Prosthetic heart valves: evaluation of magnetic field interactions, heating, and artifacts at 1.5 T

The purpose of this study was to use ex vivo testing techniques to determine the magnetic resonance imaging (MRI) safety aspects for 32 different heart valve prostheses that had not been evaluated previously in association with the 1.5-T MR environment. Ex vivo testing was performed using previously described techniques for the evaluation of magnetic field interactions (deflection angle and torque), heating [gel-filled phantom and fluoroptic thermometry; 15 minutes of MRI at a specific absorption rate (SAR) of 1.1 W/kg], and artifacts (using gradient echo and T1-weighted spin-echo pulse sequences). Thirteen heart valve prostheses displayed interactions with the magnetic field. However, these magnetic field interactions were considered relatively minor. Heating was < or =0.8 degrees C for these implants. Artifacts varied from mild to severe depending on the amount and type of metal used to make the particular heart valve prosthesis. For these 32 different heart valve prostheses, the relative lack of substantial magnetic field interactions and negligible heating indicate that MR procedures may be conducted safely in individuals with these implants using MR systems with static magnetic fields of 1.5 T or less.     

Metallic neurosurgical implants: evaluation of magnetic field interactions, heating, and artifacts at 1.5-Tesla

The purpose of this study was to use ex vivo testing to determine the magnetic resonance imaging (MRI) safety aspects for seven different metallic neurosurgical implants in association with the 1.5-T MR environment. Ex vivo testing was performed using previously-described techniques for the evaluation of magnetic field interactions (deflection angle and torque), heating (gel-filled phantom and fluoroptic thermometry; 15 minutes of MRI at a specific absorption rate [SAR] of 1.4 W/kg), and artifacts (using T1-weighted, spin-echo and gradient-echo pulse sequences). None of the metallic implants displayed interactions with the magnetic field. The highest temperature change was +0.6 degrees C for the representative implant that was evaluated. Artifacts were relatively minor. The lack of magnetic field interactions and negligible heating indicate that MR procedures may be conducted safely in patients with these neurosurgical implants using MR systems with static magnetic fields of 1.5-T or less. Furthermore, these implants may be considered for use in interventional MR procedures insofar as the MR safe qualities and relatively small artifacts would likely be desirable for such procedures.     

Magnetic resonance imaging of prosthetic heart valves

To evaluate the safety of magnetic resonance (MR) imaging of prosthetic heart valves, nine different synthetic and tissue valves were studied ex vivo. Deflection was measured in 0.35-tesla (T) and 1.5-T superconducting magnets and at the edge of the bore of a 2.35-T electromagnet in field gradients of 5, 1.1, and 6.3 mT/cm, respectively. No valve deflected in the 0.35-T magnet; six synthetic valves deflected 0.25 degrees-3 degrees in the 1.5-T magnet; all valves deflected 1 degree-27 degrees at the edge of the 2.35-T magnet. Each valve was then submerged in a vial of water and the temperature was measured immediately before and after each of two spin-echo imaging sequences in the two superconducting magnets. No significant temperature rise followed exposure in either magnet. Image distortion varied from negligible to severe in both imagers; magnitude of distortion paralleled magnitude of deflection. These data suggest that patients with present-day prosthetic heart valves can be safely imaged in present-day MR imagers and that prosthesis-induced artifacts will not interfere with interpretation in most instances.     

Programmable CSF shunt valve: in vitro assessment of MR imaging safety at 3T

A programmable CSF shunt valve was assessed for magnetic field interactions, heating (transmit-receive body radio-frequency coil; whole-body averaged specific absorption rate, 2.1 W/kg), functional alterations, and artifacts at 3T. The programmable valve showed minor magnetic field interactions and heating was not excessive (+0.8 degrees C). The function of the programmable valve was not altered by multiple exposures to the 3T scanner or from exposure to various MR imaging conditions. Therefore, this implant is safe for a patient undergoing MR imaging at 3T or less when the radiologist follows specific safety guidelines. Artifacts for the programmable valve were relatively large in relation to the size and shape of the valve; this finding may impact the diagnostic use of MR imaging if the area of interest is in proximity to this implant.     

Increased corneal temperature caused by MR imaging of the eye with a dedicated local coil.

To determine the existence of tissue heating-associated risks to the eye with magnetic resonance (MR) imaging performed at high specific absorption rates (SARs), corneal temperature was measured in 14 patients immediately before and after MR imaging performed with a 1.5-T, 64-MHz unit and a quadrature-driven body coil for radio-frequency transmission and a receive-only local coil designed for eye imaging. Fast spin-echo pulse sequences were used predominantly. Estimated peak SARs ranged from 3.3 to 8.4 W/kg. A statistically significant (P < .001) increase in average corneal temperature (32.2 degrees C +/- 0.7 before imaging, 33.1 degrees C +/- 0.6 after) was associated with MR imaging of the eye. The changes in corneal temperature ranged from 0.2 degrees to 1.8 degrees C (average, 0.9 degrees C). The highest corneal temperature measured after MR imaging was 35.1 degrees C. MR imaging performed with a dedicated local coil at the SARs studied produced elevations in corneal temperature that were physiologically inconsequential and below the temperature threshold (41 degrees to 55 degrees C) for radio-frequency radiation-induced cataractogenesis.     

Low-field magnetic resonance imaging in the evaluation of mechanical and biological heart valve function

Magnetic resonance imaging (MRI) has been frequently considered unsafe for patients with ferromagnetic implants: risks to be considered include induction of electric current, heating and dislocation of the prosthesis. Previous in vitro and in vivo studies have indicated the possibility of performing MRI examinations on patients with prosthetic heart valves. The aim of our study was to verify the presence of artifacts at the level of the prosthetic heart valve in vivo using a low-field MR unit (0.2 T) and to define the possibility of a functional analysis of the valve in patients with biomedical or mechanical prostheses. We evaluated 14 patients surgically treated for implantation of nine biological and seven mechanical aortic and mitral valves. A low-field MR unit (0.2 T) was employed using cine-MR technique on long- and short-axis view. The images were acquired on planes parallel and perpendicular to the valvular plane. Semiquantitative analysis with double-blind evaluation for definition of the extent of the artifact was performed. Three classes of artifacts were distinguished from minimal to significant. The examinations showed the presence of minimal artifacts in all biological heart valves and moderate artifacts in mechanical valves giving good qualitative data on blood flow near the valve. Analysis of the flow behind the valve showed signs of normal function in 13 prostheses and pathological findings in the remaining three. In these latter cases, MRI was able to define the presence of a pathologic aortic pressure gradient, mitral insufficiency and malpositioning of the mitral valve causing subvalvular turbulence. Nevertheless, we believe that the application of velocity-encoding cine-MR is more promising than semiquantitative analysis of artifacts.     

Metallic stents: evaluation of MR imaging safety.

OBJECTIVE: The objective of our investigation was to evaluate safety during MR imaging (i.e., magnetic field interactions, heating, and artifacts) for metallic stents. MATERIALS AND METHODS: Different types of metallic stents were tested for magnetic field interactions, heating, and artifacts using a 1.5-T MR system. Magnetic field-related translational attraction and torque were assessed using previously described techniques. Heating was evaluated using an infrared thermometer to record temperatures immediately before and after performing MR imaging using a whole-body-averaged specific absorption rate of 1.3 W/kg. Artifacts were assessed by placing the stents inside a fluid-filled phantom and performing MR imaging using fast spoiled gradient-echo and T1-weighted spin-echo pulse sequences. RESULTS: For the 10 different stents evaluated, we found no magnetic field interactions. the highest temperature change was < or = +0.3 degrees C, and the artifacts involved signal voids that would not create diagnostic problems as long as the area of interest was not positioned exactly where a particular stent was located. CONCLUSION: The findings of the safety tests indicated that the 10 different metallic stents would be safe for patients undergoing MR imaging procedures using MR systems with static magnetic fields of 1.5 T or less.     

Heating of the scrotum by high-field-strength MR imaging

Sterility can occur in mammals if spermatogenic tissue is acutely or chronically heated to levels equal to or greater than body temperature. High-field-strength MR imaging has been shown to elevate tissue temperatures, particularly if high levels of RF radiation are used. To determine if MR imaging above the recommended level for RF radiation is associated with heating of the scrotum, scrotal skin temperatures were measured in eight subjects immediately before and after MR imaging of the scrotum with a 1.5-T, 64-MHz MR scanner at mean whole-body average specific absorption rates ranging from 0.56 to 0.84 W/kg (mean, 0.72 W/kg). The average imaging time was 23 min. A statistically significant (p less than .01) increase in average scrotal skin temperature was associated with MR imaging (before MR imaging, 30.8 degrees C; after MR imaging, 32.3 degrees C). The largest change in temperature was 3.0 degrees C, and the highest temperature measured was 34.1 degrees C. MR imaging at relatively high specific absorption rates produced a statistically significant increase in average scrotal skin temperature. However, the recorded temperatures were below the threshold known to affect spermatogenesis in mammals.     

Extrusion of eye socket magnetic implant after MR imaging: potential hazard to patient with eye prosthesis.

Metallic implants or prostheses can be potentially hazardous during magnetic resonance (MR) imaging because of movement or dislodgment of the foreign object. Magnetic eye implants have been reported to exhibit strong movement when placed in water and exposed to a field of 1.5 T. The authors report a case of orbital implant extrusion possibly caused by the movement of a magnetic orbital implant during MR imaging at 0.5 T.     

High-field-strength MR imaging and metallic biomedical implants: an ex vivo evaluation of deflection forces

Ferromagnetic biomedical implants are considered a contraindication for MR imaging primarily because of the potential hazards associated with their movement or dislodgment. Many metallic biomedical implants are composed of nonferromagnetic materials and do not present a danger to patients during MR imaging. Therefore, to evaluate the ferromagnetic qualities of 36 different metallic biomedical implants (four aneurysm clips, six hemostatic clips, four dental implants, seven prosthetic heart valves, eight orthopedic prostheses, one artificial urinary sphincter, three contraceptive diaphragms, and three cerebral ventricular shunt tube connectors) not previously evaluated with a high-field-strength MR system, we measured deflection forces at the portal of the magnet of a 1.5-T MR system. Fourteen of the 36 metallic biomedical implants were determined to be ferromagnetic as indicated by their deflection in the static magnetic field. However, only the four aneurysm clips (Drake, Mayfield, McFadden, and Sundt-Kees) had sufficient ferromagnetism to warrant exclusion of patients with these implants from imaging with a 1.5-T MR system because of the possibility of movement or displacement. The calculated deflection forces for these aneurysm clips were comparable with previously reported values of certain aneurysm clips that have been designated to present a risk for patients undergoing MR imaging. Patients with 32 of 36 metallic biomedical implants tested can be safely imaged with high-field-strength MR systems.     

Spiral K-space MR imaging of cortical activation

Brain function can be mapped with magnetic resonance (MR) imaging sensitized to regional changes in blood oxygenation due to cortical activation. Several MR imaging methods, including conventional imaging and echo-planar imaging, have been successfully used for this purpose. The authors investigated spiral k-space MR imaging, implemented with an unmodified 1.5-T clinical imager, for imaging of cortical activation. A gradient-echo, spiral k-space imaging method was used to measure activation in the primary visual cortex (number sequence task), primary motor cortex (fist-clenching task), and prefrontal cortex (verbal fluency task). Comparison of conventional and spiral k-space imaging in the visual and motor cortex, in which signal-to-noise ratio, voxel size, and imaging time were matched, showed that artifacts were reduced with the spiral k-space method, while the area and degree of activation were similar. The number of sections that could be imaged in a fixed time interval was increased by a factor of four with this implementation of spiral k-space imaging compared with conventional imaging.     

Neurostimulation systems for deep brain stimulation: in vitro evaluation of magnetic resonance imaging-related heating at 1.5 tesla

PURPOSE: To assess magnetic resonance imaging (MRI)-related heating for a neurostimulation system (Activa Tremor Control System, Medtronic, Minneapolis, MN) used for chronic deep brain stimulation (DBS). MATERIALS AND METHODS: Different configurations were evaluated for bilateral neurostimulators (Soletra Model 7426), extensions, and leads to assess worst-case and clinically relevant positioning scenarios. In vitro testing was performed using a 1.5-T/64-MHz MR system and a gel-filled phantom designed to approximate the head and upper torso of a human subject. MRI was conducted using the transmit/receive body and transmit/receive head radio frequency (RF) coils. Various levels of RF energy were applied with the transmit/receive body (whole-body averaged specific absorption rate (SAR); range, 0.98-3.90 W/kg) and transmit/receive head (whole-body averaged SAR; range, 0.07-0.24 W/kg) coils. A fluoroptic thermometry system was used to record temperatures at multiple locations before (1 minute) and during (15 minutes) MRI. RESULTS: Using the body RF coil, the highest temperature changes ranged from 2.5 degrees-25.3 degrees C. Using the head RF coil, the highest temperature changes ranged from 2.3 degrees-7.1 degrees C.Thus, these findings indicated that substantial heating occurs under certain conditions, while others produce relatively minor, physiologically inconsequential temperature increases. CONCLUSION: The temperature increases were dependent on the type of RF coil, level of SAR used, and how the lead wires were positioned. Notably, the use of clinically relevant positioning techniques for the neurostimulation system and low SARs commonly used for imaging the brain generated little heating. Based on this information, MR safety guidelines are provided. These observations are restricted to the tested neurostimulation system.     

Hr imaging and vascular access ports: Ex vivo evaluation of ferromagnetism,heating, and artifacts at 1.5 t

The purpose of our study was to asacsa Futomagneh. hating. ad artifactr, associated with vabcular access ports exposed to a 1.5-T MR system. Tweaty-cight different vabcular access port. were evaluated in thb investlgation. Fmmagnetism was determined by using two previouely de techniques. Temperature changes were measured immediately before and after performing a pulse sequence on the vascular access port6 for 80 min at a speciflc absorption rate of 3.1 W/kg. Artifacts were assessed h aesoclation with the use of a fht CRASS pulse sequence. None of the vascular access ports displayed ferromnlpletim. Heating was 0.2°or less. The prerence of adhcts varied. depending on the component materials. The Lack of fcrromagnetiam and negligible heating indicates that MR imaging performed at 1.6 T or lees may be conducted dely in patients with any of the vascular access port. tested. None of the associated artifacts produced by the vascdar access port. is comddercd to paw a rubsttantial problem for MR imaging.     

Single-shot versus interleaved echo-planar MR -imaging: Application to visualization of cardiac valve leaflets

Visualization of the cardiac valves with standard magnetic resonance (MR) imaging is not adequate because of long acquisition times. Echo-planar imaging (PI) can, however, be performed with a temporal resolution (30–50 msec) comparable to that of echocardiography. The authors evaluated the feasibility of real-time imaging of cardiac valve motion with ultrafast MR techniques. Eight healthy volunteers and three patients with mitral stenosis and re- gurgitation were studied with a 1.5-T whole-body im- ager. Two different EPI sequences were assessed: a standard single-shot gradient-echo EPI (GEPI) SCquence and a fast imaging technique based on multiple-shot EPI with interleaved k-space acquisition (IGEPI). Fat-suppressed images with an in-plane resolution of 3.7 × 3.7 mm were obtained equally spaced through the cardiac cycle. Half-k-space acquisition was used. Morphologic evaluation was superior with IGEPI, owing to the better intracavitary signal homogeneity (P ≤ 0.01). and the mitral valve leaflets were easier to identify on systolic images. IGEPI provided adequate valve visibility in all three patients.     

Magnetic resonance safety testing of a newly-developed fiber-optic cardiac pacing lead

PURPOSE: To assess magnetic resonance (MR) safety for a newly developed, fiber-optic cardiac pacing lead. MATERIALS AND METHODS: MR safety was assessed for the fiber-optic cardiac pacing lead by evaluating magnetic field interactions and heating. Translational attraction and torque were evaluated using a 1.5-Tesla MR system and previously described, standardized techniques. MR imaging-related heating was assessed using a 1.5-Tesla MR system and a transmit/receive, body radiofrequency (RF) coil with the fiber-optic lead positioned to simulate an in vivo condition in a saline-filled phantom. The phantom had dimensions similar to a human subject's torso and head. A fluoroptic thermometry system was used to record temperatures on and near the electrodes of the fiber-optic pacing lead at five-second intervals immediately before and during 20 minutes of MR imaging performed at a whole-body-averaged specific absorption rate (SAR) of 1.5 W/kg. Temperatures were also recorded from a reference site during this experiment. RESULTS: Magnetic field interactions for the fiber-optic lead were minimal (deflection angle, 23 degrees; torque, +2). The highest temperature change recorded for the fiber-optic cardiac pacing lead and reference site was +0.8 degrees C. CONCLUSION: The minor magnetic field interactions and relative lack of heating for the fiber-optic pacing lead indicate that it should be safe for patients with this device to undergo MR imaging procedures using MR systems operating at 1.5-T or less and at a whole-body-averaged SARs up to 1.5 W/kg.     

Ex vivo evaluation of ferromagnetism and artifacts of cardiac occluders exposed to a 1.5-T MR system

Magnetic resonance (MR) procedures are contraindicated for patients with certain ferromagnetic biomedical implants, primarily owing to the risk of movement or dislodgment of the implants by the static magnetic field. An additional concern is the amount of artifact that the implant produces, which can affect image quality and interpretation of the examination. Therefore, an ex vivo assessment of ferromagnetism and artifact was conducted for 12 different occluders used to treat patients with patent ductus arteriosus, atrial septal defects, and ventricular septal defects, in a 1.5-T MR system. Seven of the occluders, made of 304 stainless steel, were ferromagnetic and displayed deflection forces of 248–299 dynes. Five of the implants, made of MP35n, were nonferromagnetic. Artifacts were variable and depended primarily on the type and amount of metal used to construct the implant. The authors conclude that patients with ferromagnetic cardiac occluders may undergo MR procedures approximately 6 weeks after placement of these devices, to allow tissue growth to provide additional retentive force. After this time, it is highly unlikely that the magnetic fields associated with a 1.5-T MR system are capable of moving or dislodging any of these cardiac occluders.     

Coronary arterial stents: safety and artifacts during MR imaging

PURPOSE: To investigate the safety and imaging artifacts with different coronary arterial stents and magnetic resonance (MR) imaging sequences. MATERIALS AND METHODS: The heating, artifacts, and ferromagnetism with different stents were studied with a 1.5-T MR tomograph with ultrafast gradients by using turbo spin-echo, turbo gradient-echo, and echo-planar imaging sequences. Nineteen stents, which were 8-25 mm in length and 3.0-4.5 mm in diameter, were evaluated. Stent deviation induced by the magnetic field and during MR imaging, migration, and heating caused by the radio-frequency pulses were examined. The size of imaging artifacts was measured with all the stents under standardized conditions and with six stents after their implantation into the coronary arteries of freshly explanted pig hearts. RESULTS: All except two types of stents showed minimal ferromagnetism. No device migration or heating was induced. Turbo spin-echo images had minimal artifacts; larger artifacts were seen on the turbo gradient-echo and echo-planar images. With ultrafast gradients, the artifacts on the echo-planar images were substantially reduced. CONCLUSION: The studied coronary stents were not influenced by heating or motion during 1.5-T MR imaging. Artifact size differed according to the type and size of the stent and the MR imaging sequence used. Thus, patients with these stents can be safely examined.     

MR imaging artifacts, ferromagnetism, and magnetic torque of intravascular filters, stents, and coils

Experiments were conducted in which various intravascular filters, stents, and coils were imaged using magnetic resonance (MR) spin-echo technique at 0.35 T. These devices were also evaluated for ferromagnetism (at 0.35, 1.5, and 4.7 T), magnetic torque (at 0.35 and 1.5 T), and magnetically induced migration within a plastic tube (at 0.35 and 1.5 T for the Greenfield filter [GF]). The stainless-steel GF was evaluated in vitro for its propensity to perforate canine inferior venae cavae (IVC). Magnetic force and torque at 1.5 T did not dislodge the GF or result in perforation of canine IVC by the GF. Beta-3 titanium alloy (used in a new percutaneous version of the GF) is apparently one of the best-suited metals for use with MR imaging because of its lack of ferromagnetism (up to 4.7 T) and absence of MR imaging artifacts (at 0.35 T). Devices composed of Elgiloy (Mobin-Uddin filter), nitinol, and MP32-N (Amplatz filter) alloys all created mild artifacts. Devices fashioned from 304 and 316L (GF and Palmaz stent) stainless-steel alloys created severe "black-hole" artifacts, with the 304 alloy devices also showing marked image distortion. Generally, the greater the ferromagnetism of a device, the greater its magnetic susceptibility artifact.     

Dependence of RF heating on SAR and implant position in a 1.5T MR system

PURPOSE: We evaluated radiofrequency (RF) heating of a humerus implant embedded in a gel phantom during magnetic resonance (MR) imaging for the specific absorption rate (SAR), angle between the implant and static magnetic field (B(0)), and position of the implant in the irradiation coil. METHODS: We embedded a stainless steel humerus implant 2 cm deep in tissue-equivalent loop and mass phantoms, placed it parallel to the static magnetic field of a 1.5T MR scanner, and recorded the temperatures of the implant surface with RF-transparent fiberoptic sensors. We measured rises in temperature at the tips of the implant by varying the SAR from 0.2 to 4.0 W/kg and evaluated RF heating of the implant for its angle to B(0) and its displacement along B(0) from the center of the RF irradiation coil. RESULTS: RF heating was similar for the loop and mass phantoms because the eddy current flows through the periphery of both. As the SAR increased, the temperature at the implant tip increased, and there was a linear relationship between the SAR and temperature rise. The values were 6.4 degrees C at 2.0 W/kg and 12.7 degrees C at 4.0 W/kg. Rise in temperature decreased steeply as the angle between the implant and B(0) surpassed 45 degrees . In addition, as the implant was displaced from the center of the RF coil to both ends, the rise in temperature decreased. CONCLUSION: The rise in temperature in deep tissue was estimated to be higher than 1.0 degrees C for SAR above 0.4 W/kg. RF heating was greatest when the implant was set parallel to B(0). In MR imaging of patients with implants, there is a risk of RF heating when the loop of the eddy current is formed inside the body.     

Björk-Shiley convexoconcave valves: susceptibility artifacts at brain MR imaging and mechanical valve fractures

PURPOSE: To assess the relationship between heart valve history and susceptibility artifacts at magnetic resonance (MR) imaging of the brain in patients with Bj?rk-Shiley convexoconcave (BSCC) valves. MATERIALS AND METHODS: MR images of the brain were obtained in 58 patients with prosthetic heart valves: 20 patients had BSCC valve replacements, and 38 had other types of heart valves. Two experienced neuroradiologists determined the presence or absence of susceptibility artifacts in a consensus reading. Artifacts were defined as characteristic black spots that were visible on T2*-weighted gradient-echo MR images. The statuses of the 20 explanted BSCC valves-specifically, whether they were intact or had an outlet strut fracture (OSF) or a single-leg fracture (SLF)-had been determined earlier. Number of artifacts seen at brain MR imaging was correlated with explanted valve status, and differences were analyzed with nonparametric statistical tests. RESULTS: Significantly more patients with BSCC valves (17 [85%] of 20 patients) than patients with other types of prosthetic valves (18 [47%] of 38 patients) had susceptibility artifacts at MR imaging (P =.005). BSCC valve OSFs were associated with a significantly higher number of artifacts than were intact BSCC valves (P =.01). No significant relationship between SLF and number of artifacts was observed. CONCLUSION: Susceptibility artifacts at brain MR imaging are not restricted to patients with BSCC valves. These artifacts can be seen on images obtained in patients with various other types of fractured and intact prosthetic heart valves.