Artifacts Associated with Breast MRI - Assignment Example

Artifacts Associated with Breast MRI (2) 1. Discuss the common image artifacts associated with Breast MRI and strategies used to minimize them. As in the case of other imaging modalities, breast MRI comes with a number of artifacts, in other instances, the breast MRI artifacts may take the form of pathology, or even obscure pathology and thus affect the quality of diagnosis. The nature of complexity in breast MRI often makes it difficult to recognize the artifacts and their causes. Up recognition of these artifacts, it is always possible to find means through which they can be eliminated. Some of the most common breast MRI image artifacts include ghost artifacts, aliasing artifacts, chemical shift artifacts, metallic artifacts, RF transmission artifacts (Genson et al, 2010). Ghosts artifacts The ghost artifacts are normally almost patient induced through the motion of signal producing tissues in the process of data collection (Balakrishnan et al., 2009). The motion not only produces blurred images but also a series of bright tissues, which seems to be in motion. The appearance of ghost artifacts takes the form of noise pattern that are propagated in the phase encoding (PE) direction, this occurs regardless of the motion direction of the bright tissues. The occurrence of ghosts artifacts stem from change in signal values of the fixed structures or mis-registration in image data. They therefore propagate towards the PE direction since the time taken from one PE view to the next TR is often longer than time taken between one frequency encoding (FE) to the next. Misregistration of data is therefore likely to occur between different PE views as compared to different FE views (Reeve, 2011). Figure (1): showing ghost artifact where patient coughed during the scan . To prevent the occurrence of ghost artifacts (Figure 1), the patients’ motion needs to be minimized as much as possible to immobilize the breast. Immobilizing the breast play a significant role in preventing motion since the breast may experience motion due to respiratory system. Moreover, reducing scan time and adequate explanation of the scan for the patient will contribute to reduce this sort of artifact. As well, using some equipment such as blankets and immobilization straps will play significant role in ensure the breast position which will reducing motion artifact . Aliasing artifacts Aliasing artifacts, also be referred to as wraparound or image wrap often occur in cases where the tissues producing signal go beyond the stipulated field of view in either PE or FE directions (Figure 3). Thus, the occurrence of aliasing stems from the fact that MRI needs to have a discrete signal values to enable image formation, i.e. a fixed number of FE and PE steps. The discrete number of pixels infers that the 2D or 3D reconstruction may not be able to distinguish between properly allocated signal producing tissues that appear on the scanned field of view (FOV), and the other tissues appearing outside the regions of scan field of view (Wang & Huang, 2010). The result is that the signals from tissues outside the scan field of view mixes with the signals from tissues inside the scanned field of view, the images then are added to the opposite side of the image (Figure 2 B). The aliasing artifacts often have the tendency of adding structured noise, which can affect the breast imaging details. Figure (2): (A) illustrate schematic of aliasing in phase encoding direction. (B) T2W showing aliasing artifact on the breast image where the each arm of the patient wrap on the opposite side (Harvery et al., 2007). Since the aliasing artifacts mostly occur on the PE direction (Figure 2 A) rather than the FE direction, the MR manufacturers provide technique “No phase wrap” option which is automatically doubled the number of PE step. Thus lead to eliminated or reduced the aliasing artifact (Hendrick, 2008). Furthermore, increasing the field of view (FOV) ensures that the image is fully covered which helps in compromises resolution and reducing the aliasing artifact. As well, over sampling in the PE direction but this way will lead to increase the scan time (Harvery et al., 2007). Truncation artifacts The truncation artifacts are also known as Gibbs, ringing or edge artifacts, and often occur as a result of finite sampling in MRI. The truncation artifacts often occur adjacent to images of high contrast and sharp interfaces. Due to the finite sampling of images in each of the in plane dimensions, image reconstruction with discreet tend to overshoot the true signal across the sharp interfaces hence resulting into the ringing artifacts over the interfaces. The overshooting boosts contrast on the surface, and is responsible for the dark and light banding on the sharp interfaces (Hu, 2011). Figure (4): shows truncation artifacts in the breast image at the superior edge as parallel light (Hendrick, 2008). To removing this sort of artifact (Figure 4), using a higher matrix can help in reducing the truncation artifacts, relatively, scan time needs to be increased to enlarge the matrix size on the PE direction. Chemical shift artifacts The chemical shift artifact occurs due to the presence of hydrogen nuclei in fat resonate within slightly different hydrogen nuclei in water. Resonate frequency of water amounts to 3.35 parts per million (ppm) greater than that of fat which translates to 214Hz within 1.5T. A combination of these leads to occurrence of two different types of chemical shifts. The first kind of chemical shift artifacts often takes place in all MR images due to application of encoding gradient, in the process of signal measurement (Pope, Walker, & Kron, 2010). Thus the difference in resonate frequency in water and fat of 214 Hz leads to position shift of 1.8 pixels. This chemical shift often occurs on the FE direction (Figure 5). Figure (5): showing chemical shift artifact due to shifted fat and water (Harvery et al., 2007). Among the means through which the chemical shift can be reduced is by using the wider bandwidth of the image under projection, this will reduce the chemical shift artifacts. As well, swapping the FE and PE direction can help in minimizing the chemical artifact (Harvery et al., 2007). Fat suppression is done due to its difference in the resonance frequency with water. This is achieved through the means of frequency selective pulses or by the use of phase contrast techniques. This in the end helps to reduce the fat signal which minimizing this sort of artifact (Hendrick, 2008). Metallic artifacts These are mainly triggered by the presence of ferromagnetic materials on the scanner or on the patient. Due to this, the ferromagnetic metals objects results into creation of sizable image artifacts. These are then evidenced when the distorted magnetic field leads to the creation of warped and distorted images and flaring bright signals within the vicinity of any ferocious metal in the body (Hendrick, 2008).The occurrence of these artifacts stem from the abrupt change in magnetic field due to the addition of the extra field, in the end a misallocation of the magnetic field occurs and causes the artifact (Brennan, 2008). Figure (6): showing metallic artifact on the right breast (Comstock, 2009). The occurrence of metallic artifact can be reduced through the removal of all the ferromagnetic substances from the patient before scan (Harvery et al., 2007). As well, the patient should complete the MRI consent, which equally plays a very significant in the examination process. The radiographer should inform the patient and his/her family of the importance of removing the metallic substances from their body hence preventing this type of artifacts (Durbridge, 2011). Radiofrequency transmission artifact (Zipper artifact) The radiofrequency transmission artifact takes place due to incomplete shielding of the MRI room. The radiofrequency shielding is often provided by the faraday cage, which consist of wire mesh covering the entire MRI scan room. In case of any space in the RF transmission room or the MRI scan room door left open, the discrete frequency close to the larmor frequency can emerge as distinct line in the image (Lufkin et al., 2009). These artifacts are located on the direction of the FE (Figure 7), but mostly incline on the PE direction due to the difference in amplitude of the PE views. Figure (7): T1 weighted image shows zipper artifact pointed by solid arrows (Harvery et al., 2007). To prevent the occurrence of the RF artifacts, the door to the MRI scan room should be closed during the scan process. As well, the RF mesh around the room should be properly maintained to prevent any form of incomplete shielding (Brennan, 2008). 2. Discuss the clinical usefulness of spectroscopy in Breast MRI. Magnetic resonance spectroscopy (MRS) is often performed alongside magnetic resonance imaging to get information regarding the chemical composition of the breast lesions (Cady, 2008). This information is then used in various clinical applications such as in monitoring a patient’s response to cancer therapies and ensuring that high accuracy levels are achieved in the diagnosis of the lesions. The previous magnetic resonance spectroscopy studies have provided promising results and thus led to its adoption by many organizations. It is commonly used in assessing the breast MRI protocols during treatment (Jackman & Sternhell, 2009). The initial magnetic resonance spectroscopy were conducted using phosphorus atoms, and the studies revealed that measurable variation of phospholipids was detectable and used in diagnosing and monitoring cancer treatment (Tse, 2009). In the recent times, hydrogen has been used in researching due to the high sensitivity that it portrays during diagnosis. One of the most important clinical importances of breast MRI spectroscopy is that it is used in distinguishing malignant breast before biopsy (Tse, 2009). Roebuck in their initial paper proposed that choline could be used in marking malignancy. Most preceding papers also proved the same hypothesis despite the study having been performed using different techniques. It has also been observed that MRS enhanced the specificity of the scan exam from 62% - 87%, and the addition of perfusion increased specificity to 100 percent (Morris & Liberman, 2010). Figure (8): shows the spectroscopy procedures on the breast (Kim & Jackson, 2009). The next application of breast MRS is in the prediction of response to cancer treatment. Also in the clinical setting, certain available methods like imaging and palpating relied on the changes occurring on the tumor size, which in most cases was observed to take several weeks for any change to be detected (figure 8). The breast MRS however has the capability of detecting the changes in intracellular metabolism, which would take place before any morphological change, would occur (Kim & Jackson, 2009). This makes breast MRS to be among the most effective means through which the progress of breast recovery can be detected. The very first report regarding the treatment of breast cancer was made based on total choline (tCho) measurements. In the report, Jaganathan and the colleagues noted that the total choline (tCho) resonance disappeared in nearly 89 percent of the subjects who were undergoing chemotherapy. Spectroscopy also plays an important clinical role in the establishment of the manner in which breast cancer changes occur in the body. The levels of accuracy depicted by spectroscopy in breast MRI is high as compared to the initial levels reported. This level of accuracy has improved the manner in which effectiveness of therapies and other breast cancer treatment can be monitored (Fischer, 2012). The detectable progress can then be checked to ensure that all the interventions provided are effective and beneficial to the patient. The reliability and quality of MRS data would only advance as more refinements in MR techniques and systems occur. At the moment, propagation of MRS methodology for breast studies is advancing, and the use of breast spectroscopy is becoming common in a number of state of the art clinical equipments of 1.5T or higher magnetic fields (Pomper & Gelovani, 2008). The results from various studies performed have indicated that MRS together with MRI is likely to bring a positive impact on the clinical assessment of breast cancer. Nonetheless, more experiments are still required to perfect the process and increase their use in health institutions. 3. What is Magnetic Resonance Elastography (MRE)? What information would it add to a Breast MRI examination? Magnetic Resonance Elastography (MRE) is non- invasive technique refers to the technology for qualitatively evaluating the mechanical property of tissues. The technology is considered a counterpart to other commonly used techniques such as palpation, which help in diagnosing the chemical properties of the tissues (Gilman, 2008). Palpation as a diagnostic method has been preferred because the mechanical properties of the affected tissues are often revealed (Yin, Magin, & Klatt, 2014). MRE on the other hand obtain information regarding the stiffness of the breast tissues through assessment of propagation of mechanical waves using a special magnetic resonance technique (Zhang, 2011) Magnetic resonance elastography adds a number of information to the breast MRI examination. While the breast MRI provides a detailed information about the breast tissues, the MRE further generates more information that offers more details about the tissues (Lieberman, 2013). Some of the information added includes the sheer waves in the tissues, acquisition of MR images, which depict the propagation of stimulated shear waves to create the images. Further, the MRE processes the sheer wave images generate quantitative maps of the stiffness of the tissues called elastograms as seen below in figure 9 (Schoder, 2011). The technology surrounding the use of magnetic resonance elastography has led to a significant contribution to the nature of imaging found in breast magnetic resonance. The MRE technology can be likened to palpation, used by physicians in diagnosing and characterizing diseases. MRE helps in breast magnetic resonance imaging through the acquisition of information regarding the stiffness of the breast tissues. This would therefore enable the radiographer to acquire sufficient information about breast and the infections. The quantitative tissues maps generated offers detailed information regarding the breasts illness, and therefore correct intervention is taken. Fig 9. Figure (9): Shows the MRE for patient diagnosed with 5-cm adenocarcinoma. (a) – Represents an anatomy of a breast MRI with an Outland tumor. (b) – Represent wave images which depict that the shear wavelengths in the tumor are longer than the normal glandular tissues. (c) – Represents the elastogram in which the tumor is stiffer as compared to the normal tissues. (d) – Reveals an overlay image for the stiffness map, and here there is a correlation between the magnitude of the breast image and the location of the tumor. When generating mechanical waves in the tissues, the MRE scanner uses the vibrations of a single frequency, which is generated by other external driver devices (Sinkus et al., 2012). Electrical signal used in the device is created using a signal generator triggered by the MR pulse sequence and amplified by an acoustic amplifier prior to being fed into a mechanical driver (Schoder, 2011). Measurement of tissue motion is produced by MRE based driver through an MR technique known as phase contrast MRI, after the induction of continuous harmonic motion on the tissues, the conventional MR imaging is then performed (Mariappan et al. 2009). An MR image is, therefore, obtained, and it contains the information regarding the propagation of the wave into the phase known as wave image (Linda, 2012). Two images with opposite polarity of this nature are collected then phase difference image calculated to eliminate non-motion related phase information. References Balakrishnan, A., Kacher, D. F., Gombos, E., Smith, D. N., Carretero, M., Leon, B., et al. (2009). Negative pressure fixation device to reduce motion artifacts on contrast-enhanced MRI of the breast: A clinical feasibility study. Journal of Magnetic Resonance Imaging, 30(2), 430-436. Brennan, D. D. (2008). Artifact Introduced by Radiofrequency Ablation. American Journal of Roentgenology, 186(5_Supplement), S284-S286. Cady, E. B. (2008). Clinical magnetic resonance spectroscopy. New York: Plenum Press. Comstock, C. (2009). 4-12 Effects on Breast MRI of Artifacts Caused by Metallic Tissue Marker Clips. Breast Diseases: A Year Book Quarterly, 18(4), 355-356. Durbridge, G. (2011). Magnetic Resonance Imaging: Fundamental Safety Issues. Journal of Orthopaedic and Sports Physical Therapy, 41(11), 820. Fischer, U. (2012). Practical MR Mammography High-Resolution MRI of the Breast. (2nd ed.). New York: Thieme. Genson, C. C., Blane, C. E., Helvie, M. A., Waits, S. A., & Chenevert, T. L. (2010). Effects on Breast MRI of Artifacts Caused by Metallic Tissue Marker Clips. American Journal of Roentgenology, 188(2), 372-376. Gilman, L. (2008). MRE, Alternative Breast Imaging Four Model-Based Approaches. London: McGraw-Hill. Harvey, J. A., Hendrick, R. E., Coll, J. M., Nicholson, B. T., Burkholder, B. T., & Cohen, M. A. (2007). Breast MR Imaging Artifacts: How to Recognize and Fix Them. RadioGraphics, 27(suppl_1), S131-S145. doi: doi:10.1148/rg.27si075514.  Hendrick, R. E. (2008). Artifacts and Errors in Breast Magnetic Resonance Imaging Breast MRI (pp. 187-207): Springer New York.  Hu, X. (2011). Reduction of Truncation Artifacts in Chemical-Shift Imaging by Extended Sampling Using Variable Repetition Time. Journal of Magnetic Resonance, Series B, 106(3), 292-296. Jackman, L. M., & Sternhell, S. (2009). Applications of nuclear magnetic resonance spectroscopy in organic chemistry, (2d ed.). Oxford: Pergamon Press. Kim, E. E., & Jackson, E. F. (2009). Molecular imaging in oncology: PET, MRI, and MRS. Berlin: Springer. Linda, Q. T. (2012). Developing magnetic resonance elastography (MRE) breast actuation system for detecting breast cancer: a thesis submitted for the degree of Master of Engineering in Mechanical Engineering at the University of Canterbury, Christchurch, New Zealand. London: London publishers. Lufkin, R., Anselmo, M., Crues, J., Smoker, W., & Hanafee, W. (2009). Magnetic field strength dependence of chemical shift artifacts. Computerized Medical Imaging and Graphics, 12(2), 89-96. Morris, E., & Liberman, L. (2010). Breast MRI diagnosis and intervention. New York: Springer. Pomper, M. G., & Gelovani, J. G. (2008). Molecular imaging in oncology. New York: Informa Healthcare USA. Pope, J., Walker, R., & Kron, T. (2010). Artifacts in chemical shift selective imaging. Magnetic Resonance Imaging, 10(4), 695-698. Reeve, D. (2011). MO-E-211-02: Breast MRI: Image Quality, Artifacts & Quality Control. Medical Physics, 38(6), 3719. Schoder, R. J. (2011). MR Elastography System for Breast Cancer Detection. Ft. Belvoir: Defense Technical Information Center. Tse, G. M. (2009). Magnetic resonance spectroscopy of breast tumors. New York: Nova Science Publishers. Wang, J., & Huang, H. (2010). Film digitization aliasing artifacts caused by grid line patterns. IEEE Transactions on Medical Imaging, 13(2), 375-385. Yin, Z., Magin, R. L., & Klatt, D. (2014). Simultaneous MR Elastography and diffusion acquisitions: Diffusion-MRE (dMRE). Magnetic Resonance in Medicine, 5, n/a-n/a. Zhang, B. (2011). Measurement of the shear modulus of tissue-like materials using magnetic resonance elastography (MRE). Lexington, Ky.: [s.n.]. Read More