Magnetic Resonance Imaging (MRI) is an indispensable tool in modern medical diagnostics, providing detailed images of the body's internal structures without the use of ionizing radiation. However, the quality and accuracy of MRI scans can be compromised by various phenomena known as artifacts. These are discrepancies between the displayed image and the actual anatomic structures. Among the plethora of artifacts, zipper and aliasing artifacts are particularly noteworthy due to their frequency and impact on image interpretation. This article delves into the origins, manifestations, and mitigation strategies for zipper and aliasing artifacts in MRI.

Zipper artifacts, sometimes referred to as "cross-talk" artifacts, present as unwanted stripes or lines that overlay the magnetic resonance image. These stripes can either be randomly distributed or follow a regular pattern across the image (Hargreaves, 2000). Zipper artifacts are primarily caused by the leakage of radiofrequency (RF) signals into the MRI system. This can happen if the RF shield of the MRI room is damaged or if there is an improper filter on the RF channels, allowing external RF signals to interfere with the acquired signal (Brown et al., 2014).

Aliasing, on the other hand, occurs when the field of view (FOV) is insufficient to cover the entire anatomy being imaged, leading to a wrapping or superimposition of part of the image onto the opposite side of the field. Aliasing is also known as wraparound artifact (Panych and Madore, 2008). In the frequency-encoding direction, it is often a consequence of an inadequate sampling bandwidth or a narrow FOV that fails to encompass the anatomy of interest (Deshmane et al., 2012).

To mitigate zipper artifacts, MRI technologists and radiologists must ensure that the RF shield is intact and that all cables and connections are properly filtered and grounded. Utilizing RF traps or chokes can also diminish the interference (Plewes and Kucharczyk, 2012). Furthermore, the use of saturation bands can suppress signals arising outside the area of interest, thus reducing the risk of zipper artifacts.

Addressing aliasing artifacts requires adjustments to the FOV or the application of phase oversampling techniques. Increasing the FOV to include the entire anatomy can eliminate the wrapping effect, although this may come with a trade-off in resolution or increased scan time (Dietrich et al., 2008). Applying phase-encoding steps beyond the actual size of the image matrix, known as oversampling, is another effective method to prevent aliasing without altering spatial resolution (Glockner et al., 2005).

Advanced software algorithms and hardware modifications have been developed to reduce these artifacts. Parallel imaging techniques, such as SENSE (Sensitivity Encoding) and GRAPPA (Generalized Autocalibrating Partially Parallel Acquisitions), exploit spatial information from arrays of receiver coils to correct for aliasing artifacts (Blaimer et al., 2004).

The importance of recognizing and addressing MRI artifacts is crucial as they can mimic or obscure pathology, leading to diagnostic errors. Radiologists must be able to distinguish between true pathological findings and these artifacts to avoid misinterpretation of images (Westbrook, 2008).

In conclusion, zipper and aliasing artifacts in MRI are common issues that can significantly affect image quality and diagnostic accuracy. Through a combination of preventive measures, technique adjustments, and advanced processing algorithms, the impact of these artifacts can be minimized. As MRI technology continues to evolve, the ongoing research and development in this area will be paramount in enhancing image clarity and reliability, thereby improving patient care.

References:

Hargreaves, B. A. (2000). RF artifacts in MR imaging. Radiographics, 20(6), e1-e1.

Brown, R. W., Cheng, Y. C. N., Haacke, E. M., Thompson, M. R., & Venkatesan, R. (2014). Magnetic Resonance Imaging: Physical Principles and Sequence Design. Wiley.

Panych, L. P., & Madore, B. (2008). The physics of MRI safety. Journal of Magnetic Resonance Imaging, 27(6), 1257-1262.

Deshmane, A., Gulani, V., Griswold, M. A., & Seiberlich, N. (2012). Parallel MR imaging. Journal of Magnetic Resonance Imaging, 36(1), 55-72.

Plewes, D. B., & Kucharczyk, W. (2012). Physics of MRI: A Primer. Journal of Magnetic Resonance Imaging, 35(5), 1038-1054.

Dietrich, O., Raya, J. G., Reeder, S. B., Reiser, M. F., & Schoenberg, S. O. (2008). Measurement of signal-to-noise ratios in MR images: Influence of multichannel coils, parallel imaging, and reconstruction filters. Journal of Magnetic Resonance Imaging, 28(2), 256-263.

Glockner, J. F., Hu, H. H., Stanley, D. W., Angelos, L., & King, K. (2005). Parallel MR imaging: A user's guide. Radiographics, 25(5), 1279-1297.

Blaimer, M., Breuer, F., Mueller, M., Heidemann, R. M., Griswold, M. A., & Jakob, P. M. (2004). SMASH, SENSE, PILS, GRAPPA: How to choose the optimal method. Topics in Magnetic Resonance Imaging, 15(4), 223-236.

Westbrook, C. (2008). MRI at a Glance. Wiley-Blackwell.