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Magnetic resonance imaging: Preliminary experience in orthopedic radiology
MugnelicResononce Printed in the USA.
Imaging. Vol. 2, pp. All rights reserved.
41-52,
1984 Copyright
0730.725X/84$3.00 0 1984 Pergamon
+ .&I Press Ltd.
l Research Article MAGNETIC RESONANCE IMAGING: PRELIMINARY EXPERIENCE IN ORTHOPEDIC RADIOLOGY THOMAS Department
of Diagnostic
Radiology,
H. BERQUIST, M.D.
Mayo Clinic and Mayo Foundation,
INTRODUCTION
METHODS The main objective of the study was to determine which patients were suitable for evaluation with NMR imaging (e.g., trauma patients with casts, patients with metal fixation devices or prostheses, patients with bone and soft tissue neoplasms, patients with soft tissue injuries). This necessitated evaluation of normal patients for anatomic definition and patients with known pathologic conditions. Determination of which pulse sequences, scan planes, and slice thicknesses were most helpful was also necessary. The images were obtained with a 1.5 K Gauss resistive magnet that operates at a resonance fre-
6/21/83;
ACCEPTED
7/
Minnesota
55905
quency of 6.4 MHz (Picker International, Highland Heights, Ohio). All images were obtained with the use of the head coil (30 x 32) and an image matrix of 256 x 256. Slide thicknesses of 1 and 2 cm were available at the time of this study. The currently available software allowed spin-echo (SE), repeated free induction decay (FID), and inversion recovery (IR) pulse sequences. Only single-slice reconstruction was available in the early operating stages of this unit; therefore, it was important to evaluate the imaging qualities of each sequence and to determine the time required for the various sequences (Table 1). Imaging was typically performed with the use of 180 views and a scan angle of 180” which resulted in a pulse sequence occurring at lo intervals (Figure 1). If only 90 or 45 views were obtained, data were collected every 2O and at every 4O respectively (Figure 1). Reduction of the number of views to 90 and 45 reduced the scan time by factors of 2 and 4 respectively. Figure 2 demonstrates the images obtained with the use of these parameters. The signalto-noise ratio changed significantly as the number of views decreased. The ability of 1 and 2 cm slice thicknesses to reveal lesions of the same size were also evaluated. Small high-proton density disks 0.5 cm, 1 cm, and 2 cm in diameter were studied in phantoms and attached to the calf. The detectability of these structures allowed us to determine whether 1 cm slice thicknesses were necessary routinely. If 2 cm slice thicknesses were equal to 1 cm slice thicknesses in detection and resolution, then considerable time could be saved by studying patients with the single-slice reconstruction. Another preliminary step was to evaluate orthopedic appliances and surgical clips. Magnetic properties and image degradation of the commonly used stainless steel plates, Vitallium plates and prostheses were
The clinical applications of nuclear magnetic resonance (N MR) imaging are becoming more numerous. The recent literature cortains many articles concerning imaging of the central nervous system and other parts of the body.‘.5,8.9,“.‘3 Currently available techniques involve primarily proton imaging; however, NMR permits the study of other nuclei as well (14N, 31 r, 17,, 13c, 23,,).’ Clinical investigations of NMR have not yet determined the most efficient method of imaging patients. Questions regarding the most useful pulse sequences for a given situation, or even which type of instrument (resistive, superconducting, or permanent magnets) is best suited for clinical imaging, remain unanswered. As a result, each investigator must attempt to answer these questions in determining the utility of NMR for clinical imaging. We are currently evaluating a resistive system that operates at 1.5 K Gauss (0.15 Tesla) and a resonance frequency of 6.4 MHz. The following data were obtained during a preliminary study that was initiated to determine the usefulness of this unit in orthopedic imaging.
RECEIVED
Rochester,
1 z/83.
41
42
Magnetic Resonance Imaging 0 Volume 2, Number Table
1. NMR
pulse sequences:
Number
1, 1984
of views vs scan time Scan time (s)
Pulse Sequence*
180 views
Free induction decay (90° pulse) Sequence Interval A 500 ms B 1,000 ms C 1,500 ms Spin-Echo (90”-tau- 180”) Interval tau 20 500 ms 40 60 80 A series 1,000 ms Inversion recovery (180”-tau-90”) Interval tau 200 1,000 ms 300 400 500 *Reconstruction
90 views
45 views
210
4s 90 13s
22 45 68
90
45
22
180
90
45
180
90
45
90
180
time: 30 s for all pulse sequences
Surgical clips were also studied in this regard. Metal objects were suspended in the magnet at the center of the field by means of a fine string. Position changes (motion and torque) were carefully assessed. We studied the effect of image degradation in volunteers who had the metal components attached to the lower leg near the anterior tibia. Patients with plaster and fiberglass casts were also studied to determine whether these structures affected image quality. Normal healthy volunteers were studied to determine whether anatomic structures could be adequately visualized with the resistive system. Coronal, axial, and sagittal scans of the knee, ankle, foot, and spine checked.
Scan Angle 180 Views: A = 180
Fig. 1. Diagram of scan angles and number of views. Angles can be selected up to 360” with up to 360 views.
were chosen for this initial evaluation. Finally, patients with known pathologic conditions were then studied to compare the results of NMR imaging with computed tomography (CT), routine radiography, and other available imaging procedures.
RESULTS Imuging
variables.
Evaluation of imaging variables revealed that for orthopedic purposes the SE 1,000/20 or 500/20 (90’ pulse-20 ms tau-180° pulse at a 500 or 1,000 ms interval) provided the optimal anatomic definition. Single-slice formatting required 2 minutes for the 500 ms sequence and 3.5 minutes for the 1,000 ms sequence. SE 500/20 images are satisfactory in most situations and result in reduced scan time (Table 1). Reduction of the number of views is also useful in rapid localization of lesions or anatomic areas and allows one to quickly define the area of interest for further sequence studies or to assist in alignment of coronal or sagittal imaging. Because image quality is reduced if the number of views is reduced (Figure 2). the usefulness for diagnostic purposes is limited. Moon et al.* also reported that SE sequences were useful in orthopedic imaging. In their study, two-dimensional Fourier transformation was used. New software for the resistive system at our institution will also provide this option in addition to multislice reconstruction. These changes will greatly improve image quality and reduce scan time. The pulse sequences best suited for various patho-
MRI: Preliminary experience in orthopedic
radiology
0 T. H. BERQIJIST
43
Fig. 2. Axial images of the calf. (A) IR 1,OOO/SOOwith 180 views at 180°. (B) IR 1,OOO/SOOwith 90 views at 180“. Image quality is decreased and noise is increased. V, Vitallium plate; S, stainless steel plate. (C) IR l,OOO/SOO with 45 views at 180” results in significant image degradation; however, images are satisfactory for localization purposes.
logic studies have not yet been determined. It is hoped that this will become more obvious with experience. Presently multiple sequences are necessary for the evaluation of bone and soft tissue lesions. The slice thickness obtained affects the image quality and examination time. Thicker slices naturally allow more rapid evaluation of a given area. Our initial experience demonstrated that 2 cm and 1 cm slices did not differ in ability to reveal lesions of various sizes (5 mm, 10 mm, and 20 mm). The signal-to-noise ratio was also improved with thicker slices. Therefore, at least for certain cases, contiguous 2 cm slices or 2 cm slices obtained at 15 mm intervals are all that are
required; the result is quality images with reduced examination time. As further advances are made in software, these techniques will obviously be improved. Orthopedic appliances and casts Radiologists are well aware of the artifacts created by metal objects on CT images (Figure 3). Two factors required consideration in relation to metal and NMR imaging. First, the ferromagnetic properties of orthopedic appliances needed to be studied. Second, the degree of image degradation created by these structures required evaluation. There are numerous classifications of stainless
44
Magnetic
Resonance
Imaging
Fig. 3. (A) Scout view of CT scan of lumbar spine with Harrington rod fixation of L2 fracture. (B) Axial scan demonstrates body fracture (arrows) and fragments in spinal
canal. Note starburst artifact created by metal rods.
0 Volume 2, Number
I, 1984
steel.‘*” Manufacturers of orthopedic devices use various combinations and amounts of cobalt, molybdenum, nickel, manganese, chromium and tungsten. The appliances used at each institution should be carefully checked before one examines patients who have orthopedic devices in place. We studied all of the orthopedic plates, screws, prostheses, and surgical clips commonly used at our institution and found that none exhibited ferromagnetic properties in the magnet. Changing gradients and radio frequency (RF) pulses have also been evaluated. Davis et ~1.~ studied the effects on tissue of small metal clips and prosthetic devices when exposed to magnet fields with varying gradients and RF pulses. No temperature changes were noted with small metal objects; however, significant heating of hip prostheses could be demonstrated in saline baths. We were unable to demonstrate temperature changes with large prostheses (Austin-Moore in saline) even with changing gradients.’ Metallic devices not commonly used in orthopedics have demonstrated magnetic properties. Mayfield and Heifetz aneurysm clips are ferromagnetic. New et ~1.” demonstrated significant torquing of these clips and using NMR, could even cause them to release from arteries of dogs. In an attempt to evaluate the effect of metal on imaging, we attached stainless steel and Vitallium plates (15 x 0.5 x 1.5 cm) to the lower legs adjacent to the tibia. Figure 4 demonstrates the artifact created with CT in this situation. NMR images were obtained in all planes (axial, coronal, and sagittal) and at varying pulse sequences. Scan angles were varied to determine if this had any effect on the appearance of the artifact (Figure 5). Pulse sequences, location, and the type (ferromagnetic or non-ferromagnetic) of metal present, are all considerations in the evaluation
Fig. 4. CT scan of lower legs with stainless steel and Vitallium plate; S-stainless steel plate. Note metal artifact.
plates near tibia. T, tibia; F, fibula; V, Vitallium
MRI: Preliminary
Fig. from (D), with
experience
in orthopedic
radiology
0 T. H. BERQUISI
5. (A) Scan angles beginning at 0” (A) and 90” (B) with metal in upper quadrant (rectangle). FID axial scan O” (left arrow). Note small artifact due to metal near tibia (right arrow). FID images begun at 90” (C), at 180” and at 360° and 360 views (V). (E) do not significantly affect artifact. The plate creates little if any artifact SE 1.OOO/ZO sequence with 180 views at 180” (F).
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Magnetic Resonance Imaging 0 Volume 2, Number I, 1984
Fig. 6. (A) Anteroposterior view of ankle with internal fixation of distal tibia and fibula with metal plates, screws, and K wires. (B) Coronal scan of ankle shows significant artifact (arrows) due to large amount of metal present. (C) Axial scan demonstrates tibia (T) and fibula (F) but is of little clinical value.
MRI: Preliminary
experience
in orthopedic
radiology
0 T. H. BERQUIST
47
of orthopedic patients. The scan angle would appear to have no effect on artifact formation or removal. In some cases (Figure 5 F), the metal artifact may not affect image quality. This may be an advantage over CT in the study of orthopedic patients. Two-dimensional Fourier transformation may also reduce artifacts from non-ferromagnetic objects.” Even small amounts of ferromagnetic material result in significant artifacts in NMR images.” Large amounts of nonferromagnetic metal may also reduce image quality (Figure 6). The location and amount of metal are significant in determining if images will be affected. Splints and cast material (plaster and fiberglass) do not affect an NMR image. In patients studied with NMR imaging, such material is only faintly visible (Figure 7).
Normal anatomy NMR imaging provides excellent anatomic detail, especially of soft tissue structures. SE 500/20 images of the ankle (Figure 8) clearly demonstrate the ligamentous, subcutaneous, and bony anatomy. The ligamentous anatomy of the knee can also be clearly demonstrated (Figure 9). The ligaments and cartilage are low intensity (black), and the fat and marrow are high intensity (white). Vascular structures are clearly seen (black) in the subcutaneous fat. To date, we have not satisfactorily demonstrated the menisci. These
Fig. 8. SE sagittal images of ankle. (A) Peroneal tendon (upper arrow) and long plantar ligament (lower arrow) are well demonstrated. Talus and midfoot are also clearly seen. (B) Achilles tendon (arrow).
Fig. 7. Sagittal view of wrist in plaster cast. Note that cast (upper curved arrow and lower straight arrow) is faintly visible. Lower curved arrow, distal radius.
structures can be identified in certain planes; however, more detail is necessary for clinical evaluation. The anatomy of the spine is clearly seen with NMR imaging (Figure 10). The ligaments are black on SE imaging, and the nucleus pulposus is white. The cord and vertebral bodies are also well demonstrated. The ability to differentiate the annulus from the nucleus pulposus may be of benefit in the study of patients suspected of having disk protrusions. This is, of course, dependent on the chemical makeup of the disk, which changes with age.
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Resonance
Imaging
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Fig. 9. Normal anatomy of knee. (A) SE 500/200 sagittal view. QT, quadriceps tendon; P, patella; PT, patellar tendon; PC, posterior cruciate ligament. (B) sagittal image of medial margin of knee. T, tibia; FC, femoral condyle. (C) Sagittal image of medial compartment. M, meniscus; T, tibia; PF, posterior portion of femoral condyle. (D) Lateral compartment of knee. Upper arrow, posterior meniscus, lower arrow, tibiofibular articulation. (E) Axial scan of knee demonstrating cruciate ligament (curved arrow) and vessels in subcutaneous fat (straight arrow).
MRI: Preliminary
experience
in orthopedic
radiology
0 T. H. BERQUIST
Fig. 10. Normal anatomy of spine with SE 500/20 images. (A) Sagittal view of cervical spine. Cl; upper arrows, lower small arrow, spinal cord. (B) Sagittal view of thoracic spine demonstrating disks (white) and annulus (black). Sternum (white arrow) and trachea (black arrow) are also evident. (C) Lumbar spine with disks and vertebral bodies clearly seen. Note deficient area in L5 posteriorly (due to previous surgery) and vacuum sign (upper black arrow). Rectum is evident (lower black arrow). Motion artifact due to motion of abdominal wall is evident anteriorly (open arrow).
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Magnetic
Resonance
Imaging
0 Volume 2, Number
1, 1984
A
Fig. 11. Fourteen-year-old patient with known telangiectatic osteogenic sarcoma. (A) Routine radiograph demonstrates lucent defect in metaphysis and slight periosteal elevation (arrows). (B) Axial CT scan demonstrates bony destruction and soft tissue swelling (white arrow). (C) Coronal IR 1,000/400 image demonstrates that tumor (upper arrow) extends through growth plate into epiphysis (small arrow). Tumor extends beyond bone laterally.
MRI: Preliminary experience in orthopedic
radiology
l T. H. BERQUIST
Fig. 12. Benign lipoma. (A) Routine radiograph demonstrates lucent area in soft tissues medially fat density mass in thigh. Axial SE 1,000/20 image demonstrates
Fig. 13. Coronal SE 500/20 image of ankle demonstrates slightly displaced fracture of distal fibula (arrows).
51
(arrows).
(B)
Fig. 14. Coronal SE 500/20 image of knee following ligament repair. Course of reconstruction is clearly demonstrated (arrows).
52
Magnetic
Table 2. Potential
Resonance
Imaging
applications of NMR in orthopedic radiology
Bone and soft tissue tumors Avascular necrosis Fracture healing Infection Ligament injury Postoperative complications
l Volume 2, Number 1, 1984
healing may also be useful because of the excellent medullary bone detail provided with NMR images. Other orthopedic problems such as infection and avascular necrosis can also be demonstrated with NMR.* Follow-up of surgical cases (Figure 14) is another potential use of NMR in orthopedic imaging. Table 2 lists potential uses of NMR in orthopedic radiology. CONCLUSIONS
Pathologic findings The anatomic detail noted in Figure 8 through 10 suggests that NMR imaging will be useful in the evaluation of multiple bone and soft tissue lesions. Detection of bone and soft tissue tumors has been that reported.3,8*‘3 Initial experience has demonstrated NMR may reveal such pathologic conditions at least as well as CT (Figures 11 and 12) and may provide additional information. Coronal NMR images of the femur in a patient with telangiectatic osteogenic sarcoma demonsrated a neoplasm crossing the growth plate to involve the epiphysis (Fig. 11 C). The neoplasm was not evident on theroutine radiograph or CT scan. However, coronal reconstruction was not performed with CT and the changes may have been noted if this reconstruction had been obtained. Evaluation of fractures (Figure 13) and fracture
NMR offers a new approach to imaging. To date, there are no known biologic hazards.‘4,‘5 Early experience with a resistive system has demonstrated significant potential of NMR in the evaluation of orthopedic problems. The true value of this modality compared with techniques already available is not yet completely clear. Certainly anatomic structures, especially the soft tissues, can be well demonstrated with NMR. Improvements in software with multi-slice reconstruction and two-dimensional Fourier transformation will reduce the scan time and increase the image quality with the system used at our institution. Significant study of types of units (resistive, superconducting, permanent magnets), pulse sequences, and ability to perform chemical analysis with NMR imaging units is needed before the true potential of these instruments can be known.
REFERENCES 1. Alfidi,
2
3.
4.
5.
6.
R.J.; Haaga, J.R.; El Yousef, S.J.; Bryan, P.J.; Fletcher, B.D.; LiPuma, J.P.; Morrison, SC.; Kaufman, B.; Richey, J.B.; Hinshaw, W.S.; Kramer, D.M.; Yeung, H.N.; Cohen, A.M.; Butler, H.E.; Ament, A.E.; Lieberman, J.M. Preliminary experimental results in humans and animals with a superconducting, whole-body, nuclear magnetic resonance scanner. Radiology 143: 175-181; 1982. Bottomley, P.A.; Edelstein, W.A.; Leue, W.M.; Hart, H.R.; Schenck, J.F.; Redington, R.W. Head and body imaging by hydrogen nuclear magnetic resonance. Magnetic Res. Imag. 1: 69974; 1982. Brady, T.J.; Gebhardt, M.C.; Pykett, I.L.; Buonanno, F.S.; Newhouse, J.H.; Burt, C.T.; Smith, R.J.; Mankin, H.J.; Kistler, J.P.; Goldman, M.R.; Hinshaw, W.S.; Pohost, G.M. NMR imaging of forearms in healthy volunteers and patients with giant-cell tumor of bone. Radiology 144: 549-552; 1982. Crooks, L.; Arakawa, M.; Hoenninger, J.; Watts, J.; McRee, R.; Kaufman, L.; Davis, P.L.; Margulis, A.R.; DeGroot, J. Nuclear magnetic resonance whole-body imager operating at 3.5 KGauss. Radiology 143: 169174; 1982. Crooks, L.E.; Ortendahl, D.A.; Kaufman, L.; Hoenninger, J.; Arakawa, M.; Watts, J.; Cannon, C.R.; BrantZawadzki, M.; Davis, P.L.; Margulis, A.R. Clinical efficiency of nuclear magnetic resonance imaging. Radiology 146: 123-128; 1983. Davis, P.L.; Crooks, L.; Arakawa, M.; McRee, R.; Kaufman, L.; Margulis, A.R. Potential hazards in NMR imaging: heating effects of changing magnetic fields and RF fields on small metallic implants. AJR 137:857-860; 1981.
7. Gray, J.E. Personal communication. 8. Moon, K.L., Jr.; Genant, H.K.; Helms, C.A.; Chafetz, N.I.; Crooks, L.E.; Kaufman, L. Musculoskeletal applications of nuclear magnetic resonance. Radiology 147: 161-171,1983. 9. Moon, K.L., Jr.; Hricak, H.; Crooks, L.E.; Gooding, C.A.; Moss, A.A.; Engelstad, B.L.; Kaufman, L. Nuclear magnetic resonance imaging of the adrenal gland: a preliminary report. Radiology 147: 155-160, 1983. 10 New, P.F.J.; Rosen, B.R.; Brady, T.J.; Buonanno, F.S.; Kistler, J.P.; Burt, CT.; Hinshaw, W.S.; Newhouse, J.H.; Pohost, G.M.; Taveras, J.M. Potential hazards and artifacts of ferromagnetic and nonferromagnetic surgical and dental materials and devices in nuclear magnetic resonance imaging. Radiology 147: 139-148; 1983. M.; Castle, L.; Borkowski, 11 Pavlicek, W.; Geisinger, G.P.; Meaney, T.F.; Bream, B.L.; Gallagher, J.H. The effects of nuclear magnetic resonance on patients with cardiac pacemakers. Radiology 147: 149-l 53; 1983. 12. Pykett, I.L.; Newhouse, J.H.; Buonanno, F.S.; Brady, T.J.; Goldman, M.R.; Kistler, J.P.; Pohost, G.M. Principles of nuclear magnetic resonance imaging. Radiology 143: 157-168; 1982. 13. Ranade, S.S.; Shah, S.; Advani, S.H.; Kasturi, S.R. Pulsed nuclear magnetic resonance studies of human bone marrow. Physiol. Chem. Phys. 9: 297-299; 1977. 14. Saunders, R.D. Biologic effects of NMR ing. Appl. Radial. 11: 43-46; 1982.
clinical imag-
15. Schwartz, J.L.; Crooks, L.E. NMR imaging produces no observable mutations or cytotoxicity in mammalian cells. AJR 139: 583-585; 1982.
Imaging. Vol. 2, pp. All rights reserved.
41-52,
1984 Copyright
0730.725X/84$3.00 0 1984 Pergamon
+ .&I Press Ltd.
l Research Article MAGNETIC RESONANCE IMAGING: PRELIMINARY EXPERIENCE IN ORTHOPEDIC RADIOLOGY THOMAS Department
of Diagnostic
Radiology,
H. BERQUIST, M.D.
Mayo Clinic and Mayo Foundation,
INTRODUCTION
METHODS The main objective of the study was to determine which patients were suitable for evaluation with NMR imaging (e.g., trauma patients with casts, patients with metal fixation devices or prostheses, patients with bone and soft tissue neoplasms, patients with soft tissue injuries). This necessitated evaluation of normal patients for anatomic definition and patients with known pathologic conditions. Determination of which pulse sequences, scan planes, and slice thicknesses were most helpful was also necessary. The images were obtained with a 1.5 K Gauss resistive magnet that operates at a resonance fre-
6/21/83;
ACCEPTED
7/
Minnesota
55905
quency of 6.4 MHz (Picker International, Highland Heights, Ohio). All images were obtained with the use of the head coil (30 x 32) and an image matrix of 256 x 256. Slide thicknesses of 1 and 2 cm were available at the time of this study. The currently available software allowed spin-echo (SE), repeated free induction decay (FID), and inversion recovery (IR) pulse sequences. Only single-slice reconstruction was available in the early operating stages of this unit; therefore, it was important to evaluate the imaging qualities of each sequence and to determine the time required for the various sequences (Table 1). Imaging was typically performed with the use of 180 views and a scan angle of 180” which resulted in a pulse sequence occurring at lo intervals (Figure 1). If only 90 or 45 views were obtained, data were collected every 2O and at every 4O respectively (Figure 1). Reduction of the number of views to 90 and 45 reduced the scan time by factors of 2 and 4 respectively. Figure 2 demonstrates the images obtained with the use of these parameters. The signalto-noise ratio changed significantly as the number of views decreased. The ability of 1 and 2 cm slice thicknesses to reveal lesions of the same size were also evaluated. Small high-proton density disks 0.5 cm, 1 cm, and 2 cm in diameter were studied in phantoms and attached to the calf. The detectability of these structures allowed us to determine whether 1 cm slice thicknesses were necessary routinely. If 2 cm slice thicknesses were equal to 1 cm slice thicknesses in detection and resolution, then considerable time could be saved by studying patients with the single-slice reconstruction. Another preliminary step was to evaluate orthopedic appliances and surgical clips. Magnetic properties and image degradation of the commonly used stainless steel plates, Vitallium plates and prostheses were
The clinical applications of nuclear magnetic resonance (N MR) imaging are becoming more numerous. The recent literature cortains many articles concerning imaging of the central nervous system and other parts of the body.‘.5,8.9,“.‘3 Currently available techniques involve primarily proton imaging; however, NMR permits the study of other nuclei as well (14N, 31 r, 17,, 13c, 23,,).’ Clinical investigations of NMR have not yet determined the most efficient method of imaging patients. Questions regarding the most useful pulse sequences for a given situation, or even which type of instrument (resistive, superconducting, or permanent magnets) is best suited for clinical imaging, remain unanswered. As a result, each investigator must attempt to answer these questions in determining the utility of NMR for clinical imaging. We are currently evaluating a resistive system that operates at 1.5 K Gauss (0.15 Tesla) and a resonance frequency of 6.4 MHz. The following data were obtained during a preliminary study that was initiated to determine the usefulness of this unit in orthopedic imaging.
RECEIVED
Rochester,
1 z/83.
41
42
Magnetic Resonance Imaging 0 Volume 2, Number Table
1. NMR
pulse sequences:
Number
1, 1984
of views vs scan time Scan time (s)
Pulse Sequence*
180 views
Free induction decay (90° pulse) Sequence Interval A 500 ms B 1,000 ms C 1,500 ms Spin-Echo (90”-tau- 180”) Interval tau 20 500 ms 40 60 80 A series 1,000 ms Inversion recovery (180”-tau-90”) Interval tau 200 1,000 ms 300 400 500 *Reconstruction
90 views
45 views
210
4s 90 13s
22 45 68
90
45
22
180
90
45
180
90
45
90
180
time: 30 s for all pulse sequences
Surgical clips were also studied in this regard. Metal objects were suspended in the magnet at the center of the field by means of a fine string. Position changes (motion and torque) were carefully assessed. We studied the effect of image degradation in volunteers who had the metal components attached to the lower leg near the anterior tibia. Patients with plaster and fiberglass casts were also studied to determine whether these structures affected image quality. Normal healthy volunteers were studied to determine whether anatomic structures could be adequately visualized with the resistive system. Coronal, axial, and sagittal scans of the knee, ankle, foot, and spine checked.
Scan Angle 180 Views: A = 180
Fig. 1. Diagram of scan angles and number of views. Angles can be selected up to 360” with up to 360 views.
were chosen for this initial evaluation. Finally, patients with known pathologic conditions were then studied to compare the results of NMR imaging with computed tomography (CT), routine radiography, and other available imaging procedures.
RESULTS Imuging
variables.
Evaluation of imaging variables revealed that for orthopedic purposes the SE 1,000/20 or 500/20 (90’ pulse-20 ms tau-180° pulse at a 500 or 1,000 ms interval) provided the optimal anatomic definition. Single-slice formatting required 2 minutes for the 500 ms sequence and 3.5 minutes for the 1,000 ms sequence. SE 500/20 images are satisfactory in most situations and result in reduced scan time (Table 1). Reduction of the number of views is also useful in rapid localization of lesions or anatomic areas and allows one to quickly define the area of interest for further sequence studies or to assist in alignment of coronal or sagittal imaging. Because image quality is reduced if the number of views is reduced (Figure 2). the usefulness for diagnostic purposes is limited. Moon et al.* also reported that SE sequences were useful in orthopedic imaging. In their study, two-dimensional Fourier transformation was used. New software for the resistive system at our institution will also provide this option in addition to multislice reconstruction. These changes will greatly improve image quality and reduce scan time. The pulse sequences best suited for various patho-
MRI: Preliminary experience in orthopedic
radiology
0 T. H. BERQIJIST
43
Fig. 2. Axial images of the calf. (A) IR 1,OOO/SOOwith 180 views at 180°. (B) IR 1,OOO/SOOwith 90 views at 180“. Image quality is decreased and noise is increased. V, Vitallium plate; S, stainless steel plate. (C) IR l,OOO/SOO with 45 views at 180” results in significant image degradation; however, images are satisfactory for localization purposes.
logic studies have not yet been determined. It is hoped that this will become more obvious with experience. Presently multiple sequences are necessary for the evaluation of bone and soft tissue lesions. The slice thickness obtained affects the image quality and examination time. Thicker slices naturally allow more rapid evaluation of a given area. Our initial experience demonstrated that 2 cm and 1 cm slices did not differ in ability to reveal lesions of various sizes (5 mm, 10 mm, and 20 mm). The signal-to-noise ratio was also improved with thicker slices. Therefore, at least for certain cases, contiguous 2 cm slices or 2 cm slices obtained at 15 mm intervals are all that are
required; the result is quality images with reduced examination time. As further advances are made in software, these techniques will obviously be improved. Orthopedic appliances and casts Radiologists are well aware of the artifacts created by metal objects on CT images (Figure 3). Two factors required consideration in relation to metal and NMR imaging. First, the ferromagnetic properties of orthopedic appliances needed to be studied. Second, the degree of image degradation created by these structures required evaluation. There are numerous classifications of stainless
44
Magnetic
Resonance
Imaging
Fig. 3. (A) Scout view of CT scan of lumbar spine with Harrington rod fixation of L2 fracture. (B) Axial scan demonstrates body fracture (arrows) and fragments in spinal
canal. Note starburst artifact created by metal rods.
0 Volume 2, Number
I, 1984
steel.‘*” Manufacturers of orthopedic devices use various combinations and amounts of cobalt, molybdenum, nickel, manganese, chromium and tungsten. The appliances used at each institution should be carefully checked before one examines patients who have orthopedic devices in place. We studied all of the orthopedic plates, screws, prostheses, and surgical clips commonly used at our institution and found that none exhibited ferromagnetic properties in the magnet. Changing gradients and radio frequency (RF) pulses have also been evaluated. Davis et ~1.~ studied the effects on tissue of small metal clips and prosthetic devices when exposed to magnet fields with varying gradients and RF pulses. No temperature changes were noted with small metal objects; however, significant heating of hip prostheses could be demonstrated in saline baths. We were unable to demonstrate temperature changes with large prostheses (Austin-Moore in saline) even with changing gradients.’ Metallic devices not commonly used in orthopedics have demonstrated magnetic properties. Mayfield and Heifetz aneurysm clips are ferromagnetic. New et ~1.” demonstrated significant torquing of these clips and using NMR, could even cause them to release from arteries of dogs. In an attempt to evaluate the effect of metal on imaging, we attached stainless steel and Vitallium plates (15 x 0.5 x 1.5 cm) to the lower legs adjacent to the tibia. Figure 4 demonstrates the artifact created with CT in this situation. NMR images were obtained in all planes (axial, coronal, and sagittal) and at varying pulse sequences. Scan angles were varied to determine if this had any effect on the appearance of the artifact (Figure 5). Pulse sequences, location, and the type (ferromagnetic or non-ferromagnetic) of metal present, are all considerations in the evaluation
Fig. 4. CT scan of lower legs with stainless steel and Vitallium plate; S-stainless steel plate. Note metal artifact.
plates near tibia. T, tibia; F, fibula; V, Vitallium
MRI: Preliminary
Fig. from (D), with
experience
in orthopedic
radiology
0 T. H. BERQUISI
5. (A) Scan angles beginning at 0” (A) and 90” (B) with metal in upper quadrant (rectangle). FID axial scan O” (left arrow). Note small artifact due to metal near tibia (right arrow). FID images begun at 90” (C), at 180” and at 360° and 360 views (V). (E) do not significantly affect artifact. The plate creates little if any artifact SE 1.OOO/ZO sequence with 180 views at 180” (F).
45
46
Magnetic Resonance Imaging 0 Volume 2, Number I, 1984
Fig. 6. (A) Anteroposterior view of ankle with internal fixation of distal tibia and fibula with metal plates, screws, and K wires. (B) Coronal scan of ankle shows significant artifact (arrows) due to large amount of metal present. (C) Axial scan demonstrates tibia (T) and fibula (F) but is of little clinical value.
MRI: Preliminary
experience
in orthopedic
radiology
0 T. H. BERQUIST
47
of orthopedic patients. The scan angle would appear to have no effect on artifact formation or removal. In some cases (Figure 5 F), the metal artifact may not affect image quality. This may be an advantage over CT in the study of orthopedic patients. Two-dimensional Fourier transformation may also reduce artifacts from non-ferromagnetic objects.” Even small amounts of ferromagnetic material result in significant artifacts in NMR images.” Large amounts of nonferromagnetic metal may also reduce image quality (Figure 6). The location and amount of metal are significant in determining if images will be affected. Splints and cast material (plaster and fiberglass) do not affect an NMR image. In patients studied with NMR imaging, such material is only faintly visible (Figure 7).
Normal anatomy NMR imaging provides excellent anatomic detail, especially of soft tissue structures. SE 500/20 images of the ankle (Figure 8) clearly demonstrate the ligamentous, subcutaneous, and bony anatomy. The ligamentous anatomy of the knee can also be clearly demonstrated (Figure 9). The ligaments and cartilage are low intensity (black), and the fat and marrow are high intensity (white). Vascular structures are clearly seen (black) in the subcutaneous fat. To date, we have not satisfactorily demonstrated the menisci. These
Fig. 8. SE sagittal images of ankle. (A) Peroneal tendon (upper arrow) and long plantar ligament (lower arrow) are well demonstrated. Talus and midfoot are also clearly seen. (B) Achilles tendon (arrow).
Fig. 7. Sagittal view of wrist in plaster cast. Note that cast (upper curved arrow and lower straight arrow) is faintly visible. Lower curved arrow, distal radius.
structures can be identified in certain planes; however, more detail is necessary for clinical evaluation. The anatomy of the spine is clearly seen with NMR imaging (Figure 10). The ligaments are black on SE imaging, and the nucleus pulposus is white. The cord and vertebral bodies are also well demonstrated. The ability to differentiate the annulus from the nucleus pulposus may be of benefit in the study of patients suspected of having disk protrusions. This is, of course, dependent on the chemical makeup of the disk, which changes with age.
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Fig. 9. Normal anatomy of knee. (A) SE 500/200 sagittal view. QT, quadriceps tendon; P, patella; PT, patellar tendon; PC, posterior cruciate ligament. (B) sagittal image of medial margin of knee. T, tibia; FC, femoral condyle. (C) Sagittal image of medial compartment. M, meniscus; T, tibia; PF, posterior portion of femoral condyle. (D) Lateral compartment of knee. Upper arrow, posterior meniscus, lower arrow, tibiofibular articulation. (E) Axial scan of knee demonstrating cruciate ligament (curved arrow) and vessels in subcutaneous fat (straight arrow).
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Fig. 10. Normal anatomy of spine with SE 500/20 images. (A) Sagittal view of cervical spine. Cl; upper arrows, lower small arrow, spinal cord. (B) Sagittal view of thoracic spine demonstrating disks (white) and annulus (black). Sternum (white arrow) and trachea (black arrow) are also evident. (C) Lumbar spine with disks and vertebral bodies clearly seen. Note deficient area in L5 posteriorly (due to previous surgery) and vacuum sign (upper black arrow). Rectum is evident (lower black arrow). Motion artifact due to motion of abdominal wall is evident anteriorly (open arrow).
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Fig. 11. Fourteen-year-old patient with known telangiectatic osteogenic sarcoma. (A) Routine radiograph demonstrates lucent defect in metaphysis and slight periosteal elevation (arrows). (B) Axial CT scan demonstrates bony destruction and soft tissue swelling (white arrow). (C) Coronal IR 1,000/400 image demonstrates that tumor (upper arrow) extends through growth plate into epiphysis (small arrow). Tumor extends beyond bone laterally.
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Fig. 12. Benign lipoma. (A) Routine radiograph demonstrates lucent area in soft tissues medially fat density mass in thigh. Axial SE 1,000/20 image demonstrates
Fig. 13. Coronal SE 500/20 image of ankle demonstrates slightly displaced fracture of distal fibula (arrows).
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(arrows).
(B)
Fig. 14. Coronal SE 500/20 image of knee following ligament repair. Course of reconstruction is clearly demonstrated (arrows).
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Bone and soft tissue tumors Avascular necrosis Fracture healing Infection Ligament injury Postoperative complications
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healing may also be useful because of the excellent medullary bone detail provided with NMR images. Other orthopedic problems such as infection and avascular necrosis can also be demonstrated with NMR.* Follow-up of surgical cases (Figure 14) is another potential use of NMR in orthopedic imaging. Table 2 lists potential uses of NMR in orthopedic radiology. CONCLUSIONS
Pathologic findings The anatomic detail noted in Figure 8 through 10 suggests that NMR imaging will be useful in the evaluation of multiple bone and soft tissue lesions. Detection of bone and soft tissue tumors has been that reported.3,8*‘3 Initial experience has demonstrated NMR may reveal such pathologic conditions at least as well as CT (Figures 11 and 12) and may provide additional information. Coronal NMR images of the femur in a patient with telangiectatic osteogenic sarcoma demonsrated a neoplasm crossing the growth plate to involve the epiphysis (Fig. 11 C). The neoplasm was not evident on theroutine radiograph or CT scan. However, coronal reconstruction was not performed with CT and the changes may have been noted if this reconstruction had been obtained. Evaluation of fractures (Figure 13) and fracture
NMR offers a new approach to imaging. To date, there are no known biologic hazards.‘4,‘5 Early experience with a resistive system has demonstrated significant potential of NMR in the evaluation of orthopedic problems. The true value of this modality compared with techniques already available is not yet completely clear. Certainly anatomic structures, especially the soft tissues, can be well demonstrated with NMR. Improvements in software with multi-slice reconstruction and two-dimensional Fourier transformation will reduce the scan time and increase the image quality with the system used at our institution. Significant study of types of units (resistive, superconducting, permanent magnets), pulse sequences, and ability to perform chemical analysis with NMR imaging units is needed before the true potential of these instruments can be known.
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7. Gray, J.E. Personal communication. 8. Moon, K.L., Jr.; Genant, H.K.; Helms, C.A.; Chafetz, N.I.; Crooks, L.E.; Kaufman, L. Musculoskeletal applications of nuclear magnetic resonance. Radiology 147: 161-171,1983. 9. Moon, K.L., Jr.; Hricak, H.; Crooks, L.E.; Gooding, C.A.; Moss, A.A.; Engelstad, B.L.; Kaufman, L. Nuclear magnetic resonance imaging of the adrenal gland: a preliminary report. Radiology 147: 155-160, 1983. 10 New, P.F.J.; Rosen, B.R.; Brady, T.J.; Buonanno, F.S.; Kistler, J.P.; Burt, CT.; Hinshaw, W.S.; Newhouse, J.H.; Pohost, G.M.; Taveras, J.M. Potential hazards and artifacts of ferromagnetic and nonferromagnetic surgical and dental materials and devices in nuclear magnetic resonance imaging. Radiology 147: 139-148; 1983. M.; Castle, L.; Borkowski, 11 Pavlicek, W.; Geisinger, G.P.; Meaney, T.F.; Bream, B.L.; Gallagher, J.H. The effects of nuclear magnetic resonance on patients with cardiac pacemakers. Radiology 147: 149-l 53; 1983. 12. Pykett, I.L.; Newhouse, J.H.; Buonanno, F.S.; Brady, T.J.; Goldman, M.R.; Kistler, J.P.; Pohost, G.M. Principles of nuclear magnetic resonance imaging. Radiology 143: 157-168; 1982. 13. Ranade, S.S.; Shah, S.; Advani, S.H.; Kasturi, S.R. Pulsed nuclear magnetic resonance studies of human bone marrow. Physiol. Chem. Phys. 9: 297-299; 1977. 14. Saunders, R.D. Biologic effects of NMR ing. Appl. Radial. 11: 43-46; 1982.
clinical imag-
15. Schwartz, J.L.; Crooks, L.E. NMR imaging produces no observable mutations or cytotoxicity in mammalian cells. AJR 139: 583-585; 1982.