Introduction Giant cell tumour of the tendon sheath (GCTTS) and pig- mented villonodular synovitis (PVNS) are two disorders of the synovium with similar histological findings, but with different clinical presentation and behaviour [1–7]. Depending on their growth characteristics, localized and diffuse forms of GCTTS can be recognized [3, 4]. Because localized GCTTS is a rare soft tissue tu- mour, only a few reports have described the appearance of this entity on MR images [1, 2, 4]. We report MR findings in 13 histologically proven cases of localized GCTTS (Table 1). Materials and methods Between January 1987 and September 1993, 240 pa- tients with a suspicion of a soft tissue tumour were re- ferred to our department for MR examination. From October 1993 until December 1995, another 553 soft tis- sue tumours were collected on the occasion of the Euro- pean Study on Magnetic Resonance Imaging of Soft Tis- sue Tumours. Thirteen tumours of the total amount of 793 (1.6 %) were localized GCTTS. Gender distribution was 5 males and 8 females. Age ranged from 7 to 53 years (mean age 29.6 years). Five lesions were located at flexor tendons and one at an extensor tendon of the hand. One lesion was located at the tibialis anterior tendon, one at the quadriceps ten- don, and one at the posterior subtalar joint. Two lesions were attached to the posterior cruciate ligament of the knee. One mass was seen at the flexor hallucis tendon and one at an extensor digitorum tendon of the foot. Magnetic resonance imaging examinations were per- formed on a 1.0-T system (Magnetom Impact, Siemens, Erlangen, Germany) in 5 patients, on a 0.5-T system (Magnetom 21 M + Siemens, Erlangen, Germany) in 3 patients, on a 1.5-T imaging system (Magnetom 63 SP, Siemens, Erlangen, Germany) in 3 patients, and on a 0.2-T system (Magnetom Open, Siemens, Erlangen, Germany) in 2 patients. In each patient spin-echo (SE) T1-weighted images (TR/TE 340–908/15–26) were obtained. Spin-echo T2- weighted images (TR/TE 2000–2700/80–90) or fast SE T2-weighted images (TR/TE 3500–5000/85–120) were used in 6 and 7 patients, respectively. In all 13 patients SE T1-weighted images after intra- venous gadolinium contrast administration (0.1 mmol/ kg body weight) were obtained. The MR images were reviewed by two radiologists working together. Each lesion was evaluated for mean Eur. Radiol. 7, 198–201 (1997)  Springer-Verlag 1997 European Radiology O riginal article Magnetic resonance imaging of localized giant cell tumour of the tendon sheath (MRI of localized GCTTS) L. De Beuckeleer 1 , A . D e Schepper 1 , F. D e Belder 2 , J. Van Goethem 2 , M. C. B. Marques 3 , J. Broeckx 4 , K. Verstraete 5 , F. Vermaut 6 1 Department of Radiology, Universitair Ziekenhuis Antwerpen, B-2650 Edegem, Belgium 2 Department of Radiology, MR-vereniging der Sint-Niklase Ziekenhuizen, B-9100 Sint-Niklaas, Belgium 3 Department of Radiology, Hospitais Da Universidade De Coimbra, P-3000 Coimbra, Portugal 4 Department of Medical Imaging, Salvatorziekenhuis, B-3500 Hasselt, Belgium 5 Department of Radiology, Universitair Ziekenhuis Gent, B-9000 Ghent, Belgium 6 Department of Radiology, Ho ˆpital Civil, B-7500 Tournai, Belgium Abstract. The objective of this study was to evaluate the appearance of localized giant cell tumour of the tendon sheath (GCTTS) on unenhanced and Gd-en- hanced MR images. MR images of 13 histologically proven cases of localized GCTTS were evaluated for mean size, location, homogeneity and signal intensity (SI) on both T1- and T2-weighted images, and en- hancement pattern. All lesions except 1 affected young adults. On T1- and T2-weighted images, lesions showed predominantly low SI equal to or slightly high- er than skeletal muscle. On Gd-enhanced T1-weigh- ted images, strong homogeneous enhancement was seen. These findings reflect the underlying histological composition of the lesion; haemosiderin deposition in xanthoma cells, shortening T2-relaxation time, and abundant collagenous proliferation are responsible for low SI on T1- and T2-weighted images. Strong ho- mogeneous enhancement originates from numerous proliferative capillaries in the collagenous stroma. We conclude that these characteristic MR features, to- gether with clinical information, are a valuable diag- nostic tool in offering a correct preoperative diagnosis. Key words: Tendons – Soft tissue neoplasms – MRI – Gadolinium Correspondence to: A. De Schepper