Original contributionMagic angle effect in magnetic resonance imaging of the Achilles tendon and enthesis
Introduction
The magic angle effect describes the orientational dependence of the imaging appearance of tissues with well-ordered collagen fibers, such as tendons, menisci, ligaments or hyaline cartilage [1], [2], [3], [4], [5]. In the past two decades, the magic angle effect in several types of collagenous tissues, especially tendon, has been extensively explored. The focus of many investigators has been on the orientational dependence of T2 relaxation times in short T2 tissues such as the tendon, with variable increases noted at 55° to B0 field [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. In addition to quantitative evaluation, the potential for improved visualization of tendons and ligamentous structures by using magic angle effects as a form of contrast has been introduced in the literature [12], [13].
Both the tendon and tendon attachment to the bone, referred to as enthesis, have complex internal structure and are considered to be short T2 tissues. The Achilles tendon consists largely of type I collagen fibers with a triple helical structure embedded into an extracellular matrix that contains elastins, glycoproteins and tendocytes [1]. The collagen fibers in tendon are orientated in parallel bundles leading to a highly ordered structure. The motion of water molecules binding to collagenous tissue is greatly restricted, resulting in a significant enhancement in dipole–dipole interaction which is angular dependent. The dipolar interactions result in very short T2 relaxation times, or rapid signal decay. Entheses are regions where tendons, ligaments or joint capsules are connected to the bone [20]. They are transition zones between flexible and rigid tissue. Stress concentrates at the junction between these two types of tissue which have widely different mechanical properties. Conventional clinical magnetic resonance (MR) imaging has not been helpful in demonstrating the key tissues present in normal entheses due to their short T2 relaxation times. To the best of our knowledge, there has been no magic angle study performed on entheses to date.
It is desirable to directly image short T2 tissues such as the Achilles tendon and enthesis, and measure the angular dependence of their signal intensity and T2 using the whole ankle on a clinical scanner. By using a novel form of slice selection, rapid transmit/receive (T/R) switching, radial mapping of k-space and variable-rate selective excitation (VERSE), pulse sequences with TEs 100–1000 times shorter than those currently available on clinical MR systems can be achieved [21], [22], [23], [24], [25], [26], [27]. These ultrashort TE (UTE) sequences have been applied to the Achilles tendon and enthesis, generating signals far higher than clinical sequences [28], [29], [30], [31]. In this paper, a two-dimensional UTE sequence was implemented on a clinical 3-T scanner and applied to five fresh ankle specimens at 10 to 20 angular orientations. Signal intensity, T1 and T2* relaxation times were measured. Conventional T1-, T2- and proton density (PD)-weighted fast spin echo (FSE) sequences were also applied for comparison.
Section snippets
Materials and methods
The 2D UTE sequence shown in Fig. 1 employs a specially designed half-pulse combined with a VERSE technique to synchronize RF excitation and gradient ramp down [21]. Radial projection with ramp sampling was used for data acquisition. The combination of VERSE and radial ramp sampling enables very short delay time between RF excitation and data acquisition. This delay time, referred to as TE though actually FID, is determined by the time required to turn off the RF transmitter and to enable the
Results
Fig. 2 shows axial images of an ankle specimen using PD-weighted FSE, T1-weighted FSE and UTE without and with fat saturation, respectively, at six selected orientation angles. A significant signal increase was observed for both tensile tendon and enthesis near 55° for all sequences, with UTE sequences showing less signal increase. Fig. 3 shows the signal intensity of enthesis and bone marrow fat at nine different orientation angles. Fat signal is angularly independent for all sequences,
Discussion
The Achilles tendon and enthesis consist of type I collagen fibers orientated in a parallel manner into highly ordered bundles. Water molecules binding to the collagen triple helix are ordered in-line and have restricted mobility. The structural anisotropy creates a static magnetic field which contributes further to spin–spin relaxation through angular dependent dipole–dipole interactions, resulting in a short T2 relaxation time of around 2 ms for tendon and 4 ms for enthesis when oriented 0°
Acknowledgment
The authors appreciate the grant support from GE Healthcare.
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