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Published online before print January 22, 2004, 10.1148/radiol.2303021619
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(Radiology 2004;230:709-714.)
© RSNA, 2004

Neuroradiology

Björk-Shiley Convexoconcave Valves: Susceptibility Artifacts at Brain MR Imaging and Mechanical Valve Fractures1

Maarten J. van Gorp, MD, Yolanda van der Graaf, MD, PhD, Bas A. J. M. de Mol, MD, PhD, Chris J. G. Bakker, PhD, Theo D. Witkamp, MD, Lino M. P. Ramos, MD and Willem P. T. M. Mali, MD, PhD

1 From the Julius Center for Health Sciences and Primary Care (M.J.v.G., Y.v.d.G.) and Department of Radiology (C.J.G.B., T.D.W., L.M.P.R., W.P.T.M.M.), University Medical Center Utrecht, Heidelberglaan 100, PO Box 85500, Room D01.335, 3508 GA Utrecht, the Netherlands; and Department of Cardiothoracic Surgery, Academic Medical Center, Amsterdam, the Netherlands (B.A.J.M.d.M.). From the 2000 RSNA scientific assembly. Received November 26, 2002; revision requested February 20, 2003; final revision received July 3; accepted July 28. Address correspondence to Y.v.d.G. (e-mail: Y.vanderGraaf@jc.azu.nl).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
PURPOSE: To assess the relationship between heart valve history and susceptibility artifacts at magnetic resonance (MR) imaging of the brain in patients with Björk-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.

© RSNA, 2004

Index terms: Brain, MR, 13.121411, 13.121412 • Heart, prostheses, 51.4534 • Heart, valves, 51.4534 • Magnetic resonance (MR), artifact, 13.121411, 13.121412


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Björk-Shiley convexoconcave (BSCC) heart valves (Shiley, Irvine, Calif) are associated with an increased risk of mechanical failure with potentially catastrophic consequences for the patient (1). Owing to manufacturing deficiencies, the outlet strut of the BSCC valve is susceptible to fractures at both welding points of the legs of the strut (2). Fracture of a single leg can eventually result in an outlet strut fracture (OSF) with consequent strut and disk embolization (Fig 1) (3,4). A variety of imaging and acoustic techniques for the detection of these single-leg fractures (SLF) have been suggested, but thus far they have not been implemented on a large scale (5,6). Thus, epidemiologic decision models are currently the best alternative methods for identifying high-risk valves and for use in prophylactic valve explantations to prevent calamities (7,8).



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  		Figure 1a.  (a) BSCC heart valve with large inlet strut and small outlet strut (arrows); the positions of the struts relative to the disk are shown. (b) Scanning electron microscopic image (magnification, x7.5) of an explanted BSCC valve with an SLF (arrow) of the outlet strut. (c) Scanning electron microscopic image (magnification, x60) of one leg of the outlet strut of an explanted BSCC valve with an SLF. (d) Scanning electron microscopic image (magnification, x60) of one end of a fractured outlet strut.

 


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  		Figure 1b.  (a) BSCC heart valve with large inlet strut and small outlet strut (arrows); the positions of the struts relative to the disk are shown. (b) Scanning electron microscopic image (magnification, x7.5) of an explanted BSCC valve with an SLF (arrow) of the outlet strut. (c) Scanning electron microscopic image (magnification, x60) of one leg of the outlet strut of an explanted BSCC valve with an SLF. (d) Scanning electron microscopic image (magnification, x60) of one end of a fractured outlet strut.

 


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  		Figure 1c.  (a) BSCC heart valve with large inlet strut and small outlet strut (arrows); the positions of the struts relative to the disk are shown. (b) Scanning electron microscopic image (magnification, x7.5) of an explanted BSCC valve with an SLF (arrow) of the outlet strut. (c) Scanning electron microscopic image (magnification, x60) of one leg of the outlet strut of an explanted BSCC valve with an SLF. (d) Scanning electron microscopic image (magnification, x60) of one end of a fractured outlet strut.

 


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  		Figure 1d.  (a) BSCC heart valve with large inlet strut and small outlet strut (arrows); the positions of the struts relative to the disk are shown. (b) Scanning electron microscopic image (magnification, x7.5) of an explanted BSCC valve with an SLF (arrow) of the outlet strut. (c) Scanning electron microscopic image (magnification, x60) of one leg of the outlet strut of an explanted BSCC valve with an SLF. (d) Scanning electron microscopic image (magnification, x60) of one end of a fractured outlet strut.

 
Foreign surgical or implanted material that has reached the blood circulation has been detected with magnetic resonance (MR) imaging in several studies. Very small metal and carbide particles from knives, drills, and suction tools used for neurosurgery can cause large susceptibility artifacts on postoperative MR images (9,10). Similar artifacts were found on MR images of the brain obtained in patients with prosthetic heart valves (11,12). The case reports indicate that microscopic valve particles could have been responsible for the artifacts observed. Although none of the reports is conclusive as to the true origin of the artifacts, it is suggested that brain MR imaging is sensitive enough to depict possibleembolic valve particles and therefore may serve as a noninvasive method to screen for faulty BSCC valves.

The primary purpose of this study was to assess the relationship between heart valve history and susceptibility artifacts at MR imaging of the brain in patients with BSCC valves.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Study Patients
Fifty-nine volunteers gave their informed consent to participate in this study, which was approved by the medical ethics committee of University Medical Center Utrecht. The volunteers consisted of two groups: patients with BSCC valve replacements and patients with other types of prosthetic valves. Twenty-one of the 59 patients had a history of BSCC valve replacement. Their BSCC valves had been explanted owing to fracture or for prophylactic replacement (ie, when valve was removed to prevent disease): Three valves with an OSF, seven valves with an SLF, and 11 intact valves were removed and replaced. All explanted valves (including non-BSCC devices) were replaced with a Sorin (Sorin Biomedica, Saluggia, Italy), St Jude (St Jude Medical, St Paul, Minn), or Medtronic-Hall (Medtronic, Minneapolis, Minn) heart valve.

The patients with BSCC valve replacements who were invited to participate in the study were identified by using information from a Dutch BSCC cohort follow-up study (1), in which the records of all patients in the Netherlands who had had BSCC valves were retrospectively reviewed. The only inclusion criterion for the present study was that information on the wear and fracture(s) of the BSCC valve explanted from the patient had to be available in the Dutch cohort follow-up records. The only exclusion criterion was the presence of a biomedical implant or implanted metallic device, such as a pacemaker, that is a contraindication to MR imaging. With the given inclusion and exclusion criteria, 21 patients with BSCC valve replacements were eligible for our study.

Thirty-eight patients with various other types of prosthetic heart valves were identified by reviewing the records of the cardiothoracic department of the Academic Medical Center and were invited to participate in the study. These 38 patients were all who met the criteria during the study period and gave their informed consent. Patients were selected owing to the type of implanted prosthetic valve they had: Twenty patients had Medtronic-Hall, eleven had St Jude, five had Sorin, and two had Edwards Duromedics (American Edwards Laboratories, Irvine, Calif) valves. The two patients with Edwards Duromedics valves had a history of emergency valve replacement owing to fractured disks. With the exception of the presence of an implanted device that contraindicated the use of MR imaging, there were no other exclusion or inclusion criteria.

Study participation was voluntary and had no clinical consequences for any of the patients involved. All patients underwent anticoagulant treatment at the time of MR imaging. In one patient with an explanted BSCC valve that contained an SLF, the examination could not be completed because of claustrophobia. This patient’s data were therefore excluded from further analyses. Thus, a total of 58 volunteer patients—30 men and 28 women—underwent imaging. The ages of the men ranged from 40 to 75 years (mean age, 56.5 years), and the ages of the women ranged from 45 to 75 years (mean age, 58.6 years). Twenty of the 58 volunteer patients had a BSCC prosthetic heart valve replacement, 20 had a Medtronic-Hall valve, 11 had a St Jude valve, five had a Sorin valve, and two had an Edwards Duromedics prosthetic heart valve replacement. Thirty-one valves were in a mitral position, and 27 were in an aortic position.

Assessment of Explanted Valves
The technical status of all explanted valves (BSCC and Edwards Duromedics devices)—that is, whether the devices were intact or had an SLF or an OSF—had been documented earlier, with use of scanning electron microscopy, by one of the authors (B.A.J.M.d.M.) and a physicist with metallurgical expertise. SLF was defined as a fracture encompassing the full width and thickness of the strut leg. These fractures can be present with or without substantial strut leg displacement. SLFs can sometimes be invisible to the naked eye, but they are always visible at magnification of x30 and they cannot be missed at scanning electron microscopy. OSFs of valve prostheses are easy to identify because the minor strut is broken from the metal flange and the disk of the prosthesis escapes. All explanted valves were also inspected for wear marks due to the valve-opening impact of the disk on the outlet strut (13).

MR Imaging
Protocol testing.—All MR imaging examinations were performed with a 1.5-T system (Gyroscan ACS NT; Philips Medical Systems, Best, the Netherlands) by using a quadrature head coil as the receiver. We used spin-echo and gradient-echo techniques to determine the presence or absence of susceptibility artifacts. Susceptibility artifacts are dominated by signal intensity distortion (eg, white halo) at spin-echo MR imaging and by the T2* effect at gradient-echo MR imaging. Gradient-echo MR imaging is the most sensitive technique; artifacts are detected more easily with this sequence. Because larger artifacts are also visible at spin-echo MR imaging, this technique can be used as a crude method of assessing the size of artifacts. A gap of 1 mm was used to prevent interference between adjacent sections (to make the acquisition adaptable to nonideal section profiles). It is unlikely that a relevant area of dephasing will be missed at gradient-echo MR imaging because the artifact generally extends well beyond the size of the causative particle on these images.

For unambiguous scoring of artifacts, the artifacts detected on coronal gradient-echo MR images were cross-referenced with the artifacts detected on transverse gradient-echo MR images. In the protocol-testing phase, a three-dimensional MR imaging technique also was used, but it yielded oversensitive images with many false-positive susceptibility artifacts, such as flow voids, which would have led to lower specificity. At the start of the study, the MR imaging protocol was tested, and repeated imaging in the same patient within a few months revealed the same amount of artifacts. All of the prosthetic valves were compatible with the MR imaging unit. Earlier evaluation of various prosthetic heart valves in a 1.5-T static magnetic field revealed small deflection forces, but these were minimal compared with the forces on the valves in beating hearts (14).

Patient imaging.—The final MR imaging protocol used included transverse T1-weighted spin-echo (598/15 [repetition time msec/echo time msec], 90° flip angle, two signals acquired), T2-weighted spin-echo (1,380/60, 90° flip angle, two signals acquired), and transverse and coronal T2*-weighted gradient-echo (710/27.6, 20° flip angle, two signals acquired) sequences. Other MR imaging parameters included a section thickness of 5 mm, an intersection gap of 1 mm, a field of view of 230 mm, a matrix size of 256 x 256 pixels, and 19 sections acquired at each pulse sequence.

Patient Questioning
After the MR images were obtained, all patients were asked specific questions regarding their medical history, including information about previous cerebrovascular events, hypertension, brain injury, brain surgery, comorbidities, other surgical procedures, and anticoagulant treatment, and about current and previous medications. These data were in addition to the clinical information obtained from the patients’ medical records, which were assessed before the MR imaging examinations were performed. The medical records included the following information: the presence of other implanted biomedical devices that might have contraindicated the use of the MR imaging unit, the operation characteristics of and complications associated with the given prosthetic valve, the total implantation time (ie, the total number of years the patient had had a valve implant, including the length of time he or she had had a BSCC or other type of valve implant plus the length of time he or she had had a replacement valve, if applicable), and the medical history, which could be used as a reference for the answers given by the patients.

Both the patient questioning and the medical record search were conducted by the same physician (M.J.v.G.). Patients were considered to be hypertensive if they had a clinical history of hypertension for which they were treated with antihypertensive medication. Transient ischemic attacks were defined as symptoms that were presumably caused by an ischemic cerebrovascular condition and that lasted less than 24 hours.

MR Image Interpretation
Two neuroradiologists (T.D.W., L.M.P.R.), each with more than 15 years of experience in neuroradiology and in interpreting MR images, interpreted all of the MR images and were blinded to the patients’ clinical data and valve statuses. Artifacts were assessed in a consensus reading. Artifacts were defined as round black spots and were evaluated if they were visible on the gradient-echo images. Artifacts were considered to be large when they could also be discerned on the corresponding T2-weighted spin-echo images. Artifacts that were visible on only the gradient-echo images were considered to be small. The number of artifacts (large and small) seen on the MR images obtained in each patient was documented. Structures, tissue, or other entities resembling artifacts, such as calcium, air, and small arteries (ie, flow voids) were carefully excluded.

Statistical Analyses
Differences in baseline characteristics between the patients with BSCC valve replacements and those with other types of valves were tested by using {chi}2 and two-tailed Mann-Whitney analyses. The numbers of patients with susceptibility artifacts in each of these two groups were determined, and the differences were tested by using {chi}2 analysis. Differences in the mean number of artifacts among the BSCC subgroups (ie, patients with intact valves, valve SLFs, and valve OSFs) were calculated and tested with Kruskal-Wallis analysis. Subsequently, the Mann-Whitney test was used to identify the groups that differed significantly from each other. This analysis was performed with the large and small artifacts combined and with the large artifacts only. Finally, the mean age of and mean implantation time for the patients with and those without artifacts were calculated and tested by using the two-tailed Mann-Whitney test. In these statistical analyses (SPSS, version 10; SPSS, Chicago, Ill), P <= .05 indicated a significant difference.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
 REFERENCES
 
The mean total implantation time was longer for the patients with BSCC valve replacements (11.3 years) than for those with other types of valves (8.5 years) (Table 1); the difference was statistically significant (P = .03). The patients had had BSCC replacement valves for a mean total time of 5.3 years. The patients with other types of prosthetic heart valves were older than those with BSCC valve replacements (P = .02). The majority of patients with other types of prosthetic heart valves had valve implants in a mitral position, whereas most of the valve implants in the BSCC valve group were in an aortic position (P = .01). Demographic data on the patients are given in Table 1. Figures 2 and 3 show gradient-echo MR images obtained in two patients: one patient with a BSCC valve OSF (Fig 2) and another in whom an intact BSCC valve was explanted for prophylactic reasons (Fig 3).


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  		TABLE 1. Baseline Characteristics of Patient Volunteers

 


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  		Figure 2.  Transverse T2*-weighted gradient-echo MR image (710/27.6) of the brain obtained in 58-year-old man with a history of BSCC valve OSF shows two susceptibility artifacts (arrows).

 


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  		Figure 3a.  MR images of the brain obtained in 74-year-old man in whom intact BSCC valve was removed for prophylactic reasons. (a) Transverse T2*-weighted gradient-echo image (710/27.6) shows susceptibility artifact (arrow). (b) Corresponding coronal T2*-weighted gradient-echo image (710/27.6) shows the typical dipolar distortion generated by the same artifact (arrow).

 


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  		Figure 3b.  MR images of the brain obtained in 74-year-old man in whom intact BSCC valve was removed for prophylactic reasons. (a) Transverse T2*-weighted gradient-echo image (710/27.6) shows susceptibility artifact (arrow). (b) Corresponding coronal T2*-weighted gradient-echo image (710/27.6) shows the typical dipolar distortion generated by the same artifact (arrow).

 
The numbers of patients with MR imaging artifacts in each valve group are listed in Table 2. Artifacts were seen on the MR images obtained in 35 (60%) of the 58 patients. Seventeen (85%) of the 20 patients in the BSCC valve replacement group had artifacts. Eighteen (47%) of the 38 patients in the other valve group had artifacts. The difference in total number of patients with artifacts between the BSCC group and the other valve group was statistically significant (P = .005). The mean total number of MR imaging artifacts per patient with prosthetic heart valve was 1.7 ± 2.3 (SD) (95% CI: 1.1, 2.4); there was a maximum of 10 artifacts per patient.


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  		TABLE 2. MR Imaging Susceptibility Artifacts

 
The statuses of the initially implanted prosthetic valves and the corresponding mean total numbers of artifacts seen at brain MR imaging are listed in Table 3. There were statistically significant differences in the mean total number of artifacts between the intact BSCC valve group and the BSCC valve OSF group (P = .01 for all [ie, large and small] artifacts, P = .005 for large artifacts only) and between the BSCC valve SLF group and the BSCC valve OSF group (P = .02 for all artifacts and for large artifacts only). There was no significant difference in the mean total number of artifacts between the intact BSCC valve and BSCC valve SLF groups (P > .999 for all artifacts, P = .7 for large artifacts only). The patients without artifacts were older (mean age, 61 years) than those with artifacts (mean age, 55.3 years) (P = .01). The mean total implantation time for the patients with artifacts was longer (10.2 years) than that for the patients without artifacts (8.2 years); however, the difference was not statistically significant (P = .07).


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  		TABLE 3. Statuses of Prosthetic Valves and Numbers of Susceptibility Artifacts

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
REFERENCES
 
In this study, the MR images of the brain obtained in patients with prosthetic heart valves showed susceptibility artifacts in 35 (60%) of 58 patients. Although OSF was significantly associated with high number of artifacts, the MR imaging artifacts were not restricted to patients with a history of fractured BSCC heart valves.

Cerebral areas of hypointensity seen on T2*-weighted gradient-echo MR images obtained in various patient populations have been described. These small round black spots can be identified at MR imaging in patients with chronic hypertension and appear to be associated with the occurrence of primary intracerebral hemorrhage (15). Microangiopathy is the suggested mechanism by which small intracerebral hemorrhages, or microbleeds, are generated. In a relatively recent study (16) involving 104 patients with primary intracerebral hemorrhage, microbleeds were observed in more than 50% of the cases. In addition, the presence of susceptibility artifacts could serve as a marker for both the presence of small fragile arteries and the risk of subsequent larger intracranial hemorrhages (17–20).

The risk of cerebral hemorrhage for patients with mechanical heart valves is apparent owing to the intensive anticoagulant treatment that they receive. If the artifacts that we observed were caused by microbleeds, then monitoring of anticoagulant treatment should be intensified in patients with multiple artifacts. However, the morphologic features of susceptibility artifacts typically are not representative of microbleeds, which often are smaller than the artifacts observed in this study and do not have a well-defined circular shape.

The size and shape of the artifacts that we observed suggest the presence of ferromagnetic substances, which prosthetic heart valves are also composed of. We believe that the typical dipolar shape of the susceptibility artifacts observed in this study is highly suggestive of ferromagnetic material. Wingerchuk et al (12) in their case report speculated about the origin of artifacts seen on the MR images obtained in a patient with a prosthetic heart valve, and they ruled out calcium, air, and microhemorrhages. They stated that valve wear or the surgical material used at cardiac surgery could have been responsible for these typical artifacts. In another case report, Naumann et al (11) described the same brain MR imaging artifacts in a patient with a prosthetic heart valve.

The flange of the BSCC valve is made of a chromium cobalt alloy (Haynes 25; Haynes International, Kokomo, Indiana) that contains 4% ferrite. The disk, or occluder, is made of pyrolytic carbon. Both materials have a high susceptibility for and can induce the type of artifacts observed in this study. The other types of prosthetic heart valves used in this study are made of comparable components, each with its own degree of susceptibility, and therefore may cause similar artifacts when the valve particles are released.

Another possible mechanism that may be a plausible explanation for the release of valve particles is the hydrodynamic properties of mechanical heart valves. Prosthetic heart valves have been reported to initiate gaseous microbubbles, or so-called high-intensity transient signals, that can be detected with transcranial Doppler ultrasonography (21,22). Cavitation initiation at the site where the occluder hits the stop of the valve is probably the mechanism that leads to the formation of microbubbles. Cavitation bubble initiation or subsequent collapse can induce pitting on the surface of the valve. An irregular valve surface with numerous micropores provides nucleation sites for cavitation bubbles and is more susceptible to cavitation forces. Pitting damage in explanted BSCC valves has been frequently noted and has been described in other types of valves. This could explain the presence of susceptibility artifacts in the patients with intact functional prosthetic valves in our study.

In general, biomedical devices that are implanted in the brain, such as the clips and coils used to treat aneurysms, can produce regions of signal intensity loss at brain MR imaging (23–25). Because of the thorough safety screening procedures performed before MR imaging, these susceptibility artifacts, when seen on a MR images, generally are easy to recognize and are related to the device in question.

There were several limitations to this study. First, our sample size was relatively small and may have been too small for us to discern several associations between patient or valve characteristics and outcome. Nevertheless, to our knowledge, this is the first study to focus on the brain MR imaging artifacts in patients with prosthetic heart valves. Second, the lack of preoperative brain MR images for a good comparison may have limited the conclusiveness of the results that we observed. The possibility that the artifacts that we observed on MR images were due to sources that were not investigated in this study should also be considered.

We conclude that susceptibility artifacts can be seen at MR imaging of the brain in patients with prosthetic heart valves and may, to some extent, reflect valve damage. However, the early detection of SLFs at brain MR imaging in patients with BSCC valves seems unlikely at this point.


   FOOTNOTES
 
Abbreviations: BSCC = Björk-Shiley convexoconcave, OSF = outlet strut fracture, SLF = single-leg fracture

Author contributions: Guarantor of integrity of entire study, M.J.v.G.; study concepts and design, all authors; literature research, M.J.v.G., B.A.J.M.d.M., C.J.G.B.; clinical studies, M.J.v.G., B.A.J.M.d.M., T.D.W., L.M.P.R.; data acquisition, M.J.v.G.; data analysis/interpretation, M.J.v.G., Y.v.d.G., B.A.J.M.d.M.; statistical analysis, M.J.v.G., Y.v.d.G.; manuscript preparation and editing, M.J.v.G., Y.v.d.G.; manuscript definition of intellectual content, revision/review, and final version approval, all authors


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 

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