz at 0.5 T and about 450 Hz at 3T.
Chemical shift artifact can be devided into two subtypes which is chemical shift of the first and second kind depending on the causes and origin. For the first type of chemical shift artifact is due to a spatial misregisteration of the signal for the difference in resonance frequency between fat and water. For example, the signals of fat and water protons belonging to the same volume element are encoded as being located in different voxel. The difference in precession frequencies results in the overlap of fat and water at lower frequency (bright band) and signal void (dark band) at higher frequency. The first type of CS artifact depends on several factors but mainly on magnetic field strength (B0), frequency range [bandwith (BW)] and on pixel size (matrix) (Rescinito,2009). Potential solution to reduce CS artifact is the use of fat-suppression techniques, use of long echo time (TE) or increase the readout bandwidth (BW).
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The second type of CS artifacts is due to the phase cancellation. When using gradient echo sequence with certain echo times, there is a 1800 phase shift between water and fat protons due to their Larmor frequency. at this time, when fat and water are present in the same voxel, the signal of fat and water cancel each other out in voxel with equal signal leading to a signal loss or 'black boundary' or hypointense rim around fatty tissues. Potential solution are again by using fat suppression techniques, increase the bandwidth (reduces SNR) and obviously use a specific TE for in-phase imaging. By increasing the pixel size with constant FOV also will reduce resolution thus decrease CS artifacts.
Fig. 19 Chemical shift artifact of the first and second kind. Chemical shift to the first kind (left) on the T1 w in-phase GRE image (TR 100ms, TE 4.4 ms) shows a dark rim in the posterior kidney-fat interphase. Frequency encoding direction is anteroposterior. Chemical-shift artifact of the second kind: T1 w opposed-phase GRE image (right) (TR 100 ms, TE 2.2ms) shows a surrounding dark rim at the fatty tissue interphase, independent of the frequency encoding direction (adapted from Stadler et al., 2007)
Susceptibility can be describe as the property of the matter of becoming magnetized when exposed to the magnetic field. The susceptibility artifact occur when the static magnetic field is not perfectly uniform. Such nonuniformities may be a result of imperfections in the magnetic field itself and in most cases is due to the different magnetic susceptibility of the tissue and material that are adjacent to each other such as soft tissue and air, soft tissue and metal or soft tissue and blood causing the local magnetic field to be heterogeneous. Another causes of nonuniformities is the magnetic field distortion for example there are macroscopic field gradients of the magnetic field strength at the interfaces of region of different magnetic susceptibility.
Local heterogeneity of the magnetic field causes protons to precess at different frequencies than they normally would and because of this the proton are mapped to the incorrect location in the frequency-encoding direction. This shift in the magnetic field leads to misregistration of the signal within the raw data and anatomic distortion also can occur. There are three types of substance which have different magnetic susceptibility commonly deal with in MRI which is diamagnetic, paramagnetic and ferromagnetic material. Diamagnetic substance such as water and most biological substance have negative susceptibility and slightly weaken the external field. Paramagnetic material such as platinum, titanium and gadolinium in the other hand have positive susceptibility and augment the external field. Lastly ferromagnetic material such as iron, cobalt and nickel have strong nonlinear positive susceptibility that can lead to a strong distortion of the B0 field and the linearity of the frequency encoding gradient close to the object.
Susceptibility artifacts increase with echo time and field strength and are most prominent in gradient echo and in echoplanar imaging (Heiland, 2008). To reduce susceptibility artifacts, spin echo should be use rather than gradient echo sequences. Reduction of echo time (TE) and increase of the readout bandwidth (BW) help to keep susceptibility artifacts as small as possible. The best combination of spin echo and short TE are found in fast spin echo techniques rather than in conventional spin echo. Lastly, to imigitate susceptibility effect, increasing the frequency matrix and decreasing the slice thickness can also be considered.
FIGURE 5 (a), (b)
Fig. 5a,b Magnetic susceptibility artefact. The susceptibility artefact occurs at an air-tissue interface in the bowel loops, causing local anatomical distortion with signal void (arrows, a,b) and inhomogeneous saturation of lipid signal (curved arrows, b).
Metal artifact is similar to susceptibility artifact however, this is the severe form of susceptibility artifact caused by external metallic object lying in close proximity to the subject in question and distorts the magnetic field in their surrounding. As susceptibily artifacts, it occur at the interfaces of tissue with different magnetic susceptibilities which cause local magnetic fields to distort the external magnetic field thus changing the precession frequency in the tissue. The degree of distortion is depends on the type of metal exposed and stainless steel metal proved that it have a greater distorting effect compared to titanium alloy. It also depends on type of interfaces (soft tissue-metal have high striking effect), pulse sequence and imaging parameters. Metal artifacts consists of central signal void and asymmetric margins of higher signal intensity in non-anatomic configurations.Metal artifacts are seen as an area of zero signal, often with a high intensity rim and distortion in adjacent tissue (Murray et al., 2006)
There are several factors that contributing to the degree of severity of metal artifact. The major factor is external magnetic field strength. As the external magnetic field increase, the ferromagnetism of the ferromagnetic material also will increase thus increase the metal artifact induce in the image. Secondly is the object specific factors. This include the shape of the object, orientation of long axis of the object with respect to the FEG axis and lastly the object density and chemical composition. The shape of the object will decide if any closed conducting pathways are present or not while the chemical and density composition affect the magnitude of the artifact as density of the metal present in a particular object is linearly related to the magnitude of artifact. Last factor that contribute to the severity of the artifact is the imaging technique. It depends on local field inhomogeneities and range of bandwidth used.
To reduce metal artifacts, several ways can be use including orientating the long axis of an implant or device parallel to the long axis of the external magnetic field. Another method that can be used are choosing the appropriate frequency encoding direction, using smaller voxel sizes, fast imaging sequences, increase readout bandwidth and avoid using gradient-echo imaging when metal is present. By reducing the slice thickness or expanding the FOV it will result in minimal reduce of intensity of artifact. However, if thin slice is used with increase frequency matrix, it will reduce the artifact moderately. Lastly Kaur (2006) state that 'using lower field strength MR systems. At the same time, some precautions should always be exercised. Object like scissors, keys, oxygen tanks, wheel chairs, trolleys, pens, etc. can get attracted towards the magnet iso-centre with high velocities up to 40 miles/h and can cause injuries to any individual in their path or can severely damage the magnet and its component'.
Figure 6 (a) Plain skiagram of the paranasal sinuses shows a small metal piece in the right frontal sinus (encircled). (b) Typical ferromagnetic artifacts seen on an SE T1 weighted axial image. Also the two components viz. stretching of the image and the high intensity line are marked on the image (arrows). Also pixel shift in the PEG direction was 19 pixels with
128 x128 matrix size. (image adapted from Kaur, 2007)
Generally, MRI is a very useful imaging modalities in diagnosing patients. However, artifacts is an expected outcome in MRI image. Recognizing those artifact is very essential for interpretation of image because in certain cases it may simulate disease or mask pathologic abnormalities. Even if the artifact cannot be completety removed in the image, knowledge of the radiologist is very important in radiologist as it will help to avoid misinterpretation.These artifacts may be caused either by physical limitation related to the higher field strength or by protocols used in the MRI machine itself. Many technical factor must be considered in order to choose sequence parameters that reduce the artifacts and optimize the diagnostic quality. The best way to remove some artifacts is to avoid them altogether by using up-to-date, well calibrated scanning hardware and appropriate pulse sequences, gradient waveforms and gating for application of interest. With time, the study of image artifacts will also lead to more accurate signal models and improvements in quantitative MRI.