Elsevier

Magnetic Resonance Imaging

Volume 19, Issue 2, February 2001, Pages 177-185
Magnetic Resonance Imaging

Optimization and validation of a rapid high-resolution T1-w 3D FLASH water excitation MRI sequence for the quantitative assessment of articular cartilage volume and thickness

https://doi.org/10.1016/S0730-725X(01)00292-2Get rights and content

Abstract

In view of follow up, survey and development of therapeutic strategies for osteoarthritis where cartilage deterioration plays an important role, a non invasive, reliable and quantitative assessment of the articular cartilage is desirable. The currently available high resolution T1-weighted (T1-w) 3D FLASH pulse sequences with frequency selective fat suppression are very time consuming. We have 1) optimized a high resolution T1-w 3D FLASH water excitation (WE) sequence for short acquisition time and cartilage visualization, and 2) validated this sequence for cartilage volume and thickness quantification. The spectral fat presaturation was replaced by selective water excitation. The flip angle of the WE sequence was optimized for the contrast to noise (C/Ncart) ratio of cartilage. Sagittal datasets (voxel size: 0.31 × 0.31 × 2 mm3) of the knees of nine healthy volunteers were acquired both, with the 3D FLASH WE (17.2/6.6/30°) sequence (WE) and a previously validated 3D FLASH fat saturated (42/11/30°) sequence (FS). For validation of the WE sequence, cartilage volume, mean and maximal cartilage thickness of the two sequences were compared. Reproducibility was assessed by calculating the coefficient of variation (COV %) of 4 consecutive WE data sets in the volunteers. The acquisition time was reduced from 16′30″ (FS) down to 7′14″ for the WE sequence. Image contrast and visualization of the cartilage was very similar, but delineation of the basal layer of the cartilage was slightly improved with the WE sequence. A flip angle of 30° provided the best C/Ncart ratios (WE). Reproducibility (COV) was between 1.9 and 5.9%. Cartilage volume and thickness agreed within 4% between FS and WE sequence. The WE sequence allows for rapid, valid and reproducible quantification of articular cartilage volume and thickness, prerequisites for follow-up examinations. The reduced acquisition time (50% of FS) enables routine clinical application and thus may contribute to a broader assessment of osteoarthritis.

Introduction

Osteoarthritis has a considerable clinical, social and economical impact [1], [2], [3]. As damage to the articular cartilage is a decisive factor in its pathogenesis [4], [5], the extent of cartilage damage is more and more accepted as a criterion for therapeutic decisions and new therapeutic concepts are being developed [6], [7], [8]. Consequently, there is a strong need for an easy accessible, non-invasive, accurate and reliable assessment of cartilage and MRI is considered well suited [9], [10], [11], [12], [13], [14], [15] for these demands. T1-weighted (T1-w) 3D gradient echo (GRE) sequences with spectral fat suppression (FS) have yielded good results concerning lesion detection, but correct evaluation of small focal cartilage defects is still subject to discussion [11], [16], [17], [18] and 2changes in signal intensity are not sufficiently understood due to the complex composition of articular cartilage [16], [19], [20], [21], [22], [23], [24]. Cartilage volume and thickness, however, constitute simple, global parameters to assess the long-term degradation of cartilage and its recovery. Measurement of cartilage volume and thickness has been shown to be reproducible and valid, if high resolution 3D T1-w FS GRE sequences and adequate 3D postprocessing [25], [26], [27], [28], [29], [30], [31] are applied.

However, the long imaging times of these GRE sequences have prevented a widespread routine clinical application of the technique. Imaging times range from 10′18″ to 13′30″ [17], [32] at 0.6 × 0.6 mm2, and up to 16′30″ and even 21′[17], [18], [25], [26] at 0.3 × 0.3 mm2 in plane resolution. Yet, imaging time should be kept as short as possible, especially with respect to the patients’ comfort and acceptance of the MRI-examination.

Application of selective water excitation instead of frequency selective fat presaturation has been shown [33] to shorten the acquisition time and to produce stable fat suppression. Therefore, the objective of this study was to generate a rapid cartilage dedicated high resolution T1-w 3D FLASH sequence using water excitation instead of conventional spectral fat suppression. This sequence was then adapted for cartilage visualization by optimizing its flip angle and was validated for quantification of cartilage volume and thickness.

Section snippets

Material and methods

We analyzed the cartilage of the right knees of nine healthy volunteers without any history of musculoskeletal affection or previous knee injury. Their age ranged from 22 to 68 (mean 31) years. As cartilage volume and thickness are prone to changes after physical exercise [34], the volunteers were asked to rest for 1 hour in a supine position in order to completely unload the knee before data acquisition.

The examinations were performed in a 1.5 T Magnet (Magnetom Vision, Siemens/Erlangen,

Optimization of the flip angle

The S/N ratios for cartilage, subchondral bone, fat and menisci increased with increasing FA. S/N for muscle decreased at FA’s > 30°. The S/N ratios for fat were in the same order of magnitude in FS and WE sequence. The S/N for cartilage in the WE sequence (FA 30°) was about 75% of S/N in the FS sequence. The values of S/N and C/N for cartilage vs subchondral bone, menisci, fat and muscle are shown in Fig. 1 (a,b). The C/N ratios for cartilage versus subchondral bone, fat, menisci and muscle

Discussion

The purpose of this study was to optimize a high resolution 3D FLASH sequence for visualization of the articular cartilage with a special focus on short acquisition time, and to validate the sequence for quantitative cartilage volume and thickness measurements.

In clinical routine, especially in terms of patients’ tolerance, a compromise has to be found between anatomical resolution, image contrast and acquisition time. Because high contrast between cartilage and surrounding tissues is a main

Acknowledgements

This study was in part supported by the Deutsche Forschungsgemeinschaft (DFG).

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