A comparative study at 3 T of sequence dependence of T2 quantitation in the knee

doi:10.1016/j.mri.2008.02.017

Magnetic Resonance Imaging

Volume 26, Issue 9, November 2008, Pages 1215-1220

https://doi.org/10.1016/j.mri.2008.02.017 Get rights and content

Abstract

Objective

T₂ mapping has been used widely in detecting cartilage degeneration in osteoarthritis. Several scanning sequences have been developed in the determination of T₂ relaxation times of tissues. However, the derivation of these times may vary from sequence to sequence. This study seeks to evaluate the sequence-dependent differences in T₂ quantitation of cartilage, muscle, fat and bone marrow in the knee joint at 3 T.

Methods

Three commercial phantoms and 10 healthy volunteers were studied using 3 T MR. T₂ relaxation times of the phantoms, cartilage, muscle, subcutaneous fat and marrow were derived using spin echo (SE), multiecho SE (MESE), fast SE (FSE) with varying echo train length (ETL), spiral and spoiler gradient (SPGR) sequences. The differences between these times were then evaluated using Student's t test. In addition, the signal-to-noise ratio (SNR) efficiency and coefficient of variation of T₂ from each sequence were calculated.

Results

The average T₂ relaxation time was 36.38±5.76 ms in cartilage and 34.08±6.55 ms in muscle, ranging from 27 to 45 ms in both tissues. The times for subcutaneous fat and marrow were longer and more varying, ranging from 41 to 143 ms and from 42 to 160 ms, respectively. In FSE acquisition, relaxation time significantly increases as ETL increases (P<.05). In cartilage, the SE acquisition yields the lowest T₂ values (27.52±3.10 ms), which is significantly lower than those obtained from other sequences (P<.002). T₂ values obtained from spiral acquisition (38.27±6.45 ms) were higher than those obtained from MESE (34.35±5.62 ms) and SPGR acquisition (31.64±4.53 ms). These differences, however, were not significant (P>.05).

Conclusion

T₂ quantification can be a valuable tool for the diagnosis of degenerative disease. Several different sequences exist to quantify the relaxation times of tissues. Sequences range in scan time, SNR efficiency, reproducibility and two- or three-dimensional mapping. However, when choosing a sequence for quantitation, it is important to realize that several factors affect the measured T₂ relaxation time.

Introduction

Osteoarthritis (OA) is a degenerative disease that results in morphological and biochemical changes in tissues of the joint, including cartilage, subchondral bone and bone marrow. In cartilage, it has been shown that biochemical changes in the proteoglycan and collagen matrix can precede morphological changes that happen at later stages of the disease. Therefore, detecting biochemical changes and small intracartilaginous lesions are critical for diagnosing OA and monitoring the disease progression as well as for evaluating therapeutic procedures.

Magnetic resonance imaging (MRI) has proven to be a useful noninvasive tool in imaging joint diseases. It provides multiplanar capabilities, high spatial resolution and superior sensitivity to soft-tissue details. In addition to detecting structural changes, advanced MRI techniques have been shown to have the potential of probing biochemical changes in the tissue. In particular, spin–spin or T₂ relaxation times have been correlated with increase of water contents and damage to collagen network in cartilage during OA [1], [2], [3], [4].

Although T₂ quantitation has been widely used in clinical trial due to its potential correlation with cartilage biochemistry, different scan sequences and acquisition parameters can result in different measured relaxation times. In order to develop a platform from which to diagnosis disease based on relaxation time, it is essential to identify how the quantitation of T₂ relaxation times of musculoskeletal tissues varies as different scanning parameters are used.

There are several factors that can affect T₂ quantification. Insufficient sampling of the T₂ decay curve, RF and static field inhomogeneities, stimulated echoes and T₁ effects can all attribute to the incorrect quantitation of in vivo relaxation times [5]. Currently, the most commonly used T₂ quantification sequences are spin echo (SE) and fast SE (FSE) sequences [6]. Several pulse sequences have been developed in the determination of T₂ relaxation times in the musculoskeletal system. These sequences include the multiecho SE (MESE) sequence [7] and spiral sequence [8]. Additionally, T₂ relaxation time appears to decrease slightly as the magnitude of static magnetic field increases [8], [9]. Although many clinical MR scanners operate at 1.5 T, 3 T scanners are becoming more prevalent in the clinical setting. Studies show an increase of signal-to-noise ratio (SNR) and contrast-to-noise ratio efficiencies as static magnetic field increases [8], [10], [11]. Therefore, a high-field quantitation carries with it the advantage of increased sensitivity.

Even though the differences in T₂ quantification using different sequences and parameters are known, comparisons of T₂ values based on different sequences are very limited. Maier et al. [7] and Mendlik et al. [12] have compared T₂ quantification based on SE and several multiecho sequences at 1.5 T. To the best of our knowledge, no previous studies have systematically investigated measurement differences (T₂ values, SNR and reproducibility) using sequences mentioned above (SE, FSE, MESE and spiral). In addition, with the increasing application of 3-T scanners in clinical settings, it is important to document tissue T₂ relaxation times differences at this field strength.

All of the sequences mentioned are two-dimensional (2D) acquisitions. A three-dimensional (3D) acquisition may be desired, however, in T₂ relaxation time quantitation. 3D imaging is free from artifacts caused by slice cross-talk. Therefore, 3D sequences can generally have a thinner slice thickness and, consequently, may provide a more accurate assessment of tissue relaxation properties.

The goal of this study was, therefore, twofold: (a) to evaluate the consistency of T₂ quantification in healthy musculoskeletal tissues, including cartilage, muscle, bone marrow and subcutaneous fat, in the knee using different sequences at 3 T, including SE, FSE, MESE and spiral acquisition; (b) to evaluate a new 3D T₂ quantitation method and to compare the results with currently available 2D sequences.

Section snippets

Phantoms and subjects

Three commercial cylindrical T₂ phantoms (Diagnostic Sonar, Livingston, UK) with different T₂ values spanning the expected relaxation times of tissues of interest were studied. Each phantom was scanned from three to four times to investigate the reproducibility and quantification accuracy for each sequence. The phantoms were repositioned between scans at different locations (center, left or right to the isocenter of the scanner).

The right knees of 10 healthy volunteers (6 males, 4 females; age

Phantom

The average CV of T₂ quantification was lower than 8% in all three phantoms for all sequences, showing a good overall reproducibility, as seen in Table 1. After combining data from different sequences, the average CV was the highest (5.6%) in Phantom 3, indicating the worst reproducibility. This phantom has the longest T₂ relaxation time (103 ms) and the longest T₁ relaxation time (926 ms). A high CV of 13.6% was observed in the FSE sequence with ETL=16 in this phantom. After combining data

Discussion

In this study, different sequences were used to quantify T₂ relaxation time at 3 T in musculoskeletal tissues, including cartilage, muscle, bone marrow and subcutaneous fat. A new 3D T₂ mapping technique based on SPGR sequence was also proposed. The T₂ relaxation time for each tissue or phantom was different for the different sequences (Fig. 5). However, the magnitude and the direction of sequence-dependent difference between the T₂ times were consistent across all tissues. For example, SE

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Original ContributionA comparative study at 3 T of sequence dependence of T2 quantitation in the knee

Abstract

Objective

Methods

Results

Conclusion

Introduction

Section snippets

Phantoms and subjects

Phantom

Discussion

Magn Reson Imaging

Osteoarthritis Cartilage

Severity of articular cartilage abnormality in patients with osteoarthritis: evaluation with fast spin-echo MR vs arthroscopy

AJR Am J Roentgenol

T2 relaxation time of cartilage at MR imaging: comparison with severity of knee osteoarthritis

Radiology

Human articular cartilage: influence of aging and early symptomatic degeneration on the spatial variation of T2 — preliminary findings at 3 T1

Radiology

Practical T2 quantitation for clinical applications

J Magn Reson Imaging

T2 relaxometry of the hippocampus at 3T

AJNR Am J Neuroradiol

T2 quantitation of articular cartilage at 1.5 T

J Magn Reson Imaging

Original Contribution
A comparative study at 3 T of sequence dependence of T₂ quantitation in the knee