Magnetic Resonance Imaging (MRI) Physics & Instrumentation
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TITLE : MRI IMAGE ARTIFACTS
- Name of Student: Mohamad Al-Hafiz bin Ibrahim
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Magnetic Resonance Imaging (MRI) is one of the medical imaging and diagnosis technique which widely used due to its capability to produce high resolution of cross- sectional anatomical images and high tissues contrast. Eventhough MRI has various advantageous features, but still there are numerous sources of artifacts either patient-related, signal processing-dependent and hardware (machine) related (Erasmus, Hurter, Naudé, Kritzinger, & Acho, 2004).
Definitely, artifacts can degrade the image quality and may mimicking pathology or obscure the abnormalities which can lead to misdiagnosis of MRI images. The MRI artifact can be defined as a structure or feature appearing in MRI image produced by artificial means which is not originate within the scanned object (Erasmus et al., 2004). Commonly, MRI artifacts can be caused by RF leakage, aliasing, patient motion, Gibbs, truncation, chemical shift, magnetic susceptibility and flow motion.
This artifact also known as Zipper artifact. It occurs when there are leakage of RF or electromagnetic energy generated from certain equipment into MRI system (Stadler, Schima, Ba-Ssalamah, Kettenbach, & Eisenhuber, 2007). This extrinsic RF came at a certain frequency then interferes with MRI signal produced by patient. The potential sources of the extrinsic RF are due to penetration of the RF into the shielded scanning room especially when the door is open during images acquisition (Ruan, 2013). After that, the RF will be picked up by the receiver chain of the image sub system (Zhuo & Gullapalli, 2006). This RF perhaps generated by radio, illumination or electronic device such as monitoring equipment in the surrounding (Stadler et al., 2007).
In order to overcome this artifact, the operator should identified and eliminate the possible source of the penetration. It can be done by ensure the door of the MR room remain closed during scanning, use only MR compatible MR monitor equipment, and remove the external RF source from the surrounding (Ruan, 2013).
Aliasing or wrap around artifacts can be describe as an artifacts that caused by anatomy that lies outside of field of view (FOV) mismapped within the FOV (Westbrook, Roth, & Talbot, 2011). This is because of improper selection of parameter in MR systems especially FOV. The FOV in MRI means the anatomical area that should be covered or imaged during scanning (Morelli et al., 2011). When the selected FOV is smaller than the size of area that should be imaged means the data are under-sampled (Ruan, 2013). Therefore, there are high chances for signals from the outside FOV falsely detect then create an interference with signal within FOV and encode on the reconstructed images thus ‘wrap around’ to the opposite side of image which become aliasing artifacts (Erasmus et al., 2004). Westbrook et al., (2011) state that aliasing artefact can happen along frequency encoding axis (frequency wrap) and phase encoding axis (phase wrap).
Basically, this aliasing artifacts can be eliminated through increase the sampling rate or oversampling along the frequency direction (Westbrook et al., 2011). However, high pass and low pass filter should be used as well in order to filter out frequency outside the FOV which can increase noise in image (Hiroshi, Schlechtweg, & Kose, 2009). Besides that, selection of receiver coil which unable to excite or detect the signals from anatomical tissues that lying outside the FOV also important to minimise the artifacts (Ruan, 2013). Lastly, No Phase Wrap (NPW), Phase oversampling or Fold Over supression techniques is also preferred to avoid aliasing artefact by oversamples in phase direction, thus, the phase curve get to extends and cover wider signal producing structures (Westbrook et al., 2011).
Patient motion artifact is a very most common artefact in MRI. It is caused by movement of anatomical structure during imaging sequence (Zhuo & Gullapalli, 2006). There is a broad range of examples of structure movement such as heart or arterial pulsations, respiration process, peristalsis, tremor (Parkinson’s disease) and gross movement of patient (Stadler et al., 2007). Hence, if there is a scanned anatomical part moved during the scanning, the phase gradient cannot predict and encode the signal, thus, that structures will be misplaced in phased encoding direction. As a result, it will causes MR images shown the appearances of mismapping, blurring and ghosting artefact within it (Westbrook et al., 2011).
There are several ways to eliminate or avoid the patient motion artifacts. The remedies are nullifies signal by applying pre-saturation pulses over the area which have potential to produce artifacts (Stadler et al., 2007). This way is more effective to prevent ghosting during patient swallowing. Besides that, Westbrook et al (2011) proposed that attaching a set of bellows over patient’s chest in respiratory compensation which is also known as respiratory ordered phase encoding (ROPE) might help to minimize ghosting in longer sequences while in short sequences, cooperation from patient to hold their breath during scanning is preferred.
Next,cardiac gating also plays role in reducing this kind of artefact. For example, electrocardiogram (ECG) gating used to monitors cardiac motion that trigger the excitation pulse. Hence, each excitation pulse in each slice can be timed and acquired at the same phase of cardiac cycle (Westbrook et al., 2011). In the other hand, asking for patient cooperation for keeping still, clear explanation about procedures, and optimize the patient’s comfortability are important to make them immobilize during scanning (Hiroshi et al., 2009).
Truncation artefact also can be called as Gibbs Ringing artefact (Czervionke, Czervionke, Daniels, & Hauhgton, 1988). Its happen as result of It is causes by abrupt undersampling of data that results in incorrect representation of high and low signals interfaces (Westbrook et al., 2011). That problems lead to visibility of fine lines in MR image and also respectively caused by incomplete digitization of the echo (Ruan, 2013). However, according to Erasmus et al.,(2004), alternating dark and bright lines may visible in image due to a sharp transition in signal intensity.
In order to correct this type of artefact, there are several ways that can be used. For example, increase the matrix size, 256 x 256 instead of 256 x 128 (Westbrook et al., 2011). Next, applying various filters to k-space data before Fourier transform also should be considered (Erasmus et al., 2004). Besides that, provide more phase encoding steps also preferred to make truncation or gibbs artifacts less intense and narrower (www.mr-tip.com, n.d.).
This type of artifact commonly found in MRI image of abdominal and spine imaging. Since fat and water each consist of hydrogen protons but different combination of molecules, fat contain hydrogen binds with carbon,while in water, hydrogen combine with oxygen (Westbrook et al., 2011). Hence, that different chemical environment exist shown that there are different precession frequency between fat and water which fat has lower precessional frequency rather than water (Erasmus et al., 2004). Based on the Larmor equation, precessional frequency is proportional to the strength of magnetic field (Westbrook et al., 2011). Therefore, this chemical shift can become artifact due to that difference of the precessional frequency between fat and water at higher field of magnetic strengths during the frequency encoding or slice-select directions (Ruan, 2013). That frequency is basically used to encode their spatial positions, thus, any chemical shift can lead to spatial misregistration of the MR signal (Morelli et al., 2011). MR images will show the bright or dark outlines at fat-water interfaces as the artefact.
To avoid this artefact , a few remedies should be considered such as perform scanning at low magnetic field strength, increase the receive bandwidth in keeping with good signal-noise-ratio (SNR) (Westbrook et al., 2011) . It is also suggested to use minimum FOV as possible. Lastly, swapping phase and encoding direction also may useful to reduce this artefact (Hiroshi et al., 2009).
Susceptibilty can be refer as characteristic of substance which be magnetized when exposed to magnetic field (Gary, n.d.). MRI physics explain magnetic susceptibility artifacts normally happens because of substance or material especially ferromagnetic materials and also at air-tissues interface which have different degree of magnetic susceptibility that can distort the external magnetic field when placed in large magnetic field. Besides that, the differences also lead to magnetic field inhomogeneity at the scanner region resulting in spins dephase faster and frequency shift between surrounding tissues (Zhuo & Gullapalli, 2006). Artifacts in the image will appear as bright and dark areas with spatial distortion of anatomical structures (Stadler et al., 2007).
Generally, these artifacts can be reduced by ensure all metal objects that attached to the patient has been removed before the scan. Next, spin echo sequences are more preferred to be used instead of gradient echo because it use 180° RF rephasing pulse which ideal at compensating for phase differentiation between fat and water (Westbrook et al., 2011). Since fast spin echo techniques also contribute in reduction of this type of artefact, hence, short TE is used along with spin echo (Stadler et al., 2007). Metal Artefact Reduction Sequence (MARS) technique can be used in order to minimize the size and intensity of this artifact which developed by magnetic field distortion by introducing an additional gradient following the slice gradient during frequency encoding gradient is used (Olsen, Munk, & Lee, 2000).
Flow artefact can be categorized as one kind of motion artefact which mainly caused by natural motion of liquids such as blood or cerebrospinal fluid (CSF) in the body. For example, hydrogen nuclei in blood flow within the scanned slice may trigger excitation from an incoming RF pulse, however, the signal perhaps cannot be readout due to possibility of that flowing blood have left the slice (Hiroshi et al., 2009). As a result, vessels image appear empty or low signal intensity (less bright). Generally, there are reasons of low signal intensity such as intravascular signal void by time of flight effects, first echo dephasing and fast flow (Hiroshi et al., 2009).
Nevertheless, this artifacts also can appear bright or high signal intensity. This is because of the slow blood flow (flow related enhancement), even echo rephrasing and diastolic pseudogating (Hiroshi et al., 2009).
The preferred solutions as remedies for flow motion artifacts are by reduction of phase shifts using flow compensation in order to produce gradient moment nulling, suppress the blood signal by apply saturation pulses parallel to slices and synchronization of imaging sequences with cardiac cycle using cardiac triggering (Zhuo & Gullapalli, 2006).
It is important for all operators, radiologist and engineers in MRI are able to recognize common MRI artifacts because there are a broad of range of cause that contributing to artefact. Eventhough, artifacts are unable to be totally eliminated but it can be minimized or avoided with specifics remedies in order to improve the MR image quality (Morelli et al., 2011). Therefore, basic knowledge of MRI artifacts should be learned and all MRI system operators should familiar with their MRI unit in department.
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