Magnetic resonance imaging of postoperative patients with metallic implants

Determination of the lateral extent of the subcutaneous wound bed in canine cadavers after closure of skin defects to replicate tumor excision

OBJECTIVE

To determine the extent of subcutaneous involvement after primary closure of experimental skin defects.

STUDY DESIGN

Experimental study.

ANIMALS

Eight large, mixed breed dog cadavers.

METHODS

Standardized, circular skin defects with diameters of 2, 4, 6, or 8 cm were created at the lateral thorax or abdomen by using a matrix to ensure even distribution, with eight defects of each diameter. The wound beds were covered in 60% barium sulfate paste prior to primary closure. Computed tomography and a dedicated viewing program were used to measure the distance from the edge of the subcutaneous defect to the incision at set intervals along the incision.

RESULTS

The mean ± SD maximum lateral extension was 6.4 ± 1.9, 14.1 ± 4.9, 18.5 ± 6.9, and 26.0 ± 9.6 mm for 2-, 4-, 6-, and 8-cm defects, respectively. Extension >2 cm from the incision occurred in zero of eight, one of eight, five of eight, and six of eight defects after closure of 2-, 4-, 6-, and 8-cm defects, respectively. Extension >3 cm from the incision occurred in zero of eight, zero of eight, two of eight, and five of eight defects after closure of 2-, 4-, 6-, and 8-cm defects, respectively.

CONCLUSION

The lateral extent of the subcutaneous wound bed extended past previously recommended revision margins of 2 to 3 cm in some defects measuring greater than 4 cm in diameter.

CLINICAL SIGNIFICANCE

Surgeons and radiation oncologists should consider the potential extent of the subcutaneous defect when planning revision therapy, especially after large resections.

Appraising the potential of Zr-based biomedical alloys to reduce magnetic resonance imaging artifacts

This study compared Zr-Mo alloys with commercial metallic biomaterials. It was observed that the Zr-Mo alloys exhibited favourable mechanical properties, particularly the Zr-10Mo alloy, which showed the highest strength to Young's modulus ratio among all evaluated metals. These alloys also exhibited the lowest magnetic susceptibilities, which are important for magnetic resonance imaging (MRI). However, both Zr- and Ti-based metals yielded comparable artifacts. It was concluded that the magnetic susceptibility must differ considerably to afford significantly improved MRI quality owing to the increased importance of non-susceptibility-related artifacts when comparing materials with relatively similar magnetic susceptibilities.

Biomechanical Analysis of Biodegradable Cervical Plates Developed for Anterior Cervical Discectomy and Fusion

Study Design

In-vitro biomechanical investigation.

Purpose

To evaluate the biomechanical effects of the degeneration of the biodegradable cervical plates developed for anterior cervical discectomy and fusion (ACDF) on fusion and adjacent levels.

Overview of Literature

Biodegradable implants have been recently introduced for cervical spine surgery. However, their effectiveness and safety remains unclear.

Methods

A linear three-dimensional finite element (FE) model of the lower cervical spine, comprising the C4–C6 vertebrae was developed using computed tomography images of a 46-year-old woman. The model was validated by comparison with previous reports. Four models of ACDF were analyzed and compared: (1) a titanium plate and bone block (Tita), (2) strong biodegradable plate and bone block (PLA-4G) that represents the early state of the biodegradable plate with full strength, (3) weak biodegradable plate and bone block (PLA-1G) that represents the late state of the biodegradable plate with decreased strength, and (4) stand-alone bone block (Bloc). FE analysis was performed to investigate the relative motion and intervertebral disc stress at the surgical (C5–C6 segment) and adjacent (C4–C5 segment) levels.

Results

The Tita and PLA-4G models were superior to the other models in terms of higher segment stiffness, smaller relative motion, and lower bone stress at the surgical level. However, the maximal von Mises stress at the intervertebral disc at the adjacent level was significantly higher in the Tita and PLA-4G models than in the other models. The relative motion at the adjacent level was significantly lower in the PLA-1G and Bloc models than in the other models.

Conclusions

The use of biodegradable plates will enhance spinal fusion in the initial stronger period and prevent adjacent segment degeneration in the later, weaker period.

Magnetic Resonance Imaging:Overview of the Technology and Medical Applications

The physical phenomenon of nuclear magnetic resonance (NMR) was first characterized almost forty years ago in 1946 by the simultaneous but independent experimental successes of American scientists Felix Bloch and Edward Purcell. Their discoveries prompted development of conventional NMR spectroscopy. a technique used to describe the molecular composition and behavior of chemical compounds. Twenty-five years later, in 1971, Damadian used NMR to demonstrate differences in the behavior of water in malignant and benign tissues, and he suggested that NMR possessed “many of the desirable features of an external probe for the detection of internal cancer” (7). In the same year, Lauterbur produced the first two-dimensional NMR image, a cross-sectional portrait of two tubes of water (25). The potential utility of this technique to medical imaging was obvious, and soon afterwards multiple researchers began development of clinical NMR imaging systems. The first human whole-body NMR scan was accomplished by 1977. Improvements in the scanning process and image quality continue with, as yet, no limits in sight. In this clinical context, NMR techniques have experienced a name change to the current prevailing appellation, magnetic resonance imaging (MRI).

Postoperative Assessment of Surgical Clip Position in 16 Dogs With Cancer: A Pilot Study

Metallic hemoclips or surgical staples were inserted in 16 tumor-bearing dogs at the time of surgical resection of the tumor. Orthogonal radiographs were taken immediately postoperatively and after wound healing to visualize the location and number of hemoclips or metallic staples. A shift in hemoclip/staple position was identified in nine dogs, mainly from positioning during radiography. In three dogs, an absolute shift in marker position was identified. Based on this study, it appears that the placement of surgical clips is potentially useful in identifying the tumor bed, which may be of benefit in establishing radiation treatment fields.

Artifacts of vena cava filters ex vivo on MR angiography

We evaluated magnetic susceptibility artifacts of nine types of vena cava filters in MR angiography (MRA) at 1.0T ex vivo in order to assess the filters' compatibility with MRA. Each filter (tulip filter, tulip MReye filter, stainless Greenfield filter, titanium Greenfield filter, TrapEase filter, Simon filter, LGM Vena-Tech filter, Antheor temporary filter, and Bird's nest filter) was inserted into an acrylic tube (20 or 25 mm in diameter, 15 or 30 cm in length). Gd-DTPA was poured into each tube at a concentration of 1/500 and each was placed in a water-filled container for imaging. We evaluated artifacts of the filters according to the following criteria: signal void beyond the tube, 3+; signal void within the tube but at more than one-half the diameter of the tube, 2+; and signal void within the tube but at less than one-half the diameter of the tube, 1+. We evaluated artifacts originating at the tip, intermediate portion, and distal end of the filters. We judged the artifacts as follows: tulip (3+, 3+, 3+); tulip MReye (2+, 1+, 1+); stainless Greenfield (2+, 1+, 2+); titanium Greenfield (1+, 1+, 1+); TrapEase (1+, 2+, 1+); Simon (2+, 2+, 1+); LGM (2+, 2+, 1+); Antheor (2+, 2+, 2+); and Bird's nest (3+, 3+, 3+). The numbers in parentheses refer to the degree of signal void at the tip, intermediate portion, and distal end of the filter, respectively. The tulip filter and Bird's nest filter made of 304 stainless steel caused extensive signal voids beyond the areas defined by the filters. The signal voids in the remaining seven filters were limited to within the tube. We concluded that seven of the nine filters were compatible with MRA ex vivo.

Biomedical implants and devices: assessment of magnetic field interactions with a 3.0-Tesla MR system

PURPOSE

To evaluate magnetic field interactions for 109 different biomedical implants and devices in association with exposure to a 3.0-Tesla magnetic resonance (MR) system.

MATERIALS AND METHODS

A total of 109 implants and devices (aneurysm clips, 32; clips, fasteners, and staples, 10; coils and stents, 10; heart valve prostheses and annuloplasty rings, 12; orthopedic implants, five; suture materials, 13; vascular access ports and accessories, 13; miscellaneous implants and devices, 14) were tested for magnetic field interactions at 3.0-Tesla using previously-described, standardized techniques to assess magnetic field translational attraction and torque.

RESULTS

The deflection angles and torque measurements ranged, respectively, from 0 to 16 degrees and 0 to +2 for the aneurysm clips; 0 to 90 degrees and 0 to +4 for the clips, fasteners, and staples; 0 to 47 degrees and 0 to +4 for the coils and stents; 0 to 4 degrees and 0 to +1 for the heart valve prostheses and annuloplasty rings; 0 to 12 degrees and 0 to +2 for the orthopedic implants; 0 to 13 degrees and 0 to +2 for the suture materials; 0 to 52 degrees and 0 to +4 for the vascular access ports and accessories; and 0 to 28 degrees and 0 to +3 for the miscellaneous implants and devices.

CONCLUSION

Of the 109 implants and devices assessed for magnetic field interactions at 3.0-Tesla, four (4%) are potentially unsafe based on deflection angle criteria. The implications of these results for patients undergoing MR procedures at 3.0-Tesla is discussed. Notably, these results are specific to the 3.0-Tesla MR system used for this evaluation.

Safety issues in magnetic resonance imaging

Although generally considered safe, magnetic resonance imaging (MRI) has a number of safety issues, including the effects of high magnetic fields and radiofrequency pulses on the body, and on implanted devices, the side effects of contrast agents, toxicity during pregnancy, claustrophobia, and hearing loss.

The effect of mechanical deformation on magnetic properties and MRI artifacts of type 304 and type 316L stainless steel

The purpose of this study was to evaluate the influence of composition and deformation of biomedical stainless steels on mechanical properties, magnetic properties, and MRI artifacts. Type 304 and Type 316L samples were prepared using standard wire-drawing techniques. Mechanical properties were determined using standard test methods. The amount of ferromagnetic phase present was estimated using a Severn Gage and x-ray diffraction. Magnetic field attraction and artifacts were determined using previously described techniques. The strength of both steels increased significantly with increasing deformation. None of the type 316L wires transformed to the magnetic phase. The amount of magnetic phase in the type 304 wires increased with increasing deformation. There was no magnetic field attraction, and artifacts were minimal for all of type 316L wires and the undeformed type 304 wire. Deflection and artifacts were significant for the deformed type 304 stainless steel. These results provide guidance regarding the use of type 304 and type 316L stainless steels for bioimplants. In this regard, type 316L stainless steel seems to be a more acceptable material with respect to MR compatibility.

Artifacts in magnetic resonance imaging of the spine after surgery with or without implant

Magnetic resonance (MR) imaging is often disturbed after spine surgical procedures with or without an implant. Artifacts are induced by ferromagnetic or nonferromagnetic implants and devices and by small metallic particles left by surgical instruments. All metallic artifacts can affect the quality and usefulness of postoperative spine MR examinations. The physical effects caused by the introduction of metal or other conductive materials into a magnetic field and their consequences are presented. The application to postoperative spine MR examinations and solutions to reduce artifacts are discussed.

MR imaging and metallic implants for anterior cruciate ligament reconstruction: assessment of ferromagnetism and artifact

Magnetic resonance (MR) imaging is contraindicated for patients with certain ferromagnetic implants, primarily because of potential risks related to movement or dislodgment of the devices. An additional problem with metallic implants is the potential image distortion that may affect the interpretation of the MR study. Since MR imaging is frequently useful for the evaluation of postoperative anterior cruciate ligament (ACL) reconstruction, the ferromagnetic qualities and artifacts associated with MR imaging were determined for five metallic orthopedic implants commonly used for this surgery. Only the Perfix interference screw displayed a substantial deflection force and caused extensive signal loss. Images of the knee of one patient with two Perfix screws in place were not interpretable because of the image distortion caused by these implants. Therefore, alternative nonferromagnetic implants should be considered for reconstruction of the ACL.

MR-imaging of chronic spinal cord injury. Association with neurologic function

Twenty-two para- and tetraplegic patients with chronic spinal cord injuries were examined with magnetic resonance imaging (MRI). The clinical course in the entire rehabilitation period was recorded and an attempt was made to associate the functional status of the patients with the morphologic findings on MRI. Small and large spinal cord cysts and syringomyelia, cord atrophy, and spinal stenosis were found. Additionally, in a number of patients regions of increased signal intensity within the cord, interpreted as myelomalacia, and obliteration of the intradural extramedullary space, interpreted as arachnopathy, were noted. The large number (13/22) of cystic lesions in our patients was unexpected. It was in contrast to the rate reported in autopsy studies of paraplegics which note only few cysts. Whereas a direct association of morphologic findings with neurologic symptoms and the clinical course was difficult, it was found that patients with large cysts and spinal cord atrophy generally showed no tendency to improve in spite of the measures taken during the rehabilitation period. It is difficult to decide whether the initial trauma with cord hemorrhage is limiting the chance of neurological improvement or if a sequence of events leading from hemorrhage to gliosis and cystic necrosis is the determining factor.

Safety of metallic surgical clips in patients undergoing high-field-strength magnetic resonance imaging

Use of metallic clips with ferromagnetic properties in patients undergoing a large variety of surgical procedures, and in particular, in coronary artery and other vascular reconstructions, has become increasingly popular. The safety of these commonly used surgical clips when subjected to high-intensity diagnostic magnetic resonance imaging fields is still debated. Commonly used hemostatic metallic clips manufactured by the Weck and US Surgical Corporations were tested in an in vitro system to assess their safety with regard to migration and displacement. The two tested hemostatic metallic clips proved safe and did not migrate or become dislodged when they underwent magnetic resonance imaging scans. This is in direct contrast to multiple cerebral aneurysm clips, also tested, which have highly ferromagnetic properties and were previously shown to migrate with disastrous results in patients undergoing diagnostic magnetic resonance imaging scans. This study supports the continued widespread use of small metallic hemostatic clips in the myriad of procedures in which they are presently used and illustrates the need for methods of evaluating such devices before they are clinically implanted.

Post-pneumonectomy evaluation of the chest: a prospective comparative study of MRI with CT

A comparative study of 11 pneumonectomized patients was undertaken in order to evaluate the respective advantages and drawbacks of MRI and CT in post-operative follow-up. Nine patients were healthy at the time of the study and two presented with tumor recurrence. MR examination included 500/40 ms axial, and frontal 800/40-80 ms or 1300/60-120 ms nongated spin echo sequences. MRI was slightly more efficient in identifying vascular stump and main nodal stations, and detected better than CT tumoral and metastatic spread in cancer recurrences. It was as informative as CT in evaluating postpneumonectomy space and bronchial stump. It was noncontributory in the detection of calcifications. Although clips were visible with MRI, their location was less definite than with CT, a potential pitfall when radiotherapy is planned.

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

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

Postoperative magnetic resonance imaging artifacts. Report of three cases

Artifacts are occasionally encountered on magnetic resonance imaging after operation. These may be due to minute metallic particles from neurosurgical instruments. Particles not detectable on plain x-ray films or computerized tomography scans may cause local change of magnetic resonance activity, resulting in a deceptive magnetic resonance appearance. Three illustrative case reports are presented.

Magnetic resonance imaging (MRI) in the evaluation of spinal cord injured children and adolescents

In order to determine the indications and usefulness of MRI scanning in evaluating spinal cord trauma, MRIs on 43 subacute and chronic spinal cord injured children were compared with CT myelograms and other diagnostic tests. MRI scans were superior to CT myelograms in evaluating post-traumatic syrinx, disc pathology and the physiological status of the cord. CT myelogram remains an essential study before considering spinal cord decompression. The presence of internal fixation is not a contraindication to MRI scanning.

Magnetic field effects on surgical ligation clips

Magnetic forces exerted on surgical clips and the magnetic resonance imaging distortion they create in phantoms and rabbits at magnetic field strengths of 1.5 Tesla were investigated. Results are reported for both ligation and aneurysm clips manufactured from three types of stainless steel as well as titanium, tantalum and niobium metals. Paramagnetism and eddy currents were measured in a customized moving Gouy balance. Direct measurements of other magnetic forces were carried out in a 1.5T MRI system. The titanium and tantalum clips showed the least interaction with the magnetic field, both in terms of forces exerted and the observed image distortion with the larger clips generating the larger interactions. The strongest field distortions and attractive forces occurred with 17-7PH stainless steel clips. These interactions were ferromagnetic in origin and of sufficient strength to present significant risk to patients having this type of clip present during an MRI scan.