Metal implant artifact reduction in magnetic resonance imaging Citation for published version (APA): Harder, den, J. M. (2015). Metal implant artifact reduction in magnetic resonance imaging. [Phd Thesis 2 (Research NOT TU/e / Graduation TU/e), Applied Physics and Science Education]. Technische Universiteit Eindhoven. https://doi.org/10.6100/IR783940 DOI: 10.6100/IR783940 Document status and date: Published: 01/01/2015 Document Version: Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication: • A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. 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Mar. 2023 Metal Implant Artifact Reduction in Magnetic Resonance Imaging Chiel den Harder ISBN: 978-90-5335-962-4 © 2014, Chiel den Harder The research and the technological design of the prototype presented in this thesis were performed at and made possible by: Philips Healthcare, Best, the Netherlands Printing: Ridderprint BV, www.ridderprint.nl Printing was financially supported by: Technische Universiteit Eindhoven (TU/e), Eindhoven, the Netherlands Metal Implant Artifact Reduction in Magnetic Resonance Imaging PROEFONTWERP ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de rector magnificus prof.dr.ir. C.J. van Duijn, voor een commissie aangewezen door het College voor Promoties, in het openbaar te verdedigen op woensdag 21 januari 2015 om 16:00 uur door Johan Michiel den Harder geboren te Oss De documentatie van het proefontwerp is goedgekeurd door de promotiecommissie: voorzitter: prof.dr. H.J.H. Clercx 1e promotor: prof.dr.ir. P.F.F. Wijn 2e promotor: prof.dr.ir. M. Breeuwer copromotor: dr. C. Bos (UMCU) leden: prof.dr. K. Nicolay prof.dr.ir. E.J.E. Cottaar prof.dr. A.G. Webb (LUMC) prof.dr. F.E. Lecouvet (UCL) Table of Contents 1. Introduction ....................................................................................................................7 1.1. Purpose of this thesis ...............................................................................................7 1.2. Thesis setup .............................................................................................................8 1.3. Magnetic Resonance Imaging ...................................................................................9 2. Current Issues with Metal in the MRI scanner ...............................................................21 2.1. When metal enters the MRI Scanner ......................................................................21 2.2. Clinical impact ........................................................................................................26 2.3. Currently available measures to reduce metal artifacts ..........................................30 2.4. Residual artifact .....................................................................................................36 2.5. Scan robustness issues ...........................................................................................37 3. Advanced Techniques for Metal Implant Artifact Reduction ..........................................41 3.1. View Angle Tilting ...................................................................................................41 3.2. Multi-Spectral Imaging ...........................................................................................43 3.3. Other advanced techniques ....................................................................................47 4. Solution Requirements ..................................................................................................49 4.1. Main clinical requirement .......................................................................................49 4.2. Stakeholders and their focus ..................................................................................49 4.3. Clinical requirements ..............................................................................................51 5. Off-Resonance Suppression for Multi-Spectral Imaging near Metallic Implants .............55 5.1. Introduction ...........................................................................................................55 5.2. Theory ....................................................................................................................56 5.3. Methods .................................................................................................................62 5.4. Results....................................................................................................................63 5.5. Discussion ..............................................................................................................68 6. Ripple Artifact Reduction using Slice Overlap in SEMAC .................................................71 6.1. Introduction ...........................................................................................................71 6.2. Theory ....................................................................................................................72 6.3. Methods .................................................................................................................76 6.4. Results....................................................................................................................77 6.5. Discussion ..............................................................................................................80 7. Prototype ......................................................................................................................83 7.1. Technical requirements ..........................................................................................83 7.2. Prototype specific technical requirements..............................................................87 7.3. Design ....................................................................................................................88 7.4. User Interface .........................................................................................................95 7.5. Implementation and tuning of default parameter values ........................................97 7.6. Prototype software and documentation ............................................................... 106 8. Verification and Validation of Artifact Reduction ......................................................... 107 8.1. Introduction ......................................................................................................... 107 8.2. Expected artifact reduction .................................................................................. 107 8.3. Methods ............................................................................................................... 110 8.4. Results.................................................................................................................. 113 8.5. Conclusion & Discussion ....................................................................................... 117 9. Recommendations, Outlook and Conclusion ............................................................... 121 9.1. Required and achieved artifact reduction. ............................................................ 121 9.2. Recommendations for product development ....................................................... 122 9.3. Next steps ............................................................................................................ 128 9.4. Conclusion ............................................................................................................ 130 A. References .................................................................................................................. 131 B. Summary ..................................................................................................................... 139 C. Samenvatting .............................................................................................................. 143 D. Acknowledgements ..................................................................................................... 149 E. Curriculum Vitae.......................................................................................................... 151 F. Publications ................................................................................................................. 153 G. Abbreviations and Symbols ......................................................................................... 157 1. Introduction Since the first studies of nuclear magnetic resonance (NMR) effects [1,2] and their applications for imaging [3] and diagnosis [4] in the late 1960’s and early 1970’s, Magnetic Resonance Imaging (MRI, Figure 1.1) has vastly evolved, improved, and expanded to disciplines including neurology [5,6], orthopedics [7], oncology [8], and cardiology [9]. With its proven diagnostic value for many clinical applications, and its wide range of possible tissue contrasts, MRI is a valuable imaging modality for tissue evaluation. While bone tissue is generally evaluated using X-ray imaging or Computed Tomography (CT), MRI and ultrasound (US) imaging are commonly used for the evaluation of soft tissue. Ultrasound imaging on the one hand is a widely available low-cost portable modality. MRI on the other hand provides a higher signal to noise ratio (SNR) and higher and more flexible contrast. Functionality of organs can be diagnosed using nuclear medicine or MRI. As opposed to nuclear medicine and X-ray modalities, MRI does not expose the patient to ionizing radiation. Figure 1.1: Example MRI scanner (a) and example MR image (b). Though the relevance and applicability of MRI is clear for many patients, contraindications exist. Especially metal objects may be dangerous in the scanner. Even if safe, metal compromises image quality, because it influences the magnetic field. 1.1. Purpose of this thesis An increasing number of patients are treated with joint replacements, many of which contain metal implants. This leads to the clinical need for diagnosis of potentially diseased tissue near metal, often as a consequence of complications caused by the implant itself. However, due to the influence of the metal on the main magnetic field of the scanner, MR imaging near metal is associated with image distortions and artifacts that complicate 7 Chapter 1 diagnosis using these images. Artifacts are features added by the imaging system or the imaging process that compromise the intention of imaging to provide a reliable representation of the patient’s anatomy. This thesis describes the artifacts in MR imaging due to the presence of metal and the mechanisms that cause these artifacts. An inventory is made of existing and novel artifact reduction techniques, and of the clinical and technical requirements for artifact reduction. A prototype is described that includes a selection of these techniques for evaluation. 1.2. Thesis setup This first chapter gives an overview of the thesis purpose and setup and provides a short introduction to basic MRI principles and applications. Chapter 2 provides a brief overview of safety aspects when metal objects enter the MRI scanner. The influence of metal on the signal excitation, imaging and encoding process of MRI is explained, as well as the clinical impact of the image artifacts, leading to the clinical need to reduce these artifacts. A number of currently widely available techniques may address metal artifacts to some extent, but residual artifacts remain. The mechanisms behind these techniques are described, as well as their limitations. The clinical impact of metal artifacts is described, based on interviews with radiologists and orthopedists. Recent research efforts focused on further reduction of metal implant artifacts in MRI. Chapter 3 provides an overview of advanced scanning techniques and discusses their strengths and limitations. In particular, a number of these advanced techniques enable substantial artifact reduction and imaging very close to the metal, but at the cost of a substantially prolonged scan-time. This scan-time increase needs to remain within limits to enable practical clinical use. Chapter 4 describes the requirements for a solution that meets the clinical need described in chapter 2, from the perspective of the different stakeholders. Interviews with radiologists and orthopedists formed the basis for defining the clinical requirements. Based on the requirements identified in chapter 4, modifications to the methods described in chapter 3 were proposed that resulted in novel MRI acquisition techniques. Chapter 5 explains how a tunable trade-off between scan-time and metal artifact reduction can be provided to the user. Chapter 6 describes the mechanism behind a residual artifact which is typical for one of the more recent and promising techniques, as well as a measure to reduce that residual artifact. These two chapters were published as articles in Magnetic Resonance in Medicine (MRM). Therefore, the content of these chapters –in particular the introduction and discussion– overlaps with other parts of the thesis. A prototype was built for evaluation of the most suitable advanced techniques described in chapter 3 as well as the extensions described in chapters 5 and 6. The requirements for and the description of this prototype can be found in chapter 7. Phantom experiments were used to verify that the artifact reduction obtained with the prototype’s functionality –as well as the residual artifact– behaves as expected based on theory, and to validate whether the 8 Introduction achieved artifact reduction is sufficient according to what is required to meet the clinical needs. The verification and validation are described in chapter 8. Finally, based on the validation results as well as initial experience in academic hospitals in among others Sweden [10,11] and Korea [12], chapter 9 provides recommendations for product implementation of the functionality as well as an outlook to the future. In all, the scope of the work described in this thesis is limited to the technical feasibility of the metal artifact techniques. Clinical validation of the techniques is beyond that scope, but is part of studies that are being performed using the prototype. Please enjoy reading this thesis as much as I enjoyed developing the functionality and the prototype that formed the basis for this thesis. 1.3. Magnetic Resonance Imaging An MRI examination usually consists of a number of diagnostic scans, each of which results in images of a specific contrast between different tissues. To ensure that the diagnostic images are acquired at the intended position and orientation, a low resolution survey or scout scan is acquired first, covering a sufficiently large area around the anatomy of interest. The MRI scanner operator then uses the survey images to plan the size, location and orientation of the subsequent diagnostic scans. Multiple contrasts may help for optimal visualization of different tissues or abnormalities. Images are often acquired in several orientations as this helps for optimal coverage of the anatomy of interest and for imaging the anatomy structures at the angle they are best recognized and resolved, given that the in- plane resolution is usually better than the resolution in the through-plane direction. 1.3.1. Image formation The principles of magnetic resonance image formation have been explained in many comprehensive books e.g. by Mansfield and Morris [13], Haacke and Brown [14], and Vlaardingerbroek and Den Boer [15], as well as in other material. This section only a briefly summarizes these principles and defines the terminology used in this thesis. 1.3.1.1. Magnetization and precession Tissue, as any other material, consists of atoms with a positively charged nucleus and negatively charged electrons moving around the nucleus. Spin is a property of the nucleus. In the classical mechanical model, spin may be considered a rotation of the nucleus around its axis. Its electrical charge turns into a circular electrical current, which induces a tiny magnetic field along the axis of the nucleus, effectively functioning as a tiny electromagnet. This microscopic magnetic field is called the magnetic moment of the nucleus. On a macroscopic scale, there is generally no effect of the nuclear magnetic moments, as all magnetic moments have random and independent orientations and their magnetic fields cancel mutually. The main component of an MRI scanner is a strong magnet with a field strength of a few Tesla (T), which is roughly 100,000 times as strong as the earth’s magnetic field. As a patient enters the MRI scanner, nuclear magnetic moments in the patient have a slight preference 9