|Oganes Ashikyan, MD|
Musculoskeletal imaging has seen many innovations in recent decades. While plain radiography with its superior silver grain film resolution continues to be a very important modality in evaluation of musculoskeletal disorders and represents the bread and butter of orthopedic radiology, several new technologies have been developed. High-tech successor of plain radiography, the CT scanner played an important role in imaging of musculoskeletal disorders until MR imaging, with its superior anatomical definition and relative decrease in foreign body artifact, replaced this modality. CT scanning continues to be an important modality in evaluation of patients with contraindications to MR and in certain circumstances where continuous visualization is necessary such as in kinematic CT and recently in CT fluoroscopy. Doppler sonography brought affordability to the field of musculoskeletal imaging and continues to be an ideal imaging modality in a variety of clinical situations. Its utility is enhanced when imaging of only one or two joints is necessary. In this review, we highlight recent developments in musculoskeletal radiology and examine their effect on practice of this field and in some cases their effect on health care delivery in general.
MR Imaging Overview
MRI is a continuously improving technique that has surpassed CT in its ability to define anatomy of lesions and surrounding structures. Various sequences are being used to better characterize lesions and anatomy. Spin echo imaging is the most commonly used technique.1 Both T1- and T2-weighted images are obtained in this manner. Several sequences are being used in daily practice, and new approaches to MR imaging are being continuously developed. T1-, T2-weighted imaging, proton density, fat saturated, and short tau inversion recovery (STIR) imaging techniques are the most commonly used sequences in mus-culoskeletal radiology (Figures 1, 2). Fat saturation imaging has resulted in better characterization of tissues and lesions because it suppresses high signal that arises from the bone marrow. Fat saturation images can also be obtained by the STIR technique, which involves applying RF signals in an order that results in elimination of signal from fat. The water excitation technique has been shown to decrease T1-weighted image acquisition time by as much as 50%,2 while achieving an effect similar to that of the fat saturation technique. A gradient echo image is obtained by applying a gradient rather than a uniform magnetic field.
|Jamshid Tehranzadeh, MD|
While MR imaging has superior resolution in comparison to other clinically useful imaging modalities, it suffers from greater susceptibility to motion artifact. Among recent developments that allow reduction in the effects of motion are propeller MR imaging3 and ADAPT technology.4 Both of these approaches have been developed only recently and their effect on musculoskeletal imaging is yet to be determined. In addition to the above advances that improve visualization of structures on MR images in general, novel sequences are continuously being developed for specific musculoskeletal evaluations.
Plain radiography has remained the principal modality in initial evaluation and characterization of musculoskeletal neoplasms for many years. It allows visualization of gross lesions and the effects on the bone cortex and periosteum. The visualization of the true extent of the lesion and its effects on surrounding structures, however, is often poor at best with this technology. Today, musculoskeletal evaluation of neoplasia benefits from high-resolution MR technology and increased sensitivity for screening afforded by nuclear medicine techniques. In addition to better visualization, the variety of novel MR sequences allows characterization of malignancies as well in some cases. MR imaging is capable of exhibiting specific signal for different tissues, which has made it possible to characterize pathologic lesions in certain cases. This would be further enhanced by tissue spectroscopy as our knowledge expands in this area.
|Figure 1 (a,b,c). Prelabral cyst (ganglion) in a 45-year-old male with atrophy of infraspinatus muscle. Coronal oblique fat saturation spin echo T1-weighted image (TR=4500, TE=54) (a) and axial spin echo T2-weighted image (TR=4000, TE=96) (b) show prelabral cyst at the superior glenoid notch impinging upon scapular nerve. Coronal oblique fat saturation spin echo T2-weighted image (TR=4500, TE=54) (c) shows edema due to early atrophy of the infraspinatus muscle.|
Various new MR sequences are being employed to better characterize musculoskeletal neoplastic lesions. One area of advancement involves improved ability to differentiate malignant from benign lesions. Utilization of well-established MR sequences such as T1-, T2-weighted, and proton density imaging can provide indirect evidence for presence or absence of a malignant lesion. Invasion of surrounding structures, involvement of posterior elements of the vertebra, and lack of fluid sign5 can suggest presence of spinal malignancy, for example. Recently, diffusion weighted imaging (DWI) apparent diffusion coefficient has been shown to help in differentiating collapse of the vertebral bodies associated with metastases from collapse associated with benign conditions such as osteoporosis.6,7 DWI was developed originally for visualization of cerebrovascular insults of acute onset. The technique highlights the signal attenuation based on the amount of diffusion of water molecules. In addition to regular RF pulse sequences, two strong pulses are applied to the tissue. The first pulse dephases the spins, while the second pulse returns the spins into phase. If some of the spins move out of the field of study, signal attenuation occurs. Based on the regression analysis of the signal change, apparent diffusion coefficient can be calculated.8 Diffusion is dependent on many factors, which include presence of cell membranes, characteristics of extracellular matrix, and presence of macromolecules. A high number of rapidly proliferating cells in the malignant tissues hinders diffusion and results in high signal intensity on a diffusion-weighted image.
Dynamic contrast-enhanced MRI is showing promise in evaluating tumors for viability, response to chemotherapy, and recurrence.9 Conventional MRI with contrast allows evaluation of vascularity of various structures based on the distribution of contrast. The resulting image depicts a net distribution of contrast without much differentiation between early and late enhancing structures. Fast contrast-enhanced MR imaging allows acquisition of data at various time intervals. This effectively adds a temporal dimension to the evaluation. Dynamic contrast-enhanced MR imaging allows differentiation of early enhancing structures from late enhancing counterparts. Early phase enhancement correlates with viable tumor while late phase enhancement allows visualization of normal structures and surrounding inflamed tissues. This technique allows better localization of biopsy sites. Dynamic contrast enhanced MR imaging also allows evaluation of response to chemotherapy and differentiation of true metastasis from pseudotumors.9 This techniques relies on availability of a fast MR scanner as rates of enhancement show differences only during the first few minutes after contrast injection.
|Figure 2. Bucket handle tear of lateral meniscus in a 43-year-old male. Parasagittal proton density image (TR=2000, TE=15) shows bucket handle tear of posterior horn of lateral meniscus with flipped meniscus sign (arrow). There is also bone contusion of tibial epiphysis.|
Although MR is a superior technique for detailed articular imaging, it is not suitable for screening of the whole body. Scintigraphy is the modality of choice for screening of the whole skeleton. Nuclear medicine techniques that involve introduction of radioactive tracers into the body and measurement of their accumulation in different areas are experiencing rapid development in the field of neoplasm imaging. Various radioactive materials can be used. The ability to label different compounds such as methylene disphosphonate (MDP), hexamethylpropyleneamine oxime (HMPO), immunoglobulins, and antibody fragments10 has paved the way for progress in the field of molecular imaging. In addition to the techniques that involve the administration of technetium- (99mTc-), indium 111-, and indium 123-labeled compounds and their subsequent detection utilizing gamma-camera (scintigraphy) and single photon emission computed tomography (SPECT), there is growing utilization of fluorine 18-labeled deoxyglucose (FDG) positron emission tomography (PET) in the detection of tumors. FDG allows detection of tumors with a tenfold increase in sensitivity and a spatial resolution of 6-8 mm.11
Combining the high sensitivity of nuclear medicine with reasonable visualization of anatomical detail on CT images, the application of PET-CT fusion imaging in the musculoskeletal field is increasing as well. Evaluation of such scans requires either availability of physicians trained in CT and nuclear medicine interpretation or joint interpretation of images by two specialists. Furthermore, this developing technology requires technical expertise in manipulating the fusion images. In addition to the cost associated with acquisition of hardware and software for fusion imaging, an investment of time in physician and technologist training is also necessary.
|Figure 3 (a,b). 58-year-old male with rheumatoid arthritis. Coronal spin echo T1-weighted image (TR=589, TE=16) (a) shows multiple erosions of the distal radius, ulnar styloid, and carpal bones. Coronal spin echo T1-weighted image with fat saturation post gadolinium contrast agent (TR=637, TE=9.3) (b) shows enhancement at the site of erosions and the synovial joint lining.|
MR spectroscopy is a technique that was originally developed for characterization of neurological malignancies through measurement of the amount of various chemical compounds in any given tissue. The technique has been evaluated in several studies for its feasibility to diagnose and evaluate response to therapy of musculoskeletal malignancies,12-14 but has not become a widespread practice yet.
Follow-up evaluation of response to therapy or presence of metastasis is another rapidly developing area. FDG-PET appears to be a suitable technique for this purpose.
Plain radiographs are the mainstay in orthopedic radiology. However, MRI can provide in-depth understanding of bone and soft tissue injuries in trauma cases. Among the recent advances is the better visualization of the growing skeleton in pediatric patients. Classification of the fractures through the growth plate has a well-known importance in establishing prognosis and in planning surgical therapy. The difference in prognosis is largely based on the formation of bridges across the growth plate, which restricts further growth of the bone. Spoiled gradient recalled echo (SPGR) sequence in addition to conventional imaging sequences can be used for visualization of such bridges.15 SPGR sequence involves somewhat random changes in RF phase. This technique is useful for evaluation of tissues that are normally bright on T1-weighted image, and the bony bridges are well visualized on SPGR sequences. Acquisition of intermediate and T2-weighted images remains important to allow visualization of surrounding tissues.15
Early focal changes in articular cartilage can be visualized utilizing SPGR sequences.16 Once again, early diagnoses can lead to early treatment prior to occurrence of damage to other joint structures. The SPGR technique also is helpful in postoperative follow-up of cartilage reconstruction procedures.17 Intermediate or T2-weighted spin echo images remain important for a complete evaluation.
The above examples clearly demonstrate the effect of new technology on economics. An additional sequence results in longer study times and associated increased cost. However, when taken in the context of early detection and the possibility of early intervention, especially in pediatric cases, the benefits by far outweigh the cost for the health care system in general.
Inflammatory Joint Diseases
Plain radiography can easily demonstrate erosions, periarticular changes, and bone deformities associated with inflammatory lesions. Unfortunately, these are late changes when therapy can be hoped to only relieve symptoms
or perhaps halt further destruction of joints. The MR technology once again demonstrates a tremendous step forward in the evaluation of such conditions. The utilization of gadolinium-enhanced MR sequences is one of the important recent advances in the area of musculoskeletal radiology. It is widely accepted that initial injury in such a common condition as rheumatoid arthritis takes place in the synovium of the joint. Administration of gadolinium contrast prior to MR imaging allows visualization of tissues with good vascular supply. Accordingly, synovium with its great vascular supply can be easily visualized and assessed. Fat saturation of contrast-enhanced images highlights the inflamed tissues even further (Figure 3). Objective assessment of the diseased joint can be done much earlier than with conventional radiography and the treatment can be initiated before significant damage to the joint has occurred. Fast MR units that allow temporal acquisition of data also appear to be useful in the imaging of inflammatory conditions. Since initial rates of enhancement are dependent on the vascularity of the tissue among other factors, dynamic MR techniques show promise in areas of assessment of the severity of the inflammation in the
synovium.18 The cost associated with the administration of a contrast agent adds to the cost of diagnosis. Furthermore, the radiologist must be present on site to be available for management of adverse reactions.
The Role of ultrasound
Ultrasound is a technique that allows evaluation of conditions associated with inflammatory arthritis. In many musculoskeletal evaluations, ultrasound is considered to be a technique complementary to other imaging modalities. In certain clinical situations, however, ultrasound also is becoming a procedure of choice. The relatively low cost and lack of radiation exposure make this technique a desirable alternative when evaluation of one or two superficial joints is necessary. Ultrasound is an ideal technique for the evaluation of tendon structures. As such, ultrasound can reliably detect tenosynovitis, tendonitis, tendon tears, and bursitis.19 What makes ultrasound particularly attractive in certain situations is its ability to evaluate a particular area in real time. In addition to obtaining multiplanar images, the evaluator can easily pay further attention to areas that cause discomfort in the patient. Reproduction of tenderness by the examiner gives a real-time correlation with underlying pathology. Ultrasound is becoming standard procedure in evaluation of regional musculoskeletal pain as well.20 High-frequency probes improve the image resolution and allow visualization of fine anatomy. The availability of new scanners and their relatively low cost when compared to other radiographic equipment allow the possibility of operating such units at the sites of rheumatology clinics or sport events. Because ultrasound technique has the limitation of being dependent on the experience of the operator, collaboration between radiology, rheumatology, and orthopedics departments is necessary in order to make this technology more useful, accessible, and convenient to patients.
Spatial compound sonography is a new method that combines images from sonograms at different angles into a single image. This new technique is an important development in musculoskeletal sonography because it increases quality of the image by reducing speckle and gives better tissue-plane definition.21
Because Doppler ultrasound allows visualization of blood movement, a differentiation can be made between edematous tissue and active inflammatory process. Of the two types of Doppler ultrasound, color flow Doppler and power Doppler, the latter is better suited for evaluation of small vessel blood flow in the synovium.22 Whether ultrasound can substitute for other imaging equipment in evaluation of inflammatory diseases remains a question that needs to be answered by future studies. In one recent study, qualitative assessment of vascularity by power Doppler ultrasound was shown to correlate with histopathologic assessment of the synovium in the arthritic hip joint.23 The cost of ultrasound equipment capable of obtaining flow images is somewhat higher compared to traditional ultrasound units, but it is still significantly less expensive than MR or CT equipment.
Imaging of Metabolic Disorders
In the past, a crude assessment of bone density based on measurement of lacunae and cortex was possible using plain radiographs. Later advances in the field of metabolic imaging included single and dual energy photon absorptiometry and quantitative computed tomography. Today, dual energy x-ray absorptiometry (DEXA) has largely replaced the above modalities for evaluation of osteoporotic bone. Early studies are showing promise of evaluation of osteoporosis utilizing low field pulsed nuclear magnetic resonance technology.24 This technology allows not only estimation of bone density, but also actual measurement of pores in the bone and amount of the porosity. Nuclear medicine techniques continue to be supplemental in imaging of metabolic disorders and help in evaluations when diagnosis is not easily determined based on other studies. One of the well-established uses of nuclear bone scan, however, is evaluation of the degree and extent of Paget’s disease.25
Another advance in the use of MR technology is in the operating room for interventions using non-ferromagnetic instruments. The availability of MR magnets that allow reasonable mobility and quick disassembly in the operating room, affording access to the patient between image acquisitions, has led to development of MR-guided tumor resection techniques. One study25 showed that such use of MRI resulted in significant alterations in surgery in 11 out of 31 patients with skull base tumors. This practice certainly adds to the cost of operating room equipment, but its impact on overall health care costs has yet to be determined.
It is clear that most technological advances in the area of musculoskeletal imaging have increased the cost of individual examinations. There is no argument that the cost of obtaining and evaluating a single radiograph is significantly less compared to the cost associated with obtaining the multitude of images involved in CT, MRI, nuclear, and ultrasound studies.
The cost and utilization of MR in musculoskeletal imaging are an evolving health policy issue. Downsizing the time and number of sequences by optimizing and tailoring the protocol for specific imaging can sharply decrease the turnaround time and time in the magnet. As with any other new technology, the initial cost of MRI implementation remains high. This cost is likely to decrease, however, as more experience is gained and the protocols are optimized to different clinical situations. The expense of imaging has to be evaluated with consideration of its effect on the overall health care delivery system. The cost is likely to be justified if the earlier diagnosis leads to earlier treatment, thereby conserving costs associated with long-term disability. Imaging in radiology has revolutionized the field of medicine and how the health delivery system is operated today.? The economic impact of these imaging techniques has been tremendous and beneficial to the ultimate quality of the health care system. n
Oganes Ashikyan, MD, is radiology resident, and Jamshid Tehranzadeh, MD, is professor of radiology and orthopedics, department of radiological sciences, University of California, Irvine.
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- Ostergaard M, Stoltenberg M, Lovgreen-Nielsen P, Volck B, Sonne-Holm S, Lorenzen I. Quantification of synovitis by MRI: correlation between dynamic and static gadolinium-enhanced magnetic resonance imaging and microscopic and macroscopic signs of synovial inflammation. Magn Reson Imaging. 1998;16:743-54.
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- Walther M, Harms H, Krenn V, Radke S, Kirschner S, Gohlke F. Synovial tissue of the hip at power Doppler US: correlation between vascularity and power Doppler US signal. Radiology. 2002;225:225-31.
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