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Metal Artifact Reduction Sequence: Early Clinical Applications¹

¹ From the Departments of Radiology (R.V.O., P.L.M., M.J.L., D.L.J., A.L.M., Q.S.X.), Orthopedic Surgery (P.L.M., B.M.), and Physics and Astronomy (A.L.M., Q.S.X.), University of British Columbia, Vancouver General Hospital, 855 W 12th Ave, Vancouver, British Columbia, Canada V5Z 1M9; the Vancouver Musculoskeletal Tumor Group, Vancouver, British Columbia, Canada (P.L.M., M.J.L., B.M.); and the Department of Radiology, Surrey Memorial Hospital, Surrey, British Columbia, Canada (D.L.J.). Presented as a scientific exhibit at the 1998 RSNA scientific assembly. Received March 22, 1999; revision requested May 3 and received May 28; accepted June 1. Address reprint requests to P.L.M. (e-mail: plmunk@interchange.ubc.ca).

	Abstract

Top Abstract Introduction Description of Technique Clinical Experience Current Status of Technique Conclusion References

 
Artifact arising from metal hardware remains a significant problem in orthopedic magnetic resonance imaging. The metal artifact reduction sequence (MARS) reduces the size and intensity of susceptibility artifacts from magnetic field distortion. The sequence, which is based on view angle tilting in combination with increased gradient strength, can be conveniently used in conjunction with any spin-echo sequence and requires no additional imaging time. In patients with persistent pain after femoral neck fracture, the MARS technique allows visualization of marrow adjacent to hip screws, thus enabling diagnosis or exclusion of avascular necrosis. Other applications in the hip include assessment of periprosthetic soft tissues after hip joint replacement surgery, postoperative assessment after resection of bone tumors and reconstruction, and localization of unopacified methyl methacrylate cement prior to hip arthroplasty revision surgery. In the knee, the MARS technique allows visualization of structures adjacent to implanted metal staples, pins, or screws. The technique can significantly improve visualization of periprosthetic bone and soft-tissue structures even in patients who have undergone total knee arthroplasty. In patients with spinal fixation hardware, the MARS technique frequently allows visualization of the vertebral bodies and spinal canal contents. The technique can be helpful after wrist fusion or screw fixation of scaphoid fractures.

Index Terms: Hip, necrosis, 442.44 • Hip, surgery, 442.45 • Knee, surgery, 452.45 • Magnetic resonance (MR), artifact • Magnetic resonance (MR), pulse sequences • Metallic devices • Spine, fixation devices, 30.45

	Introduction

Top Abstract Introduction Description of Technique Clinical Experience Current Status of Technique Conclusion References

 
Since the inception of magnetic resonance (MR) imaging, the presence of metal in an anatomic area of interest has proved a difficult and annoying problem for musculoskeletal radiologists. Metal produces both a large area of signal void and extensive distortion around the implant (1–6). In most instances, the signal void and distortion preclude acquisition of useful information in the immediate area of the metallic object. Unfortunately, the presence of metal also degrades computed tomographic (CT) studies; thus, when metal is present in a patient, it has been impossible to obtain useful information. For this reason, many efforts have been made to minimize metal artifact by various investigators with only modest results (2,5–15).

We have developed an MR imaging pulse sequence designed to reduce susceptibility artifacts arising from orthopedic hardware. This technique results in improved image quality and reduction of susceptibility artifact without an increase in imaging time. The technique frequently allows improved visualization of structures adjacent to the hardware and often yields diagnostically useful information (16–18). In this article, a description of the technique, clinical experience with the technique, and the current status of the technique are presented.

	Description of Technique

Top Abstract Introduction Description of Technique Clinical Experience Current Status of Technique Conclusion References

 
There are at least two problems with imaging in the presence of susceptibility-induced field gradients caused by metal implants (1,6,16,19). First, the section is distorted from the ideal planar sheet. Second, there are geometric image distortions along the frequency axis due to the inhomogeneous field.

The metal artifact reduction sequence (MARS) was optimized for reduction of metal artifact by using the following three strategies (17,18). First, the section-selection gradient and radio-frequency bandwidth were increased by 17%, and a relatively narrow (3- or 4-mm) section thickness was used. Second, the read gradient was increased. The frequency bandwidth was 31.25 kHz for a 16-cm field of view (FOV) and 62.5 kHz for a 30-cm FOV. Third, view angle tilting was employed. The view angle was 34° for the 16-cm FOV and 32° for the 30-cm FOV. Each of these three strategies contributes about equally to reduction of metal artifact. The MARS technique is presented schematically in Figure 1, and its effectiveness in reducing susceptibility artifacts is illustrated in Figure 2. The aim of the first two strategies is to make the imaging gradients as large as possible relative to the susceptibility-induced gradients produced in the tissue by the metal implants. Increasing the section-selection gradient decreases section curvature. Increasing the frequency-encoding gradient decreases geometric distortion in the image. However, the increase in the frequency-encoding gradient requires a larger image bandwidth, resulting in a decrease in signal-to-noise ratio. The signal-to-noise ratios for MARS images obtained with 16- and 30-cm FOVs were 29% and 50% lower, respectively, than that of conventional MR images, which had a frequency bandwidth of 16 kHz.

View larger version (15K): [in this window] [in a new window] [Download PPT slide]   Figure 1.   Diagram of a conventional spin-echo sequence modified for the MARS technique. Gx  = frequency-encoding gradient, Gy  = phase-encoding gradient, Gz  = section-selection gradient, RF  = radio frequency.

View larger version (83K): [in this window] [in a new window] [Download PPT slide]   Figure 2a.   Phantom study of a femoral component from total hip arthroplasty with a titanium alloy. The component was suspended in grease. (a) Coronal T1-weighted spin-echo MR image (repetition time msec/echo time msec  = 500/15, 3-mm section thickness, 1.5-mm gap, 30-cm FOV) shows extensive distortion and artifact of mixed high and low signal intensity, especially around the proximal portion of the component. (b) Corresponding MR image obtained with the MARS technique shows some residual distortion, especially around the femoral head. However, a dramatic reduction in artifact is apparent, and the remaining signal void is now easily recognized as representing a femoral component.

View larger version (80K): [in this window] [in a new window] [Download PPT slide]   Figure 2b.   Phantom study of a femoral component from total hip arthroplasty with a titanium alloy. The component was suspended in grease. (a) Coronal T1-weighted spin-echo MR image (repetition time msec/echo time msec  = 500/15, 3-mm section thickness, 1.5-mm gap, 30-cm FOV) shows extensive distortion and artifact of mixed high and low signal intensity, especially around the proximal portion of the component. (b) Corresponding MR image obtained with the MARS technique shows some residual distortion, especially around the femoral head. However, a dramatic reduction in artifact is apparent, and the remaining signal void is now easily recognized as representing a femoral component.

In effect, nearby metal produces a chronic frequency shift from that imparted by the nominal imaging gradients. If a single-axis frequency-encoding gradient were applied (ie, no tilt), this chronic frequency shift would produce an apparent spatial shift of affected spins along the frequency-encoding axis. Use of a stronger frequency-encoding gradient reduces this spatial shift but does not remove it. The application of an additional gradient along the section-selection direction with the same amplitude as the section-selection gradient restores spins in the radio-frequency–selected volume to the same band of frequencies, thus compensating for the chronic metal-induced frequency shift. Although this selected volume may be highly warped with respect to the z axis, its projection with respect to the x axis is now free of metal-induced spatial shifts.

	Clinical Experience

Top Abstract Introduction Description of Technique Clinical Experience Current Status of Technique Conclusion References

 
Our early clinical experience indicates that significant improvement in image quality in the presence of metal can be readily achieved by using the MARS technique in a variety of musculoskeletal imaging situations. The following examples illustrate a few of the potential clinical applications of this technique.

Hip
Femoral neck fractures are common injuries and are usually treated with stabilization techniques by using metal devices such as cannulated screws. Although satisfactory alignment can be achieved in most cases, postoperative complications are relatively common. Avascular necrosis of the femoral head is a significant and common complication and is difficult to detect with radiography, CT, or bone scintigraphy. The vascular supply to the femoral head consists of a circumflex femoral branch of the deep femoral artery as well as small vessels within the ligament of the femoral head and capsular vessels. After a fracture, some or all of this blood supply may be lost. Osteonecrosis may become clinically apparent weeks or months after the fracture, and plain radiographic findings are often unremarkable until frank collapse of the femoral head occurs. Use of MR imaging to detect early avascular necrosis of the femoral head is also problematic. Without use of a metal suppression sequence, it is often not possible to adequately visualize the marrow of the femoral head at MR imaging. In our experience, the MARS technique usually allows visualization of the subarticular marrow in the femoral head, thus allowing avascular necrosis to be diagnosed or excluded (Fig 3). This technique has proved particularly useful in assessing patients with persistent pain after a femoral neck fracture because early diagnosis of avascular necrosis results in reduction of weight-bearing activities by the patient and decreases the likelihood of femoral head collapse and fragmentation.

View larger version (136K): [in this window] [in a new window] [Download PPT slide]   Figure 3a.   Avascular necrosis in a 56-year-old man with hip pain 4 months after a femoral neck fracture, which was transfixed with three screws. (a) Frog-leg radiograph of the hip shows the screws in position. The fracture is in satisfactory alignment and appears well healed. The femoral head is intact with no evidence of sclerosis or collapse. (b) Coronal T1-weighted spin-echo MR image (600/15) through the hip shows extensive artifact, which precludes evaluation of the femoral head and joint space. (c) Corresponding MR image obtained with the MARS technique shows diminished artifact. A focus of avascular necrosis is clearly seen in the superior aspect of the femoral head (arrow); this finding presumably accounted for the patient's persistent and increasing hip pain despite the normal radiographic appearance.

View larger version (175K): [in this window] [in a new window] [Download PPT slide]   Figure 3b.   Avascular necrosis in a 56-year-old man with hip pain 4 months after a femoral neck fracture, which was transfixed with three screws. (a) Frog-leg radiograph of the hip shows the screws in position. The fracture is in satisfactory alignment and appears well healed. The femoral head is intact with no evidence of sclerosis or collapse. (b) Coronal T1-weighted spin-echo MR image (600/15) through the hip shows extensive artifact, which precludes evaluation of the femoral head and joint space. (c) Corresponding MR image obtained with the MARS technique shows diminished artifact. A focus of avascular necrosis is clearly seen in the superior aspect of the femoral head (arrow); this finding presumably accounted for the patient's persistent and increasing hip pain despite the normal radiographic appearance.

View larger version (172K): [in this window] [in a new window] [Download PPT slide]   Figure 3c.   Avascular necrosis in a 56-year-old man with hip pain 4 months after a femoral neck fracture, which was transfixed with three screws. (a) Frog-leg radiograph of the hip shows the screws in position. The fracture is in satisfactory alignment and appears well healed. The femoral head is intact with no evidence of sclerosis or collapse. (b) Coronal T1-weighted spin-echo MR image (600/15) through the hip shows extensive artifact, which precludes evaluation of the femoral head and joint space. (c) Corresponding MR image obtained with the MARS technique shows diminished artifact. A focus of avascular necrosis is clearly seen in the superior aspect of the femoral head (arrow); this finding presumably accounted for the patient's persistent and increasing hip pain despite the normal radiographic appearance.

View larger version (107K): [in this window] [in a new window] [Download PPT slide]   Figure 4a.   Persistent drainage from the lateral aspect of the superior thigh in a 68-year-old woman 5 years after femoral hemiarthroplasty. (a) Axial T1-weighted spin-echo MR image (800/12) shows considerable artifact around the prosthesis. (b) Corresponding MR image obtained with the MARS technique shows marked reduction of artifact, thus allowing much better assessment of the surrounding soft tissues. Note that the anterior and posterior columns of the acetabulum can be readily visualized, whereas they were completely obscured on the conventional MR image (a). (c) MR image obtained with the MARS technique several centimeters inferior to b shows the site of the drainage, which can be seen extending laterally in a low-signal-intensity track (arrows) traversing the higher-signal-intensity fat. No abnormal soft-tissue collection is apparent directly around the hip joint because spontaneous decompression has occurred. Note that the soft tissues of the thigh around the femoral component are well seen.

View larger version (113K): [in this window] [in a new window] [Download PPT slide]   Figure 4b.   Persistent drainage from the lateral aspect of the superior thigh in a 68-year-old woman 5 years after femoral hemiarthroplasty. (a) Axial T1-weighted spin-echo MR image (800/12) shows considerable artifact around the prosthesis. (b) Corresponding MR image obtained with the MARS technique shows marked reduction of artifact, thus allowing much better assessment of the surrounding soft tissues. Note that the anterior and posterior columns of the acetabulum can be readily visualized, whereas they were completely obscured on the conventional MR image (a). (c) MR image obtained with the MARS technique several centimeters inferior to b shows the site of the drainage, which can be seen extending laterally in a low-signal-intensity track (arrows) traversing the higher-signal-intensity fat. No abnormal soft-tissue collection is apparent directly around the hip joint because spontaneous decompression has occurred. Note that the soft tissues of the thigh around the femoral component are well seen.

View larger version (86K): [in this window] [in a new window] [Download PPT slide]   Figure 4c.   Persistent drainage from the lateral aspect of the superior thigh in a 68-year-old woman 5 years after femoral hemiarthroplasty. (a) Axial T1-weighted spin-echo MR image (800/12) shows considerable artifact around the prosthesis. (b) Corresponding MR image obtained with the MARS technique shows marked reduction of artifact, thus allowing much better assessment of the surrounding soft tissues. Note that the anterior and posterior columns of the acetabulum can be readily visualized, whereas they were completely obscured on the conventional MR image (a). (c) MR image obtained with the MARS technique several centimeters inferior to b shows the site of the drainage, which can be seen extending laterally in a low-signal-intensity track (arrows) traversing the higher-signal-intensity fat. No abnormal soft-tissue collection is apparent directly around the hip joint because spontaneous decompression has occurred. Note that the soft tissues of the thigh around the femoral component are well seen.

View larger version (92K): [in this window] [in a new window] [Download PPT slide]   Figure 5a.   Recurrent synovial osteochondromatosis in an obese 50-year-old woman who underwent total hip arthroplasty 2 years earlier at the time of complete synovectomy for synovial osteochondromatosis. She presented with gradually increasing hip pain. (a) Anteroposterior radiograph is unremarkable. (b) CT scan through the hip joint is of poor quality, with marked beam hardening and spray artifact precluding adequate evaluation of the periarticular soft tissues. (c) Axial T1-weighted spin-echo MR image (600/15) through the hip shows extensive artifact. Although some bowing of the soft tissues medial to the hip can be seen (arrows), a discrete mass is hard to appreciate. (d) Corresponding MR image obtained with the MARS technique clearly shows a large soft-tissue mass (arrows). The mass, which was later diagnosed as recurrent synovial osteochondromatosis, compresses the bladder, vagina, and rectum and extends posteriorly to almost come in contact with the gluteus maximus muscle. Note that the cup liner can even be partially visualized within the joint.

View larger version (147K): [in this window] [in a new window] [Download PPT slide]   Figure 5b.   Recurrent synovial osteochondromatosis in an obese 50-year-old woman who underwent total hip arthroplasty 2 years earlier at the time of complete synovectomy for synovial osteochondromatosis. She presented with gradually increasing hip pain. (a) Anteroposterior radiograph is unremarkable. (b) CT scan through the hip joint is of poor quality, with marked beam hardening and spray artifact precluding adequate evaluation of the periarticular soft tissues. (c) Axial T1-weighted spin-echo MR image (600/15) through the hip shows extensive artifact. Although some bowing of the soft tissues medial to the hip can be seen (arrows), a discrete mass is hard to appreciate. (d) Corresponding MR image obtained with the MARS technique clearly shows a large soft-tissue mass (arrows). The mass, which was later diagnosed as recurrent synovial osteochondromatosis, compresses the bladder, vagina, and rectum and extends posteriorly to almost come in contact with the gluteus maximus muscle. Note that the cup liner can even be partially visualized within the joint.

View larger version (127K): [in this window] [in a new window] [Download PPT slide]   Figure 5c.   Recurrent synovial osteochondromatosis in an obese 50-year-old woman who underwent total hip arthroplasty 2 years earlier at the time of complete synovectomy for synovial osteochondromatosis. She presented with gradually increasing hip pain. (a) Anteroposterior radiograph is unremarkable. (b) CT scan through the hip joint is of poor quality, with marked beam hardening and spray artifact precluding adequate evaluation of the periarticular soft tissues. (c) Axial T1-weighted spin-echo MR image (600/15) through the hip shows extensive artifact. Although some bowing of the soft tissues medial to the hip can be seen (arrows), a discrete mass is hard to appreciate. (d) Corresponding MR image obtained with the MARS technique clearly shows a large soft-tissue mass (arrows). The mass, which was later diagnosed as recurrent synovial osteochondromatosis, compresses the bladder, vagina, and rectum and extends posteriorly to almost come in contact with the gluteus maximus muscle. Note that the cup liner can even be partially visualized within the joint.

View larger version (135K): [in this window] [in a new window] [Download PPT slide]   Figure 5d.   Recurrent synovial osteochondromatosis in an obese 50-year-old woman who underwent total hip arthroplasty 2 years earlier at the time of complete synovectomy for synovial osteochondromatosis. She presented with gradually increasing hip pain. (a) Anteroposterior radiograph is unremarkable. (b) CT scan through the hip joint is of poor quality, with marked beam hardening and spray artifact precluding adequate evaluation of the periarticular soft tissues. (c) Axial T1-weighted spin-echo MR image (600/15) through the hip shows extensive artifact. Although some bowing of the soft tissues medial to the hip can be seen (arrows), a discrete mass is hard to appreciate. (d) Corresponding MR image obtained with the MARS technique clearly shows a large soft-tissue mass (arrows). The mass, which was later diagnosed as recurrent synovial osteochondromatosis, compresses the bladder, vagina, and rectum and extends posteriorly to almost come in contact with the gluteus maximus muscle. Note that the cup liner can even be partially visualized within the joint.

Knee
Many patients have implanted metal staples, pins, or screws adjacent to the knee joint, often as a result of previous fracture or anterior cruciate ligament reconstruction surgery. These patients may reinjure the knee and require further assessment with MR imaging for a possible recurrent internal derangement. The MARS technique is helpful in minimizing the amount of metal artifact in these patients, thus allowing visualization of important structures (Figs 6, 7). This technique is particularly helpful in patients who have undergone anterior cruciate ligament reconstruction, especially if large interference screws were used. Even in patients who have undergone total knee arthroplasty, the MARS technique can significantly improve visualization of the periprosthetic bone and soft-tissue structures such as popliteal cysts or the posterior cruciate ligament (if a ligament-sparing prosthesis was used) (3,8) (Figs 8, 9).

View larger version (116K): [in this window] [in a new window] [Download PPT slide]   Figure 6a.   Bone staples and metallic crystals in a 36-year-old man 4 years after anterior cruciate ligament reconstruction with a hamstring tendon and an "over the top" technique (ie, stapling the cranial aspect of the neoligament to the distal femur). (a) Sagittal T1-weighted spin-echo MR image (500/14) clearly shows artifact from bone staples at the top of the image. In addition, numerous small areas of low signal intensity can be seen in the posterior aspect of the suprapatellar bursa (arrows), which represent small metallic crystals deposited at the time of previous surgery (these could not be seen on radiographs). (b) Corresponding MR image obtained with the MARS technique shows marked reduction of artifact from both bone staples and metallic crystals.

View larger version (119K): [in this window] [in a new window] [Download PPT slide]   Figure 6b.   Bone staples and metallic crystals in a 36-year-old man 4 years after anterior cruciate ligament reconstruction with a hamstring tendon and an "over the top" technique (ie, stapling the cranial aspect of the neoligament to the distal femur). (a) Sagittal T1-weighted spin-echo MR image (500/14) clearly shows artifact from bone staples at the top of the image. In addition, numerous small areas of low signal intensity can be seen in the posterior aspect of the suprapatellar bursa (arrows), which represent small metallic crystals deposited at the time of previous surgery (these could not be seen on radiographs). (b) Corresponding MR image obtained with the MARS technique shows marked reduction of artifact from both bone staples and metallic crystals.

View larger version (146K): [in this window] [in a new window] [Download PPT slide]   Figure 7a.   Ligament tear in a 34-year-old woman who had undergone two anterior cruciate ligament reconstructions. She presented with symptoms suggestive of neoligament failure. (a) Lateral radiograph shows two bone staples in the distal femur and an unusual wide tunnel with sclerotic margins in the anterior aspect of the superior tibia. (b) Sagittal gradient-echo MR image (400/14, 30° flip angle) shows marked susceptibility artifact obscuring the entire intraarticular portion of the neoligament. (c) Sagittal T1-weighted spin-echo MR image (400/12) clearly shows the portion of the neoligament within the osseous tunnel (straight arrow). However, metallic crystals obscure the more cranial portion (curved arrow). (d) Corresponding MR image obtained with the MARS technique shows reduction of artifact, thus allowing more of the ligament to be assessed. The cranial portion of the ligament is noted to be torn; this finding was confirmed at surgery.

View larger version (185K): [in this window] [in a new window] [Download PPT slide]   Figure 7b.   Ligament tear in a 34-year-old woman who had undergone two anterior cruciate ligament reconstructions. She presented with symptoms suggestive of neoligament failure. (a) Lateral radiograph shows two bone staples in the distal femur and an unusual wide tunnel with sclerotic margins in the anterior aspect of the superior tibia. (b) Sagittal gradient-echo MR image (400/14, 30° flip angle) shows marked susceptibility artifact obscuring the entire intraarticular portion of the neoligament. (c) Sagittal T1-weighted spin-echo MR image (400/12) clearly shows the portion of the neoligament within the osseous tunnel (straight arrow). However, metallic crystals obscure the more cranial portion (curved arrow). (d) Corresponding MR image obtained with the MARS technique shows reduction of artifact, thus allowing more of the ligament to be assessed. The cranial portion of the ligament is noted to be torn; this finding was confirmed at surgery.

View larger version (167K): [in this window] [in a new window] [Download PPT slide]   Figure 7c.   Ligament tear in a 34-year-old woman who had undergone two anterior cruciate ligament reconstructions. She presented with symptoms suggestive of neoligament failure. (a) Lateral radiograph shows two bone staples in the distal femur and an unusual wide tunnel with sclerotic margins in the anterior aspect of the superior tibia. (b) Sagittal gradient-echo MR image (400/14, 30° flip angle) shows marked susceptibility artifact obscuring the entire intraarticular portion of the neoligament. (c) Sagittal T1-weighted spin-echo MR image (400/12) clearly shows the portion of the neoligament within the osseous tunnel (straight arrow). However, metallic crystals obscure the more cranial portion (curved arrow). (d) Corresponding MR image obtained with the MARS technique shows reduction of artifact, thus allowing more of the ligament to be assessed. The cranial portion of the ligament is noted to be torn; this finding was confirmed at surgery.

View larger version (158K): [in this window] [in a new window] [Download PPT slide]   Figure 7d.   Ligament tear in a 34-year-old woman who had undergone two anterior cruciate ligament reconstructions. She presented with symptoms suggestive of neoligament failure. (a) Lateral radiograph shows two bone staples in the distal femur and an unusual wide tunnel with sclerotic margins in the anterior aspect of the superior tibia. (b) Sagittal gradient-echo MR image (400/14, 30° flip angle) shows marked susceptibility artifact obscuring the entire intraarticular portion of the neoligament. (c) Sagittal T1-weighted spin-echo MR image (400/12) clearly shows the portion of the neoligament within the osseous tunnel (straight arrow). However, metallic crystals obscure the more cranial portion (curved arrow). (d) Corresponding MR image obtained with the MARS technique shows reduction of artifact, thus allowing more of the ligament to be assessed. The cranial portion of the ligament is noted to be torn; this finding was confirmed at surgery.

View larger version (96K): [in this window] [in a new window] [Download PPT slide]   Figure 8a.   Normal findings in an asymptomatic 59-year-old man 2 years after tricompartment total knee arthroplasty. (a) Lateral radiograph is unremarkable. (b, c) Sagittal (b) and axial (c) T1-weighted spin-echo MR images (500/12) show large areas of signal void and distortion, as well as accompanying regions of high signal intensity. (d, e) Corresponding sagittal (d) and axial (e) MR images obtained with the MARS technique show dramatic reduction of artifact, thus allowing better resolution of the soft tissues adjacent to the arthroplasty. The improved resolution is especially evident on the axial image (e).

View larger version (124K): [in this window] [in a new window] [Download PPT slide]   Figure 8b.   Normal findings in an asymptomatic 59-year-old man 2 years after tricompartment total knee arthroplasty. (a) Lateral radiograph is unremarkable. (b, c) Sagittal (b) and axial (c) T1-weighted spin-echo MR images (500/12) show large areas of signal void and distortion, as well as accompanying regions of high signal intensity. (d, e) Corresponding sagittal (d) and axial (e) MR images obtained with the MARS technique show dramatic reduction of artifact, thus allowing better resolution of the soft tissues adjacent to the arthroplasty. The improved resolution is especially evident on the axial image (e).

View larger version (133K): [in this window] [in a new window] [Download PPT slide]   Figure 8c.   Normal findings in an asymptomatic 59-year-old man 2 years after tricompartment total knee arthroplasty. (a) Lateral radiograph is unremarkable. (b, c) Sagittal (b) and axial (c) T1-weighted spin-echo MR images (500/12) show large areas of signal void and distortion, as well as accompanying regions of high signal intensity. (d, e) Corresponding sagittal (d) and axial (e) MR images obtained with the MARS technique show dramatic reduction of artifact, thus allowing better resolution of the soft tissues adjacent to the arthroplasty. The improved resolution is especially evident on the axial image (e).

View larger version (145K): [in this window] [in a new window] [Download PPT slide]   Figure 8d.   Normal findings in an asymptomatic 59-year-old man 2 years after tricompartment total knee arthroplasty. (a) Lateral radiograph is unremarkable. (b, c) Sagittal (b) and axial (c) T1-weighted spin-echo MR images (500/12) show large areas of signal void and distortion, as well as accompanying regions of high signal intensity. (d, e) Corresponding sagittal (d) and axial (e) MR images obtained with the MARS technique show dramatic reduction of artifact, thus allowing better resolution of the soft tissues adjacent to the arthroplasty. The improved resolution is especially evident on the axial image (e).

View larger version (153K): [in this window] [in a new window] [Download PPT slide]   Figure 8e.   Normal findings in an asymptomatic 59-year-old man 2 years after tricompartment total knee arthroplasty. (a) Lateral radiograph is unremarkable. (b, c) Sagittal (b) and axial (c) T1-weighted spin-echo MR images (500/12) show large areas of signal void and distortion, as well as accompanying regions of high signal intensity. (d, e) Corresponding sagittal (d) and axial (e) MR images obtained with the MARS technique show dramatic reduction of artifact, thus allowing better resolution of the soft tissues adjacent to the arthroplasty. The improved resolution is especially evident on the axial image (e).

View larger version (123K): [in this window] [in a new window] [Download PPT slide]   Figure 9a.   Multiple loose bodies within a Baker cyst in a 62-year-old man who had undergone total knee arthroplasty. (a) Lateral radiograph shows a knee arthroplasty with accompanying joint effusion and several slightly calcified areas of increased opacity (arrows) at the posterior aspect of the joint. (b) Sagittal T1-weighted spin-echo MR image (500/15) shows extensive artifact involving the metal components. At the posterior aspect of the joint, some high-signal-intensity lesions (arrows) are noted around the hamstring and gastrocnemius tendons. (c) Corresponding MR image obtained with the MARS technique shows greatly decreased artifact, thus allowing visualization of the high-signal-intensity fat within the marrow in the ossified loose bodies (arrows).

View larger version (108K): [in this window] [in a new window] [Download PPT slide]   Figure 9b.   Multiple loose bodies within a Baker cyst in a 62-year-old man who had undergone total knee arthroplasty. (a) Lateral radiograph shows a knee arthroplasty with accompanying joint effusion and several slightly calcified areas of increased opacity (arrows) at the posterior aspect of the joint. (b) Sagittal T1-weighted spin-echo MR image (500/15) shows extensive artifact involving the metal components. At the posterior aspect of the joint, some high-signal-intensity lesions (arrows) are noted around the hamstring and gastrocnemius tendons. (c) Corresponding MR image obtained with the MARS technique shows greatly decreased artifact, thus allowing visualization of the high-signal-intensity fat within the marrow in the ossified loose bodies (arrows).

View larger version (116K): [in this window] [in a new window] [Download PPT slide]   Figure 9c.   Multiple loose bodies within a Baker cyst in a 62-year-old man who had undergone total knee arthroplasty. (a) Lateral radiograph shows a knee arthroplasty with accompanying joint effusion and several slightly calcified areas of increased opacity (arrows) at the posterior aspect of the joint. (b) Sagittal T1-weighted spin-echo MR image (500/15) shows extensive artifact involving the metal components. At the posterior aspect of the joint, some high-signal-intensity lesions (arrows) are noted around the hamstring and gastrocnemius tendons. (c) Corresponding MR image obtained with the MARS technique shows greatly decreased artifact, thus allowing visualization of the high-signal-intensity fat within the marrow in the ossified loose bodies (arrows).

View larger version (90K): [in this window] [in a new window] [Download PPT slide]   Figure 10a.   Vertebral displacement in a 39-year-old man who underwent posterior fixation for progressive back pain secondary to spondylolysis and spondylolisthesis. (a) Lateral radiograph shows the posterior fixation device extending from L4 to S1 with grade II anterior displacement of L5 over S1. (b) Sagittal T1-weighted spin-echo MR image (500/15) shows geometric distortion. (c) Corresponding MR image obtained with the MARS technique shows marked improvement in the degree of distortion.

View larger version (109K): [in this window] [in a new window] [Download PPT slide]   Figure 10b.   Vertebral displacement in a 39-year-old man who underwent posterior fixation for progressive back pain secondary to spondylolysis and spondylolisthesis. (a) Lateral radiograph shows the posterior fixation device extending from L4 to S1 with grade II anterior displacement of L5 over S1. (b) Sagittal T1-weighted spin-echo MR image (500/15) shows geometric distortion. (c) Corresponding MR image obtained with the MARS technique shows marked improvement in the degree of distortion.

View larger version (115K): [in this window] [in a new window] [Download PPT slide]   Figure 10c.   Vertebral displacement in a 39-year-old man who underwent posterior fixation for progressive back pain secondary to spondylolysis and spondylolisthesis. (a) Lateral radiograph shows the posterior fixation device extending from L4 to S1 with grade II anterior displacement of L5 over S1. (b) Sagittal T1-weighted spin-echo MR image (500/15) shows geometric distortion. (c) Corresponding MR image obtained with the MARS technique shows marked improvement in the degree of distortion.

View larger version (174K): [in this window] [in a new window] [Download PPT slide]   Figure 11a.   Anterior fixation of the cervical spine from C3 to C5 in a 74-year-old man. (a) Sagittal T1-weighted spin-echo MR images (500/15) show artifact produced by the screws transfixing the anterior plate. The artifact extends into the anterior aspect of the spinal canal and distorts the anterior contours of the cord. (b) Corresponding MR images obtained with the MARS technique (500/12) show marked improvement in the demonstration of the anterior spinal canal.

View larger version (180K): [in this window] [in a new window] [Download PPT slide]   Figure 11b.   Anterior fixation of the cervical spine from C3 to C5 in a 74-year-old man. (a) Sagittal T1-weighted spin-echo MR images (500/15) show artifact produced by the screws transfixing the anterior plate. The artifact extends into the anterior aspect of the spinal canal and distorts the anterior contours of the cord. (b) Corresponding MR images obtained with the MARS technique (500/12) show marked improvement in the demonstration of the anterior spinal canal.

View larger version (137K): [in this window] [in a new window] [Download PPT slide]   Figure 12a.   Normal findings in a 35-year-old man who underwent wrist fusion with multiple bone staples. (a) Coronal T1-weighted spin-echo MR image (400/12) shows multiple areas of artifact with distortion and poor visualization of carpal bones and joints. (b) Corresponding MR image obtained with the MARS technique shows some increased blurring but a dramatic reduction in distortion and improved demonstration of the bone around the midcarpal space.

View larger version (133K): [in this window] [in a new window] [Download PPT slide]   Figure 12b.   Normal findings in a 35-year-old man who underwent wrist fusion with multiple bone staples. (a) Coronal T1-weighted spin-echo MR image (400/12) shows multiple areas of artifact with distortion and poor visualization of carpal bones and joints. (b) Corresponding MR image obtained with the MARS technique shows some increased blurring but a dramatic reduction in distortion and improved demonstration of the bone around the midcarpal space.

	Current Status of Technique

Top Abstract Introduction Description of Technique Clinical Experience Current Status of Technique Conclusion References

The principal disadvantage of the MARS sequence is an inherent element of slight blurring, which we hope to be able to minimize in the future. Geometric distortion is also not completely suppressed, although notably reduced. Owing to the larger frequency bandwidths, the signal-to-noise ratio with the MARS technique is generally lower than with conventional sequences. The decrease in signal-to-noise ratio can be overcome by increasing the number of signals acquired at the cost of increased imaging time. At present, this technique has been developed for use with only T1-weighted sequences. The technique is not commercially available at this time.

	Conclusion

Top Abstract Introduction Description of Technique Clinical Experience Current Status of Technique Conclusion References

 
The MARS technique appears to hold great promise for significantly reducing both signal void and geometric distortion surrounding metallic implants in the musculoskeletal system.

	Footnotes

 
Abbreviations: FOV  = field of view, MARS  = metal artifact reduction sequence

	References

Top Abstract Introduction Description of Technique Clinical Experience Current Status of Technique Conclusion References

 

1. Czervionke LF, Daniels DL, Wehrli FW, et al. Magnetic susceptibility artifacts in gradient-recalled echo MR imaging. AJNR Am J Neuroradiol 1988; 9:1149-1155.[Abstract]
2. Tartaglino LM, Flanders AE, Vinitski S, Friedman DP. Metallic artifacts on MR images of the postoperative spine: reduction with fast spin-echo techniques. Radiology 1994; 190:565-569.[Abstract/Free Full Text]
3. Shellock FG, Mink JH, Curtin S, Friedman MJ. MR imaging and metallic implants for anterior cruciate ligament reconstruction: assessment of ferromagnetism and artifact. J Magn Reson Imaging 1992; 2:225-228.[Medline]
4. Heindel W, Friedman G, Bunke J, Thomas B, Firsching R, Ernestus RI. Artifacts in MR imaging after surgical intervention. J Comput Assist Tomogr 1986; 10:596-599.[Medline]
5. Laakman RW, Kaufman B, Han JS, et al. MR imaging in patients with metallic implants. Radiology 1985; 157:711-714.[Abstract/Free Full Text]
6. Bellon EM, Haacke EM, Coleman PE, Sacco DC, Steiger DA, Gangarosa RE. MR artifacts: a review. AJR Am J Roentgenol 1986; 147:1271-1281.[Abstract/Free Full Text]
7. Eustace S, Hernan J, Goldberg R, et al. A comparison of conventional spin-echo and turbo spin-echo imaging of soft tissues adjacent to orthopedic hardware. AJR Am J Roentgenol 1998; 170:455-458.[Free Full Text]
8. Eustace S, Goldberg R, Williamson D, et al. MR imaging of soft tissues adjacent to orthopaedic hardware: techniques to minimize susceptibility artifact. Clin Radiol 1997; 52:589-594.[Medline]
9. Petersilge CA, Lewin JS, Duerk JL, Yoo JU, Ghaneyem AJ. Optimizing imaging parameters for MR evaluation of the spine with titanium pedicle screws. AJR Am J Roentgenol 1996; 166:1213-1218.[Abstract/Free Full Text]
10. Törmänen J, Tervonen O, Koivula A, Junila J, Suramo I. Image technique optimization in MR imaging of a titanium alloy joint prosthesis. J Magn Reson Imaging 1996; 6:805-811.[Medline]
11. Farahani K, Sinha U, Sinha S, Chiu LCL, Lufkin RB. Effect of field strength on susceptibility artifacts in magnetic resonance imaging. Comput Med Imaging Graph 1990; 14:409-413.[Medline]
12. Suh JS, Jeong EK, Shin KH, et al. Minimizing artifacts caused by metallic implants at MR imaging: experimental and clinical studies. AJR Am J Roentgenol 1998; 171:1207-1213.[Abstract/Free Full Text]
13. Rupp R, Ebraheim NA, Savolaine ER, Jackson WT. Magnetic resonance imaging evaluation of the spine with metal implants. Spine 1993; 18:379-385.[Medline]
14. Ebraheim NA, Savolaine ER, Zeiss J, Jackson WT. Titanium hip implants for improved magnetic resonance and computed tomography examinations. Clin Orthop 1992; 275:194-198.
15. Frazzini VI, Kagetsu NJ, Johnson CE, Destian S. Internally stabilized spine: optimal choice of frequency-encoding gradient direction during MR imaging minimizes susceptibility artifact from titanium vertebral body screws. Radiology 1997; 204:268-272.[Abstract/Free Full Text]
16. McGowan AJ, MacKay AL, Xiang QS, Connell DG, Janzen DL, Munk PL. Reduction of image distortion in the presence of metal (abstr) Proceedings of the Fifth Meeting of the International Society for Magnetic Resonance in Medicine. Berkeley, Calif: International Society for Magnetic Resonance in Medicine, 1997; 1973.
17. Chang SD, Janzen DL, Sallomi DF, Munk PL. MRI of spinal hardware: comparison of conventional T1-weighted sequence with a new metal artifact reduction sequence (abstr). Radiology 1998; 209(P):614.
18. Olsen RV, Janzen DL, Sallomi DF, Munk PL, MacKay AL, Xiang QS. Clinical application of a new method of metal artifact suppression on MR imaging (abstr). Radiology 1998; 209(P):612.
19. Cho ZH, Kim DJ, Kim YK. Total inhomogeneity correction including chemical shifts and susceptibility by view angle tilting. Med Phys 1988; 15:7-11.[Medline]

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