Spatial variation in T1 of healthy human articular cartilage of the knee joint

The longitudiual relaxation time T₁ of native cartilage is frequently assumed to be constant. To redress this, the spatial variation of T₁ in unenhanced healthy human knee cartilage in different compartments and cartilage layers was investigated. Knees of 25 volunteers were examined on a 1.5 T MRI system. A three-dimensional gradient-echo sequence with a variable flip angle, in combination with parallel imaging, was used for rapid T₁ mapping of the whole knee. Regions of interest (ROIs) were defined in five different cartilage segments (medial and lateral femoral cartilage, medial and lateral tibial cartilage and patellar cartilage). Pooled histograms and averaged profiles across the cartilage thickness were generated. The mean values were compared for global variance using the Kruskal–Wallis test and pairwise using the Mann–Whitney U-test. Mean T₁ decreased from 900–1100 ms in superficial cartilage to 400–500 ms in deep cartilage. The averaged T₁ value of the medial femoral cartilage was 702±68 ms, of the lateral femoral cartilage 630±75 ms, of the medial tibial cartilage 700±87 ms, of the lateral tibial cartilage 594±74 ms and of the patellar cartilage 666±78 ms. There were significant differences between the medial and lateral compartment (p<0.01). In each cartilage segment, T₁ decreased considerably from superficial to deep cartilage. Only small variations of T₁ between different cartilage segments were found but with a significant difference between the medial and lateral compartments.MRI relaxation parameters are used to evaluate cartilage degradation. T₂ has been investigated extensively and has been demonstrated to vary with water and collagen content and with collagen orientation in the different cartilage layers 1–8].The quantification of the longitudiual relaxation time T₁ of native cartilage has received less attention. In experimental studies, native T₁ has been demonstrated to correlate with mechanical properties 9] and to depend upon the macromolecular structure of cartilage 10]. However, it is frequently assumed to be constant across cartilage 11–13]. A few studies have investigated the mean values of a single compartment (10, 14–19] but have not investigated the depth-dependent variation. To our knowledge, no study has systematically compared T₁ of unenhanced human knee cartilage in different cartilage layers and in different cartilage compartments in healthy volunteers.

Table 1

T₁ of healthy human articular cartilage in the knee joint

	Sequence	T1 (ms)
	Field strength	Lateral femoral	Medial femoral	Lateral tibial	Medial tibial	Patellar
Van Breuseghem et al 16]	Combined T1–T2	449±34*				–
	IR-TSE
	1.5 T
Tiderius et al 18]	Turbo-IR	952±86	952±86	–	–	–
	1.5 T
Williams et al 14]	Turbo-IR	–		–		–
	1.5 T		916±102		819±86
	3.0 T		1146±133		1167±79
Gold et al 19]	Look-Locker	–	–	–	–
	1.5 T					1066±155
	3.0 T					1240±107
Wang et al 15]	3D GE with VFA	1004±72*				1193±108
	3.0 T
Trattnig et al 17]	3D GE with VFA	1013±89	–	–	–	–
	3.0 T

Open in a separate windowData are presented as the mean ± standard deviation. VFA, variable flip angle; GE, gradient echo; IR, inversion-recovery; IR-TSE, inversion-recovery turbo spin-echo; 3D, three-dimensional.^*Mean value averaged over the femorotibial compartment.Usually, inversion-recovery (IR) sequences have been used to measure several points in the T₁ relaxation curve. Although this technique provides ideal measurements of T₁, it is not viable in most studies that require T₁ values of a large volume within a reasonable time. Three-dimensional (3D) T₁ mapping techniques were applied for this purpose 17, 20–22].The purpose of this study was to investigate the spatial variation of native cartilage T₁ in different compartments and different cartilage layers in healthy human knee joints using a rapid 3D gradient-echo (GE) sequence with variable flip angle.