MRI

Although

Abstract

Magnetic Resonance Imaging or MRI is now ubiquitous as a diagnostic tool in clinical medicine to the extent that it is recognised by the general public; however, its use in engineering and the physical sciences is much less well known. Despite this, it has many attributes that make it an exceptionally powerful tomography technique due to the different types of contrast and information that it can be used to measure. These include but are not limited to chemical concentration, molecular diffusion and velocity, all of which can be measured locally in an image of the sample. MRI is also an inherently quantitative measurement tool with well-defined and well-understood physical principles underpinning it. This chapter is aimed principally as an introduction to MRI for engineers and will include the basic principles of data acquisition and image reconstruction. It will also consider some of the practicalities, benefits and limitations along with a few examples of applications to engineering systems.

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Magnetic Resonance Imaging

John A. Sanders, in Functional Brain Imaging, 1995

Publisher Summary

This chapter discusses magnetic resonance imaging. Clinical systems using 1.5 T magnetic fields are considered to be the high-end, high-field machines, with some able to run at 2.0 T. The flexibility built into these systems supports the latest in sequence development and application. Particularly effective for high-resolution neurologic work, these systems represent a platform for advanced users, research sites, or facilities that want an extra measure of ability to employ the latest procedures as they are developed. Midfield machines, with fields ranging from 0.5 to 1.0 T, currently make up the largest part of the instrument market. This is the result of continual improvements to image quality to the extent that the results rival those produced by high-field imagers. These systems are designed for easy and efficient clinical practice with an emphasis on consistency, throughput, convenient siting, and user interface. While the sophistication of midfield imagers approaches that of high-field systems, the implementation is geared toward efficient patient studies rather than flexibility. A number of systems operate at low and ultralow field strengths of 0.02 to 0.35 T. Often, these systems are relatively inexpensive and have very forgiving siting requirements.

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Magnetic resonance imaging

V.M. Ferreira, ... S. Neubauer, in Advanced Cardiac Imaging, 2015

6.3.1.1 Image space versus k-space

CMR is based on techniques developed within the more general field of magnetic resonance imaging (MRI). In MRI, the image can only be collected over a period of time, as the data that constitute the final image must be collected sequentially. This signal data is then stored in a matrix known as k-space, and undergoes a mathematical transformation (the Fourier transform) to generate an image. Patient motion during acquisition will result in failure of the Fourier transform to properly reconstruct the image; resulting errors can be considerable and extremely difficult to disentangle. Therefore, over the period of image collection, it is essential for that body part to be stationary. Clearly this represents a significant challenge in the case of the human heart, owing both to the beating, but also to the asynchronous and variable breathing patterns.

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Magnetic Resonance Imaging of the Pelvic Floor

J. Fielding, H.C. Ming, in Biomechanics of the Female Pelvic Floor, 2016

Abstract

Magnetic resonance imaging (MRI) advancements have improved the speed and quality of static and dynamic image acquisition. Because of this, MRI is seeing increased acceptance in the quest to better understand female pelvic floor anatomy in symptomatic and asymptomatic women.

MRI permits the assessment of structural abnormalities in multiple compartments of the pelvic floor, enabling the correlation between structure and symptom severity. MRI-based biometric markers can help to quantify a range of pelvic floor states from normal anatomy to severe anatomic anomalies.

This chapter seeks to improve understanding of the complex anatomy of the pelvic floor as well as the interpretation and reporting of MRI studies performed for this purpose.

The normal and abnormal appearances of pelvic floor descent are also discussed.

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Arthroscopic débridement in total knee arthroplasty (TKA)

M. Fosco, D. Devoti, in Surgical Techniques in Total Knee Arthroplasty and Alternative Procedures, 2015

4.4.2 Magnetic resonance imaging (MRI)

MRI is the most useful imaging modality for the diagnosis of chondral lesions even if accuracy is still controversial. One study has shown the sensitivity of MRI for evaluation of chondral lesions to be as low as 21% (Levy et al., 1995). Disler et al. (1995), however, found a 93% sensitivity and 94% specificity for detection of cartilage lesions by three-dimensional spoiled gradient-echo MRI. The sensitivity and specificity of MRI depend on the imaging sequence, as well as the grade of the cartilage lesion (Bredella et al., 1999).

Also ligament and meniscal pathologic processes are seen very well on MRI, as is soft tissue disease. Nevertheless, cartilage defects are more difficult to assess, even if the development of improved MRI machines, as well as new imaging sequences, may improve the sensitivity of MRI for identification of chondral lesions in the future.

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Whole body magnetic resonance imaging (MRI)

C.L. Hoad, ... P.A. Gowland, in Biomedical Imaging, 2014

10.5.1 Introduction to foetal and placental imaging

MRI was relatively late in entering the clinic for obstetrics compared to other areas of medicine. The main reason for this was because of the problems with unpredictable foetal motion. However, over the last two decades, the increasing availability of robust snap shot imaging techniques, particularly HASTE (Levine, 2001), has allowed MRI to enter the clinic as a tool for investigating foetuses who have been picked up as being at risk of having a congenital abnormality on ultrasound or from other investigations. However, MRI is also being increasingly used to understand the causes of compromised foetal growth and development. The following sub-sections will overview the developments in both placental and foetal MRI, as well as briefly discuss the safety issues surrounding MRI scanning in pregnancy.

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NbTi superconducting wires and applications

Pingxiang Zhang, ... Yong Feng, in Titanium for Consumer Applications, 2019

6.2  Magnetic resonance imaging (MRI)

Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to form pictures of the anatomy and the physiological processes of the body in both health and disease. MRI scanners use strong magnetic fields, electric field gradients, and radio waves to generate images of the organs in the body (see Fig. 10). Energy from an oscillating magnetic field is temporarily applied to the patient at the appropriate resonance frequency. The excited hydrogen atoms emit a radiofrequency signal, which is measured by a receiving coil [26–28].



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Fig. 10. Magnetic resonance imaging system.

(From https://baike.sogou.com/v432599.htm?fromTitle=MRI.)

MRI requires a magnetic field that is both strong and uniform. Most clinical magnets are superconducting magnets, which require a liquid helium environment. MRI consumes about 2500 t of NbTi superconducting wires (WIC wire) per year. As this type of NbTi superconductor has high copper/NbTi ratio, high RRR, and high critical current, it is not only highly reliable for MRI but also helpful to lower the cost of MRI. Bruker EAS, Mitsubishi Luvata, and WST are three main suppliers of WIC wire for MRI manufacturers.

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Diseases of the thoracic aorta and pulmonary arteries

R. Salgado, ... T. Leiner, in Advanced Cardiac Imaging, 2015

20.3.3.4 Magnetic resonance imaging

MRI is a superior imaging technique compared to CTA and ultrasound in evaluating early and late arterial wall inflammation. MRI has additional value in evaluating the large arteries in patients with large vessel arteritis, especially Takayasu arteritis [10,13,14]. However, in patients with giant-cell arteritis, the role for MRI is limited. MRI can accurately depict vessel wall inflammation and mural thickening. Furthermore, MRI can also demonstrate peri-vasculitis as soft tissue around the vessel wall. T2 inversion recovery weighted images can depict mural oedema as hyperintense signal, which correlates to active early disease [13]. Although experimental, diffusion-weighted MRI has shown promise to detect inflammatory changes in the thoracic aorta. MR angiography is suitable for the detection of late findings in aortitis such as stenosis, dilatations and aneurysms (Figures 20.9 and 20.10). Furthermore, MRI can reveal involvement of the aortic valve and quantify concomitant aortic valve regurgitation or stenosis. Edema imaging is very sensitive (sensitivity up to 94%) in detection of large-vessel vasculitis [11]. Furthermore, dynamic contrast-enhanced (DCE) MRI can accurately (sensitivity up to 86%) monitor inflammation activity with a comparable accuracy to FDG-PET examinations [11,15]. Enhancement of the vessel wall decreases in case of lower disease activity.