MRI Artifacts: Mechanism and Control

Transcription

1 MRI Artifacts: Mechanism and Control Chun Ruan Abstract A wide variety of artifacts is routinely encountered on MR images. This article presents the cause, appearance, diagnostic effect, and available remedies for the artifacts that are most frequently observed on MR images and are of greatest clinical significance. Combined with routine preventive maintenance of imaging equipment, consistent quality control, and appropriate selection of imaging parameters, awareness of the manifestations of these artifacts will allow image quality and diagnostic interpretation to be optimized. I. Introduction Magnetic resonance imaging (MRI) is widely used in medical diagnosis for its various advantageous features, such as high-resolution capability, the ability to produce an arbitrary anatomic cross-sectional image, and high tissue contrast. Unfortunately, there are many potential sources of image artifacts associated with the technology of MRI. They can potentially degrade images sufficiently to cause inaccurate diagnosis. Many MR artifacts are neither obvious nor understandable from previous experience with conventional types of imaging. While some MR artifacts are machine specific, the majority are inherent in the imaging method itself. MR imaging artifacts can be grouped into two general categories. First, there are artifacts that are hardware related. These artifacts are relatively uncommon fortunately, because they are often difficult to diagnose and usually require service personnel to correct. The second category consists of artifacts related to the patient or under operator control. This category is encountered much more commonly and may often be easily prevented or corrected once they are recognized. II. Technique and Methods A. Motion Artifacts Motion is the most prevalent source of MR imaging artifacts. As the name implies, motion artifacts are caused by motion of the imaged object or a part of the imaged object during the imaging sequence. Motion results in two effects on MR images. View-to-view effects are caused by motion that occurs between the acquisitions of successive phase-encoding steps. The inconsistent location and signal intensity of spins that move as phase-encoding data are acquired result in phase errors. When motion is periodic that is, occurs in a regular pattern the result is complete or incomplete replication of the moving tissue, commonly referred to as ghosting artifacts. These artifacts are observed along the phase-encoding direction of the image, regardless of the direction in which the motion actually occurred. Periodic physiologic motions that commonly result in ghosting artifacts include cardiac motion, respiratory motion, vascular pulsation, and cerebrospinal fluid (CSF) pulsation. 1 Motion occurring between the time of radiofrequency (RF) excitation and echo collection (i.e., within-view) results in a lack of coherent phase among the population 1

2 of moving spins at the time of echo formation. 2 This incoherence manifests as blurring and increased image noise. Unlike phase errors encountered in view-to-view motion effects, this within-view effect is expressed throughout the image. It is most frequently associated with random motion, as can occur with gastrointestinal peristalsis, swallowing, coughing, eye motion, and gross patient movement Respiratory motion Respiratory motion results in ghosting artifacts and blurring that can obscure or simulate lesions. A variety of methods have been used to reduce the effect of respiratory motion artifacts. Mechanical methods, such as use of an abdominal or thoracic binder or taking images with the patient in a prone position, are intended to restrict the amplitude of respiratory motion. However, these maneuvers often produce patient discomfort and may therefore have counterproductive effects; Signal averaging is the use of multiple data acquisitions to improve the signal-to-noise ratio (SNR) of the image ( Fig.1). In this process, the prominence of ghosting artifacts is reduced by approximately the square root of the number of signal averages obtained. Of course, imaging time increases linearly with the number of signal averages; With respiratory triggering, data are collected only during a limited portion of the respiratory cycle, usually near end-expiration, when respiratory movement is minimal. The major drawback of this technique is its marked prolongation of imaging time, because so much of the time is not being used productively for data acquisition; Respiratory ordered phase encoding involves monitoring the patient s respiratory cycle during imaging using a bellows device. Unlike respiratory triggering, however, with this method data are collected in a continuous fashion. This method does not restrict the operator s selection of TR, and it does not significantly extend imaging time; Gradient moment nulling involves the application of additional gradient pulses to correct for phase shifts among a population of moving protons at the time of echo collection. This method corrects for constant-velocity motion and helps reduce the signal loss and ghosting associated with such movement. Use of gradient moment nulling requires a nominal TE, which may preclude its use on T1-weighted pulse sequences. This method does not prolong image acquisition time; Another method to reducing the signal intensity of moving tissue is the use of fat suppression methods. With this method, not only subcutaneous fat but also mediastinal, mesenteric, retroperitoneal, and other stores of internal fat are suppressed and are thus less capable of generating ghosting artifacts. 2

3 2. Cardiac motion Figure 1. The image on the left exhibits respiratory motion as blurring of the structures as well as motion-induced ghosting. By using multiple averages motion can be reduced in the same way that multiple averages increase the signal to noise ratio. The image on the right was obtained with 16 averages. Cardiac motion produces a series of ghost artifacts along the phase-encoding direction of the image, in addition to blurring and signal loss of cardiac and juxtacardiac structures. 4 The major approach for reducing cardiac motion artifacts is electrocardiographic triggering, in which data collection is synchronized with cardiac phase ( Fig. 2). This synchronization enables cardiac tissue to be located in a consistent position as each successive phase-encoding step is acquired, resulting in increased tissue signal intensity and decreased phase errors. Other approaches include the use of fast imaging sequences that reduce the opportunity for motion during data acquisition, gradient moment nulling, and spatial RF presaturation pulses. 3. Vascular Pulsation Figure 2. The image on the left was acquired without any form of motion compensation technique for cardiac motion. The image on the right was obtained using cardiac gating. Vascular pulsation artifacts are recognized by their alignment with the responsible vessel along the phase-encoding direction of the image. These artifacts reproduce the cross-sectional size and shape of the responsible vessel, but not necessarily its signal intensity. Spatial RF presaturation pulses applied outside the field of view help reduce the signal intensity of inflowing blood and, hence reduce the resultant pulsation artifact. Other practical methods for reducing the prominence of vascular pulsation artifacts include positioning the section of interest in the middle of a multisection acquisition, thus reducing any potential entry phenomenon, and maximizing the saturation of flowing spins. B. Susceptibility Artifacts Susceptibility artifacts occur as the result of microscopic gradients or variations in the magnetic field strength that occurs near the interfaces of substance of different magnetic susceptibility ( Fig. 3). Large susceptibility artifacts are commonly seen surrounding ferromagnetic objects inside of diamagnetic materials (such as the human body). These gradients cause dephasing of spins and frequency shifts of the surrounding tissues. The net results are bright and dark areas with spatial distortion of surrounding anatomy. These artifacts are worst with long echo times and with 3

4 gradient echo sequences. Susceptibility artifacts can be made less prominent by performing imaging at low magnetic field strength, using smaller voxels, decreasing echo time, and increasing receiver bandwidth. Gradient-echo and echo-planar sequences should be avoided, because they accentuate susceptibility artifacts. The use of spin-echo and particularly fast spin-echo sequences should be considered. C. Chemical Shift Artifacts Figure 3. An axial MRI of the head in a patient with mascara on her eyelids. Susceptibility artifacts from the mascara obscure the front half of the globes. A chemical shift artifact is caused by the difference in chemical shift (Larmor frequency) of fat and water. The artifact manifests itself as a misregistration between the fat and water pixels in an image ( Fig. 4). The effect being that fat and water spins in the same voxel are encoded as being located in different voxels. The magnitude of the effect is proportional on the magnitude of the Bo field and inversely proportional to the sampling rate in the frequency encoding direction. For a constant sampling rate, the larger Bo, the greater the effect. 5 Chemical shift artifacts are typically observed along the frequency-encoding direction but can also occur along the slice-selection direction of the image. Chemical shift artifact can be reduced by performing imaging at low magnetic field strength, by increasing receiver bandwidth, or by decreasing voxel size. The artifacts tend to be more prominent on T2-weighted than on T1-weighted images. Fat suppression methods often eliminate visible artifacts, and gradient reorientation can redirect chemical shift artifacts to another portion of the image. 4

5 D. Wrap Around Artifacts Figure 4. This artifact is shown in an axial image of a kidney where the bright border along the top of the kidney and the dark border along the bottom of the kidney represent the artifact. A wrap around artifact is the occurrence of a part of the imaged anatomy, which is located outside of the field of view, inside of the field of view ( Fig. 5). This artifact is caused by the selected field of view being smaller than the size of the imaged object. Or more specifically the digitization rate is less than the range of frequencies in the FID or echo. 6 The solution to a wrap around artifact is to choose a larger field of view, adjust the position of the image center, or select an imaging coil which will not excite or detect spins from tissues outside of the desired field of view. Figure 5. The first image shows wrap-around of the back of the head on to the front of the head, where the phase-encoded direction is anterior-posterior. The second image has the phase and frequency directions reversed resulting in absence of the aliasing artifact. Oversampling was used in the frequency direction to eliminate the aliasing. E. Partial Volume Artifacts A partial volume artifact is any artifact which is caused by the size of the image voxel. It occurs when multiple tissue types are encompassed within a single voxel. 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. Volume averaging is most likely to occur in the slice-selection direction of the image, which has the largest voxel dimension. It also occurs when structures are oriented obliquely to the imaging plane and when structures move in and out of a given section during image acquisition. Volume averaging can simulate abnormalities, decrease the visualization of low-contrast abnormalities, and blur or distort affected structures. Partial volumeaveraging is usually recognized by careful analysis of adjacent images. Decreasing voxel size, particularly reducing section thickness, can be useful if further confirmation is required ( Fig.6). Three-dimensional Fourier transform imaging is particularly useful, because it provides thin sections with no intervening gaps and is conductive to reformatting in alternate imaging planes; Simple acquisition of additional two-dimensional images in alternate imaging planes is very helpful for resolving issues relating to partial volume averaging. Retrospective reformatting of two-dimensional data can also be performed using an interpolation algorithm and can be of further assistance. 5

6 Figur e 6. These two axial T1-weighted images of the head were obtained at exactly the same location, yet the second image shows the VII and VIII cranial nerves while the first does not. The reason for the vanishing nerve is explained by partial volume averaging. The first slice was obtained with a thickness of 10 mm while the second was at a thickness of 3 mm. F. Gibbs Ringing Artifacts Gibbs ringing artifacts are bright or dark lines that are seen parallel and adjacent to borders of abrupt intensity change. 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. This artifact is seen in images when a small acquisition matrix is used. Solutions include use of a higher resolution imaging matrix and filtration methods ( Fig.7). Gradient reorientation will displace the artifacts to another portion of the image. Figure 7. The fine lines visible in the image on the left are due to undersampling of the high spatial frequencies. This results in a "ringing" type of artifact following these borders in the phase direction (R to L in this image). This problem can be easily fixed by taking more samples such as the image on the right with 256 phase encodes. G. Zebra Stripes Zebra stripes can be observed along the periphery of gradient-echo images where there is an abrupt transition in magnetization at the air-tissue interface. They are accentuated by aliasing that results from the use of a relatively small field of view. Solutions include expanding the field of view, using spin-echo pulse sequences, or using oversampling techniques to reduce aliasing. H. Slice-overlap Artifacts 6

7 The slice-overlap artifact is the loss of signal seen in an image from a multi-angle, multi-slice acquisition, as is obtained commonly in the lumbar spine. If the slices obtained at different disk spaces are not parallel, then the slices may overlap. If two levels are done at the same time, e.g., L4-5 and L5-S1, then the level acquired second will include spins that have already been saturated. This causes a band of signal loss crossing horizontally in image, usually worst posteriorly. Therefore, overlap of sections within areas of diagnostic interest should be carefully avoided. I. RF Overflow Artifacts RF overflow artifacts cause a non-uniform, washed-out appearance to an image. This artifact occurs when the signal received by the scanner from the patient is too intense to be accurately digitized by the analog-to-digital converter. Autoprescanning usually adjusts the receiver gain to prevent this from occurring but if the artifact still occurs, the receiver gain can be decreased manually. J. Entry Slice Phenomenon Entry slice phenomenon occurs when unsaturated spins in blood first enter into a slice or slices. It is characterized by bright signal in a blood vessel (artery or vein) at the first slice that the vessel enters. Usually the signal is seen on more than one slice, fading with distance. This artifact has been confused with thrombosis with disastrous results. The characteristic location and if necessary, the use of gradient echo flow techniques can be used to differentiate entry slice artifacts from occlusions. K. Zipper Artifacts There are various causes for zipper artifacts in images. Most of them are related to hardware or software problems beyond the physicist immediate control. The zipper artifacts that can be controlled easily are those due to RF entering the scanning room when the door is open during acquisition of images. 7 RF from some radio transmitters will cause zipper artifacts that are oriented perpendicular to the frequency axis of image. Broad-band noise degrades the entire image, whereas narrow frequency noise produces linear bands that transverse the phase-encoding direction of the image. Solutions include identifying and removing external RF sources, ensuring that the door to the imaging room remains closed, and verifying the integrity of the magnet room enclosure and associated seals. L. Cross-Excitation Cross-excitation is caused by the imperfect shape of RF slice profiles, which leads to the unintended excitation of adjacent tissue. This excitation results in the saturation of such tissue, manifest as decreased signal intensity and decreased contrast that can hinder lesion detection. One way to avoid this artifact is to introduce an intersection gap that is 10% to 50% of the prescribed section thickness. Another method is interleaved image acquisition, in which odd-numbered sections are initially acquired, followed by acquisition of even-numbered sections. 8 Also, optimized RF pulses that have a more rectangular slice profile can be implemented. 7

8 Figure 8.The dark bands visible on the T1-weighted Spin Echo axial image are due to the intersection of other axial images through the image. These are acquired in a multi-planar fashion and thus cause pre-excitation (saturation) of the protons in the area where the slices intersect. The sagittal image illustrates where the slices were obtained and how they intersect posteriorly. M. Shading Shading artifacts manifest as foci of relatively reduced signal intensity involving a portion of the image. Abnormalities contained in the shaded portion of the MR image may be obscured. 9 There are many potential causes for this artifact, including partial volume averaging malfunction of the RF transmitter, amplifier, or receiver, excessive RF absorption, etc. 10, 11 To minimize shading artifacts, the anatomy of interest should be centered within the magnet, within the coil, and within the group of sections to be acquired. III. Conclusions Artifacts are common in magnetic resonance imaging. Most occur as a result of interactions of multiple factors, especially motion. MR artifacts primarily cause image degradation, although they can occasionally mimic pathological lesions. Some MR artifacts, such as those caused by periodic respiratory or vascular motion, may obscure important diagnostic findings. These and other MR artifacts may be minimized in some instances by proper selection of the directions of the phase and frequency encoding gradients. The ability to select and vary the direction of these gradients is a useful option in MR imaging. Investigations in the origins of MRI artifacts can not only lead to further understanding of the imaging process itself, but can also improve the quality and diagnostic accuracy of MR examination. 1 Saloner D, Flow and motion, Magn Reson Imaging Clin N Am 1999 Nov;7(4): Barish MA and Jara H, Motion artifact control in body MR imaging, Magn Reson Imaging Clin N Am 1999 May;7(2): Hedley M, Yan H, Motion artifact suppression: a review of post-processing techniques, Magn Reson Imaging 1992;10(4): Huber ME, Hengesbach D and Botnar RM, Motion artifact reduction and vessel enhancement for free-breathing navigator-gated coronary MRA using 3D k-space reordering, Magn Reson Med 2001 Apr;45(4): Altbach MI, Trouard TP and Van de Walle R, Chemical-shift imaging utilizing the positional shifts along the readout gradient direction, IEEE Trans Med Imaging 2001 Nov;20(11): Tsai CM and Nishimura DG, Reduced aliasing artifacts using variable-density k-space sampling trajectories, Magn Reson Med 2000 Mar;43(3): Mugler JP 3rd, Overview of MR imaging pulse sequences, Magn Reson Imaging Clin N Am 1999 Nov;7(4):

9 8 Clark JA 2nd and Kelly WM, Common artifacts encountered in magnetic resonance imaging, Radiol Clin North Am 1988 Sep;26(5): Jones RW and Witte RJ, Signal intensity artifacts in clinical MR imaging, Radiographics 2000 May- Jun;20(3): Mirowitz SA, MR imaging artifacts. Challenges and solutions, Magn Reson Imaging Clin N Am 1999 Nov;7(4): Mirowitz SA, MR imaging artifacts. Challenges and solutions, Magn Reson Imaging Clin N Am 1999 Nov;7(4):