Elsevier

Clinical Radiology

Volume 58, Issue 1, January 2003, Pages 1-19
Clinical Radiology

Review
Magnetic Resonance Imaging of Short T2 Components in Tissue

https://doi.org/10.1053/crad.2003.1157Get rights and content

Abstract

The most widely used clinical magnetic resonance imaging techniques for the diagnosis of parenchymal disease employ heavily T2-weighted sequences to detect an increase or decrease in the signal from long T2 components in tissue. Tissues also contain short T2 components that are not detected or only poorly detected with conventional sequences. These components are the majority species in tendons, ligaments, menisci, periosteum, cortical bone and other related tissues, and the minority in many other tissues that have predominantly long T2 components.

The development and clinical application of techniques to detect short T2 components are just beginning. Such techniques include magic angle imaging, as well as short echo time (TE), and ultrashort TE (Ute) pulse sequences. Magic angle imaging increases the T2 of highly ordered, collagen-rich tissues such as tendons and ligaments so signal can be detected from them with conventional pulse sequences. Ute sequences detect short T2 components before they have decayed, both in tissues with a majority of short T2 components and those with a minority. In the latter case steps usually need to be taken to suppress the signal from the majority of long T2 components. Fat suppression of different types may also be helpful. Once signal from short T2 components has been detected, different pulse sequences can be used to determine increases or decreases in T1 and T2 and study contrast enhancement.

Using these approaches, signals have been detected from normal tissues with a majority of short T2 components such as tendons, ligaments, menisci, periosteum, cortical bone, dentine and enamel (the latter four tissues for the first time) as well as from the other tissues in which short T2 components are a minority. Some diseases such as chronic fibrosis, gliosis, haemorrhage and calcification may increase the signal from short T2 components while others such as loss of tissue, loss of order in tissue and an increase in water content may decrease them. Changes of these types have been demonstrated in tendonopathy, intervertebral disc disease, ligament injury, haemachromatosis, pituitary perivascular fibrosis, gliomas, multiple sclerosis and angiomas.

Use of these techniques has reduced the limit of clinical detectability of short T2 components by about two orders of magnitude from about 10 ms to about 100 μs. As a consequence it is now possible to study tissues that have a majority of short T2 components with both “bright” and “dark” approaches, with the bright (high signal) approach offering options for developing tissue contrast of different types, as well as the potential for tissue characterization. In addition, tissues with a minority of short T2 components may demonstrate changes in disease that are not apparent with conventional heavily T2-weighted sequences.

Introduction

The most common method for diagnosing parenchymal disease in clinical magnetic resonance (MR) imaging involves the use of heavily T2-weighted sequences to detect an increase or decrease in long T2 components in tissue. This approach has been successful for over 20 years, and encompasses conventional spin-echo sequences, as well as newer developments such as fast spin-echo imaging, fluid attenuated inversion recovery (FLAIR), clinical EPI, diffusion-weighted imaging and susceptibility weighted imaging (Fig. 1).

In addition to long T2 components, tissues contain short T2 components. In some tissues such as tendons, ligaments, menisci and cortical bone these are the majority species. Conventional clinical methods are insensitive to these components, and so these tissues typically have a low or zero signal intensity with all pulse sequences (Fig. 2a). They include tissues that are virtually always of zero signal intensity (e.g. periosteum, cortical bone, dentine, enamel) and others in which a signal may be detectable depending on the pulse sequence used (e.g. meninges, falx) (Table 1). The lack of signal is useful diagnostically to provide a dark background against which high- signal abnormalities can be recognized, but it has meant that the options for developing tissue contrast of different types have been limited, and that these tissues have been poorly characterized in MR terms because there has been little or no signal available to manipulate with different pulse sequences.

Other tissues besides tendons, ligaments and related tissues contain short T2 components but as a minority species (Fig. 2b). These components typically arise from protons in water closely associated with macromolecules (or protons actually within macromolecules) in cell membranes and intracellular structures, and are found to some extent in all tissues. Signals from these sources are not usually detected, or poorly detected with conventional pulse sequences.

To place these observations in a quantitative context, it has generally been thought that conventional clinical MR imaging does not detect signals from tissues with T2s less than 10 ms [1]. Protons in water associated with macromolecules have T2s less than 1 ms and protons in water very closely associated with macromolecules, or actually within macromolecules, have T2s of about 10 μs [1].

Although those working in solid-state imaging are familiar with the problems of detecting signals from materials with a very short T2s, there has been little clinical work performed in this area. Over the last decade less than 20 patients have been reported using techniques that specifically detect short T2 components, and no patient studies have been described in major areas of clinical interest such as the brain, liver, pelvis and spine.

This paper describes approaches to detecting and characterizing short T2 components in tissues for clinical purposes.

Section snippets

Magic Angle Imaging

The observation that the MR signal from collagen shows a directional dependence was first made by Berensden in 1962 [2]. This is due to dipolar interactions between protons in water, which are tightly bound to highly ordered collagen. The MR signal typically shows a dependence on the term (3cos2θ−1) where θ is the orientation of the collagen fibres to the static magnetic field Bo. When θ=55° (approx.), 3cos2θ−1=0 and dipolar interactions are minimized. Fullerton et al. [3], Peto et al. [4],

Short TE Imaging (TE=2–5 ms)

In order to detect the short T2 components of tendons TE may be shortened so that the signal can be detected before it has decayed. Short TE imaging has been used by Koblik et al. [5] and Schick et al. [11] with TEs of 3 ms and 4 or 5 ms, respectively. The early studies were on normal equine tendon, and the later ones on small numbers of volunteers and patients. Two out of seven cases of Achilles tendonopathy showed abnormality with short TE sequences which were not seen with conventional longer

Ultrashort te (Ute) Imaging

The term Ute imaging has generally been applied to techniques using unconventional slice selection and radially-acquired projection reconstruction to produce images with very short TEs, typically in the range of 50–250 μs as described by Bergin et al., for lung imaging in 1991 [12], Gold et al. [13] and Schmidt et al. [14]. The radiofrequency (rf) excitation is performed using a half pulse with the data acquisition beginning at the centre of k-space and extending radially. A second rf half pulse

Long T2 Component Suppression

With tissues containing only a minority of short T2 components it is often necessary to suppress the long T2 components in order to isolate the signal from the short T2 components and thus demonstrate change in disease (Fig. 9). This may also be a useful technique for increasing conspicuity of short T2 components in relation to other normal and abnormal tissues. To date three main methods have been used to do this.

The first is a long rectangular 90° rf pulse followed by the application of

Fat Suppression

Different methods of fat suppression have been in clinical use for at least 15 years. Out-of-phase imaging (e.g. gradient echo imaging at TE=2.2 ms at 1.5 T) may produce cancellation at water–fat boundaries, which may result in a net overall reduction in background signal. This may increase the conspicuity of tissues such as tendons and ligaments when they have a high signal.

Conventional fat saturation based on frequency differences with protons in water is more effective at higher fields (e.g.

Contrast Enhancement

There has been relatively little use of intravenous gadolinium chelates in tendon and ligament studies although some authors have reported this method as the most effective technique for the diagnosis of tendonopathy [19]. With conventional imaging techniques contrast enhancement is only detectable in tendons and ligaments in regions which have abnormally increased T2s. When imaged at the magic angle however, change in signal is detectable in the normal tendon. The enhancement typically has a

Magnetization Transfer

The relationship between imaging of short T2 components and magnetization transfer imaging is of considerable interest 1, 20. Both approaches are concerned with deriving information about short T2 components. Ute imaging does this directly, and magnetization transfer does this indirectly by saturating the short T2 components in the bound pool and determining what effect this has on the detectable long T2 components in the free pool.

In tissues with a majority of short T2 components magnetization

Artefacts

The magic angle effect has received most recognition as a source of unwanted artefact in the form of spurious high signals from tendons and ligaments (or parts of them) which happen to be at 55° to Bo. With magic angle imaging, where the objective is to place these structures at 55° to achieve high signals, low signal may be seen as an apparent abnormality if the magic angle is not achieved.

The magic angle effect may be seen in different ways with Ute imaging. With difference images, the signal

Other Techniques for Increasing Signal and Providing a Greater Specificity

A branch of MR is concerned with the imaging of solid structures. Unlike the soft tissues of most interest in clinical practice, solids have extremely short T2s (of the order of microseconds) and long T1s (which may be minutes or even hours instead of milliseconds). A variety of techniques have been developed to detect signal very early after excitation, or use pulse sequences to rephase rapidly decaying signals.

Magic sandwich echo (MSE) imaging [21] is one such technique in which a multiple

Changes in Disease

A variety of pathological processes may increase or decrease the signal from short T2 components. Increases are likely in fibrosis (especially if chronic), gliosis, phases of haemorrhage, calcification and increased iron deposition. Decreases in short T2 component signals are likely with loss of tissue, loss of order in tissue, demyelination and oedema (with shift of short T2 components to become long T2 components).

Where there is no change apparent with conventional long T2 component

Examples

Examples involving normal and diseased tissues with a majority of short T2 components followed by those with a minority are illustrated: Fig. 15 shows conventional T2 weighted (a) and Flute (b) images in a case of mild disc bulging at L4/L5. At the posterior aspect of the disc there is a high signal intensity region consistent with localized scar formation (b). This is not apparent on the conventional T2 weighted image.

Fig. 16 shows a d Cute image with high signal from the meniscus and two

Discussion

Use of these techniques has shifted the lower level of detectability of short T2 components by about two orders of the magnitude from less than 10 ms to less than 100 μs. As a result, by analogy with dark blood and bright blood MR angiography, both “bright” and “dark” approaches are now available to imaging tendons, ligaments, menisci, periosteum, cortical bone and related tissues. The bright, or high-signal approach offers opportunities for different types of conspicuity between normal and

Acknowledgments

We thank the staff of the National Heart and Lung Institute, the Robert Steiner Magnetic Resonance Unit and the Department of Imaging, Hammersmith Hospitals NHS Trust including Elizabeth Burman, Angela Oatridge, Andreanna Williams, Karyn Chappell, Taigang He and Dulcie Rodrigues, as well as Diagnostic Investigations of the Spinal Conditions and Sciatica (DISCS), the Arthritis Research Council and the Golden Charitable Trust for their help.

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