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Gadolinium-DTPA-enhanced MRI in the evaluation of osteonecrosis
ClinicalRadiology(1994) 49, 645-648
Gadolinium-DTPA-Enhanced MRI in the Evaluation of Osteonecrosis R. UBEROI, G. T A I a n d P. M. H U G H E S *
Departments of Diagnostic Radiology, Derriford Hospital, Plymouth, UK and * University of Maryland, Baltimore, AID, USA We report the M R findings in three cases of osteonecrosis. Two cases were post-traumatic and the third idiopathic. The diagnosis, localization and characterization of osteonecrosis was improved in all patients by the use of Gadolinium-DTPA-enhanced, fat-saturated, Tl-weighted spin-echo imaging. We recommend the use of Gadolinium-DTPA-enhanced M R I in selective cases of osteonecrosis as it can better define the stage of disease and suitability for surgery. U b e r o i , R., T a i , G. & H u g h e s , P . M . (1994). Clinical Radiology 49, 645-648. Gadolinium-DTPA-Enhanced
M R I in the E v a l u a t i o n o f O s t e o n e c r o s i s
Accepted for Publication 19 May 1994
M a g n e t i c r e s o n a n c e i m a g i n g ( M R I ) is the m o s t s e n s i t i v e a n d specific m e t h o d f o r t h e d i a g n o s i s o f o s t e o n e c r o s i s [t,2]. T h e ' d o u b l e - l i n e ' sign, best d e m o n s t r a t e d o n T 2 - w e i g h t e d s p i n - e c h o (SE) i m a g e s , is c h a r a c t e r i s t i c o f s u b a c u t e a n d l o n g s t a n d i n g b o n e i n f a r c t i o n . M R I also a c c u r a t e l y d e t e r m i n e s the site a n d e x t e n t o f a v a s c u l a r n e c r o s i s a n d has b e e n r e p o r t e d to d i s t i n g u i s h n o n - v i a b l e f r o m r e v a s c u l a r i z i n g m a r r o w o n the basis o f the signal c h a r a c t e r i s t i c s o n u n e n h a n c e d T1- a n d T 2 - w e i g h t e d S E i m a g e s [3,4]. T h i s d i s t i n c t i o n is i m p o r t a n t to a l l o w p r o p e r m a n a g e m e n t a n d to d e t e r m i n e p r o g n o s i s . W e p r e s e n t t h r e e cases o f o s t e o n e c r o s i s in w h i c h tissue c h a r a c t e r i z a t i o n a n d s t a g i n g by u n e n h a n c e d M R I w a s i n a d e q u a t e b u t was s i g n i f i c a n t l y i m p r o v e d by t h e use o f f a t - s a t u r a t e d Gadolinium-DTPA ( G d - D T P A ) - e n h a n c e d T l - w e i g h t e d S E i m a g i n g . W e discuss the r e a s o n s f o r these findings and technical aspects of MRI which can i n c r e a s e the sensitivity to G d - D T P A e n h a n c e m e n t w i t h i n the b o n e m a r r o w a n d i m p r o v e r e s o l u t i o n w h e n i m a g i n g the hip.
the ankle demonstrated union of a talar neck fracture but no focal bony abnormality or evidence of joint degeneration. MRI demonstrated serpentine areas of low and high signal in the talus on TI- and fast T2-weighted SE images respectively (Fig. 3a, b). The marrow-fat signal elsewhere in the talus was unremarkable. The cortex was intact suggesting talar osteonecrosis rather than non-union. Marked regional differences in enhancement of the marrow following i.v. Gd-DTPA on fat-saturated Tl-weighted SE images confirmed the presence of hypo- or avascularity of the talar dome (Fig. 3c). The talar dome was biopsied, osteonecrosis confirmed, the dead bone debrided and a vascularized bone pedicle inserted. The patient is well 3 months following surgery but is not as yet fully weight-bearing. Case 3. A 27-year-old male patient sustained a fracture-dislocation of the left hip following a parachuting accident (Fig. 4a). The shearing injury to the inferior aspect of the femoral head (Pipkin type 1) initially prevented a satisfactory reduction. The delay in reduction and consideration of internal fixation of the femoral head fracture prompted a request for an MRI scan to establish the viability of the weight-bearing portion of the femoral head. The marrow signal within the detached fragment and femoral head appeared normal on Tl-weighted SE images performed 36 h following the injury (Fig. 4b). Gd-DTPA-enhanced fat-saturated Tl-weighted SE images clearly showed enhancement of the femoral head but not the detached fragment. This confirmed viability of the femoral head and also demonstrated excellent realignment of the fragment both of which prompted conservative management.
CASE REPORTS
Case 1. A 43-year-old female patient presented with a 6 month history of left hip pain. She had no relevant past medical history and received no medications in the preceding 12 months. Plain radiographs of the hip 2 weeks prior to the MRI examination were normal. MRI of the pelvis was performed using a 1.5 T superconducting system (GE-Signa) with SE sequences. Coronal T l-weighted SE images using the body coil demonstrated well-defined focal areas of low signal in the weight-bearing aspect of both hips suggestive of osteonecrosis (Fig. 1). T1- and fast T2-weighted SE images of the symptomatic hip using a pelvic-phased array coil improved resolution and demonstrated serpinginous areas of low and high signal respectively (Fig. 2a, b). Minimal enhancement was identified in this region on Gd-DTPAenhanced, fat-saturated, Tl-weighted SE images (Fig. 2c). This indicated osteonecrotic collapse which was confirmed surgically when, via a lateral approach, a vascularized bone graft was inserted into the affected segment. Case 2. A 50-year-old woman presented with a 12 month history of progressive ankle pain. She had sustained a periarticular fracturedislocation of the ankle 6 years previously. Radiographs and CT of Correspondence to: Dr P. M. Hughes, Department of Diagnostic Radiology, Derriford Hospital, Derriford Road, Plymouth PL6 8DH.
Fig. 1 Case 1. Corona/Tl-weighted (500/12) spin-echo image of the hips demonstrating focal abnormalities with well-defined low signal intensity peripheries and variable internal signal in the weight-bearing zones of both femoral heads.
646
CLINICAL RADIOLOGY
(a)
(b)
(c) Fig. 2 - Case 1. Coronal images obtained of the symptomatic hip using a pelvic phased array coil. (a) Tl-weighted (500/12) spin-echo (SE)
image demonstrating improved resolution and a central area of low attenuation within the osteonecrotic segment. (b) FSE T2-weighted (4000/95) image with fat saturation demonstrating extensive high throughout the weight-bearing portion of the femoral head. (c) GdDTPA-enhanced Tl-weighted (550/13) SE images with fat saturation showing only minor enhancementwithin the affectedsegment indicating trabecular collapserather than mesenchymalproliferation. DISCUSSION Osteonecrosis is a common entity caused by an ischaemic insult to the bone marrow. A number of predisposing or associated factors have been identified including trauma, steroid treatment, alcohol, pancreatifis and sickle cell disease. The early detection o f osteo-
necrosis in Stages 1 or 2 [5] (Table 1) is important in order to institute conservative treatment which may alleviate pain and preserve joint function. Alternative treatments such as core decompression and bone grafting remain controversial. Stages 3-5 imply subchondral collapse and require more aggressive therapeutic measures including rotational osteotomy, joint fusion or total joint replacement, Consideration of osseous devascularization is also indicated following trauma as an explanation for continuing pain and also as a guide to the most appropriate method of management, particularly when complex reconstructive surgery is considered. Early diagnosis of osteonecrosis has therefore assumed increasing importance and in this respect MRI is now well established as the premier imaging modatity in Stages 1 and 2, with a sensitivity greater than 90% [1,2]. Planar scintigraphy has a sensitivity of approximately 50% while single photon emission computed tomography (SPECT) approaches the sensitivity of MRI but remains less specific [6]. Radiographs and CT are unreliable in the early identification of osteonecrosis as the changes often only manifest themselves several months after the onset of symptoms [7]. These techniques are, however, satisfactory for the assessment of the later stages of osteonecrosis (Stages 3-5) prior to either rotational osteotomy or total hip replacement. An understanding of the anatomic and pathologic structures represented on the M R image is important as this facilitates characterization, staging of disease and therapeutic planning. The initial phase of osteonecrosis is characterized by cellular ischaemia, with death of osteocytes and stem cells within 48 h and lipocytes at 2-5 days [8,9]. Unenhanced SE images are often normal in this stage unless there is associated oedema. Failure of the marrow to enhance following injection of G d - D T P A should, however, offer an opportunity to diagnose early osteonecrosis in the absence of marrow signal abnormality on T1- or T2-weighted SE images. Successful marrow enhancement has been reported in dogs [10,1 l] but Erlemann could document enhancement of normal human bone marrow in only 44% of patients using a dynamic gradient-echo technique [12]. Fat-suppressed Tl-weighted SE images can improve conspicuity of marrow enhancement by nullifying the high signal of the marrow fat [4]. Cases 2 and 3 demonstrate the ability of this technique to differentiate viable from avascular marrow in the absence of marrow signal abnormality on unenhanced T1- and T2-weighted SE images. Fat saturation techniques can, however, produce inhomogeneity in signal intensity. To minimize the possibility of artefactual enhancement through the use of this technique, the region of interest should be located near the centre of the field of view and both pre- and post-GdDTPA-enhanced images performed. Increasing morphologic degradation of the marrow results in accumulation of necrotic debris in the intertrabecular spaces. Proliferation of mesenchymal cells and capillaries then develops at the interface with the normal bone. The mesenchymal cells subsequently differentiate to osteoblasts which lay down new bone on dead trabeculae. This represents a sub-acute or longstanding phase of osteonecrosis and is characterized on T2- and to a lesser extent Tl-weighted SE images by the 'double-line' sign. The outer low signal zone represents reactive new bone formation and the inner ring of high
647
Gd-DTPA-ENHANCED MRI IN OSTEONECROSIS
(a)
Fig. 3 - Case 2. Sagittal images obtained using a transmit/receive extremity coil. (a, b) T 1- (550/13) and fast T2-weighted (3000/95) spinecho (SE) images demonstrating serpentine areas of low and high signal respectively, throughout the talus. The high signal on the T2-weighted image does not involve or interrupt the cortex. (c) Gd-DTPA enhanced Tl-weighted (550/13) SE image with fat saturation demonstrating inhomogeneity of fat suppression but enhancement at the granulating interface and failure of the dome to enhance (arrow) relative to other regions in the talus indicating the avascular segment.
signal the granulating interface between viable and avascular marrow. The granulation tissue often fails to permeate the avascular segment but despite this the high signal of the central avascular zone on Tl-weighted sequences often persists for long periods as a result of mummification of the marrow fat [4,13]. The 'doubleline' sign was demonstrated in Case 2 but due to the mummification of fat, the avascular segment was only identified using the technique of marrow enhancement. The final reparative phase of osteonecrosis consists of an increase in osteoclastic activity which remodels the
Table 1 - Ficat classification o f osteonecrosis o f the femoral head
Stage
Findings No symptoms or radiographic findings Symptomatic. Radiographically normal Small areas of sclerosis and osteopenia Femoral head collapse. Joint space normal Femoral head collapse. Joint space narrowed Progressive degenerative joint disease
(b)
648
CLINICAL RADIOLOGY
Gd-DTPA is helpful in this context as is demonstrated in Case 1 where it established the presence and extent of microfractures and excluded any significant attempt at revascularization. Despite the considerable controversy that surrounds the management of osteonecrosis, particularly in its early stages, it remains important to establish a diagnosis and to determine the presence of revascularization or trabecular collapse. MRI is acknowledged as the most sensitive and specific technique to diagnose osteonecrosis but we feel it is worth emphasizing that the selective use of Gd-DTPA can, as demonstrated in these cases, positively influence diagnosis and staging. (a)
(b) Fig. 4 Case 3. (a) Coronal Tl-weighted (500/12) spin-echo (SE) image demonstrating a fracture through the left femoral head as a linear area of low signal. Normal marrow-fat signal either side of the fracture line. (b) Gd-DTPA-enhanced Tl-weighted SE image with fat saturation showing uniformly low signal within the detached femoral head fragment (arrow) indicating avascularity when compared with the higher signal indicating perfusion of the rest of the head and contralateral hip. Extensive enhancement is also demonstrated in the injured left adductor compartment (curved arrow).
repaired cancellous bone and during this phase structural collapse may occur. The presence of high signal within the central zone may indicate either viable mesenchymal tissue or microfractures [4]. The distinction is important, as the management of patients with viable tissue invading the osteonecrotic segment should be conservative with avoidance of weight bearing whereas rotational osteotomy is likely to be required for the latter.
REFERENCES I Glickstein MF, Burk DL Jr, Schiebler ML et al. Avascular necrosis versus other diseases of the hip: sensitivity of MR imaging. Radiology 1988;169:213-215. 2 Thickman D, Axel L, Kressel HY et al. Magnetic resonance imaging of avascular necrosis of the femoral head. Skeletal Radiology 1986;15(2):133 140. 3 Lang P, Jergesen HE, Moseley ME et al. Avascular necrosis of the femoral head: high-field-strength MR imaging with histologic correlation. Radiology 1988;169:517-524. 4 Vande Berg B, Malghem J, Labaisse MA et al. Avascular necrosis of the hip: comparison of contrast-enhanced and non-enhanced MR imaging with histologic correlation. Radiology 1992;182:445-450. 5 Ficat P, Arlet J. Diagnostic de l'osteo-necrosse femorocapitale primittive au stade 1 (stade pre-radiologique). Rev Chir Orthop 1968;54:637-640. 6 Collier BD, Carrera GF, Johnson RP et al. Detection of femoral head avascular necrosis in adults by SPECT. Journal of Nuclear Medicine 1985;26(a):979-987. 7 Resnick D, Niwayama G. Diagnosis of bone and joint disorders, 2nd ed. Philadelphia: WB Saunders, 1988:3203-3218. 8 Johnson LC. Histogenesis of avascular necrosis. In: Proceedings of the conference on aseptic necrosis of the femoral head. St Louis: National Institutes of Health, 1964:55. 9 Glimcher M J, Kenzora JE. The biology of osteonecrosis of the human femoral head and its clinical implications: the pathological changes in the femoral head as an organ and in the hip joint. Clinical Orthopaedics 1979;139:283-312. 10 Cova M, Yang YS, Tsukamato H et al. Bone marrow perfusion evaluated with gadolinium-enhanced dynamic fast M R imaging in a dog model. Radiology 1991;179:535-539. 11 Nadel SN, Debatin JF, Richardson WJ et al. Detection of acute avascular necrosis of the femoral head in dogs: dynamic contrastenhanced MR imaging vs spin-echo and STIR sequences. American Journal of Roentgenology 1992;159(6):1255-1261. 12 Erlemann R, Reiser MF, Peters PE et al. Musculoskeletal neoplasms: static and dynamic Gd-DTPA-enhanced MR imaging. Radiology 1989;171:767 773. 13 Basset[ LW, Mirra JM, Cracchiola A et al. Ischaemic necrosis of the femoral head: correlation of magnetic resonance imaging and histologic sections. Clinical Orthopaedics 1987;223:181-187.
Gadolinium-DTPA-Enhanced MRI in the Evaluation of Osteonecrosis R. UBEROI, G. T A I a n d P. M. H U G H E S *
Departments of Diagnostic Radiology, Derriford Hospital, Plymouth, UK and * University of Maryland, Baltimore, AID, USA We report the M R findings in three cases of osteonecrosis. Two cases were post-traumatic and the third idiopathic. The diagnosis, localization and characterization of osteonecrosis was improved in all patients by the use of Gadolinium-DTPA-enhanced, fat-saturated, Tl-weighted spin-echo imaging. We recommend the use of Gadolinium-DTPA-enhanced M R I in selective cases of osteonecrosis as it can better define the stage of disease and suitability for surgery. U b e r o i , R., T a i , G. & H u g h e s , P . M . (1994). Clinical Radiology 49, 645-648. Gadolinium-DTPA-Enhanced
M R I in the E v a l u a t i o n o f O s t e o n e c r o s i s
Accepted for Publication 19 May 1994
M a g n e t i c r e s o n a n c e i m a g i n g ( M R I ) is the m o s t s e n s i t i v e a n d specific m e t h o d f o r t h e d i a g n o s i s o f o s t e o n e c r o s i s [t,2]. T h e ' d o u b l e - l i n e ' sign, best d e m o n s t r a t e d o n T 2 - w e i g h t e d s p i n - e c h o (SE) i m a g e s , is c h a r a c t e r i s t i c o f s u b a c u t e a n d l o n g s t a n d i n g b o n e i n f a r c t i o n . M R I also a c c u r a t e l y d e t e r m i n e s the site a n d e x t e n t o f a v a s c u l a r n e c r o s i s a n d has b e e n r e p o r t e d to d i s t i n g u i s h n o n - v i a b l e f r o m r e v a s c u l a r i z i n g m a r r o w o n the basis o f the signal c h a r a c t e r i s t i c s o n u n e n h a n c e d T1- a n d T 2 - w e i g h t e d S E i m a g e s [3,4]. T h i s d i s t i n c t i o n is i m p o r t a n t to a l l o w p r o p e r m a n a g e m e n t a n d to d e t e r m i n e p r o g n o s i s . W e p r e s e n t t h r e e cases o f o s t e o n e c r o s i s in w h i c h tissue c h a r a c t e r i z a t i o n a n d s t a g i n g by u n e n h a n c e d M R I w a s i n a d e q u a t e b u t was s i g n i f i c a n t l y i m p r o v e d by t h e use o f f a t - s a t u r a t e d Gadolinium-DTPA ( G d - D T P A ) - e n h a n c e d T l - w e i g h t e d S E i m a g i n g . W e discuss the r e a s o n s f o r these findings and technical aspects of MRI which can i n c r e a s e the sensitivity to G d - D T P A e n h a n c e m e n t w i t h i n the b o n e m a r r o w a n d i m p r o v e r e s o l u t i o n w h e n i m a g i n g the hip.
the ankle demonstrated union of a talar neck fracture but no focal bony abnormality or evidence of joint degeneration. MRI demonstrated serpentine areas of low and high signal in the talus on TI- and fast T2-weighted SE images respectively (Fig. 3a, b). The marrow-fat signal elsewhere in the talus was unremarkable. The cortex was intact suggesting talar osteonecrosis rather than non-union. Marked regional differences in enhancement of the marrow following i.v. Gd-DTPA on fat-saturated Tl-weighted SE images confirmed the presence of hypo- or avascularity of the talar dome (Fig. 3c). The talar dome was biopsied, osteonecrosis confirmed, the dead bone debrided and a vascularized bone pedicle inserted. The patient is well 3 months following surgery but is not as yet fully weight-bearing. Case 3. A 27-year-old male patient sustained a fracture-dislocation of the left hip following a parachuting accident (Fig. 4a). The shearing injury to the inferior aspect of the femoral head (Pipkin type 1) initially prevented a satisfactory reduction. The delay in reduction and consideration of internal fixation of the femoral head fracture prompted a request for an MRI scan to establish the viability of the weight-bearing portion of the femoral head. The marrow signal within the detached fragment and femoral head appeared normal on Tl-weighted SE images performed 36 h following the injury (Fig. 4b). Gd-DTPA-enhanced fat-saturated Tl-weighted SE images clearly showed enhancement of the femoral head but not the detached fragment. This confirmed viability of the femoral head and also demonstrated excellent realignment of the fragment both of which prompted conservative management.
CASE REPORTS
Case 1. A 43-year-old female patient presented with a 6 month history of left hip pain. She had no relevant past medical history and received no medications in the preceding 12 months. Plain radiographs of the hip 2 weeks prior to the MRI examination were normal. MRI of the pelvis was performed using a 1.5 T superconducting system (GE-Signa) with SE sequences. Coronal T l-weighted SE images using the body coil demonstrated well-defined focal areas of low signal in the weight-bearing aspect of both hips suggestive of osteonecrosis (Fig. 1). T1- and fast T2-weighted SE images of the symptomatic hip using a pelvic-phased array coil improved resolution and demonstrated serpinginous areas of low and high signal respectively (Fig. 2a, b). Minimal enhancement was identified in this region on Gd-DTPAenhanced, fat-saturated, Tl-weighted SE images (Fig. 2c). This indicated osteonecrotic collapse which was confirmed surgically when, via a lateral approach, a vascularized bone graft was inserted into the affected segment. Case 2. A 50-year-old woman presented with a 12 month history of progressive ankle pain. She had sustained a periarticular fracturedislocation of the ankle 6 years previously. Radiographs and CT of Correspondence to: Dr P. M. Hughes, Department of Diagnostic Radiology, Derriford Hospital, Derriford Road, Plymouth PL6 8DH.
Fig. 1 Case 1. Corona/Tl-weighted (500/12) spin-echo image of the hips demonstrating focal abnormalities with well-defined low signal intensity peripheries and variable internal signal in the weight-bearing zones of both femoral heads.
646
CLINICAL RADIOLOGY
(a)
(b)
(c) Fig. 2 - Case 1. Coronal images obtained of the symptomatic hip using a pelvic phased array coil. (a) Tl-weighted (500/12) spin-echo (SE)
image demonstrating improved resolution and a central area of low attenuation within the osteonecrotic segment. (b) FSE T2-weighted (4000/95) image with fat saturation demonstrating extensive high throughout the weight-bearing portion of the femoral head. (c) GdDTPA-enhanced Tl-weighted (550/13) SE images with fat saturation showing only minor enhancementwithin the affectedsegment indicating trabecular collapserather than mesenchymalproliferation. DISCUSSION Osteonecrosis is a common entity caused by an ischaemic insult to the bone marrow. A number of predisposing or associated factors have been identified including trauma, steroid treatment, alcohol, pancreatifis and sickle cell disease. The early detection o f osteo-
necrosis in Stages 1 or 2 [5] (Table 1) is important in order to institute conservative treatment which may alleviate pain and preserve joint function. Alternative treatments such as core decompression and bone grafting remain controversial. Stages 3-5 imply subchondral collapse and require more aggressive therapeutic measures including rotational osteotomy, joint fusion or total joint replacement, Consideration of osseous devascularization is also indicated following trauma as an explanation for continuing pain and also as a guide to the most appropriate method of management, particularly when complex reconstructive surgery is considered. Early diagnosis of osteonecrosis has therefore assumed increasing importance and in this respect MRI is now well established as the premier imaging modatity in Stages 1 and 2, with a sensitivity greater than 90% [1,2]. Planar scintigraphy has a sensitivity of approximately 50% while single photon emission computed tomography (SPECT) approaches the sensitivity of MRI but remains less specific [6]. Radiographs and CT are unreliable in the early identification of osteonecrosis as the changes often only manifest themselves several months after the onset of symptoms [7]. These techniques are, however, satisfactory for the assessment of the later stages of osteonecrosis (Stages 3-5) prior to either rotational osteotomy or total hip replacement. An understanding of the anatomic and pathologic structures represented on the M R image is important as this facilitates characterization, staging of disease and therapeutic planning. The initial phase of osteonecrosis is characterized by cellular ischaemia, with death of osteocytes and stem cells within 48 h and lipocytes at 2-5 days [8,9]. Unenhanced SE images are often normal in this stage unless there is associated oedema. Failure of the marrow to enhance following injection of G d - D T P A should, however, offer an opportunity to diagnose early osteonecrosis in the absence of marrow signal abnormality on T1- or T2-weighted SE images. Successful marrow enhancement has been reported in dogs [10,1 l] but Erlemann could document enhancement of normal human bone marrow in only 44% of patients using a dynamic gradient-echo technique [12]. Fat-suppressed Tl-weighted SE images can improve conspicuity of marrow enhancement by nullifying the high signal of the marrow fat [4]. Cases 2 and 3 demonstrate the ability of this technique to differentiate viable from avascular marrow in the absence of marrow signal abnormality on unenhanced T1- and T2-weighted SE images. Fat saturation techniques can, however, produce inhomogeneity in signal intensity. To minimize the possibility of artefactual enhancement through the use of this technique, the region of interest should be located near the centre of the field of view and both pre- and post-GdDTPA-enhanced images performed. Increasing morphologic degradation of the marrow results in accumulation of necrotic debris in the intertrabecular spaces. Proliferation of mesenchymal cells and capillaries then develops at the interface with the normal bone. The mesenchymal cells subsequently differentiate to osteoblasts which lay down new bone on dead trabeculae. This represents a sub-acute or longstanding phase of osteonecrosis and is characterized on T2- and to a lesser extent Tl-weighted SE images by the 'double-line' sign. The outer low signal zone represents reactive new bone formation and the inner ring of high
647
Gd-DTPA-ENHANCED MRI IN OSTEONECROSIS
(a)
Fig. 3 - Case 2. Sagittal images obtained using a transmit/receive extremity coil. (a, b) T 1- (550/13) and fast T2-weighted (3000/95) spinecho (SE) images demonstrating serpentine areas of low and high signal respectively, throughout the talus. The high signal on the T2-weighted image does not involve or interrupt the cortex. (c) Gd-DTPA enhanced Tl-weighted (550/13) SE image with fat saturation demonstrating inhomogeneity of fat suppression but enhancement at the granulating interface and failure of the dome to enhance (arrow) relative to other regions in the talus indicating the avascular segment.
signal the granulating interface between viable and avascular marrow. The granulation tissue often fails to permeate the avascular segment but despite this the high signal of the central avascular zone on Tl-weighted sequences often persists for long periods as a result of mummification of the marrow fat [4,13]. The 'doubleline' sign was demonstrated in Case 2 but due to the mummification of fat, the avascular segment was only identified using the technique of marrow enhancement. The final reparative phase of osteonecrosis consists of an increase in osteoclastic activity which remodels the
Table 1 - Ficat classification o f osteonecrosis o f the femoral head
Stage
Findings No symptoms or radiographic findings Symptomatic. Radiographically normal Small areas of sclerosis and osteopenia Femoral head collapse. Joint space normal Femoral head collapse. Joint space narrowed Progressive degenerative joint disease
(b)
648
CLINICAL RADIOLOGY
Gd-DTPA is helpful in this context as is demonstrated in Case 1 where it established the presence and extent of microfractures and excluded any significant attempt at revascularization. Despite the considerable controversy that surrounds the management of osteonecrosis, particularly in its early stages, it remains important to establish a diagnosis and to determine the presence of revascularization or trabecular collapse. MRI is acknowledged as the most sensitive and specific technique to diagnose osteonecrosis but we feel it is worth emphasizing that the selective use of Gd-DTPA can, as demonstrated in these cases, positively influence diagnosis and staging. (a)
(b) Fig. 4 Case 3. (a) Coronal Tl-weighted (500/12) spin-echo (SE) image demonstrating a fracture through the left femoral head as a linear area of low signal. Normal marrow-fat signal either side of the fracture line. (b) Gd-DTPA-enhanced Tl-weighted SE image with fat saturation showing uniformly low signal within the detached femoral head fragment (arrow) indicating avascularity when compared with the higher signal indicating perfusion of the rest of the head and contralateral hip. Extensive enhancement is also demonstrated in the injured left adductor compartment (curved arrow).
repaired cancellous bone and during this phase structural collapse may occur. The presence of high signal within the central zone may indicate either viable mesenchymal tissue or microfractures [4]. The distinction is important, as the management of patients with viable tissue invading the osteonecrotic segment should be conservative with avoidance of weight bearing whereas rotational osteotomy is likely to be required for the latter.
REFERENCES I Glickstein MF, Burk DL Jr, Schiebler ML et al. Avascular necrosis versus other diseases of the hip: sensitivity of MR imaging. Radiology 1988;169:213-215. 2 Thickman D, Axel L, Kressel HY et al. Magnetic resonance imaging of avascular necrosis of the femoral head. Skeletal Radiology 1986;15(2):133 140. 3 Lang P, Jergesen HE, Moseley ME et al. Avascular necrosis of the femoral head: high-field-strength MR imaging with histologic correlation. Radiology 1988;169:517-524. 4 Vande Berg B, Malghem J, Labaisse MA et al. Avascular necrosis of the hip: comparison of contrast-enhanced and non-enhanced MR imaging with histologic correlation. Radiology 1992;182:445-450. 5 Ficat P, Arlet J. Diagnostic de l'osteo-necrosse femorocapitale primittive au stade 1 (stade pre-radiologique). Rev Chir Orthop 1968;54:637-640. 6 Collier BD, Carrera GF, Johnson RP et al. Detection of femoral head avascular necrosis in adults by SPECT. Journal of Nuclear Medicine 1985;26(a):979-987. 7 Resnick D, Niwayama G. Diagnosis of bone and joint disorders, 2nd ed. Philadelphia: WB Saunders, 1988:3203-3218. 8 Johnson LC. Histogenesis of avascular necrosis. In: Proceedings of the conference on aseptic necrosis of the femoral head. St Louis: National Institutes of Health, 1964:55. 9 Glimcher M J, Kenzora JE. The biology of osteonecrosis of the human femoral head and its clinical implications: the pathological changes in the femoral head as an organ and in the hip joint. Clinical Orthopaedics 1979;139:283-312. 10 Cova M, Yang YS, Tsukamato H et al. Bone marrow perfusion evaluated with gadolinium-enhanced dynamic fast M R imaging in a dog model. Radiology 1991;179:535-539. 11 Nadel SN, Debatin JF, Richardson WJ et al. Detection of acute avascular necrosis of the femoral head in dogs: dynamic contrastenhanced MR imaging vs spin-echo and STIR sequences. American Journal of Roentgenology 1992;159(6):1255-1261. 12 Erlemann R, Reiser MF, Peters PE et al. Musculoskeletal neoplasms: static and dynamic Gd-DTPA-enhanced MR imaging. Radiology 1989;171:767 773. 13 Basset[ LW, Mirra JM, Cracchiola A et al. Ischaemic necrosis of the femoral head: correlation of magnetic resonance imaging and histologic sections. Clinical Orthopaedics 1987;223:181-187.