Magnetic resonance imaging of the normal and ischemic femoral head

The femoral heads of 38 normal and 20 abnormal patients were evaluated using magnetic resonance imaging (MRI). The normal femoral head is surrounded by a thin, sharply defined, low-intensity cortex. The medullary cavity has a strong signal intensity in all imaging sequences due to the large content of marrow fat and hematopoietic cells. Crossing the high-intensity marrow is a thin, curvilinear, transverse, low-intensity line representing the physis and a broad, vertically oriented, low-intensity band resulting from the prominent central weight-bearing trabeculae. Anatomic structures can be identified on all scanning sequences if the examinations are technically satisfactory, although images with the best clarity and detail are obtained by using short echo times. MRI of patients with ischemic necrosis revealed areas of low intensity in the femoral heads. The abnormal areas may take the form of homogeneous regions of decreased signal intensity, inhomogeneous areas of low intensity, bands of low intensity, or rings of low intensity with higher signal intensity centrally. No correlation was found among the specific MRI patterns, the stage of the disease, the radiographic appearance, or the radionuclide bone scan findings. MRI was abnormal in all cases where the radiographs or the scintigraphs (or both) were abnormal. MRI also was abnormal in several cases where either radiographs or scintigraphs produced false-negative results.     

Avascular necrosis of the femoral head: high-field-strength MR imaging with histologic correlation

Magnetic resonance (MR) images, contact radiographs, and histologic sections of six femoral head specimens with avascular necrosis were correlated. A low-signal-intensity band or ring represented the repair tissue interface surrounding a high-signal-intensity necrotic marrow segment. Large segmental areas of low signal intensity were observed on T1-weighted images when the lesion consisted of necrotic bone with amorphous marrow debris and adjacent thickened trabecular bone with mesenchymal repair tissue infiltration. On intermediate-weighted images, however, mesenchymal repair tissue, which was located inferior to the necrotic zone, increased markedly in signal intensity, permitting distinction from low-intensity necrotic bone with amorphous marrow debris. When trabecular thickening with collapse predominated, segmental areas of low signal intensity with both sequences were found. MR signal intensities used in combination with anatomic configuration and location may provide information of potential therapeutic importance regarding tissue composition and stage of disease.     

Fat intensity within the hypointense zone on MR imaging of avascular necrosis of the femoral head

In some patients with avascular necrosis of the femoral head (ANFH), an area of signal intensity similar to that of subcutaneous fat tissue is found within or proximal to the hypointense zone of spin echo MR images. We refer to these MR findings as the "fat intensity" sign. This fat intensity sign was positive in 62 (47%) of 133 MR examinations of femoral heads with radiographically proven ANFH; this sign was negative in all hips with other diseases including degenerative joint disease, transient osteoporosis, rapidly destructive coxoarthrosis, and rheumatoid arthritis.     

股骨头坏死、修复过程的MRI及与病理对照的实验研究

目的：取得股骨头坏死的MRI与病理对照的资料。 材料和方法：我们采用液氮冷冻的方法将30只杂交狗制作成股骨头坏死模型。动物分以下10组：1天，7天，17天，23天，1个月，2个月，3个月，6个月，9个月及12个月。所有动物均进行平片（离体及活体）、CT、MR检查。观察坏死后股骨头的密度及信号改变情况。结果：1个月以内组，所有动物于活体时平片均未发现异常。离体后，17天后可见密度有轻度的增高。1个月及以后组活体及离体均可见不同程度的密度改变。2个月有囊变，3个月可见轻度塌陷。CT于17天即可发现有密度增高，1个月可见有囊变，2个月时可见有微骨折。MR除1天组未发现异常外，其他组均可见不同程度的信号改变。信号异常范围由小变大，至1个月后至整个股骨头均可见信号异常。T1加权低信号，T2加权可见混杂信号（1个月以内）、均匀高信号（1－3个月）、高信号内出现低信号（3个月以后）.组织学改变：1天仅表现脂肪细胞肥大，7－23天可见炎性细胞渗出、纤维肉芽组织开始出现。1个月以后，主要为肉芽组织修复。塌陷的股骨头病理显示坏死的骨小梁吸收和新产生的骨小梁骨折，骨性重建不完全。结论：股骨头坏死的MR表现与病理改变是一致的；MR较CT、平片敏感。     

Magnetic resonance imaging of femoral head development in roentgenographically normal patients

Magnetic resonance images (MRI) of 22 patients with roentgenographically normal hips were reviewed retrospectively and the findings categorized according to age. With increasing maturity, the MR intensity of the femoral heads on spin echo images increased, as marrow fat became a dominant tissue in the head. The femoral head pattern was relatively inhomogeneous, with a broad band of diminished intensity extending in a posteromedial to anterolateral direction, corresponding to the pattern of trabecular bone. The femoral capital epiphyses were visible in younger patients as structures of bright intensity which remained evident through early adulthood. The articular cartilage of the hip joint was noted as a distinctive halo around the femoral head. An understanding of the MR pattern of the normal hip will aid in the early recognition of pathologic conditions, such as osteonecrosis.     

Femoral capital osteonecrosis: MR finding of diffuse marrow abnormalities without focal lesions

Six painful hips in five patients were examined with magnetic resonance (MR) imaging and were found to have diffuse signal abnormalities in the marrow of the femoral head and neck, which extended into the intertrochanteric area in five cases. The abnormal regions were low in signal intensity on images obtained with a short repetition time (TR) and a short echo time (TE) and were isointense or hyperintense on long TR/TE images--findings that have been attributed by others to bone marrow edema. Edema was also seen in marrow just above the acetabulum in two cases. No focal abnormalities characteristic of osteonecrosis were seen. Osteonecrosis was subsequently shown to be present in all six femoral heads at core biopsy (three cases) or by subsequent development of focal MR abnormalities reported to be highly specific for osteonecrosis (three cases). The affected hips had been radiographically normal or subtly osteopenic and had shown intense radionuclide uptake in the femoral head at scintigraphy, with lesser abnormality in the neck and intertrochanteric region. Follow-up MR examinations of five of the six femoral heads showed the diffuse abnormalities to have been transient. Although diffuse MR abnormalities in the proximal femur are not specific, they may indicate the presence of osteonecrosis of the femoral head.     

Correlation of MRI and histomorphological findings in bone marrow oedema syndrome of the hip

Bone marrow oedema syndrome (BMOS) of the hip includes severe hip joint pain, focal loss of radiodensity in radiographs, increased uptake in bone scintigrams and the pattern of bone marrow oedema in the femoral heads in MRI scans. In 15 patients (16 hip joints) we found the clinical and radiological signs of BMOS. On T1-weighted MRI images areas of low signal intensity could be observed in the head, neck and the intertrochanteric region of the femur in various extensions. These areas showed a significant increase in signal intensity on the T2-weighted images. Because pain was resistant to conservative therapy all these patients were treated by core decompression of the femoral head in a prospective study. Bone cores were evaluated histologically using undecalcified sections and quantitative microradiography. The existence of intramedullary oedema in exactly the regions exhibiting the MRI pattern of bone marrow oedema was verified histologically; however, bone and marrow changes similar to those of early avascular necrosis (AVN) were also visible. These findings support our assumption that BMOS might be a transitory initial phase of AVN. Therefore core decompression treatment for patients suffering pain from BMOS is recommended after excluding other diseases.     

兔超早期激素性股骨头坏死模型制作及综合评价

目的制作兔超早期激素性股骨头坏死模型,同时进行影像学和病理学综合评价。方法30只新西兰大耳白兔随机分成2组：实验组20只,于臀肌注射甲泼尼龙20mg,／kg,共3次,每次间隔24h。对照组10只,与实验组相同时间点用同样方法注射等量生理盐水。在造模后第7、14、21、28、35天分别对模型实验组随机4只兔、对照组随机2只兔进行数字X线摄影（DR）、CT、磁共振成像（MRI）检查及病理学观察。结果所有动物股骨头DR、CT、常规MRI检查及大体病理均无异常改变;造模后14d开始模型实验组股骨头出现异常病理改变,主要表现为空骨陷窝增多,骨髓脂肪细胞肥大;21～35d,上述病理改变更为明显。实验组21d后股骨头MRI的短时间反转恢复序列（srnR）开始出现点片状高信号。结论通过肌肉注射激素制作兔超早期股骨头坏死模型成功率高,操作简单,可用于超早期股骨头坏死的影像学研究;MRI的STIR序列是超早期诊断股骨头坏死的敏感检查方法。     

Segmental patterns of avascular necrosis of the femoral heads: early detection with MR imaging

Thirty-two patients (64 hips) in whom avascular necrosis (AVN) of the femoral heads was highly suspected clinically were studied by magnetic resonance (MR) imaging, radionuclide bone scintigraphy, and conventional radiography. MR studies were positive for AVN in 37 hips, compared with 30 positive scintigraphic studies. In all cases in which scintigraphy and radiography were positive, MR imaging demonstrated decreased signal from the affected femoral heads, indicative of bone marrow disease. Imaging results were confirmed by biopsy or subsequent imaging appearances. In patients with negative initial scintigraphic and radiographic studies, the MR imaging criterion for a positive study was a moderately decreased bone marrow signal displaying segmental patterns within an otherwise normal-appearing femoral head on relatively T1-weighted images. In this series of high-risk patients, radionuclide scintigraphy had a sensitivity of 81%, compared with 100% for MR imaging. MR imaging should be the imaging modality of choice for early evaluation of bone marrow changes indicative of AVN.     

Detection of acute avascular necrosis of the femoral head in dogs: dynamic contrast-enhanced MR imaging vs spin-echo and STIR sequences.

OBJECTIVE. Spin-echo MR imaging has been shown to be highly sensitive in the detection of avascular necrosis. Very early avascular necrosis can, however, appear normal on MR images. We compared dynamic contrast-enhanced MR imaging with conventional spin-echo and short Tl inversion-recovery (STIR) sequences for detecting acute osteonecrosis in an animal model. MATERIALS AND METHODS. Avascular necrosis was induced unilaterally in the femoral heads of five dogs that were imaged with a 1.5-T system within 3 hr of devascularization. After standard T1-weighted, T2-weighted, and STIR images, gradient-recalled echo images, 28/5 (TR/TE) with a 45 degrees flip angle, were obtained at 6-sec intervals for 90 sec synchronous with the IV administration of 0.2 mmol of gadoteridol per kilogram of body weight at a rate of 2 ml/sec via an automated injector. Two animals were reimaged after 7 days. RESULTS. Spin-echo and STIR images did not show any acute changes in the ischemic femoral heads. In contrast, significant differences were present in the enhancement profiles of the marrow spaces in the normal and ischemic femoral heads (p = .005). Normal marrow was characterized by rapid enhancement, with an average signal intensity increase of 83% peaking at 36 sec; no measurable enhancement was seen in the marrow of the ischemic femoral head. Spin-echo images, obtained 7 days after devascularization (n = 2), showed changes characteristic of avascular necrosis. Dynamic contrast-enhanced MR images showed persistent lack of enhancement in the avascular marrow of the ischemic femoral head. A junctional zone, characterized by rapid contrast enhancement in excess of 120% without early washout, was identified at the interface between normal and avascular marrow. CONCLUSION. In this experimental model, dynamic contrast-enhanced MR imaging proved significantly more sensitive than conventional spin-echo and STIR imaging in the detection of acute avascular necrosis.     

MR imaging findings in transient osteoporosis of the hip

Purpose: The authors sought to describe the magnetic resonance (MR) imaging findings including perfusion imaging, in association with the course of acute bone marrow oedema syndrome (aBMEs), in a group of patients with acute hip pain and a final diagnosis of transient osteoporosis of the hip (TOH). Materials and methods: From 217 patients referred with a probable diagnosis of avascular necrosis (AVN) of the femoral head, we identified 42 patients who had clinical and radiographic findings not relevant to AVN. MR imaging examinations were performed on a 1.0T scanner. Perfusion imaging was performed in 20 patients. The bone marrow oedema (BME) was classified in four stages. In addition, the presence or absence of oedema in the subchondral area and the presence of other subchondral lesions were recorded. Acetabular bone marrow was also assessed for the presence of oedema. The quantitative measurements included: maximum size of the effusion, percentage of enhancement (PE) and time of peak enhancement of abnormal marrow compared to the first pass, on the perfusion images. Results: Osteopenia was present on plain radiographs in 87% of cases. The most common pattern of BME was extending to the femoral head and neck. Acetabulum was involved in 16.6%. In 22.6% the BME spared the subchondral region of the femoral head. There were two cases (4.7%) with subchondral changes. A joint effusion was noted in 33 of the 42 patients. On perfusion imaging, a delayed peak enhancement was noted in 20 patients between 40 and 65 s after the first pass of contrast. No patient had any evidence of femoral head collapse or change in sphericity on follow-up MRI. None of the patients developed avascular necrosis in a time frame of 18 months from the onset of the acute hip pain. Conclusion: The aBMEs MR imaging pattern varies and is most commonly appearing on X-rays as osteopenia. Absence of subcondral lesions, delayed peak enhancement of the abnormal marrow on perfusion images, and sparing of subchondral zone from marrow oedema are MR imaging findings highly correlated to TOH.     

Subchondral changes in transient osteoporosis of the hip

Objective. To review the subchondral changes on MR imaging in transient osteoporosis of the hip (TOH) and to consider the pathophysiology. Design and patients. MR images of 12 hips of 11 consecutive patients with TOH were retrospectively studied. The diagnoses of TOH were confirmed on the basis of previously published criteria, including decreased bone density of the femoral head and/or neck on radiographs, bone marrow edema (BME) pattern on MR images, spontaneous resolution of the symptoms and a return to normal radiodensity. Results. All 12 hips showed a BME pattern in the femoral head and/or neck. Linear patterns of very low signal intensity were identified on T1-weighted images in the subchondral area within the diffuse low signal intensity area in all 12 hips. On T2-weighted images, a low signal intensity line was observed in the corresponding area in eight hips only. These linear patterns were thought to represent subchondral fracture lines. Conclusions. The presence of a subchondral fracture may be important when considering the pathophysiology of TOH. Received: 5 April 2000 Revision requested: 11 July 2000 Revision received: 22 January 2001 Accepted: 29 January 2001     

Bone marrow edema of the hip: MR findings after core decompression.

Magnetic resonance imaging is an extremely sensitive technique for evaluation of bone marrow changes at an early stage of avascular necrosis (AVN) of the hip. We therefore examined 11 painful hips whose clinical symptoms led us to suspect idiopathic AVN. The radiographs of all of these either were normal or demonstrated a minimal decrease in radiodensity. Magnetic resonance demonstrated diffuse signal loss of the bone marrow at short TR/TE images of the femoral head in all hips, with various extensions in the head, neck, and intertrochanteric area. These regions turned iso- to hyperintense on long TR/TE images compatible with bone marrow edema (BME). Focal abnormalities characteristic for AVN were not seen in any of the cases. Radionuclide studies performed in six cases were positive. All patients underwent core decompression treatment. Hydrostatic bone marrow pressure measurement and intraosseous venography were positive in five cases when measured. Histology available in eight cases confirmed the presence of BME. Furthermore, the bone changes corresponded to those of early AVN. Follow-up examinations after core decompression with MR showed normal signal intensity in all cases. Magnetic resonance represents a viable diagnostic tool for BME and can monitor the therapeutic success of core decompression. Whether BME of the femoral head constitutes a distinct transient syndrome or represents an early form of AVN is controversial at present. Our findings support those who believe that BME may represent an initial stage of idiopathic AVN.     

MR imaging of bone marrow edema and joint effusion in patients with osteonecrosis of the femoral head: relationship to pain

OBJECTIVE: Our aim was to determine the occurrence of bone marrow edema and joint effusion and their relationship to pain in patients with osteonecrosis of the femoral head on the basis of MR imaging. MATERIALS AND METHODS: There were 71 patients with osteonecrosis of the femoral head based on characteristic radiographic and MR imaging findings. All patients had surgical confirmation of the disease. Both hips were affected with osteonecrosis in 39 patients, whereas only one hip was involved in 31 patients. The last patient underwent an arthroplasty of one hip during the study and had only one hip imaged. We evaluated a total of 110 hips in this study, of which 98 were painful. We staged osteonecrosis of the femoral head, using the classification of Steinberg et al. The 31 unaffected hips served as controls. Bone marrow edema and joint fluid were evaluated on MR images. Bone marrow edema was defined as an ill-defined area of low signal intensity on T1-weighted images with corresponding high signal intensity on T2-weighted or inversion recovery images localizing to the femoral head, neck, and intertrochanteric region. The amount of joint fluid was graded from 0 to 3. RESULTS: The peak of bone marrow edema occurred in stage III disease (72%); its odds ratio was seven times greater than that for stage I osteonecrotic hips. Effusions of a grade greater than or equal to 2 were seen most often in stage III disease (92%), compared with 10% in the control hips. With an effusion, bone marrow edema was 12.6 times greater when the hip was painful than when it was not. CONCLUSION: Both bone marrow edema and joint effusions existed with a peak occurrence in stage III disease. Bone marrow edema seems to have a stronger association with pain than does joint effusion in osteonecrosis of the femoral head.     

Ischemic necrosis of the entire femoral head and rapidly destructive hip disease: potential causative relationship

 Objective. Rapidly destructive hip disease (RDHD) is an uncommon disorder of the hip that has been considered a disease of unknown cause and distinct from ischemic necrosis of the femoral head. The objective of this study was to investigate ischemic necrosis of the femoral head as one potential cause of RDHD. Design and patients. In 600 patients who underwent MR imaging of the hip, 20 cases of ischemic necrosis involving the entire femoral head in 18 patients (3%) were retrospectively studied with routine radiography and MR imaging. All patients had surgically confirmed ischemic necrosis of the femoral head. Results and conclusions. All patients showed rapid destruction of the femoral head on routine radiography and MR imaging as compared with the gradual onset of clinical symptoms. Plain radiographs showed several bone fragments at the inferomedial aspect of the femoral head (75%), acetabular erosions (55%), eccentric depression at the lateral articular surface of the femoral head conforming to the adjacent acetabulum (35%), and mild osteoarthritis (15%). Bone sclerosis was often present at sites of impaction between the femoral head and the acetabulum. MR imaging showed marked distention of the joint capsule in all cases. In 14 of 20 cases, the contents of the joint space showed predominantly low or intermediate signal intensity on T1- and T2-weighted images. Ischemic necrosis involving the entire femoral head may represent one of the causes of RDHD.     

Chemical-shift MR imaging of the femoral head: an in vitro study of normal hips and hips with avascular necrosis

To study the conventional and chemical-shift MR images of fatty, hemopoietic, and ischemic bone marrow, four normal femurs from autopsies and six femoral heads from total hip replacement done for avascular necrosis (AVN) were imaged with a small-bore 1.4-T MR unit. T1- and T2-weighted spin-echo images, fat- and water-selective spin-echo images, and out-of-phase images were obtained. The specimens were then sectioned, radiographed, and studied histologically. The capital epiphysis and greater trochanter in normal femurs had the highest signal on T1-weighted, fat-selective, and out-of-phase images owing to their high fat content. Fat-selective and water-selective images accurately depicted regions of fat and water, whereas T1-weighted, T2-weighted, and out-of-phase images were misleading in some cases. On T2-weighted images, a double line consisting of a rim of low signal with a high-signal inner border was noted surrounding AVN lesions. Histologic correlation showed that the low-signal outer rim was due to sclerotic bone, while the high-intensity inner border was due to granulation tissue and/or chondroid metaplasia. Fractures within the six AVN lesions were best seen on T2-weighted images, but these could not be distinguished from granulation tissue. On T2-weighted images, the weight-bearing cartilage had lower signal in five of six femoral heads with AVN. We conclude that the MR appearance of AVN reflects its complex histopathology. Direct spin-echo chemical-shift imaging can help correlate MR images with histology by providing information not available from conventional or out-of-phase images.     

Idiopathic bone marrow edema lesions of the femoral head: predictive value of MR imaging findings.

PURPOSE: To determine the frequency of several subchondral magnetic resonance (MR) imaging features observed in bone marrow edema lesions of the femoral head and to determine their value for differentiation of irreversible from transient lesions. MATERIALS AND METHODS: The authors reviewed MR images of 72 femoral head lesions in 42 men and 25 women (median age, 48 years) with equivocal radiographic findings and bone marrow edema seen at MR imaging (T1- and T2-weighted images in all patients and contrast material-enhanced T1-weighted images in 39 patients). Follow-up MR images showed 57 lesions to be transient and 15 to be irreversible. The presence and size of subtle subchondral features observed on initial MR images were compared for both types of lesion. RESULTS: Lack of any additional subchondral change on T2-weighted or contrast-enhanced T1-weighted images had 100% positive predictive value for transient lesions. For irreversible lesions, presence of a subchondral area of low signal intensity at least 4 mm thick or 12.5 mm long had positive predictive values of 85% and 73%, respectively, on T2-weighted images and 87% and 86%, respectively, on contrast-enhanced T1-weighted images. CONCLUSION: Careful assessment of subchondral changes enables confident differentiation between early irreversible lesions and transient bone marrow edema lesions.     

Femoral head avascular necrosis: correlation of MR imaging, radiographic staging, radionuclide imaging, and clinical findings

To better correlate the appearance of avascular necrosis (AVN) of the femoral head on magnetic resonance (MR) images with the stage of disease, MR images of 56 proved AVN lesions were compared with staging from corresponding radiographs (n = 56), Tc-99m scans (n = 41), and grade of symptoms (n = 28). Fractures complicating AVN were seen in 28 (50%) of 56 radiographs (radiographic stages III-V). With long repetition (TR) and echo delay (TE) times, a characteristic "double line sign" consisting of high signal intensity inside a low-intensity peripheral rim was seen in 45 lesions (80%). The central region within the rim was isointense with marrow fat on both short and long TR and TE images in 20 (71%) of 28 lesions uncomplicated by fracture (stages I-II) but in only four (14%) of 28 stage III-V lesions (P less than .001). Symptoms were least severe in lesions isointense with fat and most severe in lesions with low-signal central regions at short and long TRs and TEs. The peripheral double line sign on long TR/TE images may add specificity to the diagnosis of AVN by MR imaging. A chronologic pattern of central MR signal features is presented which may allow staging of AVN by MR imaging.     

Dark t1 bone marrow

Bone marrow contains fat and water. Using magnetic resonance (MR) T1-weighted images, fat appears light gray and water appears dark gray. Black or low signal structures include normal cortical bone, bone islands, hemosiderin deposits, calcification, ossification, metal artifact, gas, and foreign material. Reactivation of bone marrow reduces the signal of fatty marrow. Additional sequences and supplementary imaging with conventional radiographs and computed tomography are often helpful in determining the nature of the low signal lesions. Genetic bone disorders such as melorheostosis and other syndromes may give low signal lesion in a typical and often diagnostic pattern. MRI is an important method of showing the fracture line as a low signal lesion on T1-weighted images and is the preferred imaging method when conventional radiographs are unhelpful and fracture is suspected. Infection will cause edema and later sclerosis and necrosis with a mixed low and high signal pattern. Infarction will cause a mixed pattern depending on the stage of the process, but this includes low signal on T1-weighted images. Ossification and calcification in primary and secondary tumors appear as low signal areas on T1-weighted images. Recognition and understanding of lesions that cause low signal on T1-weighted images is an important tool when interpreting MR images.     

Transient bone marrow edema of the hip

Transient bone marrow edema of the hip is characterized by moderate homogeneous low MR signal intensity with ill-defined margins that involves at least a portion of the femoral head. Spin echo T1-weighted images are helpful to exclude other underlying diseases (tumor, infection, necrosis from systemic origin…), for which marrow edema is secondary or no epiphyseal in location. High-resolution fat-suppressed T2-weighted or proton density images allow evaluation of the articular cartilage, subchondral bone and subchondal marrow: if the articular cartilage is abnormal, the lesion is irreversible (arthrosis or necrosis); if the subchondral bone is focally interrupted and/or if the femoral is no longer spherical, the lesion is irreversible (necrosis); if a focal linear fluid collection is present under the subchondral bone, the lesion is irreversible (necrosis). Finally, subchondral changes may provide useful prognostic information: the absence of any abnormality other than marrow edema typically indicates that complete resolution is likely; the presence of a focal T2-weighted hypointense lesion immediately next to the subchondral bone suggests an irreversible lesion, especially if it is equal to or thicker than 4mm or the joint space. In some instances, prognosis cannot be reliably determined requiring the need for follow-up imaging.