Nuclear magnetic resonance imaging in orthopaedics: principles and applications

Low Field Magnetic Resonance Imaging of Femoral Neck Fractures

Fourteen patients with cervical hip fractures were treated with internal fixation using titanium screws. The femoral head vitality was evaluated with 99Tcm-MDP scintigraphy and scintimetry within 2 weeks postoperatively and by serial low field magnetic resonance imaging (MR). Two patients with reduced radionuclide uptake (femoral head ratio <1.0) developed radiographic signs of femoral head necrosis. MR disclosed the definite area of the necrosis at 2 and 12 months after fracture, respectively. In three of the patients with a high scintimetric uptake (femoral head ratio ≥1.0), MR revealed a focal decrease of the signal intensity in the femoral head or neck at 2, 3 and 7 months after fracture, respectively. The radiographs in one of these patients were normal at 7 months after fracture. The second one showed signs of necrosis at 16 months and the last one developed delayed/non-union. With a non-ferromagnetic osteosynthesis the healing course after femoral neck fracture can be studied with low field MR equipment without disturbing artifacts. The time period between ischaemia and definite abnormalities on MR may embrace several months.

Early prediction of femoral head avascular necrosis following neck fracture

Femoral neck fracture puts at risk functional prognosis in young patients and can be life-threatening in the elderly. The present study reviews methods of femoral head vascularity assessment following neck fracture, to address the following issues: what is the risk of osteonecrosis? And what, in the light of this risk, is the best-adapted treatment to avoid iterative surgery? Femoral head vascularity depends on retinacular vessels and especially the lateral epiphyseal artery, which contributes from 70 to 80% of the femoral head vascular supply. Fracture causes vascular lesions, which are in turn the prime cause of necrosis. Other factors combine with this: hematoma tamponade effect, reduced joint space and increased pressure due to lower extremity positioning in extension/internal rotation/abduction during surgery. Head deformity is not due to direct cell death but to the repair process originating from the surrounding living bone. In post-traumatic necrosis, proliferation rapidly invades the head, with significant osteogenesis. Pathologic fractures occur at the boundary between the new and dead bone. Many techniques have been reported to help assess residual hemodynamics and risk of necrosis. Some are invasive: superselective angiography, intra-osseous oxygen pressure measurement, or Doppler-laser hemodynamic measurement; others involve imaging: scintigraphy, conventionnal or dynamic MRI. The future seems to lie with dynamic MRI, which allows a new classification of femoral neck fractures, based on a non-invasive assessment of femoral head vascularity.

A high-accuracy three-dimensional coordinate digitizing system for reconstructing the geometry of diarthrodial joints

This paper describes the design and performance evaluation of a three-dimensional (3-D) coordinate digitizing system (3-DCDS) for measuring both soft and hard biological tissue. The system incorporates a visible semiconducting laser beam and an X-Y positioning table to directly measure 3-D coordinates that define surface points. Experiments conducted to evaluate the performance of the system showed that it delivers an accuracy of 0.1 microm in the Z-direction and 1.4 microm in the X-Y plane, and an overall system root-mean-squared error (RMSE) of 8 microm on surfaces with slopes of less than 45 degrees . This error is lower than that of previously reported measurement techniques. The 3-DCDS measures 3-D coordinates of surface points uniformly separated by 500 microm in the X-Y plane. Because the 3-DCDS is automated, the coordinates are measured efficiently and the accuracy is independent of operator skill. These highly accurate coordinates can be easily incorporated into nodal values for 3-D finite element models (FEM) of diarthrodial joints. To show the use of the 3-DCDS, the 3-D surface coordinates of human menisci were measured from a cadaver specimen.

Correlating magnetic resonance imaging with the biochemical content of the normal human intervertebral disc

Magnetic resonance imaging was used to determine the T2 relaxation times of prepared proteoglycan solutions and of normal human intervertebral disc tissue from the annulus fibrosus (AF) and nucleus pulposus (NP). The collagen, proteoglycan, and water contents of the disc tissue samples were determined by biochemical assays after they were scanned. Correlations among 1/T2, collagen, proteoglycan, and water contents of the tissue samples and among 1/T2, water, and proteoglycan contents of the proteoglycan solutions were calculated. A moderate negative correlation between 1/T2 and water content was noted for the tissue samples, and a very high negative correlation was found between 1/T2 and water content for the proteoglycan solutions. The very high positive correlation between 1/T2 and proteoglycan content of the proteoglycan solutions is probably due to this negative correlation between 1/T2 and water content. There was no significant correlation between 1/T2 and proteoglycan content of the tissues. The moderate positive correlation between 1/T2 and collagen content is probably due to the high negative correlation between collagen content and water content. No significant correlation was found between the collagen and proteoglycan contents of the tissues. Thus it appears that the data confirm previous reports in the literature that the collagen of the disc tissue functions to control its water content.

Quantitation of articular surface topography and cartilage thickness in knee joints using stereophotogrammetry

An analytical stereophotogrammetry (SPG) technique has been developed based upon some of the pioneering work of Selvik [Ph.D. thesis, University of Lund, Sweden (1974)] and Huiskes and coworkers [J. Biomechanics 18, 559-570 (1985)], and represents a fundamental step in the construction of biomechanical models of diarthrodial joints. Using this technique, the precise three-dimensional topography of the cartilage surfaces of various diarthrodial joints has been obtained. The system presented in this paper delivers an accuracy of 90 microns in the least favorable conditions with 95% coverage using the same calibration method as Huiskes et al. (1985). In addition, a method has been developed, using SPG, to quantitatively map the cartilage thickness over the entire articular surface of a joint with a precision of 134 microns (95% coverage). In the present study, our SPG system has been used to quantify the topography, including surface area, of the articular surfaces of the patella, distal femur, tibial plateau, and menisci of the human knee. Furthermore, examples of cartilage thickness maps and corresponding thickness data including coefficient of variation, minimum, maximum, and mean cartilage thickness are also provided for the cartilage surfaces of the knee. These maps illustrate significant variations over the joint surfaces which are important in the determination of the stresses and strains within the cartilage during diarthrodial joint function. In addition, these cartilage surface topographies and thickness data are essential for the development of anatomically accurate finite element models of diarthrodial joints.(ABSTRACT TRUNCATED AT 250 WORDS)

Magnetic resonance imaging of the triangular fibrocartilage complex

Magnetic resonance imaging of seven fresh human cadaveric specimens was used to evaluate the integrity of the triangular fibrocartilage complex of the wrist. A variety of imaging parameters were systematically investigated, including T1-weighted images with and without contrast, long repetition times, short echo time images, and T2-weighted sequences. A variety of imaging planes were also evaluated. Wrist arthrography, dissection, and frozen coronal sections were done to substantiate our interpretations of the magnetic resonance images. T2-weighted images in the coronal plane proved to be of the greatest diagnostic value because the synovial fluid of the joint spaces serves as an excellent endogenous contrast agent. Long repetition time, short echo time sequences could be simultaneously obtained with T2-weighted sequences with the use of a multi-echo pulse sequence to provide an excellent diagnostic package in the future. On the basis of our investigation, the triangular fibrocartilage complex can be consistently and accurately evaluated with magnetic resonance imaging. As magnetic resonance imaging technology improves, wrist probes and suitable magnets should become available that will make the evaluation of triangular fibrocartilage complex abnormalities with magnetic resonance imaging clinically useful.

Estimating segment inertial properties: comparison of magnetic resonance imaging with existing methods

To explore magnetic resonance imaging (MRI) as the basis of a technique for estimating body segment inertial properties directly on subjects, MRI-based estimates were compared with those generated with more traditional methods. The sensitivity of mass, center of mass location, and moment of inertia estimates to the estimation methods and the distribution of muscle, bone, and fat tissues in the lower leg were also evaluated. A series of transverse plane images spaced 2.5 cm apart on the right lower leg of 12 adult males were taken, using MRI. Cross-sectional area data determined from the images served as the basis of the inertial calculations. MRI-based estimates for mass, center of mass location, and moment of inertia about a transverse axis through the center of mass all fell within the range of values obtained with other methods. While the center of mass position was similar for all estimation methods, MRI-based estimates of lower leg mass and moment of inertia showed considerable variability among methods. In addition, MRI mass and moment of inertia values were higher than those from most of the other prediction methods. It is suggested that the higher MRI-based estimates for these descriptors may be reasonable when age and structural differences between young adult runners and cadaver specimens for the comparison studies are considered. These results provide further support for MRI as a means of generating valid inertial property estimates.

Application of magnetic resonance imaging to ischemic necrosis of the lunate

Twenty patients (22 wrists), in whom ischemic necrosis of the lunate was suspected clinically, were studied prospectively by physical examination, conventional radiography, radionuclide bone scintigraphy, and magnetic resonance imaging. Magnetic resonance imaging showed more specificity than radiography or radionuclide bone imaging in making the diagnosis of ischemic necrosis of the lunate. Two patterns of lunate signal defect were seen on magnetic resonance imaging with focal or generalized signal loss on T1-weighted and T2-weighted images. Generalized loss of lunate signal on T1-weighted images was diagnostic for ischemic necrosis of the lunate. Focal signal loss on the radial one half of the lunate suggested early involvement. Normal or increased signal on the corresponding T2-weighted image implied an earlier stage and a better prognosis. Magnetic resonance imaging evaluation of the lunate may be useful for subclassification of Lichtman's radiographic stage 2 and can be employed to follow lunate revascularization after treatment. Magnetic resonance imaging is a low risk, noninvasive diagnostic technique, which can modify diagnosis, staging, treatment and prognosis of ischemic necrosis of the lunate.

Correlation of MRI images with histology in avascular necrosis in the hip. A preliminary study

The role of MRI in identifying the tissue level changes in the femoral head was investigated in five patients diagnosed as having avascular necrosis by radiology, scintigraphy, and MRI (0.35 Telsa). Radiographic scoring by the Ficat and Arlet system showed one hip with stage I, one stage II, two stage III, and one stage IV changes. The histologic features of core biopsy specimens obtained during decompression of the femoral heads were compared to the preoperative T1 and mixed T1/T2-weighted MR images. The cores varied with respect to their distance from the subchondral bone (7-23 mm) and length (19-45 mm). At the subchondral end of the core tract, low T1-weighted images corresponded to marrow fibrosis (5 cases) and in three of five cases to increased trabecular bone volume (TBV = 44-50%). Subjacent areas of diffusely decreased MR signal corresponded to marrow fibrosis and necrosis, with a relatively normal TBV (17-28%). The distal ends of the core tracts showed normal fatty marrow and a normal MR signal. The observations affirm that the MR signal intensity is largely reduced as a function of marrow degeneration and loss of fat content, but the signal is not predictive of particular histotypic morphologic patterns.

A review of applications of MRI in soft tissue and bone tumors

Due to excellent soft tissue contrast and multiplanar imaging capability, MRI is assuming a major role in recognition, staging, and treatment planning of soft tissue and bone tumors. Direct sagittal, coronal, and axial images permit assessment of intraosseous and extraosseous extension of tumors and their relationship to the joints and neurovascular structures, and detection of "skip" lesions. MRI allows improved detection of recurrent tumors in the presence of non-ferromagnetic metallic implants as compared to CT. In the evaluation of soft tissue tumors, MRI is more sensitive than CT and allows differentiation among fat, muscle, tendon, bone, and vascular structures based on signal characteristics. Over a period of 18 months, 100 soft tissue masses and bone tumors were evaluated using MRI. Spin echo sequences with T1 and T2 weighted images were most valuable in differentiating normal and abnormal tissues. Calculated comparative measurements of relaxation times showed no reliable difference between benign and malignant tumors.

Magnetic resonance imaging and histopathology in femoral head necrosis

We correlated preoperative magnetic resonance (MR) images and histopathology of eight femoral heads from patients with osteonecrosis. The signal intensity of the MR image was low in the area where fibrovascular tissue, disintegrated fibrovascular tissue, or amorphous necrotic material occupied the medullary space. On the other hand, the necrotic marrow without revascularization showed high signal intensity. Osteonecrosis can be detected by MR imaging as soon as a certain amount of bone marrow is replaced by fibrovascular tissue.

Medical applications of nuclear magnetic resonance imaging

In the very short time since magnetic resonance imaging (MRI) was born it has gained surprisingly rapid and enthusiastic acceptance and has speedily proliferated, particularly in the United States and Western Europe. Magnetic resonance imaging (MRI) has successfully challenged computed tomography (CT) in all areas of the body where respiratory motion does not degrade the image (Steinberg, 1986). Newer techniques using a multiplicity of approaches are starting to close the gap between CT and MRI, even in the upper abdomen where the effects of respiratory motion are most pronounced. Although MR is already widely clinically applied and is an accepted everyday diagnostic modality in most large medical centres in the United States, it is not a mature modality. It is rapidly evolving, with whole new areas opening to investigation which will vastly broaden its applications.

Magnetic resonance imaging of osteoarthritis: correlation with gross pathology using an experimental model

Conventional radiography has several limitations in the study of osteoarthritis (OA): changes occur late, they are restricted to bone, and they do not correlate well with the patient's symptoms. Magnetic resonance imaging (MRI) can visualize with great detail the soft tissue changes in and around joints. We have obtained MR images of knees using an experimental model of OA in order to study the early changes of OA, to correlate the images with the gross pathology, and to compare MR images with radiographs. The changes on MRI correlated with the gross pathological changes as assessed by Kendall's rank correlation test as follows: meniscal changes, tau = 0.58 (p less than 0.01); osteophytes, tau = 0.59 (p less than 0.05); capsular fibrosis, tau = 0.55 (p less than 0.05); and overall, tau = 0.68 (p less than 0.001). Abnormalities were evident on MRI as early as 4 weeks after the onset of the disease, which is 8 weeks before they appear on radiographs. Thus, MRI can show changes in OA earlier than radiography can, and the images correlate with the gross pathology.

T1, T2, and relative proton density at 0.35 T for spleen, liver, adipose tissue, and vertebral body: normal values

An MRI installation (Magnetom, Siemens, software version B1 of NUMARIS) working at 0.35 T was used to estimate T1, T2, and relative proton density in the spleen, liver, adipose tissue, and vertebral body in 14 healthy volunteers. Two double-echo sequences were applied for all subjects: TR = 500 ms, TE1 = 35 and TE2 = 70 ms; and TR = 1600 ms, TE1 = 35 and TE2 = 70 ms. The images were sampled in regions of interest and appropriate relaxation expressions fitted to the ROI data yielding relaxation parameters and relative proton densities. Relaxation expressions, included in standard software (Siemens), were compared to more elaborate functions, developed in parallel to this study. The latter were found more appropriate, especially for high T1 values, and gave the following mean values for the four tissues (estimated uncertainty of mean in parentheses) T1 (ms) 915(36), 428(5), 261(7), and 501(11); T2 (ms) 79.7(8.8), 51.0(0.2), 59.8(1.0), and 64.7(0.8); and corresponding relative proton density (rho, arbitrary units) 2088(136), 2182(10), 2915(49), and 2136(21). The uncertainty in the values was estimated in the fitting procedure and does not include systematic errors. The relative noise in the ROIs was about 9% and the reproducibility of the ROI mean values about 8%.

Magnetic resonance imaging of the musculoskeletal system

During the last few years, the interest and experience in magnetic resonance imaging (MRI) of the musculoskeletal system has increased rapidly. Modern equipment with good spatial resolution and multislice, multiecho technique provides detailed information on the joints, soft tissue and spine within a reasonable examination time, both with superconductive and resistive systems. Today, MRI is an established technique for examination of musculoskeletal tumors and aseptic bone necrosis and it has proven to be of great value for evaluation of diseases of the spine and spinal canal. The definite place of MRI within musculoskeletal diagnostic imaging is not yet settled, but its potential is great, and it will have an important role in the future.

Magnetic resonance. Principles and applications

Magnetic resonance is a unique, noninvasive imaging modality which allows direct, multiplanar imaging and the possibility of obtaining biochemical information in vivo. Presently, MR appears most applicable to the evaluation of central nervous system abnormalities. The high sensitivity of MR in the evaluation of intracranial pathology suggests that it may eventually replace CT for many suspected diseases, if future investigations are able to improve its specificity. As previously noted, MR may be more diagnostic than other radiologic studies in the evaluation of suspected Chiari malformation, syringomyelia, congenital abnormalities, tumors of the spinal axis, and disc space infection. In the chest, MR appears to be more accurate than CT in the determination of the extent of mediastinal tumor, but at present cannot replace CT because of the lack of experience in imaging parenchymal nodules and benign diseases. MR of the breast is promising, but the size of the lesion may prove to be a limiting factor with magnetic field strengths commonly being utilized. There are inherent difficulties in the evaluation of cardiac disease with MR, but it offers a noninvasive method of investigating congenital heart disease and may provide valuable information in suspected myocardial ischemia and altered cardiac function. MR provides a new method of evaluating the vascular system, both in terms of providing anatomic information on large and medium-sized vessels and flow analysis. In the abdomen, MR appears to be most sensitive in the evaluation of suspected hepatic masses, but as with the brain, greater specificity will be needed to replace CT. At the present time, MR offers no distinct advantage over conventional imaging modalities in the evaluation of pancreatic disease, it maybe more accurate than CT in the staging of renal cell carcinoma. Larger studies are needed to determine the role of MR in the investigation of retroperitoneal adenopathy and adrenal abnormalities. In the pelvis, MR offers the hope of earlier diagnosis of prostatic carcinoma and may replace CT for staging of prostatic carcinoma and transitional cell carcinoma of the bladder. Limited MR experience with benign disease of the female pelvis suggests that it is currently more accurately evaluated with ultrasound. MR appears to be highly sensitive and specific for the diagnosis of avascular necrosis and may provide an early clue in suspected osteomyelitis. Finally, in vivo MR spectroscopy may provide unique metabolic information that was unobtainable prior to the advent of magnetic resonance, if this proves to be technically feasible.